Preparation And Properties Of Cellulose Acetate Modified Waterborne Polyurethane Wood Coatings

Solvent-based polyurethane wood coatings have been replaced by waterborne polyurethane wood coatings due to its high volatility and high VOC.Waterborne polyurethane wood coatings not only has the advantages of non-toxicity,no pollution,good safety,but also has excellent mechanical properties.However,there are some problems with the existing wood coatings,such as poor water resistance,low hardness,and low adhesion.In order to overcome the existing defects in the existing waterborne polyurethane film,it is necessary to modify the waterborne polyurethane wood coatings.In this project,cellulose acetate(CA)was used to graft-modify waterborne polyurethane(WPU)to improve the hardness,wear resistance and water resistance of the waterborne polyurethane film.At the same time,a hydroxylated modified waterborne polyurethane is obtained,which is more easily cross-linked during use,and the hardness,wear resistance,and water resistance of the paint film can be further greatly improved by adding a cross-linking agent during use.In this project,polyurethane prepolymers were prepared by stepwise polymerization using isophorone isocyanate(IPDI),polyether diol 2000(PPG2000),and dimethylol propionic acid(DMPA)as the main raw materials.Cellulose acetate(CA)was used as a modifier and butanediol(BDO)was a small molecule chain extender to prepare a CA modified WPU emulsion.The effects of modification temperature,modification time,R value,CA content and CA acetyl content on the properties of CA modified WPU emulsions,films and coatings were discussed.The results showed that when the modification temperature was 80°C,the modification time was 2.5 h,w(CA)=6.8%,w(DMPA)= 6.8%,the acetyl content of CA was 43.50%,after adding CA,the system R value was 1.5,CA modified WPU emulsion has stable properties and slightly reduced UV transmittance.Compared with the water absorption rate of 10.53% of the unmodified PU film,the water absorption of the CA modified WPU film was reduced by 6.54%,reaching to 3.99%;The hardnessof the modified coating is 3H,which is increased by 4 levels compared to the hardness of the unmodified coating;The loss rate of the modified coating decreased from 1.83% to 0.79%;Coating adhesion level increased from 3 to 1;The number of resistance to acetone wipes was increased from 15 to 65.The CA modified WPU emulsion,film characteristics were studied by fourier transform infrared spectroscopy(FT-IR),thermogravimetric(TG),scanning electron microscope(SEM),atomic force microscopy(AFM),X-ray diffractometer(XRD),gel permeation chromatography(GPC).FT-IR proved that the reaction proceeded in the desired direction.The thermal performance analysis shows that the thermal properties of waterborne polyurethane can be improved by using CA modification.The GPC shows that the molecular weight of the emulsion increases with the addition of CA.The SEM and AFM show that with the increase of the amount of CA and the increase of acetyl content,the cross-sectional morphology of the film shows a more complex network structure.The XRD results showed that the crystallinity of WPU was lower,and the amorphous peak was the main one.When the CA was introduced,the crystallinity of the film decreased.In addition,the CA modified WPU emulsion and aziridine crosslinker(AZ)were mixed and painted to investigate the effect of the amount of aziridine crosslinking agent on the water resistance,hardness,adhesion,resistance to acetone wipes of the paint film.It was found that when the AZ/WPU=6%,the water absorption rate of the film drops to 1.75%,the hardness of coating increased from 3H to 4H,the loss rate of the modified coating decreased from0.79% to 0.66%,the adhesion level rises from level 1 to level 0,and the number of acetone resistant times increased by more than 120 times from 65 times.Through the characterization of SEM and AFM,the cross-linked cross-section showed a more complex network structure,and the micro-phase separation structure was reduced.