EFFECT OF FLAME RETARDANT ADDITIVES IN FLAME RETARDANT GRADE OF ABS
EXPLORATORY INVESTIGATION OF FIRE-RETARDANT …
35
U.S. DEPARTMENT OF AGRICULTURE • FOREST SERVICE FOREST PRODUCTS LABORATORY • MADISON, WIS. In Cooperation with the University of Wisconsin U.S.D.A. FOREST SERVICE RESEARCH NOTE FPL-0201 AUGUST 1969 EXPLORATORY INVESTIGATION OF FIRE-RETARDANT TREATMENTS FOR PARTICLEBOARD
Transcript of EXPLORATORY INVESTIGATION OF FIRE-RETARDANT …
EXPLORATORY INVESTIGATION OF FIRE-RETARDANT TREATMENTS FOR
PARTICLEBOARDU.S. DEPARTMENT OF AGRICULTURE • FOREST SERVICE FOREST
PRODUCTS LABORATORY • MADISON, WIS. In Cooperation with the
University of Wisconsin
U.S.D.A. FOREST SERVICE RESEARCH N O T E FPL-0201 A U G U S T 1969
EXPLORATORY INVESTIGATION OF FIRE-RETARDANT TREATMENTS FOR PARTICLEBOARD
Abstract
More than 80 exploratory experiments were made on the development of fire-retardant-treated particle- board using 15 fire retardant chemicals, 3 resin binders, 2 species of wood, and various types of application. Among the types of application were those employing solutions of fire retardant salts sprayed on green flakes; and others employing dry, finely divided fire retardants added after mixing binder resin and flakes. Of the boards produced and tested, several using certain borates, AWPA Type C and Type D fire retardants, and monoammonium phosphate showed promise. Properties considered were flamespread, smoke production, strength, and stability. Salts applied in solution to green wood, followed by drying, gave results superior to those obtained from the use of dry fire-retardant salts. None of the boards were tested for fire retardance and strength after cycling through changing humidities. The results of this series of experiments are released in the form of a progress report for use by others engaged in similar studies.
FPL-0201
U.S. Department of Agriculture
Introduction
As a result of increasing concern with fire safety, modern buildifig codes have developed specific requirements for fire performance of construction materials. With greater use of particleboard in building construction there is greater need for the development of an effective fire-retardant treatment for this product.
The fire-test results reported here were obtained from a single test for each set of boards. The data, therefore, should be regarded only as indicators of treatment effectiveness as a prelude to a more comprehensive study of promising combinations. The results of this series of experiments are released in the form of a progress report for use by others engaged in similar studies.
The value for each relative humidity or soak condition for each set of boards in the evaluation of dimensional stability is based upon the test of a single speci- men. Also, none of the boards were tested for fire retardance and strength after cycling through changing humidities. This must be considered in inter- preting these test results.
1Maintained at Madison, Wis., in cooperation with the University of Wisconsin.
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Use of Fire Retardants in Particleboard
Although there is appreciable knowledge on the use of fire retardants with solid wood, plywood, and textiles, there is comparatively little information on their application to particleboard. The use of fire retardants in this product involves some specific problems.
The particles composing the board are held together by the resin binder, not in the form of a somewhat continuous film as in plywood, but as a multiplicity of fine spots of adhesive which tack together the contacting particles. There is obviously a much smaller amount of adhesive per bond area in particleboard than, for example, in plywood.
The nature of the pressing operation in the manufacture of particleboard is such that precise control of the curing of the resin is essential. During the time required for the particleboard press to close to final raw board thickness, heat is being applied to the binder resin. The resin must not cure before the press has closed to board thickness. From a production standpoint, it is undesirable to slow the curing of the resin sufficiently to require a long pressing time.
Unfortunately, the chemical nature of the fire retardant and the quantity required to be effective are such that frequently the curing properties of the binder resins are affected. Fire-retardant-treated particleboard often has lower strength than untreated board.
Study Materials
The particleboards upon which this report is based were made and evaluated in two separate though similar investigations. To distinguish between the two groups of boards, since their fabricating conditions were different, the investiga- tion concerning the group made first has been designated as Study I and that concerning the second group of boards has been designated as Study II. Differ- ences in fabrication variables of the two groups are indicated in the list of fabrication specifications.
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Fabrication Specifications
The boards for this investigation were made to the following specifications:
1. Species: Douglas-fir heartwood in Study I; Douglas-fir heartwood or aspen in Study II.
2. Density: 40 pounds per cubic foot.
3. Board size: 1/2 inch by 24 inches by 28 inches, rough dimensions.
4. Particle size: 1 inch by 0.015 inch by random-width flakes.
5. Resin type: Urea-formaldehyde, 65 percent solids; phenol-formaldehyde, 43.5 percent solids; and melamine-formaldehyde, 45 percent solids. The urea and phenol resins were used in the liquid form as they were supplied by the manufacturer. The melamine-formaldehyde resin, in dry form, was dissolved in 1:15 isopropanol-water solution before using.
6. Wax sizing (where used): 1 percent wax solids, based on ovendry wood weight; applied as supplied, a 46 percent solids emulsion.
7. Mat moisture content: Adjusted to 12 percent in Study I and to 10 percent in Study II.
8. Press temperature: In Study I, 325° F. for the urea-formaldehyde and melamine-formaldehyde resins and 350° F. for the phenol-formaldehyde resin; in Study II, 300° F. for the urea- and melamine-formaldehyde resins and 325° F. for the phenol-formaldehyde resin.
9. Press time: In Study I, 15 minutes for all binders: in Study II, 8 minutes for the urea- and melamine-formaldehyde binders and 10 minutes for the phenol-formaldehyde binder.
10. Time to stops: 2 minutes in Study I; 1 minute in Study II.
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Selection of Fire Retardants
Numerous fire-retardants have been suggested for wood (14,15 ,16).2 Most of these have been water-soluble inorganic salts, a number of which have been in use for many years. Among the more recently proposed fire retardants are organic compounds which seem to lack some of the disadvantages of the inorganic water-soluble salts. Some of these newer fire retardants, which have proven quite effective for textiles and for paper, are too expensive to consider for particleboard treatment.
The fire-retardant treatments selected for these experiments include a number of the more common inorganic treatments which have proven effective for wood, and were considered economically practical for this application.
The following fire retardants were included in this investigation:
Monoammonium phosphate Diammonium phosphate Borax Boric acid Borax - boric acid, 1:1 Monoammonium phosphate - ammonium sulfate, 1:1 Zinc sulfate Aluminum sulfate AWPA type C (4) AWPA type D (4) 11-37-0 ammonium polyphosphate (12) Dicyandiamide - phosphoric acid - formaldehyde, 1:1:0.15 mole ratio Zinc sulfate - zinc silicofluoride - urea Ammonium polyphosphate (32 percent phosphorous content; proprietary
product) Boric acid - disodium borate tetrahydrate, in weight ratios of 0:100,
10:90, 20:80, and 25:75 (7)
2 Underlined numbers in parentheses refer to Literature Cited at the end of this report.
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There are several possible methods of applying fire retardants to particle- board. Selection of a particular method depends upon such considerations as the practical requirements of the manufacturing process and the characteristics of the fire-retardant-treatment chemicals.
Application of water solutions of fire retardants to the green or wet wood particles soon after the particles are made is expedient because this permits penetration of the treatment into the wood, and the water of the treating solution may be evaporated in the subsequent drying operation. The solution could be applied either by spraying or by soaking the particles.
The disadvantage of any application in solution, particularly for chemicals with rather low solubility, is the need to remove the added water in the dryer.
The fire-retardant solution could also be added at the blender where the resin and wax sizing are added to the dry particles. The disadvantage of this, however, is that the water of the treatment solution would have to be removed in the pressing operation. In addition, the added water might dilute the resin and cause excessive strike-in. It is probably desirable for the fire retardant to penetrate into the wood and for the binder resin to stay at the surface. This would be difficult to accomplish by spraying both materials on the dry particles at the blender.
It may appear convenient to add the fire retardant in solution to the finished board by soaking, applying to the board surfaces, or pressure impregnation. However, treatment of the finished board would require an additional drying operation to remove the relatively large amount of water added. Thus, it is more practical to add the fire-retardant solution to the particles rather than to the finished boards.
The fire-retardant-treatment chemical may be added as a dry powder, preferably during or immediately after the resin application, so that it will be retained on the wood particles by the tackiness of the binder. Dry application obviously eliminates the necessity of removing the water added by solution application of the fire retardant. However, many binders lack the requisite tack to retain the powder. Problems associated with dry application include lowering of the effectiveness of the binder, sifting of the chemical through the mat, and difficulty, compared with solution application, of obtaining even distribution of the treatment.
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Three methods of adding the fire retardant were investigated in these studies: (1) application of dry chemicals at the resin blender, (2) spraying the chem- icals on the wet particles, and (3) roller application of the solution on the surface of the hot board just removed from the press.
Application of dry chemicals to the wood particles in the blender immediately after spraying with resin presented problems of obtaining even distribution and of adhering all the required fire retardant to the particles. After fire test results indicated that application dry was less efficient than in solution, the dry application was used only for those chemicals having low solubility. Applica- tion of approximately 30 to 35 percent water solutions to the wet particles immediately before drying proved to be simpler, more efficient, and more practical for production, so this method was used for many of the combinations investigated.
Preliminary investigations had indicated that some of the chemicals reacted with the wood when the treated particles were oven dried: therefore, in Study I all particles treated with fire retardants in solution were air dried at room temperature. In Study II, a number of similar boards were made from particles which had been air dried at room temperature, and some others of particles which were oven dried to about 5 percent moisture content at 225° F. In produc- tion, the particles would be exposed to a higher temperature for a shorter time in a continuous dryer. In order to control the rate and uniformity of drying in a laboratory oven, however, it was necessary to use a lower drying temperature.
An attempt was made (board 18) to add the fire retardant, diammonium phos- phate in this instance, in solution to the surface of the hot hoard immediately after its removal from the press.
Fire-retardant-treatment chemical solutions for Study I, and for Study II except where noted otherwise, were made up at 30 to 35 percent concentration in water heated to 120° F. The solutions were sprayed on wood particles which were at about 30 percent moisture content. The amount of fire-retardant mate- rial added was based on dry weight of the chemical as a percentage of the weight of the ovendry wood.
All liquid materials--fire-retardant solution, binder resins, and wax emulsion-- were applied by spraying them into the mass of particles tumbling in a rotating drum. Dry fire-retardant chemicals were applied slowly to the resin-sprayed particles in the rotating drum. All materials applied dry were reduced to pass through a 100-mesh screen.
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The American Wood-Preservers’ used in this investigation were:
Association fire-retardant formulations (4)
Diammonium phosphate Ammonium sulfate Sodium tetraborate, anhydrous Boric acid
10 60 10 20
Zinc chloride Amnnonium sulfate Boric acid Sodium bichromate
35 35 25 5
Monoammonium Phosphate- Ammonium Sulfate treatment
High smoke density is a problem with particleboards treated with monoammo- nium phosphate. This may be reduced through the addition of ammonium sulfate. In Study I equal amounts of these chemicals (1:1 weight ratio) were used in three series of boards (boards 37, 38, and 39). In Study II, to attempt to reduce the effect of the sulfate on the resins, a 3:1 weight ratio of monoammonium phosphate-ammonium sulfate was used (boards 64 and 65). This treatment was added at a 13-percent level so that the level of the monoammonium phosphaete would be about 9 percent.
Boric Acid-Disodium Octaborate Treatment (7)
The boric acid-disodium octaborate solutions were made up in proportions of 0:100, 10:90, 20:80, and 25:75 at a solution concentration of 38 percent in water heated to 145° to 165° F. The disodium octaborate tetrahydrate (Na B O
2 8 13. 4H.O) was dissolved first, followed by the boric acid. The solution was main-
tained and sprayed at 145° to 165° F. Application was 18 percent, based on oven- dry wood weight.
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Zinc Sulfate- Zinc Silicofluoride- Urea Treatment
This treatment, which has been recommended as a means of insolubilizing a zinc compound within the wood, was applied at a rate of 2 pounds of zinc per cubic foot of board. The following formulation was used:
Component Weight Percent
Water 55.0 Zinc sulfate 16.5 Zinc silicofluoride 18.1 Urea 10.4
100.0
The zinc content of this formulation is 7.5 percent.
After the green (wet) wood particles were sprayed with this solution they were placed in tightly closed polyethylene bags and heated at 175° to 180° F. for 15 hours. This treatment is intended to react the zinc compounds with ammonia derived from decomposition of the urea. The particles were removed from the polyethylene bags and dried at 225° F. to about 5 percent moisture content.
Treatment with Ammonium Polyphosphate (32 Percent P)
This material is a low-solubility ammonium polyphosphate proprietary product with a phosphorous content of 32 percent. Because of the low solubility of this chemical, it was necessary to apply it as a dry powder following resin application.
Dicyandiamide-Phosphoric (9,10)
Acid Treatment
A 20-percent solution phosphoric acid, and application was used in
was formaldehyde treating
the wood particles.
amide mole
(1-cya ratio.
no- A
guanidine) 10-percent
The dicyandiamide was dissolved in water which had been heated to 120° F. With constant stirring, the phosphoric acid and formaldehyde were added in turn and the temperature was increased to 180° F. The sprayed particles were dried in a 180° to 190° F. oven to approximately 5 percent moisture content. Care was taken to insure that the oven temperature did not go above 190° F. and that the particles were not overdried.
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Evaluation of Board Properties
Three boards were made for each treatment. Four half-boards, each 13-3/4 inches by 23 inches, and one piece 13-3/4 inches by 4 inches were required for each fire test. Two half-boards were cut into specimens for mechanical and dimensional stability tests. All test specimens were unsanded.
At the beginning of Study I, control boards without fire retardants were made for reference. Table 1 gives test values ofproperties of the control boards made with urea-, phenol-, and melamine-formaldehyde resins. Because specifications are not available for fire-retardant-treated particleboard, arbitrary values were established for treated boards fabricated to the control specifications to assist in identifying the most promising treatments.
Fire Performance
The fire performance of the boards was evaluated by the 8-foot-tunnel furnace test for surface flammability (1,8) Figure 1 shows the specimen side of the test furnace. All board specimens were conditioned to equilibrium at 30 percent relative humidity before fire testing.
The 8-foot-tunnel method (1) has been developed by the Forest Products Laboratory as a research technique for measuring surface flammability. It generally ranks the flammability of materials in the same order as the 25-foot- tunnel furnace test (3), but the actual index values may differ. Both methods use index values relative to flame-spread distances traveled with respect to red oak at 100. However, because of longer exposure times in the 8-foot tunnel, fire-retardant-treated wood at the lower range of the scale frequently has a flame-spread index of about 40 as compared to 25 on the same material in the 25-foot-tunnel furnace. Therefore, one of the criteria selected for an adequate treated particleboard was a flame-spread index in the 8-foot-tunnel furnace of 40 or less. The heat contribution of similar materials generally correlates with the flame-spread distances involved in the tests, and therefore a heat contribu- tion index of 40 or less was also selected as an acceptance criterion.
Smoke density index values are established for both methods relative to the smoke density produced by untreated red oak lumber in the respective tests. In the 25-foot tunnel with a large ignition burner, both untreated red oak and fire-retardant-treated materials burn under a flaming exposure condition. However, in the 8-foot tunnel, where most of the heat is supplied by a radiant source, exposure results in flaming combustion for the untreated red oak, but nonflaming exposure for fire-retardant-treated products. Nonflaming exposure of cellulosic products results in much greater smoke production than does
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flaming combustion. In previous experience with the more effective fire- retardant chemicals, smoke index values as high as 400 were obtained in the 8-foot tunnel for products rated as 50 or less for flaming combustion in the 25-foot tunnel. Therefore, an acceptance criterion for boards produced in this study was established as a smoke index value of 400 or less in the 8-foot-tunnel test.
Strength Properties
Following methods specified in ASTM D 1037-64 (2), strength properties of the boards were evaluated by static bending and tension perpendicular to the surface. An initial strength loss of approximately 40 percent as a result of treatment was arbitrarily applied in choosing the minimum strength values in table 1 for treated boards. The 40 percent reduction was chosen as a reasonable indicator of boards showing promising strength properties after treatment. Other values could be used. For example, selectionof an approximate 20 percent initial strength reduction would reduce the number of treated boards showing promise to three (boards number 66, 67, and 68).
In Study I, nine static bending specimens and 24 tension perpendicular to the surface (internal bond) specimens were tested for each treatment. In Study II, six static bending specimens and eight tension perpendicular to the surface specimens were tested for each treatment. Specimens were cut from each set of three boards as shown in figures 2 and 3.
Dimensional Properties
Dimensional stability test specimens 3/4 inch by 22 inches were made for each set of boards. All specimens were first brought to equilibrium in an atmosphere of 30 percent relative humidity at 80° F. Then one specimen from each set of boards was exposed to each moisture treatment as follows: One oven dried to moisture freeness in a 220° F. mechanical convection oven for 24 hours; one soaked in water for 38 days; and one each exposed to 30, 60, 80, or 90 percent relative humidity at 80° F. for 30 days. A vacuum-pressure soak test (11) was performed on a specimen of each board.
Measurements were made on each specimen after the initial conditioning at 30 percent relative humidity and again after reaching equilibrium with the find condition. Length measurements to ±0.001 inch were taken from two small holes 20 inches apart in the face of the specimen, and thickness measurements were taken at five marked, equally-spaced points between the holes (11). The tabulated values are based upon a calculated ovendry condition.
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Included for controls in both Studies I and II were boards made without fire-retardant treatments (boards 1 through 4 and 44 through 49). These included a board made with each of the three resins, and in Study II control boards were made both with Douglas-fir and with aspen. One board (board 2) was made with the addition of 1 percent (solids) wax.
The relationship between properties of untreated and fire-retardant-treated commercial production boards may differ from that of the boards made in this study because of certain basic differences in the laboratory and the plant processes. For example, the higher drying temperatures and the required storage of the fire-retardant-treated and resin-sprayed particles in production will probably increase the effect of the chemicalson the wood and on the binder resin. The rapid press closure to stops used with the laboratory press may not be possible with most production presses. Increases in linear movement and thickness of approximately 50 percent as a result of treatment were arbitrarily allowed in choosing the values given in table 1.
Supplementary Investigations
Two brief investigations were made to further consider the relationship of the fire-retardant-treatment chemicals and binder resins. One involved several cure plate tests of resins alone and of mixtures of resins and fire-retardant chemicals. The other was an investigation of the effect of storage of fire- retardant-treated and resin-sprayed wood particles upon board properties.
Cure Plate Tests
Cure plate tests were performed to study the effect of several of the fire- retardant-treatment chemicals on the curing of the binder resins. Immediately before each test, a 10.0-gram sample of the liquid resin was placed in a small beaker, a weighed amount of the fire-retardant-treatment chemical was added, and the mixture was thoroughly stirred. About 2 to 4 gram of the mixture were spread on the hot surface of the plate and stroked with a spatula until the curing point was detected.
The amounts of fire-retardant chemicals in the test samples were equivalent to 2, 5, and 10 percent in the board, with the 10.0 grams of resin as equivalent to 8 percent resin solids in the board.
The apparatus used in this test was simply a laboratory hotplate with accurate thermostat control (±2° F.). A stainless steel sheet was clamped to the surface of the hotplate. The required temperatures were set by a thermocouple placed beneath a weight on the plate surface.
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A high and a low temperature were used in these tests. A low temperature of 225° F. for all resins was intended to be the equivalent of the interior board temperature, and a high temperature of 325° F. for the urea and melamine resins or 350° F. for the phenol resin was intended to be the equivalent of the board surface temperature during the pressing operation.
Effect of Storage
In the laboratory, particles are sprayed with resin and almost immediately formed into a mat and pressed into a board. In a particleboard plant under production conditions, it is necessary to hold resin-sprayed particles in a surge bin before the forming line. Therefore, it is desirable to determine the effect of storage of the fire-retardant-treated, resin-sprayed wood particles upon board properties. For this investigation, monoammonium phosphate was applied in solution at a 10 percent level for the fire-retardant treatment, and urea- formaldehyde resin was used as the binder. The resin-sprayed particles were stored at room temperature in closed polyethylene bags until they were formed into mats and pressed. Fabricating conditions follow those given for Study II of this investigation.
Observations
Results of the evaluation of the boards made in this investigation are given in tables 2 through 4. Tables 2 and 3 respectively give results for the boards made in Studies I and II. Table 4 contains data extracted from table 3 to more clearly indicate the effect of species as shown by the difference in properties of similar boards made with Douglas-fir and with aspen. Results of the brief study of the effect of storage of treated, resin-sprayed particles on board properties are given in table 5. Cure plate tests of the three binder resins in mixtures with several representative fire-retardant treatments are presented in tables 6, 7, and 8. Table 9 gives 10 combinations (extracted from tables 2 and 3) that exhibited properties equal or superior to the arbitrary limits shown in table 1.
Fire Test Properties
As the results given in tables 2 and 3 indicate, several of the fire-retardant treatments gave adequate fire test properties to the particleboards.
Monoammonium phosphate at a 10-percent level, when applied in solution to the green particles gave adequate flame-spread and heat contribution values,
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but smoke density tended to be too high. The smoke density evidently may be reduced with the addition of ammonium sulfate (boards 37, 38, 39, 64, and 65), however this salt is a strong catalyst for the urea resin.
Diammonium phosphate, applied in solution to the green wood particles, at a 5-percent level (boards 16, 62, and 63), gives good flame-spread and heat con- tribution values, but the smoke density is too high.
Treatments with boric acid and disodium octaborate in the proportions used in this investigation (boards 66, 67, 68, and 69), applied in solution at a 10- percent level, provided adequate fire test properties. However, the treatments with boric acid and borax (sodium tetraborate) in a 1:1 weight ratio (boards 19, 20, 21, 22, and 23) gave inadequate flame-spread properties.
Particleboards had adequate fire test properties when treated with either AWPA Type C or Type D formulations applied in solution at a 15-percent level (boards 29, 30, 31, 33, 34, and 35). When these treatments were applied dry at a 15-percent level (boards 32 and 36), the flame-spread was too high.
For the combinations of Fire retardants and binder resins used in this study, mechanical strength and dimensional stability of the boards were affected by the addition of the tire-retardant chemicals. Though additional factors may be involved, it is reasonable to conclude that the lower quality of the fire-retardant- treated boards is due mainly to interference of the treatment chemicals with the resin binder. Boards made with the urea-formaldehyde binder were particu- larly affected by the presence of the fire-retardant-treatment chemicals.
In an earlier exploration, the combination of borax or boric acid with phenol formaldehyde resin resulted in poor quality boards. Test specimens often split apart as they were being prepared, indicating that tensile strength perpendicu- lar to the surface was very low. The board made with the phenol-resin binder and the borax-boric acid fire-retardant treatment (board 22) had low strength properties, and it failed in the vacuum-pressure soak test. Phenol-resin-bound hoards containing the AWPA Type C and Type D fire-retardant formulations (hoards 30 and 34) were of such poor quality that test specimens failed in preparation. This may be due to the boric acid and sodium tetraborate, though some of the other chemicals in these formulations may be involved.
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ddobson
Line
ddobson
Line
The problem of using phenol-formaldehyde resins with boron compounds has been noted before (6) but the exact nature of the interference is uncertian.
The interference with the urea-formaldehyde resin by the fire-retardant treatments containing ammonium salts is probably due to the pH change caused by the decomposition products of the ammonium compound when exposed to heat of drying or pressing (5,13). A reaction between the free ammonia and the formaldehyde may also take place.
Results of the cure plate tests indicated that fire-retardant-treatment chemicals affected the curing of the particleboard binder resins. Though curing conditions in the cure plate test are somewhat different from those in the particleboard in the press, nevertheless the results of the cure plate tests indicate that the cure time is shortened by the presence of several of the fire- retardant-treatment chemicals. Therefore, it appears probable that the lower strength of boards containing fire retardants is due to the resin starting to harden before the press has closed to stops. This would account for the especi- ally low values for tensile strength perpendicular to the surface shown by many of the boards.
Cure plate tests of the phenol-formaldehyde resin mixed with borax-boric acid and with AWPA Types C and D were not possible because the resin hardened as soon as it was mixed with these materials.
Method of Application of Fire Retardant
From the limited investigation of methods of adding fire retardants to the wood particles, it appears that the best procedure is to apply the chemicals in water solution to particles having a moisture content at about the fiber satura- tion point. This method of application necessitates that water added as the solvent be removed in the subsequent drying operation, and the possible effect of heat of the dryer upon the treatment chemical should be considered. It has been suggested (7) that application in solution to the green particles is more efficient because the chemical penetrates into the wood rather than being con- centrated at the surface as it would be in dry powder operation.
The effect of drying temperature was investigated for the ammonium phos- phates since it was assumed that these would tend to decompose with the heat of drying or pressing. Results showed that the boards made from particles which had been dried at room temperature after application of monoammonium phosphate solution were usually slightly stronger than those boards made from particles which had been dried at 225° F. after treatment (compare boards 50 and 52, 51 and 53, 54 and 56).
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Boards made from particles which had been treated with dry fire-retardant chemicals after binder resin application had poorer flame-spread and heat con- tribution values than those made from particles treated with the same chemicals at the same level but applied in solution (for example, compare boards 5 and 10, 6 and 11, 21 and 24, 29 and 32, and 33 and 36). Some chemicals, however, have such low solubility that it is not possible to apply them in solution. The ammo- nium polyphosphate product (boards 74, 75, 76, and 77) is such a material.
En the application of dry fire-retardant chemicals in this laboratory investiga- tion it was difficult to retain all of the required amount of the chemical on the particles, Under plant manufacturing conditions, in which the treated particles would receive much rougher handling than they do in laboratory board fabrica- tion, retaining the dry chemicals on the wood particles would be an even more difficult problem.
Four boards made with application of the fire retardants as dry powders had acceptable flame-spread values, though smoke density was too high for all four of them. No other boards made in this investigation with dry application of fire- retardant chemicals had acceptable flame-spread properties. Fire-retardant treatments for the four boards were monoammonium phosphate at levels of 10 and 15 percent (boards 11 and 12), and ammonium polyphosphate at a 10- percent level (boards 74 and 77).
An attempt to add the fire retardant in solution to the surface of a hot board (board 18) was not successful. Penetration of the chemical into the board during cooling was inadequate, and, due either to evaporation or to absorption of water into the board, much of the solid fire-retardant chemical was deposited on the board surface. In plant production it may be possible to apply the fire-retardant treatment at this point by soaking the boards in the treatment solution as they are removed from the press. However, the necessity of removing excess water from the board as well as the concentration of the treatment in the board sur- face layer where it will be removed in sanding makes this approach rather impractical.
The strength properties of the boards containing the boric acid-disodium octaborate formulations (boards 66, 67, 68, and 69) were much better than those of other fire-retardant-treated boards made in this investigation. Additional investigation is necessary to be certain of the reason for this, but it should be noted that the pH of this strongly buffered formulation is in the range of 6.0 to 6.8 and apparently more favorable to the proper curing of the urea-formalde- hyde resin.
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Two boards were made with the addition of paraffin wax sizing to determine its effect upon fire test results. The 1 percent wax addition increased smoke density slightly in the untreated control board (board 2; compare with board 1) and considerably more in the board containing the monoammonium phosphate fire retardant (board 7 compare with board 6).
The dimensional stability test results for all boards containing fire-retardant- treatment chemicals were poorer than those for the untreated control boards. Though these test results probably indicate combinations which would perform poorly in use, those that appear satisfactory in these tests may not perform well in service. Cycling of the specimens through various humidity conditions, as would usually occur in application, might causefailure in a board that showed relatively good dimensional stability properties in these tests.
Some indication of the effect on board properties of a storage period for fire-retardant-treated particles following application of the binder resin was given by the brief supplementary investigation (table 5). A more complete investigation is required for definite conclusions, but it appears that, with the use of a monoammonium phosphate fire-retardant treatment and a urea-resin binder, the internal bond strength of the boards is decreased when there is a storage period of 2 hours or more.
To briefly study the possible effects of species on the fire-retardant-chemical and binder resin relationship, several boards in Study II of this investigation were made with aspen flakes. For each of these aspen boards there was a corresponding Douglas-fir flakeboard fabricated under the same conditions (see table 4).
Some general observations may be made in comparing the properties of similar aspen and Douglas-fir boards. Flame-spread was higher for all aspen boards than for corresponding Douglas-fir boards. The aspen boards had slightly higher flexural strength. The untreated aspen boards had somewhat lower tensile strength perpendicular to the surface than that of the correspond- ing Douglas-fir boards, but for the monoammonium-phosphate-treated boards this was variable.
Combinations of fire retardant, concentration, species, and binder resin yielding laboratory-made particleboards considered to have adequate properties before exposure to typical use conditions are shown in table 9. Four additional combinations made with monoammonium phosphate and diammonium phosphate (boards 50, 51, 54, and 63) were adequate, except for smoke density being slightly high.
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Conclusions
1. It is apparently possible to achieve adequate fire test properties with several of the treatments investigated. The most satisfactory fire-retardant treatment found in this investigation was boric acid-disodium octaborate. Fire- test properties were adequate. Tensile strength perpendicular to the surface and modulus of elasticity were approximately the same as those of the untreated board. Modulus of rupture was about 75 percent that of the untreated board.
2. Application of the fire retardant in solution to the particles before they are dried apparently is more efficient than application as a dry powder to the particles in the blender immediately after resin spraying.
3. For solution application, 10 to 15 percent of fire-retardant-treatment chemical (baaed upon the ovendry wood weight) is required for adequate fire test properties.
4. The critical problem in the development of fire-retardant-treated particle- board is the effect of the treatment chemicals upon the resin binder. Though a number of factors may be involved, the problem appears to be caused mostly by the lowering of the pH of the system by the fire retardant or its thermal decomposition products.
Since in many instances tensile strength perpendicular to the surface was greatly reduced, it seems a reasonable conclusion that the resins were being catalyzed by the lire-retardant chemicals to harden before the press was closed to board thickness.
5. It is not possible with the sort of information obtained from an exploratory investigation of this nature to predict the strength properties of a specific fire- retardant-treated board in relation to those of a similar but untreated board. Generally, the results of this investigation indicate that the addition of most of the fire retardants used in this study causes an appreciable decrease in bond strength, apparently by interfering with the proper curing of the resin. Consider- ation of the possible nature of this interference leads to the presumption that the strength properties of such board products could be improved by chemical modification of the fire-retardant-treatment chemical or, possibly, the binder resin.
FPL-0201 -17-
Literature Cited
1. American Society for Testing and Materials 1965. Tentative method of test for surface flammability of building materials
using an 8-foot (2.44 m.) tunnel furnace. ASTM E 268-65T.
2.
1964. Standard methods of evaluating the properties of wood-base and particle panel materials. ASTM D I037-64.
3. 1967. Standard method of test for surface burning characteristics of build-
ing materials. ASTM E 84-67.
4. American Wood-Preservers’ Association 1968. AWPA Book of Standards. Standards for Fire-Retardant Formulations.
P10-68.
5. Bramhall, George 1966. Fire retardant treatment of wood. A section of Fang, J. B.,
MacKay, G. D. M., and Bramhall, G. Wood fire behavior and fire retardant treatment; a review of the literature. Canadian Wood Council, Ottawa, Ontario.
6. Deppen, H. J., and Lux, B. V. 1967. The use of inorganic compounds in the production of particleboard
materials of low flammability. [German] Holz-Zentralblatt 93(107): 1671.
7. Draganov, S. M. 1967. Borates as fire retardants in particle board (2nd Ed.). U.S. Borax
and Chemical Corp., Los Angeles, Calif.
8. Forest Products Laboratory, Forest Service, U.S.D.A. 1967. Small tunnel-furnace test for measuring surface flammability.
Research Note FPL-0167.
9. Goldstein, I. S., and Dreher, W. A. 1961. A non-hygroscopic fire retardant treatment for wood. Forest Prod.
J. 11(5): 235-237.
FPL-0201 -18-
10. Goldstein, I.S., and Dreher, William 1964. Method of imparting fire resistance to wood and the resulting
product. U.S. Pat. 3,159,503.
11. Heebink, B. G. 1967. A procedure for quickly evaluating dimensional stability of particle-
board. Forest Prod. J. 17(9): 77-80.
12. Johnansen, R. W., and Crow, G. L. 1965. Liquid phosphate fire retardant concentrations, Fire Control Notes
26(2): 13-16.
13. Raknes, E. 1963. Gluing of wood pressure-treated with water borne preservatives and
flame retardants. J. of the Inst. of Wood Sci. (11): 24-44.
14. Truax, T. R., Harrison, C. A., and Baechler, R H. 1935. Experiments in fireproofing wood. 5th Prog. Rep., U.S. Forest Prod.
Lab. Rep. 1118.
15. Weiner, J., and Byrne, J. 1965. Flameproofing. The Institute of Paper Chemistry Bibliographic
Series, No. 185, Supp. 1. Appleton, Wis.
16. West, C, J., Stringham, E., Roth, L., Weiner, J., and John, B. 1959. Flameproofing. The Institute of Paper Chemistry Bibliographic
Series, No. 185. Appleton, Wis.
1.0-20FPL-0201 -19-
Ta bl e
2. --R
es ul ts of ev al ua tio ns of fir e- re ta rd an t-t re at ed pa rti cl eb oa rd s,
St ud y
Ta bl e
2. -- R es ul ts of e va lu at io ns of fir e- re ta rd an t-t re at ed pa rti cl eb oa rd s,
St ud y
Ta bl e
3. -- R es ul ts o f ev al ua tio ns of fir e- re ta rd an t- tre at ed p ar tic le bo ar ds , St ud y
II
Ta bl e
3. --R
es ul ts of ev al ua tio ns of fir e- re ta rd an t-t re at ed pa rti cl eb oa rd s,
St ud y
Ta bl
e 4.
an d
bo ar ds
(I nf or ma ti on fr om ta bl e
2)
5. -- Ef fe ct of st or ag e
of tre at ed pa rti cl es on pa rti cl eb oa rd pr op er tie s
Ta bl e
re si n1
Ta bl e
re si n1
Ta bl e
9. -- Pa rti cl eb oa rd s ha vi ng in iti al fir e
re ta rd an t
an d
m ee tin g
re qu ire m en ts of ta bl e
1
Figure 1.--Specimen side of FPL 8-foot-tunnel furnace. 1, Gas supply to main burner; 2, firebox; 3, clamp to hold down cover over test speci men; 4, gas supply to igniting burner; 5, cover over test specimen; 6, hood to collect combustion gases for temperature and smoke measure ment; and 7, photoelectric cell for smoke-density measurement.
ZM 110 169
Figure 2.--Cutting diagrams for specimens used in Study I for fire performance and strength property tests. Three boards were made with each fire-relardant treatment. Tension perpendicular to the surface (internal bond) specimens are indicated by the abbreviation I.B. M 136 175
Figure 3.--Cutting diagrams for specimens used in Study II for fire performance and strength property tests. Three boards were made with each fire-retardant treatmen-t. Tension perpendicuIar to the surface (internal bond) specimens are indicated by the abbreviation I.B. M 136 176
daJl
aJ
UU
U.S.D.A. FOREST SERVICE RESEARCH N O T E FPL-0201 A U G U S T 1969
EXPLORATORY INVESTIGATION OF FIRE-RETARDANT TREATMENTS FOR PARTICLEBOARD
Abstract
More than 80 exploratory experiments were made on the development of fire-retardant-treated particle- board using 15 fire retardant chemicals, 3 resin binders, 2 species of wood, and various types of application. Among the types of application were those employing solutions of fire retardant salts sprayed on green flakes; and others employing dry, finely divided fire retardants added after mixing binder resin and flakes. Of the boards produced and tested, several using certain borates, AWPA Type C and Type D fire retardants, and monoammonium phosphate showed promise. Properties considered were flamespread, smoke production, strength, and stability. Salts applied in solution to green wood, followed by drying, gave results superior to those obtained from the use of dry fire-retardant salts. None of the boards were tested for fire retardance and strength after cycling through changing humidities. The results of this series of experiments are released in the form of a progress report for use by others engaged in similar studies.
FPL-0201
U.S. Department of Agriculture
Introduction
As a result of increasing concern with fire safety, modern buildifig codes have developed specific requirements for fire performance of construction materials. With greater use of particleboard in building construction there is greater need for the development of an effective fire-retardant treatment for this product.
The fire-test results reported here were obtained from a single test for each set of boards. The data, therefore, should be regarded only as indicators of treatment effectiveness as a prelude to a more comprehensive study of promising combinations. The results of this series of experiments are released in the form of a progress report for use by others engaged in similar studies.
The value for each relative humidity or soak condition for each set of boards in the evaluation of dimensional stability is based upon the test of a single speci- men. Also, none of the boards were tested for fire retardance and strength after cycling through changing humidities. This must be considered in inter- preting these test results.
1Maintained at Madison, Wis., in cooperation with the University of Wisconsin.
FPL-0201
Use of Fire Retardants in Particleboard
Although there is appreciable knowledge on the use of fire retardants with solid wood, plywood, and textiles, there is comparatively little information on their application to particleboard. The use of fire retardants in this product involves some specific problems.
The particles composing the board are held together by the resin binder, not in the form of a somewhat continuous film as in plywood, but as a multiplicity of fine spots of adhesive which tack together the contacting particles. There is obviously a much smaller amount of adhesive per bond area in particleboard than, for example, in plywood.
The nature of the pressing operation in the manufacture of particleboard is such that precise control of the curing of the resin is essential. During the time required for the particleboard press to close to final raw board thickness, heat is being applied to the binder resin. The resin must not cure before the press has closed to board thickness. From a production standpoint, it is undesirable to slow the curing of the resin sufficiently to require a long pressing time.
Unfortunately, the chemical nature of the fire retardant and the quantity required to be effective are such that frequently the curing properties of the binder resins are affected. Fire-retardant-treated particleboard often has lower strength than untreated board.
Study Materials
The particleboards upon which this report is based were made and evaluated in two separate though similar investigations. To distinguish between the two groups of boards, since their fabricating conditions were different, the investiga- tion concerning the group made first has been designated as Study I and that concerning the second group of boards has been designated as Study II. Differ- ences in fabrication variables of the two groups are indicated in the list of fabrication specifications.
FPL-0201 -2-
Fabrication Specifications
The boards for this investigation were made to the following specifications:
1. Species: Douglas-fir heartwood in Study I; Douglas-fir heartwood or aspen in Study II.
2. Density: 40 pounds per cubic foot.
3. Board size: 1/2 inch by 24 inches by 28 inches, rough dimensions.
4. Particle size: 1 inch by 0.015 inch by random-width flakes.
5. Resin type: Urea-formaldehyde, 65 percent solids; phenol-formaldehyde, 43.5 percent solids; and melamine-formaldehyde, 45 percent solids. The urea and phenol resins were used in the liquid form as they were supplied by the manufacturer. The melamine-formaldehyde resin, in dry form, was dissolved in 1:15 isopropanol-water solution before using.
6. Wax sizing (where used): 1 percent wax solids, based on ovendry wood weight; applied as supplied, a 46 percent solids emulsion.
7. Mat moisture content: Adjusted to 12 percent in Study I and to 10 percent in Study II.
8. Press temperature: In Study I, 325° F. for the urea-formaldehyde and melamine-formaldehyde resins and 350° F. for the phenol-formaldehyde resin; in Study II, 300° F. for the urea- and melamine-formaldehyde resins and 325° F. for the phenol-formaldehyde resin.
9. Press time: In Study I, 15 minutes for all binders: in Study II, 8 minutes for the urea- and melamine-formaldehyde binders and 10 minutes for the phenol-formaldehyde binder.
10. Time to stops: 2 minutes in Study I; 1 minute in Study II.
FPL-0201 -3-
Selection of Fire Retardants
Numerous fire-retardants have been suggested for wood (14,15 ,16).2 Most of these have been water-soluble inorganic salts, a number of which have been in use for many years. Among the more recently proposed fire retardants are organic compounds which seem to lack some of the disadvantages of the inorganic water-soluble salts. Some of these newer fire retardants, which have proven quite effective for textiles and for paper, are too expensive to consider for particleboard treatment.
The fire-retardant treatments selected for these experiments include a number of the more common inorganic treatments which have proven effective for wood, and were considered economically practical for this application.
The following fire retardants were included in this investigation:
Monoammonium phosphate Diammonium phosphate Borax Boric acid Borax - boric acid, 1:1 Monoammonium phosphate - ammonium sulfate, 1:1 Zinc sulfate Aluminum sulfate AWPA type C (4) AWPA type D (4) 11-37-0 ammonium polyphosphate (12) Dicyandiamide - phosphoric acid - formaldehyde, 1:1:0.15 mole ratio Zinc sulfate - zinc silicofluoride - urea Ammonium polyphosphate (32 percent phosphorous content; proprietary
product) Boric acid - disodium borate tetrahydrate, in weight ratios of 0:100,
10:90, 20:80, and 25:75 (7)
2 Underlined numbers in parentheses refer to Literature Cited at the end of this report.
FPL-0201 -4-
There are several possible methods of applying fire retardants to particle- board. Selection of a particular method depends upon such considerations as the practical requirements of the manufacturing process and the characteristics of the fire-retardant-treatment chemicals.
Application of water solutions of fire retardants to the green or wet wood particles soon after the particles are made is expedient because this permits penetration of the treatment into the wood, and the water of the treating solution may be evaporated in the subsequent drying operation. The solution could be applied either by spraying or by soaking the particles.
The disadvantage of any application in solution, particularly for chemicals with rather low solubility, is the need to remove the added water in the dryer.
The fire-retardant solution could also be added at the blender where the resin and wax sizing are added to the dry particles. The disadvantage of this, however, is that the water of the treatment solution would have to be removed in the pressing operation. In addition, the added water might dilute the resin and cause excessive strike-in. It is probably desirable for the fire retardant to penetrate into the wood and for the binder resin to stay at the surface. This would be difficult to accomplish by spraying both materials on the dry particles at the blender.
It may appear convenient to add the fire retardant in solution to the finished board by soaking, applying to the board surfaces, or pressure impregnation. However, treatment of the finished board would require an additional drying operation to remove the relatively large amount of water added. Thus, it is more practical to add the fire-retardant solution to the particles rather than to the finished boards.
The fire-retardant-treatment chemical may be added as a dry powder, preferably during or immediately after the resin application, so that it will be retained on the wood particles by the tackiness of the binder. Dry application obviously eliminates the necessity of removing the water added by solution application of the fire retardant. However, many binders lack the requisite tack to retain the powder. Problems associated with dry application include lowering of the effectiveness of the binder, sifting of the chemical through the mat, and difficulty, compared with solution application, of obtaining even distribution of the treatment.
FPL-0201 -5-
Three methods of adding the fire retardant were investigated in these studies: (1) application of dry chemicals at the resin blender, (2) spraying the chem- icals on the wet particles, and (3) roller application of the solution on the surface of the hot board just removed from the press.
Application of dry chemicals to the wood particles in the blender immediately after spraying with resin presented problems of obtaining even distribution and of adhering all the required fire retardant to the particles. After fire test results indicated that application dry was less efficient than in solution, the dry application was used only for those chemicals having low solubility. Applica- tion of approximately 30 to 35 percent water solutions to the wet particles immediately before drying proved to be simpler, more efficient, and more practical for production, so this method was used for many of the combinations investigated.
Preliminary investigations had indicated that some of the chemicals reacted with the wood when the treated particles were oven dried: therefore, in Study I all particles treated with fire retardants in solution were air dried at room temperature. In Study II, a number of similar boards were made from particles which had been air dried at room temperature, and some others of particles which were oven dried to about 5 percent moisture content at 225° F. In produc- tion, the particles would be exposed to a higher temperature for a shorter time in a continuous dryer. In order to control the rate and uniformity of drying in a laboratory oven, however, it was necessary to use a lower drying temperature.
An attempt was made (board 18) to add the fire retardant, diammonium phos- phate in this instance, in solution to the surface of the hot hoard immediately after its removal from the press.
Fire-retardant-treatment chemical solutions for Study I, and for Study II except where noted otherwise, were made up at 30 to 35 percent concentration in water heated to 120° F. The solutions were sprayed on wood particles which were at about 30 percent moisture content. The amount of fire-retardant mate- rial added was based on dry weight of the chemical as a percentage of the weight of the ovendry wood.
All liquid materials--fire-retardant solution, binder resins, and wax emulsion-- were applied by spraying them into the mass of particles tumbling in a rotating drum. Dry fire-retardant chemicals were applied slowly to the resin-sprayed particles in the rotating drum. All materials applied dry were reduced to pass through a 100-mesh screen.
FPL-0201 -6-
The American Wood-Preservers’ used in this investigation were:
Association fire-retardant formulations (4)
Diammonium phosphate Ammonium sulfate Sodium tetraborate, anhydrous Boric acid
10 60 10 20
Zinc chloride Amnnonium sulfate Boric acid Sodium bichromate
35 35 25 5
Monoammonium Phosphate- Ammonium Sulfate treatment
High smoke density is a problem with particleboards treated with monoammo- nium phosphate. This may be reduced through the addition of ammonium sulfate. In Study I equal amounts of these chemicals (1:1 weight ratio) were used in three series of boards (boards 37, 38, and 39). In Study II, to attempt to reduce the effect of the sulfate on the resins, a 3:1 weight ratio of monoammonium phosphate-ammonium sulfate was used (boards 64 and 65). This treatment was added at a 13-percent level so that the level of the monoammonium phosphaete would be about 9 percent.
Boric Acid-Disodium Octaborate Treatment (7)
The boric acid-disodium octaborate solutions were made up in proportions of 0:100, 10:90, 20:80, and 25:75 at a solution concentration of 38 percent in water heated to 145° to 165° F. The disodium octaborate tetrahydrate (Na B O
2 8 13. 4H.O) was dissolved first, followed by the boric acid. The solution was main-
tained and sprayed at 145° to 165° F. Application was 18 percent, based on oven- dry wood weight.
FPL-0201 -7-
Zinc Sulfate- Zinc Silicofluoride- Urea Treatment
This treatment, which has been recommended as a means of insolubilizing a zinc compound within the wood, was applied at a rate of 2 pounds of zinc per cubic foot of board. The following formulation was used:
Component Weight Percent
Water 55.0 Zinc sulfate 16.5 Zinc silicofluoride 18.1 Urea 10.4
100.0
The zinc content of this formulation is 7.5 percent.
After the green (wet) wood particles were sprayed with this solution they were placed in tightly closed polyethylene bags and heated at 175° to 180° F. for 15 hours. This treatment is intended to react the zinc compounds with ammonia derived from decomposition of the urea. The particles were removed from the polyethylene bags and dried at 225° F. to about 5 percent moisture content.
Treatment with Ammonium Polyphosphate (32 Percent P)
This material is a low-solubility ammonium polyphosphate proprietary product with a phosphorous content of 32 percent. Because of the low solubility of this chemical, it was necessary to apply it as a dry powder following resin application.
Dicyandiamide-Phosphoric (9,10)
Acid Treatment
A 20-percent solution phosphoric acid, and application was used in
was formaldehyde treating
the wood particles.
amide mole
(1-cya ratio.
no- A
guanidine) 10-percent
The dicyandiamide was dissolved in water which had been heated to 120° F. With constant stirring, the phosphoric acid and formaldehyde were added in turn and the temperature was increased to 180° F. The sprayed particles were dried in a 180° to 190° F. oven to approximately 5 percent moisture content. Care was taken to insure that the oven temperature did not go above 190° F. and that the particles were not overdried.
FPL-0201 -8-
Evaluation of Board Properties
Three boards were made for each treatment. Four half-boards, each 13-3/4 inches by 23 inches, and one piece 13-3/4 inches by 4 inches were required for each fire test. Two half-boards were cut into specimens for mechanical and dimensional stability tests. All test specimens were unsanded.
At the beginning of Study I, control boards without fire retardants were made for reference. Table 1 gives test values ofproperties of the control boards made with urea-, phenol-, and melamine-formaldehyde resins. Because specifications are not available for fire-retardant-treated particleboard, arbitrary values were established for treated boards fabricated to the control specifications to assist in identifying the most promising treatments.
Fire Performance
The fire performance of the boards was evaluated by the 8-foot-tunnel furnace test for surface flammability (1,8) Figure 1 shows the specimen side of the test furnace. All board specimens were conditioned to equilibrium at 30 percent relative humidity before fire testing.
The 8-foot-tunnel method (1) has been developed by the Forest Products Laboratory as a research technique for measuring surface flammability. It generally ranks the flammability of materials in the same order as the 25-foot- tunnel furnace test (3), but the actual index values may differ. Both methods use index values relative to flame-spread distances traveled with respect to red oak at 100. However, because of longer exposure times in the 8-foot tunnel, fire-retardant-treated wood at the lower range of the scale frequently has a flame-spread index of about 40 as compared to 25 on the same material in the 25-foot-tunnel furnace. Therefore, one of the criteria selected for an adequate treated particleboard was a flame-spread index in the 8-foot-tunnel furnace of 40 or less. The heat contribution of similar materials generally correlates with the flame-spread distances involved in the tests, and therefore a heat contribu- tion index of 40 or less was also selected as an acceptance criterion.
Smoke density index values are established for both methods relative to the smoke density produced by untreated red oak lumber in the respective tests. In the 25-foot tunnel with a large ignition burner, both untreated red oak and fire-retardant-treated materials burn under a flaming exposure condition. However, in the 8-foot tunnel, where most of the heat is supplied by a radiant source, exposure results in flaming combustion for the untreated red oak, but nonflaming exposure for fire-retardant-treated products. Nonflaming exposure of cellulosic products results in much greater smoke production than does
FPL-0201 -9-
flaming combustion. In previous experience with the more effective fire- retardant chemicals, smoke index values as high as 400 were obtained in the 8-foot tunnel for products rated as 50 or less for flaming combustion in the 25-foot tunnel. Therefore, an acceptance criterion for boards produced in this study was established as a smoke index value of 400 or less in the 8-foot-tunnel test.
Strength Properties
Following methods specified in ASTM D 1037-64 (2), strength properties of the boards were evaluated by static bending and tension perpendicular to the surface. An initial strength loss of approximately 40 percent as a result of treatment was arbitrarily applied in choosing the minimum strength values in table 1 for treated boards. The 40 percent reduction was chosen as a reasonable indicator of boards showing promising strength properties after treatment. Other values could be used. For example, selectionof an approximate 20 percent initial strength reduction would reduce the number of treated boards showing promise to three (boards number 66, 67, and 68).
In Study I, nine static bending specimens and 24 tension perpendicular to the surface (internal bond) specimens were tested for each treatment. In Study II, six static bending specimens and eight tension perpendicular to the surface specimens were tested for each treatment. Specimens were cut from each set of three boards as shown in figures 2 and 3.
Dimensional Properties
Dimensional stability test specimens 3/4 inch by 22 inches were made for each set of boards. All specimens were first brought to equilibrium in an atmosphere of 30 percent relative humidity at 80° F. Then one specimen from each set of boards was exposed to each moisture treatment as follows: One oven dried to moisture freeness in a 220° F. mechanical convection oven for 24 hours; one soaked in water for 38 days; and one each exposed to 30, 60, 80, or 90 percent relative humidity at 80° F. for 30 days. A vacuum-pressure soak test (11) was performed on a specimen of each board.
Measurements were made on each specimen after the initial conditioning at 30 percent relative humidity and again after reaching equilibrium with the find condition. Length measurements to ±0.001 inch were taken from two small holes 20 inches apart in the face of the specimen, and thickness measurements were taken at five marked, equally-spaced points between the holes (11). The tabulated values are based upon a calculated ovendry condition.
FPE-0201 -10-
Included for controls in both Studies I and II were boards made without fire-retardant treatments (boards 1 through 4 and 44 through 49). These included a board made with each of the three resins, and in Study II control boards were made both with Douglas-fir and with aspen. One board (board 2) was made with the addition of 1 percent (solids) wax.
The relationship between properties of untreated and fire-retardant-treated commercial production boards may differ from that of the boards made in this study because of certain basic differences in the laboratory and the plant processes. For example, the higher drying temperatures and the required storage of the fire-retardant-treated and resin-sprayed particles in production will probably increase the effect of the chemicalson the wood and on the binder resin. The rapid press closure to stops used with the laboratory press may not be possible with most production presses. Increases in linear movement and thickness of approximately 50 percent as a result of treatment were arbitrarily allowed in choosing the values given in table 1.
Supplementary Investigations
Two brief investigations were made to further consider the relationship of the fire-retardant-treatment chemicals and binder resins. One involved several cure plate tests of resins alone and of mixtures of resins and fire-retardant chemicals. The other was an investigation of the effect of storage of fire- retardant-treated and resin-sprayed wood particles upon board properties.
Cure Plate Tests
Cure plate tests were performed to study the effect of several of the fire- retardant-treatment chemicals on the curing of the binder resins. Immediately before each test, a 10.0-gram sample of the liquid resin was placed in a small beaker, a weighed amount of the fire-retardant-treatment chemical was added, and the mixture was thoroughly stirred. About 2 to 4 gram of the mixture were spread on the hot surface of the plate and stroked with a spatula until the curing point was detected.
The amounts of fire-retardant chemicals in the test samples were equivalent to 2, 5, and 10 percent in the board, with the 10.0 grams of resin as equivalent to 8 percent resin solids in the board.
The apparatus used in this test was simply a laboratory hotplate with accurate thermostat control (±2° F.). A stainless steel sheet was clamped to the surface of the hotplate. The required temperatures were set by a thermocouple placed beneath a weight on the plate surface.
FPL-0201 -11-
A high and a low temperature were used in these tests. A low temperature of 225° F. for all resins was intended to be the equivalent of the interior board temperature, and a high temperature of 325° F. for the urea and melamine resins or 350° F. for the phenol resin was intended to be the equivalent of the board surface temperature during the pressing operation.
Effect of Storage
In the laboratory, particles are sprayed with resin and almost immediately formed into a mat and pressed into a board. In a particleboard plant under production conditions, it is necessary to hold resin-sprayed particles in a surge bin before the forming line. Therefore, it is desirable to determine the effect of storage of the fire-retardant-treated, resin-sprayed wood particles upon board properties. For this investigation, monoammonium phosphate was applied in solution at a 10 percent level for the fire-retardant treatment, and urea- formaldehyde resin was used as the binder. The resin-sprayed particles were stored at room temperature in closed polyethylene bags until they were formed into mats and pressed. Fabricating conditions follow those given for Study II of this investigation.
Observations
Results of the evaluation of the boards made in this investigation are given in tables 2 through 4. Tables 2 and 3 respectively give results for the boards made in Studies I and II. Table 4 contains data extracted from table 3 to more clearly indicate the effect of species as shown by the difference in properties of similar boards made with Douglas-fir and with aspen. Results of the brief study of the effect of storage of treated, resin-sprayed particles on board properties are given in table 5. Cure plate tests of the three binder resins in mixtures with several representative fire-retardant treatments are presented in tables 6, 7, and 8. Table 9 gives 10 combinations (extracted from tables 2 and 3) that exhibited properties equal or superior to the arbitrary limits shown in table 1.
Fire Test Properties
As the results given in tables 2 and 3 indicate, several of the fire-retardant treatments gave adequate fire test properties to the particleboards.
Monoammonium phosphate at a 10-percent level, when applied in solution to the green particles gave adequate flame-spread and heat contribution values,
FPE-0201 -12-
but smoke density tended to be too high. The smoke density evidently may be reduced with the addition of ammonium sulfate (boards 37, 38, 39, 64, and 65), however this salt is a strong catalyst for the urea resin.
Diammonium phosphate, applied in solution to the green wood particles, at a 5-percent level (boards 16, 62, and 63), gives good flame-spread and heat con- tribution values, but the smoke density is too high.
Treatments with boric acid and disodium octaborate in the proportions used in this investigation (boards 66, 67, 68, and 69), applied in solution at a 10- percent level, provided adequate fire test properties. However, the treatments with boric acid and borax (sodium tetraborate) in a 1:1 weight ratio (boards 19, 20, 21, 22, and 23) gave inadequate flame-spread properties.
Particleboards had adequate fire test properties when treated with either AWPA Type C or Type D formulations applied in solution at a 15-percent level (boards 29, 30, 31, 33, 34, and 35). When these treatments were applied dry at a 15-percent level (boards 32 and 36), the flame-spread was too high.
For the combinations of Fire retardants and binder resins used in this study, mechanical strength and dimensional stability of the boards were affected by the addition of the tire-retardant chemicals. Though additional factors may be involved, it is reasonable to conclude that the lower quality of the fire-retardant- treated boards is due mainly to interference of the treatment chemicals with the resin binder. Boards made with the urea-formaldehyde binder were particu- larly affected by the presence of the fire-retardant-treatment chemicals.
In an earlier exploration, the combination of borax or boric acid with phenol formaldehyde resin resulted in poor quality boards. Test specimens often split apart as they were being prepared, indicating that tensile strength perpendicu- lar to the surface was very low. The board made with the phenol-resin binder and the borax-boric acid fire-retardant treatment (board 22) had low strength properties, and it failed in the vacuum-pressure soak test. Phenol-resin-bound hoards containing the AWPA Type C and Type D fire-retardant formulations (hoards 30 and 34) were of such poor quality that test specimens failed in preparation. This may be due to the boric acid and sodium tetraborate, though some of the other chemicals in these formulations may be involved.
FPL-0201 -13-
ddobson
Line
ddobson
Line
The problem of using phenol-formaldehyde resins with boron compounds has been noted before (6) but the exact nature of the interference is uncertian.
The interference with the urea-formaldehyde resin by the fire-retardant treatments containing ammonium salts is probably due to the pH change caused by the decomposition products of the ammonium compound when exposed to heat of drying or pressing (5,13). A reaction between the free ammonia and the formaldehyde may also take place.
Results of the cure plate tests indicated that fire-retardant-treatment chemicals affected the curing of the particleboard binder resins. Though curing conditions in the cure plate test are somewhat different from those in the particleboard in the press, nevertheless the results of the cure plate tests indicate that the cure time is shortened by the presence of several of the fire- retardant-treatment chemicals. Therefore, it appears probable that the lower strength of boards containing fire retardants is due to the resin starting to harden before the press has closed to stops. This would account for the especi- ally low values for tensile strength perpendicular to the surface shown by many of the boards.
Cure plate tests of the phenol-formaldehyde resin mixed with borax-boric acid and with AWPA Types C and D were not possible because the resin hardened as soon as it was mixed with these materials.
Method of Application of Fire Retardant
From the limited investigation of methods of adding fire retardants to the wood particles, it appears that the best procedure is to apply the chemicals in water solution to particles having a moisture content at about the fiber satura- tion point. This method of application necessitates that water added as the solvent be removed in the subsequent drying operation, and the possible effect of heat of the dryer upon the treatment chemical should be considered. It has been suggested (7) that application in solution to the green particles is more efficient because the chemical penetrates into the wood rather than being con- centrated at the surface as it would be in dry powder operation.
The effect of drying temperature was investigated for the ammonium phos- phates since it was assumed that these would tend to decompose with the heat of drying or pressing. Results showed that the boards made from particles which had been dried at room temperature after application of monoammonium phosphate solution were usually slightly stronger than those boards made from particles which had been dried at 225° F. after treatment (compare boards 50 and 52, 51 and 53, 54 and 56).
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Boards made from particles which had been treated with dry fire-retardant chemicals after binder resin application had poorer flame-spread and heat con- tribution values than those made from particles treated with the same chemicals at the same level but applied in solution (for example, compare boards 5 and 10, 6 and 11, 21 and 24, 29 and 32, and 33 and 36). Some chemicals, however, have such low solubility that it is not possible to apply them in solution. The ammo- nium polyphosphate product (boards 74, 75, 76, and 77) is such a material.
En the application of dry fire-retardant chemicals in this laboratory investiga- tion it was difficult to retain all of the required amount of the chemical on the particles, Under plant manufacturing conditions, in which the treated particles would receive much rougher handling than they do in laboratory board fabrica- tion, retaining the dry chemicals on the wood particles would be an even more difficult problem.
Four boards made with application of the fire retardants as dry powders had acceptable flame-spread values, though smoke density was too high for all four of them. No other boards made in this investigation with dry application of fire- retardant chemicals had acceptable flame-spread properties. Fire-retardant treatments for the four boards were monoammonium phosphate at levels of 10 and 15 percent (boards 11 and 12), and ammonium polyphosphate at a 10- percent level (boards 74 and 77).
An attempt to add the fire retardant in solution to the surface of a hot board (board 18) was not successful. Penetration of the chemical into the board during cooling was inadequate, and, due either to evaporation or to absorption of water into the board, much of the solid fire-retardant chemical was deposited on the board surface. In plant production it may be possible to apply the fire-retardant treatment at this point by soaking the boards in the treatment solution as they are removed from the press. However, the necessity of removing excess water from the board as well as the concentration of the treatment in the board sur- face layer where it will be removed in sanding makes this approach rather impractical.
The strength properties of the boards containing the boric acid-disodium octaborate formulations (boards 66, 67, 68, and 69) were much better than those of other fire-retardant-treated boards made in this investigation. Additional investigation is necessary to be certain of the reason for this, but it should be noted that the pH of this strongly buffered formulation is in the range of 6.0 to 6.8 and apparently more favorable to the proper curing of the urea-formalde- hyde resin.
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Two boards were made with the addition of paraffin wax sizing to determine its effect upon fire test results. The 1 percent wax addition increased smoke density slightly in the untreated control board (board 2; compare with board 1) and considerably more in the board containing the monoammonium phosphate fire retardant (board 7 compare with board 6).
The dimensional stability test results for all boards containing fire-retardant- treatment chemicals were poorer than those for the untreated control boards. Though these test results probably indicate combinations which would perform poorly in use, those that appear satisfactory in these tests may not perform well in service. Cycling of the specimens through various humidity conditions, as would usually occur in application, might causefailure in a board that showed relatively good dimensional stability properties in these tests.
Some indication of the effect on board properties of a storage period for fire-retardant-treated particles following application of the binder resin was given by the brief supplementary investigation (table 5). A more complete investigation is required for definite conclusions, but it appears that, with the use of a monoammonium phosphate fire-retardant treatment and a urea-resin binder, the internal bond strength of the boards is decreased when there is a storage period of 2 hours or more.
To briefly study the possible effects of species on the fire-retardant-chemical and binder resin relationship, several boards in Study II of this investigation were made with aspen flakes. For each of these aspen boards there was a corresponding Douglas-fir flakeboard fabricated under the same conditions (see table 4).
Some general observations may be made in comparing the properties of similar aspen and Douglas-fir boards. Flame-spread was higher for all aspen boards than for corresponding Douglas-fir boards. The aspen boards had slightly higher flexural strength. The untreated aspen boards had somewhat lower tensile strength perpendicular to the surface than that of the correspond- ing Douglas-fir boards, but for the monoammonium-phosphate-treated boards this was variable.
Combinations of fire retardant, concentration, species, and binder resin yielding laboratory-made particleboards considered to have adequate properties before exposure to typical use conditions are shown in table 9. Four additional combinations made with monoammonium phosphate and diammonium phosphate (boards 50, 51, 54, and 63) were adequate, except for smoke density being slightly high.
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Conclusions
1. It is apparently possible to achieve adequate fire test properties with several of the treatments investigated. The most satisfactory fire-retardant treatment found in this investigation was boric acid-disodium octaborate. Fire- test properties were adequate. Tensile strength perpendicular to the surface and modulus of elasticity were approximately the same as those of the untreated board. Modulus of rupture was about 75 percent that of the untreated board.
2. Application of the fire retardant in solution to the particles before they are dried apparently is more efficient than application as a dry powder to the particles in the blender immediately after resin spraying.
3. For solution application, 10 to 15 percent of fire-retardant-treatment chemical (baaed upon the ovendry wood weight) is required for adequate fire test properties.
4. The critical problem in the development of fire-retardant-treated particle- board is the effect of the treatment chemicals upon the resin binder. Though a number of factors may be involved, the problem appears to be caused mostly by the lowering of the pH of the system by the fire retardant or its thermal decomposition products.
Since in many instances tensile strength perpendicular to the surface was greatly reduced, it seems a reasonable conclusion that the resins were being catalyzed by the lire-retardant chemicals to harden before the press was closed to board thickness.
5. It is not possible with the sort of information obtained from an exploratory investigation of this nature to predict the strength properties of a specific fire- retardant-treated board in relation to those of a similar but untreated board. Generally, the results of this investigation indicate that the addition of most of the fire retardants used in this study causes an appreciable decrease in bond strength, apparently by interfering with the proper curing of the resin. Consider- ation of the possible nature of this interference leads to the presumption that the strength properties of such board products could be improved by chemical modification of the fire-retardant-treatment chemical or, possibly, the binder resin.
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Literature Cited
1. American Society for Testing and Materials 1965. Tentative method of test for surface flammability of building materials
using an 8-foot (2.44 m.) tunnel furnace. ASTM E 268-65T.
2.
1964. Standard methods of evaluating the properties of wood-base and particle panel materials. ASTM D I037-64.
3. 1967. Standard method of test for surface burning characteristics of build-
ing materials. ASTM E 84-67.
4. American Wood-Preservers’ Association 1968. AWPA Book of Standards. Standards for Fire-Retardant Formulations.
P10-68.
5. Bramhall, George 1966. Fire retardant treatment of wood. A section of Fang, J. B.,
MacKay, G. D. M., and Bramhall, G. Wood fire behavior and fire retardant treatment; a review of the literature. Canadian Wood Council, Ottawa, Ontario.
6. Deppen, H. J., and Lux, B. V. 1967. The use of inorganic compounds in the production of particleboard
materials of low flammability. [German] Holz-Zentralblatt 93(107): 1671.
7. Draganov, S. M. 1967. Borates as fire retardants in particle board (2nd Ed.). U.S. Borax
and Chemical Corp., Los Angeles, Calif.
8. Forest Products Laboratory, Forest Service, U.S.D.A. 1967. Small tunnel-furnace test for measuring surface flammability.
Research Note FPL-0167.
9. Goldstein, I. S., and Dreher, W. A. 1961. A non-hygroscopic fire retardant treatment for wood. Forest Prod.
J. 11(5): 235-237.
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10. Goldstein, I.S., and Dreher, William 1964. Method of imparting fire resistance to wood and the resulting
product. U.S. Pat. 3,159,503.
11. Heebink, B. G. 1967. A procedure for quickly evaluating dimensional stability of particle-
board. Forest Prod. J. 17(9): 77-80.
12. Johnansen, R. W., and Crow, G. L. 1965. Liquid phosphate fire retardant concentrations, Fire Control Notes
26(2): 13-16.
13. Raknes, E. 1963. Gluing of wood pressure-treated with water borne preservatives and
flame retardants. J. of the Inst. of Wood Sci. (11): 24-44.
14. Truax, T. R., Harrison, C. A., and Baechler, R H. 1935. Experiments in fireproofing wood. 5th Prog. Rep., U.S. Forest Prod.
Lab. Rep. 1118.
15. Weiner, J., and Byrne, J. 1965. Flameproofing. The Institute of Paper Chemistry Bibliographic
Series, No. 185, Supp. 1. Appleton, Wis.
16. West, C, J., Stringham, E., Roth, L., Weiner, J., and John, B. 1959. Flameproofing. The Institute of Paper Chemistry Bibliographic
Series, No. 185. Appleton, Wis.
1.0-20FPL-0201 -19-
Ta bl e
2. --R
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St ud y
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1
Figure 1.--Specimen side of FPL 8-foot-tunnel furnace. 1, Gas supply to main burner; 2, firebox; 3, clamp to hold down cover over test speci men; 4, gas supply to igniting burner; 5, cover over test specimen; 6, hood to collect combustion gases for temperature and smoke measure ment; and 7, photoelectric cell for smoke-density measurement.
ZM 110 169
Figure 2.--Cutting diagrams for specimens used in Study I for fire performance and strength property tests. Three boards were made with each fire-relardant treatment. Tension perpendicular to the surface (internal bond) specimens are indicated by the abbreviation I.B. M 136 175
Figure 3.--Cutting diagrams for specimens used in Study II for fire performance and strength property tests. Three boards were made with each fire-retardant treatmen-t. Tension perpendicuIar to the surface (internal bond) specimens are indicated by the abbreviation I.B. M 136 176
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