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Compounding:
None of the elastomers discussed in the previous section have useful properties until they have been properly formulated. Although some of the science of compounding is understood, much art still remains in preparing technical rubber compositions. In this section, the science and technology of the various ingredients used in rubber are discussed.
2. 1 Vulcanization and Curing:
An elastomer, as synthesized, is basically a high molecular weight liquid with low elasticity and strength. Although the molecules are entangled, they can readily disentangle upon stressing, leading to viscous flow. Vulcanization or curing is the process in which the chains are chemically linked together to form a network, thereby transforming the material from a viscous liquid to a tough elastic solid. Strength and modulus increase, while set and hysteresis decrease. Various curing systems are used to vulcanize different types of elastomers, but complete coverage is beyond the scope of this chapter. Rather, discussion here is primarily on the curing of general purpose diene elastomers with sulfur, with only brief mention of other cure systems.
2. 1.1 Sulfur Curing:
The most widely used vulcanizing agent is sulfur. For sulfur to effectively crosslink a rubber, an elastomer must contain double bonds with allylic hydrogens. General purpose diene elastomers such as BR, SBR, NR, and IR meet this basic requirement.
Two forms of sulfur are used in vulcanization: soluble (rhombic crystals of S8 rings) and insoluble (amorphous, polymeric sulfur). Sometimes, in compounds containing high levels of sulfur, insoluble sulfur is used to prevent sulfur blooming, a process by which the sulfur migrates to the surface of a compound and crystallizes there. Blooming can occur when large amounts of soluble sulfur are used, because at high mixing temperatures, the solubility of S8 is high, enabling large amounts to dissolve, but upon cooling the solubility decreases. When the solubility limit is reached, excess sulfur blooms to the surface. Sulfur bloom reduces the "tack" of a rubber compound, a necessary property if layers of rubber are to be plied up to make a composite structure, such as a tire. Insoluble sulfur does not bloom because it disperses in rubber as discrete particles, which cannot readily diffuse through the rubber. However, above 120 °C, insoluble sulfur transforms into soluble sulfur. Thus, mixing temperatures must be kept below 120 °C to take advantage of the bloom resistance of insoluble sulfur.
Crosslinking with sulfur alone is quite inefficient and requires curing times of several hours. For every crosslink, 40 to 55 sulfur atoms are combined with the rubber. The structure contains polysulfide linkages, dangling sulfur fragments, and cyclic sulfides. Much of the sulfur is not involved in crosslinks between chains. Moreover, such networks are unstable and have poor aging resistance.
To increase the rate and efficiency of sulfur crosslinking, accelerators are normally added. These are organic bases and can be divided into five major categories: guanidines, thiazoles, dithiocarbamates, xanthates, and thiurams. Of these, the guanidine-type accelerators, such as diphenyl guanidine (DPG), give the lowest rate of vulcanization as well as a relatively slow onset of vulcanization. Delayed onset of vulcanization is a desirable feature of rubber compounds. It allows shaping processes to be carried out before vulcanization starts and the material becomes set in its final shape. Premature vulcanization is known as "scorch."
Guanidines are seldom used alone, but rather are combined with another type of accelerator. The accelerators that increase the rate of curing the most are the xanthate types. These ultra-accelerators cause crosslinking so readily that they are seldom used in solid rubber because curing would be initiated just from the heat generated while mixing. Rather, xanthates are used mainly for crosslinking rubber as a latex.
The accelerators with the widest application are the thiazoles, a subcategory of which is the delayed-action sulfenamides. Compounds containing sulfenamides may be sheared for long times without premature vulcanization (scorch). This is particularly important in the tire industry, where a compound may be mixed, repeatedly milled, and then calendered or extruded before being fabricated into a tire.
Thiurams and dithiocarbamates are considered ultra-accelerators, although they are not as active as the xanthates. Because these accelerators have a short scorch time, care must be taken to keep processing temperatures low. Some compounds with ultra-accelerators may begin curing within one day at room temperature, so they must be processed soon after mixing. Crosslinking is efficient when ultra-accelerators are used, and especially when the ratio of accelerator to sulfur is high, so that only low levels of sulfur are required for proper vulcanization.
Often, a combination of accelerators is used to obtain the desired scorch resistance and cure rate. Generally, if two accelerators of the same type are combined, then cure characteristics are approximately the average of those for each accelerator alone. However, there is no general rule when combining accelerators of different types. Moreover, the type of accelerator is much more important than the level of accelerator in controlling scorch time. Although increased levels of accelerator increase the degree of crosslinking attained, generally accelerator concentration has only a small effect on scorch time.
Accelerated sulfur curing is more efficient when the activators zinc oxide and stearic acid are added. It is thought that these additives combine to create soluble zinc ions that activate intermediate reactions involved in crosslink formation.
One instrument used to determine the kinetics of crosslinking is the oscillating disc rheometer (ODR). An oscillating rotor is surrounded by a test compound, which is enclosed in a heated chamber. The torque required to oscillate the rotor is monitored as a function of time. Another instrument to follow curing is the rotor-less moving-die rheometer (MDR), which uses thinner samples, and hence, has faster thermal response than the ODR.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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