Elastomeric blends

Blends of butadiene-acrylonitrile copolymer rubber (nitrile rubber or NBR) and PVC are among the oldest known examples of commercial elastomer/ thermoplastic blends. The shortage of natural rubber during World War II stimulated research in the USA on the compounding and modification of synthetic polymers to produce rubber-like materials. An outcome of this research was the commercial introduction of NBR/PVC blends by B.F. Goodrich in 1947 under the trade name of Geon Polyblends [Pittenger and Cohan, 1947]. The blend showed improved ozone resistance and melt processability compared to the nitrile rubber (Table 15.12). 

Ingredients and Machinery for Rubber, published annually by Bill Communications, Inc. Currently, the PVC/nitrile rubber blends worldwide consumption is estimated to be 30 kton/y. 

Nitrile rubbers are known for their oil and chemical resistance and addition of PVC improves the ozone resistance. Use of carboxylated NBR is believed to obviate the necessity of vulcanization. 

Nitrile rubber/PVC blends have reached a mature stage in their commercial usage. They face increasing competition from other thermoplastic elastomers such as the dynamically vulcanized blends of PP/EPDM and PP/NBR (Santoprene and Geolast , Monsanto-Advanced Elastomer Systems). 

Commercially important elastomeric thermoplastic alloys are dynamically vulcanized blends of polypropylene with high volume fractions of EPDM, polybutadiene rubber, nitrile rubber, and butyl rubber (Santoprene , Vyram , Geolast and Trefsin ) all currently sold by Advanced Elastomer Systems, a joint venture of Monsanto and Exxon. Another recent member of the commercial dynamically cured elastomeric thermoplastic alloys is the blend of PVC and a crosslinked ethylene copolymer (Alcryn , DuPont). The current consumption of all the elastomeric thermoplastic alloys in the USA is over 23 kton/y, with the EPDM/PP blend (Santoprene ) assuming about 90% of the market share. 

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The processing technologies for elastomeric blends, thermoplastic elastomer-based on mechanical mixing, and elastomer-plastic vulcanizates are distinctly different. Depending on the type and nature of blend, size, and their final application, a wide range of processing equipment is now in use both industrially as well as in laboratory scale preparation. 

The oldest technology involved in the elastomer blending and vulcanization process is essentially a temperature controlled two roll mill as well as internal mixers followed by an optimum degree of crosslinking in autoclave molds (compression, injection, etc.) in a batch process or in a continuous process such as continuously heated tube or radiated tubes. A few examples of laboratory scale preparation of special purpose elastomeric blends is cited here. 

A. Properties of Elastomeric Blends with Dissimilar Cure Rates... 

Jha and Bhowmick [51] have reported the development and properties of thermoplastic elastomeric blends from poly(ethylene terephthalate) and ACM by solution-blending technique. For the preparation of the blend the two components, i.e., poly(ethylene terephthalate) and ACM, were dried first in vacuum oven. The ACM was dissolved in nitrobenzene solvent at room temperature with occasional stirring for about three days to obtain homogeneous solution. PET was dissolved in nitrobenzene at 160°C for 30 min and the rubber solution was then added to it with constant stirring. The mixture was stirred continuously at 160°C for about 30 min. The blend was then drip precipitated from cold petroleum ether with stirring. The ratio of the petroleum ether/nitrobenzene was kept at 7 1. The precipitated polymer was then filtered, washed with petroleum ether to remove nitrobenzene, and then dried at 100°C in vacuum. 

Jha A. and Bhowmick A.K., Thermoplastic elastomeric blends of nylon 6/acrylate rubber Influence of interaction of mechanical and dynamic mechanical thermal properties. Rubber Chem. TechnoL, 70, 798, 1997. 

Roy Choudhury N. and Bhowmick A.K., Compatibilization of natural rubber-polyolefin thermoplastic elastomeric blends by phase modification, J. Appl. Polym. Sci., 30, 1091, 1989. 

Jha A., Bhowmick A.K., Eujitsuka R., and Inoue T. Interfacial interaction and peel adhesion between polyamide and acrylate rubber in thermoplastic elastomeric blends, J. Adhes. Sci. Technol., 13(6), 649, 1999. 

Since most polymers, including elastomers, are immiscible with each other, their blends undergo phase separation with poor adhesion between the matrix and dispersed phase. The properties of such blends are often poorer than the individual components. At the same time, it is often desired to combine the process and performance characteristics of two or more polymers, to develop industrially useful products. This is accomplished by compatibilizing the blend, either by adding a third component, called compatibilizer, or by chemically or mechanically enhancing the interaction of the two-component polymers. The ultimate objective is to develop a morphology that will allow smooth stress transfer from one phase to the other and allow the product to resist failure under multiple stresses. In case of elastomer blends, compatibilization is especially useful to aid uniform distribution of fillers, curatives, and plasticizers to obtain a morphologically and mechanically sound product. Compatibilization of elastomeric blends is accomplished in two ways, mechanically and chemically. 

Jha, A. and Bhowmick, A.K., Thermoplastic elastomeric blends of poly(ethylene terephthalate) and acrylate rubber I. Influence of interaction on thermal, dynamic mechanical and tensile properties. Polymer, 38, 4337, 1997. 

Anandhan, S., De, P.P., Bhowmick, A.K., Bandyopadhyay, S., and De, S.K., Thermoplastic elastomeric blend of nitrile rubber and poly(styrene-co-acrylonitrile). n. Replacement of nitrile rubber by its vulcanizate powder, J. Appl. Polym. Set, 90, 2348, 2003. 

Stadler R, Burgert J. Influence of hydrogen-bonding on the properties of elastomers and elastomeric blends. Makromol Chem Macromol Chem Phys 1986 187 1681-1690. 

Table 9.23 lists examples of the compatibili-zation studies conducted in laboratory TSE s, whereas Table 9.24 provides examples from the commercial patent literature. It is noteworthy that about 90% of patents on polymer blends published during the last few years, specify a TSE as the preferred compounder. Exceptions are blends formulated for oriented fibers and films e.g., with LCP) that require high die pressure and thus are usually prepared in a SSE. Similarly, elastomeric blends of either PO or PVC are preferably prepared using one of the older methods, viz. roll mill or Banbury mixer. 

In a typical formulation, an ethylene-n-butyl-acrylate-carbon monoxide (60/30/10) terpolymer (60 wt%) is melt compounded with plasticized PVC (40 wt%) in a twin-screw extruder and the ethylene terpolymer dispersion cured in situ during the mixing by catalytic amounts of a suitable peroxide (0.3%) and a bismaleimide crosslink promoter (0.2%). The extruded pellets of the elastomeric blend can be used in conventional melt fabrication processes such as profile extrusion, extrusion coating, milling and calendering of sheets, injection and/or compression molding. 

Commercial elastomeric blends of such ethylene terpolymers and PVC have been reported to have the following advantages (i) outstanding weather-ability and ozone resistance, (ii) excellent oil... 

Commercial elastomeric blends of ethylene terpolymer/PVC system (Alcryn ) are used in outdoor weather stripping, seals and gaskets, coated fabrics, pond linings and a variety of other extruded and molded goods for automotive and industrial applications. 

Table 15.14. Comparison of the typical properties of the dynamically vulcanized ethylene terpolymer/PVC blends vs. similar elastomeric blends based on polypropylene...

MAJOR APPLICATIONS Fiber, slit tape, cast and biaxially oriented film, containers and closures, automotive interior trim, appliance housings and components, component in elastomeric blends with polyethylene and olefinic rubbers. 

Grellmann, W., Heinrich, G., Kaliske, M., Kliippel, M., Schneider, K., VUgis, T. (Eds.) Fracture Mechanics and Statistical Mechanics of Reinforced Elastomeric Blends Springer-Verlag Berlin Heidelberg 2013 ISBN Hardcover 978-3-642-37909-3 and ISBN E-Book 978-3-642-37910-9 http // www.springer.com/materials/mechanics/book/978-3-642-37909-3... 

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