Unsaturated elastomer blends

Basically all conventional sulfur (or peroxide) crosslinked unsaturated elastomer blends could be classified as co-crosslinked compatibilized blend examples. Various ercamples of these combinations are discussed in Section 4.2. Other crosslinking chemistry has been noted in several references with examples discussed in the following. 

Chlorobutyl rubber is prepared by chlorination of butyl rubber (chlorine content is about 1 wt%). This is a substitution reaction produced at the allylic position, so little carbon-carbon double unsaturation is lost. Therefore, chlorobutyl rubber has enhanced reactivity of the carbon-carbon double bonds and supplies additional reactive sites for cross-linking. Furthermore, enhanced adhesion is obtained to polar substrates and it can be blended with other, more unsaturated elastomers. 

The blends are frozen in liquid nitrogen and then microtomed and stained with osmium tetraoxide, which stains only unsaturated elastomers. 

The most common blends of unsaturated elastomers are those used in various sections of automotive tires. Table 12.9 lists the important component of tires and the typical blends used for them. Much of the literature of elastomer blends reflects this important application. It is outside the scope of this chapter to discuss each of the applications. We have outlined most of the important principles used in the generation of the blends. 

The use of blends of polyolefin elastomers, such as HR and EPDM, as substantial components in blends of unsaturated elastomers is a rapidly developing area. Mouri et al. [51] have compared the properties of EPDM-NR and... 

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. 

Blends of two or more unsaturated elastomers (rubber-mbber blends) have traditionally found application in various components of the commercial and automotive tire. Specific applications are (Hess et al. 1993) ... 

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... 

Hydrogenated acrylonitrile-butadiene robber (HNBR)-Therban 3407, containing 34 % of acrylonitrile units and residual double bonds max 0.9 %, Mooney viscosity 70 was purchased from Lanxess. Dicumyl peroxide obtained from Aldrich was used as cross-linker. Co-agents consisted of nanostructured metal oxides magnesium, zinc or calcium oxide provided by Aldrich and unsaturated acids acrylic acid (Aldrich) or monoallyl maleate (synthesized by Department of Organic Chemistry- Technical University of Lodz). The composition of a typical elastomer blend was HNBR-100 phr, DCP- 3 phr, metal oxide- 3 phr, unsaturated acid-1 phr. 

Besides the mixing sequence, the distributions of carbon black in elastomer blends are affected by viscosity, degree of unsaturation, and polarity of mbbers. For... 

Materials. Ethylene-octene rubber (POE)- Engage 8150, copolymer of ethylene and 39 wt.% 1-octene, with a melt flow index (MFI) of 0.5 g/10 min (190 C, 2.16 kg) and Mooney viscosity 35, was purchased from Dow Chemical. As a cross-linker dicumyl peroxide (98%) obtained from Aldrich was used. Nanostructured metal oxides magnesium (50 nm particle size (BET)), zinc (<100 nm particle size (BET)), calcium oxide (160 nm particle size (BET)) and unsaturated acids itaconic (99%), crotonic (98%), sorbic (99%) acid were provided by Aldrich. Composition of typical elastomer blend POE-100 phr, DCP-1.5 phr, metal oxide-7 phr, unsaturated acid-2 phr. 

The majority of elastomer blends are phase separated, but of interest, as crosslinking can achieve mechanical compatibilization due to crosslinking between the phases as noted in Chapter 3. This compatibilization method can lead to unique and useful blends with a compromise in properties, offering useful commercial products as well illustrated by the applications in tire construction. The unsaturated hydrocarbon elastomers without polar functional groups are rarely miscible with each other, because no specific interactions are present to achieve the necessary thermodynamic driving force for miscibility. The few miscible examples noted generally exhibit matched solubility parameters. 

Chlorobutyl can be readily blended and cured with other, more highly unsaturated elastomers. In the adhesives and sealants area, it has been blended with both regular butyl and natural rubber and then preferentially cured through the chlorine to improve strength. The reactive chlorine will also tend to increase adhesion to many polar substrates. 

The inherent properties of polymers of the poly isobutylene family, particularly the chemical inertness, age and heat resistance, long-lasting tack, flexibility at low temperatures, and the favorable FDA position on selected grades, make these products commercially attractive in a variety of pressure-sensitive and other adhesives, in automotive and architectural sealants, and in coatings. An added dimension is achieved in the blendability of the polyisobutylene polymers with each other and with other adhesive polymers such as natural rubber, styrene-butadiene rubber, EVA, low molecular weight polyethylene, and amorphous polypropylene to achieve specific properties. They can, for example, be blended with the highly unsaturated elastomers to enhance age and chemical resistance. A description of poly isobutylene polymer family use in adhesive and sealant applications follows. 

Polyester-silicone Polyesters, thermoplastic Polyesters, unsaturated Polyester urethanes Polyester-wool blends Polyether antibiotics Polyether carboi lates Polyether elastomers... 

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