Adhesives are a substance able of holding materials together. A joint is formed among two solids is called adherends with a thin layer of adhesive among them. Mainly adhesives are made via some kind of plastic either in emulsions, solutions, liquid, powder or paste form that is allowed to solidify and harden within curing process. Adhesives have distinction of making joints in between similar and dissimilar materials. Plastic to glass and or glass to metal and metal wood can be joined via adhesives even if the strength of these joints is much less than those joints as welds. Though, the adhesive joints are effectively strong in shear and tension but weak against peeling or cleavage. This may also be maintain in sense that the strength of polymeric adhesive joint is considerably impaired because of presence of dust, grease, oil or rust. The surfaces to be joined should be as soft as possible whether only thin layer of adhesive will be necessary. Thinner the adhesive layer stronger will be the joint. The thin layers are void creep and free less. The adhesive film is generally 0.1 millimeter thick.
The adhesives are made from elastomeric, thermosetting and thermoplastic polymers. Thermoplastic adhesives are prone to loss of strength as like their soft at high temperatures. Temperature close to 100oC may not be allowed for such adhesives. Thermosetting adhesives may be employed at slightly higher temperatures. These adhesives do not soften on high temperature but until lose strength. Elastomeric adhesives are fewer sensitive to temperature even if they are similar to thermoplastic adhesives. Improved results have been acquired by blending various resins in several proportions. That type of adhesive blends is called as adhesive alloys.
The adhesives that are strong bonding and less creeping are consider as structural adhesive. Epoxy based adhesives available like liquid, solid or past and epoxy blends along with phenolics, polysulfide or nylon resins are the examples of structural adhesives. Phenolic adhesives were illustrated to be utilized between wood veneer for their resistance to water and mould growth in Section of Natural Polymers. For joining wood melamine formaldehyde mixed along with urea formaldehyde is preferably employed. This adhesive is usually available in powder form and is reconstituted by mixing along with water before application.
The adhesives based upon elastomers and thermoplastics are weaker than those based upon thermosetting plastics. They are mostly called non-structural adhesives. A lot of these, particularly elastomeric adhesives, and contain good peel strength and impact strength. Polyurethane polyster and silicone are not many bases of such adhesives.
In addition to synthetic or polymeric adhesives declared above, natural adhesives as like: starch, soya flour, animal products and dextrin and inorganic adhesives as like: sodium magnesium and silicate oxychloride are also employed. Both these classes of adhesives are however, less in strength as compared to synthetic organic adhesives.