CA1067638A - Fire retardant resin binder solutions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Novel stable-in-use aminoplast resin binder solutions comprising aqueous solutions of predominantly a urea-formalde-hyde resin having a formaldehyde to urea ratio of less than 2.0, from about 12 to 30% by weight dissolved diammonium phos-phate or mixture of diammonium phosphate and ammonium polyphos-phate and not more than 50% by weight water. Preferred aminoplast resin binder solutions also contain a methylolated melamine-formaldehyde resin. A novel process for the production of the stable aminoplast resin binder solutions containing high percentages of ammonium phosphates and suitable for production of fire-retardant particle board is disclosed.

Description

C06-12-03~9 10fà7~3~
BACKGROUND OF THE INVENTION
Urea-formaldehyde resins are widely used in the wood products industry as adhesives for plywoods and binder resins for hardboard, chipboard, particle board and the like.
Recently, due to considerations of safety and fire prevention, there has developed a demand for fire-retardant or fire-resistant plywoods and particle board or chipboard. Therefore, the prior art has sought methods for the incorporation of fire-retardant salts, and specifically such salts as ammonium phosphates and polyphosphates, into plywoods and particle board.
The prior art methods of incorporation of ammonium ; ~ phosphates into particle boards have required mixing in the dry phosphate salt or spraying a solution thereof onto the particle : i board wood furnish and drying such furnish before adding resin , binder thereto. Alternatively, it has been suggested to - incorporate the powdered dry salt into a binder resin, but such ; dry powdered sal~s are very difficult to disperse and maintain evenly dispersed in such resin solutions. Attempts to blend aqueous solutions of monoammonium phosphate, diammonium phos-phate or ammonium polyphosphates with aqueous syrups or urea-formaldehyde or melamine-urea-formaldehyde resins have been ~ , successful only in very low concentrations of the phosphates, since higher concentrations of phosphates cause the resins to ; 25 cloud and precipitate. These low concentrations are generally `~ not sufficient to confer an acceptable level of flame retard-. .~
`ir ancy on the final particle board product. Consequently, it has been desired to incorporate aqueous solutions of phosphate -~ salts into such urea-formaldehyde and melamine-urea-formaldehyde ~, '~'

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-,- . : ~ . - ' , C-(~-12-0389 c;7638 resins at cOncentrations higher than 10% by weight in order to simplify the process of production of fire-retardant particle boards.
It has now been found that certain aminoplast reslns will remaln stable to aqueous solutions of the above ammonium phosphates for adequate periods making possible a fire-retardant binder resin solution quite suitable for use in the general commercial process for the productiOn of particle board. The avallability of such blnder resin solutions makes possible the production of fire-retardant particle board pro-duced as rapidly with the same number of steps as are required for the present production of non-fire-retardant particle boards. These novel fire-retardant blnder resin solutions can be used in the same manner and with the same facility as ordinary aminoplast binder resln solutions but contai~ much hlgher concentrations of ammonium phosphates than are compati-ble with ~uch normal resin solutions. Thus, industry is enabled to produce fire-retardant particle board by the ~ame process and at the same rate of production as the non-fire-retardant particle boards. A further advantage is the factthat the novel blnder resin solutions can be produced at high - percent solids, and do not require dilution with increased amOunts of water which would, in turn, require a separate :, drying step for the wood furnish in order to eliminate same.
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As a consequence, these novel binder resln solutions and ~ process ~or their productiOn represent a decided lmprovement ; ln both economics and safety.

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. c-o6- 12- o38s 10~63~3 SUMMARY OF THE I~ENTION

The present invention is directed to the pro&e~s fOr the production Or a resin binder solution and the resin binder solution product comprising an aqueous solution o~ predomln-antly a urea-formaldehyde resin having a formaldehyde to urea ratiO of less than 2.0, and preferably about 1.2 to 1.8, pre-ferably a minor proportion of a methylolated melamine-formalde-hyde resin, rrom about 12 to 30% by weight dissolved diammonium pho!~phate or a mixture of diammonium phosphate and ammonium polyphosphate acidified with phosphoric acid and not more than 50% by weight water.
The process for production of the novel binder resln solution comprises mixing with a substantially neutral aqueous solUtlon o~ a low molecular weight urea-formaldehyde resin of a mol ratio of from about 3.0 to 3.5 formaldehyde/urea, said solution having a viscosity of from 60 to lOO~cps and a percent ~olids Or at least about 70 percent, sufricient urea to reduce the formaldeh~yde/urea ratio to from about 1.2 to 2.0 and suf-ficient of a stable concentrated aqueous solution of diammonium ~.
phosphate or diammonium phosphate, ammonium polyphosphate and phosphorlc acid havlng a percent solids Or at least 40 percent to produce a concentratlon of dissolved diammonium phosphate and ammonium polyphosphate of about 12 to 30 percent in the resulting stable binder resin solution. Preferably, from about ~5 15 to ~5 percent by weight of a methylolated melamlne-rormaldehyde resin in aqueOus solution of at least about 50 percent solids, and pre~erably more than 60 p~rcent solids, is added to the neutral aqueousurea-formaldehyde solutlon prior to mixing therewith the urea and ammonium phosphate solutlon detalled above.

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DETAILED EMBODIMENTS

The starting material to produce the desired resin binder solutions of the present invention is a low molecular weight urea-rormaldehyde resin Or from about 3.0 to 3.5 formaldehyde/urea ratio in substantially neutral aqueous solu-tlon. Such high F~U ratio, low or lightly bodied resin can be obtained by the method described in British Patent :~
Number 1, 486, 342 .
initial reaction under alkaline conditions at a temperature of at least 75C., cooling and ad~usting to an acid pH Or ~rom about 1.7 to 2.1 and reacting at a temper-ature Or from 30 to 55C. to body the resin to the desired viscosity, neutralizing the solution and coollng to amblent temperature. The desired low molecular weight or lightly bodled resin will have a viscosity Or about 60 to 100 cps at 21C. which can be obtained with only about 3 to 10 minutes of bodying reactiOn at the acid pH specified and 30 to 55C.
Thereafter the neutralized aqueous solution i8 conveniently sub~ected to distillat~on of a portion Or the water present to concentrate the UF resin solution to a high solids content Or from about 70 to 95 percent, preferably 80 to 90 percent solids. Such early concentratiOn will avoid the necessity for later concentration Or the finished binder resln or the added st~p of drying the particle board furnlsh to acceptable moisture limlts for hot pressing.
A preferred base resin comprlses such high F ~ ratlo, low molecular weight urea-formaldehyde resin above described further mixed with a methylolated melamine-formaldehyde resin ln aqueOus solution Or a solids content Or at least 50 percent, and prererably at least 60 percent, in an amount of rrOm about 10 67 ~ 3 8 15 to 25 percent by welght o~ the starting UF re~in. Such mlxed re~ln solution ls hereafter re~erred to as melamine-urea rormaldehyde re~in solutlon or MUF resln base. It ha~ been ~ound that the presence o~ ammonium phosphates in UF binder resln Rolutions tends to lead to decompositlon o~ the UF resin under hlgh temperature hot pressing conditions, but that forti-ricatlon t~ereof with the lndlcated amount of methylolated melamlne-~ormaldehyde re~ln~ overcomes thls tendency and renders the mlxed res~n base relatively stable ln the presence of ammonlum phosphates under high temperatures. De~irably the resln base wlll constltute ~rom about 30 to about 55 percent by wei~ht Or the total binder solution compositlon.
- - A dlstillation proces can be used to concentrate the -sollds content o~ the MUF ba~e resln to at least 70 percent, and preferably ~o about 80 to 90 percent, if required. If deslred, the concentratlon can be per~ormed after addltlon of the melamine-formaldehyde resin solutlon, rather than Or the startlng urea-~ormaldehyde resln as dl3cussed above.
There 1~ added to the concentrated urea-formaldehyde base resin or MUF resin base, sufficlent urea to ad~ust the FjU
ratlo Or the resultlng mixture to less than 2Ø Preferably, , - surrlcient urea is added to produce an F ~ ratlo of rrom 1.2 to 1.8. The urea can be added dry or ~ a concentrated aqueOu~
solutlon as deslred.
In order to produce a ~ire-retardant resin blnder solutlon and a partlcle board produced therewith of the highest ~ire-retardant ratin~s, it ls desirable to lnclude in the resin binder solution the hlghest concentration of ammonlum phosphates which is compatible with stability of the mixed resin solution

3 produced. However, care must be taken that the ammonlum C-~tL12-03~

phosphate content Or the binder solutlon 1~ not 80 great as to lnterfere wlth the spraylng and mixing o~ the blnder solutlon wlth the wood particle furni~h for the manufacture of partlcle boards. The amounts of ammonlum phosphate ~olutions speclfied hereln have been found to produce hlgh flre-retardant ratlngs in partlcle board pr~duct~ and to be easlly and ef~iclently applied and mlxed ln typlcal commerclal partlcle board produc-tlon processes.
It has been found that the ammonium phosphate content can be ~upplled by a variety Or compatlble aqueous solutions thereo~. Suitable sources are concentrated aqueous-~olutions from about 30 to 40 and more percent dlammonlum phosphate . .
and as nearly saturated as can be practlcally handled and stored without precipitation. A convenlent commercially avall-- 15 able solution i~ a 40 percent diammonium phosphate aqueOus :, , -801ution. Such 40 percent DAP solutlon can be used to the extent Or from about 30 to 50 parts per hundréd parts o~ base .. . .
- re~ln and urea, l.e., sufflcient to produce resin solutions - ~ containing ~rom 12 to 20 percent DAP, but ~mo~lts of up to 40 -~
parts supplying from 12 to 16 percent DAP are preferred for reasOn~ of ~ta~illty Or the binder~solutions.-~ Such 801utlon~
w~ll supply from about 2.4 to 6.5 percent by welght of the phosphate when applled at 20 to 40 parts per hundred parts or wo~d rurnlsh.
: -Even more preferred sources o~ ammonIum phosphates are aqueous solutlons of ammonlum polyphosphates slnce such ~olu-tion~ can contaln greater amounts Or the ~ho~phates prlor to 8aturatlon. Such solutions are generally commercially avail-~, .
:~ able from the fertlllzer lndustry and are complex gro~s mlx-, ~ 30 tures o~ varlou~ condensed and polyphosphates as the ammonium , -:
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C-Q~-12-o389 ~ 0 6 7 6 3 8 8alt~ produced by ammonlating a phospholeum of rrOm 95 to 12 percent total phosphate content, which phospholeums are com-merically available condensed phosphoric arld solutlons. Typl-cal ammonium polyphosphate solutions expressed by plant food content analysis are the 10-34-0 ammOnium polyphosphate avail-~ able as a 60 percent solutlon rrOm fertllizer producers and ~1-37-0 ammonium polyphosphate avallable as a 66 percent solu-- tlon from the TVA and other~. Such ammonlum polyphosphate solutlons enable one to produce blnder solutions containlng higher phosphate content~ with llttle increase in the dlluting water present.
- However, such condensed ammonium phosphate and poly--~ phosphates cannot practically be used ln blnder resins wlthout -other addltl~es because they are very strOng bu~ers in which the ammonium ion demonstrates no latency so that such polyphos-phate salts severely retard the cure of the binder resln at board presslng temperatures. Diammonium phosphate on the other hand dlsplays a latent ammonlum lon er~ect and serves-as a - latent curlng catalyst. Hence, when ammonlum polyphosphates are employed lt has been found advantageous to use DAP there-with to supply a part Or the desired phosphate content and curlng catalytic ef~ect. Furthermore, it has been ~ound that -~ the strong buffering effect Or ammonlum polyphosphate keep3 the pH Or a resin solution-contalning same too alkallne to prQmOte an acceptable curlng rate. Consequently, it has been ~ound essential to include an acid to lower the pH and phos-phorlc acld has been round most useful as a thlrd souroe Or phosphate lon. However, solutlon~ Or both ammonium polyphos-phates and dlamMonium phosphate are nOt stable ln the pre~ence 3 Or relatlvely small percentages Or phosphoric acid; as llttle .. : ;

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~ C-0~12-0389 as 6 to 15 percent o~ a 75 percent phosphoric acid wlll cause precipitatlon Or the 10-34-0 and 11-37-O ammonium polyphosph~te solutlons described above.
It has been ~ound that mixed solutlons of ammonium polyphosphate3 and diammonium phosphate are stable in the presence of much greater quantltle~ o~ phosphoric acid. There-~Ore, to produce the mixed ammonium phosphate solutlon use~ul ln a binder resin solution lt has been ~ound necessary to lnclude an amount of concentrated phosphoric acld of ~rOm about 10 to 20 perc~nt Or the total of the ammonium phosphates on a welght basis. Thls ls sur~iclent to reduce the pH o~ the phos-phate solutions to a range o~ rrOm about 4.4 to 5.4.
Typical use~ul ammonium pho~phate solutions for blending with the urea-formaldehyde or melamlne-urea-formaldehyde - 15 re31n bases are those o~ the following compositlons:
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Ingredient As Solution Conc. Parts - % Phosphate 10-34-0 APP 60% 5 - 72 30-43.2 or 10-37-0 APP 66% 50 - 72 33-47.5 DAP 40% 10 - 40 4-16 Phosphoric Acid 75~ - 10 - 18 7.5-13.5 Thus, on a 100 percent basis, such solutions can contain ~rOm 56 to 73% ammonium polyphosphate~ from 6 to 30% diammonlum phosphate and from 14 to 21~ phosphorlc acld. Such solutlons will supply ~rom about 18 to 30 percent phosphate ln the blended blnder resln; the resln wlll thus supply from about 3.6 to 10.5 percent total phosphate to the flnlshed boards when applied to 20 to 35 parts per hundred parts wood furnish.
The most prererred solutions are those contalning the ~' greater amount3 of ammonlum polyphosphates since they wlll pro-: .1 ~ 3 duce the hlghest phosphate concentratiOns ln the blended blnder .

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iO~7638 resln solution~. A qùite useful and stable ammonium phosphate solution has been found to be one of the following compositlon:
65.2 parts 10-34-0 APP - 60% solutlon 21.8 parts DAP - 40% solution 13.0 parts Phosphoric Acid - 75~ solution on a 100% basis the above solution contains 61.5~ ammonium polyphosphate, 23.2~ diammonium phosphate and 15.3% phosphor~c acid.
When referred to hereln as condensed ammonium phos-pr.atesor ammonium polyphosphates any Or the condensed ammonium phosphates can be employedl includlng the ammOnium polyphos-pnates from orthophosphoric ac~d, the ammonium metaphosphates and the ammonium ultraphosphates. Any o~ such as aqueous solu-tions can be used as a source of ammOnium phosphate in the new rire-retardant binder resin The fire-retardant particle boards produced with the ne~ blnder re~ins are made by the usual industrial prOcesses fcr such boards. These prOcesses invOlve spraying the desired a-ount of resin solution, generally from about 20 to about 40 pcrts per hundred parts o~ furnlsh, onto a dried wood particle f~-nish, thorough blending thereofJ deposit of the furnish and r~sin onto supporting cauls and pressing the wood particle ~rnish to stops at from about 20 to 30 ~ækm2 at temperatures of f~om about 135 to 177C ~Or periods of from about 3 to 8 m~utes to produce the finished particle boards.
The sample particle boards described in the examples ~clow were tested for fire-retardancy rating by the well-known S~;~lyter Burning Test described in "Fire-Test Methods", Forest F~oducts Laboratory Report No. 1443, U.S.D.A. Forest Service 3 (:9~9). When modified by the use of propane as the fuel rather c~

than the standard methane or natural ga~ such ~uel i8 noted - below. Ratlngs from the test are given in terms of rlame spread in 5 minutes as a percentage of the flame spread in the same period On standard untreated red oak lumber One inch thick.
Type I or Class I fire-retardancy is represented by flame spread ratings Or 25 or less, l.e., 25 percent Or standard, while Type II Or Class II rire-retardancy is represented by ratings of from 25 to 75, i.e. percent, based On red oak lumber as standard.
The examples below will serve to ~urther illustrate the fire-retardant prOperties of particle boards prepared with the novel binder resin solutions of the present invention.
EX~PLE I
Thls example illustrates the use of a fire-retardant binder resin solution prepared from a sOlution o~ diammonium phosphate. A urea-formaldehyde resin was prepared by reacting 100 parts of a 5 ~ aqueous formalin solution with 31.8 parts of urea at a pH of about 9.0 for 5 minutes at 80C., cooling the mixture to 60 C., distilling Orr 10~ of the water present to a temperature of about 40C. Thereafter the resln solution was ad~usted to a pH of 1.8 with sulfuric acid and the resin bodied ~or 5 minutes and neutralized. The resin solution was dis-tllled to a solids content of 89 percent, cooled, and mixed with 2 ~ by weight on the basis of the UF resin of a 65 percent aqueous solution of a methylolated melamine-formaldehyde resin (Resimene 841*). This 5:1 mixture formed the melamine-urea-` formaldehyde base resin.
There were then mixed with 48 parts of the above MUFbase resin 14 parts of urea and 38 parts of a 40 percent aque-ous solution of diammonium phosphate to produce a clear aqueous * Trade Mark ., -11-~ C-06-12-03~9 1067631~

blnder resln syrup of 80 cp~ viscosity. This binder resin solution Or only 19 percent water wa~ an excellent particle board binder stable for at least four hours to clouding or precipitation.
Particle boards were prepared in the usual manner :. .
rrom dried wood particle furnish sprayed with 20 percent and 30 percent o~ the above solution based on the weight of the dry wood rurnlsh~ su~ficient solution to supply 3.04% and

4.56~, respectively, of ammonium phosphate ln the rinished particle boards. The board~ were 1.9 cm thick with a density - of about 753 Kg/m3 produced by pressing ror 7-1/2 minutes at 163C to thickness stops under an lnltial pressure ;~ of 25 Kg~cm~ ~hese sample particle boards and a control board were sub~ected to testing by the Schlyter test described above using the indicated fuel with the fire-retardant results set out in Table 1 below. The board used as a control was prepared from a similar melamine-urea-formaldehyde base resin with no added ammonium phosphates.

Flame Board~ DAP Fuel Rating FR Type Control 0Methane 125 --1 3.04Propane 40 II
" "Methane 32-34 II
2 4.56Propane 25 Methane 23-25 ~hus, Type I ~lame retardancy can be achieved by use Or the : . .
binder resin solution containing DAP ln normal productlon pro-cesses and at reasonable application rates. All the boards 3 displayed good strengths, internal bonding and color.
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C-06-12-03 ~

iO~i7638 EXAMPLE II

In thls example the blnder resin is prepared from a 801ution 0~ ammonlum polyphosphate, dlammonium phosphate and pho~phoric acld. The mixed ammonlum pho~phate solution is pre-pared by mixing 65.2 parts o~ 10-34-0 ammonium polyphosphate 60 percent solution and 21.8 parts of a diammonil~ phosphate 40 percent solution and 13.0 parts o~ concentrated phosphoric acid 75 percent ~olution. This mixed solution was stable and dld not precipitate ror a period of longer than 10 days.
There was prepared a binder resin solutlon from the same MUF base resin described in Example I. To 12 parts of the MUF base resin there was added 3.7 part~ of urea and 13.8 parts of the above described mixed ammonium polyphosphate, diammonlum phosphate and phosphoric acid solution. The binder resin solutlon was clear, stable for at lease 4 hours and had a viscosity of about 280 cps.
Particle boards were prepared in the usual way by spraying 30 parts Or the binder resin solution onto 100 parts of dry wood particle furnish and blending ~ame before deposit-lng the furnl~h and pre3slng the board3 under the same condi-tlons a~ in Example I above. There was thus supplied a totalof 7.95~ total phosphate in the pressed particle board~. The rl~ished and cooled boards were treated in the about outlined Schlyter test using methane fuel and the result~ are set out ~ .
ln Table 2 below.
;, 25 TABLE 2 ;~ Board% Total PhosphateFlame Rating FR Ty~
1 7.95 24 2 " 25 ~ .
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', c- o6- 12- o38g ~7638 All the boards tested displayed a Type I fire~
retardant rating, good strength, internal bonding and color and represented good quality commercial particle boards.
It is clear from the above examples that fire-retardant particle boards are produced when the ammonium phos-phates are applied and coprecipitated from solution with the resin binder in amounts of from about 2.4 to 10.5 percent by weight. It is preferred that the ammonium salts be pre~ent in amOunts of from about 2.8 to 10 percent by weight.

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Claims (13)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. An improved aminoplast binder resin solution contain-ing dissolved ammonium phosphates suitable for producing fire-retardant particle board, the improvement comprising, an aque-ous solution of a urea-formaldehyde resin having a formaldehyde to urea ratio of from about 3.0 to 3.5, sufficient free urea to reduce said ratio to 1.2 to 1.8, a methylolated melamine-formaldehyde in an amount of from 15 to 25 percent by weight based on said urea-formaldehyde resin, wherein said melamine-formaldehyde and urea-formaldehyde resin solids comprise from about 30 to 55 percent by weight of said solution, from about 12 to 30 percent by weight of dissolved ammonium phosphates selected from the group consisting of diammonium phosphate and mixtures of diammonium phosphate, ammonium polyphosphate and phosphoric acid, and not more than 50 percent by weight water.

2. The binder resin solution of Claim 1 wherein the dissolved ammonium phosphate content is from 12 to 20 percent by weight diammonium phosphate.

3. The binder resin solution of Claim 1 wherein the dissolved ammonium phosphate content is from 18 to 30 percent by weight of a mixture, on a 100 percent basis, of 56 to 73 percent ammonium polyphosphate, 6 to 30 percent diammonium phosphate and 14 to 21 percent phosphoric acid.

4. A process for production of a fire-retardant amino-plast binder resin solution which comprises A. Mixing with a substantially neutral aqueous solution of a low molecular weight urea-formaldehyde resin of a mol ratio of from about 3.0 to 3.5 formaldehyde to urea, a viscosity of from about 60 to 100 cps and a percent solids of at least 70 percent, B. sufficient urea to reduce the formalde-hyde to urea ratio to from about 1.2 to 2.0, and C. a stable concentrated aqueous solution having a percent solids of at least 40 percent of ammonium phosphate selected from the group consisting of diammonium phosphate and a mixture of diammonium phosphate, ammonium polyphosphate and phosphoric acid to produce a concentra-tion of dissolved ammonium phosphate of from about 12 to 30 percent by weight in said binder resin solution.

5. The process of Claim 4 wherein there is mixed with the urea-formaldehyde resin solution of (A) from about 15 to 25 percent by weight thereof of an aqueous solution of at least 50 percent solids of a methylolated melamine-formaldehyde resin so as to produce a malamine-urea-formaldehyde resin solution of at least 70 percent solids.

6. The process of Claim 5 wherein the melamine-urea-formaldehyde resin solution is subjected to distillation to concentrate same to a solids content of at least 70 percent.

7. The process of Claim 5 wherein the solution of ammonium phosphate is a stable solution of diammonium phos-phate of at least 30 percent concentration.

8. The process of Claim 5 wherein the solution of ammonium phosphate is an aqueous solution of from 56 to 73 percent ammonium polyphosphate, from 6 to 30 percent diammon-ium phosphate and from 14 to 21 percent phosphoric acid.

9. The process of Claim 5 wherein the urea-formaldehyde resin solution has a solids content of at least 85 percent wherein the methylolated melamine-formaldehyde resin solution has a solids content of at least 60 percent and is present in an amount of about 20 percent of the weight of the urea-formaldehyde resin, wherein the solution of ammonium phosphate is a 40 percent solution of diammonium phosphate and the mixed binder resin solution has a viscosity of from 70 to 300 cps and a solids content of at least 75 percent.

10. The process of Claim 5 wherein the urea-formaldehyde resin solution has a solids content of at least 85 percent, wherein the methylolated melamine-formaldehyde resin solution has a solids content of at least 60 percent and is present in an amount of about 20 percent of the weight of the urea-formaldehyde resin, wherein the solution of ammonium phosphate comprises about 65 parts of a 60 percent 10-34-0 ammonium polyphosphate solution, about 22 parts of a 40 percent diammonium phosphate solution and about 13 parts of a 75 percent phosphoric acid solution, and the mixed binder resin solution has a viscosity of from about 70 to 300 cps and a solids content of at least 75 percent.

11. A fire-retardant particle board comprising wood particles bonded with an aminoplast resin binder of Claim 1 and coprecipitated with said resin binder from about 2.4 to 10.5 percent by weight of the total weight of the particle board, of an ammonium phosphate selected from diammonium phosphate and mixtures of diammonium phosphate, ammonium polyphosphate and phosphoric acid.

12. The fire-retardant particle board of Claim 11 wherein the aminoplast resin binder is melamine-urea-formaldehyde resin and the coprecipitated ammonium phosphate is diammonium, phosphate in an amount of from about 2.4 to 6.5 percent by weight.

13. The fire-retardant particle board of claim 11 wherein the aminoplast resin binder is a melamine-urea-formaldehyde resin and the coprecipitated ammonium phosphate is a mixture of diammonium phosphate, ammonium polyphosphate and phosphoric acid in an amount of from 3.6 to 10.5 percent by weight.