" " wisno wood furniture finishing: Polyurethane adhessives in the woodworking

Saturday, March 2, 2013

Polyurethane adhessives in the woodworking

Polyurethane adhesives (polyurethane glue) contains the urethane resin as the main ingredient. There could be some variation in the formulation depending on the type of urethane used. In the application the urethane resin is mixed with the isocyanate to produce the crosslinking polymerization to form strong hard and insoluble bond. The uses of PU adhesive in the woodworking industry may be not very much, but the glue provides some advantages that make it is useful for some special purposes. The glue is often used to do: gluing for the “relatively” wet wood,  gluing with a short drying time and gluing for wood laminated with low-pressure press.
PU glue has some advantages such as: water resistant, strong, and fast drying. This glue also has low viscosity that make it is quite easy to be applied on the wood. The glue setting is happened in a relatively short time which is useful to do the gluing in a short time. It can do the gluing for the working condition when the clamping press is not available.  But the most preferred property of the glue is the ability to fill and close the gap and cavity at the joint. The PU glue can help the woodworker to address the voids or gaps problem that are often left out at the wood joint. The isocyanate can react with the water in the wood to form a solid mass that will harden and fill the existing gaps in the joint. The PU adhesives is the best glue for joint the end grain. The strength of the glue and its ability to fill any voids on the surface could be a good combination to address the problem of gluing the end grain.
For the regular wood gluing the glue may be not the best choice. It is expensive and rather difficult in handling; the application and the storage. The glue could give the strong and durable bond. But the wood is an anisotropic material (material that is not homogeneous) with high variation properties make the wood gluing become more complex. In glue testing, the bond failure might be not caused by the failure of the polyurethane adhesive itself but because of the condition of the wood. In the glue testing for most of wood gluing, the good result can be achieved by using the cheaper glue.
The polyurethane glue is two components that require perfectly mixing in the application process. The bond failure could also could be caused by the imperfectly mixing. The glue use the isocyante, a reactive substance. Isoyanate easily reacts with water, so he should kept at the closed container to prevent reaction with air from the humid air.

Tips in using PU glue
  • Use the glue selectively.
The PU glue is expensive and somewhat difficult to be handled. The inappropriately used could be "the luxuries process" that lead to expensive cost. The glue should be used when the cheaper glue does not work. Do some trial and test to find the best glue for some specific process
  • Do the proper process and handling.
A polyurethane adhesive is the two components glue that is relatively difficult to be handled and applied. Read the technical guidance carefully to get the maximum benefit from it. Avoid to storage this glue in too much amount so we don’t get dizziness problem with the storage.
  • Preparation of the wood.
The success of the wood gluing depends on preparation of the wood. Make sure that the wood pieces are dry enough before they are glued and unified. Although technically the glue could be used for the relatively wet wood, it does not mean that we can glue the wet wood carelessly. We may be able to get a good adhesion at the wood joint, but we still have risk to get the problem caused by the wet wood such as: the wood cracked, bend, the fungus problem, etc.
Make precision cuts and slices as good as possible. Although the glue can fill the gap, but of course this is not the best option to eliminate the gap at wooden connections. Using glue to fill the gap will leave the disconnected grain and pores to spoil the wood looked.

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