Sustainable Development Approaches through Wooden Adhesive Joints Design

Abstract

Over recent decades, the need to comply with environmental standards has become a concern in many industrial sectors. As a result, manufacturers have increased their use of eco-friendly, recycled, recyclable, and, overall, more sustainable materials and industrial techniques. One technique highly dependent on petroleum-based products, and at the edge of a paradigm change, is adhesive bonding. Adhesive bonding is often used to join composite materials and depends upon an adhesive to achieve the connection. However, the matrices of the composite materials and the adhesives used, as well as, in some cases, the composite fibres, are manufactured from petrochemical products. Efforts to use natural composites and adhesives are therefore ongoing. One composite that has proven to be promising is wood due to its high strength and stiffness (particularly when it is densified), formability, and durability. However, wood must be very carefully characterised since its properties can be variable, depending on the slope of the grains, irregularities (such as knots, shakes, or splits), and on the location and climate of each individual tree. Therefore, in addition to neat wood, wood composites may also be a promising option to increase sustainability, with more predictable properties. To bond wood or wooden composite substrates, bio-adhesives can be considered. These adhesives are now formulated with increasingly enhanced mechanical properties and are becoming promising alternatives at the structural application level. In this paper, wooden adhesive joints are surveyed considering bio-adhesives and wood-based substrates, taking into consideration the recent approaches to improve these base materials, accurately characterise them, and implement them in adhesive joints.

Keywords: bio-adhesive; literature review; mechanical behaviour; sustainable development; wood adhesive joint.

Figure 1

The most significant families of bio-adhesives discussed in this paper.

Figure 2

Cross-sections in wood that indicate two common defects.

Figure 3

Orthotropic directions of wood.

Figure 4

A scheme of the four-point bending test configuration for determining the shear properties of wood.

Figure 5

A scheme of the wood delignification and densification processes.

Figure 6

Bio-adhesives used for wound closure by (a) joining the tissue beneath the surface, (b) joining the two sides of the injury, or (c) combining both (adapted from [92]).

Figure 7

A scheme of the test specimen for determining the strength of wood adhesive joints according to EN 302-1 (2013).

Figure 8

Simplified schemes of the (a) DCB and (b) ENF test configurations for studying the fracture behaviour of wood adhesive joints.