Preparation Of Polyurethane-Based Layered Double Hydroxide Nanocomposites And Its Application In Leather-making

Retanning is able to strengthen and supplement the properties of wet bule,which determines the performance and style of the finished leather.The traditional waterborne polyurethane tanning agent(WPU)is widely used in the retanning process because it can maintain the good feels and natural grain of the leather.However,WPU has little improvement in the filling and mechanical properties of the leather,and cannot give the leather a special functionality,which seriously affects its application in leather-making.Therefore,it is one of the future development directions of leather to modify the traditional WPU,improve performance of crust leather and endow it special functions.Based on it,polyurethane-based layered double hydroxides nanocomposite tanning agents were prepared by nano-modification technology in order to improve the retanning property of WPU and the dyeing property of crust leather,while impart flame retardancy and increase the additional value of the leather product.The results are shown as follows:(1)A series of polyurethane-based LDH nanocomposites(WPU/LDH)with partially intercalated structures are prepared by physical blending method.The structures of WPU/LDH are characterized by dynamic laser scattering(DLS),fourier-transform infrared microspectroscopy(FT-IR),X-ray diffraction(XRD)and transmission electron microscop(TEM).The application results show that when the LDH dosage is 2%,the mechanical properties and dyeing properities of crust leather are optimal.But the crust leather show the best flame retardant performance,when the amount of LDH is 8%.Compared with WPU retanned leather,the damage length of WPU/LDH-8%retanned leather reduces by 74.2%and the limit oxygen index(LOI)increases from 23.6%to 26.0%.At the same time,it is explored that the effect of LDH on the burning process.The results show that LDH is beneficial to the formation of a dense carbon layer during the burning process,thereby preventing further damage to the leather by heat radiation.However,WPU/LDH-8%shows poor integrated application performances,so it needs to be further modified.(2)The sulfonated calixarene(SCA)is used as a crosslinking agent to modify WPU because of its multiple tanning groups.The results show that the optimal preparation process are:the simultaneous addition of SCA,polyethylene glycol(PEG)and 2,2-Bis(hydroxymethyl)butyric acid(DMBA),nSCA:nPEG=3:10,R(n-NCO/n-OH)value=0.8.The application results show that the introduction of SCA can enhance the thickening rate,softness,mechanical properties and dyeing performance of crust leather,but it can not improve the flame retardancy.Then,LDH is added to the SCA-WPU,and the SCA-modified polyurethane-based LDH nanocomposites((SCA-WPU)/LDH)are obtained by physical blending method.The application results show that when the LDH dosage is 4%,the dyeing performance of crust leather is the best,and the flame retardancy reaches the optimum at the LDH amount of 8%.But the retanning property dose not change with the introduction of LDH Compared with SCA-WPU retanned leather,the damage length of(SCA-WPU)/LDH-8%retanned leather reduces by 33.3%and the LOI value increases from 22.7%to 24.0%,which is lower than WPU/LDH-8%.This is because LDH does not from the intercalated structures in(SCA-WPU)/LDH.The dispersion of LDH in the matrix determines its application performance.Therefore,it is proposed to optimize(SCA-WPU)/LDH structure.(3)The SCA intercalated LDH(SCA-LDH)are prepared by co-precipitation method.The structures of SCA-LDH are characterized by FT-IR,XRD and TEM.The results show that SCA enters the LDH layer and the interlayer distance increase from 0.76 nm to 2.06 nm.The polyurethane-based SCA-LDH nanocomposites(WPU/(SCA-LDH))with all intercalated structures are prepared by physical blending method.The application results show that the retanned leather has the best thickening rate,softness,mechanical properties and flame retardancy.When the amount of LDH is 8%,the damage length reduces by 53.5%,and the LOI increases from 24.2%to 28.1%.Moreover,the best dyeing performance of crust leather is observed at the LDH dosage of 10%,and its dye absorption rate increases from 90.6%to 95.7%.According to the integrated comparison,WPU/(SCA-LDH)-8%has the best application performance.Finally,the application performance of commercially available WPU,WPU/LDH,(SCA-WPU)/LDH and WPU/(SCA-LDH)retanned leather are compared across the board.The sequence of comprehensive performance is:WPU/(SCA-LDH)>WPU/LDH>(SCA-WPU)/LDH>commercially available WPU.