Product Containing High Density Skins on a Low Density Core and Method of Manufacturing Same

The present invention relates to a fiberboard panel having a relatively low density core or central portion and having relatively high density skins or surface portions. More particularly, the present invention relates to a man-made board having high density skins or surface portions as an integral part of a relatively low density core wherein the high density skins are produced by contacting the surface fibers of a relatively low density consolidated board with a skin forming chemical, and thereafter hot pressing the board in a post-press or second pressing operation at a temperature of at least 525.degree. F. to densify the surface portions into smooth, stiff, hard, dense skins.

The process of the present invention relates to a "post-press" or second press operation for creating thick, hard, integral surface portions on at least one and preferably both surfaces of a man-made fiberboard substrate. A handleable mat is first manufactured by consolidating a cellulosic fiber-containing composition to form a consolidated fiberboard having a density of less than 35 pounds per cubic foot. The handleable mat can be consolidated by drying a mat formed by depositing a water slurry of fibers onto a water pervious support member, as is common in the manufacture of insulation board, or the mat can be consolidated under heat and pressure until substantially dry in a first press operation. This consolidated mat is thereafter "post-pressed" to form dense surface layers for stiffness and strength, while retaining the relatively low density core or central portion. The density of the core is substantially predetermined when the mat is initially consolidated. For the purpose of the present invention, a "post-press" or "post-consolidation" pressing operation refers to a press step performed on the handleable mat after consolidation thereof.

It is necessary to form the fiberboard of the present invention in two separate operations. The first operations forms a consolidated mat and sets the core density while the second or "post-press" operation creates the denser surface portions. It is quite surprising that a "post-press" or second pressing operation is effective in densifying a surface of a consolidated mat while leaving the density of the core material substantially unchanged. Prior to the present invention, it was thought that the qualities and physical characteristics of a board would be finally determined by the conditions under which the board is initially consolidated and by the raw materials used to form the board, because of the bonding occuring during consolidation.

Much of the bonding necessary for cohesion and strength in a man-made board occurs during the consolidation of the board during hot-pressing. Prior to drying the board made by the wet process, the board is weak, but the bonding forces created during the final consolidation are generally quite powerful. Generally the same bonding effect occurs in the final consolidation of a board made by the dry process. Prior to hot-pressing, the loosely disassociated cellulosic fibrous product is quite weak, but after hot-pressing the mat into its final configuration, it is very powerfully held together by the bonding which occurs during hot-pressing. Hot-pressing during consolidation causes a welding or coalescing of the cellulose fibers at the surface of the product so that the surface portion consists of wood remade in modified form. It is very difficult to densify or restructure the surface of a consolidated product without again destroying these fiber-to-fiber surface welds. It is especially difficult to densify only a surface portion of a consolidated board in an amount sufficient to substantially increase the strength and stiffness of the product while retaining the lightweight core material to provide a lightweight product. The process described herein accomplishes these results.

A prior application, assigned to Masonite, Ser. No. 537,254 filed Aug. 23, 1974, discloses and claims a method of post-press embossing consolidated, man-made boards to effectively and accurately reshape the surface of a completed panel without surface defects such as surface fiber tearing. In accordance with Ser. No. 537,254 filed Aug. 23, 1974, a previously consolidated panel is post-press embossed at a pressure of at least 1000 psi to create a surface design, or to permanently redesign the surface of a consolidated product without destroying the interfiber bonding established at the surface of the board during consolidation. The product thus formed does not include a relatively low density core or central portion.

In accordance with the present invention, it has been found that the surface density and physical characteristics of a relatively low density consolidated mat can be altered in a post-press operation to harden, smooth, and densify the surfaces thereof when at least the surface fibers of the mat are contacted with a skin forming chemical. During the post-press operation, at a temperature of at least 525.degree. F., the skin forming chemical reacts to stiffen and strengthen the cellulosic substrate, such as low density fiberboard, to provide the necessary strength for the intended purpose of the substrate while retaining a relatively low density, lightweight central core of the material. In this manner, the resulting post-pressed substrate is very lightweight while having strength and stiffness properties equal to or better than much heavier boards.

An object of the present invention is to provide a lightweight fiberboard panel having hard, dense surface skins thereon.

Another object of the present invention is to provide a lightweight cellulosic fiber-containing article having a hard, dense skin on at least one surface thereof.

Another object of the present invention is to provide a lightweight fiberboard, having a central core of material with a density in the range of 10-35 pounds/ft..sup.3, having strength and stiffness properties equivalent or better than hardboard having a density in the range of 45-65 pounds/ft..sup.3

Another object of the present invention is to provide a lightweight man-made board having design-fidelity and paint hold-out properties equal to or better than hardboards manufactured in accordance with existing technology.

Another object of the present invention is to provide a method of providing an integral, structural skin on one or more surfaces of a cellulosic fiber-containing consolidated product by including a skin forming chemical within at least the surface fibers of the consolidated product, and, thereafter, post-pressing the chemically treated consolidated board in a second-pressing operation at a temperature of at least 525.degree. F.

In accordance with an important feature of the present invention, it has been found that particular chemicals will provide hard, dense surface skins to a handleable mat when the surface fibers are contacted therewith and the mat is thereafter "post-pressed" at a temperaure of at least 525.degree. F.

In accordance with another important feature of the present invention, the technology disclosed herein has been developed to provide a low density, strong cellulosic fiber-containing product, for example, fiberboard which has sufficient strength for end uses such as wall panelling, siding, and moldings. The low density, high strength product is produced by first manufacturing a low density substrate having strength sufficient to be handled in manufacture, including a skin forming chemical in at least the surface fibers of the substrate and thereafter post-pressing the substrate to develop dense outer layers or skins on the exterior surfaces.

Surprisingly, it has been found that the skin created by post-pressing a low density consolidated product having a skin forming chemical in at least the outer surface fibers thereof creates a hard, dense surface which, if desired, effectively and permanently reproduces embossing plate details on the surface of the product and creates a smooth outer surface having excellent holdout of coating materials, such as paint. The post-pressing operation restructures the surface fibers contacted with the skin forming chemical to provide exceptional strength and stiffness properties to a lightweight panel.

In accordance with the present invention, a high strength fiberboard product having hard, dense skins or surface portions thereon is manufactured in two steps. The first step comprises manufacturing a relatively low density consolidated, handleable mat, using either the wet or dry process as known in the manufacture of man-made boards; the second step comprises post-pressing the consolidated mat after first treating at least the surface fibers of the mat with a chemical to form a skin on the surface of the product.

The method of producng a consolidated mat is well known as presently used in producing man-made boards such as hardboard, chipboard, particle board, panelboard, acoustical board, insulation board, and the like. In the wet process, the raw material is uniformly blended in a head box with copious quantities of water to form a slurry. The slurry is deposited onto a water-pervious support member, generally a Fourdrinier wire, where much of the water is removed leaving a wet mat of cellulosic material. The wet mat is then dried to consolidate, as in the manufacture of insulation board, or can be transferred from the pervious support member and consolidated under heat and pressure to form the board. Typically, pressures of from 400 to 500 psi and temperatures up to about 400.degree. F. are encountered in hot-press consolidation of a man-made board manufactured by the wet process. The dry process is similar to the wet process except that the cellulosic fibers are first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin, and are then randomly distributed into a mat by air blowing the resin-coated fibers onto a support member. In the dry process, the mat is pressed at temperatures up to about 450.degree. F. and pressures less than about 1000 p.s.i. to cure the thermosetting resin and to compress the mat into an integral consolidated structure.

The handleable mat produced in accordance with the first step of the present invention is manufactured in a conventional manner, using conventional cellulosic fiber stock. To achieve the full advantage of the present invention, the handleable mat should have a density, after consolidation, in the range of 10-35 pounds per cubic foot, preferably in the range of 15-30 pounds per cubic foot. Panels of varying thicknesses having densities within this range can be produced in accordance with known technology to provide lightweight core materials on which a surface skin can be developed in accordance with the following disclosure.

Handleable mats have been produced having a density as low as ten pounds per cubic foot and a thickness of one and one-half inches. Mats having densities as low as ten pounds per cubic foot are useful as the handleable mat in producing products in accordance with the principles of the present invention. In fact, in accordance with the presention invention, any low density mat (less than 35 pounds per cubic foot) is considered to be handleable if it can survive the trimming, cutting, stacking, packing, shipping, and unloading operations necessary to produce fiberboard. All such handleable mats are useful in accordance with the principles of the present invention.

The consolidated mat containing a skin forming chemical is pressed in a post-pressing operation at a temperature of at least 525.degree. F. to develop a surface layer therein called a "skin", defined as an outer layer having a higher density than a thickness of material over which it is formed. It was found in early investigations that heat and pressure alone (without a skin forming chemical) would not form a thick skin on the surface of a low density panel. Skin thickness was very thin regardless of the temperature and compression. Table I shows that post-pressing alone, without pre-treatment with a skin forming chemical, does not produce a skin.

Various chemicals were evaluated on the surface of low density mats to determine which chemicals provide a high density skin on the surface of a low density mat when the chemically treated, consolidated mat is "post-consolidation" hot-pressed at a temperature of at least 525.degree. F. The following chemicals set forth in Table II provide a high density skin of at least 0.020 inch thickness on the surface of a low density (10-35 pounds/ft.sup.3) mat when the consolidated mat, containing the chemical, is hot-pressed at a temperature in the range of 525.degree.-650.degree. F.:

Mixtures of any two or more of the chemicals listed in Table II can also be used as the skin forming chemical in accordance with the principles of the present invention.

The following group of chemicals taken from Table II provide stronger skins when a consolidated low density mat is post-pressed in accordance with the present invention: ammonium thiocyanate, thiourea, dicyandiamide, hexamethylenetetramine, MASONEX (Trademark of Masonite Corporation), sucrose and mixtures thereof. Derivatives of urea such as guanamine, alkylureas (i.e. methylurea, ethylurea), lower alkylthioureas, methylolurea, biuret, inorganic acid salts of urea, and mixtures thereof are useful in forming skins. The remainder of the chemicals listed in Table II and mixtures thereof form weaker skins.

The chemicals listed in Table III were evaluated and found not to develop skins on the surface of low density consolidated mats when "post-consolidation" pressed at a temperature of at least 525.degree. F.:

Ammonium Chloride

Ammonium Hydroxide

Dimethylol Urea

Aluminum Sulfate

Zinc Chloride (Hygroscopic)

Sodium Carbonate

Potassium Dichromate

Sodium Nitrate

Aniline

Melamine

Butylamine

Glycerin

Resorcinol

Formamide

Formic Acid

Ammonium Acetate

Raw Linseed Oil Without Dryer

Quaternary Ammonium Silicate

Starch

Hydrochloric Acid

Nitric Acid

Phosphoric Acid

In attempting to achieve thicker surface skins on the handleable mat during post-pressing it was found that the skin forming chemical should deeply penetrate the surface fibers of the handleable mat prior to post-pressing. Surface spraying therefore led to the impregnation method of incorporating the chemical into the handleable mat.

It has been found that when the skin forming chemicals are impregnated into the surface of a handleable mat after initial consolidation of that mat, a much thicker skin can be produced by vacuum impregnating the chemical to provide a skin thickness in the range of 0.060 inch.

In testing various degrees of chemical penetration by vacuum impregnating the handleable mat after initial consolidation, it was found that increasing the depth of chemical pentration beyond that required to produce 0.060 inch skins, for example, at a level of 100-150 pounds of chemical per thousand square feet, does not improve the physical properties of the panel, nor does it increase the skin thickness beyond 0.070 inches.

Typical post-consolidation press conditions an developing a skin in accordance with the present invention are as follows:

Platen Temperature--550.degree. F. (Both Sides)

Closing Speed--Fast as Possible

Holding Time at Caliper--30 Seconds

Compress--0.100 inch

Pressure--200 p.s.i.

Opening Speed--Fast as Possible

The skin forming chemical can be included in overlay fibers in manufacturing the handleable mat. For example, a handleable mat can be produced by dry forming the mat in three layers from three separate head boxes with the skin forming chemical blended into the two surface layers, and consolidating to produce a handleable mat having the skin forming chemical in two distinct surface layers but not in the central core. The amount of chemical blended into the overlay or surface layers regardless of the method of manufacture should be at least 5% by dry weight of surface layer contacted with the chemical, and preferably in the range of 10-15% by dry weight of surface layer contacted. Resin binders in the surface layers can be reduced to about one half the normal amount (i.e. about 4%) since post-consolidation hot-pressing at a temperature of at least 525.degree. F. substantially increase the strength of the surface layers.

To provide for better surface smoothness and paint holdout, a handleable mat produced as described by the overlay process can be surface treated with the skin forming chemical in an amount of about 5-15 pounds of skin forming chemical per thousand square feet by spraying an aqueous solution of the chemical onto the surface of the mat just prior to post-pressing.

In addition to being stiffer and stronger than hardboard having a much greater density, the boards having skins produced in accordance with the present invention are superior with respect to hardness, embossing fidelity, and paint holdout. "Design fidelity" or simply "fidelity" as used herein is a measure of the accuracy of reproduction of the design of an embossing plate onto the surface of a consolidated mat. The panel produced in accordance with the present invention permits deeper embossing patterns because of the lower density. "Paint holdout" is the ability of a panel to keep paint on its surface without a significant amount striking into the panel.

The amount of pressure used in post-pressing should not go beyond 300 p.s.i. so that a relatively low density core or central portion of the board can be maintained. Some caliper reduction must result during post-pressing to apply both heat and pressure necessary for thick skin development. Contact (unregistrable) pressure is sufficient for slight caliper reduction. It has been found that as the fibers are compressed during the post-pressing operation, the density increased in the overlay layers and, to some extent, in the core material. Thereafter, the core begins to resist compression. Accordingly, the face or overlay layers will compress more than the core.

The handleable mat having a density of less than 35 pounds per cubic foot is "post-pressed" at a temperature of at least 525.degree. F. To achieve the full advantage of the present invention, the temperature of "post-pressing" should not exceed 650.degree. F. It is preferred to "post-press" at a temperature in the range of 525.degree.-575.degree. F. After chemical impregnation, the consolidated mat is generally dried to about 10% or less moisture prior to post-pressing.

To maintain a relatively low density core material in the fiberboard product, the pressure used in "post-pressing" should not exceed about 300 p.s.i. Pressures can vary depending upon the density of the handleable mat--with lower density mats (i.e. 10-20 pounds per cubic foot) it is preferred to use pressures on the order of 30-150 p.s.i. in post-pressing. It is always preferred to use a "post-press" pressure less than 200 p.s.i. The time of post-pressing is not critical and preferably is in the range of 5-60 seconds. With higher temperatures in the range of 575.degree.-650.degree. F., it is desirable to remove the product from the post-press operation within about 30 seconds to avoid charring or decomposition of the surface skins.

The amount of skin forming chemical necessary to form a sufficient skin thickness for the purpose of the present invention generally is at least about 5% by weight of the fibers contacted. Amounts vary depending upon the particular chemical, as set forth in Table II, to provide a relatively thick skin. The weight percent of skin forming chemical needed is at least 5% based upon the dry weight of the surface material in which the chemical has penetrated when combined by the impregnation process. When combined by the overlay process, the weight percent of skin forming chemical needed is at least 5% based upon the dry weight of overlay material.

To achieve the full advantage of the present invention, the amount of skin forming chemical incorporated into the handleable mat should be in the range of 6-20% based on the dry weight of the material contacted therewith, and preferably in the range of 9-17% by weight.

The product formed by the process of the present invention includes a hard, dense, smooth skin having a density greater than the density of the consolidated mat on which the skin is formed. The composition of the skins has not been determined by analysis. The base layer or core material underlying the skin has a density of 10-35 pounds per cubic foot, so that lightweight products of varying density can be manufactured having hard skins thereon.

Although the present invention is described in terms of particular constituents, and ranges thereof, and manner of making and using the same, it is recognized that departures may be made therefrom within the scope of the invention as defined in the appended claims.