The dictionary defines an adhesive as "a substance capable of holding materials together by surface attachment." This is a simple definition for a material that is the basis for a multi-billion dollar industry with more that 750 companies competing for a share of the market. It is estimated that 50 of those companies are responsible for 50% of the sales dollars in the adhesive industry.

Loctite Corporation is one of the top 50 adhesive companies that has had a major impact on the technology of adhesives in today´s high tech world. To gain an appreciation of our company´s impact on adhesive technology, we need to look at the history of adhesives and how adhesive touches our everyday life.

The first evidence of a substance being used as an adhesive dates back to 4000 B.C. Archaeologists studying burial sites of pre-historic tribes found foodstuffs buried with the deceased in broken pottery vessels that had been repaired with sticky resins from tree sap. Archaeologists have also uncovered statues from Babylonian temples that have ivory eyeballs glued into eye sockets. This tar like glue has held for almost 6000 years.

The first references in literature concerning glue and the art of glue appears about the year 2000 B.C. Simple procedures for making and using animal glue were written.

The period of time between 1500-1000 B.C. gave further proof that glue had become a method of assembly. Paintings and murals showed details of wood gluing operations. A casket removed from the tomb of King Tut shows the use of glue in its construction. Our museums today contain many art objects and furnishings from the tombs of Egyptian Pharaohs that are bonded or laminated with some type of animal glue.

The next period of activity is from 1-500 A.D. when the Romans and Greeks developed the art of veneering and marquetry, which is the bonding of thin sections or layers of wood. From this art, the making of animal and fish glues were refined and other types of adhesives were developed, such as an adhesive from egg whites to bond gold leaf. In addition to egg whites, other natural ingredients were used to prepare glue such as blood, bones, hide, milk, cheese, vegetables and grains. The Romans were one of the first to use tar and beeswax to caulk the planking in boats and ships.

A study of history shows the use of glue fell into disuse until about 1500-1700 A.D. when adhesives were used in the building of furniture. Some of the greatest furniture and cabinet makers of all times used adhesives in their products--names you will still recognize today like Chippendale and Duncan Phyfe.

Another notable name in history who may owe his notoriety, at least in part to adhesives: about the year 1000 A.D. Genghis Khan overcame all attackers because of the exceptional power and range of the weaponry his men carried. Bows were made from laminated lemon wood and bullhorn bonded with an adhesive whose formula has been lost in antiquity.

The secret of violins made by Antonio Stradivari was the adhesive process used to laminate his specially treated woods. His methods have also been lost in antiquity, and have not been rediscovered even with today´s sophisticated analytical methods.

 In about 1700, the widespread use of glue brought about some rapid changes in the history of adhesive. The first commercial glue factory was started in Holland to manufacture animal glue from hides.

About 1750, the first glue patent was issued in Britain for a fish glue. Patents were then rapidly issued for adhesives using natural rubber, animal bones, fish, starch, milk protein (casein). By 1900, the U.S. had a number of factories producing glue from the afore mentioned bases.

The Industrial Revolution caused an explosion in technical breakthroughs which resulted in new materials becoming available for use in formulating adhesives. The first plastic polymer to be synthesized was cellulose nitrate, a thermoplastic material derived from the cellulose of wood. Its first use was in the manufacture of billiard balls, which had been made of ivory. The era of plastics began with the introduction of Bakelite phenolic, a thermoset plastic, in 1910. Within a year, adhesives using phenolic resin were put on the market. The 1920´s, 30´s and 40´s saw many new plastics and rubbers synthetically produced, many out--of an urgent necessity--developed during World War II. Although adhesives have been known for about 6000 years, most of the technology of adhesives has been developed during the last 100 years.

The development of plastics and elastomers has rapidly advanced the development of adhesives and has given formulators a wide variety of products that can change and improve various properties of adhesives such as flexibility, toughness, curing or setting time, temperature and chemical resistance.

Adhesives touch our lives every day. They are never more than an arm´s length away, even though we may not be aware of their presence. A description of some of the more common types of adhesives and their uses should make you more aware of how adhesives touch your life.

Animal Glues - made from the protein extracted from the bones, hide, hoofs and horns of animals by boiling. The extract is cooked to form a gelatin material. The gelatin can then be reliquified with heat, which gives it quick setting properties. It´s major use has been in the wood and furniture industry. If you have seen a heated glue pot with a brush in it, it was probably an animal glue. Animal by-products from meat processing have been the source of supply for this type of glue. Also, the source of jokes about old Dobbin being past his prime and only good for the "glue pot."

Fish glue is a similar protein-based glue made from the skins and bones of fish. An exceptionally clear adhesive can be made from fish and was the first adhesive used for photographic emulsions for photo film and photo resist coatings for photoengraving processes.

Casein glue is made from a protein isolated from milk. The extraction process creates an adhesive that is waterproof. Its first use was in bonding the seam of cigarette paper. It provides a fast setting bond that requires very little adhesive, one gram of adhesive can bond 2000 cigarettes.

Starch - a carbohydrate extracted from vegetable plants such as corn, rice, wheat, and potatoes. Probably better known as paste. Major use area is in bonding paper and paper products such as bookbinding, corrugated boxes, paper bags, wallpaper paste (non-removable), also used as a sizing in textiles. The laundry uses starch on your shirt collars, to stiffen and give shape to your shirt.

Cellulose adhesive is made from a natural polymer found in trees and woody plants. It is the adhesive used on the cellophane wrapper on cigarette packs, the adhesive on decals we put on windows, and, interestingly enough, the adhesive used on the strippable wallpaper we have in our homes that allows us to remove the paper easily.

Rubber-based solvent cements are adhesives made by combining one or more rubbers or elastomers in a solvent. These solutions are further modified with additives to improve the tack or stickiness, the degree of peel strength, flexibility, the viscosity or body. Rubber-based adhesives are used in a wide variety of applications such as: contact adhesive from plastic laminates like counter tops, cabinets, desks and tables. It is adhesive on pressure-sensitive tapes used as floor tile adhesive and carpeting adhesive. Self sealing envelopes and shipping containers use rubber cements. Solvent based rubber adhesives have been the mainstay of the shoe and leather industry.

Epoxies are adhesive systems made by a complex chemical reaction. Various resins are made synthetically by reacting two or more chemicals. The resultant resin can then be reacted or cured by the addition of another chemical called a hardener, or catalyst. The basic epoxy resin systems are further modified to change their physical properties by the addition of such things as flexibilizers for impact resistance and flexibility, diluents or solvents to reduce the viscosity fillers and reinforcements like glass fiber, alumina, silica sand, clay, metal powders and flakes to change properties such as heat and electrical resistance, fire retardance, strength and adhesion to certain substrates or materials.

Epoxy adhesives can bond a wide variety of substrates with high strength particularly metals. They have been used to replace some traditional metalworking methods of joining like nuts and bolts, rivets, welding, crimping, brazing and soldering. High strength epoxies are used to construct rotor blades of helicopters, attach aluminum skins to the struts of aircraft wings and tail sections. Those of you who ski may know that your skis are laminates of plastics, wood, and metal joined with an epoxy. If you are a golfer, the heads of your clubs are bonded with an epoxy.

Hot melt adhesives are thermoplastic polymers that are tough and solid at room temperature, but are very liquid at elevated temperatures. The origin of hot melts probably started with the use of sealing wax used to seal documents and letters with a signature ring or stamp, but the art of hot melts was not pursued until the 1960´s.

Other adhesives that you may have heard of that represent higher technology and/or complicated chemical processes include:

RTV Silicone adhesive are a rubber like polymer called polydimethsiloxanes. RTV stands for room temperature vulcanizing, or simply a rubber which cures at room temperature. Silicone rubber adhesives are made from a complicated process that turns elemental silicon metal made from sand (silica) into a rubbery polymer. When cured, silicone rubber adhesives/sealants have excellent resistance to heat (500-600ºF) and moisture which makes them exceptionally suited for outdoor weathering applications, such as sealant and caulking compounds in the construction industry. Because of its exceptional properties, silicone adhesive has been used in some exotic applications such as the soles of the boots worn by the first astronauts to walk on the moon. Silicone adhesive/sealants are used to seal windows, doors and portholes on the space shuttle and many satellite missiles. A special silicone adhesive is used to bond the heat shield tiles on the space shuttle.

Anaerobic adhesives are derived from methacrylates, a monomer related to acrylic or more commonly known as Plexiglas. The word anaerobic really applies to microbes that become active or live in the absence of oxygen. While anaerobic sealants do not contain microbes, they become active and cure or polymerize in the absence of oxygen. These adhesives were developed by Professor Vernon Krieble, the founder of American Sealants Company which is now known as Loctite Corporation.

The basic ingredient in anaerobic adhesives is a monomer from the acrylic family, a special liquid of small molecules which can combine chemically to form a polymer or group of molecules. The molecules of an anaerobic monomer contain two carbon atoms that are double bonded to each other. The "active" ingredient in this monomer is called a free radical. This is a molecule that has an affinity for carbon, but prefers to react with oxygen. As this free radical reacts with oxygen molecules, it forms a stable liquid like the original monomer.

A special ingredient in this chemical composition continually produces free radicals as they react with the oxygen and are used up. As you can see, there is an unlimited supply of catalyst being produced.

If no oxygen is present, the free radical then reacts with the carbon of the double bonded atom of the monomer molecule. A reaction is caused which makes that molecule react with another molecule and begin a chain reaction which produces a solid polymer chain

Some of you may already know that metal has a place in this reaction process. The presence of metal speeds the polymerization process by causing the production of free radical catalysts to increase greatly. The increased amount of catalyst in the monomer causes polymerization to occur more quickly.

This anaerobic reaction has been stated in a fairly simple maner, but it is really a very complex chemical reaction using certain ingredients in a parts per million basis. Other reactions can take place at the same time as curing is occurring, such as crosslinking of the polymer chains which makes the resultant solid resin insoluble in a solvent or by the use of heat.

Anaerobic adhesives are very versatile and are used in a wide variety of applications. The very first application of an anaerobic adhesive was to lock and seal a screw in the carburetor of an automobile engine. Today, anaerobic adhesives not only lock all types of threaded fasteners, they are used to seal pipe fittings, retain bearings, pulleys, and gears to shafts as well as sealing flanged surfaces to replacing cut gaskets. The major users of these adhesives are the automotive, truck, construction and farming equipment companies. The actual market for these adhesives is anywhere that fasteners, gaskets, bearings or any mechanical device that needs to be secured or sealed are used in both OEM or MRO areas.

Cyanoacrylates are extremely rapid curing adhesives known as instant adhesive or Super glue. These adhesives were discovered by accident by Dr. Harry Coover´s group at Eastman Chemical Company while doing some basic research on characterizing certain polymers derived from a chemical called ethylene.

A cyanoacrylate adhesive is a very rapid curing adhesive also from the acrylic family tree, but having a completely different cure system. Cyanoacrylate monomer is made from a complex chemical process. The monomer produces a very reactive polymerization. The reaction or polymerization process is stabilized and the monomer kept in the liquid state by the addition of a small amount of an acid stabilizer material.

When a drop of cyanoacrylate adhesive is put on the surface of a part, the acid stabilizer molecules react with the water molecules present on the surface of the part from the relative humidity in the air. The reaction of the water and acid causes the acid stabilizer to be neutralized. The cyanoacrylate molecules then react with each other and form polymer chains without crosslinking.

The amount of stabilizer molecules in cyanoacrylate is very small--measured in parts per million--and very little moisture molecules are required to cause rapid polymerization. Cyanoacrylates begin to form polymer chains immediately on contact with the water vapor on the surface of the part. If parts are moved during initial contact, the polymerization process and polymer chains are stopped. The process must start again at a new catalyzed site.

Cyanoacrylate adhesive can be made from different acrylate monomers, such as methyls, ethyls, butyl, isopropyl, etc. These molecules differ in size and adhesives made from them exhibit different physical properties. Methyls are the smallest molecule and seem to work best on metal and rubber parts while ethyls work best on plastic parts. Many modifications can be made to the monomers to alter or improve their properties as adhesives. They can be toughened with rubber such as in Black Max"!, thickened such as Quick Gel™ or formulated to have low odor, resistance to thermal cycling, or less sensitivity to surface conditions which tend to stabilize the adhesive and slow down the cure, such as in the PRISM™ series of products.

Cyanoacrylates are very rapid curing and provide high bond strengths on plastic and rubber materials. The versatility of these adhesives make them highly useful in all industries. Some of the larger application areas are in electronics for printed circuit board wires and components, and in medical technology for disposable plastic medical devices. Other OEM applications exist in the toy, small and large appliance, automotive, and cosmetic packaging. MRO applications exist in all industries for repair of all rubber and plastic parts as well as some metal parts. The consumer market is a large volume user of cyanoacrylate adhesives for repairing everything in the home from wallpaper tears to broken toys to torn and false fingernails.