The effect of surface chlorination on the mechanical properties of a CR vulcanizate.

In a particular project, it was found necessary to laminate stainless steel sheets to a chloroprene rubber (CR) vulcanizate while maintaining the total thickness of the laminate to within close tolerance. The CR base being only slightly polar and therefore unlikely to bond strongly if at all to the steel sheets, it was decided to increase the polarity of the surface of the vulcanizate by chlorination.

Chlorine is an unpleasant and dangerous chemical to handle, and concentrated solutions are not normally available. It was therefore decided to use a solution of calcium hypochlorite as a chlorinating agent. This material is available in preweighed (3 g) tablets for household use.

The CR sheets were obtained by curing in a semi-positive mold to close tolerance in thickness. After cooling, they were immersed in a solution of 150 g of calcium hypochlorite dissolved in 10 liters of water at 60[degrees]C for 40 minutes. The CR sheets were rinsed in water and allowed to dry in air.

The adhesive used was an epoxy resin activated by DETA suitably diluted with MEK in order to minimize the thickness of the glue line. This resulted in a very strong bond in which the rubber consistently tore before the bond failed.

It is well known that CR compounds cured with zinc oxide and ethylene thiourea or its derivatives absorb water to a considerable degree as a result of the in situ production of the very deliquescent zinc chloride. It was therefore feared that the chlorination process would change the mechanical properties of the compound, particularly the low strain modulus, called the "zero" modulus.

Accordingly, tensile strips were tested before and after chlorination and their zero modulii calculated (ref. 1). This was considered to be the most indicative property for objects which would be used in applications of very low amplitude oscillations. The results of the tests showed that the absorption of water for the short period of inmaersion was negligible and the mechanical properties only slightly affected. The results are shown in table 1.

Conclusion

Chlorination of CR vulcanizates is a surface process which enhances the polarity and does not affect the bulk of the rubber.

Table 1 - properties of CR compound before and
after surface chlorination

                   Before                 After

Tensile strength   1.778 kgf/[cm.sup.2]   1.693 kgf/[cm.sup.2]
Elongation           273                    211%
Zero modulus       .3258 kgf/[cm.sup.2]   .3244 kgf/[cm.sup.2]

References

(1.) Z. Rigbi, "On the determination of the zero modulus of rubber," Kautscuk + Gummi Kunststffe, Vol. 33, p. 1,011, 1991.

Zvi Rigbi, Dept. of Chemical Engineering, Technion IIT, Haifa, Israel