The structure and properties of the yarn and fabric were optimised considering the characteristics of filter fabrics (Table 1). Different types of surface treatment of polyester lead to different types of etching due to chemical modification. The surface becomes smooth, which leads to deterioration of wettability.
Table 1
Structure characteristics of filters.
| Mass of filter, g | Mass of composition, g | Thickness, mm | Area density, g/m2 | Bulk density, g/cm3 |
Polyester | 0.72 | - | 0.315 | 128 | 2.28 |
Padded polyester | 0.87 | 0.14 | 0.333 | 155 | 2.61 |
Covered polyester | 1.08 | 0.36 | 0.340 | 192 | 3.17 |
The Finishing process happens due to the mechanism shown in Fig. 1. Partially esterified melamine can be linked covalently to polyester fabrics and styrene-acrylic binder by curing at 140–150 0C through trans-amidation. At these temperatures, the primary amino groups of the melamine can react with the carboxylic acids groups of styrene-acrylate and with accessible ester groups at the surface of the polyester fibre.
Polymer matrix and textile material can stabilise nanoparticles. Such stabilisation provides fixing the nanoparticles to each other or fixing the particle layer on the support layer. Such treatment enabled a diversity of textile materials and their pore geometries. High flow resistance is possible during filtration if the particles used as fillers are selected by size.
Surface chemical modifications of finished polyester fabric were determined by an FTIR analysis in the range of 500–4000 cm− 1. Figure 2 shows the FTIR spectra obtained for the untreated polyester, padded polyester and covered polyester. In untreated polyester, the presence of ester, alcohol, anhydride, aromatic ring, and heterocyclic aromatic rings can be seen. An absorption peak at the wavelength of 1710 cm− 1 is related to the stretching vibration of the carbonyl group (C = O), the peak at 700 cm− 1 attributed to the out-of-plane bending vibration of the C-H on the benzene ring, and the peak at 1014 cm− 1 is assigned O-H out-of-plane bending in terminal carboxylic groups 13. This is a reason that there is still alcohol and anhydride as residual reactants left in the polyester. The carboxyl, ester, anhydride and alcohol groups showed that the polyester fabric was not pure. There is not much change in the padded polyester since the intensity and position of absorption peaks were not changed too much. Cleary can be seen an absorption bend of C = O vibration is shifted to the left (1760 cm− 1), indicating the adhesion of acrylic acid on the surface, and it introduced carboxyl groups 14. Covered polyester shown an absorption peak at 2957 cm− 1 assigned to stretching vibration of C-H groups, intensive band at 1449 cm− 1 is due to bending vibrations of -CH2 groups; the band at 1162 cm− 1 corresponds to stretching vibrations of C–O–C, which indicates the self-cross-linking in the polymer composition on the surface of polyester due to the etherification of carboxyl groups. The wavelength of C = O shifts to the left (1729 cm− 1), indicating the increase in the absorption intensity due to the introduction of acidic groups. Also, the proportion of the oxygen-containing groups such as C = O, –C–OH and –COOH increased on the surface of the treated fabric. These results may be attributed to the fact that active groups of polymer nanocomposite react with active –O–C = O (carboxylic) groups of polyester fabric, resulting in the formation of oxygen-containing polar groups on the fabric surface. The introduction of oxygen-containing polar groups on the fabric surface changes the nature of the surface. Evidently, the intensity of peaks of covered polyester is sharpened over the untreated and padded, indicating acrylic acid makes bonding with polyester fibres in the process.
The observed absorption bands (Fig. 2b, c) at 608.6 cm− 1 indicate the stretching vibrations of Zn-O in the covered and padded polyester sample, so it signifies that the ZnO nanoparticles are presented 15.
Water absorption test
Water absorption (retention) test results of the filter materials are given in Fig. 3. The standard polyester fabric absorbs a minimal amount of water (0.8 %) into its structure. Water absorption decreases dramatically at padded polyester. Polymer nanocomposite applied to the polyester surface made a significant contribution to the water absorbency of filter media. It was detected that the differences between the water absorption values of padded and covered polyester occurred because of the low adsorption property of the polymer composition that is due to their ability to decrease the porous diameter and change fabric structure.
The structure of the surfaces
The structure of the coated surfaces was characterized by scanning electron microscopy (SEM). The SEM images were used to investigate the change in the surface morphology of the untreated, padded and covered polyester fabric, as shown in Fig. 4 (A-C). The surface of untreated polyester (A) is smooth and distinct. The polyester surface becomes rough due to the applying of the polymer nanocomposite. The surface of the covered polyester consists of big particles of unmixed acrylic dispersion, that's why pores are not regular (B). The padded polyester (C) shows a drastic change in the fibre surface morphology, with the presence of voids and pores. On the surface of padded polyester, single ZnO nanoparticles can be clearly seen. The wires are flatter, as they are covered by polymer composition.
The SEM images show that the structure of treated polyester is relatively bulky, and the structure of the polymer composition is relatively dense. Due to the fact that polymer composition consists of ZnO nanoparticles, on the surface of textile can be formed cross-linked structure which has crosslinked units with components of composition and polyester fibres. As a result, the structure became crisscross and intertwined, so the filter pore size decreased. This factor can’t be ignored, as the combination of polymer nanocomposite surface and polyester substrate will increase the filtration resistance and improve the filtration efficiency. Produced layer decreases the possibility for the penetration of the dye molecules into the fibres and lets them rather adsorbed on the surface.
The result of the elemental mapping performed on the surface of the padded polyester fabric sample is shown in Fig. 5. The nanoscale ZnO particles can be clearly seen well distributed on the surface of polyester. The particle size plays a primary role in determining their adhesion to the fibre. It is reasonable to expect that the largest particle agglomerates will be easily removed from the fibre surface. In contrast, the smaller particles in the polymer composition will penetrate deeper and adhere strongly to the fabric matrix.
Hydrophobicity
Hydrophobicity improved by modification of the filters using polymer composition. The results of the studied filters are shown in Fig. 6. When the polyester textile material was deposited with polymer composition, the contact angle was dramatically increased, indicating that the hydrophobicity of the surface was greatly improved.
Untreated textile material exhibits poor hydrophobicity with water contact angle (WCA) 30.6 0. Covered polyester has good hydrophobic properties, with a measured WCA of 100.00. Padded polyester has poor hydrophilic properties WCA 60.80.
This finding is in contradiction with the results of polymer nanocomposite film formed on the glass surface reported in previous studies. The polymer film exhibit smoothness surface 16. Polyester fabric is characterised by hairiness which forms an unregular surface. As the polymer film texture is created on the surface of textile fiber, polymer nanocomposite build-up hierarchical structures and increases surface roughness 17, which significantly improve the hydrophobic properties of treated polyester fabric18. The primary reason that produced filters realises hydrophobicity is that polluted water droplets can be stably supported on the hierarchical structure of the filter surface, and dyes could form pockets in the interface.
Low-temperature nitrogen adsorption-desorption isotherms
Figure 7 shows the nitrogen adsorption-desorption isotherms obtained at 77 K for polyester textile material prepared with different treatment techniques. The covered and padded polyester exhibit steep type III isotherms, indicating the occurrence of macroporous. Additionally, the development of mesoporosity is indicated by the pronounced desorption hysteresis loops that appear for samples. Covered polyester shows a wide hysteresis loop, and the desorption curve is steeper than the adsorption branch, indicating that the samples have various pore types and pore diameter distributions. Padded polyester characterised with narrow hysteresis loop. Meanwhile, untreated polyester shows open-wedge pores.
The surface areas of the filter available to the nitrogen vapour were calculated according to the BET equation. The calculated surface area values are presented in Table 2. It is pointed out that the surface area available to the nitrogen vapour is greatly dependent upon the way of polyester treatment. Polymer nanocomposite film was also tested and showed a non-porous structure with low specific surface values of 8.7 m2/g. A small hysteresis loop can be seen, which indicates the presence of some pores or holes in the network.
Table 2
Characteristics of the produced filters surface
| Surface area SBET, m2/g | Total pore volume Vtot, cm3/g | Pore size, nm |
Polyester | 5.9 | 0.036 | 3.4 |
Padded polyester | 85.2 | 0.065 | 3.4 |
Covered polyester | 44.6 | 0.028 | 3.1 |
Polymer composition | 8.7 | --- | --- |
Pore Size Distribution
The proper selection of filter material is an essential factor in achieving efficient filtration. Pore size measurements Fig. 8 shows the distribution of the pore size.
The average pore size of untreated polyester is ~ 3.4 nm, covered polyester is ~ 3.08 nm, and padded polyester is ~ 3.4 nm. Total pore volume is 0.036 cm3/g, 0.028 cm3/g and 0.065 cm3/g, respectively. The pore size of covered polyester is smaller compared with untreated and padded polyester. Moreover, the average pore size of untreated polyester is the same as padded. Two more maximums of pore distribution 4.2 nm and 12.4 nm at padded polyester, indicate that pores are not regular. The surface of the polyester substrate consists of a large number of fibres that can interweave and pile up with each other, contributing to the formation of some large pores between the fibres. On the other hand, the accumulation of fibres on the surface may result in partial blockage of the pores and, therefore, some small pores formed.19
Filtration experiment
The possibility of using such fabrics for filters was explored with methylene blue and methyl orange solution. The size of the methylene blue molecule is around 13.82 Å20. The methyl orange molecules have a larger size ~ 26.14 Å 21. Considering the length of the dyes molecules and the dimension of the pores in filters, organic dyes can easily enter into the pores. It was shown that the concentration of MB decreased from 100 ppm to 60 ppm using the padded polyester, and the content of MO declined from 100 ppm to 40 ppm assisting the covered polyester (Fig. 9–10). These observations indicate that organic dyes of different nature can be effectively removed from the water by using the suitably processed textile filter. It is evident that the covered technology almost totally encloses the pores. Meanwhile, padded technology decries the ratio between fibres and form the solid covering of the pores.
and solutions after filtration
Filtration efficiency
It was noticed that thru the filtration process, the movement of the dyes particles typically deviates from the water flow, especially as they approach the fibre. During the filtration Brownian diffusion, the electrostatic effect and the gravity effect happened. The electrostatic effect firmly attaches the particles to the surface of the fibres. Results are presented in Table 3.
Table 3
Results of filtration of organic dyes.
| pH ZPC | MB | MO |
Equilibrium quantity that can adsorb on the filter mg/g in t 120sec | Removal rate (R), % | Equilibrium quantity adsorb on the filter mg/g in t 120sec | Removal rate (R), % |
Polyester | 6.81 | 0.69 | 20 | 0.520 | 15 |
Padded polyester | 6.29 | 1.14 | 40 | 1 | 35 |
Covered polyester | 6.65 | 0.81 | 35 | 0.92 | 60 |