Ebonite Curing

The detailed structure of ebonite is not known but it is believed that the same structures occur in the rigid material as have been suggested for vulcanised rubber. There will, however, be far more S-containing structures per unit volume and the ratios of the various structures may differ. The curing reaction is highly exothermic. 

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. 

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)...

NR can yield a hard rigid thermoplastic with excellent chemical resistance when cured with over 30 phr of sulphur. Such a product is termed ebonite. 

Table 3.1 Relative critical cure time for ebonite...

In spite of a high proportion of sulphur used, blooming does not occur in fully cured ebonite vulcanizates. 

While considering swelling characteristics of ebonite two facts are noteworthy 1) high sulphur and adequate cure reduce both the... 

If we take a rubber-sulphur ratio of 68 32 in an ebonite compound and then cure it at 155°C, the vulcanization coefficient increases practically to a constant value of 47 after about five hours and the uncombined sulphur decreases during the first four hours. This state may be called a full cure in the chemical sense. There is also a reduction in volume of about 6%. It is known that after the combination of the first few percent of sulphur, the material passes through a leathery stage with low strength and poor resistance to oxidation and with time passing a true ebonite is formed with increased impact strength. 

Some of the technical properties of ebonite are appreciably affected by the method of cure. In open steam curing, the external pressure is limited to a few atmospheres of steam pressure. The direct... 

Ebonite compounds shrinks to about 6% during cure and as such due allowance for a change of dimensions has to be made for this during manufacture. The ebonite dust and the mineral fillers added for controlling the exothermic reaction serves to reduce this shrinkage. 

In open steam cure of linings and tubes and rods it is important that the ebonite does not flatten under self weight. Addition of ebonite dust and mineral fillers or reclaimed rubber increases the stiffness of the green stock and reduces the tendency for loss of shape. 

Hot water cures can be used for articles that are not affected by immersion in hot water. This method is useful for thick walled articles and rubber lined equipment and especially most suitable for ebonite compositions. Direct contact with water produces better heat transfer than with hot air or steam. Consequently this system gives less deformation of products during cure. 

The calendering process and its conditions are developed or modified according to the requirements of subsequent operations and the purpose for which the sheet is used. Thus for sheets which are to be open cured, such as in chemical plant lining and custom built items such as inflatables and ebonite pipes, roll coverings for paper and steel mills, the calendering needs to be more exact than the sheets which are used for blank preparation for molding of... 

When fully cured ebonite is exposed for a considerable period to the action of sunlight in the presence of humid air, the black surface changes its colour and assumes a green or brown tint. These changes are superficial since the colour can be restored by buffing of the surface layer. 

Exothermic - A chemical reaction in which heat energy is liberated. In tank linings, very thick layers of ebonite compounds may exotherm during cure. 

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