Structural adhesives are capable of sustaining load-bearing joints for a long period of time. They are composite systems containing several components that are available in various forms, including liquids, pastes, cartridges and films. The major factors in the selection of an adhesive for a specific application are the characteristics of the materials being joined and operational requirements, viz., strength, temperature variation, moisture resistance and whether disassembly will be required for inspection, repairs or replacement.
The most widely used structural adhesives are epoxies, acrylics, anaerobics, cyanoacrylate, silicones, and urethane.
Epoxy resin, a reaction product of acetone and phenol, are the most popular adhesives used today. Epoxies may be cured (cross-linked) by the addition of catalyst, (hardener) resulting in a room-temperature cure or by exposure to heat. Cross-linking between polymer chains adds strength, heat and solvent resistance. Most structural adhesives are cross-linked.
One of the advantages of the epoxies is that they consist of 100 % solids, so there is no problem of solvent evaporation and shrinkage when joining impervious surfaces. All the basic desirable qualities of epoxies namely, high adhesion, tensile strength, rigidity, creep resistance, easy curing, and heat resistance can be enhanced by additives. New formulations are being constantly created to meet special needs.
Slow epoxies need 24 hours, but fast ones, now available, harden in an hour or two. Incidently, heat speeds up hardening. Put the articles in a plastic bag and keep in the sun or keep the article near an electric bulb for faster curing.
Like epoxy, urethane adhesives are available in one or two part systems as solid hot melts, liquids or pastes or as solvent systems. The basic urethane is a thermoplastic material and is often used in that form. The distinguishing feature between urethane or polyurethane and epoxy is its basic flexibility and high peel strength. It is usually cured at room temperature. It is preferable to epoxy when greater elongation or shorter cure time is needed, or if the joint is to be subjected to prolonged vibration or other cyclic stresses.
The new generation acrylics offer flexible bonds with superior peel and impact resistance and are challenging epoxies for bonding supremacy. They are two-part adhesives, but are not mixed for most applications. The resin is applied to one surface and the accelerator to the other. Once they are mated, handling strength is typically achieved in a few minutes. Curing can be done at room temperature.
The prime advantage of these adhesives is their rapid are usually within seconds. Cure is brought about by contact with moisture or akali. Almost any surface has sufficient moisture to effect a cure. They are costly that amount used is small since a very thin blue line is required. The adhesive is in and watery in its basic form which allows it to wick into tight spaces by capillary action but limits the gap-filing ability to about 0.13 mm. The accelerators-primers increase the practical gap-filing ability to about 0.75 mm. Usede for crack injection routing.
Anaerobic adhesives are so called because they harden satisfactorily only in the absence of atmospheric oxygen. Hence they must be stored in the presence of air. When in liquid form, they are capable of flowing into the most crevices, and thus whole of free space between assembled components is minute reading and completely filled with fluid. After the fluid has cured and formed tough wedge, the assembly is sealed because there is virtually no shrinkage and therefore, no passage for gases or liquids.
Instant adhesives are best for joining broken pieces of articles, which can contour into each other. When things break clean and without chips, this is the glue you can use. This glue must not touch your thumb or finger, because if they pressed each other, they would join tight. It is to be noted that these adhesives only harden if air is excluded as would happen if two faces fit snug into each other. If air could leak in between the faces of the articles to be jointed I e. into the joint itself, this glue will not work.
6. Hot Melts
They may be formulated to form flexible or rigid bonds and offer sufficient strength for low stress assembly applications. These adhesives are copolymers of polyethylene with poly vinyl acetate, polypropylene nylon, or polyester.
7. RTV Silicone Adhesives/Sealants: Described in separate topics
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