Vulcanization and curing

Tire Cord. Melamine resins are also used to improve the adhesion of mbber to reinforcing cord in tires. Textile cord is normally coated with a latex dip solution composed of a vinylpyridine—styrene—butadiene latex mbber containing resorcinol—formaldehyde resin.. The dip coat is cured prior to use. The dip coat improves the adhesion of the textile cord to mbber. Further improvement in adhesion is provided by adding resorcinol and hexa(methoxymethyl) melamine [3089-11 -0] (HMMM) to the mbber compound which is in contact with the textile cord. The HMMM resin and resorcinol cross-link during mbber vulcanization and cure to form an interpenetrating polymer within the mbber matrix which strengthens or reinforces the mbber and increases adhesion to the textile cord. Brass-coated steel cord is also widely used in tires for reinforcement. Steel belts and bead wire are common apphcations. Again, HMMM resins and resorcinol [108-46-3] are used in the mbber compound which is in contact with the steel cord to reinforce the mbber and increase the adhesion of the mbber to the steel cord. This use of melamine resins is described in the patent Hterature (49). 

Approximately 10 billion pounds per year of elastomers are used in the United States. Crosslinking is a requirement if elastomers are to have their essential property of rapidly and completely recovering from deformations. The terms vulcanization and curing are used synonymously with crosslinking. Crosslinking is achieved either by chemical reaction or physical aggregation depending on the elastomer. 

Chemical cross-linking is given many names depending on the particular area of application. For instance, for hair, the term setting is often associated with the breakage and subsequent reformation of thiol cross-links. For tires, the terms vulcanization and curing are associated with the formation of sulfur-associated chemical cross-links (Figure 2.19). 

Vulcanization and Curing. Originally, vulcanization implied heating natural rubber with sulfur, but the term is now also employed for curing polymers. When sulfur is employed, sulfide and disulfide cross-links form between polymer chains. This provides sufficient rigidity to prevent plastic flow. Plastic flow is a process in which coiled polymers slip past each other under an external deforming force when the force is released, the polymer chains do not completely return to their original positions. 

Vulcanization and curing processes Concentration changes in chemical reactions Thermal heating under load Fluid permeation, drying... 

Uses Vulcanizing and curing agent for chlorosulfonated and crosslinked PE heat stabilizer for electrical grade plastisols, phonograph records, and electrical insulation... 

Rheological characterization, which wonld include measurement of compound viscosity using a Mooney Viscometer, indicating ease of processing, vulcanization and curing kinetics, and rheological information obtained from a capillary rheometer. 

Butyl and Halobutyl Rubber. Butyl mbber is made by the polymerization of isobutylene a small amount of isoprene is added to provide sites for curing. It is designated HR because of these monomers. Halogenation of butyl mbber with bromine or chlorine increases the reaction rate for vulcanization and laminates or blends of halobutyl are feasible for production of mbber goods. It is estimated that of the - 100 million kg of butyl (UR) and halobutyl (HIIR) mbber in North America, over 90% is used in tire apphcations. The halogenated polymer is used in the innerliner of tubeless tires. Butyl mbber is used to make innertubes and curing bladders. The two major suppHers of butyl and halobutyl polymers in North America are Exxon and Bayer (see ELASTOLffiRS,SYNTHETIC-BUTYLrubber). 

This is an activator-starved formulation and so is highly sensitive to the presence of nonmbbers that are capable of activating or accelerating vulcanization, and Table 2 illustrates the cure behavior of different grades of SMR (28). Cup lump grades show the highest state of cure and fastest rate of cure, whereas the stabilized grade, SMR CV, shows the lowest state of cure and slowest cure rate. 

Sodium nitrate is also used in formulations of heat-transfer salts for he at-treatment baths for alloys and metals, mbber vulcanization, and petrochemical industries. A mixture of sodium nitrate and potassium nitrate is used to capture solar energy (qv) to transform it into electrical energy. The potential of sodium nitrate in the field of solar salts depends on the commercial development of this process. Other uses of sodium nitrate include water (qv) treatment, ice melting, adhesives (qv), cleaning compounds, pyrotechnics, curing bacons and meats (see Food additives), organics nitration, certain types of pharmaceutical production, refining of some alloys, recovery of lead, and production of uranium. 

Two kinds of monomers are present in acryUc elastomers backbone monomers and cure-site monomers. Backbone monomers are acryUc esters that constitute the majority of the polymer chain (up to 99%), and determine the physical and chemical properties of the polymer and the performance of the vulcanizates. Cure-site monomers simultaneously present a double bond available for polymerization with acrylates and a moiety reactive with specific compounds in order to faciUtate the vulcanization process. 

These materials are reviewed elsewhere in this book except RTV. The RTV (room temperature vulcanization) silicone plastic is a very popular type. It solidifies by vulcanization or curing at room temperature by chemical reaction, made up of two-part components of silicones and other elas-tomers/rubbers. RTV are used to withstand temperatures as high as 290° C (550°F) and as low as —160° C (—250°F) without losing their strength. Their rapid curing makes them... 

The torque continues to increase, until there is no more signihcant rise. At this point the compound is vulcanized, and this maximum torque value is designated by the symbol Mh- The last major piece of data to be extracted is the time it takes to complete the cure, known as the cure time (t x). 

The vulcanization recipe wasi EPDM 100, stearic acid 1.0, zinc oxide 5 0, accelerator M 0.5, TMTD 1.5, HAF 50, sulfur 1.5 phr. Peroxide-curing of E-P copolymers was carried out with a modified procedure of (6) EPR 100, dicumyl peroxide 3, zinc oxide 3, TMTD 0.5, HAF 50, sulfur 0.2 phr, the mixture being masticated for 20 minutes at 0-50°C and cured at lb0°G. 

The influence of ZnCFO concentration (3,0 5,0 7,0 phr) on formation of properties complex of the unfilled rubber mixes and their vulcanizates on the basis of isoprene rubber of the following recipe, phr isoprene rubber - 100,0 sulfur - 1,0 di - (2-benzothiazolyl) -disulfide - 0,6 N, N -diphenylguanidine - 3,0 stearic acid - 1,0, was carried out in comparison with the known activator - zinc oxide (5,0 phr). The analysis of Rheometer data of sulfur vulcanization process of elastomeric compositions at 155°C (fig. 5) shows, that on crosslink density and cure rate, about what the constants of speed in the main period (k2) testify, they surpass the control composition with 5,0 phr of zinc oxide. Improvement of the complex of elastic - strong parameters of rubbers with ZnCFO as at normal test conditions, and after thermal air aging (tab. 1), probably, is caused by influence of the new activator on vulcanization network character. So, the percent part of polysulfide bonds (C-Sx-C) and amount of sulfur atoms appropriating to one crosslink (S atoms/crosslink) in vulcanizates with ZnCFO are decreased, the percent part of disulfide bonds (C-S2-C) is increased (fig. 62). 

When chemical bonds between neighboring molecules are introduced, a raw elastomer is converted into a rubber vulcanizate, which is essentially a three-dimensional network structure (see Figure 5.4). The process is referred to as vulcanization or curing or, more accurately, as cross-linking. Cross-linked elastomer, or rubber vulcanizate, is capable of large reversible deformations within a broad temperature range and does not dissolve, but swells, in solvents and other liquids. 

Table 1 Formulations and curing characteristics of organoclay-filled XNBR vulcanized at 160°C...

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