Bonding of Plastics

Plastics are bonded to one another or to other materials. They differ widely in their affinity for bonding. Some plastics are easy to bond (e.g., PVC-U, ABS, PS), whereas others can be bonded only after special preparation (e.g., PTFE). On account of the low polarity and surface tension of certain plastics (e.g., polyolefins) there are still no adhesives for pipe joints which are durably resistant to shear strain and pressure. The choice of adhesive is determined by the substrate and the mechanical, thermal, and chemical requirements [69] - [75]. 

Poly(Vinyl Chloride). Rigid PVC (PVC-U) and chlorinated PVC (PVC-C) pipes, sheets, and films are used worldwide in the construction industry. They are joined by adhesives consisting of PVC solutions in a mixture of solvents, which operate by the principle of diffusion bonding. The bond is established by diffusion of the adhesive into the surfaces and the temporary dissolution of the PVC (cold swell welding). 

PVC adhesives, usually consisting of solutions of post-chlorinated PVC in methylene chloride, are used in the case of inorganic acids as flow-through media in PVC pipe because of their better chemical resistance. Since PVC adhesives have no gap-filling properties a special application technique is required. This priming method is little used today. 

Contact adhesives based on polychlorobutadiene or polyurethane are used for large-area bonds between PVC-U and PVC-C sheets and wood or metal. 

Vessels are often lined with thin PVC-U and PVC-C films (1-2 mm thick). Contact adhesives are used with usual application techniques. For bonding cooling tower films, special adhesive solutions, which are harmonized in viscosity and thixotropy with the processing techniques (roll or pot application), analogous to PVC pipe adhesives, are used. 

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Cellulose Graft Copolymers for Potential Adhesive Applications Bonding of Plastics to Wood... 

Encouraging results on the bonding of plastics to wood using tailor-made cellulose-polystyrene graft polymers as compatibilizers or interfacial agents may offer a new approach to the engineering of wood-plastic products with improved mechanical and physical properties for a variety of applications. It also holds the potential of opening up new markets for renewable resources in the form of woody materials. For example, polystyrene production is currently 3.9 billion... 

Anonymous, "3 Prime Factors in Adhesive Bonding of Plastics," Plastics Design and Processing, 8(6) 10-22 (June 1968). 

On the other hand, especially in bonds of plastics to other materials, the thermal resistance of the plastics and adhesives is equivalent, for which reason such applications are bound to grow in importance. 

With their close resemblance to the properties of most plastics, polyurethanes are particularly well-suited for bonding of plastics such as PC and PMMA. The adhesives are ductile, have a low glass transition temperature, a lower inherent strength level than EP adhesives, and a highly variable potlife. 

Polyvinyl acetate and polyvinyl acetate copolymers (EVA, PVAC for low-cost adhesive bonding of plastics to wood, paper, leather, and metals)... 

Owing to their limited thermal resistance, bonding of plastics, in particular of thermoplastics, requires certain precautionary measures to avoid deformation of the adherends (use hot-melt adhesives with low processing temperature, e.g., on a polyamide basis). 

Grinding, brushing or sanding (with the exception of the above-mentioned Saco method) do not cause chemical modifications of the material s surface. A clean surface results with a characteristic structure corresponding to the composition of the material, as shown in Figure 7.6. Therefore, physical and chemical pretreatment methods are aimed at the chemical modification of the surfaces. Thus, on the one hand it is possible to further enhance the adhesive forces for extremely high demands on bonded joints, and on the other hand, to make poorly bondable material (e.g., plastics) bondable at all. Since physical methods are mainly used in bonding of plastics, they are described in Section 9.2.4. 

For bonding of plastics, some supplementary information on their behavior in comparison to metals is required. The essential difference is the fact that metals are generally insoluble in organic solvents. Various plastics, however, especially thermoplastics, are soluble in such solvents or at least swellable in the surface area. The result is a particular way of bonding, which is not possible with metals (Section 9.2.5). 

A precondition for the bonding of plastics is the knowledge of the adhesive characteristics. Trade names and product marking according to Table 9.1 help identify the adhesive in the case of missing indications, it is difficult or almost impossible for laymen. The following two criteria may serve to distinguish between thermoplastic and thermoset material ... 

The diffusion bonding method described in Section 9.2.5 cannot be applied to the bonding of plastics to metals, since the metal surfaces are not swellable by means of organic solvents. Bonded joints are only possible after an adequate surface treatment with the known reactive adhesives. 

Rule 2 is not always applicable to bonding of plastics. As mentioned in Section 2.1.1, cured adhesive layers can be compared to plastics as far as their strength is concerned. Then, under tensile stress, the adhesive layer would not be the weakest link in case (a) of Figure 11.4, since adherends and adhesive layer have comparable strengths. Thus, such butt joints are possible and also common for bonded plastic joints. 

Urethane structural adhesives have been very popular for bonding of plastics to plastics and plastics to metal by the transportation industry. As an example, more than 90% of the bonds in Fords experimental graphite car were urethane adhesives rather than rivets or welds. Urethane structural adhesives have been used by the transportation industry since 1969. It is estimated that 5 million pounds of urethane structural adhesives were used in the transportation industry in the United States in 1979. Some of the advantages of urethane adhesives as used by the transportation industry are shown in Table IX. 

Table 2.9 gives a Kst of various adhesives and typical applications in bonding of plastics. This table is not complete, but it does give a general idea of what types of adhesives are used and where. It should be remembered, however, that thousands and thousands of variations of standard adhesives are available off the shelf. The computer may shape up as an excellent selection aid for adhesives. Selection is made according to the combination of properties desired, tack time, strength, method of appKcation, and economics (performance/cost ratio). 

Melt adhesives are based on thermoplastics, but usually contain a number of other components. The most commonly used melt adhesives are based on ethylene-vinyl acetate (EVA) copolymers, but polyethylene, polyesters, polyamides, and thermoplastic rubbers (e.g., styrene-butadiene block copolymers) are also used (see Adhesive Bonding of Plastics in Chapter 2). 

The adhesive bonding of plastic assembly components such as instrument panels, spoilers, spare wheel boxes, roof parts, trim assembles, and fenders is generally with one- or two-component polyurethanes. Frequently, a primer is used as pretreatment to improve adhesion. Adhesive tape systems can provide good results for the bonding of dash panels, trim lines, insignia parts, and rear view mirrors, for example. 

Window lifter rails Roofs and sun roofs Rigid roof linings Bonding of plastic components Bonnets Tailgates... 

Haberer, C., Adhesive Bonding of Plastics to Metal in the Automotive Industry, Science and Technology, Industry-University Short Course Program, CEI Europe, 1984, Chap. 11. 

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. 

Factors most likely to influence adhesive selection are listed in Table 7.13. However, thermosetting adhesives such as epoxies, polyurethanes, or acrylics are commonly used for structural application. The adhesive formulations are generally tough, flexible compounds that can cure at room temperature. The reasons that these adhesives have gained most popularity in bonding of plastics are summarized in this section. 

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