Rubbery nylons

Random copolymerization of aliphatic monomers develops elastomeric or rubbery nylons by reducing interchain attractions and lowering the decree of crystallinity by destruction of chain r ularity. Random copolymerization of ahphatic monomers generally decreases both meehanieal and thermal properties of nylorts but, in some cases, it allows rubber-like behavior in the copolymer. 

A -Substitution. Replacement of the hydrogen atom in the —CONH— group by such groups as CH3 and -CH20CH3 will cause a reduction in the interchain attraction and a consequent decrease in softening point. Rubbery products may be obtained from methoxymethyl nylons. These materials are considered in more detail in Section 18.9. 

When thermoplastics are reinforced with short fibres, both stiffness and strength may be increased, but these improvements are accompanied by a reduction of the ultimate strain for high concentrations of fibres [1-5]. This brittle behaviour is incompatible with growing engineering applications in which tlie parts are subjected to cyclic loadings or impacts, as in automotive under-the-hood applications. Thus, very tough nylon thermoplastics with enhanced creep and impact resistances have been produced by the introduction of a rubbery phase to the... 

Problem 2.5 Explain the fact that conversion of the amide groups -CONH-in nylon to methylol groups -C0N(CH20H)- by reaction with formaldehyde, followed by methylation to ether groups -CON(CH20CH3)- results in the transformation of the fiber to a rubbery product with low modulus and high elasticity. 

Other, non-rubbery polymers can be made to increase their crystalline proportion permanently by stretching under appropriate conditions. For example, heating nylon 6,6 to a temperature between its transition temperature and the melting point of the crystalline regions followed by stretching causes additional crystallisation with the crystallites being aligned with the direction of the extension. The oriented crystalline filaments or fibres that... 

In reaction injection moulding (RIM), two monomers are injected into a mould, where polymerisation and crosslinking occur. It is used mainly with polyurethanes to make large automotive panels. The chemistry of polyurethanes was described in Chapter 4. Other systems used include a block copolymer between a crystalline polyamide (nylon 6) and a rubbery polyether (polypropylene oxide). In principle, any polymerisation reaction that can be substantially completed after about 30 s in the mould is a candidate for RIM. 

The low hot-wet strength of acrylics can be attributed to the maimer in which water plasticizes the unique laterally bonded acrylic fiber structure. The water lowers the Tg to approximately 70°C, but this is not sufficient to account for the extremely low modulus near the boil, since other fibers including nylon are also highly plasticized by water. However, these fibers contain a well-defined, stable, three-dimensional crystalline phase that is thought not to be penetrated by the water. Therefore, they can act to reinforce the fiber and limit the drop in modulus at temperatures above the Tg, where the amorphous phase has become rubbery. The crystalline phase in the acrylic fiber is highly imperfect (as was discussed in Section 12.4) and can probably be easily penetrated and plasticized by the water. 

At the end of Table 9 we find under d) the rubbery product polyethylene tetrasulphide and under e) the so-called S5mthetic polyamides (Nylon), the latter being condensation, products between diamines and dicarboxylic acids. In contrast to the others, the types d) and e) are linear polymers. 

Effect of water on the adhesive. The influence of water on the adhesive is generally reversible, so that any deterioration in, say, mechanical properties is recovered upon drying. The extent of this influence depends upon the adhesive s composition. All polymers absorb greater quantities of water when above their Tg, so that rubbery materials tend to show greater water absorption than rigid adhesives. Interestingly, the key position in structural metal-to-metal adhesives for airframe construction is occupied by epoxy-nylon adhesives, some of which display water uptakes of the order of 14%. 

---