| In recent years,the wide application of formaldehyde-based adhesives has not only aggravated society’s dependence on fossil resources,but also caused a series of environmental problems such as formaldehyde release.Looking for green and renewable biomass resources to produce adhesives has become a research hotspot.As a widely used biomass based adhesive,defatted soybean flour based adhesive has some shortcomings such as poor water resistance and gluing performance.To solve these problems,the preparation of biobased adhesives with good water resistance and high bonding strength has practical significance.In this thesis,DFX-DSF derived adhesive was firstly prepared from diformylxylose(DFX)and defatted soybean flour(DSF).The influence of DFX two-step pre-reaction conditions(pre-reactionⅠ,pre-reactionⅡ,adhesive p H value)on the gluing properties of DFX-DSF derived adhesives was investigated,and further the pre-reactionⅠ(DFX amount,reaction time,reaction temperature,acid amount),pre-reactionⅡ(reaction time,reaction temperature,formaldehyde:urea molar ratio),and hot-pressing conditions(hot pressing time,hot pressing temperature,glue amount)were also optimized.Finally,the DFX pre-reactionⅠconditions was determined as follows:5 g DFX,5 g deionized water,42.5μL 37%HCl,170°C,35 min.DFX Pre-reactionⅡconditions were:0.35 g urea(formaldehyde:urea molar ratio,10:1),30°C,60 min.Hot pressing conditions:160°C,1.2 MPa,10 min and glue amount 250 g/m~2.According to these conditions,the dry and wet strength of the plywood reached 1.2MPa and 1.3 MPa,respectively,which meet the national standard of class II board(wet strength is higher than 0.7 MPa).The characterization of gas chromatography-mass spectrometry showed that in pre-reaction I,DFX was firstly converted to xylose through acid catalyzed formaldehyde removal,and then converted to furfural.Finally,furfural was condensed to form a prepolymer.Fourier transform infrared spectroscopy characterization showed that the prepolymers contained a large number of aldehyde groups.Therefore,in pre-reactionⅡ,urea could react with the prepolymers to form polymers through nucleophilic addition reactions.The residual aldehyde groups in such polymer could further crosslink with active groups such as amine groups in DSF during hot pressing to form a network structure,which can improve the water resistance,bonding performance and thermal stability of the adhesives.X-ray photoelectron spectroscopy analysis showed that,compared to prepolymers,there were more C-O bonds in DFX-DSF adhesives,which was beneficial to building more hydrogen bonds between adhesive and polar groups on the surface of the veneers,enhancing the bonding strength of the plywoods.The characterization of adhesive glue lines indirectly proves that DFX-DSF derived adhesive has certain fluidity,which can penetrate into the cracks of wood boards and then cure to form adhesive nails,which will firmly bond plywood together.However,the formaldehyde emission of the plywood prepared by DFX-DSF derived adhesive only reached the E2 level of the national standard of GB/T 9846.3-2004,and the mildew resistance level was the4th level of the building materials industry standard(JC/T 2039-2010),which limited its application in indoor environment.Second,amine and polyvinyl alcohol(PVA)were screened as additives of DFX-DSF derived adhesives in this thesis.To reduce the content of free formaldehyde in the adhesives and retain the mechanical properties of adhesives,melamine was selected as an appropriate additive.Subsequently,the pre-reactionⅡ(reaction time,reaction temperature,formaldehyde:amine additives contain amine groups molar ratio)and hot-pressing conditions(hot pressing time,hot pressing temperature,glue amount)were optimized.The appropriate pre-reactionⅡconditions were determined as follows:0.35 g urea,0.24 g melamine(formaldehyde:melamine contain amine groups molar ratio,10:1),60°C,120 min.Hot pressing conditions were:160°C,1.2MPa,8 min and glue amount 250 g/m~2.According to the above conditions,the dry and wet strength of the plywood reached 1.2 MPa and 1.3 MPa,respectively.Meanwhile,the formaldehyde release of the plywood met the E1 level in the national standard of GB/T 9846.3-2004,and the mildew resistant grade was the 4th level in the building materials industry standard(JC/T 2039-2010).Corresponding plywoods were suitable for interior decoration.In pre-reactionⅡ,melamine could react with formaldehyde to form hydroxymethyl compounds through nucleophilic addition.Hydroxymethyl compounds further condensed with hydroxymethyl urea to form a polymer with the chemical structure similar to melamine-urea-formaldehyde(MUF)resin.As such,more C-C and C=C bonds were introduced to DFX-DSF derived adhesive,which eventually improved thermal stability and adhesive performance of the adhesive.Because of the possessing of a large number of hydroxyl groups,PVA are easy to react with formaldehyde to form hemiacetal,accomplishing the task of capturing free formaldehyde in the DFX-DSF derived adhesive.However,the stability of ether bond in the structure of acetal is poor,and it is easy to decompose and release formaldehyde.Finally,the mass ratio of DFX to DSF in adhesives was regulated in this thesis,and the mass ratio of DFX to DSF was determined to be 1 according to the gluing properties and formaldehyde emission degree of adhesives.On this basis,the hot-pressing conditions(hot pressing pressure,hot pressing temperature,hot pressing time and sizing amount)of DFX-DSF derived adhesives were regulated.Considering the bonding strength and formaldehyde release amount of plywood,the hot-pressing conditions were determined as 1.4 MPa,120°C,8 min and glue amount of 140 g/m~2.The dry strength and wet strength of the plywood prepared under the above conditions reached 2.8 MPa and 1.1 MPa,respectively.The formaldehyde release reached the E1level in the national standard of GB/T 9846.3-2004,and the mildew resistance grade was the 3rd level in the building materials industry standard(JC/T 2039-2010).When the mass ratio of DFX to DSF was 1,the DFX-DSF derived adhesive performed better fluidity.At the same time,the adhesive could effectively penetrate into the cracks of the wood board,and cure into a firm adhesive nail during hot pressing,which guaranteed better bonding strength.The aldehyde groups in the prepolymers could react with the amine group in DSF through the Schiff base reaction,which can not only improve the water resistance of the adhesive,but also increase the cross-linking networks in the adhesive.In general,DFX-DSF derived adhesives with good bonding performance and low formaldehyde release were prepared through a two-step pre reaction using DFX and DSF as raw material and melamine as additive.The characterizations of DFX-DSF derived adhesive and plywoods showed that the aldehyde and amine groups in the adhesive formed a cross-linked network with water resistance,and the adhesive infiltrated into the cracks of the wood boards and cured as adhesive nails,which provided good mechanical properties of the plywoods.The plywoods prepared by DFX-DSF derived adhesives met the national standard of class II board,and have a low degree of formaldehyde release and certain anti-mildew characteristics.In general,DFX-DSF-derived adhesives not only improve the utilization value of biorefinery and oil industry by-products(DFX and DSF),the prepared plywoods also have certain application potential,which provides a theoretical basis for the efficient preparation of new bio-based wood adhesives. |
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