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AMTS-SWP-0002-A-2008
AMTS STANDARD WORKSHOP PRACTICE _________________________________________ Raw Materials
Reference Number: AMTS_SWP_2_2008 Date: December 2008 Version: A
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Contents 1 Technical Terms ................................................................................... 3 2 Scope ..................................................................................................... 3 3 Primary References .............................................................................. 3 4 Typical Composite Materials ............................................................... 3 4.1 Structural fibres and fabrics .........................................................................3 4.1.1 Glass fibres ....................................................................................4 4.1.2 Carbon fibres..................................................................................5 4.1.3 Aramid (KevlarTM) fibres.................................................................5 4.1.4 Other fibre types.............................................................................6 4.1.5 Types of fabrics..............................................................................6 4.2 Sandwich structure core materials ..............................................................8 4.2.1 Foams ............................................................................................8 4.2.2 Honeycombs ..................................................................................8 4.2.3 Woods ............................................................................................9 4.3 Resin systems..............................................................................................9 4.3.1 Epoxy resin systems ....................................................................10 4.3.2 Polyester resin systems ...............................................................10 4.3.3 Polyurethane resin systems.........................................................11 4.3.4 Vinyl-ester resin systems.............................................................11 4.4 Hardeners ..................................................................................................12 4.5 Applications of resin systems ....................................................................12 4.5.1 Laminating resins .........................................................................12 4.5.2 Bonding resins .............................................................................12 4.5.3 Gelcoats .......................................................................................12 4.5.4 Casting resins ..............................................................................13 4.6 Resin additives (fillers)...............................................................................13 4.6.1 Cotton flocks ................................................................................13 4.6.2 Carb-o-sil......................................................................................14 4.6.3 Micro balloons ..............................................................................14 4.7 Prepregs.....................................................................................................15
5 Processing of Composites................................................................. 15 6 Logistics .............................................................................................. 16 6.1 Procurement of Raw Materials ..................................................................16 6.1.1 Vendor Evaluation........................................................................16 6.1.2 Selecting Raw Material ................................................................17 6.1.3 Selecting Consumables ...............................................................17 6.1.4 Recording of actions – taking into stock......................................17 6.1.5 Quality Control of Materials .........................................................17 6.2 Storage and Issuing of Raw Materials.......................................................17
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1 Technical Terms Hygroscopic:
Affinity for moisture
2 Scope The purpose of this document is to outline different composite materials and their use as well as guide composite part manufacturers in the procurement of these raw materials. It covers the following: • Typical composite structural fibres, resin systems, additives as well as core materials used in sandwich structures. • Special processing requirements of resin systems • Basic logistics concerning raw materials
3 Primary References J.S.U. Jonker & J.P. Schümann, Training Manual – Composites, Jonker Sailplanes CC, 2007. A.C. Marshall, Composite Basics, Marshall Consulting, 1994.
4 Typical Composite Materials Typically, basic composite parts or components are made up of at least two parts – a reinforcement material substrate e.g. fibreglass, carbon fibres or aramid (Kevlar™) fibres and a resinous binder. The relatively brittle and firm resin matrix transfers forces acting on the part to the load-capable flexible fibres. The low weights of both resin and fibres lend these parts extremely high strength-to-weight ratios. Furthermore, simple composite “skins” may be fixed to the top and bottom of so called “core materials” to form sandwich structures, e.g. composite beams, still holding true to exceptional strength characteristics of these parts.
4.1 Structural fibres and fabrics
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When compared to an unreinforced cured resin system, the mechanical characteristics of reinforcement fibres are tremendously higher. The mechanical performance of a composite part is therefore dictated in most part by that of the structural fibres. The following factors determine the ultimate mechanical properties of a cured composite part: • Basic mechanical characteristics of the reinforcement fibres • Bond / surface interaction between resin system and fibres • Amount of fibres per volume in the composite (fibre count) • Orientation of fibres in cured part Note:
The bond between fibres and resin can be improved by surface treatment. This is especially important when bonding composite parts. See SWP 12 on Adhesive Bonding.
As mentioned above, the orientation of fibres play an important role. In almost all cases reinforcement fibres are available in different types of weaves, making up a fabric. The following are typical commercially available reinforcement fibres and weaved fabrics:
4.1.1 Glass fibres Many unique chemical compositions of glass fibres are manufactured worldwide. These different compositions are designated by an alphabet letter and each display varying mechanical and chemical properties. The most common are: • E-glass • C-glass • S-glass • H-glass. The E denotes high Electrical resistance, the C Chemical resistance, S high Strength characteristics and the H stands for Hollow glass (extremely light fibres). Many more types of glass fibre exist, too numerous to all be mentioned here. Refer to a supplier’s catalogue and datasheets for fibre-specific information. As an example the properties of the already mentioned E-, C- and S-glass are listed below:
Property
E-glass
C-glass
S-glass
2540 kg/m3 6.5
2490 kg/m3 6.5
2480 kg/m3 6.5
3.447 GPa 2.620 GPa 1.724 GPa 72.395 GPa
3.309 GPa 68.948 GPa
4.585 GPa 4.447 GPa 2.413 GPa 85.495 GPa
Physical
Density Hardness Mechanical
Tensile strength 25°C Tensile strength 350°C Tensile strength 550°C Young’s Modulus
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Chemical Composition
Silicon oxide Aluminium oxide Iron oxide Calcium oxide Magnesium oxide Sodium oxide Potassium oxide Boron oxide
54.3% 15.2% 17.2% 4.7% 0.6% 8.0%
64.6% 4.1% 13.2% 3.3% 7.7% 1.7% 4.7%
64.2% 24.8% 0.21% 0.01% 10.27% 0.27% 0.01%
Table 4.1: Properties of single-fibre E-, C- and S-glass
4.1.2 Carbon fibres Carbon fibres are usually made by taking strands of poly-acrylanitrile (PAN) in the form of multifilament yarn and then oxidizing, carbonizing and graphitizing it to form carbon fibre filaments. These are then usually given a surface oxidation treatment to promote bonding with resins. Other properties of carbon fibre include: • High thermal conductivity • Conducts electricity • Electrically opaque to radio waves
4.1.3 Aramid (KevlarTM) fibres Also a relative newcomer, aramid fibre has many of the characteristics of carbon fibre but sets itself apart from carbon and glass with its unique degree of toughness. Several types of aramid fibres can be found commercially. Some benefits include: • Electrically non-conductive • Heat resistant • Transparent to radio waves One big drawback of using aramid fibre in a composite part is that it cannot be sanded after curing. Be sure to leave gaps between the edges of aramid fabrics and part ends. Important:
Special shears, sharpened at a specific angle are needed in order to cut aramid fabrics correctly.
Aramid fibres are hygroscopic (absorbs moisture from the atmosphere) and UV sensitive. Fibres should therefore be stored in a dry, darkened storage room. Moisture will adversely affect its bonding properties with resins. It should also be noted that aramid only bonds satisfactorily with epoxy and vinylester resin systems. Using aramid fabrics with polyester resins is not recommended – poor interlaminated bonding can be expected. 5
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4.1.4 Other fibre types Many exotic, experimental and classified fibres are in existence around the world, each having unique properties still being explored and tested. Many of these fibres are either very rare or very expensive and are therefore beyond the scope of this manual.
4.1.5 Types of fabrics Different fabrics are mainly recognised by varying strand thickness and type of weave. Consult with manufacturers on their unique fabrics with different yarn thicknesses and weaves for specific purposes. In general, the following types of woven fabric can be found commercially:
Plain weave
Twill weave
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Satin weave
Basket weave
Leno weave
Mock Leno weave
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4.2 Sandwich structure core materials Sandwich structures are made by “sandwiching” a core material between two skins. The basic idea behind these structures is to significantly increase a structure’s bending stiffness whilst only marginally increasing the overall weight. With this in mind there are 2 basic characteristics a core material should display: • Low weight • High volume This basically means any core should have a low density (other than the skin material consisting of heavier resin and fibres). Several compounds are suitable as cores and can be placed under 3 main categories: • Foams • Honeycombs • Woods Refer to SWP 18 on Sandwich Structures, where core materials are discussed in greater detail.
4.2.1 Foams Foamed plastic materials are affordable and easy to use as cores. The mechanical and physical properties of different foams vary greatly and their specific datasheets should be consulted for more detail. Examples are: o o o o o o o Important:
PVC foam Polystyrene foams Polyurethane foams Polymethyl methacrylamide foams Styrene acrylonitrile (SAN) co-polymer foams Metallic foams Other thermoplastics Polystyrene “bead foam” is not suitable for use as a sandwich core material. This is foam made by exposing polystyrene granules to steam which then expand in a mould. The bonds between these beads are weak and varied. Air might also become trapped in the structure. These factors make this type of foam unusable.
4.2.2 Honeycombs Composite honeycombs are made from a variety of materials. Used mostly in the aerospace industry, honeycombs can also be found in stage flooring sandwich structures, marine vessels. Of all the core materials, honeycomb has the best compressive strength (next to balsa, see section 4.2.3)
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Examples are: o o o o o o
Aluminium honeycomb Nomex honeycomb Thermoplastic honeycomb Glass fibre / plastic honeycombs Carbon fibre / Kevlar honeycombs Stainless steel, titanium and super-alloy honeycombs
4.2.3 Woods Balsa wood offers good strength whilst having a very low density. If the grain is orientated perpendicular to the sandwich skins, balsa wood’s compressive strength is better than most honeycombs. Examples are: o o o o o o o o
Balsa (most common because of low density) Cedar Spruce Mahogany Redwood Pine Fir Many others
Woods are normally cheaper than foams, but prone to the attack from insects, mildew and will deteriorate when exposed to moisture. Proper sealing and treatment is therefore necessary where woods are used.
4.3 Resin systems Typically material composites include at least two parts – a reinforcement material substrate e.g. fibreglass, carbon fibres or aramid (Kevlar™) fibres and a resinous binder. The sole purpose of the hardened (cured) resin system is to keep the fibres in place and along their correct orientation. Thus a resin should also be able to chemically connect to the different layers of material. Resin systems fall in the thermosetting plastic category can be classified under the following groups, according to their chemical composition: • Epoxy resin • Polyester resin • Vinyl-ester resin • Polyurethane resin Other binding materials categories (for information purposes) are: 9
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Thermoplastic (e.g. Perspex) Phenolics Unsaturated polyesters Silicones Polyimides
In this SWP only the thermosets will be discussed.
4.3.1 Epoxy resin systems Epoxy resins are nearly transparent after curing. They are commercially available in hardware stores for small scale repairs as well as in large quantities (resin and different hardeners) for aerospace and marine applications. Epoxies are used as either a structural matrix material reinforced with fibres (glass, carbon, aramid, boron) or as a structural adhesive. Some properties of epoxy resin systems: • • • • • • •
Resin-to-hardener ratio is usually between 1:1 and 5:1 Excellent chemical- and corrosion resistance Excellent thermal properties Better mechanical properties compared to polyester resins Good gap filling properties when mixed with additives (see section 4.4) Offers excellent adhesive properties (including to polyester resin surfaces) Low shrinkage compared to Polyester resins.
• •
Gelcoat does not readily adhere to epoxy surfaces Deteriorates when exposed to UV light
4.3.2 Polyester resin systems Polyester resins may have a slightly yellow, transparent colour and are also known as thermosetting plastics (will set at high temperatures.) Because of their sensitivity to UV light and degradation over time, polyester resins are often coated with a protective layer. Properties of polyester resin systems: • • • •
The hardener and accelerator agents are pre-mixed in the resin - the system only requires a catalyst to set off the reaction. MEKP (Methyl-ethyl-ketone-peroxide) is used as the abovementioned catalyst Usually requires only 2% catalyser by weight Offers good resistance against chemicals, corrosion and exposure to the environment
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Flame retardant (self-extinguishes) Very easily processed in low cost equipment Usually cheaper than epoxy systems Typical shelf life of less than 6 months Storage containers for these resins must be tightly closed to slow down the natural hardening process Hygroscopic (draws in moisture from surrounding air) High shrinkage compared to epoxies.
4.3.3 Polyurethane resin systems Polyurethane is widely used in flexible and rigid foams, heavy duty adhesives and sealants, fibres and hard plastic parts. Products containing polyurethane are often referred to as "urethanes,” but should not be confused with the specific substance, urethane (ethyl carbamate). Polyurethane system properties: • • • • •
Offers excellent thermal insulation Resists the spreading of flames Results in parts with high strength-to-weight ratios Easily processed Usually cheaper than epoxies
4.3.4 Vinyl-ester resin systems Vinyl-ester resins may have a coloured tint, ranging from green to blue to purple. They are also slightly more transparent then polyester resins and flows more easily. A Vinyl-ester system is a good alternative to a polyester or epoxy resin system, having inferior characteristics to those of epoxies, but better compared to those of polyester. Properties: • • • • • • •
Vinyl-ester resins are more flexible than polyester resins Also catalyzed with MEKP, at a similar mixing ratio Better corrosion and temperature resistance Better strength properties Resists water absorption They degrade faster than polyester resins Shelf life less than three months
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4.4 Hardeners Hardeners are substances or a mixture added to a plastic composition to promote or control the curing action by taking part in it. Resins are sometimes referred to as “Part A” while hardeners are referred to as “Part B”. The reaction can normally not be controlled by modifying the mixing ratios. Mixing ratio must be used as per the manufacturer’s datasheets. Different hardeners can be found for specific types of resins. As explained above in section 4.3, some resin systems only require a catalyzing agent. Furthermore different hardeners (as in epoxy systems) differ normally only in respect to the pot life of each different hardener-resin mixture.
4.5 Applications of resin systems The different applications for which specific resins can be used allow us to reclassify different sub-types of resin systems. Therefore common applications will be listed and described in the following sections, along with a few examples of resins available on the market.
4.5.1 Laminating resins Laminating resins easily wet any cloth, due to their low viscosity. They also chemically connect to the weave, resulting in a strong composite material. Examples of laminating resins include: • • • • •
SP Systems Ampreg 20, 22, Axon Epolam 2015, 2020, 2022 Axon Epolam 2080 (high temperature epoxy) Hexion L285 (LBA aircraft certified system) NCS and Scott Bader Polyester resins
4.5.2 Bonding resins Bonding materials have to have a higher viscosity to prevent the material from flowing off the area being bonded. A few examples: • • •
SP Spabond 345 Axson H 9940 Laminating resins (mixed with cotton flocks and carb-o-sil, see section 4.4)
4.5.3 Gelcoats 12
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Gelcoats can be divided into spray-paintable gelcoats and mould surface gelcoats. Whereas the mould surface gelcoat is usually black or some darker colour, the spray-painting Gelcoat can have any colour, although it is frequently white. Spray-paint Gelcoat: Properties: Thick when mixed according to standards but can be diluted. Examples: • • • •
SP 127 Hexion, T 35 Azko Nobel, Schwabellack NCS Ultragel P1075
Mould surface Gelcoat: Properties: Develops a hard and durable surface. Examples: • • •
Axon GC1050 Hexion F 200/F 15 (polishable surface) Hexion F 260/F 16 (non- polishable surface)
4.5.4 Casting resins Casting materials can be used for both high and low density foams or to form polyurethanebased rubbers. Examples: • • •
Axson F16 Axson 3034 Axon 5056
4.6 Resin additives (fillers) By augmenting a resin-hardener mixture with a variety of additives, a wide range of different properties can be obtained. These additives work well when mixed with epoxy resin systems
4.6.1 Cotton flocks This additive is made from natural cotton and appears as fine fibres. A mixture of cotton fibres and epoxy is referred to as “flox”. Other names: “Micro fibres”, “Cotton flakes”
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The mixture is used in structural joints and in areas where a very hard, durable build-up is required. Preparation: Flox is normally mixed with 1 part cotton and 1 part carb-o-sil to one part resin. Effects: Turns resin into a tough bonding compound Examples where used: • Bonding of parts • Filler material • To create chamfers when needed
4.6.2 Carb-o-sil Carb-o-sil or names are: • • •
fused quartz is a non-crystalline form of silicon dioxide, also called silica. Other Aerosil (German) Fused silica Colloidal Silica (SP Systems)
Carb-o-sil can be used to reduce the flow of epoxies on vertical surfaces, as well as for filling pinholes. Uses: • Carb-o-sil can be mixed with epoxies or gelcoats to modify the flow characteristics • Carb-o-sil can be mixed with microballoons or cotton flocks to give non-sag properties to fillers.
Effects: It decreases the viscosity of resin. Examples where used: • Sealant in water tanks • As a bonding adhesive when mixed with laminating resin and other fillers (see 4.3.1) • A filler material, when combined with microballoons. • Sealing paste on vacuum bag edges
4.6.3 Micro balloons Micro balloons are hollow spheres made from either glass or phenolic. The differences are:
Other names Colour Particle Size
Glass Micro balloons
Phenolic Micro balloons
Microballoons White 40-80 microns
Glass bubbles Reddish/brown 50 microns 14
Raw Materials Density Sandability Waterproof Cost
AMTS-SWP-0002-A-2008 250g/litre Good Moderate Expensive
200g/litre Moderate Good Less expensive
Also known as glass bubbles, micro balloons are used in composites to fill polymer resins for specific characteristics such as weight, sandability and sealing properties. The term “micro” or “micro balloons” was applied to the mixture of solid microspheres and epoxy early in the development of composite structures. Although microspheres have been replaced by glass bubbles, “micro” is still commonly used to refer to a micro balloon and resin mixture. Effects: Lightens resin and eases its processing and application Refer to SWP 13 on Composite Repairs for detail on mixing microballoon fillers.
4.7 Prepregs Prepreg materials can be different weaves or unidirectional fibres pre-impregnated with a specific resin system. They are available for purchase from any reputable composite materials supplier and come packaged between plastic films or wax paper. Importantly, prepregs should be stored in freezers or freeze rooms below -18°C. Storing prepregs at higher temperatures will severely shorten their shelf life. Also called the cure-by date, the shelf lives of prepregs differ and are available in their respective datasheets, and they should be used and cured before/on this date. Important:
Care must be taken when handling prepregs in order to prevent contamination and workers should always wear gloves when handling prepregs.
See SWP 8 on Prepregs for further information on the storing, preparation, processing and uses of prepreg.
5 Processing of Composites Processing refers to the material preparation, mixing of a resin and its respective hardener as well as application of the mixture (laminating, bonding etc.) and trimming Refer to the following SWP for details on the processing •
SWP 15 on Dimensioning and Cutting of fibres
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SWP 7 on Mixing of Resins, including: o
Temperature and humidity
o
Mixing quantities
o
Mixing ratios
•
SWP 14 on Wet Lay-up
•
SWP 9 on Curing of Composites
•
SWP 13 on Composite Repairs
6 Logistics The following section briefly outlines the basic actions involving raw materials. With the characteristics of different materials known, it becomes a whole new playing field to select, evaluate, procure and store raw materials. The figure below illustrates the typical flow of raw material in any organization:
Figure 6-1: Material flow process
6.1 Procurement of Raw Materials 6.1.1 Vendor Evaluation The importance of selecting reputable vendors that are knowledgeable on the required raw materials and can provide repeatability on orders regarding certain products should be stressed. It is good practise to perform a vendor evaluation to determine the following: 1. Reliability 2. Competence and knowledgeable in composite raw materials 3. Stock sufficient quantities 16
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4. Storage facilities to enable them to store as per manufactures specification 5. Quality control process in place 6. Ability to provide Technical support
6.1.2 Selecting Raw Material To select raw materials that will perform as expected and required is crucial. Viewing and understanding their technical data sheets is expected from resident engineers. The instructions on handling, preparation and application should be understood by technicians. Their skills should also be honed to include all materials involved.
6.1.3 Selecting Consumables Understanding the performance characteristics of consumables and their price vs. performance is important: • Selecting consumables that compliment the work being carried out • Knowing where compromises can be made to save production costs yet maintain quality (tricks of the trade)
6.1.4 Recording of actions – taking into stock The following are important points which should be addressed in a composite parts workshop to ensure organization, traceability and recordkeeping: • Stocktaking • Recording of batch numbers, expiry dates etc. • Real-time stock lists • Restocking before running out
6.1.5 Quality Control of Materials Ensure materials received are what were ordered; check that resins have not yet expired when they arrive, check for serious blemishes on boards etc. • Check expiry dates • Dispose of expired materials in an appropriate manner • Check prepreg ‘out-life’
6.2 Storage and Issuing of Raw Materials Refer to the manufacturer’s datasheet of the resin system being used for details on storage conditions and shelf-life. Epoxy resins and hardeners can be stored for at least 12 months in their original, tightly sealed containers. Prepregs have very strict rules regarding the shelf-life and storage specification. Some resin systems may begin to crystallize when stored below a certain temperature. Before processing it should be heated up and mixed thoroughly to remove crystals. See SWP 35 on Handling and Storage of Raw Materials.
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