Optimized Preparation Of PAE Resin And The Structure-activity Mechanism For PAE Modified Formaldehyde-free Wood Adhesives

China is the largest producer and consumer of wood-based panels in the world,and the adhesives for wood-based panels are still in the majority of formaldehyde-based adhesives.There is an urgent need to develop and apply high-quality aldehyde-free adhesives and panels,because of the 14 th Five-year Plan put forward construction of healthy China.Currently,the adhesives used in the production of formaldehyde-free wood-based panels are mainly soybean protein adhesives and isocyanate adhesives.The soybean protein adhesives is a biomass-based adhesives prepared by extracted soybean meal.However,due to the unsatisfactory bonding strength,water resistance and mildew resistance,the application of soybean protein adhesives in wood-based panels industry is limited.In addition,the isocyanate adhesives exhibit disadvantages such as high cost,high activity,complex production technology and equipment requirements,which restricting the application of isocyanate adhesives as the substitute for formaldehyde-based adhesives in a large scale.In view of the deficiencies of soybean protein adhesives and isocyanate adhesives,this study focused on the chemical structure design,industrial preparation and cost optimization of PAE resin modifier for formaldehyde-free wood adhesive,including the ECH dosage,the type of termination acid,the ratio of acid to amine and the degree of polymerization of PAA prepolymer.As a result,two kinds of high performance and low cost formaldehyde-free wood adhesives with PAE resin as the core modifier were prepared and the synergistic mechanism of PAE resin which is the cross-linking agent for substance.The detailed research contents and results are shown as following:(1)The amount of ECH has an important effect on the structure and properties of PAE resin and PAE-modified soybean protein based adhesives.The efficient crosslinking between PAE resin and soybean protein depends on the amount of azetidine group which was formed by ECH through alkylation.The insufficient ECH could not completely react with all the free amino groups of PAA prepolymer,resulting in low content of azetidine group,large amount of terminate acid and poor water resistance of modified soybean adhesives.Increasing the ECH dosage could effectively increase the content of azetidine group in PAE resin,improve the crosslinking activity of modifier,and enhance the water resistance of modified soybean adhesives.The type of terminated acid affects its reaction efficiency with amino groups.The PAE resin with the highest content of azetidine group when ECH dosage was 45%,because of the high acidity of sulfuric and combined ability with amino groups.For formic acid,due to the weak acidity,more acid dosage was needed in reaction,and when ECH dosage was 50%,the optimal modification effect could be obtained.Moreover,the formic acid as mildew inhibitor could protect PAE modified soybean protein adhesive from mold infection in 15 days.The aged wet bond strength of plywood prepared by F-PAE50 modified soybean protein adhesive reached 1.53 MPa,which was 39.1% higher than that of commercial PAE modified soy glue.(2)Increasing the AA dosage in the synthesis of PAA prepolymer can further reduce the synthesis cost on the basis of ensuring the modification ability of PAE resin.The content of azetidine group and p H value in PAE resin shown a trend of continuous decrease with increasing AA dosage.More AA improved the storage stability of PAE resin,but reduced the bonding performance of modified soybean adhesive.The excessive AA in PAE45 could effectively unfold the spherical structure in soybean protein to release the embedded active groups,which increasing the chance of chemical reaction between internal groups in protein and modifier,and ensuring the bonding strength of cured adhesive layer.The results showed that the ratio of DETA/AA was 4/5,which was the extreme point.At this time,the bonding property of the modified soybean adhesive could meet the performance requirements of Class Ⅰplywood,and the boiled wet bond strength reached 1.16 MPa.Meanwhile,the economic costs of PAA45 prepolymer,PAE45 resin and modified soybean adhesive were 6 080CNY/t,2050CNY/t and 3 175CNY/t,and reduced by 5.37%,14.58% and 9.29%,respectively.(3)On the basis of the most economical raw material ratio(DSTA/AA was 4/5),increasing the degree of polymerization of PAA prepolymer was contribute to increase the molecular weight of PAA resin and the branching coefficient,and generates a significantly affected of the content of active groups and modified ability of PAE resin.Increasing the holding temperature and prolonging the holding time could improve the polymerization degree of PAA prepolymer,lengthen the molecular chain,increase the molecular weight and the viscosity of PAA resin,reduce the degree of self-polymerization of PAA resin,which intensify the continuous collision of active terminal groups and increase the amount of azetidine groups and improve the modification ability of PAE resin.However,the PAA prepolymer branched because of over polymerization,resulting in inhibited the effective grafting of ECH and reduced the concentration of azetidine group.At the same time,the excessive viscosity inhibited the effective collision between modifier and soybean protein,leading to a significantly decreased water resistance of modified adhesive.The PAA-H20 prepolymer prepared at 175-180℃ for 20 min had the most appropriate polymerization and molecular weight,and the PAE-H20 resin modifier with the highest content of azetidine in the further synthesis.Thus,the modified soybean protein adhesive could use as a structural wood adhesive,and the soaked wet bond strength of plywood was 1.99 MPa,which was 15.7% higher than that of commercial sample.(4)PAE resin,diphenylmethane diisocyanate(p MDI)resin and montmorillonoid(MMC)filler were employed to constitute MPM adhesive which is a new low-cost water-based isocyanate adhesive.MPM adhesive could rapidly be cured under the thermal process to form a dense and stable glue layer via both chemical cross-linking and organic-inorganic hybrid.The modifier could improve the crosslinking density and heat resistance by chemical cross-linking between PAE resin and p MDI resin.Furthermore,the PAE resin with long straight chains and low branched degree could inhibit the reaction of p MDI with water to prolong working life(>5h).The incorporation of MMC filler into PM resin was not only reduce the cost but also improved the hygrothermal resistance and bonding durability via organic-inorganic hybrid.In addition,the over-penetration of PM resin on the wood surface was prevented due to the MMC clay embedded into the adhesive layer.The plywood bonded by MPM adhesive withstood 7hygrothermal aging cycles,the aged wet bond strength reached 1.73 MPa with a strength reduction ratio of 18.4% and the cost was 4 000CNY/t.