Plastic adherend

Solvent-Based Adhesives—In these the adhesive flows because it is dissolved in an appropriate solvent, and solidification occurs on evaporation of the solvent. Good bonds are usually formed if the solvent attacks or actually dissolves some of the plastic adherend to produce a solvent-welded bond. 

Plastic adherend Pretreatment/preparation Adhesive types... 

Table 7.10 lists common recommended surface treatments for plastic adherends. These treatments are necessary when plastics are to be joined with adhesives. Specific surface treatments for certain plastics and their effect on surface property characteristics are discussed in Sec. 7.6. Details regarding the surface treatment process parameters may also be found in ASTM D-2093 and various texts on adhesive bonding of plastics. An excellent source of information regarding prebond surface treatments is the suppher of the plastic resin that is being joined. 

Table 9.11 lists common recommended surface treatments for plastic adherends. These treatments are necessary when plastics Eire to be joined with adhesives. Solvent and heat welding are other methods of fastening plastics that do not require chemical alteration of the surface. Welding procedures win be discussed in another section of this chapter. The effects of plastic suT ce treatments decrease with time. It is necessary to prime or bond soon after the surfaces are treated. Some common plastic materials that require special physical or chemical treatments to achieve adequate surfaces for adhesive bonding are listed in the following sections. 

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. 

ASTM D2651 describes practices that have proved satisfactory for preparing various metal surfaces for adhesive bonding or coating. Surface preparations for aluminum alloys, titanium alloys, and copper and copper alloys are included in this standard. ASTM D2093 describes recommended surface preparations for plastic adherends. 

The principles involved in producing bonds of optimum strength and durability are the same, irrespective of the nature of the adherends. The surface must be free of contamination, must be receptive to the adhesive (or primer, if used), and the adhesive/adherend interface formed must be stable to environmental exposure. It is the last mentioned factor that is of most concern in a discussion of the durability of adhesive bonds, particularly for metallic adherends. The prebond treatment of the metal surface is of paramount importance in determining joint performance. With wood and plastic adherends, long-term performance is usually determined by other factors, provided that good initial joints are formed. 

The contribution of plastic yielding to the measured peel force has been analyzed for an elastic-plastic adherend (144,145). If the adherend has a thickness greater than about EGJay", where ay is the yield stress, then no plastic deformation occms and the peel force is unaffected. But if the adherend is thinner... 

Further evidence for a critical water concentration comes from Ohno et al. [36] for joints of mild steel bonded to PMMA with an acrylic dental adhesive, immersed in water at 37°C. Water entered the bonds by diffusion through the plastic adherend, and the steel surface inside the joints could be visually examined through the PMMA and adhesive. After immersion, the joints were subjected to 20 thermal cycles between liquid nitrogen (-196°C) and water at 40°C, which showed that the interface was broken by water when its concentration reached 48% of the equilibrium concentration in PMMA. No changes were visible on the steel surface at the 48% water level, but at 95%, small white spots appeared and the surface then gradually turned black due to corrosion. 

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. 

The most common means of joining some plastics is to employ what has been called a plastic dope or plastic cement. The type will vary depending on the plastic adherend to be joined, since the best choice of plastic dope for that adherend will be a solvent blend of that adherend. PVC, ABS, polystyrene, and polycarbonate, for example, form durable bonded joints through the use of solvent adhesives made by blending suitable solvents with solid plastic of the same composition as the adherend to be joined. After the solvent dissipates from the bondline, the joint is virtually continuous plastic across the interface. This is often referred to as a welded plastic joint. 

The most common general method of treating plastics is to abrade the surface and solvent-wipe away the debris. The abrasion tends to remove any detachable contaminants from the surface as well as roughen it, so that mechanical interlocking forces can develop at the interface. The author has shown that, for a variety of engineering-grade structural plastics, this simple surface preparation can adequately prepare the plastic adherends for durable joining... 

S.7.2 Plastic adherends. Many plastics and plastic composites can be treated by simple mechanical abrasion or alkaline cleaning to remove surface contaminants. In some cases, it is necessary that the polymeric surface be physically or chemically modified to achieve acceptable bonding. This applies particularly to crystalline thermoplastics such as the polyolefins, linear polyesters, and fluorocarbons. Methods used to improve the bonding characteristics of these surfaces include... 

For plastics, we do have the D-3929, Practice for Evaluating the Stress Cracking of Plastics by Adhesives Using the Bent-Beam Method. It recognizes that some adhesives may interact with plastic adherends in such a way as to induce areas of weakness leading to stress cracking. 

The removal of all traces of oil and grease from the adherend is essential for nearly all bonding applications. This can be accomplished using one or other of the following procedures. If plastic adherends are being used then careful consideration of both solvent and temperature must be made. 

Conventional X-ray techniques are of little use on metal-to-metal bonded joints since the polymeric adhesive is much less dense than the adherends. Metallic fillers can be used to enhance the contrast and show tapering or voids. However, the density of fibre reinforced plastics adherends is of a similar order to the adhesive and so X-rays can be used, by choosing a suitable energy and flux. For honeycomb-cored panels. X-rays are used for checking the position of the core and whether it has been locally crushed or otherwise damaged. 

This chapter describes the methods used for the preparation of specific plastic adherends, emphasizing practical techniques. These procedures provide relatively simple ways to obtain strong reproducible adhesive bonds that readily fit in commercial processes. To add to the usefulness of this chapter, the author has made liberal citation of commercial plastics when describing treatment techniques. It is difficult to describe surface preparation methods of specific plastics without identifying them. After all, plastic manufacturers are the most frequent sources of basic preparation methods. 

To achieve successful coating adhesions, coating formulations must have chemistries which are covalently bondable to the chemical surface properties of the plastic adherend to be coated. In order to meet this objective it is necessary to determine the substrate s receptivity to thermal or non-thermal pretreatment, as well... 

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