- Definition of an adhesive
Environmental/Safety Issues for Solvent-based Adhesives
Alternatives: Waterbased Adhesives
Drying and Curing
As part of an overall project to provide industry with better access to information on pollution prevention research, the Pacific Northwest Pollution Prevention Resource Center (PPRC) has undertaken literature reviews on alternative solvent-free adhesive technologies that have the potential to replace conventional ozone-depleting and solvent-based adhesive systems.
This article is an edited version the first of the series of technology reviews being undertaken as an element of the Northwest Pollution Prevention Network that PPRC is coordinating. The technology reviews synthesize current research on P2 technologies, explore cost and technical issues, identify areas where further research is needed, and provide extensive bibliographies and glossaries. The full version of this review is also available on PPRC's web site at http://www.pprc.org/pprc, where there will be links to relevant projects in PPRC's research projects database and to relevant web sites. I urge you to visit this site to see the complete version. -MLD
Definition of an adhesive (back to top)
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 joining 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. Adhesives often provide advantages over mechanical fastening techniques as a result of flexibility, versatility, weight reduction, or labor savings.
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. 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," 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). Of these pressure sensitive adhesives, labels and decals are the most likely to have the highest growth rate, followed by tapes and miscellaneous products.
Environmental/Safety Issues for Solvent-based Adhesives (back to top)
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. 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. As an example, the National Risk Management Research Laboratory estimates that 26,000 metric tons (MT) of TCA are used in adhesive formulations and 5,000 MT of halogenated solvent, primarily TCA, are consumed annually in cleaning adhesive spray application equipment. Cleaning equipment actually generates the largest amount of liquid solvent wastes during the adhesive application process.
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.
Alternatives: Waterbased Adhesives (back to top)
In response to regulatory drivers described above, manufacturers have developed reformulated products with either reduced HAP-emitting solvent content or HAP-free content. The end-use market trend appears to lean toward zero-emission adhesives such as waterbased adhesives, already representing nearly 50% of the market in 1994.
Waterbased adhesives use water as the carrier fluid, with the adhesive particles suspended in water, reducing the adhesive's viscosity so that it can be applied to various substrates at varying thicknesses. Evaporation of the carrier fluid during the set and cure stages typically occurs in large ovens. Evaporation and cure also can take place in the open under ambient, non-thermal conditions. It is important to note that not all waterbased adhesives are 100% solvent-free, but may contain some VOCs as assistants to the water base for proper viscosity or fluid control. Waterbased adhesives have been available since the 1970s. They are formulated from rubber compounds (as the base material), with various additives such as synthetic hydrocarbon resins or pine sap derivatives to increase strength characteristics.
Water is a relatively high surface tension material, and as a result, waterbased adhesives work well on other high surface tension materials like paper. Waterbased adhesives are best applied in longer production runs, as opposed to batch runs, since waterbased adhesives take longer to equilibrate on the backing material. A longer run results in an increase in the usable end product and ensures a profit on the run.
Waterbased adhesives have some limitations that must be recognized. Waterbased adhesives do not work well with backings that have a low surface energy, such as plastic films, metal foils, vinyls and foams. Waterbased adhesives also have lower performance characteristics than solvent-based adhesives, including:
- lower peel strength at room temperature
- lower shear strength at high temperatures
- less flexibility in adhesion to a broad range of backings
- lower humidity resistance
Evaluating a switch to any alternative solvent-free adhesive requires the close scrutiny and effective teamwork of numerous parties to resolve any technical issues. The successful application of new adhesive technology requires the input of multiple decision makers, in particular product development engineers, adhesive suppliers, application equipment vendors, and end product customers.
- Product development engineers determine the adhesive performance specifications such as thickness, required surface finish or substrate preparation, and structural bonding strength.
- Adhesive suppliers can best determine which products will perform to designed performance specifications. They will also know specifics about adhesive-to-substrate compatibility issues that are important to the product development engineers.
- Application equipment vendors know the limitations and capabilities of their equipment. Issues such as waste generation, energy consumption, corrosiveness of adhesives, and routine equipment maintenance can be addressed by these specialists.
- Customers ultimately determine the overall quality required for the product being manufactured. A few of the most important issues regarding waterbased adhesives include:
- performance requirements, including bond strength, durability, and process adaptability
- application issues
- curing technology
- mixing options. It should be noted that both process adaptability and application techniques include the incorporation of manufacturing issues such as equipment and plant layout.
Performance Requirements (back to top)
Ultimately, the performance of an adhesive depends upon the material's ability to durably and successfully bond substrates in a manner that is acceptable to the end user. Among the durability issues are:
- Working load and types of stress on the joint
- Service temperature range
- Expected life
The use of adhesives in the manufacture of goods requires an analysis of the application limitations of the adhesive, in order to ensure that the production line is not adversely affected by the method chosen. The production issues that need to be addressed for adhesives include:
- Adhesive fixture time and positioning desired
- Reaction methodology
- Sag and flow properties
- Manual or automated assembly
- Maintenance requirements for the system
An important performance requirement to evaluate is the adhesive coating thickness before and after application. Both solvent-based and waterbased pressure-sensitive adhesives lose 50%-70% of their applied thickness while drying and curing. Consequently, different adhesives will require different amounts to be applied to achieve the desired coating thickness. In turn, that could have production and environmental consequences, such as longer drying times, dryer requirements and/or increased air emissions.
If a considerable amount of solvent-based application equipment is already in place, a manufacturer may favor waterbased adhesives over other alternatives because they can be applied by existing equipment. This avoids the costs of new application equipment, downtime for new maintenance procedures and training, and unfamiliarity with new equipment. However, chemical properties of the adhesive and equipment must be compatible. For example, waterbased adhesives may require the use of stainless steel piping and tanks to prevent rusting from occurring.
Drying and Curing (back to top)
Drying and curing of waterbased adhesives relies on large ovens to evaporate carrier fluids and cure the adhesives. Variables that affect drying and cure time include the oven characteristics: source of heat, temperature, residence time in the oven, circulation and air velocity. Due to the lower volatility characteristic of water carrier fluids, waterbased adhesives may take longer to dry than solvent-based adhesives. However, infrared (IR) heaters may be added to the ovens to reduce the drying time.
Depending on production needs, dry adhesive can be mixed with water at the manufacturing site or the formulation can be purchased pre-mixed. The former method requires close attention to achieving the proper adhesive formulation. Although it is possible that errors may occur while mixing, and batches of waste adhesives might be generated as a result, it may be more cost-effective to ship smaller quantities of dry adhesive than premixed adhesive formulations. There are also several advantages to purchasing pre-mixed adhesive in reusable totes:
- Less chance of mixing errors
- Supplier is responsible for adhesive performance
- Reusable totes can be easily located throughout the facility and require little hard mounted plumbing
- Additional mixing can occur in the shipping container for adhesive customization
- Totes are returned to the supplier and refilled to reduce clean-up and packaging disposal wastes.
By Mark Drukenbrod