VULCANISATION TIME

It is now common practice to use sulphur in conjunction with several other additives. First amongst them are vulcanisation accelerators, of which there are many types. In the absence of an accelerator about 10 parts of sulphur is required, the vulcanisation time may be a matter of hours and much of the sulphur is... 

Figure 10.17 Dependency of T2 relaxation time as a function of the actual vulcanisation time during accelerated sulfur vulcanisation of EPDM [179]. The line has been included to guide the eye. The vulcanisation temperature was 413 K (140 °C)...

The time it takes for the temperature of a sample to stabilise in an NMR probe can also be determined in real-time XH NMR Tj relaxation experiments, because at vulcanisation temperatures Tj is only slightly affected by a moderate crosslink density [22, 180, 181]. The T2 data obtained in the aforementioned experiments were used for determining an increase in the density of chemical crosslinks upon vulcanisation time (Figure 10.18) [179]. The method can also be used in kinetic studies of the vulcanisation of filled and oil-extended rubbers. 

Table 3 The vulcanisation time (igo), 100% modulus (SEioo), tensile strength (TS), elongation at...

Considerably shorter cycle times in comparison with conventional rubber materials Vulcanising times of 5-6 seconds per millimetre wall-thickness with a mould temperature of 200 C... 

Mould sealing, which is characterised by high rigidity and parallelism of the mould platen, as well as fast opening and elosing speeds enable a rapid injection cycle. Due to the fast vulcanisation time of LSR, 4 to 5 second cycles are not rare. 

An important requirement of the crosslinking systems for injection moulding is high processing safety because this enables the injection temperature to be raised, thus improving the flow. In addition the vulcanisation time must be short so that economic production cycles can be achieved. 

Until now the use of crosslinking systems based on thiuram disulphide for compounds which reach high temperatures has been difficult owing to the inadequacy of the scorch times. However, Leibbrandt has drawn attention to the development of systems comprising tetramethyl thiuram disulphide (or preferably dimethyl diphenyl thiuram disulphide), N-morpholinyl-2-benzothiazyl sulphenamide, a special retarder (Vulkalent E), and 0-5-0 8 phr of sulphur which, especially in the case of NBR, combine high processing safety with short vulcanisation times. The thiuram disulphide can be replaced by other sulphur donors (see Table 3 and Fig. 9). 

The possibility was demonstrated of using the accelerator Disulfal MG as a replacement for the Altax or Sulphenamide Ts traditionally used in formulations of vulcanisates for mechanical rubber goods, provided that the appropriate adjustment was made to the composition of the vulcanising group and the vulcanisation time. 2 refs. (Full translation of Kauch.i Rezina, No.6, 1996, p.33)... 

Secondary accelerators of a given formula, which are useful in combination with a primary accelerator selected from sulphenamides and thiazoles, in the sulphur-vulcanisation of rubber compositions, are disclosed. Also disclosed are sulphur-vulcanised rubber compositions which are vulcanised with a primary accelerator and in the presence of one or more of the disclosed compoimds. The present approach achieves a clear improvement in the vulcanisation time as compared to a system without a secondary accelerator and, further, when compared with other secondary accelerators, the scorch time is improved by these secondary accelerators. 

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