Dissimilar materials, joining adhesive bonding

In adhesive bonding process, material, termed as substrate, is joined by placing and solidifying an adhesive between the adherend surfaces to produce an adhesive joint. It is also possible to join similar as well as dissimilar materials by adhesively bonded joint. Proper surface preparation must be taken care of the substrates prior to bonding for better durability of the joint [6],... 

Plastics that are readily bonded with induction methods include aU grades of ABS, nylon, polyester, polyethylene, polypropylene, and polystyrene, as well as those materials often considered more difficult to bond such as acetals, modified polyphenylene oxide, and polycarbonate. Reinforced thermoplastics with filler levels up to 65 percent have been joined successfully. Many combinations of dissimilar materials can be bonded with induction welding processes. Table 8.5 shows compatible plastic combinations for electromagnetic adhesives. Thermoset and other nonmetallic substrates can also be elec-tromagnetically bonded. In these applications the bonding agent acts as a hot-melt adhesive. 

Bonding permits the joining of dissimilar materials, and since adhesives are generally dielectric materials, their use minimizes the possibility of electrolytic corrosion when different metals are joined. 

A variety of techniques is available for joining plastics to themselves or to other materials—mechanical fastening, welding, and bonding with adhesives—and all are used extensively. Each method has both advantages and drawbacks for plastics, bonding might be said to be preferable when they are to be joined with dissimilar materials. 

Dissimilar materials, including plastics, are readily joined by many adhesives, provided that proper surface treatments are used. Adhesives can be used to join metals, plastics, ceramics, cork, rubber, and combinations of materials. Adhesives can also be formulated to be conductive. The focus of this book is on adhesives for bonding plastics, thermosets, elastomers, and metals. 

Cementing n. Joining plastics to themselves of dissimilar materials by means of solvents (dopes, or chemical cements. Dope adhesives See solvent cementing), comprise a solvent solution of a plastic similar to the plastic to be joined. Chemical cements, the only type suitable for thermosetting plastics, are based on monomers or semi-polymers or semi-polymers that polymerize in the joint to form a strong bond. 

Cyanoacrylates have wide appeal as industrial product assembly adhesives because they rapidly form strong bonds on so many dissimilar materials. They are particularly useful in joining rigid, impervious substrates which will not be subjected to high heat, high impact, outdoor use, or long-term moisture exposure. They are also useful for bonding flexible adherends to... 

Dissimilar materials can be joined, e.g., aluminum-to-paper, iron-to-copper. When two metals are bonded, the adhesive separates them and prevents corrosion. When the two adher-ends are markedly different in coefficient of thermal expansion, a flexible adhesive lessens the stress due to temperature change. Laminates of dissimilar materials can give combinations superior to either adhered alone for example, a polyethylene-cellophane composite has the heat-sealability and water-resistance of the former plus the grease-resistance and print-ability of the latter. 

The ability to join dissimilar materials De Bruyne originally had in mind the use of adhesives to join combinations of metal, wood and Gordon Aerolite. However, it is just as valid today when considering the structural bonding of the range of fibre reinforced composites that are now used in... 

Joining of similar or dissimilar materials (adherent) by the application of a natural or synthetic substance (adhesive) to their mating surfaces which subsequently cures to form a bond (see 7.15F). 

The last two items of the earlier list involve the behaviour of primers, in part chemical, in part physical, as forms of adhesives sticking the bulk adhesive to the actual metal, wood or other adherend. When dissimilar materials are to be joined, an adhesive suitable for one may not be so for the other. In these circumstances a primer for one of the substrates is chosen to be both suitable for it and compatible with the adhesive. Phenol-formaldehyde condensation products are often thus used particularly when the adhesive itself contains such material dispersed in another polymer, e.g. Redux775 (Bonded Structures, Ltd) or various epoxy-phenolic or nitrile-phenolic adhesives. Most important members of the class of coupling agents are the reactive silanes (siloxanes) and titanates. These materials have achieved their main use in the treatment of glass fibres for the manufacture of glass reinforced plastics. 

In many non-structural applications, adhesives are used to bond engineering plastics without the need for any surface preparation and indeed this brings out the benefit of using the adhesive, especially when bonding dissimilar materials (where ultrasonic welding or other joining methods may not be possible). 

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