PUR (reactive polyurethane) hot melts is another type of hot melt glue L.D. Davis distributes. PUR glues are a more recent development for perfect binding and are largely different from the EVA and SBR (styrene butadiene rubber) hot melts.
They polymerize when applied so that they form much stronger bonds than traditional hot melts. They also possess superior flexibility and can withstand a larger range of temperatures. They are currently much more expensive than traditional EVA hot melts (2-4X) but application rates are much lower. PUR is clear, less of it needed to create a solid bond and it yields stunningly beautiful products. PUR glues are an excellent choice when bonding difficult substrates when immediate tack is needed, especially in certain bookbinding applications where traditional animal glues are not the correct product for the specific binding process.
Unlike traditional hot melt glues, PUR glues have to be left to cure for 24 hours. PUR glue bonds occur when the chemicals in the glue draw moisture out of the air; technically making PUR glues a moisture-curing glue product. PUR glues are incredibly strong and flexible and work beautifully as a book binding glue on difficult binding applications. The result is a strong, long-lasting bond even on difficult to bind substrates.
PUR glues require a special equipment setup as the glue cannot come into contact with air during application
Physical Properties of PUR Glue
- The major limitations for hot melts are the suspect nature of hot melt bonds at temperatures over 150° F due to its thermo plasticity and safety issues due to the high temperatures used.
- PUR glue requires a 24-hour curing period. During this time, PUR books must remain exposed to natural air in order for the bonding chemicals in PUR to cure
The Major Advantages of PUR Glues
- PUR glues are excellent for tacking tough glue binding jobs
- Excellent immediate (green) tack
- Quick achievement of final bond strength
- Great adhesion to many surfaces
- Excellent shelf life
- Great flexibility
- Lay flat friendliness