The influence of surface hydrophilicity on the adhesion properties of wet fabrics or films to water

Abstract

The adherence of wet fabrics to the skin brings much discomfort. The relationship between hydrophilicity and adhesion properties of fabric materials is investigated. A theoretical expression is given to describe the relationship of adhesion force, water contact angle (WCA), and radius of fabric–liquid interface. The adhesion force grows with decreasing WCA and increasing radius of the fabric–liquid interface. With the help of atmospheric pressure plasma jet (APPJ) treatment, the hydrophilicity of the fabric materials is improved, accompanied by reduced WCA, roughened fiber surfaces, as observed by scanning electronic microscope (SEM), and increased Oxygen/Carbon (O/C) atomic ratio and polar bonds, analyzed by X-ray photoelectron spectroscopy (XPS). In accordance with the theoretical conclusion, the APPJ treated fabrics have a much larger maximum adhesion force and longer adhesion duration with water, indicating more discomfort resulting from the increase of hydrophilicity when they are wet. To minimize the discomfort caused by wet adhesion, less hydrophilic fabric surfaces may be preferred.

Keywords

adhesion property wet fabric comfort hydrophilicity APPJ surface modification

When inevitably getting wet by sweat, urine, body exudate, rain, sea water or pool water, and so on, fabrics become sticky and usually adhere to the skin, restricting body movements, and causing physical and mental discomfort,¹ in addition to exhibiting poor appearance. According to our questionnaire survey, eight out of 28 interviewees (23 females, five males, age 19–35, height 150–177 cm, and body weight 41–70 kg), often experienced discomfort because their clothes were wetted by sweat or other liquids. Nineteen of them experienced the discomfort sometimes, while only one of them was never bothered by this situation. Among those feelings such as adherence, restrictiveness, wetness, dripping, coldness, and ugliness that may cause discomfort when wearing wet clothes, adherence ranked top in the survey. If there is no adherence between fabrics and the skin, no force would be applied to the skin and many of the other uncomfortable feelings may not occur. Adhesion of fabrics and films to a wet surface could also be of interest to the end users of many industrial textiles and films, such as medical textiles^2–4 and geotextiles.⁵ Therefore, it is important to look into the adhesion properties of wet fabrics and find a way to adjust the adherence between the fabric and human skin.

Wet fabric adhesion to skin has been studied based on a mass spring model of a wet fabric being pulled up from a plane through the center,⁶ and on a back-propagation neural network to connect instrumental measurements to sensory evaluation indices, including sticky feeling.⁷ Vertical^8,9 and horizontal^10–14 testing methods have also been developed to measure the wet adhesion properties of fabrics and films.

The adhesion of a wet fabric can be explained by the presence of the liquid bridge(s) between the fabric and the skin, or between the fabric and a large volume of liquid.^11,13 The liquid bridge holds the fabric and the skin together. The configuration and adhesion force of a very small liquid bridge (within the volume of a few µL) have been reported between two rigid surfaces, such as two parallel surfaces, two spheres, or a sphere and a plane surface.^15–17 However, a liquid bridge of a larger scale (could be more than 16 mL) between flexible materials such as fabrics are quite different, and are rarely reported. Previous studies^11,13,18 showed that the adhesion properties of wet fabrics are intricately related to tensile and bending properties, hydrophilicity, structure of the material, and surface tension of the liquid. The influence of hydrophilicity of the material on the adhesion properties has not been independently and systematically studied, and thus remains unknown.

The atmospheric pressure plasma jet (APPJ) treatment is well acknowledged as an efficient clean source for surface modification,^19–23 without changing the overall bulk properties of the original material.^{19,20,24–26} With short time exposure to the plasma, the chemical reactions and physical changes take place only on the surface layer, within a few tens of nanometers,²⁵ making the treatment harmless to the mechanical properties (including deformability of stretching and bending) of the bulk material, but very effective for surface layer modification. With the right technique and proper parameters, the APPJ treatment is suitable to modify fabric materials and to improve their hydrophilicity.^{19–21,27,28}

In this study, a theoretical expression was deduced to describe the relationship between adhesion force, F, water contact angle, $θ_{a}$ , and radius, R, of the fabric–liquid interface. The APPJ treatment was adopted to alter the hydrophilicity of woven, knitted, and nonwoven fabrics and a film in order to study the effect of surface hydrophilicity change on the wet adhesion properties of different materials. Water contact angle, surface morphology, and surface chemical composition were measured before and after the APPJ treatment to determine the hydrophilicity change of the fabrics. The adhesion properties of the fabrics to water were also determined using the adhesion force measurement system developed previously.^10,11

Theoretical modeling

Our previous research¹¹ revealed that the adhesion force applied to the fabric can be expressed as follows

F = \frac{γ π R^{2}}{h} (cos θ_{a} + 1) + γ π R \sin θ_{a}

(1)

With a small and constant separating speed of the fabric from the liquid, the separation can be regarded as a quasi-static process, and the influence of the viscosity and turbulence of the liquid is negligible.¹³ The pressure difference, $Δ p$ , between air and liquid at the air–liquid–fabric border (point A in Figure 1) can be expressed in two ways: Young–Laplace equation or Pascal's law

Δ p = γ (\frac{1}{R_{1}} + \frac{1}{R_{2}}) (R_{1} > 0, R_{2} < 0)

Δ p = ρ gh

Figure 1.

The geometric relationships in the fabric–liquid system.

The two expressions could be assumed equal

γ (\frac{1}{R_{1}} + \frac{1}{R_{2}}) = ρ gh h = R_{1} (1 + cos θ_{a})

(2)

γ (\frac{1}{R_{1}} - \frac{\sin θ_{a}}{R}) = ρ g R_{1} (1 + cos θ_{a}) \frac{ρ g}{γ} (1 + cos θ_{a}) {R_{1}}^{2} + \frac{\sin θ_{a}}{R} R_{1} - 1 = 0 R_{1} = \frac{γ}{2 ρ g (1 + cos θ_{a})} [\sqrt{\frac{\sin^{2} θ_{a}}{R^{2}} + \frac{4 ρ g}{γ} (1 + cos θ_{a})} - \frac{\sin θ_{a}}{R}]

(3)

Putting equations (2) and (3) into equation (1)

F = \frac{γ π R^{2}}{h} (cos θ_{a} + 1) + γ π R \sin θ_{a} = \frac{γ π R^{2}}{R_{1}} + γ π R \sin θ_{a} = \frac{2 π ρ g (1 + cos θ_{a}) R^{3}}{[\sqrt{\sin^{2} θ_{a} + \frac{4 ρ g}{γ} (1 + cos θ_{a}) R^{2}} - \sin θ_{a}]} + γ π R \sin θ_{a}

(4)

Within the ranges of $0 \leq θ_{a} < π and 0 < R \leq 35 mm$ , a 3D map surface (see Figure 2) is derived through expression (4), showing the relationship between adhesion force, F, water contact angle, $θ_{a}$ , and radius, R, of the fabric–liquid interface. As shown in Figure 2, a larger R leads to a faster growth of F. On the other hand, F decreases with the increase of $θ_{a}$ , and approaches 0 when $θ_{a}$ is around π. In the region of larger $θ_{a}$ , F falls faster.

Figure 2.

The theoretical relationship between F, $θ_{a}$ , and R.

According to the theoretical relationship, in order to reduce the adhesion force, F, between the fabric and the liquid, a larger water contact angle, $θ_{a}$ , or a more hydrophobic surface is preferable in fabric design.

Experimental

Materials

The parameters and images of the sample fabrics and a film are shown in Table 1 and Figure 3, respectively. The fabrics and the film were selected to represent typical structures of materials used as apparel and industrial textiles. Before any treatment or test, the materials were soaked in acetone for 30 min, multiple rinsed with deionized water until the conductivity of the rinsed water < 1 µs/cm, and then oven dried at 80 ℃ for 2 h.

Table 1.

Parameters of the fabric materials

Material	Structure	Component	Areal density (g/m²)	Linear density of the yarn (tex)	Warp density (/mm)	Weft density (/mm)
A	Transparent film	Polyethylene (PE)	8.2	−	−	−
B	Technical face of spunbonded and thermo-dot bonded nonwoven fabric	Polypropylene (PP) filament	21.0	−	−	−
C	Plain weave	Polyethylene terephthalate (PET) staple	130.7	14.8 × 2 (plied yarn) for both warp and weft yarns	2.5 ends	1.8 picks
D	Technical rear of weft plain stitch	PET filament	151.6	9.0 (draw textured yarn)	2.1 stitches	3.7 stitches

Figure 3.

Appearance of the samples (a) PE film, (b) nonwoven PP fabric, (c) woven PET fabric, (d) knitted PET fabric under magnification of ×100.

Plasma treatment

The samples were treated with the afterglow of atmospheric pressure plasma jet (APPJ) generated by a Surfx Atomflo-A400L plasma system (Redondo Beach, CA, USA) equipped with a 50 mm-wide linear beam source. The parameters of the plasma treatment operation are listed in Table 2. The samples were exposed to the open air during the APPJ treatment. All tests were completed within 7 days after the APPJ treatment.

Table 2.

Parameters of the plasma treatment

Treatment parameters	Specifications
Primary/secondary gas	99.999%He / 99.5%O₂
Flow rate (L/min)	30.0/0.30
RF power (W)	100
Source-to-substrate distance (mm)	3
Scan speed (mm/s)	5
Times scanned	3
Ambient temperature (℃)	18 ± 2
Relative humidity (%)	55 ± 5
Pressure (atm)	1

Hydrophilicity measurement

The hydrophilicity was evaluated by measuring the surface water contact angle with a KRUSS EasyDrop system (Model FM40Mk2, Hamburg, Germany). During the test, a 3 µL deionized water drop was released at 3.5 mm above the sample, which was recorded by a video camera at a frame rate of 25 fps. The frame of the moment when the water drop was in contact with the sample for 1 sec was extracted and the apparent contact angle was measured. For each material, five specimens were tested, and the average contact angle was calculated. The room temperature and relative humidity were 20 ± 2℃ and 65 ± 5%, respectively.

Surface morphology analysis

The surface morphology was studied by a Hitachi S-4800 scanning electronic microscope (SEM). The specimens were platinum-coated prior to performing the SEM observation.

Surface chemical composition analysis

The surface chemical compositions were analyzed to gain insight into the changes induced by APPJ treatment, using an X-ray photoelectron spectroscopy (XPS) system (Model Thermo ESCALAB 250, Thermo Fisher Scientific Inc., Waltham, MA, USA) with an Al Kα (hν = 1486.6 eV) X-ray source and 150 W power. The take-off angle was 45 °.

Adhesion measurement

The adhesion properties were evaluated by the adhesion force measurement system developed in our laboratory^10,11 (see Figure 4). In that system, the horizontally placed sample ascended from the deionized water below at a constant speed. The adhesion force (measurement error of ± 0.1 mN) for the sample clinging to the water surface was measured and a curve of the adhesion force versus time was obtained. The parameters of the adhesion measurement are listed in Table 3.

Figure 4.

Testing system for adhesion force measurement.

Table 3.

Parameters of the adhesion measurement

Parameters	Specifications
Size of the sample (mm)	120 × 120
Diameter of the sample holder (mm)	70
Circumferential pre-tension (mN)	1372
Depth of immersion below water surface (mm)	3
Ascending speed (mm/min)	50
Wetting duration (min)	< 3
Temperature of air and liquid (℃)	20 ± 2
Relative humidity (%)	65 ± 5

Results and discussion

Water contact angle

The apparent water contact angles of the four different materials decreased universally after surface modification with APPJ treatment (see Figure 5). One-way analysis of variance (ANOVA) showed there were significant differences between the contact angles before and after APPJ treatment (P < 0.05). The decrease percentage ranged from 3.3% to 19.9%. The decrease of the contact angles indicated that the materials became more hydrophilic after the APPJ treatment.

Figure 5.

Average contact angles of the materials before and after the APPJ treatment.

As the materials were not ideally smooth and flat, and the fabrics even had very porous structures, the APPJ treatment could penetrate the material,^20,29,30 leading to numerous wicking tunnels. The overall improvement of hydrophilicity might be more significant than the decrease percentage of the static surface water contact angle.

SEM analysis

The surface morphology of the materials changed after APPJ treatment, which created lots of micro and nano roughness (see Figure 6). Granules and dents appeared on the originally smooth surface of Material A (PE). Protrusions, dents, and small pores are visible on the fibers of Material B (PP) after APPJ treatment. For Material C (PET), granules, plaques, and cracks occurred after the treatment. Granules, coatings, and branches were formed on the fiber surface of treated Material D (PET). The activated particles from the plasma induced etching and chemical reactions on the surface of the materials, resulting in those morphological changes, which could contribute to the hydrophilicity of the materials.

Figure 6.

Surface morphology of Materials A–D before and after APPJ treatment.

XPS analysis

XPS analysis of the material surfaces showed a significant difference in the chemical composition before and after APPJ treatment, as shown in Table 4. The atomic ratio of O/C was raised by 175%, 260%, 15.8%, and 29.3% for materials A, B, C, and D, respectively, after the APPJ treatments. The much larger increase of O/C ratios for the PE film and the PP fiber (A and B) were due to the fact that these two materials had little oxygen originally on their surfaces, while the PET fibers (C and D) had much higher oxygen content to start with. The bombardment of activated particles in He/O₂ plasma induced chemical reactions with substrate molecules, resulting in surface oxidation accompanied by chain scission and active site substitution with other atoms or molecules, especially oxygen in the air.^20,31 The oxidation of the material surface usually leads to more polar chemical groups with better hydrophilicity.

Table 4.

Surface chemical analysis determined by XPS on Materials A–D before and after APPJ treatment

		Atomic concentration (%)			Atomic ratio	Relative area of different chemical bonds (%)
Material	Condition	O	C	N	O/C	C–C/C–H	C–O/C–OH	C = O	O–C = O
A	Before	3.96	95.67	0.37	0.04	100	0	0	0
	After	9.61	90.12	0.27	0.11	95.79	4.21	0	0
B	Before	4.74	94.92	0.34	0.05	100	0	0	0
	After	15.16	84.54	0.30	0.18	93.64	6.36	0	0
C	Before	27.26	72.07	0.67	0.38	47.12	32.80	3.58	16.50
	After	30.31	68.99	0.70	0.44	39.39	32.52	8.77	19.32
D	Before	28.64	69.88	1.49	0.41	85.47	9.80	0	4.74
	After	33.88	64.33	1.79	0.53	73.66	21.52	0	4.82

For further analysis, the XPS spectra were deconvolved with XPSPEAK software. The percentages of chemical bonds determined by curve-fitted spectra for C1s are presented in Table 4. The content of C–C/C–H was significantly decreased and oxidized polar groups were increased or added to the material surfaces after APPJ treatments. The material surfaces could potentially have carbon-containing peaks corresponding to C–C/C–H (∼284.8 eV), C–O/C–OH (∼286.3 eV), C = O (∼287.4 eV), and O–C = O (∼288.8 eV).^29,32–34 For Material A (PE) and Material B (PP), APPJ treatment created 4.21% and 6.36% of C–O/C–OH, respectively. For Material C (PET), the content of C = O was increased by 5.19% and that of O–C = O by 2.82%. For Material D (PET), a 11.72% increase of C–O/C–OH occurred. The creation and increase of oxygen-containing groups during the oxidation process of APPJ treatment contributed fundamentally to the improvement of surface hydrophilicity.^20,31

Adhesion property analysis

The maximum adhesion force and the duration time for materials with different structures made of different components all increased a lot after the APPJ treatments (see Figure 7). The maximum adhesion force increased by 21.7%–55.1% and the duration time increased by 21.4%–60.5% for the different materials. Compared with the PE film (A), the percentage decrease of apparent water contact angles, the percentage increase of adhesion force and adhesion duration, were much larger for the PP and the two PET fabrics (B, C, and D). A larger adhesion force indicates that the material would stick more tightly to a wet surface. A larger duration time indicates that the material would linger on the wet surface for a longer time before detaching. After treatment with APPJ, a larger force and a longer travelling distance or duration time would be required, leading to increased work for separating the adhering material from the water surface.

Figure 7.

Curves of adhesion force versus time for different materials before and after APPJ treatment.

As discussed in our previous study, the adhesion property could be related to hydrophilicity, deformability, structure of the material, and surface tension of the liquid.^11,13,18 Since the APPJ treatment only increased the hydrophilicity of the materials, with little change in other characteristics, such as deformability or structure, the above results showed a positive relationship between the hydrophilicity of the material surface and the adhesion force, which agrees with the theoretical conclusions of equation (4) quite well. The increase of hydrophilicity leads to a larger adhesion force and also a longer adhesion duration, which may cause more discomfort with wet fabric materials. However, in many applications, such as adhesion of pasting film or geotextiles to soil, larger adhesion force and longer duration of adhesion before detaching could be advantageous.

Conclusions

A theoretical expression was derived to describe the relationship between adhesion force, F, water contact angle, $θ_{a}$ , and radius, R, of a fabric–liquid interface, and verified with experiments, using a PE film, a PP nonwoven fabric, a PET woven fabric, and a PET knitted fabric. In accordance with the theoretical relationship, the experiments showed that more hydrophobic materials had smaller maximum adhesion forces and also substantially shorter adhesion durations than more hydrophilic materials (with smaller water contact angle, rougher surfaces, larger O/C atomic ratios, and more polar chemical bonds on surfaces). In order to minimize wet adhesion and discomfort, attention should be paid to decreasing the hydrophilicity of the fabric surface in contact with human skin.

Nomenclature

Adhesion force on the fabric.

θ_{a}

θ_{b}

Contact angle at the top or bottom of the liquid bridge.

Δ p

Pressure difference between air and liquid, with air pressure higher than liquid pressure.

Surface tension of liquid.

R_{b}

Radius of the top or bottom of the liquid bridge.

R_{1}

Radius of circle Q which is on the profile of the liquid–vapor interface.

Density of the liquid.

Height of the liquid bridge.

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 Scientific Research Fund of Zhejiang Provincial Education Department (grant number Y201432037), and the Zhejiang Top Priority Discipline of Textile Science and Engineering & Engineering Research Center of Clothing Technology of Zhejiang Province (grant number 2013KF11).

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