Chlorination* is of particular benefit to natural rubber. During storage and use, unprotected natural rubber products are exposed to molecules of oxygen or ozone in the air, both of which can cause a breakdown in the rubber surface structure. This can result in the rubber becoming discolored and brittle. Rubber elastic bands are often found to break under comparatively little stretching after being left for some weeks on a desk top due to this.
In chlorination, chlorine gas is combines with the materials of the product to produce a surface layer more resistant to the harmful effects of oils, acids and agents that cause progressive cracking and eventual breakdown of unprotected rubber. It can be compared to the benefit of electroplating metal to prevent oxidation by chemical and effects of the atmosphere and in the same way that plating enhances a metals surface, chlorination can beneficially improve perception of a rubber products’ quality by reducing its surface friction coefficient. In effect, it takes away the rubbery texture and creates a silky surface finish.
Chlorine is a halogen and halogen gas fluorine, is similarly used to produce the non-stick protective surfaces such as Teflon ™ on pans and other cooking utensils. In contact with a natural rubber product, chlorine reacts with each surface molecule to form a structure that restricts future access to oxygen or ozone to the rubber and increases its barrier to absorption of oils and greases. Acidic substances combine less easily with a chlorinated surface. For these reasons, good quality household gloves and many other domestic products are usually chlorinated.
Synthetic rubbers have different surface structures but equally benefit from chlorination. In many cases their tensile strength and potential extensibility is somewhat lower than natural rubber and for that reason more easily physically damaged. Chlorination not only reduces the coefficient of surface friction, but also improves surface appearance and handling characteristics. Residual free sulphur bloom, a powdery surface appearance similar to natural yeast on grapes, during long-term storage of rubber products can also be delayed.
If a product is to be used in contact with skin, a valuable further benefit of chlorination is the removal or substantial restriction of potentially dermatitic chemicals from the rubber. All rubber products contain a residual organic accelerator which, if present, above a very low level can cause contact dermatitis in people, sensitization does not occur, and at the same time largely remove or deactivate potentially harmful proteins form natural rubber products. Also, chlorinated products can be defined as “clinically clean” at the end of the process.
Chlorination therefore can be very beneficial to the quality of rubber products but it must be carried out with great care and at the right level for the product involved. Special processing equipment is used to deliver the correct amount of gas for the correct time of exposure. Great care and experience are important factors in successful chlorination.
*Courtesy of Cotswold Floor Machines, Gloucester, UK