By: Chen, Hung-En, Chen, Jui-Chin, Yao, Wei-Hua, Chen, Cheng-Chi
Textile Research Journal, Jun 2006

Abstract

The steeped procedure was used to investigate the physical properties and crosslinking structure. It was found that the values of the N content, dry crease recovery angle (DCRA), and wet crease recovery angle (WCRA) of the treated fabrics with any specific steeped procedure were higher than those of normally treated fabrics. For a given number of crosslinks per anhydroglucose unit (CL/AGU) the crosslinking length (CL length) values for the steep-pad-dry-cure process were higher than those for pad-dry-cure process. All the DCRA and WCRA values for the steep-pad-dry-cure process were higher than those for pad-dry-cure process at a given resin concentration, tensile strength retention (TSR), and CL/ AGU. The surface distribution of crosslinking agent on the finished fabrics for pad-dry-cure process was higher than that for the steep-pad-dry-cure process. The expansion degree of the steep-pad-dry-cure treated fiber is slightly higher than that of the pad-dry-cure process. Those results obtained from pictures of SEM, crosslinking structure, and surface distribution of fibers/ fabrics treated by the two processes clearly show that the crosslinking agents could diffuse and self-condensate in the treated fabrics to obtain an inner and longer crosslink under the steeped procedure so as to have good DCRA and WCRA.

Key words crosslinking, distribution, steeped process, CL length, CL/AGU

A previous report by the authors [1] showed that some physical properties were improved by the polymerization of crosslinking agents in the treated fabrics. Franklin et al. [2] revealed that the use of dimethyloldihydroxyethyleneurea and acrylic acid (DMDHEU/AA) under a redox catalyst system could improve some physical properties of the finished cotton fabrics. They suggested that the polymerization occurred between the yV-methylol compound and acrylic acid during the curing process. Some previous studies [3-5] pointed out that the changes to the finishing process, swelling state of the crosslinked fibers, and crosslinking agents could affect the crease recovery properties of the crosslinked fabrics.

The authors have found it interesting that the dry and wet crease recovery properties of the treated fabrics are increased with the use of a steeped procedure before a traditional pad-dry-cure process for a longer time period and higher temperature. The detailed information about agent distribution, crosslinking structures, and physical properties of the treated fabrics under a steeped procedure are of great interest to them. They have examined the crosslinking of cotton fabrics by using the steep-pad-dry-cure and pad-dry-cure processes with respect to the measurement of nitrogen and formaldehyde content, distribution of crosslinks per anhydroglucose unit (CL/AGU), length of crosslinks, some physical properties, and the surface distribution of the crosslinking agent of the treated fabrics.

Experimental

In this study, desized, scoured, and bleached cotton fabric 20s 20s end (60) and picks (60) were used. The crosslinking agents used were dimethyldimethyloldihydroxy-ethyleneurea (DMDMDHEU) and acrylic acid (AA). Acrylic acid, hydrogen peroxide, and other chemicals were all reagent grade.

The fabric samples were steeped with freshly prepared mixtures of DMDMDHEU (2, 4, 6, and 8% w/w) and acrylic acid at mole ratio of 1 : 1 in the presence of hydrogen peroxide [H^sub 2^O^sub 2^ initiator used were 0.017,0.033,0.050, and 0.067% H^sub 2^O^sub 2^ (35% v/v), separately] under a N^sub 2^ environment at 70C for 60 minutes. The ammonium sulfate catalyst (0.1 times the used amounts of DMDMDHEU) was added to the solutions and stirred for about 5 minutes. The fabric samples were padded twice at approximately 100% wet pickup. Padded fabrics were dried at 80C for 5 minutes, cured at 160C for 3 minutes, soaped, washed, and dried.


Formaldehyde and nitrogen determinations were made using chromatropic acid [6] and Kjeldahl methods, respectively. The tensile strength of the warp yarns was measured on an Instron tensile tester of KAO-TIEH, AT-7010 Dl (Machinery Industrial Co. LTD., Taichung, Taiwan). ASTM standard D 1295-67 was used to determine the dry and wet crease recovery angles.

Treated fibers were brought to the boil in a 50% (by volume) aqueous solution of methanol containing 0.5% wetting agent (Triton X-100) for 1 hour and allowed to cool and soak overnight in this wetting solution. The wet fibers were laid on a glass slide and covered immediately with nitrocellulose, which had been dissolved in acetone. The cross-sections of the treated fibers were observed by scanning electron microscopy.

For the investigation of agent distribution on the treated fabrics, the various treated fabric samples were dyed with C. I. Direct Red 81 at 80C for 50 minutes (dye concentration: 0.1 g/1, NaCl: 10 g/1, liquor ratio equals to 1 : 60). The KIS values (depth of color of the various treated fabrics) were measured at a CS-5 Chroma-Sensor (Applied Color System, U.S.A.). The [D]^sub f^ values (g dye/kg cotton) were determined spectrophotometrically with an U-3010 spectrophotometer (Hitachi, Japan) after extraction of the dyed fabrics with pyridine/water (1/3) at 60C.

Results and Discussion

The dry crease recovery angle (DCRA), wet crease recovery angle (WCRA), and tensile strength retention (TSR) of the finished fabrics under different resin concentrations with pad-dry-cure and steep-pad-dry-cure processes are listed in Table 1. The data in Table 1 shows that the values of DCRA and WCRA of the treated fabrics with specific steeped procedure were higher than those of normal treated fabrics and that the DCRA and WCRA values of the treated fabrics gradually increased with the increasing amount of resin concentration used. The TSR values decreased in all cases. Table 1 also shows the values of DCRA, WCRA, and TSR of the treated fabrics for different resin concentrations in the bath. For a given resin concentration all the DCRA, WCRA, and TSR values for the steep-pad-dry-cure process were higher than those for the pad-dry-cure process.

The relationships between DCRA, WCRA, and TSR of the finished fabrics for the two processes are shown in Figures Ia, b, and c, separately. From the relationships between DCRA and WCRA of the finished fabrics (Figure Ia), it can be seen that the WCRA values of the treated fabrics for the steep-pad-dry-cure process were higher than those for the pad-dry-cure process at a given DCRA. Figures Ib and c show the plots of DCRA and WCRA versus TSR of the finished fabrics separately. For a given value of TSR, all the DCRA and WCRA values of the treated fabrics for the steep-pad-dry-cure process were higher than those for the pad-dry-cure process. The higher DCRA and WCRA values for the steep-pad-dry-cure process may be caused by the different amounts of resin-bonded, crosslinking agent distribution, and crosslinking structure on/in the treated fabrics.

The nitrogen and formaldehyde contents of the cotton fabric crosslinked with varying resin concentrations for the steep-pad-dry-cure process and the pad-dry-cure process are presented in Table 2. As expected, the values of nitrogen and formaldehyde content showed a gradual increase with increasing resin content in the bath in all cases. The nitrogen content of the finished samples for the steep-pad-dry-cure process were slightly higher than those for the pad-drycure process at a given resin concentration in the pad bath, but the formaldehyde contents were opposite (shown in (a) and (b) of Figure 2 separately). The results for nitrogen and formaldehyde contents at a same resin concentration support the hypothesis that the higher DCRA and WCRA of the steep-pad-dry-cure treated fabrics were not mainly caused by the amounts of resin bonded.

The number of crosslinks per anhydroglucose unit (CL/ AGU), length of crosslinks (CL length), and the mole/ AGU of nitrogen and formaldehyde of the finished fabrics listed in Table 2, which were obtained using the methods of Frick et al. [7, 8], indicate that both increased as the concentration of the resin in the bath increased for both processes. The curvilinear relationship between the length of crosslinks and CL/AGU of the treated fabric samples for all the crosslinking agents (Figure 3) was similar to that reported in the authors previous study [9], For a given number of CL/AGU the CL length values for the steep-pad-dry-cure process were higher than those for the pad-dry-cure process. The authors suggest that the longer CL length for the steep-pad-dry-cure process was caused by the higher self-condensation of the crosslinking agents in a steeped bath.


Figures 4a, b, and c, respectively, reveal the relationships between the values of DCRA, WCRA, and TSR and the values of CL/AGU of the various treated fabrics, which show that the values of DCRA and WCRA for the steep-pad-dry-cure process were significantly higher than those for the pad-dry-cure process; however, the values of TSR for the steep-pad-dry-cure process were somewhat lower than those for the pad-dry-cure process at the same value of CL/AGU. The higher DCRA and WCRA values for the steep-pad-dry-cure process may be attributed to the beneficial distribution of crosslinking agents under the steeped procedure. The slightly lower TSR may be caused by the degradation of cellulose molecules in the presence of hydrogen peroxide at a relative higher temperature and for a longer time period during the steeped procedure.

The KIS and [D]^sub f^ values of the fabrics treated with the steep-pad-dry-cure process and pad-dry-cure process are also listed in Table 1, and are plotted in Figure 5 according to the method described by Rowland et al. [10,11], and the linear relationships are similar to their results [11]. K is the coefficient of absorption, S is the coefficient of scattering, K/S is the color intensity calculated from the equation K/S = (K/S)^sub dye^ - (K/S)^sub white^, and [D]^sub f^ (g dye/kg cotton) is the dye content of the finished fabric, determined spectrophotometrically after extraction with pyridine/water (1/3) at 60C. Color intensity is an inverse measure of crosslinking on fabric surfaces and the dye content is an inverse measure of total crosslinking content through fabric thickness. The K/S results are average for the two sides of the fabric samples.

A negative staining condition agent distribution follows the rule that the lower the value of K/S at a specific level of dye content and the lower the dye fixation on the surface of the fabric, the higher the concentration of crosslinks on the surface of the fabric and the greater the agent distribution on the fabric. Figure 5 reveals lower values of logA75 for the pad-dry-cure-treated fabrics at the same value of log [D]f, i.e., the surface distribution of crosslinking agent on the finished fabrics for the pad-dry-cure process was slightly higher than that for the steep-pad-dry-cure process. The lower surface distribution of crosslinking agents for the steep-pad-dry-cure process may have been caused by the higher swelling of cotton cellulose fibers during the steeped procedure that could improve the penetration of crosslinking agents in the fibers. On the other hand, the improved WCRA (the WCRA value for the steep-pad-dry-cure process minus the WCRA value for the pad-dry-cure process) percentages were higher than those of DCRA at a given resin concentration calculated from Table 1. This result agrees with the result of Figure 5, namely the inner distribution of the crosslinking agent of the treated fabric with the steep-pad-dry-cure process.

The expansion patterns (SEM) of the cross-section of the 4% resin treated with the steep-pad-dry-cure process and the pad-dry-cure process are shown in Figures 6a and b, separately. These figures show that the expansion degree of the steep-pad-dry-cure treated fiber was slightly higher than that of the pad-dry-cure process-treated fiber. This result agrees with the observation that the values of crosslinking length for the steep-pad-dry-cure process were higher than those for the pad-dry-cure process (Figure 3).

The results obtained from pictures of SEM, crosslinking structure, and agent surface distribution of the steep-paddry-cure process and the pad-dry-cure process-treated fibers/fabrics clearly show that the crosslinking agents could self-condensate and inner crosslink under the steeped procedure and thereby yield good DCRA and WCRA.

Conclusions

In this study, the authors used the steeped procedure to investigate the physical properties and crosslinking structure of treated cotton fabric. It was found that the values of N content, DCRA, and WCRA of the treated fabrics with any specific steeped procedure were higher than those of normal treated fabrics. At a given resin concentration, all the DCRA, WCRA, and TSR values for the steep-pad-dry-cure process were higher than those for the pad-dry-cure process. For a given value of TSR all the DCRA and WCRA values of the treated fabrics for the steep-pad-dry-cure process were higher than those for the pad-dry-cure process. The CL length values for the steep-pad-dry-cure process were higher than those for the pad-dry-cure process for a given number of CL/AGU. The values of DCRA and WCRA for the steep-pad-dry-cure process were higher than those for the pad-dry-cure process. However, the values of TSR for the steep-pad-dry-cure process were somewhat lower than those for the pad-dry-cure process at the same value of CL/AGU. The surface distribution of crosslinking agent on the finished fabrics for the pad-dry-cure process was slightly higher than that for the steep-pad-dry-cure process. The expansion degree of the steep-pad-dry-cure-treated fiber was slightly higher than that of the pad-dry-cure process-treated fiber.


Literature Cited :

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6 Sello, S. B., and Lewin, M., "Handbook of Fiber Science and Technology: Volume I Chemical Processing of Fibers and Fabrics Part B," Ch. 2, Marcel Dekker, Inc., New York, 1984.

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