Adhesives for Wood and Lignocellulosic Materials. R. N. Kumar
and their interactions. The uniqueness of wood as an adherend by virtue of possessing a hierarchical structure has already been dealt with in detail in Chapter 1. In this respect, wood differs significantly from other substrates such as metals, plastics, elastomers, etc. Surface science, rheology, materials science, surface chemistry and surface morphology, organic chemistry, polymer science and polymer characterization, and solid mechanics and interaction between polymers and wood—all contribute to the development and understanding of the adhesion phenomenon.
Further, these studies will enable
The identification of practical problems and root causes for adhesion failure and provide practical preventive solution from the knowledge of the nature of wood–adhesive interaction, i.e., a scientific approach to troubleshooting.
Optimization of the performance of existing adhesives and to develop new adhesives to meet stringent environmental regulations.
The increase in the durability of bonded wood products by precisely understanding the role of internal and external stresses to which wood bond lines are subjected.
The development of new technologies based on the insights gained from the knowledge of the basic principles, which can be applied efficiently for bonding difficult to bond wood species and preservative treated wood. This is so that the carbon sequestration is possible for prolonged periods of time to reduce the global warming potential of wood products.
2.2 Definitions
We should first define the terms adhesive, adhesion cohesion, and other related terms in order to understand their individual role in determining the effectiveness of bonding.
2.2.1 Adhesion
Adhesion is defined as the state in which two surfaces are held together by interfacial forces that may consist of valence forces or interlocking action, or both. Adhesion is further classified as mechanical adhesion and specific adhesion. Specific adhesion between two surfaces is caused by the valence forces of the same type as those that give rise to cohesion, as opposed to mechanical adhesion in which the adhesive holds the parts together by an mechanical interlocking.
2.2.2 Cohesion
Cohesion is defined as the internal strength of an adhesive as a result of a variety of interactions within the adhesive.
2.2.3 Adhesive
ASTM defines an adhesive as a substance capable of holding materials together by surface attachment.
2.2.4 Adherend
Adhered, also called a substrate, is defined as a body that is held to another body by an adhesive used interchangeably. Various descriptive adjectives are applied to the term adhesive to indicate certain characteristics as follows: (1) physical form, that is, liquid adhesive, tape adhesive, etc.; (2) chemical type, that is, silicate adhesive, resin adhesive, etc.; (3) materials bonded, that is, paper adhesive, metal–plastic adhesive, can label adhesive, etc.; (4) condition of use, that is, hot setting adhesive, room temperature setting adhesive, etc.
2.2.5 Bonding
Bonding is the joining of two substrates using an adhesive. According to DIN EN 923, an adhesive is defined as a non-metallic binder that acts via adhesion and cohesion. ASTM D907-06 defines an adhesive as “a substance capable of holding materials together by surface attachment”. A material attached using adhesive is called an adherend.
2.2.6 Adhesive, Assembly
Adhesive, assembly—an adhesive that can be used for bonding parts together, such as in the manufacture of a boat, airplane, furniture, and the like. Note: The term assembly adhesive is commonly used in the wood industry to distinguish such adhesives (formerly called “joint glues”) from those used in making plywood (sometimes called “veneer glues”).
2.3 Mechanism of Adhesion
The role of an adhesive for wood is to transfer and distribute loads between components, thereby increasing the strength and stiffness of wood products [6].
This is achieved through the following three basic types of adhesion:
1 Specific Adhesion—Bonding between the adhesive and the adherend is due to chemical reaction.
2 Mechanical Adhesion—occurs due to mechanical anchorage.
3 Effective Adhesion—combines specific and mechanical adhesion for optimum joining strength.
One should distinguish between adhesion and cohesion.
Cohesion as defined earlier is the attraction of molecules and groups within the adhesive (or other material) that holds the adhesive molecules together. The combination of adhesion and cohesive strength determines the bonding effectiveness. An adhesive bond fails if either the adhesive separates from the substrate (interfacial adhesion failure) or the adhesive breaks apart (cohesive failure). The adhesive and cohesive strengths of some adhesives are high enough that the cohesive strength of the substrate fails before the adhesive bond.
2.3.1 Specific Adhesion
Specific adhesion involves the bond created by chemical means, rather than mechanical, as a result of the molecular attraction between the surfaces in contact. This can be ionic, covalent, or induced by any other intermolecular forces (Figure 2.1), as described below:
Figure 2.1 Potential energy diagram for different forces [4].
(a) Coulombic (ionic) or hydrogen bonding
Hydrogen bonds occur in molecules that have H–F, H–O, and H–N bonds. Basically, this strong intermolecular force is due to strong dipole–dipole forces.
Besides the above, there can exist non-covalent and non-electrostatic interactions (apolar interactions) between neutral atoms and molecules [2, 3]. However, they are not as strong as Coulombic (ionic) or hydrogen-bond interactions. They are ubiquitous and are always attractive between like particles.
(b) Apolar interactions
There are three types of intermolecular forces that occur in chemical compounds. These forces cause molecules or groups of molecules to be attracted to one another, thus affecting many of their properties. Collectively known as the van der Waals forces, these electrodynamic intermolecular forces originated from three distinct interactions. These are (a) Keesom (permanent–permanent dipoles) interaction (b) Debye (permanent-induced dipoles) force, and (c) London dispersion force (fluctuating dipole-induced dipole interaction) [2]. While these three kinds of interactions have distinct origins, they have in common the fact that their interaction energies decay rapidly with the sixth power of the interatomic or molecular distance. See Sections 2.3.1.1 to 2.3.1.3.
The London dispersion force is the weakest, followed in increasing strength by dipole–dipole forces and then hydrogen bonding. Lewis acid–base interactions can also occur (discussed later) [3].
The mathematical relationships for the various potential energies are given below: