Phenolic resins for the wood industry have been refined for the past twenty years to the current "state of the art" and as such, perform to a well defined standard. The recent development of isocyanates have provided wood adhesives that are far faster curing than phenolics and are replacing phenolics on an ever increasing basis due to their faster cure rates, even though they have negative health and economic aspects.
However, phenol-formaldehyde resins remain widely used as adhesives and binders in many wood products, including structural wood products such as plywood, particleboard, fiberboard, hardboard and oriented strandboard. The productivity of most mills manufacturing structural wood products using liquid phenol-formaldehyde resole (PF) binders is limited by the cure speed of the binder in the hot press. This is because of the inherently slow thermal cure of these products, compared to other commonly used binders, and because of the need to eliminate moisture from the system during curing.
Plywood is a glued-wood panel that is composed of relatively thin layers, or plies, with the grain of adjacent layers at an angle to each other (usually 90.degree.). The usual constructions have an odd number of plies to provide a balanced construction. If thick layers of wood are used as plies, often two corresponding layers with the grain directions parallel to each other are used; plywood that is so constructed often is called four-ply or six-ply. The outer pieces are faces or face and back plies, the inner plies are cores or centers, and the plies between the inner and outer plies are crossbands. The core may be veneer, lumber or particleboard, the total panel thickness typically being not less than one eighth inch nor more than two inches.
In general, the plies are dried to remove moisture to a level which is compatible with gluing. The plies are coated with a liquid glue, front and/or back as appropriate, with a glue applicator. Heat and pressure are applied in a hot press to cure the glue and bond the panels together to form the plywood.
Dry process composition board is a common form of composite panel. It may be made from wood fibers. In the manufacture of the board, raw wood is broken down to a fibrous form, sprayed with an appropriate adhesive, and then formed into a mat by a sifting or dry forming technique. This mat is then subjected to a high pressure and an elevated temperature to compact the mat to the desired density, commonly 40-60 lbs./ft..sup.3 In this hot pressing operation, the high temperature causes the resin to harden and to form an adhesive bond between the fibers.
In the preparation of particles used to make particleboard, a variety of materials may be employed. The board may be formed from a homogeneous type of particles. That is, all of the particles may be flakes, or all of them may be fibers. The board may be formed from a single layer or it may be multilayered, with fine surface flakes applied over a core of coarse flakes, or there may be a coarse flake core having an overlay of fibers on each of its surfaces. Other combinations are also used.
In the manufacture of particleboard, an aqueous solution of a synthetic resin binder is sprayed on the wood particles in an amount of from about 6 to about 10 parts of resin solids per 100 parts of dry wood. The resin-treated particles are then formed into a mat, and compacted in a hot press to the desired density. This type of panel is usually made to have a density in the range from about 35 lbs./ft..sup.3 to about 45 lbs./ft..sup.3 Typically, the thickness of particleboard would fall in the range from about one-eighth inch to two inches.
This type of process is quite versatile. Materials that would otherwise be waste materials can be formed into desirable products. For example, planer shavings can be formed into useful particleboard by this process, used alone, or in combination with other wood particles.
The mat process has been refined and improved, so that it is now common to make a multiple-ply board. For example, three forming heads may be used. Each head effects the placement of flakes, fibers or particles that have had resin and wax sprayed onto them, on a moving wire, or caul plate. The first forming head lays down a fine surface material, the second lays down a coarser material for the center layer of the board, and the third head lays down another outer layer of fine surface material.
In addition to the mat-forming hot pressing process, an extrusion process is now in use. In this process, a mixture of wood particles, resin and a wax size is forced through a die to make a flat board. The extrusion process is commonly used for captive production by companies which produce the resulting composite panel for use in furniture cores.
Some modern processes make use of a combination of press curing with hot platens and heat generated by radio frequency electricity. This combination permits rapid curing with a minimum press time.
While the dry process techniques for manufacturing composite panels are entirely dependent on synthetic resin adhesives, there are wet process techniques that can be used to make panels without any synthetic resin adhesive. However, often in actual practice the manufacturer of a wet process panel such as a hardboard will add a small amount of a synthetic resin binder in order to improve the properties of the product so that it can be used in demanding applications. Often the proportion of resin binder used is on the order of one-tenth to one-twentieth of the proportion used in the dry process.
In the mat-forming stage of the wet process, a slurry of fibers is drained on a screen to form a wet mat. Often the mat is produced as an endless ribbon and cut into the desired panel size for curing.
In the manufacture of hardboard, the wet mat is treated somewhat differently than in the dry process. The wax emulsion, for example, is added in the wet end of the mat-forming machine. Enough emulsion, generally of paraffin was, is used to add from about 0.3% to about 3.0% of wax to the fibers, dry basis. Similarly, when a resin binder is added in the wet process, it is generally added to the fiber slurry before the mat is formed. It may be precipitated onto the fibers by acidifying the slurry with alum.
Wet process techniques are often also used in the production of insulation board. This kind of product emphasizes a low density structure that combines thermal insulating and sound-absorbing properties in a composite panel type of product. With the addition of synthetic resins and other additives, properties such as surface quality, strength and moisture resistance of insulation boards can be improved.
Normally, a phenolic resin is modified in its molecular weight to effect cure. Increasing the molecular weight will decrease the time required for complete cure; however, as the molecular weight increases, the mobility or flowability of the polymer is reduced. A phenolic resin must be mobile at the time of cure to be able to wet the adjoining substrate. Moisture is also an aid to phenolic resin flow; therefore, flowability is affected by the amount of adhesive applied and the dwell time allowed prior to hot-pressing. As long as sufficient moisture is present, the polymer will demonstrate adequate flowability; however, as the adhesive sets on the wood substrate, moisture will migrate away from the glue line and into the wood. If the dwell time prior to hot-pressing is excessive, or if conditions favor the absorption of water by the wood such as high ambient temperatures or low moisture wood, the resin will not show adequate flowability nor the ability to wet the substrate, and an inferior bond will result.
When a resin is designed for a given application, the molecular weight is usually selected by compromise to allow for sufficient flow of the resin with the constraints imposed by the user's process. If a faster cure is required, then a reduction in resin flow must be tolerated or acommodated by change in the manufacturing operations.
Increased formaldehyde concentration improves the cure speed of phenolic resins, but leads to excessive formaldehyde emissions.
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