Materials rubbery

Rubbery materials are usually lightly cross-linked. Their properties depend on the mean distance between cross links and chain rigidity. Cross linking can be quantified by the use of functions derived from graph theory, such as the Rao or molar Hartmann functions. These can be incorporated into both group additivity and QSPR equations. 

As the author pointed out in the first edition of this book, the likelihood of discovering new important general purpose materials was remote but special purpose materials could be expected to continue to be introduced. To date this prediction has proved correct and the 1960s saw the introduction of the polysulphones, the PPO-type materials, aromatic polyesters and polyamides, the ionomers and so on. In the 1970s the new plastics were even more specialised in their uses. On the other hand in the related fields of rubbers and fibres important new materials appeared, such as the aramid fibres and the various thermoplastic rubbers. Indeed the division between rubbers and plastics became more difficult to draw, with rubbery materials being handled on standard thermoplastics-processing equipment. 

Rubbery materials are often incorporated into rigid amorphous thermoplastics to improve their toughness but it is a moot point whether or not they should be... 

Low-density polyethylene has a gas permeability in the range normally expected with rubbery materials Table 5.11). This is because in the amorphous zones the free volume and segmental movements facilitate the passage of small molecules. Polymers of the Phillips type (density 0.96 g/cm ) have a permeability of about one-fifth that of the low-density materials. 

Because of the in-chain ring the Tg is as high as -i-35°C and the polymer is therefore not rubbery at usual ambient temperatures. If, however, the polymer is blended with an aromatic oil or certain ester plasticisers a rubbery material is obtained. Because of the ability of the polymer to take up large quantities of oil the Tg of a polymer-oil blend can be as low as -60°C. Such polymer-oil blends can also incorporate very large amounts of filler. 

BRYDsoN, J. A., Rubbery Materials and their Compounds, Applied Science, London (1988)... 

Plasticised PVC, referred to below as PPVC, is used in a wide variety of applications. Originally a substitute for natural rubber when the latter material became difficult to obtain during World War II, it is frequently the first material to consider where a flexible, even moderately rubbery, material is desired. This arises from the low cost of the compounds, their extreme processing versatility, their toughness and their durability. 

In order to produce a rubbery material the polymer must have a flexible baekbone, be suffieiently irregular in structure to be non-crystalline and also contain a site for cross-linking. These are of course requirements applicable equally to any potential elastomer whether or not it contains fluorine. 

Over the past 40 years there have been a number of developments that have resulted in the availability of rubbery materials that are thermoplastic in nature and which do not need chemical cross-linking (vulcanisation or setting) to generate elastomeric properties (see also Section 11.8 and 31.2). This approach has been extended to the fluoroelastomers. 

Following the success in blending rubbery materials into polystyrene, styrene-acrylonitrile and PVC materials to produce tough thermoplastics the concept has been used to produce high-impact PMMA-type moulding compounds. These are two-phase materials in which the glassy phase consists of poly(methyl methacrylate) and the rubbery phase an acrylate polymer, usually poly(butyl acrylate Commercial materials of the type include Diakon MX (ICI), Oroglas... 

To overcome brittleness these materials are sometimes blended with rubbery materials and with polyurethanes. These polymers may contain unsaturated groups, particularly at the chain ends, so that graft structures may be produced rather than simple mixtures. 

In the 1940s ICI introduced a material marketed as Vulcaprene made by condensing ethylene glycol, adipic acid and ethanolamine to a molecular weight of about 5000 and then chain extending this with a diisocyanate. This rubbery material found some use as a leathercloth and is dealt with further in Chapter 25. 

In 1975 Wacker-Chemie introduced silicones under the name of m-polymers. These are also room temperature curing liquid polymers which give rubbery materials on cross-linking and are available both as one- and two-component systems. Their particular feature is that they contain dispersions of copolymers such as those of styrene and n-butyl acrylate in the shape of rods or rice grains in the fluid silicone polymer. A small amount of the organic copolymer is also grafted onto the silicone backbone. 

All three types of material have now been available for some years and it is probably also true that none have yet realised their early promise. In the case of the thermoplastic elastomers most of the commercial materials have received brief mention in earlier chapters, and when preparing earlier editions of this book the author was of the opinion that such materials were more correctly the subject of a book on rubbery materials. However, not only are these materials processed on more or less standard thermoplastics processing equipment, but they have also become established in applications more in competition with conventional thermoplastics rather than with rubbers. 

For this reason, many attempts have been made over the years to produce a rubbery material which has a network structure over a useful temperature range but which, if heated further, loses this structure. In many cases this involves a form of cross-linking that is said to be heat fugitive. In Section 3.4 four types of heat-fugitive cross-link were identified, namely ... 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

In Table 1, drawn up by the author, of abbreviations in common use those in bold type are in the main schedule of BS 3502. In this list the names given for the materials aie the commonly used scientific names. This situation is further complicated by the adoption of a nomenclature by the International Union of Pure and Applied Chemistry for systematic names and a yet further nomenclature by the Association for Science Education which is widely used in British schools but not in industry. Some examples of these are given in Table 2. Because many rubbery materials have been referred to in this book. Tables 3 and 4 list abbreviations for these materials. 

Table 3 Standard abbreviations for rubbery materials (based on ISO Recommendation and ASTM D 1418)...

Table 4 Miscellaneous abbreviations used for rubbery materials...

Rubbery materials are defined by ASTM (D1566) as those which will have less than 50% permanent set after one minute when recovering from a strain of 100% applied for one minute. Of the many rubbery materials available, block copolymers are by far the most common used in hot melts. 

Two-pack polyurethanes Also called urethanes, these materials are similar to two-pack epoxies in that they can be formulated to provide different properties. They can be made into foams or soft, rubbery materials, as well as very hard, tough, abrasion-resistant coatings. 

Along with such materials as plastics, adhesives, fibers, and coatings. rubber is polymeric in nature. Such materials consist of long chains, with molecular masses generally of the order of 50,000 to 500,000 g/mol. Common rubbery materials—often called elastomers— include automotive tires and rubber bands. 

It must be above its glass transition temperature, which means that the polymer chains have sufficient thermal energy to move freely. Many rubbery materials have glass transition temperatures around 200 K, below which they are glassy, like plastics. 

Common rubbery materials consist of butadiene and styrene statistical copolymers, written poly(butadiene-sfaf-styrene). The butadi-... 

The major applications of rubbery materials today include automotive tires, rubber bands, tubing of various kinds, electric wire insulation, elastomeric urethane fibers for undergarments, and silicone rubber. Such types of polymers are important materials in our 21st-century world. 

The solidity of gel electrolytes results from chain entanglements. At high temperatures they flow like liquids, but on cooling they show a small increase in the shear modulus at temperatures well above T. This is the liquid-to-rubber transition. The values of shear modulus and viscosity for rubbery solids are considerably lower than those for glass forming liquids at an equivalent structural relaxation time. The local or microscopic viscosity relaxation time of the rubbery material, which is reflected in the 7], obeys a VTF equation with a pre-exponential factor equivalent to that for small-molecule liquids. Above the liquid-to-rubber transition, the VTF equation is also obeyed but the pre-exponential term for viscosity is much larger than is typical for small-molecule liquids and is dependent on the polymer molecular weight. 

In reality the ideal elastic rubber does not exist. Real rubbery materials do have a small element of viscosity about their mechanical behaviour, even though their behaviour is dominated by the elastic element. Even so, real rubbers only demonstrate essentially elastic behaviour, i.e. instantaneous strain proportional to the applied stress, at small strains. 

Several aspects of the deformation and fracture of rubbery materials are reviewed in this chapter. They reveal some important gaps in our present understanding of the behavior of simple mbbery solids. 

Brydson J.A., Thermoplastic mbbers. Rubbery Materials and Their Compounds, Elsevier, London, 1988, 312. 

As early as 1895, the synthesis of polydichlorophosphazene was attempted by H.N. Stokes by thermal ring-opening polymerization of hexachloro-triphosphazene [(NPCl2)3]. The product obtained by H.N. Stokes was a high-molecular weight cross-linked rubbery material called inorganic rubber which is insoluble in all solvents and hydrolytically decomposes into phosphates, ammonia, and hydrochloric acid in the presence of moisture. Because of its insolubility and hydrolytic instability, the polymer found no technological application and remained as a laboratory curiosity. 

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