Joining mechanisms Bonding, solvent

The solvent-borne NR adhesives show an important mechanical component in the bonding process, and therefore bulk mechanical and rheological properties (addition of fillers is quite effective) are important. In fact, these adhesives are mainly suitable when at least one of the surfaces to be joined is water-porous (paper, concrete, leather, textiles). 

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. 

Thermosetting plastics (e.g., epoxies diaUyl phthalate polyesters melamine, phenol and urea formaldehyde and polyurethanes) are joined either mechanically or by adhesives. Their thermosetting nature prohibits the use of solvent or thermal welding processes however, they are easily bonded with many adhesives. 

Certain thermoplastic substrates may be joined by methods other than mechanical fastening or adhesive bonding. By careful application of heat or solvent to a thermoplastic substrate, one may liquefy the surface resin and use it to form the bond. With thermal or solvent welding, amface preparation is not as critical as with adhesive bonding. The bond strength is determined by diffiision of polymer from one surface into another instead of by the wetting and adsorption of an adhesive layer. 

Polyimide parts can be joined with mechanical fasteners. Self-tapping screws must be strong enough to withstand distortion when they are inserted into the polyimide resin which is very hard. Polyimide parts can be bonded with epo adhesives. Only abrasion and solvent cleaning is necessary to treat the substrate prior to bonding. Tbe plastic part will usually have a hi er thermal rating than the adhesive. Thermosetting polyimides carmot be heat welded or solvent cemented. 

Adhesive bonding Adhesive bonding uses a separate material at the joint interface, which binds either chemically or mechanically to the substrate. The adhesive may be chemically or thermally reactive or may bond on evaporation of a carrier (water or solvent). Most synthetic or natural fabrics (woven and nonwoven) independent of synthetic fibre content. It can also be used to join garments to nontextile accessories (glitter, foil, etc.). 

Thermoset plastics are cross-linked by chemical reaction so that their molecules cannot slip by one anotber. They are rigid when cool and cannot be softened by the action of heat. If excessive heat is applied, thermoset plastics will degrade. Consequently, they are not weldable. Because of their chemical resistance, they cannot be solvent cemented. Thermoset plastics are usually joined by either adhesive bonding or mechanical fastening. Typical thermosetting plastics are epoxy, urethane, phenolic, and melamine formaldehyde. 

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. 

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