| Poly(vinyl alcohol) (PVA) is widely used in textile industry as a sizing agent. But cotton fabrics treated with PVA have to be desized to meet the subsequent processing (dyeing, printing and finishing) requirements. In conventional desizing process, cotton fabrics are washed with hot water and sometimes NaOH or oxidant such as H2O2 is used to remove PVA. It is not only water and energy consuming but the wastewater from the textile plant contains large quantities of PVA and oxidant agents, resulting in serious environmental pollution. Obviously, this conventional desizing process is unable to meet requirement of environmental protection and quite adverse to the sustainable development of textile industry. For this reason, the application of environmentally friendly low-temperature plasma is of great interest to the textile industry. If plasma treatments could degrade PVA in the desizing process, both chemical agents and PVA concentration in the wastewater would be significantly reduced. Meanwhile, the consumption of energy and water could be greatly decreased. Therefore, it has great environmental and economical benefits.However, most of the previous plasma treatments were carried out at low pressure, which can not be integrated into the production line due to existence of the vacuum system. It is thus economically infeasible for industrial treatment of low cost products such as most of the textiles. On the other hand, the recently developed non-equilibrium low temperature surface treatment at atmospheric pressure can be directly added on to the processing line. So far, there are two main types of atmospheric pressure plasma equipments, namely dielectric barrier discharge (DBD) and plasma jet in the market. DBD is capacitive and the treated materials have to pass through a narrow slit between two polar plates, while plasma jet has no such limitation in substrate shapes. One of the main differences between a low-pressure plasma treatment and an atmospheric pressure plasma treatment is that in atmosphere, the substrate material may absorb significant amount of water which may potentially influence the plasma treatment effects. But it does not need to be taken into account in low pressure plasma treatment since all the moisture will be removed before the treatment chamber could reach the required degree of vacuum. The existence of water molecules could lead to a more complicated interaction between active species in plasma and substrate surface, which could alter the microstructures and mechanical properties of materials.Poly(vinyl alcohol)(PVA) is a water-soluble synthetic polymer and its macromolecular structure and physical properties change significantly when in contact with liquid water or high humidity due to the loosening of the intermolecular bonds. It is proposed that the glass transition temperature (Tg) of PVA will drop from 68°C to below 20°C when the moisture content increases by more than 10%. Clearly, water greatly affects the internal structure, physical and mechanical properties of PVA. However, recent researches only focus on atmospheric pressure plasma desizing effect of PVA on cotton fabric and no systematic study has been reported about how moisture pre-existed in PVA could influence the desizing of PVA by means of plasma treatments.This research is aimed to employ the atmospheric pressure plasma treatment on PVA size removal and study the influence of moisture on the desizing effect. The effect of moisture on etching and solubility of PVA film by atmospheric pressure plasma treatment is studied and the desizing effect of cotton fabrics with different moisture regain is discussed systematically based on the results of various surface analysis, namely scanning electron microscope (SEM), atomic force microscopy (AFM), x-ray diffractometry (XPS), dynamic mechanical thermal analysis (DMA), x-ray diffraction analysis (XRD), wicking height, weight loss and solubility measurement.Firstly, the influence of various processing parameters on etching rate of PVA film by APPJ is investigated, including gas type, gas flow rate, output power, treatment duration, jet to substrate distance, moisture content and the underlining material. The etching rate increases as the output power, the oxygen flow rate, the flow rate of helium/oxygen mixed gas and the moisture regain increase. As the treatment duration and the helium flow rate increase, the etching rate increases initially and then decreases. The etching rate decreases as the jet to substrate distance increases and diminishes when the distance is above 6 mm. Meanwhile, the difference in thermal conductivity of underlining materials (plastic sheet, steel sheet and aluminium plate) has little effect on the etching rate. After the plasma treatment, the surface of the PVA film becomes rough and more polar groups are introduced, resulting in enhanced hydrophility.To investigate the relationship between the moisture and the solubility of the PVA film during plasma treatment, atmospheric pressure plasma jet (APPJ) is used to treat PVA films with moisture regain (MR) of 2.45%, 9.32% and 78.31% corresponding to 10%, 65% and 98% relative humidity (RH), respectively. Three substrate underlining materials including plastic sheet, steel sheet and aluminum plate are selected. Helium/oxygen mixture is used as the working gas. The treatment nozzle temperature is about 60°C, the output power is 100 W and the sample moving speed is about 2 mm/s. Another group is annealed at 140°C for 20 min to discern the thermal effects from those due to plasma treatment. It is found that the surface oxygen concentration increases for the plasma treated films with 65% and 98% RH and decreases for the plasma treated films with 10% RH. Among the three plasma treated samples, the one with 98% MR has the highest etching rate but the lowest solubility, especially for the samples underlined with plastic sheet during the plasma treatment. The decrease of solubility could be resulted from plasma enhanced surface crystallization of PVA as shown in SEM images and XRD analysis.To investigate the influence of moisture absorption on the effectiveness of plasma desizing, PVA sized cotton fabric is chosen as a model system. Placed onto aluminum plate, samples with three relative humidities (RHs) (10%, 65% and 98% respectively) are treated with APPJ using mixed gas of helium/oxygen and then subjected to water washing at 60°C. Solubility measurement reveals that the percent desizing ratio (PDR) increases and the washing time is shortened. Fabric with 10% RH had the highest desizing efficacy and the fiber surfaces are nearly as clean as the unsized fibers after 64 s exposure plus 20 min washing. The improvement of PDR is due to etching induced weight loss, macromolecular chain scission as well as increased oxygen-containing polar groups. After the plasma treatment, the wicking ability of cotton fabrics is improved with the removal of wax and pectin on fabric surface caused by bombardment of plasma species. In addition, no significant decrease in single yarn tensile strength is observed for the plasma treated yarns.The influence of moisture on low-stress mechanical and surface properties of treated cotton fabric is also studied. Experimental results indicate that after the plasma treatment, surface properties such as surface friction (MIU) and surface roughness (SMD) increased for all three treated fabrics, especially for fabric with 98% MR as verified by AFM results. The enhancement of surface friction (MIU) increased interyarn or interfiber friction, hindered fabric stress deformation and recoverability and consequently led to an increase in low-stress mechanical properties, such as tensile, shear and compression properties.Finally, the relationship between DBD plasma treatment time and desizing effect for a cotton fabric is determined and compared with the conventional pre-treatment process. Four groups are treated by argon/oxygen DBD with treatment durations of 1, 2, 4 and 6 min respectively, followed by hot (60°C) and/or cold (25°C) wash. It is found that plasma treatment time has a significant influence on the etching effect and desizing efficacy. SEM shows that PVA on the fabric surface is broken into pieces after short time treatment. The fiber surface becomes somewhat rough and even micro-cracks appear as the treatment time prolongs due to intensive plasma etching. Solubility measurement reveals that plasma treatment increases PVA solubility in hot washing but less effective in cold washing. Wicking tests indicate that the capillary heights of plasma treated fabrics increase significantly due to the destruction of continuous cuticle layer, even larger than that of conventional scouring. The whiteness of the treated fabrics improves slightly. The yarn tensile strength test results show that short DBD exposure time has no negative effect on fabric tensile strength while the long treatment time may slightly reduce the fabric tensile strength. Therefore, the DBD treatment may assist the conventional desizing and meet scouring requirements. |
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