Glue used in Exterior and Marine Plywood Manufacture
Adhesives used in exterior and marine plywood manufacture is a source of great confusion among budget conscious amateur boatbuilders. Here are some details. Also read my page on Resorcinol type glues which are sometimes used in marine grade plywood. If your plywood has a very dark or purply glue it is possibly resorcinol or more likely a closely related Phenol Formaldehyde glue.
Marine Glues, what is WBP
Glues used in exterior and marine plywood that will be exposed to water and weather is described as WBP (Weather and Boil Proof). This only means that a plywood sample glued with this adhesive does not delaminate after a prolonged period of boiling. It is NOT a description of the type of glue. WBP only describes its characteristics. Several glues have proved to be WBP.
Most WBP glues are either melamine or phenolic glues. Lower quality melamine plywood can sustain 4-8 hours boiling without delaminating. Higher end melamine can last between 10-20 hours in the boil test.
Phenolic plywood is considerably more resistant (24-72 hours). The best phenolic glue can be regarded as essentially permanent adhesives and are used in the higher grades of Marine Plywood.
Various organizations have set up standards for the testing of glues and marine plywoods must conform in order to be able to claim conformance to the marine plywood standards.
It is likely that under sustained boiling, plywood plies glued with high end phenolic adhesive, will fail before the glue. It is therefore important to consider the ply quality and manufacture quality along with the quality of the adhesive. No amount of quality WBP will keep a poorly manufactured panel made with inferior plies together.
Cost is an important factor in the choice of adhesive. Certain materials such as Epoxy might be acceptable as a glue, but are not used because of their high price. Ease of use is another factor that comes into play. It is more difficult to use adhesives which have 2 components or have short pot life.
In Canada the Canadian Plywood Association (CANPLY) certify plywoods. Although no specifically marine plywood is manufactured, exterior plywood uses Phonol Formaldehyde as a component of its glue. Its process is typical of quality marine plywood manufacture.
Phenol Formaldehyde is a two part synthetic glue. Phenol (C6H5OH) is reacted with formaldehyde (CH2O) under specific temperature conditions and produces a thermosetting resin (heat set resin). This resin has properties that are completely distinct from phenol or formaldehyde. The formaldehyde is converted to stable methylene linkages with minimal formaldehyde outgassing. These do not break down under exterior conditions. Phenolic resin is often supplied in a water solution premixed with (caustic soda) a catalyst. Soda Ash and fillers and extenders (such as bark and wheat flour) improve gluing characteristics.
Plywood is manufactured by bonding layers of wood veneers with the phenolic resin glue. The panels are heated to 150°Centigrade (300°F) under pressure of about 200 psi. Polymerizing (curing and hardening) then occurs. The result is an inert and boil-proof bond.
What is MR plywood
Like the term WBP (Weather and Boil Proof) the MR (Moisture Resistant) term describes a characteristic of glue rather than a specific formulation. MR glue manufactured plywood can withstand immersion in cold water for several days without delaminating. Exposure to hot water will cause some panels to delaminate within 30 minutes. These panels are perfect for interior or furniture applications but have no use for boats.
Film Faced Plywood
These plywoods are made with a phenolic impregnated paper covering or replacing the face veneers. They are produced in great quantity for use in concrete forms, where the smooth outside allows for easy release. They are also used for signs, furniture and any application where ease of finish or even a pre finish is useful.
The adhesive used is often WBP (definitely moisture resistant MR). Where Film Faced Plywood is most different from Marine Plywood is in the quality of the plies. The film faced ply is more likely to have voids, knots, joins including butt joins and repairs. The quality and strength of the veneers needs not be as good as marine plywood.
Marine vs Exterior
Both marine and exterior plywood are regulated so that the adhesives used in their manufacture can withstand severe conditions including boiling for several hours.
The main differrence is in the quality of the plies used. Number of knots, joins, repairs, voids are all strictly regulated in marine ply and less so in outside ply. For more details see my Marine plywood page.
This web site reflects my personal ideas and doesn't represent anyone else's point of view.