Discolouration of Natural Rubber Latex Products

Manufacturer: Can you please explain why products made from natural rubber latex tend to discolour more than synthetic latices?

John Woon (Senior Latex Consultant): Good question. One major causative agent is the naturally occurring enzyme called polyphenol oxidase in the so-called non- rubbers. This tends to darken the coagulated or dried rubber when exposed to air (i.e. oxygen).

Another reason could be the presence of residual "Frey Wyssling" particles which lead to discolouration due to their carotenoid nature. And this accounts for the yellowish colour.

As shown in the diagram, FW = Frey Wyssling Particle, RP = Rubber Particle, L = Lutoid, LVW = Latex Vessel Wall.

Poorly leached products made from natural rubber latex would also tend to darken on storage.

                                                                                                                                                  
                                                                                                                                            

Air Permeability of Bladders based on Natural Rubber

Manufacturer: We are manufacturers of bladders that require good air impermeability. However we found that natural rubber latex while giving superior resilience that is one of the product specification, it fails in air impermeability. We would appreciate your advice in guiding us to solve this problem at a cost.



John Woon (Senior Latex Consultant): I'm sorry to let you know that the best polymers for the best air impermeability, such as Polychloroprene, Nitrile and Butyl do not give you the resilience you need from natural rubber.

I suggest you conduct laminate dipping with your NR latex with one of the synthetic latices I mentioned, my personal choice being Polychloroprene.

Details of the recipes for both NR latex and Polychloroprene latex compounds and the manufacturing processes are being sent to you separately.

                                                                                                      

Stiffness of Latex Medical Tubing

Manufacturer: We are manufacturing medical tubing based on natural rubber latex. We are required to increase the stiffness of the tubing without adding filler. Please advise us how we could achieve this.



John Woon (Senior Latex Consultant): You have to aim at having a tighter cure by increasing the levels of ZnO and sulphur. A combination of ZDEC and ZMBT would also increase the modulus and hence the stiffness.

If you need too achieve this without changing your existing curative recipe, you can look into the addition of synthetic latex such as high styrene SBR or MMA grafted latex.