Modification of PP Fabric with Polyols by the Plasma Composite Technique

Plasma technology is a new modification technology for fiber and textile which can improve the hygroscopicity of fiber and the dyeing property of textile. Depending on the type of dopant used, two main classes of treatments can be distinguished: grafting and coating. This paper mainly adopted the grafting way to activate and etch the sample surface [45].

PP fabric (4cm×8cm) was treated on glow-discharge plasma apparatus (Omega DT-03), which was supplied by Suzhou Optics Plasma Co., Ltd (China). Oxygen was used in the plasma pretreatment. The electrode distance was 4.5cm. The entry flow (200sccm, 250sccm, 300sccm), discharge power (50W, 100W, 150W, 200W, 250W, 300W), and treatment time (90s, 120s, 150s, 180s, 210s, 240s) were set as variables for the experiment.

Three 5wt% polyol solutions (sorbitol, maltitol, PEG-800) were individually prepared by dissolving the crosslinker in distilled water (100mL). The pH value was adjusted to 7–8 for the EGDE solution, and the pH value was 6–7 for the TTMA solution. Then the plasma-treated PP fabric was dipped into the polyol solution for a grafting effect. It was a single factor variable experiment, whose variables were the amount of crosslinkers (2%, 4%, 6%, 8%, 10%) (o.w.f), temperature (AMB, 50°C, 80°C), and time (8h, 12h, 16h, 20h, 24h). After the experiment, the samples were heated at 120°C for 3min, then rinsed with water, and finally dried again.

The surface chemical composition of the samples was determined using a Fourier transform infrared spectrometer (IR affinity, Shimadzu, Japan). The KBr tablet method was used for sample preparation. The samples were cut up and ground in a mortar with KBr, then placed in a press for tablet pressing. The samples were then put onto the spectrometer for testing. The scanning range was 500 ~ 4000cm⁻¹, the number of scans 200, and the resolution was 4cm⁻¹.

Reactive dyes (reactive yellow KE-4R, reactive scarlet B-3G, reactive deep blue K-R) were used to dye the PP fabrics at a bath ratio of 1:50. The PP fabric was dyed at 60°C for 30 minutes, and NaCl and Na₂CO₃ were added. After that, the PP fabric was dyed at 90°C for 30 minutes, then washed and dried. The dye-uptake was calculated by Equation 1, where A₁, A₂ are the absorbances of the dye solution before and after dyeing, N₁, N₂ are the dilution factors of the dye solution before and after dyeing. The absorbance was tested with an ultraviolet spectrophotometer of UV-1750 (Shimadzu, Japan). The colour depth was evaluated according to the K/S value, and the value was tested with a computerised colour matching instrument (CM-3600A, MINOLTA, Japan).

The grafting rate was used to analyse the immobilisation results of the polyols. Equation 2 is the calculation formula of the grafting rate, where G is the grafting rate; M₀ is the weight of PP after plasma treatment, and M₁ is the weight of PP after graft modification.

An S-3400N scanning electron microscope was used to observe the surface morphology changes of the original sample, the sample after plasma treatment, and the sample after modification. Before the SEM observation, the samples were coated with a carbon film, and then with a gold film (5nm) by ion sputtering. The voltage was 20kV, the working distance 11.8 mm, and the magnification was 2000x.

Loose fiber was suspended in H₂O to test the hydrophilicity of PP fabrics. First of all, the test sample is separated into fibers and marked. After that, the fiber was put in a beaker with 100mL of water for 3 min, and then the fiber settling velocity was observed and recorded. To avoid contingency, each group was tested three times.

The level of moisture release of PP fiber was used to characterise its moisture retention. The samples were firstly completely dried, and then they were placed in a humidity chamber under normal conditions (25 °C, 65% RH) and high humidity conditions (30 °C, 95% RH) successively for 80min. Lastly, the samples treated with high humidity were taken out and put into normal conditions (25 °C, 65% RH) again for 160 min. The sample was weighed for a random period of time. Moisture regain is defined by Equation 3, in which W_m is the weight (g) of the moisturised sample, and W_d is the weight (g) of the sample dried in an oven (100°C, 2 h): (3) Moistureregain(%)=Wm−WdWd×100 {\text{Moisture}}_{{\text{regain}}} \left( \% \right) = {{{\text{W}}_{\text{m}} - {\text{W}}_{\text{d}} } \over {{\text{W}}_{\text{d}} }} \times 100

The antistatic performance of the modified PP was analysed according to the half-life value. The samples (6cm×8cm) were treated by a fabric static tester (YG (L) 342D, Shandong Textile Research Institute (China)). The test conditions were 20 °C, and the relative humidity was 35%. The sample was placed in a high voltage electrostatic field when the charge was stable, and then the voltage was naturally attenuated through a grounding metal station. When the voltage value had decayed to the half, the corresponding time was recorded.

The original PP lacks polar functional groups, but the chemical composition of its surface will change after plasma treatment. Figure 3 shows the infrared spectra of PP fiber before and after O₂ plasma treatment. Obviously, the characteristic peak of -OH and -C=O groups at 3340 cm⁻¹ and 1716 cm⁻¹ appeared successively in the case of plasma (O₂)-PP. This shows that the PP surface is oxidised after oxygen plasma treatment. This is consistent with the emergence of new groups after the oxygen plasma treatment of paper [47]. Therefore, the amount of -COOH and -OH can be used to evaluate the effect of PP fiber treatment with O₂ plasma.

Fig. 3

Table 1 shows absorption values of the characteristic peak in single variable plasma experiment. It can be seen that the absorption value of hydroxyl and carbonyl groups reaches the maximum with the treatment condition of 250W, 180s and 300sccm. The value does not rise with an increase in power to 300W or discharge time to 240s. It may be that the newly formed functional groups were been further disrupted under high-power processing for a long time. Therefore, 250W, 180s and 300sccm are considered as the optimal process of O₂ plasma treatment. The PP obtained by the optimal plasma treatment were used to graft polyols.

The effect of O₂ plasma treatment on PP fiber can also be evaluated by a colouring experiment with reactive dyes because the reactive groups of dyes react with the O-containing groups (-OH or -COOH) of plasma (O₂)-PP fibers in the form of covalent bonds. Figure 4 shows photos of the original sample and PP fabric dyed with three dyes after O₂ plasma optimal treatment. Evidently, the effect of three dyes on plasma treated fabric is much better than that of untreated fabric. This indicated that PP fabric treated with O₂ plasma has a good dye permeability and bonding force. The colouring performance of reactive yellow dye is the best. Thus, reactive yellow dye is used to dye the PP after plasma treatment. The colouring performance of plasma treated PP was analysed by testing its K/S value and dye uptake, the results of which are shown in Table 1. It can be seen that the dyeing results were the best when the plasma treatment conditions were 250W, 180S and 300sccm. This is consistent with the conclusion of IR characteristic peaks, indicating that the more the amount of -COOH and -OH on the surface of PP, the better the colouration effect of PP will be.

Fig. 4

To investigate the grafting effect at low temperature and to prevent the self-polymerisation of polyols at high temperature, 80°C was set as the upper limit of grafting. The effect of the reaction temperature on the grafting rate was investigated under the condition of 4% crosslinker for 12h. Clearly, the grafting rate increased with the increase of temperature (Figure 5a), which is consistent with the result of other papers [42, 44]. It indicates that high temperature is conducive to the reaction of hydroxyl with epoxy and aziridine groups. The grafting rate of TTMA at 80°C was similar to that at 50°C, therefore 50°C was selected as the reaction temperature for TTMA. The grafting rate of EGDE increased with the increase of temperature, hence 80°C was selected as the reaction temperature for EGDE.

Under conditions of 80°C, 4% EGDE crosslinker usage, and 50°C, 4% crosslinker usage, the influence of time on the grafting rate is investigated. The corresponding result is presented in Figure 5b. The grafting rate of EGDE increased with the increase of reaction time, but did not increase after 16h. TTMA reached the maximum grafting rate when the reaction time was 12h, and then the grafting rate decreased, which was because TTMA self-polymerised with the increase of the reaction time. Therefore, 16h and 12h were selected as the reaction time for EGDE and TTMA. The less equilibrium time for TTMA may be due to the reaction of -COOH on the plasma (O₂)-PP surface with aziridine crosslinker being much faster than that of -OH on the plasma (O₂)-PP surface with the EGDE crosslinker at low temperature.

Polyols grafting experiments were carried out with various amounts of the crosslinker under conditions of 16h at 80°C for EGDE, and 12h at 50°C for TTMA. Figure 5c shows the results of the crosslinker amount with respect to the grafting rate. It can be seen that the grafting rate presents an increasing trend with the increase of the amount of crosslinker until 8% EGDE and 6% TTMA, respectively. Then, with the increase of the amount of crosslinker, the grafting rate did not change much. Therefore, a crosslinker dosage of 8% EGDE and 6% TTMA were separately selected. Comparatively, the polyol (no matter if it is sorbitol, maltitol, or PEG) grafting rate using EGDE as the crosslinker is less than for TTMA. This is because TTMA is more active and easier to react with carboxyl and hydroxyl groups. In addition, it was found that the grafting rate of three polyols followed the same order of PEG>maltitol>sorbitol, no matter whether it was EGDE crosslinker or TTMA crosslinker. This sequence may be caused by the different molecular weight of polyols, as the larger the molecular weight, the higher the grafting rate.