CHAPTER 1 1:0 INTRODUCTION In recent years, the natural fibres have attracted substantial importance as potential structural material. The attractive plus point of natural fibre is in terms of industrial usage which has made its availability more demanding. Keeping this in view the present work has been undertaken to develop a polymer matrix composite (epoxy resin) using bagasse fibre as reinforcement and to study its mechanical properties and performance. The composites are prepared with different volume fraction of bagasse fibre.

Kenya is a country which largely depends on agriculture. Farm products constitute the separate joints and bones of the backbone of the country’s economy out of which natural fibre has been a waste product which remains largely untapped. In recent years the natural fibre epoxy composites has attracted substantial importance as a potential structural material.

The mechanical properties of several types of epoxy systems are designed based on the chemical structure, network structure and morphology aiming at cryogenic applications such as cryogenic wind tunnels, cryogenic transport vessels, support structures in space shuttles and rockets. In these applications they are often under cyclic loading. The attractive features of natural fibres like jute, sisal, coir and banana have been their low cost, light weights, high specific modulus, renew ability and biodegradability.

Natural fibres are lingo-cellulosic in nature. These composites are gaining importance due to their non-carcinogenic and bio-degradable nature. The natural fibre composites can be very cost effective material especially for building and construction industry. However in many instances residues from traditional crops such sugarcane bagasse or from the usual processing operations of timber industries do not meet the requisites of being long fibres. Bagasse contains about 40% cellulose, 30% hemi-cellulose, and 15% lignin. The present use of bagasse is mainly as a fuel in the sugar cane mill furnaces, for example in Mumias Sugar Company based in Kenya, Western province. It is felt that the value of this agricultural residue 1

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can be upgraded by bonding with resin to produce composites suitable for building materials. Keeping this in view the present work has been undertaken to develop a polymer matrix composite (epoxy resin) using bagasse fibre as reinforcement with volume fractions 10, 20, 30 & 40% and to study its mechanical properties. In the next part of this project the main emphasis was laid on the experimental work relating to the mechanical behaviour of this composite. Kenya is endowed with an abundant availability of natural fibres such as Jute, coir, sisal, pineapple, ramie, bamboo, banana etc. We majorly focus on the development of natural fibres composites primarily to explore value-added application avenues. Such natural fibre composites are well suited as wood substitutes in the housing and construction sector. The development of natural fibre composites in Kenya should be adapted on basis of two pronged strategy of preventing depletion of forest resources as well as ensuring good economic returns for the cultivation of natural fibres. The developments in composite material after meeting the challenges of aerospace sector have cascaded down for catering to domestic and industrial applications. Composites, the wonder material with light-weight; high strength-to-weight ratio and stiffness properties have come a long way in replacing the conventional materials like metals, wood etc. The material scientists all over the world focused their attention on natural composites reinforced with Jute, Sisal, Coir, Pineapple etc, the primarily reason being to cut down the cost of raw materials. Usually the fibre reinforcement is done to obtain high strength and high modulus. Hence it is necessary for the fibres to possess higher modulus than the matrix material. So the load is transferred to the fibre from the matrix more effectively. Fibre reinforced composites are popularly being used in many industrial applications because of their high specific strength & stiffness. Due to their excellent structural performance these composites are gaining potential also in tri-biological applications. The physical properties of natural fibres are mainly determined by the chemical & physical composition, such as structure of fibres’ cellulose content, angle of fibrils, cross section and by the degree of polymerization. Only a few characteristics values but especially the specific mechanical properties can reach the compensable values of traditional fibres. The application of natural fibres as reinforcing materials in composite materials require as just for glass fibre reinforced composites, a strong adhesion between the fibre and the matrix regardless of whether a traditional polymer(thermoplastic or thermosetting) matrix, a biodegradable polymer matrix or 2

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cement is used. The mechanical and other physical properties of the composites are generally dependent on the fibre content, which also determines the possible amount of coupling agents in the composite. An important property of natural fibres to be used as reinforcements is their availability in large quantities. For several more technical oriented applications, the fibres have to be specially prepared or modified regarding, homogeneity of the fibre’s properties, degrees of elementarization and degumming, degree of polymerization and crystallization, good adhesion between fibre and matrix, moisture repellent properties and flame retardant properties. Nowadays natural fibres are very fast replacing the traditional manmade fibres as reinforcements they have several advantages over manmade fibres, which include; plant fibres are renewable and their availability is more or less unlimited, when natural fibre composite were subjected to at the end of their life cycle, to a combustion process or landfill the amount of CO2 released of the fibres is neutral with respect to their assimilated amount during their growth. The abrasive nature is lower which leads to advantages regarding technical material recycling, natural fibre reinforced plastics by using biodegradable polymers as matrix are the most environment friendly materials.

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1.1 OBJECTIVE The main objective of this research was to study and evaluate the mechanical and physical properties of sugarcane bagasse as reinforcing fibres in epoxy resin matrices. The effects of sugarcane bagasse fibre reinforcement on epoxy resin matrix was investigated with respect to the fibre content and their orientations on the following composite strength properties;



Tensile Strength



Tensile Modulus of Elasticity



Flexural Strength



Flexural Modulus of Elasticity

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CHAPTER 2 2.0 WHAT IS A COMPOSITE? The most widely used meanings are the ones which have been stated by; 

Jartiz defines it as ”Composites are multifunctional material systems that provide characteristics not obtainable from any discrete material. They are cohesive structures made by physically combining two or more compatible materials, different in composition and characteristics and sometimes in form”. The weakness of this definition resided in the fact that it allows one to classify among the composites any mixture of materials without indicating either its specificity or the laws which should given it which distinguishes it from other very banal, meaningless mixtures.



Beghezan defines it as “The composites are compound materials which differ from alloys by the fact that the individual components retain their characteristics but are so incorporated into the composite as to take advantage only of their attributes and not of their short comings”, in order to obtain improved materials.



Kelly very clearly stresses that the composites should not be regarded simple as a combination of two materials. In the broader significance; the combination has its own distinctive properties. In terms of strength to resistance to heat or some other desirable quality, it is better than either of the components alone or radically different from either of them.



Van Suchetclan explains composite materials as heterogeneous materials consisting of two or more solid phases, which are in intimate contact with each other on a microscopic scale. They can be also considered as homogeneous materials on a microscopic scale in the sense that any portion of it will have the same physical property.

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2.1 WHY STUDY COMPOSITES? Over the past years composite materials, plastics and ceramics have been the dominant emerging materials. The volume and number of applications of composite materials have grown steadily, penetrating and conquering new markets relentlessly. Modern composite materials constitute a significant proportion of the engineered materials market ranging from everyday products to sophisticated applications. While composites have already proven their worth as weight-saving materials, the current challenge is to make them cost effective.

The efforts to produce economically attractive composite components have resulted in several innovative manufacturing techniques currently being used in the composites industry. It is obvious, especially for composites, that the improvement in manufacturing technology alone is not enough to overcome the cost hurdle. It is essential that there be an integrated effort in design, material, process, tooling, quality assurance, manufacturing, and even program management for composites to become competitive with metals.

2.2 CHARACTERISTICS OF THE COMPOSITES Composites consist of one or more discontinuous phases embedded in a continuous phase. The discontinuous phase is usually harder and stronger than the continuous phase and is called the ‘reinforcement‘or ‘reinforcing material’, whereas the continuous phase is termed as the matrix. Properties of composites are strongly dependent on the properties of their constituent materials, their distribution and the interaction among them. The composite properties may be the volume fraction sum of the properties of the constituents or the constituents may interact in a synergistic way resulting in improved or better properties. Apart from the nature of the constituent materials, the geometry of the reinforcement (shape, size and size distribution) influences the properties of the composite to a great extent.

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The concentration distribution and orientation of the reinforcement also affect the properties. The shape of the discontinuous phase (which may by spherical, cylindrical, or rectangular crosssanctioned prisms or platelets), the size and size distribution (which controls the texture of the material) and volume fraction determine the interfacial area, which plays an important role in determining the extent of the interaction between the reinforcement and the matrix.

Concentration, usually measured as volume or weight fraction, determines the contribution of a single constituent to the overall properties of the composites. It is not only the single most important parameter influencing the properties of the composites, but also an easily controllable manufacturing variable used to alter its properties. 2.3 COMPONENTS OF A COMPOSITE MATERIAL In its most basic form a composite material is one, which is composed of at least two elements working together to produce material properties that are different to the properties of those elements on their own. In practice, most composites consist of: 

A bulk material (the ‘matrix’) which is the binder.



A reinforcement of some kind, added primarily to increase the strength and stiffness of the matrix.

2.4 CLASSIFICATION Composite materials can be classified into many categories depending on the type of matrix material, reinforcing material type etc. According to the type of matrix material they can be classified as follows: 

Metal matrix type composites: MMC are composed of a metallic matrix (Al, Mg, Fe, Co, Cu)



Ceramic matrix composites: CMC is a material consisting of a ceramic combined with a ceramic dispersed phase.



Polymer matrix material: PMC are composed of a matrix from thermosetting (unsaturated polyester, epoxy) or thermoplastic (nylon, polystyrene) and embedded glass carbon, steel or Kerler fibres (dispersed phase).

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Some of the major advantages and limitations of resin matrices are shown in Table below: ADVANTAGES Low densities Good corrosion resistance Low thermal conductivities Low electrical conductivities Translucence Aesthetic Color effects

DISADVANTAGES Low transverse strength Low operational temperature limits

Table1. Advantages and disadvantages of polymer matrix. Generally, the binders (polymer matrices) are selected on the basis of adhesive strength, fatigue resistance, heat resistance, chemical and moisture resistance etc. The resin must have mechanical strength which should cluster with that of the reinforcement. It must be easy to use in the fabrication process selected and also stand up to the service conditions.

According to the type of reinforcing material type they can be classified into the following categories: 

Particle composites Particle reinforced composites consist of a matrix reinforced by a dispersed phase in the form of particles. It can be either of random orientation or preferred orientation.

FIG.1 PARTICULATE FIBRE COMPOSITE

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

Fibrous composites-Short fiber: they consist of a matrix reinforced by a dispersed phase in the form of discontinuous fibers either of random or preferred orientations.

Fig 2:(a) SHORT FIBRE COMPOSITE



Long fiber composites - they consist of a matrix reinforced by a dispersed phase in the form of continuous fibers. They can be either unidirectional or bidirectional.

(b) LONG FIBRE COMPOSITE



Laminate composites-when a fiber reinforced composite consists of several layers with different fiber orientations, it is called multilayer composite.

We also have;

FIG. 3(a) FLAKE COMPOSITE

(b) FILLER COMPOSITES

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The two major classes are; 2.4.1 Particulate Composites As the name itself indicates, the reinforcement is of particle nature (platelets are also included in this class). It may be spherical, cubic, tetragonal, a platelet, or of other regular or irregular shape, but it is approximately equiaxed. In general, particles are not very effective in improving fracture resistance but they enhance the stiffness of the composite to a limited extent. Particle fillers are widely used to improve the properties of matrix materials such as to modify the thermal and electrical conductivities, improve performance at elevated temperatures, reduce friction, increase wear and abrasion resistance, improve machinability, increase surface hardness and reduce shrinkage.

2.4.2 Fibrous composites A fibre is characterized by its length being much greater compared to its cross-sectional dimensions. The dimensions of the reinforcement determine its capability of contributing its properties to the composite. Fibres are very effective in improving the fracture resistance of the matrix since a reinforcement having a long dimension discourages the growth of incipient cracks normal to the reinforcement that might other wise lead to failure, particularly with brittle Matrices Man-made filaments or fibres of non-polymeric materials exhibit much higher strength along their length since large flaws, which may be present in the bulk material, are minimized because of the small cross-sectional dimensions of the fibre. In the case of polymeric materials, orientation of the molecular structure is responsible for high strength and stiffness. Fibres, because of their small cross- sectional dimensions, are not directly usable in engineering applications. They are, therefore, embedded in matrix materials to form fibrous composites. The matrix serves to bind the fibres together, transfer loads to the fibres, and protect them against environmental attack and damage due to handling.

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CHAPTER 3.0 LITERATURE REVIEW 3.1 MATRIX Many materials when they are in a fibrous form exhibit very good strength property but to achieve these properties the fibres should be bonded by a suitable matrix. The matrix isolates the fibres from one another in order to prevent abrasion and formation of new surface flaws and acts as a bridge to hold the fibres in place. A good matrix should possess ability to deform easily under applied load, transfer the load onto the fibres and evenly distributive stress concentration.

3.2 REINFORCEMENT The role of the reinforcement in a composite material is fundamentally one of increasing the mechanical properties of the neat resin system. All of the different fibers used in composites have different properties and so affect the properties of the composite in different ways. For most of the applications, the fibres need to be arranged into some form of sheet, known as a fabric, to make handling possible. Different ways for assembling fibres into sheets and the variety of fibre orientations make it possible to achieve different characteristics. Agro-based fibres are classified according to what part of the plant they come from. Five different fibre classifications are;



Bust or stem fibres , which are fibrous bundles in the inner bark of the plant stem running the length of the stem.



Leaf fibres, which run the length of the leaves. Seed-hair fibres.



Core, pith or stick fibres, which form the low density, spongy inner part of the stem of certain plants. All other plant fibres not included above.

Examples of bust or stem fibres include; jute, flax, hemp, kenaf, ramie, roselle and urena. Leaf fibres include; bananas, sisal, henequen, abaca, pineapple, cantala , caroa, mauritius and phormium. Seed-hair fibres include; coir, cotton, kapok, and milk weed floss. Core fibres represent the centre or pith fibres of such plants as kenaf and jute and can represent over 85% of the dry weight of these plants. The remaining fibres include; roots, leaf segments, flower heads and seed 11

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3.3 MATERIALS USED 3.3.1 BAGASSE FIBER-REINFORCEMENT

Figure 4: Bagasse fibre Sugarcane bagasse is a residue widely generated in high proportions in the agro-industry. It is a fibrous residue of cane stalks left over after the crushing and extraction of juice from the sugar cane. Bagasse is generally gray-yellow to pale green in colour. It is bulky and quite non uniform in particle size. The sugar cane residue bagasse is an under utilized, renewable agricultural material that consist of two distinct cellular constituents. The first is a thick walled, relatively long, fibrous fraction derived from the rind and fibro-vascular bundles dispersed throughout the interior of the stalk. The second is a pith fraction derived from the thin walled cells of the ground tissue. The main chemical constituents of bagasse are:  Cellulose and hemicelluloses; They are present in the form of hollow cellulose in bagasse which contributes to about 70 % of the total chemical constituents present in bagasse.  Lignin; It acts as a binder for the cellulose fibres and also behaves as an energy storage system. Bagasse consists of water, fibre and small quantities of solids in solution in the following proportions. Water 46-57 %( mean50%), Fibre 43%-53 %( mean 47%), Solids in solution (sugar) 2%-6 % (mean 3%). It is a composition within certain limits as variable and depends in the varieties, their maturity, the harvest technique and efficiency of milling. By definition the fibre of the bagasse is the component which is insoluble in water. It consists of mainly cellulose pentosens and lignin. Cellulose is a polysaccharide having the general formula (C6H10O6)n and the main constituent of vegetable tissue. According to its degree of solubility in caustic soda, cellulose is classified as;  ∞-Cellulose, portion insoluble in a 17.5%solution caustic soda at 20 oC 12

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

β- Cellulose in a 17.5 % solution of caustic soda but easily precipitated when the solution is acidified.



α- Cellulose, like in 2 and not precipitated by acids but by alcohol.

The soluble cellulose, alpha and beta are called hemi-cellulose. Pentosans are a form of hemicellulose which on hydrolysis yield xylose and arabinose and heating with steam is important industrially and can be expressed as follows; C5H8O4 → (Pentosan)

C5H10 → C5H9O2 (pentose)

(furfural)

The third costituent of the fibre is lignin, a substance with a high molecular weight and which structure and size have not yet been satisfactorily established being richer in carbon, cellulose, and lignin is used essentially for combustion. On average bagasse composition can be estimated as follows;

Composition

Raw bagasse

Fibre

pith

Alpha (α) Pentosans Lignin Ash

35.0% 27.0% 21.0% 1.5%

0.5% 22.0% 26.0% 49.0%

35.0% 29.0% 21.0% 0.2%

Table 2. Composition of bagasse Physically bagasse fibre is considered to be made up of 60% true fibre and 40% pith.The true fibre has an average length of 1.5mm and length to diameter ratio of 70.The pith has a mean length of 0.3 mm and length to diameter ratio of 5. Physical composition varies within narrow limits. As far as calorific value is concerned, moisture as is the most important factor. A bagasse at 45% moistire is a result of exellent milling whereas, 52% moisture is poor milling efficiency with low heating value. In general, bagasse moisture of 48% is the reality today with standard sugar mills.

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Apart from water, bagasse contains; 

Fibre- The components of which are carbon, hydrogen and oxygen. Some sucrose (1-2%)



Mineral salts



Extraneous matter (soil, rocks etc)

Mechanical harvesting does bring soil and crushed stones with the cane; these decreases calorific value of bagasse. Bagasse coming from mills at 48% moisture does not keep well. It is subject to fermentation and chemical reaction that can bring the outbreak of slow internal combustion upto complete combustion. When bagasse is stored in a room, one must prevent the entry of droughts if internal combustion has already started, otherwise, spontaneous combustion will occur. Bagasse dried to moisture content of less than 30% can be stored upto one year. A process developed in Brazil called bagatex 20 using an enzyme to dry the bagasse to less than 20% moisture content is worth to note. Below are physical properties of bagasse. TYPE OF FIBRE

CELLLULOSE (%)

FIBRE DIMENSION (mm)

LIGNIN MEAN (%) LENGTH

MEAN WIDTH (mm)

Cotton Seed flax Hemp Abaca Sisal Kenaf Jute Coniferouswood Esparto Papyrus Sugarcane bagasse Cereal straw Rice straw Deciduous Coir

85-90 43-47 57-77 56-63 47-62 44-57 45-63 40-45 33-38 38-44 32-37 31-45 28-38 38-49 35-62

0.7-1.6 21-23 9-13 7-9 7-9 15-19 21-26 26-34 17-19 16-19 18-26 16-19 12-16 23-30 30-45

25 30 20 6.0 3.3 2.6 2.5 4.1 1.9 1.8 1.7 1.5 1.4 1.2 0.7

0.02 0.02 0.022 0.024 0.02 0 0.02 0.025 0.013 0.012 0.02 0.023 0.008 0.03 0.02

Table3:Physical properties of bagasse fibre 14

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Below are the mechanical properties of some natural fibre;

FIBRE TYPE

sugarcane sisal

jute

Water reed N/A

coconut Bamboo

FIBRE LENGTH (MM) FIBRE DIAMETER (MM) RELATIVE DENSITY MODULUS OF ELASTICITY GPA ULTIMATE TENSILE STRENGTH MPA ELONGATION AT BREAK % WATER ABSORPTION %

N/A

N/A

175300

50-100

N/A

0.2-0.4

N/A

0.10.2

N/A

0.1-0.2

0.05-0.4

1.12-1.15

N/A

N/A

1.121.15 19-26

1.5

13-26

1.0210.5 26-32

15-19

180-290

275570

250350

70

120200

350-500

N/A

3-5

1.2

10-25

N/A

70-75

60-70

1.51.9 N/A

N/A

130180

N/A

5

33-40

Table4: Mechanical properties of commonly used natural fibres N.B N/A-Properties not readily available or not applicable

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3.3 .2 EPOXY RESIN Plastics can be broadly classified into two classes namely thermoplastics and thermosets. Thermoplastics are anisotropic, high molecular weight strong solids and do not crosslink. They soften on heating but upon cooling regains their original mechanical properties Typical examples in clued nylon 6-6, polypropylene and polycarbonates among others. They yield and undergo large deformation before final fracture. Their mechanical properties are however depended on the temperature and applied strain rate so they creep under constant load. This means that in a composite system there will be a redistribution of the load between the resin and fibres during deformation.

Thermosetting resins on the other hand are isotropic and brittle. They harden by a process of chemical cross-linking and do not melt on heating. Examples in this category include polyester, epoxy, phenolics, silicons and polyamides. Epoxy resins may be defined as resins in which chain in extension and cross-linking occurs through the reactions of epoxide group.

This epoxy has outstanding properties which are utilized as the matrix material. 

Excellent adhesion to different materials.



High resistance to chemical and atmospheric attack.



High dimensional stability.



Free stresses.



Excellent mechanical and electrical properties.



Odorless, tasteless and completely nontoxic.



Negligible shrinkage.

The majority of epoxy resins used in composites are manufactured by the reaction of epichlorhydrin with materials such as bisphenol A or aromatic amines as it has been noted below.

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Liquid Epoxy Resin This is a reaction product of epichlorohydrin and bisphenol A. One of the epoxies available in the markets from Dow Chemical Company is; D.E.R. 330 Epoxy Resin 

This is a liquid epoxy resin processed to provide low viscosity without the use of diluents. The physical strength, toughness, excellent adhesion, chemical resistance and low shrinkage properties have established liquid epoxy resins as major raw materials for high quality solvent-free coatings, linings, industrial flooring, grouts and concrete reinforcements. They have also found application in the fields of tooling, encapsulation, adhesives, filament winding and laminates.



D.E.R. 330 Resin can also serve as a basis for advanced polymers for a variety of solvent-borne, water-borne and UV-curable resins. A wide variety of curing agents is available to cure this liquid epoxy resin at ambient conditions. The most frequently used are aliphatic polyamines, polyamides, and modified versions of these. If anhydride or catalytic curing agents are employed, elevated temperatures cures are necessary and long post-cures are required to develop full end properties.

Epoxy resins contain a reactive oxirane structure

This is commonly referred to as“epoxy” functionality. Liquid epoxy resins are converted through these reactive epoxy sites into tough, insoluble, and infusible solids. The simplest possible epoxy resin derived from the reaction of bisphenol A and epichlorohydrin is (2,2-bis[4-(2'3' epoxy propoxy) phenyl] propane), commonly called the diglycidyl ether of bisphenol

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The higher molecular weight homologs are represented by the following theoretical structure:

Generic Bisphenol A Based Epoxy Resin Chemical Structure With increasing molecular weight, another reactive site — the OH group is introduced. This group can react at higher temperatures with anhydrides, organic acids, amino resins, and phenolic resins, or with epoxide groups (when catalyzed) to give additional cross-linking.

In high melting point solid resins, “n” may be as high as 18.

3.4 MODIFICATION ON EPOXIES

Some additions are added to the epoxy resin for various reasons though the major reasons are to reduce cost and to improve workability. They are grouped in three forms namely; reactive diluents, modifiers and fillers. Each form has specific functions as explained below. 3.4.1 REACTIVE DILUENTS A reactive diluent is used primarily to reduce viscosity. Adding reactive diluents also permits higher filler loading and gives better wetting and impregnation. Preferably, the diluents should react with the curing agent at approximately the same rate as the resin, contribute substantial viscosity reduction at low concentrations, and be nonreactive with the resin under normal storage conditions. 18

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Reactive diluents in common use are:

Butyl Glycidyl Ether (BGE) (Molecular weight - 130)

C12-C14 Aliphatic Glycidyl Ether (Molecular weight -242-270)

Cresyl Glycidyl Ether (CGE) (Molecular weight - 165)

2

- Ethylhexyl Glycidyl Ether (Molecular weight-186

3.4.2 MODIFIERS Epoxy resins may be modified for several reasons: 

To enhance physical properties, such as impact strength and adhesion.



To alter viscosity,



To improve life, lower exotherm, or reduce shrinkage.

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

To lower the cost of the formulation.

Butyl glycidyl ether which is a diluent produces maximum viscosity reduction. However, excessive exposure to these products may present serious health hazards, hence a need for adherence to safety precautions. The higher molecular- weight reactive diluents (like the C12-C14 aliphatic ethers), are safer to work with, but not quite as efficient as the former. Figure 1 shows the viscosity-diluents concentration relationship for representative epoxy resins Effect of Diluents on Epoxy Resins

Figure: 5

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Resin Modifiers Resin modifiers are used to improve: 

Mechanical and thermal shock resistance,



Increase elongation, and obtain higher impact strength and flexibility.

Usually there is some sacrifice of physical strength, electrical properties, chemical or solvent resistance, or elevated temperature performance. We have two types of modifiers namely; reactive modifiers and Non-reactive modifiers.

Reactive epoxide-type modifiers The reactive modifiers from Dow Chemical Comany include; 

D.E.R. 732 Aliphatic diepoxides



D.E.R. 736 flexible epoxy resins

Mono-functional epoxide compounds (such as C12 - C14 Glycidyl Ether. Such compounds can be used at ratios up to 1:1 to obtain a flexible cured composition. They have the added advantage of being shelf stable when blended with the resin. The low viscosity and light color of D.E.R. 732 and D.E.R. 736 resins offer viscosity reduction in epoxy formulations without affecting color of cured compositions. These advantages are not found with most other flexibilizers.

Figure 1 above shows the effect on viscosity of increasing amounts of flexible resin in blends with D.E.R. 331. Modifiers which may be reactive as curing agents are often used. Common among these are polysulfide polymers, triphenyl phosphite, and various polyamides. The latter react readily with the epoxy. Nonreactive modifiers They include; 

Dibutyl phthalate.



Nonylphenol.



Pine oil and



Glycol ethers.

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Nonreactive modifiers are not used extensively, as they cause reduction in cured resin properties. When used, their more common function is to lower cost. Chief requisites is that they should; 

Be compatible with the resin before and after cure,



Not vaporize or foam during cure.



Not migrate excessively from the cured composition.

3.4.3 RESIN FILLERS The use of fillers in epoxy compositions can lower costs, reduce exotherm, extend life, and achieve improvement in one or more of the cured resin properties like;  Improved machinability  Improved abrasion resistance  Improved impact strength chopped glass other fibrous materials  Improved electrical properties mica silica powdered or flaked glass  Improved thermal conductivity metallic fillers coarse sand alumina  Improved anti-settling and flow.

Most fillers reduce the coefficient of thermal expansion and shrinkage in proportion to the amount of filler rather than the type of filler used. Such improvements are usually achieved at the sacrifice of other mechanical properties. Fine granular fillers –When fine granular fillers are used the following properties are affected. They include; Tensile strength. Flexural strength. Impact strength Medium-weight granular fillers may be used at quite higher loadings They include; powdered aluminum, alumina, and silica, Fibrous and flake fillers. They impart high viscosities at low filler loadings They include ; Chopped glass strand, Glass flake, Mica. 22

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Heavy fillers may be loaded at ratios up eight times that of medium weight granular loadings. They include; Powdered iron, iron oxide, Coarse sand. Generally the finer particle size fillers are easier to incorporate, and have fewer tendencies to settle. Coarse and heavy fillers tend to settle and cake on standing unless some light-weight filler or antisettling agent is also incorporated. Fumed silica compounds are effective as anti-settling and thixotropic agents.

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CHAPTER 4 4.0 MECHANICAL PROPERTIES Two important mechanical properties of any resin system are its tensile strength and stiffness. The two fibres below show results for tests carried out on commercially available epoxy resin, vinyl-ester, and polyester cured at 20oC and 80oC

3.5 3 2.5 2 Series 1 1.5

Series 2

1 0.5 0 Epoxy

polyster

Vinylester

Figure 6 Tensile modulus comparison graph After a cure period of seven days at room temperature it can be seen that typical epoxy will have higher properties than typical polyester and vinyl ester for both strength and stiffness. The beneficial effect of a post cure at 80OC for five hours can also be seen. Also of importance to the composition designer and builder is the amount of shrinkage that occurs in a resin during and following its cure period. Shrinkage is due to their resin molecules rearranging and reorienting themselves in the liquid and semi-gelled phase. Polyester and vinyl ester require considerable molecular arrangements to reach their cured state and can show shrinkage up to 8%. The different nature of the epoxy reaction, however, leads to very little rearrangement and with no volatile by products being evolved: typical shrinkage of an epoxy is reduced to around 2%. 24

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The absence of shrinkage is , in part, responsible for the improved mechanical properties of epoxies over polyester, as shrinkage is associated with built in stresses that can weaken the material. Also shrinkage through the thickness of a laminate leads to ‘print through of the pattern of the reinforcing fibres , a cosmetic defect that is difficult and expensive to eliminate.

4.1 DEGRADATION FROM WATER INGRESS An important property of any resin, particularly in a Marin environment is its ability to withstand depredation from water ingress. All resins will absorb some moisture, adding to laminates weight, but what is more significant is how the absorbed water affects the resin and resin fibre bond in a laminate, leading to a gradual and long term loss in mechanical properties. An epoxy laminate immersed in water for a period of one year will retain around 90% of its inter-laminar shear strength. Whereas both polyester and vinyl reins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures, as a result, a thin polyester laminate can be expected to retain only 65% of its inter-laminar shear strength for the same period. The elevated temperature soaking gives accelerated degradation properties for the immersed laminate. 4.2 GELATION, CURING AND POST - CURING On addition of the catalyst or harder a resin will begin to become more viscous until it reaches a state when it is no longer a liquid and has lost its ability to flow. This is the gel point. The resin will continue to harden after it has gelled, until, at some time later, it has obtained its full hardness and properties. This reaction itself is accompanied by the generation of exothermic heat, which, in turn speeds the reaction. The whole process is known as the curing of the resin. The speed of cure is controlled by the amount of accelerator in the epoxy resin and by varying the type, not the quantity, of hardener in an epoxy resin.

Generally polyester resins produce a more severe heat and faster development of initial mechanical properties than epoxies of a similar working time. With both the resin types, however, it is possible to accelerate the cure by the application of heat, so

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that the higher the temperature the faster the final hardening will occur. This can be most useful when the cure would otherwise take several hours or even days at room temperature. A quick rule of thumb for the accelerating effect of heat on a reaction is that 10oC increases in temperature will roughly double the reaction rate. That is, if a resin gels in a laminate in 25minutes at 20oC it will gel at about 12 minutes at 30oC, provided no extra heat occurs.

Curing at elevated temperatures has the added advantage that is actually increases the end mechanical properties of the material, and many resin systems will not reach their ultimate mechanical properties of the material, unless the resin is given this post cure, which increases the amount of cross linking of the molecules that can take place. To some degree this post cure will occur naturally at warm room temperatures, but higher properties and shorter post cure times will be obtained if elevated temperatures are used. This is particularly true for material’s softening point for glass Transition temperature (Tg), which, up to a point, increases with increasing post curve temperatures. 4.3 ADHESSIVE PROPERTIES Adhesive properties of the resin system are important in realizing the full mechanical properties of a composite. The adhesion of the resin matrix to the fibre reinforcement or to a core material in a sandwich construction is important. Epoxy system offer the best performance of all the three resins considered here. Polyester resins generally have the lowest adhesive properties of the three. On the other hand, vinly-ester resin shows improved adhesive properties over polyester. Due to this performance property, epoxy resins are frequently found in many high strength adhesives. This is due to their chemical composition and the presence of polar hydroxyl and ester group. As epoxies cure with low shrinkage the various surface contacts set up between the liquid resin and the adherents are not disturbed during the cure The adhesive properties of epoxy are generally useful in the construction of honey comp cured laminates where the small bonding surface area means that maximum adhesion is required. The strength of bond between resin and fibre is not solely depended on the adhesive property of the resin system but is also affected by surface coating on the reinforcement fibres.

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4.4 MICRO-CRACKING The strength of a laminate is usually thought of in terms of how much load it can withstand before it suffer composition failure. This ultimate or breaking strength is the point it which the resin exhibits catastrophic breakdown and the fibre reinforcements break. However, before this ultimate strength is achieved, the laminate will spread through the resin matrix. This is known as ‘transverse microcracking’ and, although the laminate has not completely failed at this point, the breakdown process has commenced. Consequently, engineers who want a long-lasting structure must ensure that their laminate do not exceed this point under regular service loads.

The strain that a laminate can reach before micro-cracking depends strongly on toughness and adhesive properties of the resin system. For brittle resin systems, such as most polyester, this point occurs a long way before laminate failure, and so severely limits the strains to which laminates can be subjected. As an example, recent tests have shown that for a polyester/glass roving laminate, micro-cracking typically occurs at about 0.2% strain with ultimate failure not occurring until2.0% strain. This equates to a usable strength of only10% of the ultimate tensile strength.

As the ultimate strength of a laminate in tension is governed by the strength of the fibres, these resin micro-cracks do not immediately reduce the ultimate properties of the laminate. However, in an environment such as water or moist air, the micro-crack laminate. This will then lead to an increase in weight, moisture attack on the resin and fibre sizing agents, loss of stiffness and with time eventually drops in ultimate properties.

Increased resin/fibre adhesion is generally derived from both the resin chemistry and its compatibility with the chemical surface treatments applied to fibres. Here the well known adhesive properties of epoxy help laminates achieve higher micro-cracking strains. As it has been previously mentioned, resin toughness can be hard to measure, but is broadly indicated by its ultimate strain to failure.

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A set of properties of epoxy resins is given in table below which was compelled by JONSON(1979) from Manufactures literature. PROPERTY

VALUE

UNITS

Density

1.1-1.4

MgM-3

Young’s modulus

2-6

GNM-2

Poisons ratio

0.38-0.4

ν

Tensile strength

35-100

Mpa

Compressive strength

100-200

Mpa

Elongation to break

1-6

%

Thermal conductivity

0.1

WM-1 oC

Coefficient of thermal expansion(α)

60

10-6 oC

Heat distortion temperature

50-300

o

Shrinkage on curing

1-2

%

C

Table 5; Typical properties of epoxy resins used in composite materials(After Johnson 1979)

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CHAPTER 5 5.0 MECHANICAL PROPERTIES OF EPOXY RESIN/BAGGASSE COMPOSITES The test piece was subjected to a tensile load. Testing was done under room atmospheric conditions. A gauge length of 60mm was maintained for all test pieces. This was set as the distance between the gripping jaws before loading commenced. The specimens were tested in tension until failure, with the main parameters observed being the tensile modulus and strength. A complete load deflection plot to fracture was obtained for all specimens. Tensile strength was obtained by dividing the maximum load the composite would withstand by the original cross-sectional area. δuts = Where 6uts is the ultimate tensile strength in N/m2, Pmax is the maximum load in (N) and A the initial cross sectional area (mm2). Cross sectional area was calculated by taking an average of three readings of the pieces measured by a veneer caliper accuracy of 0.05mm the strain calculations were done with the assumptions that the strain in the machine member was negligible compared to the specimens strain. The tensile modulus of elasticity was obtained from the slope of the linear portion of the stress-strain curve. It was as the ratio of the increment stress to the corresponding increment strain.

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5.1 Flexural strength

Figure 7: 3 point bending test Flexural strength, also known as modulus of rupture, bend strength, or fracture strength, a mechanical parameter for brittle material, is defined as a material's ability to resist deformation under load. The transverse bending test is most frequently employed, in which a rod specimen having either a circular or rectangular cross-section is bent until fracture using a three point flexural test technique. The flexural strength represents the highest stress experienced within the material at its moment of rupture. It is measured in terms of stress, here given the symbol σ. In this project work we used a rectangular cross-section.

Flexural and Direct Tensile Strengths The flexural strength would be the same as the direct tensile strength if the material was homogeneous. In fact, most materials have small or large defects in them which act to concentrate the stresses locally, effectively causing a localized weakness. When a material is bent only the extreme fibers are at the largest stress, if those fibers are free from defects, the flexural strength will be controlled by the strength of those intact 'fibers'.

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However, if the same material was subjected to direct tension then all the 'fibers' in the material are at the same stress and failure will initiate when the weakest fiber reaches its limiting tensile stress. Therefore it is common for flexural strengths to be higher than direct tensile strengths for the same material. Conversely, a homogeneous material with defects only on it surfaces (e.g. due to scratches) might have a higher direct tensile strength than flexural strength. 5.2 FLEXTURE OF BAGASE EPOXY RESIN COMPOSITES

Introduction

Fig. 8 - Beam of material under bending. Extreme fibers at B (compression) and A (tension)

Fig. 9- Stress distribution across beam When an object formed of a single material, like a wooden beam or a steel rod, is bent (Fig. 8), it experiences a range of stresses across its depth (Fig. 9). At the edge of the object on the inside of the bend (concave face) the stress will be at its maximum compressive stress value. At the outside of the bend (convex face) the stress will be at its maximum tensile value. These inner and outer edges of the beam or rod are known as the 'extreme fibers'. Most materials fail under tensile stress before they fail under compressive stress, so the maximum tensile stress value that can be sustained before the beam or rod fails is its flexural strength.

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5.3 MEASURING FLEXURAL STRENGTH This can be achieved using three point and four point loading as illustrated below; 5.3.1 THREE POINT BENDING TEST

Fig. 10- Beam under 3 point bending

For a rectangular sample under a load in a three-point bending setup (Fig. 10):

σf =

for a rectangular cross section

P is the load (force) at the fracture point L is the length of the support span b is width d is thickness

σf=

π

for a circular cross section

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5.3.2 FOUR POINT BENDING TEST

Fig. 11 - Beam under 4 point bending For a rectangular sample under a load in a four-point bending setup where the loading span is one-third of the support span i.e. (1/3L)

σ= P is the load (force) at the fracture point L is the length of the support (outer) span b is width d is thickness If the loading span is 1/2 of the support span (i.e. Li - 1/2 L)

σ= If the loading span is neither 1/3 or 1/2 the support span for the 4 pt bend setup (Fig. 11):

σ=

(

)

Li is the length of the loading (inner) span 5.3.3 INTERFACE It has characteristics that are not depicted by any of the component in isolation. The interface is a bounding surface or zone where a discontinuity occurs, whether physical, mechanical, chemical etc. The matrix material must “wet” the fibre. Coupling agents are frequently used to improve wettability. Well “wetted” fibres increase the interface surfaces area. To obtain desirable properties in a composite. Failure at the interface (called debonding) may or may not be desirable.

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5.3.4 THE FLEXURAL INTER LAMINAR SHEAR STRENGTH (ILSS) OF THE COMPOSITE This is the maximum shear stress that a material can withstand before it ruptures, it is calculated using the equation

σm = Where: σm is the ILSS, P= is the load, b =is the width and t= is the thickness of the specimen under test. The maximum tensile stress was found out form the equation.

τm = Where τm is the maximum tensile stress and L is the gauge length.

εf= 5.3.5 CALCULATION OF FLEXURAL MODULUS EF

Ef= Where; σf = Stress in outer fibers at midpoint, (MPa) εf = Strain in the outer surface, (mm/mm) Ef = flexural Modulus of elasticity,(MPa) P = load at a given point on the load deflection curve, (N) L = Support span, (mm) b = Width of test beam, (mm) d = Depth of tested beam, (mm) D = maximum deflection of the center of the beam, (mm) m = The gradient (i.e., slope) of the initial straight-line portion of the load deflection curve,(P/D), (N/mm

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CHAPTER 6

6.0 PARALLEL ALIGNED FIBRE COMPOSITE

6.1 STIFFNESS PARALLEL TO THE FIBRE

FIG.

When we consider the behavior of continuous and parallel aligned fibre i.e. a composite that is loaded in the direction of fibre alignment direction, it is assumed that the fibre matrix interfacial bond is very good such that the deformation of both matrix and the fibres is the same (an isostrain situation). Under these conditions the total load sustained by the composite Fc is equal to the loads carried by the matrix phase Fm and fibre phase Ff. i.e. Fc = Fm+Ff

(1)

We know stress is given by, F= σ A

(2)

Hence we can write

σc= σmAm+ σfAf

(3)

Now dividing through by Ac which is the total cross-sectional area we get,

σc=σm

+ σf

(4)

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Where,

and

are the area fraction of the matrix and the fibre phase respectively.

If the composite, matrix and fibre phase length are all equal then

=

And so (4) becomes

σc=σmVm+ σfVf

(5)

And basing on on iso-strain assumption,

Ec=εm=εf

(6)

And if each term above is divided by its respective strain it becomes, σmVm

σfVf

(7)

= εm + εf

Further if composite matrix and fibre deformation are elastic, then

σ ε

=Ec;

=Em ;

σ ε

=Ef

(8)

Where; E is the modulus of elasticity for respective phases.

Putting the E’s into above yields Ecl=EmVm+E fVf

(9a)

Ecl=Em(1-Vf) + EfVf

(9b)

Since the matrix contains only matrix and fibre phases, Vf+Vm=1

(10)

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The Ecl is equal to the volume fraction weighted average of the moduli of elasticity of the fibre and the matrix phases. It can also be shown for longitudinal loading that the ratio of the load carried by the fibre to that carried by the matrices is;

=

6.2 LONGITUDINAL TENSILE STRENGTH

Normal strength is taken as the maximum stress on the stress strain curve,

If we assume εf < εm which is the usual case then the fibres will fail before the matrix. Once the fibre have fractured the majority of the load that was borne by the fibres is now transferred to the matrix. This being the case it is possible to adapt the expression for the stress on this type of composite into the following,

σ*cl =σ*m(1-Vf)+σ*fVf

Here, σ* is the stress in the matrix at failure and σ*f is the stress of fibre at failure

6.3 STIFFNESS PERPENDICULAR TO THE FIBRE

FIG

A continuous and oriented fibre composite may be loaded in the transverse direction, i.e. the load is applied at 90 o angle to the direction of fibre alignment

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For this situation the σ to which the composite as well both phases are exposed is the same, or

σc= σm = σf=σ

(13)

This is termed as iso-stress state, also, the strain or deformation of the entire composite εc is

Ec= εmVm+ εfVf

(14)

But, since =

(

)

+

(

)

Where Ect is the modulus of elasticity in the traverse direction. Now dividing through by σ yields

=

+

This reduces to,

Ect=

(

)

6.4 DISCONTINUOUS AND RANDOMLY ORIENTED FIBRE COMPOSITE

When fibres are not perfectly aligned with the direction of modulus estimate, an orientationefficiency factor k is added to the equation (14)

Ec= KVfεf+Vmεm

0