Effect of silica nanoparticles and BTCA on physical properties of cotton fabrics

Silica nanoparticles particles were synthesized from rice hulls and characterized. The particles were found to be amorphous in nature, ranging in size from 50 to 100 nm. The concentration of silica nanoparticles, pH and curing time were taken as independent variables to design the experiment. Box-Behnken method has been used to derive the experimental plan and fifteen experiments were conducted. Regression equations have been formed with the dependent and independent variables and the results of all possible combinations have been derived. The combination of optimized concentration of BTCA and SHP were used as crosslinking agent and catalyst respectively and silica nano particles were used to enhance the physical properties of the cotton fabric. The effect of pH and curing time on physical properties were analysed by FTIR studies. By ranking method the best combination of process parameters were identified. From this study, it was observed that higher concentration of silica nanoparticles with BTCA improve the crease recovery angle and tensile strength. FTIR studies revealed that the increase of pH and curing time increases the ester carbonyl band intensity ratio.


Introduction
Finishing is a process step to add value to the textile product. It is classified into two categories such as physical finishes and chemical finishes. Functional finishes are one of the chemical finishes incorporated into the textile materials to enhance the functional and comfort properties 1 . Nanotechnology has the potential to create new bulk materials with new bulk properties in textile coating and finishing 2 . Nano particles can provide high durability for fabrics, because of its larger surface area-to-volume ratio and high surface energy. These can present better affinity for fabrics and leading to an increase in durability without affecting their breathability or hand feel [3][4][5] .
Cotton is poor in crease resistance due to its free hydroxyl groups. To enhance the crease recovery property of cotton, the free hydroxyl groups are either removed or cross linked by applying formaldehyde and non formaldehyde based cross linking agents. The hazardous nature of the formaldehyde cross linking agents is replaced by non formaldehyde cross linking agents such as citric acid and BTCA (1,2,3,4-butanetetracarboxylic acid). The decrease of whiteness index of cotton fabric treated with citric acid tends to increase the usage of BTCA.
The mechanism of cross linking of cellulose with polycarboxylic acids is given in Figure 1. The mechanism of polycarboxylic acids is carried out in two steps by cellulose esterification. In the first step the formation of a cyclic anhydride intermediate by the dehydration of two carboxyl groups is carried out. In the second step the ester crosslinks are formed by the reaction between the anhydride with cellulose 6 . Polycarboxylic acid esterifies cellulose through the formation of a five-membered cyclic anhydride intermediate by the dehydration of two adjacent carboxyl groups 7 . The formation of cyclic anhydrides at lower temperatures is slow without catalyst, but the catalyst sodiumhypophosphite (SHP) accelerates the cyclic anhydride process 8,9 . High curing temperature and high BTCA concentration reduce the tensile strength and increase the crease recovery angle 10 . The whiteness index of the treated fabrics seems to decrease as the concentration of cross linkers increases [11][12][13][14][15] . The quantity of the cyclic anhydride intermediate formed in a cotton fabric increased as the pH of a finish bath was reduced from 4.5 to 1.5 [16,17] .
The thermodynamic affinity of fibres form noncovalent interactions such as dipolar-dipolar and hydrogen bond, should be the key interactions for the adhesion of silica particles. The Si-O-cellulose covalent bonds in coated cellulose fabrics, make less contribution towards the adhesion of particles. Moreover, the cellulose in cotton (∼98%) fabric plays a decisive role in the intensity of covalent bond between the cellulosic hydroxyl group and -Si-OH in silica particles 21,22 . The bond formation can improve the crease resistance property of the cotton fabrics. The crease recovery finishing process with DMEU, tetraethoxysilane (TEOS) and isoproponal at various volumes improved the crease resistance and tensile strength 23 .
In most of the techniques to synthesis the nano silica, the base material is taken from the chemically available precursors such as tetraethoxysilane (TEOS) by sol-gel method 23 . But in the case of thermal degradation, the source material taken from natural resource such as rice hulls is the simplest and cost effective method for large scale production of silica nano particles 24 . In this research work, the synthesized silica nanoparticles are coated on the surface of the fabric with BTCA and SHP to analyse the crease recovery and other physical properties of the fabric and also the process parameters are optimized.

Quality factor and ranking method
Quality factor has been derived for fabric properties which consist of the importance of each property given by this experiment. Higher priority has been given to crease recovery angle and surface coating processes generally reduce the strength behavior and permeability of the fabric. Hence it is considered next to crease recovery respectively. Equation 2 has been used to calculate the quality factor. The quality factor has been ranked and it is given in Table 4. Where X = quality factor • a = crease recovery angle in degrees • b = whiteness index • c = tensile strength in kg • d = tearing strength in g • e = flexural rigidity in g.cm -1 • f = weight loss due to abrasion in g • g = air permeability in cm 3 .cm -2 /s

Characterization of silica nanoparticles
The synthesized silica nanoparticles were characterized by X-ray Diffractometer (XRD) using CuK α (λ = 1.54 Å) as a radiation source. The XRD patterns of the silica nanoparticles obtained from rice hulls is shown in Figure 2. The powder diffraction pattern indicates a broad peak at 2θ = 22°, which reveals the amorphous nature of the silica nanoparticles. Further, the XRD pattern confirms the absence of any ordered crystalline structure.
High Resolution Transmission Electron Microscope (HRTEM) was used to characterize the particle size and the image of nano particles are shown in Figure 3. It shows that the particles size is below 100 nm.

Analysis of physical properties
The physical properties of the coated and uncoated fabrics were analysed and are given in Table 1. Regression equations were derived for each property and are given in Table 3. The results of all possible combinations of process parameters have been derived and are given in Table 4.

FTIR analysis of coated fabric
The infra red spectroscopy data demonstrated that ester cross linkages in a finished cotton can be examined and evaluated on a semiquantitative basis by measuring the ester band intensity and carbonyl band intensity ratio (ester/carboxylate). The carbonyl band intensity ratio (ester/ carboxylate) is a function of the average number of ester groups formed for each BTCA molecule, so it represents the effectiveness of this bonded BTCA molecule as a crosslinking agent 17 .

Experimental
Amorphous silica nanoparticles were prepared by thermal degradation method from natural resource of rice hull. The rice hulls were rinsed, dried and aged at 80 °C for 1 hour and converted to micro particles. The particles were refluxed with 10% HCl for 30 minutes and dried at 80 °C. The residue was kept in a muffle furnace at 750 °C for 3 hours to convert nano particles. The synthesized silica nanoparticles were characterized by X-Ray Diffractometer (LabX XRD-6000) to find out the nature of the particles. Scanning Electron Microscope (HITACHI S-3400 SEM) and High Resolution Transmission Electron Microscope (FEI Quanta FEG 200) are used to find out the size of the silica nanoparticles.
Three levels and three variables were selected to design the experiment by Box-Behnken method and it is given in Table 1. Fifteen different combinations of experiments were derived and conducted, each combination has different process parameters such as concentration of silica nanoparticles, pH and curing time and it is given in Table 2. In each experiment, the pad solution contains different concentration of silica nanoparticles (1, 1.5 and 2%), 1,2,3,4-butanetetracarboxylic acid (BTCA) 6.5% and sodium hypophosphite (SHP) (both from Sigma-Aldrich) 4.5% with a fabric softener of silicone emulsion( 1%, Finox HSS), acrylic binder (Texacryl binder ELT) and wetting agent (0.1%, Lissapol-N). The pH (1.52, 2.39 and 3.01) of the each experiment varied as per the experimental plan. The prepared solution has been coated on the surface of cotton fabric (134 ends.inch -1 , 64 picks.inch -1 , 120 g.m -2 , 40 × 40 seconds count and plain weave) to impart crease recovery property by pneumatic padding mangle with the material-to-liquor ratio of 1:5 and 100% wet pickup was maintained for all experiments. Then, it was squeezed and dried at 85 °C for 5 minutes and subsequently cured at 180 °C at different curing intervals of 60, 120 and 180 seconds. The cured fabrics were treated with 0.1 M NaOH solution at room temperature for two minutes to convert the free carboxyl groups to carboxylate, rinsed with cold water and dried at 80 °C for 5 minutes.
A quadratic polynome was used to analyze the relationship of each response with the three independent variables. The multiple polynomial regression equation was used to derive the regression co-efficients to analyse the fabric properties and is given in Equation 1.
where b 0 , b i , b ii and b ij are the co-efficients of the regression equations, i, j are integers and Y is the response of the dependent variable. The physical properties of treated and untreated fabrics were tested as per ASTM test methods such as crease recovery angle (D 1295-67), tensile strength (D 5035-95), tearing strength (D 1424-96), weight loss due to abrasion (D 4966-98), air permeability (D 737-96), flexural rigidity (D 1388-08) and whiteness index (E 313-98) and the results are given in Table 2.  carboxyl carbony band at 1725 cm -1 [17] .The conversion of carbonyl to carboxylate reaches maximum when the pH of the BTCA solution is 3.01 17 . When the carbonyl band intensity ratio increases, the cross linking increases. It is evident that the crease recovery angle has been increased proportionally. Choosing the optimum pH range will reduce the quantity of BTCA for desirable fabric performance, thus reducing the cost of non formaldehyde agents 17 . Figure 4 shows the FTIR spectroscopy of silica nanoparticles and BTCA coated fabrics with different pH and curing time. From the figure it was observed that the carbonyl band at 1722 to 1735 cm -1 represents the carbonyl of ester while the band at 1577 to 1586 cm -1 and it represents the carbonyl of carboxylate 17 . The above two are the basic forms of the free carboxylic acids. Table 5 shows that the values of ester carbonyl band intensity and carbonyl band intensity ratio. The values for the above are 0.516 to 0.545 and 0.619 to 0.636 respectively. By increasing the curing time from 60 to 180 seconds and the pH value of 1.52 to 3.01 with the BTCA concentration of 6.5%, the ester carbonyl band intensity and carbonyl band intensity ratio were increased. Carboxyl is the predominant species in the BTCA solution when the pH ranges from 1.52 to 2.39 which is demonstrated by the intense

Influence of process parameters on whiteness index
Citric acid is one of the nonformaldehyde cross linking agents, the cotton fabric yellowing caused by citric acid at elevated temperatures and the yellowness increases as curing temperature, curing time, and CA concentration increase. Using sodium hypophosphite as a catalyst causes less fabric yellowing than monosodium phosphate. Therefore, yellowing caused by citric acid and other hydroxyl multi functional carboxylic acids can probably be attributed to the formation of unsaturated polycarboxylic acids 26 .
High acidity of chemicals is a source of causing fabric tendering and yellowing 27 , hence the pH increases the acidity level reduces and the whiteness of the fabric has been increased and it is shown in Figure 6. The increase in curing time and silica nanoparticles concentration gradually reduces the whiteness index of the fabric. The reduction of whiteness index up to 19% is due to higher concentration of silica nanoparticles, higher curing time and lower pH.

Influence of process parameters on tensile strength
Cross linking between cellulose molecules causes stiffening of the cellulosic macromolecular network and fibre embrittlement, thus reducing the mechanical strength of the treated cotton 28 . The increase in pH improves the tensile strength of the fabric due to its decrease of acidic nature of the solution. The acidity of the cross linking treatment had a severe effect on the reduction of strength of the treated cotton fabrics and trapping of nano metal oxide particles

Influence of process parameters on crease recovery angle
Nanometregrade metal oxides such as TiO 2 , Ag, MgCl 2 , SiO 2 and ZrO 2 are used as a co-catalyst along with non formaldehyde cross linking agents to improve the crease recovery properties of cotton fabric 25 . As the particle size of the TiO 2 is in nano-scale, the nano TiO 2 could fill the amorphous region of the CellOH, and hence, the presence of nano TiO 2 inside the fibre would probably restrict the molecular movement of cellulose 25 .
Silica nanoparticles intrude more easily into the interior of cotton fibre and adhere more tightly to the fibre structure 21 . It was observed that, the increase of concentration of silica nanoparticles contributes more to increase the crease recovery angle of the fabric due to restriction of the molecular movement in the fibre structure than pH. During the curing process, the polycarboxylic acid reacts with the cellulose molecules of cotton fabric most probably through the formation of cyclic anhydrides as reactive intermediates, which, in turn, esterify cotton cellulose. Due to increase of ester carbonyl band intensity ratio while increasing the pH from 1.52 to 3.01, the crease recovery property has also been increased 17 . The increase in curing time increased the crease recovery angle due to the formation of more esterification of cellulose and the Figure 5 shows its contribution is minimum when compared to silica nanoparticles and pH.    improves the strength of the fabric 27 . Hence, the higher concentration of nano metal oxides results increases the strength of the cotton fabric and it is shown in Figure 7 The increase of curing time reduced the tensile strength of fabric due to more esterification (acid dehydration at high temperatures and high curing time) and cross linking, when it was treated with BTCA/SHP. The same trend has been followed for tear strength.
The investigated process parameters showed insignificant effect on properties such as flexural rigidity, weight loss due to abrasion and air permeability

Conclusions
The silica nanoparticles were synthesized by thermal degradation method from natural resource such as rice hull and the particles are amorphous in nature with the size of 50 to 100 nm. Silica nanoparticles particles along with optimized concentration of BTCA of 6.5% as a cross linking agent and optimized concentration of SHP of 4.5% as a catalyst were applied on the cotton fabric as per the experimental plan. The BTCA and SHP accelerates the esterification of cellulose to form more number of cross links between the molecules. The effect of curing time and pH on ester carbonyl intensity ratio has been studied and the ester carbonyl intensity ratio significantly increases while increasing the pH value and curing time. The combination of crosslinking of cellulose and trapping of nano silica particles significantly increases the crease recovery angle of the fabric. The higher pH level and lower curing time significantly retains the whiteness index of the fabric, since the curing process contributes maximum to reduce the whiteness index of the fabric. The tensile strength of the fabric was significantly increased due to the intreraction of nano silica with cotton fibres and other process parameters. The weight loss of the fabric was increased in the range of 1.33 to 2.83%. The influence of the process parameters on flexural rigidity and air permeability characteristics of the fabric was observed. There is no significant difference and specific trend. The concentration of silica nanoparticles of 2%, pH of 3.01 and curing time of 60 seconds observed as best combination to impart crease recovery finish. The Fourier Transform Infra Red spectrometer (FTIR) studies revealed that the formation of ester carbonyl band and carboxylate band intensity ratio. Both were increased by increasing the pH vale from 1.52 to 3.01 and the curing time from 60 to 180 seconds. The increased crease recovery angle of the fabric is due to the formation of more cross links, silica nanoparticles trapping and also noticed that the increase in ester carbonyl intensity from 0.516 to 0.547 and carboxylate band intensity ratio from 0.619 to 0.637.