|ADVANTAGES OF WATER BASED CONTACT ADHESIVES
No Fire or Explosion Hazard
Little or No Health Hazard
Little or No VOC
More Coverage Per Gallon
DISADVANTAGES OF WATER BASED CONTACT ADHESIVES
Not Freeze/Thaw Stable (in most cases)
Adhesive Break Down in Piston Pumps and Regulators
In the past, water based contact were only discussed or considered when there was a solvent crunch, such as in 1973, and shops would go back to solvent based contact when the crunch was over. We are now seeing a major push to water based contacts, because of the EPA restrictions on the VOC emissions and the phase out of 1,1,1 trichloroethane.
In water based contact adhesives, there is usually 3-5% of solvent. The solvent is used by the raw material suppliers to make the emulsion and this small amount of solvent does not effect the non-flammability. However, the solvent does emit an extremely small amount of VOC's (Volatile Organic Compounds), but levels are not high enough to be concerned with, and in some cases, not SARA reportable depending on the solvent used. However, we use V M & P Naphtha which is not SARA reportable.
Workers will not develop headaches, dizziness, nausea, or be affected with any of the problems associated with long term over exposure to solvent based adhesives.
Most water based contacts average a solid content of 49-50%. This will result in higher coverages per gallon, compared to solvent based contacts, but not 3-4 times the coverage boasted by some companies. Water based contacts spray flat like paint, while solvent based adhesives spray a pebbly or cob-webbed pattern, which produces highs and lows for the adhesive to grab to. With water based spraying flat, more adhesive has to be applies to insure enough adhesive is on the surface of the laminate and the substrate to achieve a good bond. It is recommended that 2.0 dry grams of adhesive be applied to both surfaces using a solvent based contact, while it is recommended that 3.0 dry grams of adhesive be applied to both surfaces using a water based contact. By applying 3.0 dry grams, you will notice an improvement in coverage, but not by 3-4 time the square footage of a solvent based contact. Please see the coverage example:COVERAGE EXAMPLE
(454grams/lb X Wt/gal X % Solids) / # of Dry Grams Applied
This will yield the number of sq. ft. per gallon single coverage (all DHPL or all substrate). Divide the sq. ft. per gallon single coverage by 2 to receive the sq. ft. per gallon completed bond. 454 is the number of grams in a pound and is a constant. The number of dry grams applied for a solvent based adhesive should be 1.8 - 2.0 dry grams hand spraying, and a minimum of 1.5 dry grams with an automatic spray. The number of dry grams applied for water based adhesive should be 3.0 - 3.5 dry grams.Flammable Spray Grade 6.7 lbs/gal 20.0% solids
(454 X 6.7 X .20) / 2.0 = 304.18 Sq. Ft. per gallon single coverage
(304.18) / 2 = 152.09 Sq. Ft. per gallon completed bond
($6.90 cost/gal) / 152.09 = $0.0454 cost/sq. ft. completed bond
Water Based 9.1 lbs./gal 49.0% solids
(454 X 9.1 X .49) / 3.0 = 674.80 Sq. Ft. per gallon single coverage
(674.80) / 2 = 337.40 Sq. Ft. per gallon complete bond
($17.75 cost/gal) / 337.40 = $0.0526 cost/sq. ft. completed bond
Based on these figures, there is a increase in coverage of 122% by using water based contact compared to flammable solvent based contact. Pricing figures used are direct industrial prices and would be higher through a distributor or retailer. This example also demonstrates that even though water based is much higher in cost per gallon compared to a solvent based, with the additional coverage that you receive, the FINAL cost per sq. ft. with water based is in the same ballpark of solvent based costs.
The carrier in water based contacts is water. Water is slower to evaporate than solvents, both flammable and non- flammable. If water based is allowed to air dry, the dry times will vary according to the temperature and humidity, just as it is with solvent based adhesives. The two variables needed to speed the dry time are AIR MOVEMENT and HEAT. Air movement will significantly reduce the amount of time required for the adhesive to dry. Heat combined with the air movement will also help speed the dry time. However, you do not want to use heat alone because this could cause the adhesive to form a skin and trap water which will soak into the board and possible result in a warp.
Warping can be a problem with water based contact adhesives since putting water into one side of the board can cause a change in the dimension of the board. To stabilize the board and prevent warping, adhesive and a backer sheet should be applied to the other side of the board. the chance of warping can be reduced by preheating the substrates.
Most water based contacts are not freeze/thaw stable. This means that one cannot allow them to freeze and then bring them back to room temperature, agitate, and then use the product. Once frozen, water based contacts are ruined, they cannot be reprocessed and they have to be disposed of. However, with new technology, some companies are starting to manufacture some water based contacts that are somewhat freeze/thaw stable. In contrast, solvent based contacts generally will not freeze. Instead, when they get cold enough, they will phase, meaning the solids will settle out of solution. One can warm the adhesive to room temperature, agitate, and the product can be used again.
The pumping equipment, fluid tips and needles, fluid channels, and all connections used with water based contact must be stainless steel. Piston pumps cannot be used with most neoprene water based contacts because they are not mechanically stable, and the shearing action of the pump causes the adhesive to break down. Instead, a diaphragm pump is recommended. Pressure pots and cup guns are okay. Fluid regulators on larger systems will also cause a shearing action which will eventually cause the adhesive to clog the regulator.
Water based contacts also have a problem with misting. By atomizing at 60-80 psi, a fog will be floating in the spray area and the air caps on the guns will constantly become clogged. To reduce this problem, it is strongly recommended that and HVLP gun be used, either by conversion gun or turbine system.
Water based contacts are a viable alternative to solvent based. Most are made with neoprene rubber, which has a proven track record and will produce strong permanent bonds. The application methods are essentially the same and the only change is in the production scheduling, unless a method to force dry the adhesive is implemented.Tim Mizell