Optimised fluorine free polysiloxane based water repellent for cellulosic fabric

Abstract

This research focuses on creating a new water-repellent finish for textiles utilising a sol-gel process and the combination of 3-glycidyloxypropyltrimethoxysilane (KH560) and gamma-Methacryloxypropyltrimethoxysilane (KH570) as a precursor which was then modified with branched silicone through sol-condensation. A fluorine-free, new-generation polysiloxane-based water repellent was prepared and analysed to achieve the best water repellent characteristics. Different parameters and conditions were tested during the preparation of chemicals to achieve this. After preparation, all the chemicals were characterised using various scientific analysing tools such as Fourier Transform Infrared radiation, thermogravimetric analysis, particle size analysis, contact angle and water-repellent testing. Following the particle size investigation, it was discovered that the particle size distributions for these samples were homogenous and advantageous, allowing for more efficient contact and penetration with the textile fibres. The results of this study confirm the existence of functional groups that can contribute to the desirable characteristics of the compounds to repel water with one coupling agent, KH570:KH560 = 0.02 mol:0.04 mol, showing particularly good water-repellence performance.

Keywords:

• Cellulosic fibre
• water repellent finish
• silicone oil
• environmental friendly
• polysiloxane

1. Introduction

Water repellence is one of the most common functional properties needed for protective clothing that does not affect comfortability and wearability. Water-repellent textiles have many applications, including industrial, consumer, and apparel (Holmquist et al., 2016; Khatton et al., 2022). This repellence can be achieved by coating a thin surface layer of water-repellent chemicals onto textile fibres or fabric. It can be done through modification of the surface energy of textiles with minimal effects on other functional properties like strength, flexibility, breathability, and softness (Chowdhury, 2018; Chowdhury et al., 2018; Mukhopadhyay & Midha, 2008). When a water-repellent chemical is applied to a cellulosic fibre, a monomer present in the water-repellent chemical is absorbed into the fibres with the help of an initiator mixture. It makes the fabric water-repellent by forming polymeric chains and graft bonds inside the textile structure (HM Fahmy et al., 2019; Mukhopadhyay & Midha, 2008; Tragoonwichian et al., 2011). Further, these textiles with modified surfaces can be fabricated using a low monomer add-on with interpenetrating components and a homogenous distribution of that monomer into the fibres (Ferrero et al., 2008).

Water-repellent finishes can come in many forms, such as fluids, emulsions, and resins. Methyl hydrogen polysiloxane (MeSiOH) is a very popular water-repellent finish, but it has many risks as it is quite reactive and requires careful handling (Warrick et al., 1979). Silicon Hydride (SiH) compounds rapidly evolve into hydrogen gas when they come in contact with strong bases, amines, and primary alcohols, which can form flammable and explosive mixtures in the air (Guillier et al., 2000). The inherent risk involved with these compounds make them unpopular and unattractive for water-repellent finishing operations (Speier & Hook, 1958). Compounds based on paraffin oil with silicone are another extensively used water-repellent finishing agent but were insufficient in protecting textiles from grease and oil stains (Nadi et al., 2018; Teli, 2018). This led to the development of fluorocarbon polymers (FCPs). Carbon-fluorine’s relatively low reactivity and high polarity impart unique characteristics to fluorocarbon polymers. FCPs are applied via the pad dry-cure technique, wherein the substrate aligns the fluorocarbon segments of the polymers, thereby reducing the tendency of soil, oil, and water to adhere to the fibres of the substrates. This is desired because fibres, along with wool and silk, tend to be hydrophilic and are therefore prone to staining faster than synthetic fibres (Dams & Hintzer, 2016; Rieber & Loffler, 1974). However, FCP consists of perfluorinated carbon chains incorporating a polymer backbone with perfluoro groups as side chains keep the fibres from staining. While organic fluorine-based agents exhibit excellent water and oil repellence, they pose concerns regarding high cost, poor degradability, bioaccumulation, and potential human and environmental hazards (Cousins et al., 2019). Some existing fluorochemicals are made with C8 carbon backbone chains that can release per fluorooctanesulfonate (PFOS), perfluorooctanoic acid (PFOA), and other toxic and hazardous materials (Ogawa & Soga, 1996); therefore, C6 based fluorocarbons were introduced, though their repellence performance and durability are less than C8 based ones.

On the other hand, silicone-based water repellents are known for their environmental friendliness, cost-effectiveness, and superior water repellence; making them the preferred choice for market application (HM Fahmy et al., 2016; Fahmy et al., 2022; Özek, 2018). The sol-gel method is widely regarded as the simplest and most efficient approach for preparing silicone water repellents. This method offers several advantages, including achieving a large contact area, controlling the coatings’ morphology, and shorter processing times (Amiri & Rahimi, 2016; C. Wei et al., 2016). Consequently, the sol-gel method holds significant economic potential. In most studies on silicone water repellents, researchers utilise modified silica sols containing linear alkyl silicon derivatives, such as oxyalkylene segments or aliphatic hydrocarbon groups. For instance, Torstein Textron et al. employed the sol-gel method to hybridise silica sol with linear alkyltrial alkoxysilanes of various carbon chain lengths. This resulted in an organic-inorganic hybrid water-repellent material with excellent water repellence and antistatic properties (Textor & Mahltig, 2010). Another study by Daoud successfully prepared a super-hydrophobic silicon coating for fabrics using a hydrolysis and polycondensation reaction of hexamethylenetrimethoxysilane and tetraethoxysilane (Daoud et al., 2004).

In this study, a novel water repellent was synthesised by using a mixture of 3-glycidyloxypropyltrimethoxysilane (KH560) and gamma‐Methacryloxypropytrimethoxysilane (KH570) as a precursor which was then modified with branched silicone through sol-condensation. During the preparation of chemicals, different parameters and conditions were optimised and the branched silicone-modified silica sol formed a film on the fabric’s surface, exhibiting good water repellence.

2. Experimental section

2.1. Synthesis of water repellents

The preparation process involves several stages. Figure 1 represents the synthesis reaction for the water-repellent chemicals. In the first stage, 40 g of factory-grade methyl hydrogen silicone oil 203 and 0.2 ml of isopropylplatinic acid, serving as a catalyst, are mixed thoroughly in an isopropanol solvent using magnetic stirring and in a nitrogen-protected triple-necked flask. The mixture is stirred continuously while 17 g of allyl glycidyl ether is added dropwise over 10 min. This controlled addition ensures proper incorporation of the allyl glycidyl ether. 1.425 g of chloroplatinic acid is then added to the mixture, and the temperature is raised to 80 °C for 7 h, where the excess allyl glycidyl ether is removed under reduced pressure. In the second stage, a nitrogen-protected four-necked flask is used. 11.8 g of the coupling agent KH 560 and 100 ml of isopropanol are added and homogeneously mixed at 35 °C. Then, the epoxy silicone oil is added dropwise over 10 min, followed by a 30-minute reaction period. The pH is checked and adjusted using glacial acetic acid, and the insulation reaction continues for 1 h. In the third stage, deionised water, 20 μl of dibutyltin dilaurate (a catalyst), and a coupling agent (KH560, KH570, or a mixture) are added dropwise. The amount of coupling agent used was as follows:

Figure 1. Generalised synthesis reaction for the water-repellent chemicals.

• KH570 = 0.06mol

• KH560 = 0.06mol

• KH570:KH560 = 0.02mol:0.04mol

• KH570:KH560 = 0.03mol:0.03mol

• KH570:KH560 = 0.04mol:0.02mol

The reaction is allowed to proceed for 7 h. The resulting mixture is filtered to remove insoluble impurities and the solvent is then removed. Finally, the sample weight is determined and three times its weight in deionised water is added. The mixture is emulsified at 40 °C in a water bath. The hydrolysis of silane coupling agents like KH560 and KH570 involve a chemical process that converts these agents into forms that can more readily bond with substrates. When the silane coupling agents are exposed to water, a hydrolysis reaction occurs. The alkoxy groups in the silane agents react with water, leading to the formation of silanol groups (-Si-OH). The general reaction can be represented as: $$\text{R‐Si}{\left(\text{OR}\prime \right)}_3{\text{+ 3H}}_2\mathrm{O}\to \text{R‐Si}{\left(\text{OH}\right)}_3\text{+ 3R}\prime \text{OH}$$

Here, R represents the organofunctional group in the silane (which differs between KH560 and KH570), and OR′ represents the alkoxy group.

2.2. Water-repellent finishing

The formulation of the self-emulsified water repellent involves using a 0.5% finishing solution of the prepared chemicals. The pad-dry method is employed to apply the water-repellent finish onto woven cotton fabric (10 cm × 10 cm). During the pad-dry process, a take-up percentage of 70% is maintained and the fabric is dried and cured at 180 °C for 2 min to set the water-repellent finish. This thermal treatment helps crosslink, or bond, the finishing agents to the fabric fibres, enhancing the durability and effectiveness of the water-repellent finish.

2.3. Characterisations

Fourier Transform Infrared (FTIR) analysis was conducted using a Tensor 27 spectrophotometer from Bruker, which has a spectral range of 7500 to 370 cm⁻¹, a resolution better than 1 cm⁻¹, and a wave number accuracy greater than 1 cm⁻¹ at 2000 cm⁻¹. Thermogravimetric Analysis (TGA) was performed using a DTG-60AH Thermogravimetric Analyzer from the Day Benji Jin Company. The particle size of the prepared chemicals was determined using a Zetasizer Nano ZSP instrument from Malvern Analytical, UK. The washability of the treated fabrics was done using ISO 6330 textile domestic washing and drying procedures in horizontal-drum front-loading machines. Additionally, the wettability of the treated fabrics was assessed through contact angle measurements using a DSA30 contact angle system from Kruss. Finally, the spray rating was determined ausing an LLY-13 Water Spray Meter from Laizhou Electronic Instrument Co., Ltd. according to the AATCC 22 method.

3. Results and discussion

3.1. Infrared spectroscopy

The Fourier Transform Infrared Radiation (FTIR) spectrum was utilised to prove the presence of anticipated functional groups in the prepared chemicals. Figure 2 represents the spectrum of Infrared radiation of the prepared water-repellent chemicals. All the prepared water repellents’ FTIR spectrums exhibit distinctive featured peaks of a band transpired at about 1400 cm⁻¹ and 1570 cm⁻¹, which are referred to as -OH and -NH bending, respectively. Consistent stretching of -CH₃ groups can be recognised in the alkyl chains by the peak near 2960, 2920 and 2850 cm⁻¹. The peak near 1100 cm⁻¹ proves the presence of -C-O-C-. The peak near 1020 cm⁻¹ refers to -Si-O-, and the peak near 2150 cm⁻¹ and 800 cm⁻¹ is -Si-H-. They are asymmetric vibration absorption peaks, indicating that a network has formed in the silica sol film structure. The peaks near 1260 cm⁻¹, 910 cm⁻¹ and 800 cm⁻¹ at the absorption of the Si-CH₃ peak indicate that the branched polysiloxane has been attached to the silica sol. The peak near 3450 cm⁻¹ is the stretching vibration absorption peak of O-H, indicating that it is in epoxy silicone oil. The opening of the epoxy group results in the formation of a hydroxyl group, as represented by the -N-H peak. In summary, the FTIR spectrum analysis confirms the presence of various anticipated functional groups in the prepared water-repellent chemicals, including -OH, -NH, -CH₃, -C-O-C-, -Si-O-, -Si-H, and Si-CH₃ groups. These functional groups contribute to the desired properties and performance of the water-repellent chemical.

Figure 2. Infrared spectrum of water repellent (a) using coupling agent KH560, (b) using coupling agent KH570, (c) using coupling agent KH570:KH560 = 0.01 mol:0.02 mol, (d) using coupling agent KH570:KH560 = 0.02 mol:0.04 mol, (e) using coupling agent KH570:KH560 = 0.03 mol:0.03 mol, and (f) using coupling agent KH570:KH560 = 0.04 mol:0.02 mol.

3.2. Thermogravimetric analysis

The thermogravimetric (TG) curves show two separate weight loss phases for all the prepared chemicals, as seen in Figure 3. The elimination of water molecules and small amounts of excess solvent from the gel causes the first step to occur between 100 °C and 200 °C. Due to the solvent’s volatilisation upon heating, there is weight loss. At 340 °C, the second weight loss stage becomes noticeable and is predominantly related to the thermal decomposition of the product’s -Si(CH₃)₃ groups. These groups undergo decomposition as the temperature rises, which causes weight loss. The polysiloxane chain continues to degrade at temperatures over 500 °C, and an oxidation reaction occurs. Temperatures above 600 °C create an inorganic, very temperature-resistant phase, specifically a SiO₂ solid. The heat-stable SiO₂ remains after the complete decomposition of the organic phase materials. As a result, weight loss is no longer occurring during this period, and the TG curves tend to become parallel. A rise in the residual rate seen on the TG curve reflects the sample’s increased inorganic phase concentration. The mass that remains after decomposition likewise rises as the proportion of the inorganic phase does, indicating a higher residual rate. The data in Figure 3 provides insight into the sample’s thermal decomposition behaviour by showing weight loss trends at various temperature levels and the conversion of organic components into heat-stable SiO₂.

Figure 3. TG curves of the prepared water-repellent chemicals coating.

3.3. Particle size analysis of water-repellent emulsion

The self-emulsifying, hybrid, silica sol water repellent’s particle size distribution was determined. The particle size distribution of the synthesised emulsion utilising various coupling agents and their ratios is shown in Figure 4. When KH560 was the only coupling agent used, the particle size distribution was mostly centred at 163.4 d.nm. However, when coupling agent KH570 was employed solely along with the mixtures of KH570 and KH560 in various ratios, such as KH570:KH560 = 0.01 mol:0.02 mol, KH570:KH560 = 0.02 mol:0.04 mol, KH570:KH560 = 0.03 mol:0.03 mol, and KH570:KH560 = 0.04 mol:0.02 mol, the particle sizes distributed around 196.4 d.nm, 163.2 d.nm, 117.4 d.nm, 139.8 d.nm, and 140.7 d.nm, respectively. The particle size distribution of coupling agents KH560, KH570, and KH570:KH560 = 0.01 mol:0.02 mol showed a double peak pattern. However, the particle size distribution showed a single peak for KH570:KH560 = 0.03 mol:0.03 mol, KH570:KH560 = 0.02 mol:0.04 mol, and KH570:KH560 = 0.04 mol:0.02 mol. In every instance, the distribution of particle sizes showed a completely normal distribution.

Figure 4. Particle size distribution of the water-repellent emulsions (a)using coupling agent KH560, (b) using coupling agent KH570, (c) using coupling agent KH570:KH560 = 0.01 mol:0.02 mol, (d) using coupling agent KH570:KH560 = 0.02 mol:0.04 mol, (e) using coupling agent KH570:KH560 = 0.03 mol:0.03 mol, and (f) using coupling agent KH570:KH560 = 0.04 mol:0.02 mol.

The self-emulsified water-repellent emulsion’s uniform particle size distribution allows for effective interaction and penetration with the fibres. Additionally, the effectiveness of the water-repellent treatment is improved by the substantial contact area between the emulsion particles and the fibres. The self-emulsified water-repellent emulsion’s uniform particle size distribution and advantageous infiltration characteristics contribute to its improved performance and simplicity of application on textile substrates, enabling efficient and uniform coverage of the fibres.

The most effective coupling agent in terms of a water-repellent finish was determined by conducting an investigation of the particle size distribution for several coupling agents and their ratios. The tested coupling agent, KH570:KH560 = 0.02 mol:0.04 mol, produced the best results out of all the coupling agents. The particle size distribution showed a peak centred at about 117.8 d.nm when this particular coupling agent ratio was applied, indicating a relatively narrow and homogenous particle size distribution. This implies that the emulsion produced by this ratio of coupling agents will probably have the best characteristics for repelling water. The coupling agent ratio KH570:KH560 = 0.02 mol:0.04 mol is therefore suggested as the best option for producing an efficient, water-repellent finish based on examining the particle size distribution. It is crucial to remember that selecting the ideal coupling agent also depends on other aspects, such as substrate compatibility and desired water-repellent performance. A thorough evaluation of the water-repellent finish cannot be based solely on the particle size distribution. It is advised to do additional tests and evaluations, such as water-repellence testing, durability analysis, and compatibility assessments with the textile substrate to select the optimal coupling agent for the water-repellent finish. These experiments will give a more thorough insight into how each coupling agent performs in producing the ideal water-repellent finish.

3.4. Surface analysis

A smooth, uncoated textile surface with distinct fibrous structures and grooves can be seen in the scanning electron microscopy image of the untreated fabric. This structure is characteristic of untreated fabrics, where the natural texture and properties of the fibres are apparent due to the absence of any coating. The natural roughness of the fibres, as indicated by the grooves along them, may have an impact on how moisture and other substances interact with the fabric. The surface topography of the fabric has significantly changed after treatment, as seen in the SEM image. The extra roughness and less noticeable grooves suggest that the fibres are covered in a layer of the chemical that repels water. Water-repellent finishes, which frequently produce micro- and nano-scale structures on the fabric surface to enhance hydrophobicity through the lotus effect, are characterised by this roughness. The uniform coating on the treated fibres indicates a complete and even application of the water-repellent treatment, which is necessary for efficient and long-lasting performance.

A comparison of the surface morphology pre- and post-treatment, as shown in Figure 5, demonstrates the significant influence the chemical finish has on the fabric. The surface morphology of the treated fabric shows distinctive changes consistent with successful water-repellent treatments. Reduced wettability and increased water resistance of the fabric are caused by both the presence of a uniform coating and the rougher surface of the fabric. The SEM images are enhanced by the schematic illustration, which offers a theoretical framework that validates the observed surface morphology. The physical characteristics seen in the SEM images, such as the improved roughness and uniform coating, are explained by the chemical structures and bonding patterns shown in the illustration. Understanding the underlying mechanisms that give the treated fabric its water repellency depends on this synergistic relationship between its chemical structure and physical morphology.

Figure 5. (a) Surface image of the untreated fabric, (b) surface image of the treated fabric, (c) magnified surface image of the treated fabric, (d) magnified surface image of the treated fabric, (e) schematic illustration of the prepared chemical coupled with the fabric.

3.5. Contact angle analysis

The contact angle is a numerical indicator of surface wettability and offers important details about how a liquid interacts with a solid surface (Bayer & Megaridis, 2006). A fabric’s hydrophilic nature is generally shown by a smaller contact angle, which denotes a higher affinity between the liquid and the fabric surface. A higher contact angle, on the other hand, denotes a lesser affinity and a hydrophobic behaviour, where the liquid prefers to condense on the surface rather than spread out (Voronov et al., 2008; D. W. Wei et al., 2020). A liquid formulation with a 0.5% concentration was used to cure white woven cotton fabric. The aim was to give the fabric water-repellent properties. The static contact angle of the treated fabrics was measured and studied to determine the efficiency of the produced chemicals.

The experimental findings in Table 1 reveal the contact angles observed for fabrics treated with various formulations at different time intervals. Remarkably, even after 5 min of testing, all treated materials displayed hydrophobic properties (Figure 6). This shows that all of the chemicals used in the formulation successfully provided the fabric with water repellence. Additionally, though the contact angle for all the treated fabrics reduced significantly over time, it remained higher than the crucial value of 90 degrees, which is frequently used to distinguish between hydrophilic and hydrophobic surfaces. This indicates that the treated materials’ water-repellent characteristics were kept and remained to be effective. The coupling agents KH570 and KH560, at a ratio of 0.02 mol:0.04 mol, displayed the largest contact angle among the formulations investigated. This shows that this repellent exhibited better water-repellent abilities compared to other formulations. The increased resistance to water penetration suggested by the larger contact angle indicates an optimal level of water repellence attained by this coupling agent combination.

Figure 6. Contact angle of treated fabric (a) water repellent using KH560, (b) water repellent using KH570, (c) water repellent using KH570: KH560 = 0.01 mol: 0.02 mol, (d) Water repellent using KH570: KH560 = 0.02 mol: 0.04 mol, (e) Water repellent using KH570: KH560 = 0.03 mol: 0.03 mol, (f) Water repellent using KH570: KH560 = 0.04 mol: 0.02 mol.

Table 1. The contact angle of different prepared chemicals.

Sample name	Contact angle
	After 1 min	After 3 min	After 5 min
Water repellent using KH560	107.19	104.48	104.48
Water repellent using KH570	97.84	96.53	90.00
Water repellent using KH570: KH560 = 0.01 mol :0.02 mol	115.38	113.29	113.29
Water repellent using KH570: KH560 = 0.02 mol :0.04 mol	130.01	124.12	121.59
Water repellent using KH570: KH560 = 0.03 mol :0.03 mol	111.32	110.42	109.93
Water repellent using KH570: KH560 = 0.04 mol :0.02 mol	111.32	110.83	107.19

3.6. Spray rating analysis

The spray ratings of various water-repellent formulations used on samples are provided in Table 2. The efficiency of the water repellence treatment is measured by the spray rating, which describes how well the treated fabric repels water in a spray test. Higher spray ratings indicate better water repellence (Gargoubi et al., 2020). The table presents the initial spray ratings of the samples treated with different formulations. The samples treated with water repellents using coupling agents KH560 and KH570 individually achieved initial spray ratings between 80 and 90. However, after 10 washes, the sample treated with KH560 slightly decreased its spray rating to 80. On the other hand, the sample treated with KH570 showed a more significant decrease to 70 after 10 washes.

Table 2. The spray rating of treated fabrics with all the prepared chemicals before and after washing.

Sample name	Spray Rating
	Initial	After 10 washes
Water repellent using KH560	80–90	80
Water repellent using KH570	80	70
Water repellent using KH570: KH560 = 0.01 mol :0.02 mol	100	80–90
Water repellent using KH570: KH560 = 0.02 mol :0.04 mol	100	90–100
Water repellent using KH570: KH560 = 0.03 mol :0.03 mol	100	90
Water repellent using KH570: KH560 = 0.04 mol :0.02 mol	100	90

The samples treated with different ratios of KH570 and KH560 exhibited higher initial spray ratings. The sample first attained a spray rating of 100 when treated with a ratio of 0.01 mol KH570 to 0.02 mol KH560, which remained high after 10 washes at 80–90. Similar to this, after 10 washes, the samples treated with ratios of 0.02 mol KH570 to 0.04 mol KH560 and 0.03 mol KH570 to 0.03 mol KH560 both maintained high spray ratings of 90–100 and 90, respectively. The treated sample initially reached a spray rating of 100 with a ratio of 0.04 mol KH570 to 0.02 mol KH560; however, this rating dropped to 90 after 10 washes.

Overall, samples treated with formulations combining KH570 and KH560 showed improved spray ratings compared to samples treated with separate coupling agents. The formula with a ratio of 0.02 mol KH570 to 0.04 mol KH560 showed the best initial spray rating of 100 and maintained good water repellence even after 10 washes 90–100.

4. Conclusion

In conclusion, water repellence is a crucial, functional property for protective clothing. Numerous water-repellent compounds and treatments have been created to provide fabrics with this characteristic. The sol-gel technique is a quick and effective method for creating silicone water repellents and its benefits include faster processing times and control over coating morphology. Researchers have developed organic-inorganic hybrid water-repellent materials using modified silica sols incorporating linear alkyl silicon derivatives with high water-repellence qualities. This study’s novel water-repellent formulation was developed using a mixture of 3-glycidyloxypropyltrimethoxysilane (KH560) and gamma-methylacryloxypropyltrimethoxysilane (KH570) as precursors and modified with branched silicone through sol-condensation. FTIR investigation proved that the synthesised water-repellent compounds exhibited various functional groups, including -OH, -NH, -CH3, -C-O-C-, -Si-O-, -Si-H, and Si-CH3. The synthesised compounds’ thermal decomposition behaviour was shown by TG analysis. The most efficient finish was achieved by coupling agent KH570:KH560 = 0.02 mol:0.04 mol, according to the particle size analysis of the water-repellent emulsion. The emulsion had a fairly uniform and narrow particle size distribution, making it easier to interact with and penetrate textile fibres. The water-repellent treatment provided good water-repellence on the fabric surface, according to the contact angle analysis and spray rating. This study shows how water-repellent textiles can be obtained through synthesising and using novel water-repellent compounds demonstrated promising outcomes.

Disclosure statement

The authors have no competing interests to declare relevant to this article’s content.

Additional information

Funding

Partial financial support was received from British Women International through the FfWG Crosby Hall Fellowship Award