Saturated elastomer blends

BIMS-NR blends as sidewall components. In many of the applications, the saturated elastomer is considered a polymeric antioxidant for the diene rubber. It is believed that the higher molecular weight polyolefins are better in these applications due to limited interdiffusion and a more stable morphology. Some of the benefits in tensile properties and abrasion resistance of the blends may be due to the interdiffusion of high molecular chains of dissimilar elastomers across the phase interface. Significant advances have been made in modifying the structure of polyolefin elastomers to increase the compatibility to unsaturated elastomers. Tse et al. [50b] have shown that uncompatibilized blends of saturated elastomers and unsaturated elastomers are possible if the former contains substantial amounts (>12%) styrene residues. This is expected to be an important area of development in the future with the advent of new synthesis procedures for polyolefins. 

Before reviewing in detail the fundamental aspects of elastomer blends, it would be appropriate to first review the basic principles of polymer science. Polymers fall into three basic classes plastics, fibers, and elastomers. Elastomers are generally unsaturated (though can be saturated as in the case of ethylene-propylene copolymers or polyisobutylene) and operate above their glass transition temperature (Tg). The International Institute of Synthetic Rubber Producers has prepared a list of abbreviations for all elastomers [3], For example, BR denotes polybutadiene, IRis synthetic polyisoprene, and NBR is acrylonitrile-butadiene rubber (Table 4.1). There are also several definitions that merit discussion. The glass transition temperature (Tg) defines the temperature at which an elastomer undergoes a transition from a rubbery to a glassy state at the molecular level. This transition is due to a cessation of molecular motion as temperature drops. An increase in the Tg, also known as the second-order transition temperature, leads to an increase in compound hysteretic properties, and in tires to an improvement in tire traction... 

A section of a nylon blend with a saturated elastomer was prepared by room temperature microtomy with no staining, as shown in the STEM image (Fig. 4.17A). Unfortunately, the section... 

Most elastomers that are used for nylon modification contain a small amount of maleic anhydride (0.3 to 2%). In the melt blending process, these elastomers react with the primary amine end groups in nylon, giving rise to nylon grafted elastomers. These grafts reduce the interfacial tension between the phases and provide steric stabili2ation for the dispersed mbber phase. Typically, thermally stable, saturated mbbers such as EPR, EPDM, and styrene—ethylene/butylene—styrene (SEBS) are used. 

Blends of Saturated Hydrocarbon Elastomers (Uncompatibilized Blends)... 

Krishnamoorti, R. in Miscibility of Blends of Saturated Hydrocarbon Elastomers. Rubber Division, Proceedings of the American Chemical Society, Nashville, TN, Sept. 29-Oct. 2, 1998, Paper No. 33, 1-14. 

The most prevalent approach to achieve long-lasting and nonstaining ozone protection of rubber compounds is to use an inherently ozone-resistant, saturated backbone polymer in blends with a diene rubber. The ozone-resistant polymer must be used in sufficient concentration (minimum 25 phr) and must also be sufficiently dispersed to form domains that effectively block the continuous propagation of an ozone-initiated crack through the diene rubber phase within the compound. Elastomers such as ethylene-propylene-diene terpolymers, halogenated butyl mbbers, or brominated isobutylene-co-para-methylstyrene elastomers have been proposed in combination with NR and/or butadiene rubber. 

As a result of its saturated polymer backbone, EPDM is more resistant to oxygen, ozone, UV and heat than the low-cost commodity polydiene rubbers, such as natural rubber (NR), polybutadiene rubber (BR) and styrene-butadiene rubber (SBR). Therefore, the main use of EPD(M) is in outdoor applications, such as automotive sealing systems, window seals and roof sheeting, and in under-the-hood applications, such as coolant hoses. The main drawback of EPDM is its poor resistance to swelling in apolar fluids such as oil, making it inferior to high-performance elastomers, such as fluoro, acrylate and silicone elastomers in that respect. Over the last decade thermoplastic vulcanisates, produced via dynamic vulcanisation of blends of polypropylene (PP) and EPDM, have been commercialised, combining thermoplastic processability with rubber elasticity [8, 9]. 

In order to disperse low concentrations of CPs in thermoplastics and elastomers, it is necessary to prepare highly structured fine aggregates to permit percolation in the polymer blend. With better polymerization techniques and new blending methods, the percolation threshold decreases. Indeed, a critical concentration as low as 1.5vol% and a saturation... 

Acrylic-styrene-acrylonitrile terpolymers, ASA, are produced by simultaneous polymerization of styrene and acrylonitrile monomers in the presence of an acrylic rubber (polybutylacrylate). Thus, the material has a two-phase strucmre similar to ABS. However the elastomer phase in ASA is saturated and thus significantly more resistant to oxidative degradation. ASA is used in applications requiring good weatherability, mostly with PVC. The blends are primarily extruded for exterior trims and window profiles applications. 

Callan and coworkers studied the distribution of carbon black in 50/50 blends of different rubbers and found that the black affinity decreases in the order of BR, SBR, CR, NBR, NR, EPDM, and HR. Transfer of black takes place from saturated rubber master batches to those of unsaturated rubbers. These conclusions are in agreement with the findings of Sircar and Lamond (1973). Vonwinkel (1969) showed that carbon black accumulates in the BR from an analysis of 50/50 binary blends. The carbon black retention in each phase was estimated from TEM micrographs of the blend. The ranking of carbon black affinity for different elastomers was in the order SBR > BR, CR, NBR > NR > EPDM > CIIR, HR. Cotton and Murphy, 1988, in a complementary experiment, have determined the carbon black distribution for seven different carbon blacks in preblended SBR-BR and SBR-NR blends. The data is shown in Table 12.8. The carbon blacks differ in the surface structure and size they ranged in surface area (CTAB) from 43 to 136 m /g. For all cases of SBR-NR blends, the carbon black was preferentially located in the SBR phase. 

Other compounds commonly used in vulcanization, in addition to sulfur and accelerators, are zinc oxide and saturated fatty acids such as stearic or lauric acid. These materials are termed activators (as opposed to accelerators). Zinc oxide serves as an activator, and fatty acids are used to solubilize the zinc into the system. Rubber formulations can also include fillers such as fumed silica and carbon black, and compounds such as stabilizers and antioxidants. Further complicating the situation is the engineering practice of blending various elastomers to obtain the desired properties. 

Rovel Styrene-acrylonitrile copolymer, SAN, blended with saturated olefinic elastomer, EPR Dow Chem. Co. 

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