Optimum cure time

Fig. 2. Cure curve from oscillating disk rheometer where A represents scorch safety B, cure rate C, state of cure D, optimum cure time and E, reversion.

A significant revision to the SMR scheme was introduced in 1991 in response to consumer desire for greater consistency in natural mbber (13). Other producing countries have similar specification schemes (14), as does the International Standards Organization (ISO) (15). An example of the specifications for TSR is given in Table 1 for the present Standard Malaysian scheme. Except for SMR 5, rheograph and cure test data (delta torque, optimum cure time, and scorch) are provided. 

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. 

Lack of flexibility in regulating scorch and optimum cure time... 

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... 

Mix number3 Mixing temperature (°C) Organoclay loading (phr) Roc (dNm) Scorch time t2 (min) Optimum cure time t9 (min)... 

In the case of thermal crosslinking, the corresponding curatives were added to each of the EPDM and CR rubbers on a two-roll laboratory mill at room temperature. A constant friction ratio of 1.2 was maintained between the milling rolls. Approximately 2 mm sheets were drawn out from the mill and cured in a hot press at a pressure of 10 MPa and a temperature of 170°C and 160°C for EPDM and CR, respectively, for their corresponding optimum cure time (f90). 

It should be noted that AM is only a measure of an apparent crosslink density of compounds. It is beyond the scope of the present work to investigate in detail the effective crosslinking (physical and chemical). However, for a qualitative assessment it can be concluded that the apparent crosslink density decreases or is influenced by the E-beam irradiation of PTFE powder. PTFE500kGy-EPDM composites show much lower AM and hence lower apparent crosslink densities. It can be inferred that the state of cure and crosslinking efficiency are strongly dependent on irradiation dose. Table 3 shows the optimum curing time (f90, time required to reach 90% of the AM) as a function of PTFE loading and irradiation dose for different PTFE-filled EPDM composites. 

Table 3 Optimum curing time of PTFE-filled EPDM composites...

Fig. 28 Optimum cure time (t90) and delta torque (AM) of EPDM gum and PTFE-EPDM composites...

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. 

The point on the curve which has a tangent based on a slope of 10 lb increase in tensile strength per minute of cure denotes an optimum cure time of 46 minutes. 

For polymers with high ethylene content, under optimum cure time, tensile strength varies from 20 MPa to 125 MPa. Values for compression set can be as low as 1%. The most striking feature of both EPM and EPDM vulcanisates is their excellent resistance to oxygen and ozone, with the latter being slightly less resistant. 

Optimum cure time (t9o) (minutes) Time to reach optimum cure level... 

In general, the masticated NR and EPDM were mixed together with other ingredients including compatibilizers and homogenizing agents in an internal mixer or open mills. The rubber compound is cured in an electrically heated press at 160 °C for optimum cure time, which is determined by rheometer. In order to study the... 

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). 

Step 2 - From the ODR plot, determine the optimum cure time (time B in Figure 3) ... 

The curing reaction rate of NR/CNT nanocomposites was found to be lower than that of pure NR, at the same temperature. The absorption of curatives on CNT was hypothesized to cause higher curing activation energy and lower reactivity. " As a consequence, the optimum cure time of NR increased. The absorption of curatives onto was also attributed to the presence on CNT surface of carboxylic acid groups and quinone oxygen atoms. 

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