Preparation And Properties Of Polyvinyl Alcohol-based Porous Materials Cross-linked By Metal Oxides/Metal Ions

Polyvinyl alcohol（PVA）formaldehyde porous material（foam）has super water absorption capacity and water retention capacity.After water absorption,PVA porous material is soft,elastic and biocompatible.It is widely used in medical polymer materials and cleaning industry.In order to ensure the mechanical properties of PVA formaldehyde porous materials during the use process,adding excess paraformaldehyde as a crosslinking agent in the preparation process can be used as a crosslinking agent to cause condensation reaction.The resulting unwashed formaldehyde in the subsequent washing PVA foam is very difficult,and the formaldehyde residue in the foam is too high,which will cause great harm to the human body during the use.Therefore,the synthesis of a PVA composite porous material with low formaldehyde content,excellent water absorption,water retention and mechanical properties has important theoretical value and practical application value.Based on this,the acidic silica sol,nano Fe₃O₄ and other metal oxides are used as the green cross-linking agent of PVA condensation reaction to replace the interaction between partial paraformaldehyde and PVA macromolecules to form a cross-linked network structure.This greatly reduces the amount of paraformaldehyde.Later,the foam is easy to clean the formaldehyde,thereby preparing PVA composite porous material with low formaldehyde content,so that it can be used in the medical industry health-care and environmental protection.Meanwhile,new PVA composite porous materials were prepared by adding new surfactant and mechanical stirring foaming method instead of the starch foaming method widely used in the market.The latter postprocessing technology is complex,and the PVA foam is easy to generate bacteria.On this basis,the structure,properties and formaldehyde residue of PVA composite foam were characterized and tested.（1）Pure PVA formaldehyde porous material was prepared by adding new surfactant and mechanical stirring foaming method.The optimum preparation conditions of PVA formaldehyde porous material were determined by orthogonal experiment.Infrared spectrum shows that PVA reacts with formaldehyde and there are a large number of hydrophilic hydroxyl groups on the surface of the material.SEM shows that the material has interconnected 3D porous structure.The water absorption of PVA formaldehyde porous material is 851%,and it can hold water for 4h at a constant temperature of 50℃;The tensile stress-strain curve shows that the maximum tensile stress of the material is 0.208 MPa,the elongation at break is153.67%,and the recovery rate of the material after 10 compression cycles is 94.17%;Acetylacetone Spectrophotometric method was used to test formaldehyde residue in foam80.56mg/L.（2）Using acidic silica sol as a green cross-linking agent for PVA foams,instead of partial paraformaldehyde,PVA condensation reaction was carried out.The surface of Si O₂ in silica sol contains a large number of hydroxyl groups,which form a 3D network when heated under acidic conditions,and Si-OH can react with hydroxyl groups on PVA to form Si-O-C bond,so as to form a semi interpenetrating network structure.When the amount of acid silica sol is 10%,the performance of silica sol/PVA porous material is the best.SEM shows that silica sol/PVA porous material has interconnected 3D network porous structure,with pore size of 50-100μm and uniform distribution,which is less than 150-200μm of pure PVA formaldehyde porous material;The water absorption of silica sol/PVA porous material is 1042%,and it can hold water for 8h at 50℃;The tensile stress-strain curve and compressive stress-strain curve show that the silica sol modified material has better mechanical properties.Acetylacetone spectrophotometric determination of formaldehyde in foam was 34.23mg/L,which was significantly lower than that in pure PVA foam 80.56mg/L.（3）Nano Fe₃O₄ is used as a green cross-linking agent for PVA foams to replace partial paraformaldehyde for PVA condensation reaction.Nano Fe₃O₄ generates Fe³⁺in sulfuric acid（50wt%）system.In this way,Fe₃O₄ and Fe³⁺coordinate and complex with-oh on PVA molecular chain to form double crosslinking,which greatly improves the crosslinking degree of PVA macromolecules and reduces the amount of paraformaldehyde.SEM shows that nano Fe₃O₄/Fe³⁺/PVA composite foam has interconnected 3D porous structure with a pore size of50-100μm and relatively uniform,and the pore diameter is less than that of PVA formaldehyde porous material;When the paraformaldehyde dosage is 20%and the nano Fe₃O₄ dosage is 3%,the water absorption rate of the composite foam is 1131%,and the water retention is 8h at 50℃.The tensile stress-strain curves and compressive stress-strain curves show that nano Fe₃O₄/Fe³⁺/PVA composite foam has better mechanical properties.Acetylacetone Spectrophotometric method was used to test the residual formaldehyde content in foam 25.36mg/L,which was significantly lower than that in pure PVA foam 80.56mg/L.（4）Silica sol and nano Fe₃O₄ were used as green composite crosslinking agent to condense with PVA macromolecules to replace part of paraformaldehyde.The optimum preparation conditions of silica sol/Fe₃O₄/PVA porous material are as follows:Take 12.5g1888 PVA,add 87.5g distilled water,dissolve at 90℃until clear and transparent.After cooling,add 1.5g paraformaldehyde,5.0 g sulfuric acid（50 wt%）,0.1 g surfactant,10g HP4010silica sol and 3g nano Fe3O4.The foaming speed of high speed mixer is 3000 r/min,stirring time is 2 min,and the mixture is placed in the air blast oven at 50℃for 12 h.Acetal reaction.After curing,the porous silica sol/Fe3O4/PVA was obtained by washing with distilled water for3h in accordance with the actual industrial production process.SEM shows that the composite modified PVA porous material has interconnected three-dimensional porous structure,and the pore size distribution of the composite modified material is 500nm-50μm;The water absorption of the composite modified material is 1186%,and the water can be kept for 8.5h at a constant temperature of 50℃;The tensile stress-strain curve shows that the maximum tensile stress of the material is 0.273Mpa,the elongation at break is 169.8%,and the recovery rate of the material after 10 compression cycles is 97.83%;Acetylacetone Spectrophotometric method was used to test the residual formaldehyde content in foam 10.32mg/L,which was significantly lower than that in pure PVA foam 80.56mg/L.In conclusion,the acidic silica sol and nano Fe₃O₄are used as green crosslinking agents for the preparation of PVA composite porous materials,which not only solves the problem of large amount of formaldehyde in the traditional PVA foam,but also further improves the water absorption and water retention properties and mechanical properties of the materials.The addition of surfactant to the mechanical stirring foaming process is simple and has important application value.