by: Ken Kroeger, PH.D., L.D. Davis Industries, Inc.

The Cuyahoga River in Northeastern Ohio was ignited by a spark in 1969. The fact that there was so much oil-soaked debris and sludge floating on the river to sustain a fire was an environmental wake-up call for the American people. Many cite the fire as an impetus for the formation of the Environmental Protection Agency in 1970 and the passage of the Clean Water Act in 1972. These events helped lead adhesive manufacturers away from solvent-based adhesives (adhesive polymers dissolved in organic solvents) and accelerated the development of water-based adhesives that could perform similarly. In terms of reducing the environmental impact of adhesives, this has been the most significant development in adhesive technology. The blaze on the river 41 years ago has led to the situation today in which adhesives have a relatively mild impact on the environment.

In this second installment of the two-part series about adhesives in the bindery, the environmental impacts of animal glue, resin emulsion, and hot melt adhesives will be compared and contrasted. Keep in mind the overall perspective that all of the adhesives used in the bindery today have a much reduced environmental impact compared to solvent-based adhesives used in decades past. The complete lifecycle of adhesives will be considered: their manufacture from raw materials, use in the bindery, and the post-consumer recycling or disposal of bindery products.

It’s recommended that you read the first part of this series in the Winter 2010 issue of The Binding Edge since it discusses the nature and composition of these adhesives.

Animal Glues

The major raw materials used to manufacture animal glues are technical gelatin, glycerin, Epsom salt, sugars, and water. Technical gelatin is produced industrially by the chemical treatment (hydrolysis) of collagen protein contained in the hides and bones of cattle and pigs. The chemicals used are not environmentally hazardous (usually slaked lime), but gelatin production produces a large amount of waste water that must be treated to reduce the solids content. While the raising of farm animals has a large environmental impact, the main economic drivers of farm animal production are meat and leather production. Therefore, technical gelatin production is only responsible for an extremely small fraction of this environmental impact. Glycerin is produced as a byproduct of cooking oil and soap production. Epsom salt is generally obtained from mines. The sugars used to produce animal glues are produced from corn, a renewable resource. The technical gelatin and Epsom salt are generally obtained overseas, which adds to their carbon footprint.

The use of recycled materials greatly reduces the environmental impact of animal glue production. The production of pharmaceutical capsules (both hard and softgel capsules) produces a significant amount of gelatin “scrap.” This “scrap,” which contains high-quality pharmaceutical gelatin, can be recycled by using it in place of technical gelatin to produce animal glues. Through contracts with pharmaceutical companies, L.D. Davis Industries uses this “scrap” as the source of approximately 50 percent of the gelatin it uses for animal glue production. This recycling effort saves thousands of metric tons of this “scrap” from reaching landfills every year.

Animal glues are used in the bindery by applying a moderate heat to maintain the adhesive at 150°F. Animal glues generally contain no volatile organic compounds (VOCs) and clean up easily with water.

Since even dried animal glues easily disperse in water, they produce absolutely no problem for a repulping process in the recycling of bindery products. In addition, all of the major raw materials used to manufacture animal glues (gelatin, glycerin, Epsom salt, and sugars) are highly biodegradable. When a bindery product ends up in a landfill, the animal glue will biodegrade within weeks of exposure to water.

Resin Emulsions

The major raw materials used to manufacture resin emulsion adhesives are polymer emulsions, polyvinyl alcohols, and water. Most of the polymers contained in the emulsions, as well as the polyvinyl alcohols, are ultimately synthesized from acetic acid or ethylene produced in a very energy-intensive process of cracking petroleum. In addition, a large percentage of the petroleum must be shipped from the Middle East, which further increases the carbon footprint. There are no recycled materials used in manufacturing resin emulsions.

Resin emulsion adhesives are used in the bindery at ambient temperature so no heating energy is used. While resin emulsions generally contain no VOCs, they sometimes require an organic solvent for cleaning.

Since most dried resin emulsion adhesives will soften and only partially disperse in water, they produce minor problems during the repulping process. Resin emulsions are not biodegradable. While polyvinyl alcohols are biodegradable, the polymer component of the polymer emulsions is not.

Hot Melts

The major raw materials used to manufacture hot melt adhesives are the base polymer, tackifying resins, and waxes. Most of the base polymers, tackifying resins, and waxes are ultimately synthesized from petroleum using energy-intensive processes. In addition, a large percentage of the petroleum must be shipped from the Middle East, which further increases the carbon footprint. Generally, there are no recycled materials used in manufacturing hot melts.

Hot melt adhesives are used in the bindery by applying a large amount of heat to maintain the adhesive at 350°F. However, there are low-temperature hot melts designed for safety and energy efficiency that can be used as low as 250°F. Hot melts contain low or no VOCs. Since hot melts are not soluble in water, clean-up requires a wax, an organic plasticizer, or an organic solvent.

Since hot melts will not disperse in water, the presence of hot melts in bindery products seriously affects their recyclability. During the repulping process, the hot melts produce “stickies” that hamper the process. Key to the success of a repulping process is having a method to separate the “stickies” from the pulp to prevent compromising paper pulp quality. In addition, all of the raw materials used to manufacture hot melts (base polymers, tackifying resins, and waxes) are generally inert and, therefore, not biodegradable.

Conclusion

This two-part series of articles has described the history, composition, uses, and environmental impact of animal glues, resin emulsions, and hot melts used in the bindery. This second installment has compared and contrasted different aspects of the environmental impact associated with these adhesives.

The animal glue, resin emulsion, and hot melt adhesives used in the bindery today all have a much reduced environmental impact compared to solvent-based adhesives used in the past. Those solvent-based adhesives had very high VOC content, were not biodegradable and, if discarded into a landfill leaching solvents, would produce toxic soil and ground water contamination.

Animal glues are the friend of the environment. They are the adhesives of choice when high wet tack is needed and the materials to be bonded are absorbent and relatively easy to bond.

Resin emulsions are the adhesives of choice when a large amount of wet tack is not needed, the materials to be bonded are absorbent, and the materials are more difficult to bond than what animal glues can handle.

Hot melts are the adhesives of choice when excellent immediate (green) tack is needed and the materials to be bonded are more difficult to bond than what animal glues can handle.

Acknowledgement

The author would like to thank Mark Katsaros of H.B. Fuller Company for his assistance.

Dr. Ken Kroeger has been the research and development manager for L.D. Davis Industries for the past 13 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 and distributing multiple lines of hot melts for the graphic arts, rigid box, packaging, and other industries. For more information, call (800) 553-3284, email Dr. Kroeger at [email protected], or visit www.lddavis.com.