Performance And Enhanced Mechanism Of Modified Soy Protein Adhesive

In order to improve the initial adhesion,water resistance and bonding strength of the soybean protein adhesive,a urea-formaldehyde resin(UF)with good initial adhesion was employed to modify soy protein isolate(SPI)adhesive in this research,the prepress and adhesion performance of resultant adhesives were measured.Furthermore,in order to the solve the formaldehyde emission issue,the condensed tannins(CTs)was used to mimicking mussel protein’s DOPA structure and brought catechol structures into a soy protein isolate(SPI)system to develop hexamine-modified,acid-modified and alkaline-modifeid adhesives.The chemical structur,thermal performance,morphology,and physical characteristics were tested to analyse the reaction and adhesion mechanism of the resulting adhesives.When using these adhesives to fabricate indoor plywood,the wet shear strengths were tested to evaluate the water resistance and adhesionperformances.The main conclusions were summarized as followed:(1)The prepress and wet shear strength of plywood bonded by SPI adhesive mixed with UF resin could be improved due to UF resin’s high initial adhesion.The adhesive cured by oxalic acid and ammonium chloride compounded curing agent had the highest wet shear strength after prepressing,which was 1.46MPa and increased by 192%compared to native SPI adhesive without prepressing.(2)The CTs were modified with hexamine to increase the adhesion performance of SPI adhesive and promote the crosslinking reaction between tannin molecules,which could enhance the crosslinking density of SPI adhesive system.Also the oxidized quinone structure reacted with peptide of SPI molecules and formed covalent bonds,which form a compact structure,good water resistance and thermal stability.The highest wet shear strength of plywood was 1.03 MPa,which increased by 91%compared to native SPI adhesive.(3)The acid-modified CTs possessed un-oxidized catechols in the B ring,which could be introduced into SPI molecules completely with the potential ability to enhance bonding strength of the resulting adhesives and form strong hydrogen bonds with adhered surface functional groups(wood-hydroxyl).The perfect wet shear strength of plywood was 1.07 MPa,which increased by 45%compared to acid modified SPI adhesive.(4)In the alkaline condition there existed the oxidation reaction in the B ring of the CTs,which obtained an ortho-quinone and could react with SPI amino groups through crosslinking and covalent reactions.These reactions generated an ideal adhesive which could form electrostatic and covalent interactions with adhered surface functional groups to fabricate indoor plywood,the wet shear strengths enhanced to 0.95 MPa,which increased by 32%compared to alkaline modified SPI adhesive.