IE960403A1 - Method and apparatus for manufacturing composite cellulosic¹articles with screen backing, and product thereof - Google Patents

Abstract

A method of producing a pressed cellulosic composite article includes providing a cellulosic mat on a screen backing in a pressing apparatus. The screen backing is preferably made of a plastic material. The cellulosic mat is preferably made by a dry process. The method allows short press cycles without surface roughness caused by moisture present during decompression. Apparatus for performing the method is also disclosed.

Description

METHOD AND APPARATUS FOR MANUFACTURING COMPOSITE CELLULOSIC ARTICLES WITH SCREEN BACKING, AND PRODUCT THEREOF BACKGROUND OF THE INVENTION Field of the Invention The invention relates generally to the manufacture of composite cellulosic articles. More particularly, the invention relates to the manufacture of composite cellulosic articles having a smooth exterior surface.

Brief Description of Related Technology Composite cellulosic materials or articles can advantageously be used in the manufacture of many different articles. Composite cellulosic materials can be molded to have various shapes and sizes and to have various design and structural features that may or may not have been available with products made from natural wood or other materials. Methods for producing man-made cellulosic composites are disclosed, for example, in U.S. Patent No. 5,367,040 (November 22, 1994) to Teodorczyk and U.S. Patent No. 4,514,532 (April 30, 1985) to Hsu et al.

A variety of types of cellulosic (e.g., wood) composite materials exist, including flat or contoured boards of pressed cellulosic material and/or boards of bonded cellulosic sheets (e.g., plywood). Examples of useful man-made boards can be referred to by the following terms: (a) fiberboards, such as hardboard, softboard, and medium density fiberboard and (b) chipboards, such as particleboard and oriented strandboard.J OPEN TO PUBS Composites of these boards (i.e., articles! UIC INSPECTO F.R flALS.......Uip i SECTION AND RULE 23 JNL No............0F.i?.W% - 2 comprising two or more of these materials) can also be useful.

Many different methods of manufacturing these cellulosic composites are known in the art. Methods for manufacturing fiberboard include (a) wet felted/wet pressed or wet" processes, (b) dry felted/dry pressed or dry processes, and (c) wet felted/dry pressed or wet-dry processes.

Synthetic resins, such as phenol-formaldehyde resins, are often used as binders in these processes.

Generally, in a wet process, cellulosic fillers are blended in a vessel with large amounts of water to form a slurry. Useful cellulosic materials include, for example, woody material that is subjected to fiberization to form wood fibers.

The slurry preferably has sufficient water content to suspend a majority of the wood fibers. The slurry is deposited along with a binder material, such as resin binder, onto a water-pervious support member, such as a fine screen or a Fourdrinier wire, where much of the water is removed to leave a wet mat of cellulosic material having, for example, a moisture content of about forty weight percent to about sixty weight percent. The wet mat can then be consolidated under elevated temperature and pressure to form a composite article.

A wet-dry forming method can also be used to produce cellulosic composite materials. A wetdry method generally begins by blending cellulosic material in a vessel with a large amount of water. This slurry is then blended with a binder material. 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 k96040 3 - 3 cellulosic material having a water content of about forty weight percent to about sixty weight percent, for example. This wet mat is then transferred to one or more zones where much of the remaining water is removed by evaporation to form a dried mat having a moisture content of less than about twenty weight percent, and typically less than about ten weight percent. The dried mat is then consolidated under elevated temperature and pressure to form the composite article.

In a dry felted/dry pressed, or ’’dry, method, the cellulosic filler is generally transported in a gaseous stream or by mechanical means. Cellulosic fibers can be first coated with a binder material, for example in a blowline blending procedure. The resin-coated fibers can then be randomly formed into a dry mat by air blowing the fibers onto a support member. The dry mat can have a moisture content of less than about thirty weight percent, and typically has a moisture content of less than about ten weight percent (e.g., about four to about ten percent). (One or more evaporation or drying steps can be performed to attain the desired moisture content.) The dry mat is then consolidated under elevated temperature and pressure to form the composite article.

One disadvantage of the above-described methods is that the pressing of a mat can undesirably produce an article (e.g., a board) which has surface defects such as a rough exterior surface. A rough surface, sometimes referred to as blistered surface, can be caused by moisture which is present during consolidation. More particularly, this problem can be caused during decompression of the pressing apparatus, · when water vapor escapes the - 4 mat (sometimes called degassing) and exits through the surfaces of the mat. This problem is particularly evident when the mat is pressed at relatively high press temperatures and relatively short press times (e.g., about ninety seconds or less).

In the past, attempts to overcome the above-described problem have typically been made by attempting to reduce moisture in the board at the time of press opening. It would be advantageous for the board to be moisture-free, or as close thereto as possible, prior to decompression. In the past, this moisture content has been minimized by increasing the time over which the press cycle is carried out (e.g., by increasing press time) and/or by reducing the moisture content of the cellulosic fibers in the mat at the start of the press cycle. Increasing press time will generally minimize the amount of moisture present in the mat when decompression begins. However, increasing the length of the press cycle to reduce board moisture content is generally undesirable when working on a commercial scale, due to the increased production time (and resulting cost) that this adds. Further, it is often not possible to effectively dry the cellulosic material to a moisture content that is low enough to sufficiently reduce the possibility of moisture-induced surface defects (e.g., to less than about six weight percent).

Therefore, it is desirable to provide a method for producing composite cellulosic articles having satisfactory surface quality, for example an article with a smooth exterior surface without blistering. It is also desirable provide a method for producing composite- cellulosic articles which is 960403« - 5 efficient and utilizes a relatively short press cycle. It is further desirable that such a method does not require undue effort to reduce the moisture content of cellulosic material prior to the consolidation step.

SUMMARY OF THE INVENTION It is an object of the invention to overcome one or more of the problems described above.

Accordingly, the invention provides a process for producing a composite cellulosic material that is efficient and effective and eliminates or reduces the problems described above.

The inventive process generally includes forming a mat including a cellulosic material and a binder resin. The mat preferably has a moisture content of less than about 30 weight percent. The mat is provided onto a screen backing. The screen backing is preferably formed from a plastic material. The screen and mat are provided on the interior of a pressing apparatus, and the mat is then consolidated under elevated temperature and pressure in the pressing apparatus.

The invention preferably allows the use of relatively short pressing cycles (e.g., 90 seconds or less), as opposed to longer press cycles usually required when pressing mats of cellulosic material. The invention also allows the use of cellulosic material having a higher moisture content than can be used without the invention, while still permitting relatively short press cycles. The invention also preferably provides an efficient and effective method for manufacturing composite - 6 cellulosic articles. The invention also provides apparatus for performing the inventive process, as well as a composite article produced by the inventive process.

Further objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken in conjunction with the appended claims.

BRIEF DESCRIPTION OF THE DRAWING The figure is a perspective view of cross section of an embodiment of a two layer screen utilized in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION According to the inventive process, a mat is formed from materials including a suitable cellulosic material (e.g., wood fibers) and a binder resin. The mat is provided onto a screen (sometimes referred to as a ’’screen backing·' or screen backer) that is preferably formed from a plastic material. The mat preferably has a moisture content of less than about 30 weight percent. The screen and mat are provided on the interior of a pressing apparatus. The mat is then subjected to elevated temperature and pressure in the pressing apparatus, so as to cure the binder resin and consolidate the cellulosic material into a solid, integral structure.

The use of the screen in accordance with the invention will reduce subsequent surface defects on the surface of the pressed material, which is particularly advantageous when using relatively 900403 - 7 short press cycles, e.g., 90 seconds or less.

Surface defects, such as surface roughness, are typically the result of moisture or water vapor expanding when the pressure is reduced on the press platens as the press is opening (sometimes referred to as '’degassing") . If this moisture is released from the mat prior to decompression, these surface effects will be minimized. The screen provides a pathway for the moisture or water vapor to travel to the periphery of the mat and escape throughout the press cycle. By the time the platens are pulled apart, the composite has already been substantially degassed. The result is a smoother surface than when degassing occurs during decompression.

Various preferred methods for producing a composite cellulosic article in accordance with the invention will now be described. However, those of skill in the art will be able to make modifications based on the description provided herein. For example, the inventive process contemplates variations in pressing temperature, press cycle, selection of materials, and sequence of the procedure.

The inventive process is preferably used in conjunction with a method for producing a fiberboard product, such as a hardboard panel or doorskin product, due to the surface qualities that such final products generally require. However, the inventive process can be similarly used in the manufacture of other types of cellulosic composites, for example those cellulosic composites disclosed above.

The inventive process is preferably used in conjunction with a dry felted/dry pressed method, such as the dry process, described above, because the - 8 problems to which the invention is addressed are particularly apparent in such a method. However, the inventive process can also be used in conjunction with a wet felted method (with either a wet or dry pressing procedure), in order to produce an article with a smooth surface at shorter press times than would be required while not using the invention. However, a dry-felted/dry-pressed method is preferred due to the shorter press times that are generally required, which makes the dry method particularly advantageous for use on a commercial scale.

The inventive process preferably begins by providing a suitable cellulosic material. Suitable cellulosic material can be obtained, for example, by subjecting wood to a fiberization process (for example, in a pressurized refiner), as is known in the art, to produce wood fiber. The moisture content of the fibers used with the invention is preferably in the range of about four to about ten weight percent. The wood fiber can have a density in the range of about twelve to about 20 pounds per cubic foot, for example.

In addition, a suitable binder resin is provided. Many suitable binder resins are known in the art and include, for example, various modified and unmodified phenolic-formaldehyde thermosetting resins. Suitable resin binders are disclosed in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 15, pp. 176-208 (2nd Ed., 1970) and U.S. Patent No. 5,367,040 to Teodorczyk (November 22, 1994), the disclosures of which are hereby incorporated herein by reference.

A mat is formed of the cellulosic material and binder resin, for example according to one of •60405 - 9 the methods described above. For example, the wood fibers can be blended with the resin in a blowline blending procedure or by mechanical means. A suitable procedure is described in commonly assigned, copending U.S. application serial number 08/342,778 filed November 21, 1994 (Teodorczyk), the disclosure of which is hereby incorporated herein by reference. The amount of binder resin used in the process is generally less than 20 weight percent (based upon the weight of the wood fiber), but can vary depending upon the other process parameters and intended use of the final product. Preferably, the resin is added at about one to about 15 weight percent.

Preferred formulations include (a) about one to about five weight percent phenolic resin, (b) about one to about three weight percent wax sizing, and (c) the remainder, i.e., about 92 to about 98 weight percent, wood fiber, wherein the weight percents are based upon the total formulation.

The moisture content of the mat which is formed is preferably less than about 30 weight percent, and more preferably less than about ten weight percent, based upon the dry weight of the fibers. The moisture content of the mat is most preferably about the same as the moisture content of the wood fibers themselves (e.g., about four to about ten weight percent). The fiber mat preferably has a thickness, for example, of about three inches and a basis weight of about 0.640 pounds per square foot of surface area of the mat. However, it should be understood that these proportions are variable depending upon the desired final product and that those of skill in the art will be able to make modifications based upon the desired final product. - 10 If desired, as is known in the art, the moisture content of the cellulosic fibers (either before or after formation of the mat) can optionally be reduced by evaporation in a drying step. Such a drying step typically includes the application of heat. Care must be taken to avoid excessive drying, which can reduce the efficiency of the binder resin and cause defects in the final product.

The fiber mat is then placed on the screen backing. The screen can support the cellulosic mat and can separate the mat from the lower press platen, while allowing moisture to exit the mat during pressing via the screen openings. The screen backing is a generally planar filter medium having a plurality of strands, cables or fibers defining a plurality of openings or voids. The strands of the screen preferably have a thickness in the range of about ten mils to about thirty mils, and are more preferably approximately twenty mils (about 0.508 mm) thick.

As stated above, the screen backing is preferably made of a plastic material. The term '•plastic'· is intended to include both synthetic and natural materials which are capable of being formed into an article having a plurality of intersecting strands. Plastics typically include one or more high polymers which can be optionally combined with other known ingredients. Examples of plastics can be found, for example, in Hawley's Condensed Chemical Dictionary, llth Ed. 1987 (see, e.g., pages 924-25), the disclosure of which is hereby incorporated herein by reference. Although many materials are suitable for the screen backing of the invention, high temperature resistant polyesters are preferred.

I 96040J - 11 The plastic screen preferably includes two screen layers. Although a variety of screens are suitable for use with the invention, the two layers of the screen are preferably woven together so that there is a pathway within the screen through which moisture can travel. The screen openings formed by the two layers are preferably out of phase from one another (i.e., the openings formed by the strands of the one screen layer are not located directly over the openings formed by the strands of the second screen layer). A two layer screen enhances the ability of the screen to diffuse moisture, relative to a one-layer screen, for example because of the above pathway.

A particular preferred screen is illustrated in the figure and includes a first, upper layer of equally spaced parallel strands (generally designated 10) that is placed directly on top of, or superimposed on, a preferably identical second, bottom layer of parallel strands (generally designated 20). The strands forming these first two layers 10 and 20 (e.g, those designated 11-15 and 21-25, respectively) are generally linear and spaced apart by about ten mils to about fifty mils. A third series of equally spaced weaving strands 40 is transverse, and preferably perpendicular to the first two layers 10 and 20 and weaves the first two layers together by winding, weaving, or twisting through the first two layers in a repeating pattern.

In the preferred screen, each weaving strand 40 of the third series has a pattern which repeats itself beginning at every fifth strand of the first two layers. For example, a first strand 11 of the first layer 10 is located directly above a - 12 second strand 21 of the second layer 20. These two strands can be referred to as a strand pair 31. At each strand pair, the weaving strand 40 can be located at one of three positions: (1) wound over the top of the first strand, (2) disposed between the two strands, or (3) wound below the second strand. The first strand pair 31 can be described as being adjacent a second strand pair 32, which in turn is adjacent a third strand pair 33 and then a fourth strand pair 34 and a fifth strand pair 35, and so on. Each strand of the strand pair 31-35 is equally spaced from the other strand of the pair by about ten mils to about fifty mils, for example.

In the preferred screen, a strand 40 of the third series (the weaving strand) is wound over the top strand 11 of the first strand pair 31, as shown in the figure. At the location of the second strand pair 32, the weaving strand 40 travels between the pair of strands 12 and 22. At the third strand pair 33, the weaving strand 40 is wound below the bottom of the lower strand 23 of the third strand pair. Immediately after the third pair 33, the weaving strand 40 turns upwardly again and, at the fourth strand pair 34, the weaving strand 40 is again located between the pair of strands 14 and 24. At the fifth strand pair 35, the pattern begins to repeat, and the weaving strand 40 is again wound over the top strand 15 of the strand pair 35 (as in the case of the first strand pair).

Preferably, each weaving strand 40 is one position out of phase from the adjacent weaving strand 40A (a portion of which is visible in the figure) so that pattern of every other (i.e., every third) weaving strand repeats. For example, at the first strand pair 31, when one weaving strand 40 is 900403 - 13 wound over the top of the strand pair, the adjacent weaving strand 40A will be located between the strands of the first strand pair. Similarly at the second strand pair 32, when the weaving strand 40 is between the strands 12 and 22, the adjacent weaving strand 40A will be wound over the top of the strand 12, as shown in the figure.

The overall surface area of the screen should be larger than the surface area of the mat, so that the screen extends outside the peripheral edges of the mat on all sides.

Useful screens are available from Drytex Inc. of Jonesboro, Georgia (sold under the trade name Enertex) and Albany International of Appleton, Wisconsin. After providing the mat on the screen backing, the screen and mat are placed on the interior of a pressing apparatus. (However, the process could be altered by first placing the screen backing in a pressing apparatus, followed by providing the mat on the screen. Alternatively, the mat could be initially formed directly onto the screen.) A pressing apparatus useful in manufacturing man-made composite materials such as fiberboard sheets generally includes an upper press platen or die and a lower press platen or die.

The screen forms and occupies a space between the mat and the lower press platen. The mat will preferably remain entirely on top of the screen during pressing; for example, the wood fibers of the mat will preferably not enter into plug any of the openings of the screen. It may be desirable to have the screen extend outside the pressing apparatus in order to affix the screen to the pressing apparatus.

Once the screen and mat are loaded in the pressing apparatus, the· mat is subjected to elevated 9β0403 - 14 temperature and pressure for a press cycle. A press cycle includes the steps of (a) closing the press, (b) holding at an elevated temperature and pressure, and (c) decompression of the press. At decompression of the press, the pressed mat preferably is moisture-free, or as close thereto as possible. One total press cycle can take place during a period in the range of about 20 seconds to about 20 minutes, for example. However, the present invention is adapted to provide beneficial results even with short press cycles of about 90 seconds or less. The press cycle is preferably in the range of about 30 seconds to about 90 seconds and, more preferably, about 60 to about 90 seconds.

When using the screen of the invention, the press pressure is preferably in the range of about 1000 psi to about 1200 psi, e.g., about 1112 psi. A relatively high pressure is generally required in order to press the mat to a desirable density, because the screen will absorb some of the applied pressure. (In comparison, pressing without the screen is typically performed at about 850 psi.) These pressures are variable, however, based upon the type of screen that is used and the type of composite that is desired, for example. The press temperature (measured by the temperature of the press platens) is preferably in the range of about 300°F to about 450°F (about 149°C to about 232°C). Higher press temperatures are generally not preferred because such temperatures can cause the degassing of the moisture to be even more disruptive to the surface of the mat and thus make moisture removal prior to decompression even more critical. However, higher press temperatures can be used under some circumstances and these pressing conditions are 99049J - 15 variable depending upon the desired final product. Those of skill in the art will be able to make modifications based upon the desired final product.

When exposed to this heat and pressure, the thermosetting resin will be cured and the mat will be compressed into an integral consolidated structure. Depending on the shape of the press platens, the mat can be pressed into a flat panel or molded into any desired shape (e.g., that of a paneled doorskin). The use of a plastic screen is advantageous because it can stretch and conform to the shape of the die cavity. In the press, the mat can be compressed, for example, to a thickness of about 0.125 inch, which is a standard industry thickness for doorskin panels. After pressing, the article preferably receives a surface treatment of linseed oil, often referred to as tempering. '· As described above, prior pressed composites often suffered from surface defects, such as roughness or blisters, that resulted from water vapor expansion when the pressure is reduced on the interior of the pressing apparatus as the press is opened (degassing). Defects can be caused because during decompression because this water vapor can contain a large amount of energy which allows the vapor to force its way through the top of the pressed mat. To produce a product with a very smooth surface, it is desirable to have already minimized the moisture in the pressed composite by the time the pressure on the interior of the press is reduced and the press is opened (the aforementioned decompression step).

The inventive method and apparatus provides means for removing moisture from the mat and interior of the press. The invention overcomes - 16 the problems described above by providing a pathway for the escape of moisture during the pressing of cellulosic mats. As stated above, the screen forms and occupies a space between the mat and the lower press platen. The space formed by the screen will preferably allow moisture to escape from the mat (more particularly the bottom side thereof) during hot pressing. A substantial portion of the moisture of the mat escapes from the mat during pressing and can travel through the voids of the screen located between the fiber mat and the bottom die or platen. In this way, moisture can escape from the mat prior to decompression, even with relatively short press cycles, thereby avoiding surface defects. The presence of the screen also allows for the press to be opened more rapidly than would otherwise be possible, because a substantial portion of the moisture has been eliminated prior to decompression.

The inventive process can be utilized to produce many types of cellulosic composite materials, for example molded composite wood doorskins. It is highly preferred to use the inventive process to produce articles wherein only one exterior surface is visible in the final product, because the screen backing will likely mark the surface of the article which rests on the screen. Although the pressing of a mat generally will produce a board surface with moisture-induced roughness, particularly when consolidated at a relatively high press temperature and relatively short press times, the inventive process preferably avoids any moisture-induced roughness and produces an article which has a smooth outer surface.

The foregoing description is given for clearness of understanding only, and no unnecessary 9604001 - 17 limitations should be understood therefrom, as modifications within the scope of the invention will be apparent to those skilled in the art.

Claims (29)

CLAIMS:

1. A method of producing a composite cellulosic article, comprising the steps of: (a) forming a mat comprising cellulosic material and a binder resin; (b) providing said mat on a plastic screen; and (c) consolidating said mat under elevated temperature and pressure.

2. The method of claim 1, wherein: said screen comprises at least two screen layers.

3. The method of claim 1, wherein: said consolidating step is about 90 seconds or less in length.

4. The method of claim 1, wherein: said consolidating step has a length in the range of about 60 seconds to about 90 seconds.

5. The method of claim 1, wherein: the moisture content of said mat is less than about 30 weight percent, based upon the dry weight of said cellulosic material.

6. The method of claim 6, wherein: the moisture content of said mat is less than about 10 weight percent, based upon the dry weight of said cellulosic material. 990403 - 19

7. The method of claim 6, wherein: the moisture content of said mat is in the range of about 4 to about 10 weight percent, based upon the weight of dry cellulosic material.

8. The method of claim 7, wherein: the elevated temperature in step (c) is in the range of about 300°F to about 450°F.

9. The method of claim 8, wherein: said consolidating step is about 90 seconds or less in length.

10. The method of claim 1, wherein: said screen is made of a material comprising a heat resistant polyester.

11. The method of claim 1, wherein: said screen comprises a multiplicity of strands about 10 to about 30 mils thick.

12. The method of claim 1, wherein: said screen comprises a first screen layer and a second screen layer, each of said screen layers defining a plurality of screen openings; and said screen openings of said first screen layer are out of phase from said screen openings of said second screen layer.

13. The method of claim 1, wherein: the elevated temperature in step (c) is in the range of about 300°F to about 450°F. 96040S - 20

14. The method of claim 1 wherein: the elevated pressure in step (c) is at least about 1000 psi.

15. A method of producing a composite cellulosic article, comprising the steps of: (a) forming a dry mat comprising cellulosic material and a binder resin, said mat having a moisture content of less than about 30 percent by weight; (b) providing said dry mat on a screen; (c) consolidating said mat under elevated temperature and pressure; and (d) providing said mat and said screen on the interior of a pressing apparatus prior to step (c) .

16. The method of claim 15, wherein: said screen comprises at least two layers.

17. The method of claim 15 wherein: said screen is made of a plastic material.

18. The method of claim 15, wherein: said consolidating step is about 90 seconds or less in length.

19. The method of claim 15, wherein: said consolidating step has a length in the range of about 60 seconds to about 90 seconds.

20. The method of claim 15, wherein: the moisture content of said mat is less than about 10 weight percent, based upon the dry weight of cellulosic material. -

21. 21. The method of claim 20 wherein: the elevated temperature in step (c) is in the range of about 300°F to about 450°F; and said consolidating step is about 90 seconds or less in length.

22. The method of claim 15, wherein: the elevated temperature in step (c) is in the range of about 300°F to about 450°F.

23. An article produced by the method of any one of claims 1-5 or 15-20.

24. An apparatus for producing a composite cellulosic article, comprising: (a) a pressing apparatus comprising a top platen, a bottom platen, and means for applying a pressure of about 1000 psi or greater; and (b) a plastic screen backing disposable in said pressing apparatus and adapted for supporting a mat of cellulosic material.

25. The apparatus of claim 24, wherein: said screen backing is made of a material comprising a heat resistant polyester.

26. The apparatus of claim 24, wherein: said screen comprises at least two screen layers.

27. The apparatus of claim 26, wherein: said screen comprises a first screen layer and a second screen layer, each of said screen layers defining a plurality of screen openings; and 96040J - 22 said screen openings of said first screen layer are out of phase from said screen openings of said second screen layer.

28. A method according to Claim 1 or 15 of producing a composite celulosic article, substantially as hereinbefore described.

29. An apparatus acccording to Claim 24, substantially as hereinbefore described.