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The number of types of continuous fibre used in thermoset- and thermoplasticmatrix composites is on the rise, particularly with the arrival of natural fibres like hemp, jute, flax and sisal.
Aramid fibre was rapidly tested when it emerged in 1970, and used to reduce weight in comparison to composites reinforced with glass fibre. Aramid fibre is now well known for its advantages and disadvantages, and carbon fibre is giving it some stiff competition.
An aromatic polyamide
The word aramid is a contraction of aromatic polyamide. There are two main types of aramid, with distinctly different chemical formulae:
The manufacturing process includes several steps:
The continuous fibres available on the market have yarn counts ranging from 200 dtex to 2,400 dtex for fibres and from 3,000 dtex to 5,000 dtex for roving. They are multifilament fibres with 130-5,000 filaments, and are processed either in the form of dry and/or wet prepregs and UDs, or using filament winding.
Ninety percent of aramid fibre is produced by two companies, which are Dupont and Teijin Aramid. Also to be cited Khimvolokno (Belarus) and Kolon Industries (South-Korea). Dupont markets its para-aramid fibres under the names of Kevlar 29® (low modulus) and Kevlar 49® (high modulus) and its meta-aramids under the name of Nomex® (continuous fibre or paper).
Teijin markets its para-aramid fibres under the names of Twaron® and Technora® (copolymer) and its metaaramids under the name of Teijin Conex®. Other aramid fibre manufacturers include Kolon Industries (South-Korea) and Khimvolokno (Belarus). Dupont produces in South Carolina (US), Great Britain and Japan; Teijin in the Netherlands (Emmen) and Japan.
Continuous para-aramid fibres are low density, with very good tensile properties. They are used in the following applications:
For helicopters, the maximal use was reached with the stability skin in sandwich structures for aircraft central fuselage (Super Puma MK2).
The good impact behaviour allowed this fibre to be used in specific applications such as crashworthiness structure or composite armoured seats, both applications on military helicopters.
Short para-aramid fibres are used in thermoplastic injection applications to give the materials better abrasion resistance.
Meta-aramid fibres have lower tensile strength, but are more stable thermally (fire resistance) and chemically. These fibres are used for thermal protection, e.g. in personal protective equipment or industrial filters, and especially in the form of paper for electrical insulation and making honeycomb core materials for sandwich structures.
Nomex and Kevlar paper for sandwich structures
The paper consists of a mix of short fibres and fibrils that are combined during a standard wet paper coating operation. This results in a low-density paper with low mechanical strength. A second hot calendering operation gives the paper the required mechanical and dielectrical properties for use in electrical, electrotechnical, and honeycomb composite applications.
The honeycombs manufactured according to the standard process: printing sheeting, stacking bonding, expansion, dipping(phenolic resin), curing, slicing, according to the diagram below are available in a range of densities, sizes and cell shapes.
Unlike metals, Nomex provides corrosion resistance, in particular against the galvanic corrosion caused by metal inserts, and allows the use of carbon skins. Other definite advantages are its low thermal expansion and conductivity and high heat and fire resistance.
Kevlar paper is more saturable than Nomex paper, the phenolic resin penetrates during the dipping step the cell walls of the honeycomb becoming the matrix resin of a true composite paper honeycomb. The composite nature of this impregnated para-aramid paper contribues largely to the overall improved perfprmance of the honeycomb cores and the corresponding sandwich panels and parts.
The current production of continuous aramid fibres (both low and high modulus) worldwide is estimated at about 60,000 metric tons (MT)/year, only 4% of which goes into composite applications. This represents 2,400 MT/year, compared to 40,000 MT/year of carbon and five million MT/year of glass fibre.
The development of para-aramid fibres (especially high-modulus) has been slowed down by certain disadvantages that have turned up gradually with their use. These disadvantages include poor compression strength, microcracking due to the high thermal expansion coefficient in the width direction, high moisture regain, and problems with processing (cutting, machining, finishing, surface aspect).
In addition, these fibres face stiff competition from carbon, which has much better mechanical properties and is available at lower cost. As a result, there is currently little to report in the way of major developments for composites applications.