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Reinforcement types

Although many textile yarns fibres can be used for the matrix reinforcement only four principal classes dominate which include:

  • Glass fibres
  • Carbon (graphite) fibres
  • Aramid fibres
  • Boron fibres

Others fibre types include natural polymers such as cellulosics (jute, flax and cotton) and synthetic polymers (polyamide, polyethylene, polypropylene).

In all fibrous composite technologies, a proper fibre finishing or application of a coupling agent is essential. These are surface coating applied to the reinforcement to protect it from damage during processing, aid in processing and to promote adhesion to matrix. They can be film forming organics and polymers, adhesion promoters (like silane coupling agents) or chemical modifiers (like silicone carbide on boron fibres)

The graph below gives an appreciation of the costs for each of the reinforcements shown. Again, there are a number of variants within each generic family, some of which may exceed in price the upper bound shown. Generally the mechanical properties and environmental resistance, particularly temperature resistance, increase with increasing cost. The range of prices for the individual fibre types is dependent on variations in quality, which dictate the strength and durability, of the fibre, and the costs for the different types of surface treatment which are applied to improve the bonding of the reinforcement to the resin.

Glass fibre

Glass fibre is the commonest reinforcing material used in polymer matrix composites. These have high tensile strength but low modulus compared with other fibres.

Typical variants are:

E-glass

E-glass accounts for 90% of the glass fibre market and is used mainly in a polyester matrix.

The 'E' in E-glass stands for 'electrical' and was intended to indicate that the material had low electrical conductivity. The major driver, however, for its command of the market is due to the fact that it is the cheapest glass fibre and is therefore the preferred material in general purpose products.

Continuous and chopped E-glass are widely used in product manufacture. Its advantages are relatively low cost combined with high tensile strength and modulus, with individual filament strengths around 3500 MPa and modulus around 80GPa. Elongation-to-break is nearly 5%. Two types are available one contains boron, the other boron free.

The ultimate use temperature of E-glass is around 500oC. The maximum service temperature for the composite will, however, be dictated by the matrix material.

The corrosion resistance of E-glass without boron is approximately seven times the corrosion resistance of the boron-containing E-glasses. Boron free E-glasses have approximately a 10% higher dielectric constant than boron containing E- glasses when measured at room temperature making them less suitable for electronic circuit boards and aerospace applications.

ECR-glass

ECR-glass was manufactured specifically to resist acid and alkali exposure. These fibres are boron-free with a modified structure to enhance short and long term acid alkali resistance. Their mechanical properties are similar to E-glass but they have less weight loss based on sulphuric acid exposure. The enhanced corrosion resistance results in an increase in cost.

S-glass, R-glass and Te-glass

S-glass is 10-15% stronger than E-glass, but is otherwise similar in its properties to E-glass and R-glass. It is boron free and heat resistance is better than that of E-glass. S-glass fibres have modified silicate network giving an increase in mechanical properties. S-glass is typically selected for many structural applications because due to its enhanced mechanical and temperature performance. It should be noted, that higher melt temperatures are required for S-glass, requiring more process energy making these fibres more expensive than E-glass.

Class Density (kg/m3) Tensile Strength (MPa) Young’s Modulus (GPa) CTE (10-6/k) Strain to failure (%)
E-glass 2620 3450 81 5.0 4.9
S-glass 2500 4590 89 5.6 5.7

Silica/quartz

Silica / quartz glass fibres have increasing silicon oxidewith enhanced high temperature performance. Applications include communication and satellite equipment for protection against electrical discharges.

D-glass

D-glass fibres are low volume specialist fibres with low dielectric constant which can be 40% lower than E-glass, They are used in circuit boards because of their low dielectric constant.

These variations of glass can be obtained supplied in several different forms including:

  • Rovings
  • Woven rovings
  • Chopped strand mat
  • Sheet moulding compound

Carbon fibre

Carbon fibre is 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:

Young's Modulus / GPa Tensile Strength / GPa Strain to Failure / %
PAN-based High Modulus 350-550 1.9-3.7 0.4-0.7
PAN-based Intermediate Modulus 230-300 3.1-4.4 1.3-1.6
PAN-based High Strength 240-300 4.3-7.1 1.7-2.4

Aramid fibre

Aramid fibres have 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%

Boron Fibre

Boron fibre actually predates carbon fibre as a high-modulus reinforcement material. The cost of boron, however, has seen its demise, with its replacement with carbon fibre. They do not differ greatly from glass fibre in tensile strength, but can have modulus five times that of glass. Since the objective of reinforcement is to stiffen, this is a significant advantage. Their use is confined to niche markets, where the modulus advantage over carbon fibre is critical.