| Acorn shell is the outer shell of the fruit acorn of Fagaceae plant,also know as oak cup.Acorn shell are rich in resources,with annual output of 6 million to 7 million tons,but they have never been effectively utilized.When the acorn fruit matures,a large number of acorn shells are abandoned,become forestry waste,resulting in serious waste of resources.There are many polyphenols such as tannin in acorn shell,which can be used as raw materials for the preparation of biomass-based wood adhesives.It can not only promote the multifunctional utilization of acorn shell resources,but also reduce the dependence of traditional wood adhesive industry on fossil resources.At the same time,it is of great significance to the income growth of farmers in mountainous areas.In this paper,sawtooth oak(Quercus acutissima Carruth.)hopper shell was taken as the research object,and its application in phenolic resin adhesive and protein-based adhesive was explored.(1)Liquefaction and degradation of acorn shell at different liquid to material ratios(phenol/acorn shell)resulted in liquefied products with substitution rates of 10~40wt%.The liquefied products were used as the raw materials to synthesis the acorn shell-based phenolic resin(APF).(2)The potential of AR resin as a raw material for the modification of proteinbased adhesives was investigated by synthesising an acorn shell-based resin(AR).The properties of the adhesives were characterised using FTIR,13C-NMR,TG,DSC,XRD,SEM.The main research contents and achievement are as follows:(1)The FTIR results showed that the small molecules of phenolic rings in acorn shell could participate in the reaction system of phenol and formaldehyde after liquefaction and degradation by phenol,formed a co-condensation structure linked by methylene bridge bonds(-CH2).The DSC results showed that the curing temperature of the 20wt% substitution rate APF resin was reduced from 135.71 °C to 132.78 °C compared to the pure phenolic resin.The bonding strength and formaldehyde emission tests showed that the bonding strength of APF resin with 20~40wt % substitution rate was greater than 0.70 MPa,and the formaldehyde emission was less than 0.50 mg/L,which met the requirements of Class I plywood.13C-NMR and FTIR results showed that in the process of 40wt%APF resin synthesis,after adding different metal ion catalysts(Na+,Zn2+,Mg2+),the reaction activity of the ortho-benzene ring in the resin synthesis system was improved,and the adjacent hydroxyl methylation reaction and condensation reaction were promoted.The gel time and DSC results showed that the curing rate of 40wt%APF resin with metal ion catalyst increased,and the curing temperature decreased from 140.18 °C to 133.28 °C,137.42 °C and 126.26 °C,respectively.The bonding strength increased from 0.85 MPa to 1.17 MPa,1.04 MPa and 1.13 MPa,respectively.(2)The soybean isolate protein adhesive(SPI)was cross-linked with the AR resin synthesised from acorn shells.FTIR and SEM analysis showed that the addition of AR resin could destroy the secondary structure of protein to a certain extent,and react with the active groups(-NH2,-COOH,-OH,etc.)on the molecular chain of soybean protein to form a crosslinked structure,thereby enhancing the water resistance of the SPI adhesive.XRD results showed that when the addition amount of AR resin was 20wt%(based on the mass of soybean protein isolate),the protein peptide chain was expanded,the crosslinking density increased,and the crystallinity of the modified adhesive decreased from 17.46% to 14.47%.SEM results showed that after the addition of AR resin,the gap on the fracture surface of the adhesive decreased,which could effectively reduce the water invasion and improve the water resistance of the adhesive.The test results of adhesive residual rate and bonding strength showed that the water-resistant bonding strength of different modified adhesives met the requirements of Class II plywood(≥0.70 MPa).The water-resistant bonding strength of the adhesive with20wt%AR addition was the highest,from 0.55 MPa to 1.07 MPa.These results indicate that acorn shell has great potential as a modified raw material for protein-based adhesive.(3)A synergistic modification of an inexpensive soybean meal protein adhesive(SM)with AR resins,chemical denaturants and epoxy compounds was carried out.FTIR results showed that the amide characteristic peak of the SM adhesive protein peptide chain was displaced after the addition of Na OH denaturant.XRD results showed that the addition of Na OH and AR resin disrupted the α-helical and β-folded structures in the protein structure,and the crystallinity of the modified adhesive(SM/Na OH/AR)decreased from 16.76% to10.73% compared to the pure SM adhesive.It was shown that the folded and distorted peptide chains of the protein were broken by Na OH,which could expose more active functional groups to react with the AR resin and increase the crosslink density of the adhesive.The results of adhesive strength and residual rate showed that the residual rate and adhesive strength of modified adhesive increased from 70.28% and 0.35 MPa to 82.23% and 1.14 MPa respectively after adding sodium hydroxide and AR resin into soybean meal protein adhesive.On top of this,the addition of epoxy compounds(CA)has again improved the performance of the adhesive,with SM/AR and SM/Na OH/AR bonding strengths increasing to 1.16 MPa and 1.17 MPa respectively. |
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