Preparation And Durability Mechanism Of Superhydrophobic Coatings With Repellence To Multiphase Accretion

Multiphase matters,such as vapor,grease,ice,and snow,tend to accumulate on the traditional solid surface,causing condensation,fouling,and icing phenomena.It will reduce the cleanliness and mass/heat transfer property of surfaces,which induces negative influences on the work efficiency and safety of the related equipment.Superhydrophobic coatings that are capable of resisting accretion of matters in diverse phases（for example,anti-condensation,anti-frosting,icephobicity,self-cleaning,corrosion protection,etc.）and meanwhile endowing scalable preparation are essential to numerous practical applications,such as aero/marine engineering,petrochemical engineering,biology,architecture,and heat transfer.However,there are several problems existing in the study of superhydrophobic coatings:（1）the preparation relies on organic solvents,which violates environment protection and application safety;（2）coatings are short of durability,including environmental and mechanical stability;（3）it appears mutually exclusive to design one coating that resists multiphase accretion simultaneously,especially for salt fog and solidified mortar.These drawbacks have been widely regarded as the bottleneck in restricting the large-scale application of superhydrophobic coatings.Extensive attempts have been made to mitigate these problems,yet it remains challenging to synergistically achieve all aforementioned goals.In this paper,we gave an insight into the environmental protection property,durability,and resistance to multiphase accretion of superhydrophobic coatings.Under the effect of the rheological additive,the concentrated solution containing superhydrophobic particles was dispersed in water uniformly to prepare the waterborne superhydrophobic nanocoating.Meanwhile,the mechanism of its repellence to multiphase accretion was investigated.Then,the functional groups on waterborne superhydrophobic nanoparticles were optimized.After mixing with waterborne resin,they drove the shrinkage of the resin in the film formation process,forming a reinforcement effect.To demonstrate it,the mechanical and environmental durability was systematically studied,as well as the mechanism of protection of superhydrophobic mechanism by micro-structure.To further improve the superhydrophobic particles,a superhydrophobic cell was designed with unique physical and chemical properties.By introducing the cells to modify and strengthen various resins,the mechanical and superhydrophobic properties of the coating were studied.Also,the system analyses on the durability and resistance to multiphase accretion were conducted.Based on the micromechanics,the durability mechanism was revealed,laying a good foundation for the practical application of superhydrophobic coating.The main results are divided into the following aspects:（1）By adding the rheological agent of modified urea polyurethane,superhydrophobic nanoparticles could be dispersed in water uniformly to prepare waterborne superhydrophobic coatings with resistance to multiphase-matter accretion.Under the effect of the rheological agent,the rheological property of the waterborne system was controlled,which endowed the pseudoplastic flow and improved the anti-sagging and anti-settling properties.As a result,the concentrated solution containing superhydrophobic nanoparticles could be dispersed in water uniformly,and then was sprayed onto various substrates to prepare waterborne superhydrophobic nanocoatings with a porous structure consisting of nanopores and sub-micropores.The resultant coating not only possessed both superamphiphobicity and anti-reflection property,but also displayed resistance to multiphase-matter accretion,such as anti-condensation,anti-frosting,icephobicity,and so on.Compared to the reported waterborne superhydrophobic coating,the surface coverage of condensate dewdrops,frost thickness,and ice adhesion strength were reduced by 43%,40%,and 83%,respectively,as well as delaying the icing time by 700%.Moreover,vapor could be utilized to spontaneously clean the dust on the coating.It has been proved that the surface structure of the waterborne superhydrophobic nanocoating presented a high aspect ratio（ten times that of the control waterborne superhydrophobic coating）.It could impede the vapor permeation and facilitate the occurrence of phase change on the pore side according to the frictional drag equation,leaving the pocket air and generating Laplace pressure.Consequently,desorption of condensate dewdrops and frost were promoted and the vapor accretion was mitigated.（2）By introducing anionic fluorocarbon surfactant,superhydrophobic nanoparticles with hydrophilicity at ambient temperature were prepared,realizing the homogenous dispersion in waterborne resins.The waterborne superhydrophobic coatings with mechanical durability could be obtained.Based on the design concept of micro-structure armor,the waterborne superhydrophobic nanoparticles with heterogeneous chemistry were dispersed into waterborne resins,which promoted the shrinkage of the latter and caused the phase separation.After heat treatment to remove the hydrophilic groups on particles,the waterborne superhydrophobic composite coatings were successfully prepared with network micro-structure.Due to the phase separation,superhydrophobic nanoparticles were prevented from the excessive package by waterborne resins,endowing the coating with repellence to various liquids with a surface tension ranging from 23.8 to 72.1 m N/m,including water,ethanediol,olive oil,hexadecane,and so on.Meanwhile,the frame of the network structure could tolerate external force to protect the embedded superhydrophobic nanoparticles.After repeated abrasion,the Si content loss and roughness reduction of the coating were only 3%and 33.6%,respectively,demonstrating both stability of surface chemistry and structure.Therefore,the coating could withstand 725 cycles of friction tester abrasion under a 250-g load,improving the mechanical durability by 3-25 times compared to the reported superhydrophobic coatings.（3）A binary cooperative superhydrophobic cell was designed,consisting of rigid porous diatomite（microshell）with heterogeneous chemistry and fluorinated silica（nanoseeds）.After modifying the resins,the cells on the one hand provided the mechanical shield covering the resistance to mechanical damages and self-repairing.On the other hand,they could form chemical bonds with the active groups of resins to construct a compact structure in the coating.Besides,the superhydrophobic nanoseeds provide global functionality.Therefore,cellular coatings with excellent superhydrophobicity and mechanical durability were obtained.As shown in the results,the fracture strength and hardness of the resultant coating were more than 10 and 15 times higher than that of the conventional superhydrophobic coatings,respectively.Additionally,the wear resistance and impact resistance were improved by 30-100 times and 120 times,respectively.Based on the Griffith-Irwin-Orowan theory,a model,which was consistent with the experimental results,was derived to predict and explain the reinforcement effect of basic cells.That is,the elasticity modulus and interfacial bonding strength of the coating were improved,thereby reducing the crack size and enhancing the plasticity.The cellular design exhibited universality,that could be applied to various resins and even waterborne resins.（4）Ultra-durable superhydrophobic cellular coatings exhibited strong resistance to multiphase accretion.For the gas,they could repel hot vapor and maintain superhydrophobicity and low moisture absorption in a long-term high humidity environment.Compared to the control coatings,the moisture-proof property was improved by 183%,and even 408%.Meanwhile,the cellular coating suppressed the condensation and frosting behaviors,and facilitated the desorption of the dewdrops and frost.For the liquid,the cellular coating could resist the adhesion of the mortar slurry in high viscosity state.Even after repeated pressing and various mechanical damages,the mortar slurry could slide away completely,showing a roll-off angle less than 30°.For the solid,the solidified mortar could not adhere to the coating surface,exhibiting ultra-low adhesion strength（<0.1 k Pa）.The ice adhesion strength was also as low as～20 k Pa,meaning that the long-term resistance to solid accretion of superhydrophobic coatings was enhanced by more than 6 times,and even 300 times.For corrosive ions,even after mechanical abrasion and 150-day immersion in saltwater,the cellular coating possessed an impedance of 10⁹-10¹⁰Ω·cm²,which was four orders of magnitude higher than conventional coatings.Besides,the duration was prolonged by more than 5 times.Similarly,the coating protected the metal substrate from corrosion after 50 days of neutral salt spray.The cellular coating endowed the heat-exchanger with anti-condensation,anti-frosting,and self-cleaning properties.Compared to the commercial hydrophilic heat-exchanger,the efficiencies were improved by 8.2%and even 65%under various operating conditions.The findings provide theoretical and technological support to the practical applications in the fields of anti-fouling,icephobicity,anti-corrosion,energy conservation and emission reduction,and so on.