Xu, Weilin
ABSTRACT
1,2,3,4-Butanetetracarboxylic acid (BTCA) is confirmed to be an effective crosslinking agent with sodium hypophosphate (SHP) catalyst for washing durability improvement of cotton fabrics treated with fluorocarbon resin. By FTIR analysis, hydroxyl groups (--OH) are confirmed as a water repellency agent, so the resin can be theoretically crosslinked with the surface of the cotton fibers. The water repellency of the sample treated with fluorocarbon resin and 8% BTCA is much higher than the sample treated only with fluorocarbon resin. This kind of difference can be seen especially after fifty washing cycles and subsequent heat treatments. ESCA analysis confirms that the F/O ratio on the fabric surface changes dramatically after fifty washing cycles and subsequent heat treatment. The F/O of the sample treated with fluorocarbon resin and 8% BTCA is almost twice that of the fabric treated with fluorocarbon resin only. Crosslinks can restrict F loss and transfer into the inner part of the fibers. At the same time, this kind of treatment can effectively improve the crease resistance of cotton fabrics.
Water repellency is an important property for some functional fabrics, and fluorocarbon resin is the most effective treating agent. To improve the washing durability of water repellency, some crosslinking agents are usually used along with the repellency agents. Crosslinking agents are usually small molecules containing several functional groups capable of reacting with some active groups in the polymer, such as hydroxyl groups in cellulose.
Traditional crosslinking agents used in cellulose are N-methylol resins or their derivatives. Some of these are prohibited by some governments because the treated fabric will emit formaldehyde during use. In 1988, Welch [4], reported that tetracarboxylic acids, 1,2,3,4-- butanetetracarboxylic acid (BTCA) in particular, are able to form effective crosslinks in cotton fabrics when salts of certain phosphorus-containing acids are used as catalysts [5, 6]. Polycarboxylic acids have been confirmed as the most promising formaldehyde-free crosslinking agents for cotton cellulose among the various new reagents investigated [1, 4-8]. It is now clear that cellulose esterfication with a polycarboxylic acid proceeds first to form a cyclic anhydride, and then to form an ester with the -OH group in the cellulose macromolecule. Polycarboxylic acids have also been used as crosslinking agents for wood pulp cellulose to improve the wet strength and dimensional stability of paper [9].
Sato [2] used some traditional crosslinking agents in the fluorocarbon resin treatment of fabrics. The water repellency of the fabrics treated with fluorocarbon resin decreases significantly with washing, but recovers with subsequent heat treatment. The reason for the decrease with washing is thought to be mostly due to the rotation of the hydrophobic fluoroalkyl groups into the polymer substrate to repel the hydrophobic washing conditions. When crosslinks are formed between the fiber surface and the water repellency film formed by the agents, they can restrain the rotation of the fluoroakyl groups into inner the part of the fibers during washing, which improves washing durability. The objectives of the work we report here are to analyze the effect of BTCA as a new kind of nonformaldehyde crosslinking agent in improving the washing durability of fabrics treated for water repellency by fluorocarbon resins.
Experimental
1,2,3,4-Butanetetracarboxylic acid (BTCA) was purchased from Aldrich Chemical Company. Sodium hypophospfite (SHP) was analytical grade. Fluorocarbon resin TG-490 was supplied by Dakin Company, Japan. Undyed 100% plain cotton fabric (142.0 g/m^sup 2^) was desized, scoured, and bleached by the supplier. The treated samples for the waterproof test were 25 X 25 cm.
BTCA and SHP concentrations are expressed according to the weight of the agent in the water solution. The fabric was treated in the solution comprising 8% fluorocarbon resin and different concentrations of BTCA and SHP, giving a range of concentrations in the treatment solutions. The fabric was then passed through squeeze rolls, again wet with treating solution, an again passed through squeeze rolls to give a specified wet pickup (approximately 80%). The fabric was predried at 85 deg C for 10 minutes and cured in a second oven for 2 minutes at 180 deg C.
A Nicolet FTIR 20 SXB was used to analyze the spectrum of the agent. Resolution for the infrared spectra was 4 cm^sup -1^, and there were thirty-two scans for each spectrum. Standard methods were used to measure the conditioned wrinkle recovery angle (ASTM-1295-67); the WRA of the control sample was 124 deg (w + f). Water repellency of the samples was evaluated according to a spray test method (JIS L-1092 5.2): 250 ml water was sprayed on the fabric fitted on a 20 cm diameter circular frame inclined 45 deg to the horizontal. In order to quantify water repellency changes in the samples, water repellency was evaluated according to the water weight gain (WWG) of a water absorbent paper, which was located directly under the test specimens. The paper had excellent water absorbing properties: when water was transmitted by the fabric, the paper absorbed it very quickly. The higher the WWG of the paper, the poorer the water repellency of the specimen.
Results and Discussion
HYDROXYL GROUP CONFIRMATION IN FLUOROCARBON RESIN
FTIR analysis of the fluorocarbon resin revealed whether there were hydroxyl groups in the fluorocarbon resin used for the water repellency treatment. Before analysis, the fluorocarbon resin was dried in an oven under low pressure at 50 deg C for 3 days, placed over CaCl^sub 2^ for one week, then quickly analyzed by FTIR The results are shown in Figure 1. The absorbing intensity around 3300 cm^sup -1^ is very strong, indicating the presence of active hydroxyl and other similar groups in the agent. Due to the existence of these hydroxyls in the agent, the hydroxyl groups can be crosslinked by BTCA with the hydroxyl in the fibers and can also form effective crosslinks between the fiber surface and the repellency film formed by the fluorocarbon resin during heat treatment.
EFFECT OF BTCA ON WASHING DURABILITY
Water repellency of the samples treated by the fluorocarbon resin and different concentrations of BTCA and SHP are expressed by the water weight gain (WWA) of the water absorbing paper. The results are shown in Table I. There is almost no effect of BTCA on the water repellency before washing; all the WWG values of the water absorbent paper are around 0.4 g, indicating that all the fabrics have excellent water repellency properties. However, even after one washing (according to the AATCC washing standard) and air drying, the water repellency expressed by WWG shows a great difference due to the effect of the crosslinking agent. For example, the wwc of fabric treated without BTCA is 16.5 g, but the wwG of fabric of treated with 8% BTCA is only 4.7 g. When the samples are heat treated at 160 deg C after washing, the water repellency of the samples treated with BTCA in the solution are better than that of the fabric treated without BTCA. After the fabric is washed fifteen times (Table II) and then heat treated at 160 deg C, the WWG for the sample treated with 8% fluorocarbon resin only (fabric 1) is 13.6 g, but for fabric 7 treated with 8% BTCA and 4% SHP, the tested WWG is only 4.4 g. This effect is more evident as the washing times increase. When the samples are washed fifty times and then heat treated, the control sample (fabric 1) shows a WWG of around 20.3 g, but sample 7 shows a WWG of around 4.8 g, indicating that the water repellency is greatly improved when the crosslinking agent is applied together with the water repellency agent.
WRA IMPROVEMENT OF TREATED FABRIC
Since BTCA and the catalyst can also penetrate into the fibers, crosslinking can take place, thereby improving the crease recovery of the fabric. Data for fabrics treated with different concentrations of BTCA are shown in Table III. The WRA values of the fabrics treated at low concentrations of BTCA increase very slowly, which is probably due to some crosslinks being formed in the repellency film and between the film and the fiber surface. As washing times increase, the WRA values of the treated fabrics decrease to some degree.
ESCA ANALYSIS
In order to analyze the surface chemical composition, we made ESCA measurements (Table IV). The results show that N content is almost zero, and the main component elements are F and C, with small amounts of O. More F in the fabric surface indicates better water repellency, and more O in the surface indicates a hydrophilic character of the fabric. So the ratio of F to O (F/O) can be used to express the water repellency of the fabric. From Table IV, we see that before washing, the F/O for the sample treated along with 8% BTCA (sample 7) is only slightly higher than that of sample 1, treated without BTCA, in agreement with the water repellency results in Table I. When the fabric had been washed fifty times and air dried, the F/O for the fabric treated with BTCA (sample 7) is only a little higher than that of sample 1, but after a 160 deg C X 3 min treatment, the F/O for sample 7 is much higher than that for sample 1. Although consistent with water repellency data, there is no linear relation between F/O and water repellency values. A slight increase in F/O will lead to large increase in water repellency. After adding BTCA and the catalyst to the fluorocarbon resin solution, a water repellency film formed during the heat treatment is closely attached onto the fiber surface. It has been suggested that decreased water repellency is mostly due to mechanical action when the fiber is rinsed in water, inducing the water repellency film to loosen from the fiber surface [2, 3]. At the same time, during washing, F can also diffuse into the inner part of the fiber, decreasing surface water repellency. However, apparently this kind of transfer can be reversed during the high temperature treatment (160 deg C x 3 min) (see Table IV). Crosslinks formed by BTCA can restrict F diffusion and transfer into the fiber, improving the film strength and adhesion to the fiber surface.
Conclusions
We have evaluated the effect of polycarboxylic acid (BTCA) crosslinking on the washing durability of water repellent fabrics treated by fluorocarbon resin. We have appraised the water repellency of the treated samples according to the water weight gain (WWG) of water absorbing paper located under the treated samples during water repellency testing. Washing durability of water repellency can be greatly improved when the fabric is treated with 8% fluorocarbon resin in the presence of a certain quantity of BTCA and SHP. By ESCA analysis, the F/O in the surface of the samples treated with 8% BTCA and the catalyst is almost twice that of the samples treated only with fluorocarbon resin. This indicates a crosslinking agent in the treatment can crosslink the water repellency film onto the fiber surface effectively and restrict F diffusion and transfer into the inner part of the fiber.
Literature Cited
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Manuscript received August 29, 2000; accepted December 12, 2000.
WEILIN XU
Wuhan Institute of Science & Technology, Wuhan, 430073, People's Republic of China
TIENWEI SHYR
Department of Textile Engineering, Feng Chia University, Taichung, Taiwan
