Glues made from natural ingredients have been used for gluing wood for thousands of years. Protein glues are the oldest glues. In good conditions natural glues can create durable glue joints, but they endure weather and microorganisms poorly.
- Glutin glue was made from animal waste, such as leather, bones and fish.
- Caseine glue was made from whey.
- Albumen glue was made from the dried blood of bovine animals.
Vegetable proteins have also been used to make wood glues. Soybeans have been one of the most important raw materials for this.
Synthetic glues have almost completely replaced natural glues in wood gluing, because several of their properties are better than those of natural glues:
- Easier to prepare for spreading condition
- Harden faster and
- Endure microorganisms better than natural glues.
Glue that is ready for use consists of adhesic resin, hardener and solvent. In addition, filling material, additives and decelerators are added if need be. Their purpose is to prevent excessive absorption of glue, increase viscosity so that the glue is spreadable, improve the filling properties of glue, decrease its surface tension and soften the glue joint.
Additives themselves have adhesive properties. Starch; rye and wheat flour are the most common adhesive ingredients of additives. Starches loose their adhesive capability as they age.
Filling materials serve to make the viscosity of the glue suitable and lower its prize. The most common filling materials are chalk and wood flours, and kaoline. Filling materials and additives must not change the acidity of glue.
Glues can be divided into groups according to several different properties, for example
- Hardening temperature: cold-setting and hot-setting glues,
- Production method: polycondensate, polymerizate and polyaddition glues
- Hardening method: physically and chemically setting glues
- Water resistance of the glue joint
Physically setting glues harden as the solvent evaporates from the joint. These glues include, for example, PVAc glues, close contact glues, and hot melt glues.
Chemically setting glues harden because of a chemical reaction. Chemically setting glues start hardening when the hardener is added. These glues include, for example, urea, phenol, resorcinol, polyurethane and epoxy glues.
There are many different types of glues and their properties vary according to which raw and additional materials are used. Therefore, it is very important to read carefully the instructions for use, mixing and work safety that the manufacturer has provided. Before the actual use, it is advisable to do test gluing with a new glue type. You should be especially careful when handling glues that include unhealthy or easily inflammable substances.