Introduction

Purified wool wax, known as “lanolin”, is a highly viscous natural matter composed of fats and oils. This lubricant represents about 10–25% of the sheared greasy wool (Yao and Hammond 2006; Flockhart et al. 1998). The hydrophobic nature of lanolin makes it valuable in various industrial fields, such as lubricants, plastics, rustproof coatings, paints, and inks. It can easily combine with a variety of materials to be employed for cosmetics and pharmaceuticals due to its strong emulsification and penetration properties. Its adhesion nature makes it an excellent substance to be applied as a plasticizer in adhesives and resins (Edman and Möller 1989). Spray coating technology has been introduced as a simple, cheap, and non-contact process to coat surfaces with high speed, insignificant agglomeration, and at a decreased quantity of excess coating paste aerosol (Moridi et al. 2014; Huang et al. 2004; Aziz and Ismail 2015; Khattab et al. 2018a, b, c, 2019; Cheng et al. 2018; Girotto et al. 2009). Room Temperature Vulcanizing (RTV) silicone can be cured at ambient conditions employing a catalyst, such as dibutyltin dilaurate. It is distinguished by good resistance to chemicals, temperature, ageing, and acid/base conditions. It also possesses low viscosity and shrinking, high-quality hardness, good mechanical stress, and easy to apply. RTV has been applied in aviation and printing technologies, electronics, and casting procedure of diverse materials, such as epoxy and polyester resins, low melt-temperature alloys/metals and urethane, wax, and gypsum (Sabri et al. 2015; Labouriau et al. 2015a, b; Heredia-Guerrero et al. 2018; Kumar et al. 2016; Labouriau et al. 2015b).

A hydrophobic surface has attracted a significant consideration due to potential applications in maritime industry, such as drag reduction, antifouling and self-cleaning (Zhang et al. 2019a, b; Liu et al. 2019; Zimmermann et al. 2008). In recent years, various publications have been introduced describing several methods, some of which are complicated, for the development of hydrophobic surfaces on a variety of substrates such as metals, wood and textiles (Hoefnagels et al. 2007; Xue et al. 2013; Zhang et al. 2013, 2019; Xue et al. 2008). Hydrophobic coatings have been highly studied through different physical and chemical techniques, such as lithography, electrospinning, plasma or chemical etching, chemical vapor deposition, self-assembly, and sol–gel methods. However, methods able to introduce mechanically durable water-repellent surfaces employing simple and cost-effective techniques are limited. Some of the above mentioned approaches are mainly characterized by being time-consuming and multiple-step process, which are not practical for developing water-repellent coating for engineering purposes (Rehan et al. 2018; Khalil-Abad and Yazdanshenas 2010; Wang et al. 2011; Xue et al. 2017; Abbas et al. 2015; Abdelmoez et al. 2016). A number of attempts describing a facile approach does not introduce satisfactory hydrophobic surface of suitable mechanical durability. Ogihara et al. reported the creation of hydrophobic surfaces by spraying SiO2 nanoparticles suspended in an alcohol (Ogihara et al. 2011, 2012, 2013).

Textile industries are as wide and diverse as the goods it affords with inventive applications. There are global competitive manufacturers of textile products, such as yarns, fabrics, furnishings and apparels. High performance textiles can be defined as the textile merchandises that are produced for non-aesthetic purposes, where their functions are the main decisive factor (Khattab et al. 2017, 2018a, b, c; Khattab and Helmy 2019; Aboutalebi et al. 2014; Holme 2007). The major purpose of high performance protecting textiles, such as water-repellent, fire-retardant, and antimicrobial garments, is to improve human safety (Abdelrahman and Khattab 2019; Zimmermann et al. 2008; Chen et al. 2016; Oulton 1995). Hydrophilicity usually reduces the diversity of textile applications, especially in packaging, crafts and transportation. A water-repellent surface with a contact angle > 150° and a sliding angle < 10°, forced by lotus phenomenon, has been always a research hot mark for most recent decades due to its remarkable waterproof ability, self-cleaning nature, and antifouling property (Qiang et al. 2017; Chauhan et al. 2019; Ma et al. 2018; Xue et al. 2019). The generation of nano- and micro-hierarchical structures results in a surface roughness and a low surface energy. This nano- and micro-hierarchical morphology can be recognized as sufficient approach to achieve a hydrophobic phenomenon, which is the key function to generate waterproof fabrics. Fluorinated compounds have been used as one of the major applied materials to introduce a hydrophobic surface. However, this approach is not effective due to the toxicity and high cost of those fluorinated materials. Recently, research efforts have been taken into consideration to approve environmentally friendly processing toward water-repellent textiles (Pan et al. 2018; Xu et al. 2013; Si and Guo 2016; Bian et al. 2014; Cao et al. 2017).

In this work, we introduce a cheap and simple process to prepare mechanically durable water-repellent surface on a viscose fabric. In this method, viscose fibers were spray-coated by RTV silicon rubber/lanolin solution. By varying the concentration of RTV silicone rubber solution in petroleum ether, a hierarchical surface with high static contact angle can be produced.

Experimental

Materials

Light-weight 100% plain weave viscose fabrics (150 g m−2) were kindly supplied from Misr Company for Spinning and Weaving, El-Mahalla El-Kobra, Egypt. Purified wool wax (lanolin) was extracted in our labs from wool fleece (Madara and Namango 2014). Decoseal silicon rubber RTV-2540 was supplied by A.D.M. Chemical Industries (ADMICO), Egypt. Petroleum ether (b.p. 60–80 °C; AR Grade) was purchased from Sigma-Aldrich. Reagents were purchased at highest purity available and were employed without further purification.

Spray-coating of viscose fabrics

Lanoline (hereafter LA) 15 wt% (weight of lanoline to volume of petroleum ether) was dissolved in petroleum ether. Silicone rubber (hereafter RTV) was then dissolved in LA/ether solution to prepare solutions with five different concentrations which varied from 3 wt% (RTV-3), 7 wt% (RTV-7), 10 wt% (RTV-10), 12 wt% (RTV-12), to 17 wt% (RTV-17). Viscose substrates were spray-coated by the prepared different concentrations of RTV/LA/ether solution. The spray-coating process of the prepared solutions was carried out under ambient conditions using Lumina automatic spray gun STA-6R (Fuso Seiki Co. Ltd., Tokyo, Japan) bearing spray nozzle with orifice size (1.00 mm). The spray nozzle was moved back and forth over the viscose substrate at a speed of ~ 3 cm/s. It was equipped with pressurized air as a carrier gas at 250 KPa, whereas pressure of inflow air was recorded by a pressure gauge. Distance between viscose fabric and spray nozzle was 20 cm. The solutions were sprayed intermittently with a flow rate of 10 mL/min. In general, the spraying course was conducted until the solution covered the entire viscose fabric. The samples were then air-dried for 30 min on a clean flat surface to evaporate the solvent.

Characterization

Contact angle measurements

Water contact and sliding angles were measured on OCA-15EC (Dataphysics GmbH, Germany) with software. Contact angle properties were carried out with 10 μL drops of triple distilled water. Viscose substrates were connected to glass cover slips using double sided adhesive tape to create a planar surface.

Ultraviolet protection factor

The ultraviolet protection factor (UPF) was automatically calculated according to Australia/New Zealand standard AS/NZS-4399:1996 method employing UPF calculation system of UV/Vis spectrophotometer as reported in the standard AATCC Test Method 183:2010-UVA Transmittance.

Fourier transform infrared spectroscopy

FTIR spectra were measured using the transmission mode on FTIR spectrophotometer (Nexus 670; Nicolet, United States) in the range of 4000–400 cm−1 with spectral resolution of 4.0 cm−1. The viscose samples were placed in direct contact with the detector during measurement.

Field emission scanning electron microscope

The morphological properties of both blank and spray-coated viscose samples were tested using field emission scanning electron microscope (FE-SEM) on a Quanta FEG-250 (Czech Republic). The elemental content was investigated by surface Energy-dispersive X-ray analysis (EDAX) unit (TEAM-EDX Model) connected to the electron microscope. The distribution of particles’ diameter was measured using Image J software program saved on scanning electron microscope.

Whiteness index

The level of whiteness of untreated and treated samples was determined by studying color strength (K/S) and color coordinates (L*, a*, b*) on a spectrophotometer with pulsed xenon lamp as light source (Ultra Scan Pro, Hunter Lab, United States), and illuminant of D65 with 10° observer, measurement area of 2 mm and d/2 view geometry. An average of five reads at different locations of the tested viscose fabric was recorded and then was taken as the main value.

Bending stiffness

The bending stiffness of viscose fabrics was assessed according to the British Standard method 3356:1961 using Shirley stiffness tester. The bending length was reported as an average of five recorded readings at five different positions for each sample.

Burst strength

The burst strength of both blank and spray-coated viscose fabrics was measured according to the Standard method ASTM 3786, JIKA (Toyoseiki). The burst strength was measured as an average of five recorded readings at five different positions for each sample.

Air-permeability test

The air-permeability exam of viscose samples was reported using the standard ASTM D-737 employing Textest FX 3300 with 100 Pa pressure gradient. The air-permeability was recorded as an average of five readings at five different positions for each viscose substrate.

Durability test

To study the durability of the finished fabrics, the spray-coated specimens were washed for 1, 5, 10, and 20 laundering cycles according to the standard AATCC 61-1989 method. The treated viscose sample (5 × 15 cm) was placed in a launder-o-meter and accelerated machine laundering with a detergent solution (200 mL) at a temperature of 40 ± 3 °C for 45 min. The UPF efficacy as well as the contact angle measurements was assigned as indicators for the durability of the proposed treatment.

Results and discussion

Treatment of viscose by RTV/LA

This work was devoted to render hydrophobic surface to viscose fabrics using lanolin, extracted from raw wool fleece, in combination with silicon rubber. Mixtures of lanoline (LA) 15 wt% and silicone rubber (RTV) was prepared at five different concentrations of RTV from 3 wt% (RTV-3), 7 wt% (RTV-7), 10 wt% (RTV-10), 12 wt% (RTV-12), to 17 wt% (RTV-17), where the weight percentage was considered to the volume of petroleum ether. Viscose fabrics were spray-coated by the prepared RTV/LA solutions. The spraying process was performed until the solution completely covered the whole surface of the viscose fabric followed by air dying.

Contact and sliding angles

Figure 1 and Table 1 show the results of the static contact angle measurements of untreated viscose (Fig. 1a), lanolin (only)-treated viscose (Fig. 1b), RTV (only)-treated viscose (Fig. 1c), and RTV/LA-treated viscose (Fig. 1d). It was observed that the water contact angle of viscose fabric increased when it was treated with either lanolin or silicon rubber to nearly the same degree from 101.5° for blank/untreated viscose to 131.0° LA (only)-loaded viscose, 134.5° RTV (only)-loaded viscose and 141.8° for RTV-LA. Thus, further increase in contact angle of viscose fabric was attained upon treatment with lanolin followed by silicon rubber; by virtue of a synergetic effect of the aforementioned two hydrophobic reagents. Similarly, the static contact angles of the treated fabrics were measured after 20 washing cycles. We can conclude from Fig. 1e,f,g that washing of viscose-treated fabrics LA (only)-loaded, RTV (only)-loaded and RTV-LA substrates respectively, resulted in acceptable decrease in their water contact angles to 121.7°, 120.8° and 131.0°, respectively.

Fig. 1
figure 1

Static water contact angles of the untreated viscose fabric (a), LA-treated fabric (b), RTV-10-treated fabric (c), LA/RTV -10-treated fabric (d), LA-treated fabric, then 20 washing cycles (e), RTV-10-treated fabric then 20 washing cycles (f), and LA/RTV-10-treated fabric then 20 washing cycles (g); LA concentration was fixed at 15 wt%

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Table 1 Static water contact angles of treated as well as untreated viscose fabric
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A rough surface was fabricated via spray-coating of RTV-LA applied on viscose surfaces of RTV-3, RTV-7, RTV-10, RTV-12 and RTV-17 with static water contact angles between 112.7°, 128.6°, 148.1°, 135.8° and 119.8° depending on RTV concentration between 3 wt% (a), 7 wt% (b), 10 wt% (c), 12 wt% (d), and 17 wt% (e), respectively, as depicted in Table 2. At low RTV content (3 wt% and 7 wt%), the surfaces demonstrated a fairly rough morphology compared to uncoated pristine sample. Accordingly, the fibrous surface was covered by a thin layer of RTV-LA. Upon increasing RTV content (12 wt% and 17%), in addition to covering the viscose fibrous surface, the surface pores located among the viscose fibers were also filled by RTV-LA and the roughness was accordingly decreased. Thus the surface roughness at high RTV content was decreased due to increasing RTV-LA film thickness. Because increasing RTV-LA content led to increasing the quantity of particles on the substrate surface and the surface roughness was accordingly reduced, the presence of optimum RTV-LA content was considered (Fig. 2). Consequently, the entire surface of the viscose fibers was fairly covered by hydrophobic RTV-LA, whilst the surface pores among fibers were not filled by RTV-LA (Abdelrahman and Khattab 2019; Nouri and Saadat-Bakhsh 2017; Seyedmehdi et al. 2012).

Table 2 Static water contact angles of the spray-coated viscose at different concentrations of RTV; LA concentration was fixed at 15 wt%
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Fig. 2
figure 2

SEM images demonstrate the surface roughness of the treated viscose fabrics at different RTV concentrations a RTV-3, b RTV-10, and c RTV-17

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The sliding angle was also employed as an additional factor to assess the quality of the water-repellent viscose surface as summarized in Table 2. When the LA/RTV solutions were applied to spray-coat viscose fabrics, the wetting behavior was shifted as the sliding angle was increased as a function of RTV concentration. There have been a variety of methods in literature reporting the generation of water-repellent surface using toxic and/or expensive materials, which were then applied via sophisticated processing and time-consuming processes (Zhu 2018). Therefore, it is a technical challenge to establish hydrophobic substrates by applying our simple and cheap spray-coating technique in absence of any complicated instrumentations. Moreover, this simple technique can be employed for potential large-scale manufacturing of hydrophobic products for a variety of practical applications, such as water-repellent tents.

Ultraviolet protection factor (UPF)

The protective effect of cloths against the harmful ultraviolet rays is of great importance for people safety. The UPF values of untreated as well as treated viscose fabrics were assessed and the results were summarized in Table 3. Treatment of viscose fabric with lanolin (LA only) or silicon rubber (RTV only) enhanced its resistance to transmission of ultraviolet rays to 30.7% or 38.5%, respectively. The synergetic action of lanolin and silicon rubber (RTV-LA) on viscose fabric enhanced its UPF to 53.8%. A remarkable decrease in the UPF values after 20 washing cycles was monitored for either lanolin(only)-treated samples or silicon rubber(only)-treated viscose fabrics. On the other hand, samples treated with LA/RTV lost only one-third of their UPF efficacy after 20 wash cycles; presumably due to encapsulation of viscose fibrils with LA/RTV layer which can withstand the effect of washing. This indicates that the treatment of viscose with RTV-LA mixture is more durable, particularly when compared to lanolin (only) or silicon rubber (only)-treated fabrics. These findings are in harmony with the results of water contact angle measurements.

Table 3 The UPF of untreated as well as LA and/or RTV-treated viscose fabric
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Morphological properties of spray-coated viscose

The morphological characterization of RTV-LA incorporated onto viscose surface was investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX). Studying the fiber morphology of the treated viscose fabrics in comparison with the untreated fabric would account for the hydrophobic nature of lanolin and or silicon rubber treated fabrics. Figure 3 shows the Scanning Electron Micrographs (SEMs) of untreated as well as treated viscose fabrics. Figure 3a shows the SEM of the normal pattern of untreated viscose fabric. Treatment of viscose fabrics with lanolin resulted in coating of the fiber with an even uniform hydrophobic layer of lanolin; which rationalizes the induced water-repellency to lanolin-treated viscose fabrics (Fig. 3b). Treatment of viscose fabrics with silicon rubber resulted in formation of a heterogeneous layer on the fiber surface, as indicated in Fig. 3c. Comparing SEM images in Fig. 3 proves that the treatment of viscose fabrics with lanolin together with silicon rubber made an even homogenous layer, which is responsible for the water-repellency induced nature of the treated fabrics (Fig. 3d). The elemental content in at% and wt% of the treated viscose substrate RTV-10 concentrations, at three different positions, are demonstrated in Fig. 4 and Table 4. The elemental content of the spray-coated viscose was quite the same confirming the homogenous distribution of RTV-LA on viscose. The creation of nano/micro hierarchical structures led to a surface roughness of low surface energy, and consequently resulted in the hydrophobic phenomenon as shown in Fig. 5.

Fig. 3
figure 3

SEM images of a blank, b LA-treated, c RTV-10-treated, and d LA/RTV-10-treated viscose fabrics

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Fig. 4
figure 4

EDAX diagrams of lanolin/silicon rubber-treated viscose fabric at three different positions on surface of the treated viscose sample (RTV-10); area 1 (top) and area 2 (bottom)

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Table 4 Elemental analysis (wt%) at three different positions of the treated viscose sample (RTV-10)
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Fig. 5
figure 5

SEM image of LA/RTV-10-treated viscose fabric displaying the nano/micro-hierarchical morphology

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Fourier transform infrared spectra

FT-IR Spectra was used to prove the substantivity of viscose fabric to lanolin. The characteristic bands within the FT-IR chart of viscose fabric are shown in Fig. 6 shows. The characteristics broad band for the stretching vibration of hydroxyl group appeared at 3313 cm−1. The C–O stretching vibration appears as a sharp intense band at 1017 cm−1.

Fig. 6
figure 6

FT-IR chart of untreated viscose fabric, lanolin-treated viscose fabric, and RTV-treated viscose fabric

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In the FT-IR chart of lanolin-treated viscose fabric, we can observe that a new sharp weak band appeared at 1736 cm−1 which corresponds to the carbonyl group stretching vibration found in the esters found in lanolin.

Physico-mechanical and colorimetric properties

The major reason of using spray-coating procedure was to present a smooth hydrophobic layer with lower optimal surface roughness, while keeping the fabric’s breathability and flexibility. Results of air-permeability for treated as well as untreated viscose fabrics, summarized in Table 5, reveals that coating process did not considerably affect on air-permeability by increasing RTV-LA content.

Table 5 Air-permeability of treated viscose samples at different total content of RTV (concentration of LA is 15 wt%)
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The effect of treatment of viscose fabric with lanoline/silicon rubber system on some of its inherent physical properties was investigated and the results thereof were summarized in Table 6. It is clear from the data in Table 6 that viscose fabrics lost about 20% of its degree of whiteness upon treatment with lanolin/silicon rubber system. On the other hand, no appreciable effect of this treatment on the burst strength of viscose fabrics, was traced after being treated with lanolin and silicon rubber. On the contrary, the bending length of the treated fabrics in the warp and weft direction was significantly enhanced relative to the untreated sample. In general, the hydrophobic property imparted to the treated viscose fabrics has had no negative impact on the inherent physic-mechanical properties of viscose.

Table 6 Physico-mechanical properties of untreated as well as LA/RTV-treated viscose fabrics (concentration of LA is 15 wt%; and that of RTV is 10 wt%)
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To evaluate the color changes owing to LA-RTV layer, both of CIE Lab color coordinates (L*, a*, b*) and color strength (K/S) were reported and presented in Table 7. As was anticipated, only negligible color shifts were monitored after treating viscose with the said reagents. This was verified by the very close values of both color strength and color coordinates which were recorded for treated and untreated viscose fabrics.

Table 7 Coloration measurements of untreated as well as treated viscose fabrics at different concentrations of RTV (concentration of LA is 15 wt%)
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Conclusion

Intumescing coating was incorporated onto viscose fabric employing a mixture of the environmentally friendly RTV silicone rubber and lanoline to impart water-repellent property. The results were demonstrate by investigating the surface morphological properties of the spray-coated viscose using Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The static water contact angles were monitored in the range between 112.7° and 148.1°, while the sliding angles were between 5° and 13°. Lanolin, extracted from wool sheep, was found to be an appropriate candidate for imparting hydrophobicity to viscose fabric; a property which is obliged in definite textile applications. The hydrophobocity is even enhanced by post treatment of lanolin-treated viscose fabric with silicon rubber. This new application of lanolin would open the door to new era of utilization of lanolin in industries other than cosmetics. The obtained hydrophobic viscose fabrics exhibited adequate protection against ultraviolet rays as well as enhanced comfortability, without adverse deterioration in the inherent physico-mechanical properties of viscose. The lanolin/silicon rubber-treated viscose fabrics lost less than one-fifth of its ultraviolet protection efficacy after 20 wash cycles. The comfort features of spray-coated viscose were also evaluated to show satisfactory stiffness and air-permeability.