Cross-linking of rubbers

The function of cross link requires no elaborative discussions in the matter of product design since the implications of the same on the physical properties are quite obviously mentioned in many textual treatises on cross linking of rubbers. The cross link density directly affects physical properties such as heat build up, tear strength and elongation, too. 

This historical development of the radiation technology of polymers is reviewed in this outline. The important applications of this technology are divided into two classes - large scale processes such as cross-linking of rubbers and plastics and specialized sophisticated processes such as microlithography. The initial fundamental studies that led to these applications are outlined and the slow process of commercialization is emphasized in this review. 

Chain reactions, essentially polymerizations, can be achieved with medium doses, as a result of the chemical amplification by purely thermal processes of radiation-induced initiation (Scheme 2). Processes involving single steps or short kinetic chain length reactions require much higher doses.This is generally the case for the radiation cross-linking of rubbers and thermoplastics. 

The problem is made more complicated by the inversion of phases taking place in the system in the course of the process and by the cross-linking of rubber macro-molecules occurring at elevated temperatures. 

Radiation cross-linking affects different characteristics of polymers like mechanical behaviour, chemical stability, thermal and flame resistance. Until now, radiation cross-linking is limited to only a few industrial applications cross-linking of rubber or polymers for tyres, cables, pipes (e.g. in under floor heating systems), and heat-shrinkable tubes. Nevertheless, there exist industrial facilities like electron accelerators and gamma plant. Some of these radiation sources are operated by research institutes. 

The second example of a polymer reaction is the industrial cross-linking of rubber by vulcanization sketched in Fig. 3.50. The process was invented already in 1839 by C. N. Goodyear without knowledge of its chemical stracture. Natural rubber is cis-poly(l-methyl-1-butenylene) or polyisoprene with a low glass transition temperature of about 210 K. Its structure and those of other rubbers are given in Fig. 1.15. The addition of sulfur in the form of Sg rings and heating causes the vulcanization. Of the listed cross-hnks in Fig. 3.50, only the left example is an efficient network former. The sulfur introduces about 1 cross-link for each of 50 S-atoms used. Modem vulcanization involves activators and accelerators for increased efficiency. The detailed mechanism is rather complicated and not fully understood. 

The cross-linking of rubber with sulfur is called vulcanization. Cross-linking bonds the chains together to form a network. The resulting product is called a thermoset, because it does not flow on heating. 

Figure 3.3 Comparison of experiment (points) and theory [Eq. (3.39)] for the entropy elasticity of a sample of cross-linked natural rubber. [From L. R. G. Treloar, Trans. Faraday Soc. 40 59 (1944).]...

The use of stabilisers (antioxidants) may, however, have adverse effects in that they inhibit cross-linking of the rubber. The influence of phenolic antioxidants on polystyrene-SBR alloys blended in an internal mixer at 180°C has been studied. It was found that alloys containing 1% of certain phenolic antioxidants were gel-deficient in the rubber phase.The gel-deficient blends were blotchy in appearance, and had lower flow rates compared with the normal materials, and mouldings were somewhat brittle. Substantial improvements in the impact properties were achieved when the antioxidant was added later in the mixing cycle after the rubber had reached a moderate degree of cross-linking. 

The term ABS was originally used as a general term to describe various blends and copolymers containing acrylonitrile, butadiene and styrene. Prominent among the earliest materials were physical blends of acrylonitrile-styrene copolymers (SAN) (which are glassy) and acrylonitrile-butadiene copolymers (which are rubbery). Such materials are now obsolete but are referred to briefly below, as Type 1 materials, since they do illustrate some basic principles. Today the term ABS usually refers to a product consisting of discrete cross-linked polybutadiene rubber particles that are grafted with SAN and embedded in a SAN matrix. 

From Figure 16.13 it will be seen that a minimum of about 20% cross-linked nitrile rubber is required in order to obtain tough products. For high-impact... 

Properties and Applications of Cross-linked Polyurethane Rubbers... 

Indications are that cross-linking of dimethylsilicone rubbers occurs by the sequence of reactions shown in Figure 29.9. 

Properties and applications of cross-linked polyurethane rubbers... 

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

In natural rubber, the cross-linking of these radicals is hindered because of the bulkiness of the methyl side group. Consequently, these radicals prefer to disproportionate and cleave. This reduces the molecular weight and natural rubber softens on ageing. 

These materials have characteristics of both rubbers and thermoplastics. At room temperature they behave like cross-linked rubbers, but at elevated temperatures the cross-links effectively disappear (they are said to be heat fugitive) and the material may be processed as a thermoplastic. Unlike truly cross-linked (vulcanised) rubbers, these materials may be capable of disolution in solvents, although not necessarily at room temperature. 

Silicones are a large group of compounds that include large polymers containing silicon. Depending on the formula and the degree of polymerization and cross-linking of the polymers, they may be slippery liquids, waxes, or rubbers. 

Dibutyltin diacetate, dilaurate, and di-(2-ethylhexanoate) are used as homogeneous catalysts for room-temperature-vulcanizing (RTV) silicones. The dialkyltin compounds bring about the cross-linking of the oligomeric siloxanes, to produce flexible, silicone rubbers having a host of different uses, such as electrical insulators and dental-impression molds. Recent work has also shown (560) that various dibutyltin dicar-boxylates catalyze both the hydrolysis and gelation of ethyl silicate under neutral conditions. 

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