| intern summary |
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American Converters Eliminates Methylene
Chloride Based Adhesives
Substitutes Eliminate Regulatory
Compliance Burdens without Disrupting Production
| Company |
American Converters,
Inc. Fridley, Minnesota |
| Results |
Eliminated
31,580 pounds of air emissions and 8,000 pounds
of hazardous waste from methylene chloride. |
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Process Background
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.
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.
Processing Considerations
- 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?
Economics
- 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.
Options Implemented
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.
More Information
MnTAP has a variety of technical assistance services
available to help Minnesota companies reduce and manage their
industrial waste. If you would like assistance or more information
about MnTAP’s Intern Program, call 612.624.1300 or 800.247.0015
from greater Minnesota.
This project was conducted in 1997 by MnTAP
intern Scott Solberg, a chemical engineering senior at the
University of Minnesota, Duluth.
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