Similarly,the maximum adsorption capacity was also observed at 200 rpm.After that,there was no further uptake of the drug molecules.Increasing the agitation speed to a certain level,expanded the contact of sorbate and adsorbent,which results in adsorption peak.Further increase in agitation speed intensified the turbulence and produce eddy currents which inversely affects the adsorption process; because adsorption is a reversible phenomenon.The interplay between surface chemistry and surface roughness has been an active research topic for enhancing the hydrophobicity of cellulose based materials.The woody core of the hemp plant known as shiv has gained interest in the building industry during the recent years for production of lightweight composites.Hemp shiv based composites have interesting properties such as thermal,hygroscopic,mechanical,acoustic and biodegradability.Hemp shiv are generally very porous with low density tending to absorb large amounts of water.Previous studies have reported that hemp shiv not only has higher water absorption rate but also absorb high amounts of water in the very first minutes compared to different plant materials.Moreover,the presence of cellulose,hemicellulose and lignin in bio-based materials contributes to the presence of hydroxyl groups in their structure.
This leads to certain disadvantages of using bio-based materials making them incompatible with hydrophobic thermoset/thermoplastic polymers.High moisture uptake also encourages colonial fungal growth resulting in cell wall degradation and lower durability of the material.The major constituents of industrial hemp shiv are: cellulose,indoor hydroponic grow tent hemicellulose,lignin and other components such as extractives and ash.Cellulose is a semi crystalline polysaccharide consisting of linear chain of several D-glucose units linked together by glucosidal bond.Cellulose contains free hydroxyl groups,and since they form the major structural component of hemp shiv,they are responsible for the extreme hydrophilic behaviour.One of the mechanism to convert cellulose–based material from hydrophilic to hydrophobic involves chemical modification to block the hydroxyl groups of the cell wall thereby reducing water sorption sites.Treatments include acetylation,silanization and in situ polymerization that involve incorporation of materials into the cell wall blocking the voids accessible to water molecules.Other treatments methods that are known to enhance the water repellence are plasma etching,lithography,electrospinning and sol-geltreatment that endow thematerial with nano-scale surface roughness.Chemical pre-treatment of natural plant materials have reported better bonding with polymer matrix interface due to improvement of their hydrophobic characteristics.There is a need to develop a novel treatment method for hemp shiv to enhance its water resistance thereby improving the shiv-binder interfacial adhesion and reduce its susceptibility to decay.The solgel technique is a highly versatile method to deposit silica based coatings possessing single or multi functionality.These thin mesoporous coatings have high structural homogeneity and their adhesion can be tailored to different substrates.
Sol-gel based hydrophobic and water repellent coatings have been investigated on different bio-based materials such as wood and cellulosic fibres,however for hemp shiv this is the first time.The reactive hydroxyl groups present in the polysiloxane network of the sol-gel combine with the hydroxyl groups of cellulose through a covalent bond.This study successfully delivers a sol-gel modified hemp shiv material of hydrophobic character through a simple and inexpensive,one step dip-coating method.Hemp shiv used in this study was received from MEM Inc.,manufacturer of ecological materials based in Rimouski,Canada.Tetraethyl orthosilicate and hexadecyltrimethoxysilane were obtained from Sigma-Aldrich.Anhydrous ethanol was purchased form Commercial Alcohols,Canada.Hydrochloric acid and nitric acid were obtained from Anachemia,VWR,Canada.All chemicals were used as received without further purification.It is known that HDTMS may not be able to penetrate the outer surface layers of the cell wall due to its high molecular weight.Due to this,the hydrophobicity would be compromised and it can be predicted that the coating might not be robust.Moreover,using only HDTMS would be highly expensive and would not be of interest to the construction industry.For these reasons,it was considered inappropriate to make a comparative study using purely HDTMS.The silica based sol-gel was synthesised by hydrolysis and condensation of TEOS in ethanol and water.The reaction was catalysed using 0.005 M acid.Two sets of silica sols were prepared based on the difference in concentration of ethanol.The first set of formulations were prepared stirring 1 M TEOS in a mixture of 4 M water and 4 M ethanol.For the preparation of the second set of formulations,1 M TEOS was added to 4 M water and 16 M ethanol.After the preparation of both sets of silica formulations,the hydrophobic agent HDTMS was added in concentrations of 0.5–4 wt% of the sol.These mixtures of silica sol and HDTMS were stirred at 300 rpm for at least 20 min before performing the dip-coating process.All the sols were prepared at 40 ◦C and atmospheric pressure.The sols were allowed to cool down to room temperature and the pH was recorded.The sols aged for 48 h in closed container at room temperature before the dip-coating process.Gelation took place in-situ in which pieces of hemp shiv were dipped in the sol for 10 min and then carefully removed and transferred onto a Petri dish.
The samples were placed at room temperature for one hour and then dried at 80 ◦C for one hour.A schematic illustration of the HDTMS modified silica sol-gel coating is shown in Fig.1.As for the preparation of the pure sol-gel specimen,the sol aged in a container at room temperature until gel point.The gel-point was taken as the time when the sol did not show any movement on turning the container upside down.The gel-time and pH for all the prepared sols are reported in Table 1.The water contact angle of uncoated and coated hemp shiv samples were measured using a contact angle meter.The sessile drop method was employed and the contact angle was determined on at least three different positions for each sample.The volume of the water droplets was 5 l for the contact angle measurements.The average value was adopted as a final value.Images were captured and analysed using the FTA32 Video 2.0 software.All the measurements were performed at room temperature.The surface elemental and chemical composition of the samples were analysed using XPS.Prior to XPS analysis,samples were oven-dried at 80 ◦C for 96 h.XPS spectra of uncoated and sol-gel coated hemp shiv were recorded with an X-ray photoelectron spectrometer.All spectra were collected using a monochromatic Al K X-ray source operated at 300W.The lateral dimensions of the samples were 800 m × 400 m,corresponding to those of the Al K X-ray used,and probing depth was approximately 5 nanometres.For each sample,two spectra were recorded: survey spectra recorded for apparent composition calculation; and high resolution C1s,O1s and Si 2p spectra recorded to obtain information on chemical bonds.Calculation of the apparent relative atomic concentrations is performed with the CasaXPS software.Peak fitting is performed with CasaXPS,which automatically and iteratively minimizes the difference between the experimental spectrum and the calculated envelope by varying the parameters supplied in a first guess.
The water contact angle was determined as soon as the water droplet encountered the sol-gel coated hemp shiv surface.The sol-gel coatings with high HDTMS loadings and varying concentration of ethanol are compared in Fig.2.It can be seen that uncoated shiv has an extremely hydrophilic surface and water droplet sinks into the substrate reducing the WCA in a short time.The sol-gel coatings yield hydrophobicity to the hemp shiv by maintaining a stable contact angle over 60 s.Considering the coating compositions with 4% HDTMS loading listed in Table 1,it was observed that ethanol diluted sols performed better in terms of providing hydrophobicity to hemp shiv surface compared to undiluted sols.Sol B-1 and sol B-5 coatings had higher contact angles compared to sol A-1 and sol A-5 coatings.Ethanol helps the HDTMS to be fully dissolved in water thereby promoting the hydrolysis reaction.Fig.2 shows the WCA measurements of sol coatings containing 4 wt% HDTMS.Sol A-1 and sol A-5 contain only 4 M ethanol whereas sol B-1 and sol B-5 contain 16 M of ethanol.At 4 wt% HDTMS concentration,using 16 M of ethanol favours the hydrolysis of HDTMS.In this way HDTMS molecules are able to self-assemble on the silica network,hence providing enhanced hydrophobicity to the material.In general,it was observed that sol-gel coatings with HNO3 as catalyst perform slightly better in terms of hydrophobicity than coatings with HCl as catalyst.The changes in water contact angle as a function of HDTMS loading is presented in Fig.3.The contact angle measurements had a standard deviation between 1.1◦ and 6.0◦.The hydrophobic performance of the coatings is not reduced on lowering the HDTMS loading down to 1%.Surfaces coated with sol B series showed good water repellence with contact angles ranging between 96◦ to 108◦.The samples were analysed for their surface micro-structure and roughness by the Vision64 software using a Robust Gaussian Filter and a short wavelength cut-off 0.025 mm.The use of such filters helps to reduce the anatomical influence and optimizes the roughness profile data for evaluation of the sample surface.The robust Gaussian filter avoids the distortions produced by some filters when applied in profiles with deep valleys.Mean surface roughness was calculated according to ISO 4287.Sa gives the description of the height variations in the surface and it is the most widely used parameter to measure the surface roughness profile of the sample.The surface roughness parameters for sol-gel coated hemp shiv with 1% and 4% HDTMS loadings are shown in Fig.4.The influence of different sol-gel coatings on the surface roughness of hemp shiv can be seen in Fig.5.The 3D surface roughness profile showed that the sol A-5 coating on the hemp shiv lowered the surface roughness providing a smoother surface as seen in Fig.5b.The non-uniform coating was also cracked,trim tray which in turn can facilitate water penetration into the hemp shiv.
On the other hand,sol A-7 enhanced the surface roughness of hemp shiv.Overall ethanol diluted sol-gel coatings had enhanced the surface roughness of hemp shiv.Sol B-5 had the highest mean surface roughness as seen in Fig.5c.Roughness parameters alone cannot describe the surface morphology and therefore microscopy analysis is beneficial to improve surface evaluations.The morphology of the uncoated and solgel coated surfaces was studied by scanning electron microscopy.Fig.6 shows the micrographs of hemp shiv surface before and after modification with different sol-gel coatings.Sol A-5 and sol B-7 formed a thick coating layer and changed the morphology of the shiv surface.This resulted in coating with major cracks which could be a result of shrinkage after drying the treated sample.On the other hand,sol A-7 and sol B-5 showed uniformly coated surfaces without significantly altering the morphology of the hemp shiv.Conventional SEM techniques proved unsuccessful in determining the coating thickness,but SEM-FIB imaging of an early iteration of the formulation measured a thickness in the range 160–180 nm.It is expected that the current formulations would have a similar thickness.The surface chemical composition was determined by X-ray photoelectron spectroscopy.A low-resolution survey scan determined the atomic percentage of various elements present at the sample surface.The relative elemental composition of the uncoated and sol-gel coated hemp shiv surface is listed in Table 2.The main elements detected for uncoated hemp shiv were carbon and oxygen.Small amounts of other elements were present either possibly arising from the epidermal cell wall or from contamination during sample preparation.
The sol-gel coated hemp shiv additionally showed high content of silicon arising from the silica based membrane on the surface.A high-resolution scan was performed on the C1s region for the uncoated and sol-gel coated hemp shiv samples to determine the type of oxygen-carbon bonds present.The chemical bond analysis of carbon was performed by curve-fitting the C1s peak and deconvoluting it into four sub peaks corresponding to unoxidized carbonC1,and various oxidized carbons C2,C3 and C4.The sol-gel coatings were functionalised using HDTMS as the hydrophobic additive during the sol-gel synthesis.The co-precursor method of sol-gel synthesis was followed based on the simplicity of the process.In the sol-gel process,TEOS is hydrolysed and condensed to form a SiO2 network which is covalently bonded to cell wall through the hydroxyl sites of cellulose present in the hemp shiv.On addition of hydrophobic agent as a co-precursor during the sol-gel processing,the hydroxyl groups on the silica clusters are replaced by the Si C16 groups through oxygen bonds as illustrated in Fig.9.The hydrophobicity of the sol-gel coatings is due to the attachment of these long alkyl chains on the silica network thereby providing water resistance to the hemp shiv surface.Overall,the acid catalysed sol-gel coatings enhance the water repellence of hemp shiv making the surface hydrophobic.