Silk fabrics have low wet and dry resiliency. Hence the fabrics wrinkle easily during home laundering or when wet ^{(1, 2)}. To improve these performance properties, silk fabrics are given chemical treatment known as durable press finishing. Durable press chemical finishes applied to silk fabrics in the presence of appropriate catalyst impart wrinkle resistance and smooth drying properties ^{(3, 4)}.

The release of formaldehyde vapours is another problem with those agents. The most likely used cross linking agents in crease resistant finishes have been N- Methylol agents or N- Methyl amides because of their efficiency and low price. Formaldehyde free cross linking agents for producing crease resistant properties are of interest to replace N-methylol compounds for crease resistant finishes ^{(5, 6)}. Polycarboxylic acids which are non formaldehyde reactants are possible replacement for conventional finishing reactants. The main advantage of polycaboxylic acids is that they are formaldehyde free, do not have bad odour, and produce very soft fabric hand ^{(7, 8)}.

Based on the above premise, an attempt has been made to try and assess the effect of polycarboxylic acids on silk with respect to its crease resistance behavior.

Materials and methods

Materials and their specification

Fabrics

Two different kinds of silk fabrics, namely, Mulberry and Tassar Silk were used for the experiment.

Chemicals used

1. Cross linking agents - Glyoxal, Citric acid and DMDHEU

2. Catalysts used - Aluminum Sulphate, Magnesium Chloride & Sodium Hypophosphate

3. Softener

Methods

Degumming of silk

Silk fabrics were first degummed using soap (8 gpl) for two hours at a temperature of 90º C after which they were washed and dried. They were then treated with the cross linking agents as explained below

Application of Glyoxal

Both the fabrics were treated with 3 different concentrations of glyoxal, viz, 5%, 10% & 15% (owf)

Recipe for padding bath is as follows:

# Glyoxal 5%, 10%, 15% (owf)
# Aluminium Sulphate 3% (owf)
# Softener 2% (owf)
# Liquor Ratio 1:20
# Temperature - Room Temperature

The fabrics were padded for about half an hour and then passed through the padding mangle for 10% expression. The fabrics were then tried and cured at 120ºC. They are then washed and dried.

Application of Citric Acid

Both the fabrics were treated with 4 different concentrations of citric acid, viz, 6%, 8%, 10% & 15% (owf).

Recipe for padding bath is as follows:

# Citric Acid - 6%, 8%, 10% & 15% (owf)
# Sodium hypophospate 6% (owf)
# Softener 2% (owf) # Liquor Ratio 1:20
# Temperature - Room Temperature

The fabrics were padded for about half an hour and then passed through the padding mangle for 10% expression. The fabrics were then tried and cured at 120ºC. They are then washed and dried.

Application of Glyoxal

Both the fabrics were treated with 2 different concentrations of DMDHEU, viz, 6% & 10% (owf)

# DMDHEU 6% &10% (owf)
# Magnesium Chloride 6% (owf)
# Softener 2% (owf)
# Liquor Ratio 1:20
# Temperature - Room Temperature

The fabrics were padded for about half an hour and then passed through the padding mangle for 10% expression. The fabrics were then tried and cured at 120ºC. They are then washed and dried.

Dyeing

The fabrics were dyed by using acid dye and then washed and dried at room temperature. Recipe for dyeing bath as follows:

# Dye 4% (owf)
# Acetic acid 5% (owf)
# Glauber's salt 10% (owf)
# Liquor Ratio 1:40
# Temperature - Room Temperature
# Time - 1 hour

Testing methods

Sample Preparation

Samples from both the fabrics were taken and the following tests were conducted:

Ambient testing conditions:

RH 65 + (or) - 2%
Temperature 25 + (or) - 2^ºC

Crease recovery

Crease Recovery is quantitatively measured in terms of crease recovery angle using Eureka crease recovery tester. The sample size taken for testing was 2" x 1"

Tensile strength

The tensile strength was determined using Eureka tensile strength tester. The gauge length selected was 20 cm X 5 cm and ravel strip method was adopted.

Abrasion resistance

The abrasion resistance was determined using "Martindale abrasion tester". In the instrument a multidirectional movement is given to the fabrics, which in turn is mounted on top plate, and abraded against emery paper.

Washing fastness

The washing fastness was determined using "Washing Fastness Tester". In the instrument the fabrics are treated with 5% soap solution at 40^ºC.

Rubbing fastness

Adequate number of pieces of 5 X 5 cm undyed bleached cotton were rubbed on dyed material of size 14 X 5 cm using crock metre. They were rubbed about 10 seconds with a downward force of 900 g on the finger.

Results and Discussion

The results of various treatments and tests have been tabulated and they are discussed suitably and suitable conclusion has been drawn.

Crease Recovery Angle The crease recovery angles of control and treated Mulberry and Tassar fabrics are presented in Table 4.1 and depicted graphically in Figures 4.1(a), 4.1 (b) and 4.1 (c) respectively. There is a significant improvement in the crease recovery angle with all three treatments. The crease recovery angle increases from 130^º to 141^º with glyoxal, from 130^º to 140^º with citric acid treatments.

The DMDHEU although shows an increasing trend, the increase are much lower compared to other two methods. Maximum increase is observed with 5% of glyoxal, 6% of citric acid and 10% of DMDHEU. It is observed that as the concentration of chemical increases the crease resistant angles decreases significantly. When the concentration of citric acid was lower the crease recovery angle found to be increased with increasing concentration of citric acid, this was due to the increase in the cross linkages between silk polymer molecules.

When the concentration was too high, the finish reacted too severely with the fibre and formed a thick layer on the surface of the fabric. This can reduce the resiliency and increases the specific density of the fabric. For the same reason, too much finish reduced the whiteness of the fabric.

Tassar exhibits a remarkable improvement in the crease recovery properties with all the treatment as compared to mulberry. The increase in crease resistance is much higher at 81^º (glyoxal), 86 (citric acid) and 91^º DMDHEU from 71^º (untreated samples). The decrease in crease recovery angle with increase in concentration is observed here. This may be due to the difference in the basic structure and constituents of tassar and mulberry fabric, and also these fabrics have greater interlacing points per unit area.

Effect of catalyst

The action of above eco-friendly chemicals is enhanced by the use of catalyst. In the present study two catalysts, ie, Aluminum Sulphate and Magnesium Chloride were used. The addition of aluminium sulphate catalyst and softeners to finishing bath can help in full swelling of silk fabric and assist the penetration of finishing agent into the fibres. So that cross linking positions are increased resulting in considerable increase in the crease recovery angle. When magnesium chloride is used as catalyst for applying 5% concentration of glyoxal the crease recovery angle is reduced from 135^º to 115^º. This is due to the presence of magnesium chloride catalyst and less fabric weight gain, besides it results in fabric yellowing. There may be some effect on formation of cross linking also.

Effect of curing temperature and time

At higher curing temperature the increase in stiffness, yellowness and decrease in whiteness index results with irrespective of the finishing conditions used. This shows that higher temperature and long curing time are not favourable. In order to improve the resiliency and wash durability with a complimentary limited scarifies to other properties of silk, curing temperature at 120^ºC to 130^ºC with curing time of 120 seconds found to be most suitable one.

Tensile Strength

The tensile strength values of control and treated mulberry and tassar fabrics are presented in Table 4.2 and depicted graphically in Figures 4.2(a), 4.2 (b) & 4.3 (c).There is a significant decrease in tensile strength, which was found in all the three treatments.

In case of mulberry, tensile strength decreases from 45 kg to 41 kg with 5% glyoxal, from 45 kg to 40 kg in 6% citric acid treatments. No doubt this is substantial strength retention was found after the treatment. There is only 12% loss in tensile strength results in fabric treated with glyoxal and citric acid at 5% and 6% concentrations respectively, with increase in concentration decrease in tensile strength results. This may be due to acid catalyzed depolymerisation during the curing process. The magnitude of fabric strength loss may be affected by the pH of the acid solution applied to the fabric and also cross linking system become inefficient when the pH increased to 3.65. However 12% loss in tensile strength is not an acceptable value to take into consideration.

In case of Tassar fabric significance difference in tensile strength loss was observed with all the three treatments with different concentrations. From the results it is observed that the tensile strength drops from 35 Kg to 10 Kg with 5% glyoxal and 35 Kg to 4 Kg with 6% citric acid. The considerable decrease in tensile strength was also found in DMDHEU. It is observed that with increasing concentration of chemical increase in tensile strength loss occurs. This is due to embrittlement and molecular degradation of tassar fabric.

Abrasion Resistance

The weight loss percentage of control and treated mulberry and tassar fabrics are presented in Table 4.3 and depicted graphically in Figures 4.3(a), 4.3 (b) & 4.3 (c). In case of mulberry there is a significant increase in weight loss percentage with all the three treatments, the weight loss percentage increases from 3% to 5% with 5% glyoxal and 6% citric acid and 10% DMDHEU. It is observed that as the concentration of chemicals is increased the weight loss per cent increases, ie, abrasion resistance decreases. The abrasion resistance affected to a large extent at 15% concentration of glyoxal and citric acid. This may be due to Abrasion resistance is associated with tensile strength, which in turn associated with extent of cross-linking. However, additives may have an important play on this property.

In case of tassar there is no significant decrease in the weight loss percentage of fabric treated by 5% glyoxal, 6% citric acid and 10% DMDHEU. However with increase in concentration results will increase in weight loss percentages, ie, decrease in abrasion resistance. Abrasion resistance is affected greatly to extent at 15% concentration of glyoxal, 15% concentration of citric acid and 10% concentration of DMDHEU. This may be due to abrasion resistance is associated with tensile strength which in turn associated with cross-linking. However, additives may have an important role to play on this property.

Dye uptake behaviour

The fabrics were dyed with an acid milling dye. Significant difference was found visually in dye uptake of treated and controlled Mulberry and Tassar fabrics. Much work was not carried on this due to lack of time and available facility.

Washing fastness

The fabrics treated with 5% glyoxal ( aluminium sulphate), 6% citric acid, 0% DMDHEU and controlled were tested for wash fastness, the grey scale rating of fabrics was found to have 4 for fabrics treated with 5% glyoxal ( aluminium sulphate), 6% citric acid and 4/5 for untreated fabric 10% DMDHEU treated fabric.

Rubbing fastness

The fabrics treated with 5% glyoxal (aluminium sulphate), 6% citric acid, 10% DMDHEU and controlled were tested for Rubbing Fastness, the grey scale rating of fabrics were found to have 4 for fabrics treated with 5% glyoxal (aluminium sulphate), 6% citric acid, 10% DMDHEU treated fabric and 4/5 for untreated fabric.

Conclusion

The optimum concentration for treatment with glyoxal and citric acid were found to be 5% and 6% respectively. The aluminum sulphate was found to be the most suitable catalyst for glyoxal.The optimum curing temperature and curing time were found to be 120^ºC and 120 seconds. Eco-friendly crease resistant finishes were found to have less deleterious effect on mechanical properties, ie, tensile strength, abrasion resistance etc, compared with DMDHEU. The percentage of increase in crease recovery angle was found to be more in tassar fabric than with mulberry fabric. The work can be extended with using different catalysts and different crease resistant agents. Formaldehyde release can be extensively studied to know about the nature of various finishes. It can also be extended on other fabrics like cotton, viscose, etc.

References

Andrew: Wrinkle Resistant Cotton and Formaldehyde Release, Colorage, 41, 87-93 (1995).

Dr S A Shah and K S Taraporewala: Manmade Textile in India (1995).

Reinhardt R M, Bhattacharya N, Doshi B A, Sahasrabuddha A S and Ministry P R: Citric Acid Treatment of Dyed Cotton Fabrics, American Dyestuff Reporter, 84, 17-20 (1995).

Reinhardt R M, Bhattacharya N, Doshi B A Sahasrabuddha A S and Ministry P R: A Comparison of BTCA and DMDHEU Cross-linking Treatments on Dyed Cotton Fabrics - American Dyestuffs Reporter, 83, 80-90 (1994).

Welch C M and Peters J G: Mixed Polycarboxylic Acid Mixed Catalyst in Formaldehyde Free Durable Press Finishing, Textile Chemist and Colorist, 29, 22-72 (1997).

Brodmann G L: Performance of Non-formaldehyde Release from Durable Press Fabrics, Textile Chemist and Colorist, 14, 100-106 (1982).

Welch C M and Andrews B A K: Ester Crosslinks: A Route to High Performance Non-formaldehyde Finishing of Cotton, Textile Chemist and Colorist, 21, 13-17(1989).

Peterson H: The Chemistry of Crease Resistant Crosslinking Agents, Review of Progress in Coloration and Related Topics, 17, 7-22 (1987).

Note: For detailed version of this article please refer the print version of The Indian Textile Journal December 2007 issue.

M Parthibana,
Faculty of Fashion Technology,
PSG College of Technology, Coimbatore 641 004.
E-mail: parthi_mtech@yahoo.com

M Ramesh Kumar,
Faculty of Textile Technology,
SSM College of Engineering, Komarapalayam,
Tamil Nadu 638 183.
E-mail: rk_textile@yahoo.co.in