TYPES of ADHESIVES
Adhesives join materially and dimensionally different materials. Adhesives can penetrate deep groves, closely spaced sides and other difficult to access forms to create a joint. Adhesives are very low viscosity or thin body materials so take up very little space. Even their overlapping joints do not add much to the thickness of the assembly. Adhesives allow uniform stress distribution, unlike screws and nuts which create localized stress points. Adhesive Joints may be designed as required, to be elastic or rigid. Adhesive joints can be demountable and very clean on removal.
Adhesives form a joint at ambient temperature and at very low processing temperature. Adhesive joining does not affect crystallographic structure of metals or plastics. Adhesives can create very extensive, multi layered laminar compositions without physically cutting or puncturing the materials.
Adhesives bonding are surface sensitive joining system. Adhesives require elaborate surface treatments, specific application conditions, curing procedures and sometimes considerable expense of time for setting. Screws, nut-bolts and other mechanical fixings nearly involve substantial mass of the material being joined, so often can join materials with weak surface components. But for adhesive bonding the integrity of the surface component, with the rest of the material is extremely important. Inspection of the joint is difficult. Joint design becomes very critical, compared with other mechanical and thermal processes. The adhesive itself may corrode the materials it is joining, or induce stresses during curing.
Many of the adhesive products are used as sealants, masking pastes, caulking compounds, bulk fillers, water proofing materials, cracks and crevices filler, temporary to permanent holding compounds (such as for metal sheets, glass, etc.).
Adhesives have many formulations and forms, both of which are closely linked to the technique of applications, material surface types and shape configurations. Adhesives can be broadly defined by what they join, such as paper, wood, metals-ceramics-glasses, fibers-fabrics, plastics-elastomers, and biological entities like skin-bones-hairs. Adhesive joining technologies also are a method of categorization, such as liquid-paste spread, hot glues, tapes or patches, ambient temperature curing, baking, heat curing, fusing, surface softening or dissolving, pressure softening, instant bonding, contact adhesive, etc.
One of the largest markets for adhesives is for wood, paper and fibres. Wood adhesives are of basically two types: 1 Used for joining wood components by carpenters and workshops, these are Polyvinyl Acetate (PVA) and Acrylic emulsion-based adhesives, 2 Used for plywood composite manufacturing such as plywoods, particle boards, etc., such as the Amino resins made from Urea, Phenol and Melamine processed in combination with Formaldehyde.
For wood joinery workshops, and packing and labelling fields two major types of adhesives are used, Adhesives based on solvents and based on water. Water-based systems are mainly in the form of emulsions. Water-based formulations of Polyvinyl acetate (PVA), Ethylene vinyl acetates (EVA) and Methacrylate, are widely used. Solvent adhesives include acrylics and styrene-butadiene (SB) latex. Acrylic resins are used for removable and permanent pressure sensitive applications.
Neoprene or Polychloroprene was the first synthetic elastomer adhesive. It is widely used in shoe manufacturing. It is also used as a contact adhesive for attaching large surfaces and very quickly. It provides water and heat resistant bonds. Epoxy adhesives are widely used for structural applications. Epoxies offer reliable and good adhesion to all substrates. Urethane adhesives are environmentally tough, abrasion resistant and flexible adhesives. Instant adhesives such as cyanoacrylates provide an instantaneous bond and are called super glues. These are available as liquids, pastes, and gels, that set at normal or elevated temperatures. These are used for repair work of pipes, toys, ceramics, etc.