MXPA00012802A - Method for making composite board using phenol formaldehyde binder - Google Patents

Abstract

Wood composite boards are produced from a wood fiber treated with a slow curing, low alkilinity phenol formaldehyde (PF) resin. The resin treated wood fiber is consolidated and cured in a press using steam injection. The slow curing nature of the resin prevents pre-cure of the resin. The press using steam injection speeds curing so that boards can be produced in press cycles comparable to the curing of wood fiber treated with other resins in a press with no steam injection.

Description

METHOD FOR PREPARING COMPOSITE TABLES USING FENOL-FORMALDEHYDE AGLOMERANTE FIELD OF THE INVENTION The present invention relates in general to methods for the manufacture of a composite board such as a particle board, a fiber board, a chip board or the like and more particularly to a method for preparing composite boards using a board. Phenol formaldehyde binder. The boards are prepared from wood particles, chips and / or fibers treated with a curable or hardenable phenol formaldehyde resin.

BACKGROUND OF THE INVENTION Composite wood products such as boards can be formed by consolidating a loose conglomerate of lignocellulosic materials under heat and pressure, until the individual lignocellulosic elements adhere to each other to form a solid product of the wood type. The lignocellulosic materials can take the form of wood materials such as particles, chips, fibers and / or the like and it is understood that these terms are used interchangeably herein. Typically, the materials forming the conglomerate are treated with a binder such as a resin before applying heat and consolidation, to enhance the adhesion of the materials and to improve the resulting properties of the finished product.

The consolidation of the conglomerate is generally carried out in a press. A conventional press for consolidating a composite conglomerate of binder-treated wood into a particular molded configuration such as a board, includes two opposing press plates spaced apart to define a mold cavity. At least one of the dishes is heated by conduction such as through the use of electric heating resistors or by passing a hot fluid or a gas medium such as steam through ducts located in the body of the dish. Through contact with the conglomerate, the heat is transferred from the plate to the conglomerate by conduction. This process is known as hot pressing. Urea formaldehyde (UF) or isocyanate resin (MDI) have typically been the binder of choice in the hot pressing of composite wood products due to the low curing temperatures, the reasonably short pressing cycles and the higher properties imparted to the finished product in short press cycles. Recently, due to the significantly lower cost in use, attention has been directed to methods that employ phenol formaldehyde (PF) resins. However, the properties of the hot-pressed composite products with PF resins are inferior to those prepared with UF or MDI resins, and the pressing time for the PF resins is typically significantly longer.

Therefore, it is known that some resins having for example rapid cure rates or high curing temperatures, provide composite wood products with inferior properties when produced in a conventional press by hot pressing. US Patent No. 4,850,849 to Hsu et al. Discloses that prior art presses are not capable of producing sufficiently high temperatures within a reasonable time frame to achieve curing of binders such as phenol formaldehyde resin. Additionally, it is believed that the slow transfer of heat by conduction from a conventional plate to a conglomerate, particularly a thick conglomerate, produces temperature differentials through the thickness of the conglomerate. Temperature differentials can cause for example the resin and the fibers at or near the surface of the conglomerate adjacent to the hot plate to be exposed to excessive heat, while the core materials of the conglomerate can be exposed to insufficient heat. The difference in temperature across the thickness of a conglomerate during curing in a conventional press can therefore lead to over-curing and / or under-curing of portions of the thickness of the conglomerate, which will result in structural and / or unsightly imperfections in the finished product. Resins with fast curing rates or with high curing temperatures are particularly susceptible to the negative effect on the temperature difference in the curing of the resin through the thickness of the conglomerate. For the foregoing reasons, phenol formaldehyde resins have generally been considered unsuitable for producing thick composite boards in a conventional press. Also, although conventional plates have been successful in the preparation of fiberboard products using only heat of conduction (hot pressing), the current manufacturing demands require faster cycle times in the press and the use of stronger resins at high temperature to produce product of thicker and denser fiber boards highly detailed. It is known that some of the disadvantages of conventional press plates can be overcome by providing or injecting steam directly into a conglomerate through modified press plates provided with doors for steam injection for that purpose. This is generally known as pressing with "steam injection". The steam passes from the injection doors to the interstitial spaces between the particles of wood, chips and / or fibers that form the conglomerate, thus transporting the heat quickly and uniformly to the core of the conglomerate. Steam injection has several advantages. The steam injection press speeds the curing of the typically dimensioned boards using conventional resins, thus significantly shortening the pressing cycles. Pressing with steam injection also allows the use of high temperature curing resins which are not typically suitable for use in conventional presses and which can be cheaper, safer and / or result in a stronger bound product. And steam injection allows the consolidation and curing of relatively thick composite boards that either are not properly cured in a conventional press, or are not cured quickly enough to provide a product with competitive cost. Therefore, steam injection is known to speed the curing of resins and improve the quality of the product and shorten the production time for wood composite products, particularly products that have thick dimensions. The benefits and advantages of steam injection can be significantly enhanced by carrying out the injection in a sealed press, ie a press that isolates the press cavity from the surrounding atmosphere. This can be done by sealing the perimeter of the cavity. Alternatively, the entire press can be isolated in a sealed chamber. A sealed press reduces or significantly eliminates the loss of valuable steam and facilitates the injection of steam into the conglomerate at high pressures. In relation to binders that are cured at moderate temperatures such as urea formaldehyde resin binders (UF) or isocyanate resin (MDI), phenol formaldehyde resin binders require high temperatures for curing and, therefore, require cycles of longer pressing to effect curing through the full thickness of a composite board profile. Because the press cycle time is considered to be an important factor in determining the manufacturing economy of wood composite products, resins that require longer press cycle times have been avoided due to the additional time required to cure the resin. It is believed that the longer pressing cycles required by the high curing temperature of a resin could be counteracted by rapid heating of a quick curing resin with steam injection or with preheating followed by injection of steam to cure the resin. Nevertheless, rapid heating, either by high pressure steam injection or by a combination of preheating and high pressure steam injection is known to cause the quick curing resin to be pre-cured. It is known that the use of a slower curing resin avoids the pre-curing of the resin in a processing equipment adapted to treat wood fibers with resin before the formation of a conglomerate for consolidation. U.S. Patent No. 5,629,083 to Teodorczyk discloses the formation of composite wood products with a slower curing PF binder to avoid pre-curing in a process for the application of resin to wood fibers prior to the formation of the conglomerate. A journalistic publication by Ernest W. Hsu entitled A Practical Steam Pressing Technology for Wood Composites, Proceedings of the Washington State University International Particleboard / Composite Materials Symposium, Pullman, Washington, April 10, 1991, describes high-curing resins Temperature such as phenol formaldehyde resins can be cured over a range of reasonable pressing times by injection of steam into a sealed press. A summary of the conference attributed to Ernest W. Hsu entitled Comparison of Fiberboards Bonded with PF and UF Resins (1995), describes that the pressing times for fiber boards linked with phenol formaldehyde resin can be substantially reduced and therefore can be comparable to those of the fiber boards linked with UF, by manipulating the temperatures of the fiber conglomerate, the molecular weight distribution of the PF resin and the pressing parameters. The preheating of a conglomerate of a wood composite is known to reduce the pressing time and prevent the pre-curing of the surface layers of the conglomerate in the pressing cycle. US Patent No. 3,649,396 to Carlsson discloses preheating a material with a stream of saturated steam at a temperature close to the curing temperature of the binder to shorten the pressing time, and to prevent premature curing of the surface layers of the conglomerate in the press. Carlsson also describes that pre-curing should be avoided in preheating. US Patent No. 5,246,652 to Hsu et al, discloses that good bond strength of a phenol formaldehyde binder can be achieved by steam injection. Patent 5,246,652 to Hsu et al, describes a method for preparing wood compositions bound with phenol formaldehyde resin that have better resistance to biological attack and fire. Patent 5,246,652 to Hsu does not distinguish between slow-curing and fast-curing phenol formaldehyde resins. Despite the indication of Hsu that good adhesion strength of a phenol formaldehyde binder can be achieved by steam injection, and that the high temperature curing resins can be cured over a reasonable pressing time range by steam injection, the use of phenol formaldehyde resins in steam pressing proved to be generally unsatisfactory, particularly for commercial applications. The generally unsatisfactory results are attributed to a resistance of low internal adhesion or without consistency in the consolidated product (see Lim et al, US Patent No. 5,217,665). As indicated above, phenol formaldehyde resins are significantly less expensive to use. Therefore, there is a need for a method for preparing composite boards using phenol formaldehyde resin in a reasonable pressing time so that the products consistently have suitable properties such as for example high internal adhesion strength, dimensional stability, durability, etc.

SUMMARY OF THE INVENTION The present invention consists of a method for producing composite wood boards, particularly outdoor quality boards, from wood fibers treated with a slow-curing, low-alkalinity phenol formaldehyde (PF) binder. The method includes the steps of forming a conglomerate from wood fibers treated with a phenol formaldehyde binder of low alkalinity, and slow curing, preheating the conglomerate and curing and consolidating the treated conglomerate by a combination of high steam injection pressure, plate heat and plate pressure. In the present invention, the pre-curing is avoided by the use of a slow-curing PF resin, and short pressing cycles are achieved by counteracting the slow curing regime and the high curing temperatures of the PF resin with rapid heat transfer. of high pressure steam injection. The pressing cycles can be further shortened by preheating the conglomerate. Therefore, composite boards linked with PF can be produced in pressing cycles comparable to those of the tables linked with UF or MDI.

DETAILED DESCRIPTION According to the present invention, a wood composite board is produced from a conglomerate formed by wood fibers treated with a low-alkalinity, slow-curing phenol formaldehyde (PF) binder. The conglomerate is cured and consolidated in a pressing cycle including preheating followed by steam injection. The wood fiber produced by conventional means is treated with a phenol formaldehyde resin of low alkalinity and slow curing, not cured. Examples of suitable commercially obtainable resins include GP99C28 and GP58C38, both manufactured by Georgia Pacific Co. of Atlanta, Georgia, GP58C38 exhibited particularly good results. In the preferred embodiment, the resin has a curing temperature of 380 ° C. However, the curing temperature of the resin is influenced by variables including but not limiting the type of material treated, the particle size, the thickness of the conglomerate, the moisture content, etc. In the context of this invention, a slow curing resin is a resin that has a gelling time in boiling water of more than 20 minutes. The gelling time in boiling water is determined by a conventional resin test that measures the cure rate of the resin at 100 ° C. The gelling time in boiling water is used to establish the relative curing rates of various resin types and formulations. However, the cure rate of the particular resin is influenced by external factors, including the materials to which it is applied, the thickness of the resin coating, the thickness of the article being cured, the humidity, etc. Therefore, a slow-curing PF resin could have a gelling time in boiling water of just under 20 minutes.

Preferably, the gelling time in boiling water is in the range of 20-60 minutes. The resin preferably has an alkalinity of less than 2.5% to provide absorption properties of low water content in the resulting composite table. The resin has a pH below 10. The lignocellulosic material treated with resin is formed into a fibrous conglomerate. The fibrous conglomerate is loaded onto a press adapted for steam injection. Preferably, the press is of the type having a press cavity defined between the opposing press plates. The press plates are heated to a temperature higher than the curing temperature of the resin. Additionally, at least one of the press plates is adapted to allow steam injection. Preferably, the fibrous conglomerate is preheated to a temperature of 100 ° C or more to prevent condensation of subsequent vapor applications in the conglomerate. The fibrous conglomerate can be preheated by for example exposing the conglomerate to a hot gas such as steam in a preheating chamber before loading the conglomerate into the press. Alternatively, the conglomerate can be loaded into the press cavity and preheated by exposure to steam or heat conduction from the press plates forming the press cavity. In a first press preheating operation, said press remains open while low pressure steam is introduced into the bottom of the conglomerate until the upper surface of the conglomerate reaches a temperature of 100 ° C which indicates the penetration of steam through the thickness of the conglomerate. Alternatively, the press cavity is sealed and the conglomerate is subjected to a period of rest while heat is brought from the press plates to the conglomerate to convert the moisture of the conglomerate to steam. The subsequent ventilation of the accumulation of the vapor pressure in the conglomerate purges the conglomerate of an excess of moisture and air and ensures that the heat will penetrate uniformly through the thickness of the conglomerate preferably to raise the temperature of the conglomerate to at least 100 ° C. In another alternative method of preheating, the press cavity is sealed and the conglomerate is subjected to a low pressure steam discharge for example at 344.7 kPa. Again, the subsequent ventilation of the vapor pressure accumulated in the conglomerate purges the conglomerate of excess moisture and air and ensures that the heat provided by the low pressure steam will penetrate uniformly through the thickness of the conglomerate to preferably raise the temperature of the conglomerate. conglomerate up to at least 100 ° C. This initial preheating of the conglomerate is followed, in a closed sealed press, of a high pressure steam injection cycle sufficient to cure the PF resin. In the preferred embodiment, it is supplied at a pressure of 1.378.8 kPa for 50-90 seconds to bring the temperature of the conglomerate to 193 ° C. However, the steam may be supplied at a pressure of 689.4 kPa or more for 30-120 seconds. The conglomerate can be consolidated under pressure either before, during or after the injection of steam under high pressure. The programming of the consolidation under pressure in relation to the high pressure steam injection is selected so as to provide a desired density profile throughout the thickness of the board. A uniform density profile is obtained by injecting steam into the conglomerate before closing the press. A density profile that exhibits high density surfaces in a lower density core is obtained by injecting steam after fully consolidating the conglomerate. Through the control of the steam injection programming in relation to the programming of the pressure consolidation, any number of density profiles can be obtained. After the consolidation and curing of the resin, the sealed press is ventilated to relieve the vapor pressure accumulated in the cured and consolidated conglomerate. The press opens and the composite table is extracted. Tables of a thickness of 1.27 cm of sample were prepared in a conventional press by known methods, and in a sealed press using a PF resin according to the method of the present invention. A comparison of the properties is summarized in Table 1 below. The patterns of the American Hardboard Association are listed in the column to the right of the table.

TABLE 1 The "one hour boil dilation" is a test used by the inventors to determine the relative durability of a composite table by calculating the percent change in thickness of the board after submerging a 2.54 cm by 30.48 cm sample from the table in boiling water for one hour. After removing it from the boiling water, the thickness of the sample in the table was measured and compared with the thickness of the sample in the table before boiling. The difference between the measurements is used to calculate the percentage change. The results of the comparative data of Table 1 demonstrate that sealed pressed product samples prepared with PF resin according to the present invention exhibited improved (lower) boiling dilation, improved rot resistance, specific gravity (density) plus low, reduction or elimination of post-pressing humidification, and significantly shorter pressing times. The reduction or elimination of post-pressurized humidification is an important advantage of the present invention over conventional pressing. Fluctuations in the moisture content of a product of a composite table after the manufacturing know that cause undesirable dimensional changes such as for example linear expansion or buckling of the product. During typical end-use exposures, products collect and lose moisture based on environmental factors such as humidity, rain, currents, etc. To avoid undesirable dimensional changes in an end-use exposure, typically, composite table products are moistened after being pressed by conventional methods to increase the content by means of product moisture to an appropriate level for a particular climatic or geographic area in order to to minimize the fluctuation of moisture content. The post-press humidification adds moisture content to the products of composite tables. Post-pressurization humidification is particularly important for products produced in conventional hot plate presses that have substantially all the moisture "evaporated" during pressing and therefore leave the press with almost 0% moisture. The ideal moisture content of composite wood products should typically be 7% (with a range of 2%) in environmentally dry areas and 12% or more in environmentally humid areas. As indicated above, the boards produced according to the present invention have a moisture content of 4-8%. Therefore, the boards produced in accordance with the present invention are particularly suitable for indoor or outdoor applications in a variety of climates with little or no post-press humidification. The applications contemplated for the products of tables include, but are not limited to, tables for decoration, for fences, for coatings, for construction, as components of windows and doors, as coatings for the furniture industry, for pallets and containers, for molding interiors and works in sawmills, ornamental products such as covered terraces, enclosures, and panels for walls and wall systems. It is understood that although they are not specifically mentioned, numerous other applications are also contemplated.

Although the preferred embodiments of the invention have been described for illustrative purposes, those skilled in the art will appreciate that many additions, modifications and substitutions are possible without departing from the scope and spirit of the invention defined by the appended claims.

Claims (15)

1. CLAIMS 1. A method for manufacturing a composite wood product comprising the steps of: forming a conglomerate comprising wood particles treated with an uncured, slow curing phenol formaldehyde binder where the binder has an alkalinity lower than 2.5% and a pH lower than 10; consolidate the conglomerate; supplying a quantity of steam to the conglomerate at a pressure and for a period of time sufficient to cure the binder; and ventilate the excess pressure of the conglomerate.
2. 2. A method for manufacturing a composite wood product comprising the steps of: forming a conglomerate comprising wood particles treated with an uncured, slow curing phenol formaldehyde binder where the binder has an alkalinity of less than 2.5% and a pH less than 10; placing said conglomerate in a press cavity defined between first and second press plates; seal the press cavity; consolidate the conglomerate by moving at least one of the first and second press plates towards the other of the first and second press plates; providing a quantity of steam to the conglomerate through at least one steam door, said quantity of steam being supplied at a pressure and for a period of time which is sufficient to cure the binder; ventilate the excess pressure of the conglomerate before opening the sealing of the press cavity; and open the press cavity.
3. 3. A method for preparing a composite wood product comprising the steps of: forming a conglomerate comprising wood particles treated with an uncured, slow curing phenol formaldehyde binder where the binder has an alkalinity of less than 2.5% and a pH less than 10; placing said conglomerate in a press cavity defined between first and second press plates; close the press cavity; consolidate the conglomerate completely by moving at least one of the first and second press plates towards the other of the first and second press plates to a final compression position; providing a first quantity of steam for the conglomerate through at least one steam door in the first press platen, said first quantity of steam being supplied at a pressure in the range of 172.35 to 517.05 kPa and for a period of time in the range of 30 to 120 seconds; ventilating said first quantity of steam from the conglomerate through said at least one steam door in said first press plaque so as to purge excess air from the conglomerate; supplying a second quantity of steam to the conglomerate through the at least one steam door in the first press platen, the second quantity of steam being supplied at a pressure in the range of 689.4 to 1723.5 kPa and at a temperature sufficient to cure the binder; ventilate the excess pressure of the conglomerate before opening the press cavity; and April the press cavity.
4. 4. A method for preparing a composite wood product comprising the steps of: forming a conglomerate comprising wood particles treated with an uncured, slow curing phenol formaldehyde binder where the binder has an alkalinity of less than 2.
5. 5% and a pH less than 10; preheat the conglomerate; consolidate the conglomerate in a press cavity; and providing a quantity of steam to the conglomerate at a pressure and at a temperature and for a period of time sufficient to cure the binder. The method for preparing a composite wood product according to claim 4, comprising the step of: venting the excess pressure of the conglomerate after curing the binder.
6. 6. The method for preparing a composite wood product according to claim 4, further comprising the step of sealing the press cavity before consolidating the conglomerate.
7. 7. The method for preparing a composite wood product according to claim 6, which further comprises the step of ventilating the press cavity after consolidating the conglomerate.
8. The method for preparing a composite wood product according to claim 4, wherein the preheating step further comprises the step of exposing the conglomerate to steam in a preheated chamber.
9. The method for preparing a composite wood product according to claim 4, wherein the preheating step further comprises the steps of placing the conglomerate in the press cavity and providing a quantity of steam for the conglomerate.
10. The method for preparing a composite wood board according to claim 9, wherein the amount of steam is provided at a high pressure.
11. The method for preparing a composite wood board according to claim 9, wherein the amount of steam is provided at a pressure lower than 689.4 kPa.
12. The method for preparing a composite wood board according to claim 9, wherein the amount of steam is provided at a pressure of 344.7 kPa.
13. The method for preparing a composite wood product according to claim 4, wherein the amount of steam is provided at a pressure equal to, or greater than 689.4 kPa over a period of 30-120 seconds.
14. The method for preparing a composite wood product according to claim 4, wherein the amount of steam is supplied at a pressure of 1.378.8 kPa for a period of 50-90 seconds.
15. 15. The method for preparing a composite wood product according to claim 4, wherein the amount of steam is provided at a pressure and for a period of time sufficient to bring the temperature of the conglomerate to 193.3 ° C.