American Converters Eliminates Methylene Chloride Based Adhesives
Substitutes Eliminate Regulatory Compliance Burdens without Disrupting Production
American Converters (AC) is a job shop that specializes in fabricating foam components. Products range from packaging to cushioning used on recreational vehicles. They employ about 90 people.
American Converters eliminated 31,580 pounds of air emissions and 8,000 pounds of hazardous waste from methylene chloride.
The fabricating process begins with large pieces of foam, called buns. The buns are cut smaller then converted into the desired final shapes. Many foam components are joined with an adhesive to a substrate of either plywood, corrugated cardboard, rigid plastic or another piece of foam.
Methylene chloride based adhesives made up the largest portion of the adhesives used by American Converters. Over 4,170 gallons of these materials—each containing in excess of 65 percent methylene chloride—were consumed each year. Methylene chloride acts as the adhesive’s carrier solvent, transporting the adhesive and keeping it from bonding until it is applied.
Methylene chloride based adhesive is applied to each component by either a spray gun or a mechanical roller. After allowing enough time to evaporate most of the solvent, the two pieces are pressure-fitted together to complete the product.
American Converters uses spray application the most. But when the part is a complicated configuration, for example the cushioning component of an instrument storage container, the adhesive is roller applied. This way only the bottom surface is coated, giving a neater, cleaner appearance in the final assembly.
Incentives for Change
The goal of American Converters was to eliminate the use of methylene chloride based adhesives in their joining processes. The major driver for this project was a decision by the Occupational Safety and Health Administration (OSHA) to lower the limit on worker exposures to methylene chloride due to it being a potential carcinogen. If the company was to comply with the rule while continuing to use methylene chloride adhesives, programs for routine air monitoring and medical surveillance of affected employees would have to be implemented. In addition, significant—and costly—modifications to the plant ventilation system would be necessary to keep the methylene chloride levels below 25 parts per million (ppm).
The company needed the alternatives to meet physical, process and economic criteria. The considerations fell into the following categories:
Physical Properties of the Adhesive
- Foam tear. Two parts are joined and cured, then pulled apart. Success is gauged by the extent that after a set time the foam tears or the bond releases.
- Initial bonding (tack time). This is the time that adhesive applied parts need to be exposed to air before they can be brought together and have their bond withstand light handling. Typically, if a sprayed foam piece can be folded and remain joined, the initial bond is adequate.
- Open time. The maximum amount of time that adhesive applied parts can be exposed to air before joining. If open time is exceeded, bond strength can decrease.
- Dry time. The amount of time required to evaporate the majority of the carrier solvent from joined parts. Waterbased adhesives have longer dry times than the more volatile solvents, like methylene chloride.
- Method of application. How will the adhesive be applied to the part? Sprayed, rolled or other?
- Ease of application. How easily does the material cover a surface? Are there cleanup issues?
- Production rate. How does the cycle time for parts joined with the alternatives adhesives compare to current practices?
- Applied material cost. How much will the alternative cost, considering material cost per applied square foot and increases or decreases in labor input?
- Capital for new equipment. What will be the total capital requirement for implementing the alternative? Costs will include roller applicator, spray guns, local ventilation and spray booths.
Two-part Waterbased Adhesive. Spray operations use the majority of methylene chloride based adhesives at American Converters. The spray method is favored because it produces an even layer of adhesive at an efficient rate and a low cost. A two-part waterbased adhesive was selected to replace the methylene chloride based spray adhesive.
Two-part waterbased adhesive tests produced physical results that nearly equalled their methylene chloride based counterpart. Almost instantly after spraying, parts could be assembled and have enough strength for light handling. After one hour at ambient conditions, foam tear occurred at the edges of the part, reflecting an increase in bond strength.
Because water evaporates relatively slow, maximum foam tear properties took longer to achieve. Drying methods were needed to enhance evaporation. Parts made up of closed foams tend to trap moisture and are handled differently than open-cell foams. When closed-cell foams are being joined together, the foam pieces are set under a heat lamp for a short period prior to joining, evaporating a significant quantity of water. After open-cell foams are joined, either forced air circulation or a pass through a heat tunnel is necessary to enhance foam tear properties. With either drying method, changes to productivity were insignificant. The company had the drying equipment in their plant so no purchase was necessary. But, extra floor space was needed to accommodate the heating equipment for these processes.
To implement the two-part waterbased adhesives, equipment was needed to mix the two components prior to spraying. Based on feedback from the operators on gun function and handling, and on the ease of applying adhesive, internal mix, high-volume low-pressure (HVLP) spray guns were selected. The spraying results in the fine droplets that need to be collected from the air. American Converters designed and constructed their own spray booths to address this issue. The basic design included a fan to pull the contaminated air through a filter media then discharge the cleaned air back into the plant. Four booths were required, each having two spray guns. The total capital outlay to implement the two-part waterbased adhesives was estimated at $20,000. The overall cost of this adhesive was about 30 percent higher than the methylene chloride material.
Hotmelt Adhesive. Cold rollers applied methylene chloride adhesive to parts with special configurations. The equipment required significant cleaning and maintenance, mainly because of the incompatibilities between the methylene chloride and the aluminum roller. A hotmelt adhesive was selected as the alternative for roller application.
Hotmelts are melted and applied as liquids. Once applied and the glue is cooling, open time is limited. If the hotmelt cools too much, its coating ability and tackiness will be low, greatly impacting the final bond strength and foam tear properties. For American Converters, less open time was a benefit, allowing parts to be assembled within seconds. The hotmelt they selected is very versatile, bonding a variety of substrates. The material also had low webbing tendencies; the liquid glue does not cling to itself, spanning die cuts and leaving noticeable traces across them. Low webbing ensures clean and neat glue joints.
American Converters purchased equipment to roller dispense the hotmelt adhesive. Because hotmelts have a tendency to give off a small amount of fumes during heating, a ventilation hood was placed over the unit to direct the fumes outside. The total capital outlay to implement the hotmelt system, including the hotmelt dispenser and the ventilation hood was estimated at $9,000. The overall cost of the hotmelt is almost twice that of the methylene chloride based material. But, the increase is offset by eliminating the downtime and cleanup effort required to maintain the methylene chloride system.
Results and Benefits
American Converters was able to identify alternatives to using methylene chloride based adhesives. The alternatives selected met the bonding criteria and maintained productivity at previous levels.
The process changes eliminated their need to comply with the new OSHA methylene chloride standard. In addition, the need for an air permit and annual reporting for the Toxic Release Inventory is eliminated. Their hazardous waste generator status will be reduced to Very Small Quantity Generator (VSQG).
OSHA estimates that for a company the size of American Converters the annual cost to comply with the methylene chloride standard would be about $40,000. Add to that figure the savings associated with reducing other regulatory burdens and eliminating the need to make costly improvements to the plant ventilation system.
The capital outlay and higher raw material costs appear to offset the benefits from eliminating the regulatory costs, making the change a neutral financial trade-off.
Additional Options Reviewed
The intern evaluated several other potential joining technologies. The few additional alternatives that were given any serious consideration are listed below.
Pressure Sensitive Adhesive (PSA) Tapes. Acrylic based PSA tapes yielded favorable results with respect to the company’s physical criteria. However, the high applied cost limited serious consideration.
One-part Waterbased Adhesives. These materials exhibited extremely long tack times and were found unsuitable for American Converters. Heat could be used to reduce the time, but the results were poor compared to the two-part waterbased adhesive.
Alternative Solvents Adhesives. Adhesives exist that contain a solvent comparable to methylene chloride with similar volatility and other traits. These materials were not seriously considered; their use was seen as a short term solution. American Converters was concerned that these presently unlisted solvents could become listed at a future date.