0.90 – 0.91 gm/cm3
Because of its low specific gravity, polypropylene yields the greatest volume of fibre for a given weight. This high yield means that polypropylene fibre provides good bulk and cover, while being lighter in weight. Polypropylene is the lightest of all fibres and is lighter than water. It is 34% lighter than polyester and 20% lighter than nylon. It provides more bulk and warmth for less weight
The specific gravities of common fibres are given in Figure 1.
Lowest thermal conductivity of any natural or synthetic fibre (6.0 compared to 7.3 for wool, 11.2 for viscose and 17.5 for cotton). Polypropylene fibres retain more heat for a longer period of time, have excellent insulative properties in apparel, and combined with its hydrophobic nature keeps wearer dry and warm. Warmer than wool.
The thermal conductivity of various fibres is shown in Figure 3.
The generation of static electricity on textiles is a complex and, to some extent, a subjective problem. Practical experience shows that polypropylene does not exhibit a static behavior in most normal circumstance and if a problem does occur it can easily be controlled by the use of normal textile anti-static treatments during processing. Anti-static agents can also be incorporated in the polymer to reduce static build up.
Resistant to Bacteria and Micro-organisms
Like other synthetic fibres – nylon, acrylic and polyester – polypropylene fibres are not attacked by bacteria or micro-organisms; they are also moth-proof and rot-proof and are inherently resistant to the growth of mildew and mold.
Recyclable, ecologically friendly. Incinerates to trace ash with no hazardous volatiles.
Effect of Heat
The melting point of polypropylene is about 165°C and while it does not have a true softening point temperature, the maximum processing temperature of the fibre is approximately 140°C. Prolonged exposure to elevated temperatures will cause degradation of the fibre, but anti-oxidants are incorporated in polypropylene fibres to protect them during processing and at normal service temperatures.
Wool, cotton, acrylic and viscose rayon fibres do not melt although they are ‘tendered’ to varying degrees by exposure to high temperatures. Nylon and polyester are thermoplastics that have maximum processing temperatures in excess of 220°C. Thus polypropylene has the lowest softening point of all these materials. Nevertheless, this temperature is sufficiently high to enable the fibre to be processed satisfactorily in almost all normal manufacturing processes. The lower softening temperature is of little consequence in service except that, when ironing, it is necessary to control the temperature so as not to exceed 120°C.
The shrinkage of polypropylene fibre is controlled by its manufacturing conditions. At textile processing temperatures, which do not normally exceed 130°C., the shrinkage varies between 2.5% to 5%, but figures as high as 30% can be obtained, or by conditioning the fibre or yarn, as low as 0.5%
Effect of Extreme Cold
Remains flexible at temperatures in the region of -55°C.
Polypropylene fibre burns and presents much the same risks as most other man-made textile fibres. It is difficult to ignite, and is defined as combustible but not highly inflammable. It can, however, be rendered flame-retardant by the incorporation of additives.
Resistance to Water
The water absorption of polypropylene fibre is about 0.3% after 24 hours immersion in water, and thus its regain – the amount of water absorbed in a humid atmosphere – is virtually nil (0.05% at 65% RH, 21 °C.)
The regain of various fibres is shown in Figure 4.
Effects on Strength
Water has no effect on the strength of polypropylene fibres, whereas cotton increases in strength when wet and the strength of viscose rayon falls quite markedly when wet.
The tenacities of various fibres are compared in Table 1.
Because polypropylene fibres absorb hardly any moisture their dimensions do not alter with changing humidity or when they become wet. In this respect they are superior to all other fibres, although polyester is another fibre little affected by moisture.
Resistant to Staining
Because of their extremely low water absorption, polypropylene fibres resist water-borne stains better than any other fibres. And since the fibre does not accumulate static though friction during use, it does not attract as much dirt or dust as other man-made fibres.
Polypropylene is hydrophobic and will not absorb water in the fibre. Water “wicks” away from the skin and through the fabric to the face for quick evaporation.
Fade-resistant. Polypropylene is dope-dyed (melt-dyed), which is the most “colourfast” of any colouring methods, and produces fibres and yarns that are colourfast, washfast, lightfast and fade-resistant. The colour is incorporated within the fibre itself.
Resistance to Sunlight
Strength, colour fastness and degradation can be effectively protected by means of stabilizers.
Effect on Strength
Ultraviolet (UV) absorbers and stabilizers are incorporated in polypropylene fibres to give them the required degree of UV resistance and stability. All fibres are degraded by sunlight, the effect varying with their physical form. Polypropylene fibres that are not stabilized against UV light will lose strength on exposure to sunlight, but the stabilizing systems commonly used in polypropylene fibres impart a degree of UV resistance suitable for most applications.
The normal method of colouring other fibres is by dyeing. The minimum colour fastness rating required for fibres in many applications is 4 – 5 (BS 1006) and ratings in the range of 4 – 6 are normal. However, polypropylene fibre is coloured during production by pigmentation – often called ‘dope dyeing’ or ‘melt dyeing’. The pigments used for polypropylene fibres give very good light fastness ratings of 7 and higher.
Since polypropylene molecular chains have no polar functional groups (active sites for chemical bonds, or dyesites) and relatively high crystallinity (50-65%), dye molecules cannot be chemically attracted to the fibres. Adsorbed dye molecules which interact with the fibre surface by weak van der Waal’s force are easily washed away because of polypropylene’s hydrophobic properties.
However, polypropylene fibre is dope-dyed (spun-dyed) by the manufacturer in virtually unlimited colour choices.
Chemical and dry-cleaning resistance
Polypropylene has the best resistance of any common fibre to the action of most types of chemicals and is attacked only by the most aggressive acids and oxidizing agents. Although this excellent chemical resistance may be needed only infrequently it is of value in certain industrial applications. However, some chemicals, although not attacking the polypropylene itself, may destroy the colour. The fibre is unaffected by most acids, alkalis, and salts.
Effect of Acids
Excellent resistance to most acids except chlorosulphonic and concentrated sulfuric acid.
Effect of Alkalis
Excellent resistance with the exception of some oxidizing agents.
Effect of Bleaches and Solvents
Excellent resistance. However, chlorinated hydrocarbons cause swelling at room temperature and dissolve polypropylene at 71 °C. and higher.
Certain solvents, particularly halogenated solvents, are absorbed by polypropylene and swell it. Thus the properties are changed in the presence of the solvent but are regained when the solvent evaporates. While the fibre is softened by the solvent, shrinkage can occur if the fibre has not been annealed. This can happen during dry cleaning and hence it is important that polypropylene fibres, yarns or fabrics, intended for use in applications where dry cleaning will be used, are heat-annealed to prevent complaints in service.
Chlorine bleach can be used as a cleaning agent without fading the colour nor degrading the fabric.
A summary of the chemical resistance of polypropylene to various substances is available in our Chemical Resistance Guide.
The abrasion resistance of fibres, unlike some other properties such as tenacity or modulus, is not a fundamental property and hence comparisons between the abrasion resistance of fibres are only useful if they truly represent performance in the application in question and are carried out on fabrics of identical construction.
The abrasion resistance of polypropylene approaches that of nylon and is superior to that of other fibres, and is good even when wet.
The good abrasion resistance of polypropylene is well established both by independent tests and by service trials. The results are given in figure 5.
Excellent – recovers well from bending. Excellent low temperature flexibility.
Fibre is identified by fusing, shrinking, and curling away from a flame leaving a hard, round, dark-coloured bead. Fibre floats on water and is soluble in hot toluene or xylene.