950
Ind. Eng. Chem. Res. 1996, 35, 950-953
Interaction of meso-1,2,3,4-Butanetetracarboxylic Acid with Phosphorus-Containing Catalysts for Esterification Cross-Linking of Cellulose Cletus E. Morris,* Nancy M. Morris, and Brenda J. Trask-Morrell Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, P.O. Box 19687, New Orleans, Louisiana 70179
In the presence of catalysts, polycarboxylic acids such as meso-1,2,3,4-butanetetracarboxylic acid (BTCA) can cross-link cellulose in a process believed to involve cyclic anhydrides as intermediates. This study was undertaken to explore, in the absence of cellulose, interactions between catalysts and polycarboxylic acids that might explain the catalysts’ role. Aqueous solutions containing equimolar amounts of BTCA and a catalyst (NaH2PO4 or NaH2PO2) were evaporated to constant weight. The residues were subjected to thermal analysis and to infrared spectroscopic analysis after heating. As the BTCA-NaH2PO4 residue was heated, the BTCA remained complexed with phosphate until a monoanhydride formedsthe same monoanhydride that formed when BTCA alone was heated to a higher temperature. In the BTCA-NaH2PO2 system an anhydride apparently formed at an even lower temperature than in the phosphate-containing system but then reacted with the hypophosphite. The discovery that some alkali metal salts of phosphorus-containing inorganic acids catalyze the crosslinking of cellulose by polycarboxylic acids (Welch, 1988) has led to industrial interest in this formaldehyde-free process. Research on the esterification cross-linking process and its use to impart durable smooth-drying properties to cellulosic fabrics has been reviewed by Welch (1992, 1994) and La¨mmermann (1992). A consensus has developed that cyclic anhydrides of the polycarboxylic acids are intermediates in the esterification, the mechanism for the first stage of the process being
A second stage of anhydride formation from the product of the first stage and that anhydride’s reaction with a hydroxyl group in another cellulose chain is believed to complete the cross-linking process. This mechanism explains the observation that the effective cross-linking agents are acids containing at least three carboxyl groups or compounds that can form such polycarboxylic acids during a preliminary reaction. Evidence supporting a mechanism involving a cyclic anhydride intermediate has been obtained from thermoanalytical and spectrometric studies (Trask-Morrell et al., 1990, 1991; Trask-Morrell and Andrews, 1991; Yang, 1991, 1993a). No consensus has been reached on the role of the catalyst, however. It has been suggested that the catalyst may accelerate both the formation of anhydride and its reaction with cellulose (Welch, 1992), that its chief role is to accelerate anhydride formation (Yang, 1991), and that its chief role is acceleration of the anhydride-cellulose reaction (Yang, 1993b). Various anhydride-catalyst reaction products have been proposed as the acylating agents (Brown and Tomasino, 1991; La¨mmermann, 1992). Knowledge about the ac* To whom correspondence should be addressed. FAX: (504) 286-4271.
This article not subject to U.S. Copyright.
tual role of the catalyst would facilitate the search for a less expensive and more environmentally acceptable replacement for the most effective catalyst known at present, sodium hypophosphite. The most effective of the esterification cross-linking agents that have been studied is meso-1,2,3,4-butanetetracarboxylic acid (BTCA). In the course of investigating BTCA’s unusual solubility properties (Morris et al., 1992a), we observed that the addition of either sodium hypophosphite or NaH2PO4 (another catalyst) improved the stability of metastable solutions of BTCA in water and that the solids deposited by these solutions upon standing contained the salts as well as BTCA. This suggested that BTCA formed complexes with these catalysts in the solutions and that BTCA-catalyst complexes may be deposited when solutions of these agents are dried onto fabrics during textile finishing processes. In this study, we sought information about a possible role of these catalysts in anhydride formation by investigating their interactions with BTCA when BTCAcatalyst solutions were evaporated to dryness and the residues were heated in the absence of cellulose. Morris et al. (1992b) found only water and CO2 in the gases evolved from a similar BTCA-NaH2PO2 residue at temperatures up to 220 °C. The present study focused on the nonvolatile products. Experimental Section Aqueous solutions containing equimolar amounts of BTCA and either NaH2PO4 or NaH2PO2sall of reagent gradeswere evaporated to constant weight at reduced pressure and temperatures below 60 °C. The residues were ground in a mortar to ensure homogeneity. Galbraith Laboratories performed Karl Fischer analyses for water in the residues. (Names of companies or commercial products are given solely for the purpose of providing specific information. Their mention does not imply recommendation or endorsement by the U.S. Department of Agriculture over others not mentioned.) Thermal analyses were performed with a TA Instruments 2100 Thermal Analyzer under a dynamic nitrogen atmosphere. Thermogravimetric (TG) data were
Published 1996 by the American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 951
obtained with the Automated Hi-Res TGA 2950 system. The samples were held at 60 °C for 3 min, heated at 50 deg/min with a resolution setting of 5 (except as otherwise noted) to 300 °C, and held at 300 °C for 3 min. Differential scanning calorimetric (DSC) data were obtained at a heating rate of 4 deg/min. Some TG runs were terminated at specific temperatures, and the residual material was removed promptly for Fourier transform infrared (FT-IR) analysis. Materials for FT-IR analysis were prepared also by placing portions of a residue from an evaporation in an oven preheated to 175 °C and heating them at 174-182 °C for various times after the oven temperature returned to 175 °C during 8-12 min. FT-IR spectra of the products in KBr disks were obtained on a Digilab FTS-40 spectrometer, equipped with a deuterated triglycine sulfate detector, a watercooled source, and a dry air purge. One hundred scans of each sample were collected at a resolution of 4 cm-1. A monoanhydride was prepared by the method of Auwers and Jacob (1894) by heating BTCA in an oven at 190-193 °C for 30 min, whereupon it melted and lost 6.60% of its weight (86% of the theoretical loss for monoanhydride formation). Recrystallization of the residue from methyl ethyl ketone gave the monoanhydride as colorless crystals: mp 238-239 °C (with gas evolution) when contained in a capillary tube that was placed in a 217 °C bath and heated at 4 deg/min (literature mp 232 °C); FT-IR 3236.5, 1838, 1769, 1726, 1414, 1261, 1173, 1151.5, 1126, 1051, 974, 799, 633, 463 cm-1. Results and Discussion Monoanhydride Prepared from BTCA. Crystallization from water of the monoanhydride prepared by heating BTCA alone gave the racemic (2RS,3RS) form of 1,2,3,4-butanetetracarboxylic acid. This compound decomposed (rapid gas evolution) at 206-207 °C when the tube containing it was placed in a 205 °C bath heated at 4 deg/min; if inserted at a lower temperature, the evolution of gas was not apparent and the product melted near 238 °C, as did the monoanhydride. This confirmed the report (Auwers and Jacob, 1894) that BTCA isomerizes to the racemic form as it forms a monoanhydride and implies that cellulose cross-linked by BTCA should be expected to be an ester of the racemic mixture rather than the meso form. Interaction of BTCA with NaH2PO4. The thermogravimetric analysis (TGA) curve for BTCA in Figure 1 shows maximum rates of weight loss at 203 and 241 °C, the maximum at the lower temperature evidently representing formation of the cyclic anhydride. It is evident from a comparison of this curve with the one for the BTCA-phosphate residue that the presence of NaH2PO4 led to significant weight losses at substantially lower temperatures. TGA data for the BTCAphosphate residue are presented in Table 1. The DSC thermogram contained endotherms at 88, 187, 208, and 228 °C (melting endotherm: mp 224.5-226.5 °C with gas evolution), corresponding to the four stages of weight loss. The endotherm at 208 °C may be associated with the phosphate: in DSC thermograms for NaH2PO4‚H2O there were endotherms at 213-215 °C, attributed to formation of disodium pyrophosphate. The thermal event near 187 °C is of special interest with respect to a possible relationship to cellulose crosslinking, since 180 °C is a commonly used curing tem-
Figure 1. Thermogravimetric analysis of (a) BTCA, (b) BTCANaH2PO4 residue, and (c) BTCA-NaH2PO2 residue. Table 1. Weight Losses during Thermogravimetric Analysis of BTCA‚NaH2PO4‚1/2H2Oa temp at max rate of loss, °C
weight loss,b %
93-94 186-188 199-201 218-220
2.25 3.63 4.33 38.40
a Based on analysis indicating 2.47% H O. b Mean loss for two 2 runs.
perature when cotton textiles are treated with these agents. Statistical analysis of data from textile finishing experiments has shown that, when equimolar amounts of BTCA and NaH2PO4 were applied, a minimum curing temperature of 181 °C was required to give fabric the desired level of wrinkle resistance (Brown and Tomasino, 1991). The TGA weight loss near 187 °C was less than the calculated 4.96% for loss of 1 mol of water. However, the total weight loss in the range 175-212 °C was 8.08%, equivalent to 1.6 mol of H2O, and close to the theoretical loss (1.5 mol of H2O) for formation of a monoanhydride from BTCA and pyrophosphate from the phosphate. Figure 2 shows FT-IR spectra of the BTCA-phosphate residue after drying to remove the water of crystallization and after TGA analyses terminated at various temperatures. The numerous differences between the spectrum of the oven-dried product and that of anhydrous BTCA (Morris et al., 1992a) indicate that the former is a BTCA-phosphate complex. A pair of bands characteristic of an anhydride, at 1840 and 1765 cm-1, appeared even after this residue was heated only to 168 °C. The substantial increase of these bands when the residue was heated to 187 °C makes it reasonable to conclude that the TGA event at that temperature represented anhydride formation. As the anhydride bands increased, the carboxyl CdO band at 1688 cm-1 decreased and was replaced by one at 1726 cm-1. A shift of this magnitude in the carbonyl frequency for a carboxylic acid commonly signifies a change in the manner whereby COOH groups are associated by hydrogen bondingswith each other or another group. The spectrum of the residue that had been heated to 201 °C was very similar to that of the monoanhydride prepared by heating BTCA alone, the differences being the presence in the former of broad, relatively weak bands that probably should be attributed to the phosphorus compound.
952
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996
Figure 2. Infrared spectra of solids remaining after (a) drying BTCA‚NaH2PO4‚1/2H2O in an oven at 130-142 °C or (b-d) TGA analyses of the same residue terminated at 168, 187, or 201 °C; (e) spectrum of the monoanhydride of (2RS,3RS)-1,2,3,4-butanetetracarboxylic acid. The analysis ended at 168 °C was conducted at a resolution setting of 3.
Figure 3. Infrared spectra of solids remaining after TGA analyses of the BTCA-NaH2PO2 residue terminated at (a) 160, (b) 179, and (c) 182 °C.
Table 2. Weight Losses during Thermogravimetric Analysis of BTCA‚NaH2PO2‚nH2Oa temp at max rate of loss, °C
weight loss,b %
163 178
3.19 17.43
a Based on analysis indicating 4.17% H O, n ) 0.78. b There also 2 was a 0.88% weight loss during the equilibration at 60 °C.
All of the thermoanalytical and spectroscopic data obtained in this study were consistent with a mechanism wherein phosphate facilitates the reaction of BTCA with cellulose by lowering the temperature at which a monoanhydride can be generated from BTCA, forming a complex with BTCA in solution and remaining associated with it until the anhydride is formed. The only unusual aspect of this catalyzed reaction is the concurrent isomerization of the BTCA, since that reaction also appears to occur at the lower temperature. Interaction of BTCA with NaH2PO2. It is evident from Figure 1 that substitution of hypophosphite for the NaH2PO4 led to a very substantial weight loss at an even lower temperature. The only endotherm in the DSC thermogram for the BTCA-hypophosphite residue was centered at 173 °C (mp 172-174 °C). TGA data for this residue are presented in Table 2. The weight loss at 163 °C should be attributed to loss of water of crystallization, since the sum of the weight losses at that temperature and during equilibration at 60 °C was close to the Karl Fischer water content. The weight loss at 178 °C was more than 3 times the theoretical 5.36% loss for conversion of the BTCA in this residue to a monoanhydride. Figure 3 shows FT-IR spectra of the BTCA-hypophosphite residue after TG analyses terminated at 160, 179, and 182 °C. The total weight losses during these analyses were 0.76%, 5.71%, and 11.01%, respectively. The weight loss by 179 °C was 1.54% more than the Karl Fischer water content. At this point a pair of bands indicating anhydride formation appeared in the spectrum, at 1840 and 1782 cm-1, but they were weaker than those in the spectrum of the BTCA-phosphate residue after being heated only to 168 °C during a thermal analysis. Despite an additional weight loss,
Figure 4. Infrared spectra of solids remaining after heating the BTCA-NaH2PO2 residue in an oven at 174-182 °C for (a) 2, (b) 7, and (c) 14 min.
these bands were no longer evident after the BTCAhypophosphite residue was heated to 182 °C. The band at 1084 cm-1 due to the H-P-H wagging vibration in hypophosphite also disappeared as the residue was heated to 182 °C. (This band was at 1088 cm-1 in the spectrum of NaH2PO2‚H2O.) During the same heating interval the band at 2399 cm-1 due to the P-H stretching vibration broadened and shifted. As shown in Figure 4, there were similar changes in the spectrum of the BTCA-hypophosphite residue as the time it was heated in an oven was extended from 2 min to 7 and 14 min. The weight losses after these heating times were 4%, 9%, and 10%, respectively. The similarity of spectrum c in Figure 4sfor the residue heated in airsto spectrum c in Figure 3sfor the residue heated under nitrogensindicates that air oxidation of P-H groups was not a factor in the reactions leading to changes in the spectra. A likely explanation of these data is that hypophosphite catalyzes formation of an anhydride from BTCA, at an even lower temperature than does NaH2PO4, but also reacts with the anhydride that is formed, the reaction going to completion after sufficient heating. Kasˇpa´rek (1963, 1968) reported that in very dilute solutions, at temperatures above 100 °C, the vigorous
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 953
reaction of potassium hypophosphite with carboxylic acid anhydrides can be controlled to obtain acylphosphinates in 13-23% yield:
Acknowledgment We thank Edwin A. Catalano for obtaining the FTIR spectra. Nomenclature
However, since the P-C bonds in these products are reported to be very stablesunder both acidic and basic conditions (Kasˇpa´rek, 1968)san acylphosphinate would not be expected to act as an acylating agent in the cellulose cross-linking process. It is possible that any hypophosphite-anhydride reaction that may occur when cellulose is treated with BTCA and hypophosphite is a side reaction rather than part of the cross-linking mechanism. Higher reactivity of the anhydride with cellulose than with hypophosphite to form an acylphosphinate might minimize the latter reaction when cellulose is cross-linked with BTCA. Yang (1993b) reported that, when poly(maleic acid) was applied to cotton cellulose and heated, the product’s infrared spectrum contained anhydride bands at 18481850 and 1777-1781 cm-1. He found that the intensity of the band at 1777-1781 cm-1 decreased in the presence of hypophosphite, especially at the highest curing temperature. He attributed this decrease to catalysis of the anhydride-cellulose reaction by the hypophosphite, possibly without considering that anhydride might be consumed by reaction with hypophosphite, and concluded that “the chief role of sodium hypophosphite is possibly the acceleration of the esterification of cellulose by the anhydride intermediate.” In view of Kasˇpa´rek’s reports, and especially of our findings, this conclusion is untenable. Yang previously (1991) had obtained evidence that, even in the absence of a catalyst, succinic anhydride esterified cellulose at 100 °C and to some extent even at room temperature. Whether hypophosphite catalyzes the reaction of an anhydride with cellulose remains in question. Summary and Conclusions When heated near its melting point, BTCA formed a monoanhydride. Thermal and FT-IR spectroscopic analyses showed that a residue prepared by evaporating an aqueous solution containing equimolar amounts of BTCA and NaH2PO4 formed the same monoanhydride when heated to a lower temperature, near that commonly used in processes for cross-linking cellulose with these agents. The data indicated that the phosphate formed a complex with BTCA and remained associated with it until the anhydride formed. These observations can account for the effect of NaH2PO4 in facilitating the esterification cross-linking of cellulose by BTCA, since a cyclic anhydride is believed to be an intermediate in the esterification. Interactions in the BTCA-NaH2PO2 system upon heating were more complex. The hypophosphite in a similar residue containing these compounds apparently catalyzed formation of an anhydride from the BTCA at an even lower temperature than did the phosphate, but FT-IR data suggested that NaH2PO2 also reacted with the anhydride formed. The nature and effects of a hypophosphite-anhydride reaction in this system remain to be established, however; it should not be assumed that the reaction is part of the cellulose crosslinking process.
BTCA ) meso-1,2,3,4-butanetetracarboxylic acid DSC ) differential scanning calorimetric FT-IR ) Fourier transform infrared TG ) thermogravimetric TGA ) thermogravimetric analysis
Registry No. (The following registry numbers were supplied by the authors.) BTCA, 4534-68-3; NaH2PO4, 7558-80-7; NaH2PO4‚H2O, 10049-21-5; NaH2PO2, 7681-53-0; NaH2PO2‚H2O, 10039-56-2; Na2H2P2O7, 7758-16-9. Literature Cited Auwers, K.; Jacob, A. Stereoisomeric Butanetetracarboxylic Acids. Ber. Dtsch. Chem. Ges. 1894, 27, 1114-1132. Brown, R. O.; Tomasino, C. Catalysis of 1,2,3,4-Butanetetracarboxylic Acid in the Durable Press Finishing of Cotton Textiles. Book Pap.sInt. Conf. Exhib., AATCC 1991, 168-185. Kasˇpa´rek, F. The Reaction of Potassium Hypophosphite with Acetic Anhydride. Monatsh. Chem. 1963, 94, 809-813. Kasˇpa´rek, F. P-Acyl Hypophosphites. Z. Anorg. Allg. Chem. 1968, 362, 205-209. La¨mmermann, D. New Possibilities for Non-Formaldehyde Finishing of Cellulosic Fibres. Melliand Textilber. 1992, 73, E105E107, 274-279. Morris, C. E.; Morris, N. M.; Trask-Morrell, B. J. Variation in Solubility and Crystal Form of meso-1,2,3,4-Butanetetracarboxylic Acid. Ind. Eng. Chem. Res. 1992a, 31, 1201-1203. Morris, N. M.; Trask-Morrell, B. J.; Andrews, B. A. Kottes. Safety of Sodium Hypophosphite Catalyst Under Textile Finishing Conditions. Text. Chem. Color. 1992b, 24 (6), 27-29. Trask-Morrell, B. J.; Andrews, B. A. Kottes. Thermoanalytical Characteristics of Polycarboxylic Acids Investigated as Durable Press Agents for Cotton Textiles. J. Appl. Polym. Sci. 1991, 42, 511-521. Trask-Morrell, B. J.; Andrews, B. A. Kottes; Graves, E. E. Spectrometric Analyses of Polycarboxylic Acids. Text. Chem. Color. 1990, 22 (10), 23-27. Trask-Morrell, B. J.; Andrews, B. A. Kottes; Graves, E. E. Catalyst Effects Found in Thermal and Mass Spectrometric Analyses of Polycarboxylic Acids Used as Durable Press Reactants for Cotton. J. Appl. Polym. Sci. 1991, 43, 1717-1726. Welch, C. M. Tetracarboxylic Acids as Formaldehyde-Free Durable Press Finishing Agents, Part I: Catalyst, Additive, and Durability Studies. Text. Res. J. 1988, 58, 480-486. Welch, C. M. Formaldehyde-Free Durable-Press Finishes. Rev. Prog. Color. Relat. Top. 1992, 22, 32-41. Welch, C. M. Formaldehyde-Free DP Finishing With Polycarboxylic Acids. Am. Dyest. Rep. 1994, 83 (9), 19-20, 22, 24, 26, 132. Yang, C. Q. FT-IR Spectroscopy Study of the Ester Crosslinking Mechanism of Cotton Cellulose. Text. Res. J. 1991, 61, 433440. Yang, C. Q. Infrared Spectroscopy Studies of the Cyclic Anhydride as the Intermediate for the Ester Crosslinking of Cotton Cellulose by Polycarboxylic Acids. I. Identification of the Cyclic Anhydride Intermediate. J. Polym. Sci., Polym. Chem. 1993a, 31, 1187-1193. Yang, C. Q. Infrared Spectroscopy Studies of the Effects of the Catalyst on the Ester Cross-Linking of Cellulose by Poly(carboxylic acids). J. Appl. Polym. Sci. 1993b, 50, 2047-2053.
Received for review August 29, 1995 Accepted November 22, 1995X IE950540I
Abstract published in Advance ACS Abstracts, February 1, 1996. X
