Advantages of Prevulcanised latex

Manufacturer: You seem to be an advocate of Prevulanised latex. Why?

John Woon: From the point of view of a latex products manufacturer, the use of prevulcanised latex simplifies a great deal the whole manufacturing process. The seemingly exaggerated and oft-repeated cliche, "Just add water to the PV and start running the machine" actually holds some truth.

Depending on what products are being made, the prevulcanised latex could be used as it is (as in the case of toy balloons after the addition of pigments) or after some dilution with water to achieve a final latex solid content of as low as 30% (as in the case of examination gloves). What one needs to do next is to mix for about 30 minutes and allow enough time, usually 16 to 24 hours for deaeration (i.e. for air bubbles to escape) before the latex is ready for dipping, casting, extrusion, spraying, painting, coating etc.

No maturation period is required as in the case of post-vulcanisable latex compound where a "maturation" stage is almost always a prerequisite for making reasonably good quality latex products.

"Maturation" is a stage when sufficient time must be allowed for both the naturally occurring and added surfactants and fatty acid soaps to reach an equilibrium. Also, a controlled degree of vulcanization must take place during this stage before the latex compound is ready to be used. To use the latex compound too early or too late would result in under-curing and over-curing respectively.

Generally speaking, unlike a prevulcanised latex, post-vulcanisable latex compound would have a marching curve immediately after compounding in terms of the degree of vulcanization. (Figures 5 and 6)

Hence, in the case of post-vulcanisable latex compound, it is more difficult to prevent situations of over-curing when cracking and tearing of, for instance, gloves and condoms are frequently encountered. This is attributed to the fact that the tensile strength reaches a peak before reclining as the cross-link density increases. In short, post-vulcanisable latex compound has short shelf-life of usually from 2 days to 2 to 3 weeks depending on the curative formulation. On the other hand prevulcanised latex generally has a very much longer shelf-life of 6 to 9 months.

Therefore less stringent process controls are required for prevulcanised latex. Also, very often, one finds the viscosity of such compound increasing with time, unlike prevulcanised latex. This is basically a result of zinc ammine thickening. This involves the dissolution of zinc oxide by ammonia in the presence of ammonium salts releasing zinc ammine complex ions which in turn would react with the stabilisers on the latex particles namely the fatty acid soaps and proteins forming insoluble zinc soaps and proteinates. The end result is the loss of latex stability accompanied with increasing viscosity.

It goes without saying that the vulcanization stage is not required for prevulcnaised latex although, in practice, an oven is still required to accelerate the drying.

Another important advantage is the low residual chemicals, particularly the accelerators. This results in a "cleaner" latex compound with low toxicity level which is of utmost importance for the manufacturing of medical devices such as gloves, baby teats, condoms, catheters and medical tubings. Testing of these articles have been carried out by medical device manufacturers taking into account their destination and service conditions. This includes chemical analysis of extracts, skin irritation, skin sensitization, muscle implantation, pyrogeneity, cell cytotoxicity etc.

Experience shows that latex medical articles made from prevulcanised latex are in most cases biologically less active than those made from typical post-vulcanisable latex compounds. Biological activity is usually caused by ingredients that have not fully reacted in the process of vulcanization. These could migrate into human bodily fluid such as fluid from mucous membranes, blood, saliva and other physiological fluids.

Study has been done to compare the efficiency of leaching of films made from prevulcanised latex and post-vulcanisable latex compound particularly during the so-called post-leaching or dry leaching process. The findings indicate that a prevulcanised latex film offers more efficient leaching when post-leaching is carried out.

However, there are some disadvantages of prevulcanised latex. These are the inherently darker colour and the higher tackiness.