Man-made consolidated cellulosic articles, such as fiberboard, may be molded to have either planar or three-dimensional shapes and embossed with various designs and patterns, such as a wood grain appearance of natural wood. Fiberboards are a well known and widely used class of consolidated cellulosic articles that include such materials as hardboard, soft board, and medium-density fiberboard (MDF). Chipboards such as particleboard, medium-density particleboard, and oriented strandboard (OSB) constitute another useful class of consolidated cellulosic articles. Composite structures of these boards are also useful.
Various processes have been practiced to produce consolidated composite articles such as those mentioned above. The principal processes for the manufacture of consolidated composite articles include wet felted/wet pressed or “wet” processes; dry felted/dry pressed or “dry” processes; and wet felted/dry pressed or “wet-dry” processes.
Generally, in a wet process, cellulosic materials such as fibers (e.g., fibrillized wood fibers) are blended in a vessel with large amounts of water to form a slurry. The slurry preferably has sufficient water content to suspend a majority of the cellulosic fibers and preferably has a water content of at least ninety weight percent of the wood fibers. (All percentages disclosed herein are by weight, unless specifically stated otherwise.) The slurry is combined with a synthetic resin binder, such as for example a phenol-formaldehyde resin, and deposited onto a water-pervious support member, such as a fine screen, where much of the water is removed to leave a wet mat. The resulting cellulosic material has a moisture content, for example, of about fifty weight percent based on the dry weight of the cellulosic material. The wet mat is transferred from the pervious support member to a press and consolidated under heat and pressure to form the consolidated composite article, such as a door facing or other desired article.
A wet-dry forming process typically includes blending cellulosic raw material, e.g., wood fibers, in a vessel with large amounts of water having a pH of less than seven to form a slurry. This slurry is then blended with a resin binder. As in the wet process described above, the blend is then deposited onto a water-pervious support member, where a large percentage of the water is removed, thereby leaving a wet mat of cellulosic material having a water content of about fifty weight percent, for example. This wet mat is then transferred to an evaporation zone where much of the remaining water is removed by evaporation. The dried mat preferably has a moisture content of less than about thirty weight percent based on the dry weight of the cellulosic material. The dried mat is then transferred to a press and consolidated under heat and pressure to form the consolidated composite article.
In a dry process, the cellulosic material is generally conveyed in a gaseous stream or by mechanical means rather than a liquid stream. The cellulosic material may be first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin. The cellulosic material is then randomly formed into a mat by air blowing one or more layers of the resin-coated cellulosic material onto a support member. The mat may optionally be subjected to pre-press drying. The mat, typically having a moisture content of less than about thirty weight percent and preferably less than about ten weight percent, is then pressed under heat and pressure to cure the thermosetting resin and to compress the mat into an integral consolidated article.
In the processes described above, the mat is typically consolidated in a press between upper and lower press platens. After compression, the resulting molded article may include a surface intended to be exteriorly disposed, such as a door facing exterior surface. The aesthetic appearance and physical properties of the exterior surface may play important roles in evaluating the quality and functionality of the article. However, the compression process sometimes results in an article having a surface with undesirable qualities. For example, the surface of the article removed from the press may possess imperfections such as cracks, voids and high porosity. Without wishing to be necessarily bound by any theory, it is believed that these defects are generated when the consolidated material “sticks” to the platens as the platens release. These imperfections not only adversely affect the aesthetic quality of the door skin, but also deleteriously affect important properties, such as reducing surface hardness. Even if sufficient press release is achieved, the surface quality of the article may still be inadequate, given the high surface porosity often present in a consolidated cellulosic article.
In order to provide the desired surface characteristics to the finished product, a sealer or finish coat may be applied to the molded article after removal of the molded consolidated composite article from the press. Often, the surface color is also desirably altered by the precursory step of applying a primer to the surface of the molded article, thereby providing a ready-to-finish surface on the composite articles conditioned to accept paint or stain, for example. However, conventional sealers do not adequately fill the relatively large surface cracks, voids, and pores sometimes present in consolidated cellulosic articles. As a result, the consolidate cellulosic articles, even after receipt of a surface treatment, may possess a poor surface seal and high water sensitivity and permit moisture penetration. Articles such as exterior door facings are often exposed to large amounts of water (e.g., rain and high humidity). The penetration of moisture into a poorly sealed surface of a consolidated cellulosic article can lead to fiber swelling, which can manifest as cosmetic blemishes and blistering, making the article commercially unacceptable to discriminating consumers.