Vulcanization temperature

However, when the vulcanization temperature was increased to 190°C, it was observed that the peaks in the copper and sulfur profiles no longer coincided. Instead, the peak in the sulfur profile coincided with the peaks in the zinc and oxygen profiles. These results indicated that at higher vulcanizing temperatures, zinc sulfide formed in abundance while formation of copper sulfide decreased. 

Changes observed in the composition of the rubber/brass interphase correlated well with results of adhesion tests carried out on brass-plated steel wires embedded in blocks of rubber [46]. The force required to pull the wires out of the blocks decreased steadily as vulcanization temperature increased. This effect was especially pronounced when the specimens were aged at elevated temperature and humidity for several days before the wires were pulled out of the rubber blocks. 

Figure 6 Light transmission of quenched cured polyethylene as a function of the sample thickness (vulcanization temperature 160°C, time 15 min.). 1-sample of normal polyethylene 2 mm thick 2, 3, and 4-quenched cured samples of different thickness 2-1 mm, 3-2 mm, 4-4 mm.

Aside from the sulfur, the sulfur bearing compounds that can liberate sulfur at the vulcanization temperature can be used as vulcanizing agents. A few sulfur donors are given in Table 14.5, which include some compounds like dithiodimorpholine (DTDM), which can directly substitute sulfur. Others, like tetramethylthiuramdisulhde (TMTD), can act simultaneously as vulcanization accelerators. The amount of active sulfur, as shown in Table 14.5, is also different for each compound. Sulfur donors may be used when high amount of sulfur is not tolerated in the... 

The thermal stability of peroxides can be expressed in terms of their half-hfe (ti/a)- Half-life values can be estimated in solution utilizing the technique of differential thermal analysis. These values, or more precisely the temperatures at which their half-life is equivalent, provide an indication of practical vulcanization temperatures [49] (Table 14.29). 

Possibility to apply high vulcanization temperatures without reversion... 

Ethylene-propylene-diene terpolymers (EPDM), with their inherent complexity in structural parameters, owe their tensile properties to specific structures dictated by polymerization conditions, among which the controlling factor is the catalyst used in preparing the polymers. However, no detailed studies on correlation between tensile properties and EPDM structures have been published (l,2). An unusual vulcanization behavior of EPDMs prepared with vanadium carboxylates (typified by Vr g, carboxylate of mixed acids of Ccj-Cq) has been recently reported Q). This EPDM attains target tensile properties in 18 and 12 minutes at vulcanization temperatures of 150 and l60°C respectively, while for EPDMs prepared with V0Cl -Et3Al2Cl or V(acac) -Et2AlCl, about 50 and 0 minutes are usually required at the respective vulcanization temperatures, all with dieyclopentadiene (DCPD) as the third monomer and with the same vulcanization recipe. This observation prompted us to inquire into the inherent structural factors... 

Since in a normal vulcanization system at least part of the sulphur combines with the rubber during vulcanization, an obvious way of following vulcanization is to measure the decrease in free sulphur. This method is not used extensively since it is well known that the combination of free sulphur does not correlate well with the development of cross links or other physical properties. In addition to this the analytical procedure is lengthy and costly. However free sulphur determinations are often made on finished products as a means of checking for uniformity of the product and to estimate the degree of cure. Figure 8.6 below shows the rate of sulphur combination at different vulcanization temperatures for a typical... 

It can be seen that every type of vulcanization system differs from every other type in the kind and extent of the various changes that together produce the vulcanized state. In the vulcanization processes, consideration must be made for the difference in the thickness of the products involved, the vulcanization temperature and thermal stability of the rubber compound. The word cure to denote vulcanization is believed to have been coined by Charles Goodyear and the same has been a recognized term in rubber industry circles [2]. The conditions of cure will vary over a wide range according to the type of vulcanizate required and the facilities available in a rubber factory. Many factors must be predetermined, including the desired hardness of the product, its overall dimensions, the production turnover required and the pretreatment of the rubber stock prior to vulcanization. Hardness will normally be determined by the composition of the stock but it can also be influenced by the state of cure. 

The vulcanization temperature must be chosen in order to produce a well cured product having uniform and optimum physical properties in the shortest possible time. The term temperature coefficient of vulcanization can be used to identify the relationship between different cure times at different temperatures. With this information optimum cure times at higher or lower temperature can be estimated for many rubber compounds with known coefficient of vulcanization. For approximately most rubber compounds the coefficient of vulcanization is 2. This indicates that the cure time must be reduced by a factor of 2 for each 10°C increase in cure temperature or if the temperature is reduced to 10°C, the cure time must be doubled. 

All rubber products exhibit shrinkage after cure, mainly due to the thermal expansion which occurs at vulcanization temperature. Moulded rubber goods are never as big as the moulds in which they are cured. The difference between the dimensions at room temperature of the finished goods and of the mould expressed as a percentage is called the shrinkage from mould dimensions. 

Reactivity with Tetrazoles. Reactive tetrazoles were used for the synthesis of rubbers having an appreciable ageing protection [237]. 2-(Subst.)phenyl -5-(3,5-di-tert-butyl-4-hydroxyphenyl)tetrazole (181) can be compounded without thermolysis with BR or IR. At vulcanization temperatures, 181 is thermolysed and reactive nitrileimine 182 undergoes 1,3-dipolar addition to C = C double bonds. 183 is formed with yields from 70 to 85%, depending on the character of R in 181 (the activity series R = CH3 < H < Cl). Mechanistic model experiments were performed with 181 (R = Cl) and styrene, the latter was isued as a model instead of IR [237]. l-(3-Chloro-phenyl) -3-(3,5-di-/ert-butyl-4-hydroxyphenyl)-5-phenyl-2-pyridazoline was isolated in 44% yield and medhanism of the attachement of 183 to IR was thus confirmed. 

Special methods of incorporation processing history has an essential effect on conductivity shear imposed during mixing causes a fracture of secondary carbon aggregates increased temperature during mixing may preferentially form rubber-carbon bonds rather than the carbon-carbon bonds required for conductivity vulcanization temperature may affect recovery of broken connections between carbon-carbon bonds talc reduces melt viscosity which results in a smooth surface of extruded and calendered products as well as reduced wear of equipment ... 

An interesting, indirect application of the statistical theory to filled rubbers has been published by Wolff (/33). In recent years increasing use has been made of oscillating disc rheometers to follow the course of vulcanization. These instruments operate at low frequencies and at vulcanization temperatures, so that torque recorded at full cure is a good measure of an equilibrium shear modulus and hence proportional to v. Wolff finds for polymers filled with furnace blacks and cross-linked with dieumyl peroxide ... 

The compression strength oc dependence on vulcanization temperature mode according to experimental data is ... 

The response surface of vulcanization mode is illustrated in Figure 2.78. Analysis of the data shows that the maximum compression strength is reached at a vulcanization temperature in the area of 110°C over 7-7.5 hours. 

It is easy to verify that the maximum tension strength at a bend is reached at a vulcanization temperature of 90°C over 8 hours, at a temperature of 110°C over 7 hours, and at a temperature of 130°C over 6 hours. 

Thus, the area of optimum vulcanization temperatures is 90°C-115°C and the optimum duration of heat treatment is 6-8 hours (Figure 2.79). 

In light of this, a search for ways of reducing vulcanization temperature is gaining importance. Research shows that lowering the curing temperature of a RubCon mixture can be achieved in two ways ... 

FIGURE 2.78 The compression strength oc dependence of vulcanization temperature mode. (Reprinted from O. Figovsky and D. Beilin, Building Materials Based on Advanced Polymer Matrix Review, J. Scientific Israel Technological Advantages 10, no. 3 (2008) 1-119. With permission.)... 

Reactivity of QI with compounded NR at vulcanization temperatures results in two concerted processes [4,24], More than 30% of the QI was reduced into CB AO 11 or 83, another part was bound into the polymeric substrate. The... 

Although a free radical source is frequently used with a maleimide vulcanizing agent, at high enough vulcanization temperatures, the maleimides react with the rubber without the need for a free radical source. This could occur as shown here ... 

---