Handbook of Adhesives

Adhesives are social substances. They unite materials, creating a whole that is greater than the sum of its parts. They are small in volume compared to the metals, glass, wood, paper, fibers, rubber, and plastics that they join together; but just as enzymes, hormones, and vitamins are required for individual well-being, the adhesives are recognized as essential to the health of our industrial society.

In the quarter century between publication of the first and third editions of Handbook of Adhesives, the adhesives industry has shown exciting development and diversity. It has grown from 3 billion pounds worth $650 million in 1962 to over 10 billion pounds valued at $5.5 billion in 1987. This chapter will concentrate on the contributions made by adhesives to industrial growth in the United States since the second edition of this Handbook, reviewing both the types of adhesives and applications employed by the adhesive-using industries.

Adhesion is the interaction that develops between two dissimilar bodies when they are contacted. Adhesion is thus a multidisciplinary science dealing with the chemistry and physics of surfaces and interfaces as well as the mechanics of deformation and fracture of adhesive joints. In this overview, these various aspects of adhesion are discussed. We begin by describing the general types of adhesive bonds. This is followed by sections on solid surfaces and their characterization, interfacial properties, surface treatment, and finally a discussion of the mechanics of adhesive joints.

Adhesive bonding is unique among structural fabrication methods in having surface and interface properties so dominant in controlling bond formation and bond performance properties. A single molecular layer of contaminant can prevent proper wetting by the adhesive, or a weak substrate boundary layer can provide the “weak link” for premature failure. For these reasons, much adhesive application effort and technical study has been focused on the adhesive-substrate interface. With metals, surface characteristics are largely determined by the nearly always present oxide layer; this layer must be strongly adhering and resistant to humidity, etc., or it must be removed or specifically replaced by a more controlled oxide surface. With polymeric surfaces, successful bonding requires removal of contaminants, such as mold release residues, etc. In some cases, weak, low molecular weight fragments, which can be pushed to the polymer surface during crystallization processes, etc., must be removed.

Selecting a proper adhesive for a given bonding application can, at times, appear to be an overwhelming task, but it need not be. The purpose of this chapter is to help in the selection process by: 1.Listing and briefly describing key material and system factors which should be considered during adhesive selection.2.Outlining steps in the preliminary adhesive selection process.3.Describing basic test methods which are useful in screening the candidate adhesives, once they have been chosen.

Animal glue has been used as an aqueous adhesive and size for thousands of years, and in more recent times has also found acceptance as a protective colloid, flocculant, and adhesive composition.

The casein, soy protein, and blood adhesives are often considered as a family of adhesives because they have some properties in common. Although similar, the chemical structures of these three proteins are different enough to give unique properties to the adhesives which are made from each. This chapter will primarily cover the casein adhesives, but will touch also on some of the properties common to the three protein sources and the use of combinations of casein-soy, casein-blood, and casein-soy-blood in the same adhesive. In the matter of terminology, casein adhesive will be used as a broad term embracing all the adhesives based on casein; but for the casein-lime-sodium salt wood glue, the colloquial term casein glue will be used.

Starch is a natural polymer, available in very large quantities and at relatively low and stable prices. It consists of glucose units chemically bound together so as to form a nonreducing polyhydroxy material. Because of the many hydroxyl groups, starch has a high affinity for polar substances such as water or cellulose. Starch can be reduced to low molecular weight sugars by enzymes called amylases, or by acid hydrolysis.

This chapter reflects the changes in the technology of one of the natural materials which was used as an adhesive long before modern synthetic polymers and which still maintains an important place. The chapter includes a few formulations from the first edition of this Handbook,1 others have been revised. More importantly, over the second and the present editions the introduction of natural rubber grafted with methyl methacrylate has been accompanied by greatly improved grading systems for the more simple product as well as chemically stabilized and chemically depolymerized products. Reclaimed rubber must now be regarded as obsolescent in the adhesives industries of advanced countries, partly because of its uncertain composition but partly on economic grounds; it may still have a place in autarchic economies.

Butyl rubber and polyisobutylene are elastomeric polymers1 used quite widely in adhesives and sealants both as primary elastomeric binders and as tackifiers and modifiers. The principal difference between these polymers is that butyl is a copolymer of isobutylene with a minor amount of isoprene, which introduces unsaturation, while polyisobutylene is a homopolymer.

Nitrile rubbers are broadly defined as copolymers of a diene and a vinyl unsaturated nitrile. This chapter will focus primarily on emulsion-polymerized copolymers of 1, 3-butadiene and acrylonitrile, which represent the bulk of the commercially available nitrile elastomers.

Adhesives have long represented a market, albeit relatively small, for styrene-butadiene rubbers. The original commercial SBR was used successfully in this application and today all of the SBRs, solution types as well as hot and cold emulsion types, are used in a variety of adhesive compositions. They are used by adhesive formulators as latexes or as solid rubbers.

Thermoplastic rubber is truly a useful and versatile class of polymer. It has the solubility and thermoplasticity of polystyrene, while at ambient temperatures it has the toughness and resilience of vulcanized natural rubber or polybutadiene. This characteristic results from its unique molecular structure. Visualize the simplest thermoplastic rubber molecule: a rubbery midblock with two plastic endblocks. This situation is pictured schematically in Fig. 1 where the diamonds represent monomer units in the plastic endblocks and the circles represent monomer units in the rubbery midblock. Such a molecule is called a block copolymer.

This chapter is based on the chapters which appeared in the two previous editions of this Handbook.1, 2 As stated in the second edition, it is our intent to cover the broad topic of carboxylic copolymers as employed in, or useful for adhesive applications.

Neoprene, or polychloroprene was the first synthetic elastomer used by the adhesives industry, and is one of the most versatile materials ever developed as a backbone for elastomeric adhesives. Neoprene combines rapid bond strength, development with good tack or auto-adhesion, and resistance to degradation by oils, chemicals, water, heat, sunlight, and ozone. It is popular in numerous areas such as shoe sole attachment, furniture construction, automobile assembly, and a variety of construction applications.

The term polysulfide polymers referred at one time exclusively to the high-sulfur-containing polymers as manufactured by the Thiokol Chemical Corp. From 1928 to 1960 they were the only high-sulfur polymers available. The solid polysulfide polymers contained 37–82% bound sulfur, while the liquid polymers contain approximately 37%, which gives them their unique chemical properties.

Phenolic resins have played an important role in industrial advancement for over 80 years. The term phenolic is applied to those materials formed during the condensation reaction between phenol or substituted phenols and formaldehyde. Although Adolph Baeyer1 first reacted phenol and an aldehyde in 1872 to produce a resinous material, and Arthur Smith was issued the first phenolic resin patent in 1899,2 it is Leo H. Baekeland who is considered the creator of the phenolic resin industry. He published a series of papers3, 4 beginning in 1905, and established the Bakelite Company in the U.S. in 1910. This eventually became a division of the Union Carbide Company in 1939.2 Over the years many scientists have helped make phenolic resin products an integral part of modern life.

Amino resins are prepared by reacting formaldehyde with a compound containing the amino group −NH₂.

Structural adhesives based on epoxy resins were first introduced in 1950 and their use has grown steadily since. Epoxy resins are reactive with a number of different curing agents and yield a wide variety of products with different cure requirements and end-use performance. Epoxy resins cure with no evolution of byproducts, have low shrinkage and adhere to many different substrates. Although epoxy adhesives represent only a small part of the total adhesives market, they are unequalled in performance where high strength and endurance properties are critical.

The most important polyurethane adhesive components continue to be toluene diisocyanate (TDI) (I)*, diphenylmethane-4,4’-diisocyanate (MDI) (VIII), polymethylene polyphenyl isocyanate (PAPI) (XV), and triphenylmethane triisocyanate (Desmodur R) (III) together with various polyester and polyether glycols. One review points out that polyester based polyurethanes have emerged as the forerunner over polyether systems because of their inherently higher cohesive and adhesive properties.54 Nevertheless, poly(ether-urethane) adhesive compositions unquestionably have useful adhesive properties.78

Polyvinyl acetate has been available commercially in the United States since the 1930s. Growth was slow until the 1940s, when polyvinyl acetate emulsions were introduced. The volume of resin consumed has since grown from a negligible amount in 1945 to 1.7 billion pounds currently.

Poly(vinyl alcohol) (PVOH) is a water-soluble synthetic resin. It is produced by the hydrolysis of poly(vinyl acetate); the theoretical monomer, CH₂=CHOH, does not exist. Discovery of PVOH was credited to German scientists W.O. Herrmann and W. Haehnel in 1924, and the polymer was commercially introduced into the United States in 1939.1

The adhesive industry in the U.S. has had a remarkably steady growth over the past 15 years. Hot melts, in particular, have grown steadily from 100 million pounds in 1970 to over 400 million pounds in 1985. The annual growth rate over this total period averages above 10%. It is rather obvious then that hot melt adhesives have been the major growth segment in the adhesive industry, with the aqueous and solvent-based products showing a decline. The growth rate for polyolefin and ethylene copolymer based hot melt adhesives has slowed down until currently it approximates the growth in GNP. Emphasis in the adhesive industry, however, is still largely centered on the hot melt products, in particular those based on new block copolymers. While it is obvious that in one sense the hot melt adhesive market is maturing, there are still many new opportunities opening up.

Polyvinyl acetal resins are known for their excellent adhesion to a variety of surfaces. While the principal applications are adhesives for glass and metal, polyvinyl acetals provide structural adhesion for paper, fiber and plastics. They contribute flexibility and toughness to coatings. Excellent pigment binding capability has led to their use in printing inks, electrographic toners, and magnetic tape. The polyvinyl acetal polyvinyl butyral is almost exclusively used in the manufacture of laminated safety glass where optical clarity along with structural and adhesive performance is required.

In 1901, at Tuebingen, Germany, a doctoral candidate named Otto Rohm published a thesis describing liquid condensation products obtained from the action of sodium alkyoxides on methyl and ethyl acrylate. He also discussed the chemical nature of the polymer materials formed simultaneously in these reactions. With this work, Dr. Rohm put in motion a chapter of chemical history which blossomed in the ensuing half century into a significant commercial factor in the adhesives, plastics, coatings, and other industries. During this half century, a number of processes have reached commercial utilization for the manufacture of the acrylate monomers and for the variety of polymer materials which are derived from those monomers.

Anaerobic adhesives are single-component liquids or pastes which can be stored for prolonged periods of time at room temperature in the presence of oxygen but harden rapidly to form strong bonds when confined between surfaces that exclude air. Researchers at General Electric identified the first anaerobic adhesive in the late 1940s.1 They discovered that tetraethylene glycol dimethacrylate, oxygenated by heating at 60–80°C in the presence of bubbled air, remained liquid when cooled as long as aeration was maintained. However, when the air bubbling was discontinued or when the liquid was pressed as a thin film between glass microscope slides, rapid crosslinking occurred forming a solid polymeric material.

Alkyl cyanoacrylate adhesives are unique among the many classes of adhesives, in that they are the only single component, instant bonding adhesives that cure at ambient conditions without requiring an external energy source. This characteristic, coupled with an ability to bond a wide variety of diverse and dissimilar substrates, has made them the ideal adhesives for numerous bonding applications. Though moderately high in bulk cost, they are very economical to use in practice because generally only one drop is required per bond and the nearly instantaneous room temperature cure makes fixtures, ovens, and expensive radiation sources unnecessary.

Hot melt adhesives have been known for centuries. Historically, mixtures of natural waxes, rosin, pitch, and other naturally occurring substances were used alone or in mixtures to produce sealing compounds for a variety of applications. It was not, however, until the early 1950s that hot melts based on synthetic polymers appeared in the marketplace.

High temperature organic adhesives are required for joining metals, ceramics, plastics, and composites to themselves and to each other. These adhesives, and other materials that are required to exhibit good adhesion, are needed for use in a variety of applications in the aerospace, automotive, computer, electrical, household, and oil industries. The common requirement is thermal stability; but stability under other environmental conditions is also needed. In some applications, the use temperature is not the determining factor. Stability at high temperatures encountered during various processing steps (e.g., soldering) to fabricate a component is the important requirement. These processing temperatures can be significantly higher than the actual use temperature. In this article, high temperature organic adhesives are defined as materials that exhibit usable stength after long term aging (i.e., thousands of hours) at 232°C or short term exposure (i.e., minutes) at 538°C and higher.

Silicones are synthetic polymeric materials that possess an extraordinarily wide range of physical properties. They can be low- or high-viscosity liquids, solid resins, or vulcanizable gums. As a class of substances, silicones are characteristically very resistant to extremes of temperature, to ultraviolet and infrared radiation, and to oxidative degradation. They display an unusual combination of organic and inorganic chemical properties that are due to their unique molecular structure of alternating silicon and oxygen atoms; this polysiloxane chemical structure is common to all silicones.

The development and increasing use of organofunctional silanes has closely paralleled the growing use of composite materials in expanding and diverse applications. From the crude laminates developed to meet the material shortages of World War II, composite materials have evolved into sophisticated products that are required to meet ever increasing performance demands. In addition to providing a wide range of mechanical properties, many of today’s composites are asked to furnish specific electrical characteristics, lend themselves to various fabrication techniques, maintain their properties after exposure to hostile environments, and accomplish this in an economically viable manner. The challenge is indeed great.

Surface treatment of substrates has been an important factor in obtaining well bonded structures. Modifying or treating the substrate surface is usually essential for achieving a bond that will survive long term stress and exposure to environmental conditions. In composites technology, the term coupling agents has been used to designate chemicals that are used to treat the surface of fillers and reinforcements in order to obtain optimum physical properties and for long-term retention of physical properties. Coupling agents are chemical molecules with dual functionality, wherein one part of the molecule will adhere to one surface, e.g., filler or reinforcement, while another part of the molecule provides a bond to the other material, e.g., the polymer matrix. Thus, a bonded bridge is formed between two different materials.

Elastomer-based adhesives are widely used in industrial and household applications. Pressure sensitive tapes and labels, hot melt packaging adhesives, disposable products, construction adhesives, and hot melt bookbinding adhesives are just a few of the adhesive systems which have shown rapid market growth in recent years. Other types of elastomer-based adhesives have been developed for high-strength structural applications required by the aircraft, automotive, and construction industries. The wide range of properties available in these adhesive systems is due in part to: (a) the variety of properties obtainable in natural and synthetic elastomers and (b) the many modifying materials such as tackifying resins, reinforcing resins, fillers, plasticizers, and curing agents which may be incorporated into the adhesive formulation.

Modern products exploit many of the inherent design and manufacturing advantages of plastics. Although designing with molded plastic often allows a reduction in the number of parts needed, usually some assembly operations remain. Sometimes the product is too complex to mold in one piece, or it may require properties available only in metal, ceramic, elastomers, or other materials.

When J. B. Dunlop made his first pneumatic tire in 1888, Irish flax was used as the reinforcing material. Cotton, however, quickly replaced flax because of material cost savings, and cotton was used in tires until World War II. It was used with no adhesive treatment since the mechanical interlocking brought about by protruding filament ends was enough to adhere cotton cord to the rubber. However, as tire performance requirements became more demanding it was necessary to use man-made fibers as reinforcing materials.

The bonding of wood dates back to the ancient Egyptian Pharoahs, for whom artistic inlaid wood veneers, bonded with animal glues, were created by skilled artisans for the purpose of adornment. Today bonded wood products serve as a major structural component of shelter and furnishings, which continue to be cherished for aesthetic in addition to practical purposes. Wood bonding technology has evolved into a complex interdisciplinary science involving fields of chemistry, engineering, and materials science as well as wood science and wood technology. The size and shapes of wood to be bonded are limited only by man’s imagination, ranging from microscopic fibrous material in hardboard to huge structural laminated members. The adhesives are no longer primarily animal glues but rather synthetic polymeric resins formulated and characterized with the assistance of state-of-the-art chemical instrumentation.

Sealants and caulks are used to fill joints, gaps and cavities between two or more similar or dissimilar substrates. Sealants and caulks seal these discontinuities in structures for economy, convenience, and functional necessity. Their purpose is to isolate and control conditions, such as water and weather, to optimize the functioning of the structure being sealed. Today the number of applications for sealants and caulks in construction, industrial and consumer markets is growing. These materials are required to seal and adhere to the appropriate mating surfaces over a wide range of temperatures, environmental stress and joint movement conditions. These surfaces to be sealed include a wide variety of glass, concrete, masonry, wood, steel, aluminum, and plastic substrates.1–11, 87, 98, 102

Pressure-sensitive adhesives (PSAs) are materials which in dry form are aggressively and permanently tacky at room temperature and firmly adhere to a variety of dissimilar surfaces upon mere contact without the need of more than finger or hand pressure.1 They are widely used in familiar, everyday products such as masking tapes and office tapes, finger bandages and labels.

Modern civilization would not be possible without bonded abrasive products. By the year 1825 sand, emery, and even diamond were being bonded together with shellac for use in abrasive sticks and wheels. Rubber bonded wheels were introduced in 1857, the sodium silicate and the vitrified bond just after the Civil War,1 and the phenolic resin bond in 1923. The metal bond was introduced for diamond wheels in 1940.

A coated abrasive consists of a backing substrate coated with an abrasive mineral (grit), which is bonded to the backing using an adhesive system. There can be several layers of adhesives used in the manufacturing process. The primary adhesive coat, called the make coat, is used to bond the grain onto the backing. A top coat of adhesive is applied after the make coat and grain coating. This is called the size since it is used to reinforce the grain and insure an adequate abrasive bond. Reactive fillers are also incorporated into the size formulation to enhance the grinding performance of coated products.

Glues and sealants have been used in construction since Biblical times; but the synthetic adhesives achieved prominence only after World War II. Even in recent decades, however, changes have been dictated by new adhesive materials and new building materials.

The use of adhesives in the electrical industry is rapidly expanding because adhesives are being developed having characteristics suited to the unique needs of electrical/electronic market. Also the electrical industry has gained faith in the reliability of adhesives.

Polymeric materials are normally excellent insulators. Most of the resins, such as epoxies, that are used in today’s best adhesives are prized for their ability to insulate metals and other surfaces from both heat and high electrical voltages. But for many important industrial applications, particularly in the electronics industry, it is necessary that an adhesive be able to conduct either heat or electricity, or both. Conductive adhesives, therefore, owe their conductivity as well as their cost and most other physical properties to the incorporation of high loadings of metal powders or other special fillers of the types shown in Table 1.

Adhesives are used extensively by the aerospace industry for bonding structural components of aircraft (both military and commercial), missiles, and satellites. Sealants are used in joints around windows, in fuel tanks, etc.; hot melts and pressure sensitive adhesives are utilized in aircraft interiors (primarily in fabricating decorative panels); while thermosetting adhesives are used to bond load bearing structural components.

The automotive use of adhesives is nearly as old as the industry itself. The early wood and canvas bonding agents have been replaced by formulations capable of holding metal, glass, plastics, rubber, and a variety of fabrics to themselves, to each other, and to painted surfaces. They are routinely used in structural, holding, and sealing applications. Automotive adhesives have become increasingly sophisticated and capable over the past twenty years. This trend has been driven by a need to bond new weight-saving and/or corrosion-resistant materials in an ever more health and cost conscious environment. The most significant trends since the previous edition of this Handbook have been the increased use of plastics and galvanized steel, a shift toward the robotic application of adhesives and sealants, and the use of nondestructive testing and statistical quality control techniques.

Many formulations of reactive adhesives require mixing of the resin and hardener in a critical ratio. This chapter deals with the basic designs of equipment for this purpose.

The dispensing of adhesives and sealants (hot, warm, or cold) has been established as a proven market for the application of robotics. Robots are being used across industries for applying adhesive and sealants to increase quality and to reduce labor and material costs. The automotive segment has led the way for the application of robotics. This chapter will deal with that segment and some of the criteria associated with automating particular applications, emphasizing the relationships between the application objectives and the adhesive/sealant dispensing equipment. Successful applications take into consideration the capabilities and limitations of the adhesive/sealant materials, the dispensing equipment, the tooling and fixtures and the robot.