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Crosslinking of Poly(Vinyl Alcohol) Via Bis(\Beta-Hydroxyethyl) Sulfone

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Crosslinking of Poly(Vinyl Alcohol) Via Bis(\Beta-Hydroxyethyl) Sulfone Polymer Journal, Vol. 36, No. 6, pp. 472—477 (2004) Crosslinking of Poly(vinyl alcohol) via Bis( -hydroxyethyl) Sulfone y Kazuhiro KUMETA,Ã;Ãà Ichiro NAGASHIMA,Ãà Shigetoshi MATSUI,à and Kensaku MIZOGUCHIÃ; ÃDepartment of Materials Science and Chemical Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan ÃÃNitivy Co., 300 Zenzaemon, Fujieda 426-0053, Japan (Received November 4, 2003; Accepted March 29, 2004; Published June 15, 2004) ABSTRACT: Poly(vinyl alcohol) (PVA) was crosslinked via bis( -hydroxyethyl) sulfone (BHES) at elevated tem- peratures to improve the water resistance. The best conditions for the crosslinking such as temperature, heat-treatment period, and amount of BHES or catalyst were screened out. Water resistance of the crosslinked PVA was characterized in terms of swelling and weight loss in water. The addition of BHES into PVA followed by heating at elevated temper- ature resulted in successful formation of crosslinking structure. The crosslinking occurs above 160 C and takes shorter period at high temperature than low temperature. The crosslinking catalyst, sodium carbonate, acts more effectively at 160 C than at 200 C. The results from thermal analysis using a differential scanning calorimetry revealed that the cat- alyst functions against BHES to lower the temperature for the crosslinking reaction and that even the crosslinked films have the PVA crystalline. The crosslinking procedure using BHES and sodium carbonate is applicable to form ion-ex- change fibers based on PVA with high water resistance. [DOI 10.1295/polymj.36.472] KEY WORDS Poly(vinyl alcohol) / Crosslinking / Thermal Post-treatment / Bis( -hydroxyethyl) Sulfone / PVA is a typical hydrophilic polymer and can be plied heat-treatment methods to crosslinking PVA readily blended with additives such as particles or oth- via glyoxal.3 In this case, the process requires an acid- er polymers. Therefore, PVA has potential feasibili- ic catalyst. Another case using heat-treatment to cross- ties to be matrices forming functional products with link PVA deals with water-soluble isocyanates or iso- strong hydrophilicity. However, PVA sometimes re- cyanates in an emulsion form.4 The isocyanates react quires crosslinking formation for practical applica- with hydroxyl group in PVA to form urethane bonds. tions in water since PVA is a water-soluble polymer. Our previous study used poly(acrylic acid) as a cross- Industries conventionally heat-treat PVA products linking reagent for PVA to produce crosslinking net- to enhance water-resistance. Such heat-treated PVA work via ester bonds.5 The PVA crosslinked by has only physical crosslinking based on crystalline heat-treatment exhibited strong water resistance. structure. So formed physical crosslinking may lose However, this crosslinking method is inapplicable to in hot water. When PVA is blended with other sub- PVA used as a matrix for cationic materials. stances or has insufficient degree of saponification, In this study, we investigates the crosslinking of the crystallization by the heat-treatment may fail to PVA via bis( -hydroxyethyl) sulfone (BHES) by form; PVA can be dissolved in water even at room means of thermal post-treatment. BHES is known as temperature. Chemical modifications are also em- a shape-stabilizer for cotton.6 The reaction of BHES ployed to improve the water resistance of PVA. The with the hydroxyl groups in cotton was extensively modifications involve the acetalization of the hydrox- studied by Tesoro et al.7 BHES is non-ionic, water- yl group in PVA by mono-aldehyde like formaldehyde soluble, and miscible with PVA. This reagent seems to enhance hydrophobicity and by di-aldehyde such as to have feasibilities as a crosslinking reagent in wide glutaric aldehyde or glyoxal to form intermolecular range of applications. BHES is expected to react with crosslinking. The chemical crosslinking is effective the hydroxyl group in PVA. However, there are few to improve the water resistance of PVA.1,2 However, reports regarding the reaction with PVA and its appli- the acetalization processes require heavy-duty equip- cations.8 Therefore, we conducted fundamental re- ment since the post-treatment for the crosslinking uses search on the crosslinking conditions of PVA via strong acid and volatile reagents. If the crosslinking of BHES to enhance the water resistance. Finding proper PVA by a simple heat-treatment is viable, then the conditions may result in practical use for functional processes to obtain PVA-based products with high materials such as ion-exchange fibers. water resistance become industrially valuable. How- ever, little information on such methods and their ef- fectiveness has been open to public. Ding et al. ap- yTo whom correspondence should be addressed (Tel & Fax: +81-53-478-1192, E-mail: [email protected]). 472 Crosslinking of PVA via Bis( -hydroxyethyl) Sulfone brous samples were post-treated at 200 C for 10 min. EXPERIMENTAL Characterization Materials Thermal analysis for BHES, sodium carbonate, and A completely saponified PVA (DENKA F12) was PVA was performed with a differential scanning cal- used after sufficient washing with water. The degree orimetry, DSC, (Shimadzu DSC60) at a scanning rate of saponification and the degree of polymerization of 10 K/min. were more than 99.9 mol % and 1,200, respectively. The effectiveness of crosslinking was evaluated in Bis( -hydroxyethyl) sulfone (BHES) solution in water terms of swelling and weight loss of the PVA films (47%) for film preparation and solid BHES for ther- in water at 30 and 98 C. A heat-treated film was ini- mal analysis were kindly supplied from Meisei Chem- tially weighed (wini) and immersed in water at 30 C ical Works, Ltd. Sodium carbonate, Na2CO3, obtained over night or in hot water at 98 C for 5 h, followed from Wako Pure Chemical Industries, Ltd. was used by weighing (wwet). The water was chilled to room as a crosslinking catalyst. Ion-exchange resin (IER temperature to weigh the swollen film (wwet). The film DIAION SK-116) was pulverized with a ball mill be- was again dried on a hot plate for 5 min to measure the fore use. weight (wdry). Degree of swelling, SR, and weight loss were defined as follows, respectively. The SR and Film Preparation weight loss reflect the degree of crosslinking and the Blend ratios of PVA/BHES in films were 100/0, gel fraction for crosslinked film. 99/1, 95/5, 90/10, 85/15, and 80/20. The amount SR ¼ wwet=wdry ð1Þ of catalyst added, sodium carbonate, was 0, 0.1, 0.2, and 0.4 wt % based on solid films. PVA aqueous solu- Weight loss (%) ¼ð1 À wdry=winiÞÂ100 ð2Þ tion and BHES aqueous solution were mixed to pre- Heat-treated ion-exchange fibers with and without pare 8 wt % solution of solid in water. Desired amount BHES were subjected to the above mentioned meas- of the catalyst was added into the solution if necessa- urements for swelling and weight loss in hot water ry. The solution was cast onto a glass plate to form at 98 C. Ion-exchange capacity of the fibers was also 100 mm films in solid form and dried under ambient determined before and after the immersion in the hot conditions. The resulting films were thermally treated water. We employed a general method to determine in an air oven at a fixed temperature in the range of the capacity of strong cation-exchange resins.9 Degree 120 to 200 C for a certain period up to 60 min. Fig- of swelling, SR, and weight loss of the ion-exchange ure 1 represents an expected reaction between PVA fiber were determined using the same procedure for and BHES. films mentioned above. Preparation of Ion-exchange Fiber RESULTS AND DISCUSSION The ratio of components in spinning dope, PVA/ IER/BHES/Na2CO3/H2O, was 19/19/3.6/0.07/60 Heat-treatment Conditions by weight. The spinning dope was prepared with a Figure 2 shows plots of degree of swelling, SR, and kneading machine and extruded from a nozzle into weight loss, respectively, for PVA films with BHES hot air at 130 C to obtain fibrous samples. A control (10 wt %) and without BHES as a function of heat- spinning dope solution without BHES and sodium car- treatment temperature from 120 to 200 C. The sam- bonate was also prepared by the same manner. The fi- ples were exposed to the heat for 10 min. There is no significant difference in the SR values against the heat-treatment temperature between these two films when the films were equilibrated to the water at 30 C. The SR values decrease with the temperature. Generally, the crystallinity of PVA increases with the heat-treatment temperature.10 Therefore, no differ- ence in the SR values between these two films implies that the water resistance of PVA at 30 C depends mainly on the amount of crystalline structure in the films. On the other hand, the PVA films without BHES completely dissolved in the hot water at 98 C except for a sample cured at 200 C. However, the weight loss of the film containing BHES starts Figure 1. Expected reaction between PVA and BHES. to drop between the temperatures from 140 to Polym. J., Vol. 36, No. 6, 2004 473 K. KUMETA et al. 100 100 without BHES 80 80 immersion at 98 °C 60 60 ° Heat-treatment: with BHES heat-treatment:160 C 10min 40 40 Weight loss (%) Weight Weight loss (%) Weight immersion at 30 °C 180°C 20 with BHES 20 without BHES 0 0 200°C 9.0 0204060 Heat-treatment:10 min Heat-treatment time (min) 8.0 Figure 3. Weight loss of PVA film with 10 wt % BHES at 7.0 with BHES 98 C vs. heat-treatment time. 6.0 immersion at 98 °C 5.0 on degree of the crosslinking whereas that at 30 C in- SR (g/g) volves the effect of the amount of crystalline structure.
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