Study On Synthesis And Applications Of Nano-Silica Modified Two-component Waterborne Polyurethane

Waterbrne polyurethane?WPU?is safe and environmently friendy because its dispersion medium is water.However,it exhabits lower water resistance,mechanical properties and thermal stability as it owns the hydrophilic group.Two-component waterborne polyurethane?2K-WPU?can be comparable to solvent-based polyurethane because 2K-WPU has higher crosslinking degree which is prepared by the polyurethane polyol?component A?and curing agent.But 2K-WPU also has some disadvantages,such as longer drying time,more bubbles during film formation and worse water resistance.In order to improve the 2K-WPU,Nano-silica was used as the modifier,which is the rigid particle with lower price.On the basis of this,two-component waterborne polyurethane topcoat was prepared with great application value.Nano-silica has large surface energy and particles are easy to agglomerate.In this research,two different methods were used:“polyol dispersion method”and“prepolymer chain extension method”.The optimal method and amount.of nano-silica was obtained by comparing the properties of 2K-WPU film prepared by the two different methods.?1?Nano-silica was modified by 3-aminopropyltriethoxysilane?KH550?to obtain the aminated nano-silica?modified nano-silica?.FTIR showed organic groups had been successfully grafted to the surface of nano-silica.The content of KH550 on the dispersion properties of modified nano-silica was investigated by sedimentation experiment.Resuts showed that the dispersion was best when the amount of KH550 was 70%of that of nano-silica.SEM and TEM showed that the dispersion of nano-silica improved a lot.?2?Using the unmodified nano-silica as the modifier,throuth polyol dispersion method,by which nano-particles tended not to aggregate due to the large steric hindrance when nano-particles were dispered in the polyol,nano-silica modified two-component waterborne polyuretahe was prepared by the reaction between nano-silica modified polyurethane aqueous dispersion?component A?and the hydrophilic-modified polyisocyanate curing agent?component B?.With nano-silica dispered in polypropylene glycol,which reacted with isophorone diisocyanate?IPDI?,along with the hydrophilic chain extender?DMPA?,small molecule chain extender?BDO?,hydroxy terminal blocking crosslinker agent?TMP?as main raw materials,polyurethane aqueous dispersion?component A?was synthesized.FTIR showed nano-silica had been inserted into the polyurethane molecular chain.With the increase of nano-silica contentration,the viscosity of the polyurethane polyol emulsion became larger,and theappearance gradually changed from transparent to milky white.When the content of nano-silica was 2%,the properties of composite film reached optimum.The water absorption decreased from 37.7%to 8.9%;the water contact angle increased from 74.6°to 100.4°;the tensile strength increased from 10MPa to 23MPa,and the elongation at break gradually decreased;TGA showed a significant improvement in thermal performance when the weight loss was 10%,the pure film needed 279.9?,but the modified film needed 287.7?;XRD showed that the crystallinity of the composite film decreased after nano-silica was added;SEM showed that most of the nanoparticles were dispered well in the polyurethane.?3?Using the grafted nano-silica as the modifier,through the prepolymer chain extension,two-component waterborne polyurethane was synthesized by the reaction between nano-silica modified polyurethane aqueous dispersion?component A?,which was prepared by the–NH₂ of the SiO₂-NH₂ on the surface reacting with the–NCO of the polyurethane prepolymer,and the hydrophilic-modified polyisocyanate curing agent?component B?.FTIR tracking had confirmed that modified SiO₂ was chemically bonded with polyurethane successfully.With the increase of the modifier,the transmittance of polyurethane polyol dropped,but the zeta potential exhibited a high value,which showed the stability of polyurethane polyol was well.When the concentration of modified-SiO₂ was 0.8%,the performance reached the best.The tensile strength increased from 10 MPa to 28 MPa;the water absorption decreased from 37.7%to 6.3%;the water contact angle increased from 74.6°to 123.5°;TGA showed the theremal property of composite film improved evidently;the pencil hardness of the coating film increased from HB to 3H;the coating film also showed good acid and alkali resistance.SEM and TEM showed that the nanoparticles were well dispersed in polyurethane,there were few nano particles aggregating.The nano particles had good compatibility with polyurethane molecular chains.?4?The main film forming substance was nano-silica modified polyurethane polyol prepared by prepolymer extension chain method,which was mixed with Pigment and filler,common coating additives,hydrophilic curing agent to form nano-silica modified two-component waterborne polyurethane topcoat.The optimal originally process conditions and usage of raw materials were obtained by single factor experiment.On basis of this,orthogonal experiments were carried out to optimize experimental formula.Resaults showed that the performance of topcoat reached optimal when PVC?pigment volume concentration?was 14%,the amount of polyol was 62%,n?-NCO/-OH?=1.6,the amount of titanium dioxide was 22%,and the amount of defoamer was 0.12%.The paint film appeared bright and almost had bubbles.The glossiness?60°?could reach 91,the pencil hardness was 3H,the surface drying time was24min,the curing time was 3h,the impact resistance is 50Kg·cm,the fineness was 30?m.Italso had excellent water and chemical resistance.In a word,the topcoat had excellent application value.