W&WP February 2000
When Are PUR Hotmelts PURfect for Woodworking Applications?
Polyurethane hotmelts promise increased bond strengths and increased cold, heat and moisture resistance.
PUR hotmelts' strong bond and their ability to adhere a variety of substrates may provide benefits for the woodworking industry. But are PURs right for your shop?
W&WP asked two industry experts to give us their view on the strengths and weaknesses of PUR hotmelts and their uses. One, Jim Auber of Nordson Corp., is an expert on the application equipment of PURs. The second, Dan Pikula of Franklin International, is an expert on the material itself. Here is how each responded to a series of questions on PURs.
D.P.: Polyurethane reactive hotmelt adhesives offer the advantages of conventional hotmelt products combined with higher bond strengths and improved thermal, creep, solvent and water resistance. Similar to conventional hotmelts, PUR hotmelts are applied to the substrates in a molten state. Upon cooling, the adhesive solidifies providing an immediate bond and development of strength. In the second stage, which is unique to PURHMs, the adhesive begins to react with moisture from the atmosphere and within the substrates. Irreversible chemical bonds are formed over a period of time ranging from hours to days, and bond strength builds proportionally. This distinguishes the adhesive as a "thermoset" as opposed to a "thermoplastic" and it is this thermosetting character that gives the system many of its excellent performance properties. Thus, while conventional woodworking adhesives provide mechanical bonding, PUR hotmelts provide both mechanical and chemical bonding
Typical application equipment can be a pail or drum melter. These systems will have a heated platen and a piston or gear pump to pump the molten material out and deliver it to the point of application. These systems are good for high volume applications or when the preferred packaging is in 5 gallon pails or 55 gallon drums.
A tank unit can also be used. These are systems with a heated, closed tank and a gear pump. Either system should have features built in to care for the adhesive such as precise temperature controls, temperature setback, release coating on wetted surfaces and an inert gas blanket to keep the ambient air away from the adhesive.
A heated nozzle dispenses a bead between the substrates to be bonded. The substrates are hand-positioned and can be repositioned during the adhesive's open time. Once the adhesive has "set", the bonded substrates are ready for immediate processing and finishing.
PUR hotmelts can also be applied by slot die coat, extrusion or swirl spray, and are dispensed from drum, pail or 2-kilogram "unloaders" for flat and intricate substrates. For panel lamination, a hotmelt coater followed by a nip roller is preferred.
Q: What are the major uses of PUR hotmelts?
PUR is also very tolerant of the many different materials used in edge banding: HPL, solid surface, solid surface laminates, PVC, wood veneer, and solid lumber to all core material.
Every month more PUR formulations with slightly different performance characteristics are being introduced into the market by the major adhesive companies. Competition for market share will continue to foster this kind of diverse product offering and support varied uses for PUR in the market. It will also help in bringing PUR prices down.
Have something to say? Share your thoughts with us in the comments below.