Preparation And Properties Of Carbon Dioxide Based Waterborne Polyurethane Adhesive

As an important part of veneer wood-based panel processing industry,adhesive environmental protection and their properties are crucial for veneer processing industry.The environmental pollution caused by veneer wood-based panel mainly comes from the veneer fabric and adhesive,in which the amount of adhesive is far more than that of fabric and it is the main source of pollution.At present,formaldehyde"trialdehyde adhesive"is still dominant for decorative wood-based panels due to its high cost performance,and the prepared products have the risk of releasing free formaldehyde and volatile organic compounds.Formaldehyde-free aqueous polymer-polyisocyanate API adhesive is limited in practical use due to short construction period,and its price is high;The relatively environmentally friendly bio-based adhesives not only have the problem of poor water resistance,but are still in the research stage in the field of wood-based panel coating.There is a lack of systematic research on the development of core resins for environment-friendly adhesives used in wood-based panel finishes.With the rapid development of wood-based panel industry,the demand for adhesives is also increasing,and the problems of excessive resource consumption and environmental protection caused by rapid economic development are becoming increasingly prominent.Carbon dioxide is a renewable,abundant and inexpensive source of carbon.The efforts of scientific researchers make the preparation of biodegradable CO₂-based oligomer polyol（CO₂-polyol）become a mature technology,which provides the possibility of extending the application of CO₂to the polyurethane industry.Water-borne polyurethane with high solid content was prepared by using CO₂-polyol as the soft part of polyurethane.The low temperature film formation modification of ester-based waterborne polyurethane was realized by using CO₂-polyol with amorphous structure.The mesh structure of CO₂-based ternary alcohol was used to toughen the linear ester based waterborne polyurethane.The effects of the dosage of modifier and the content of hydrophilic base on the properties of the emulsion and film were discussed.The adhesive properties of various emulsions as the main resin used in veneer wood-based panels were investigated.This study aims to reduce the greenhouse effect of CO₂and also reduce the dependence of raw materials on petrochemical resources,in order to prepare water-based polyurethane adhesives that meet certain bonding needs and are environmentally friendly and biodegradable with low aldehyde.The main research results are as follows:（1）CO₂-based waterborne polyurethane resin with solid content up to 52wt%and viscosity as low as 142 m Pa.s was prepared using CO₂-based diol as soft segment.The average particle size of the emulsion was 200nm and the emulsion was stable after 6 months at room temperature.Compared with polyester waterborne polyurethane（PBA-CWPU）,CO₂-based waterborne polyurethane shows better water resistance,hydrolysis resistance and oxidation resistance.This special performance is due to the synergistic effect of carbonate bond and ether bond in the polymer structure.It was found that the particle size and size distribution of the emulsion and the mechanical properties of the film could be adjusted by adjusting the loading amount of hydrophilic group.In addition,the amorphous structure of CO₂-based diols leads to the relatively weak initial adhesion of CO₂-based waterborne polyurethane adhesives,and the initial adhesion is relatively low for the MDF of PVC film vacuum blist-forming finishes.（2）A series of polyester waterborne polyurethane resins were prepared with CO₂-diols as modifier.The effects of CO₂-diols on the dispersion and stability of the emulsion were studied by Zeta potential,particle size change,appearance and storage stability of the emulsion.The results showed that the mixture of CO₂-diols greatly improved the dispersion and stability of polyester waterborne polyurethane emulsion,and no precipitation was found after 6 months at room temperature.The study of film formation showed that the introduction of CO₂-diols broke the regular structure of polyester polyols,and the film formation performance of the film at low temperature was excellent.The complete film could be formed by mixing 5wt%CO₂-diols at low temperature of 3℃.The above results are attributed to the soft ether bond and low glass transition temperature of CO₂-diols.In addition,compared with polyester water-based polyurethane,The resin modified with CO₂-diols not only has excellent water resistance,hydrolysis resistance and other properties,but also has a higher initial adhesion,which is expected to be used in the plastic absorbing process of decorative wood-based panels.（3）A series of polyester waterborne polyurethanes were prepared with CO₂based ternary alcohol as modifier.The toughening modification,film formation modification and water resistance modification of linear polyester waterborne polyurethane were realized by using the network structure of CO₂-based ternary alcohol.The mechanical properties showed that with the addition of ternary alcohol,the fracture mechanism of the film gradually changed from brittle failure to ductile failure,and the toughness of the film was enhanced.The elongation at break of the film was up to 995%when the ratio of ternary alcohol was 10wt%.The AFM results showed that the composite ratio of 6wt%showed the most uniform microstructure,the highest value of the film was only 21.9nm,and the root mean square roughness Rq was0.129nm.In terms of water resistance improvement,the water absorption rate of the CO₂-based trianol modified film with the same proportion of 2wt%was only 64.2%after soaking in deionized water for 120 min,which was lower than that of the CO₂-diol modified film（73.6%）.In addition,compared with CO₂-diol modified waterborne polyurethane rein,the waterborne polyurethane resin modified with CO₂-based ternary alcohol can obtain higher initial adhesion,which can ensure its application in the plastic-absorbing process of wood-based panels.（4）The adhesives with various emulsions as the main resin were respectively applied to PVC film veneer plywood and PVC film vacuum veneer MDF,and the results showed that the bonding effect was good.The test results of surface bonding strength of PVC film veneer plywood are as follows:Among CO₂-based waterborne polyurethane adhesives,In terms of the influence of isocyanate,only HDI-WPU is relatively low,which fails to meet the standard requirements（LY/T 1279-2020）;In terms of the influence of hydrophilic content,the bonding strength of CO₂-WPU-4 and CO₂-WPU-5 were 0.59MPa and 0.65MPa,respectively,which met the standard requirements.The adhesive strength of adhesives modified with CO₂-diol was up to the standard requirements.The surface adhesive strength of 30wt%CO₂-diol combined PU₃₀CO₂to PVC film veneer was up to 1.13MPa,which was 135%higher than that of unmodified PU₀CO₂adhesive（0.48MPa）.Similarly,the surface bonding strength of the series of adhesives modified with CO₂based ternary alcohol to the veneer plywood exceeds the standard requirements.The bonding strength test results of PVC film vacuum finishing MDF are as follows:in terms of the influence of isocyanate,only MDI-WPU adhesive on the surface bonding strength of PVC film vacuum plastic absorbing finishing MDF reaches the standard requirements（0.56mpa）;The surface bonding strength of PU₅CO₂,PU₁₀CO₂,PU15CO₂and PU₂₀CO₂adhesives to PVC film vacuum plasticizing MDF were 0.52MPa,0.57MPa,0.61MPa and 0.63MPa,respectively,which met the standard requirements.The surface bonding strength of adhesives modified with CO₂-based ternary alcohol to PVC film vacuum plastic-absorbing medium fiber board was 0.65MPa,0.71MPa,0.75MPa,0.7MPa and 0.61MPa,respectively,which reached the standard requirements.