Polyurethane Reactive Hot Melt (PUR) has become an important fastener for manufacturers. It is used in thousands of assembly applications worldwide from woodworking to aerospace.
PUR Hot Melts are such a valuable product because they are an easily processed single component adhesive that bonds very quickly to a wide range of substrates then cross links to form an extremely strong flexible bond that is not easily reactivated. PUR’s offer good strength and flexibility at high and low temperatures with a wide range of substrates to include plastics, metals, glass and wood.
PUR Hot Melt cures with moisture found in the air and the substrates. It is for this reason that PUR must be kept in air tight desiccated containers. The containers PUR’s are available in are 310 ml aluminum cartridge, 2 Kilo slug sealed in a foil bag, 5 Gallon/20 Liter pail and 55 Gallon/ 200 liter drum.
Cartridge guns are used to process PUR in cartridge form. The front of the cartridge is pierced and the nozzle adapter attached. The cartridge is inserted in the gun and heated. Cartridge guns can dot glue, extrude beads, swirl and spray. The maximum melt rate for these guns is 2 cartridges per hour and the use of a cartridge pre-heater can accelerate the melt rate. Actual output rate using these tools can exceed 1 cartridge per minute depending on the adhesive viscosity.
For “Hands Free” assembly cartridges are also being manually opened, the adhesive removed and cut in half and inserted into nitrogen pressurized melt tanks for extrusion. Specially designed heated pressure vessels are pressurized with nitrogen to keep moisture away for the PUR and to create adhesive flow through check valve nozzle tips. PUR in 2 Kilo slugs is also being pared down in size and processed in the heated nitrogen pressure vessel extruders.
Standard hot melt systems with small reservoirs and customized lids are also being used to process PUR. Very low flow regulators are used to nitrogen blanket the PUR in the reservoir keeping any moisture from the adhesive. Regular system purges are important to insure long term performance
Another system used to process PUR in slug for is a customized reservoir with dry air purge feature. After inserting the 2 Kilo slug these systems desiccated the air in the reservoir is replace with desiccated air and held at negative pressure.
The most cost effective way to process PUR, if your usage allows, is in pails or drums with pail or drum unloaders. These systems force a heated platen into the container, effectively purging all air between the adhesive and platen and the adhesive. Used properly pail and drum unloaders seal the pail or drum without the need for dry air or nitrogen blanketing.
Polyurethane Reactive Hot melt should be considered for the fast, permanent bonding of substrates. This adhesive can eliminate the need to clamp or hold a product in compression and delay production.