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ADHESIVES AND SEALANTSADHESIVEis a compound that adheres or bonds two items together so as to form a single unit.may come from either natural or synthetic sources. Adhesives and sealants virtually touch every part of our lives. Some modern adhesives are extremely strong, and are becoming increasingly important in modern construction and industry.

Mostly all materials have at least two common characteristics:Cohesiveness- the ability of particles of a material to cling tightly to one another and Adhesiveness- the ability of a material to fix itself and cling to an entirely different material.1. gluesa. Animal Glue Available in either solid or liquid form. Solid glue is melted and applied hot. It is slow setting and allows time for adjustment to the glue joint. Animal glue has excellent bonding properties with wood, leather, paper or cloth developing up to 12,000 psi in. shear. It has moderate resistance to heat and good resistance to cold but poor resistance to water. It cures by air drying at room temperature.

b. Blood Aluminum Gluea special animal glue made for use particularly with leather and paper. It has only very moderate bonding/power with wood. It is usually sold as a dry powder which is mixed with water. It has fair resistance to both heat and cold but poor resistance to water, will dry from 150 to 200F.c. Casein Glue made from protein materials, is a dry powder to be mixed with water. It has good bonding powder for wood-to-wood or paper-to-wood applications and will develop the full strength of the wood in most situations.Casein glue has good dry heat resistance and moderate resistance to cold. It has moderate resistance to water but does not perform well when subjected to high humidity or wetting. and drying cycles. It is subject to attack from molds, fungi, and other wood organisms, will dry to as low as 35F with moderate pressure.

d. Starch and Dextrin gluesavailable in both dry and liquid state, the dry glue being mixed with water. They have good bond with paper or leather and fair bond with wood, but strength does not compare with those of animal or casein glues. They have fair resistance to heat and cold but poor resistance to water. They dry at room temperature.

e. Asphalt Cementsare thermoplastic' materials made from asphalt emulsions or asphalt cutbacks. They have a good bond to paper and concrete and are used mainly for roofing applications and for laminating layers of wood fiberboard. They have relatively poor resistance to heat but good resistance to cold and good water resistance.

f. Cellulose Cementsare thermoplastic in nature and have good bond to wood, paper, leather or glass, developing up to (400 psi in shear with wood. They have moderate resistance to both heat and cold and good resistance to water. A common solvent is ethyl acetate. Cellulose cement cures by air drying and setting.

g. Chlorinated rubber adhesiveis usually a liquid; it has good bond for paper and fair bond with wood, metal or glass. Strength does not compare with animal or casein glues.It has moderate resistance to heat, cold, and water but poor resistance to creep. It cures by drying at room temperature. The usual solvent is ketone.

h. Natural-rubber adhesives are usually latex emulsions or dissolved crepe rubber. They have a good bond with rubber or leather and fair bond with .wood; ceramics, or glass, developing strengths of about 350 psi in tension with wood. They haw fair resistance to heat and cold, good resistance to water, but poor resistance to creep. Room temperature is sufficient for drying.

i. Urea formaldehyde resin glues available in powder-form to be mixed with water, and in liquid form, which requires the addition of a hardener.They are thermosetting in nature, with excellent bond to wood, leather, or paper having a shear strength of up to 2,800 psi. They have good resistance to heat and cold and fair resistance to water. Creep resistance is good. Wood welding can be done by applying a high-frequency electric current directly to joint for rapid curing.j. Melamine Resins thermosetting glues manufactured as a powder with a separate catalyst. They have excellent bond with wood or paper resistance to heat, cold, creep, and water are all excellent. Melamine resins are cured under hot press at 300F. Melamine- formaldehyde resin glues are manufactured as a powder mixed with-water and may be either hot setting or intermediate-temperature setting types.l. Resorcinol resinsare usually made as a liquid with a separate catalyst. They have good bond with wood or paper, developing shear strengths up to 1,950 psi with wood.They have very good resistance to heat, cold and creep and are generally used where a water proof joint is required. Some cure at room temperatures, while others require moderate heat, up to 200f.

m. Epoxy resinsare thermosetting in nature, manufactured in liquid form with a separate catalyst. The amount of catalyst added determines the type of curing required. They have excellent bond with wood, metal, glass and masonry and are widely used in the manufacture of laminated curtain-wall panels of various kinds. They are also used in making repairs to broken concrete. They have excellent resistance to both heat and cold, while creep resistance and water resistance vary widely, depending upon how the glue is compounded. sealantsa material used to prevent some form of fluid from escaping its container or providing a leak proof barrier. Ex. Fixing of glass window frames in aluminum channel.Seven major end uses define adhesives sealants industry : construction, packaging, transportation, rigid and non-rigid bonding, consumer products and tapes. The top three sectors represented by construction, packaging and transportation drive the market.

2. SEALERS

Sealing compounds are products which are used to seal the surface of various materials against the penetration of water or other liquids or in some cases to prevent the escape of water through the surface. To do this they must have some adhesive qualities and the ability to fill the surface pores and form a continuous skin on the surface to which they are applied. In many applications, the adhesion should be permanent, while in others it need only to be temporary.Types of Sealers

a. Liquid asphalteither in cutback form or as an asphalt emulsion. Uses are, to coat the outer surface of concrete below ground level to prevent the penetration of water to the interior through pores in the concrete. b. Polysulflde polymers this has excellent adhesive qualities, are highly flexible, and maybe applied either by hand or by spray. They are being used in exterior wails of foundations, between two-course concrete slab floors, on roof decks, as swimming- pool waterproofing and under roof flashing,

c. Solution of sodium silicateused to seal the inside surface of concrete liquid containers.The sodium silicate forms a gel-like film on the surface to prevent water penetration.

d. Wax compoundsmade in the form of emulsions to be sprayed over the surface of newly placed concrete. The wax oxidizes to form a continuous film prevents the evaporation of water from concrete in this case the adhesion is only temporary. As the wax continues to oxidize it becomes hard and brittle and flakes or is worn off the concrete by traffic.e. Liquid siliconesare used as sealers over concrete, brick and tile masonry to prevent the penetration of water into the surface.The absorption of water by masonry walls often leads to staining and efflorescence. The silicone sealers are particularly valuable for such applications because they are colorless and do not affect the appearance of the wall.

f. Oils and turpentines sealers used to seal wood surfaces before the application of paint or varnish. They penetrate into and are absorbed by the wood fibers so that the vehicle in paints and varnish will not be similarly absorbed. Similar sealers are used to seal wood which will not be painted against moisture penetration.

g. Synthetic plastic productssealers for wood which form a film over the surface and allow better bonding of synthetic lacquers to wood.

h. Thin solutions of animal and casein glues are used to coat the surface of plaster and gypsum board under paint. These products are commonly known as wall sizing.i. Epoxy-resin formulationused as sealers over concrete, wood, or. old terrazzo surfaces before epoxy-resin terrazzo is applied. The thin liquid adheres to and seals the old surface and provides good bond for the new application. Similar sealers are used under concrete surface repairs.

End.REFERENCE: ARCHITECTURAL BUILDING MATERIALS 3RD ED. BY GEORGE S. SALVAN