People working in the plastic coating industry are well aware of the problems pertaining to proper adhesion. Due to its excessively smooth surface, paints or coats do not adhere to the surface unless it’s treated. Untreated plastic surfaces contain low levels of surface energy which leads to poor bond formation with the adhesive. Improved adhesion on plastics results from the increased bonding strength that essentially depends on the liquid and substrates chemical nature.
Adhesion of material surfaces to coats, inks and paints etc. largely depends on the wettability of a particular surface. Wettability is known as a ratio of surface energy that exists between the substrate and liquid. Improved wettability would mean that a material is more likely to accept the changes made on its surface unlike the unresponsiveness shown through certain chemicals. Consequently, an adhesion problem will only occur if the wetted material has a surface energy level (measured in Dyne) that’s lower than that of the liquid.
The following methods can be used to obtain improved adhesion on plastics:
Promoted Adhesion
For improved adhesion, promoters like coating compositions, primers and tie-coats are largely used. That’s because these promoters use forces of attraction for bonding, bond covalently, and have chemical similarities that encourage adhesion.
In order to support adhesion, the promoter has to search for the functionality of a chemical existing within the plastic that’s ready for binding or bonding together with the promoter. Residual functionality on a reasonable level available for binding in thermoset surfaces will increase the promoter’s performance. Although, polymers having no or low residual functionality can create a problem during adhesion promotion. However, there are a number of other ways for promoting adhesion on plastic surfaces.
Silanes are a very popular group of promoters that are available for particular combinations of coatings and plastics. Adhesion coating can be difficult on substrates made from hydrocarbon thermoplastics and polyolefins. Therefore, promoters like polychlorinated polyolefin are used along with these substrates to help the coating bind with the plastic surface on chemical similarity. Moreover, they have a propensity to migrate to the interface of the substrate due to their relatively hydrophobic nature.
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