Zinc oxide application in the textile industry: surface tailoring and water barrier attributes as parameters with direct implication in comfort performance

Materials

Woven 100% polyester fabric was donated from the Condor Factory, Bucharest. The woven characteristics were as follows: poly(ethylene terephthalate) complex fabric (two weft plus one warp); F:Diagonal 2/1 Z:R; canvas, thickness 0.23 mm. ZnO powder 99.5% assay, with a molecular mass of 81.38 g, was supplied by the ENSAIT Laboratory, France.

Suspension preparation

Surface modification was performed using ZnO particle dispersion in two different media as follows:

ZnO particle dispersion in a water medium;

The impregnation suspension with controlled active agent concentration was made by mixing the dispersion media for several minutes with the appropriate amount of ZnO and with 4–5 drops of Vitexol (a nonionic foam suppressant for finishing woven fabrics, made from a mixture of alkoxylates), in order to avoid foaming.

The substrate was 100% PES textile material strips of 5×10 cm of known weight.

The technique used was padding. Padding is an operation of finishing through impregnation of a textile material; it is performed using a liquid or a paste, followed by wet-picking, by exposing the textile between hydroextraction cylinders, for a partial removal of the liquid or paste from the textile surface. Padding was performed on a Wemer Mathis AG laboratory machine.

Coating of the PES fabric

Firstly, dispersions with appropriate viscosity of ZnO powder (3%, 5%, 7% wt), and a binder agent, Appretan TT.FR liq from Clariant 10% wt, in water and methanol respectively have been prepared. Appretan is a hard, coarse disperse, vinyl acetate dispersion for handle modification of apparel fabrics and knitwear, bonding of nonwoven and wadding and coating of technical textiles. It has a nonionic character. Binder properties are dependent upon the Tg of the polymer. Due to its low Tg, vinyl acetate is not that stiff, and it has the advantage of low cost.

The dispersions were deposited onto the PES support using a micrometer double blade wet film applicator EQ-Se-KTQ-150D, for the deposition of microstructural coatings. The thickness of the coating was controlled by the width of the two blades. A width of 0.05 mm was used. Micrometer head accuracy is 10 microns.

The samples were dried and heated at 120℃ for 6 minutes. The samples were prepared as follows:

100% PES textile material modified with 10% Appretan solution (for reference);

The description and denomination of each prepared sample (samples specifications) are presented in Table 1.

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Table 1.

Specifications of the prepared samples

Modification 1	Modification 2	Modification 3
0 – PES	0 – PES	0 – PES
1 – PES + acrylic binder	6 – PES + 1% ZnO + methanol	10 – PES + 1% ZnO + methanol/48 hours
2 – PES + 1 % ZnO	7 – PES + 3% ZnO + methanol	11 – PES + 3% ZnO + methanol/48 hours
3 – PES + 3% ZnO	8 – PES + 5% ZnO + methanol	12 – PES + 5% ZnO + methanol/48 hours
4 – PES + 5% ZnO	9 – PES + 7% ZnO + methanol	13 – PES + 7% ZnO + methanol/48 hours
5 – PES + 7% ZnO

Characterization methods

Surface characterization

SEM characterization of the prepared samples as presented in Table 1 is shown in Figure 1. OPEN IN VIEWER

The surface of 100% PES fabric is smooth as one can see in Figure 1(a). Small impurities are present. Figure 1(b) shows PES fibers modified only with Appretan 10% solution. They have similar aspects compared to the unmodified fibers.

Figures 1(c) and (d) show the surface of 1% ZnO-polyester composite obtained from water and methanol dispersions. It can be observed that PES fibers are inhomogeneously coated with ZnO. Water dispersion seems to lead to a rougher coating, which can be explained by the fact that methanol solvent is somewhat less polar than water which favors better ZnO coating quality on fibers. This affirmation is supported also by the fact that SEM images of the PES supports coated with ZnO dispersed in methanol as solvent (Figure 1(d), (f), (h) and (j)) show different morphology of deposited ZnO than samples coated with ZnO dispersed in water. Figures 1(e) and (f) show the surface of 3% ZnO-PES composite obtained from water and methanol dispersions. As observed from these images, increasing the ZnO content leads as expected to better covering of the PES surface. Further increase at 5% ZnO leads to a coating morphology as shown in Figures 1(g) and (h) for ZnO-PES composite obtained from water and methanol dispersions. Figures 1(i) and (j) show the surface of 7% ZnO-PES composite obtained from water and methanol dispersions. It can be observed that larger concentration leads to agglomerations and a diminished coating quality. Figure 1(f) reveals a random distribution of ZnO small size agglomerations, with undispersed visible particles. Consequently, it can be observed that a higher concentration of finishing agent does not result in an improved efficiency of surface treatment with respect to a better deposition of particles (Figure 1(i)). Comparing Figure 1(c) (the lowest concentration of ZnO suspension) and Figure 1(i) (the highest concentration of ZnO suspension), there is an obvious difference in surface coating in terms of aspect and quality. By analyzing Figures 1(e) and (g), it can be seen that the treatment with a 5% concentration of ZnO suspension induces a better coating of the fiber surface than in the case of finishing with 3% ZnO suspension.

These results can be explained considering the higher solubility of ZnO in water than methanol. In an aqueous medium, ZnO powder nanoparticles partially dissolve and subsequently recrystallize onto existing ZnO particles in the suspension trough an Ostwald ripening process. By increasing the ZnO concentration in the suspension, saturation of dissolved ZnO concentration can be achieved faster. Growth processes is less obvious and coating homogeneity and uniformity improve. Using methanol as a solvent, the covering effect is superior, in terms of uniformity for all concentrations. This analysis suggests that ZnO-PES composite fibers morphology can be controlled via deposition conditions for tuning of a specific property required for the application. As an example, for a larger surface to volume area, essential in photocatalytic applications, ZnO in water treatment may be a better option than ZnO in methanol. Obviously, larger ZnO concentration in the suspension leads to lager fiber loading as observable from the respective images corresponding to increasing ZnO concentration for each solvent.

Roughness of the ZnO coating on PES fibers was studied by AFM method. As an example, Figure 2 presents 3D AFM images of surface topography of the 7% ZnO in water and methanol solvents coatings onto PES fibers. Also included is the image for 7% ZnO plus methanol PES composite treated for 48 hours. OPEN IN VIEWER

Fibers

Fiber local surface roughness was evaluated and approximate layer thickness was obtained for each concentration and each solvent. Local roughness was found to range 10-650 nm for fibers modified with ZnO in water suspension and slightly lower for ZnO in methanol suspension.

Comparing samples treated with the same concentration of ZnO suspension, but applied in different conditions, it was observed that ZnO in water dispersion corresponded to rougher surface coverage than ZnO dispersed in methanol. These observations are in accord with SEM characterization results. Figure 3 presents the example of the case of 7% ZnO powder dispersed in water (Figure 3(a)), in methanol (Figure 3(b)) and at a treatment duration of 48 hours (Figure 3(c)). One can notice the differences between the cross-sectional profiles in terms of increasing layer thickness. OPEN IN VIEWER

As estimated from the cross-section analysis, for the specific sample, the average Ra of layer thickness is 630–650 nm. Thickness layer was found to vary from approximately 1 µm to approximately 7 µm with increasing the ZnO concentration in water and from approximately 700 nm to approximately 6 µm for ZnO in methanol. 48 hours passive ZnO in methanol treatment leads to much lower thickness than the direct impregnation case for similar concentrations.

The EDX analysis of ZnO-PES composites was performed. Due to the fact that a high energy electron beam damages the fibers quickly, this analysis has only qualitative value in our case. A typical EDX spectrum (for the sample prepared from 3% ZnO in methanol dispersion with the highest Zn content) is presented in Figure 4. The inset presents also the compositional analysis in mass (Wt%) and atomic (At%) percentages. The values are approximate values due to the limitations of the material in the specific analysis conditions. OPEN IN VIEWER

Wetting properties: CA measurements

DCA can be defined when the three phase line is in controlled motion. DCAs are divided into advancing and receding CAs, meaning CAs measured when the three phase line is in controlled movement by wetting the solid using the liquid or by withdrawing the liquid over a pre-wetted surface, respectively. ²⁰

In the case of chemical heterogeneity the surface has domains with different CAs. For example, when wetted with water, hydrophobic domains will pin the motion of the contact line as the liquid advances thus increasing the CA. When the water recedes, the hydrophilic domains will hold back the draining motion of the contact line, thus decreasing the CA. From this analysis, it can concluded that, when tested with water, advancing angles will be sensitive to the hydrophobic domains and receding angles will characterize the hydrophilic domains on the surface. DCAs were measured for all the samples prepared as described above. Water was used as the measuring liquid. In this experiment, it was found that surface roughness is only one of the main reasons causing the CA value modification, especially for the receding CA. In fact, the water molecules should overcome the forces of the chemical bonds when adsorbing or leaving the surface of fibers.

As one can see in Figure 5, comparing the three different experimental procedures of treatment of 100% PES samples with ZnO powder dispersed in methanol, the one treated for 48 hours had the strongest change in CA values. The maximum CA value for these specific samples reaches 120°, meaning a hydrophobic surface, when the ZnO concentration in dispersion was 3%. The lowest values of CA are achieved for impregnation of 100% PES samples with ZnO powder dispersed in water. The largest CA value (approximately 100°) for these preparation conditions corresponds again to a concentration of 3% ZnO in dispersion. Impregnation of 100% PES samples with ZnO powder dispersed in methanol reaches a maximum point (approximately 118°) at the same 3% ZnO concentration in dispersion. These observations lead to the conclusion that 3% ZnO concentration in any of the preparation conditions corresponds to the maximum CA and the best surface hydrophobicity. The use of methanol as a dispersion medium leads to slightly higher values of CA with the best hydrophobicity for the 48 hour-prepared samples. Larger ZnO concentrations lead to hydrophilic surfaces as well as lower concentrations. These results are comparable with the results presented by Popescu et al. ²⁰ for ZnO nanoparticles deposited by pulsed laser deposition method in optimized conditions. OPEN IN VIEWER

Finally, TGA analysis was performed. In order to determine the thermal behavior of ZnO-PES composite fabrics samples, their thermal decomposition was examined by using TGA under a nitrogen environment. Figure 6(a) and (b) shows the TGA curves of ZnO-PES composite fabrics prepared from the various ZnO concentration dispersions. In the case of samples resulted from water dispersions, the TGA curves showed that the untreated polyester fabric decomposed at a maximum rate at around 370℃ (nitrogen environment) as expected based on the fact that polyester fibers display good heat resistance. ZnO presence onto the fibers leads to a dramatic decrease in decomposition temperature (Td); for example, the sample prepared from 5% ZnO in dispersion begins decomposition at 150℃. This may advance the observation that the coating with ZnO does not improve the thermal stability of PES fibers but the opposite. The fabrics treated with the highest ZnO concentrations (samples 3–5) showed a two stage decomposition pattern which became more obvious at higher concentrations of ZnO. OPEN IN VIEWER

According to similar studies, the material decomposition occurs in two major steps separated by a development of a residue around 470℃ which progressively and totally degrades up to 560℃. ²¹ In our case, the first initial decomposition between 120℃ and 400℃ could be explained by the thermal decomposition due to random scission of the ester linkage and pyrolysis of the simple esters. Newly formed end groups decompose by secondary reactions following the second stage of decomposition between 400℃ 470℃ and 560. ²² The control PES sample is thermally stable up to 380℃, the temperature at which the neat thermoplastic loses 5% of its weight. Another possible explanation for the second stage decomposition between 400℃ and 470℃ accentuate with increasing ZnO concentration may be the reduction of Zn in the composite. The Td of 100% PES textile material treated with 3%, 5% and 7% concentrations of ZnO are 230℃ and 415℃, respectively, much lower than the untreated sample. Subsequently, the treatment of PES fabric with ZnO induced a decrease of thermal stability of PES support and the stability diminished with the increase of ZnO powder concentration. Regarding the coated fabrics, PET-ZnO composites reveal a thermal resistance at high temperature starting to degrade at 370℃ up to 460℃ where the material decomposition is slowed by a stabilization. This may be associated with ZnO presence as an oxygen source for the PES fibers thermal degradation.

According to graph plotting of the thermogravimetric curves (Figure 6(a) and (b)), it is noticeable that the control sample is the most stable with respect to thermal decomposition; taking into account the theoretical basis of polyester thermal stability, the Td starts at 370℃. Thermal stability of ZnO-PES composite fibers seems to be almost the same in both cases of ZnO dispersed in water and methanol (short and 48 hours). The highest Td among these (being thermally stable up to 300℃) appears to be the samples denoted 8 – PES plus 5% ZnO plus methanol, 10 – PES plus 1% ZnO plus methanol/48 hours and 12 – PES plus 5% ZnO plus methanol/48 hours, in Table 1.