Plasticity, adherend adhesive

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. 

In addition, the adherend strength is an important influence, which in many plastics accounts for only approximately 10% of the strength of metal materials. Since, due to the chemical relatedness of plastics and adhesive layers, the same or similar strength values can be assumed, butt joints are feasible, in contrast to metals (Chapter 11, 2nd rule). 

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. 

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. 

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. 

Modified or second generation acrylic engineering adhesives are more complicated than those described above. In modified acrylic adhesives, polymerization between adherends leads to impact resistant plastics with adhesion to the adherends. The following formulation of Brigas and Muschiatti illustrates an early modified acrylic engineering adhesive ... 

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... 

Petrie EM. Handbook of plastics and elastomers. New York, NY McGraw-Hill 2000. DeLollis NJ. Adhesives, adherends, adhesion. 3rd ed. Huntington, NY Robert E. Krieger Publishing Co. Inc 1985. 

Fig. 2. Schematic of energy dissipation in a commonly used peel test. The energy dissipation can occur in the adhesive and/or the adherends. The extent of energy dissipation depends on the elasto-plastic properties of the adhesive and the adherends under the test conditions as well as the local stresses and strains near the crack tip.

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