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Facts of the Matter Richard Brill |
Tackling a sticky subject
that’s as old as mankindAccording to legend, Icarus used wax as the adhesive for his wings, and despite his poor choice of material, the story illustrates that the history of adhesives is as old as mankind.
There is no doubt people have been looking for ways to stick things together from the dawn of human history.
Glues and adhesives are never more than an arm's length away, and even though we might not always be aware of their presence, they are the basis of a multibillion-dollar industry.
Adhesives have been known for at least 6,000 years, but most of the technology has been developed in the last 100 years. The variety and types of sticky products have grown in an attempt to keep up with the variety of things we want to stick, but even our modern technology has yet to produce the perfect adhesive with properties that will suit every application.
The difference between glue and adhesive is merely technical, if not trivial. Most people use the terms interchangeably. Glues are derived from natural plant and animal sources. Adhesives are based on synthetic products. All glues and adhesives are polymers and bond to surfaces not by chemical reaction, but by filling irregularities on the bonding surfaces; the glue actually sticks to itself chemically and to surfaces physically.
The oldest known glue, found near the Dead Sea in Israel, was carbon-dated to around 8,200 years ago. Elsewhere, archaeologists have uncovered prehistoric sites with broken pottery vessels that had been repaired with sticky resins from tree sap. Babylonian statues from 4000 B.C. have ivory eyeballs glued into eye sockets with a tarry resin. By 2000 B.C. the Egyptians were using hide glue to make furniture and for painting.
Paintings and murals from between 1500 and 1000 B.C. show details of wood gluing operations, and a casket removed from the tomb of King Tut was glued in its construction. Museums contain many art objects and furnishings from the tombs of Egyptian pharaohs that are bonded or laminated with some type of animal glue.
Animal glues are made from the collagen-based protein extracted from bones, hide, hoofs and horns. The extract is cooked to form a thick gelatinous material, which can be re-liquified by heating, giving it quick setting properties. Its major use is in the wood and furniture industry. The major source for this type of glue is byproducts from meat processing and is the basis of jokes about the old horse being past its prime and headed for the "glue factory."
The ancient Greeks and Romans developed the art of veneering, which is the bonding of thin sections or layers of wood. This spurred the development and refining of other varieties of animal and fish glues and other types of adhesives, such as a derivative of egg whites that was used to bond golf leaf. Other natural ingredients such as blood, milk, cheese, vegetables and grains were also used to prepare glue. The Romans were among the earliest to use tar and beeswax to caulk the planking in ships.
Around 1000 A.D., Genghis Khan built an empire that covered all of Asia, aided by the exceptional power and range of bows that were supposedly made from laminated lemonwood and bullhorn bonded with a powerful adhesive whose formula has been lost in antiquity.
Otherwise, not much is known about the uses of glue until the middle of the second millennium. In the period 1500-1700 A.D., some of the greatest furniture and cabinet makers used proprietary adhesives in the building of products such as Chippendale.
One of the secrets of Stradivarius violins was supposedly the adhesive used to laminate the specially treated woods. The methods have been lost in antiquity and have not been rediscovered, even with today's sophisticated analytical methods.
The modern era of adhesives and glues began around 1700 A.D., when 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 that made an exceptionally clear adhesive, a derivative of which would later be used for photographic emulsions and photoengraving processes.
In the ensuing years, patents were issued for adhesives using natural rubber, animal products, fish, starch and milk protein (casein). By 1900 the United States had a number of factories producing glue from these various bases.
Many different types of glues and adhesives are on the market today. They fall into three main groups: natural glues, nonreactive and reactive synthetic adhesives.
Natural glues (rice and wheat pastes, fish glue, hide glue and casein glue) cure primarily by loss of moisture. Casein is a minor exception. It is made from milk curd but cures by loss of moisture and by chemical reaction.
Starch-based glue, better known as paste, is made from carbohydrates extracted from plants. Its major use is in bonding paper and paper products, as in "library paste," in bookbinding, corrugated boxes, paper bags and permanent wallpaper application. It also is used as a sizing in textiles and as a laundry product to stiffen fabrics.
Synthetic polymers have rapidly advanced the science of adhesives and have given rise to a variety of products that improve properties of adhesives, such as flexibility, toughness, curing or setting time, and resistance to temperature and chemicals.
Nonreactive synthetic glues such as PVA (polyvinyl acetate), contact cement and hot-melt glue are formulated from synthetic ingredients but cure much like natural glues by releasing water, solvent or heat.
Both white and yellow glues are PVAs. There are two types: low grade (craft, school or hobby glue) and high grade (professional/industrial glue). PVAs are useful for an assortment of jobs around the house and in carpentry, and as additives to increase the strength of mortar and cement in various applications. These are water-based products but are not water soluble once the moisture has escaped. The water is a carrier that prevents polymerization when it is present.
Rubber cements combine one or more rubber or synthetic elastomers in an organic solvent, which are modified with additives to improve the tackiness, the degree of peel strength, flexibility, viscosity or body.
Rubber-based adhesives are used in a variety of applications, such as contact cement for counter tops, cabinets, desks and tables, on pressure-sensitive tapes, sticky notes, self-sealing envelopes and shipping containers. Solvent-based rubber adhesives have also been the mainstay of the shoe and leather industry.
Hot-melt glues are designer plastics with home and industrial applications. They are used industrially to seal cases and cartons; virtually every corrugated box has flaps that were bonded with a hot-melt adhesive.
Reactive synthetic adhesives include resorcinol, polyurethane, epoxy and cyanoacrylate, which cure primarily by chemical reactions rather than by evaporation of a carrier or solvent.
Resorcinol is a toxic chemical that is mixed with formaldehyde to make a contact cement that resists water and heat. It is used to make plywood and other laminated wood products. It retains the unpleasant odor of formaldehyde and is now being questioned as a possible hazardous material because of the possible long-term health effects of formaldehyde.
Polyurethane adhesive is useful for water-resistant exterior wood bonding, laminating, veneering and for bonding nonwood materials, but it develops full strength only in well-fitted, tightly clamped joints. Newer formulas cure faster than older ones and are among the strongest and most durable of all adhesives.
Epoxies are complex chemical systems that are cured by reaction with another chemical called a hardener, or catalyst. Epoxy resins of various kinds are formulated and modified for impact resistance, flexibility, viscosity. Reinforcements such as fiberglass, alumina, silica sand, clay, metal powders and flakes are added to change properties such as heat and electrical resistance, fire resistance, strength and adhesion to certain substrates or materials.
Epoxy adhesives bond to a variety of materials (particularly metals) with high strength. In many applications they replace traditional methods of joining like nuts and bolts, rivets, welding, crimping, brazing and soldering. High-strength composite epoxies are used to construct rotor blades for helicopters and to attach aluminum skins to the struts of aircraft wings and tail sections. Skis are laminates of plastics, wood and metal joined with an epoxy. The heads of golf clubs are bonded with an epoxy, and common polyester epoxy is used in combination with fiberglass and foam to make surfboards and other marine sports equipment.
Cyanoacrylates are extremely rapid-curing adhesives marketed as "superglue." These compounds were discovered by accident when chemists at Eastman Chemical Co. accidentally glued their Plexiglas microscope slides together. They also bond readily to skin and have been used with some success in surgery to replace sutures.
The holy grail of chemists working on adhesives is to synthesize the adhesives used by marine mollusks such as mussels and barnacles. How these creatures are able to stick underwater to such diverse surfaces as rock, wood, epoxy and metal remains one of the great biological mysteries.
Researchers discovered that bivalves extract iron atoms from sea water and use it to join proteins together, linking the fibrous molecules into a strong, adhesive mesh that will stick to virtually anything, even Teflon.
In addition to using the knowledge to develop safer and stronger alternatives for surgical and household adhesives, the researchers are looking at how to combat it to prevent damage to shipping vessels and the accidental transport of invasive species, such as the zebra mussel that has ravaged the Midwestern United States.
Understanding these bioadhesives could potentially lead to a whole new category of designer materials with new degrees of plasticity, strength and adhesiveness for household, structural and biological uses. There is the possibility that some of these materials could be made sensitive to electrochemical potential, thereby creating new applications for electronic materials.
The next time something breaks and you need to glue it, there is a good chance that the right glue for the purpose is out there somewhere. The problem is matching the adhesive to the necessity and then finding the right one on the shelves of your local home improvement store.
Richard Brill, a professor at Honolulu Community College, teaches earth and physical science and investigates life and the universe. His column is published on the first and third Sundays of every month. E-mail questions and comments to rickb@hcc.hawaii.edu
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu