Glue is one of the oldest and most versatile adhesives. From the white glue used in household crafts to the commercial-grade glues utilized in furniture and even automobile manufacturing, glue is part of your everyday life.
Understanding how glue is made and the differences between glue types can help you decide on the best material for your applications. If you are looking to produce glue of your own, understanding the way it's made will also help you to choose the right glue mixing equipment to suit your needs.
What is Glue?
Glue is any adhesive that is applied in liquid form and dries hard to hold materials together. Technically, true glues are made from organic compounds like animal collagen. However, many products marketed as glue are in fact synthetic adhesives made with polyvinyl acetate (PVA) emulsions. These synthetic adhesives are also sometimes referred to as gums or cements.
Other types of adhesives, including epoxy, caulk, and sealant, have many similarities with glue. However, they are created with specific jobs in mind and have specific additives that provide these task-specific qualities; glue is a more multi-purpose adhesive and thus has a more basic recipe.
What is Glue Made Of?
Early glues were made of collagen harvested from animals. Muscle, bone, sinew and other tissues release collagen when boiled; this collagen is sticky when wet but dries to a hard, water-insoluble substance.
Traditionally, horses were the most common source of glue ingredients for many years. This is because horses are large animals, with bodies full of collagen-producing tissues, and they were once pervasive in human society. Now that horses are no longer used for day-to-day transportation and agricultural needs, they are not as commonly used as ingredients in glue manufacturing. Cattle and pigs are more frequently used, with bone and hide glues made from leftovers of commercial meat packing being the most common.
Other organic glue ingredients include casein, blood albumin, fish swim bladders and vegetable starch. One of the better-known vegetable-based glues is made from gum arabic derived from the sap of the acacia tree.
Synthetic "glues" or adhesives are generally made from a combination of polyvinyl acetate (PVA), water, ethanol, acetone and other substances. Water is used to modify the glue's consistency; other ingredients control the rate at which the glue dries out.
How Glue is Made Today
Organic glues are made by first washing animal hides, bones, and other tissues to remove dirt and debris. The animal tissues are then soaked in water to begin softening them and start the process of releasing collagen.
After soaking, they are treated with lime and acid to facilitate the breakdown of tissue and release more collagen. The acid is later removed, and the collagen-and-water mixture is cooked down to produce a thick, gelatin-like substance that can be further purified and treated with other chemicals and ingredients as necessary to create the type of glue desired.
How Synthetic Polymer Glue is Made
The chemical compounds in synthetic glue are isolated through chemical reactions. The base glue ingredient behind synthetics, vinyl acetate, is made through a chemical reaction of acetylene and acetic acid. In the presence of a catalyst, the molecular weight will increase, creating a polymer. A similar chemical process is utilized to create vinyl paint.
The polymer, called polyvinyl acetate, can be mixed into an emulsion to create a glue of the appropriate texture. PVA glues must have additives to prevent them from drying out immediately so that they can remain shelf stable.
Emulsification and Mixing Equipment
Appropriate mixing equipment is crucial to creating a strong, durable adhesive. Glues are emulsions, made by mixing collagen or polymers into a solvent, such as water. An appropriate level of agitation is crucial to ensuring the emulsion mixes adequately. Otherwise, the glue may settle once bottled, rendering it ineffective.
One type of glue mixing is low shear mixing, which utilizes a low-speed propeller or turbine to slowly fold the polymer into the solvent. This process is time-consuming, with a vat of glue taking 12-24 hours to mix in some cases.
A more efficient option is a high shear disperser, which typically utilizes a high shear dispersion blade. It works essentially like a large kitchen blender, drawing material up from beneath the mixing head and thrusting it through the blades in order to thoroughly break apart polymer particles and mix the emulsion.
Some situations may require an even more high-speed option, like a high-pressure homogenizer or colloid mill. These utilize a combination of shearing, agitation, and high pressure to emulsify adhesives.