Superior Processing natural rubber

Table 4. Grades of Superior Processing Natural Rubber...

The third category of natural rubbers are the specialty materials, which include liquid low molecular weight rubber, methyl methacrylate grafted polymers, oil-extended natural rubber, deproteinized natural rubber, epoxidized natural rubber, and superior-processing natural rubber. 

Superior Processing natural rubber is a polymer in which a proportion of the latex used in its preparation has been vulcanized in the latex phase prior to coagulation of the rubber to form the sheet or bale. 

Butyl rubber is one product formed when isobutylene is copolymerized with a few percents of isoprene. In the Exxon process an isobutylene-methyl chloride mixture containing a small amount of isoprene is mixed at — 100°C with a solution of AICI3 in methyl chloride. An almost instantaneous reaction yields the product, which is insoluble in methyl chloride and forms a fine slurry. Molecular weight can be controlled by adding diisobutylene as a chain-transfer agent. Increased catalyst concentration and temperature also result in lowering molecular weight. The product can be vulcanized and is superior to natural rubber. A solution process carried out in C5-C7 hydrocarbons was developed in the former Soviet Union.471,472... 

Chemical pretreatments with amines, silanes, or addition of dispersants improve physical disaggregation of CNTs and help in better dispersion of the same in rubber matrices. Natural rubber (NR), ethylene-propylene-diene-methylene rubber, butyl rubber, EVA, etc. have been used as the rubber matrices so far. The resultant nanocomposites exhibit superiority in mechanical, thermal, flame retardancy, and processibility. George et al. [26] studied the effect of functionalized and unfunctionalized MWNT on various properties of high vinyl acetate (50 wt%) containing EVA-MWNT composites. Figure 4.5 displays the TEM image of functionalized nanombe-reinforced EVA nanocomposite. 

Processing aid-80, a masterbatch in the form of pressed crumb consisting of an 80 20 blend of crosslinked to ordinary natural rubber. The correct proportions of vulcanised latex and field latex are blended, coagulated and the resulting crumb pressed into 100 lb bales. The use of PA 80 confers Superior Processing properties on any natural or styrene-butadiene rubber with which it may be mixed. See Superior Processing Rubber. 

Many workers have studied and published correlations between various types of plastimeter, often to show that they do not agree and to illustrate the superiority of the instrument which supposedly agrees best with processing behaviour. Several comparisons are included in the literature already noted and other examples are shown in Figure 6.6. Figure 6.6a shows the relatively close correlation obtained between two compression instruments, the Wallace rapid and Williams plastimeters, for materials of similar flow characteristics (plasticised natural rubber). When such rubbers are compared on two basically different instruments (compression and extrusion) the... 

Before World War II (1939-1945) natural rubber was used for practically all rubber applications. Natural rubber is preferred in many products because of its superior building tack, green stock strength, better processability, high strength in non-black formulations, hot tear resistance, retention of strength at elevated temperatures, better resilience, low heat build-up, fatigue resistance, and better dynamic properties. Rubber products are broadly classified as dry rubber products and latex based products [60]. 

As a result of the concurrent progress on the polymerization side, Ludwigshafen and Leverkusen agreed in July 1929 to build a semi-technical works plant for Buna at Knapsack, alongside the carbide works. This plan was blocked by Carl Krauch of Oppau, largely because he wanted to wait until Oppau s methane-to-acetylene electric arc process was ready. A few months later, the Buna program was effectively halted by the onset of the Depression, which soon reduced natural rubber prices to minimal levels. When the production of synthetic rubber was revived in Hitler s Third Reich, the weak Buna, which was a sodium-polymerized polybutadiene, had been displaced by the superior copolymers of butadiene with styrene (Buna S) and acrylonitrile (Buna N or Perbunan). 

The discovery of this outstanding synthetic rubber dates back to 193<4, which corresponds both with the date of the basic U.S. patent of Konrad and Tsohunkur and with its first commercial production in Germany, as "Perbunan". This was part of the intensive effort going on in the laboratories of the I. G. Farbenindustrie Company in Germany at that time to develop synthetic rubbers which were superior, both in properties and process, to the sodium-polymerized polybutadienes (Buna rubber) which was then in production. Out of this effort, of course, came the butadiene-styrene copolymers (Buna S), which were the precursors of SBR, still the dominant synthetic tire rubber today. It was most fortunate that, because of the close contacts between IGF and the Standard Oil Co. prior to World War II, all the necessary information on the emulsion copolymerization of butadiene with styrene, and other monomers, was available when this country was suddenly out off from its main supply of natural rubber on Itecerober 7, 19A1. 

Polybutadiene vulcanizates (see Table 18.1 for typical properties) are superior to those of natural rubber with respect to resilience, heat build-up and abrasion resistance. These properties are particularly significant in tyres. On the other hand, polybutadiene vulcanizates have lower tensile strength and tear resistance and polybutadiene tyres have relatively poor road-adhesion in wet conditions. For these reasons and to aid processing, butadiene rubbers are generally used in blends with natural or styrene-butadiene rubbers such blends usually contain less than 50% polybutadiene. Because of their use in tyre production, butadiene rubbers have become significant tonnage rubbers (Table 18.2). 

Because of its superior resistance to oUs and hydrocarbons. Neoprene is preferred to natural rubber for pumping oil (tar) sand slurries, oil shales, pulp and paper, and phosphate and in moly circuits when oils are used as flotation reagents. A special curing process is applied when molding equipment parts out of Neoprene to give it excellent resistance to water absorption and swelling, and suitable thermal resistance for use up to a temperature of 120°C (248°F) with a nominal hardness of 60 Shore A. 

Still further modifications of natural rubber are the superior processing or SP rubbers. Here a portion of the latex is vulcanized before coagulation. In a sense, the vulcanized globules serve as small particle reinforcement of the rubber. Such rubbers help keep close dimensional tolerances or smooth finish requirements. Extrusion and/or calender output is improved, and die swell is lessened in lightly loaded compounds. In the interest of economical overseas shipments from the Far East, masterbatches of these rubbers, called Processing Aids (PA), are used. For example, PASO is a rubber that has 80 parts of vulcanized rubber and 20 parts of raw rubber. A softer material called PA57 has 57 parts of vulcanized rubber, 14 parts of raw rubber, and 29 parts of oil. 

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