The PVA FAQ

Anyone know the what's why's and wherefore's of when and when not to use PVA ?

 It seems to be used in all building applications with a certain amount of gay abandon. What does it do, when should you use it ?

PVA ( polyvinyl acetate ) has many DIY uses;

As an adhesive, It will bond wood, paper, leather, expanded polystyrene, mortar, brick and most other porous materials to themselves and to each other. It makes a reasonable border and edge adhesive for wallpaper.

As a priming agent, Diluted 3 or 4 to 1, it prepares plaster, brickwork and concrete for tiling, rendering, painting or papering.

As a mortar additive. It improves water retention to ensure good curing of the mortar, even in thin layers, and makes the mortar "stickier" so that if you are rendering, the render tends to stay put until it sets. Use 3 or 4 to one diluted PVA as the gauging water.

As a dust proofer for concrete floors, Sweep the floor and brush over with 3 or 4 to one PVA. An old floorbrush is useful for large areas. PVA is not fully waterproof as the dried resin, but will resist temporary wetting well. In a set mortar it is unaffected by water to any gross degree.

There are also EVA (ethylene vinyl acetate) and PVP (polyvinyl propionate) bonding agents. Neither of these is very much better in the water resistance stakes, and seem to be more common in mainland Europe. For the best water resistance, acrylic or SBR (styrene butadiene rubber) bonding agents are the choice.

Some years ago, about 1985, PVA bonding agents moved from 55% TSC (total solids content) to 44% TSC. TSC is a misnomer, as bonding agents contained 10% of the resin weight as plasticiser. The purpose of the plasticiser was twofold, firstly it acted as a coalescing solvent (lowering the minimum film formation temperature), and to make the dried film flexible. Back then the plasticiser was dibutyl phthalate, but this has been found to be an oestrogen emulator. Of course when the solids content went down the water content went up. The viscosity was therefore made higher in one of three ways, By adding a thickener, by upping the molecular weight of the polyvinyl alcohol in the aqueous phase, or by changing the particle sizing of the emulsion.

PVA can be detected by the use of Iodine, either as an alcoholic tincture, or better, as a solution in aqueous Potassium Iodide. The PVA will turn brick red on contact with these reagents. Either that, or the builder who swore blind that he had primed the wall will. PVA is essentially non-toxic, although it can be irritating in the eyes. The hazchem notification used to read "Caution - may be very sticky". Imagine 20 tons of PVA spilt on a motorway.
 

When PVA dries will it redissolve, and why does it turn white?

This is generic for PVA, EVA PVP, and acrylic emulsions. Minus the bit about coalescing solvent, it is true for natural and synthetic rubber latices. The polymer comes in the form of an aqueous emulsion, i.e. polymer globules in water. There are various compounds to keep the emulsion stable, so that creaming and settling tend not to happen. As the water dries off the polymer forms a continuous film, which does not tend to redisperse. Most of these polymers tend to reabsorb water, which makes them whiten and reduces their strength to some degree. Another consideration is the minimum film formation temperature (MFFT) If a polymer emulsion loses water below its MFFT, it will not form a cohesive film. Almost all DIY polymer emulsions contain what is called a coalescing solvent, which is in quantity sufficient to lower the MFFT to about 4 degrees C. Some also contain a plasticiser to make the film less brittle. When possible the manufacturers use a solvent which performs both functions.
 

I have seen powdered PVA about. How do they do that?

This is a spray-dried version of PVA, known in the trade as redispersible PVA. It is widely used in cementitious compounds to promote adhesion. The various manufacturers are undestandably reticent about the exact details of the process, as this seems to be the key to producing a quality product.
 
 

Copyright notice: With the growth of the net, copyright law has become very complicated. You may reproduce this FAQ in any form from stone tablets to putting it on your homepage. You may distribute it to absolutely anyone. You may even add your own bits to it. However, if you remove my name from it, or try to pass it off as your own work, I will be very, very angry with you.

To the index

John Schmitt <john49@mdx.ac.uk>

Last updated 19/12/00