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Preventing cratering

Prevention of cratering

 

The causes of cratering are diverse. Cratering can occur, for example, from overspray: if fine spray droplets of an overspray of a different coating material fall onto a freshly coated surface that is still liquid and mobile, these droplets can cause cratering if their surface tension is less than that of the surrounding coating film. Surface tension differences of 1-2 mN/m are already sufficient to cause spreading of the overspray droplets and, in consequence, cratering. If the surface tension of both materials is the same or if the spray mist has a greater surface tension, there are no spreading processes and therefore also no cratering. Small dust particles falling onto the liquid coating can provoke the same effect as overspray droplets. Craters can also be the result of a contaminated substrate (e.g. fingerprints or similar). If this contamination has a lower surface tension, it will cause cratering when the coating is applied. The occurrence if these effects is a special case of a poor substrate wetting. By using a silicone additive, the surface tension is lowered and the coating is therefore much less susceptible to disturbances or disruptions, irrespective of whether they originate from the environment (overspray, dust particles, etc.), from the substrate (contamination), or even from the coating itself (gel particles). The coating process can be carried out more reliably through the use of silicone additives. In this case, when the greatest possible reduction in surface tension is needed, the use of silicone additives as anti-cratering agents is preferred.

Polyacrylates have barely any influence on the surface tension and are therefore ineffective as anti-cratering additives.

Cratering