Polar specialty elastomers

The major reason that butyl rubber is used in the tire industry is its superior resistance to air permeability, as weii as other gases. Butyi has approximately 13 times greater resistance to air permeability than natural rubber. This makes it ideal for use in making tire innerliners and inner tubes. While some polar specialty elastomers can also provide good air permeability resistance, they are far more expensive and generally do not possess the right combination of other needed properties. 

DOP and other phthalate plasticizers are commonly used in rubber compounds based on polar elastomers such as nitrile rubber or polychloroprene, for example. DOP is used in these specialty elastomer-based compounds because like dissolves like. In the language of thermodynamics, these elastomers have a solubility parameter similar to that of DOP. A polar plasticizer is used with a polar elastomer because they are compatible with each other, and the plasticizer has sufficient affinity for the elastomer that It does not bleed (or bloom) to the surface of the rubber product. 

One of the most difficult elastomer families to plasticize with any confidence in the results is polychloroprene. The CR grades respond differently to plasticizers than most elastomers due to the ability of polar plasticizers and aromatic process oils to attack the molecular backbone of the CR. In addition, plain unsaturated vegetable oils (e.g., rapeseed, corn, and castor), which are relatively incompatible in most specialty elastomers, are seemingly quite compatible in CR up to about 30 phr. 

A specialty class of carboxyl containing elastomers are the telechelic ionomers. In these systems the carboxyl functionality terminates both ends of the polymer chain. Such polymers range in molecular weight from 1500 to about 6000. These materials can be prepared via several synthetic routes involving anionic or free radical initiated polymeri-zation(32-34). Recently, telechelic sulfonate ionomers of poly-isobutylene have been synthesized(35). These systems offer an unusual opportunity to assess the influence of chain length, chain architecture, cation type, and the Influence of polar additives on ionomer properties. 

The nitrile elastomers group is the largest consumer of the specialty synthetic plasticizers. It is primarily the acrylonitrile content of the NBR that needs to be managed from the processing, extraction/swell, and low-temperature flex points of view. Because NBR can be highly polar, care must be taken to compound with highly polar glycol ether esters for the maximum efficiency, compatibility, extraction requirements, and low-temperature properties. 

Silicone rubber and, in general polar polymers, are by nature materials of choice for preparing silica filled systems however limited to niche applications, with respect to the range of properties that such specialty polymers may offer. In order to develop optimum reinforcing performance with more common diene elastomers, silica must be chemically treated as we will see below, because contrary to carbon blacks, silica particles do not develop spontaneous strong interactions with nonpolar polymers. It is nevertheless interesting to see that, even with comparable size and structure, pure silica does not affect the mechanical properties of vulcanized rubber compounds in the same manner as carbon black. 

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