Polyurethane adhesive is a type of glue used in book binding and woodworking. Polyurethane is a class of polymers that are created from a chain of organic units, joined together with urethane links. There are a wide range of polyurethane adhesive products available. Different chemical combinations result in different properties. The chemical composition can be alerted to produce different levels of stiffness, density and hardness.
The major advantage of a polyurethane adhesive is that it is water resistant. This property makes it the glue of choice for book binders and book repair firms. The glue has a very low viscosity level, cures within seconds and has a pliable state in room temperature conditions. When used in terms of adhesives, the cure time is the length of time required before the adhesive has bonded the two items together.
As an adhesive, polyurethane was first developed in 1985. The first version had very specific properties. The adhesive was made available in 1988 and had a high viscosity, low starting solidity level, but had a three day wait period before it would cure.
In 1996, the second generation of polyurethane adhesive was developed. The cure time was reduced to less than three days, while retaining the low starting solidity levels and the high viscosity. Further work was undertaken to reduce the cure time while maintaining the features that made the adhesive useful.
The next major improvement occurred in 2000. At this stage, the cure time was reduced to a maximum of 16 hours and a minimum of six hours. The solidity was improved and the viscosity lowered to make it more commercial viable.
The fourth generation was made available in 2007. In this version, the viscosity is much lower, the solidity remains the same as in 2000, but the cure time is just a few seconds. This change is crucial to both commercial success and long term product usefulness.
In North America, polyurethane adhesive is sold under the brand Gorilla Glue™ or Excel™. Sales of polyurethane adhesive have expanded outside the specialized book binding industry. These products can now be found in any hardware store for general consumer use.