Study On Preparation And Anticorrosion Properties Of Acid-catalyzed Polysilsesquioxane Coatings

Polysilsesquioxanes refer to a kind of polysiloxanes with a formula of(RSiO_3/2)_n, where R is hydrogen,alkyl,aryl,alkylene groups or other organic groups.The main chains of polysilsesquioxanes are composed of Si-O structures,while the side chains consist of organic groups which attach to the silicon atom.Such unique structures contain both inorganic and organic parts,resulting in their outstanding thermal, chemical,mechanical,and electronic properties.They have been widely used as coatings,engineering plastics and electronic materials.Polysilsesquioxanes used as coatings are commonly synthesized via sol-gel process.It can be divided into two steps.The first step is the preparation of prepolymers based on acid-catalyzed hydrolysis and condensation of organotrialkoxysilanes or organotrichlorosilanes.The second step is the further condensation of the prepolymer in the film forming process.These polysilsesquioxanes can be used as anticorrosive coatings because of their hydrophobic properties.The frequently used catalysts in the sol-gel process of siloxane precursors are small molecular inorganic acids such as hydrochloric acid, acetic acid and nitric acid,which usually remain in the coatings as free molecules, causing negative influences on the corrosion resistant property of polysilsesquioxanes.In this paper,we used phytic acid as a catalyst for the first time to prepare polysilsesquioxane coatings.Phytic acid(C₆H₁₈O₂₄P₆),a natural and innoxious organic big molecule compound,consist of 6 phosphate carboxyl groups.This peculiar structure makes phytic acid have powerful chelating capability with many metal ions, forming a protective coating at the surface of the metals.The objective of this research was to explore the possibility of phytic acid-catalyzed siloxane precursors for preparation of corrosion resistant polysilsesquioxane coatings,by investigating the microstructure and properties of phytic acid-catalyzed polysilsesquioxanes with aid of ¹³C NMR,²⁹Si NMR,GPC,SEM,electrochemical techniques and salt spray test, respectively.The microstructure and properties of hydrochloric acid-catalyzed polysilsesquioxane films were also studied for the sake of comparison.It was found that phytic acid could catalyze the sol-gel reaction more efficiently than hydrochloric acid,causing greater molecule weight of polysilsesquioxanes.Potentiodynamic polarization,EIS and salt-fog praying measurements showed the phytic acid-catalyzed polysilsesquioxane coatings had better corrosion resistant performance than the hydrochloric acid-catalyzed polysilsesquioxane coatings.In addition,we found when the concentration of phytic acid is equal to 0.010-0.014mol/L,and the concentration of TEOS is equal to 0.055-0.01mol/L,optimal anticorrosive property of polysilsesquioxane coatings can be obtained.Besides,in this paper,we tried to prepare polysilsesquioxanes via sol-gel process of methyltriethoxysilane(MTES),3-glycidoxypropyltrimethoxysilane(KH560) and tetraethoxysilane(TEOS) using phytic acid,tannic acid and 1-hydroxyethylidene-1,1-diphosphonic acid(HEDPA) as the catalysts,respectively. The objective of this research was to compare the influence of different kinds of acid catalyst on the microstructure and corrosion resistant property of polysilsesquioxane coatings with the aid of ¹³C NMR,²⁹Si NMR,GPC,SEM,electrochemical techniques and salt-spray test,respectively.The microstructure and anticorrosion property of hydrochloric acid-catalyzed polysilsesquioxanes were also studied for the sake of comparison.It was found that phytic acid catalyzed polysilsesquioxane coatings have the best anticorrosive property,HEDPA catalyzed polysilsesquioxane coatings took the second place,tannic acid catalyzed polysilsesquioxane coatings took the third place,HCl catalyzed polysilsesquioxane coatings took the forth place.In order to improve the adhesion and compactness of the film,the poly(methyl methacrylate) modified polysilsesquioxanes were successfully synthesized by using KH570 as a coupling agent.In this work,KH570 is first copolymerized with methyl methacrylate monomer by using AIBN as initiator.Subsequently,the as-prepared copolymer is then cohydrolyzed with MTES and TEOS to afford chemical bondings to the forming silica networks,giving a series of PMMA-Modified Polysilsesquioxanes.Their microstructure and corrosion resistant performance were investigated by ²⁹Si NMR,GPC,SEM and electrochemical techniques,respectively. Furthermore,the effect of PMMA content and PMMA molecular weight were also studied via electrochemical techniques.It was found that PMMA-modified polysilsesquioxane coatings had excellent corrosion resistance performance compared to polysilsesquioxanes only.