RESEARCH
Built-in catalysts yield improved cottons Tertiary amines catalyze reactions between cellulose and epoxides to give cottons having high dry- and wet-crease properties Chemical reactions of cellulose have come into sharper focus as a result of efforts to improve the physical properties of cotton fabrics having permanent-press quality. In their work aimed at modifying cotton with epoxides, U.S. Department of Agriculture chemists Donald M. Soignet, Ruth R. Benerito, John B. McKelvey, and physicist Truman L. Ward have found that tertiary nitrogen groups, when added to cellulose, act as "built-in" catalysts for further chemical modification of cottons. This work is being done at USDA's Southern Utilization Research and Development Division (SURDD) in New Orleans, La. The use of amines (particularly tertiary amines known to cure epoxide resins) as catalysts for cellulose-epoxide reactions has been studied by chemists in other laboratories, such as those at Rohm and Haas, Union Carbide, and Shell Chemical. However, these studies did not lead to the addition of many epoxides to cotton because the amines did not catalyze the epoxide cellulose reaction. According to Mr. Soignet, many epoxides which do not react with cotton using external base catalysts do react with cotton containing built-in basic groups. Some epoxides which react with cotton impart different properties to fabrics when the terminal tertiary nitrogen group (catalyst) is within the cellulose matrix than when it is simply an external catalyst. Major effort at SURDD has been with epichlorohydrin because of its relatively low cost, compared to that of other epoxides, and because of the extensive work done there on mechanisms of epichlorohydrinamine reactions. Details of these reactions had been unclear and controversial because the sequence of reactivity of the chlorine atom and the oxirane group of epichlorohydrin had not been known previously. Physicochemical evidence indicates that the tertiary amine initially attacks the oxirane ring, which releases the chlorine only after an induction period. Selection of a cotton fabric containing tertiary amine groups as built-in catalysts for cellulose-epoxide reactions was the natural outgrowth of these fundamental studies. Now, the material used in this cellulose work is di52 C&EN APRIL 3, 1967
ethylaminoethylcellulose (DEAE cotton) . This substance is well known to textile chemists and is often used in ion exchange fractionations. It is made by adding /J-chloroethyldiethylamine hydrochloride to cotton in sodium hydroxide. Repeated treatments raise the nitrogen content of the cotton fabric well above 1%. The tertiary nitrogens of DEAE cotton are weak-base anion exchangers, Mr. Soignet says. Their titration with hydrochloric acid gives curves similar to those which result if a weak base such as ammonium hydroxide is titrated. Only after quaternization of the tertiary amino group does its titration curve appear similar to that obtained when a strong base such as sodium hydroxide is titrated. With some epoxides, DEAE cotton apparently forms a quaternary salt. Mr. Soignet suggests that the hydroxy! ion can catalyze the opening of the oxirane ring. Alternatively, a proton originally attached to the nitrogen in the DEAE cotton may move to the epoxide to form an intermediate containing a protonated oxirane group. Two types of reactions of DEAE cotton with monoepoxides are possible. The intermediate containing a pro-
tonated oxirane group can react to form quaternary groups in the cellulose. It also can react to give a linear graft with the DEAE cotton to yield a product in which tertiary amino groups are not quaternized. Statistically, the tertiary amine is twice as likely to form. The extent to which tertiary amino groups convert to quaternary groups in this reaction is found by titration with hydrochloric acid and from the exchange capacity of the DEAE cotton. Diepoxides can react with cellulosic hydroxyls of DEAE cottons to crosslink the cellulose chains, react with amino groups to cross-link chains, or react with a combination of both amino and cellulosic hydroxyl groups, Mr. Soignet says. Evidence for the reaction of epoxides with tertiary amino groups to form quaternary groups is obtained from the potentiometric titration curves of the finished fabrics. Quaternary nitrogen groups titrate as strong-base anion exchangers; DEAE-cellulose itself titrates as a weak-base anion exchanger. With difunctional reagents, crosslinking of the cellulose chains changes the properties of the cotton fabric. Mr. Soignet can estimate the degree of
CROSS-LINKAGES. Donald M. Soignet, Truman L. Ward, Dr. J. B. McKelvey, and Dr. Ruth R. Benerito (left to right) propose a structural model for DEAE cottons which has three types of cross-linkages of DEAE-cellulose with diepoxides
cross-linking by measuring improvements in crease-recovery angles, and by electron microscopy of ultrathin cross section of finished fibers. From a practical standpoint, epichlorohydrin is especially valuable because the treated fabric has both high dry- and wet-crease resistance. Other bases, such as caustic soda, usually result in etherified cottons which have high wet-crease resistance, but show little improvement in dry-crease resistance. Previous work at SURDD to react epichlorohydrin and cotton with external caustic soda resulted in fabrics with only high wet-crease resistance. Some epoxides graft onto cellulose at the sites of the tertiary amino groups in DEAE cotton. These compounds include epichlorohydrin, vinyl cyclohexene diepoxide, and butylene oxide. Some compounds, such as phenyl glycidyl ether and styrene oxide, polymerize; they are catalyzed by the tertiary amino groups. Those compounds which polymerize, but do not cross-link, increase the flex abrasion resistance of the treated cotton fabric by as much as 20 times that of untreated fabric. Whether these polymers form inside or outside the fiber is unknown. Polymers such as acrylates, silicones, and polyethylenes now used to add abrasion resistance are not covalently bound to cellulose. The epoxide reactions with DEAE cottons have also been studied at SURDD with both basic and acidic external catalysts. As expected, various epoxides add significantly to the DEAE cottons in 8% sodium hydroxide, but titration curves show that no quaternization of the amino groups occurs. With a zinc tetrafluoroborate catalyst, the fabric weight gains because the epoxide additions (except for butadiene diepoxide) to DEAE cottons are much below those of untreated cottons. A lack of strong anion exchange characteristics shows no quaternary nitrogen groups. These studies at SURDD revealed that butadiene diepoxide imparts wetand dry-crease resistance to cotton fabrics under more varied conditions than do other common mono- and diepoxides. Without an external base catalyst, butadiene diepoxide will react with DEAE cotton to give wet- and dry-crease resistance as high as that given to unmodified cotton with an external base catalyst. With a zinc tetrafluoroborate catalyst, butadiene diepoxide gives somewhat greater improved wet- and drycrease resistance to unmodified cotton than it does to DEAE cotton. Flex abrasion resistance of these kinds of modified cottons increases, but not nearly as much as it does with poly-
SURDD chemists modify cotton with epoxides Monoepoxides form DEAE cellulose intermediates
A
Cell-OOH 1 CH^N : H "bH + R.CHCH2C2H5
