Improve Performance of Soy Flour-Based Adhesive with a Lignin-Based Resin.

Xiaochun Zhang,Yuding Zhu,Youming Yu,Jiangang Song

doi:10.3390/polym9070261

Xiaochun Zhang, Yuding Zhu + Show 2 more

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https://doi.org/10.3390/polym9070261

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Abstract

A lignin-based resin (LB) was used to improve the performance of soy flour-based adhesives. Soy flour (SF), polyamidoamine-epichlorohydrin (PAE), and LB were used to develop a plywood adhesive. The solid content and viscosity of the adhesive, the functional groups, the thermo-stability, and the crystallinity of the cured adhesives were characterized, and the performance of the resultant adhesive was evaluated by fabricating three-ply plywood. Results showed that the LB and PAE mixture used to modify the SF adhesive improved both dry and wet bond strength by 66.3% and 184.2%, respectively. Therefore, the PAE improved the wet bond strength, and the LB improved the dry bond strength. The improvement was attributed to: (1) the reaction of LB/PAE with the functions of the soy protein to form a cross-linking network; (2) a polycondensation reaction between the LB molecules improved the crosslinking density of the adhesive to form an interpenetration structure with cross-linked proteins; and (3) the easy penetration of the LB into the wood surface that enhanced interlocking between the wood and adhesive. Furthermore, the denser structure created by the LB and the PAE mixture improved thermal stability and decreased the crystallinity of the cured adhesive. The use of the LB and the PAE mixture increased the solid content by 35.5%, while still making its viscosity acceptable for industrial applications.

Highlights

Formaldehyde-based resins— melamine-urea-formaldehyde resin—are widely used to fabricate plywood panels [1]; they are non-biodegradable and petroleum-derived, resulting in environmental concerns regarding their preparation and use [2,3]
The use of the lignin-based resin (LB) and the PAE mixture increased the solid content by 35.5%, while still making its viscosity acceptable for industrial applications
28,510 to 687,500 mPa·s after the addition of LB (Adhesive b). This was thought to have occurred as the LB contained free sodium hydroxide (NaOH), which is effective at denaturing proteins, so it unfolded the soy protein molecules and exposed the inner functional groups [18]

Summary

Introduction

Formaldehyde-based resins— melamine-urea-formaldehyde resin—are widely used to fabricate plywood panels [1]; they are non-biodegradable and petroleum-derived, resulting in environmental concerns regarding their preparation and use [2,3]. The development of adhesives based on ecological and renewable natural resources is crucial. Soy flour has been used as a wood adhesive for decades, as it is a renewable, abundant, readily available, and inexpensive raw material [4]. Its poor water resistance has limited the application of soy flour-based adhesives. Many attempts have been made to improve the water resistance of soy flour-based adhesives, including protein denaturing agent modification and crosslinker modification. Denaturing agents such as alkalis [5], urea [6], and sodium dodecyl sulfate (SDS) [7] can unfold protein molecules to expose their internal hydrophobic groups that improve the water resistance of the adhesive

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