Hot Melt Adhesives
Definition of an adhesive
Adhesives are widely used in many industry sectors, where their use is essential in manufacturing thousands of everyday products. There are many kinds of formulations and methodologies capable of performing bonding or the joining of various substrates. The adhesive material "must be capable of wetting the surface to which it is applied, at least for an instant, be used in a relatively thin layer, so that it forms a joint capable of transmitting stress," and be both strong and lightweight. (Ref. 1) Adhesives often provide advantages over mechanical fastening techniques as a result of flexibility, versatility, weight reduction, or labor savings. (Ref. 2)
Nearly 80 percent of the demand for all adhesives comes from the packaging and construction industries. Table 1 describes several industrial sectors and applications in which adhesives are commonly used.
|Construction||Manufacture and installation of laminated wood panels, prefabricated beams, wall panels, general building construction; installation of flooring, tile, carpeting, ceiling panels and wall coverings.|
|Consumer goods||Manufacture of office supplies, hobby and model supplies, and stationery.|
|Nonrigid bonding||Bonding of woven and non-woven fabrics; manufacture of athletic shoes, rugs, filters, books, and sporting goods.|
|Packaging||Manufacture of cartons, boxes and corrugated boards; bags, envelopes, disposable products (diapers, paper products); cigarettes; and labels and stamps.|
|Rigid bonding||Manufacture of appliances, electronics, household products and furniture.|
|Tapes||Manufacture of all tapes, including those used for surgery, packaging, industrial applications, consumer applications and masking applications.|
|Transportation||Aircraft and aerospace structural assemblies; automotive, truck, boat, and bus assembly; mobile home manufacturing.|
Adhesives Market Trends
The need for high-strength bonding materials is expected to grow substantially over the next several years. U.S. demand for adhesives is expected to reach 14 billion pounds and a market value of $9 billion by 2001. (Ref. 4) In the U.S., the packaging and construction industries are responsible for 80% of adhesives demand, and the packaging market continues to grow.
Within the packaging industry, corrugated boxes are "the single largest adhesive-consuming product within the sector," (Ref. 4) and pressure sensitive adhesives are also used extensively in this industry. U.S. consumption of pressure sensitive products is projected to grow at an average annual growth rate of 7-8% (from 1996 to 2001). (Ref. 5) Of these pressure sensitive adhesives, labels and decals are the most likely to have the highest growth rate, followed by tapes and miscellaneous products. (Ref. 5)
Environmental/Safety Issues for Solvent-based Adhesives
There are significant environmental issues associated with the use of solvent-based adhesives. Solvents such as toluene, methyl ethyl ketone (MEK), and trichloroethane (TCA, also known as methyl chloroform), typically act as carrier fluids for the bond forming materials comprising a conventional solvent-based adhesive formulation. These solvents are volatile organic compounds (VOCs), which makes them ideal as a carrier fluid, but also causes environmental and safety concerns. Solvents such as toluene, MEK and TCA are subject to regulation as Hazardous Air Pollutants (HAPs) under the 1990 Clean Air Act Amendments (CAAA), because they are suspected of causing cancer, birth defects, or nervous system damage, and are emitted (at varying degrees) during application, product life and disposal. (Ref. 1) Additionally, TCA is a halogenated compound that depletes the stratospheric ozone layer; U.S. production and importation of TCA ended in 1996, as part of the phase out required by amendments to the 1987 Montreal Protocol. VOCs also contribute to the formation of ground-level ozone smog that can cause respiratory damage, and are regulated under Occupational Health and Safety Administration (OSHA) standards as well.
The solvent carrier fluid is designed to evaporate after application to the substrate and during the curing process. Most solvent air emissions occur during application. Other solvent air emissions occur during the storage, transfer, formulation, coating, drying and curing, and equipment cleaning process stages. For example, before the ban on production of ozone-depleting solvents, the Source Reduction Research Partnership estimated that 26,000 metric tons (MT) of TCA were used in adhesive formulations and 5,000 MT of halogenated solvent, primarily TCA, were consumed annually in cleaning adhesive spray application equipment. (Ref. 2) Cleaning equipment actually generates the largest amount of liquid solvent wastes during the adhesive application process. (Ref. 6)
Environmental statutes and international treaties affecting adhesives include:
- U.S. Clean Air Act and 1990 amendments Regulates ground-level ozone smog precursors such as VOCs. This Act also regulates 188 chemicals that emit HAPs. At least 50 solvents used in adhesives are found among these 188 chemicals. The Ozone Depletion rules of this Act ban the manufacture and use of certain substances, including TCA (see above)
- United Nations Economic Commission for Europes 1979 Convention on Trans-boundary Air Pollution Commits to reducing VOC emissions 30 percent by year 2000.
The U.S. Environmental Protection Agency (EPA) also has an initiative that targets specific sectors, mandating compliance with certain Maximum Achievable Technologies (MACT) Standards, and the adhesives sector is one of the MACT target categories. Generally, a MACT standard does not ban a HAP from use, but sets a level of control designed to protect public health. EPA has not yet established the MACTs for the adhesives sectors, but there is a congressionally mandated deadline for setting these standards. (Ref. 7)
Drying and Curing
Drying and curing of solvent-based adhesives relies on large ovens to evaporate carrier fluids and cure the adhesive. Eighty-five to 90 percent of the carrier fluid must be evaporated during drying and curing. This requirement leads to longer process times. Additionally, ovens consume plant floor space that could be used for other production activities.
In response to regulatory drivers described above, manufacturers have developed reformulated products with either reduced HAP-emitting solvent content or HAP-free content. (Refs. 8, 9) The end-use market trend appears to lean toward zero-emission adhesives such as hot melt adhesives. The hot melt adhesives have been projected to grow at a rate of 8-12% per year. (Ref. 10)
Continue on to the hot melt adhesives defined page of the Hot Melt Adhesives Technology Review.
Return to the introduction of the Hot Melt Adhesives Technology Review.
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