Performance of Durable Press Finish on Cotton with Modifi ed DMDHEU, Citric Acid, BTCA and Maleic Acid

Cotton fabrics possess the inherent property to form wrinkles under stress. To overcome this problem, conventionally, selective cross-linking agents are applied via the pad-dry-cure method at a high curing temperature. Research carried out in this fi eld has identifi ed invariable deterioration in the mechanical properties of fi nished cotton fabric. In this study, four diff erent cross-linking agents, i.e. modifi ed dimethylol dihydroxy ethylene urea (DMDHEU), citric acid, 1,2,3,4 butanetetracarboxylic acid (BTCA) and maleic acid, were applied on cotton fabric through selection of combinations, using the Box-Behnken experimental design. It was established that DMDHEU shows the best improvement in the crease recovery angle along with the highest durable press (DP) rating with a poor retention of strength. The citric acid shows the average strength retention as well as an acceptable improvement in crease recovery. BTCA shows the best strength retention, but the poorest crease recovery. The maleic acid also shows an average strength retention with a crease recovery superior to that with BTCA.

Tekstilec, 2018, 61 (4), [289][290][291][292][293][294][295][296][297] Performance of Durable Press Finish on Cotton with Modifi ed DMDHEU, Citric Acid, BTCA and Maleic Acid into desirable wrinkles, i.e. for the smartness of a fabric, and undesirable wrinkles, which need to be prevented [3]. Th e wrinkling behaviour of cellulosic fabrics may be directly linked to the mobility of free hydroxyl groups in the polymer chain, and the frequent breaking and reforming of hydrogen bonds [4,5]. As the hydroxyl groups in the amorphous region are far apart and since hydrogen bonds operate at short distances, these hydroxyl groups remain unbound. When a cloth is folded and pressed, some of the hydrogen bonds at the boundary of crystalline and amorphous regions break (hydrogen bonding forces are fairly weak). Simultaneously, free hydroxyl groups in the amorphous region approach other free hydroxyl groups and when they are suffi ciently close to each other, they bind. Th ese newly formed hydrogen bonds bind the molecules and prevent unfolding; thus, forming crease marks [6]. Th e creasing behaviour of a cotton fabric can be reduced by reducing or totally masking the hydrogen bond formation capacity of hydroxyl groups. Th is is generally done by imparting the creaseresist fi nish in which the hydroxyl groups of adjacent macromolecules react with multi-functional crosslinking agents as shown in Figure 1 [7,8].  [8] Th e mechanical and certain other properties of cellulosic fi bres are aff ected by cross-linking, which is refl ected in the performance of fabrics. Th e stability of fi bres in any assembly is a consequence of the combination of internal hydrogen bonding and Van der Waals forces [9]. Th e introduction of covalent cross-links into cellulosic fi bres has two important eff ects, i.e. i) it reduces the mobility of chain molecules to move laterally [10], and ii) it extends longitudinally under stress [10,11]. Studies have shown that the unavoidable side eff ect of cross-linking fi nishes is a reduction in elasticity and fl exibility of cellulosic fi bres, next to a considerable decrease in abrasion resistance, tear and tensile strength [12]. A rule of thumb states that an increase in the wrinkle recovery angle of 10° corresponds to the loss in abrasion and tear strength of about 7% [13]. Th is can be illustrated by considering an imaginary fi bre with no lateral forces between chains. It would be mechanically weak, since when under stress, chain molecules would slide past one another. Th e introduction of cross-links would increase the load required to break the fi bre due to the increased inter-chain cohesion. However, aft er a certain limit, the introduction of more cross-links would restrict the movement of chains to such an extent that the proportion of chains capable of resisting the applied stress would decrease and the tensile strength would fall. Cotton develops a number of H-bonding during use, which in turn increases its strength. Th e crosslinking of anti-crease reagents with cellulose replaces a number of H-bonds and imparts stiff ness, hence reducing the fi bre chain movement on loading [9,14,15]. Th e application of statistical tools has been of paramount importance for a thorough study of the process with a minimum bias. Th ese tools help evaluate a process more comprehensibly with a remarkably smaller number of samples. Th e Box-Behnken design is one of such statistical tools frequently used. Th e Box-Behnken designs usually have fewer design points than the central composite designs and are thus less expensive to run with the same number of factors. For six factors, Box-Behnken requires 54 observations and this is the minimum of the central composite design. In contrast, the central composite designs can fi t a full quadratic model. Th ey are oft en used when the design plan calls for sequential experimentation, as these designs can include information from a correctly planned factorial experiment. Unlike the central composite designs, the Box-Behnken designs never include runs where all factors are at their extreme setting, such as all of the low settings. Th erefore, if the experimental region is such that extreme points are a problem, then there are some advantages to Box-Behnken. Otherwise, they both work well.

Performance of Durable Press Finish on Cotton with
Modifi ed DMDHEU, Citric Acid, BTCA and Maleic Acid Th e Box-Behnken designs can effi ciently estimate the fi rst-and second-order coeffi cients; however, they cannot include the runs from a factorial experiment. Th e Box-Behnken designs always have 3 levels per factor, unlike the central composite designs, which can have up to 5. In this study, a pre-treated cotton fabric was fi nished with four diff erent crease-resist fi nishes, i.e. modifi ed DMDHEU, citric acid, BTCA and maleic acid, via the Box-Behnken design. Th ese four reagents were selected for the study mainly due to the extensive research on them, showing their promising capability to impart an anti-crease fi nish on cotton compared to that with other chemicals. DMDHEU being carcinogenic must be eloped from the industries with suitable alternate reagents. A durable press rating as well as the mechanical properties of fi nished cotton was compared to assess the performance of each fi nishing process.

Finishing of cotton fabric
Th e anti-crease fi nish liquor, prepared with desired factors (parameters) and levels using the Box-Behnken experimental design as mentioned in Tables 1-4, was imparted to cotton on a laboratory padder with 70-80% expression (owf) with modifi ed DMDHEU, citric acid, BTCA and maleic acid systems. All fi nished fabrics were dried at 80 °C and cured under stretched conditions at varying curing temperatures and times according to the design. Magnesium chloride, trisodium citrate and sodium hypophosphite were used as catalysts for respective fi nishes as detailed in Tables 1-4. Th e fi nished samples were passed through fi ve cyclic washing and drying processes before the testing for the DP rating.

Evaluation
Th e fi nished cotton was evaluated for its DP (durable press)/smoothness appearance rating, using the AATCC DP replica (AATCC test 124:2006). In the test method, a platform with fi ve DP fi nished cotton replicas are available side by side with fi ve diff erent types of durable press ratings from 1-5, 1 being the poorest and 5 the best DP performance with no crease marks. Th e fi nished samples were compared visually with these standards to assess the grade. Th e fi nished cotton with the best DP was further evaluated for the total crease recovery angle (TCRA, AATCC Test 66-2003), tensile strength (ASTM D5035) and tear strength (ASTM D 2261).

Experimental design
Th e Box-Behnken factorial design with six factors and three levels (3 6 ) was selected for this study. Th e six factors were the concentrations of reagent, catalyst, silicone soft ener, polyethylene emulsion along with the time and temperature of curing. Th e three levels were chosen based on the industrial application parameters with the central point at 0 and equidistant two levels on each side of 0. Th e Box-Behnken factorial 3 6 research statistical design resulting in 54 runs with six replicates at the central point was used to evaluate the functional characteristics of anti-crease fi nishes. Various parameters were included in the research design and their actual levels as mentioned in Tables 1-4.    3 Results and discussion

Durable press ratings of fi nished cotton
Cotton fi nished with a set of parameters according to the respective experimental design was evaluated for its DP rating. Th e DP rating of the unfi nished fabric was found to be as poor as 1.5. Fabrics fi nished with anti-crease fi nishes showed an improvement in the DP rating. However, some sets of parameters lagged in either optimum temperature, or the concentration of catalyst or reactant, and could not produce the desired DP ratings. For further discussions, all the sets of runs with DP rating ≥ 3.5 were selected. Tables 5-8 highlight the set of parameters showing DP ratings equal to 3.5 or higher.  13  60  20  20  25  160  5  3.5  14  60  20  15  20  160  4  4  15  60  20  10  25  160  3  3.6  16  60  20  20  15  160  3  4  17  60  20  15  20  160  4   Th e samples with a higher concentration of the fi nish and higher temperature requirements are not suitable from the industrial viewpoint and cost. Moreover, a higher temperature and a higher concentration of the fi nish may lead to a greater degradation of cellulose. Since the runs with lower energy and chemical requirement also show good DP ratings, the samples prepared with more energy and chemical requirement were eliminated for further study. Th e combinations requiring less energy and a lower concentration of the fi nish were hence evaluated for a further test to evaluate the total crease recovery angle, tensile strength and tear strength. Th e test values for the unfi nished cotton (control) were thus: tensile strength 580 N, tear strength 10.4 N, total crease recovery angle 132° and DP rating 1.5 (Tables 9-12).   1  50  50  0  5  170  2  4  2  50  50  20  5  170  4  3.8  3  50  50  20  5  170  2  3.8  4  50  50  10  10  180  3  3.5  5  60  40  20  5  160  3  3.5  6  60  50  0  10  170  4  3.5  7  60  50  20  0  170  4  3.7  8  60  50  10  5  170  3  3.5  9  60  50  10  5  170  3  3.5  10  60  40  20  5  180  3  3.75  11  70  50  0  5  170  2  3.6  12  70  50  20  5  170  3  3.5 Tekstilec, 2018, 61(4), 289-297      Th is selection was based mainly on the improvement in the crease recovery angle. Further, it was considered that the selected combination should not show a severe loss in the strength. Consequently, a compromise for crease recovery was conducted in the case of some fi nishes to ensure acceptable physical properties.

Conclusion
Th e fi nal selection shows that DMDHEU showed the best improvement in the crease recovery angle along with the highest DP rating, whereas the strength retention was poor in the case of DMDHEU. BTCA showed the best strength retention but the poorest crease recovery. Th e maleic acid also showed average strength retention with crease recovery superior to BTCA. Th e citric acid showed the average strength retention as well as an acceptable improvement in crease recovery. To conclude, the best combinations of parameters for diff erent alternative fi nishes have been suggested in this work and the physical characteristics of fi nished cotton presented.