An adhesive that is applied to two surfaces, allowed to dry, and which provides an instant and permanent bond when proper pressure is used. Contact adhesives are used in many applications to bond a variety of substrates. They are used extensively in the woodworking industry to bond decorative high pressure laminates to particle board and plywood. Contact adhesives are available as extremely flammable, flammable 20 degree flash (consumer grade), non-flammable solvent based, and water based. You also have your choice of brush grades and spray grades.

1. Brush - Animal hair or solvent resistant brush that is 2" to 4" wide
2. Roller - Short nap phenolic roller
3. Spray - Spray equipment, fluid pressures, and atomization pressures will depend on the contact adhesive being used. Fluid hoses for solvent based contact adhesive should be solvent resistant, and for water based contact adhesives, the fluid hose should be nylon or rubber coated nylon and the fittings should be stainless steel. Spray guns for water based should be HVLP

Copper and brass should not be used to transfer or contain contact adhesives, and they should NEVER be laminated with any contact adhesive.

Never use aluminum to transfer or contain non-flammable contact adhesives.

There are four procedures that are necessary to achieve good bonds when working with contact adhesives:

When spraying solvent based contact adhesives, you should apply 80% coverage (2.0 dry grams/sq. ft.) in the body of the substrates, with near 100% coverage around the edges. Brushing and rolling contact adhesives, you should apply 100% coverage (3.0 dry grams/sq. ft.). To tell if you have applied enough adhesive by brushing and rolling, when the adhesive is dry and ready to bond, the adhesive should have a uniform glossy appearance with light reflecting on it. Any dull areas indicate insufficient adhesive from not applying enough, soak-in, or a combination of both. Dull areas should be re-applied and allowed to dry.

Water based contact adhesives should be applied at 100% coverage, whether you brush, roll, or spray. When dry, the adhesive should have a uniform glossy appearance with light reflecting on it. Dull areas require another coat of adhesive.

When applying contact adhesives to porous materials such as plywood and edges, it is a good idea to apply two coats. Apply the first coat and allow to dry. This will act as a sealer. When dry, apply the second coat and allow to dry properly before bonding. This helps to insure that the adhesive does not soak-in below board fiber and that you have the proper amount on the surface to achieve a good bond. It IS NOT recommended to apply one heavy coat of contact adhesive to porous materials. By applying one heavy coat, you could trap solvents which would lead to bubbling and/or edge lift. The solvents can penetrate into the porous substrate, and since contacts dry from the outside in, form a skin which could fool you into believing that the adhesive is dry and ready for bonding. Once bonded, the solvent will try to escape and seek the weakest relieve point in the bond and produce bubbling and/or edge lift.

Contact adhesive must be completely dry before bonding. The proper way to check for dryness is to take a piece of brown kraft paper and press into the adhesive and lift up. Any adhesive transfer to the paper or legginess indicates that the adhesive requires more time to dry. When completely dry, the paper should pull away clean without any transfer or legginess. Since people don't keep brown kraft paper laying around their shop or house, the method to check for dryness is to use the back of your fingers and follow the same procedures as with using kraft paper. If you have heavy areas of adhesive on the surface to be bonded, press the back of your fingers into the adhesive and twist. If it has formed a skin, this will tear it open and allow you to notice that the adhesive requires more dry time.

Never use the surface of your fingers or the palm of your hands to check for dryness. These are some of the oiliest parts of your body and you can transfer skin oil to the surface of the adhesive, which will act as a barrier between the two glue lines when you make your bond.

Once the adhesive is dry, you have a certain amount of time called "open time," that you can index the materials together and with proper pressure get a good bond. On most solvent based contact adhesives, you will have a one hour "open time," and on most water based contact adhesives, you will have at least a two hour "open time."

During the drying process in periods of high humidity, you can receive "blushing" with solvent based contact adhesives. The "blush" is caused from the rapid evaporation of the solvents, which causes the temperature in the immediate area to drop. When the temperature reaches the dew point, moisture will form on the surface of the adhesive. After applying solvent based contact adhesive, place your hand underneath the high pressure laminate (the decorative face), and it will feel cool. Sometimes it will not only feel cool, but also wet. If there is moisture on the decorative face of the laminate, then there is moisture on the glue line itself. You can also feel the moisture on the glue line by running the back of your finger on it, and it will feel slick. You can also notice a color change in the adhesive. A red adhesive will turn pinkish and a clear adhesive will turn cloudy. Once the "blush" has formed, the solvent cannot penetrate the moisture, and the moisture will act as a barrier between the two glue lines. You must remove the water and allow the remaining solvents time to flash off. The best way to remove the water is to use a fan and allow a gentle breeze to blow across the surface. Once the moisture is removed and you give the solvents time to flash off, you can make your bond. If you do not remove the water and go ahead and make the bond, it will create a suction effect much like sticking a wet paper towel on the wall. As long as it is wet, it will stick, but when the water evaporates off, it will fall, or in the case of laminate, it will delaminate. You can help eliminate "blushing" by using a hot spray system.

Remember in the drying process that temperature also plays a role. The warmer the temperature, the faster the adhesive will dry and vice versa. If an adhesive dries in 5 minutes at 70 degrees F, it will take 20 minutes at 50 degrees F. It is also advisable to keep the substrates to be bonded and the adhesive at or above 65 degrees F.

Once you have carefully positioned the pieces and made the initial bond, you MUST apply proper pressure to insure good film fusion. A pinch roller is recommended, however small or home shops are not going to have this piece of equipment. The next best method for applying pressure is by using a J roller, which is a 3" rubber roller with a metal handle. The best one that I have seen is a Gundlach V300. You what to apply a minimum of 25 psi.

A rubber mallet or a mallet and a block of wood will not apply sufficient and uniform pressure, which will result in poor bonds. The only area that gets any pressure is the area in direct contact with the mallet head. Large metal linoleum rollers will also not provide the proper pressure. For example, if a linoleum roller weighs 100 lbs., but is 15" wide, you will only receive 6.7 psi of pressure. To get the proper minimum amount of pressure (25 psi) with this type of roller and width, it would have to weigh 375 lbs.

Remember, once dry, most solvent based adhesives have a one hour "open time," and most water based contacts have at least a two hour "open time." If you exceed the "open time," the adhesive can normally be reactivated by applying a thin coat of adhesive, wiping with an approved solvent, or heating adhesive to 150 degrees F.

• WEAK OR SPOTTY BONDS
• Insufficient adhesive - Apply more adhesive
• Insufficient pressure - Apply more pressure
• Exceeded open time - Shorten open time
• Low temperature - Raise temp., force dry, hot spray

• WEAK OR LEGGY BONDS
• Insufficient drying - Allow longer dry time
• Blushing - Use air movement and extend dry time, hot spray, force dry.
• Low temperature - Raise temp., force dry, hot spray.

• EDGE LIFT
• Insufficient adhesive - Apply more adhesive
• Insufficient pressure - Apply more pressure
• Insufficient dry time - Extend dry time

• BUBBLING
• Insufficient dry time - Extend dry time
• Voids in core - Check core quality
• Improper lamination - Laminate from center out to avoid trapping air pockets

• WARPING
• Stress built in - Check pressure application
• Unequal expansion of substrates - Acclimate, review choice of substrate.
• Laminating one side only with water based - Laminate both sides, preheat.

• SEAMS SHRINKING OR LIFTING
• Unequal expansion of substrates - Acclimate substrates.
• Insufficient adhesive - Apply more adhesive.
• Insufficient pressure - Apply more pressure.

• ADHESIVE PEELING FROM SUBSTRATE
• Surface contamination - Clean surface
• Adhesive/substrate incompatible - Review adhesive choice.
• Substrate contains plasticizer - Review adhesive choice.

Hope that this information helps everyone in working with contact adhesives.