Elastomer blends formation

Another area of recent interest is covulcanization in block copolymers, thermoplastic rubbers, and elasto-plastic blends by developing an interpenetrating network (IPN). A classical example for IPN formation is in polyurethane elastomer blended acrylic copolymers [7]. 

Compatibilization along with dynamic vulcanization techniques have been used in thermoplastic elastomer blends of poly(butylene terephthalate) and ethylene propylene diene rubber by Moffett and Dekkers [28]. In situ formation of graft copolymer can be obtained by the use of suitably functionalized rubbers. By the usage of conventional vulcanizing agents for EPDM, the dynamic vulcanization of the blend can be achieved. The optimum effect of compatibilization along with dynamic vulcanization can be obtained only when the compatibilization is done before the rubber phase is dispersed. 

N. Tokita, Analysis of Morphology Formation in Elastomer Blends, Rubber Chem. Technol., 50, 292-300 (1977). 

These concepts for formation of miscible blend of elastomers with similar or near equivalence of solubility parameters require the components to be similar in properties. Thus a wide variation in the properties of the elastomer blends by changing the relative amounts of the two elastomers is not typical since it is unlikely that, for example, a nonpolar polyolefin elastomer and a polar elastomer like acrylate would be similar in solubility parameters. This relative invariance in the properties of the blend compared to the components is an inherent limitation on the basic, economic, and technological need for elastomer blends, which is to generate new properties by blends of existing materials. Similar or near equivalence of solubility parameters can be difficult to predict from chemical structure. For example, chemically distinct 1,4-polyisoprene and 1,2-polybutadiene are miscible, but isomeric 1,2-polybutadiene and 1,4-polybutadiene are immiscible. It is illustrative of this concept that an apolar hydrocarbon elastomer and a highly polar elastomer such as an acrylate cannot have, under any practical structural manifestation for either, a similar solubility parameter and thus be miscible. 

The formation of miscible rubber blends slows the rate of crystallization (Runt and Martynowicz, 1985 Keith and Padden, 1964) when one of the components is crystallizable. This phenomenon accounts for data that show lower heats of fusion that correlate to the extent of phase homogeneity (Ghijsels, 1977) in elastomer blends. Additionally, the melting behavior of a polymer can be changed in a miscible blend. The stability of the liquid state by formation of a miscible blend reduces the relative thermal stability of the crystalline state and lowers the equilibrium melting point (Nishi and Wang, 1975 Rim and Runt, 1520). This depression in melting point is small for a miscible blend with only dispersive interactions between the components. 

Intensive properties of the blend components that dominate vulcanizate properties of the elastomer are improved if the compatibilizer is the predominant fraction of the elastomer blend. Davison et al. (1982) describe the formation of... 

The detailed morphology of elastomer blends depends on (1) the mixing procedure, (2) the rheology of the blend components, and (3) the interfacial energy. As with other polymer blends, the elastomer of lower viscosity tends to be the continuous phase [26a-b]. Cocontinuous blend morphology is observed only for elastomers with similar viscosities. The viscoelastic forces developed during the formation of the compounded blend from two rheologi-... 

The EP thermoplastic elastomers are distinguished from the crossUnked analogues, which are not thermoplastics since reforming is impossible. A very important thermoplastic elastomer is comprised of a blend of an EP copolymer with an ethylene-propylene-diene (EPDM) terpolymer. This latter material is, of course, a crosslinkable thermoset however, these materials can be processed as thermoplastics if the crosslinkable component is present at low enough concentration to be present as an isolated phase. Melt-processing causes the formation of chemical bonds within the isolated rubber phase, a process called dynamic vulcanization. A commercial example of this type of material is Santoprene [4] manufactured by Advanced Elastomer Systems. Other blends of noncrosslinkable TPEs with crosslinkable materials are used commercially. These materials are classified as elastomer blends and are the subject of Chapter 12. 

Tokita, N. (1977) Analysis of morphology formation in elastomer blends. Rubber C3tem. Tech.. 50 (2), 292-300. 

Another procedure for synthesis of polsrmer blends is by formation of interpenetrating polymer networks. A network of one polymer is swollen with the other monomer or prepol5mier after that, the monomer or prepolymer is crosslinked (63). In contrast to the preceding methods used for thermoplastics and uncrosslinked elastomers, blends of reactoplastics are prepared by this method. Phase Structure Development in Molten State. 

Similar results were obtained by shear blending of two synthetic elastomers (45). The formation of a block or graft copolymer during the process of mixing butadiene rubber (SKB) and styrene-containing rubber (SKS-30A) was postulated by Slonimskii and Reztsova (49). They claimed that the anomalies observed in the dependences on composition of the mechanical properties of a mixture of two mutually insoluble rubbers after vulcanization may be reduced by increasing the part played by the mechanical mixing (inert atmosphere, reduction of radical acceptors, intensity of mechanical action). 

The competition between the phase separation and crosslinking processes opens up the possibility of controlling pattern formation and the structure of the resulting material by varying the reaction temperature and the blend composition. There is considerable effort to modify the useful properties of elastomers by introducing a coagent that is copolymerized with the elastomer and contributes positively... 

The ceramizing capacity of polyboroxosiloxane elastomer provides a new P-free intumescent mechanism [57], The combustion of PP-polyboroxosiloxane 3 1 blend filled with 1.25% organo-modified SEP (OSEP, Pangel B40 product of Tolsa Ltd.) and 1.25% melamine borate lead to the formation of white foamed ceramic structure shown in Figure 13.15. 

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