The Ideal Glue–How Close Are We?

The first widely used dependable adhesives were animal glues. These glues had low water resistance, however, and required heating before mixing and application. In 1903 the first starch adhesive was developed into a practical wood glue. This adhesive, though not water resistant, was easily mixed, and was the basic wood adhesive in the United States until WW I. Casein glue was developed during WW I, providing an easily applied and moderately water-resistant adhesive. During the same period, the U.S. Forest Products Laboratory developed blood albumin adhesives, which were found to be superior to casein glues in water resistance, but required hot pressing, which restricted their acceptance. In the early 1920’s seed meal glues were developed from the protein fraction of soybean meal. Seed meal glues were the lowest cost, the most dependable, and the most popular adhesives available from 1925 to 1940. The synthetic phenol-formaldehyde and urea resin adhesives, developed during the same period, were the first glues to provide a glue joint as strong as the wood. Due to their weather-resistance, the phenol-resin glues became the basic adhesives of the exterior softwood plywood industry. The urea resin glues, with their clear glue line and easy application, became the basic adhesives of the furniture industry. Melamine resin glues represent a later development in synthetic resin adhesives. Though the adhesive has very high strength qualities, it has gained little popularity, due to high cost and high curing temperatures required. The most recent developments in the adhesives field have been the formulation of adhesives based on resorcinol and polyvinyl resins. Resorcinol-based adhesives have gained popularity as strong, water-resistant glues capable of curing at moderate temperatures. The poly-vinyls are thermoplastic adhesives, affording a degree of flexibility desirable in many applications. Other notable developments have been the development of polyester and epoxy resins, particularly useful in bonding wood to metals and other materials. The further development of contact-setting and pressure-sensitive adhesives continues to offer potential for improvement in bonding techniques. No single type of adhesive available at present meets all the requirements tor an ideal glue. Further development should be directed toward achieving the basic properties and operating characteristics desired, paying particular attention to applicable developments in other related fields of research.

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