Ask Dr. Dave / Columns

Ask Dr. Dave

October 2007

Question: It has been written that sealant should only adhere to two surfaces, not three, and not to the backer. Is this because it creates a neutral air pressure space between the sealant and the backer, giving the sealant more support in case of an exterior/interior air pressure differential?

Answer: One of the fundamental design principles for a moving joint is to allow a flexible sealant to expand and contract. Thus, adhesion is required only to the two sides of the joint that will move. In fact, corner joints often have an installed release tape to prevent adhesion to the third side of the joint. The joint movement capability of a sealant is also best exploited in a wide joint rather than a narrow one. Joints that are too deep cause severe stresses and should be avoided or made shallower with a backing material. It is important to prevent adhesion to this backer in order to continue getting true two-sided adhesion.

Question: I have noticed in instructions for polyester putties that it is recommended to first apply a thin layer of putty and let it cure before applying a thicker layer on top. I always assumed that this recommendation was simply to ensure one was getting good enough contact with the substrate by carefully applying a thin layer. We are now developing a two-component, water-based putty, and I see that even if we get good initial contact with the substrate (usually wood or steel) the thinner layers always adhere better than the thicker ones. Any thoughts on why this is?

Answer: In order to get good adhesion to a surface, one needs to first achieve a good wetting of the surface with the adhesive. A thin coating will take longer to gel and cure than a thick one, and this allows time for thorough spreading and wetting of the surface. Another problem one will encounter when applying thick layers of coatings or the putties you describe is that you will get excessive shrinkage that will create stresses at the bonding surface. By applying a thin coating, these stresses are minimized while adhesion is maximized.


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