Designing breathable superhydrophobic cotton fabrics

Abstract

Superhydrophobic cotton fabrics were prepared through radiation induced graft polymerization by applying a series of alkyl methacrylates as the functional monomers. We found that the superhydrophobic cotton fabrics exhibited considerable resistance to air permeability whereas the water vapour transport is almost the same as pristine cotton. The mechanism is that the air permeability is mainly determined by the thickness and pore size of the fabric, while the lower degree of interaction between the grafted cotton fabrics and water molecules along with an enhanced capillary effect are the key factors responsible for the high water vapour transmission rate. The combination of opposing properties – a decreased air permeability and ideal water vapour transmission rate, creates a breathable and comfortable fabric as a dressing material.

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Introduction

Superhydrophobic surfaces have garnered immense academic and commercial interest owing to their wide potential for application in various spheres, including as self-cleaning surfaces,^1,2 oil/water separation,^3–7 water collection,^8–10 and corrosion-resistant surfaces.^11–13 Clothing is regarded as the “second skin” of human beings. Therefore, its function and comfort are parameters of primary focus in the textile field.¹⁴ Significant progress has been made to develop superhydrophobic fabrics with good anti-contamination and self-cleaning properties using low-surface-energy monomers.^15–17 In our previous studies, we had prepared a washing-durable superhydrophobic cotton fabric (SCF) by a radiation-induced graft polymerization technique. The SCF could maintain its properties even after 50 accelerated laundering cycles.¹⁸ Recently, a cotton fabric that is abrasion durable, self-healing and superhydrophobic has been reported.¹⁹ Studies on the durability^20,21 and comfort²² of the superhydrophobic fabrics are necessary for broadening their use in areas such as sports and industrial applications and as foul-weather-protective textiles. In this regard, it is imperative to improve the breathability of these fabrics.

According to the literature, clothing comfort and functionality can be divided into the following independent parameters: water and wind proof, tactile comfort (including softness, antistatic nature, and prickle sensation), moisture/sweat management and warmth/temperature control.²² Herein, thermal and moisture handling properties of textiles during intense physical activities have been regarded as major factors in the comfort performance. Higher the water vapour permeability, higher the comfort level of the fabric.²³ Hence, superhydrophobic fabrics should be designed such that they exhibit breathability with respect to water vapour, while ensuring protection against wind and water.²⁴ Considerable efforts have been made to improve the comfort of clothing. Palanikkumaran et al. developed a breathable coating based on poly(vinyl alcohol) on cellulosic fabric. This coating exhibited a water vapour transmission rate (WVTR) of 2165 g m⁻² 24 h⁻¹ which is 50% of the value exhibited by uncoated cotton fabric.²⁵ In another study, a copolymer was used to develop a smart and breathable cotton fabric, whose breathability changed with the ambient temperature.²⁶

In the case of superhydrophobic fabrics, there is a contradictory requirement that the inter yarn spaces should be as small as possible to yield maximum protection against wind and rain, whilst the fabric's outer surface should be non-absorptive and hydrophobic to minimize wetting by rain.²⁷ The present approach to designing hydrophobic and breathable textiles mainly focuses on the microporous polymer coating on the fabric. However, this leads to poor mechanical durability because of the weak van der Waals forces between the hydrophobic coating and the fabric. The post-finish softness and smoothness of hydrophobic textiles is also quite poor. Therefore, it's essential to address the issues related to the breathability, durability, and softness of waterproof textiles.

In this study, we attempt to develop a series of breathable superhydrophobic cotton fabric by covalently introducing alkyl methacrylates onto cotton substrates through radiation induced graft polymerization. The grafted fabrics exhibited low air permeability, which decreased rapidly with an increase of grafting. On the other hand, the water vapour transmission rates of the fabrics were as high as that of the pristine cotton. The reason is that the air permeability of fabric is mainly related to its porosity, thickness and pore diameter, and the thickness will increase and pore size will decrease significantly after graft polymerization. Whereas the permeation of water vapour through a perforated structure obeys Fick's law (absorption, diffusion and desorption mechanisms), and the capillary action are also the key factors affecting water vapour transport. The grooves on the surfaces of the grafted cotton samples are deeper and narrower than that of pristine cotton, and therefore leading to stronger capillary effect to transfer water vapour quickly. The water-proof finish have effectively decreased the degree of interaction between the surface of the fabric and water molecules, inducing a fast wicking of the vapour out of the fabric.

To the best of our knowledge, there have been few studies reporting on the synthesis of superhydrophobic surfaces with WVTR values as high as the unmodified cotton along with a decreased air permeability.^27–29 Furthermore, the alkyl methacrylate monomers used are more environmental friendly than other low-surface-energy materials such as fluoro-monomers. Finally, the fact that radiation induced graft polymerization procedure is simple and cost-effective.

Experimental

Materials

Six series of alkyl methacrylates were used as low surface energy functional monomers (Fig. 1), including methyl methacrylate (MMA, x = 1), ethyl methacrylate (EMA, x = 2), n-propyl methacrylate (PMA, x = 3), n-butyl methacrylate (BMA, x = 4), n-hexyl methacrylate (HMA, x = 6) and n-lauryl methacrylate (LMA, x = 12).

Fig. 1 Graft polymerization process for synthesizing superhydrophobic fabrics.

Radiation-induced graft polymerization

Fig. 1 depicts the procedure for synthesizing the superhydrophobic fabrics by radiation induced graft polymerization. The cut cotton fabrics and methanol solutions with a defined alkyl methacrylate monomer concentration were put into glass tubes and ultrasonic for 15 min and then purged with N₂ for 10 min to remove the O₂, and subsequently irradiated by γ-ray in a ⁶⁰Co facility for 30 kGy. After that the samples were extracted for 72 h by ethyl acetate to remove the residual monomers and homopolymers. Finally, they were dried under vacuum condition at 60 °C for further measurements.

The degree of grafting (DG) is calculated according to the following equation:³⁰

DG = (W_g − W_o)/W_o × 100% (1)

where, W_o is the weight of the sample before graft polymerization and W_g is the weight of the sample after graft polymerization, respectively.

Surface characterization

The Fourier transform infrared (FT-IR) measurement was taken on a Nicolet Avatar FT-IR spectrometer (Thermo Nicolet Company, USA).

Scanning electron microscopy (SEM, S4800) were used to study the morphology of cotton fibrous structures. Samples were conducted at the electron energy of 20 kV and current of 10 μA.

The water contact angles (CA) were determined on an Attention Theta system (KSV Instruments Ltd., Finland) by placing a 5 μL droplet of deionized water onto the substrate.

The Air permeability were determined to study the change of the physical parameters before and after graft polymerization. Samples were tested on the Kawabata air permeability (KES-F8-AP1) tester with a test pressure difference of 100 Pa according to GB/T 5453-1997. Each sample was measured for six times on different places. The total thickness of the samples were also tested using a Vernier calliper.

The WVTR was tested using the upright cup test method according to GB/T 12704.2-2009. The samples were housed in an environmental chamber at the air temperature of 38 ± 0.5 °C and relative humidity of 50% and the air velocity was 2.8 ± 0.25 m s⁻¹. Circular specimens, 7.4 cm in diameter, were cut from each fabric and placed on an aluminium cup which was filled with 34 mL of distilled water.

Results and discussion

Superhydrophobic characterization

A low free energy surface is required for a super-hydrophobic surface to be obtained, and in our studies this can be realized by modifying the cotton substrate with alkyl methacrylate compounds.

After γ-ray radiation induced graft polymerization, the cotton fabrics contain hydrophobic poly(alkyl methacrylate) chains, which repel water effectively. The DG was determined by the gravimetric method, which increases readily with the monomer concentration. The Fourier transform infrared (FT-IR) spectra in Fig. 2a showed that all the grafted cotton fabrics exhibit an absorbent band at approximately 1729 cm⁻¹ which is attributed to the characteristic C=O stretching vibration of the ester groups on the graft chains. In contrast, the C=O signal was not present in the spectrum of pristine cotton, confirming that the alkyl methacrylate was grafted successfully onto the cotton fabric samples. The grafted density of the poly(alkyl methacrylates) on the cotton fabric was also calculated. As shown in Fig. 2b, the grafting density of poly(alkyl methacrylates) increases as the degree of grafting increases, suggesting that the coverage of the graft chains on the surface is increasing.

Fig. 2 (a) The FT-IR spectra of the samples. (b) Grafting density of poly(alkyl methacrylates) onto cotton fabrics versus degree of grafting.

The water contact angle on the grafted cotton fabric surface was measured. The CA eventually increased with DG, and became greater than 150° after the DG increased beyond a threshold value, suggesting that the surface of the cotton fabric was sufficiently covered by the graft chains, which was defined as the critical DG and grafting density required for superhydrophobicity, shown as Table 1. Experimental data revealed that longer the alkyl chain, lower DG required to exhibit superhydrophobicity. For instance, the Cotton-g-PLMA sample which has the longest alkyl group (x = 12) among the monomers, the critical DG and grafting density was about 10% and 0.0564 g cm⁻³ respectively, whereas it is approximately 35% and 0.1499 g cm⁻³ for the Cotton-g-PEMA (x = 2). The reason is that the long chain alkyl methacrylate has lower surface energy than the short one, which can be seen from water contact angles on smooth homopolymer films (Table 1). The cotton fabric grafted with poly(alkyl methacrylate) with shorter alkyl chain requires higher grafting density to achieve full coverage of the fabrics, whereas longer alkyl chain length resulted in lower critical grafting density. While the CA of the MMA-grafted cotton fabric sample did not exceed 150° due to the DG should be much higher to reach superhydrophobicity. However, it's difficult to reach high DG because of the high reactivity of MMA and consequently a greater number of homopolymers was formed after radiation. The result indicates that the DG, along with the chain length of the alkyl group, have an appreciable effect on the superhydrophobicity. Therefore, controlling the length of the polymer chain seems to be an ideal way of preparing highly water-repellent materials.

Table 1 The critical grafting density for superhydrophobicity and the contact angle of the homopolymer as function of the alkyl chain length (i.e., x value)

Sample	Alkyl chain length (x)	Critical DG (%)	Critical grafting density (g cm^−3)	CA of homo-polymer film (°)	Wetting state
Pristing cotton	—	—	—	—	Hydrophilic
Cotton-g-PMMA	1	41	0.1664	68.3	Hydrophobic
Cotton-g-PEMA	2	35.6	0.1499	77.9	Superhydrophobic
Cotton-g-PPMA	3	33.6	0.1485	84.1
Cotton-g-PBMA	4	30.2	0.1174	88.4
Cotton-g-PHMA	6	26.6	0.1136	93.5
Cotton-g-PLMA	12	11.8	0.0564	101.1

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Air permeability

Cotton fabric is assumed to be a pseudo-2D porous material with multi-scale pore structure (Fig. 3a) described as follows:³¹

Fig. 3 Schematic of the pore structure of woven cotton fabric.

(1) The micron pores between the yarns in the warp and wept directions (classified as p1). Typically the pore size larger than 5 μm, as can be seen from the SEM image (Fig. 4a).

Fig. 4 SEM images of pristine cotton fibre (magnification: 100×, 1000×, and 50 000×, respectively).

(2) The sub-micron pores between the fibres within a yarn (classified as p2). These pores range from 100 nm–10 μm (SEM image in Fig. 4b).

(3) The nano pores within individual fibres (classified as p3), owing to the natural porous structure of the cotton fabric which are almost round with the diameter less than100 nm (Fig. 4c).

Thus, the cotton fabric can be considered to be constituting of capillaries arranged in a parallel fashion across the thickness direction. The flow rate of air through the fabric can be determined using the Hagen–Poiseuille equation:³²

where Q is the air permeability per unit area, K is the number of capillaries per unit area, r is the radius of the capillaries, and L is the length of the capillaries, which can be regarded as the thickness of the fabric. Further, ΔP is the pressure difference between the two surfaces of the fabric, which was assumed to be 100 Pa during our experiment, and η is the viscosity of air and it's a constant at the room temperature.

On the basis of the classification system mentioned above for the pores of cotton fabric, the equation can be rewritten as following:

where K_p1 and A_p1, K_p2 and A_p2, K_p3 and A_p3 are the number and area of the pores which are denoted as p1, p2, p3, respectively.

The equation indicates that the air permeability Q is a function of the thickness and surface porosity of the three kinds of pores. As shown in Fig. 5, the fabric thickness increased from 187 μm to approximately 250 μm with increasing DG, suggesting that the passage for air transmission will increase after grafting, and therefore induced to lower air permeability. The relationship between the Q values of six grafted samples and the DG is also studied. Fig. 6 showed that the air permeability decreases dramatically with increasing DG and is independent of the alkyl chain length.

Fig. 5 Increase in the thickness of the grafted samples as function of DG values.

Fig. 6 Dependence of air permeability on DGs.

In order to elucidate this mechanism, the pore size of the fabric samples were measured using mercury porosimetry. As shown in Fig. 7, the obtained differential pore size distribution was in accordance with the three kinds of pores (p1, p2, p3). It was found that after a degree of graft polymerization, the peak intensity showed a slightly decrease at pore size larger than 5 μm. Further, the peak intensity less than 100 nm reduced clearly and were nearly non-visible, whereas the intensity of some sub-micron peaks ranging from 100 nm to 5 μm (p2) became stronger, i.e., at 150 nm and 35 μm respectively. Additionally, some new peaks were present at 200 nm to 1 μm range. The above results indicated that the micron pores between the yarns get narrower with shifting to sub-micron size after graft polymerization, and the nano pores within individual fibres were probably blocked by the graft chains. Therefore, the increased thickness combined with decreased pore size were the crucial reasons for the reduced air permeability of grafted cotton fabrics.

Fig. 7 Pore size distribution of the pristine cotton and Cotton-g-PLMA samples.

Water vapour transmission rate (WVTR)

Generally, the waterproof micro porous textiles have tiny holes on their surface smaller than a rain drops but larger than water vapour molecule, offering high resistance to liquid water penetration but allow water vapour to transport at the same time.³³ In our experiment, the water vapour transmission rate (WVTR) of the grafted cotton fabric samples with different DG values were measured using the GB/T 12704.2-2009 method at 38 °C and 50% humidity to mimic the conditions under which the human body starts to sweat. First, about 34 mL distilled water was put into an aluminium cup and weighed after being balanced for 1 h. And then weighed at intervals of 2 h. The WVTR values were calculated from the change in the weight in the 2 h period using the following formula (4):where Q_wvt is the WVTR (g m⁻² h⁻¹), ΔW is the change in the weight of the water sample contained in the cup, A is the test area of the specimens, and t is the time duration (in this case, t is 2 h).

The WVTR value of the pristine cotton was assumed as 1, then the normalised WVTR value was obtained using the following formula:

A linear relationship was obtained between the normalised WVTR and DG, as shown in Fig. 8a, WVTR values of the SCF samples were nearly the same as the hydrophilic cotton and did not change with an increase of DG, indicating that the water-repellent finishes did not affect the vapour resistance of the fabrics. To investigate the effect of the alkyl chain length on the WVTR value, tests were performed on the cotton samples grafted with different alkyl methacrylates with the DG around 30% (Fig. 8b). The results suggest that the WVTR of grafted cotton fabrics is also independent on the alkyl chain length. In addition, this result is in contrast to previous studies, which reported that the WVTR value decreases after the cotton was treated with waterproofing finish (Fig. 9).^25,26

Fig. 8 (a) Normalized WVTR values as function of the DG value. (b) Relationship between the WVTR values and the chain length of the grafted alkyl methacrylate.

Fig. 9 Water vapour permeability values of the grafted cotton fabrics compared to that of the pristine cotton in this study, and ref. 25 and 26.

The result seems to be contradictory between the air permeability and water vapour permeability. In fact, water vapour transport through the micro porous fabric employs much more complex mechanisms than air permeability.³⁴ The air permeability takes place in convection condition, where larger surface of clearance is good for air transmission. While water vapour permeability takes place in condition of free convection, which is governed not only by porosity and pore diameter, but also by its base fibre moisture properties and the molecular mechanism (Fick's law), and is also depending on the capillary action.^27,35–37

The pristine cotton fabric exhibits excellent water vapour absorption ability but a poor water diffusion rate because of a large number of hydrophilic hydroxyl groups on the cellulose molecules which can form strong hydrogen bonds with the water molecules that act as “stumbling blocks”. After being subjected to graft polymerization, the hydrophilic surface of cotton fabric turns hydrophobic, meaning a frictionless interface has been formed on the fabric surface³⁸ at the same time, water molecules preferentially form strong hydrogen bonds.³⁹ Therefore, the weak attraction between water molecules and the smooth interface combined with the hydrogen bonds formed between the water molecules account for the very rapid flow of water vapour through the fabric.

In addition to the effect of roughness structure of the interface, the capillary effect also plays an important role in conducting water molecules out of the fabric.^27,40 The spaces between the fibres effectively form capillaries, and the narrower the spaces between the fibres, the greater the ability of the textile to wick moister.⁴¹ The efficiency of yarn wicking is depending on the size, volume and number of capillary spaces within the fibre bundle. After graft treatment, the grooves on the single fibre become deeper and narrower than those in pristine cotton (Fig. 4c and 10), which can enhance the flow of water vapour transfer to the environment. Therefore the capillary effect induced by the grooves facilitates speeding up of water diffusion and desorption ability.

Fig. 10 SEM image of a single cotton fibre with Cotton-g-PLMA (DG = 20.1%).

Conclusions

Waterproof, windproof, and breathable cotton fabrics were prepared by covalently grafting non-fluorinated methacrylate monomers onto cotton substrates by simple radiation induced graft polymerization procedure. The superhydrophobicity of the grafted cotton fabrics is determined by the low surface energy poly(alkyl methacrylates). When the surface of the cotton is covered sufficiently by the graft chains, the cotton fabric becomes superhydrophobic. The cotton fabric grafted with poly(alkyl methacrylate) with shorter alkyl chain requires higher grafting density to achieve full coverage of the fabrics, whereas longer alkyl chain length resulted in lower critical grafting density. The air permeability of the SCFs decreased with an increase of DG value and was independent with the alkyl chain length of the monomer. This can be attributed to the combination of the increase of thickness and decrease of the pore size in the cotton fibres after grafting. While the WVTR values of the grafted fabrics were as high as that of the pristine cotton, owing to the reduced interaction between the surfaces of the fabrics and water molecules, along with the stronger capillary effect of the deep grooves formed between the fibres after grafting. The fabricated SCFs solve the problem of designing breathable and comfortable clothing materials that are resistant to wind-driven rain while allowing water vapour to pass through the fabric. The technique investigated in this study should lead to the development of various types of fabrics that are comfortable, and can be produced without causing significant environmental pollution.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (51473183 and 11475246).

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