Study On Blending Modification Of New Formula Urea-formaldehyde Resin And Isocyanate

Isocyanate is a typical aldehyde-free adhesive,in which the presence of the isocyanate group makes it highly reactive,its small molecular volume,easy to penetrate into the pores of the material,and can react with a variety of materials to produce chemical bonding,so it has a good water-resistant gluing performance,and in recent years in the man-made board industry is increasingly widely used.However,due to the expensive,gluing veneer easy to glue and sticky board and other issues,to a certain extent limit the application of isocyanate in the veneer type of man-made panels.Urea-formaldehyde resin is one of the most used formaldehyde adhesives,which has the advantages of low cost,simple synthesis process and fast curing speed,but its low water-resistant gluing strength,especially poor resistance to boiling water,limits its application in water-resistant gluing.This study combines the advantages of both resins,from the reaction mechanism of urea-formaldehyde resin and isocyanate,according to the structural characteristics and reaction properties of the two resins,design a synthetic process of urea-formaldehyde resin with high content of mono-hydroxymethyl urea,and prepare adhesives by blending the synthesized new process of urea-formaldehyde resin with isocyanate,and initially discuss the preparation and application process of the blended adhesives.The main research contents are:（1）to prepare the new urea-formaldehyde resin by changing the traditional material addition method,and to study its synthesis process and basic properties;（2）to study the effect of the molar ratio of formaldehyde to urea in the new process urea-formaldehyde resin on the basic properties,structure,thermal stability,microscopic morphology and gluing performance of the blended adhesive;（3）to study the effect of the addition ratio of the new process urea-formaldehyde resin on the basic properties,structure,thermal stability,microscopic morphology and gluing properties of the blended adhesive.（4）to study the basic properties,structure,thermal stability,microscopic morphology and bonding properties of the blended adhesive prepared from urea-formaldehyde resin under different synthesis processes by comparing with the urea-formaldehyde resin of conventional processes.The main findings are as follows:（1）The urea-formaldehyde resin synthesized by the new process has a higher content of monomethylurea compared with the conventional process urea-formaldehyde resin.By dehydration and drying,it can be blended with isocyanate to prepare stable blended adhesives.（2）When the addition ratio is the same,the higher the molar ratio of the new process urea-formaldehyde resin and isocyanate blends the greater the viscosity of the prepared adhesive.Infrared spectra showed that polyurea structures were generated in the blended adhesives.The molar ratio of 1.1 of the new process urea-formaldehyde resin blended with isocyanate prepared adhesives with the best resistance to boiling water gluing performance and the highest thermal stability.（3）When the molar ratio of new process urea-formaldehyde resin is 1.1,the viscosity of the adhesive prepared by blending with isocyanate increases and the permeability decreases with the increase of the addition ratio of new process urea-formaldehyde resin.Infrared spectra showed that the content of isocyanate groups in the cured blended adhesive decreased with the increase of the addition ratio of urea-formaldehyde resin of the new process;when the addition ratio was 20%and the hot pressing time was 6 min,the prepared plywood reached the maximum boiling water resistance bonding strength（1.69 MPa）,which exceeded the requirements of the national standardⅠplywood;the thermal stability of the blended adhesive under this preparation process was also the best;scanning electron microscopy The results show that the addition of new process urea-formaldehyde resin increases the toughness of isocyanate after curing.（4）When the addition ratio is less than 20%,the adhesive prepared by blending conventional urea-formaldehyde resin with isocyanate has higher boiling water strength than that prepared by blending new urea-formaldehyde resin with isocyanate;when the addition ratio is more than 20%,the result is the opposite.The reaction between conventional urea-formaldehyde resin and isocyanate starts after blending,which affects the stability of the blended adhesives and leads to shorter application period and increased process difficulty.Infrared spectroscopy showed that the water in the conventional urea-formaldehyde resin consumed part of the isocyanate group;the thermal stability of the adhesive prepared by blending the urea-formaldehyde resin with isocyanate in the new process was higher than that of the adhesive prepared by blending the urea-formaldehyde resin with isocyanate in the conventional process;the results of scanning electron microscopy showed that the CO₂generated by the reaction between the water in the conventional urea-formaldehyde resin and the isocyanate caused more pores in the adhesive layer after curing,which affected the adhesive strength and water resistance.In terms of production application,the new process of preparing adhesives by blending urea-formaldehyde resin with isocyanate can effectively solve the problems of high cost of isocyanate alone,the tendency of glue-through and sticky board in gluing veneer,and the low water resistance of urea-formaldehyde resin alone.When the molar ratio of urea-formaldehyde resin of the new process is 1.1 and the addition ratio is 20%,the overall performance of the blended adhesive is the best.If further control of production costs is considered,the addition ratio can be increased to about 30%.The blended adhesive in this study can effectively improve the board performance,reduce the cost,and be suitable for water-resistant gluing of veneer type boards in high temperature and high humidity environment,which can help to further expand the application scope of isocyanate in the production of wood-based panels and effectively enhance the application value of urea-formaldehyde resin.