A significant problem in the manufacture of wood-based composite products that are exposed to the exterior and extreme interior environments is that upon exposure to variations in temperature and moisture, the wood can lose water and shrink, or gain water and swell. This tendency to shrink and/or swell can significantly limit the useful lifetime of most exterior wood products, such as wooden doors, often necessitating replacement after only a few years. The problem is particularly prevalent in areas of high moisture (e.g., Hawaii) or in climates that are extremely hot or dry (e.g., Arizona). Shrinking and swelling can also be a problem when the wood is exposed to a wet environment during construction, or upon exposure to the dry heat used indoors in the winter.
A possible solution to the problem of moisture gain and loss in wood exposed to the elements includes covering the wood with paint and/or other coatings that act as a barrier to moisture. Still, such coatings tend to wear off with time, leaving the wood susceptible to the environment.
Rather than treating the unit at the site of installation, it may be preferable to manufacture products that exhibit increased resistance to moisture gain and loss. For example, increasing the amounts of resin content or decreasing the amount of wood fiber used in a door can increase resistance to water gain and water loss. However, such modifications can be associated with significantly increased production costs. Other options include the use of metal or fiberglass doors, but such doors are not always as aesthetically pleasing as wood doors and may have other performance problems associated with the use of these materials.
Alternatively, doors, and other structural units, may be covered with a wood-containing water-resistant layer. For example, doors may be covered with a thin-layer wood composite known as a doorskin. Doorskins are molded as thin layers to be adhesively secured to an underlying door frame to thereby provide a water-resistant outer surface. Doorskins may be made by mixing wood fiber, wax, and a resin binder, and then pressing the mixture under conditions of elevated temperature and pressure to form a thin-layer wood composite that is then bonded to the underlying door frame.
Wood composite doorskins are traditionally formed by pressing wood fragments in the presence of a binder at temperatures exceeding 275° F. (135° C.). The resin binder used in the doorskin may be a formaldehyde-based resin, an isocyanate-based resin, or other thermoplastic or thermoset resins. Formaldehyde-based resins typically used to make wood composite products include phenol-formaldehyde, urea-formaldehyde, or melamine-formaldehyde resins. Phenol-formaldehyde resins require a high temperature cure and are sensitive to the amount of water in the wood since excess water can inhibit the high temperature cure. Urea and melamine-formaldehyde resins do not require as high of a temperature cure, but traditionally do not provide comparable water-resistance (at the same resin content) in the doorskin product.
As compared to doorskins made using phenol-formaldehyde resins, doorskins that utilize high-temperature pressed isocyanate resin binder display increased surface strength. However, these doorskins exhibit decreased porosity to adhesives and thus, do not bond well to the underlying doorframe. Also, isocyanate-bonded wood composites made using currently available methods and compositions do not consistently exhibit sufficient resistance to environmentally-induced swelling and/or shrinking to be commercially useful. Thus, there remains a need for a commercially viable method to produce a thin-layer wood composite that displays resistance to shrinking and swelling. Such thin-layer wood composites are useful to protect doors and other wood-based structures exposed to the environment.