Preparation Of Raw Lacquer-based Nanocomposite Coatings And Their Effect On Wood Properties

Raw lacquer is a natural and renewable paint produced from the lacquer tree.Raw lacquer has high hardness,strong corrosion resistance,high gloss and elegant beauty after film formation.However,natural raw lacquer has the disadvantages of long drying time,insufficient mechanical properties,poor alkali resistance,high brittleness and easy photoaging degradation,which cannot meet the requirements of rapid drying and durability of contemporary wood coatings.In addition,lacquer-coated wood is easy to fade and degrade when exposed to ultraviolet light,and it is also prone to wet expansion and deformation in humid environments,which seriously limits the use of lacquer-coated wood outdoors.For the consideration of sustainable development and environmental protection,a series of nano-composite coatings were prepared by using raw lacquer as matrix,nano-Al₂O₃ and CNF-P as nano-fillers,and nano-particles were chemically pretreated by KH550 to make them uniformly dispersed in raw lacquer.The hardness,impact resistance,adhesion,color,gloss,water contact angle,ultraviolet shielding performance,corrosion resistance,thermal stability and cross-section morphology of the composite paint film were characterized to determine the ideal addition amount of nanoparticles.Then,the optimal modified nano-Al₂O₃/raw lacquer and modified CNF/raw lacquer composite coatings were applied to wood surface finishing to investigate the effects of finishing treatment on moisture resistance（surface hydrophobicity,water absorption,moisture content and dimensional stability）and aging resistance（microscopic morphological changes,hydrophobic changes,chemical changes and color changes）of wood.A simple and green modification method was provided to improve the performance of raw lacquer and expand its application in outdoor finishing wood.The main findings are as follows:（1）The surface hydrophobic chemical modification of nano-Al₂O₃ was performed using silane coupling agent KH550 to obtain modified nano-Al₂O₃ with higher dispersibility stability,which was dispersed into the raw paint to prepare nano-composite raw paint coatings with different nano-Al₂O₃ contents（0 wt%,0.5 wt%,1 wt%,2 wt%,3 wt%and 4 wt%）.The addition of modified nano-Al₂O₃ improved the hardness,gloss,impact resistance,adhesion,hydrophobicity and alkali resistance of the paint film.When the content of nano-Al₂O₃ was 1wt%,the best thermal stability and mechanical properties of the composite film were achieved:the hardness reached 6 H and the impact resistance reached 50 kg·cm,higher than 3 H and 40kg·cm of the raw lacquer.In addition,the light transmittance of both raw lacquer and nano-composite lacquer film in the UV region was 0,which had good UV shielding ability.The SEM results showed that the addition of nano-Al₂O₃ filled the internal pores of the lacquer film,thus the composite lacquer film had good dense property.（2）CNF-P ethanol dispersion solution was prepared by ultrasonic dispersion of KH550 modified redispersible cellulose nanofibers（CNF-P）,and then it was fully blended with raw lacquer to prepare modified CNF/raw lacquer nanocomposite coatings.When the addition of modified CNF was 10 wt%,the composite lacquer film had the best overall mechanical properties,with a hardness of 5 H,an adhesion of grade 3,and an impact resistance of 48 kg·cm.The water contact angle of the composite lacquer film decreased gradually with the addition of CNF,and the water contact angle of the lacquer film at 20 wt%CNF was 81.87°,which was 5.7°less than that of the raw lacquer film.In addition,the drying time of the nanocomposite coating was greatly shortened with the addition of modified CNF.When the addition amount of CNF was 20%,the drying time of the composite film was 170 min shorter than that of the raw lacquer.The addition of CNF improved the degree of polymerization of the lacquer film.The SEM results showed that the internal structure of the modified CNF/raw lacquer composite film was dense and uniform.（3）Compared with the wood coated with raw lacquer,the wood coated with 1%modified nano-Al₂O₃/raw lacquer（Q-1A）and 10%modified CNF/raw lacquer（Q-10F）had higher moisture resistance and aging resistance.The water contact angle of Q-1A coated wood was 82°,which was 10°higher than Q-10F and 12°higher than lacquer.After 172 h of water absorption,the water absorption of untreated tung wood was as high as 160.12%,while the water absorption of Q-1A and Q-10F coated tung wood was only 25.04%and 32.17%,which was 13.73%and 6.6%lower than that of pure raw lacquer coated tung wood,respectively.When in a humid environment,the dimensional stability of Q-10F coated wood was the highest,showing the smallest volume swelling rate.The changes in moisture-absorbing and moisture-proof properties of cedar before and after finishing with raw and modified lacquers remained basically the same as those of tung wood.（4）The color,hydrophobicity,surface morphology and chemical changes of raw and modified lacquer-coated wood during aging were tested to analyze the aging mechanisms of modified and raw lacquers and their effects on the resistance of wood to UV oxidative degradation.The results showed that the UV aging resistance of both Q-1A and Q-10F coated wood was better than that of raw lacquer coated wood,and the Q-1A coated wood was the most tolerant to UV light.After artificial UV accelerated aging for 480 h,the total color difference on the surface of Q-1A coated wood was 11.88,which was 46.94%lower than that of raw lacquer,and the Q-1A coated wood surface had the best UV discoloration resistance stability.In addition,the water contact angle of Q-1A coated wood was the largest and the decrease was the smallest,which was only 27.66%compared with that before aging.The SEM results showed that the surface of both raw and modified lacquer coated wood showed small pores and became rougher after aging,and the IR results indicated that the degradation of Q-1A coated wood surface was the lowest,and the addition of modified nano-Al₂O₃ slowed down the degradation of lacquer phenol side chains and improved the UV aging resistance of the lacquer film.