Joining of Thermoplastics

Thermoplastic materials are joined by either solvent cementing, thermal fusion, or by means of adhesives. Solvent cementing is the easiest and most 

UHMWPE Ultra high molecular weight polyethylene 

The major disadvantages of solvent cementing are the possibility of the part s stress cracking and the possible hazards of using low vapor point solvents. Adhesive bonding is generally recommended when two dissimilar polymers are joined because of solvent and polymer compatibility problems. 

A universal solvent should never be used. Solvent cements that have approximately the same solubility parameters as the polymer to be bonded should be selected. Table 2.7 lists the t)tpical solvents used to bond the major polymers. 

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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. 

The most important application of hot-plate welding is in the joining of thermoplastic water, gas and effluent pipes, where it is often referred to as butt-fusion welding. It is also used by the automotive industry in the manufacmre of fluid reservoirs and in the welding of PVC door and window frames. 

Hot-bar welding is widely used in industry, mainly in the joining of thermoplastic films having a thickness of less than 0.5 nun. It could be used to weld thin composites and has the potential to tack prepregs. It is used extensively within the packaging industry for producing plastic bags. 

All the advantages and disadvantages of the discussed procedures are summarized in Table 1. Since reliable data for composites do not exist up to now, the assessment of the different procedures is mainly based on data on joining of thermoplastics without reinforcing structures. This was extended by our preliminary results achieved on reinforced thermoplastics. 

Jauss M, Emmerich R and Eyerer P (1997) Joining of thermoplastic composites by bolts and microwave welding, in Eleventh International Conference on Composite Materials (Ed. Scott, M L), Woodhead Publishing, Cambridge, UK, VI-65-VI-73. 

Kagan, V., Innovations in Laser Welding Technology State of the Art in Joining of Thermoplastics and Advances with Coloured Nylon for Automotive Applications , SAE World Congress Proceedings, Paper 2002-01-0716 (2002)... 

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

RF (radio frequency) welding Utilizes specific bands of radio frequency waves which are directed through specially constructed tooling to form localized melting/joining of certain dielectric thermoplastic materials. Can be used to form hermetic seals. Also known as high frequency or dielectric welding. 

The reinforcement of thermoplastics complicates welding, solvent joining and adhesive bonding. 

Heat sealing may be defined as the joining together of thermoplastic film by the application of heat and pressure. In practice, although it is a general term (known also as thermal sealing ) usually it refers to a technique in which either one or a pair of temperature-controlled elements is employed. 

Unlike the thermosetting resins, the thermoplastic resins will soften on heating or on contact with solvents. They will then harden on cooling or on evaporation of the solvent from the material. This is a result of the noncrosslinked chemical structure of thermoplastic molecules. The following are important characteristics of thermoplastic resins that can affect their joining capability. 

Heat sealing is usually applied to the joining of pliable plastics sheet (less than 50 mils thick) and is limited to use on thermoplastic materials. The heat may be provided by thermal, electrical, or sonic energy. A wide variety of heat sealing systems are available. 

Acyl cations are involved as propagating species in the synthesis of poly-(ether ketone)s. Poly (ether ketone)s are a class of thermoplastic crystalline polymers that have many desirable properties that make them useful as high-performance engineering materials [153,154]. The poly(ether ke-tone)s with the most useful properties are actually para-linked poly(aryl-ether ketone)s (PAEKs). They have excellent chemical resistance to oxidation and hydrolysis, high thermal stability, and many useful mechanical properties. Unlike some other materials with similar properties they are readily melt processable using conventional equipment. In addition, their mechanical properties are not affected deleteriously by most solvents. These polymers are usually crystalline. PAEKs contain arene groups joined by ether and carbonyl linkages. For example, two commercial poly-(ether ketone)s are PEK and PEEK (Fig. 36). 

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. 

The use of plastics and their composites is rapidly increasing in numerous areas. However, the final assembly of products is often limited by the capability of existing joining techniques. The ability of ICPs, especially polyaniUnes, to absorb electromagnetic radiation and convert it into heat introduces another application in the welding of thermoplastics and thermosets [210]. 

Evidence suggests that the failure behavior of thermoplastics is much the same as for thermoset composites. High joint efficiencies can be obtained with suitable consideration to the joint design, fastener type, and environmental factors. In addition to mechanical and adhesive joining, thermoplastic composites can also be heat welded depending on the concentration and type of fillers within the composite. 

A method of joining similar forms of thermoplastic materials using heat. 

In the seaming of articles, use of hot melts is limited to relatively simple operations such as joining of straight edges. The thermoplastic material can be incorporated into the fabric or there is a wide range of films, tapes, nets and even coaled threads that can be sandwiched between layers of non-thermoplastic material. 

Seam welding n. Any stitchless procedure for joining fabrics based on the use of thermoplastic resins or the direct welding of thermoplastic materials. Seam welding is an alternative to conventional needle-and-thread seaming operations that is extremely popular in the non-woven field. 

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