May 19 2003
Background
Polyaramid Polyparaphenylene Terephthalamide ( Kevlar / Twaron ) is an infusible, wholly aromatic polymer that can strictly be described as nylon T,T - but rarely is. Manufactured only as a fibre (by solution spinning), it has a very high thermal stability and temperature and flame resistance.
In contrast to its chemical isomer Nomex®, the tensile properties of Polyaramid Polyparaphenylene Terephthalamide ( Kevlar / Twaron ) are up to an order of magnitude greater than those of normal textile fibres, because of a high degree of molecular orientation resulting from its stiff linear molecules and their propensity for forming liquid crystals in the spinning solution.
In common with the small number of other highly oriented materials, these Polyaramid Polyparaphenylene Terephthalamide ( Kevlar / Twaron ) fibres have a (small) negative co-efficient of thermal expansion in the axial direction.
Applications
Polyaramid Polyparaphenylene Terephthalamide ( Kevlar / Twaron ) fibres are quite widely used in composites which are lighter than those based on carbon fibre and electrically insulating. Their mechanical properties are generally inferior; more specifically they have a high specific tensile strength approaching that of carbon fibre composites, but quite low strength in compression.
In addition to composites, applications of Polyaramid Polyparaphenylene Terephthalamide ( Kevlar / Twaron ) include protective clothing and body armour, friction products, elastomer reinforcement (e.g. hoses and drive belts), ropes and cords and as high strength high modulus fabrics e.g. high performance sailcloths.
Key Properties
The key properties of Polyaramid Polyparaphenylene Terephthalamide ( Kevlar / Twaron) are tabulated below.
Table 1. Key properties
| Chemical Resistance |
| Acids - concentrated |
Poor |
| Acids - dilute |
Fair |
| Alcohols |
Good |
| Alkalis |
Good |
| Aromatic hydrocarbons |
Good |
| Greases and Oils |
Good |
| Halogens |
Good |
| Ketones |
Good |
| Mechanical Properties |
| Tensile modulus ( GPa ) |
59 - 124 |
| Tensile strength ( MPa ) |
2760 |
| Physical Properties |
| Density ( g.cm-3 ) |
1.44 |
| Resistance to Ultra-violet |
Fair |
| Thermal Properties |
| Coefficient of thermal expansion ( x10-6 K-1 ) |
-2 along axis |
| Lower working temperature (°C ) |
-200 |
| Specific heat ( J.K-1.kg-1 ) |
1400 |
| Thermal conductivity ( W.m-1.K-1 ) |
0.04 @ 23 |
| Upper working temperature (°C ) |
180 - 245 |
| Properties Polyaramid Fibre |
| Property |
Value |
| Material |
|
Kevlar 29® |
Kevlar 49® |
Kevlar Ht (T129) |
| Specific Modulus |
cN/tex |
4000 |
8300 |
5200 |
| Specific Tenacity |
cN/tex |
190 |
190 |
235 |
| Density |
g.cm-3 |
1.44 |
1.45 |
1.44 |
| Extension to break |
% |
3.7 |
1.9 |
3.6 |
| Modulus |
GPa |
58 |
120 |
75 |
| Shrinkage @ 100 °C |
% |
|
0.02 |
|
| Tenacity |
GPa |
2.76 |
2.76 |
3.32 |
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