Factors Affecting Latex Gel Strength

Manufacturer: In our process we need to maximize the gel strength of our latex compound in order to reduce our rejection rate. Please give us your advice and comment on the factors affecting the gel strength of latex. We are using both prevulcanized latex (PV) and post-vulcanisable latex compounds and calcium nitrate as our coagulant.

John Woon (Senior Latex Consultant): To begin, you need to realize the fact that the gel strength of an unvulcanised latex is generally superior to that of the compounded latex be it prevulcanised or post-vulcanisable. Hence the higher the dregree of cross-linking, the lower would be the gel strength. This means that a PV with lower modulus would give better gel strength compared to one with higher modulus.

Also, with a post-vulcanisable latex compound, one has to monitor the cure state carefully as too advance a cure would lead to lower gel strength and hence higher chances of cracking of the latex flim.

Gel strength is believed to be dependant on how readily the rubber particles could coalesce to form an integrated film.

Generally, the gel strength could be maximised by using the highest possible latex solid content with highest coagulant strength coupled with the shortest possible latex dwell time. However in practice, very short latex dwell time is some times difficult to achieve. Also, too short a latex dwell time would lead to uneven thickness of the latex film.

Other factors include the colloidal stability of the latex compound, especially the chemical stability which in turns depends on the level of ZnO, fixed alkali and other surfactants in the latex, not forgetting of course the former temperature. (As agreed, detailed remedial actions for maximum gel strength is omitted here)