Wood is an extremely desirable construction material because it is inexpensive, durable, lightweight, relatively weather resistant, and easily fashioned into useful end products. However, certain of the natural characteristics of wood must be overcome for man to make use of many potential wood products. For instance, wood has knots, splits, and pitch pockets that mar its surface. Wood has high and low density growth rings that are difficult to hide. Wood swells and shrinks dramatically with moisture variation. Wood also has soluble extracts that may leave the surface of the wood and participate in undesired chemical reactions with substances at the wood surface.
Consequentially, a number of methods have been used to take advantage of wood's desirable characteristics while overcoming the undesirable characteristics. In the specific case of plywood, the methods include: selecting higher quality veneers for exposed panel surfaces; removing and repairing knots and pitch pockets; repairing splits; sanding the veneers to impart a smoother surface; and applying overlays.
Overlays for plywood, and similar substrates, have proven relatively effective in hiding veneer repairs, helping provide a smoother surface, reducing shrink and swell, and providing a surface seal that prevents water from entering and wood extracts from exiting through the surface. However, in recent years the quality of available logs and, correspondingly, veneer quality have dramatically dropped. Among the factors promoting this drop in quality are the following: (1) virtually all of the fine grain, slow growing, and relatively defect-free old growth timber has been harvested, being replaced by fast growing and knotty second growth timber, due to the fact that timber harvest rates have exceeded sustained yield rates and due to the dramatic increase in log export volume to the Orient; (2) the desire to retain for posterity the small amount of old growth timber left; and (3) the discovery that some species (e.g., the spotted owl) can exist only in old growth forests. As a result of the drop in wood quality, the situation has gone from one where overlays were readily and easily used to upgrade high quality veneers to one where even maximum overlay efforts via conventional methods may lead to undesired surface characteristics due to the lower quality of the underlying veneer.
In an effort to circumvent this problem, a defect-free substitute for outside plywood veneers was developed. As discussed in U.S. Pat. No. 4,210,692 issued to Bohme et al. in 1980, sawdust, pulp, and 0.2 to 4.0% resin are formed into a sheet of essentially homogeneous material, known as Lebonite, which is then subjected to a hot press. The result is a veneer of excellent surface smoothness that may be employed as the surface ply in plywood. While not an overlay per se, its purpose is identical. However, a chief disadvantage of a Lebonite veneer is that it possesses virtually no water resistance. Consequently, its use has been restricted to interior applications.
A chief area of application for plywood overlays is with concrete forms. This application requires that the surface of the form possess a very smooth finish so as to impart a similar finish to the surface of the concrete. Additionally, it is imperative that the overlay surface possess abrasion resistance and, more importantly, water resistance. Clearly, Lebonite could not be used in the concrete forms application due to its poor water resistance. While the desired surface properties are possible with Formica overlays and the like, they have proven too expensive for concrete form application. Instead, sheets of paper impregnated with resin material, usually phenolic resin, have been used to create the desired overlay.
For years, most concrete forms were overlaid with a single cured cellulose sheet impregnated with medium density phenolic resin (hereinafter MDO, which stands for medium density overlay). The same is true today. However, this overlay possesses only moderate abrasion resistance and water resistance. Accordingly, concrete forms bearing this overlay are capable of only approximately 20 reuses before an intolerable level of degradation of the form occurs.
In an effort to increase the number of reuses possible with concrete forms, an overlay was developed that consisted of two uncured cellulose sheets impregnated with high density phenolic resin (hereinafter HDO, which stands for high density overlay). When used on the higher quality veneers abundantly available in the past, this overlay resulted in a concrete form capable of as many as 100 reuses before intolerable degradation occurs. The increase in reuses was directly attributable to the much improved abrasion resistance and water resistance of this overlay.
HDO and MDO are defined under U.S. Products Standards (PS1-83), which state that cellulosic overlay sheets having 22 to 45% phenolic resin are MDO, whereas those having greater than 45% phenolic resin are HDO. Since phenolic resin is a thermosettable material, it has a fixed final structure after curing has taken place. MDO sheets are traditionally completely cured during fabrication. Thus, their resin component will not remelt and flow when subsequently heated during adherence to the plywood substrate. Additionally, an adhesive must be employed to provide adherence. In contrast, HDO sheets are not cured during fabrication, so their resin component will remelt and flow during subsequent heating. It is this characteristic that makes HDO sheets self-binding, thus eliminating the need for an adhesive.
As the quality of veneer has decreased, the ability of the double HDO sheet overlay to adequately cover veneer defects has also decreased. The result has been a drop in surface smoothness, abrasion resistance, water resistance, and number of reuses previously possible with the double HDO sheet overlay.
In an attempt to retain the increased level of reuses and surface quality, an overlay consisting of two HDO sheets over a single MDO sheet has developed in the last three or four years. This laminate overlay is known in the plywood industry as a Hi-Med overlay. While the Hi-Med overlay is roughly twice as costly as a single MDO sheet or a double HDO sheet overlay, it has produced concrete forms of moderate smoothness capable of approximately 50 reuses on the typically available lower grade veneers of today.
A single sheet of HDO having a very high resin content (nearly 70%) over a single sheet of MDO was attempted as a concrete form overlay. This attempt was unsuccessful because the extremely high resin content led to extensive overlay shrinkage. The shrink factor simply could not be overcome, and this approach was quickly abandoned in favor of the Hi-Med overlay.
When concrete forms bearing a Hi-Med overlay are produced, the overlay and the underlying plywood substrate of the concrete form are usually formed together in a one-step manufacturing process. In this one-step process, the two HDO sheets over a single MDO sheet that make up the Hi-Med overlay are positioned upon a caul, which is a thin sheet of polished aluminum. The HDO sheet distant from the MDO sheet is in contact with the caul. If an overlaid plywood panel measuring four feet by eight feet is the desired product, the caul measures roughly 52 inches by 101 inches, and has a thickness of 0.04 inches. The purpose of the caul is to help provide as smooth an outer surface to the overlay as is possible. To keep the overlay from sticking to the surface of the caul, the caul is treated with a release agent. Often, a solution of approximately 10% stearic acid in isopropyl alcohol is used.
Using 5-plywood as an example, after the two HDO and one MDO sheets are laid down on the treated caul, the following items are chronologically stacked over the sheets: a layer of low water content adhesive, a face ply, a layer of glue (production grade), a core ply, a layer of glue, a center ply, a layer of glue, another core ply, a layer of glue, a back ply, a single HDO sheet as a backer sheet used to control warp, and a second treated caul. The backer sheet helps control warp by counteracting the warp caused by the overlay at the opposite surface of the solid substrate. Since an HDO sheet is self-binding, no lower water content adhesive would be required. If an overlay surface is desired on both sides, the backer sheet is replaced by a layer of low water content adhesive and the two HDO and one MDO sheets making up the second overlay. As is obvious, it is standard practice for the highest quality veneers to be used as the outermost layers of the plywood substrate over which is laid an overlay.
Normally, the low water content adhesive is prefabricated onto one side of the MDO sheet used in the overlay, thereby eliminating the need for the application of the adhesive layer. A low water content adhesive is required to adhere the overlay because the water present turns to steam as heat is applied, causing blisters to form under the overlay, thereby adversely affecting overlay surface characteristics. Conversely, the water in the production grade glue between wooden plys (veneers) is more readily absorbed by the wood and, therefore, not a problem. In a large-scale, commercial production setting, the entire above-described composite housed between the cauls is then inserted as a unit into a conventional plywood hot press. The plywood hot press usually consists of a bank of polished steel platens having some 12 to 50 openings, each opening designed to accommodate the aforementioned composite surrounded by cauls. Each platen has steam distributed internally in order to expose the composite to a temperature of approximately 300.degree. F. The platens are connected via a hydraulic ram system so as to impart pressures of roughly 200 psi gauge to the composite. At pressures much above 200 psi gauge, wood of the type commonly used in plywood (fir, etc.) begins to compress. At pressures significantly less, proper bonding will not occur. In the case of automatically operated plywood hot presses, the cauls often interfere with the automated machinery. As a result, such production facilities normally have one or more separate overlay hot presses devoted to plywood that is to be overlaid. These overlay hot presses tend to be of lower production capacity and tend to require a moderate amount of manual operation.
If plywood alone is manufactured, the residence time within the hot press is roughly 4 to 6 minutes, which corresponds to the time required to adequately cure the production grade glue between veneers. Six minutes is normally required for 3/4 inch plywood, which is the most common thickness used for concrete forms. However, when a Hi-Med overlay is used, the residence time must be increased to approximately 9 minutes in order that the uncured resin present in the two HDO sheets may be cured.
Overlay shrinkage that occurs during curing is a major concern. Severe levels of shrinkage can cause face checks in the concrete forms. Face checks are splits in the overlay, and sometimes the underlying plywood, that may occur during fabrication or during later field use. Overlay shrinkage can also cause warping of the underlying plywood. To help combat shrinkage, the finished concrete forms are immediately exposed to a flow of air, often through the use of high speed fans, in order to quickly cool the overlays. Despite such efforts, shrinkage remains a serious problem.
A primary disadvantage of using the one-step manufacturing process with Hi-Med overlay and lower quality veneer is the corresponding lower quality characteristics obtained at the overlay surface. This occurs because the uncured resin in the HDO sheets begins to melt and freely flow over the surface of the underlying MDO sheet when first exposed to the hot press. The uncured resin remains in this semi-liquid state until sufficient time has passed for the curing process to begin. It is not until this point that the uncured resin becomes cross-linked, thereby establishing the final structure that the outermost layer of the overlay will take. As a result of this free-flowing aspect of the uncured resin, the outermost layer of the overlay (formed by the two HDO sheets) tends to mirror the surface of the underlying MDO sheet. While the MDO sheet contains cured resin, and thereby principally maintains its structural identity to form a bridge between the underlying veneer of the plywood substrate and the outermost layer of the overlay, it does not act as a complete bridge. Because high amounts of heat and pressure are exerted on the MDO sheet in the hot press, the MDO sheet does tend to pick up some of the surface irregularities of the underlying veneer of the plywood substrate. When the uncured resin in the outermost layer of the overlay is cured, thereby becoming thermoset, this outermost layer also tends to mirror the surface irregularities of the veneer, albeit to a somewhat lesser degree.
A two-step manufacturing process has developed that provides an overlaid concrete form with surface quality superior to that obtained under the one-step manufacturing process. The two-step process consists of the manufacture of plywood alone in a manner virtually identical to that described above. Because lower quality veneer is the source of the problem, the outermost veneer surface is repaired after the plywood substrate, or panel, has been separately manufactured. Repair generally consists of removal of knots and pitch pockets and filling in splits, removed knots, and removed pitch pockets. After the repair efforts, the outermost veneer is then sanded to smooth the surface. The repaired plywood panel must then be placed between two sets of overlay component sheets, or between one such set and a backer if a one-sided overlay is desired. This composite is then placed between a pair of treated cauls and reinserted into the hot press to allow the formation of the overlay through the curing process.
While this two-step manufacturing process does result in an overlay with surface characteristics superior to those obtained with the one-step process, there are many disadvantages. The veneer repair necessary with the two-step process is extremely labor intensive. Further, the two-step process requires two trips through the hot press machinery. Because the plywood alone requires roughly 6 minutes of hot press residence time, and the overlay requires roughly 9 minutes to cure during the second trip through the hot press, a total hot press residence time of about 15 minutes is required. This incremental 6 minutes of hot press residence time over that required in the one-step manufacturing process leads to extreme production bottlenecks and/or increased manpower requirements at the hot press machinery. In short, the production costs for the two-step process are significantly greater than that for its one-step counterpart.
Another important application for cellulose sheets impregnated with resin material is as an overlay for superstructure surfaces in marine environments (boats, etc.). This application requires both aesthetics and functionality from the overlay, whereas functionality alone was the chief requirement in concrete form applications. With the higher grade veneers of the past, a single uncured cellulose sheet impregnated with polyester resin (hereinafter poly sheet) was used as the overlay. As veneer quality dropped, an overlay consisting of a single poly sheet over one MDO sheet was fairly recently developed. Cellulose sheets impregnated with polyester resin are much more expensive than sheets impregnated with phenolic resin. However, in this environment where aesthetics and structural permanence are important, the higher cost is warranted by the superior surface characteristics achievable with the polyester resin. Given the cost justifications, a single sheet of MDO is used to help further isolate the polyester resin layer from the surface of the underlying plywood substrate.
The one uncured poly sheet over one cured MDO sheet overlay is readily produced by the one-step process described above. However, the curing time for the poly sheet is significantly less (4 to 6 minutes). Shrinkage is also a concern with poly sheet overlays, but to a lesser degree. Just as with concrete form overlays, a two-step process for producing this overlay also exists, but at significantly increased costs. This overlay aimed at marine superstructure surfaces must also compete with Formica overlays, which possess excellent surfaces qualities, but are much more expensive. If the surface qualities of poly sheet overlays continue to decline as veneer quality decreases, a good portion of their market will be lost to Formica overlays, even though Formica is significantly more expensive.
As a result of the reasons detailed above, there has been a long-felt need for an overlay applicable to plywood and the like that effectively bridges underlying irregularities, exhibits superior surface characteristics, minimizes shrinkage concerns, and is relatively cost effective. This invention is directed to satisfying this need.