Molecular weight mastic

An entirely new concept was iatroduced iato mbber technology with the idea of "castable" elastomers, ie, the use of Hquid, low molecular-weight polymers that could be linked together (chain-extended) and cross-linked iato mbbery networks. This was an appealing idea because it avoided the use of heavy machinery to masticate and mix a high viscosity mbber prior to mol ding and vulcanization. In this development three types of polymers have played a dominant role, ie, polyurethanes, polysulftdes, and thermoplastic elastomers. 

Natural rubber displays the phenomenon known as natural tack. When two clean surfaces of masticated rubber (rubber whose molecular weight has been reduced by mechanical shearing) are brought into contact the two surfaces become strongly attached to each other. This is a consequence of interpenetration of molecular ends followed by crystallisation. Amorphous rubbers such as SBR do not exhibit such tack and it is necessary to add tackifiers such as rosin derivatives and polyterpenes. Several other miscellaneous materials such as factice, pine tar, coumarone-indene resins (see Chapter 17) and bitumens (see Chapter 30) are also used as processing aids. 

Figure 11.16. Efficiency of mastication of rubber at different temperatures. Molecular weights (M) measured after 30-minute mastication of 200 g natural rubber in a size B laboratory Banbury...

By rolling on a two-roll mill the molecular weight of the polymer can be greatly reduced by mechanical scission, analogous to that involved in the mastication of natural rubber, and so mouldable materials may be obtained. However, bulk polymerisation is expensive and the additional milling and grinding processes necessary make this process uneconomic in addition to increasing the risk of contamination. 

Generally, systems developed in the USA favour a combination of polyethylene with either butyl-rubber or hot-applied mastic adhesives, the latter consisting of a blend of rubber, asphalt and high molecular weight resins. In European and Far East coating plants, epoxy type primers and hard ethylene copolymer adhesives have been successfully employed. 

The molecular weight of natural rubber (NR) can be altered by mechanical means. Mastication of NR results in a reduction of molecular weight. In the distant past, NR was commonly subjected to mechanical degradation on open mills for extended periods of time to improve processability. 

Low-molecular weight chains do not experience enough shear force to induce scission. Watson et al." demonstrated (by the intrinsic viscosity characterization of masticated NR) that the limiting molecular weight for the shear-induced degradation is in the order of 0.7-1.0 X 10 Frenkel independently speculated that shear-induced cleavage occurs near the midpoint of the polymer chain. 

Lower the viscosity of natural rubber as a result of breaking of the molecular chains to enable problem-free compounding with saving of cost and time. Without the presence of oxygen, mastication would not be possible. The consequence of mastication is a reduction of average molecular weight. 

NR requires a certain degree of mastication (reduction in molecular weight) to facilitate processing, although the advent of constant viscosity grades, and oil extended grades has substantially reduced the need for mastication. 

The LP grades are liquid polymers used in sealant and mastic applications, and are formed by breaking down a high molecular weight polymer in a controlled manner. The liquid polymer again has mercaptan end groups. 

Grinding or milling causes degradation of many polymers. The process of mastication of natural rubber involves a mechanically initiated, autoxidative degradation which lowers the molecular weight to a level where the material is easier to process on a commercial scale. 

A block copolymer may initially have the simplest structure P—R, but, as the molecular weight of the homopolymers falls during mastication, multisegment block copolymers are formed, by mechanical scission of initial reaction products. 

Table 12. Polymerization of styrene by poly(methyl acrylate) mastication (30% of monomer). Effect of rubber molecular weight on interpolymer composition (/i)...

Polymers are very soft materials which are not easily milled under normal conditions. The mastication of natural rubber is the reduction of molecular weight by milling or cutting, the process being invented by Thomas Hancock as early as 1820. Even today this is a major industrial process in the tire and rubber industry. Other polymers are milled under reduced temperature or even in liquid nitrogen to achieve a controlled molecular mass. The degradation of several polymers has been investigated by Dimitrov et al. [104]. 

Ammonium polyphosphates, on the other hand, are relatively water insoluble, nonmelting solids with very high phosphorus contents (up to about 30%). There are several crystalline forms and the commercial products differ in molecular weights, particle sizes, solubilities, and so on. They are also widely used as components of intumescent paints and mastics where they function as the acid catalyst (i.e., by producing phosphoric acid upon decomposition). They are used in paints with pentaerythritol (or with a derivative of pentaerythritol) as the carbonific component and melamine as the spumific compound.22 In addition, the intumescent formulations typically contain resinous binders, pigments, and other fillers. These systems are highly efficient in flame-retarding hydroxy-lated polymers. 

Three polybutadiene/poly isoprene diblocks were provided to us by Paul Rempp of CRM, Strasbourg. Details of the molecular weights and the expected diene microstructures were discussed earlier (1,13) and are summarized in Table I. Homopolymers of corresponding microstructures and molecular weights (1,13) are also described in Table I. The molecular-weight distributions of the homopolymers are considerably broader than those of the block polymers, owing to a mastication procedure (13) carried out on these samples to lower their molecular weights. To insure that" the microstructures of the homopolymers were well matched to the constituent blocks in the copolymers, 60-MHz NMR spectra were obtained on a Varian T-60 apparatus for the diblocks, the individual homopolymers, and three equivalent homopolymer blends (13). 

Hypalon does not break down during mastication in the sense that natural rubber and Neoprene do. These rubbers undergo an irreversible chemical change (i.e., a reduction in molecular weight) when they are worked in the mixing mill. 

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