Rubbery materials chemically modified

Another way of modifying unsaturated PHAs in the bulk is by crosslinking of the material. This has been accomplished by either chemical reaction with sulfur or peroxides [109, 110], or by radiation curing [91, 111]. In all cases, crosslinking altered the ultimate material properties drastically, yielding a true rubbery material. The advantages of applying rubbers from crosslinked PHAs over the use of current rubbers will be elaborated in Sect. 4.5. 

Characterise Chemically Modified Rubbers and Rubbery Materials... 

One approach to addition of elastomeric tougheners is to select a modifier which is initially soluble in the resin (22) but precipitates as small rubbery spheres as curing progresses. However, this procedure has always resulted in depression of thermal mechanical properties. Alternatively, an immiscible material can be dispersed in the resin by use of physical and chemical interactions of the solvents and catalysts so that a fine dispersion of rubber particles is produced in the epoxy resin prior to cure. It is also possible to produce a stable dispersion by a reactive blending process without the use of a catalyst or solvents. 

Dufresne et al. studied stress vs strain curves (nominal data) for the chitin whiskers/unvulcanized NR evaporated composites, shown in Figure 14.12."" The polymeric matrix is in the rubbery state and its elasticity from entropic origin is ascribed to the presence of numerous entanglements due to high molecular weight chains. They further observed that the incorporation of anhydride and isocyanate modified chitin whiskers into NR lead to composite materials with improved mechanical properties. The study of the morphology of these nanocomposites leads to the conclusion that the various chemical treatments improve the adhesion between the filler and the matrix (Figure 14.13). However in some cases there is loss of performance, which could be due to the partial or total destruction of the three-dimensional network of chitin whiskers assumed to be present in the unmodified composites. 

Chlorinated polyethylene (CPE) n. Any polyethylene modified by simple chemical substitution of chlorine on the hnear backbone chain, CPEs range from rubbery amorphous elastomers at 35-40% Cl to hard, semicrystalline materials at 68-75% Cl. They are sometimes included with chlorinated natural and butyl rubbers under the term chlorinated rubbers. Certain CPEs are used as modifiers in PVC compounds to obtain better flexibihty and toughness, particularly low-temperature toughness, greater latitude in compounding, and ease of processing. 

These materials are used to improve thermal and mechanical shock, increase elongation, and obtain higher impact strength and flexibihty. Usually, there is some sacrifice of physical strength, electrical properties and chemical or solvent resistance, and elevated temperature performance. Flexible epoxy resins or monofunctional epoxide compoimds are examples of reactive epoxide-type modifiers. They can be used at ratios up to 1 1 to obtain a flexible and rubbery cured epoxy compound. They are shelf-stable when blended with the resin. 

Synthetic rubbers, discussed in Section 15.16, are often copolymers chemical repeat units that are employed in some of these rubbers are shown in Table 14.5. Styrene-butadiene rubber (SBR) is a common random copolymer from which automobile tires are made. Nitrile rubber (NBR) is another random copolymer composed of acrylonitrile and butadiene. It is also highly elastic and, in addition, resistant to swelling in organic solvents gasohne hoses are made of NBR. Impact-modified polystyrene is a block copolymer that consists of alternating blocks of styrene and butadiene. The rubbery isoprene blocks act to slow cracks propagating through the material. 

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