Urethane adhesives can be purchased as a single-component product that is pre-mixed or a two-part glue that needs to be mixed before use. The structural adhesive industry calls these non-reactive adhesives or reactive adhesives. Additives can also be mixed with both types of urethane adhesives to make them stronger.
In the non-reactive class of adhesives, urethane glues can be water-based or solvent-based. A third type is called a hot melt, which is related to a solvent urethane adhesive. The difference is that the adhesion properties react as it cools. This glue is suitable for laminating two surfaces when a light coat of glue is needed. The solvent and water-based products are applied to both sides of the material the user wants to bond.
A reactive urethane adhesive typically contains a polymer base and a curing agent. When the two substances are mixed together, a chemical reaction takes place that gives the glue its strength. One-component adhesives use moisture in the air as a curing agent. A reactive hot-melt product contains a low melting point and converts to a solid as it cools, and also reacts to moisture in the atmosphere.
Urethane adhesives are considered strong and flexible. They can expand and shrink, and are generally good on areas with joints, such as hardwood floors. Depending on their intended uses, urethane adhesives can be purchased in thick or thin formulas, and can be applied with a putty knife, trowel, caulking gun, or brush. Some of the glues set quickly, while others have an extended working time before they set.
These adhesives are effective when gluing two items made of different materials, such as concrete and wood. Windshields in many vehicles are kept in place with urethane adhesive. They are also commonly used in shoe industries, construction, on woodworking projects, and on ceramics. Certain types of urethane glues are able to penetrate porous surfaces, like fabrics.
Some people prefer urethane adhesives because they resist abrasion and chemicals. They are also good for outside projects because they are resistant to salt water and temperature fluctuations. Most of these adhesives can withstand temperatures up to 250°F (121°C).
For health reasons, proper ventilation is recommended while using an urethane adhesive over a long period of time. Care should be taken to keep the product away from eyes and skin. The manufacturers also caution against using these glues near heat, flames, or sparks that could cause ignition.
