Effect of dual catalyst on the maleic acid cure of eri silk fabric
To study the role of esterification catalyst and free radical polymerization catalyst for the pad-dry-cure technique of silk fabric with maleic acid, the silk fabric was treated with maleic acid in absence of either of the two catalysts with varying concentrations of Aloe vera gel @ 5% to 15% (w/v), (see table 2). In each experiment maleic acid dose levels were maintained at 10% (w/w). Treatment of silk fabric in the presence of peroxodisulfate as the free radical polymerization catalyst only resulted in poor weight gain and wrinkle recovery angle with retention of a high order of tensile strength. Such effects appear to be the consequence of only graft copolymerization induced by ammonium peroxodisulfate (as shown in the chemical reaction scheme 2.a. (i) and (ii), 2.b. and 2.c. In Figure 1 in the introduction section) and limited self catalyzed esterification reaction effected only at a high temperature of drying and curing. Silk fabric finished with maleic acid in presence of only esterification catalyst also resulted in poor weight gain with only marginal improvement in wrinkle recovery angle with high retention of tensile strength inconsequent to the establishment of ester linkages under the influence of esterification catalyst (as shown in reaction scheme 1 in Figure 1) with limited thermally induced graft copolymerization of maleic acid in absence of free radical polymerization catalyst. Under the influence of two catalysts taken together (ammonium persulphate and trisodium citrate) for the maleic acid cure of silk, substantial weight gain and wrinkle recovery angle is achieved. Retention of tensile strength, however, suffers for the maleic acid cure of silk under the influence of dual catalyst system was noted in our study. However, to increasing the antimicrobial efficiency, the tear strength and tensile properties were brought down due to a reduction in chain flexibility after graft polymerization under dual catalytic effect at higher temperature curing. Although the crease recovery angle was increased to some extent at an initial concentration of Aloe vera gel up to 10% (w/v) and thereon, it started decreasing as the flexibility of chain molecules reduces at a higher concentration of Aloe vera gel i.e. 15% (w/v) and so on. Results in Table 2, clearly shows the retention or improvements in weight gain, wrinkle recovery angle and tensile strength are optimal on pad-dry-iron-cure of silk with maleic acid under the influence of a dual catalyst system.
Effect of variation of batching time and pH variation
In each experiment, the maleic acid dose level was maintained at 10% (w/w) for batching at 300C room temperature for 60 minutes. In the case of the dual catalyst system and subsequent drying by heating at 950C for 5 minutes, followed by curing at 1400C for 5 minutes, there is a notable weight gain, wrinkle recovery angle, tear strength retention, breaking load retention and elongation at break. However, bending length remained level for the entire batching time. The batching for an extended time distinctly favour higher incorporation of maleic acid moieties on silk by ammonium persulphate induced graft copolymerization. Initial peroxodisulfate induced homopolymerization of maleic acid, to increase extents over increasing batching periods, at ambient temperature 300C and further polymerization of free maleic acid and silk bound maleic acid moieties during subsequent drying at 950C cause an overall change in environment and proximity of the hydroxyl groups of silk and carboxyl groups of the unbound or silk bound maleic acid or poly (maleic) acid moieties that finally causes an enhanced degree of trisodium citrate catalysed esterification and further chain polymerization leading to substantial cross-linking during curing at 1400C as revealed by the relevant data for wrinkle recovery in Table 3.
The esterification reaction that assumes more prominence at the high processing temperature (1400C) in the final stage appears to be somewhat dependent on the initial batching time. An increase in batching time favours improved transformation of the grafted maleic acid /poly (maleic) acid units to ester moieties at the high curing teperature1400C under the influence of the esterification catalyst in the final stage of processing. Optimum batching time (45-60) minutes also allows improved diffusion of finishing agent maleic acid within the chain molecules of silk.
Relevant data for the change of pH indicate that under neutral condition (pH 7), optimum grafting and esterification leading to much-improved wrinkle angle and substantial weight gain are achieved with no loss of breaking strength and with more than 90% retention of tear initial fabric. In the case of the moderate acidic condition, (pH 5.6), moderate improvement in extensibility with more than 80% retention of the tear strength of the initial fabric was achieved.
Again, moderate alkaline conditions, (pH 8-9) result in poor retention of breaking strength (<70%), and tear strength(<75%), despite substantial weight gain much as a consequence of weakening of the silk fibre in the fabric by alkali attack. Under the slightly acidic condition, (pH 5.6), improvement in wrinkle recovery angle is comparatively poor even though tear strength and breaking strength retention are good, (pH 7), therefore, apparently provides the most optimum condition of the finishing process.
The antibacterial activity was quantitatively assessed against gram-negative bacteria viz. E. coli: Strain No. ATCC 9637 and gram-positive bacteria viz. S. aureus: Strain No. ATCC 6538, according to the AATCC 100-2004 test method. The bacterial reduction percentage (%) of the treated eri silk fabric against gram-negative E. coli and gram-positive S. aureus on treated fabric with 10% (w/v) Aloe vera gel is observed to be 97 and 91, respectively. The results of IR analysis suggests that esterification of maleic acid with the hydroxyl group of silk effectively accomplished in presence of trisodium citrate and free radical polymerization with simultaneous grafting of poly (maleic acid) on silk effective accomplished in presence of ammonium persulphate on pad-dry-cure of maleic acid-treated silk fabric. In the presence of two catalysts during maleic acid treatment both the reaction became prominent resulting in cross-linking of protein molecules promoting wrinkle recovery of silk.
It is also noteworthy that since more cross-linking takes place when the reaction between the fibres and maleic acid is more active with a higher power or longer time, the treated fibres become hardened and straightened, resulting in a greater loss of tensile strength of fabrics. Furthermore, long time enhances the hydrolysis of fibres in acid catalysts thereby reducing the tensile strength of the finished eri silk fabric.
This work is aimed at establishing optimum condition for the application of maleic acid evaluating attainable changes or improvements in the fabric nature and properties including crease-resistance, stiffness, strength, and moisture regain anti-microbial, properties. Results of such studies are reported in the present article.
Effect of Aloe vera gel concentration
Figure 2 shows the effect of Aloe vera gel concentration from 5-15% (w/v) on the performance properties of eri silk, viz., wrinkle recovery angle and tear and tensile strength of the treated fabric. The finishing baths were prepared to contain Ammonium persulphate 1% and trisodium Citrate: 6%, the fabrics treated thus with 100% pick up were dried and then exposed to curing at 1400C for 5 minutes. It is clear (Figure 2) that the wrinkle recovery angle of the treated fabrics which were cured was pronounced as Aloe vera gel concentration increased up to 10% (w/v) and then decreased sharply whereas, there was a notable increase in weight gain with the increase of dose level of Aloe vera gel concentration.
The enhancement in wrinkle recovery angle of the finished fabrics by increasing Aloe vera gel concentration suggests that Aloe vera gel performed two functions: (1) it reacts with maleic acid in the fibre molecules; (2) Aloe vera gel undergoes cross-linking with the fabric to form a network matrix. The water-soluble Aloe vera gel with its low molecular weight penetrates the fibre more easily promoting anti creasing in the treated eri silk fabrics. Water-soluble Aloe vera gel generates an ether reaction with the hydroxyl groups in the fibres, forming a two-dimensional structure that improved the crease resistance of the fabrics. Decrement in wrinkle recovery angle by increasing the Aloe vera gel concentration above 10% (w/v) could be associated with increased basicity of the finishing environment at higher Aloe vera gel concentrations. Logically, basicity would stand as an inverse function to the acidity of the catalytic system of the cross-linking peptide molecule with maleic acid under the dual catalytic influence. Lower catalysis would certainly lead to decreased wrinkle recovery angle. With respect to tensile strength, on the other hand, penetration or encapsulation of Aloe vera gel molecules would improve the strength properties of the treated fabrics. As shown in Figure 2, the tensile strength and elongation at break increased by increasing Aloe vera gel concentration up to 10% (w/v) which tends to decrease thereafter. Rigidity conferred on the structure of silk by the inclusion of Aloe vera gel through various interactions with silk and maleic acid may account for the decrease in tensile strength at higher Aloe vera gel concentrations and also, tear strength retention shows a monotonic fall with increases of Aloe vera gel dose level. It is also probable that higher concentrations of Aloe vera gel n create more fibres bridging and are more likely to cause stress accumulation thereby decreasing the tensile strength. Breaking load increased by increasing Aloe vera gel concentration up to 10 g/1 which tends to decrease thereafter. Rigidity conferred on the structure of silk by the inclusion of Aloe vera gel through various interactions with silk and maleic acid may account for the decrease in tensile strength at higher Aloe vera gel concentrations and also, tear strength retention shows a monotonic fall with increases of Aloe vera gel dose level. It is also probable that higher concentrations of Aloe vera gel create more fibres bridging and are more likely to cause stress accumulation thereby decreasing the tensile strength.
Pore size analyser of treated and untreated sample
Pore Size Analyser Report like smallest pore diameter (micron), largest pore diameter (micron), mean flow pore diameter (micron), as well as first bubble point diameter (micron), is given below in Table 4.
The distilled water of surface tension 72 mN/m was used for wetting the samples and test pressure was kept at 0.5 bar. The smallest and largest flow pore size of the untreated sample was measured as 7.89 and 336.21 micrometres, respectively. The smallest and largest flow pore size of the treated sample were found as 7.68 and 332.22 micrometres, respectively when the sample is treated with 5% Aloe vera gel. The mean pore diameter was found 232.52 micrometres for the same sample. The smallest and largest pore size of the treated sample with 10% Aloe vera gel was found to be 7.55 and 321.28, respectively and the mean pore diameter was observed as 219.03 micrometres. It is observed that there is a slight decrease in the mean pore diameter after-treatment of the 5% Aloe vera gel and the decrement is more significant after the treatment of the 5% Aloe vera gel. This may be attributed due to the add-on of mass on the fibre surface blocking the pores. The presence of the Aloe vera coating on the fibre surface can be evidenced by SEM images.
Evaluation of antibacterial property of textile fabric:
The antibacterial activity was quantitatively assessed against gram-negative bacteria E. coli: Strain No-ATCC 9637 and gram-positive bacteria S. aureus: Strain No-ATCC 6538), according to the AATCC 100-2004 standard test method. The test microorganism is grown in liquid culture. The concentration of the test microorganism is standardized. The microbial culture is diluted in a sterile nutritive solution. Untreated and treated fabric swatches are inoculated with microorganisms. The inoculation is performed such that the microbial suspension touches only the fabric. Bacteria levels on both untreated and treated fabrics are determined at ‘time zero’ by elution in a large volume of neutralizing broth, followed by dilution and plating. A control is run to verify that the neutralization/elution method effectively neutralizes the antimicrobial agent in the fabric. Additional inoculated control and test fabrics are allowed to incubate, undisturbed in sealed jars, for 24 hours. After incubation, microbial concentrations are determined. Reduction of microorganisms relative to initial concentrations and the control fabric is calculated. Per cent reduction of bacteria by the specimen treatments was calculated using the following formula: R= 100 (B – A)/B where R is % reduction A is the number of bacteria recovered from the inoculated treated test specimen swatches in the jar incubated over desired contact period. B is the number of bacteria recovered from the inoculated treated test specimen swatches in the jar immediately after inoculation (at ‘0’ contact time).
The inoculation is performed such that the microbial suspension touches only the fabric. The photographs of bacterial growth on untreated and maleic acid-treated samples in presence of Aloe vera gel with varied concentration i.e. 5% (w/v) and 10% (w/v) under dual catalytic effect are also given in Figure 3.
Maleic acid treatment when suitably done with Aloe vera gel caused a substantial reduction in the growth of microorganism in treated samples assessed in terms of colonies recovered. The finished is normally tested for antibacterial properties and finish stability is tested even after 10 washing cycle after washing the fabric with non-ionic detergent at mild Alkaline PH. From the result, it can be inferred that the eri silk fabric finished with Aloe vera showed more than 90% antimicrobial property against both the bacteria. Even after 10 wash, it shows more than 80% antimicrobial efficiency. This may be due to significant loss of active ingredient of Aloe vera after 10 machine washes. In other words, cross-linking are deteriorated paving the way for the active ingredients to leach out from the fabric during washing.
FTIR analysis
The FTIR spectra of untreated silk fabric and eri silk treated with maleic acid and Aloe vera gel under conventional curing are shown in Figure 4. A broad absorption band over 3200 cm-1 characteristic of hydrogen-bonded (N-H) stretching vibration and an absorption band in the range of 1621 cm-1 to 1637 cm-1 characteristic of amide stretching are common to all spectra (Das et al., 2014). Two notable absorption bands at 1316.14 cm1 and 1426.65 cm-1 appearing in different intensities in the spectrum of unmodified silk [spectrum 1 of Figure 4] are characteristic of carboxylate anion stretching and phenolic (-OH) bending, respectively. Carboxylate anion stretching accounts for the presence of a free carboxylic acid group at the end of polypeptide chains and phenolic (-OH) bending accounts for the presence of residues of tyrosine fractions of amino acids in the unmodified silk. The strong absorption band at 1202.68 cm-l and 1156.89 cm-l also appears in the spectrum of unmodified silk and is attributed to (C-N stretching) vibration of amine groups present at the end of polypeptide chains of silk. In the spectrum of unmodified silk, the absorption band at 1621.16 cm1 (ester stretching) and 962.44 cm-1 (vinyl unsaturation) are practically not existent.
The maleic acid finish on eri silk in presence of esterification catalyst i.e. trisodium citrate only however result in intensification of the absorption band at 1510.19 cm-1 characteristics of vinyl ester stretching (spectrum 4 in Figure 4), substantial weakening of absorption band at 1510.19 cm-1 characteristic of unsaturation present in the vinyl group as expected (spectrum 5 in Figure 4). However, maleic acid finish on eri silk under the influence of dual catalyst system (spectrum 4 of Figure 4) results in weakening of the band at 1023.74 cm-1 due to significant disappearance of the vinyl group unsaturation during final stage polymerization induced by heat and catalyst action along with sharp intensification of the band at 1621.14 cm-1 due to stretching with retention of the band corresponding to 1426.65 cm-1 for carboxylate (anion) stretching. Silk treated with maleic acid and Aloe vera gel (spectrum 5) exhibited a decrease in absorbance intensity at 1636.69 cm-1 and 1426.65 cm-1 after the curing method as compared with untreated silk sample. A decrease in intensity at 1636.69 cm-1 and 1426.65 cm-1 could be attributed to a decrease in the total number of hydroxyl groups through crosslink formation between silk and maleic acid. Substantial weakening/disappearance of band 1426.65 cm-1 corresponded to phenolic (-OH) bending due to significant disappearance of phenolic (-OH) groups. Silk Proteins are known to attach to Aloe vera gel through carboxylate ions which show antimicrobial potential. The results of the IR analysis are in tune with the mechanism proposed.
FTIR spectra of treated fabric showed that the intensity of this band is a measure of the total quantity of ester group created in the finished eri silk fabrics. The FTIR spectrum (spectrum 4 and 5 of Figure 4) of Aloe vera treated fabric showed a little shift of ester peak from 1623.50 (spectrum 4 of Figure 4) cm-1 to 1621.16 cm-1 (spectrum 5 of Figure 4) and also the intensity of this peak is lowered as compared to that of only maleic acid-treated fabric (spectrum 4 of Figure 4). This indicates a decrease in the average number of ester groups formed in presence of Aloe vera. The lower intensity peak of Aloe vera with cross-linking agent treated eri silk is due to the interaction of Aloe vera active compounds with some of the amine (-NH) groups of the eri silk and also interaction with the free –COOH groups of carboxylic acid molecules which are supposed to form ester linkage with eri silk in absence of Aloe vera compounds. Hence, the extent of degree of direct chemical cross-linking between eri silk and maleic acid via ester linkage is effectively less in Aloe vera treated samples as some of the –NH groups of eri silk are actively occupied by some of the – OH groups of Aloe vera ingredients. Thus active ingredients of Aloe vera containing –OH groups in their chemical structure can easily form H-bonding with the either –NH groups of eri silk structure or chemically react with the maleic acid during curing of Aloe vera and silk molecules.
SEM Image Analysis
Surface deposition of finishing chemicals is depicted below in Figure 5 using the following SEM image.
Figure 5 shows SEM images (a) untreated degummed silk (b) degummed silk treated with 5% Aloe vera gel (c) degummed silk treated with 10% Aloe vera Gel. In the SEM image of untreated degummed silks, the smooth well-separated filament of degummed silk appears with no surface deposition of any chemical agent. However, fibrillation of silk fibroin affected due to the degumming treatment can be traced in some portion of the silk fibre in the SEM of the degummed silk. Treatment of silk with 5% Aloe vera gel resulted in the deposition of Aloe vera gel on the surface of the silk fibre as seen in the micrograph b. Such deposition of Aloe vera gel as appears in micrograph b, however, is less frequent, few and incapable of giving uniform distribution of such Aloe vera gel on the surface of the silk. Silk when treated with 10% Aloe vera gel shows a much more uniform and frequent distribution and presence of Aloe vera gel that covers almost all the surface area of the silk fibres which however retained by the silk fibre even when the silk was post washed following a method described in the experimental section. Aloe vera gel retained by the silk fibres appears to be capable of conferring antimicrobial properties to the silk fibres. And the extent of such functional properties offered by the silk fibres in our experiment reported in this study appears to be in line with the deposition of Aloe vera gel in consequence of the different treatment of Aloe vera gels described in the present study.