LECTURE NOTES Dental Material Science I
DENTAL TECHNOLOGY DIPLOMA
General Introduction Welcome to Dental Materials Science. Please have the notes with you during lectures, when the material will be further explained. Although you may find taking some notes is useful to give an extra view on some points, these notes cover all the material you will need to pass the module. You will not need to take any notes during the lecture, which will go over the material in these notes again by a Powerpoint presentation, or on the board. You may also find it useful to purchase the recommended materials science textbook, or to read the reference textbooks in the library. These books have been listed in the resources section which follows shortly. These notes have also been designed to allow you to study and learn the material away from class. Being able to cover the material at your own speed and with your own pattern of learning is beneficial for many students. To help with this, there are questions typical of some of those asked in the examinations at the end of each section. These allow you to check your knowledge of each section as you proceed.
Module Overview This module has been designed to build your knowledge of atoms, material structure, chemical bonding, and properties of materials. This knowledge leads to a better understanding of the chemical, physical and mechanical properties of materials. In later sections of the course this knowledge will be valuable in understanding the reasons for using a particular dental restorative material, and the techniques necessary to fabricate it.
i Dental Material Science
Recommended Textbook Title: Author: Publisher: ISBN:
Dental Materials properties and Manipulation. R.C. Craig, J.M.Powers, & J.C. Wataha. Mosby 0-323-02520-X
Reference Textbook Title: Author: Publisher: Pub Place: Pub Date: ISBN:
Dental Materials: Properties and Selection. O’Brien, William, J. Quintessence Publishing Company Chicago 1989 0867151994
Your college may have a copy that you can borrow or you can purchase it yourself.
ii Dental Material Science
iii Dental Material Science
iv Dental Material Science
LEARNING OUTCOME 1
CLASSIFICATION OF MATTER.
Assessment criteria: You will have achieved this learning outcome when you can: •
Distinguish the different states of matter.
•
Classify matter as elements, mixtures or compounds.
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TOPIC 1 - Classification of Matter.
Recommended Time - 2 hrs
Introduction In explaining to you the properties and use of various dental materials, we need first to understand what material, or matter, is. Scientists describe matter as belonging to different types in order to help understand its properties, For example, we often divide solid materials into metals, plastics (polymers) or ceramics, a classification we will learn more about later. Topic 1 introduces you to some of the ways scientists describe and classify matter. You will learn about: •
States of matter (ways in which matter can exist)
•
Classification of matter as elements, mixtures or compounds.
Matter Matter is defined as anything which has mass and occupies a volume. Mass is the amount of material present. For example, when you see bubbles in a liquid, the bubbles have a volume, and the mass of the air can be determined. At the same time, you can observe differences between a gaseous and a liquid state of matter. This simple observation shows that the same matter can to exist in different states. Changing from one to another state of matter is a reversible, physical change. We can change water (a liquid form of hydrogen oxide) to ice (a solid form of the same compound) by cooling it sufficiently. We can change the ice back into water by heating it
States of Matter Matter can exist in one of 4 forms: (i) Solid. (ii) Liquid. (iii) Gas. (iv) Plasma Plasma is a rare state of existence for matter on this planet and needn’t bother us much for dental work, but it is important to understand the other three.
The differences in the behaviour of matter as solid, liquid and gas is caused by the behaviour of its atoms in that state. For example, water can exist as a solid (ice) below 00C, as a liquid above 0oC, and as a gas if heated above 1000C. What causes the properties of these different states of matter is the how mobile the atoms are each state. In the solid state, the atoms or molecules are fixed in position due to strong forces between molecules. Because the molecules or atoms remain in these fixed positions, the only movement possible is vibration. Every solid has a fixed volume and a fixed, definite shape. However, when the solid is heated, the atoms or molecules react to the extra energy by vibrating with increased frequency and amplitude. They are still held firmly in place, however, and cannot break free of the forces holding them in place until their energy becomes great enough. At this point, the matter becomes a liquid. We say that it has reached its melting point. The forces between molecules are much weaker in liquids so the particles have greater mobility. Liquids are able to flow, a property due to the constant motion of their particles relative to one another. This is why they have no definite shape. The particles have only limited movement, however. They cannot move apart much, so that liquids have a constant, or fixed volume at any one temperature. Most particles of a liquid are held within the liquid due to forces of attraction between molecules (surface tension), but particles can gain enough energy to escape and form a vapour. This is called evaporation. As a liquid is heated, more and more particles evaporate until the temperature reaches the boiling point and a complete change of state from liquid to gas occurs. In the gaseous state the particles are in constant motion and free to move in any direction. As a result, gases are not only fluid (like liquids, they have no fixed shape), they also have no fixed volume. As the particles in a gas regularly collide and rebound from each other at high speed, they move around until they fill the whole container. If they are not contained they will fly away due to their highly mobile state. As gases are heated, the heat energy is transferred to increased motion (velocity) of the particles, and can be observed as an increase in pressure. At room temperature, if a substance appears as a liquid then it has a melting point below room temperature. If a substance is a gas at room temperature then it has a boiling point below room temperature.
Kinetic Theory of Matter This theory explains how matter can be changed from one state to another. We know that all matter is be made up of tiny particles (atoms or molecules.) At any temperature above absolute zero, these particles are in a state of constant motion. The amount of motion of a substance in any state of matter is due to its particles responding to the available energy. Heat is a form of energy. If a substance is COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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liquid at room temperature, its particles are mobile, or in the liquid state. Its melting point is below room temperature. For water, with a melting point of 00C, there is enough energy at 200C to keep water particles mobile and able to flow. As the temperature of water is decreased the mobility of the molecules decrease until they cannot move fast enough to overcome the forces of attraction between them. The substance then changes state to become a solid. The molecules are no longer free to move, but they are still able to vibrate in positions fixed relative to each other. If we decrease the temperature further, particle vibration decreases until -2730C is reached. This is referred to absolute zero, since at this point all vibration stops. The temperature at which we observe changes of state are exact for pure substances. If we change the purity of the substance, the melting point and boiling point will also change.
Homogenous and Heterogeneous Matter We use these terms describe the composition and properties of matter. For example, if we have pure water in a container, then a sample taken from anywhere in the liquid will have the same composition, and properties. Because of this, only one value of a property is needed to fully describe pure matter. It is said to be homogenous, its properties are identical at any point in it. If two elements or substances are mixed together, such as oil and water, they will quickly separate. We are able to easily separate the two distinct parts by physical means. This is called heterogeneous matter. The properties will not be the same at any point in the body. They will depend on which of the mixture components has been sampled.
Elements, Mixtures and Compounds A pure substance is composed of only one type of atom or molecule, and it will have homogeneous properties. An atom is the smallest part of an element that still has the properties of that element A pure substance composed only of one sort of atom is called an element. A pure substance made from one sort of molecule is called a compound. A compound is a pure substance made by combining two or more elements in a fixed proportion by weight. A molecule is the smallest part of a compound that still has the properties of that compound When a second element is introduced to a first, the chemical and physical properties will change. For example, pure water melts at 00C and boils at 1000C.
If we dissolve salt in the water, its composition changes. It is no longer pure. If we re-measure its melting and boiling points, they will have changed also. The difference in composition and properties between pure water and salt water explains the difference between a pure substance and a mixture. The properties of a mixture are a mixture of the properties of the individual substances. As the amounts of each substance vary, so will the properties. Mixtures do not have fixed melting points or boiling points; they change with composition. There is no chemical reaction involved in their formation so they are still easily separated. In the case of salt water, the water can be evaporated to form pure water leaving pure salt water behind. If two or more pure elements are made to react together chemically, then the result will be the formation of a compound. The compound formed will be a pure substance and will be different in composition and properties to the original elements. An example of forming a compound is the combustion of pure hydrogen in the presence of oxygen to form pure water. The compound formed is in a different state of matter at room temperature, does not resemble its gaseous reactants, and cannot easily be reversed to hydrogen and oxygen. A chemical reaction is needed to do that. Another example is mixing pure iron and sulphur together. Only when the mixture is heated will a chemical reaction occur to form a new compound, iron sulphide. This has different composition and properties to its reactants.
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ELEMENTS, MIXTURES & COMPOUNDS – Practical Assignment I AIM To examine some of the properties of elements, mixtures and compounds.
METHOD (1) (i)
Weigh out 5g of iron (Fe) and 3g of sulphur (S).
(ii)
Place these two elements in a mortar and grind them into a fine powder with a pestle.
(iii)
Spread this powder on a piece of filter paper and examine it with a magnifying glass. Is it uniform throughout (homogeneous) or is it non-uniform (heterogeneous)?
(iv)
Pass a magnet under the filter paper and observe what happens. Is there any separation? Does this indicate a homogeneous or heterogeneous material?
(v)
Fill a clean test tube to a depth of approximately 10 mm with the ground material and shake it up. Is there any separation? Half fill the test tube with water and shake. Is there any separation?
METHOD (2) (i)
Place 10mm of the ground material into a Pyrex test tube. Heat it over a bunsen flame in a fume cupboard. Note any reactions.
(ii)
Allow the mass in the test tube to cool. Remove it form the test tube and grind it up in the mortar and pestle. Place some of the ground powder on a filter paper and examine it with a magnifying glass. Compare the results to those obtained in 1(iii).
(iii)
Test the powder with a magnet as in 1(iv) and compare the results.
(iv)
Place some of the material obtained in part 2 in a test tube to a depth of 10 mm and repeat 1(v). Compare the results.
RESULTS It would be helpful to record all your observations of the results in a tabulated form.
CONCLUSIONS Consider the results you have written down. Do they indicate that we have an element, a mixture , or a compound at each stage of the experiment? What do the results show you about In light of your discussion draw some conclusions about elements, mixtures and compounds.
Check Your Progress Self Evaluation Questions Listed below are questions which will help you to review Topic 1, Write your answer to each question on the lines below the question. You can check your answers with the ones given at the end of this topic. Q1.
Q2.
Q3.
List three (3) different states of matter. (i)
_____________________
(ii)
_____________________
(iii)
_____________________
Describe the motion of particle of matter in each state you listed in Q1. (i)
_____________________________________________________
(ii)
_____________________________________________________
(iii)
_____________________________________________________
Briefly describe the Kinetic Theory of Matter. _______________________________________________________________ _______________________________________________________________ _______________________________________________________________
Q4.
Define the following terms: (i)
Homogeneous
(ii)
Heterogeneous
(iii)
Element
(iv)
Mixture
(v)
Compound
•
•
•
•
•
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Suggested Responses for Topic 1
Q1.
(i)
solid
(i)
liquid
(ii)
gas
Q2.
(i) (ii) (iii)
Vibration of particles only – rigidly restrained. Particles have constant motion and can flow. Particles are free to move in any direction.
Q3.
All matter consist of particles which are in a constant state of motion. Changes in temperature increase or decrease motion and cause changes of state.
Q4.
Define the following terms: (i)
Homogeneous
(ii)
Heterogeneous
(iii)
Element
(iv)
Mixture
• • •
•
A pure substance composed of only one type of atom.
•
A substance containing with two or more elements or compounds combined in no fixed proportions. It shows the properties of each of its components. The components are easily separated by physical means (heterogeneous) This is a pure substance made by chemically combining atoms of two or more elements in fixed proportions by weight. A compound has its own chemical and physical properties, different from those of its components. It is still homogeneous.
•
(v)
Compound
chemical properties are uniform only one physical distinct property throughout the material. show two or more different property which allows separation easily by physical means
•
LEARNING OUTCOME 2
DISTINGUISH BETWEEN METALS AND NON-METALS, USING KNOWLEDGE OF ATOMIC STRUCTURE.
Assessment Criteria: You will have achieved this learning outcome when you can: • Name and describe sub-atomic particles. • Describe the arrangement of sub-atomic particles within the atom. • Relate the atomic number of an atom to its structure. • Classify atoms as metals or non-metals based on their structure. • Name, write the symbol for and describe the atomic structure of the first twelve elements. • Name, and write the symbol(s) for the elements within the periodic table that are of interest to dental restoration study.
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TOPIC 2 - Atomic Structure.
Recommended Time - 3 hrs
Introduction We now understand that materials can be classified according to their different behaviour, both physical and chemical. To understand why materials are different from each other, we first must understand some basic chemistry about how matter is made, and how this influences its properties. This topic starts the process of understanding the basic structure of materials by introducing you to the structure of atoms, under the following headings: • • • • • •
The type and nature of sub atomic particles. The arrangement of sub atomic particles in an atom. The relationship between atomic number and structure. The classification of atoms, including the division into metals or non metals. Important properties of the first twelve elements. Identifying elements that are present in dental materials.
History of Atomic Theory (just read this, not examinable) Precisely what goes to make up matter or substances is a problem that has fascinated scientific philosophers for centuries. Early philosophies considered matter to be made from of four elements: earth, fire, air and water. Around 400 BC Greek philosophers proposed that matter consisted of tiny indivisible particles called atoms. In the early 1800’s John Dalton proposed a revolutionary new approach: • All elements are made up of atoms • Atoms cannot be created or destroyed (they are indivisible) • Atoms of different elements may combine with atoms of another element in definite ratios. • Atoms of one element are different from atoms of another element. By 1820, laboratory experiment had found the presence of smaller particles in atoms, which suggested the presence of sub-atomic particles. Atoms could be taken apart, contrary to Dalton’s ideas.
In 1911, Ernest Rutherford confirmed the presence of sub-atomic particles, and made the following conclusions: • Each atoms has a nucleus which is positively charged. • Most of the atomic mass is contained in the nucleus. • The nucleus is surrounded by an almost empty space that makes up the rest of the atom. • Negatively charged electrons are present in this space around the nucleus. The negative charge on the electrons balances the positive charge of the nucleus. In 1913, Niels Bohr suggested that the nucleus contains two different types of sub atomic particles. This gave rise to the modern atomic theory.
Modern Atomic Theory Bohr conclude his atomic theory as: •
Atoms consist of subatomic particles
•
The nucleus contains protons (+ charge) and neutrons (no charge).
•
A cloud of electrons (- charge) orbits the nucleus.
•
The volume of the nucleus is extremely small compared to the volume of an atom.
•
The atom is electrically neutral since the number of electrons = number of protons.
Properties of Subatomic Particles Subatomic Particle Electron
Symbol
Charge -1
Mass (grams) 9.07x10-28
Mass a.m.u 0.00055
e-
Proton Neutron
Location outside nucleus
p+
+1
1.672x10-24
1.0073
inside nucleus
n
0
1.672x10-24
1.0087
inside nucleus
All elements are made up of different combinations of these subatomic particles. The number of each type of sub-atomic particle in an atom can be determined from the information about that particular element contained in the Periodic Table. This is a table of all the known elements and their basic properties, arranged in order of atomic number.
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e
e n n p
n n
n
pn e
Electron orbits
p n
Nucleus
e
The arrangement of particles in an atom. PROTONS (+ charge), NEUTRONS (no charge) and ELECTRONS (- charge).
Atomic Number and Mass Number What makes the difference between elements? To determine this we have to look at the arrangement of subatomic particles that make up each atom. This arrangement is different for each element. The information can be determined from the Atomic Number and the Mass Number: Atomic number = number of protons in the nucleus Mass number = number of protons plus neutrons in the nucleus As each atom must be electrically neutral, the number of electrons must be equal to the number of protons, which is the atomic number. If you look at the table of elements, you will see, for instance;
Carbon
Atomic Number 6
Mass Number 12
From this. we can work out that that carbon has: 6 protons (atomic number) 6 neutrons (mass number minus atomic number) 6 electrons (number of electrons = number of protons)
Element Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen
Atomic Number (Z) 1 2 3 4 5 6 7 8
Mass Number (A) 1 4 7 9 11 12 14 16
We know now that the central nucleus contains the protons and neutrons, but we don’t know how the electron orbits are arranged. For example, carbon has 6 protons (atomic number) and to be electrically neutral, must have 6 electrons. Logically there must be some consistent arrangement of these electrons, because they are moving around the nucleus without crashing into each other,
Electron Structure Bohr described electrons as moving in fixed circular orbits around the nucleus, rather like planets orbiting around a sun. This is why his description is often referred to as the “Planetary Model” of an atom. These orbits, shells, or orbitals are different distances away from the nucleus, so each electron in a different shell must have a different energy. The electrons in the furthest orbit form the nucleus have the most energy, while those closer to the nucleus have less energy. Bohr identified each electron shell with a number, n. The shell closest to the nucleus had n=1 which is the lowest energy level. The next shell, n=2, has a higher energy level and so on for n=3,4,5,6. It seems logical that the outer shells, being larger, could hold more electrons. In fact, it turns out that the maximum number of electrons which can fit in each shell is governed by the formulae: Maximum Number of electrons in any shell “n” = 2n2 n can be 1,2,3,4,5,6. For example in the first electron shell, n=1.and n2 = 1. The maximum number of electrons which can be fitted into that shell is 2, because 2x(1)2=2. For the second shell n=2. n2 = 4 The maximum number of electrons in this shell is 8, because 2x(2)2 = 8 For the third shell n = 3. n2 = 9. The maximum number of electrons in the third shell is 18 because 2x(3)2 = 18
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Consider the case of carbon. As we discussed earlier, it has six electrons. Its electron configuration can thus be calculated as 2 electrons in the 1st shell (n=1) 4 electrons in the 2nd shell (n=2) The second shell could hold a maximum of eight electrons, but carbon only has six. After putting two into the first shell there are only four left, so the second shell can only have the remaining four electrons in it. Under normal conditions electrons in their shells are referred to as in their “ground state.” If atoms are heated, electrons gain energy and they may jump to higher energy levels. When dropping back to the ground state, they may re-emit the same amount of energy. If we gave each shell a number to identify it, this could become confused with the number of electrons in the shell, so chemists have identified each shell with a letter instead The closest shell to the nucleus is called K. The next is L followed by M, N etc. The following table shows the electron configuration for the first twelve elements. Remember, the maximum number of electrons is expressed by 2n2.
Element (chemical symbol)
Number of electrons
Maximum number in shell
Hydrogen (H)
1
2
Electron Configuration K,L,M,N 1
Helium (He)
2
2
2
Lithium (Li)
3
2, 8
2, 1
Beryllium (Be)
4
2, 8
2, 2
Boron ( B)
5
2, 8
2, 3
Carbon (C)
6
2, 8
2, 4
Nitrogen (N)
7
2, 8
2, 5
Oxygen (O)
8
2, 8
2, 6
Fluorine (F)
9
2, 8
2, 7
Neon (Ne)
10
2, 8
2, 8
Sodium (Na)
11
2, 8, 18
2, 8, 1
Magnesium (Mg)
12
2, 8, 18
2, 8, 2
Names and symbols of elements As you can see from the table above, each different element has been given a name by its discoverer, and a symbol of one or two letters made up from the element’s name or from its Latin name. The elements are named after greek or roman gods, scientists, countries, or anything else which took their discoverer’s fancy at the time. The symbols are used as a type of short-hand to represent the elements in chemical formulae and equations. For the purposes of this course, you should know the names and symbols of the first twelve elements, and their symbols, and those of another fourteen common elements or those of interest in dental work. These are set out in the next table Element Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium
Symbol H He Li Be B C N O F Ne Na Mg
Element Gold Silver Palladium Platinum Iron Cobalt Nickel Chromium Tin Copper Aluminium Lead
Symbol Au Ag Pd Pt Fe Co Ni Cr Sn Cu Al Pb
Element Chlorine Sulfur Phosphorus Mercury Zinc Calcium
Symbol Cl S P Hg Zn Ca
Periodic Table You will notice that the first twelve elements in the table have been listed in order of their Mass Number, which is the way that they were listed as the number of elements being discovered grew. The normal way of considering all the elements at once is the Periodic Table. Scientists in the 1800’s discovered that some elements had very similar chemical properties to each other, even though their atomic and mass numbers were different. To find out why this was so, they arranged groups of elements with similar properties in columns and rows and looked for patterns in their properties or the numbers. In this way they made early versions of a periodic classification. The modern Periodic Table lists the elements in a series of boxes arranged in columns and rows.. In the horizontal row, elements increase in atomic number from left to right. Each box contains important information about the element: Atomic number, or “Z” Mass number, or “A” Chemical symbol Electron configuration COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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Originally, elements with similar chemical properties had their boxes arranged in a vertical column. This arrangement is still used. For example, Helium, Neon, Argon, Krypton and Xenon are all inert gases and are found in a column on the right hand side of the periodic table. Look at the periodic table shown below. Although it is more than seventy years old, little has changed except for the addition of a few extra heavy elements with atomic weights above 105. Note that elements which have very obvious metallic properties are found on the left of the table. The less “typical” metals are found in the middle, the transition metals. Just before the non-metals on the right, separated by the stepped vertical line, there are elements which have some properties of both metals and non-metals, the “metalloids” such as arsenic, or silicon. Why should these obvious groupings occur? What determines the difference between metals and non-metals, so that arranging the elements in order of atomic number and similar chemical properties will reveal it? It turns out that the chemical properties of any element are controlled mainly by the number of electrons in its outer shells. This is a most important fact.
Properties of Metal and Non-Metals We now know, from a study of the types of chemical reactions they undergo, that what distinguishes metallic from non-metallic elements is the number of electrons they have in the outermost shell of their atoms. We call these valence electrons. An element will behave as a metal if it easily loses, or donates, one or more electrons when forming chemical bonds. Metals thus are those elements with
only a few electrons in their outer shell. Look at the second column from the left in the periodic table. Use the atomic numbers to work out how many electrons each one has, and you’ll find they all have two electrons in their outermost shell. Non-Metals are elements that have outer electron shells that are close to being full. They readily accept electrons during chemical bonding. A comparison of the common properties of metals and non metals is shown in the following table: Metal Have 1, 2, or 3 valence electrons. Lose electrons easily. Form compounds with non metals. High electrical conductivity. High thermal conductivity. Malleable and Ductile.
Non Metal Have 4, or more valence electrons. Tend to gain electrons. Form compounds with metals. Low electrical conductivity. Low thermal conductivity. Non ductile (brittle).
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Check Your Progress Self Evaluation Questions With a self-paced learning package like this one, we provide regular opportunities for you to check your knowledge as you go. You’ll find a set of questions for you to answer, and so review your knowledge, at the end of each topic. Listed below are questions that will help you to review Topic 2. Write your answer to each question on the lines below the question, and when you have finished you can check your answers with the ones given at the end of this topic.
Q1
Give the name and charge of each of the particles that make up an atom (i)……………………………………………………………………………
Q2
(i)
……………………………………………………………………………
(ii)
……………………………………………………………………………
(ii)
………………………………………………………………………….
Define what is meant by the following terms. (i) Atomic Number ……………………….……………………………………. ………………………………………………………………………………… (ii) Mass Number ……………………………………………………………….. …………………………………………………………………………………...
Q3
The maximum number of electrons in any shell with number “n” is calculated from which formula?………………………………………………………….
Q4.
List three (3) properties of metals and non metals. Metals
Non Metals
(i)
(i)
(ii)
(ii)
(iii)
(iii)
Suggested Responses for Topic 2
Q1
Q2.
Give the name and charge of each of the particles that go to make up an atom. (i)
proton (+)
(ii)
electron (-)
(iii)
neutron (0)
Complete the following sentences:. (iii)
The Atomic Number of an atom is The number of protons in the nucleus.
(iv) The Mass Number of an atom is The number of protons plus the number of neutrons in the nucleus. Q3.
The maximum number of electrons in any shell with number “n” is calculated from the formula :- Maximum number =2n2 where n = shell number
Q4.
List three (3) properties of metals and non-metals. Metals
Non-Metals
(i)
1,2 or 3 valence electrons
(i)
4 or more valence electrons
(ii)
lose electrons easily
(ii)
tend to gain electrons
(iii)
malleable
(iii)
brittle
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So, how did you go with the topics under Learning outcome 2? Did you answer all the questions correctly? Do you feel confident in being able to meet the assessment criteria listed under the learning outcome? If you did, congratulations and please proceed to Learning outcome 2. If you answered a question incorrectly or you had difficulty with any of the activities, go back and have a look at the information again. If any part of this module is not clear, it is very important to contact your teacher and discuss this with him or her before you start Learning outcome 3.
LEARNING OUTCOME 3
RELATE CHEMICAL REACTIONS AND BONDING TO THE STRUCTURE OF MATTER.
Assessment criteria: You will have achieved this learning outcome when you can: •
Describe primary and secondary bonding in matter.
•
Classify the bonding in different materials.
•
Write balanced chemical equations to describe chemical reactions.
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TOPIC 3 - Chemical Bonding and Reactions
Recommended Time - 3 hrs
Introduction The way that elements are combined to make compounds often has much to do with their properties as possible dental materials. This topic introduces you to important information about:
Nature and types of primary and secondary bonds in matter.
Specific types of bonding in materials.
Writing and balancing chemical equation to represent reactions.
If you think about what might happen as two atoms are brought closer together, it is obvious that what they do will be controlled by their outer electron shells, which are the first parts to come together. We now know that atoms will be in a lower state of energy if their outer electron shells are full of electrons. If they are close to another atom, they can achieve this condition by obtaining extra electrons from it and filling their outer shell, or by giving electrons to it, so as to empty their outer shell and expose the full shell next to it, or by sharing electrons with the other atom so that both have a full outer shell. When atoms do this, they very often bond together to form a new substance, a compound
Valency The combining power of an atom is known as its valency, or valence, and is an important property of each different element. It is determined by the number of electrons the atom will acquire, give away, or share during chemical bonding. electrons, as these high energy electrons are the ones involved in chemical bonding to from compounds. Metallic atoms have 1, 2, or 3 valence electrons which they lose easily during bonding to form a positive ion. An Ion is an atom that has been given a positive or negative charge by losing or gaining electrons. Non metallic atoms tend to gain electrons during bonding, so the number of vacant site in the outermost shell is their valency. Examples are shown in the following table. These elements form negative ions. Also in the table are a
number of negative ions and one positive ion which are made from a number of atoms, not one. These negative or positive groups of atoms are called “functional groups”, because they can function as, or take the place of, a single atom or ion. Positive valency ions
Negative valency ions
Lithium
Li1+
Carbon
C4-
Beryllium
Be2+
Chloride
CI1-
Boron
B3+
Carbonate
CO32-
Sodium
Na1+
Fluoride
FI3-
Magnesium
Mg2+
Hydroxide
OH1-
Aluminium
AI3+
Oxide
O2-
Potassium
K1+
Nitrate
NO31-
Calcium
Ca2+
Sulfate
SO42-
Hydrogen
H1+
Phosphate
PO43-
Iron
Fe2+ or Fe3+
Copper
Cu1+ or Cu2+
Silver
Ag1+
Gold
Au1+
Ammonium
NH41+
Chemical Bonding in Compounds The simplest form of bonding which occurs to form a compound is shown by nonmetals such as oxygen, hydrogen or nitrogen. Two gas atoms like these can bond together to produce a stable diatomic molecule. The chemical formulae shows this: Oxygen Chlorine Nitrogen Hydrogen
-
O2 CI2 N2 H2
Diatomic gases
For bonding to occur between metals and non-metals, an ion may have to be formed. This is a charged atom, an atom with some electrons missing, or some extra electrons added. For example, aluminium has 3 valence electrons, but bonding is not possible until an ion has been formed by removing these. Al (element) Na (element)
→ →
Al3+ + 3 electrons Na1+ + 1 electron
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An ion of aluminium has been formed form the element by losing 3 electrons. Similarly Sodium loses 1 electron to form an ion. As an example of a non-metal, the element chlorine can gain one electron to form a chloride ion for bonding. Cl (element) + e
→
Cl1-
Ions of opposite charges attract each other, and may join together to form an ionic compound. An example of an ionic compound is sodium chloride Na1+ + Cl1-
→
NaCl
The ions involved are Na1+ and Cl1- As they both have the same valency they are going to react in a 1:1 ratio, that is one ion of sodium (Na1+) will react with one chloride ion (Cl1-) to form the new compound NaCl. Another example is the reaction between zinc ions (Zn2+) and chloride ions (Cl1-). In this case the valency of zinc allows it to react with two chloride ions Zinc has two electrons to lose, and each chlorine atom can only pick up one of them, so the ratio of Zinc ions to Chlorine ions in the new compound is 1:2. (Zn2+) + Cl1-
→
ZnCl2
Notice how we use the abbreviated symbols given to the elements to describe the chemical reaction and also to construct a formula for the new compounds formed. How do these ions, in their fixed ratios, come together to form what we see as large amounts of matter, material, or substances? To answer this question, we must discuss chemical bonding, what holds atoms together
Primary Chemical Bonds 1.
Ionic Bonds The stability and strength of these bonds can be explained by looking at the formation of sodium chloride we discussed above. Sodium has 1 electron in its outer shell which it will freely give up so that its next innermost shell is full, a more stable arrangement. Chlorine needs 1 electron to fill its most outermost shell, similarly becoming more stable. When these two elements react, electrons are taken from the sodium to the chloride atoms. The positive sodium ions and negative chloride ions formed have a strong electrostatic attraction to each other, and are held in place in a fixed structure to from a solid compound (at room temperature.)
This newly formed compound, sodium chloride, normal table salt, is a solid, normally small clear crystals. Its properties are very different from those of the reactive shiny metal which is sodium and the green poisonous gas, chlorine. This difference proves the formation of a new substance. Ionic compounds such as sodium chloride have high melting points, are usually solid at room temperature and are soluble in water. They conduct electricity when they melt or when dissolved in water because they separate into ions (dissociate) which can carry an electric current.
Steps in ionic bond formation:
2.
i)
formation of sodium ion
ii)
chlorine accept electron
iii)
elecrostatic attraction forms new compound
→ Na1+ + 1 electron Cl + 1e → CI1Na+ + Cl- → NaCI Na
Covalent Bonds Atoms may achieve stable electron configurations by sharing electrons with adjacent atoms rather than donating or accepting them like elements which form ionic bonds. The gases we discussed earlier – oxygen, hydrogen and carbon, all share electrons to fill their outer shells, which makes them more stable. Sharing outer shell electrons between atoms makes a new outer shell which encloses both atoms. A molecule of a new substance is created, with its atoms held together by a covalent bond. For example, consider a hydrogen atom with its single electron.
Two hydrogen atoms pair together to become a diatomic. molecule. To do this, they share their two electrons, which form a single orbital around both atoms. The orbital has a figure-eight shape.
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We can write this as an equation, showing the reagent atoms and their product molecule.
→
H. + H.
H–H
(hydrogen gas molecule, formula H2)
Other non metallic atoms can share atoms to form covalently bonded molecules. For example a molecule of water (formula, H2O) is formed from two atoms of hydrogen and one of oxygen.
H
H. + H + O
O
→
H–O–H
H
(water)
The bond which holds the atoms together in a molecule forms because the positive nucleus of each atom is strongly attracted to the cloud of shared electrons surrounding the atoms. A more extensive example of covalent bonding is found in methane, CH4. Here carbon, shares each of its four outer shell electrons in a bond with one of four separate hydrogen atoms. This results in eight electrons in a complex orbital around the carbon, and two around each hydrogen atom. All the atoms have an effectively full outer shell. The “structural” diagram for this is simpler, but does not show each electron.
H | H−C−H | H
H
H
C
H
H
Atoms undergoing covalent bonding may also share more than 2 electrons. Sharing four electrons, for example, produces a double bond An example can be seen in the gas ethylene. There is a double bond between the two carbon atoms, which also share an electron in turn to each of two hydro atoms. Each carbon atom has thus shared its outer four electrons with another four from different atoms to give a total number of eight in its outer shell. The ethylene molecule forms a very reactive gas.
H H \ / C=C / \ H H The atomic or electronic diagram shows us the structure of each bond
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H
H
C
H
3.
C
H
Metallic Bonds This type of bonding is only formed between metallic atoms. There are not enough valence electrons in metals to share between atoms to make a true covalent bond. So, each atom contributes its valence electron to form a loose ‘cloud’ of electrons. These electrons are not associated with any of the positive metallic ions formed, but are free to move between them. This produces a strong electrostatic attraction between the positively charged ions and the ‘cloud’ of mobile electrons. Atoms
Electron cloud
The strength of the metallic bond produces very close packing of the ions in a regular lattice arrangement. The freely moving electron cloud produces good electrical conductivity, and the close packing gives metals their typically high densities.
4.
Secondary Chemical Bonds Secondary bonds are formed by the attraction of weak intermolecular forces between dipoles. A dipole is part of a covalently bonded molecule where charge is not evenly distributed. An example is water. The hydrogen atoms are not bonded to form a linear molecule, in fact the bond angle between the hydrogen is only 104.50 degrees instead of 180 degrees.
O H
H
104. 50 + This produces a weakly positive electrostatic charge on one side of the molecule and a weak negative charge on the opposite side. Attraction is now possible between oppositely charged poles of other polar molecules.
Chemical Reactions and Equations During the previous paragraphs, we have been coming close to describing the reaction between elements to from new compounds not just in words, but also by using the symbols which are the shorthand identification for the elements. Now we will expand this idea so that you can learn how scientists describe complicated chemical reactions in a brief and precise way. Chemical reactions involve substances which are present before the reaction. These substances are called reactants, or reagents The reaction produces new substances formed in a chemical change which are called products. There are many different types of chemical reactions, but for our purposes we only need to consider the general type described by the following form. Two or more elements, or one or more compounds (the Reagents) react or break down to make one or more different elements or compounds (the Products)
REAGENT + REAGENT → PRODUCT + PRODUCT Balancing Chemical Equations
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Chemical equations can tell us not only what elements and compounds are involved, but also how much of each one is needed, and how much is produced. For them to do this, they need to be Balanced. We say that an equation is balanced if it tells us tell us the exact truth about the reaction. It must show the correct reagents and products, and the correct amounts of each one. We test for a balanced equation by using the principal that we cannot make (or lose) atoms. Therefore exactly the same number of atoms of each element involved should be present in the reagent compounds as is in the product compounds. Where atoms combine in different ratios the correct number of atoms is important to complete the reaction. Balancing is achieved as a number of steps: 1.
2. 3. 4.
Write the equation for the reaction in words, making sure that the correct reagents and products are specified. In many cases, such as your examinations, the equation will be given to you already in symbols, in which case you cam leave out this step. Express the products and reactants as the correct chemical symbols and formulae, and that the correct formulae are used for each one. Count the number of atoms of each type of element in the reagents and in the products. Balance the elements one at a time, using as many steps as needed.
Example Hydrogen burns in oxygen to make water Step 1 Step 2
Step 3
Step 4A
Step 4B
Oxygen + Hydrogen
→
Water
O2 + H2 → H2 O Note The formula for oxygen and hydrogen are correct; we know that these gases are normally present as diatomic molecules. Also, the formula for water is correct, we know it is made up from two atoms of hydrogen and one atom of oxygen Remember the molecular formulae for these in previous pages? Note. There are two atoms of hydrogen in the reagents and in the product. The equation is balanced for hydrogen. There are two atoms of oxygen in the reagents, but only one atom of oxygen in the products. The equation is not balanced for oxygen O2 + H2 → 2 H2 O Note. Now we have doubled the number of water molecules produced, we have balanced the equation for oxygen (two atoms in reagents and products) but it is unbalanced for hydrogen. O2 + 2 H2 → 2 H2 O Note. Now both oxygen and hydrogen are balanced. There are two atoms of oxygen and four atoms of hydrogen on both sides of the equation.
The equation now shows the same number of each type of atom among the reagents as among the products. We say that it is a balanced equation. Note that we cannot change the small numbers below the line in each formula, such as the 2 in H2 O. These numbers are part of the formula; they show us the exact numbers of each atom in a molecule of a compound. To change them would be to change the compound into another one. For example, we could change H2O to H2O2, but this would be changing the compound from water to hydrogen peroxide, an entirely different compound! What we have to do is increase or reduce the number of molecules of each compound, or atoms of each free element until the equation balances. We show this variation by altering the large numbers placed before the formula for the compound or element. For example, H2O means one molecule of water, containing two hydrogen atoms and one oxygen atom. 2H2O means two molecules of water, therefore four atoms of hydrogen and two atoms of oxygen. For clarity, these numbers are bold, italic, and bigger in the equations above. Example: Sodium metal reacts with water
→
Step 1:
Sodium+ water
Step 2:
Na + H2O
Step 2
Note. There is one atom of sodium on each side of the equation, and one atom of oxygen. But there are two toms of hydrogen in the reagents and three in the products.
Step 4A:
Na +2H2O
→
Step 4B
Na + 2H2O
→ 2NaOH + H2
Step 4C
2Na + 2H2O →
→
Sodium Hydroxide + hydrogen gas
NaOH + H2
NaOH + H2
NaOH + H2
Now we will try a harder equation to balance: Balance the reaction for calcium hydroxide and nitric acid.
→
Step 1:
Nitric acid + Calcium Hydroxide
Step 2:
HNO3 + Ca(OH)2
Step 3
Note The reagents have 3xH, 5xO, 1xN, 1xCa. The products have 2xH, 7xO, 1xN, 1xCa
Step 4A:
2HNO3 + Ca(OH)2 →Ca(NO3)2 + H2O
→
Calcium nitrate + water
Ca(NO3)2 + H2O
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Step 4B
2HNO3 + Ca(OH)2 →Ca(NO3)2 + 2H2O Note4xH, 2xN, 1xCa, 8x0 → 4xH, 2xN, 1xCa, 8x0 Same number of atoms on both side. – it balances!
Activity 1 – Student Exercise
Try balancing the following equations. The answers are at the end of this unit. 1.
C2H6 + O2 → CO2 + H2O
2.
Ca + HCI
3.
KOH + AI(NO3)3
4.
H2SO4 + AI(OH)3
5.
FeCI2 + Na3PO4
→
Fe3(PO4)2 + NaCI
6.
CaCO3 + H3PO4
→
Ca3(PO4)2 + H2O + CO2
7
Mg + O2
→
→
CaCI2 + H2
→
KNO3 + AI(OH)3
→
H2O + AI2 (SO4)3
MgO
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Activity 1 - Answersheet
1.
2C2H6 + 7O2
2.
Ca + 2HCI
→ →
4CO2 + 6H2O H2 + CaCI2
6.
→ 3H2SO4 + 2AI(OH)3 → 3FeCI2 + 2Na3PO4 → 3CaCO3 + 2H3PO4 →
7.
Mg + O2
3. 4. 5.
3KOH + AI(NO3)3
→
MgO
3KNO3 + AI(OH)3 6H2O + AI2(SO4)3 Fe3(PO4)2 + 6NaCI Ca3(PO4)2 + 3H2O + 3CO2
PHYSICAL & CHEMICAL CHANGE = Assignment 2 AIM To observe and describe changes of properties of various materials and to classify them as: a) b) c)
physical chemical allotropic change
An allotropic change is a special form of physical change where the material changes its solid state structure, that is, the arrangement of atoms used to put it together.
METHOD 1.
2.
3.
(i)
Dissolve a small amount of ammonium dichromate in water in a test tube. Record what is happening. Retain the solution.
(ii)
Ignite a small quantity of dry ammonium dichromate in an evaporating basin in a fume cupboard. Note any reactions, sparks evolved, colour changes or volume changes. Dissolve some of the reaction product in water and compare with 1 (i).
(i)
Gently heat (in a fume cupboard) a small quantity of napthalene in a crucible until melting occurs. Allow to cool, and note all changes.
(ii)
Reheat strongly (in a fume cupboard) projecting the flame down onto the material. Note all reactions.
(i)
Place a lump of limestone (calcium carbonate) in a test tube and add a few drops of water. Note any reactions.
(iii)
Add a few drops of hydrochloric acid to the test tube in 3(i). Note any reactions.
4.
(i)
Dissolve some salt (sodium chloride) in 5 ml water. Evaporate to dryness. Note the changes that have occurred.
5.
(i)
Heat (in a fume cupboard), a test tube 1/3 full of sulphur. Note any changes in state, viscosity and colour as the sulphur reacts to the heating. Note each change carefully. (Do not heat to strongly as sulphur will ignite).
(ii)
Warm the sides of the test tube and pour the ,molten sulphur into a beaker of water. Compare the properties of the solid produced with the original sulphur. Note the changes that have occurred.
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RESULTS Record all the results in tabulated form. Include chemical equations where applicable.
CONCLUSION To come to some conclusions about what has happened, you need to look at the results you observed, and decide whether there was a physical reaction, a chemical reaction, or an allotropic reaction in each case.
Check Your Progress Self Evaluation Questions Listed below are questions which will help you to review Topic 3, Write your answer to each question on the lines below the question. You can check your answer with the one given at the end of this topic.
Q1.
Fill in the missing words (i) (ii)
Metallic atoms have ……………………………valence electrons. Non metallic atoms have ………………………valence electrons.
Q2.
How do metallic ions form from a metal atom? (i) Metallic ions are formed by ………………………………………………………………………….. (ii) Give an example: ……………………………………………….
Q3.
Write an equation to represent an ionic bond. …………………………………………………………………………………
Q4.
What type of force holds ionic compounds such as sodium chloride together? …………………………………………………………………………………
Q5.
Covalent bonds are characterized by ………………………………………….
Q6.
Write an equation which gives an example of covalent bonding. …………………………………………………………………………………..
Q7.
Describe metallic bonding. ………………………………………………………………………………….. …………………………………………………………………………………..
Q8 Describe Secondary bonds……………………………………………………... ………………………………………………………………………………………….. Q9.
Balance this equation: C3H2COOH + O2
→
CO2 + H2O
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Suggested Responses for Topic 3
Q1.
(i) (ii)
1, 2 or 3. 4 or more.
Q2.
(i) (ii)
Losing their valence electrons to gain a positive valence. Al → Al3+ + 3e
Q3.
Sodium plus chlorine +
-
Na + CI
→
→
salt
NaCI
Q4.
A very strong electrostatic attraction between the positive sodium ion and the negative chloride ion.
Q5.
Sharing electrons to fill the outer shell.
Q6.
H + O + H
Q7.
The positively charged metallic ions are strongly attracted to a freely moving negative electron cloud.
Q8
Weak intermolecular forces of attraction between oppositely charged dipoles.
Q9.
C3H2 COOH + 5O2
. . . .
→
: :
H O H (water)
→
4CO2 + 4H2O
So, how did you go with the topics under Learning outcome 3? Did you answer all the questions correctly? Do you feel confident in being able to meet the assessment criteria listed under the learning outcome? If you did, congratulations and please proceed to Learning outcome 4. If you answered a question incorrectly or you had difficulty with any of the activities, go back and have a look at the information again. If any part of this module is not clear, it is very important to contact your teacher and discuss this with him or her before you start Learning outcome 4.
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LEARNING OUTCOME 4
MATERIALS CLASSIFICATION, ALLOYS, POLYMERS OR CERAMICS.
Assessment criteria: You will have achieved this learning outcome when you can: •
Describe the bonding in alloys, polymers or ceramics.
•
Categorise common dental materials as alloys, polymers or ceramics.
TOPIC 4 - Structure of Materials
Recommended Time - 1 hr
Introduction Dental applications involve some of the most fascinating and varied materials used nowadays, but, since no one type of materials possess all the desired properties for a particular dental application, we have to use a range of different materials, or combinations of them. . Later in the Diploma of Dental Technology the modules you study will include discussions of the properties and uses of Dental Metals, Dental Polymers, and Dental Ceramics. First, however, we need an introduction to these groups of materials, which we will find in this topic. This topic introduces you to important basic information about: •
The bonding which constructs alloys, polymers and ceramics.
•
Classifying dental materials as alloys, polymers or ceramics.
Classification of Materials Materials used in dental applications can be divided into 4 families: (i) (ii) (iii) (iv)
metals and their alloys ceramics and glasses low and high molecular weight polymers and elastomers composites
Each of these material groups has specific properties which make them useful as dental materials. There is also great variation for these properties. For example, metals and ceramics show very limited flexibility (they are comparatively stiff, or rigid,) whereas polymers can be compounded to give the rubbery behaviour necessary in impression materials.
Metals (metallic bonding) We can explain the chemical behaviour of materials by the number of their outer shell electrons. The mechanical properties of materials are caused by the way that COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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atoms are arranged in order to make bulk material, We call this arrangement the structure of the material. The mechanical and physical properties of metals can be explained in terms of their metallic bonding. As discussed earlier in this unit, positive metallic ions are held rigidly in a close packed crystalline lattice structure, around which an electron cloud freely moves. This electron cloud can transmit energy with little loss, and so produces the characteristic metallic properties of electrical and thermal conductivity. If light is shone on a metal surface, it is reflected, producing a characteristic lustre. Most metals have high melting points, which is explained by the strength of the metallic bond. The ions need much heat energy to overcome electrostatic bonding forces and break free of each other, change state and become liquid. In terms of mechanical properties, most metals are as tough and ductile. They can be stressed below a certain limit and return to their original dimensions when the stress is released (elastic behaviour) or they can be stressed above their elastic limit and become permanently deformed (this plastic behaviour makes them formable, a useful property). Deformation without fracture is possible because layers of the crystalline arrangements can slip past one another under stress, another property of the metallic bond structure.
Polymers (covalent bonding) Organic materials involve covalent bonding which involves sharing electrons They commonly form large molecules or macromolecules.by a process of repeated joining of a basic group of atoms, called polymerisation. These molecules may be many hundreds of thousands of atoms in size. In may polymers the molecules take the form of long chains of atoms, where the atoms are joined by covalent bonding, but the chains are only held to each other by weaker secondary bonds. Other polymers may have very large three-dimensional structures of atoms. When polymers melt the molecules separate from one another and move independently. Polymer melting points are much lower than those of metals or ceramics because only secondary bonds need to be broken. For the same reason. the strengths of polymers will also be much lower than that of metals or ceramics. . The rigidity of polymers is also lower than that of metals or ceramics. However, the low weight of most of the atoms in polymer molecules, and their relatively large spaces between chains makes polymers much less denseThe bonding of polymer atoms in chains or rings is strong, but the secondary bonds between chains are weak unless there is covalently bonded crosslinking. Increased temperature causes separation of the chains to allow each one to vibrate more. This phenomenon gives polymers much higher thermal expansion than metals or ceramics. Also, water can penetrate the weak bonding between chains, producing a susceptibility to swelling and degradation.
Ceramics (ionic bonding) A simple definition of a ceramic is, “A compound of metal ions and non-metal ions with ionic bonding,” although there are some ceramics, such as glasses, with covalent bonding. A simpler definition of a ceramic is a material whose structure has been caused by firing it. Either way, their high strength bonds make ceramics very stable, with high melting points and rigidity (stiffness.) They are poor conductors, since the electrons donated by metal ions are strongly held by the non-metallic ion. On melting the crystal structure of the compound separates making the ions mobile, so ceramics can carry electric currents. when molten. Ceramics are also characterised by their high hardness and brittleness, as well as outstanding resistance to high temperatures. These properties are due to the electron behaviour of the constituent ions. Also due to this electron behaviour, ceramics are usually electrical and thermal insulators. Ceramics may be transparent (window glass) or coloured by absorption of ions as well as the suspension of pigments. This property is again due to the absence of free electrons in the material. Colouring is important to dental ceramics as their final shading must be matched to the patients natural tooth colour and stains. Hardening and strengthening of ceramics is possible by incorporating other ionic compounds like alumina (A1203). This increases their strength and rigidity. A big disadvantage of ceramics is their brittleness. Unlike metals, and a number of polymers, the strong ionic bond and crystalline structure does not allow any localised movement under stress. At higher stresses, the only response possible from the structure is to fracture. A brittle failure results, with a sudden release of energy. Many ceramic articles are made by firing powder until the particles fuse, because the ceramic melting points are inconveniently high for casting, unlike many metals. The spaces between the particles results in small entrapped voids which are hard to remove. Ceramics are poor heat conductors, so the outer surface cools faster, placing it under a tensile stress which produces small cracks. Although very strong in compression, the combination of surface cracks and internal defects like porosity makes ceramics much weaker in tension than many metals. As a result, ceramics are most suitable for use in applications where they are stressed in compression. Ceramics for dental applications are selected for their high strength and ease of processing. Their brittleness can be improved by forming a type of composite material when they are fused to the surface of a reinforcing metal structure which has a much greater toughness.
Composites (covalent bonding matrix – metallic or ionically bonded fillers). A composite is a material with two or more distinct phases. One phase is usually much harder than the other, and more brittle. The softer phase is generally COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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tougher. The combination of materials has properties that each of the separate ones cannot match. For example, human teeth can be repaired by a composite of a setting acrylic polymer with hard glass (ceramic) particles mixed in with it. This gives the composite the following properties: (i) (ii) (iii) (iv) (v)
good strength and toughness good bonding to natural teeth wear resistance rapid setting (polymerised by light) ease of use
Check Your Progress Self Evaluation Questions
Listed below are questions which will help you to review Topic 4. Write your answer to each question on the lines below the question. You can check your answer with the script section at the end of this topic.
Q1.
List four (4) families of material (i) ______________________________ (ii) ______________________________ (iii) ______________________________ (iv) ______________________________
Q2.
List three (3) properties of metals (i) ______________________________ (ii) ______________________________ (iii) ______________________________
Q3. Define what is meant y a “polymer” ___________________________________________________ ______________________________________________________________ Q4.
List three (3) properties of polymers (i) _____________________________ (ii) _____________________________ (iii) _____________________________
Q5.
Crosslinking is _______________________________________________
Q6.
Ceramics are _______________________ bonded.
Q7.
List 3 properties of ceramics (i) _____________________________ (ii) _____________________________ (iii) _____________________________
Q8.
The biggest disadvantage of ceramics is their ______________________ which is due to ______________________________________________
Q9.
Composites have ______ phases, one of which is ____________ and the other __________ COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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Suggested Responses for Topic 4
Q1.
i) ii) iii) iv)
metals and alloys ceramics and glasses polymers and elastomers composites
Q2.
i) ii) iii)
high melting point metallic bonding high density
Q3.
organic materials characterised by long chains which are covalently bonded
Q4.
i) ii) iii)
Q5.
chemical bonding between polymer chains
Q6.
ionically
Q7.
i) ii) iii)
Q8.
brittleness, surface cracks and internal defects
Q9.
two, hard, soft
low density low softening points covalent bonding
poor electrical conductors brittle high hardness
So, how did you go with the topics under Learning outcome 4? Did you answer all the questions correctly? Do you feel confident in being able to meet the assessment criteria listed under the learning outcome? If you did, congratulations and please proceed to Learning outcome 5. If you answered a question incorrectly or you had difficulty with any of the activities, go back and have a look at the information again. If any part of this module is not clear, it is very important to contact your teacher and discuss this with him or her before you start Learning outcome 5.
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LEARNING OUTCOME 5
RATIONALISE THE USE OF SELECTED MATERIALS ACCORDING TO THEIR PROPERTIES.
Assessment criteria: You will have achieved this learning outcome when you can: •
Classify the properties of materials as chemical, physical or mechanical.
•
Relate selected properties of materials to their use in dental technology applications.
•
Define and calculate specific properties using data for selected dental materials.
TOPIC 5 - Properties of Materials
Recommended Time - 5 hrs
Introduction Now that we know about the basic differences between types of material, we need to start looking at their properties. Selection of materials for dental uses involves matching these properties against those needed for the particular application. In order to specify what properties we need, we must start by considering what properties there are, and which ones are relevant to the particular use. This topic introduces you to important basic information about: •
Classification of chemical, physical and mechanical properties useful for dental applications.
•
Properties which are desired in various dental materials.
•
Calculating specific properties from data obtained by testing dental materials.
Property Classification The properties of materials can be classified into three categories: (i) (ii) (iii)
(i)
chemical physical mechanical
Chemical Properties
The chemical properties of a material describe the types of chemical reactions that it will undergo in various circumstances. Some physical properties are also linked to the chemical structure of the material. The chemical properties of elements include valency and reactivity., stability, corrosion resistance, acidity or alkalinity, and composition. Of particular dental importance, are the reactions the material may have with human tissue. COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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(ii)
Physical Properties
The physical properties of a substance are describe how it reacts to the physical universe. Often, these properties are used to identify a substance. Examples are boiling point, melting point, electrical conductivity, or density.
(iii) Mechanical Properties These properties describe how substances react to applied forces. They are often measured by destructive testing, such as by tensile, compressive or impact tests. Typical properties include tensile or compressive strengths, stiffness, (rigidity) hardness, brittleness, fatigue resistance and impact toughness. These properties are usually related to how the atoms are arranged in a substance. Before looking at the properties needed for specific dental applications, we need to examine some of them more fully.
(i)
Chemical Properties
Bonding -
The adhesion of one substance to another is an important chemical property. For example, if they cannot bond to the natural tooth, then restorative materials would be useless.
Wetting -
The wetting characteristics determine if molecules are compatible in terms of their bonding energy. Good wetting produces high strength bonding. The surface tension of the material is a measure of this energy. If materials like solders, enamels or adhesives do not wet a surface, they will not join to it.
Stability -
Material in the oral environment must not react or change in any way which alters its properties. If the material absorbs fluid and swells, or is attacked by oral fluids then its usefulness is limited.
Toxicity
Ideally dental materials are fully “bio-compatible” That is, they can become integrated with human tissue without unfavourable results. Any material used certainly should not cause trauma or tissue damage to the patient. Care must be taken when some degradation may occur, or where certain materials may be toxic or reactive to patient or technician if used in certain ways.
(ii)
Physical Properties
Dimensional -The material should not shrink or contract and so cause discomfort Stability for the patient and so diminished usefulness of the appliance.
Rheology -
This is the behaviour of materials moving under stress, such as when mixing, or when being squeezed out of a tube. The flow and viscosity of some materials used to make dental appliances are also important.
Density -
This is the mass per unit volume of a material. Polymers have low densities and metals have high densities. This property needs consideration when selecting materials, as it controls the weight of an appliance. It may also control how much the material costs
Melting Point -
Boiling Point -
The temperature at which a substance changes from solid to liquid state. It is important in determining how easy a metal is to cast, because it partly determines the energy needed for melting it.
This is the temperature at which a substance changes from liquid to gaseous states. It will affect how a material is to be processed, or how it may react in dental use.
Optical Properties - There are a number of these, such as colour (accurately defined and quantified), opacity and reflectance. These determine how light is transmitted, absorbed, or changed in wavelength on meeting the material. This controls its appearance, which is dentally important. These properties are vitally important to dental ceramics. Coefficient Of Thermal Expansion - Almost all substances expand as their temperature rises, and contract if cooled. As a result they change size with temperature, and the amount can be significant to the accuracy of metal castings, polymer restorations, and ceramics. In a mixture of two different materials (such as dental metal/ceramic restorations) two different rates of expansion can set up destructive stresses on cooling Comparison of the varying amounts by which different materials expand is given by their coefficient of expansion. This is the amount a material will expand for every degree Celsius its temperature rises and for every unit of its original length. So, dental chromium alloys have a coefficient of linear expansion, (called “α”). of 15 x 10-6 mm per mm of original length, per oC rise in temperature. We use the formula: L1 = Lo + Lo x α x ∆T, where L1 = Final length, after expansion or contraction Lo = Original length, before expansion or contraction α = coefficient of linear expansion of the materials ∆T = Change in temperature COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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Substances also have expansion coefficients of area (β) and volume (γ) To illustrate the importance of this property, consider a dental chromium/cobalt alloy partial denture casting measuring 75mm across. This would solidify at 1425oC, and after solidifying it will contract by 1.6mm (15 x 10-6 x 1400 x 75 mm) as it cools to room temperature. Unless some allowance is made for this (which is done, as you will learn in the Module “Removable Alloy Partial Dentures”) such a casting can become significantly undersized. Specific heat This is a measure of the capacity of a material to absorb energy, while changing temperature. It is measured as the energy required to raise the temperature of one unit of mass of the substance by one degree. So, for example, calories per gram per degree Celsius (Cal/g/oC) Metal with high specific heat takes longer heating (more energy) from a gas torch to reach the same temperature as a metal with lower specific heat. Viscosity
Surface Tension
This is a measure of the resistance to flow of a fluid. The unit in the metric system is the Poise. For example, a fluid like honey has a higher viscosity than water. Fluids with high viscosity flow less under pressure. An example where the viscosity is important in dental work is where the fluid must be poured to shape, such as an impression material. This can be a difficult property to understand, but is of vital importance in dental work whenever a fluid needs to wet and flow over a surface. Surface tension is a measure of the force with which one material is attracted to another at their surface, and is related to the chemical phenomenon of wetting. Look at the diagrams below. An atom within the body of a liquid; it is evenly surrounded by other atoms of the same type. Their attraction for it is evenly distributed, which holds the atom in one place. On the surface of the liquid, however, the same atom is partly surrounded by atoms in the surface of whatever substance the liquid is touching. The force on the atom is now unbalanced; it may be attracted towards the new material surface, or back into the liquid, depending on how much it is attracted to atoms in the new surface. This imbalance means that the liquid will be attracted to, or repelled by a different material. Such a force mean that liquids will wet a new surface such as a solid, and spread across it, or they will not wet it, and withdraw from it.
An atom in the centre is pulled evenly in all directions by other atoms
An atom at the surface is pulled inwards or outwards. This creates a “surface tension”
The results of surface tension are seen in the capacity of surfaces to pull liquid upwards against gravity. Examples are liquid soaking into finely porous material, or in sap rising in trees Latent Heat This is the amount of energy, expressed in calories per gram, which a substance takes up or gives off when it changes state. So, there is a latent heat of fusion, needed to melt material already at its melting point, and a latent heat of vaporisation, needed to boil material already at its boiling point. Material with a high specific heat, high latent heat of fusion, and high melting point, needs much more energy to melt it, say for casting, than material with lower values for these properties
(iii) Mechanical Properties To give satisfactory service in the oral environment, any material needs to have sufficient strength to withstand the stresses involved. A number of properties like the strength of a material can be measured from a Tensile Test. To carry out this test, we take a material specimen of suitable size. We measure the length to be tested, and the cross-sectional area at right angles to the applied force. We apply a gradually increasing force to the material, and measure the increase in length which occurs and the increasing force needed to continue extending the material. The test machine will gives us an applied force, measured in Newtons in the metric system, and an extension of the specimen, in millimetres. Before describing the results from such a test, we need to consider the units we are measuring. We need to understand exactly what a Force, and a Stress are. COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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A force is defined as applying energy to a substance, so that it tends to move. Applying a force, f, to a body of mass m, produces an acceleration, a F=mxa A Force can be applied to a body in a number of ways. If we apply a linear force, the body is in tension, or in compression
TENSION, A TENSILE FORCE
COMPRESSION, A COMPRESSIVE FORCE
Other forces include bending, twisting and shear forces
A BENDING FORCE
TORSION, A TWISTING FORCE
SHEAR, A PARTING FORCE
A typical test graph for a tensile test is shown on the next page, but what is plotted on the diagram is not the force and extension the test machine measured, but the related properties of stress and strain. Stress is defined as the force applied to a substance, divided by the area of material across which the force is applied. Strain is the resulting change in size of the body of material (say, its extra length) divided by the original size We need stress and strain because they are properties of the material independent of the size of the specimen used. Consider two steel bolts, one with a cross-section area of ten square millimetres, and one with a cross-sectional area of twenty square millimetres. It will take twice as much force to break the bigger bolt in a tensile test, not because the steel in it is stronger, but because there is twice as much of it. But if we divide the force used to break each bolt by the area of the bolt, we will get the identical stress.. The unit of stress in the metric system is the Pascal, which is defined as a Newton of force per square metre of area. Unfortunately, the Pascal is such a small amount of stress that we commonly find ourselves measuring things in millions of Pascals, or “ MegaPascals” Conveniently for our test, if we measure the force in Newtons, and the area of cross-section of the specimen in square millimetres, the resulting stress comes out in MegaPascals. Similarly, we want to measure as a material property, not the extension produced, but the strain, or percentage extension, which won’t vary with the original length of the specimen. A
B
It takes twice as much force to break piece A than piece B, but if we divide the force by the cross-section area of the piece, the resulting stress experience by the material is the same
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The next diagram is a graph of the stress against strain that happens when a piece of material is subjected to a tensile force
Maximum, or Ultimate Stress Elastic limit or Yield Point
C B
Breaking Stress D
S t r e s s
A Elastic region
Plastic region % Elongation
Now, let us see what the important areas on this diagram can tell us about the mechanical properties of the material tested, Elastic Limit This is sometimes called the Yield Point of the material, point B on the diagram It is the stress at which the material stops behaving elastically and starts behaving plastically. That means that, if we were to remove the test force below the point B, the material would return to its original length (behave elastically) After the stress at point B, if we remove the applied force, the material will not return to its original length. It will have become plastically, or permanently deformed. So, the Yield Point or Elastic Limit, is the value of the stress at point B, is a measure of how easy it is to deform (say bend, or stretch) the material. Elastic Modulus -
Alternatively called stiffness or rigidity or the modulus of rigidity, or Young’s Modulus, after its inventor, it describes how much material may elastically distort in use. In our diagram, the rigidity of a material is shown by the slope of the elastic part of the
test line, A-B. The rigidity would be found from the stress at the point B divided by the strain at the point B. In the diagrams below, a) shows the stress/strain relationship for a material of high rigidity, such as a chromium alloy, whereas b) shows the curve for a material of low rigidity, such as a polymer. The rigidity of a material is a measure of how stiff it is, how much it resists deflection
stress
stress
strain a) High rigidity material
Ultimate Tensile Strength -
strain b) Low rigidity material
This is the maximum stress a material can withstand, the stress at point C on the test diagram. The Ultimate Tensile Stress or U.T.S. is a measure of how much stress a material will carry before it breaks. This is what is called strength
Plasticity -
Often called ductility, this is a measure of the amount of deformation a material can withstand up to its failure at point D. Brittle materials, such as glass have low plasticity, whereas ductile materials have high plasticity. A completely brittle material would have a stress/strain diagram like b) below, whereas a ductile material would show a curve like a), and give a large value of strain at the breaking point Ductility is thus a measure of how much we can change the shape of a material before it breaks.
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stress
stress
strain at breaking
strain
strain
a) Stress/ strain curve for ductile material
b) Stress/strain curve for brittle material (no plastic deformation
Resilience - This is the ability of a material to store elastic energy and release it when the force is removed. This can be considered as the area under the elastic part of the curve, shaded in the diagram below. Resilience is the amount of energy a material can store before deforming, obviously an important property for assessing material for making springs
resilience stress strain
Toughness- This the amount of energy a material can absorb without fracture. On a stress/strain diagram it is represented by the total area under the curve. Toughness can be seen as the total amount of energy a material will store before breaking
stress
toughness
strain Fatigue -
This is the resistance of a material to failure by repeated stressing and unloading at a level of force lower than is required to break the
material by one application e.g. mastication. Think of it as cumulative damage Hardness -
Hardness is related to wear resistance, an important property which can control the useful life of a material in dental application. Hardness is defined as the resistance of a substance to indentation. It is measured by making a test indentation in the substance using an indenter of specific shape and size, and applying a set load to it. After the indentation is made, measuring its depth or width and referring to a set of charts will give a relative hardness number on the scale measured by the particular test method. Notice that, unlike strength, hardness is not a property which allows us to do any further calculations about what forces a substance will resist. Hardness is just a comparative figure. There are three basic hardness tests whose results might be found in materials textbooks; BRINELL, ROCKWELL, AND VICKERS The Brinell test uses a round steel ball about 12mm in diameter as an indenter. In use this indenter is gradually forced into the surface of the test specimen by applying weight gradually, controlled by a hydraulic piston. It makes a circular indentation whose diameter is measured using a small microscope. Looking up a set of tables using the indentation diameter and the test load give s a Brinell Hardness Number, or BHN.
`
Because this test makes a fairly large indentation, it is not much use on thin or small specimens, although it is statistically more valid on large ones. It also cannot test substances which are harder than the hardened steel ball. The Vickers test uses a similar, but smaller machine, and a much smaller indenter made of industrial diamond, so it can test any material. This indenter is in the shape of a four-sided pyramid, so it leaves a square-shaped dent, whose size is easier to measure. Again, the size of this dent and the test load are used to look up a hardness result, the Vickers Hardness Number, or VHN. The Vickers test is fussy, and harder to use, but it can test any material. It is probably the one most commonly found in dental textbooks to indicating the hardness of a material. The Rockwell test uses a range of different sizes steel balls and diamond cone indenters for material of differing hardness. It measures the depth, not the width of the indentation, and so can give a direct reading of hardness from its dial, without any need for looking up tables. Although it is hard to compare its results with other hardness tests, it is the quickest and easiest to use. Its results COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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are given, for example, as 55Rc, meaning a hardness of 55 on the Rockwell C scale.
Examples of mechanical and physical properties of various materials In the previous section the properties of polymers, metals and ceramics were discussed in general terms. Now that we know what the particular properties mean, we should consider the values of important properties to show the difference between metals, polymers and ceramics. Melting point The melting point of metal ranges up to that of tungsten at 3410OC, with many of them melting in between 500 and 1500OC. Ceramics have a similar maximum to metals, over 3000OC, but many of the common ones soften above the melting point of most metals. Polymers are generally much lower, only silicone polymers softening at above 4000C Density Metals are the densest materials. The common ones vary from as low as 2.5g/cm3 for aluminium to Lead at 11g/cm3. Gold has a very high density of 19.6g/cm3 and Platinum is the highest at 21.45g/cm3. Chrome alloys have density about 8.5g/cm3. Ceramics have lower densities, from 2.5g/cm3 to 5, and polymers are generally between 0.9g/cm3 and 2.2g/cm3 unless foamed or filled. Hardness Human teeth have a hardness in healthy enamel of about 300V.H.N.. Metals can be as low in hardness as 60V.H.N., for lead, to about 600V.H.N. for tool steels. Dental gold alloys are about 250 VHN and chrome alloys about 400V.H.N.. Dental polymers are much softer, generally about 30 or 40V.H.N. Ceramics are harder, though some minerals are very soft because of their structure. Ceramics vary from about 600 V.H.N for silicates up to 3500V.H.N. or more for abrasives. Coefficient of expansion Natural teeth have a coefficient of expansion typically about 11 x10-6mm/mm/OC, varying from 7 to 14. Ceramics generally have a slightly lower value, but can be made to match human teeth fairly easily. Metals generally have slightly higher values, with chrome alloys at 15x10-6mm/mm/OC, but can be made to match tooth expansion. Polymers have much higher expansion coefficients, up to 300 or more x10-6 mm/mm/OC, although mineral filled dental composites are much lower, they do not have values a slow as that for human teeth.
Strength Metals have a tensile strength varying from below 100 Mpa (MegaPascals) to as high as 2300Mpa for heat treated high alloy steels. By comparison, only the strongest of polymers get over 100Mpa. Ceramics are weaker in tension than metals, and difficult to test, but the strongest are comparable to metals. The compressive strength of ceramics can get over several thousand Mpa, but they are normally tested in a “flexural strength” test Ductility Ceramics have no ductility, being brittle, they fail under stress without any permanent change of shape. Polymers can extend by many hundreds of percent if they are above their transition temperature. Metals have ductility ranging from fairly brittle at a few percent extension to the formable metals such as brass and aluminium which can be extended by 40 or 50%.
Application of Dental Materials – Properties. Considering the properties in the previous couple of pages, it is obvious that no one material will completely match the properties of human dental materials. The task then becomes one of selecting the best material for a particular application by considering all of its properties, including cost and appearance. In this section we will look briefly at some typical dental materials and relate the properties of the materials used to their purpose. The properties of all these materials will be examined in much greater detail in later sections of the course in the appropriate theory section. Consequently the following introduction to dental materials is intended to be read for information only, and would not be examinable in the Materials Science Exam 1)
Impression Materials
The first step in many dental applications is to make a working copy of the patient’s oral structure. For a material to suitably record oral morphology, it must be fluid during application, and set in a reasonable time. These properties are related to its chemical and physical properties. The material should not be toxic or allergenic to the patient or the dental technician, and should not be unpleasant in any way during setting (taste, smell, surface feeling). The material should be easily removed from the mouth without distortion. After removal the material should be dimensionally stable until the model of the patient’s dentition can be taken from it. The impression it has taken must be accurate in dimension and in surface detail. This means that the impression material must be fluid enough to flow over and COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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duplicate surface detail, but not so fluid that it is difficult to use or flows out of the tray into the patient’s throat. Although non-elastic, or rigid, materials have been used in the past, the impression materials used nowadays are elastic. That means that they can be elastically deformed sufficiently to be distorted over undercuts in the shape of the patient’s dentition when being removed. Elastic materials
-
polyether polysulphide addition silicone condensation silicone hydrocolloid Water based alginate
Tooth replacement materials
2)
The forces of mastication necessitate high strength and wear resistance in restorative materials. They must be strong under compressive and shear stress, and have a high hardness. The ability of a material to remain rigid (elastic modulus) and not distort is also important as damage to other surfaces may result if a restoration distorts in use, even if it returns to shape later.. Corrosion resistance, dimensional stability, good aesthetics, and ease of handling and ease of polishing are important. Porosity and moisture uptake are not desirable, for hygienic reasons. Commonly used materials are: • • • •
3)
polymethyl methacrylate (acrylic) dentures ceramics of various types –commonly called “porcelain” Precious metal alloys, gold and palladium based (crowns, bridges and copings, partial dentures) base metal alloys, cobalt, nickel and titanium alloys (crowns and copings, partial dentures)
Restorative Materials
These materials are used to repair or restore natural tooth structures. The material must have good wetting characteristics so that high bond strengths to the natural structure can occur. The expansion and contraction of the material should be similar to the bonded material, otherwise fatigue failure may be a problem. Ease of use and moisture (saliva) tolerance is important for fitting.
Where special adhesives and surface treatments are necessary the process should not be toxic and present minor discomfort to the patient. Working times should be as suitable, and the aesthetics should be pleasing. Typical materials are: • amalgam • composites • acrylic
4)
Casts, Model and Die Materials
These materials are required for creating replicas of the patient’s dentition over which the final appliance can be constructed.. Both of these applications require strength and abrasion resistance, and the commonly used material is a specially hardened gypsum “stone.” Casts have to be made from this, “type IV stone” for high strength to withstand clamping forces. Die materials need abrasion resistance to withstand damaging during wearing. Both requirements necessitate materials which are easy to use, reproduce detail well, and have a low setting expansion. Additives are used to control setting expansion and time. Other materials include: • acrylic resins • polyester and epoxy resins
5)
Investment Materials
To produce a cast metal appliance, a negative image must be created as a cavity in a mould made from a refractory material to withstand the high temperatures during casting. Such materials are called investment materials and consist of a binder and refractory particles. .Refractory particles are necessary to withstand the heat of casting and cause expansion. Modifying agents are used to control properties and produce setting and thermal expansion – the total of which has to be equal the solid state contraction of the alloy as it cools. Cristobalite and quartz are commonly used to produce the desired thermal expansions. Typical materials are: • gypsum bonded investment – low melting point alloys • phosphate bonded investment – high melting point alloys 6)
Dental Castings
Reproducing a lost tooth structure can easily be done by replicating the shape in wax and investing to form a mould. The mould is then filled with an alloy with has the following properties COMM SERV HEALTH TOURISM HOSP ESD© PARAMEDICAL STUDIES PROGRAM AREA
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• • • • • • • • • • • •
corrosion resistance ready flow in the molten state ease of cleaning and polishing no toxicity compatibility with current materials hardenability (for example, gold/copper alloys) strength toughness hygienic aesthetic appeal compatibility with dental porcelain ease of melting and casting
Two types of alloys are used: i) precious metal casting alloys (various alloys of Au, or Pd) ii) base metal casting alloys (alloys of Ni,Cr,Co or Ti)
Implant Materials
7)
These special materials are used where there is available bond to support the implant. The alloy used must have • • • • • • •
biocompatibility corrosion resistance high strength and elastic modulus reasonable price compatibility to dental coatings (hydroxyapatite) ease of processing and installation excellent wear resistance
Typical materials are: titanium based alloys.
MATERIALS SCIENCE -SURFACE TENSION The property of SURFACE TENSION has been defined as the capacity of surface of material to wet, or be attracted to the surface of another. In practice this can be seen by the tendency of liquids to be pulled up the inside of thin tubing. PRACTICAL METHOD Place some water in a beaker. Select a thin bore glass tube, and measure its inside diameter by using the thin wire gauge supplied. Place this tube in the liquid in the beaker. From the side of the beaker, observe how far the liquid in the tube is raised above the surface of the liquid in the beaker. This is a measure of the surface tension of the water for glass tubing. Repeat the experiment using methylated spirits instead of water. Does methylated spirits have a higher or lower surface tension than water? Rinse the beaker and the tubing, and repeat the experiment with a couple of drops of detergent in the water. What effect has the detergent had on the surface tension of water on glass. Repeat the experiment using water with a few drops of WAXIT in it. WAXIT is a proprietary surface tension reducer used in dental technician’s work. What is its effect on surface tension? • • •
cobalt based alloys ceramics
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Check Your Progress Self Evaluation Questions
Listed below are questions which will help you to review Topic 5. Write your answer to each question on the lines below the question. You can check your answer with the script section at the end of this topic.
Q1.
List three (3) types of material properties: (i) ______________________________ (ii) ______________________________ (iii) ______________________________
Q2.
Define the following properties: (i) bonding __________________________________________________ (ii) toxicity __________________________________________________ (iii) dimension stability _________________________________________ (iv) melting point _____________________________________________ (v) elastic modulus ____________________________________________ (vi) resilience _________________________________________________ (vii) plasticity _________________________________________________
Q3
Which type of material shows extensive plastic deformation in a tensile test, ductile, or brittle _________________________________________________
Q4
What property is given by the slope of the elastic part of the tensile test curve?_______________________________________________________
Q5
If the maximum force which can be resisted by a piece of material is 3000 Newtons (3kiloNewtons, or3kN) and it is 10mm square in cross section, what is the stress applied to it? ___________________________________
Q6
A material has a strength of 2000 Mpa, melts at 1500OC, and has a density of 9g/cm3. Is it a polymer, a ceramic, or a metal?…………………………...
Q7
A material has a softening temperature of 1500C, a density of 1.0g/cm3 , and a strength of 40Mpa. Is it a ceramic, a metal, or a polymer?…………...
Suggested Responses for Topic 5
Q1.
(i) (ii) (iii)
chemical physical mechanical
Q2.
(i) (ii) (iii) (iv) (v) (vi) (vii)
bonding adhesion of a material to a substrate toxicity the ability of a material to cause tissue trauma dimensional stability material should not absorb fluid and swell melting point temperature state changes from solid to liquid elastic modules - rigidity or stiffness of a metal or alloy resilience - the storage and release of elastic energy plasticity amount of deformation before failure
Q3
Ductile
Q4
Rigidity, Modulus of elasticity
Q5
30Mpa (stress = force/area, =3000N/100mm2
Q6
A metal
Q7
A polymer
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So, how did you go with the topics under Learning outcome 5? Did you answer all the questions correctly? Do you feel confident in being able to meet the assessment criteria listed under the learning outcome? If you did, congratulations and please proceed to Learning outcome 6. If you answered a question incorrectly or you had difficulty with any of the activities, go back and have a look at the information again. If any part of this module is not clear, it is very important to contact your teacher and discuss this with him or her before you start Learning outcome 6.
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LEARNING OUTCOME 6
RATIONALISE THE USE OF IMPRESSION AND MODELMAKING MATERIALS ACCORDING TO THEIR PROPERTIES.
Assessment criteria: You will have achieved this learning outcome when you can: •
List, describe and explain the important properties of alginate impression Relate selected properties of materials to their use in dental technology applications.
•
List, describe and explain the important properties of plaster, stone and die stone as used for making dental models
•
TOPIC 6 - Properties of Materials used to make dental models
Recommended Time - 2 hrs
Introduction Now that we know about the basic differences between types of material and their properties, we need to start looking at the first ones used in dentistry. The procedure their properties which begins the construction of a dental appliance is usually the taking of an impression of the surface of the patients dentition. The next step is to make from this impression a model in solid material of the patient’s dentition. This is used as a basis for constructing the appliance. This topic introduces you to the properties which control the techniques of using: Gypsum products •
Alginate impression materials.
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GYPSUM PRODUCTS The major materials used to make models are types of gypsum; calcium sulfate dihydrate. This is a mineral with 2 molecules of water in the structure of its crystals, combined with one molecule of calcium sulfate, CaSO4٠2 H2O. When this material is heated, or calcined, it loses three molecules of water of crystallisation for every two molecules of gypsum, to form plaster of Paris, (CaSO4)2·H2O. The chemical formula for this is often wrongly described as CaSO4٠1/2H2O, which gives the material its common name, Calcium sulfate hemi-hydrate. When it is mixed with water, plaster of Paris forms gypsum again, and sets. The resulting solid makes very accurate models; it is reasonably strong, it is cheap, and it sets fairly quickly. It is convenient to use and non-toxic. It is, however, brittle. Like all brittle materials, its strength in tension is much lower than its strength in compression. The actual ratio is about 1:5. As a result it has little resistance to tensile and bending loads, and has a low shock resistance. In practice this means that set gypsum products such as models will fracture if dropped, and the surface is easy to chip or flake during repeated use. Chemistry of Plaster and stone To understand how this material should be used, we need to know how it is made, how it mixes with water and how it sets. The term “plaster” covers not only dental plaster, but also stone, and improved stones. These products are categorised in order of increasing strength, for example by the American Dental Association, as: • • • •
Impression plaster, ADA type I Model Plaster, ADA type II Dental Stone, ADA type III High strength Dental Stone, ADA type IV
These products are actually chemically identical. The difference between plaster, medium strength plaster (stone) and high strength plaster (improved stone, or die stone) is the size and shape of the their particles of calcium sulfate hemi-hydrate. This difference, however, can have a great effect on the strength of the material. The particles vary in size and shape because of the way they are formed as the plaster sets. Plaster is made by heating gypsum between 120OC and 180OC. The gypsum gives off water, and the eventual product at room temperature is calcium sulfate hemi-hydrate. 2CaSO4٠2H2O → (CaSO4)2·H2O +3 H2O This is the process is called calcination, or calcining Stone Particles of plaster made by calcining are roughly the same shape and size as the ground particles of gypsum from which they have been made. However, if the process is carried out by heating gypsum with superheated steam, at about 120OC for several hours, followed by drying, the formation of the particles of plaster is controlled by crystallisation. They are less porous, more geometrically shaped and more even in size and shape than particles of dry calcined plaster. Stone is also reground after calcining, making a certain amount of fine powder particles as well. The stone particles with some fines make a powder whose particles pack together more closely, increasing the bulk density of the dry powder. When the stone powder is mixed with water and reset, the resulting gypsum has smaller crystals because it starts to set from more points. It is also
more densely packed, and this resulting “stone” has a compressive strength double that of “dental” plaster, or model plaster. a)
b)
c)
THE “BULK DENSITY OF a) PLASTER, b) STONE, and c) DIE STONE. Each cylinder contains 100g of calcium sulfate hemi-hydrate
High-strength Stone By using chemical additions to further control the shape and size of the crystals of the hemi-hydrate grown during calcining, the stone produced can be made stronger again. “Modified” or “die”stones such as this will have a compressive strength four times that of dental plaster. Although the particle size and strength of stone and modified stone is different from plaster, they all set in the same way when mixed with. For this purpose, they can all be considered as one material. The setting reaction When one of the powder products plaster, stone or die stone, is mixed with water, it eventually sets as gypsum. (CaSO4)2·H2O + 3H2O → 2CaSO4·2H2O + Heat his makes a solid mass of interlocking crystals of gypsum. Obviously, this reaction is the reverse of the calcining reaction, but that only tells us what the product is, not how the reaction works. Plaster has low solubility in water, and dissolves slowly, although it is still more soluble than gypsum. As a result, when plaster powder is mixed with water, it takes time to dissolve. Eventually the amount of calcium sulfate in the water becomes high enough that gypsum, because of its lower solubility, begins to form solid crystals. As it does so, a little more plaster dissolves, and more gypsum crystals form. Because of this slow dissolution and resetting process, plaster sets gradually. This is a valuable property because it gives technicians some time for mixing and pouring the plaster into a mould before it starts to set. Understanding this process gives us information about how to control it better. There are a number of variables which must be properly controlled by the technician before the best possible product can be ensured. 73
Setting stages Observing the setting of gypsum will show how it should be used. Mixing water with powder initially produces a liquid “slurry,” a fluid paste. The proper mixture will be a little too stiff to flow easily, but it will flow readily if assisted by vibration. As solid gypsum crystals form the mixture becomes too viscous to be poured, although it remains sufficiently plastic to be moulded by applying pressure, say with the hands. Eventually all the water present is used, and the surface of the plaster loses its glossy wet look. This stage, called “gloss-off,” is often used by technicians to indicate that the plaster has set. In fact there will continue to be some smaller crystals growing in between the others for some time. The solid mass will not gain its full strength until it is completely dry. Because of the gradual way the crystals form, plaster also changes dimensions during setting. There is actually a shrinkage in the liquid stage as setting starts, but this is not large enough to be obvious. Once gloss-off is reached, the growing crystals of di-hydrate push against each other and cause a slight expansion of the final solid bulk. Mixed with the correct amount of water, plaster will expand about 0.3% on setting, and stones about 0.1%. If the plaster is in contact with an excess of water during the later stages of setting (after gloss-off) it will expand further. The final crystals are grow in an excess of water rather than coated with a thin film of it. They are not restrained by pushing against each other as they grow. This is commonly, but not altogether correctly, called hygroscopic expansion. It can be achieved by immersing the setting plaster block in water after the gloss-off stage. This technique is used to cause compensatory setting expansion of investments mixtures.
Additives Commercial plaster and stone mixtures generally have a number of additives: • •
Accelerators. These make the gypsum set faster. Fine dry plaster powder is one, as are a number of ionic salts such as sulfates and chlorides, in low concentrations. Retarders: These slow gypsum setting. Included are acetate, borate, tartrate and citrate salts, and any colloid, such as gelatine.
The amount of accelerators and retarders in plaster products is carefully controlled because many of them also reduce the setting expansion and the strength of the set product Because they are added as a mixture to the dry powder, it becomes important that the plaster in a container is evenly mixed before using it.
Water to Powder ratios, W:P The ratio of water to plaster powder in the original mixture has a number of effects on the setting process, and on the properties of the set plaster The amount of water needed for plaster to set usefully depends on two things: •
The amount of water needed for all the plaster particles to dissolve and precipitate as gypsum
•
The amount of water that is needed by the technician to make a pourable liquid mixture, with each particle of powder exposed to water in the time available.
Theoretically plaster only needs a W:P ratio of 0.186 (0.186g water for each 1g of plaster) to set fully. However, a mixture made to this ratio of water would be too dry to mix evenly. Some particles would remain dry and unset. In order to make a mixture which is smooth and even, and can be poured into a mould, W:P ratios about 0.5 or 0.6 are needed for plaster. Dental stone needs a W:P ratio of about 0.33, and die stones need about 0.25. The smaller particles in the stones allow full and even mixing with less water than is needed for plaster. Although exposing plaster to an excess of water during the final stages of solidification may increase the setting expansion, using a higher initial W:P ratio will result in reduced setting expansion.
Strength of plaster products As stated above, stone and die stone have much greater strength than plaster, due to their smaller and better packed crystals. STRENGTHS OF SET PLASTER Product
Compressive strength, MPa Tensile strength, MPa
Plaster Stone Die stone
25 65 80
4 7 10
Note that the results in the above table are typical of fully dry plaster. If the plaster is still wet, or has been re-wet after drying, its strength in both tension and compression will be halved! Only a small amount of added water is necessary to produce this effect on dry plaster. Increasing the W:P ratio (making a runnier mix) will substantially reduce the strength of the set plaster products because it makes the resulting set plaster more porous. EFFECT of W/P RATIO ON THE STRENGTH OF SET DRY PLASTER
Water/ Plaster Ratio 0.45 0.60 0.80
Compressive strength, MPa 26.2 17.9 11.0
Hardeners For some purposes the brittleness of plaster and its tendency to flake at the surface can be a nuisance. In such cases, such as making dies, hardeners can be used. These are either preparations painted on the plaster surface, or additives made to the plaster during mixing. 75
Strictly speaking these are not hardeners, as the hardness of the plaster is not affected. They are better described as strengtheners, or tougheners Surface hardeners. These are usually polymers dissolved in solvent. Cyanoacrylate, acrylic, and polystyrene. have all been used. It is not hard for the technician to make his or her own in a small nail-polish bottle, though dissolving the polymer in a solvent, such as acetone, should be done in a well-ventilated space. The mixture is applied with a small brush to the wearing surfaces of the model. The mixture should be thin enough that it will soak into the pores of the plaster and set there, binding the crystals together. It is not intended to add the polymer as a surface layer of added thickness (unless this is necessary, to create a later spacing effect) An alternative to polymer hardeners is to use colloidal silica in water or alcohol based solutions. This is the same material which is used to set phosphate and silicate-bonded investments. Provided it is used thin enough to avoid a “paint-on” layer, it penetrates and sets in the surface of the plaster as a gel, making the plaster much tougher Mix-in hardeners Again polymer solutions or colloidal silica can be used, though the polymers will need to be dissolved in a water compatible solvent such as an alcohol. The addition of colloidal silica liquid to the plaster at the same time as the water is mixed will add 20 to 40% to the compressive strength of the plaster. There is a risk in using high concentrations of this liquid with the powder. Depending on what they are carried in, some of the silica colloids can invert and set, apparently due to the pH of the plaster mix. It can cause greatly decreased setting rates for the plaster. If you are using some left-over investment fluid rather than a specific gypsum hardener, make a trial mix first.
USING PLASTER PRODUCTS In the light of the above information, a number of points can be made about good practice in using plaster products in dentistry: • •
• • • • •
It is important to mix the plaster and water in a smooth fashion, not beating it. Any air entrained in the plaster can result in bubbles that will remain after setting and weaken it. Mix plaster with water by sifting the powder onto the water surface. That way every powder particle is more evenly wetted. Pouring the water over a heap of powder will result in the outside of the heap starting to set first. It will be difficult to mix evenly. This problem will be much more pronounced with large amounts of plaster. Using mechanical mixing machines will assist in producing easy and even mixing within the available working time. If the plaster starts to set before you have poured it into the impression, adding extra water will not correct matters. It will still set, just as quickly, but now you will have a lumpy set material with uneven texture and low strength. Adding more water than recommended to a mix produces a lower strength solid. For example, a die stone with a W:P ratio of about 0.2 will have maximum strength at that value. Mixing it to a ratio of 0.25 will make the resulting solid roughly 40% weaker. If an extremely accurate set product is desired, i.e. a setting expansion or contraction of 0%, use a slightly higher W:P ratio. However, the product will be slightly weaker. If the strongest possible set product is desired, use a slightly lower W:P ratio. However, the setting expansion will be increased.
•
•
Store plaster in an airtight bag and keep the lid on the container. Atmospheric moisture will set plaster gradually if it is exposed. What most commonly happens is that the moisture wets the surface of the powder and sets some of it, working from the outside inwards on each particle. The result is a powder made from half-set particles. This will make a gritty, lumpy mix, and sets to form a weaker solid. The accelerators and retarders in plaster may settle out over a long period of storage, particularly where there is some vibration. This will produce plaster where material at the top of the tin will have different setting properties from material at the bottom. It is good practice to invert, or shake stored tins every few weeks to avoid this.
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PRACTICAL ASSIGNMENT - DENTAL GYPSUM PRODUCTS
AIM To examine some of the physical and mechanical properties of dental gypsum products
METHOD Prepare test cylinders of plaster by setting 60g of plaster in the plastic moulds supplied. These moulds are tapered slightly to make it easier to remove the set plaster, which may fit rather tightly because it expands a little during setting. The moulds may be also be greased lightly to make it easier to remove the set plaster. Experiment (1) – The effect of Water/Plaster ratio Make the specimens in the lists below, for plaster, stone and die stone. These are based on using the same amount of plaster, but with a different water amount for each sample. Make sure that there are no air bubbles in the plaster mould, as this would reduce the subsequent test strength. If the mixture is too dry to make a mould, note this and comment. After removal of the plaster specimen from the mould, write your group name on it with a pencil to avoid confusion during later testing While the material is setting, observe:a) The setting time, using a surface needle such as a Vicat or Gilmore. b) The consistency, or fluidity of the mix. c) The suitability of the mix for dental use After setting, measure d), The effect of W/P ratio on the strength of the set mixture by keeping the set specimens for a week, then testing their compressive strength using a hydraulic pressure frame. Before placing the specimens in the machine, make sure that the top and bottom surfaces are flat and level by scraping them with a plaster knife. Samples i) Plaster
ii) Stone
W/P Ratio 0.25 0.30 0.50 0.60
Plaster 60g 60g 60g 60g
Water 15ml 18ml 30ml 36ml
0.25 0.30 0.50 0.60
60g 60g 60g 60g
15ml 18ml 30ml 36ml
0.20 0.25 0.30 0.50 0.60
60g 60g 60g 60g 60g
12ml 15ml 18ml 30ml 36ml
iii) Die stone
Experiment (2) – The effect of water temperature Prepare six plaster cylinders using a W/P ratio of 0.5 (60g water, 30g plaster,) but using water at different temperatures for each one. Use water at:i) Room temperature (about 20oC) iv) Water at 70oC o ii) Water at 30 C v) Water at 90oC iii) Water at 50oC vi) Water at boiling point Record the setting time for each mixture, and plot a graph of setting time versus temperature.
Experiment (3) – Accelerators and Retarders Make a specimen at W/P ratio 0.5 (60g plaster, 30ml water,) but dissolve 0.5g of potassium sulfate in the water before mixing. Repeat this experiment, but using sodium chloride, borax, and sodium citrate as the additives. By comparing the setting time of this specimen with that of the 0.5 W/P ratio specimen without additions from Experiment (1) determine which chemicals are accelerators, and which are retarders. Experiment (4) – Setting expansion
Using the expansion micrometer provided, measure the setting expansion of plaster, stone and die stone. Set the micrometer to zero, which will leave a 100mm trough length. Grease the sides of the trough, and pour the plaster mixture in. Immediately after the plaster has become rigid (Gloss-off) back the micrometer end block off a couple of millimetres to allow the plaster to expand. After 1 hour, close the micrometer onto the end of the plaster block and measure any expansion.
RESULTS Record the results from each experiment in a table DISCUSSION /CONCLUSION Compare the results from each experiment with what you would expect from previous theory sections. What significance do these results have for dental use of plaster products?
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IMPRESSION MATERIALS The next material we consider is the “Impression” material, the substance that is used to take an impression of the patient’s oral structure. This can be one of two types, rigid or flexible, , sometimes (wrongly) called non-elastic and elastic. The rigid ones were the original materials, now no longer used, and the more recent materials are flexible, or elastic Rigid impression materials: Plaster Zinc oxide/Eugenol Impression compound Flexible impression materials: Alginate Agar Polyether Polysulfide Silicone (addition) Silicone (condensation) The difference between the two groups is that the rigid impression materials cannot not be flexed to any extent after they have set without breaking them. Flexible, or elastomeric, materials have can have a large amount of elastic, or rubbery behaviour. After a force is applied to flex them, they will return to their original shape when the force is removed. In use rigid materials cannot be used to take an impression of any dental surface which contains any undercuts, or reverse angles, because the only way they can be removed from such a surface is to break them into pieces and subsequently re-assemble the bits. While this used to be done when rigid impression materials were the only ones available, the more recent flexible materials have taken over because they can be used more easily for all types of impressions. In this part of Materials Science we will look at the agar and alginate materials. The synthetic polymer materials will be dealt with at a later stage in the course.
Hydrocolloid materials, Agar and Alginates Agar is the first of the flexible, or elastic impression materials to be used in dentistry. It is cheap, but complicated to use, and difficult to disinfect satisfactorily. It has been replaced by the synthetic rubber polymers for oral impressions, but still finds occasional use as a material for duplicating from stone models. Agar and alginates are colloid materials. Colloids are one member of a group of materials called suspensions. These are constructed of a very fine suspension of one substance in another. The particles of the first substance are so fine that they do not settle. For example, mist, or clouds, or steam are a colloidal suspension of water droplets in air, sometimes called an aerosol. (liquid in gas suspension) Smoke is a suspension of solid particles in air (solid in gas suspension), Margarine and hand cream are suspensions of oil droplets in water, called an emulsions (liquid in liquid suspension).
Agar is made from a natural material derived from seaweed, which forms a colloidal suspension of solidified particles in water. Above a certain temperature, the particles are separated in the water, and the suspension behaves like a thick liquid. Below this temperature
the particles join together and form a brush or fibril structure, long molecules with a structure like tiny combs. The water is contained in the spaces between these fibres, and the material now behaves like a rubbery solid. In the solid form, this suspension is called a Gel, in the liquid form it is called a Sol.
Interlocking fibril structure of colloidal material The change from a gel to a sol and from as sol to a gel takes place at about 43OC, just above mouth temperature so that the material can be heated to form a thick fluid, then placed in the mouth on a water-cooled tray where it will become a sol and take an impression. Because the change from gel to sol structures involves a lot of movement of atoms to make or break up the fibril structure, the change is slow. Often the liquefaction temperature will be well above the gelation temperature, because the material is warming up while it liquefies. Agar is an accurate impression material. The elastic recovery is 98.8%. This means that the material will regain 98.8% of its shape to after the elastic strain of being removed from undercuts. The flexibility is rated at 11, a reasonable value which indicates that the material is not too stiff to remove from undercuts. Like most polymers agar will flow, that is, it will gradually change shape with time under a light load. This is a property like the flow of a liquid, and leads to materials like colloids being referred to as “semi-solids” It is thus recommended to remove agar (and any other flexible impression material) rapidly rather than gradually, to minimise flow. The mechanical properties of agar are dependent on the amount of water in its structure. More water decreases the strength and stiffness, and increases flexibility and flow. The usual mixture of agar is supplied as a gel containing 15% agar, and a little borax, potassium sulfate, preservatives and colouring agents. The Borax strengthens the mix. The potassium sulfate is there to act as an accelerator on the surface of the subsequent stone model poured in the agar. If the sulfate were not there, the water in the gelled agar would act to slow down the setting of the surface of the plaster, leaving wet, weak patches. The major problem with agar is that it is dimensionally unstable. Because its structure contains a large and variable amount of water, it will easily change shape and dimensions depending on the water content of the air around it. The expansion of agar placed in contact with water is called imbibition, and its shrinkage if placed in air of less than 100% humidity is called syneresis. The uncertainty of its dimensional stability means that models must be poured into agar immediately after it has set.
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Agar is cheap, accurate (if poured up immediately), and reusable. However, it needs a water boiler to heat it, and water-cooled trays. Its re-useability cannot be applied to oral impressions because of the risk of infection transfer. Its equipment cost, and its dimensional instability have led to its being used nowadays only for model duplications. For oral impressions, alginates and the rubber impression materials have superseded it.
Alginate Alginate is a synthetic colloid based on the compound, sodium alginate. This undergoes a chemical reaction and sets with a similar molecular structure to agar, but it has increased strength and reduced shrinkage due to the presence of a large amount of fillers included. These large amounts of filler also make set alginate stiffer and stronger than agar. The alginate is supplied as a powder containing about 15% sodium alginate, about 10% gypsum (calcium sulfate di-hydrate), some phosphates or carbonates as retarders and 70% inert filler, usually a silicate powder. When the powder is mixed with water, the sodium alginate dissolves immediately. The gypsum dissolves slowly, Setting occurs when the calcium sulfate reacts with the sodium alginate to form calcium alginate, which is insoluble in water. It forms as a set solid and includes a lot of water in its structure. The phosphate present slows the reaction by using up some of the calcium sulfate. Sodium alginate + calcium sulfate → calcium alginate↓ + sodium alginate Because it sets by a chemical reaction, not the physical gelation of agar, alginate sets more quickly and does not need complicate cooling trays. Because it sets by a chemical reaction which is not easily reversed, alginate is not re-usable. Mainly because of its ease of use, alginate has taken over from agar as a flexible impression material. It has a lower elastic recovery of 97.2%. Like agar, however it has only a moderate resistance to tearing, and this strength is reduced if too much water is used in the mixture. Because alginate is a colloidal material, its structure still contains considerable amounts of water. Like agar it will suffer from substantial shrinkage after use due to syneresis, The model must have the modelling material poured into it immediately after setting. Not only do agar and alginate tend to shrink after setting, but if they are placed in contact with variable amounts of water, they may not only shrink, but also distort as different areas experience different amounts of syneresis or imbibition USING ALGINATE IMPRESSION MATERIAL •
Alginate needs to be mixed with care to avoid lumps which form grainy patches after setting; it is, however, relatively fast setting, so more than one minute mixing is unadvisable, even for large mixes.
•
Cooling the mixing water will prolong the setting time, warming the water will shorten it. This is because the alginate reaction is a chemical reaction, and speeds up as the temperature is raised. In contrast, agar’s physical setting reaction is slowed by heating.
•
Using extra water in the mix, above that recommended, will increase the flexibility, but decrease the stiffness and tear resistance of the set product. Conversely, less water and more powder makes a stronger, but stiffer product.
•
It may not always be possible to pour the model immediately after the alginate has set. In such cases the dimensional change of the alginate can be reduced, or even prevented, by storing the impression in an atmosphere saturated with water. This is usually achieved by putting it in a plastic bag together with a wet rag, but it is better done by putting the wet rag or paper into a container with a perforated lid, made from a polish tin or spice jar. In this case the impression is surrounded by uniformly moist air, but cannot come into full contact with extra water in the wet rag, which might cause localised expansion of some part of the impression, and so produce distortion. This procedure usually minimises the alginate shrinkage for a few hours, but it is hard to know if it has worked. It is still much better practice to pour the model immediately.
•
To reduce the chances of subsequent distortion, the set impression should be kept in the tray and not have any weight placed on it.
Agar will usually contain a small amount of accelerator for the plaster which will be set on it. Like agar, its water content can otherwise cause unset patches on the surface of the subsequent model. This small amount of accelerator can settle in the storage container with time or vibration during storage, so it is a good idea to mix the powder before each use. Do this by shaking the closed tin. Stirring the powder in an open tin can is a potential long-term health hazard, as the filler is usually some form of fine silicate powder and should not be breathed.
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