Early man had to rely on natural, unprocessed glues found for adhesive applications. 200,000 years ago, stone flakes held together by birch bark tar were used by humans. Tree resin, two-part mixtures of plant gum and iron oxide and starch-based glues were widely used by later civilizations to bind stone to wood, paper to fabric and in furniture making.
The first commercial glue plant was founded in Holland in 1690. Glue manufacturing became an industry during the 18th and 19th centuries, but not until the 20th century did the science of glue making take off with the advent of new resins, plastics and an increased understanding of the science of adhesion. Now, for each person in the U.S., forty pounds of adhesive is manufactured each year for use in clothing, shoes, cars, construction, paper products and countless other products.
Natural and Synthetic Adhesives
Vegetable or animal-based glues are still in widespread use because they are relatively inexpensive to manufacture and are environmentally friendly. Glues made from natural materials tend to have lower resistance to moisture, heat and chemicals than do synthetic adhesives, however. Furthermore, synthetic adhesives can be formulated with specific properties required for ease of application and durability in their intended application.
Natural Glue Manufacture
Animal-based glues are derived from waste animal remains such as skin, bones and hooves. To transform these into an adhesive, they are first washed and soaked. This stock is cooked in water, cooled and re-cooked to break down the collagen into raw glue. Chemicals, such as hydrochloric acid remove impurities in the final stages. Further heating or drying brings the glue to a specific viscosity.
Starch-based adhesives are made by extracting long-chain carbohydrates from seeds, tubers and the roots of plants such as corn, cassava, rice or wheat. The resulting pastes are mainly utilized for paper and cardboard products. Since they biodegrade quickly and come from renewable sources, they are considered environmentally friendly.
Methyl Cellulose Adhesive
Cellulose, derived from trees and other woody plants, may be used directly to create adhesives. Typically, however, it is chemically processed to create methyl cellulose glues, which have superior polymer binding properties. These are used mostly on labels, decals, wallpaper and other paper products.
To create methyl cellulose, raw cellulose is combined with a highly alkaline solution such as sodium hydroxide and carefully heated. Cellulose hydroxyl groups are replaced by methoixide during the chemical process, which provide a stronger bond. It is also used as an emulsifier thickener in the food industry since it is non-toxic and hypoallergenic.
Rubber glues belong to the elastomer class of adhesives, a large adhesive family with many applications. Natural rubber has limited properties as an adhesive, but when processed and combined with additives its properties are adjusted to provide superior adhesion to specific substrates, better temperature resistance and higher cohesive strength.
Manufacturing rubber-based adhesives is generally a process of dissolving natural rubber with solvents, adding resins and other compounds and using chemical catalysts or heat to create particular polymers. The most common polymer-based adhesives made from rubber include polychoropene, styrene, polyisobutylene, polysulfide, polyamide, silicone and polyurethane.
Because of safety and environmental concerns regarding some synthetic glue manufacturing and application processes, the adhesive industry is seeing a trend toward so-called eco-adhesives. These are based on renewable materials that are non-toxic and require no special disposal procedures.
To be eco-friendly, high solid solvents, low in volatile organic compounds, are utilized in formulations. These adhesives are less hazardous to manufacturing personnel and non-toxic for end consumers since they are non-flammable and do not contain off-gassing ingredients such as formaldehyde-based binders.
There is now “green labeling” from several construction and environmental associations that identify eco-friendly adhesives and the products that use them.
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