Plastics surface pretreatment

Adhesion promoters are the substances that improve adhesive strength of paints in terms of its resistance against mechanical separation from the painted surface. A large number of different chemical adhesion promoters are available. These include silanes, silicones, titanium compounds, zirconates, amides, imines, phosphates, and specially modified polymers. Furthermore, there are binders, plasticizers, and additives, which have the secondary effect of providing good adhesive strength. Adhesion promoters can be used as additives to the paint formulation, or can be employed in the form of a surface pretreatment. 

The described reactive adhesives are suitable for bonding of nearly all metal and nonmetal materials used in industry, trade and even in the private sector. They are characterized by good to very good adhesive properties on correspondingly prepared surfaces (Section 7.1.2) as well as by stress-related strengths. Some plastics, especially polyethylene, however require special measures regarding their surface pretreatment Here, we refer to Section 9.2. 

Surface - smooth metals, plastics, ceramic, glass depending on manufacturing and surface pretreatment... 

Activators Chemical compounds able to trigger chemical reactions not possible without such compounds (e.g., activators as means of pretreatment of hard-to-bond plastic surfaces with anaerobic adhesives). In contrast to catalysts, activators participate directly in chemical reactions. 

Atmospheric pressure plasma Physical-chemical procedure for surface pretreatment, in particular of plastics. It is based on the development of an ionized gas atmosphere by high voltage, and leads to the formation of active surfaces. In contrast to the low-pressure plasma, it works at atmospheric pressure. 

Corona process Method for surface pretreatment of plastics, based on the entrapment of reactive atoms from the gas phase in the surface of plastics by high-voltage discharge. 

Flame treatment Surface pretreatment method, especially for plastics, by means of an acetylene, propane or butane flame burning in excess oxygen. Results in improved surface wettability by the adhesive due to the chemical entrapment of oxygen atoms in the polymer surface. 

Plasma Gas state (also called fourth dimension) from a mixture of electrically charged ions and neutral atoms/molecules. Results from plasma discharge mainly used for surface pretreatment of plastics (q.v. atmospheric plasma). 

For assessment of the surface pretreatment, lap shear tests were performed after 14 days of cataplasma storage for metals, and seven days of cataplasma storage for fiber-reinforced plastics (FRPs) cataplasma storage means that samples are stored at 70 °C and 100% r.h. The results indicate that CLP is most... 

Pretreatment of plastic surfaces is necessary for the following reasons ... 

The surfaces of many plastics and rubbers have low surface energies (Table 3.1) such that wetting by an adhesive is inhibited unless special surface pretreatment processes have been employed. However, plastics which contain polar groups such as PVC, nylons and acrylics are bondable with a minimum of surface treatment. 

Polyolefins, such as polyethylene, polypropylene and polymethyl pentene, as well as polyformaldehyde and polyether, may be more effectively treated with a sodium dichromate-sulfuric acid solution. This treatment oxidizes the surface, allowing better wetting by the adhesive. Flame treatment and corona discharge have also been used. Table 7.20 shows the relative joint strength of bonded polyethylene and other plastic substrates pretreated by these various methods. 

With plasma treatment, surface wettability can be readily induced on a variety of normally non-wettable materials as shown in Table P. 5. Certain polymeric surfaces, such as the polyolefins, become crosslinked during plasma treatment. The surface skin of polyethylene, for example, will become crosslinked so that if the polymer were placed on a hot plate of sufficient heat, the interior would turn to a molten liquid while the crosslinked outer skin held a solid shape. Other polymers have their critical surface energy affected in different ways. Plasma-treated polymers usually form adhesive bonds that are two to four times the strength of nontreated polymers. Table P.5 presents bond strength of various plastic adherends pretreated with activated gas and bonded with an epoxy or urethane adhesives. 

A wide range of surface pretreatment procedures has been developed for different plastic adherends available in the current market. As with other adherends mentioned above, it is the purpose of any surface pretreatment to establish a surface condition for good wetting by the adhesive. 

Cleaning will remove oily soils and mold release agents, but additional pretreatment may be needed on certain plastic surfaces to ensure adhesion. Many of the plastic substrates are chemically inert and wUl not accept coatings and finishes because of their poor wettability. 

Treatment of certain polymeric surfaces with excited inert gases greatly improves the bond strength of adhesive joints prepared from these materials. With this technique, called plasma treatment, a low-pressure inert gas is activated by an electrode-less radio-frequency discharge or microwave excitation to produce metastable species which react with the polymeric surface. The type of plasma gas can be selected to initiate a wide assortment of chemical reactions. In the case of polyethylene, plasma treatment produces a strong, wettable, cross-linked skin. Commercial instruments are available that can treat polymeric materials in this manner. Table 7.10 presents bond strength of various plastic joints pretreated with activated gas and bonded with an epoxy adhesive. 

Although in many applications no form of pretreatment is employed for the substrate materials prior to adhesive bonding, to attain the maximum in joint performance some form of surface pretreatment for the substrate materials being joined is almost always necessary. Adhesives are available which will join substrates which are covered in light machine or protective oils or low molecular weight plasticizers but some control over the state of the surface is still usually necessary. For example, the type of protective oil needs to be matched to the adhesive and its thickness needs to be controlled, so that the adhesive can dissolve and displace the oil. Further, the reproducibility and service life of such adhesive joints is still invariably greatest when some form of surface pretreatment is employed. 

As mentioned earlier, other plastics such as poly(vinyl chloride), poly (ethylene terephthalate), polyacetals, nylons and polyimides do not present such a severe problem to the adhesives technologist as do fluorocarbon polymers or polyolefins. Nevertheless, in order to obtain very high joint strengths approaching the cohesive strength of the substrate, some form of surface pretreatment is often necessary. 

Figure 8.7 Effect of immersion in water at 40 °C on the tensile lap-shear strength of glass fibre reinforced plastic (GRP) substrates bonded with a two-part acrylic adhesive [40]. The surface pretreatment was simply an abrasion/solvent-cleaning process.

---