US 3383274 A
Description (OCR text may contain errors)
United States Patent 3,383,274 FLAMEPROOFING 0F CQNSTRUCTION MATERIAL Don W. Craig, Anderson, Califi, assignor to US. Plywood-Champion Papers Inc., a corporation of New York Filed Jan. 6, 1965, Ser. No. 423,698
' Claims. (Cl. 161-162) The present invention is broadly concerned with a process for the manufacture of flameproof wood articles of superior qualities and strengths, as well as with the manufactured articles per se. The invention is more particularly concerned with the improved production of particle board, hardboard, bagasses, and other types of fiber boards possessing excellent fire resisting and other high quality physical properties. The invention is especially concerned with a high quality board which comprises in combination a salt of ammonia, a synthetic resin, and a wood particle material; and with its method of manufacture. In accordance with a specific adaptation of the present invention, a superior fire resistant particle board is produced by the technique of adding crystals of ammonium salts at a critical time to the wood particles and resin, whereby these salts effectively distribute themselves throughout the mass of the board.
It is known in the art to utilize ammonium salts in order to modify and to attempt to render cellulose materials non-inflammable. However, one great commercial manufacturing problem is that it is very difficult to secure a thorough and even distribution of the fire retardant chemical within the board and particularly in sufficient quantity or concentration to render the board substantially non-inflammable. Another problem is that the treated material or board in many instances does not retain its mechanical strength and its desired surface quality and texture when these salts are added. Techniques presentiy employed for the commercial production of fire retardant or fire resistant particle board is to first manufacture the board in one stage and thereafter in a subsequent stage treat the board with chemicals in order to render the board non-inflammable or fire resistant. For example, one method presently used is to dissolve the fire retardant salt in an aqueous solution and then soak the wood board in this solution in a bath which is usually under pressure. Pressures which are generally employed are in the range of from about atmospheric to 150 lb./sq. in. while the temperatures of the soaking operation are in the range of about ambient F. to 190 F.
One ditficulty with this technique is that it is very difficult if not impossible to posiiively and accurately control the amount of chemical which is added to or impregnated in the wood. Another disadvantage is that extensive relatively heavy equipment, such as the soaking equipment, for the effective and efficient handling, treating and conditioning of the boards is relaiively expensive. Furthermore, this soaking technique requires prolonged periods of treating and drying which lowers production, thus making the process expensive and undesirable. Also, when the treated board is dried, fire retardant chemicals are deposited on the surface of the board as the water or other solvent evaporates, which detracts from the surface quality. If sanding is required due to loss of surface quality, the fire retardant material on the surface is removed, which decreases the fire resistivity of the board. On the other hand, if the efilorescence material is left on the surface or if the concentration of the fire retardant material at the surface of the wood is high, then difficulties are experienced in finishing the surface to the desired degree of luster. In many instance also the dimensional stability is impaired as well as the resistance of the board to wetting.
Another commercial technique presently utilized, particularly in the manufacture of fire retardant particle board, is to soak the wood particles with the fire retardant material prior to the gluing stage or step. In some instances when the wood particles used have'a high moisture content in the range from 30 to by weight, and when the fire retardant chemical is highly water soluble the dry chemicals have been sprinkled on the wood chips, flakes, or particles prior to the gluing step and the mixture allowed to stand until the diffusion of the chemicals into the particles is complete. Whenever this method of fire retardant treatment is done prior to gluing, the wet wood material must be dried before gluing, and as a result the fire retardant chemicals are deposited on or near the wood surface as described hereinbefore. Also, at the high concentrations required for effective treatment, the surface coating of fire retardant chemical interferes substantially with the effective bonding of the wood particles by the glue or resin. In some instances, this may be compensated for by using higher concentrations of the hinder, or more expensive resins, or longer pressing times, or a combination of these factors, all of which substantially increase manufacturing costs.
Thus, it has been discovered that when this wet technique is utilized, namely, the impregnation of the formed particle board or particles with the fire retardant solution, loss of strength results, necessitating increased fabrication and finishing costs. In order to partially reduce the severe strength losses which result due to the soaking treatment, a higher than normal concentration of the waterproof resin is used on the binder. Also, surface roughing and variability of swelling which occurs after the treated panels are dried results in heavy material losses during the sanding operation. This also substantially increases the cost of the operation.
The present invention produces a particle board of high quality and of very superior fire resistant qualities. The fireboard of the present invention also has excellent mechanical and surface properties which are secured at a minimal cost. The particle board of the present invention is characterized by having excellent physical properties as well as having excellent dimensional stability. Furthermore, excellent surface finishing and overlaying properties are secured.
The fire retardant product of the present invention in essence involves a composition board comprising preferably three distinct layers having an ammonium salt distributed in optimum concentrations through the layers. This composition board of the present invention is secured by a unique improved process which may be readily understood by reference to the drawing illustrating an embodiment of the same.
Reference is made to the drawing which is a diagrammatical sketch or diagrammatical flow plan illustrating the manufacture of a three-layer or three-ply fire retard.- ant panel. Wood flakes or fiber particles designed for the manufacture of a lower face ;layer are introduced into drier and storage silo 4 by means of line or conveyor means 1. Wood flakes or fiber particles designed for the manufacture of a top face layer are introduced into drier and storage silo 6 by means of line or conveyor means 3. The wood material for the manufacture of particle boards for the respective faces may be of various sizes and configurations, but preferably have dimensions of about 0.05 to 0.75" Width and 0.25 to 1.5" lengths and thicknesses of about 0.01 to 0.05 inch.
A wide range of wood particle species and sizes can be utilized as face material. The choice is predominantly determined by what is available and offers high strength properties, soft wood flakes that are cut from plywood peeler cores are quite suitable. Another possibility is refined fibers from hogged lumber and plywood residues or planer shavings. The material selected depends upon the desired panel properties, e.g., long flakes and fibers provide high strength, small fragments produce a smooth surface. The effect of surface density on flame spread is not clear, however, bond strengths and density as related to ability of a panel to retain a layer of char is important. A smooth surface is also beneficial toward reduced flame spread, therefore weak fibers which tend to cause a hairy or fuzzy sanded surface should be avoided.
Driers 4 and 6 are maintained at temperatures in the range from about ambient to 175 F. for a time period of about 3-5 minutes. (Face material drying is negligible for most raw materials.) The conditions are so adjusted that the moisture content of these chips are from about 17 to 23 percent by weight moisure, preferably at about 20 weight percent moisture. These particles therefore are relatively moist.
Resin or binder A is introduced and mixed with the wood chips removed from drier 4 by means of line 37 in blender 50, while resin or binder C is introduced and mixed with the wood chips from drier 6 by means of line 39 in blender 51. Resins A and C are mixed by any suitable means with the prepared, moist wood material such as with a rotating blender and stationary spray nozzle or any suitable mixing means.
Wood material suitable for the manufacture of the core or center layer is introduced into drier by means of line 2. Generally, these particles are of a different configuration than the face material. For example, the particle size core-plywood material may range from about 0.25 to 1.5 inches in length to 0.05 to 1.0 inch in width to 0.01 to 0.2 inch in thickness.
Because contributions to flame spread and strength property are minimal in the core layer, material is determined by availability, economics and ease of refining, and not so much by strength. This means that small material and weed species can be utilized. Flakes provide the tightest core layer and therefore will have slower release of pyrolysis products than a particle core. Density effects are negligible outside of consolidation of a char layer. Resin B is mixed with the chips Withdrawn from drier 5 by means of line 38 in blender 52. Here again any suitable means of thoroughly mixing the chips with the resin may be employed.
The ammonium phosphate crystals are added as a dry powder to the mixture of resin chips by means of lines 10, 11 and 12 at zones 7, 8 and 9, respectively. These salt crystals are preferably sprinkled onto the face wood particles and core wood particles by metering or control systems 40, 41, 42 which accurately proportions the rate of crystal salt addition to the calculated bone dry wood passing along conveyor belts or equivalent means 7, 8 and 9.
The salt addition occurs immediately after the wood material and binder emerges from the blenders where the resin is mixed with the wood chips. The addition of the dry crystals of salt should be within about minutes, preferably within 2 minutes.
The salt crystals are added as soon as possible after the resin addition and with as much agitation or tumbling as possible to effect maximum distribution and adherence of the salt to the moist and tacky wood material. If little mixing is achieved in the transfer of the glued material to the forming line spreader, then a separate mixing stage should be introduced. The effectiveness of the salt and wood mixing is critical, the length of time in mixing or after mixing until panel pressing is of little consequence. Since the glued material is tackiest just out of the blender, this is the time to add the salt crystals. Thus, the immediate addition of the fire retardant salt crystals to the freshly glued wood material is very important in obtaining the required dispersion and adhesion of the crystals to the wood.
The mixtures of crystal, wood particles and resin then flow into hoppers 13, 14 and 15 respectively onto weighing conveying belts or transfer points 16, 17 and 18 respectively. The mixtures then flow through hoppers 19, 20 and 21 onto conveying belts or caul plate which passes on at felting points 22, 23 and 24 respectively. The use of these mixing hoppers and transfer points and conveyor belts secures even distribution of the crystals throughout the mass of glued, moist wood particles.
The face material is at ambient temperature upon entering the blender. The only heating in the blender is produced by the resin which is sprayed at 32 to 42 C. and therefore is negligible. The same resin temperatures are used in spraying the core material. This material may be warm from the storage silo in the range from to F. but never over F. The wax sprayed 011 the core (approx. 1%) is heated to 265-285 F. and contributes some heat but no attempt is made to heat the wood material during blending.
The salt crystals are not added to the blender because of plugging from salt gumming up the resin during spraying and because of ammonia fumes.
The respective chip mixtures are spread in layers on a caul plate at felting points 22, 23, and 24 and then passed into a prepress 25, for biscuit consolidation. (Press=l00- p.s.i.). The prepressed panel is then easily loaded into the hot press 26. These presses are operated at temperatures in the range of about 320 F. to 375 F., preferably at a temperature of 350 F. for a time period of about 5 to 16 minutes, as for example about 10 minutes. The pressures employed are in the range of about 225 to 400 lbs/sq. in., preferably at about 350 lbs/sq. in.
The panels are removed by means of line or conveyor 27. The finished panel is conveyed by means of line 27 and given a hot stack, at zone 28 for about 20 to 30 minutes. The boards may be cooled in zone 29 and then passed to storage 30. Under certain conditions the boards may bypass the hot stack and passed directly to storage 30.
As pointed out, after the pressing cycle is completed, the hot panels are immediately preferably given at least a 20 minute stacking while hot. This hot stacking not only furthers the curet of the phenolic binder, but increases the length of time the ammonium crystals are exposed to a hot and humid atmosphere. Thus, by the time the panels are cooled and have reached equilibrium moisture content conditions, the highly effective distribution of the fire retardant chemicals throughout the wood particles will have been completed, thereby producing a very excellent flameproof wood structural member of high mechanical strength.
In the panels, under normal equilibrium moisture conditions, a large percentage of the ion components that were in solution during pressing will have reprecipitated as a thin effiorescence on the fiber surfaces and within the cellular structure of the fibers as the moisture leaves the panel. Thus, excellent distribution and intimate contact of the fire retardant chemical with the wood is secured after the majority of the resin bonds between the wood particles have been made. Thus, this avoids or overcomes almost entirely the interference and losses in strength of the panel which result when the wood is treated With the fire retardant material before gluing.
The thicknesses of the face panels or plies as the core layer or ply may be varied appreciably. Generally, the thicknesses of the face veneers are in the range of about to 7 inch, such as A; inch, while the thickness of the core ply is in the range of about A to 1 /2 inches such as about inch.
As pointed out heretofore, the present invention comprises a combination of the use of an ammonium salt in conjunction with a resin, preferably a synthetic resin and a wood material as, for example, wood chips. The preferred salts are the ammonium salts, preferably ammonium phosphates such as diammonium phosphate. Other less satisfactory salts are, for example, monoammonium phosphate, ammonium bromide, ammonium sulphate, and the like.
With respect to the salt crystals, it is important that the size be controlled if the final product is to have the salt effectively distributed throughout the mass and have excellent bond strength as well as good surface qualities. For example, with a three-ply panel construction, it is preferred that the face veneers have very fine salt materials while the core layer will tolerate coarser salt crystals. The preferred results are obtained by grinding the salt crystals to less than 20 mesh (Taylor Standard screen scale) for the core ply and using the fines which pass through a 35 mesh screen for the face veneers or plies. The use of fines of salt in the critical face layers not only gives excellent distribution and adherence of one ply to another, but also solubilizes in the mass more completely during pressing. This leaves virtually no surface pitting following the sanding of the panel.
While various binders or resins may be used, it is preferred to use an alkali catalyzed liquid phenol-formaldehyde resin in combination with diammonium phosphate salts. However, the physical properties of this resin must satisfy certain definite requirements. The further the resin is advanced or cross-linked, the shorter will be the time requirement during pressing and the lower will be the loss in bonding efliciency as a result of resin migration into the wood. At the same time, the higher the resin viscosity, the lower will be the spraying efliciency. Of course, the resin may be heated during spraying to reduce viscosity, but this approach is limited by the resin advancement that will occur during storage.
With regard to viscosity, the maximum viscosity that can be effectively sprayed should be used. Usually this means a spraying viscosity of about 150-200 c.p.s. at 25 C. Thus a. resin of over 200 to 400 c.p.s. at 25 C. can be heated up to 42 C. to reduce the spraying viscosity to the desired level without drastically shortening the pot life of the resin in the stand tanks. Therefore, viscosity used will depend on the capabilities of the spraying equipment.
The resin solids or resin concentration used depends on the moisture content of the wood material at the blenders and the final moisture content desired before pressing and the amount of resin to be added. For wood moisture contents of and 3% and resin additions of 12 and 6.5% (based on ODW) for face and core material respectively, the final moisture content of approximately 33.5 and 8.5% will be achieved by using a resin solids concentration of 40%. The range for a desired operation would be 37 to 43%. Where resin is added during fiber refining, the resin solids may go as low as 12%.
This method is limited by the amount of resin sprayed on the wood and the moisture tolerance of the particle board during pressing. For process variables and equipment used in this invention, the resin advancement must be kept between 150 and 400 centipoises and preferably about 175 centipoises at C. and at a spraying temperature of about 32 to 42 C. Thus, the viscosity of the resin when sent through the spraying equipment should be 150 to 200 c.p.s. to obtain effective resin distribution and rate of application. This will change somewhat according to the equipment used and the amount of resin to add within a given period of time.
The concentration of the resin used in one operation was at 43% resin solids based on the total weight of the solution. The sodium hydroxide concentration of the resin was also carefully controlled within 4.5 to 5.4%. Since the ammonium salts have a buffering action or slowing of the cure rate of the resin, more caustic must be added than is ordinarily used in the phenolic bonded particle board as for example about 0.5 to 2.5%.
The alkali is usually added as 50% sodium hydroxide. The concentration and amount added is taken into account when calculating the resin solids and amount of water added. For this process, the desired alkali content is present when the resin is purchased in the concentrate form (approx. 50% R.S.); only water is added during advancement cooking. However, above a certain caustic level, i.e., above about 5% of resin solids added, in the finished panel the swelling and water absorption properties and the stability of the resin bonds are noticeably degraded. The sodium hydroxide concentration used in combination with Douglas Fir, Western Pine, and White Fir wood material is about 4.7%. The use of less acidic wood species permits a decrease in the caustic concentration.
With woods of nearly neutral or slightly acidic pH, the sodium hydroxide concentration will still have to be maintained at a comparatively high level due to the buffering of the ammonium salt. A sodium hydroxide concentration above about 4% but not more than about 5% would encompass all changes due to species acidity (percent based on resin solids).
The three layer or three veneer construction particle board of this invention affords a high :degree of flexibility in producing a superior and strong fire retardant commercial product. This is secured by controlling the material which makes up each individual layer. It is possible and very desirable to add higher concentrations of the fire retardant chemical or salt in the face layer than in the core layer, or only in the face layer. The diammonium phosphate or an equivalent salt or crystal may be used in both of the face layers and also in the core layer.
The amount of fire retardant salt used may be varied appreciably *but it is preferably in the range from about 15 to 30 wt. percent, preferably in the range of about 20 to 25 wt. percent as, for example, about 22 wt. percent based on the oven dry weight of the wood particles in the panel. At a 15% by wt. percent diammonium phosphate concentration in the core layer, (range 10 to 20% by wt.), and 22% diammonium phospate in the face layer plus the use of high quality cut Douglas Fir for the face flakes (range 20% to 30% by wt.), a high density face layer is produced which is very effective in resisting flame spread and retarding the escape of inflammable pyrolysis products from within the panel.
A particular adaptation of the present invention is to have a varied moisture content in the face layer or face ply which differs from the moisture content of the core ply. It is preferred that the moisture content in a glued face ply prior to pressing be in the range from about 25 to 40 wt. percent as, for example, about 33 wt. percent and the resin solids at about 11% or in the range of 8 to 15% based on the oven dry weight of the wood. The face material moisture content will be greater than bone :dry and probably 8 to 25%, and preferably 20%. When the resin is added, the water present brings up the moisture content according to the resin solids used. In this way the moisture content is controlled. The low cost White Fir and Western Pine coarse particles for the core are approximately 7 to 11 wt. percent moisture content after gluing as, for example, about 9 wt. percent moisture content and about 5 to 9 wt. percent solids as, for example, about 7% resin solids.
The core material is usually dried to 3% before gluing. After the resin is applied the moisture content is raised according to the resin solids. The term bone dry wood (BDW) is used to designate the base upon which the amount of resin added or wood used is calculated. This is because the moisture content of the wood is constantly changing during processing.
The use of a higher moisture content in the outer layers with respect to a fi-ameproof member provides a very important advantage over single layer boards or plies where the moisture is uniformly distributed throughout the panel. When the moisture content is only in the face panel, the steam generated will pass through the face flakes and into and out through the core layer, causing dissolution and disass-ocia-tion of the ammonium salts. The ions formed after solvati-on of the crystals then diffuse into the fiber structure. This etfect is greatest in the face material where the flame spread is the most critical. The moisture in the face layer also plasticizes the wood to form a very smooth, high density face layer with excellent physical properties.
There are no further requirements after pressing insofar as fire retardant treatment is concerned. Very little degradation or material loss is incurred from efilorescence on the surface of the panel because most of the moisture in the panel is driven into the center and out of the edges of the core during pressing. No further time is required in the board forming process as the fire retardant materials are added and the glue materials are transferred to the storage bin. The total cost of fire retardant treatment is far below the cost of other processes.
Thus, the present invention is concerned with the use of particular type crystals which are applied to a mixture of wood flakes and glue at a predetermined time after the glue and wood flakes are mixed. By this technique, the diffusion of the crystals such as the diammonium phosphate crystals throughout the entire mass of the wood and resin is secured while, at the same time, the mechanical strength and other desirable characteristics of the panel remain unimpaired. Furthermore, by the use of particular size crystals in the face veneers and the use of relatively coarse crystals in the core, additional mechanical features are secured. In addition, by the control of the moisture content prior to pressing in the face veneers as compared to the total moisture content in the core, additional unexpected desirable features are secured.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. A fiameproof wood article of superior quality and strength which comprises a lower face layer of adhered small dimensioned wood particles having uniformly distributed therethrough crystals of an ammonium salt taken from the group consisting of ammonium phosphates, ammonium bromide and ammonium sulphate, a core layer of adhered large dimensioned wood particles having uniformly distributed therethrough crystals of said ammonium salt, a top face layer of adhered small dimensioned wood particles having uniformly distributed therethrough crystals of said ammonium salt, said layers being rigidly adhered to one another by a resin binder.
2. Article as defined by claim 1 wherein the particles in said lower face layer and said top face layer have dimensions of about .05 to .75" in width, about .25 to 1.5" in length and about .01 to .05 thick; and wherein the particles in said core layer have dimensions of about .05 to 1.0" in width, about .25 to 1.5" in length and about .01 to .2" thick and wherein said salt is an ammonium phosphate salt and the core particles are always of greater dimensions than the face layer particles.
3. Article as defined by claim 1 wherein said salt is diammonium phosphate.
4. Article as defined by claim 3 wherein the size of the salt crystals in the face layers are less than about 20 mesh, and the size of the salt crystals in the core layer are about 2035 mesh.
5. Article as defined by claim 4 wherein the moisture content in the face layers is in the range of about 20 to 40 wt. percent and wherein the moisture content in the core layer is in the range of about 7-11 wt. percent.
6. Article as defined by claim 1 wherein said particles are adhered by means of an alkali catalyzed liquid-phenolformaldehyde resin, and wherein said ammonium salt is an ammonium phosphate salt.
7. Article as defined by claim 1 wherein said face layers are about to thick, wherein said core is about A to 1%" thick, and wherein said article contains from about 15 to 30 wt. percent of an ammonium phosphate salt distributed therethrough.
8. Article as defined by claim 1 wherein said face layers contain from about 20 to 30 wt. percent of an ammonium salt distributed therethrough, and wherein said core layer contains from about 10 to 20 wt. percent of an ammonium salt distributed therethrough.
9. Article as defined by claim 8 wherein said face layers contain about 22 wt. percent of an ammonium phosphate and wherein said core contains about 15% by weight of an ammonium phosphate.
10. Process for the production of a strong, high quality flameproof particle board which comprises, (1) mixing relatively small dimensioned wood particles with a binding resin, (2) mixing relatively large dimensioned wood particles with a binding resin, (3) directly thereafter adding dry crystals of an ammonium salt taken from the group consisting of ammonium phosphates, ammonium bromide and ammonium sulphate to the mixture of relatively small dimensioned wood particles and resin, and to the mixture of relatively large dimensioned wood particles and resin, said ammonium salt being present in a higher concentration in said small dimensioned wood particles and a lower concentration in said large dimensioned wood particles, (4) thereafter positioning a core layer of relatively large dimensioned wood particles and resin intermediate a top face layer and a bottom face layer of said relatively small dimensioned particles and resin, (5) thereafter pressing the same, whereby a three layer particle board is secured having the ammonium salt crystals distributed therethrough.
11. Process as defined by claim 10 wherein said resin is an alkali catalyzed liquid-phenol-formaldehyde resin, and wherein said salt is an ammonium phosphate salt.
12. Process as defined by claim 10 wherein said salt is an ammonium phosphate salt, wherein the concentration of the salt in said face layers is in the range of between 20 to 30% by weight and wherein the concentration of the salt in said core layer is in the range from about 10 to less than 20% by weight.
13. Process as defined by claim 10 wherein said particles are dried to a moisture content between about 17 to 23% by weight before mixing with said resin and wherein said layers are pressed at a temperature in the range of about 320 F. to 375 F. and at a pressure in the range from about 225 to 400 lbs/sq. in.
14. Process as defined by claim 13 wherein the drying step and the concentration of the resin are controlled to have a moisture content in said face layers in the range from about 20 to 40% by weight, and wherein the moisture content in the core is in the range from about 7 to 11% by weight in the finished product.
15. Process as defined by claim 10 wherein the viscosity of said resin when mixed with said particles is in the range from about to 200 cps.
References Cited UNITED STATES PATENTS 2,859,187 ll/l958 Ropella 161-168 2,891,019 6/1959 Ericks 260l72 3,050,424 8/1962 Schmitt 16l--403 ROBERT F. BURNETT, Primary Examiner.
W. J. VANBALEN, Assistant Examiner.