Design, Preparation And Properties Of Chemical Moisture Resistant Films

Polyurethane foam（Polyurethane foam,PUF）is widely used because of its low relative density,high specific strength,good sound insulation and shockproof performance,low thermal conductivity,chemical corrosion resistance and excellent heat preservation performance.However,rigid PUF is extremely sensitive to moisture and slowly absorbs water vapor in the air,which plasticizes and expands and deforms the foam,resulting in poor dimensional accuracy and poor performance,which seriously affects the installation and usage stability.Therefore,there is an urgent need to construct a high moisture barrier interface to solve this problem.However,there are still many challenging technical problems and core basic scientific issues in the design of efficient moisture barrier for hydrophilic porous polymer materials,mainly including the factors influencing the moisture absorption of polyurethane foam,the process and mechanism of moisture absorption,and the need to build moisture barrier coatings based on the mechanism of moisture absorption.The research shows that there are no mature cases at home and abroad to learn from,so there is an urgent need to propose a design strategy for efficient moisture barrier interface,so as to provide a new idea for the design,performance improvement and safe service of hydrophilic porous polymer materials.To address the above-mentioned urgent scientific and technical problems,this study investigates the moisture-absorbing behavior,moisture-absorbing influencing factors and mechanisms of rigid polyurethane foam with the aim of constructing an efficient moisture-barrier coating on the surface of polyurethane,and constructs chemically reactive films and chemical-physical synergistic films on the surface of polyurethane foam based on the moisture-absorbing mechanism to study its moisture-barrier performance.The main research contents are as follows:（1）A series of rigid polyurethane foams with different compositions and densities were synthesized,and the effects of various influencing factors including chemical composition,microstructure,density,external temperature and humidity on the hygroscopic properties of polyurethane foams were investigated in detail,and the effects of hygroscopicity on the size and mechanical properties of polyurethane foams were systematically discussed.The results show that polyurethane foam is easy to adsorb water molecules in the air,and the experimental results of moisture absorption of foam show that the moisture absorption of foam can be roughly divided into three stages,which are rapid moisture absorption period,stable moisture absorption period and moisture absorption saturation period.In the initial stage of moisture absorption,the foam conforms to the second law of Fick.The mechanical properties of the foam were slightly reduced by moisture absorption,and the hydrophobically modified PU-5 had the lowest moisture absorption rate（0.616%）and the lowest dimensional change rate（0.118%）,but its compressive strength was poor,only 10.548 MPa;PU-6containing fluorosurfactant had the smallest reduction in compressive strength after moisture absorption（1.63%）,and the moisture absorption rate and dimensional change rate were0.738%and 0.168%;PU-4 containing ammonia ether cross-linker has the largest compressive strength,with moisture absorption rate and dimensional change rate of 0.818%and 0.150%,respectively,and the compressive strength decreased by 4.6%after moisture absorption.Considering that the application environment requires more important changes in dimensional and mechanical properties after moisture absorption,PU-4 is the best system.In addition,the moisture absorption rate increased significantly with the increase of temperature and humidity,and was three times higher when the temperature increased from 25℃to 45℃,and the same change was observed when the humidity increased from 40%to 80%.（2）The polyurethane foam contains more hygroscopic groups,such as-OH（free water）at3600 cm^-1,-OH（bound water）at 3330 cm^-1,-OH（free water）at 1640 cm^-1,HN-（bound water）at 3298 cm^-1and-C=O（bound water）at 1704 cm^-1.In addition,low field NMR shows that the HN-（bound water）and-C=O（bound water）at 1704 cm^-1are changing in the foam under the influence of moisture,and the sequence of changes is 3298 cm^-1→3330 cm^-1→3600 cm^-1→1640 cm^-1→1704 cm^-1.The foam was found to contain four states of water by low-field NMR,with the highest proportion of bound water in the initial stage and the highest proportion of strongly bound water in the hygroscopic saturation stage.It was found that the hygroscopicity of PUF mainly comes from the formation of hydrogen bonds between oxygen atoms and water molecules through theoretical simulations.In the initial stage of moisture absorption,water molecules are very likely to enter the porous PU block and accumulate on the PU block surface at a low rate.After the water covers the surface of the porous block（including the top and inside of the PU block）,the water molecules will accumulate more water molecules through water-water interactions.Therefore,moisture absorption saturation is reached.（3）Chemically reactive moisture barrier films were successfully prepared using the spraying method.The results showed that the best performance of the films was obtained when the magnesium oxide content was 5%and the film thickness was 60μm.The tensile strength of the films decreased by 51.88%,Young’s modulus by 25.93%and elongation at break by 53.80%after incorporation of chemically reactive fillers.Finally,the coating was sprayed on the surface of polyurethane foam and tested for moisture permeability after dry encapsulation treatment,and it was found that the chemical moisture barrier coating could reduce the moisture permeability of polyurethane foam from 27.66%to 22.02%.（4）On the basis of PU+5%MgO film,the moisture barrier films with synergistic effect of chemical reaction and physical barrier were successfully prepared by doping graphene oxide（GO）and modified graphene oxide（hGO）as fillers,respectively,and the results showed that the moisture permeability of the films was minimized and the moisture barrier performance was best when the content of GO physical barrier filler was 0.5%.Both of them can improve the mechanical properties of the film while making the moisture permeability of the coating decrease.Compared with the blank foam,PU+5%MgO+0.5%GO coating decreased the moisture permeability of the foam by 41.43%,and PU+5%MgO+0.5%hGO coating decreased the moisture permeability of the foam by 32.07%.However,the addition of GO and hGO reduces the adhesion of the coating to the substrate,and the scratches of the GO-infused coating are deeper and more severe than the peeling of the GO-infused coating.