Vulcanization optimum cure

Table 6 Vulcanization Rate Constant (k) and Optimum Cure Time (T90)...

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. 

Influence of the ZnCFO contents (3,0 5,0 7,0 phr) on crosslink kinetics of the modelling unfilled rubber mixes from NBR-26 of sulfur, thiuram and peroxide vulcanization of recipe, phr NBR-26 - 100,0 sulfur - 1,5 2-mercaptobenzthiazole - 0,8 stearic acid - 1,5 tetramethylthiuramdisulfide - 3,0 peroximon F-40 - 3,0, is possible to estimate on the data of fig. 7. As it is shown, the increase of ZnCFO concentration results in increase of the maximum torque and, accordingly, crosslink degree of elastomeric compositions, decrease of optimum cure time, that, in turn, causes increase of cure rate, confirmed by counted constants of speed in the main period (k2). The analysis of vulcanizates physical-mechanical properties testifies, that with the increase of ZnCFO contents increase the tensile strength, hardness, resilience elongation at break and residual deformation at compression on 20 %. That is, ZnCFO is effective component of given vulcanization systems, as at equal-mass replacement of known zinc oxide (5,0 phr) the cure rate, the concentration of crosslink bonds are increased and general properties complex of rubber mixes and their vulcanizates is improved. 

The comparative estimation of efficiency of zinc oxide and ZnCFO similar concentrations (3,0 5,0 7,0 phr) as the agents of metaloxide vulcanization system was carried out on example of modelling unfilled elastomeric compositions from chloroprene rubber of recipe, phr chloroprene rubber - 100,0 magnesium oxide - 7,0. Kinetic curves of rubber mixes curing process at 155°C are shown on fig. 8. The analysis of the submitted data testifies, that at increase of zinc oxide contents vulcanization kinetics is changed as follows the scorch time and optimum cure time are decreased, the cure rate is increase. Vulcanization... 

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. 

Figure 10.3 Practical vulcanization chart for optimum cure.

In the one-stage vulcanization process, NR and EPDM are first masticated separately and then mixed with each other. Additives such as ZnO, stearic acid, carbon black (or silica), and process oil are added. The mix thus obtained is allowed to cool to room temperature. Finally, coupling agent known as DIPDIS and sulfur are added to the mix on the cooled mill. The stocks are finally cured under pressure at 160°C (32-33). In the two-stage process, NR and EPDM are first masticated separately. Then, additives such as ZnO, stearic acid, DIPDIS, and sulfur are incorporated in the EPDM. The compounded EPDM mix is then heated at 160°C in the hydraulic press for the predetermined time to yield the grossly undercured mix. The undercured mix is then blended with NR to the required blend ratio. The blend compound is finally vulcanized to the optimum cure time values (32-33). 

As noted [15], when a compounded rubber is subjected to a plasticity measurement at a high enough tanperature and for a long time, it will cure, and consequently a plasticity test can be used as a test for scorch or rate of cure. Thus, the Mooney viscometer is used to measure scorch (i.e., the onset of vulcanization), and an oscillating disc rheometer will measure the plasticity of the compound before the onset of cure, as weU as the iuCTease in stiffness as curing takes place. Moreover, tests for scorch and rate of cure should be distinguished from tests for degree of cure or optimum cure measured on the vulcanized material. 

The scorch time, which is the measure of premature vulcanization, and the optimum cure of vulcanizates increased. ... 

This is followed by vulcanization to obtain vulcanized composite sheets using a compression moulding in hydraulic press. The cure time for the vulcanization is generally determined using a Monsanto rheometer and optimum curing times (tgo) is calculated. The sheeted rubber compounds are conditioned at a tem- perature of 25 °C for 24 h in a closed container before cure assessment. 

It should give a plateau-type cure, i.e. significant properties such as modulus should, after an appropriate delayed action period, rapidly reach their optimum values and retain these values even after prolonged further cure. This should enable thick sections to be evenly vulcanized and prevent reversion due to further heating after the optimum cure has been achieved. 

The maximum elongation at a break is given by Eq. (70). The optimum cure and elongation become (vi + v2)/fV=0.01 and a= 10, respectively. This means that the optimum degree of vulcanization is of the same order as that of the pseudolink B. In other words, point C is located close to the transition point, B. 

A vulcanization time of 20 min at 150°C ensured that the mixes were at or slightly beyond optimum cure. 

Sample preparation. All ingredients were mixed in an open two-roll laboratory mill at room temperature. The rotors operated at a speed ratio of 1 1.4. Rubber compounds were vulcanized in an electrically heated hydraulic press. The compounds were cured into 0.25 mm thick films at 150 °C for 15 min according to the optimum cure time ( 90) derived from the curing curves previously determined by means of a Rubber Process Analyzer (RPA2000 Alpha Technologies). Rectangularshaped specimens were mechanically cut out from the film samples. 

Vulcanization is a very complex reaction and involves activators for the breakage of die sulphur ring (Sg) and accelerators for the formation of sulphur intermediates, which facilitate sulphur-to-double bond crosslinking. Elastomer vulcanization by sulphur wifliout any accelerators takes several hours and is of no commercial importance. By using accelerators in the sulphur curing system, die optimum curing time can be decreased to as little as 2-5 min. 

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