Adhesion Issues Using Construction Adhesive and Fire-Retardant-Treated MDF

      Getting panels to stick can be tricky, especially if the panels are burnished or chemically treated. Here is some info on adhesive choice and application methods.October 17, 2011

Iím using LN-903 Liquid Nails Multipurpose Construction Adhesive in Las Vegas, NV to glue MDF Panels to leveling pads and furred out strips (also made of MDF), and encountering are having numerous delaminations. We are also using it to glue finished trim to backer strips at door locations. Even after one week of clamping, we are experiencing creep and adhesion failure in almost 100% of installations. All products are fire treated and the trim products have sealer and catalyzed finish on both sides.

Upon inspection the cured Liquid Nails has a soft bubble gum consistency and parts from the substrates easily with a sharp chisel leaving an almost clean surface. Normally we would apply liquid nails, adhere the panel, and screw it off where fasteners would not show. Panels continue to delaminate from the walls - sometimes a week to ten days later. We have tried applying adhesive to the MDF and pressing it against the furring strips them pulling it away and applying more adhesive to those witness spots. It doesn't seem to make a difference. Has anyone else experienced these problems working in a hot environment?

Forum Responses
(Adhesive Forum)
From Gene Wengert, forum technical advisor:
It sounds like you are expecting this construction adhesive to cure and get hard, but that is not characteristic of construction adhesives. They do remain soft and pliable and are affected by heat. Liquid Nails has a 43 page booklet on the use of their various adhesives; it is an excellent source of info.

Do you put the Liquid Nails on in a serpentine pattern, then push the panel against the adhesive beads, then pull the panel off and allow the solvent to quickly evaporate and then put the panel back on (per LN instructions)? LN-910 and LN-710 are designed for paneling, so you might want to try them rather than LN-903.

Sometimes when MDF is manufactured, a wax is added to increase water repellency. Sometimes the surface is burnished from the heat in manufacturing. Try touch sanding (very light, one pass) the MDF prior to gluing in order to obtain a better wood surface for gluing.

From contributor Z:
Few glues will stick to a sealed and catalyzed finish.

From contributor A:
We use elastomeric adhesive to glue back sealed wood flooring. We learned that the finish on the bottom needs scratched with 120grit paper to get any adhesion out of the adhesive. With a good scratch in the back seal we get a very adequate bond. When testing we will pry the material from the substrate and either tear the wood or subfloor and not peel the glue from the wood.

From the original questioner:
I spoke to a Liquid Nails rep this morning and he covered the scuffing and humidity issues. Our MDF panels have a fire treated rating and that might be some of the problem. He said his product acts like water based latex adhesive and needed air circulation to allow the water in it to evaporate. He had no explanation as to why it was curing in a crystalline form except to note that any adhesive which squeezes out is basically wasted.

The solvent based Liquid Nails we could use almost to "weld" part together. This product needs close surface contact. He mentioned the bridging qualities are less than the solvent based Liquid Nails though he claimed the cured bond was stronger. Our current temp and humidity is around 105 and 11% humidity inside the building. In application it is almost as if this adhesive flashes and "skins over" if conditions are not optimal. Unfortunately the contract specifies a low VOC adhesive. Polyurethane was sticking to both the MDF and the back finished products but requires an exception in the contract.

From Gene Wengert, forum technical advisor:
LN-710 is low voc.

From contributor R:
Not sure if this applies to your glue but we have had issues in the past laminating HPL to fire rated MDF. The issue was that the PVA (water based glue) we used was not compatible with the fire rated board. We were told that the fire rated board had a chemical in it that caused the water in the glue to be absorbed much faster causing a bad glue bond.

From contributor W:
My first thought was the fire retardant treatment. On treated plywood I notice a dusty crystalline residue which can negatively affect anything sticking well. We try to use mechanical fasteners where ever possible, although most of our FRT work has been doors, jambs, and adjacent trim.

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