In examining potential composites systems both the manufacturing route and end use need to be considered. Like metals we need to consider two stock materials, cast and wrought. Mouldings can be of two types, one where the fibres are prepositioned and the resin - generally a thermoset, (though there is no reason why a thermoplastic could not be used) is injected into the preform - mould and allowed to cure (Resin Transfer Moulding) or where the two constituents of the resin (one or both containing short fibres) are rapidly mixed and injected into the mould (Reaction Injection Moulding). Wrought products such as bar and tube can be fabricated by pultrusion, while shell structures can be fabricated by diaphragm forming. In both cases the feed stock is in the form of pre-impregnated fibres, where the polymer is precoated onto the fibres before they are wound into tape (unidirectional) or woven into mat.
| Material | E, GPa | UTS, MPa | K, MNm-3/2 | Tglass, °C | Tmelt, °C | Tmax, °C |
| PVE | 1.5 to 4 | 40 to 80 | 1 to 3 | - | - | 260 |
| PEI | 3.3 | 93 to 197 | 10 | 200 | 217 | 170 |
| PAI | 2.5 | 70 to 100 | 2.2 | 220 | 355 | 260 |
| PEEK | 3.6 | 90 to 170 | 7.5 | 143 | 334 | 250 |
| PPS | 4.8 | 80 to 150 | 2 | 93 | 285 | 240 |
| POLYESTER | 1-4 | 30-70 | 0.5 | - | - | - |
| Epoxy | 2-5 | 40-80 | 1 | - | - | - |
| Nylon pA | 2-3 | 60-120 | 3-5 | - | - | - |
| Polymer | abbr. | type | Manufacturer |
| poly(Vinyl Ester) | PVE | Thermoset | (Dow Chemical) |
| poly(AmideImide) | PAI | Thermoplastic | (TORLON - Amoco Chemicals) |
| poly(EtherImide) | PEI | Thermoplastic | (ULTEM - General Electric Plastics) |
| poly(EtherEtherKetone) | PEEK | Thermoplastic | (VICTREX PEEK - ICI) |
| poly(phenyleneSulphide) | PPS | Thermoplastic | (Philips Petroleum) |
In order to chose suitable polymer matrices we need to understand what - at the molecular level - is responsible for for the mechanical properties such as elasticity and strength. We must also be aware of the difference between thermosetting polymers and thermoplastic polymers and the glass transition temperature.
A schematic stress -strain plot for an amorphous thermoplastic above (i) and below (ii) its glass transition temperature.
The polymer may be visualised as a number of coils of rope which are tangled together. The chains may or may not contain polorisable groups such as N-H or O-H bonds and may or may not contain side groupings which distort the linearity of the molecules. As the polymer is loaded the interchain bonds stretch and the moleculs slide past each other, trying to uncoil - a process known as reptition. The stronger the interchain bonds, the stiffer the polymer. Thermosetting polymers, in which the chains are bound together by C-C bonds are generally stiffer than thermoplastics in which the interchain bonds are Hydrogen bonds. Nonetheless the stiffness is low because the C-C bonds in the chains are not aligned along the testing direction and are more likely to act as a stiff hinge.
Below the glass transition temperature the interchain bonds limit motion of the chains, above Tg the bonds effectively melt leaving the polymer chain s free to move past each other giving a more 'plastic' or viscoelastic response. For thermosets and thermoplastics below Tg releasing the stress will cause the molecules to spring back to their original positions - If the stress is large enough to cause yield then yielding will be limited. Plastic deformation is easiest in the simplest molecules and impossible in highly cross-linked polymers. Yielding can occur by SHEAR BANDING or CRAZING.
Shear bands are regions of intense local deformation parallel to the direction of maximum shear - it may be possible for the shear bands to align parallel to the applied stress creating a hig strength neck and for a period of extended necking/ductility to occur as the polymer draws out. In the complex polymers this is not the case and rarely happens in thermosetting polymers. The yield criteria is best represented by a Von Mises criterion
Just to complicate things, the shearing process is relatively simple in tension where there is a dilattory component in the applied stress, ie. the mean stress is positive, but more difficult in compression:
To account for this problem the yield criterion is modified and the polymer is usually stronger in compression than in tension for biaxial tension this reduces to.
which delays yielding to a higher stress when the mean stress is compressive. This equation is elliptical when a is zero and distorted when a is positive.
Crazing is an alternative and competitive mechanism of permanent deformation in polymeric materials. Crazing becomes more favourable as the temperature is lowered. However, since the crazing process produces a hole in the polymer, ie. an increase in volume, it cannot occur under compression. Crazes appear like cracks whichrun perpendicular to the applied stress.
The craze is basically a narrow crack that is bridged at intervals across its surface by fibrils of polymer chains. The chains are highly drawn (aligned) in the direction of the maximum principal stress and serve to keep the craze crack from opening. The craze extends with little increase in thickness. Overall ductility is low as fracture occurs by the breakage of the fibrils and the macroscopic extension of the crack. (note: deformation within the craze itself is very large). Crazing occurs based on the magnitude of s1-s2>0 ie. a strain based criterion.
There are two types of thermosetting matrix, polyester and epoxy, both of which have been reinforced with glass, carbon and kevlar fibres. Polyester is the simplest of the thermosets and is generally made by reacting dibasic acids such as maleic anhydride or phthalic anhydride with dihydric alcohols such as ethylene glycol (antifreeze) in approximately equal amounts. The resulting polymer is a short chain polymer with a molecular weight of about 5000 (about 35-40 repeat units or mers) and is a stable liquid. The liquid resin is set into an amorphous solid by cross linking the polyester chains to each other. The cross linking occurs by the addition of a small monomer molecule such as styrene. The monomer, like the active sites on the polyester chain has an unsaturated C=C bond and it is this that provides the bridge between the polyester chains.
The monomer is usually mixed with the polyester to form 'fibreglass' resin. The two components will react but at room temperature the reaction may take years and needs to be initiated by the addition of a catalyst (loosely referred to as a hardener) such as methyl ethyl ketone epoxide at about 0.5 to 1% by volume. Once the reaction is initiated, the cross linking generates sufficient heat to further catalyse the reaction and the resin gels (within about 10 minutes) the cross linking continues and full strength is generated after about 24 hours.
| Chemical | mole fraction | kg/100kg resin |
| phthlalic anhydride | 0.2 | 28.86 |
| maleic anhydride | 0.15 | 19.11 |
| propylene glycol | 0.2 | 14.83 |
| ethylene glycol | 0.15 | 12.10 |
| styrene | 0.3 | 30.00 |
Vinyl Ester resins differ from ester resins in that the ester resin has one unsaturated group per mer unit, i.e. about 30 to 40 cross linking sites per polyEster molecule while the vinyl ester has only two unsaturated groups, one at each end of the molecule. Polyesters, being more cross-linked are more brittle than their vinyl ester counterparts.
In order for the composite to function properly there must be a chemical bond between the matrix and the re-inforcing fibres in order that the applied load (applied to the matrix) can be transferred to the fibres (which are expected to do all the work. However, the bond must not be too strong since the toughness of the composite comes from such sources as fibre pullout and fibre-matrix interfacial fracture. In 'fibre glass' the fibre is inorganic while the matrix is organic and the two do not bond readily unless the fibres are treated to modify their surface.
Silica (SiO2) is hygroscopic ie. it absorbs water onto its surface where the water breaks down into hydoxyl (-OH) groups. It is impossible to avoid the water especially as the surface modifier or ‘size' is applied in a water based solvent. It should also be stressed that water reduces the strength of SiO2 by a stress-corrosion-cracking mechanism. The coupling agent takes the form of a silane (R-SiX3) where R is an organic radical that is compatible with the polymer matrix (it may even react with the matrix polymer; for this reason styrene groups are favoured for polyesters while amine groups are preferred for epoxies) and X is a hydrolisable organic group such as an alcohol. The most common silane couplant is tri-ethoxy-silane.Heat will force the elimination of water between the -OH pairs at the hydrated silica surface and the silane as well as between the adjacent silane moecules.
Epoxy resins are much more expensive than polyester resins because of the high cost of the precursor chemicals most notably epichlorohydrin. However, the increased complexity of the 'epoxy' polymer chain and the potential for a greater degree of control of the cross linking process gives a much improved matrix in terms of strength and ductility. Most epoxies require the resin and hardner to be mixed in equal proportions and for full strength require heating to complete the curing process. This can be advantageous as the resin can be applied directly to the fibres and curing need only take place at the time of manufacture. - known as pre-preg or pre impregnated fibre.
Epoxy polymers are made by reacting epichlorohydrin with bisphenol-A in an alkaline solution which absorbs the HCl released during the condensation polymerisation reaction.
Each chain has a molecular weight between 900 and 3000 (about 3 to 10 mers) with an epoxide grouping at each end of the chain but none within the polymer chain. The epoxy is cured by adding a hardner in equal amounts and being heated to about 120°C. The hardners are usually short chain diamines such as ethylene diamine. Heat is usually required since the cross linking involves the condensation of water which must be removed in the vapour phase.
