The effect of dielectric barrier discharge plasma treatment on the air drag force of polyacrylonitrile,polyethylene,polypropylene and polyethylene terephthalate yarns

Abstract

In this paper, atmospheric pressure dielectric barrier discharge (DBD) plasma was used to treat four kinds of chemical fiber yarns, namely polyacrylonitrile (PAN), polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET), for which the air drag force, moisture regain and yarn diameter were measured. Compared with untreated samples, the air drags of treated PAN, PE, PP and PET yarns increased maximally by 10.7%, 43.0%, 40.0% and 18.9%, respectively, in the present experiments. A two-way analysis of variance and correlation analysis were performed to study the effect of plasma treatment on air drag and other properties of these four chemical fiber yarns. The combination of X-ray photoelectron spectroscopy analysis and the determination of the DBD plasma component can further enhance the understanding of the difference between DBD plasma treatment processes among these four chemical fiber yarns. This study provided the experimental basis to improve the air-jet weaving efficiency of chemical fiber fabrics.

Keywords

chemical fiber yarn dielectric barrier discharge plasma air drag force moisture regain two-way analysis of variance

In recent decades, low-temperature plasma has become known as a clean and dry surface modification technique for various polymeric and natural fibers without affecting their bulk characteristics.¹ This technology provides an attractive means of on-line treatment with low quantities of starting materials and energy saving. Dielectric barrier discharge (DBD) plasma is one of the common plasma surface modification techniques, which has several advantages including simple equipment, easy operation, running at ambient pressure, etc. DBD plasma treatment can provide different types of functionalized surface or multifunctional surface via introducing excited or polar groups or even a new polymer layer, which depends on the type of gas used in the plasma reactor (air, oxygen, nitrogen, etc.) as per the applications, for example, those used in apparel sectors, including polyethylene terephthalate (PET), cotton, wool, silk, etc.^2–5 Some interesting results have been obtained in the improvement of the wettability, dyeability and other desired properties by the use of atmospheric DBD plasma.^6,7

Besides the study mentioned above, up to now, little research was found about how to improve air-jet weaving efficiency through plasma modification of chemical fiber yarns. The air-jet loom is a widely used shuttle-less weaving machine at present, of which the main disadvantage is high air consumption.⁸ Therefore, research works have mainly focused on the improvement of air-jet weaving efficiency through the following two routes: (1) optimizing the operational parameters of nozzles and the air-supply system;^9–11 (2) optimizing the geometric parameters of the main nozzle and the sub-nozzle.^12,13 In fact, the air drag of the yarn is caused by the viscous effect between the air flow and yarn surface when the yarn moves in the air flow.¹⁴ Our previous research indicated that the change in both chemical and physical surface properties of PET yarn via DBD plasma treatment could affect its air drag. Compared with the untreated sample, the air drag of the treated PET yarn could increase by 18.9% maximally.¹⁵ However, further study needs to be conducted to verify the universality of this technique for other chemical fiber yarns, which can not only provide optimum processing parameters of plasma treatment for air-jet weft insertion of chemical fiber yarns, but also develop a new route for the cleaner industrial production of chemical fiber fabrics.

In this paper, several chemical fiber yarns with distinct chemical structures, namely polyacrylonitrile (PAN), polyethylene (PE), polypropylene (PP) and PET yarns, were utilized as experimental samples, which were subjected to the atmospheric DBD air plasma treatment under the same processing condition. The variation trends of air drag force for these four chemical fiber yarns treated with different dosages of DBD were compared. A two-way analysis of variance (ANOVA) and correlation analysis were performed to study the effect of plasma treatment on air drag and other properties of these four chemical fiber yarns. X-ray photoelectron spectroscopy (XPS) analysis was performed to study the characteristics of the DBD surface modification. Moreover, the analysis of the DBD tail gas component absorbed with deionized water could further enhance the understanding of the mechanism of air drag improvement via DBD plasma treatment.

Experimental details

Experimental setup and plasma treatment details of yarn samples

A schematic diagram of the DBD plasma treatment platform is shown in Figure 1. It mainly consists of a high-frequency alternating current (AC) power supply (CTP-2000K, Coronalab, Nanjing Suman Electronic Co., Ltd, China) and a DBD plasma reactor. The reactor was a rectangular hollow section quartz tube with a length of 400 mm and a discharge gap of 4 mm, generating DBD in atmospheric pressure air under ambient conditions. Stainless fine wire meshes (300 mm in length) were used as high-voltage (HV) and ground electrodes, placed on the top and bottom sides of the quartz tube. The HV electrode was connected to the power supply. A winding device (speed adjustable) was used to drag the yarn through a small circular opening at the center of the sealed cap fixed at the right-hand side of the reactor. The outlet of the reactor was connected to a tail gas absorber.

Figure 1.

Schematic diagram of the dielectric barrier discharge (DBD) plasma treatment setup.

Each yarn sample was cut into lengths of 2 m. The gas flow rate in the plasma reactor was set at 400 mL/min. When the reactor discharge was stabilized, the samples were dragged by the winding device through the reactor to complete the plasma treatment. To eliminate experimental error, the control sample was also pulled through the reactor. The discharge power (W) can be calculated by Equation (1)

W_{p} = UI cos θ

(1)

where U (V) is the voltage and I (A) is the current that is detected by the voltage and current meter and θ is the phase difference between voltage and current. The value of θ is 45 ° (measured by the digital oscilloscope) in the present experiments. The treatment power (W_p) ranged from 20 to 30 W with an interval of 2 W. The treatment time (T) was set as 3, 6, 15 and 30 s. Table 1 shows the combination of treatment power and treatment time in the present experiment.

Table 1.

The combination of treatment power and treatment time in the present experiment

Power (W) Time (s)	20 (W_p1)	22 (W_p2)	24 (W_p3)	26 (W_p4)	28 (W_p5)	30 (W_p6)
3 (T₁)	T ₁ W _p1	T ₁ W _p2	T ₁ W _p3	T ₁ W _p4	T ₁ W _p5	T ₁ W _p6
6 (T₂)	T ₂ W _p1	T ₂ W _p2	T ₂ W _p3	T ₂ W _p4	T ₂ W _p5	T ₂ W _p6
15 (T₃)	T ₃ W _p1	T ₃ W _p2	T ₃ W _p3	T ₃ W _p4	T ₃ W _p5	T ₃ W _p6
30 (T₄)	T ₄ W _p1	T ₄ W _p2	T ₄ W _p3	T ₄ W _p4	T ₄ W _p5	T ₄ W _p6

Materials

Figure 2 shows the chemical structures of these chemical fiber yarns. The characteristics of PAN, PET, PE and PP are 369, 166, 173 and 444 dtex, respectively, and were purchased at a local textile store. The specific terms used in the field of the textile industry and their meanings can be consulted in a textile glossary.

Figure 2.

Chemical structures of (a) polyacrylonitrile, (b) polyethylene, (c) polypropylene and (d) polyethylene terephthalate.

Air drag force measurement

The air drag force of yarn samples was measured by the yarn tension testing platform, including an air-supply system and a yarn tension tester. The detailed measurement procedure can be found in our previous work.¹⁵

X-ray photoelectron spectroscopy characterization of chemical fiber yarns

The element analysis of treated and control samples was conducted by XPS. The XPS spectra were recorded using a Kratos Axis Ultra DLD spectrometer (Shimadzu, Japan). The detailed procedure can be found in our previous work.¹⁵

Moisture regain measurement of chemical fiber yarns

According to GB/T9995-997, the moisture regains (M) of the untreated and treated yarn samples were tested by the infrared heating oven drying method,¹⁶ which can be expressed by Equation (2)

M % = \frac{(W_{m} - W_{d})}{W_{d}} \times 100 %

(2)

where W_d is the dry weight of the yarn samples and W_m is the wet weight of the same samples after saturation at standard humidity.

Compared with M of untreated yarn, the growth rate (R_m%) of moisture regain of treated yarn, which could be considered as the contribution to the improvement of wettability after DBD plasma treatment, was expressed by Equation (3)

R_{m} = \frac{M_{i} - M_{0}}{M_{0}} \times 100 %

(3)

where M_i is the moisture regain of the plasma-treated yarn and M₀ is that of the same sample without any treatment.

Yarn diameter measurement

The yarn diameter was measured with a wide-field fluorescence microscope (MacroZoom Z16, Leica). Owing to the elasticity of the fiber bundle, the yarn diameter varied with the change of tension. The detailed measuring method can be found in our previous work.¹⁷ Each sample was prepared with three copies and the measurements were performed at 30 different locations on each copy randomly, of which the average was taken.

Determination of reactive species in DBD plasma flow under different treatment powers

The plasma tail gas was absorbed with 50 mL deionized water (see Figure 1). The absorbing time was 10 minutes for each sample and the flow rate was set at 400 mL/min. According to the potassium indigotrisulfonate method, dissolved ozone in aqueous solution was identified by ultraviolet-visible (UV-Vis) spectrophotometry (TU-1810) at the wavelength of 600 nm.^18,19 The ${NO}_{2}^{-}$ and ${NO}_{3}^{-}$ in the absorbing solution were detected via ion chromatography (883 Basic IC plus, Metrohm).²⁰

Results and discussion

The effect of yarn diameter on air drag of the four chemical fiber yarns

The characteristics of untreated PAN, PE, PP and PET are 369, 173, 444 and 166 dtex, respectively, of which the diameters are 313.20, 186.88, 389.06 and 251.61 µm, correspondingly. Under the same air-supply condition, the air drags of untreated PAN, PE, PP and PET yarns were 39.24, 9.64, 19.44 and 23.77 cN, respectively (Table 2). Obviously, the air drag of PP, which has the largest yarn diameter, only ranked third; the air drags of the other three yarns increased with the growth of diameter; however, the increases of air drags were out of proportion to the rise of their diameters. Equation (4) gives the air drag force (F_d) of yarn

F_{d} = \frac{1}{2} C_{d} ρ π d (U - V)^{2} L

(4)

where C_d is the drag coefficient, ρ is flow density, d is the diameter of the yarn, U and V are the velocity of the flow and the yarn, respectively, and L is the length of the yarn immersing in the jet flow. The results indicated that the yarn diameter is only one of the factors to affect air drag.

Table 2.

The air drag forces and yarn diameters of the four untreated chemical fiber yarns

	PAN	PE	PP	PET
Air drag force (cN)	39.24 ± 1.49	9.64 ± 0.39	19.44 ± 1.07	23.77 ± 0.93
Yarn diameter (µm)	313.20 ± 5.64	186.88 ± 3.36	389.06 ± 7.56	251.61 ± 4.59

PAN: polyacrylonitrile; PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate.

The effect of DBD plasma treatment on yarn diameter and air drag

After DBD plasma treatment, the diameters of these four types of yarns changed slightly (Table 3), among which the average diameters of PAN, PE and PP increased by 0.04 µm (0.12%), 4.75 µm (2.54%) and 4.99 µm (1.28%), respectively, while that of PET decreased by 0.59 µm (–0.24%). Relvas et al.⁵ reported a similar phenomenon when they processed quiscal fibers with plasma, which may be due to the deposition of radicals or chemical groups generated due to plasma treatment or significant etching and oxidation at higher plasma doses. In this study, the difference in yarn diameter variation among the four chemical fiber yarns may attribute to the difference of resistance to plasma oxidation due to the different chemical structures of chemical fiber yarns. Under the same air-supply condition, the air drags of these four treated chemical fiber yarns changes obviously, among which the average air drags of PP, PE, PET and PAN increased by 34.59%, 13.49%, 3.83% and 2.53%, respectively. Obviously, the increase of air drags for these four chemical fiber yarns were much larger than the growth of their diameter, especially for PP. Therefore, it is implied that the drag coefficient C_d also affected the increase of air drag. As reported by other authors, the surface properties of the yarn sample could influence C_d.^15,21 In this study, in the given flow field, the variation of air drag was mainly attributed to the change in chemical characteristics and surface roughness of the sample surface via DBD plasma treatment. Our previous work has verified that this plasma treatment could generate nano-grooves on PET surfaces along the length of yarn,¹⁵ which might cause drag reduction;²² meanwhile, the grafted polar groups via plasma could enhance the air drag force (see the schematic diagram in Figure 3). The two-way ANOVA for the effect of T and W_p on diameter (d ) for PET, PE, PP and PAN yarns was carried out (Table 4). Both T and W_p had no significant effect on the d value of these four chemical fiber yarns; no significant interaction was observed between T and W_p for PET, PE, PP and PAN. The results indicated that the treatment energy might not be strong enough to affect the diameter effectively under most treatment conditions in the present experiment.

Table 3.

The effect of dielectric barrier discharge plasma treatment on yarn diameter and air drag

	PAN	PE	PP	PET
Average diameter after plasma treatment (µm)	313.24 ± 15.98	191.63 ± 9.39	394.05 ± 26.40	251.02 ± 13.56
Growth rate of average diameter (%)	0.12	2.54	1.28	–0.24
Average increment of air drag (cN)	0.99 ± 0.31	1.30 ± 0.36	6.72 ± 1.95	0.78 ± 0.26
Growth rate of average air drag (%)	2.53	13.49	34.59	3.83

Note: The average data represent the averages of corresponding data of all the plasma-treated samples for each type of chemical fiber yarn.

PAN: polyacrylonitrile; PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate.

Figure 3.

Mechanism of the air drag variation via plasma.

Table 4.

Two-way analysis of variance for the effect of treatment time (T) and power (W_p) on diameter (d)

Factor		Type III sum of squares	Degree of freedom	Mean square	F	Significance
PET	T	212.409	3	70.803	1.478	0.232
	W _p	458.990	5	91.798	1.916	0.109
	T*W_p	875.880	15	58.392	1.219	0.291
	Error	2300.137	48	47.920
PE	T	413.819	3	137.940	1.505	0.225
	W _p	607.569	5	121.514	1.326	0.269
	T*W_p	669.931	15	44.662	0.487	0.936
	Error	4400.000	48	91.667
PP	T	195.153	3	65.051	0.598	0.619
	W _p	737.569	5	147.514	1.356	0.258
	T*W_p	1191.264	15	79.418	0.730	0.743
	Error	5222.000	48	108.792
PAN	T	274.778	3	91.593	1.422	0.248
	W _p	562.500	5	112.500	1.747	0.142
	T*W_p	983.389	15	65.559	1.018	0.454
	Error	3091.333	48	64.403

PAN: polyacrylonitrile; PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate.

The difference in the effect of DBD plasma treatment power and treatment time on air drag among the four chemical fiber yarns

In order to explore the difference in the effect of DBD plasma treatment power and treatment time on air drag among the four chemical fiber yarns, each type of yarn sample was subjected to the same plasma treatment (see the Experimental setup and plasma treatment details of yarn samples section). Subsequently, the air drags of treated samples were compared, for which the difference and possible mechanism were analyzed (Figure 4). All the 24 samples for each chemical fiber yarn were divided into four groups based on the level of T, including T₁W_p1–6 (3 s), T₂W_p1–6 (6 s), T₃W_p1–6 (15 s) and T₄W_p1–6 (30 s) (see Table 1).

Figure 4.

The air drag increases of the four chemical fiber yarns in different groups: (a) T₁W_p1–6; (b) T₂W_p1–6; (c) T₃W_p1–6; and (d) T₄W_p1–6. PAN: polyacrylonitrile; PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate.

Figure 4(a) shows the air drag treated for 3 s via DBD plasma, in which the average increases of air drag for PAN, PE, PP and PET were 0.54 cN (95% confidence interval (CI): 0.39–0.69), 0.38 cN (95% CI: 0.25–0.51), 0.40 cN (95% CI: 0.27–0.53) and 0.60 cN (95% CI: 0.38–0.83), respectively. The maximal increases of air drag for PAN, PE, PP and PET were 1.01 cN (24 W), 0.56 cN (20 and 28 W), 0.90 cN (24 W) and 1.79 cN (26 W), respectively (PET > PAN >PP > PE). Compared with the other three yarns, the ester groups in PET molecules were more easily broken by plasma; therefore, the chain scissions in the PET backbone would generate more active sites for polar group grafting, which meant PET yarns had the maximal increase in air drag. By contrast, the backbones of PAN, PE and PP were relatively more stable, and hence fewer polar groups were grafted in such a short treatment time, which led to the weaker interaction between polar groups on fiber surfaces and water molecules in the air flow. As the backbones of PAN, PE and PP have a similar chemical structure, the difference in air drag might mainly be attributed to yarn diameter; when the plasma treatment was inadequate, the yarn with a larger diameter would have more surface area exposed to plasma, and hence would be more easily grafted with more polar groups, which may be the reason that the increase in air drag of PAN was larger than those of PP and PE. When treated at 20 W, the air drags of PAN, PP and PET were all lower than those of untreated samples.

As can be seen in Figure 4(b), when the treatment time was extended to 6 s, the average increments of air drag for treated PAN, PE, PP and PET yarns were 2.20 (95% CI: 1.36–3.04), 1.77 (95% CI: 1.15–2.39), 2.22 (95% CI: 1.33–3.11) and 1.12 cN (95% CI: 0.62–1.62), respectively. The maximal increases of air drag for PAN, PE, PP and PET were 4.26 cN (28 W), 3.14 cN (24 W), 4.04 cN (26 W) and 3.59 cN (26 W), respectively (PAN > PP > PET > PE). Compared with the samples in the T₁W_p1–6 (3 s) group, with the increase of treatment time, the growth of air drag for each chemical fiber yarn improved significantly, which further indicated that the plasma treatment with relatively longer time could graft more polar groups on fiber surfaces, and hence further improved the drag coefficient C_d and air drag. However, owing to the different chemical characteristics of yarns, each type of yarn would have selectivity for different plasma reactive species, which can be verified such that the maximal increase of air drag among different chemical fiber yarns corresponded to different treatment powers. When treated at 20 W, the air drags of PE and PP were lower than those of the untreated samples.

As can be seen from Figure 4(c), when the treatment time was extended to 15 s, the average increments of air drag among treated PAN, PE, PP and PET yarns were 0.78 (95% CI: 0.51–1.05), 2.94 (95% CI: 2.12–3.76), 6.45 (95% CI: 5.48–7.42) and 1.12 cN (95% CI: 0.50–1.74), respectively. The maximal increases of air drag for PAN, PE, PP and PET were 1.96 cN (26 W), 4.15 cN (24 W), 7.77 cN (24 W) and 4.04 cN (26 W), respectively (PP > PE > PET > PAN). Obviously, with the further increase of treatment time, the air drag of PAN yarns decreased dramatically while those of PP and PE increased significantly. As the cyano groups in the PAN backbone possess a certain degree of polarity, the moisture absorbance of PAN is higher than those of the other three chemical fiber yarns. In addition, the cyano groups are relatively more stable, which may limit the number of polar groups grafted on PAN via plasma; the longer treatment time may decrease the number of cyano groups through chain scissions to reduce the wettability of the yarns. Therefore, plasma treatment with shorter time may be more suitable for PAN yarns to improve air drag. Compared with PE, the methyl groups from PP could be easily broken by the bombardment of high-energy electrons or the oxidation of reactive species, forming methyl radicals (•CH₃). As methyl radicals are reactive, they could easily take part in the reaction with other plasma species,²³ which may produce more polar groups grafted on the PP surface, and hence increase C_d and air drag. The average increment of air drag for PET decreased but the maximal increase of air drag improved obviously, compared with those in the T₂W_p1–6 (6 s) group; when the treatment power surpassed 28 W, the air drag was lower than that of the untreated sample. This indicates that when treated for a longer time, the plasma operating parameters for PET need to be controlled in a reasonable range.

As shown in Figure 4(d), when the treatment time was extended to 30 s, the average increments of air drag for treated PAN, PE, PP and PET yarns were 0.46 (95% CI: 0.33–0.59), 0.11 (95% CI: –0.23–0.46), 2.47 (95% CI: 1.43–3.51) and 0.30 cN (95% CI: –0.19–0.80), respectively. The maximal increases of air drag for PAN, PE, PP and PET were 1.01 cN (24 W), 2.92 cN (24 W), 5.53 cN (24 W) and 4.49 cN (24 W), respectively (PET > PE > PP > PAN). These four chemical fiber yarns all showed negative growth in air drag, among which PET showed that when treatment power exceeded 26 W, PE showed that when treatment power surpassed 28 W and PP and PAN showed that when treatment power surpassed 30 W.

Overall, the air drag of different yarns changed with the variation of plasma treatment conditions. Meanwhile, their own chemical characteristics and molecular structure also affected the air drags. In the present experiment, the optimal plasma treatment condition for PAN is 24–30 W, 6 s; for PE is 22–26 W, 6 s and 20–24 W, 15 s; for PP is 24–26 W, 6 s and 20–30 W, 15 s and 22–24 W, 30 s; for PET is 26–28 W, 6 s and 26 W, 15 s, and 24 W, 30 s.

The two-way ANOVA has been performed to study the effect of treatment time (T) and power (W_p) on the air drag (F_d) of PET, PE, PP and PAN yarns. As can be seen from Table 5, both T and W_p had significant effects on F_d for PET, PE, PP and PAN yarns (P < 0.001); a significant interaction (P < 0.001) was found between factors for each chemical fiber yarn. The results indicated that time, power and their combination could effectively affect the F_d for these four chemical fiber yarns.

Table 5.

Two-way analysis of variance for the effect of treatment time (T) and power (W_p) on air drag (F_d)

Factor		Type III sum of squares	Degree of freedom	Mean square	F	Significance
PET	T	11.298	3	3.766	31.349	<0.001
	W _p	46.623	5	9.325	77.619	<0.001
	T*W_p	98.162	15	6.544	54.474	<0.001
	Error	5.766	48	0.120
PE	T	97.240	3	32.413	165.743	<0.001
	W _p	30.681	5	6.136	31.377	<0.001
	T*W_p	35.570	15	2.371	12.126	<0.001
	Error	9.387	48	0.196
PP	T	351.645	3	117.215	411.330	<0.001
	W _p	74.641	5	14.928	52.386	<0.001
	T*W_p	48.631	15	3.242	11.377	<0.001
	Error	13.678	48	0.285
PAN	T	41.970	3	13.990	34.357	<0.001
	W _p	22.041	5	4.408	10.826	<0.001
	T*W_p	24.959	15	1.664	4.086	<0.001
	Error	19.545	48	0.407

PAN: polyacrylonitrile; PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate.

Correlation analysis between the variation of air drag and diameter for different chemical fiber yarns

The results in the The effect of DBD plasma treatment on yarn diameter and air drag section indicated that the plasma could vary the yarn diameter, which may be attributed to the deposition or etching of radicals generated due to plasma treatment on the four chemical fiber yarn surfaces.⁵ In order to investigate the effect of yarn diameter on air drag, the correlation analysis between them has been performed for each type of chemical fiber yarns in different groups, including T₁W_p1–6 (3 s), T₂W_p1–6 (6 s), T₃W_p1–6 (15 s) and T₄W_p1–6 (30 s). Before the analysis, these two factors were normalized and represented by the growth rate of air drag (R_d) and the growth rate of diameter (R_φ).

Table 6 shows the correlation analysis between R_d and R_φ for PET, PE, PP and PAN yarns, where r represents the correlation coefficient and s represents significance. Taking PET yarns as an example, when the treatment time was relatively short (3 and 6 s), the air drag was positively correlated to diameter, but not significantly; when the treatment time was extended to 15 and 30 s, the correlations were both significant at the 0.05 level (two tailed). The results indicated that the shorter time plasma treatment had less effect on the PET yarn diameter (R_φ ranged from –0.4% to 1.6% for 3 s and from –0.5% to 1.0% for 6 s), which could not obviously affect the air drag; the longer time treatment could lead to a relatively obvious change in yarn diameter (R_φ ranged from –0.7% to 2.4% for 15 s and from –3.9% to 2.9% for 30 s), which could influence the air drag. The correlation analysis for PE, PP and PAN generally had similar results. Therefore, the improvement of air drag in the T₂W_p1–6 (6 s) group may be attributed to the different degrees of radical deposition or etching for each type of chemical fiber yarn.

Table 6.

Correlation analysis between R_d and R_φ for polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP) and polyacrylonitrile (PAN) yarns

Group	PET		PE		PP		PAN
Group	r	s	r	s	r	s	r	s
T ₁ W _p1–6	0.808	0.052	0.425	0.521	0.651	0.163	0.337	0.512
T ₂ W _p1–6	0.553	0.255	0.821^a	0.045	0.728	0.095	0.698	0.126
T ₃ W _p1–6	0.896^a	0.016	0.863^a	0.025	0.826^a	0.042	0.837^a	0.035
T ₄ W _p1–6	0.874^a	0.023	0.857^a	0.030	0.839^a	0.033	0.828^a	0.041

Correlation is significant at the 0.05 level (two tailed).

The effect of plasma treatment on moisture regain for different chemical fiber yarns

To further investigate the effect of DBD plasma on air drag, the moisture regains of the four chemical fiber yarns treated by plasma were measured. The commercial moisture regains of PET and PAN are 0.4% and 2.0%, respectively, and those of PP and PE both approach zero. After plasma treatment, their moisture regains all increased obviously (Figure 5). When the treatment time was 3 s, compared with the untreated samples, the moisture regains of PAN, PE and PP remained unchanged while that of PET increased by 33.3% under the treatment power of 20 W; with the increase of power, the moisture regains of PAN, PE, PP and PET improved obviously, of which the maximal growth rates R_m were 29.0% (24 W), 900.0% (28 W), 600.0% (24 W) and 180.0% (26 W), respectively, and the average growth rates were 13.5% (95% CI: 7.6–19.4), 300.0% (95% CI: 141.6–458.4), 180.8% (95% CI: 92.2–269.4) and 90.6% (95% CI: 58.9–122.3), respectively. When the treatment time was extended to 6 s, the moisture regains of PAN, PE, PP and PET samples all increased with the improvement of treatment power, for which the maximal R_m values were 36.4% (28 W), 600.0% (28 W), 580.0% (26 W) and 350.0% (26 W), respectively, and the average growth rates were 24.7% (95% CI: 17.0–32.4), 375.0% (95% CI: 277.5–472.5), 279.3% (95% CI: 173.2–385.4) and 187.5% (95% CI: 125.6–249.4), respectively. When the treatment time was extended to 15 s, the maximal R_m values of PAN, PE, PP and PET samples were 60.0% (24 W), 1800.0% (22 W), 800.0% (24 W) and 400.0% (24 W), respectively, and the average growth rates were 24.2% (95% CI: 12.8–35.6), 1175.0% (95% CI: 810.7–1539.3), 408.3% (95% CI: 277.6–539.0) and 237.5% (95% CI: 175.7–299.3), respectively. When the treatment time was 30 s, the maximal R_m values of PAN, PE, PP and PET samples were 37.5% (24 W), 1200.0% (22 W), 240.0% (24 W) and 530.0% (24 W), respectively, and the average growth rates were 18.2% (95% CI: 11.8–24.6), 475.0% (95% CI: 275.5–674.5), 123.3% (95% CI: 69.0–177.6) and 226.8% (95% CI: 117.9–335.7), respectively. Overall, the variation tendency of moisture regain was very similar to that of air drag for each type of chemical fiber yarn.

Figure 5.

The R_m values of the four chemical fiber yarns processed under different treatment conditions: (a) polyacrylonitrile; (b) polyethylene; (c) polypropylene; and (d) polyethylene terephthalate.

The results above may imply that the variation of air drag was mainly due to the grafting of polar groups and the roughening of the fiber surface, which could affect the viscous effect between air flow and the fiber surface, and hence improve air drag. The explanation for the positive influence of plasma treatment on moisture absorbance of yarns has been suggested by many researchers.^24–27 They proposed that the plasma treatment could create a rough surface, which provides more capacity for capturing the moisture from the air and could also facilitate the penetration of moisture into the treated fibers and the number of polar groups grafted on the fiber surface would increase; thus, these polar groups could make the plasma-treated chemical fiber surfaces become more hydrophilic compared to the untreated ones.

A two-way ANOVA was applied to verify the effect of treatment time (T) and power (W_p) on moisture regain (M). As shown in Table 7, both T and W_p had a significant influence on M (P < 0.001) for PET, PE, PP and PAN yarns; a significant interaction (P < 0.001) was found between factors for each chemical fiber yarn. The results indicated that T, W_p and their combination could effectively affect the M across the four chemical fiber yarns.

Table 7.

Two-way analysis of variance for the effect of treatment time (T) and power (W_p) on moisture regain (M)

Factor		Type III sum of squares	Degree of freedom	Mean square	F	Significance
PET	T	3.860	3	1.287	7.340	<0.001
PET	W _p	10.663	5	2.133	12.167	<0.001
	T*W_p	8.595	15	0.573	3.269	<0.001
	Error	8.413	48	0.175
PE	T	10.267	3	3.422	183.581	<0.001
	W _p	1.202	5	0.240	12.895	<0.001
	T*W_p	3.521	15	0.235	12.590	<0.001
	Error	0.895	48	0.019
PP	T	3.235	3	1.078	453.191	<0.001
	W _p	0.737	5	0.147	61.926	<0.001
	T*W_p	0.666	15	0.044	18.671	<0.001
	Error	0.114	48	0.002
PAN	T	2.121	3	0.707	110.969	<0.001
	W _p	0.413	5	0.083	12.979	<0.001
	T*W_p	0.711	15	0.047	7.439	<0.001
	Error	0.306	48	0.006

PAN: polyacrylonitrile; PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate.

XPS characterization of yarn samples

The untreated samples and the samples with maximal air drag for each chemical fiber yarn were characterized by XPS to investigate the variation of surface chemistry. Due to the limited space, the C1s narrow scan spectra of only PAN and PET were analyzed (Figure 6). Figures 6(a) and (b) show the spectra of untreated and treated PAN samples, in which three peaks can be observed, located at 284.5 eV (C-C/C-H), 286.6 eV (-CN/C-O) and 288.9 eV (O = C-O), respectively.²⁸ The ester groups may be donated from the impurity, for example, methyl acrylate, in the fiber. As can be seen, compared with untreated sample, the -CN/C-O content increased by 10.0% after plasma treatment, indicating that the plasma etching generated active sites on the C-C backbone via chain scission, enabling O-containing polar groups to graft to substrate. Figures 6(c) and (d) show the spectra of untreated and treated PET samples, in which three peaks can be observed, located at 284.5 eV (C-C/C-H), 285.8 eV (C-O) and 288.2 eV (O = C-O), respectively.²⁹ After plasma treatment, the C-O content reduced from 27.6% to 16.9%, indicating the breaking of ester bonds, while the O = C-O content increased by 3.8%, indicating the grafting of carboxyl groups on the substrate.

Figure 6.

C1s narrow scan spectra of polyacrylonitrile yarns: (a) untreated yarn; and (b) treated yarn with maximal air drag. C1s narrow scan spectra of polyethylene terephthalate yarns: (c) untreated yarn; and (d) treated yarn with maximal air drag.

Table 8 shows the quantification analysis of untreated samples and the samples with maximal air drag for each chemical fiber yarn. As can be seen, the O1s content of PAN, PE and PP all improved obviously after plasma treatment, which increased by 3.2%, 4.7% and 4.4%, respectively. This indicates that plasma treatment could generate active sites on all three yarn surfaces through C-C bonds breaking or hydrogen abstraction, then the O-containing radicals would undergo chemical bonding with the substrate to complete the grafting process. By contrast, the O1s content of PET decreased by 3.7%, which may attribute to the breaking of ester bonds. The N1s content of PE, PP and PET increased by 0.7%, 0.6% and 0.9%, respectively, implying that the nitrogen element was also grafted onto the substrates. The decrease of N1s content on the PAN surface may be attributed to the increase of O1s content, which reduced the relative amount of the N1s element.

Table 8.

Quantification analysis of untreated samples and samples with maximal air drag for four chemical fiber yarns

Elements		C1s	N1s	O1s
Element concentration (%)
PAN	Untreated	72.4	14.9	12.6
PAN	Treated	73.3	10.9	15.8
PE	Untreated	94.5	0	5.5
PE	Treated	89.1	0.7	10.2
PP	Untreated	95.1	0	4.9
PP	Treated	90.2	0.6	9.3
PET	Untreated	77.6	0	22.4
PET	Treated	80.4	0.9	18.7

PAN: polyacrylonitrile; PE: polyethylene; PP: polypropylene; PET: polyethylene terephthalate.

Correlation analysis between air drag and moisture regain for different chemical fiber yarns

The XPS results indicated that the DBD plasma treatment grafted polar groups to the yarn surfaces. In our previous work, we proved experimentally that the grafted polar groups would enhance the air drag.³⁰ Meanwhile, the grafted polar groups could also lead to the improvement of moisture regain.³¹ Therefore, in order to discuss the mechanism of air drag improvement from another perspective, the correlation between the air drag and moisture regain was analyzed. The increments of air drag and moisture regain after plasma treatment were chosen as factors. Before correlation analysis, these two factors were normalized, represented by the growth rate of air drag (R_d) and growth rate of moisture regain (R_m). The correlation analysis between R_d and R_m in different groups, namely T₁W_p1–6 (3 s), T₂W_p1–6 (6 s), T₃W_p1–6 (15 s) and T₄W_p1–6 (30 s), was performed for each type of chemical fiber yarn, where r represents the correlation coefficient and s represents significance.

Table 9 shows the correlation analysis between R_d and R_m for PAN, PE, PP and PET yarns. The air drags of PAN yarn samples were positively correlated to moisture regain, but not significantly, especially when the treatment time was 15 s. The air drag of PE yarns had a poor correlation with moisture regain under different treatment times. By comparison, the R_d value of PP yarns was significantly correlated with R_m. When the treatment time was 3, 6 and 15 s, the correlation was significant at the 0.05 level (two tailed); when the treatment time was 30 s, the correlation was significant at the 0.01 level (two tailed). For PET yarn samples, when the treatment time was 3 s, R_d was negatively correlated with R_m, while the correlation was significant at the 0.05 level (two tailed) when the treatment time was 30 s. The results above reflect not only the difference in moisture absorbance and viscous effect of the air–yarn interface among the four chemical fiber yarns caused by the grafting of polar groups, but also the difference in the response of air drag to the distinct chemical properties of the chemical fiber yarns under the same treatment condition. It is also implied that the grafting of polar groups is not the only factor to affect air drag.

Table 9.

Correlation analysis between R_d and R_m for polyacrylonitrile (PAN), polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET) yarns

Groups	PAN		PE		PP		PET
Groups	r	s	r	s	r	s	r	s
T ₁ W _p1–6	0.730	0.099	0.115	0.829	0.851^a	0.032	–0.314	0.545
T ₂ W _p1–6	0.701	0.121	0.509	0.303	0.914^a	0.011	0.666	0.149
T ₃ W _p1–6	0.070	0.896	0.163	0.757	0.833^a	0.039	0.653	0.160
T ₄ W _p1–6	0.797	0.058	0.250	0.630	0.921^b	0.009	0.858^a	0.029

Correlation is significant at the 0.05 level (two tailed).

Correlation is significant at the 0.01 level (two tailed).

The determination of reactive species in DBD plasma under different treatment powers

In order to discuss the differences in air drag among PAN, PE, PP and PET caused by different treatment conditions, the plasma species have been detected under different treatment powers. Owing to the diversity of reactive species, of which the half-lives vary widely, generated by DBD,³² it is still difficult to on-line measure short-lived reactive species. In the present experiment, only the long-lived reactive oxygen species were detected to conjecture the reaction process and mechanism under different treatment powers (Table 10). As can be seen in Table 10, the ozone output varied obviously under different powers, in which the output peaked in the power range from 22 to 26 W, while the output was limited when the power was lower than 22 W or higher than 26 W. Generally, during the atmospheric DBD process, atomic O and O₃ are generated from O₂ in the air stream,³³ which may cause the oxidation and grafting on yarn surface. The NO₃⁻ output increased with the improvement of treatment power and was maximized at 28 W (32.29 mg/L) then decreased to 7.75 mg/L at 30 W. Compared with the NO₃⁻ output, the NO₂⁻ output was relatively higher at 20 and 30 W, while that in the range of 22–28 W was limited or undetectable. The gross output of NO₃⁻ and NO₂⁻ peaked in the range of 26–30 W. The results indicate that the DBD plasma treatment may be distinguished by a low-power “ozone mode” and a high-power “NO_x mode” based on the plasma power density,³⁴ owing to the fact that nitrogen is more stable than oxygen. With the increase of treatment power, the number of microdischarge channels in the reactor would improve,³⁵ which would enhance the bombardment of high-energy electrons and induce the ionization of working gas, and hence the components of reactive species would vary. The chemical fiber yarns with different chemical characteristics would have different selectivities for reactive species, which meant that the optimal treatment conditions for each yarn were different.

Table 10.

The outputs of ozone and NO_x under different treatment powers

Treatment power (W)	20	22	24	26	28	30
O₃ output (mg/L)	9.28	59.02	36.36	21.69	2.00	7.54
${NO}_{3}^{-}$ output (mg/L)	3.43	4.29	7.59	29.16	32.29	7.75
${NO}_{2}^{-}$ output (mg/L)	4.51	0.00	0.56	0.00	2.56	8.91
${NO}_{3}^{-} + {NO}_{2}^{-}$ output (mg/L)	7.93	4.29	8.14	29.16	34.85	16.67

Conclusions

In this paper, atmospheric DBD plasma was used to treat PAN, PE, PP and PET yarns under different treatment conditions. The plasma treatment obviously changed the air drag, wettability and diameter of these four yarns. The maximal increases in air drag for PAN, PE, PP and PET were 4.26 cN (10.9%), 4.15 cN (43.0%), 7.77 cN (40.0%) and 4.49 cN (18.9%), respectively, in the present experiments. The two-way ANOVA and correlation analysis results indicated that different chemical structures of the yarn samples may affect the properties of yarns (e.g. air drag, moisture regain) to different degrees. According to the XPS analysis and wettability and yarn diameter measurements, it is indicated that the atmospheric DBD plasma could produce hydroxyl, carboxylic groups and N-containing polar groups grafted on the yarn surfaces, which would alter the air drag, wettability and yarn diameter of the samples. The component analysis of the reactive species in DBD plasma flow indicates that the high-energy electrons, excited species and radicals participated in the oxidation and chain scissions on fiber surfaces. This study provided the experimental basis to improve the air-jet weaving efficiency of chemical fiber fabrics.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by projects funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the National High Tech Research and Development Program of China (“863” Program, No. SQ2009AA06XK1482331) and “Jin Shan Ying Cai” and “Chang Shan Ren Cai” Programs (Zhenjiang, Jiangsu, China).

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