Triclosan and silver ions leach out or move away from the applied surface and work on leaching mechanism at a slow yet sustained controlled rate to provide protection against microbes (Kut et al. 2005). Triclosan is a non-ionic agent, therefore, it is assumed that it doesn’t form chemical bonds with cellulose. It has low molecular weight and acts like a disperse dye with high exhaustion rate that diffuses into the fibre like polyester and nylon (Gao and Cranston 2008).
Silver particles are incorporated in synthetic polymers before extrusion and during its use it diffuses onto the surface of the fibre where it forms silver ions in the presence of moisture and acts against microbes. Release rate of silver from the fibre can be influenced by the physical and chemical characteristics of the fibre as well as the amount of silver in the fibre (Gao and Cranston 2008). N9 Pure silver is in nonionised metallic state. It doesn’t leach out from the fabric and acts against the microbes through contact with surface.
Triclosan and silver agents were evaluated for “Zone of inhibition” test which provided qualitative analysis of the extent to which antimicrobial agents effectively migrated onto the agar and diffused outward. Figure 1 represents the zone of inhibition for E. coli bacterium with both triclosan and silver based antimicrobial agents marked as C1 and C2 respectively. The same was done using S. aureus bacterium and results were same as for E. coli. It was found that Triclosan leached out to great extent and had more zone of inhibition whereas silver had a very low zone of inhibition due to inability of functional silver to diffuse through nutrient agar because it binds with proteins in the agar (Swofford 2010). This test wasn’t done for chitosan as it forms bonds with cellulose and remains incapable to leach out.
Fig. 1Representing the “Zone of inhibition” for triclosan and silver based antimicrobial agents against E. coli (C1 triclosan based and C2 silver based antimicrobial agents)
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The finished cotton fabrics were evaluated for antimicrobial activity against S. aureus and E. coli before and after each laundering cycle up to five times. The results for all three agents with two different application methods are presented in Table 2. It was found that for triclosan and silver based finishes no substantial differences were noticed in antibacterial activity against both selected bacteria after curing. No significant difference was found among both application methods for triclosan and silver whereas the results for chitosan showed better activity after curing. There was 100 % reduction in antimicrobial activity in both the methods with triclosan (60 g/L) against both bacteria without laundering. It retained its antimicrobial activity up to 99.98 % even after 5th laundering cycle. For further study, the concentration of triclosan was formulated from 5 g/L onwards. Silver at 10 g/L showed comparatively same reduction rates for both application methods against both bacteria. It showed 95.65 and 92.5 % reduction rates without any laundering and after five laundering showed 94.44 and 91.14 % reduction rates for S. aureus and E. coli respectively. N9 pure silver based agent had only silver particles dispersed in water and it required no curing as it does not possess crosslinking groups; concentration of it was selected from 3 g/L onwards.
Table 2 Evaluation of antibacterial activity of finished cotton in various padding methodsFull size table
Both triclosan and silver are applied on the surface of the cotton fabric. Due to their low molecular weight they can leach out. For N9 pure silver it remains intact on the fibre surface, but it also leaches out as can be seen from Fig. 1 on testing its efficiency against microbes on agar plate. Chances are that these agents remain coated on the surface as well as get trapped inside the cellulose convolutions due to application of force during padding and act until their reservoir ends with usage and repeated launderings.
From Table 2 the launderability results for pad-dry and pad-dry cure methods for all three agents against both microbes showed that curing was not required for triclosan and N9 pure silver to bind these on cellulose as no such difference in the antimicrobial activity was found for both the padding techniques up to their 5th laundering cycles. Therefore, it could be concluded that curing doesn’t show any influence on fixation as well as durability of triclosan and silver agents.
These finishes didn’t require any specific application technique and could be easily applied by pad-dry method. Obviously, for further studying the effect of concentration, pH and launderability on antimicrobial activity, these were applied only by pad dry method.
Chitosan at 5 g/L showed comparatively poor activity against both bacteria. Loss in antimicrobial activity was obtained after first laundry cycle to a great extent. The reduction rates were better i.e. 68.38 % as compared to 43.98 % for S. aureus and 51.39 % as compared to 37.74 % for E. coli with and without curing steps respectively. Up to 5th laundry the activity was reduced to 48.97 and 29.9 % for S. aureus and E. coli respectively even after curing. This could be due to removal of a superficial layer of chitosan from the fabric. Structure of chitosan resembles to that of cellulose and binds with cellulose through H-bonds and Van der Waal’s forces; curing could have resulted in formation of more bonds with cellulosic structure. Hence, for further finishing of cotton with chitosan and to see its durability on antimicrobial activity pad-dry-cure method was applied with curing at 150 °C for 5 min.
All the three antimicrobial agents were applied on cotton at different concentrations at pH 6, and antimicrobial activity of finished cotton fabrics on percentage reduction in CFU against S. aureus and E. coli are shown in Figs. 2, 3, and 4 for triclosan, silver and chitosan respectively. It was observed that regardless of triclosan concentration, the finished cotton fabrics showed over 90 % reduction in number of CFU against both E. coli and S. aureus, indicating excellent antimicrobial activity of triclosan at even low concentration of 5 g/L (Fig. 2). Reduction increased up to >99.99 % with increase in its concentration at 20 g/L and beyond that 100 % reduction was obtained against both bacteria. This confirmed that triclosan was noticeably effective in reducing bacterial growth tremendously for both Gram positive and Gram negative bacteria even at lower concentrations. Silver finished cotton fabrics (Fig. 3) also showed a reduction rate of >90 % against both E. coli and S. aureus bacteria at 10 g/L and the reduction increased up to 99.99 % with increase in concentration of silver at or beyond 30 g/L. Silver at lower concentrations did not show any remarkable reduction. At 5 g/L concentration it showed only around 45.89 and 47.76 % reductions in CFU for E. coli and S. aureus respectively. At 3 g/L silver the antimicrobial activity was around 21 % against both bacteria. Therefore, it was concluded that silver based antimicrobial finish required higher concentration i.e. at least 10 g/L to show required antimicrobial activity.
Fig. 2Effect of triclosan concentration on % reduction in CFU of Gram positive and Gram negative bacteria
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Fig. 3Effect of silver concentration on % reduction in CFU of Gram positive and Gram negative bacteria
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Fig. 4Effect of chitosan concentration on % reduction in CFU of Gram positive and Gram negative bacteria
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The reduction rate of chitosan finished fabrics (Fig. 4) was somewhat lower than that with triclosan and silver finishes. It was observed that for chitosan it exhibited good antimicrobial activity when its concentration was increased from 3 to 10 g/L and after that there was no remarkable improvement in its activity against both selected microorganisms. It showed antimicrobial activity above 90 % for S. aureus and above 80 % for E. coli at 10 g/L respectively. At 20 g/L the reduction rate increased up to 94.62 and 88.73 % for S. aureus and for E. coli. With increase in concentration of chitosan, the viscosity of the bath also went on increasing posing difficulty on smooth application with proper penetration. That is why effectiveness of chitosan was studied up to 20 g/L. Another problem with chitosan was stiff handle of fabric with increased concentration. It may be concluded that both triclosan and silver based finishes showed markedly higher reduction rates and were easy to apply on cotton whereas chitosan was difficult to apply because of viscous bath and showed comparatively less activity. Chitosan produced stiff handle to the fabric. Almost 100 % protection against both microorganisms was achieved with triclosan (20 and 30 g/L) and silver (30 and 40 g/L). Chitosan concentration at 15 g/L was comparatively easier to apply and showed almost same activity as that at 20 g/L. These concentrations were further studied for evaluating effect of repeated laundering on durability of finish. Effect of pH was also studied for both triclosan and silver finishes at 30 g/L.
Effect of pH on antimicrobial performances against S. aureus and E. coli was studied with triclosan and silver at 30 g/L each. The antimicrobial activity at 4, 5, 6 pH for both these agents are presented in Fig. 5. It was observed that in case of triclosan, pH did not have any negative impact on its antimicrobial performance against both bacteria but silver based finish showed slight negative impact on its antimicrobial performances against both bacteria and fall in pH from 6 to 4, it got reduced from 99.99 to 91.87 and 92.33 % for S. aureus and E. coli respectively. N9 pure metallic silver was used for finishing which in turn possibly show high tendency to get ionised due to reaction with acid with successive fall in pH, i.e. increase in acidity of bath and was not exhausted on cotton rather showed affinity towards acid. Therefore, pH 6 was accepted as the optimum for both agents. As chitosan dissolves at pH around 5–6 its performance against change in pH wasn’t conducted.
Fig. 5Effect of pH on antimicrobial activity of triclosan and silver based antimicrobial agents on cotton fabric
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Cotton finished with triclosan (20 and 30 g/L), silver (30 and 40 g/L) and chitosan (15 g/L) were evaluated to study durability of finish after repeated laundering. Figure 6 shows the changes in antimicrobial activity of finished cotton without laundering and after each laundering up to five cycles. The results for both triclosan and silver based finishes were consistent and were found to be extremely effective against both S. aureus and E. coli bacteria. They exhibited slight decrease in their antimicrobial activities even after five repeated laundering cycles; whereas, chitosan finished laundered fabrics showed drastic decrease in antimicrobial activity probably because of removal of finish from fabric as chitosan forms weak binding with cellulose and loss of its cationic nature under alkaline conditions of laundering. In case of triclosan, finished cotton durability was maintained up to five laundering cycles and showed >98 and >99 % reduction rates for 20 and 30 g/L finished fabrics respectively against S. aureus and E. coli, showing high antimicrobial effectiveness and durability properties. Similar was the case with silver as it maintained its antimicrobial properties >99 % reduction rates for 30 and 40 g/L against S. aureus and E. coli respectively. From Table 2 it was found that at 10 g/L silver finished cotton showed >90 and >94 % reduction in CFU of E. coli and S. aureus by 5th laundry cycle. Chitosan (15 g/L) finished cotton showed more than 87 and 94 % reduction rate for E. coli and S. aureus respectively without any laundering. After first laundering there was significant loss in its antimicrobial activity. By the end of 5th laundry cycle there was substantial decrease in its activity to 46 and 55 % respectively for E. coli and S. aureus.
Fig. 6Antimicrobial activity of antimicrobial finished cotton on repeated launderings
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It may be concluded that both triclosan and silver based finished cotton showed excellent antimicrobial activity above for both 98 % against E. coli and S. aureus bacteria even after 5th laundry cycle but for chitosan it was almost reduced to half. It is highly recommended to use binding agents for crosslinking chitosan with cellulose to obtain good durability results.
The launderability results for all three agents as shown in Fig. 6 were statistically analysed for each agent with ANOVA. In case of triclosan, concentration, laundry cycles and bacteria along with interaction effect of laundry cycle with concentration showed statistical significant difference (p < 0.05) in % reduction of CFU with their percentage contribution as 21.1 % with concentration, 61.5 % with laundry cycles and 17.3 % due to interaction effect of concentrations of antimicrobial agent and laundry cycles. In case of silver, its concentration, bacteria and laundry cycle along with interaction of concentration with bacteria and laundry cycles showed statistically (p < 0.05) significant differences in % reduction of CFU. Percentage contributions on results were found to be 9 % with concentration and 72.72 % with laundry cycles. In case of chitosan for 15 g/L finished cotton fabrics launderability results showed significant difference (p < 0.05) for bacteria and laundering cycles on % reduction in CFU with 91.8 % contribution by laundering cycles. These results prove that laundering cycles had significant effect on antimicrobial activity for each agent.
Physical properties of finished cotton are summarized in Table 3. None of three antimicrobial agents showed any significant deterioration in tensile strength; 95 % or more retention of tensile strength was reported for both warp and weft. Tear strength marked no change and rather slightly increased up to 10 % for chitosan finished cotton due to jamming of yarns with its layer. Air permeability was decreased from 17.96 to around 16 cm3/cm2/s for both triclosan and silver finished fabrics both and 14.8 cm3/cm2/s for chitosan finished fabric due to blocking of the fabric pores with its layer. Total crease recovery angle (TCRA), stiffness, whiteness index of finished fabric remained almost unchanged for triclosan and silver based finishes but with chitosan decrease in whiteness index and increase in stiffness of fabric to a great extent from 2.3 to 2.88 for warp and from 1.52 to 1.94 for weft.
Table 3 Physical properties for antimicrobial finished cottonFull size table
It was concluded that triclosan and silver based finishes did not alter the physical properties of cotton whereas chitosan affected the same and showed some negative impact on physical properties of finished cotton. With chitosan, stiffness increased to a great extent, air permeability decreased tremendously due to blocking of the fabric pore structure, TCRA decreased due to more add-on of finish on fabric, slight decrease in whiteness index but tear strength got increased compared to other finishes.
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