Thermoplastics solvent welding

Thermoplastics. There are five elastomeric membranes that are thermoplastic. Two materials, chlorinated polyethylene (CPE) and polyisobutylene (PIB), are relatively obscure. Thermoplastic materials can be either heat-fused or solvent-welded. In contrast to Hypalon and uncured EPDM, this abiHty to fuse the membranes together remains throughout the life of the material. However, cleaning of the membrane surface after exposure to weather is required. Correct cleaning procedures for specific membranes are available from the individual manufacturer. 

Often it is necessary to join two or more components of plastics to produce a particular setup or to repair a broken part. For some thermoplastics solvent welding is applicable. The process uses solvents which dissolve the plastic to provide molecular interlocking and then evaporate. Normally it requires close-fitting joints. The more common method of joining plastics, however, is to use heat, with or without pressure. Various heat welding processes are available. Those processes in common commercial use are described here. 

Solvents can be used to soften thermoplastic materials, which, if placed together, will then completely fuse when the solvent evaporates. The main disadvantage of solvent welding is the risk of some of the solvent reaching surfaces other than those being joined and leaving a mark. 

Diffusion. This theory proposes that adhesive macromolecules diffuse into the substrate, thereby eliminating the interface, and so can only apply to compatible polymeric substrates. It requires that the chain segments of the polymers possess sufficient mobility and are mutually soluble. The solvent welding of thermoplastics such as PVC (polyvinyl chloride), softened with a chlorinated solvent, is an example of such conditions being met. Diffusion will also take place when two pieces of the same plastic are heat-sealed. The joining of plastic service pipes for carrying gas and water makes use of the diffusion mechanism. 

Solvents, or solvents containing small amounts of hodying resin, are used for bonding thermoplastic resins and film adhesives. An example is toluol, which can he used to soften and dissolve polystyrene molded articles to allow joining the softened pieces. Ketones can be used to bond PVC films in a similar manner. A small amount of resin can be used to thicken the solvent so that a sufficient amount would stay in place to dissolve the substrate. It should be noted, however, that solvent welding of molded plastics can cause stress cracking and weakening of the structure as the parts age. ... 

Welding processes are of two main types thermal and solvent. By careful application of heat or solvent to a thermoplastic substrate, one may liquefy the surface resin and use it to form the bond. The bond strength is determined by diffusion of polymer from one snr-face into another instead of by the wetting and adsorption of an adhesive layer. It is possible to weld plastics of different types. However, for both thermal and solvent welding, the success of the process will be heavily determined by the compatibility of the polymers being joined. 

Compatibility is an issue of relevance in a number of aspects of adhesion. Perhaps, the most obvious is the selection of solvents in the making of a Solvent-based adhesive, or for Solvent welding of thermoplastics. In both cases, solubility parameters can be employed. 

Solvent welding and solvent cementing are widely used techniques for the joining of thermoplastic polymers. A small amount of solvent along the joint promotes interdiffusion of polymer chains between the substrates. This creates a permanent weld, with no additional phases or potentially weak interfaces. A solvent cement is a solution of the polymer being joined. This also promotes interdiffusion, and leaves a single-phase joint when the solvent has evaporated. 

Solvent bonding is suitable for all amorphous plastics. It is used primarily on ABS, acrylics, cellulosics, polycarbonates, polystyrene, polyphenylene oxide, and vinyls. Solvent welding is not suitable for crystalline thermoplastics. It is not effective on polyolefins, fluorocarbons, or other solvent-resistant polymers. Solvent welding is moderately effective on nylon and acetal polymers. Solvent welding cannot be used to bond thermosets. 

Methylene chloride is employed in the manufacture of polycarbonate resin used for the production of thermoplastics. It is used as a solvent in the production of cellulose triacetate which serves as a base for photographic film. Other applications include its use in the solvent welding of plastic parts, and as a releasing agent to prevent the manufactured part from permanently bonding to the mold. 

Many thermoplastics can be welded together using organic solvents or combinations of solvents. In solvent welding, the solvent penetrates between closely fitted parts, dissolves the surface, and causes them to fuse together. When the solvent dries, strong, durable bonds are formed based on the interpenetration of the polymer chains. 

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