IMPROVED METHOD OF MAKING DIMENSIONALLY STABLE COMPOSITE BOARD

Abstract: Disclosed is a method of making a dimensionally stable composite board product made from a mixture of particles of a cellulose material and binder and a composite board so produced by such method. Dimensional stability is in reference to the resistance to thickness swelling when the board is subjected to high humidity or moisture conditions. The method and composite board displaying the attribute of improved dimensional stability involves subjecting the parti­ cles of cellulosic material to a pressurized steam treatment and then making the composite board under heat and pressure. When compared to conventional composite board that has not been subjected to the pretreatment, the difference in thickness swelling is significant.

Description: The present invention relates to an improved process of making synthetic board and boards produced therefrom wherein the final product i.e., the formed board has improved dimensional stability under varying moisture conditions and more particularly to pretreating the ligno cellulosic materials and using the pretreated material in making the boards. The technologies of manufacturing wood-based composites have been continuously improved. It is no longer an imagination but a reality that wood-based composites can be produced stronger and stiffer than plywood, solid wood and laminated wood. The production rate has also been significantly increased through the advances in resin technologies. However, in many applications, wood-based composites are much inferior to plywood, solid wood and laminated wood due to lack of dimensional stability. Therefore it is not exaggerated to have a statement "the most severe drawback of wood-based composites is lack of dimensional stability". For panel products, the mat is usually formed in such a way that the grain direction of furnish is normally parallel to the panel surfaces and the pressure direction is perpendicular thereto. The furnish is compressed in the thickness direction. Consequently, the thickness direction is the most unstable direction in wood-based panels. The thickness swelling of wood-based composite panels consists of reversible and irreversible swelling when the panels absorb water or moisture. The former is due to the hygroscopic nature of wood and the latter is due to the springback of compressed wood. The reversible swelling is normally less than the solid wood because the hygroscopicity of wood is reduced by heat during hot pressing. The irreversible swelling is the main cause of instability of wood-based composites. Therefore, the irreversible swelling must be radically reduced in order to improve the dimensional stability of wood-based composites drastically. Irreversible swelling results from the release of pent-up internal stresses in the composite upon adsorbation of water or moisture. Therefore it is reasonable to believe that highly stable composites can be produced if the composite is made in such a way that internal stresses are minimized during pressing. Thickness swelling of wood-based composite board is undesirable particularly where such boards are used in exterior applications and other applications where uncontrolled moisture conditions exist. The dimensional stability of a composite board or panel is normally determined by measuring the thickness swelling of the panel and/or linear expansion following controlled exposure to moisture. Conventional wood-based composite boards or panels can experience a thickness swelling ranging from 10 to 25% of the panel's thickness following a horizontal 24 hour cold water soak and which can range from 20 to 40 percent if subjected to a vertical 24 hour cold water soak. When subjecting a conventional panel to a 2 hour boiling period followed by a 1 hour cold water soak, thickness swelling in the range of 50 to 60 percent can be anticipated. As a result, the use of conventional composite boards and panels as a construction material is limited to installations and environments where the moisture conditions are controlled or anticipated in advance so as to take preventative steps. As a consequence, wood-based composites are regarded as undesirable for exterior applications and particularly ground contact applications because of differential dimensional changes between the wet and dry portions of the material below and above the ground. The moisture and moisture cycling effect experienced by composite panels subjected to variations in humidity or exposure to water also contribute to the breakdown or degradation of the panel rendering it unfit as a construction material for the purpose intended. Indeed, building contractors are reluctant to use wood-based composite panels as a flooring or sub-flooring since the marginal edges of a panel can exhibit greater thickness swelling over its central portion which detracts from a substantially planer abutment joint with neighboring panels. The dimensional stability i.e. thickness change of waferboard or other composites can be improved by increasing the resin content, press time or press temperature. Increases in resin content dramatically increase the production costs and therefore is undesirable. Increasing press time also is undesirable from a production cost point of view and therefore not considered effective. Increase of press temperature is effective but results in a fire hazard and therefore again is undesirable. EP-A-016l766 (published 21.11.85) discloses a process for converting lignocellulose into a reconstituted composite product which includes the steps of: (i) steam-heating the lignocellulose to release hemicellulose, for long enough to hydrolyze the hemicellulose to free sugars and other decomposition products, and (ii) pressing the treated material for long enough to transform and thermoset the decomposition products into a polymeric adhesive for the lignocellulose material. A principal object of the present invention is to provide a process for producing highly stable wood-based composite board without resorting to high pressure or high temperature treatments and without increasing resin content or resorting to special high-cost resin binders. Another object of the present invention is to provide a process for producing highly stable and bond durable products and products produced by such process which can be further treated with preservatives, fire retardants or other chemicals without causing significant damage to strength and excessive thickness swelling. The invention provides a method of making synthetic board comprising: a) subjecting particle form cellulosic material i.e. chips and the like to the action of saturated steam and pressure for a time period in the range of 1-4 minutes and a pressure in the range of 24-15 bar (350-225 psi) to mobilize the lignins and hydrolyze the hemicelluloses; b) adding a binder to the cellulosic material c) forming a mat wherein at least some layers of the mat are formed from said treated material and d) subjecting said mat to heat and pressure to form a composite board, said formed board having improved dimensional stability compared to synthetic boards formed in a conventional manner without pretreatment of the cellulosic material. Unexpectedly, applicants have discovered considerable improved dimensional stability of the so formed composite board where the starting material has been pressure treated, i.e. pretreatment of the furnish before forming the rigid board, under the conditions according to the invention. At this time is not known precisely what takes place in the process but as previously mentioned there is the unusual result obtained of improved dimensional stability. It is believed the purpose of treating furnish is to hydrolyze and pyrolize the highly hydroscopic hemicellulose, hydrolyze lignins to smaller molecules to facilitate flow during pressing and mobilize lignins to surfaces to reduce the resistance during hot pressing. While not specifically known at this time it is believed the following theories and facts might be applicable. 1. A plastic flow of lignin in situ during hot pressing results in low pent-up internal stresses within the product. 2. Steam can hydrolyze lignins and reduce the molecular size of lignins. 3. Smaller molecular size of hydrolyzed lignins permits flow in situ more easily. 4. Control of steam pressure (or temperature) and treatment time can properly hydrolyze lignins and hemocelluloses without causing significant damage to celluloses. 5. Steam can mobilize lignins to wood surfaces, reduce the rigidity of wood and thus reduce the resistance (i.e., less pent-up stresses) during hot pressing. 6. Steam at high temperature (150°C to 180°C) can hydrolyze and pyrolize hemicelluloses which are the most hygroscopic components and thus reduce the reversible swelling Steam and pressure treatment of fibrous material to form a board dates back to the early 20's in what is known as the Masonite R process. Such process is a multi-stage temperature-pressure process wherein the chips are exploded through a die or restricted orfice resulting in a pulp called gun stock. In the present process there is no explosion but instead merely a heat-pressure treatment of the stock. In carrying out the invention furnish i.e. wood chips or the like is placed in a steam treatment unit such as a high-pressure autoclave or a high pressure steam cylinder whereafter the same is closed and injected with steam under pressure which maybe saturated steam or dry steam for a short period of time. In utilizing saturated steam the pressure is 15-24 bar (225 to 350 psi) and the time of the process of course is dependent upon the pressure. The time may, for example, be seconds at high pressures such as 24 bar (350 psi) and minutes for lower pressures such as 15 bar (225 psi) or high temperature such as 240° C for higher dry steam. After the pressure treatment, the steam pressure is released and the treated furnish removed from the pressure vessel. The pretreated furnish is thereafter formed into a composite board under pressure and heat. A binder such as a phenolic resin in amounts conventionally used is normally included in the mat prior to the heat-pressure treatment. The steam pressure (temperature) and treatment time can be varied to have an optimum combination. For example, treatment time can be as short as 1 minute for steam pressure of 22 bar (320 psi) or treatment time can be as long as 4 minutes to have a proper treatment for steam pressure of 15 bar (225 psi) . In general, the degree of treatment increases linearly with increasing treatment time. Also, there is a rule of thumb that the degree of treatment can be doubled by a rise in steam temperature of 10°C, a temperature co-efficient common to many chemical reactions. The following specific examples will further illustrate the practice and advantage of the present invention. Example 1 Waferboards, measuring 1.3 x 61 x 61 cms (½" x 24" x 24") were fabricated with the following parameters. 1. wafers: commercial disk-cut wafers 2. wafer thickness: normally 0.07 cm (0.027 in.) 3. wafer length: 3.8 cm (1.5 in.) 4. resin type and content: powdered phenolformaldehyde resin, 2.25% 5. wax type and content: slack wax, 1.5% 6. mat moisture content: 3.5% 7. press time: 5 min. including 11 sec. daylight close 8. press temperature: 400°F (205°C) To make stable boards, wafers were treated with 15 bar (225 psi) pressure of steam for 2, 3 and 4 minutes before drying. For control, the boards were made with wafers without steam treatment. The results of this experiment are shown in Table 1. TABLE 1. Thickness Swelling of the Waferboard Made From the Regular Wafers and Those Treated with Saturated Steam at 15 bar (225 psi) * Vertical Soak, - Specimen Size 10cm x 10cm (4" x 4") - Measured at 3 points along the lines which are 1 inch in from the top and bottom edge, 1, 2 and 3 inches from one end Example 2 Panels were prepared in the similar manner as Example 1 except the differences specified in Table 2. The results are shown in Table 2. TABLE 2. Thickness Swelling of the Waferboards (½ inch thick) Made From the Wafers Which Were Treated with Saturated Steam at 17 bars (250)psi for 4 Minutes Example 3 Panels were prepared in the similar manner as Example 1 except as follows: Board Thickness: 1.1 cm (7/16") Resin Content: 2.25% in face layers and 2.5% in core Construction of Boards: Three layers The results are shown in Table 3. Example 4 Particleboards, measuring 1.6cm x 61cm x 61cm (5/8" x 24" x 24") were prepared with the following parameters. 1. Particles: fine particles for face layers coarse particles for core 2. Resin type: urea formaldehyde resin 3. Resin content: face: 8.5% core: 5.5% 4. Ratio of formaldehyde to urea: 1.6 5. Press temperature: 177°C 6. Press time: 3 minutes 7. Pretreatment of particles: control: no pretreatment Steam treatment: for 4 minutes at 225 psi The results are summarized in Table 4. The mat of material from which the boards are formed may be multilayered, for example, consisting of a core with two outer layers. The core layer may be made up from chips which have been pretreated, i.e. by pressure and steam or, alternatively, the two outer layers may be made of chips of the pretreated cellulosic material. If desired all three layers, of course, can be made of the pretreated material. In the instance where the core only is made of the pretreated material and the outer layers are not, a further post-treatment can be effected by applying heat to the formed composite board at any time to stabilize the outer layers. In the foregoing the invention has been described by way of example with respect to pressure-steam treatment of wood chips and forming boards from the same. The process, however, in its broadest aspect involves pressure-steam treatment of ligno cellulosic material irrespective of its physical form. The material herein may be and is referred to as furnish. Furnish is wafers, flakes, particles and/or fibers of wood. These are obtined by conventionally processing trees by chippers, refiners, hammer mills, digesters, autoclaves and/or driers. Fiber preparation is one of the most important steps in the process for fiber characteristics which have a predominant effect on the properties of final products. In general, wood chips are processed through a digester system usually consisting of a continuous digester and then discharged into a pressurized refiner. The pressure used in the digester is ranged from 7 to 10 bars g (100 to 150 psi g) for a few minutes (e.g. 2 to 10 min.). The products made from the fibers generated by this process are dimensionally unstable when they are exposed to a high humidity environment or water. That dimensional stability is dramatically improved by treating the wood fibers with moderately high pressure steam. The wood chips can be processed through a refiner and/or defibrator in a conventional manner and the pressure steam treatment can be done before or after the defibration and/or refining process. There is, however, a minor drawback to pressure-steam treating a large quantity of loose fibers in a treatment vessel because of volume (the bulk density of fibers is very low, approximately one pound per cubic foot) but this can be overcome by compacting the loose fibers prior to pressure-steam treatment and then dispersed after treatment. Steam pressure treatment before defibration is more practical and, thus, preferred. The dimensional stability of the final products can be further improved by subjecting the products to a high humidity environment (such as 90 percent relative humidity) for a predetermined time. This conditioning process will allow the products to expedite most of the irreversible linear expansion in a short period of time without roughening board surfaces or significantly impairing the board quality. This can be done just because the products made from the fibers prepared by the present invention are stable. While specified embodiments of this invention have been disclosed herein, those skilled in the art will appreciate that changes and modifications may be made therein without departing from the concept and scope of this invention as defined in the appended claims.

Claims: A method of making synthetic board comprising a) subjecting particle form cellulosic material i.e. chips and the like to the action of saturated steam and pressure for a time period in the range of 1-4 minutes and a pressure in the range of 24-15 bar (350-225 psi) to mobilize the lignins and hydrolyze the hemicelluloses; b) adding a binder to the cellulosic material; c) forming a mat wherein at least some layers of the mat are formed from said treated material and said binder; and d) subjecting said mat to heat and pressure to form a composite board, said formed board having improved dimensional stability compared to synthetic boards formed in a conventional manner without pretreatment of the cellulosic material. A method as defined in Claim 1 wherein said binder comprises a powdered phenol-formaldehyde resin. A method as defined in Claim 1 including adding a binder to the treated particles prior to the heat and pressure treatment. A method as defined in any one of Claims 1-3, wherein the particles are wood chips.

Inventors: HSU WU-HSIUNG ERNEST, HSU, WU-HSIUNG ERNEST
Applicants: FORINTEK CANADA CORP.
Application year: 1985
Main classification: B27N1
IPC codes: B27N1