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There are a relatively small number of polymer types that can be utilised in the fabrication of a composite. However, it must be remembered that within each class many grade variations exist. The selection of any polymer system will be a function of the design criteria including operating environment, cost, fibre type and manufacturing method.
The common resins available include:
Polyester usually dissolved in styrene to reduce its viscosity for ease of fabrication. A catalyst such as organic peroxide is added to produce polymerization (solidification) with the application of heat around 100°C to 160°C. The material exhibit significant mould shrinkage, which can be reduced by the addition of a thermoplastic additive.
Polyester is a widely used matrix material, generally in conjunction with glass fibre reinforcement. It is used primarily for low performance applications and has limited high temperature performance. Strong bonding between the matrix and glass fibre can be achieved provided a silane coupling agent is used. Other types of reinforcement fibre are generally not used because of inadequate bonding to the resin.
Vinyl esters are dissolved in styrene in order to reduce viscosity and facilitate fabrication into a composite structure. Catalysts are required to cure the material. No additional agents are required in order to produce high adhesion to glass fibres. The product has more flexibility and greater fracture toughness than a polyester resin. Heat deflection temperatures up to 200°C can be obtained.
A liquid resin which is cured by the addition of a curing agent at room or higher temperature conditions. Epoxy resins have a wide range of properties. They can be rigid or flexible with different temperature resistance, with some able to withstand continuous use up to 250°C. Advantages of epoxy resins over polyester and vinyl ester resins include lower mould shrinkage, low volatility during curing, good environmental and solvent resistance, and very good adhesion to most reinforcement materials. They are widely used in structural aerospace applications.
Bismaleimide can exist in solution or hot-melt resin form. These materials have high temperature resistance and withstand temperatures of 300°C. When used with carbon fibre reinforcement they are considered to be advanced composites because they have superior performance to epoxies in hot/wet conditions, but there are limitations. They are normally very brittle and prone to micro-cracking, although, as with epoxies, this tendency can be reduced by the incorporation of a thermoplastic component.
Polyimide exhibits better performance than bismaleimide in high temperature, wet conditions, and appear to be the most widely used resins for high temperature applications. It should be noted that service temperature will depend strongly on the nature of the application with some grades capable of sustained periods at temperatures over 300°C. The limitation of polyimide is that it is very brittle.
The curing mechanism is a condensation reaction. This tends to produce voids in the cured component. Additional reactions can be used instead to reduce or eliminate this problem.
Although many cheap textile yarns can be used for the matrix reinforcement only four principal classes dominate which include:
- Glass fibres
- Carbon (graphite) fibres
- Aramid fibres
- Boron fibres
Glass fibreis the commonest reinforcing material used in polymer matrix composites. These have high tensile strength but low modulus compared with other fibres.
Typical variants are:
- S-glass, R-glass and Te-glass
The different types of glass are supplied in several different configurations including:
- Fibreglass rovings
- Sheet moulding compound
- Woven rovings
- Chopped strand mat
Carbon fibreis the reinforcement material of choice for "advanced" composites, Carbon fibre exhibits excellent fatigue resistance which do not suffer from stress rupture compared with glass or aramid fibres. Carbon fibres are supplied in tows and may vary from 1000 fibres per tow to hundreds of thousands per tow.
Untreated carbon fibres do not wet easily, so adhesion to the matrix must be achieved by mechanical interference coupled with surface treatment and chemical bonding between the fibre and the matrix.
Carbon reinforced composites are often used for low strength applications requiring good electrical properties due to the high conductivity of carbon fibre. Most carbon fibres are derived from polyacrylonitrile, but for even higher conductivity, fibres derived from pitch can have three times the conductivity of copper.
Carbon fibre properties depend on the structure of the carbon used. Typically they come defined as standard, intermediate and high modulus fibres. Indicative materials properties are:
|Standard modulus||Intermediate modulus||High modulus|
|Tensile Strength||3450-4830 MPa||3450-6200 MPa||3450-5520 MPa|
|Young's Modulus||220-241 GPa||290-297 GPa||345-448 GPa|
|Elongation at break||1.5-2.2%||1.3-2.0%||0.7-1.0%|
Aramid fibreshave the highest strength to weight ratio compared to other commercially available fibres. Kevlar manufactured by DuPont is one familiar brand name. Aramid fibre exhibits similar tensile strength to glass fibre, but can have modulus at least two times as great. Aramid is very tough allowing significant energy absorption but, compared to carbon, it is lower in compressive strength and has poorer adhesion to the matrix. It is also susceptible to moisture absorption.
Aramid fibre properties depend on the structure used and can be tailored for high toughness or high modulus. Indicative materials properties are:
|Kevlar 29 High toughness||Kevlar 49 High modulus||Kevlar 149 Ultrahigh modulus|
|Tensile Strength||3.6 GPa||3.6-4.1 GPa||3.4 GPa|
|Young's Modulus||83 GPa||131 GPa||179 GPa|
|Elongation at break||4%||2.80%||2.0%|