Polyurethane refers to any polymer that is made up of repeating chains of organic units joined by urethane links. They form as a result of complex chemical reactions which take a variety of forms. Some of these resulting polymers are made into polyurethane adhesives. These can often be extremely strong, and have become increasingly applicable for household and industrial use.
Polyurethane adhesives can be used to bond together a wide range of materials, including wood, metal, concrete, glass, and plastic. Whether the material is porous or non-porous is of little importance, unlike with most other adhesives. Polyurethane adhesives are used especially often as woodworking glues. They have the advantage of being very water-resistant, much more so than other glues used with wood. The United States has only seen these types of adhesives on the market since the 1990s, but they have been widely available in Europe for much longer.
The bookbinding industry has benefited greatly from the advent of polyurethane adhesives. They allow papers to be bound together that could not be bound before, because of grain direction or being coated in certain ways. Cost-effectiveness with these glues is also notable, because only a very thin layer need be applied, compared to older techniques. It can theoretically be effective in a thickness of only 0.0004 inches (0.01 mm), but it is impossible to apply a layer much thinner than 0.002 inches (0.05 mm).
Whatever they are used for, polyurethane adhesives have many valuable properties. They are very resistant to extreme temperatures, and remain stable up to 210 degrees Fahrenheit (100 degrees C). When cured, they are highly flexible and yet resist being dissolved by even harshly acidic or alkaline chemicals. They are also highly flexible, making them excellent for use in places that receive frequent vibration or shock, such as in construction applications.
Polyurethane adhesives come in several common forms. The first is as a household or light industrial adhesive which is made to bond wood or ceramic materials, among others. There are also polyurethane gels that come loaded in a cartridge like silicone or caulk, to be used in building and repairing homes, furniture, boats, and stone fabrications. These are made to be able to bond any two materials as long as one of them is porous. Some heavy-duty polyurethane adhesives also come in cartridges, and while these have a longer drying time, they can be up to 25% stronger.