Study On The Construction Of Functional Waterborne Polyurethane Coatings Regulated By Nano Cellulose And Its Performance Influence Mechanisms

Wooden panels need protection due to natural defects such as flammability and perishability,and waterborne polyurethane（WPU）is an eco-friendly wood coating widely used for wood panel protection and decoration;however,the shortcomings of WPU itself,such as heat resistance,mechanical and antibacterial properties and poor flame retardancy,have limited its scope of application.In order to improve these defects,with the help of nanocellulose of high specific surface area,very high modulus and the presence of a large number of hydroxyl groups on the surface,it was used as a coupling agent and stabilizer,and nanocellulose prepared by steam explosion pretreatment was compounded with other micro-nanoparticles and WPU and based on the study of compatibility and homogeneity of nanocellulose blended with WPU and other micro-nanoparticles,a functional WPU/nanocellulose composite was constructed,which well solved the problems of easy aggregation and non-uniform dispersion of each particle in WPU.The effects and mechanisms of each particle on the antibacterial,antistatic,flame retardant,wear resistance and mechanical properties of the composite materials were systematically analyzed.Thus,with no significant impact on transparency performance and greatly improved film adhesion,a functional WPU coating method was explored finally and applied to wood panels to provide the corresponding properties and improve their service life and added value.The main contents and research results are shown as follows:（1）Based on the fact that steam explosion can significantly change the thermal stability of fiber and affect its flame retardancy,four different types of nano cellulose were prepared by combining bleaching and phosphorylation treatment and finally high-pressure homogeneous milling:MFC（treated only by steam explosion）,BMFC（treated by steam explosion and bleaching treatment）,PMFC（treated by steam explosion and phosphorylation）and PBMF（treated by steam explosion,bleaching and phosphorylation）.The experiments showed the thermal stability of MFC and BMFC prepared by steam explosion was greatly improved,with maximum weight loss temperature(T_max)rising from 345°C to 368°C and maximum weight loss rate(R_Tmax)dropping sharply to 1.25%/min;while for PMFC and PBMF not only have good thermal stability,but also have excellent flame retardancy,with the intensity of the main p HRR（the peak of heat rate release）dropping significantly from 200 w/g to 45 w/g,Tp HRR（the temperature at p HRR）decreased from 350°C to 250°C,THR（the total heat release）andΔh（the heat of combustion）also decreased from 11.9 k J/g and 13 k J/g to 2.9 k J/g and 4.6k J/g,respectively,and the residual percentage was all around 40%.（2）Different types of WPU/nanocellulose composites were prepared based on the compatibility between nanocellulose and WPU.It was found that nanocellulose had a positive effect on the thermal stability performance,flame retardancy and mechanical properties of WPU/nanocellulose nanocomposites.The T_5%（initial pyrolysis temperature）,T_max and residue of WPU/nanocellulose composites were greater than those of WPU and increased with the addition amount up to 303°C,405°C and 9.55%,respectively,but the addition of nanocellulose had little effect on the R_Tmax of the composites.Although the different types of nanocellulose showed different combustion behavior of WPU/nanocellulose composites,all of them improved the flame-retardant properties of WPU.The addition of nanocellulose also improved the tensile strength and Young’s modulus of each composite film,while decreasing the elongation at break.The maximum values of tensile strength and Young’s modulus for different types of composite films（WM,WB,WPM,WPB）increased by 77.3%and 329.5%,156.3%and 267.6%,74.8%and 330.7%,98%and 429.5%,respectively,while the minimum values of elongation at break decreased by 82.3%,78%,88%and 92.5%,respectively.（3）WPU/nanocellulose/Ag NPs composites were prepared by using nanocellulose as a coupling agent and combining the compatibility between nanocellulose and WPU coatings.It was found that Ag NPs improved the antibacterial properties,thermal stability,flame retardant properties,and mechanical properties of WPU/nanocellulose/Ag NPs composites to different degrees.WPU/nanocellulose/Ag NPs composite films had antibacterial effects on Enterococcus faecalis,Escherichia coli and Micrococcus luteus,and the best antibacterial effects were observed when the content of Ag NPs were all 5%,WMG5,WBAG5,WPMAG5,and WPBAG5 showed sterilization rates of 90.6%,99.5%,89.6%,90%,99.3%,89%,79.4%,99.6%,84.8%,89.4%,99.6%,83.4%for different strains,respectively.While the tensile strength and Young’s modulus of WPU/nanocellulose/Ag NPs composite films increased significantly with the increase of Ag NPs content and reached the maximum value at 5%of Ag NPs addition,at which time the tensile strength and Young’s modulus of WMAG5,WBAG5,WPMAG5,and WPBAG5 increased by 50%and 101.7%,78.5%and 114.6%,148.3%and197.8%,55.1%and 94.2%,respectively,compared with the corresponding control groups,while the elongation at break decreased significantly with the increase of Ag NPs content.（4）Combined with the above research,WPU/nanocellulose/magnesium hydroxide and WPU/nanocellulose/copper nanoparticle composites were prepared by introducing magnesium hydroxide particles and copper nanoparticles,respectively,using nanocellulose as a template,and applying them with WPU/nanocellulose/Ag NPs composites to wood panels to improve the flame retardant properties,wear resistance,antibacterial properties and antistatic properties of the panels.The experimental results showed that the size of the antimicrobial circle diameter of the panels is closely related to the amount of Ag NPs added,when the content of Ag NPs from 0-5%,the diameter of the antimicrobial circle becomes larger with the increase of Ag NPs,the diameter of the antimicrobial circle is the largest and the antimicrobial effect is the best when the addition ratio is 5 wt%;the antistatic properties of wood-based panels were also improved with the addition of Ag NPs,and the surface resistance of the specimens was reduced from11.66 lg/Ωto 3 lg/Ωwhen the Ag NPs content reached 70%and 80%,thus bringing the panels to conductor level and making the panels antistatic;the flame retardant performance of wood panels also improves with the use of WPU/nano-cellulose/magnesium hydroxide composite coating,when the addition of magnesium hydroxide is 20%,30%and 40%,the panels change from B3 flammable material to B2 combustible material,and when the addition is 50%and60%,the flammability class of the panels change to B1 non-combustible material.The use of WPU/nano-cellulose/copper nano-particle composite coating improved the wear resistance of the plates,with the increase of copper nano-particle content,the wear amount of the plates gradually decreased,when the nano-copper was added up to 80.0 wt%,the wear of the plates reached the minimum value of 0.15 g,which was 15.7%lower than the control group,with the best wear-resistance effect.