Adhesives can be grouped into five classifications based on chemical composition. Natural adhesives include vegetable- and animal-base adhesives and natural gums. They are inexpensive, easy to apply, and have a long shelf life. They develop tack quickly, but provide only low strength joints. Most are water soluble. They are supplied as liquids or as dry pow-ders to be mixed with water. Casein-latex adhesive is an exception. It consists of combinations of casein with either natural or synthetic rubber latex. It is used to bond metal to wood for panel construction and to join laminated plastics and linoleum to wood and metal. Except for this type, most natural adhesives are used for bonding paper, cardboard, foil, and light wood.

Thermoplastic adhesives can be softened or melted by heating and hardened by cooling. They are based on thermoplastic resins (including asphalt and oleoresin adhesives) dissolved in solvent or emulsified in water. Most of them become brittle at subzero temperatures and may not be used under stress at temperatures much above l50°F (65°C). Being relatively soft materials, thermoplastic adhesives have poor creep strength. Although lower in strength than all but natural adhesives and suitable only for noncritical service, they are also lower in cost than most adhesives. They are also odorless and tasteless and can be made fungus resistant.

Elastomeric adhesives, based on natural and synthetic rubbers, are available as solvent dispersions, latexes, or water dispersions. They are primarily used as compounds which have been modified with resins to form some of the adhesive "alloys" discussed below. They are similar to thermoplastics in that they soften with heat but never melt completely. They generally provide high flexibility and low strength and, without resin modifiers, are used to bond paper and similar materials.

Thermosetting adhesives soften with heat only long enough for the cure to initiate. Once cured, they become relatively infusible up to their decomposition temperature. Although most such adhesives do not decompose at temperatures below 500°F (260°C), some are useful only to about 150°F (65°C). Different chemical types have different curing requirements. Some are supplied as two-part adhesives and mixed before use at room temperature; some require heat and/or pressure to bond.As a group, these adhesives provide stronger bonds than the other three groups. Creep strength is good and peel strength is fair. Generally, bonds are brittle and have little resilience and low impact strength.

Alloy adhesives are adhesives compounded from resins of two or more different chemical families, e.g., thermosetting and thermoplastic, or ther- mosetting and elastomeric. In such adhesives the performance benefits of two or more types of resins can be combined. For example, thermosetting resins are plasticized by a second resin resulting in improved toughness, flexibility, and impact resistance.

Paste adhesives are usually water solutions of starches or dextrins sometimes mixed with gums, resins, or glue to add strength, and containing antioxidants. They are the cheapest of the adhesives, but deteriorate on exposure unless made with chemically altered starches. They are widely employed for the adhesion of paper and paperboard. Much of the socalled vegetable glue is tapioca paste. It is used for the cheaper plywoods,postage stamps, envelopes, and labeling. It has a quick tack, and is valued for pastes for automatic box-making machines. Latex pastes of the rub-off type are used for such purposes as photographic mounting, as they do not shrink the paper as do the starch pastes. Glues are usually water solutions of animal gelatin, and the only difference between animal glues and edible gelatin is in the degree of purity. Hide and bone glues are marketed as dry flake, but fish glue is liquid. Mucilages are light vegetable glues, generally from water-soluble gums.

Rubber cements for paper bonding are simple solutions of rubber in a chemical solvent. They are like the latex pastes in that the excess can be rubbed off the paper. Stronger rubber cements are usually compounded with resins, gums, or synthetics. An infinite variety of these cements is possible, and they are all waterproof with good initial bond, but they are subject to deterioration on exposure, as the rubber is uncured. This type of cement is also made from synthetic rubbers which are self-curing. Curing cements are rubber compounds to be cured by heat and pressure or by chemical curing agents. When cured, they are stronger, give better adhesion to metal surfaces, and have longer life. Latex cements are solvent solutions of rubber latex. They provide excellent tack and give strong bonds to paper, leather, and fabric, but they are subject to rapid disintegration unless cured. In general, natural rubber has the highest cohesive strength of the rubbers, with rapid initial tack and high bond strength. It also is odorless. Neoprene has the highest cohesive strength of the synthetic rubbers, but it requires tackifiers. Gr-S rubber (styrene-butadiene) is high in specifr adhesion for quick bonding, but has low strength. Reclaimed rubber may be used in cements, but it has low initial tack and needs tackifiers.

Pyroxylin cements may be merely solutions of nitrocellulose in chemical solvents, or they may be compounded with resins, or plasticized with gums or synthetics. They dry by the evaporation of the solvent, and have little initial tack, but because of their ability to adhere to almost any type of surface they are called household cements. Cellulose acetate may also be used. These cements are used for bonding the soles of womens shoes. The bonding strength is about 10 lb/in² (0.07 MPa), or equivalent to the adhesive strength of the outer fibers of the leather to be bonded. For hotpress lamination of wood the plastic cement is sometimes marketed in the form of thin sheet.

Polyvinyl acetate-crotonic acid copolymer resin is used as a hot- dipadhesive for book and magazine binding. It is soluble in alkali solutions,and thus the trim is reusable. Polyvinyl alcohol, with fillers of clay and starch, is used for paperboard containers. Vinyl emulsions are much used as adhesives for laminates.

Epoxy resin cements give good adhesion to almost any material and are heat-resistant to about 400°F (204°C). An epoxy resin will give a steel-to-steel bond of 3,100 lb/in² (22 MPa), and an aluminum-to-aluminum bond to 3,800 lb/in² (26 MPa). Some pressure-sensitive adhesives are mixtures of a phenolic resin and a nitrile rubber in a solvent, but adhesive tapes are made with a wide variety of rubber or resin compounds.

Furan cements, usually made with furfural-alcohol resins, are strong and highly resistant to chemicals. They are valued for bonding acid-resistant brick and tile.

Structural adhesives have come to mean those adhesives used to bond metals to other metals, to wood, or to rigid plastics, where bond strength is a critical requirement. They are generally of the alloy or thermosetting type. Three of the most commonly used are the modified epoxies, neo-prene-phenolics and vinyl formal-phenolics. Modified epoxy adhesives are thermosetting and may be of either the room-temperature curing type,which cure by addition of a chemical activator, or the heat-curing type.They have high strength and resist temperatures up to nearly 500°F (260°C). Neoprene-phenolic adhesives are alloys characterized by excellent peel strength, but lower shear strength than modified epoxies. They are moderately priced and offer good flexibility and vibration absorption. Vinyl formal-phenolic adhesives are alloys whose properties fall between those of modified epoxies and the thermoset-elastomer types. They are supplied as solvent dispersions in solution or in film form.

Acrylic adhesives are solutions of rubber-base polymers in methacrylate monomers. They are two-component systems and have characteristics similar to those of epoxy and urethane adhesives. They bond rapidly at room temperature, and adhesion is not greatly affected by oily or poorly prepared surfaces. Other advantages are: low shrinkage during cure, high peel and shear strength, excellent impact resistance, and good elevated temperature properties. They can be used to bond a great variety of materials, such as wood, glass, aluminum, brass,-copper, steel, most plastics, anddissimilar metals.

Ultraviolet cure adhesives, of Loctite Corp., are anaerobic structural adhesives formulated specifically for glass bonding applications. The adhesive remains liquid after application until ultraviolet light triggers the curing mechanism.

A ceramic adhesive developed by the Air Force for bonding stainless steel to resist heat to l500°F (816°C) is made with a porcelain enamel frit iron oxide, and stainless-steel powder. It is applied to both parts and fired at l750°F (954°C), giving a shear strength of 1,500 lb/in² (10 MPa) in the bond. But ceramic cements that require firing are generally classed with ordinary adhesives. Wash-away adhesives are used for holding lenses,electronic crystal wafers, or other small parts for grinding and polishing operations. They are based on acrylic or other low-melting thermoplastic
resins. They can be removed with a solvent or by heating.

Electrically conductive adhesives are made by adding metallic fillers, such as gold, silver, nickel, copper, or carbon powder. Most conductive adhesives are epoxy-based systems, because of their excellent adhesion to metallic and nonmetallic surfaces. Silicones and polyimides are also frequently the base in adhesives used in bonding conductive gaskets to housings for electromagnetic and radio-frequency interference applications.