Chemical Functionalisation of Cotton Fabric to Impart Multifunctional Properties

A cotton fabric was functionalised using the nanoparticle vapor deposition (NVD) and molecular vapor deposition (MVD) techniques to impart super hydrophobic/oleophobic properties. The NVD method was used to deposit a layer of Al2O3 nanoparticles onto the fabric surface. MVD led to the deposition of a functional layer of (tridecafl uoro-1,1,2,2,-tetrahydrooctyl)trichlorosilane (FOTS). The nanoparticles deposition increased the surface roughness, leading to higher contact angles when compared with the surfaces functionalised only with FOTS. FTIR spectra showed the presence of peaks corresponding to fl uorocarbon chains and Al2O3 on functionalised samples. Surface free energies of the samples were calculated. Low hysteresis and dynamic contact angles higher than 150° were obtained for water and organic liquids. Tetrabutyl orthotitanate (Ti(OC4H9)4) was used to functionalise fabrics to impart self-cleaning and UV protection properties. Furthermore, the functionalisation with monochlorotriazyl-β-cyclodextrin molecules introduced cavities on the fabric surface, which were used to perform the inclusion of antimicrobial agents.


Introduction
Super hydrophobic surfaces are attractive for many applications, where water repellency, anti-sticking, self-cleaning and anti-fouling are desired or required [1][2][3][4].Super hydrophobic surfaces exist in nature, however, various surfaces are fabricated based on natural surface structures [5], e.g. rose petals can be considered super hydrophobic [5,6].A surface is said to be hydrophilic when the static water contact angle is below 90°.A surface is hydrophobic when the contact angle is between 90° and 150°.However, a surface is considered super hydrophobic when the contact angle is higher than 150° [7].On super hydrophobic low adhesive surfaces, a liquid droplet rolls off the surface as the interaction liquid-solid is minimised.Th is property is called "self-cleaning" or "lotus eff ect" since the rolling liquid droplet can "clean-up" the surface as it rolls over surface dust or particles [8].
In general, a surface is considered super hydrophobic when exhibiting a static water contact angle greater than 150° and a sliding angle smaller than 10°.Scientists have studied the structures of super hydrophobic surfaces to understand the behaviour of highly hydrophobic surfaces.Natural super hydrophobic surfaces consist of 20-40 μm particles, each covered by smaller-scaled, rough surfaces [9].Th e nanoparticle structure and the surface roughness have been reported to lead to a super hydrophobic property.To impart super hydrophobic properties, a surface can be functionalised in two ways.Th e fi rst approach consists of using low-surface-energy materials (such as fl uorocarbons) to create a rough coating.However, preparing rough surfaces with fl uorocarbons is difficult to achieve.İn the second approach, a rough surface can be modifi ed with low-surface-energy materials [9].In this study, a vapour phase reaction is used to form nanoparticles on the fabric surface, which create a rough surface.Other functional properties Noureddine Abidi 1, 2

Materials
Desized, scoured and bleached cotton fabrics (yarn linear density: warp/weft = 16.4 tex/14.7 tex; mass per unit area = 118.7 g/m 2 ) were used in this study.Th ey were purchased from Testfabrics, Inc. (Testfabrics, West Pittston, PA, USA).Water (deionised in a Milli-Q plus system from Millipore to reach the fi nal resistivity of 18.2 MΩ⋅cm) and varying volumetric concentrations of isopropyl alcohol (IPA) were used to assess the hydrophobic properties of the treated cotton fabrics.

Super hydrophobic treatment
Th e treatment procedure used was reported in our previous work [10,11].Th e chemicals used for the treatments were: water, (tridecafl uoro-1,1,2,2,-tetrahydrooctyl)trichlorosilane (FOTS, (Cl) 3 Si(CH 2 ) 2 (CF 2 ) 5 (CF 2 ) 3 ), a blend of bifunctional trichlorosilanes and trimethylaluminum (TMA, Al(CH 3 ) 3 ).Th ree treatments were performed.In the treatment A, the fabric was pretreated with N 2 -plasma, followed by the MVD deposition of FOTS (MVD FOTS layer).In the treatment B, the fabric was pretreated with N 2plasma followed by the ALD deposition of Al 2 O 3 and then by the MVD deposition of FOTS (ALD Al 2 O 3 + MVD FOTS layer).In the treatment C, the fabric was pretreated with N 2 -plasma followed by the MVD deposition of trimethylaluminum nanoparticles, then the MVD deposition of a bifunctional trichlorosilane blend and fi nally the MVD deposition of a FOTS layer (NVD Al 2 O 3 + MVD bifunctional tricholorosilane + MVD FOTS layer).Th e dynamic contact angle measurements were performed using an FTA 1000 instrument (First Ten Angstroms, Portsmouth, VA).A 26-gauge needle was used to dispense a drop of the liquid (5-8 μL) onto the fabric surface.To calculate the contact angles, the Laplace fi t method was used.To measure the advancing and receding angles, the needle was brought into close proximity to the fabric surface and the test liquid was pumped for 20 s until the drop reached the size of approximately 30 μl.
Subsequently, the liquid was pumped in at the same rate until the drop detached from the needle or all of the liquid returned to the syringe.

Self-cleaning and UV protection
Th e cotton fabric samples were also dipped into titania nanosols (prepared from Ti(OC 4 H 9 ) 4 ) and passed through a two-roller laboratory padder (BTM 6-20-190) at the speed of 3.66 m/min and air pressure of 2.76 × 105 Pa.Th e padded fabric samples were then dried at 60 °C for 10 min and then cured at 150 °C for 5 min.Th e samples were subjected to a hydrothermal treatment by boiling them in water for 1 h.Aft erwards, the samples were dried, ironed and fi nally conditioned in a laboratory maintained at 65 ± 2% relative humidity and 21 ± 1 °C for at least 24 hours before performing the analysis.

Functionalization with cyclodextrin
MCT-β-CD was graft ed to the cotton fabrics.Diff erent MCT-β-CD concentrations in water (5, 10, 15, 20, 25, 30 w/w %) were prepared and stirred for 5 min.Th en, Sodium Carbonate (Na 2 CO 3 ) was added to the solution (x/4, x is the amount of MCT-β-CD).Th e solution was stirred for 5 min.Th e pH of the solution was around 11.5.Th e cotton fabric samples were dipped into the solution, soaked for 5 min and passed through a two-roller laboratory padder.Th e wet pick-up was around 100%.Th e padded fabric samples were dried at 50 °C for 10 min.Th e samples were thoroughly rinsed with water and dried.

Super hydrophobic treatment
Figure 1 shows a droplet of the dye solution, which diff uses inside the fi bre when placed on the untreated control cotton fabrics.However, when the fabric is functionalised with the MVD technique, inside the fi bres and forms a bead on the surface FOTS (Figure 2), the dye solution does not diff use (Figure 3).Th is means that the droplets of water will be easily rolled off the fabric.Th e diff erence between the advancing and receding contact angles is called "hysteresis" and can be described as the change in the adsorption of liquid on a solid as a consequence of the change in the surface energy [12] or the "roughness of the surface" [13].
Figure 4 shows the advancing and receding contact angles of the cotton fabric aft er the treatment C, which shows low hysteresis.Th e treatment B on the same fabric results in high hysteresis (Figure 4).Any heterogeneities, even at the molecular scale (e.g. a diff erent length of the chain), and surface defects will lead to hysteresis in advancing and receding contact angle measurements [8].Th e advancing and receding contact angles are also aff ected by the shape of the tip of the molecule chains that are emerging from the surface.Th e hysteresis of the samples was measured using distilled water with the above-mentioned procedure.

Self-cleaning and UV protection
Th e cotton fabric was successfully modifi ed by titania nanosols prepared by means of the sol-gel process using tetrabutyl orthotitanate (Ti(OC 4 H 9 ) 4 ) as an active ingredient.Scanning Electron Microscopy showed the presence of a titania dioxide fi lm on the fi bre surface (Figure 5).Th e photocatalytic properties of the titania-nanosol-treated cotton fabric were investigated.Th e results showed that the stains of coff ee and red wine were successfully decomposed by the exposure of the stained fabric to UV radiation (Figure 6).Th e exposure of TiO 2 particles to the photons of energy equal to or greater than the band gap energy results in the promotion of an electron from the valance band to the conduction band of the particle.Th is creates a region with a positive charge (hole, h+) in the valance band and a free

Functionalisation with monochlorotriazyl-β-cyclodextrin
Figure 7 illustrates the reaction mechanism between cellulose macromolecules and MCT-β-CD.Th e cyclodextrin derivative is fi xed to the cotton by a nucleophilic substitution reaction between the hydroxyl groups of the cellulose chain and the chlorotriazine ring.Th e hydrochloric acid is released as a byproduct of the reaction.Phenolphthalein forms inclusion compounds with cyclodextrin.When phenolphthalein is included in the CD cavity in alkaline solutions, it is transformed from the red-coloured dianion form to its colourless form.Th is characteristic makes phenolphthalein a very eff ective colorimetric indicator to confi rm that cyclodextrin cavities are available to form inclusion compounds.Th e control and treated samples were immersed into a solution containing phenolphthalein.Th e maximum absorbance of phenolphthalein in an alkaline solution is at 552 nm.As shown in Figure 8, when the amount of graft ed MCT-β-CD on the fabric is increased, the absorbance at 552 nm is decreased.Triclosan has very powerful antimicrobial properties.Th e measurement of the released amount of triclosan was performed as follows: fi rst, triclosan was included in the cavities and then the amount of molecules released in ethanol was monitored by the UV absorbance of the solution.Figure 9 shows the results of the release of triclosan in ethanol by measuring the absorbance at 282 nm as a function of the MCT-β-CD concentration in the solution.Th e results show that triclosan molecules are present in the control aft er rinsing.Th is is due to the low solubility of triclosan in water and ethanol-water solution.It is important to point out that the amount of triclosan released by the MCT-β-CD treated samples is higher than the untreated sample.For example, the average amount of triclosan released in ethanol by the samples treated with 30% MCT-β-CD concentration in the solution is enough to achieve a molar concentration by 3.5 times higher than the control.
Absorbance at 282 nm  MCT-β-CD treated cotton fabrics were immersed in triclosan solutions and rinsed.To test the ability to remove triclosan that is included in the cavities, the samples were washed in ethanol.Figure 10 shows the Colony Forming Unit of S. aureus of the control and treated cotton fabrics.Th e untreated cotton fabric (Sample A) does not exhibit any antimicrobial properties.However, the cotton fabric treated with MCT-β-CD and loaded with triclosan molecules (Sample B) has a very strong antimicrobial activity against S. aureaus.When the fabric is washed with ethanol to remove triclosan (Sample C), the fabric does not retain its antimicrobial performance.Th is indicates that triclosan can be easily removed from the cavity, which allows the cavities to be available for other guest compounds.

Conclusion
A cotton fabric was successfully functionalised to impart multifunctional properties.Super hydrophobic properties could be imparted using the NVD and MVD techniques.Th e self-cleaning and UV protection properties could be imparted using tetrabutyl orthotitanate by means of the sol-gel process.Th e self-cleaning property was due to the photocatalytic properties of TiO 2 particles formed on the fabric surface.Th e UV protection was attributed to the scattering eff ect of the TiO 2 particles.Th e cotton fabric was also functionalised with β-cyclodextrins.Th is created free cavities on the fabric surface that could be used for inclusion of different compounds that do not in general exhibit affi nity to cellulose.

Figure 4 :
Figure 4: Dynamic water contact angle of functionalised cotton fabric

Figure 8 :
Figure 8: Phenolphthalein inclusion -absorbance at 552 nm as function of MCT-β-CD concentration in solution expressed in w/w %

Figure 9 :
Figure 9: Triclosan release -absorbance at 282 nm as function of MCT-β-CD concentration in solution expressed in w/w%