Basics of Surface Wetting & Pretreatment

Plastics adhesion problems are wide-spread throughout industry. A major component of these problems is due to the fact that many plastics have chemically inert and nonporous surfaces with low surface tensions. That is, most plastics are hydrophobic and are not naturally wettable. These properties, although advantageous to the design engineer, often result in secondary assembly and decorating concerns-bonding, printing, coating and painting. Surface pretreatments on today's high performance engineering resins solve many adhesion problems while increasing bond strength performance.

As a general rule, acceptable bonding adhesion is achieved when the surface energy of a substrate (measured in dynes/cm) is approximately 10 dynes/cm greater than the surface tension of the liquid. In this situation, the liquid is said to "wet out" or adhere to the surface. Surface tension which is a measurement of surface energy is the property, due to molecular forces, by which all liquids through contraction of the surface tend to bring the contained volume onto a shape having the least surface area. The higher the surface energy of the solid substrate relative to the surface tension of a liquid, the better its "wettability", and the smaller the contact angle. Figure 1.

Figure 1

The degree or quality of treatment is affected by the cleanliness of the plastic surface. The surface must be clean to achieve optimal pretreatment. Surface contamination such as silicone mold release, dirt, dust, grease, oils, and fingerprints inhibit treatment. Material purity is also an important factor. The shelf life of treated plastics depends on the type of resin, formulation, and the ambient environment of the storage area. Shelf life of treated products is limited by the presence of low molecular weight components such as antioxidants, plasticizers, slip and antistatic agents, colorants and pigments, stabilizers, etc. Exposure of treated surfaces to elevated temperatures increases molecular chain mobility. The higher the chain mobility the faster the aging of the treatment. Polymer chain mobility in treated materials causes the bonding sites created by the treatment to move away from the surface. These components may eventually migrate to the polymer surface, therefore it is recommended to bond, coat, paint, or decorate the product as soon as possible after pretreatment.

Surface pretreatments are used to increase surface energy and improve the wetting and adhesive properties of polymer materials. A variety of pretreatment processes are used in industry including RF cold gas plasma, electrical (corona discharge), and flame plasma. Each method is application specific and possesses unique advantages and potential limitations. All of these processes are characterized by their ability to generate a "gas plasma" - an extremely reactive gas consisting of free electrons, positive ions, and other chemical species. In the science of physics, the mechanisms in which these plasmas are generated are different but their effects on surface wettability are similar. Plasmas can be conceptualized as a fourth state of matter. If sufficient energy is supplied, solids melt into liquids, liquids vaporize into gases, and gases ionize into plasmas.

Free electrons, ions, metastables, radicals and UV generated in plasma regions can impact a surface with energies sufficient to break the molecular bonds on the surface of most substrates. This creates very reactive free radicals on the polymer surface. These free radicals can form, cross link, or in the presence of oxygen react rapidly to form various chemical functional groups on the substrate surface. Polar functional groups which can form and enhance bondability include carbonyl (C=O), carboxyl (HOOC), hydroperoxide (HOO-), and hydroxyl (HO-) groups. Even small amounts of reactive functional groups incorporated into polymers can be highly beneficial to improving surface characteristics and wettability.