Conventional elastomers

TPEs from thermoplastics-mbber blends are materials having the characteristics of thermoplastics at processing temperature and that of elastomers at service temperature. This unique combination of properties of vulcanized mbber and the easy processability of thermoplastics bridges the gap between conventional elastomers and thermoplastics. Cross-linking of the mbber phase by dynamic vulcanization improves the properties of the TPE. The key factor that controls the properties of TPE is the blend morphology. It is essential that in a continuous plastic phase, the mbber phase should be dispersed uniformly, and the finer the dispersed phase the better are the properties. A number of TPEs from dynamically vulcanized mbber-plastic blends have been developed by Bhowmick and coworkers [98-102]. 

The diisocyanates and polyols are reacted to form a high molecular weight hydroxyl terminated millable gum. These millable gums are compounded and processed as conventional elastomers, both sulphur and peroxides being used to cure the polymers. Here again, polyether and polyester types are available, and the differences between these two types referred to above also apply here. 

The thicker lines represent the sequences of hard urethane segments, and the clusters of these effectively act as crosslinks, making the material act as a conventional elastomer. When the temperature is raised high enough, the clusters disassociate and the material can be made to flow when subsequently cooled, the clusters can reform and the material again exhibits elastomeric properties. Thus these materials show elastomeric behaviour at room temperature, but can be processed as thermoplastics. Hence the name of the material class - thermoplastic elastomers. 

The styrenic thermoplastic elastomers are the only type which are fully compounded in the manner of conventional elastomers. In this case, however, the addition of carbon black, or other fillers, does not give reinforcement. Additions of polystyrene, or high impact polystyrene, and oil are used to vary hardness and tear strength, and fillers can be used to cheapen the material. Other added polymers, e g., EVA, can be used to increase ozone resistance. These materials also require antioxidants for protection during processing and service life, and the poor UV stability restricts their use in outdoor applications. 

Vulcanization, or cross-linking of elastomers, is technically the most important process for conventional elastomers. During that process, strong chemical bonds are formed between molecules, thus restraining their mobility. As pointed out earlier, a three-dimensional network is formed. The cross-linking of elastomeric molecules is a random process typically, one cross-link is formed per 100 to 200 monomeric units. 

Fillers are not normally used in polyurethanes to bulk out the product, because they reduce the properties too drastically. This is in contrast to conventional elastomers, where they can be used to reinforce the product. Ultra fine silica is used as a thixotrophic filler in trowelable polyurethanes. 

Tiquid rubbers are polymeric products used primarily as adhesives, sealants, castable rubbers, and rocket propellant binders. The name liquid rubber comes from the properties of flowing at room temperature and curing to rubbery networks. These materials offer advantages over conventional elastomers in their ease of handling and processing as they can be readily pumped and mixed in low power (relative to normal rubber) equipment with resultant savings. 

A liquid rubber must be more than just a low molecular weight counterpart of a conventional elastomer. Because of molecular weight, functionality, and network-formation requirements, a liquid rubber needs to be considered as a polymeric entity of its own. This point has been shown by an analysis of the network formed upon vulcanization of a liquid rubber (3). This analysis compares and contrasts two classes of liquid rubbers terminally functional and randomly functional prepoly-... 

There are certain advantages and disadvantages to this type of material compared to conventional elastomers. The advantages concern the existence of reversible physical cross-links. When a TPE is heated, the cross-links disappear if the temperature increases beyond the Tg of the hard phase or above the of the hard domain if it is crystalline. In contrast, conventional elastomers display thermostable structure. The physical nature of... 

Because of low cost, high heat deflection temperature (HDT (104°C)), notched impact resistance, improved low temperature impact and flexibility, weather resistance, flame retardancy, and impact resistance, the PP/EPDM blend has got a widespread applications. These unique characteristics of this thermoplastic elastomer blend make it an attractive alternative to conventional elastomer in a variety of markets such as automobile industries, wire, cable insulator, automobile bumpers and fascia, hose, gaskets, seals, weather stripping, among others. 

The market of PP/EPDM blends has grown dramatically because of its recycling abihty and processability by conventional thermoplastic processing equipment. The unique characteristics of thermoplastic elastomer made it an attractive alternative to conventional elastomers in a variety of markets. Liu et al. showed from the experimental blends (53) that materials cost reduction of between 30% to 50% is possible in comparison to commercial products if one applies the PP/EPDM blends to the construction of a basketball court, a tennis court, and a roller hockey rink, which were estimated around 7000, 14,000, and 40,000, respectively. The cost comparison took into account the percentage of rubber or PP used in experimental blend, the exponential factor for a scale-up process and the overall surface area of the specific applications. Among many possible application of this blend two readily feasible applications are roofing and flooring. 

The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is the presence of a cross-linked network structure. As we have seen from the above discussion, in the case of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible, insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which are the most widely used thermosets, consist essentially of two classes of thermosets. These are the condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or with urea or melamine (aminoplastics or amino resins). 

Thermoplastic block copolymeric elastomers are also of interest as dielectric elastomer materials. These polymers differ from conventional elastomers in that they possess physical crosslinks rather than chemical ones. In these polymers... 

Crosslinking liquid crystalline polymers yield materials with exceptional properties due to the coupling between elastic properties and mesomorphous behavior. These compounds, especially the mesogenic elastomers, have attracted considerable attention in recent years. Like conventional elastomers, they can sustain very large deformations causing molecular extension and orientation, but they can also exhibit spontaneous distortions, some memory effects, an unusual mechanical response, and coupling between mechanical, optical and electric fields. 

At high temperatures the product CT is independent of the temperature as is predicted by the theory and the network behaves similar to conventional elastomers. For comparison the CT-values of natural rubber (NR) are plotted in both figures. The negative values of CT (Fig. 5) indicate that the mesogenic side chains orient more or less perpendicular to the deformed network chain. Only in this case the polarizability perpendicular (qj ) to the axis of the polymer segment is greater than parallel (a P to this axis and therefore Aa( CT) = 0. 

As expected, the monol content of the EO-capped PPG diols has a significant effect on the elastomer processability. The Shore hardness of the conventional elastomer was only 57A after 16 hours cure at 100 C, whereas the elastomer based on ultra-low monol PPG had a hardness of 78A. It took three weeks for the conventional... 

Among Ihe most commercially prevalent of these alloys is the PVC-NBR blends, the properties of which approach those of thermoplastic elastomers. Since nitiile elastomers are well established in conventional elastomer applications such as hoses, belts, and calendared goods, these alloys are found in many of the same fabricated products. NBR s are available in a variety of molecular weights, gel levels, comonomer rahos, and in physical forms ranging from powders to crumb rubber to slabs or bales of rubber. Alloys of these polymers are commonly found in automotive dashboard skins, wire and cable compounds, water and fuel hoses, and oil resistant boots and outerwear, among others (83). 

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