Wood glue is a general term for adhesives engineered to create a strong, lasting bond between wood surfaces. Since wood is porous and expands and contracts, these glues must form a bond that is stronger and more flexible than the wood itself. The composition varies widely, ranging from synthetic polymers to natural proteins, which explains why different products are suited for different applications.
Composition of Water-Based Polymer Glues
The most common wood glues found in homes and workshops are water-based polymer emulsions, primarily Polyvinyl Acetate (PVA) and Aliphatic Resin glues. The main ingredient in both is the synthetic polymer polyvinyl acetate, an aliphatic rubbery compound that forms the actual bond. Water acts as a carrier or solvent, keeping the adhesive in a liquid, workable state.
The glue hardens not through a chemical reaction but through the physical process of water evaporating into the air and the wood itself. As the water leaves the emulsion, the polyvinyl acetate particles coalesce, forming a solid, interlocking film that adheres to the wood fibers. This process is known as solvent evaporation curing.
Aliphatic resin glues, often recognizable by their yellow color, are a modified version of PVA glue. These modifications typically involve additives that improve properties like heat and moisture resistance compared to standard white PVA glues. Plasticizers are also included to maintain the cured film’s flexibility, preventing it from becoming too brittle.
Composition of Reactive Chemical Glues
Reactive chemical glues cure through a chemical process, resulting in a thermosetting polymer highly resistant to heat and moisture. This category includes Polyurethane and Epoxy wood adhesives. Unlike water-based glues, these materials undergo an irreversible change in their chemical structure during curing.
Polyurethane glues are composed of isocyanate prepolymers. These glues require moisture, either from the air or within the wood, to initiate curing. The isocyanate group reacts with water to form a rigid polymer, often expanding slightly as it cures to fill minor gaps. This reaction transforms the liquid adhesive into a durable, moisture-resistant solid.
Epoxy adhesives are sold as a two-part system: a resin and a hardener. The resin component is usually an epoxide polymer, while the hardener is a compound like an amine. Once the two components are mixed, a polymerization reaction begins, creating a strong, cross-linked thermosetting polymer. This reaction is exothermic, meaning it releases heat. The resulting bond offers exceptional strength and gap-filling capabilities.
Composition of Natural and Protein Glues
Traditional wood glues are derived from natural sources, relying on proteins to form the adhesive bond. While less common in general consumer use today, these compositions remain relevant in specialized woodworking and restoration. The two primary types are hide glue and casein glue.
Hide glue is made from rendered collagen, the primary structural protein found in the connective tissues, hides, and bones of animals. Collagen is a long, fibrous polymer molecule composed of amino acids. This glue is applied hot and hardens as it cools and the water evaporates, forming a slightly elastic network held together by hydrogen bonds.
Casein glue is derived from casein, the main protein found in mammalian milk. This protein is supplied as a dry powder and mixed with an aqueous alkaline solvent, such as lime or sodium hydroxide, to dissolve and activate it. Casein glues were historically valued for providing a stronger bond than many early animal glues and offering moderate water resistance once cured.