Vulcanization processes

Scorch resistance is usually measured by the time at a given temperature required for the onset of crosslink formation as indicated by an abrupt increase in viscosity. The Mooney viscometer is usually used [4]. During this test, fully mixed but unvulcanized rubber is contained in a heated cavity. Imbedded in the rubber is a rotating disc. Viscosity is continuously measured (by the torque required to keep the rotor rotating at a constant rate) as a function of time. The temperature is selected to be characteristic of rather severe processing (extrusion, calendering, etc.). 

Both the rate of vulcanization after the scorch period and the final extent of vulcanization are measured by devices called cure meters. Many workers contributed to this development [6]. Widely used cure meters are oscillating disc rheometers of the type introduced by the Monsanto Company in about 1965. The development of the oscillating disc rheometer, largely through the efforts of R. W. Wise, was the beginning of modern vulcometry, which has become standard practice in the industry. Before the development of the cure meter, it was necessary to measure mechanical properties of many specimens 

FIGURE 4 The effect of heat history (processing) on scorch safety. 

In order to measure the vulcanization characteristics, the rubber is enclosed in a heated cavity (Fig. 5). Imbedded in the rubber is a metal disc that oscillates sinusoidally in its plane about its axis. Vulcanization is measured by increase in the torque required to maintain a given amplitude (e.g., degrees of arc) of oscillation at a given temperature. The torque is proportional to a low strain modulus of elasticity. Since this torque is measured at the elevated temperature of vulcanization, the portion of it due to viscous effects is minimal. Thus it has been assumed that the increase in torque during vulcan- 

Newer versions of the cure meter have been introduced (e.g., Fig. 6). The cavity is much smaller and there is no rotor. In this type of cure meter, one-half of the die (e.g., the upper half) is stationary and the other half oscillates. These instruments are called moving-die rheometers. The sample is much smaller and heat transfer is faster. Also, because there is no rotor, the temperature of the cavity and sample can be changed more rapidly. In either case (oscillating disc or moving die), torque is automatically plotted against time. Such a chart is shown in Fig. 7. 

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Sulfenamide accelerators generally requite less fatty acid because they release an amine during the vulcanization process which acts to solubilize the zinc. Guanidines and similar amine accelerators also serve to both activate and accelerate vulcanization. 

The thiophthalimide (CTP) and sulfenamide classes of retarders differ from the organic acid types by thek abiUty to retard scorch (onset of vulcanization) without significantly affecting cure rate or performance properties. Much has been pubUshed on the mechanism of CTP retardation. It functions particularly well with sulfenamide-accelerated diene polymers, typically those used in the the industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with additional sulfenamide to complete the vulcanization process. If the MBT initially formed is removed as soon as it forms, vulcanization does not occur. It is the role of CTP to remove MBT as it forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior. 

The NO nitrosating agents present in the atmosphere are often due to air poUution. High surface area fillers such as carbon black absorb NO and Hberate it during the vulcanization process. Of course, this is the process where NO is most likely to be in contact with the various accelerators. 

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. 

The vulcanization process established by Goodyear in 1839 is still currently in use. Sulphur vulcanization takes place in three stages. 

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

The oldest technology involved in the elastomer blending and vulcanization process is essentially a temperature controlled two roll mill as well as internal mixers followed by an optimum degree of crosslinking in autoclave molds (compression, injection, etc.) in a batch process or in a continuous process such as continuously heated tube or radiated tubes. A few examples of laboratory scale preparation of special purpose elastomeric blends is cited here. 

The energy required to produce a tire is only about 10 percent of that consumed while in use on an automobile overcoming rolling resistance during 40,000 miles of seiwice. The majority of the manufacturing energy expended, 60 to 70 percent, is for the mold during the vulcanization process. 

Vulcanization was carried out in one and two stages with and without carbon black. In one-stage vulcanization, all the ingredients were mixed and the compound was vulcanized in the mold for optimum cure time as shown by Monsanto rheometer curves. In the two-stage vulcanization process, all the ingredients except XNBR were mixed and heated for a short time, followed by mixing with required amount of XNBR and heating for rest of the time required for optimum vulcanization. 

Rubber elasticity has a long-standing history. Ancient Mesoamerican people were processing rubber by 1600 BC [1], which predated development of the vulcanization process by 3500 years. They made solid rubber balls, sofid and hollow rubber human figurines, wide rubber bands to haft stone ax heads to wooden handles, and other items. 

It is beyond our control how the cross-links are spaced along the polymer chains during the vulcanization process. This extraordinary important fact demands a generalization of the Gibbs formula in statistical mechanics for amorphous materials that have fixed constraints of which the exact topology is unknown. Details of a modified Gibbs formula of polymer networks can be found in the pioneering paper of Deam and Edwards [13]. 

The MFAs on heating (during the vulcanization process) decompose into two constituents— the diamine part functions as an accelerator while the fatty acid part acts as a flow promoter-cum-lubricant. 

In addition to the two major processes, cross-linking and chain modification (or cyclization), chain scission doubtless occurs also to varying degrees during conventional vulcanizations. Processes of this nature are not difficult to envisage in the presence of free radicals. The radical intermediate (II) may, for example, undergo /3-fission as follows ... 

Workers in tire-manufacturing facilities may have a heightened potential for health hazards since the rubber vulcanization process can involve exposures to 77-hexane (Graham et al. 1995). 

Clustering of crosslinks can be explained by a kinetic chain reaction occurring through the C=C double bonds. Crosslinking by the conventional vulcanization process with sulfur has been shown by NMR to proceed through the allylic hydrogen atoms. 

Problems may arise if P.B.15 1 is to be used in natural rubber, because the presence of free copper affects not only the vulcanization process, but also considerably affects the fastness of the product to aging. P.B.15 1 is thus considered a rubber poison. The free copper content in a pigment should therefore never exceed 0.015%. Commercial types are available which have been tested accordingly. 

Despite of 150-year s history of vulcanization process, it is impossible to consider that fundamental and applied researches in direction of vulcanization systems perfection are completed. For today one of the ways of rubbers properties improvement is the synthesis and application of the new chemicals-additives, including, vulcanization active, that is connected, first of all, with reduction of global stocks of zinc ores as basic raw material for reception of traditional activator - zinc oxide. Besides, modem increase of industrial potential and the accumulation of big quantity wastes derivate the problems of ecological character, which require the emergency decision. Therefore creation of resourcesaving technologies of the new compounds reception from products of secondary raw material processing has paramount importance. 

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). 

Figur 5. Rheometer data of sulfur vulcanization process of modeling unfilled elastomeric compositions on the basis of isoprene mbber at 155°C with ZnCFO as the activator ...

That is, the analysis of the received results, has shown an opportunity of equal-mass replacement of the traditional activator - zinc oxide on the new polymer - inorganic composite (5,0 phr) at maintenance of a high activation level of sulfur vulcanization process of rubber mixes on the basis of diene isoprene rubber and improvement of the physical-mechanical properties complex of their vulcanizates. 

Measuring Vulcanization. The formation of a three-dimensional structure during vulcanization increases the stiffness (modulus) of the compound. Therefore, following the modulus increase versus cure time provides a continuous picture of the vulcanization process. Oscillating disk rheometers provide a useful method to do this (17). In this test, a preweighed sample of uncured mbber is placed into a preheated cavity containing a conical rotor. The cavity is closed and the rotor is set to oscillate within the mbber sample. As vulcanization proceeds, the compound s resistance to rotor movement increases and this resistance is followed as a function of time, thereby generating a continuous profile of cure behavior. These cure curves,... 

It was suggested that stearic acid, which consist of small molecules, enters into the gallery of the layered silicate structure during the mixing and vulcanization processes, thus paving the way for intercalation of the rubber chains. 

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