For both environmental and economic reasons, there is broad interest in recycling rubber and in the continued development of recycling technologies. The use of postindustrial materials is a fairly well-established and documented business. Much effort over the past decade has been put into dealing with of end-of-life tires from landfills and vacant fields. It is only in the last few years that more business opportunities for recycled rubber have come to the forefront. Reclaiming rubber has gained increasing interest, more so in Europe than in North America. In those areas, much work has been done to refine the processes used. The major form of recycled rubber is still ground rubber. This is produced either by cryogenic, ambient, or wet grinding. The material is then used neat with sulfur/curatives, binders, or cements. The binders are normally moisture curable urethanes, liquid polybutadienes, or latex to produce items such as mats, floor tiles, and carpet undercushion. Recycled rubber is still used as tire derived fuel, but less so than 10 years ago. Another outlet is as an additive to asphalt. Recycled rubber can be used in the plastics industry, for which much development is being done. Large particle size ground rubber or chips are used in civil engineering applications, landscaping, or artificial turf. In terms of applications, most use is outside of the conventional rubber industry. Cost factors are still addressed in the tire industry. As of 2012, approximately 8–10% recycled material is used in tires. The biggest obstacles to further adaption are safety factors and property loss. Better methods are needed for treating or modifying the rubber surface and for regenerating the rubber through devulcanization. Devulcanization gives the highest quality recycled material in terms of processing and properties. However, shortcomings to devulcanization are reduced process safety and odorous chemicals that are required at present.

For both environmental and economic reasons, there is a continuing broad based interest in recycling of scrap rubber and development of recycling technologies. The use of post- industrial scrap is established as a systematic business. However, the disposal and reuse of scrap tires remains a serious environmental concern and a business opportunity. The method for reclaiming rubber utilizing aqueous alkaline solutions has been abandoned in North America because of environmental pollution hazards. The focus of more recent research is to apply processes that do not generate disposal hazards and that might be carried out directly in the product manufacturer's factory. The major process at the present time is to utilize the scrap rubber as a very finely ground crumb. Crumb is produced either by ambient temperature mechanical grinding or by cryogenic shattering. In general, the crumb rubber is combined with virgin elastomer compounds to reduce cost. However, there is some loss in physical properties and performance. This factor has motivated the search for cost effective in-situ regeneration or devulcanization of the scrap rubber to provide superior properties. Some progress has been achieved utilizing mechanical shear, heat and other energy input, and a combination of chemicals such as oils, accelerators, amines, etc. to reduce the concentration of sulfur crosslinks in the vulcanized rubber. The major application of scrap rubber, particularly as crumb, is outside the conventional rubber industry. More than half of the scrap is burned for its fuel value for generation of electricity and as a component in cement production. The utilization in extension of asphalt in road construction is now recognized to provide superior road performance and reduced cost. The simple use of crumb rubber as a component in artificial turf is developing into a significant industry. Rubber crumb is now widely utilized in rubber products such as mats, floor tiles, carpet undercushion, etc., where the crumb is rebonded using polyurethane or latex adhesives. Other applications, such as in landfill, concrete, thermoplastic blends, pyrolysis to generate carbon black and chemicals, are discussed. The tire industry does utilize a significant proportion of fine crumb rubber in tire compounds. This is likely to not increase much due to the concern about tire performance and safety. However, there is a serious interest by tire manufacturers to increase the use of scrap tire rubber, if the recycled rubber could be regenerated to improve compatibility and performance in tire compounds.

1. The vulcanization system based on nitrosophenols (quinone monoximes) is not primarily based on the formation of a urethane group. Evidence is presented that the crosslink consists of carbon—carbon, carbon—sulfur, and urea groups. Urea groups can be identified in reaction products which also contain the free phenol group of the nitrosophenol (oxime); urethane groups are identified in products which also contain quinone groups. 2. The presence of zinc dithiocarbamate is conducive to the formation of the amino group which is the functional group on which the crosslinking reaction with diisocyanate is based. The dithiocarbamate also acts as a sulfur donor. The dialkyl amino group of the dithiocarbamate is bound into the network. 3. Nitroso compounds which have no tautomeric oxime structures show no vulcanization in combination with free diisocyanate. 4. The level of vulcanization of various substituted phenols, when used in combination with a diisocyanate precursor, is dependent on their thermal decomposition termperatures, but such a dependence is not apparent when these compounds are used in combination with free diisocyanate. 5. A reaction mechanism is suggested for the vulcanization with oximes and diisocyanates which does not require the splitting of their adducts into their components but instead into ionic fragments. This mechanism explains why monoximes lead to vulcanization whereas nitroso compounds do not. 6. Attempts to effect premature vulcanization should be directed towards delaying the formation of the diisocyanate/oxime adduct since it provides the vulcanization-active intermediates. Towards this goal it has been shown that a diisocyanate precursor can be applied but then a thermally stable nitroso compound (oxime) is required. 7. Tautomerism between O-derivatives of oximes, nitrones, and oxaziridines can be assumed to provide a working hypothesis for the discovery of new N-O-containing crosslinking agents for unsaturated polymers.

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SynopsisAn attempt is made to distinguish properties of elastomers by types. "Basic properties of materials" or "network properties" in elastomers are properties which either increase or decrease from the liquid to the solid state of materials or over the range of the "elastomeric plateau" of elastomers.From these are distinguished properties that exhibit characteristic maxima and are therefore "maximum properties" or bivalued properties. Mechanical failure properties show the characteristics of "maximum properties." The maxima in "maximum properties" generally do not coincide. This noncoincidence of the maxima with a change in a "basic property of a material" has major theoretical and practical implications, for example, it is the cause of the crossovers in the relative performance rating of materials under different test conditions. The implications of this noncoincidence of the failure property maxima on the relevance of correlations between these properties are discussed. A change in the testing conditions is reflected in a shift of the optimum value in a "basic property of a material" with respect to a specific "maximum property." Data and certain conclusions in the literature are interpreted on the basis of this concept. Examples of the limitations of the validity of mathematical relationships are presented. Also, a defmition of the term "state of cure" is proposed and a suggestion for the rating of severities of test equipment and applications of elastomeric materials recommended. The effect of increased degrees of crosslinking for a series of polymers and crosslinking agents is assessed. It is suggested that the "mechanisms" of failure properties will remain elusive if their rationalization is attempted on the basis of other failure properties, e.g., the mechanism of abrasion on that of tear strength or cut growth. The main purpose of this proposal is to provide support for a drastic reduction in laboratory testing by identifying those properties which can lead to different relative ratings in routine evaluations and actual applications. A more empirical approach to materials evaluations is recommended based on the calibration of laboratory instrumentation with respect to specific applications. A de-emphasis of routine evaluations of materials on the basis of their "maximum properties" seems to be justified.