Vulcanization Time

The total time of vulcanization is split into two steps  

The time taken to heat the rubber to curing temperature, and 

The time to cure the rubber after the curing temperature has been attained. 

No one curing process posesses all the virtues and those which may appear to be most desirable may be rejected on grounds of initial cost and maintenance of the equipment. For example even an autoclave curable rubber lined vessel can be cured in open steam at atmospheric pressure and this method can be adopted while the autoclave is down for repairs and maintenance. 

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Geon and Seo [47] also determined the effect of vulcanization time on the adhesion of natural rubber to brass-plated steel. For relatively short times, there was a peak at the end of the copper profile that corresponded well with a peak in the sulfur profile. Similarly, peaks in the zinc and oxygen profiles corresponded well. These results showed that copper sulfide and zinc oxide mostly formed at short times but some evidence for formation of zinc sulfide was also obtained. For long times, the peak in the sulfur profile no longer corresponded with that in the copper profile. Instead, the peak in the sulfur profile corresponded to the peak in the zinc profile. It was concluded that the formation of zinc sulfide increased substantially at long times. An increase in vulcanization time correlated well with a decrease in the force required to pull brass-plated steel wires out of rubber blocks. 

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. 

Cure characteristics and reinforcement parameter (aF) - The cure characteristics were determined using the RPA 2000 dynamic curemeter (Alpha Technologies). The increase in torque at a frequency of 0.833 Hz and 2.79% strain was measured. The optimal vulcanization time (f90) of the samples was determined and used for curing of the samples in the press. 

The vulcanization times for ebonites are very long when compared with those of soft rubber. 

The classical means for following vulcanization by physical methods is to vulcanize a series of sheets for increasing time intervals and then measure the stress strain properties of each and plot the results as a function of vulcanization time. A modification of this test generally called a rapid modulus test is widely used in the industry as a production control test. A single sample taken from a production batch of compounded rubber is vulcanized at a high temperature and its tensile modulus is measured. Temperatures as high as 380°F are used to reduce the vulcanization test time to only a few minutes. Any modulus value deviating from a predetermined acceptance limit indicates that the batch is defective and is to be rejected. 

In view of the high cost of mould and relatively large consumption of steam and electricity, economy means the shortest possible vulcanization time. Moulding time is directly proportional to the thickness of the product and inversely proportional to the temperature. 

The measurement of pre-vulcanisation properties is given in ISO 289-238. This is essentially a matter of running a test on a fully compounded material (including curatives) as in ISO 289-1 until the viscosity reaches a specified number of units above the minimum (usually 5 units). The time to this point is designated as the scorch or pre-vulcanization time. The temperature of test is chosen to suit the process in question. 

The system Cl-butyl-cis-polybutadiene has been studied in some detail because it was suitable for the developed differential swelling technique and because this system of blends vulcanized with zinc oxide, sulfur, and thiuram disulfide first revealed the presence of interfacial bonds. This curative system has the feature of a flat cure —i.e.y the two homophases are vulcanized rapidly, and the crosslinked density does not increase radically as vulcanization time is prolonged. This is observed in Table IV by swelling and extractable levels of a series of crosslinked networks cured at increasing times and swollen in a common solvent, cyclohexane. 

This situation is reversed in the case of ENB-EPDM (Table 17) since, after high vulcanization times, relevant fractions of polysulfidic and disulfidic links are still present, while the overall cross-link density is rather low in relation with the unsaturation content. 

Inorganic accelerators and metallic oxides (usually zinc) are essential for satisfactory cure. Organic accelerators, introduced in the early 1920s, notably shortened vulcanization time. 

Barker L.R. 1973. Effect of temperature on vulcanization time. J. Institution Rubber Industry. 7 121-23. 

Component B is prepared as follows To 100 parts of component A, add 4 parts of a 10% solution of a dithiocarbamate accelerator. The mixed adhesive will vulcanize at room temperature or at a higher temperature, if required. Typical vulcanizing time at room... 

Different types of accelerators impart vulcanization characteristics that differ with respect to both scorch resistance and crosslinking rate. Figure 7.11 is a map of accelerator system characteristics. Within groups or types, differences can be obtained by choosing the individual accelerators. In the group of benzothiazolesulfenamides, the scorch resistance and vulcanization time increase in the order TBBS or CBS, MBS, DCBS. 

In thick-walled moldings, the vulcanization time - even in injection moldings -is within the >3 min range, in C > 6 min. The relaxation of the orientations may be complete (requiring only seconds at higher temperatures). 

Figure 37-1. Network density ft in amount of network chains per unit volume vulcanizate as a function of the vulcanization time for the vulcanization of natural rubber with 2 phr sulfur, 5 phr zinc oxide, 1 phr 2>mercaptobenzthiazole, and 1 phr stearic acid at 140 C. Cl, main cross-linking reaction Cii, long-term crosslinking A chain degradation. (After D. A. Smith.)...

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