by: Ken Kroeger, Ph.D.
Adhesives are used in many different operations in the bindery. In a pejorative sense, adhesives are the Rodney Dangerfield of the bindery (they get no respect) because while adhesive performance is very critical to finished product quality, adhesives are almost always a very low cost component. For example, in the production of an 800-page college textbook, the total cost of adhesives is approximately $0.06:
- $0.0134 for animal-glue casemaking adhesive to wrap and bond the cover to the chip board
- $0.0025 for hot melt for back lining to reinforce the spine
- $0.0082 for hot melt for gluing off to flexibly reinforce the book block after sewing
- $0.0092 for a liquid resin adhesive for end-sheet tipping
- $0.0264 for a liquid resin adhesive for casing-in, in which the book block and end sheets are bonded to the cover
This six cents is just a fraction of the total material cost (including the paper, printing inks, cover wrap, chip board, etc.), but the performance of the adhesives is as important as any of the components to the quality of the finished textbook.
In this first part of a two-part series about adhesives in the bindery, an overview is presented of the three major categories of adhesives used: animal glues, resin emulsions and hot melts.
Animal glues are the adhesives with the oldest historical record of use. Evidence of their use is dated at least to 1350 B.C. and, indeed, animal glue was used to bond wooden decorations on the tomb of King Tutankhamun! Animal glues (an unfortunate moniker) also are commonly called hide glues in the dehydrated form, gelatin-based adhesives, and cake glues since they are usually produced in a gelled cake form (see Figure 1). Animal glues are water-based adhesives using gelatin protein as their adhesive polymer. Gelatin is the same material used to make Jell-O™, marshmallows, and pharmaceutical capsules. It is produced industrially by the chemical treatment (hydrolysis) of collagen protein contained in the hides and bones of cattle and pigs.
Simple mixtures of gelatin (hide glue) and water have been used in woodworking for thousands of years and in book binding for at least 200 years. Modern animal glues contain not only gelatin and water but also performance modifiers such as plasticizers, sugars, salts, surfactants, defoamers, and biocides. These glues are sold in gelled cake form and are liquefied for use by heating to 150°F. The performance of modern animal glue formulations is much improved over the simple gelatin/water mixtures of the past. The formulations are tailored for each application by modifying parameters such as the level of tack, open time (speed of set), and viscosity. For example, L.D. Davis Industries produces over 100 different animal-glue formulations with open times ranging from two seconds to three minutes.
In the bindery, animal glues are used in hardcover case making, perfect binding, and back lining. The major advantage of animal glues, and the reason they’re still being used 60 years after the advent of synthetic adhesives, is that they possess excellent initial wet tack. In other words, when an animal-glue adhesive film is applied, it is immediately very tacky. This property allows the use of animal glues when a glued turned edge is needed. The major limitation of animal glues is their lack of specific adhesion. Animal glues form only mechanical bonds. This means that they adhere well to porous surfaces but poorly to some coatings and all nonporous substrates such as metals and plastics. Animal glues are outperformed in some ways by synthetic adhesives but persist because of their unique property of excellent wet tack, which remains unmatched by any synthetic aqueous adhesive.
Adhesives based on resin emulsions have a very brief history compared to that of animal glues. Up until the 1940s, the major adhesives in use were natural polymers dispersed in water such as starches, dextrins, and animal glues. After World War II, scientists extended the technology for the emulsion polymerization of synthetic rubber (developed for the war effort) to the production of plastics. This gave birth to adhesives based on resin emulsions. These adhesives are commonly called liquid glues, white glues, or cold glues because of their appearance (see Figure 1) and use at ambient temperature. Resin emulsions are stable dispersions of polymer microparticles (0.1-1.0 micrometer in size) in water. In other words, they are mixtures of tiny plastic particles in water. The polymer microparticles can be composed of “homopolymer,” which is usually polyvinyl acetate produced by the polymerization of vinyl acetate. Alternatively, the polymer microparticles can be composed of “copolymer,” which is formed when vinyl acetate is copolymerized with ethylene (vinyl acetate ethylene) or an acrylate (vinyl acetate acrylate). The copolymers possess better adhesive properties and are more flexible than the homopolymer, but they are more expensive to produce. In addition, rubber-based polymer emulsions can be used to formulate pressure-sensitive adhesives.
Resin emulsions adhesives are formulated by blending one or more resin emulsions with modifiers such as plasticizers, surfactants, defoamers, polyvinyl alcohols, thickeners, fillers, adhesion promoters, and biocides. Various types of adhesives can be tailored in this way for properties such as speed of set, viscosity based on the application method (roller, extrusion nozzle, spray, silk screen, or brush), specific adhesion required, and level of lay-flat needed to avoid warping. The most common uses for these adhesives in the bindery are casing-in and end-sheet tipping. The major advantages of these liquid glues are their good specific adhesion to many surfaces and the ease of use due to their ambient application temperature. The major drawbacks are the lack of immediate wet tack and the need for at least one of the two bonding surfaces to be absorbent. When a bonding situation is encountered where excellent wet tack is not necessary and the surfaces are absorbent, liquid glues are usually the best choice.
Hot melts also have a relatively brief history, with commercial use limited to the past 50-60 years. Hot melt adhesives are polymer-based and are 100 percent solids (contain no solvents or water). They are thermoplastic, which means they are solids at room temperature but liquefy upon heating and solidify again upon cooling. This property gives hot melts a fast setting speed if desired. The lack of water and solvents allows hot melts to attain peak bonding strength very quickly (as soon as they cool).
Hot melts are typically composed of four major components: polymer, tackifying resin, wax, and antioxidant. The polymers used for most hot melts utilized in the bindery are ethylene vinyl acetate (EVA) and styrene block copolymer (SBC, for pressure-sensitive hot melts). The polymer component of a hot melt determines its strength, toughness, and flexibility. Tackifying resins control the adhesion and hot tack properties of hot melts. They can be synthetic hydrocarbons or natural pine rosin or rosin derivatives. Waxes are selected for controlling the open time and set speed of the hot melt. Antioxidants are added to improve the pot life (preventing charring and other oxidation reactions). Plasticizers and/or diluents also are sometimes added to improve the flow characteristics and modify the viscosity and open time of the hot melt. Hot melts are normally applied at a temperature of 350°F, but lower temperature versions are available and can be applied as low as 250°F. Application methods include roller, extrusion nozzle, and spray. Hot melts bond mainly by mechanical means in that the polymeric components wet out and penetrate the bonding surfaces prior to solidifying as they cool. When adhering to thermoplastic surfaces (plastics, polyolefin laminates, and some coatings), bonding is very similar to welding due to the temperature of the hot melt when it’s applied.
PUR (reactive polyurethane) hot melts are a more recent development for perfect binding and are largely different from the EVA and SBC hot melts described above. They polymerize when applied so that they form much stronger bonds than traditional hot melts. They also possess superior flexibility and can withstand a larger range of temperatures. PUR is currently much more expensive than traditional EVA hot melts (2-4X), but application rates are much lower.
The most common uses for these adhesives in the bindery are perfect binding spine glue, side glue, gluing off/up, and back lining. The major advantages of hot melts are the excellent immediate (green) tack, the quick achievement of final bond strength, very good adhesion to many surfaces, and excellent shelf life. The major limitations for hot melts are the suspect nature of the hot melt bond at temperatures over 150°F due to its thermoplasticity and safety issues due to the use of high temperatures. Hot melt adhesives are an excellent choice when bonding difficult substrates where immediate tack is needed.
Since World War II there has been a great development of adhesive technology, providing the bindery with a large variety of adhesives – an embarrassment of riches. For every adhesive need in the bindery, an animal glue, resin emulsion, or hot melt can be selected to meet all desired criteria for tack level, open time, set speed, flexibility, bond strength, application method, and temperature stability.
In part two of this series, we’ll investigate the environmental impact of adhesive use in the bindery.
The author would like to thank Kevin Rodeck of IFS Industries for his assistance.
Dr. Ken Kroeger has been the research and development manager for L.D. Davis Industries for the past thirteen years and is an adjunct professor of chemistry and physics at Wingate University. He has an undergraduate chemistry degree from the University of Dayton and a Ph.D. in Chemistry from the University of Colorado at Boulder. In addition to adhesives, Dr. Kroeger has experience working in the fields of macromolecular X-ray crystallography, atmospheric chemistry, isotope separation, and explosive and pyrotechnic chemistry. L.D. Davis Industries, Inc. is an 84 year-old family-owned company manufacturing both animal glues and resin emulsion adhesives, as well as distributing multiple lines of hot melts for the graphic arts, rigid box, packaging, and other industries. For more information, call (800)883-6199 or visit www.lddavis.com.