Plastics joining adhesive bonding

The major disadvantage of solvent cementing is the possibility of stress cracking or crazing of the part and the possible hazards of using low-vapor-point solvents. When two dissimilar plastics are to be joined, adhesive bonding is generally desirable because of solvent and polymer compatibility problems. 

Additional details on joining plastics by adhesive bonding, direct heat welding, indirect heat welding, frictional welding, solvent cementing, or mechanical fastening can be found in numerous places. The best source of information is often the plastic resin manufacturers themselves. They often freely offer related recipes and processes, because it is in their interest that their materials successfully find implementation in joined components. 

Adhesive bonding is the most versatile of these joining processes, since all classes of plastics and other materials can be joined. Adhesive bonding of plastic involves several of the joining interactions previously described, depending upon the characteristics of the adhesive and the substrates being joined. 

L. J. Hart-Smith, Design of Adhesively Bonded Joints, in Joining Fibre-Reinforced Plastics, F. L. Mathews (Editor), Elsevier Applied Science, London and New York, 1987, pp. 271-311. 

Reference was made earlier to difficulties of wetting plastics surfaces, and to problems in this regard compared with hydrophilic materials like glass, metals, and timber. With some plastics very effective bonds are possible, but as a group, mainly for this reason, they are amongst the most difficult to join by adhesives. 

However, even with these potential difficulties, adhesive bonding can be an easy and reliable method of fastening one type of plastic to itself, to another plastic, or to a nonplastic substrate. Pocius et al. provides an excellent treatise on the use of adhesives in joining plastics.51... 

When two dissimilar plastic foams are to be joined, which is rarely done, adhesive bonding is generally preferable because of solvent and polymer incompatibility problems. Solvents used to cement plastics should be chosen with approximately the same solubility parameter (5) as the plastic to be bonded. The solubility parameter is the square root of the cohesive energy density (CED) of the liquid solvent or polymer. CEDs of organic chemicals are primarily derived from the heat of vaporization and molecular volume of the molecules, and are expressed as calories per cubic centimeter (cal/cm ). Literature sources provide data on 6 s of a number of plastics and resins (2) (3) (4). 

Thermosetting Foam Substrates Most thermosetting plastics are not particularly difficult to bond. Obviously, solvent cementing is not suitable for bonding thermosets to themselves, since they are not soluble. In some cases solvent solutions can be used to join thermoplastics to thermosets. In general, adhesive bonding is the only practical method of joining a thermoset to itself or to a non-plastic material. Epoxies or modified epoxies are the most widely used adhesives for thermosets (1). 

Landrock, A.H., Effects of Varying Processing Parameters in the Fabrication of Adhesive Bonded Joints, Part XVIII, "Adhesive Bonding and Related Joining Methods for Structural Plastics—Literature Survey," Picatinny Arsenal Technical Report 4424 (November 1972). 

Adhesive Bonding - A method of joining two plastics or other materials in which an adhesive is applied to the parts surfaces. Bonding occurs through mechanical or chemical interfacial forces between the adhesive and adherend and/or by molecular interlocking. Surface preparation of the adherends and curing of the adhesive may be required. 

Joints are potential failure sites. This applies whether they are adhesively bonded or mechanically fastened, and whether they join two reinforced plastics sections, or one reinforced plastics component and one constructed from another material. 

With plastic materials, the designer also has a greater choice of bonding techniques than with many other materials. Thermosets must be adhesively bonded or mechanically joined, but most thermoplastics can also be joined by solvent or heat welding. Additionally, plastic parts can be designed for assembly by means of molded-in, snap-fit, press-fit, pop-on, and threaded fasteners so that no additional adhesives, solvents, or special equipment is required. 

Adhesive bonding presents several distinct advantages over other methods of fastening plastic substrates. These are summarized in Table 7.5. Adhesive bonding is often preferred when different types of substrates (e.g., metals to plastics) need to be joined, when high-volume production is necessary, or when the design of the finished part prohibits the use of mechanical fasteners. 

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. 

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