Study On Preparation And Application Of Aldehyde-free Wood Adhesive Based On Maleamic Acid Copolymer

China is the largest country of wood-based panel（WPB）production,consumption and international trade in the world,which accounts for more than50%of the world’s annual WPB production.Nowadays,formaldehyde-based resin is heavily used in the production of WBP,which is the primary source of indoor formaldehyde pollution and severely affects people’s health,work and living quality.As the related regulation on indoor formaldehyde content became increasingly stringent,the development of formaldehyde-free adhesive is of great significance to solve the problem of formaldehyde pollution of WBP.Furthermore,it can promote the transformation and upgrading of WPB industry of China and improve its competitiveness.Although a variety of formaldehyde-free adhesives have been developed,including soybean protein,starch,isocyanate,thermoplastic resin,waterborne polyurethane,epoxy resin,lignin adhesives and inorganic adhesives,there are still some problems,such as high cost,unsatisfactory bonding performance and complex gluing process.Based on the mentioned background,this thesis proposed the preparation of maleic anhydride copolymer microspheres by self-stabilized precipitation polymerization,and then prepared two kinds of water soluble maleamic acid copolymer through gas-solid reaction by using the characteristics of its high specific surface area and porous structure.After systematically studying the reaction feature of conversion of maleamic acid groups or cleavage of weak bond in main chain of these two kinds of copolymer at high temperature,two water-based formaldehyde-free adhesives with low production cost and simple gluing process were developed.The main research contents and results of this thesis are as follows:1.A method for preparing maleimide copolymer microspheres from maleic anhydride copolymer microspheres by solvent-free reaction was developed.Firstly,monodisperse styrene-maleic anhydride（SMA）microspheres with diameters of 226 nm,452 nm,543 nm and 728 nm were prepared by self-stabilized precipitation polymerization.Then,three kinds of styrene-maleamic acid copolymer（SMAA）microspheres were synthesized by gas-solid reaction using gaseous ammonia,methylamine and ethylamine as ammoniating reagents,which were noted as NH₃-SMAA,N-methyl SMAA and N-ethyl SMAA,respectively.The results show that the gas-solid reaction rate is affected by the specific surface area,pore size distribution of SMA microspheres and molecule size of gaseous amine.When the particle size of SMA microspheres is 226 nm or NH₃ is used as reagent,the gas-solid reaction can be finished within 5 min.However,for the agglomerated SMA particles that prepared by solution polymerization,100%ammoniated product could not be obtained even if the reaction time was extended to 2 hours.Styrene maleimide copolymer（SMI）microspheres were prepared by further heating SMAA microspheres.The imidization of SMAA microspheres is affected by whether the system is hermetic,the type of branched chain connecting to amide group and microspheres size.In hermetic system,when SMAA microspheres were heated to 140℃,the product was SMI with excellent heat resistance（imidization degree over 98%）,while in the open system,it was a mixture of SMA and SMI.The higher the temperature,the faster the reaction rate.The imidization kinetics curve showed that N-methyl SMAA had the fastest imidization rate when heated in a hermetic environment.At 180℃,the maleamic acid group in N-methyl SMAA could be completely converted to the imide group within 4 hours,while NH₃-SMAA had the slowest rate,and the reaction time needed to be extended to about 6 hours.Compared with SMA microspheres,SMI microspheres have better thermal properties.The glass transition temperature of NH₃-SMI is 240℃,and the fastest thermal decomposition temperature of N-ethyl SMI is 445℃,which is much higher than the glass transition temperature（210℃）and the fastest thermal decomposition temperature（390℃）of SMA.The solubility test showed that N-methyl SMI and N-ethyl SMI could be dissolved in acetone,but NH₃-SMI could not be dissolved in acetone.The imide group in NH₃-SMI could form an intermolecular H bond with water molecule,allowing it to be dissolved in water/acetone mixture（mass ratio=1/5-2/5）.The static water contact angle of SMI film is between 86°and100°,which is higher than that of SMA film（68°）.This method for the preparation of SMI microspheres is simple,low-cost and solvent-free,which is expected to be applied to heat-resistant coatings and composites.2.A water-soluble green aldehyde-free adhesive for WBP production based on maleamic acid copolymer was developed.The reaction mechanism of styrene-maleamic acid copolymer（SMAA）with wood during hot pressing was studied.The results of FTIR,acid-base titration and water contact angle measurements show that SMAA can simultaneously generate SMA and SMI under hot pressing condition,and the higher the temperature,the faster the reaction rate.When dried SMAA was pressed at 180℃for 10 min,most of amic acid groups（85%）in maleic acid copolymer were converted to imide groups,and a small part of them（15%）were converted to anhydride groups.However,when pressing dried SMAA at 160℃or 140℃for 10 min,the degree of imidization of amic acid groups decreased to 38%and 8%,respectively,and no anhydride reaction occurred.The aqueous solution of SMAA（25%wt.）was also hot-pressed at 180℃,160℃and 140℃for 10 min.It was found that the presence of water could reduce the temperature of anhydration.At 160℃,14%of amic acid groups converted into anhydride groups,and this value increased to27%at 180℃.The formed anhydride groups can react with hydroxyl groups in wood to become the covalent binding site between polymer and wood.At 160℃and 180℃,70%and 80%of anhydride groups became ester bonds,respectively.The water contact angle measurement showed that the water contact angle of particle board surface would increase with the increase of pressing temperature,which was 105°at 140℃,113°at 160℃,and 118°at 180℃.Cross section SEM,ATR-FTIR and water contact angle test of fiberboard show that there is a 30μm waterproof surface layer coated on fiberboard,which can effectively prevent the infiltration of water and give WPB excellent waterproof performance.Finally,the effects of pressing temperature and polymer content on the properties of WPB were systematically investigated.When the content of SMAA is 15%of the mass of fiber or shavings,MDF and particleboard that meet can the national standard requirements were successfully produced after hot pressing at 180℃for 5 min.The internal bonding strength of fiberboard is 0.66 MPa,the internal bond strength after boiling for 2 h is 0.37 MPa,and the thickness expansion rate after soaking in water at room temperature for 24 h is 13%.The internal bonding strength of particleboard is 0.7 MPa,and the internal bond strength after boiling for 2 h is 0.15 MPa.The thickness expansion rate after soaking in water at room temperature for 2 h and 24 h is 6%and 18%,respectively.When the sizing amount of SMAA for plywood is 30 g/m²,which is much lower than that of urea formaldehyde resin（>70 g/m²）,the dry bonding strength of plywood is 1.15MPa,the wet bonding strength is 0.93 MPa,still superior to the national standard requirement of 0.7 MPa.Therefore,in view of the low production cost and sizing amount,maleamic acid copolymer is expected to be used as a new type of aldehyde-free wood adhesive to replace urea-formaldehyde resin in production of various WBP.3.A new method for preparation of core-shell,vesicle-like and rod-like particles based on maleamic acid-α-methyl styrene copolymer（MAA-alt-AMS）macro-initiator was developed,and the core-shell nanoparticles were successfully used for cold bonding of wood.Firstly,MAA-alt-AMS macro-initiator with emulsification and initiation was synthesized.Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry（MALDI-TOF MS）analysis verified the existence of"AMS-AMS"units in MAA-alt-AMS.The free radical trapped experiment confirmed the ability of"AMS-AMS"units to break and generate radicals when MAA-alt-AMS was heated above 70℃and the proportion of"AMS-AMS"units in total units was 2.71%.The decomposition activation energy of MAA-alt-AMS is 62.81 k J/mol,which is lower than that of traditional thermal initiators.Then,the emulsion PISA of butyl acrylate（BA）and methyl methacrylate（MMA）was successfully initiated by MAA-alt-AMS,and nanoparticles with MAA-alt-AMS as shell and BA-ran-MMA as core were obtained.The nanoparticles had good homogeneity and very low dispersion coefficient（PDI<5%）.Moreover,by adding ethanol to reaction medium and adjusting its p H to alkaline,vesicle-like and rod-like heteromorphic particles can be obtained.The effects of monomer ratio,MAA-alt-AMS amount,reaction temperature and system concentration on the polymerization process were studied in detail.Increasing the proportion of MMA,the amount of MAA-alt-AMS,reaction temperature and system concentration can improve the polymerization yield,especially when MAA-alt-AMS is 40%of the monomer mass and the system concentration is 50%,the polymerization yield is close to100%.TEM,DLS and GPC tests were used to track the growth process of nanoparticles.It was found that nanoparticles grew rapidly before 60 min and could grow to 160 nm.After 60 min,particle size only increased by 15 nm.The molecular weight of nanoparticles also increased with the reaction time.DSC test shows that there are two glass transition temperatures of nanoparticles,one is from MAA-alt-AMS shell,which is 162℃.The other is derived from the BA-ran-MMA nucleus,which can be adjusted by adjusting the ratio of BA/MMA（-21℃to 50℃）.The lapped-shear experiment confirmed that the nanoparticles had a good ability to bonding wood.At room temperature,the nanoparticles with BA/MMA=1/1 had the best wood bonding performance,and the dry bonding strength was up to 3.5 MPa.