Influence of He/O 2 atmospheric pressure plasma pretreatment on sizing adhesion strength and breaking elongation of sized cotton rovings

Abstract

This study investigated the influence of pretreatment conditions of helium/oxygen (He/O₂) atmospheric pressure plasma, including treatment duration, oxygen flow, and distance between the nozzle and the sample (DBNS), on sizing properties of cotton roving. Results indicate that plasma treatment can effectively improve the surface roughness, static friction coefficient, and the wettability of raw cotton fibers, as well as the absorption ability of cotton rovings for starch size. Consequently, sizing adhesion strength (SAS) and breaking elongation (BE) of the roving sized by starch are greatly influenced by the treatment. They first rise and then slightly drop with the increase of treatment duration or oxygen flow, but decrease with the elongation of the DBNS. Compared with a roving without plasma pretreatment, pretreated rovings can possess 59% and 36% improvement of SAS and BE, respectively, by a chosen plasma treatment condition, i.e. 15 s of treatment duration, 1.5 mm of DBNS, 30/0.3 L/min of He/O₂, and 40 W of the power.

Keywords

helium/oxygen atmospheric pressure plasma cotton roving sizing adhesion strength wettability friction coefficient surface morphology

Sizing adhesion strength (SAS) is a reflection of the adhesive capacity of a size to the yarn.^1,2 Proper SAS can combine the size and the yarn tightly without size cracking-off during the weaving, which helps improve the yarn’s resistance to repeated interaction with an external force and makes the weaving process run smoothly. Breaking elongation (BE) reflects the toughness of the sized yarn to a mechanical impact and high BE is beneficial to lower end breakage and improve the weaving efficiency.³ The surface properties of the yarn, the performance of the size, and the sizing technique play equally important roles in determining SAS and BE during the sizing process, including wetting, absorption, and permeation of the size into the yarn.^4–6

Starch, which is a natural, abundant, and environmentally friendly material, has great potential in the sizing field, although it has quite a number of disadvantages, including poor fluidity and formation of low tenacity size film, etc.⁷ To conquer these weaknesses, starch-based sizes are usually modified at the molecule level,^8,9 or mixed with other sizes such as polyvinyl alcohol (PVA), polyacrylamide (PAM), and some agents including plasticizers and lubricants.^10–12 However, chemical sizes such as PVA and PAM are facing prohibition because of high COD (chemical oxygen demand) in their desizing wastewater, which can result in severe pollution of the natural environment.^13–15 Therefore, a promising way to improve the tenacity of the starch film is to add some eco-friendly small molecule agents into the starch. The agent, such as glycerin, can effectively enhance the tenacity of the size film, but it also brings some side effects such as low SAS and BE.^16,17

Recently, plasma treatment has been widely used as an effective and eco-friendly way to modify the surface properties of textile materials.^18–23 Plasma process parameters can play a key role in determining the properties of plasma-treated fabrics.²⁴ In this study, cotton rovings were pretreated by helium/oxygen (He/O₂) atmospheric pressure plasma (APP) with different treatment duration, distance between the nozzle and the sample (DBNS), and oxygen flow in order to explore the influence of plasma treatment conditions on SAS and BE of sized rovings. Thereby, a promising sizing technology combining APP pretreatment and green sizing instructions to improve sizing properties in the absence of PVA could be developed. The morphology and chemical composition of the cotton fiber surface were examined by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The surface friction coefficient and wettability of cotton fiber were tested using a Y151 fiber friction tester and DMC-AD31S contact angle analyzer, respectively. A roving impregnation method was adopted to evaluate SAS and BE. The cross section of sized rovings was stained by iodine solution to observe the extent of size permeation.

Experimental details

Materials

Raw cotton fibers (with diameters of 15–20 µm), rovings (with linear density of 640 tex), and fabrics (60^s, plain stitch knitting) were provided by Shandong Sanyang Textile Group. Both rovings and fabrics were made from raw cotton fibers without any chemical pretreatment before use. Rovings were used for SAS and BE test with considerate operation in case of drafting. Fabrics were cut into samples with dimensions of 52 × 10 mm² for wettability measurement. All fiber, roving, and fabric samples were conditioned in the standard environment of 20℃ and 65% relative humidity (RH) for 24 h before use. Corn starch (industrial grade, with density of 1.499–1.513 g/cm³, moisture content ≤14.0%, and purity ≥99.0%) was used in the roving impregnation method. Both iodine and potassium iodide used in this study are chemical pure.

Plasma treatment

Cotton fiber, roving, and fabric samples were put on the movable stage and pretreated using an Atomﬂo-TM250 APP treatment apparatus (Surfx Company, USA), as shown in Figure 1. The apparatus employs helium and oxygen as carrier and reactive gas, respectively, and the required O₂/He ratio is not more than 1%. The plasma source with a capacitively coupled electrode design can produce a stable and homogeneous glow discharge at 13.56 MHz radiofrequency.²⁵ The discharge extended out through a nozzle at the end of the electrodes forming a plasma jet. A large quantity of oxygen atoms, metastable oxygen molecules, and ozone at a gas temperature between 50 and 300℃ depending on discharge power are produced during the discharge.²⁶ A theoretical model conﬁrms the existence of a stable discharge region and provides estimated electron densities of 0.2–2 × 10¹¹cm³ and characteristic electron energies of 2–4 eV. These plasma parameters indicate the unique advantage of an APP jet for producing a large quantity of chemically active species while maintaining a non-thermal discharge.²⁶ In this study, samples were treated in the afterglow of the discharge at a power of 40 W with different treatment durations, DBNS values, and oxygen flows.

Figure 1.

Schematic diagram of Atomﬂo-TM250 APP treatment apparatus.

SEM and AFM analysis

Surface morphology of cotton fiber before and after plasma treatment was examined by SEM (JSM-5600LV Model, Japan). Samples were gold sputtered in vacuum condition to obtain a better electrical conductivity. The magnification was set at 5000×. The surface topology and roughness were analyzed by AFM (Multimode Nanoscope IIIa, Digital Instrument, USA) with the Pocoscan5 software to calculate the surface roughness.

Friction coefficient test

The static friction coefficient of the cotton fiber samples can be calculated by equation (1) in which m can be determined by reading a torsion balance on a Y151 type fiber friction tester (Changzhou 2^nd Textile Device Co., Ltd, China). Equation (1) is as follows²⁷

μ = 0.733 [lg f_{0} - lg (f_{0} - m)]

(1)

where μ is the friction coefficient, f₀ is the mass of the tension clamp (100 mg), and m is the reading of the torsion balance.

XPS analysis

Surface chemical composition of cotton fibers before and after plasma treatment was examined by XPS (ESCALAB 250, Thermo Electron VG Scientific, USA) with an X-ray source of Al K_α (1486.6 eV) and a power of 150 W. The pressure in the chamber was 10⁷−10⁸Pa and the take-off angle was 45°. The pass energy was 20 eV and the scanning step size was 0.05 eV. The deconvolution curve fitting of C 1 s peaks was performed using XPSPEAK software.

Wettability measurement

Wettability of cotton fabrics was tested using a DMC-AD31S contact angle analyzer (Kyowa Interface Science Co., Ltd. Japan) with the sessile drop method.²⁸ A drop of 2 µL distilled water was dropped onto the fabric surface with a microliter syringe and observed through an optical microscope equipped with a video camera and a computer with image capturing and processing software. The waiting time before the measurement of water contact angle (WCA) was 1000 ms after dropping the water. The mean angle value on both sides of the drop was adopted as the useful WCA. For the drop completely being absorbed into the fabric within 1000 ms, the absorption process was recorded by the video camera. The period of time it takes to permeate into the fabric can be calculated by the software and is taken as water absorption time (WAT). An average of at least six measurements both for WCA and WAT were tested for each sample.

SAS and BE test

The SAS test was conducted by using a roving impregnation method.²⁹ Roving samples were carefully wound onto an aluminum frame (165 × 50 mm²) to avoid being drafted. Then the frame was immersed in the size solution (temperature 95℃, molar concentration 1%) of corn starch for 5 min. After being taken out of the size solution, sized roving samples were dried at ambient conditions. Afterwards, the samples were cut off from the frame and conditioned in the standard environment of 20℃ and 65% RH for 24 h. Breaking strength and breaking elongation were tested by using an XL-1B tensile testing machine (Shanghai New Fiber Instrument Co., Ltd.) as SAS and BE of sized roving samples. Each data reported was an average result of 30 samples.

Observation of cross sections of sized rovings

In order to assess the extent of permeation of the size into the roving, a 0.1 mol/L iodine solution was utilized to stain the cross sections of the sized rovings which were then observed under a KH-1000 type stereo microscope.

Result and discussion

Surface morphology, roughness, and friction property

The surface morphology of cotton fibers with different plasma treatment conditions are displayed in Figures 2 to 4. As can be seen from Figures 2 and 3, the surface roughness of cotton fibers was first enhanced and then decreased with an increase of treatment duration or oxygen flow. A previous study showed that, for He/O₂ APP treatment, singlet delta metastable oxygen and oxygen atoms in afterglow had been identified to be responsible for the etching of Kapton films or other polymers.³⁰ For the case of longer plasma treatment duration or higher oxygen flow in this study, more active singlet delta metastable oxygen and oxygen atoms would interact with the fiber surface and lose their kinetic energy, causing fiber surface heating and thereby more chemical etching.³¹ However, with further increase of treatment duration and supply of oxygen, the outmost cuticle layer of raw cotton fiber could be over etched and removed to a certain extent, leading to the decrease of surface roughness.³²

Figure 2.

SEM images of cotton fibers APP treated with different durations: (a) 5 s, (b) 15 s, and (c) 25 s (He/O₂: 30/0.30 L/min, DBNS: 1.5 mm, power: 40 W).

Figure 3.

SEM images of cotton fibers APP treated with different O₂ flow rates: (a) 0 L/min, (b) 0.30 L/min, and (c) 0.45 L/min (treatment duration: 15 s, He flow rate: 30 L/min, DBNS: 1.5 mm, power: 40 W).

Figure 4.

SEM images of cotton fibers APP treated with different DBNS: (a) 6 mm, (b) 3 mm, and (c) 1.5 mm (treatment duration: 15 s, He/O₂: 30/0.30 L/min, power: 40 W).

Figure 5 shows the surface mean roughness (R_a) of APP-treated cotton fibers with the extension of treatment duration. It demonstrates that R_a has the same variation trend with etching effect of fiber as time extended shown in Figure 2 and confirms the above analysis about etching. An increased surface roughness with the drop of DBNS from 6 to 1.5 mm can be observed, as shown in Figure 4, which is due to less de-excitation and energy loss of active singlet delta metastable oxygen and oxygen atoms at smaller DBNS values.³⁰

Figure 5.

Surface mean roughness of the cotton fiber surface treated with different plasma durations (He/O₂: 30/0.30 L/min; DBNS: 1.5 mm; power: 40 W).

Static friction coefficient of the fiber surface could be greatly influenced by the APP etching, as shown in Figure 6. It was found that static friction coefficient exhibits the same changing tendency with R_a as time extended, showing a good reflection of the etching and surface roughness. Enhanced surface friction helps to improve cohesion among fibers, which is beneficial to the improvement of sizing properties.

Figure 6.

Static friction coefficient of the fiber surface treated with different plasma durations (He/O₂: 30/0.30 L/min; DBNS: 1.5 mm; power: 40 W).

XPS and wettability analysis

C1s deconvolution analyses were performed using XPSPEAK software, as shown in Figure 7. Table 1 shows the XPS elemental analysis of control and APP treated cotton fibers with different treatment durations. Table 2 demonstrates the detailed deconvolution and wettability information of the control and APP treated cotton fibers. From Table 1, a notably increased O/C ratio, from 26.4 to 82.3, is observed with the extension of APP treatment duration. As can be seen from Table 2, the content of C–O, C = O, and O–C = O increases while C–C/C–H drops with time elongation. This means that the fiber surface has effectively been oxidized by APP treatment.³³ During the treatment, an army of free radicals could be formed on the fiber surface under the bombardment of plasma particles and subsequently reacted with the active oxygen species, introducing polar groups of C–OH, C = O, and O-C = O into the molecular chains of cellulose.^34,35 Both the oxidation and introduction of polar groups greatly promote the hydrophilicity of cotton fabric. As shown in Table 2, the WCA of the control fabric sample is 135.4°. After 15 s plasma treatment, the WCA decreases markedly to 4.2°, as shown in Figure 2(b). For samples treated longer than 20 s, the water droplets are absorbed rapidly into the fabric within 1000 ms, indicating the cotton fiber is endowed with superior wettability by plasma treatment. Besides surface oxidation and introduction of polar groups, the dewaxing was also confirmed to be responsible for the tremendously improved hydrophilicity.³⁶

Figure 7.

Deconvoluted XPS C ls core level spectra for (a) control and APP treated cotton ﬁbers with different treatment durations: (b) 5 s, (c) 10 s, (d) 15 s, (e) 20 s, (f) 25 s, and (g) 30 s (He/O₂: 30/0.30 L/min; DBNS: 1.5 mm; power: 40 W).

Table 1.

XPS elemental analysis of control and APP-treated cotton fibers with different treatment durations (He/O₂: 30/0.30 L/min; DBNS: 1.5 mm; power: 40 W)

Sample	Chemical composition (%)					Atomic ratio (%) O/C
Sample	C1s	O1s	N1s	Ca2p	Si2p	Atomic ratio (%) O/C
Control	73.4	19.4	3.6	1.4	2.2	26.4
Sample 1 (5 s)	73.3	19.5	3.6	1.4	2.2	26.6
Sample 2 (10 s)	71.3	23.8	1.7	0.8	2.4	33.3
Sample 3 (15 s)	69.6	24.5	2.4	1.2	2.3	35.2
Sample 4 (20 s)	66.6	30.8	1.2	0.3	1.1	46.2
Sample 5 (25 s)	56.1	37.7	2.3	1.5	2.4	67.2
Sample 6 (30 s)	51.9	42.7	1.2	2.0	2.2	82.3

Table 2.

XPS and wettability analysis of the cotton fibers (He/O₂: 30/0.30 L/min; DBNS: 1.5 mm; power: 40 W)

Group	Treatment duration (s)	Relative percentage of C 1 s (%)				WCA (°)	WAT (ms)
Group	Treatment duration (s)	C − H/C − C (285 eV)	C − OH (286.6 eV)	C = O (288 eV)	O − C = O (289 eV)	WCA (°)	WAT (ms)
Control	0	62.7	29.6	6.6	1.1	135.4	/
Plasma treated	5	61.7	29.2	8.1	1.0	104.5	/
	10	59.5	21.9	10.3	8.3	40.7	/
	15	57.1	25.9	11.0	6.0	4.2	/
	20	55.3	20.6	12.6	11.5	0.0	965
	25	43.8	26.7	17.6	12.0	0.0	666
	30	32.1	29.5	21.2	17.2	0.0	200

WCA: water contact angle; WAT: water absorption time.

SAS and BE

According to the adhesion theory of Frank H. Chung, the adhesion between two materials is governed by the intimate molecular contact and their mutual attractive force.³⁷ The roughness improved by plasma treatment has an effect of “mechanical interlocking” on the interface between the fiber and the size film, which brought more contacting area, resulting in the enhancement of van der Waals force between them. The introduction of polar groups such as C–O, C = O, and O–C = O to the fiber surface by plasma treatment not only improves the wettability of the roving, enabling the size to more easily permeate into it, but also promotes hydrogen bonding between the fiber and the size, leading to a favorable adhesion force between them. The SAS is effectively raised by the increased van der Waals force and hydrogen bonding between the fiber and the size. Because SAS determines the transmission efficiency of tensile stress in the sized roving, BE also is improved with the increase of van der Waals force and hydrogen bonding by APP treatment.³⁸

Treatment duration

SAS and BE versus different plasma treatment durations are shown in Figure 8. Both SAS and BE increase from 0 to 15 s and start to drop with time extension. From the above analysis, it can be deduced that SAS and BE are evidently influenced by the etching and oxidation of APP treatment. At time of 15 s, the fiber is endowed with peak roughness, friction coefficient, and good wettability with WCA of 4.2°. Therefore, SAS and BE present peak values at this time. Compared with the substantial increase of SAS, the improvement of BE is relatively small from 0 to 15 s. The reason is that BE is determined by a lot of factors such as the size film, fibers in the roving, and their interactions, etc., apart from SAS. Raised SAS only makes better transmission efficiency of tensile stress in the sized roving, which is not sufficient to enhance BE greatly.

Figure 8.

SAS and BE versus different plasma treatment durations (He/O₂: 30/0.30 L/min; DBNS: 1.5 mm; power: 40 W.)

Oxygen flow

SAS and BE versus different oxygen flows are presented in Figure 9. SAS and BE increase greatly when oxygen flow rises from 0 to 0.30 L/min. The number of singlet delta metastable oxygen and oxygen atoms is gradually increased as oxygen flow grows, giving rise to better etching and wettability of the fiber surface. Consequently, SAS and BE are effectively improved by the raised etching and wettability. When the oxygen flow is over 0.30 L/min, i.e. the O₂/He ratio is more than 1%, SAS and BE decrease because of unstable plasma discharge impairing the APP treatment effect.³⁰

Figure 9.

SAS and BE versus different oxygen flow rates (He flow rate: 30 L/min; DBNS: 1.5 mm; treatment duration: 15 s; power: 40 W).

DBNS

Figure 10 shows the change of SAS and BE with the increase of DBNS. It can be seen that both SAS and BE evidently drop when DBNS is raised from 1.5 to 6 mm. Generally speaking, fewer plasma particles can etch and oxidize the fiber surface because of energy loss at larger DBNS values. During the travel to longer distances, most of the energy of reactive plasma particles, such as singlet delta metastable oxygen and oxygen atoms, in afterglow will be consumed by their de-excitation which happens within approximately 100 µs. Consequently, the number of reactive particles is greatly lessened resulting in insufficient etching and oxidation effects. Infrared emission measurements showed that only a little singlet delta metastable oxygen can persist for a long lifetime of about 100 ms with a longer journey.³⁰ Therefore, behind the drop of SAS and BE with increase of DBNS, energy loss of plasma particles caused by de-excitation is found to be the main reason.

Figure 10.

SAS and BE versus different DBNS values (He/O₂: 30/0.30 L/min; treatment duration: 15 s; power: 40 W).

From above analysis, a chosen plasma treatment condition for the improvement of SAS and BE can be achieved as follows: 15 s of treatment duration, 1.5 mm of DBNS, 30/0.3 L/min of He/O₂, and 40 W of the power. SAS and BE can be notably increased by 59% and 36%, respectively, under the chosen conditions.

Analysis of cross sections of sized rovings

Figure 11 illustrates the cross section of the cotton rovings after being stained by iodine. During the sizing process, the size solution gradually permeates from the outside into the roving. The circular dark blue area positively correlates with the absorption ability of the roving for the size after plasma treatment. It can be observed that the color darkens and its area grows with an increase of APP treatment duration, which implies that the absorption ability of the roving for the starch size has been greatly promoted by the plasma etching, oxidation, and introduction of polar groups with time extension. Meanwhile, it can be noticed that the other side of the roving has smaller and lighter stained area than the side facing the plasma treatment, showing a worse treatment effect. Although some plasma particles, such as singlet delta metastable oxygen and oxygen atoms, in afterglow could go through the roving, their energy and reactivity would decrease to some extent because of the de-excitation mentioned above. Rovings used in this study have diameters of approximate 2 mm, which means the other side of the roving is 2 mm further away than the side facing the plasma treatment. Besides, some reactive particles could probably be blocked by the roving and hardly penetrate through. For these reasons, the other side of the roving would have a worse APP treatment effect and thereby influence the size permeation, as shown in Figure 11.

Figure 11.

Photographs of roving cross sections stained by I/KI solution: (a) control and (b)–(g) samples with plasma treatment durations of 5, 10, 15, 20, 25, and 30 s, respectively. The magnification is 70× and plasma treatment conditions are 30/0.30 L/min of He/O₂, 1.5 mm of DBNS, and 40 W of power.

Conclusion

In this study, cotton fiber, roving, and fabric samples were pretreated under different plasma treatment conditions. The surface roughness, the static friction coefficient, the wettability of raw cotton fibers, and the absorption ability of the cotton roving for the starch size are significantly improved by He/O₂ atmospheric pressure plasma pretreatment. Plasma treatment conditions play a crucial role in the enhancement of SAS and BE. With the increase of treatment duration or oxygen flow, SAS and BE first increase and then decrease, while with the increase of DBNS, both SAS and BE drop. When the roving samples are treated under the chosen condition, namely 15 s of treatment duration, 1.5 mm of DBNS, 30/0.3 L/min of He/O₂, and 40 W of the power, SAS and BE could be notably improved by 59% and 36%, respectively, compared with the control group. The etching and introduction of polar groups by plasma treatment are found to be the main reason behind the improvement of SAS and BE, which gives great potential to improve sizing properties in textile applications.

Footnotes

Declaration of conflicting interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Industrial Research Project for Public Welfare of Zhejiang Province (grant number 2014C31073).

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