Ebonites curing temperature

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3. 

In autoclave curing best results can be obtained with high pressure and curing temperatures, i.e., at 3 to 4 kg/cm2 pressure and 134 °C to 144 °C temperature. The usual procedure followed is to subject the rubber lined vessel to air pressure of about 2 kg/cm2 at the beginning of the cure cycle which ensures good contact between metal and rubber and then introduce steam and raise pressure and temperature to complete the cure. Typical cure cycles for ebonite and soft rubber are represented in the curves as shown in Figures 6.2a and 6.2b. 

If the sheet is made thicker, the central reactive layer is thicker and hence more heat is generated and heat loss is retarded. If the generation of heat exceeds loss the system becomes unstable. If the sheet is made thinner, the increased loss quickly overtakes the less heat generation and the temperature rise is small. Therefore, the curing temperatures of ebonite are very important. A reduction from 130 °C to 120 °C moves the heat loss upwards and the heat generation downwards thus reducing temperature rise. 

Figure SO.3. Variation of internal temperature during cure of ebonite stocks containing 0, 20, 50 and 100 parts of ebonite dust per 100 parts (rubber and sulphur). (After Scott, see bibliography)...

One of the recent contributions to rubber technology has been the development of methods whereby both natural and synthetic, both cured and uncured, may be bonded directly to metal surfaces. Earliest methods used to fasten rubber to metal were mechanical. Ebonite was also used in early work, the ebonite surface giving a strong adhesion to the metal while uniting with the rubber on the other side. Ebonite as a bonding agent reduces the flexibility of the unit as a whole, especially when the rubber section itself is not very thick. Secondly, ebonite is thermoplastic and the tenacity of the bond falls off rapidly even at moderate temperature. Recently, other modified derivatives of rubber have become avail-... 

The use of up to 35 phr sulfur with a butraldehyde-monobutylamine such as (Vanax 833) type accelerator may be used to produce a higher temperature resistant ebonite, primarily for submarine battery cases. This approach has virtually disappeared having been replaced with either plastics or nitrile with higher amounts of an acrylic monomer and a peroxide cure. 

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