PVA Emulsion Plasticised with Polyvinyl Alcohol

Manufacturer:  Most of our range of adhesives are based on PVA (polyvinyl acetate) stabilised with polyvinyl alcohol. Can you comment on our choice of the base emulsion, effect of it's molecular weight, pH, particle size, solvent tolerance and plasticisation?
 
John Woon (Senior Latex and Rubber Consultant):  PVA (polyvinyl acetate) stabilised with polyvinyl alcohol is widely used as the based polymer for adhesives with satisfactory performance. It usually has quite a high inherent viscosity which allows it to be compounded into low-cost adhesives with high water content. But you have to bear in mind that emulsion with maximum solid content and low viscosity is required for fast drying and setting.

Extra care has to be taken when using it for bonding transparent films or sheets since it has quite poor gloss and clarity.

Although high molecular weight is desirable for high cohesive strength of the bond, it would give poor heat sealing property due to high softening point. Particle size is important in that the penetration of the particles into porous substrates would be lower and hence resulting in weaker bond if the particle size is too coarse.


pH of PVA emulsion ranges usually from 4 to 5 due to the acetic/acetate effect. Although you could make it more alkaline especially when adding this to natural rubber latex (pH of about 10.0) as an additive or a blend, there would be a gradual drift towards the lower range of pH due to hydrolysis of the polymer on storage.


Solvent resistance should be sufficiently high since we often add some level of solvent to enhance the initial tack.


Plasticiser is usually added to impart flexibility of the dried adhesive. However, you should not exceed certain level beyond which the bond strength would be reduced.


If you don't already know, PVA is sometime added to adhesives based on natural rubber latex as a reinforcing agent.