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Analysis Of Application Methods Of Superhydrophobic Surfaces

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SOL-Gel Processing

Dong et al describes a method to synthesize iodine butyl-n-sulfonate amino polysiloxane {(IB-N-SA) PDMS} using poly(4-iodobutoxy) methylsiloxane and guanidine sulfamate. The cotton fabric was then soaked in a bath of (IB-N-SA) PDMS, zirconium oxide chloride, and urea at room temperature for four minutes under the proper pH condition. The samples were padded with two nips and two dips. The fabric was then dried at 100oC for three minutes and cured at 150oC for four minutes. The fabric experienced an increase in contact angle from 88. 37o to 124. 49o. The fabric cannot be labeled as superhydrophobic; however, it can be classified as hydrophobic. The hydrophobicity was a result of interactions between the cotton fabric and methyl groups, creating a polymer film. Using a limited oxygen index test, the treated fabric increased in value from 18. 0% to 30. 9%. Additionally, a vertical burn test showed that untreated cotton fabric was completely destroyed while cotton treated with (IB-N-SA) PDMS showed decomposition of the fibers while releasing an incombustible gas, improving the flame retardancy of the fabricZhang et al uses a sol-gel process utilizing tetraethyl orthosilicate, ammonium hydroxide, ethanol, a solution of poly(dimethyldiallylammonium chloride) {poly-DMDAAC}, and (heptadecafluoro-1, 1, 2, 2-tetradecyl) trimethoxysilane. Firstly, silica particles were synthesized by hydrolyzing tetraethyl orthosilicate with ammonium hydroxide and ethanol. These silica particles were then dispersed into a solution of poly-DMDAAC. The cotton fabric was then submerged into the poly-DMDAAC solution for 20 minutes, allowing the fabric to gain a charge. After a de-ionizing water wash, the cationic fabric was submerged into an electronegative silica particle bath for 20 minutes, then washed again with de-ionized water. This process was repeated twice in order to form multiple layers of a hydrophobic film. Thirdly, the fabric was immersed in a solution of (heptadecafluoro-1, 1, 2, 2-tetradecyl) trimethoxysilane for one hour, allowing this molecule to graft onto the polymer film. The fabric treated with this poly-DMDAAC/silica method and grafted with (heptadecafluoro-1, 1, 2, 2-tetradecyl) trimethoxysilane had a maximum contact angle of 157o.

Zhang also investigated the durability for industrial applications. His team exposed the fabric to ambient air for one month, immersed the fabric in tap water for 24 hours, and washed the fabric in a washing machine. The ambient air caused no visible change, keeping the contact angle above 150o. The immersion test proved that the fabric remained superhydrophobic. The washing machine test caused mechanical and chemical degradation to the treated fabric, decreasing the contact angle to 149o and damaging the density and uniformity of the silica coating. All in all, this method transforms the wettability of cotton fabric to superhydrophobic and maintains decent stability.

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Fluorinated di/triblock copolymerization

Shi et al synthesized two fluorinated diblock copolymers: poly[2-(perfluorohexyl) ethyl acrylate]-block-poly[(triisopropyloxysilyl)propyl methacrylate {PFHEA-b-PIPSMA} and poly[2-(perfluorohexyl)ethyl acrylate]-block-poly(glycidyl methacrylate) {PFHEA-b-PGMA}. 3-(Triisopropoxysilyl) propyl methacrylate (IPSMA) was synthesized using a mixture of 3-(trimethoxysilyl)propyl methacrylate, isopropanol, and p-toluenesulfonic acid as a catalyst. The solution was cooled, saturated with sodium bicarbonate, extracted with hexane, and dried over magnesium sulfate. Poly(2-(perfluorohexyl) ethyl acrylate) (PFHEA) was synthesized from a solution of ethyl α-bromoisobutyrate, 2-(perfluorohexyl)ethyl acrylate, N, N, N’, N”, N”-pentamethyldiethylenetriamine (PMDETA), and trifluorotoluene (TFT). Both diblock copolymers used PFHEA as a macroinitiator. PFHEA-b-PGMA is synthesized with PFHEA, glycidyl methacrylate, and TFT. The copolymer is then precipitated with hexane and dried under vacuum. On the other hand, PFHEA-b-PIPSMA was synthesized in a solution of PFHEA, bipyridine, IPSMA, and TFT. The copolymer is then precipitated with methanol and dried under vacuum. The two fluorinated diblock polymers acted differently when applied to cotton textiles. The PFHEA-b-PIPSMA coating decreased stability and only provided a modest amount of hydrophobicity.

Conversely, PFHEA-b-PGMA provided higher durability and a high contact angle greater than 150o. Li et al used fluorinated triblock azide copolymers containing poly(ethylene glycol) {PEG}, poly(2, 2, 3, 4, 4, 4-hexafluorobutyl acrylate) {PHFA}, and poly(methyl acrylate-co-4-azidophenyl methacrylate) {P(MA-co-APM)} blocks using reversible addition-fragmentation chain-transfer (RAFT) polymerization. Firstly, PEG was dissolved in methylene dichloride with S-1-dodecyl-S’-(α, α’-dimethyl-α”-acetic acid) trithiobonate and 4-(dimethylamino)pyridine as a catalyst. The precipitate, PEG5000-CTA, is removed with dicyclohexylurea. PEG5000-CTA is then polymerized in a petroleum ether with 4-azidophenyl methacrylate (APM), benzoyl peroxide (BPO), N, N-dimethylaniline (DMA), methyl acrylate (MA) and 1, 4-dioxane, creating PEG5000-b-P(MA-co-APM). This is then polymerized in another petroleum ether with BPO, DMA, 2, 2, 3, 4, 4, 4-hexafluorobutyl acrylate (HFA), and 1, 4-dioxane to produce a triblock azide copolymer, PEG5000-b-P(MA-co-APM)-b-PHFA. The cotton fabric was then soaked in a solution of the triblock azide copolymer and dried at 80oC for ten minutes. The fluorinated azide triblock copolymer modified the cotton fabric to have a contact angle of 155o. They also discovered that the fluorinated polymer chain gave the cotton textiles have enhanced erosion resistance to acids, bases, and organic solvents.

Electrospinning

Electro spinning is a fiber production method that uses electric force to draw charged threads of polymer solutions onto the fabric fibers. There are three main parts: a feeding system, a high-voltage power supply, and a grounded collector. A needle is connected to a syringe, of which has a very high voltage supply attached to it, creating a Taylor cone, a conical shape elicited by an electrical field acting in opposition to the surface tension of the droplet. The droplet elongates and creates a jet of solution which creates a web of fibers from a polymer solution.

The literature on electrospinning shows that the electrospun webs can enhance surface roughness, increasing the self-cleaning hydrophobicity of the material Kaplan et al used an electrospinning technique to create superhydrophobic polyester mesh for use in biomedical materials. Electrospinning is a fiber production method that uses electric force to draw charged threads of polymer solutions onto the fabric fibers. 5-benzyloxy-1, 3-dioxan-2-one was used to copolymerize D, L-lactide and (epsilon)-caprolactone, creating poly(1, 3-glycerol carbonate-co-caprolactone) and poly(1, 3-glycerol carbonate-co-lactide). The polymer is precipitated into cold methanol and dried for 24 hours. Depending on the polyester used for electrospinning, a different solvent is needed: chloroform/methanol for polylactocaprone (PCL) or tetrahydrofuran/N, N-dimethylformamide for poly(lactic acid-co-glycolic acid) (PLGA). The dissolved polymer and solvent are then loaded into a syringe pump and electrospun or electrosprayed onto the polyester. The electrospun polymer, when synthesized in a 10% weight-weight percent of PCL and poly(glycerol carbonate) (PGC), exhibited a contact angle of 143o. When synthesized in a 30% weight-weight percent of PCL/PGC, the material exhibited a contact angle of 150o, and 160o when synthesized with a 50% weight-weight percent of PGC/PCL.

One-pot mist copolymerization

Xi et al used a solution of tert-butyl peroxybenzoate (TBPB), a free radical initiator, lauryl methacrylate (LMA), 2-isocyanatoethyl methacrylate (IEM), and ethylene glycol dimethacrylate (EGD). The solution was dissolved in hexane and atomized onto the cotton fabric using an air compression-type atomizer. His one-pot mist polymerization created a thin layer with advanced morphology on the surface of the cotton fabric. His team achieved a maximum contact angle of 157o. Upon further testing, the superhydrophobic copolymer film endowed the cotton fabric with abrasion resistance, laundering durability, and heat induced healing abilities. He and his team found no significant impact on the cotton fabric’s flexibility, water absorptivity, and vapor permeability. Wang et al used a graft-polymerization technique with LMA as a monomer, just as Xie did. His team used a solution of LMA and EGD dissolved in ethanol. He then completed mist polymerization in two steps. Firstly, ammonium ceric nitrate, an initiator, was atomized using an air compression-type atomizer, then applied to the cotton fabric for two minutes. Secondly, various concentrations of monomer solutions were atomized and applied to the cotton fabrics then washed with ethanol to remove excess monomer. The maximum contact angle he managed to experimentally find was 151. 9o. He found that cotton textiles inherently allowed capillary action to drive water droplets through the fabric. His cotton fabric samples that were treated with poly(lauryl methacrylate), (PLMA), had weakened capillary action, increasing the superhydrophobicity of the surface. After testing laundering durability and mechanical stability, the textiles he washed showed a decrease in the water contact angle after five washes with detergent; however, it is noted that after 35 washes with out detergent, no change in WCA was recorded. By adding a third monomer, IEM, the laundering durability and mechanical stability increased.

Pad-dry-cure application

Xue et al prepared synthetic polyacrylate soap-free latex (FPA) and ammonium polyphosphate (APP). Polymethacryloxypropylsilsesquioxane (PSQ) latex were synthesized using 3-(trimethoxysilyl) propyl methacrylate (KH-570) as a catalyst with sodium hydroxide to polycondensate PSQ seeds. These seeds, combined with various monomers and potassium persulfate (KPS), underwent emulsion polymerization to synthesize FPA latex. The outer shell of the mixture consisted of dodecafluoroheptyl methacrylate (DFMA), butyl acrylate (BA), and methyl methacrylate (MMA). The pretreated cotton fabric was submerged in two baths: a poly(ethylimine) solution and an FPA/APP bath. After removing the treated fabric, the fabric went through a two-roll padder, dried in a 100oC oven, and cured at 170oC. By increasing the amount of DFMA, the contact angle increased from 152o to 160o.

Additionally, APP demonstrated at superhydrophobic contact angle, yet due to the hydrophilic ammonium ions, the contact angle decreased slightly. His team also found that the FPA/APP coating provided a decrease in cellulose decomposition, increasing the fabric’s thermal stability. Moreover, the coated fabric showed self-extinguishing properties and resistance to washing. Li et al synthesized a cross-linked acrylate monomer through emulsion polymerization and applied to the cotton textile through a pad-dry-cure process. His team synthesized 2, 2’-sulfonyldiethanol. This compound was combined with anhydrous cyclohexane, triethylamine, and propenoyl chloride to make 2-[(2-hydroxyethyl) sulfonyl] ethyl acrylate (HSEA). To make the polymer, a novel perfluorinated acrylate copolymer (NPAC), to be applied through pad-dry-cure, HSEA was combined with perfluorodecyl acrylate, octadecyl acrylate, sodium lauryl sulfate (SLS), and ammonium persulfate. The fabric was then submerged in an aqueous solution of NPAC. After submersion, the were padded with two dips and two nips, dried for three minutes, and cured at 170oC. Li used two tests to determine the water repellency of his samples: the water/isopropyl alcohol test and hydrocarbon resistance test method. In both cases, the textile treated with NPAC has superior and more efficient water repellency due to cross-linkage. Additionally, the cross-linkage mechanism improved the washing durability.

Applications

As mentioned before, by adding these superhydrophobic polymers, cotton fabrics experience increased flame retardancy, self-cleaning, and improved mechanical stability; however, cotton textiles expand into a scope encompassing more applications. Superhydrophobic cotton fabrics also have oil/water separation capabilities, anti-icing, and self-healing properties. Importantly, the increase in oil leakage and potential danger to marine and aquatic life has increased the need for an efficient and quick method to separate water and oil. Superhydrophobic cloth can also double over as a superoleophilic material capable of separating the two phases. The treated fabric acts as filtration membrane, capable of separating and capturing various oils, for example toluene, chloroform, and kerosene, both on the surface and under the water.

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