Rubber, natural specific properties

Benzothiazines have been used in industry as dyes and as antioxidants of rubber and natural elastomers. They have been used widely to prepare azodye compounds with specific properties, such as water insolubility, which is useful in the dyeing of polypropylene fibers, and increased stability to gas fading.72,73 Several reports are concerned with the antioxidant properties of benzothiazines,74-77 and their use as color photography developers.78... 

Natural rubber from Hevea brasiliensis is a natural polymer composed of an association of poly(cz5-l,4-isoprene) [poly(2-methyl-1,3-butadiene)] and biological elements, giving it highly specific properties. Originating from the Amazon Basin, Hevea was already booming in Asia at the turn of the twentieth... 

The mesostructure covers both the macromolecular scale (dimension, conformation and architecture of the macromolecules) and also the supramole-cular scale (complex aggregates between macromolecules). It is this last point that is the key to the specific properties of natural rubber. This associative... 

Several elastomers can be used in rubber-based adhesives. The elastomer provides the backbone of the adhesive, so the main performance of the adhesive is provided by the rubber properties. However, several specific properties for application are imparted by adding other ingredients in the formulations. The most common elastomers used in rubber-based adhesives are natural rubber (NR), butyl rubber (BR) and polyisobutylenes, styrene-butadiene rubber (SBR), nitrile rubber (NBR) and polychloroprene rubber Neoprene) (CR). 

Some of the key properties of PAREL elastomer vulcanizates are summarized in Table I. They have excellent low teiqierature properties excellent dynamic properties, which are much like those of natural rubber good ozone resistance and good heataging resistance. This interesting combination of properties is leading to substantial specialty markets in such applications as automotive engine mounts. Vulcanization and stabilization studies on PAREL elastomer, as well as additional specific properties of PAREL elastomer, are reported in this book by Boss. 

With the exception of natural rubber, all the above polymers are synthetic products. Although this book will deal with the properties of synthetic materials only, we have to be aware of the decisive role played by polymers in nature. Control of life processes is based on two polymer species, nucleic acids and proteins. The specific property of these polymers is that they form stable microscopic objects, mainly as the result of the action of intramolecular hydrogen bonds. The stable, specifically ordered surface of the proteins provides the high selectivity and catalytic potential used in biochemical reactions selectivity and catalytic activity disappear when the globular molecular shape is destroyed at elevated temperatures or upon the addition of an active chemical agent. The synthetic polymers discussed in this book do not possess the potential to form a unique molecular conformation as single chains and, therefore, do not show any biochemical activity. 

The inherent properties of polymers of the poly isobutylene family, particularly the chemical inertness, age and heat resistance, long-lasting tack, flexibility at low temperatures, and the favorable FDA position on selected grades, make these products commercially attractive in a variety of pressure-sensitive and other adhesives, in automotive and architectural sealants, and in coatings. An added dimension is achieved in the blendability of the polyisobutylene polymers with each other and with other adhesive polymers such as natural rubber, styrene-butadiene rubber, EVA, low molecular weight polyethylene, and amorphous polypropylene to achieve specific properties. They can, for example, be blended with the highly unsaturated elastomers to enhance age and chemical resistance. A description of poly isobutylene polymer family use in adhesive and sealant applications follows. 

The types of processing and physical property tests that are required to assess the important characteristics referred to above are also continually cited in the later sections of this book dealing with the use of waste rubber crumb in rubber products, thermoplastics, and thermosets, for the same reasons (Chapter 7). Many other specific property tests (e.g., acoustic) are also referred to in this book but, unfortunately, there is insufficient space here to cover them. A reasonably detailed section on the characterisation of rubber crumb is provided in Chapter 6, Section 6.4, because understanding the nature of this material is as important as understanding the properties of devulcanised rubber when it comes to its re-use applications. 

The chemical nature and molecular weight of the rubber will greatly determine the properties of elastomeric adhesives. However, some common characteristics can be found in most of the rubber base adhesives. The elastomeric adhesives show the following specific features in assembly operations. 

Many polymer items are designed specifically to make contact with other materials. Where surface contact is concerned, two key properties are coefficient of friction and abrasion resistance. Polymers used in such applications include ultra high molecular weight polyethylene, polyacctal, fluorinated polymers, and natural and synthetic rubbers. Examples that we routinely come across include furniture upholstery, bushings and gears in office equipment, and bicycle tires. Industrial uses include the outer cover of electrical cables, and pipes that convey abrasive liquids such as slurries and powders. 

The first synthetic rubbers to be commercially available in United States were Thiokol (1930) and Neoprene (1931). Both of these are still being produced commercially because they have special properties that are not matched by natural rubber. Various types of synthetic rubbers were introduced during (1939-43) World War II. After world war, stereo rubbers have been made using stereo specific catalysts. 

Knowledge of the physical properties of materials is essential for design, specification and quality control, and the particular nature of rubbers demands that specific test procedures, rather than methods for materials in general, are used to measure almost all of the properties. The importance of the subject of rubber testing to industry and to research is witnessed by the large number of national and international standards which have been produced. 

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