PRODUCT AND PROCESS DEVELOPMENT
Reactions of Tetra kis( hydroxyrnethyl)phssphoniurn Chloride WILSON A. REEVES
AND
JOHN D. GUTHRIE
Soufhern Regional Research laborufory, New Orleans, La.
TETRAKIS(HYDR0XYLIETHYL)PHOSPHOSIUM
chloride, (HOCH,)4PCI, is a rather unusual and little-investigated organophosphorus compound that undergoes many interesting reactions. This paper calls attention to its polymer-forming properties and especially to polymers formed with nitrogencontaining compounds. The use of this compound for making polymers opens up a vast field of new polymels that are flameresistant. Tetrakis(hydroxymethy1)phosphonium chloiide (THPC) was first reported in 1921 by Hoffman ( I ) . At that time he was not certain of its structure, but 9 years later (8)he established the structure and made some of its derivatives. I t is a crystalline compound that is soluble in water and lower aliphatic alcohols, but insoluble in most common organic solvents. I t is potentially a relatively cheap material. It is made in about 907, yields (based upon formaldehyde or hydrochloric acid) by ieaction of phosphine with formaldehyde and hydrochloric acid a t room temperature (4). The crystalline product can be isolated by evaporating the aqueous solvent at a temperature not euceeding about 100’ C. There is considerable similarity between tetrakis( hydroxymethy1)phosphonium chloride and aqueous formaldehyde, in which state the formaldehyde may be considered to react like methylene glycol. The hydrouymethyl groups of methylene glycol are attached to an electronegative atom, just as each of the hydroxymethyl groups of THPC is attached to the electronegative phosphorus atom. As the reactions are described, its similarity to aqueous formaldehyde will be noted. It also undergoes reactions characteristic of primary alcohols. I t s value as a flame-retarding agent was first recognized in a study of the chemical and physical properties of aminized cotton (6). It reacted with the amine groups of aminized cotton t o produce a flame-resistant chemically modified cellulose. The reaction was carried out by moistening aminized cotton with a dilute aqueous solution of THPC and then drying at about 100” C. The treated fabric contained phosphorus and nitrogen but no halogen. The phosphorus introduced was attached by a stable link, as it wm not removed by boiling 1% sodium hydroxide solution. The reaction might be represented as a condensation between the amino group and hydroupmethyl groups:
cellulose
-
II
I
OCHzCHzN-CH2-P-CH2-N-CHzCHz0 - cellulose I
CHz 0 H
Reacton with amines, phenols and polybasic acids, and anhydrides produces useful polymers
Polymerization with Amines. TetrakiP(hydrosyinethyl)pli(~~phonium chloride reacts with monomeric amines, primary and secondary, even more readily than with nminized cotton; tris(hydroxymethy1)phosphine oxide also undergoes reaction n-ith these amino compounds. With the monomeric amines and THPC the reaction is exothermic and takes place readily at room temperature-for example, cetylamine in alcoholic solution is converted almost instantly to a product insoluble in alcohol when an alcoholic solution of THPC is added to the amine. The magnitude of the potential use of this “phosphoruscontaining formaldehyde” was made more evident when it n-as found that THPC, like formaldehyde, would react with urea to form insoluble polymers. Although this reaction does not proceed with the rapidity indicated with the above amines, it does proceed readily. The primary reaction product may be represented as follon-s: 0
I n the above representation the phosphonium structure present in the crystalline compound was converted to a phosphine oxide when it reacted with the amine, Phosphonium compounds in general undergo paraffinic decomposition to phosphine oxides when heated:
64
Hoffman (2) demonstrated that tetrakis( hydro\j methy1)phosphonium chloride could be converted to tris(hydroxymethv1)phosphine oxide (THPO), by alkali hydroxides or neutral carbonates like barium carbonate. Instead of the normal paraffinic derivatives-that is, methanol along 15 ith the phosphine oxide-Hoffman showed the decomposition products to be hydrogen and formaldehyde. THPC is converted to THPO xith triethanolamine in mildly acidic media at room temperature. Considering these facts, it is likely that THPC is converted to the phosphine oxide during reaction in alkaline. neutral, 0 1 mildly acidic solutions or when heated. Thus, under these conditions the compound acts as a trifunctional monomer. Severthelese, it is possible that even under these conditions some of the molecules react and become stabilized in the phosphonium form. The ionic halogen could then be exchanged for a hydrouyl group i n alkaline media.
(HOCHa)4PCl
0
I1
1I + HzNCNHz
-+
H O
(HOCH?)z-P-CHa~-C-h;Rz
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
+ HCl + HCHO + HZ+ H20 Vol. 48, No. 1
PRODUCT AND PROCESS DEVELOPMENT or
FLAME-RESISTANT POLYMERS
Water-soluble products can easily be isolated, but their structures have not been verified. They usually contain halogen. The final hard insoluble resinous products usually contain no more than trace quantities of halogen. To make these resins, the monomers are usually heated for about an hour a t 100' C. or for a few minutes a t 140' to 150" C. The ease of the reaction is dependent to a considerable extent upon pH; the reaction proceeds best in a pH range of from 3 to 6. Tetrakis(hydroxymethy1)phosphonium chloride contains a "built-in" catalyst, hydrochloric acid, but in the presence of an additional amine catalyst the reaction rate is increased considerably. Hexanolamine (4-amino-4-methyl-2-pentanol) is an effective catalyst. Time required for a solution of THPC and urea to gel a t a boiling temperature was reduced from 60 minutes to 9 minutes by addition of hexanolaniine. Concentration and molar ratio of reactants also influence gel time. The presence of urea tends to maintain a mildly acidic condition, which favors the formation of a phosphine oxide structure in the final resinous ploymers. Polymers with urea can be made over a very wide range of molar ratios, as the monomers have polymer-forming functionalities greater than 2. Highly insoluble polymers that contain 15% phosphorus have been made. THPC-urea reaction products are modified by the carbonyl group adjacent to the amine radical and accordingly differ in many respects from the corresponding reaction products with alkyl amines. THPC-urea polymers closely resemble urea-formaldehyde reaction products, except that these phosphorus-containing polymers are flame- and glowresistant. Melamine, a cyclic trimer of cyanamide, reacts with THPC in mildly acidic solutions, producing initially water-soluble methylol phosphorus derivatives. Apparently, from one to six of the hydrogens on a molecule of melamine can react to produce methylol phosphorus derivatives. These initial reaction products are water-soluble, but if they are left in mildly acidic solutions they readily polymerize to highly insoluble products, even at room temperature. Melamine polymerizes with THPC more readily than urea. The resultant polymers are, in general, more insoluble than the THPC-urea polymers. Water-soluble methylolmelamines made by condensing melamine and formaldehyde are very useful in making polymers with THPC. These melamine derivatives make it possible to use aqueous solutions without having to prepare the water-soluble intermediate reaction product of THPC and melamine. As with melamine, mixtures of THPC and methylolmelamine polymerize a t room temperature, but can be stabilized for several hours a i t h triethanolamine. Such Stabilized solutions are useful in producing flame-resistant cotton fabrics (6). The fabric is impregnated with the water- soluble monomers, then dried and cured a t an elevated temperature. A temperature of about 150" C. for 4 to 5 minutes is generally sufficient to cure these polymers. Very interesting polymers of another type are made with ethylenimine. The initial reaction can be illustrated as an addition of ethylenimine to a hydroxymethyl group with the formation of a primary amine: (HOCH2)aPCl
+ CHZCHI N ''
+
B
made by reacting this organophosphorus compound with urea, melamine, phenol, etc., will serve in many industrial uses
The amine readily condenses with additional hydroxymethyl groups to form highly branched polymers. A unit of such polymers can be represented as follows:
\
0
N-CHg
The rate of reaction with ethylenimine is much greater than any other reactions described above. These materials in aqueous solution react with violence a t room temperature, and at 5" to 10" C. they will reach the gel point in 1 or 2 minutes. When the gels are heated a t above 140" C. for a few minutes they become light brown in color and brittle, but the brittleness disappears within a few minutes when they are exposed to air a t about 25" C. The apparent absorption of moisture converts these resins to spongelike resilient materials. Polymerization with Phenols. THPC reacts with phenolic compounds to form a variety of phosphorus-containing compounds and polymers. Reaction occurs with those phenolic compounds in which hydrogen atoms are attached to the phenolic nucleus in the 2, 4, or 6 position with reference to the hydroxyl group. Substituent groups have the same influence on reactions as has been found with phenol-formaldehyde reactions. In phenol-formaldehyde reactions, two are most important: the formulation of nuclear methylol phenols, and the formation of polynuclear methylene derivatives (8, p. 237). The formation of nuclear methylol phenols is generally considered to proceed by first forming a hemiformal; then the unstable hemiformal rearranges to the methylol phenol: H 0
H
v u
U
The formation of polynuclear methylene derivatives may be indicated as follows:
H
,
0
H
o
c .o I
--CHzOH
+
I
--CHzOH *
H 0
H 0
0
(HOCH2)a8-CH2-O-CH2CH2-KH2 January 1956
3. HCHO 4-HCI 4- Ha INDUSTRIAL AND ENGINEERING CHEMISTRY
65
PRODUCT AND PROCESS DEVELOPMENT It is not feasible to formulate an intermediate hemiformal between phenols and THPC, but the reaction may be represented as corresponding to the formation of polynuclear methylene derivatives from phenols and formaldehyde:
Polymer formation probably proceeds through esterification, as indicated in the following equation:
+ u20 + n c I + n c ~ o + n ~ Chlorendic acid ( 1,4,5,6,7,7-he~achlorobicyclo[2,2,1]-5-heptene2,3-dicarboxylic acid)
+ H20 + HC1 + HCHO + Hz Under strongly acidic conditions some of the phosphonium structure is likely to occur, but in mildly acidic, neutral, or alkaline conditions the phosphine oxide structure will prevail. Further condensation could bridge two phenol nuclei:
In these reactions with polybasic carboxylic acid and anhydride, THPC reacts more like a polyhydric alcohol than like aqueous formaldehyde. However, reactions between formaldehyde and organic acids have been reported. Paquot and Perron ( 3 ) sucreeded in malting formaldehyde react xith palmitic acid at about 300" C. in an autoclave for 5 hours. They obtained the methyl and ethyl palmitates. Schopff ( 7 ) found that formaldehyde reacted with benzoic acid in the presence of concentrated sulfuric acid to produce methylene-3,3'-dibenzoic acid. Walker ( 8 , p. 273), however, states that aromatic acids show little tendency to react with formaldehj-de under neutral, alkaline, or mildly acidic conditions. Experimental polymerization reactions with THPC are described
Resin formation results from reactions of this type in which methylol phosphorus compounds and phenolic compounds condense to form complex molecules. To make these molecules even more complex, some of the formaldehyde that is released from the THPC molecule during reaction probably also reacts with the phenolic nucleus. These polvniers resemble the corresponding products made from phenols and formaldehyde, but because of the presence of the phosphorus, they are rather flameresistant. Most of the polymers made from THPC and phenols are infusible resins, but fusible polymers can be made with THPC and substituted phenols even though THPC has a functionality greater than 2. For example, the reaction products \T-ith p-Ledbutylphenol are generally fusible. Reaction with phenols occurs over a very Ride range of pH conditions. Products have been obtained from solutions ranging from strongly acidic to mildly basic solution. Amines, like triethanolamine or ammonia, are good catalysts. Polymers with Polybasic Acids and Anhydrides. THPC reacts with polybasic carboxylic acids and anhydrides to form phosphorus-containing alkydlike polymers. Both aliphatic and aromatic acids react with THPC under conditions suitable for reaction of glycerol with the corresponding acids and anhydrides. 66
A considerable number of polymerization reactions have been carried out, but only a few typical examples are given here. With Cetylamine. T o 3.6 grams of cetylamine (0.016 mole) dissolved in 29 ml. of n-arm (approximately 40' C.) 6270 ethyl alcohol were added 2 grams of 95% THPC (0.010 mole) and 0.35 gram of sodium carbonate dissolved in 80% ethyl alcohol. A greaselike polymer precipitated rrithin 1 minute., The supernatant liquid was decanted and the greaselike product xas washed several times in hot 95% ethyl alcohol. After the first hot wash, the polymer was converted to a mealy product which resembled cetylamine in appearance. It was dried a t 55" C. The yield of 3.8 grams vas 78%, if THPC is assumed to be converted to phosphine oxide during polymerization. The polymer melted between 70' and 90' C. It contained 5.5% phosphorus and 4,36% nitrogen. With Urea. Four grams of 95% THPC (0.02 mole) ivas dissolved in 10 ml. of wat'er. The pH n-as adjusted to 5 with a concentrat,ed solution of sodium carbonate, and then 1.5 grams of urea (0.02 inole) ivas dissolved in the solution. It !vas boiled at constant volume for 55 minutes, at, which time gelation occurred. I n another experiment 4.0 grams of 95% THPC was dissolved in 10 ml. of water and the pH was adjusted to 5, as above. 4 Amino-4methyl-2-pentanol (1.2 grams) was added to the solution and i t was then boiled a t constant volume for 20 minutes. Urea (1.5 grams) x-as added and the solution was again boiled. It gelled in 9 minutes. The gel was heated in an oven a t 110' C. for several minutes in order to cure the polymer. The brittle polymer was then ground in a mortar and ext,racted several times with hot water. It contained 14.7% phosphorus, 21% nitrogen, and about 0.1% chlorine. The resin was insoluble and would not support a flame. With Urea and Melamine. Melamine (3.3 grams) was added to a solution containing 9 grams of THPC. The mixture, a h k h had a pH of less than 2, was varined to about 60" to 70" C. and as soon as t,he melamine had dissolved, 9 grams of urea and 0.9 gram of sodium acetate were added. The solution was brought to a boil and boiled for 15 minutes at constant volume. The clear solution was cooled and then poured into 60 ml. of 95% ethyl alcohol. A rather viscous residue formed. The alcoholic supernatant liquid was decanted, another 60 ml. of 95% ethyl alcohol
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
PRODUCT AND PROCESS DEVELOPMENT was added to the residue, and the mixture was stirred vigorously for 3 to 4 minutes to convert the viscous residue to a white flaky product, which was filtered off and dried a t 60" C. Its dry weight was 12.6 grams. The product was very soluble in water. Dilute sodium hydroxide caused immediate polymerization of aqueous solutions of the product into a water-insoluble resin. The water-soluble product contained 30% nitrogen, 7.82% chlorine, and 9.44% phosphorus. A water solution of the product could be polymerized to an insoluble resin by heating. This resin was also very flame resistant. With Melamine. Twenty grams of THPC, 6 grams of melamine, and 2.0grams of sodium carbonate were stirred into 35 ml. of water. The pH of the mixture wm approximately 7. \$"hen it was stirred and heated to 65" C. the pH fell to 6. I t was heated at 65" C. just to the point where the clear solution would not become turbid when cooled at 25' C., which required approximately 15 minutes. At this point a drop of the solution would dissolve in water but would form a white precipitate in 95% ethyl alcohol. This solution was divided into two equal parts, A and B. Part A was poured into an excess of 95% ethyl alcohol. The white precipitate that formed was removed by filtration. It was readily soluble in water. Some of the redissolved material was placed upon a watch glass and heated a t 110' C., when a hard, clear, insoluble resin formed. Part B was allowed to stand overnight a t 25" C., during which time it became very viscous. When a portion of the viscous product was poured into water a white precipitate came down; when a portion was heated a few minutes at 110" C. it gelled and became insoluble. With Ethylenimine. A cold (5" to 10" C.) solution of 3.36 grams (0.078 mole) of ethylenimine in 10 grams of water was mixed with a cold (5' to 10' C.) solution of 5 grams of THPC (0.026 mole) in 5 grams of water. An exothermic reaction occurred and within 2 minutes a cleai gel formed, which was soluble in water, volume for volume. Heating of the gel to 150" C. for several minutes produced a hard brown polymer which was insoluble in water or alkaline solutions. After standing at about 25' C. for 20 minutes, the polymer became resilient and had a moisture regain of 21%. It contained 6.2% phosphorus and 14.9% nitrogen on the dry basis. When 4 grams of this resilient polymer was mixed with 30 grams of water, the polymer swelled until it occupied the entire volume. In a similar experiment 2.24 grams of ethylenimine reacted with 5.0 grams of THPC to form a polymer of similar physical properties. It contained 5.7% phosphorus and 12.5% nitrogen on the dry basis and had a moisture regain of 18.5%. With Phenol. Phenol (7.6 grams, 0.08 mole), 95% THPC (8 grams, 0.04 mole), and water (10 ml.) were warmed gently and stirred until the reagents were in solution. The solution was divided into two equal parts. To part B was added 0.05 gram of sodium carbonate to raise the p H to 5. Portion A was left at pH 1. Both portions were boiled gently on a hot plate. Five minutes after boiling was started, sample A began an exothermic reaction-white fumes were expelled and before the sample could be cooled to stop the reaction, polymerization had proceeded to a point where the entire product ws a hard brown, brittle resin. Sample B began a less vigorous reaction 7 minutes after boiling was started. The beaker containing it was placed in cool water
January 1956
to slow down the reaction and it stopped just as a clear gel had formed. The gel waB broken up and heated until it was tan colored. Samples A and B were both ground in a mortar, washed with distilled water, and dried. Sample A contained 0.7% phosphorus; sample B contained 1.4% phosphorus. With Phloroglucinol. To 4.0 grams (0.02 mole) of THPC in 20 ml. of water was added 3.2 grams (0.02mole) of phloroglucinol dihydrate. Heating on a steam bath did not cause solution. Solution occurred when it was boiled 10 minutes on a hot plate. After 26 minutes of boiling, a viscous straw colored product resulted, which was soluble in water and in ethyl alcohol. The soluble product was heated in an oven at 110' C. until it became reddish brown. The brittle resin was ground in a mortar, then soaked in water for 16 hours. The water was decanted, and the resin was further extracted by heating for 30 minutes in boiling water and filtering. It was then rinsed twice with warm water. The dry polymer contained 6.01% phosphorus and 5.95% chlorine. When the above reaction was carried out in ethyl alcohol, the brittle resin contained 9.26% phosphorus and 5.77% chlorine. In another case the THPC and phloroglucinol were dissolved in 50% ethyl alcohol and heated for 10 minutes to form a viscous product. Ammonium hydroxide was added with stirring to raise the pH to about 5. An exothermic reaction began and within about 1 minute the entire mass solidified. After grinding and washing, the polymer cohtained 9.91% phosphorus and 3.32% nitrogen. With Chlorendic Anhydride. Eighteen grams of chlorendic anhydride (1,4,5,6,7,7-hexachlorobicyclo[2,2,1]-5-heptene-2,3-dicarboxylic anhydride) and 6 grams of THPC were mixed, then heated on a hot plate at low heat for about 10 minutes. When the molten mass had cooled, it was a clear, hard, brittle resin which was soluble in acetone but insoluble in water or toluene. The resin contained 2.5% phosphorus. Acknowledgment
The authors wish to express their appreciation to the following members of this laboratory: Samuel M. Stark, Jr., for phosphorus determinations, Julian F. Jurgens, for nitrogen determinations, and Elizabeth R. McCall, for chlorine determination. literature cited
(1) Hoffman, A., J. Am. Chem. SOC.43, 1684 (1921). (2) Ibid.,52, 2995 (1930). (3) Paquot, C., and Perron, K., Bull. 8oc. chim. (5) 5 , 855 (1948). (4) Reeves, W. A., Flynn, F. F., and Guthrie, J. D., J . Am. Chem. SOC.77, 3923 (1955). (5) Reeves, W. A.. and Guthrie, J. D., U. S. Dept. Agr., Agr. Ind. Chem. Bull. AIC-364 (1953). (6) Reeves, W. A., McMillan, 0. J., Jr., and Guthrie, J. D., Tezfile Research J . 23, 527 (1953). (7) Schopff, M., Ber. 27, 2321 (1894). (8) Walker, J. F., "Formaldehyde," Reinhold, New York, 1953. RBCEWED for review February 21, 1955. ACCEPTBD November 7, 1955. Division of Polymer Chemistry, 125th Meeting, 4CS, Kansas City, Mo., March 1954. Work supported in part by funds supplied by the Office of the Quartermaster General, Department of the Army, and conducted under the general supervision of the Research and Development Center, Natick, Mass.
INDUSTRIAL AND ENGINEERING CHEMISTRY
67
