Hydrophobicity of Hemp Shiv treated with Sol-gel Coatings 3

13 This is the first time sol-gel technology is used in the treatment of hemp shiv to develop 14 sustainable thermal insulation building materials. The impact on the hydrophobicity of hemp shiv 15 by depositing functionalised sol-gel coatings using hexadecyltrimethoxysilane (HDTMS) has been 16 investigated. Bio-based materials have tendency to absorb large amounts of water due to their 17 hydrophilic nature and highly porous structure. In this work, the influence of catalysts, solvent 18 dilution and HDTMS loading in the silica sols on the hydrophobicity of hemp shiv surface has been 19 reported. The hydrophobicity of sol-gel coated hemp shiv increased significantly when using acid 20 catalysed sols which provided water contact angles of up to 118° at 1% HDTMS loading. Ethanol 21 diluted sol-gel coatings enhanced the surface roughness of the hemp shiv by 36% as observed 22 under 3D optical profilometer. The XPS results revealed that the surface chemical composition of 23 the hemp shiv was altered by the sol-gel coating, blocking the hydroxyl sites responsible for 24 hydrophilicity.


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Wettability of a solid surface is governed by a combination of chemical composition and geometric 32 structure of the surface [1,2]. The interplay between surface chemistry and surface roughness 33 has been an active research topic for enhancing the hydrophobicity of cellulose based materials. Previous studies have reported that hemp shiv not only has higher water absorption rate but also 42 absorb high amounts of water in the very first minutes compared to different plant materials [7]. It is known that HDTMS may not be able to penetrate the outer surface layers of the cell wall due 97 to its high molecular weight [18]. Due to this, the hydrophobicity would be compromised and it can 98 be predicted that the coating might not be robust. Moreover, using only HDTMS would be highly 99 expensive and would not be of interest to the construction industry. For these reasons, it was 100 considered inappropriate to make a comparative study using purely HDTMS. 101 102 103 2.2 Preparation of the hydrophobic coatings 104 The silica based sol-gel was synthesised by hydrolysis and condensation of TEOS in ethanol and 105 water. The reaction was catalysed using 0.005M acid (HCl/ HNO3). Two sets of silica sols were 106 prepared based on the difference in concentration of ethanol. The first set of formulations (sols 107 A) were prepared stirring 1M TEOS in a mixture of 4M water and 4M ethanol. For the preparation 108 of the second set of formulations (sols B), 1M TEOS was added to 4M water and 16M ethanol. 109 After the preparation of both sets of silica formulations, the hydrophobic agent HDTMS was added 110 in concentrations of 0.5-4 wt% of the sol. These mixtures of silica sol and HDTMS were stirred at 111 300 rpm for at least 20 minutes before performing the dip-coating process. All the sols were 112 prepared at 40 °C and atmospheric pressure. The sols were allowed to cool down to room 113 temperature and the pH was recorded. 114

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The sols aged for 48 hours in closed container at room temperature before the dip-coating 116 process. Gelation took place in-situ in which pieces of hemp shiv were dipped in the sol for 10 117 min and then carefully removed and transferred onto a Petri dish. The samples were placed at 118 room temperature for one hour and then dried at 80 °C for one hour. A schematic illustration of 119 the HDTMS modified silica sol-gel coating is shown in Figure 1. 120 121 As for the preparation of the pure sol-gel specimen, the sol aged in a container at room 122 temperature until gel point. The gel-point was taken as the time when the sol did not show any 123 movement on turning the container upside down. The gel-time and pH for all the prepared sols 124 are reported in Table 1

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The water contact angle (WCA) of uncoated and coated hemp shiv samples were measured using 130 a contact angle meter (First Ten Ångstroms USA, FTA200 series). The sessile drop method was 131 employed and the contact angle was determined on at least three different positions for each 132 sample (coated substrate). The volume of the water droplets was 5µl for the contact angle 133 measurements. The average value was adopted as a final value. Images were captured and 134 analysed using the FTA32 Video 2.0 software. All the measurements were performed at room 135 temperature (24 ± 1 °C). 136 137 7

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The topography and surface roughness of the samples was obtained using a 3D optical 139 profilometer (Bruker Nano GmbH Germany, ContourGT-K series). The surface roughness was 140 measured over an area at 0.25*0.30 mm 2 in non-contact mode at 20X magnification. Vision 64 141 on board software was then employed to analyse these data and calculate the roughness 142 parameters. The readings were taken on at least three different positions for each sample and 143 the average value was reported as the final value. 144 145 2.5 X-ray photoelectron spectroscopy (XPS) 146 The surface elemental and chemical composition of the samples were analysed using XPS. Prior 147 to XPS analysis, samples were oven-dried at 80 °C for 96 hours. XPS spectra of uncoated and 148 sol-gel coated hemp shiv were recorded with an X-ray photoelectron spectrometer (Kratos Axis 149 Ultra, UK). All spectra were collected using a monochromatic Al Kα X-ray source operated at 300 150 watts. The lateral dimensions of the samples were 800 microns × 400 microns, corresponding to 151 those of the Al Kα X-ray used, and probing depth was approximately 5 nanometres. For each 152 sample, two spectra were recorded: (i) survey spectra (0-1150 eV, pass energy 160 eV, and step

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Roughness parameters alone cannot describe the surface morphology and therefore microscopy 232 analysis is beneficial to improve surface evaluations. The morphology of the uncoated and sol-233 gel coated surfaces was studied by scanning electron microscopy (SEM). Figure 6 shows the 234 micrographs of hemp shiv surface before and after modification with different sol-gel coatings. Sol 235 A-5 and sol B-7 (Figures 6b and 6e)

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The surface chemical composition was determined by X-ray photoelectron spectroscopy. A low-263 resolution survey scan determined the atomic percentage of various elements present at the 264 sample surface (Figure 7). The relative elemental composition of the uncoated and sol-gel coated 265 hemp shiv surface is listed in Table 2. 266 267    Table 3. 289 The ratio of Cox/unox has dropped significantly for sol-gel coated hemp shiv indicating that the 290 carbon oxygen bonds have decreased on the surface of the samples.  The C1s high resolution spectra with the deconvoluted peaks for uncoated and sol-gel coated 296 surfaces are represented in Figure 8. The C1 peak represents carbon-carbon or carbon-hydrogen 297 bonds whereas C2, C3, and C4 peaks possess carbon-oxygen bonds.

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The sol-gel coatings were functionalised using HDTMS as the hydrophobic additive during the 308 sol-gel synthesis. The co-precursor method of sol-gel synthesis was followed based on the 309 simplicity of the process. In the sol-gel process, TEOS is hydrolysed and condensed to form a 310 SiO2 network which is covalently bonded to cell wall through the hydroxyl sites of cellulose present 311 in the hemp shiv. On addition of hydrophobic agent as a co-precursor during the sol-gel 312 processing, the hydroxyl groups on the silica clusters are replaced by the -Si-C16 groups through 313 oxygen bonds as illustrated in Figure 9. The hydrophobicity of the sol-gel coatings is due to the 314 attachment of these long alkyl chains on the silica network thereby providing water resistance to 315 the hemp shiv surface.