Bio-Based Adhesives and Evaluation for Wood Composites Application

Abstract

There has been a rapid growth in research and innovation of bio-based adhesives in the engineered wood product industry. This article reviews the recent research published over the last few decades on the synthesis of bio-adhesives derived from such renewable resources as lignin, starch, and plant proteins. The chemical structure of these biopolymers is described and discussed to highlight the active functional groups that are used in the synthesis of bio-adhesives. The potentials and drawbacks of each biomass are then discussed in detail; some methods have been suggested to modify their chemical structures and to improve their properties including water resistance and bonding strength for their ultimate application as wood adhesives. Moreover, this article includes discussion of techniques commonly used for evaluating the petroleum-based wood adhesives in terms of mechanical properties and penetration behavior, which are expected to be more widely applied to bio-based wood adhesives to better evaluate their prospect for wood composites application.

Keywords: adhesive evaluation; bio-based adhesives; lignin; protein plants; starch; wood composites.

Figure 1

The three structural units of lignin [25], reproduced with permission from Elsevier.

Figure 2

Formation of aliphatic aldehyde groups onto phenolic compounds (n = 1 or 2, R₁ and R₂: CHO, OMe or aliphatic chain substituents) [57], Reproduced with permission from Elsevier.

Figure 3

Chemical structure of amylose and amylopectin [66], Reproduced with permission from Elsevier.

Figure 4

Reaction pathway for the synthesis of starch-based adhesive [88], Reproduced with permission from Elsevier.

Figure 5

Possible cross-linking reaction of starch-based wood adhesive in the presence of hexamethoxymethylmelamine (Cymel 323) [91], Reproduced with permission from Elsevier.

Figure 6

The schematic structure of starch-based wood adhesive (a) with sodium dodecyl sulfate (SDS); (b) without SDS [101], Reproduced with permission from Elsevier.

Figure 7

The chemical reaction between canola protein and glycidyl methacrylate [106], Reproduced with permission from Elsevier. R: Possible functional groups (COOH, SH, OH, NH₂).

Figure 8

Schematic of wood adhesive (a) shear strength test [137], Reproduced with permission from Elsevier; (b) pull-off strength test [130], Reproduced with permission from Elsevier; and (c) three-point bending strength test.

Figure 9

(a) Illustration of numbers of filled fibers (FFN), numbers of filled vessels (FVN), maximum penetration depth in fibers (MPD_f), and maximum penetration depth in rays (MPD_r) [157], Reproduced with permission from Springer; (b) states of adhesives filled in vessels [149], Reproduced with permission from Springer.