Composite yarns properties

If everyone liked the same thing we would have run out of it, whatever it was, long ago. So it is with textile polymers. There are an almost infinite number of end uses, each of which demands a particular combination of fiber/yarn/fabric/composite-construction properties, but one of the most common features of textile polymer products is that they are all orthotropic, i.e., they have elastic properties with considerable variation of strength in two directions perpendicular to one another. This element of elasticity has a profound impact on fiber toughness and also on the time dependent properties of the polymer product. 

A filament yarn (or a flat yarn) is a continuous tow that has been twisted to aid future processing. The direction of twist in a yarn is designated as Z or 5 twist—Z twist if the spirals around the axis slope in the same direction as the middle portion of the letter Z and S if the spirals slope in the same direction as the middle portion of the letter S (Twist in Appendix 1). A Z twist is normally applied to a single tow, whereas plied yarns tend to have S twist. A twisted tow can give improved composite mechanical properties, such as flexural strength. 

Recently, new compounding methods have been investigated to produce long, natural fiber-reinforced thermoplastic pellets and improve composite mechanical properties [31, 32]. For example, pellets have been formed by melt impregnation of continuous natural fiber yarns by pultrusion followed by cooling and chopping. Another method involves commingling of continuous forms of natural and synthetic fibers that are then heated, consolidated, and chopped. 

Fiber reinforced composites, depending on the properties needed, can be fabricated in three different ways. Very short fibers can be used as filler, short fibers can be organized with random orientation and long fibers can be laid in one direction to form unidirectional composites. Short staple fibers may also be twisted together to form continuous yams to fabricate unidirectional composite laminates similar to those made using long fibers. Several unidirectional laminates may be combined by layering in different directions to form laminar composites. Yarns may also be woven or knitted into fabrics to form similar laminar composites. 

Jou G.T., East G.C., Lawrence C.A. and Oxenham W. (1996), The physical properties of composite yarns produced by an electrostatic-filament charging method. Journal of the Textile Institute, 87,78-97. 

Reinforcing fibers can be modified by physical and chemical methods. Physical methods, such as stretching [22], calandering [23,24], thermotreatment [25], and the production of hybrid yarns [26,27] do not change the chemical composition of the fibers. Physical treatments change structural and surface properties of the fiber and thereby influence the mechanical bondings in the matrix. 

The newest advances in C-C composite materials have resulted from fiber improvements, such as modulus increases of two to three times and diameter decreases of more than 50% in the last ten years. Weaving techniques have also been improved, so that increased fiber content and reproducible distribution of filaments and yarns (down to 0.75 mm center-to-center spacing between yarns in 3D composites) are possible. However, because the interrelationship of the filaments and yarns with the bonding matrix is not well understood, the improved fiber properties have not yet been fully translated into a corresponding magnitude of improvement in C-C composites. 

Mechanical Properties and Fracture Behavior. Until recently, little was known about the sequence of fracture behavior that occurs in the 2D composites that are used to fabricate large C-C structures, such as the exit cones of rocket engines. In such composites, the yarns in the cloth are held together by a carbon matrix derived from a phenolic resin filled with carbon particles. 

The fibers or the yarn or rovings made therefrom can be processed to fleeces or mats (non-oriented semi-finished product) and textiles, lattices or meshes (oriented semifinished products) and can be utilized as such e.g. for thermal insulation or as filter materials, or in composites with other materials e.g. for fiber-reinforced polymers, metals or ceramics. Fibers are generally marketed after surface treatment (chemical modification, annealing, smoothing) to optimize their application and processing properties. 

J. Kajaks and S. Reihmane. Thermal and water sorption properties of polyethylene and linen yarn production waste composites. In 2nd International Wood and Natural Fibre Composites Symposium, Universitat Gesamthochschule Kassel, Institut fur Werkstoff-technik, Kassel, Germany, 1999, pp. 39/1-39/5. 

Textile materials can be used in moist wound management as fibres themselves (advanced fibres such as alginate and chitosan fibres), or conventional/advanced fibres can be modified or coated with various substances such as honey or hydrogels to obtain special properties such as ultra-absorbency, drag release, etc. In general, textiles used in wound-dressing products come in all possible forms, including fibres, nanofibres, filaments, yarns, and woven/knitted/non-woven and composite materials. 

Inter-yarn fibre and yam volume fractions. Within the meso-scale modelling paradigm, yams are considered as continuous soUd medium with effective properties that are primarily governed by the fibre volume firaction (FVF). The correct overall FVF (o-FVF) is an obligatory feature of any acceptable model as it has to match fabric areal density, resin content and specific composite weight. The o-FVF is determined as the product of the yam volume firaction (YVF) and the local intra-yam FVF (iy-FVF). The latter depends on yam compressibility, pressure used for preform consolidation and even matrix viscosity. 

Table 7.6 Physical properties of composite and ring-spun yarns... 

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