Aminated modification of shellac resin and properties of its waterborne coatings
This study focused on investigating the preparation method of shellac resin modified by p-amino benzoic acid and the examination of the properties of water-based coatings formed by combining amino benzoic acid modified shel-lac resin and four types of polymers.Firstly,the effect of the amount of p-amino benzoic acid on the thermal life of the modified shellac resin was explored,and the amount of amino benzoic acid was determined.Then the properties of the modified shellac resin,such as acid value,hydroxyl value and color index,were tested.The structure of the modi-fied shellac resin was characterized by the Fourier transform infrared spectroscopy(FT-IR),carbon-13 nuclear mag-netic resonance spectrometry(13C-NMR),and differential scanning calorimetry(DSC).The experimental results showed that the p-amino benzoic acid was mainly grafted into shellac resin by esterification.The consumption of hy-droxyl group during esterification reduced the thermal life of the modified shellac resin,and the introduction of amino group increased the melting point of modified shellac resin.The amino molar concentration of the modified shellac re-sin could reach 1.90 mmol/g,and the thermal life could be extended from 3.9 min to 11.1 min.The performance tests of waterborne coatings made by compounding modified shellac resin with different types of polymers,such as polyeth-ylene glycol(PEG),polyvinyl alcohol(PVA),polyacrylamide(PAM),hydroxypropyl methyl cellulose(HPMC),showed that the adhesion of the modified shellac resin crosslinked with glyoxal was significantly higher than that of the modified shellac resin,indicated that the modified shellac resin could improve the curing crosslinking degree of the film through the carbonyl ammonia reaction of glyoxal.Thus,the adhesion of shellac resin on different substrate surfaces was improved,among which the adhesion of metal substrate can reach 5.61 MPa,and the adhesion was increased by 92.12％.The colloidal solution of modified shellac resin added with PVA and PEG had better ductili-ty and uniform coating.The impact resistance,salt spray resistance and water resistance of shellac resin could be im-proved to 120 cm,72 h and 720 h,respectively.The introduction of polyethylene glycol polymer could also improve the bending flexibility while maintained the water resistance and salt spray resistance of the film.Hydroxypropyl meth-yl cellulose was an insoluble polymer in cold water,and the salt spray resistance of the film could be improved com-pared with that of modified shellac(M-SHL)and glyoxal crosslinked modified shellac resin.In conclusion,this re-search explored a new path for the modification of shellac resin based on the carbonyl ammonia reaction,providing new raw materials and ideas for the further development and utilization of shellac resin.
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