By Kent Pitcher
This article describes the gluing of high density plastic laminates (i.e. Formica, Debar, Nevamar, Micarta, Pionite and the like) to plywood, lumber core, and particleboard core panels. Such laminations are widely used for table, chest, sink, cabinet and counter tops.
One of the major problems encountered in bonding high pressure plastic is telegraphing of defects from the underlayment or substrate. This telegraphing is generally accentuated by the high gloss surface on the plastic.
Elimination of telegraphing generally requires either better surface preparations, as with particleboard, or crossbanding with at least one ply of a defect free, close-grained species of veneer, such as poplar or basswood, between the core and plastic. This veneer may be bonded to the core in a separate operation, the assembly sanded and then the plastic face and backing sheets applied.
Sometimes bonding plastic directly to lumber core, particleboard core or rough cross banding will highlight any voids, knots or glue lines present. When bonding the plastic and the veneer to the underlayment in one operation, the pressure required must be carefully controlled for it can cause any open defects, glue lines and/or rough grain in the veneer to telegraph.
Foreign matter in the glue line will also show through the plastic. Straining the glue mix to remove any sawdust, dirt or dried glue before putting it into the spreader is recommended. Also, brushing the plastic with a soft-bristle cleaning brush to remove sanding dust or clinging wood and dirt particles is suggested.
The gluing surface of the plastic should be sanded for best adhesion. The manufacture of high density plastic laminates utilizes extremely high pressures and temperatures. This results in two hard glazed surfaces. The non-decorative surface which is bonded requires some sanding to roughen the surface to achieve maximum adhesion. A fine finished sanding should be used, since coarse or rough sanding has a tendency to telegraph.
To prevent warping, a less expensive backing sheet should be used to balance the construction. The backing sheet must have the same rate of expansion to heat and the same moisture barrier properties as the decorative face. Most manufacturers of decorative face plastic provide a matching backing sheet.
Pressures required to bond plastic are relatively low, usually 25 to 50 p.s.i., although occasionally 100 p.s.i. will be reached. As pressure is increased, the tendency to telegraph also increases. Pressing can be done by any of the conventional methods; hot press, cold press, or contact depending on the adhesive system employed.
Several hot press urea-formaldehyde resin formulations are used to bond plastic. Because of the expansion differentials between the plastic and most substrate materials, platen temperatures above 190F are not recommended. In addition, higher temperatures may cause blistering of the plastic face sheet due to moisture present.
Improved wetting of the dense plastic surface by urea resins can be accomplished by addition of Craze-Proofing Liquids. Where a craze-resistant bond is required, an internally plasticized urea resin can be used. Phenol-resorcinol resins are rarely used for hot pressing, but if employed in this manner, they will provide a waterproof bond.
In cold pressing plastic laminates, urea-formaldehyde, polyvinyl acetate emulsions and urea-polyvinyl acetate mixed adhesives, are regularly employed. Phenol-resorcinol adhesives are utilized, but to a lesser degree. For craze-resistance and improved wetting, either a craze-proofing liquid is added to a conventional urea resin or an internally plasticized resin may be used.
It is often desirable to utilize an adhesive with a high wet tack or quick set. In such instances, a polyvinyl acetate emulsion adhesive is recommended. Where both the tack and setting speed of a polyvinyl and the water resistance and economics of a urea are desired, a blend mixture of the two adhesives can be used.
Casein adhesives can be used where plant temperatures are below 65F, but utilization is somewhat limited because high moisture introduced into the glue line that may cause telegraphing through the plastic surface.
Both solvent and water based contact adhesives are utilized to bond plastic laminates. The general directions for using these systems is to apply the adhesive to surfaces to be glued by applicator, roller, spreading or spraying, and allowing to dry. The surfaces are then mated and pressure applied sufficient to bring the surfaces into intimate contact with each other. This is accomplished with nip rolls, and air or hydraulic presses. Contact cements are extensively used to bond plastic edge banding.
Note: Best results are obtained when the gluing surface of the laminate has been sanded. Both adhesive and stock should have a temperature of at least 70F when bonding high pressure laminates.
The following is a description of the troubles often encountered in high pressure plastic laminating and some possible causes:
Telegraphing - Improper underlayment or too low a grade. Laminating pressure too high. Unsanded or rough sanded underlayment or plastic. Foreign matter in glue line or on face of plastic. Laminating directly to core stock (lumber or particle- board) without x-banding.
Loose corners or edges - Core material not sanded evenly. Non-uniform pressure due to poorly aligned platens or nip rolls. Plastic not thoroughly sanded. Too low a laminating pressure Improper loading in press.
Blisters under plastic face - Core material not sanded evenly. Plastic not thoroughly sanded. Air entrapped (contact adhesive). Too low a laminating pressure. Excessive assembly time. Insufficient glue spread.
Warping - Improper backing sheet. Core material not balanced. Too high a gluing temperature (hot pressing).
Adhesive on plastic faces - Too thin an adhesive mix. Too heavy an adhesive spread. Improper stacking in press.
Slippage - Improper stacking in press. Too heavy an adhesive spread. Too thin an adhesive mix. Improper and uneven pressure.Carelessness - spots on decorative surface must be washed off with water while soft. Wax decorative surface to prevent sticking.
This article was provided by Kent Pitcher of Custom-Pak Adhesives, Inc.
Comment from contributor A:
Edgebanding with hpl... I find that most hot melt glue will not work with particleboard, but works fine with plywood and MDF.