Elastomers high consistency

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

High-consistency thermosetting medical grade silicone elastomer compounds are prepared from high molecular weight polydior-ganosiloxanes compounded with high-surface fumed silica (approximately 400 mVg). Silica is the only material known that adequately reinforces silicone elastomer. 

Polymers II a-f were found by X-ray diffraction to be noncrystalline amorphous materials. Similar structured polymers were prepared for free radical vulcanization by the introduction of vinyl crosslinking sites. The polymers were formulated Into high consistency elastomers reinforced with silica and were free radical vulcanized. The properties for only lib and Ild are shown In Table I with a commercial elastomer prepared from polymethyl (3,3,3-tr1flu-oropropyl)s11oxane (LS) shown for comparison. Also included Is an elastomer prepared from the following copolymer (III),... 

For example, in heat-cured mbber systems a common loading with synthetic silica is around 30%. For economic reasons, precipitated silicas are used in increasing volumes in silicone elastomers such as high-consistency mbber (HCR), liquid silicone mbber (LSR), and two-component room temperature vulcanized elastomers (RTV2). The reinforcement provided by precipitated silicas is only slightly lower than that provided by fumed silicas. However, fumed silicas are necessary in silicone sealants and other high-end applications because of their desirable properties like low moisture content. 

The silicone elastomers most commonly used for medical applications are the high consistency (HC) and liquid injection molding (LIM) types. The former is most often peroxide cured and the latter platinum cured although there are variations. Both materials are similar in properties. LIM offers greater advantages to the medical device molder and is gaining in popularity. This form of silicone may become the molder s material of choice within the next few years. 

High consistency silicone elastomer consists of methyl and vinyl substituted silicones with aromatic and fluorinated functional groups in some formulations. For the most part, they are peroxide crosslinked. Items are usually compression or transfer molded (Tables 4.8). 

Table 4.8 Typical Properties of High Consistency (HC) Silicone Elastomers...

The mechanical properties of highly filled elastomers have led to their use as solid propellants in rocketry. Composite propellants consist of elastomers highly filled with inorganic oxidiser. Mixing is effected in an uncrosslinked state in which the polymer still has a low MW and the consistency of a viscous fluid. The compounded mixture is cast or extruded into the desired shape and hardened by polymerisation... 

High molecular weight polydlmethylsiloxane with 3,000 to 10,000 repeat units containing less than 1% of vinyl functionality are the basis of most heat cured high consistency elastomers. Phenyl groups are added to inhibit crystallization at low temperature and Improve radiation resistances. 

Andrejewski, D., Polyflow a treatise on inverse die/mandrel design for high consistency silicone elastomer, Proc. of the ANTEC97 conference, ed. Society of Plastics Engineers, Brookfield CT, (1997), vol. 1, 308-314... 

Nitrile mbber finds broad application in industry because of its excellent resistance to oil and chemicals, its good flexibility at low temperatures, high abrasion and heat resistance (up to 120°C), and good mechanical properties. Nitrile mbber consists of butadiene—acrylonitrile copolymers with an acrylonitrile content ranging from 15 to 45% (see Elastomers, SYNTHETIC, NITRILE RUBBER). In addition to the traditional applications of nitrile mbber for hoses, gaskets, seals, and oil well equipment, new applications have emerged with the development of nitrile mbber blends with poly(vinyl chloride) (PVC). These blends combine the chemical resistance and low temperature flexibility characteristics of nitrile mbber with the stability and ozone resistance of PVC. This has greatly expanded the use of nitrile mbber in outdoor applications for hoses, belts, and cable jackets, where ozone resistance is necessary. 

The elastomers consist of very high moleculcU weight (-0.5 X 10 ) linecu gums cross-linked after fabrication. In order to achieve such polymers it is necessary that very pure difunctional monomers be employed since the presence of monofunctional material will limit the molecular weight while trifunctional material will lead to cross-linking. Where dimethylsilicone rubbers are being prepared, the cyclic tetramer, octamethylcyclotetrasiloxane, which may be obtained free from mono- and trifunctional impurities, is often used. This tetramer occurs to the extent of about 25% during the hydrolysis of dichlorosilanes into polymers. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

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