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Polyurethane adhesive (PU) has been popular in Europe for years because it offers advantages not found in other products. It has been used for laminating chopping boards, as it is food safe in its cured state. Although it will never replace PVA wood glues, PU is a genuine advance in the adhesive field, as it is designed to bond dissimilar materials. The most important point in working with a PU is the moisture content of the workpiece. PU needs moisture to cure; insufficient moisture will result in a poor bond. Joints should be tight for best bonding.

Physical Properties

Shelf Life: About 3 years unopened, 12 months opened.
   
Color: Dark brown.
   
Viscosity: Comparable to medium syrup.
   
Clean-Up: Allow glue to dry, then scrape, sand or cut excess glue. Wet clean-up is not recommended. Wear gloves when using. Dried glue is harmless, but will remain on skin for 2 to 3 days. To help soften any dried glue on the skin, apply hand cream or lotion regularly. Do not spill glue on clothing, carpet, or other items where glue removal may cause damage.
   
Solvent: Use acetone or lacquer thinner to clean equipment, etc.

Functional Properties

Gorilla glue is waterproof, has a long open time, bonds dissimilar materials effectively, creates a rigid glue line, absorbs solvent-based stains well, and sands without clogging abrasives. It has the strength and durability of an epoxy, but is a one-part system. It can be used for interior and exterior applications, except for gap-filling bonds where structural integrity is a factor. It is suitable for marine applications. For underwater uses, dried glue joints must be covered with a protective coating (e.g., paint, varnish).

Outdoor Work: PU will easily endure the regular soakings and high humidity to which outdoor projects are exposed. This glue also possesses good resistance to the effects of ultraviolet light. Gorilla glue is less water resistant than epoxy, but passes the same ANSI test as a Type I PVA (e.g., Titebond III®). It is considerably more water resistant than a Type II PVA (e.g., Titebond II®).

Bonds Dissimilar Materials: Few glues can bond wood securely to non-porous materials like metal and plastics. PU can. This makes it ideal for assembling wooden pen kits and perfume atomizers, applying melamine to plywood, etc.

Working Temperature: To ensure proper curing, use in temperatures above 40°F (5°C). Works best at 68°F (20°C).

Working/Open Time: This glue has a rated open time of up to 15 minutes, making it useful for projects demanding complicated assembly.

Clamping Time: 3 to 4 hours but, for best results, leave for 24 hours. Use moderate clamping pressure (about 30 to 60 psi at the point of contact). Maximum water resistance is achieved after 5 to 7 days at 68°F (20°C). Do not overclamp, especially with dense hardwoods, as this may cause too much glue loss due to squeeze-out.

Shear Strength: A PU glue bond is as strong as the wood; the wood will fail before the bond will fail. However, joints should be tight (less than 0.003" gap) for a solid, invisible line and maximum strength.

Application: The spread rate is 1/2 oz per sq. ft. for most applications. If you have never used a PU glue before, we recommend testing application on scrap wood until you are familiar with how little glue is required. There is no need to wipe the excess. Just allow it to dry and simply scrape or sand off. For very porous materials, spread glue on both surfaces. Clamp pieces together within 20 to 30 minutes. Allow glue to dry and then sand or scrape excess.

Gorilla glue has a special formula for increased bonding to oily, exotic woods. For best results, remove any surface oils by wiping with a quick-drying solvent. Roughening the surfaces will also enhance bonding.

Expansion: All PU glues expand when they dry – the glue reacts with water and forms carbon dioxide bubbles (vesicles). Gorilla glue foams in about 20 minutes.

Rigid Glue Line: Applications involving bent laminations or cross-grain joints subject the glue line to constant side pressure from competing tensions in adjoining pieces of wood. Glues that are flexible even after curing (such as PVA) can allow "joint creep" – a small shift between joint members over time – resulting in the formation of a ridge along the glue line. PU doesn't allow joint creep because it is completely rigid after curing. It is not affected by temperature changes and will not become brittle with time.

Stainability: This glue will accept wood stains more readily than most adhesives. However, water-based aniline stains do not soak into this glue nearly as well. Excess glue should not leave spots. However, if you notice a spot while applying a stain, lightly sand it and continue staining.

Sands Without Clogging: As you sand a joint glued with PU, you'll find that the hardened squeeze-out breaks up into a powder that's easily cleaned from all sandpapers. Residual glue will not stain wood.

Use with Biscuits or Dowels: PU can be used with compressed biscuits and dowels. Apply the glue to the slot rather than to the biscuit. Dip the biscuit into water and immediately insert it into the slot. Use the same procedure with dowels.

Use with Veneers: PU can be used; however, make sure you do not have any air pockets between the veneer and the substrate. The glue will foam up in the pocket, creating a rise in the surface.

Special Notes

Polyurethane glue has some drawbacks, namely the inability to fill gaps as securely as it appears to and the need for moisture to be present within a glue joint to trigger the curing process. The glue also expands as it dries. For first-time users, it is always best to test on scrap wood first.

Shelf Life: Gorilla glue has a shelf life of approximately 3 years in its sealed bottle and stores for up to a year once opened. To keep glue fresh and strong, squeeze excess air out of the bottle after use, tightly secure the cap, and store in a cool place, away from direct sunlight. PU doesn't freeze but should be removed from cold storage two to three days before use to allow glue to reach room temperature (lower temperature will not affect bonding strength, but it will slow drying and curing time). If there is an inadequate seal on an opened bottle, hardening will sometimes occur because of moisture in the atmosphere. You can store the bottle upside down so that any hardening will take place at the bottom of the container and not at the neck.

Not a Gap Filler: PU expands and forms a hard foam as it cures. This creates the illusion that it fills joint gaps well, but don’t be fooled. Our tests show that as gap size increases past 0.003", joint strength drops considerably because the foam that spans this gap contains many weak, hollow spaces. This glue should not be used to fill gaps in structural bonds; e.g., load-bearing beams.

Needs Moisture to Cure: Wood with a moisture content lower than 10% requires the addition of water to trigger the curing of PU. If you don’t have a moisture meter to verify this minimum content, brush or spritz water on one side of each joint. This is especially important during the winter, when relative humidity is low, and your wood is particularly dry. If the wood joint fails the next day, it is either due to lack of moisture or use of old glue.

Safety Precautions: HARMFUL IF SWALLOWED OR INHALED. May cause sensitization by inhalation, irritating to eyes and skin. Use in well-ventilated room. Avoid eye and skin contact. Will stain skin. Wear protective gloves, such as nitrile, natural rubber or polyvinyl alcohol (PVA) chemical-resistant gloves. Wear safety glasses or face shield. Do not use product while wearing contact lenses.

First Aid

Contains: 4,4'-Diphenylmethane diisocyanate + isomeres + homopolymer CAS #9016-87-9.
   
Ingestion: Severe irritation of the gastrointestinal tract may occur and should be treated symptomatically. Call a poison control center or ambulance.
   
Eye Contact: Flush eyes with water for at least 20 minutes, occasionally lifting eyelid. Seek medical attention.
   
Skin Contact: Wash thoroughly with soap and water. Get medical attention if sensitization develops or persists.
   
Inhalation: Move to fresh air. Seek medical help if any problem persists.
 
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