Degradation, thermoplastic rubber

Selecting a Plasticizer for High Performance Adhesives. The ideal plasticizer for use with thermoplastic rubbers in high performance adhesives is one which is completely insoluble in the endblock phase, completely miscible with the midblock phase, and low in cost. Low volatility, low viscosity, low density, and resistance to degradation are also desirable characteristics. Various hydrocarbon oils whose average solubility parameters are below those of the midblocks, but not too far below, satisfy these requirements reasonably well. 

Adequate protection during solution mixing, application to the substrate, solvent evaporation, and storage of thermoplastic rubber formulations can be provided by a combination of the chemical stabilizers listed in Table 6. A high temperature drying step for solvent evaporation may require special consideration, but degradation is not usually a problem in such a case due to the short exposure times involved. 

Jha, A. and Bhowmick, A.K., Thermal degradation and ageing behaviour of novel thermoplastic elastomeric nylon-6/acrylate rubber reactive blends, Polym. Degrad. Stab., 62, 575, 1998. 

Al-Malaika, S. and Amir, E.J., Thermoplastic elastomers Part III—Ageing and mechanical properties of natural rubber-reclaimed rubber/polypropylene systems and their role as solid phase dispersants in polypropylene/polyethylene blends, Polym. Degrad. Stab., 26, 31, 1989. 

Kumar, R.C., Fuhrmann, I., and Kocsis, J.K., LDPE-based thermoplastic elastomers containing ground tire rubber with and without dynamic curing, Polym. Degrad. Stab., 76, 137, 2002. 

Natural rubber is a polymer of isoprene- most often cis-l,4-polyiso-prene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials is found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. Some natural rubber sources called gutta percha are composed of trans-1,4-poly isoprene, a structural isomer which has similar, but not identical properties. Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both, i.e., if it is heated and cooled, it is degraded but not destroyed. 

Other transitions such as degradation and phase separation may be also observed during the formation of the polymer network. Degradation is usually present when high temperatures are needed to get the maximum possible conversion. Phase separation may take place when the monomers are blended with a rubber or a thermoplastic, to generate rubber-modified or thermoplastic-modified polymer networks. In these cases, formulations are initially homogeneous but phase-separate during the polymerization reaction. This process is discussed in Chapter 8. 

Thermal and Chemical Stability. In addition to load-bearing properties, tire reinforcement must be able to resist degradation by chemicals in cured rubber and heat generation. The most critical degradant depends on the material in use. Most thermoplastic reinforcements are either modified direedy or stabilized with additives to offset some, mosdy thermal, degradation (32,33). 

Thermoplastic elastomers (TPEs) with blocks of polydiene rubber are subject to degradation at the carbon-carbon double-bond sites and require proper stabilization. In SIS block copolymers, chain scission is the predominant degradation mechanism. In an SIS block copolymer, the addition of a more effective stabilizer, AO-3, alone or blended with a secondary antioxidant, PS-1, can provide a significantly superior performance over AO-1 alone or with PS-1. Resistance to discoloration after static oven aging at 80°C (176°F) is improved dramatically (Fig. 5). Viscosity stabilization (melt flow index stability) (Fig. 6) is also improved drastically using AO-3/PS-1. 

Quite different polymers can be obtained from butadiene by polymerisation through the 1,2 position, 1,2-PB is no longer a rubber but a semi-crystalline thermoplastic polymer rather similar to PP which has some potential as an environmentally degradable plastic (Chapter 5). 

In the same manner as blends of thermoplastics or thermoplastic/ elastomer, irradiated blends of elastomers can undergo chain scission due to degradation or cross-linking, depending especially on the dose range. For example, Zurina et al. i measured tanS versus temperature for 50/50 epoxidized natural rubber (ENR-50)-EVA blends by DMA. At 60 kGy the irradiation-induced cross-link enhanced the Tg of the blend, whereas at higher dose (100 kGy), the Tg decreased due to the occurrence of oxidative degradation that broke the cross-link structure. 

Cyanoacrylates polymerize by anionic mechanism initiated by moisture or basic ions. The polymers formed tend to be more brittle than those formed from other acrylics, and the bond-lines are usually not resistant to degradation by moisture. Adhesive formulations are singlecomponent, fast-curing products, however, and suited to many hard-to-bond surfaces such as rubber and many thermoplastics. 

The main advantage of such triblock copolymers is that they can be moulded and recycled simply by heating the material above the glass transition temperature of polystyrene, unlike classical vulcanised rubbers, which cannot be reused without degradation as they are chemically cross-linked. Such SBS-based materials are called thermoplastic elastomers. However, for various reasons, including cost, the commercial use of such polymers is rather limited compared with the use of natural and classical synthetic rubbers. 

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