Fiber-polymer systems

Power-law expressions are still nsed to describe snch polymer-fiber melts. Typical power-law parameters for selected fiber-polymer systems are shown in Table 4.7. Semiempirical expressions based on Eq. (4.23) have also been developed, as well as models based on energy dissipation. A complete review of these correlations is beyond the scope of this text, and the interested reader is referred to reference 9 for a more complete review of viscosity in fiber-reinforced polymer melts. 

For many fiber-polymer systems (L/D) is in the range 10 50. From this analysis, it is evident that fiber length is important to the development of maximum tensile properties in the composite. It is also apparent that changes in the composite tensile strength are monotonically dependent on fiber concentration. 

A number of polymers are suitable as matrix material, each with its own advantages and disadvantages, with wide differences in properties, and the selection of a given carbon fiber-polymer system must be made after a thorough analysis of its suitability for the application. 

In terms of scientific research, a large variety of fiber-polymer systems have been (and are still) studied, with new compositions constantly investigated. However, in terms of industrial realizations, the list of useful systems is reduced to a few tens, as described in Table 7.1. 

As one might expect, any fiber-polymer system is unique, not only because of the differences in the respective properties of the components, but also because the fiber surface properties and the polymer chemistry have important effects. Not all aspects are yet understood and any generalization would indeed be abusive. A clear demonstration of this fact is offered when comparing the effect of a given type of short glass fiber (E-glass), at constant loading (30%) in different thermoplastics (Table 7.2). 

Polymer systems have been classified according to glass-transition temperature (T), melting poiat (T ), and polymer molecular weight (12) as elastomers, plastics, and fibers. Fillers play an important role as reinforcement for elastomers. They are used extensively ia all subclasses of plastics, ie, geaeral-purpose, specialty, and engineering plastics (qv). Fillets are not, however, a significant factor ia fibers (qv). 

An important direct use of phosgene is in the preparation of polymers. Polycarbonate is the most significant and commercially valuable material (see Polycarbonates). However, the use of phosgene has been described for other polymer systems, eg, fiber-forming polymeric polyketones and polyureas (90,91). 

Another approach in chemical finishing is to use reagent systems that are reactive with themselves but only to a limited extent or not at all with the fiber substrate. An example of such approaches are in situ polymer systems that form a condensed fiber system within the fiber matrix (1,2). A third type of approach may be the deposition of a polymer system on the fiber substrate. Once deposited, such systems may show a strong affinity to the fiber and may be quite durable to laundering. Polyacrjiate and polyurethane are examples of durable deposits on cotton, which last through numerous launderings (3). 

Synthetic polymers have become extremely important as materials over the past 50 years and have replaced other materials because they possess high strength-to-weight ratios, easy processabiUty, and other desirable features. Used in appHcations previously dominated by metals, ceramics, and natural fibers, polymers make up much of the sales in the automotive, durables, and clothing markets. In these appHcations, polymers possess desired attributes, often at a much lower cost than the materials they replace. The emphasis in research has shifted from developing new synthetic macromolecules toward preparation of cost-effective multicomponent systems (ie, copolymers, polymer blends, and composites) rather than preparation of new and frequendy more expensive homopolymers. These multicomponent systems can be "tuned" to achieve the desired properties (within limits, of course) much easier than through the total synthesis of new macromolecules. 

Covalent Bonds. Fiber-reactive dyes, ie, dyestuff molecules containing reactive groups, are adsorbed onto the fiber and react with specific sites (chemical groups) in the fiber polymer to form covalent bonds. The reaction is irreversible, so active dye is removed from the equiUbrium system (it becomes part of the fiber) and this causes more dye to adsorb onto the fiber to re-estabflsh the equiUbrium of active dye between fiber and aqueous dyebath phases (see Dyes, reactive). 

Internal and External Phases. When dyeing hydrated fibers, for example, hydrophUic fibers in aqueous dyebaths, two distinct solvent phases exist, the external and the internal. The external solvent phase consists of the mobile molecules that are in the external dyebath so far away from the fiber that they are not influenced by it. The internal phase comprises the water that is within the fiber infrastmcture in a bound or static state and is an integral part of the internal stmcture in terms of defining the physical chemistry and thermodynamics of the system. Thus dye molecules have different chemical potentials when in the internal solvent phase than when in the external phase. Further, the effects of hydrogen ions (H" ) or hydroxyl ions (OH ) have a different impact. In the external phase acids or bases are completely dissociated and give an external or dyebath pH. In the internal phase these ions can interact with the fiber polymer chain and cause ionization of functional groups. This results in the pH of the internal phase being different from the external phase and the theoretical concept of internal pH (6). 

Ladizesky, N.H. and Ward, I.M. (1983). A study of the adhesion of drawn polyethylene fiber/polymer resin systems. J. Mater.. Sci. 18, 533-544. 

In using Eq. (6.10) to predict / , of a given composite system it is important that the said failure mechanisms all exist. If any one mechanism is apparently absent the corresponding toughness term must be excluded from the / t equation. It is also worth emphasizing that / , varies linearly with reciprocal of the frictional shear strength of the interface, i.e. l/tf, with the lower limit of (1 — Ff)/fm when if approaches infinity. This relationship has been shown to apply to many carbon fiber polymer matrix composites (CFRPs) (Harris et al., 1971 Beaumont and Phillips,... 

STYRENE-MALEIC ANHYDRIDE. A thermoplastic copolymer made by the copolymerization of styrene and maleic anhydride. Two types of polymers are available—impact-modified SMA terpolymer alloys (Cadon ) and SMA copolymers, with and without rubber impact modifiers (Dylark ). These products are distinguished by higher heat resistance than the parent styrenic and ABS families. The MA functionality also provides improved adhesion to glass fiber reinforcement systems. Recent developments include lerpolymer alloy systems with high-speed impact performance and low-temperature ductile fail characteristics required by automotive instrument panel usage. 

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