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.



Q: How do PUR hotmelts differ from other hotmelts used in the woodworking industry?



J.A.: Traditional hotmelts are thermoplastics that form a bond when they cool. PUR hotmelts are high-performance adhesives that create a much stronger bond. PUR adhesives form a bond in two ways, they cool and form an initial bond and in addition they form a molecular crosslink. They absorb moisture from the wood and from the air to form a permanent bond. PUR adhesives, however, generally do not form a green tack (initial bond) quite as quickly as conventional hotmelts.

-- Jim Auber, market manager product assembly for Nordson Corp. of Duluth, GA.

PUR hotmelts can be applied in virtually all the same ways as conventional hotmelts.

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

-- Dan Pikula, Franklin International, market development manager, Columbus, OH.



Q: How are they applied?



J.A.: PUR hotmelts can be applied in virtually all the same ways as conventional hot melts. They can be applied in wide or narrow slot patterns, both vertically or horizontally, be applied by a glue spreader, and be extruded from application heads in single or multibead patterns, etc. It is important that the operator use the proper hotmelt application equipment. The equipment must be closed up to the point of application to protect the adhesive from excess exposure to the moist atmosphere. The operating area should also be vented to exhaust any vapors from the glue.

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.



D.P.: The application of PUR hotmelts in the woodworking industry usually is carried out with cartridge-holding guns. These easy-to-use applicators are operated manually, heated electrically and use compressed air. The adhesive, contained in aluminum cartridges, is brought to working temperature in the gun.

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.

PUR hotmelt adhesives are suitable for a variety of applications including for use in building laminated household doors.

Q: What are the major uses of PUR hotmelts?



J.A.: PUR hotmelts are widely used today in profile wrapping applications for furniture, picture frame, door and window profiles, kitchen counter top lamination production, postforming applications, desk and table top lamination applications, prefabricated wall and divider lamination, door lamination applications, and wall panel lamination in the RV industry. PUR hotmelts are rapidly becoming commonplace in edgebanding applications.



D.P.: PUR hotmelts are prevalent in the automotive, transportation, bookbinding, door assembly, and woodworking industries bonding substrates such as wood and wood products, steel, aluminum, ABS, polycarbonate, polystyrene, vinyls, HPL and fiberglass. PUR hotmelts are increasing in popularity in the woodworking industry not only because of their excellent bonding capabilities, but also because of their contribution to improved process throughput. They eliminate the need for nails, staples, screws and other mechanical fasteners as well as the follow-up steps of puttying and sanding. Examples where PUR hotmelts are used in the woodworking industry include: bonding decorative mouldings, cornerblocks, base rails and mitre joints. PUR hotmelts also bond to many painted, lacquered and finished surfaces.



Q: What are some of the advantages of PUR hotmelts?



J.A.: Once a polyurethane crosslinks and forms a permanent bond, it can never be broken again. The bond usually becomes stronger than the substrate itself. This gives them extremely good heat and moisture resistance properties which makes them ideal for kitchen and bathroom counter tops, window and door applications and the RV industry. Polyurethanes are also good at sticking diverse substrates such as vinyl, paper, film or foil to metal, wood, PVC, and HPL to MDF, particle board, and honeycomb core materials.



D.P.: PUR hotmelts exhibit numerous desirable bonding features simultaneously. They are easy to process, are solvent free (no VOCs), require no drying time, have high initial and final bond strengths, have good high and low temperature properties and exhibit excellent adhesion to a wide variety of substrates. Assemblies prepared with PUR hotmelts can be reworked, machined or finished immediately after bonding.



Q: What are its drawbacks?



J.A.: As stated earlier, PUR hotmelts do not form an initial bond as quickly as conventional hotmelts. They are thus not suited for ultra high line speed applications, at least when additional operations will be performed on the work piece soon after bonding. PURs should also be used properly. Proper equipment and ventilation should be used.



D.P.: PUR hotmelts cost more per pound than other adhesives such as PVAs, conventional hotmelts, and contact cements. However, when taking into account that PUR hotmelts are 100% solids, will increase productivity, reduce labor costs, improve first pass quality and provide excellent product performance, an overall system cost savings should be realized.



Q: When does it make sense to use PUR hotmelts in edgebanding applications?



J.A.: Edgebanding applications are using more and more hotmelt PUR materials. Many producers and their customers are demanding higher performance from their adhesive. They want a much higher level of bond strength and a reduction in delamination problems regarding edge banding. This is especially true with kitchen and bath counters and any surface exposed to heat, moisture and UV light.

The strong bond that is formed when using PUR hotmelts helps eliminate delamination problems in edgebanding applications.

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.



D.P.: PUR hotmelts should be used in edgebanding applications when high thermal, moisture and/or solvent resistance is needed. They should also be considered when bonding dissimilar, difficult, rigid or non-porous substrates and when a quick bond line set-up is desired.



Q: What developments do you see on the horizon?



J.A.: PUR use is growing rapidly in the woodworking and furniture industry. Europe is three to five years ahead of North America on the introduction of PUR into many woodworking applications. Looking at Europe as an example, we can be sure this growth will continue here. Furthermore, many office and home furniture companies are having to extend warranties out to 10 years to remain competitive. The high performance they get using PUR hotmelts helps these producers achieve these term warranties.

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.



D.P.: PUR hotmelts will continue to increase in popularity as the woodworking industry searches for ways to improve product quality, increase throughput and lower costs.