painting elastomeric sealants - stretched to the limit

Architect's memo 50: February 1988

Elastomeric construction sealants are immensely important products; indeed many modern building practises would not have developed without the availability of these tough, adhesive, resilient materials. So why does the paint industry hate them?

There are many times that, for aesthetic reasons, it is desirable to paint these materials and there lies the rub. Many sealants contain plasticisers to achieve their required physical properties. These materials are generally content to stay within their domains inside the sealant but deep down they can have a roving eye and a faithless heart. They can become fatally attracted to paint films lying in intimate contact on the sealant's surface and migrate into them.

This loss of plasticiser is never sufficient to jeopardise the properties of the sealant but it can drastically affect the paint film. The paint binder can swell and soften causing some gloss changes in sheen paints but more importantly, a drastic lowering of the paint's dirt resistance. Dirt particles stick to the softened film, bringing exactly the attention to the joint that was sought to be avoided by painting in the first place.

Paints and glazes have different susceptibilities to the effects of plasticisers with harder paints being less susceptible. However, therein lies the second rub.

If an elastomeric sealant, coated with an excellently adhering coating is stretched beyond the elastic limit of the coating, the coating will crack. Due to the excellent adhesion of the coating small tears will occur in the sealant, at the site of the cracks, to accommodate the movement. Just as a rubberband will become severely weakened when a small notch is cut in its side, so is the sealant weakened by these crack-induced notches. Further, normal expansions of the joints can lead to these notches propagating deep into, and eventually through the sealant, leading to joint failure.

It seems generally agreed that, although some elastomeric sealants are paintable from the perspective that paint will adhere to them, best elastomeric sealant performance is achieved by leaving them unpainted. Coating with high-build elastomeric coatings is a possible solution but these soft coatings are at risk should the sealant contain a plasticiser with wanderlust.

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