Main Chain Modification of Natural Rubber Vulcanisation

Students: We are a group of students pursuing a course in Polymer Science and Technology. We are interested in finding more information about the effect of "cyclic sulphide" groups formed during vulcanisation of natural rubber. Any advice from you would be highly appreciated.

John Woon (Senior Latex Consultant): Earlier work conducted by rubber scientists and technologists indicated that conventional sulphur vulcanisation would result in the formation of cyclic sulphide structures (or side groups) along the rubber network chain. This is sometimes referred to as main chain modification which is a wasteful by-product.

By "conventional sulphur vulcanisation", I mean a vulcanisation system where sulphur is added in the curative formulation as opposed to "Efficient Vulcanisation system" or "EV system" where a sulphur donor such as TMTD is used in place of sulphur.

The effect of this cyclic sulphide structure that is of interest to the rubber technologists is its ability to resist the tendency of the rubber molecular chains to crystallise when exposed to low temperature storage. The rubber product therefore remains flexible at low temperatures. The effect is less evident with EV system.

It might interest you to know that, fortunately, the cyclic sulphide structure does not prevent significantly the strain-induced crystallisation. This means that you still observe the high tensile strength of natural rubber.

                                                                                                                                             

"Clastogenicity", "Mutagenicity" and "Teratogenicity" of Rubber Chemicals

Manufacturer: When reading about the toxicity of rubber chemicals, I often come across the terms, "Clastogenicity", "Mutagenicity" and "Teratogenicity". Can you please explain the meaning of these terms?

John Woon (Senior Latex Consultant): Interesting questions. I had mentioned this before but briefly, I would like to make the following comment again:

1) Clastogenicity
"Clastogen" refers to a general class of chemicals which produce changes or alterations in the cell chromosomes.

2) Mutagenicity
'Mutagens" refer to those chemicals which tend to induce changes in the genetic materials carried by both the male and female reproductive cells.

Please take note that mutagenic chemicals are also carcinogens. Hence some cancer tests are actually tests of the mutagenicity potential.

3) Teratogenicity
"Teratogens" are chemicals which interfere with the normal development of the foetus during pregnancy and hence leading to malformation of the baby.

                                                                                                                                           

Natural Rubber Latex Thread - Resistance to Dyeing

Manufacturer: We are manufacturer of natural rubber thread using latex extrusion process. We have received complaints from our customers that their garments lost the elasticity property after the dying process. What could have caused this problems and how can we eliminate or minimize this problem?

John Woon (Senior Latex Consultant): I believe this could be due to the following reasons:

1) The nature and type of the dye itself. Some inorganic dyes contain transition metals such as copper, manganese and iron which act as "pro-oxidants" that catalyses the oxidation of the rubber. If this is what your customers are using, I suggest they switch to organic type of dyes.

2) The condition of the dyeing process which entails pH, temperature and time. Often the dyeing process is done at alkaline pH of 9 - 12 which would tend to extract the accelerators and antioxidant from the thread leading to poor resistance to oxidation.

Of course, the higher the temperature and the longer the time, the worst would be the situation. I therefore recommend that the dyeing process be carried out at a pH approaching neutral, at lowest possible temperature and shortest possible time.

Also, make sure that water used for washing does not contain trace amount of copper.