Liquid Coating or Glue Retraction (Also Known As Crawling) Of Applied Liquids Onto A Solid Surface
The quality of any coating or adhesive application is adhesion depends on how well the surface is prepared. The resin system must be able to properly 'wet out' the surface to form a continuous film. Surfaces demonstrating poor wettability prevent the liquid resin from spreading into a stable continuous film. The coating retraction or crawling is caused by a mismatch of surface tension between the coating and the substrate to which the coating is applied. Poor surface wetting yields poor film appearance. Upon cure, the coating easily delaminates and peels from the surface since the applied coating did not wet the surface enough to create a strong adhesion.
Wetting is the ability of a liquid to interface or 'wet out' a solid surface; its dynamics are expressed as surface tension. Crawling is the term used to describe how the liquid coating, whether epoxy coating, commercial paints, or printing inks, tends to retract from the solid substrate to which it is applied.
Crawling, also called aligaotring is a uniform de-wetting effect: the surface energy of the solid substrate is lower than the surface tension of the liquid paint. The resulting effect is that the liquid paint does not thoroughly wet the substrate: the liquid retracts and forms droplets, leaving part of the substrate uncoated.
A general statement is that wetting becomes more critical when the substrate's surface energy is low and/or when the liquid paint's surface tension is high. A substrate with low surface energy is called hydrophobic, implying that its surface cannot interact with the applied coating or adhesive. Unmodified plastics, for example, are materials that have low surface energy or LSE.
Before application, the substrate must be clean of grease, oils, or old coating. In some cases, the substrate must be modified in such a way that the surface energy of the substrate is raised, implying that the substrate is changed from hydrophobic to more hydrophilic. Substrate treatment is often applied to plastics before coating. Another approach, which is most often used to prevent crawling, is to lower the surface tension of the liquid resin by adding a wetting agent or solvents. However, solvents and wetting agents can affect critical properties like hardness, clarity, recoatability, and general aesthetics.
A surface that demonstrates low surface tension, like Teflon, UHMW, HDPE-type plastics, waxed surfaces, or an oily surface, prevents the liquid resin from wetting the substrate and causing poor adhesion.
In the same respect, if the surface is coated from a previous application (varnished, oil-stained, or clear-coated), the epoxy adhesion is limited to the adhesion quality of the primary coating applied. Remove any loose or old peeling coating before application to avoid delamination. Remove the old coating by mechanical sanding or power washing so the applied epoxy is in direct contact with the base substrate. This will produce a much stronger bond than applying the epoxy over an existing or old coating application.
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23 окт 2024
