Fibre description of Polyester fibres (PES) / Polyamide fibres (PA) / Acrylic fibres (PAC) / Polypropylene fibres (PP) / Elastane (EL)
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- Polyester fibres (PES)
Polyester fibres (PES) are made of linear makromolekules containing at least 85% of an ester in the chain. To date three polyester polymers are commercially available:
- polyethylene terephthalate (PET), which is based on ethylene glycol
- polybutylene terephthalate (PBT), which is based on butyl glycol
- and polytrimethylene terephthalate (PTT), which is based on trimethylene glycol.
However, only one of these three polymers, PET, has so far been widely applied in the textile industry. PET fibres have a very high degree of crystallinity, which allows for excellent heat-resistance and other mechanical properties. On the other hand, this compact structure inhibits the diffusion of the colourants into the fibre during dyeing. As a result, PET fibres cannot be dyed below 100°C, unless dyeing accelerants (so-called carriers) are used. Carriers are harmful for the environment and in many cases are toxic for humans. Their use is now in decline, but has been common practice for many years. Carrier-free dyeables PES fibres are now available on the market. They include the “modified PES fibres” and the PTT fibres. “Modified PES fibres” are based on PET. Thanks to physical and chemical alterations made to the structure of the fibre during the manufacturing process, they show lower crystallinity, which makes them easier to dye without need for carriers. Because of their high cost, however, these fibres are applied only in specific segments of the market. As concerns PTT, this aromatic polyester (polymethylene terephthalate) is made by the polycondensation of 1,3-propanediol and terephthalic acid. The high cost of synthesis of 1,3-propanediol has for many years prohibited the commercialisation of this fibre. Recently Shell pursued a new lower-cost synthesis route for the production of this monomer, which has led to renewed interest for PTT polymers [178, VITO, 2001].
On polyester fibres in general, it is also worth mentioning that during the polycondensation reaction, cyclic oligomers with very low water-solubility can be formed (1-3% on the weight of the fibre [77, EURATEX, 2000]. These oligomers tend to migrate to the surface of the fibre during dyeing, thus negatively affecting level dyeing and rub-fastness properties.
- Polyamide fibres (PA)
The starting polymer comes from the polycondensation reaction between a diamine and a dicarboxylic acid. According to the number of carbon atoms of the end-product one can speak of PA 6,6 or PA 6.
PA 6,6 is made by thermal polycondensation of equimolecular amounts of adipic acid and 1,6-hexamethylendiamine. The equilibrium condensate contains small amounts of monomers and cyclic dimmers (2%).
PA 6 is made by polymerisation of the cyclic monomer epsilon-caprolactame. The caprolactame content in the polymer can be reduced to 0,2% by extraction with hot water. During the melting process for the production of the fibre (melt spinning), the caprolactame content rises again and is partially emitted during the following thermal treatments.
- Acrylic fibres (PAC)
The polymer is obtained by radical polymerisation of acrylonitrile in aqueous emulsion or in solvent. The obtained polymer, made of 100% acrylonitrile (also called PAN), gives fibres with insufficient dye-binding capability, due to the high glass transition temperature (above 100°C). For this reason this polymer is no longer used in the textile industry. Acrylic fibres (PAC), commonly found on the market today, are anionic copolymers containing 85-89% of acrylonitrile, 4-10% of a non-ionic comonomer (vinyl chloride, vinyl acetate, methyl acrylate) and 0,5-1 % of ionic comonomers containing sulphonic or sulphate groups.
Dry and wet spinning techniques can be used for the production of the fibre. When dry spinning is used the polymer is dissolved in dimethylformamide (DMF). If the fibre is manufactured through wet spinning, besides DMF, dimethylacetamide, dimethylsulphoxide, ethyl carbonate and aqueous solutions of inorganic salts or acids are also used. Residues of these solvents (0,2-2% of the weight of the fibre) are found in the waste water from pretreatment.
- Polypropylene fibres (PP)
The isotactic polymer is used for fibre production. Due to the tertiary carbon atom, PP is very sensitive to high temperature and oxidation. Alkylated phenols pr p-xylolys, together with sulphides or thio-derivates (dilauryl- or distearyl thiopropionate) are used as stabilizers. Substanes with benzotriazol groups, Ni complexes, anthrachinon derivates and steric hindered diamines are used as UV-absorbents. These low molecular weight substances remain in the PP fibres and are considered as possible emission-relevant substances.
- Elastane (EL)
Elastane fibres are made out of an elastomer containing at least 85% of polyurethane (PU). For the production of the fibre (dry spinning) the polymer is dissolved in dimethylacetamide. Residues of this solvent remain in the fibre (<1% on the weight of the fibre) and are found in the waste water from pretreatment. To reduce the high adhesive characteristics and to guarantee sufficient gliding properties during processing, preparation agents are applied to the fibre (approximately 6-7% add-on). These auxiliaries contain 95% silicone oils and 5 % surfactants. The high percentage of silicone oils will cause environmental concerns during pretreatment of the textile material, when these substances have to be removed.
Litertaure: BAT for the Textile Industry, July 2003
