Amino Resin Adhesives

Normal reaction of amino resins proceeds under acidic conditions. The reaction demands hydrochloric acid as a catalyst, which is produced according to the following reaction  

In kapur, oak, and other wood species this reaction is retarded. Hydrochloric acid formation is delayed by the presence of a 1 % hot-water extract of kapur in the resin. This extract contains tannin and lignin-like substances (2). The pH can be lowered by ethanol extract in Quercus alba and Q falcata. This changes the gel time by 40%, but in turn this causes other problems (38). 

In Tectona grandis (teak) extractives are responsible for three kinds of gluing problems (14) 1) Alcohol- and benzene-soluble compounds (11.5% of dry wood) hinder glue penetration into wood 2) formation of hydrochloric acid is delayed by hot-water-soluble compounds (the inhibitors originate in Ca, Mg, and K salts of acidic sugars in the methanol insolubles of the hot-water extract) and 3) the solidified glue is hydrolyzed by the methanol-soluble fraction of the hot-water extracts. 

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It was also found that the lignin-resol resin adhesives satisfied the JIS requirements for non-volatile content, pH value, viscosity, gel time, and dry and wet adhesion strength. Furthermore, the low temperature curability typically found in amino resin adhesives could also be achieved. Thus, it can be concluded that an effective utilization of lignin is possible with simultaneous improvement of the properties of resol resin adhesives. 

The methylol compounds formed are fairly stable under neutral or alkaline conditions, and many amino resins (adhesives, for example) are simply mixtures of these methylolated monomers and low molecular... 

The urea-formaldehydes (UFs) are the most important and most used class of amino resin adhesives. Amino resins are polymeric condensation products of the reaction of aldehydes with compounds carrying aminic or amidic groups. Formaldehyde is by far the primary aldehyde used. The advantage of UF adhesives are their (1) initial water solubility (this renders them eminently suitable for bulk and relatively inexpensive production), (2) hardness, (3) nonflammability, (4) good thermal properties, (5) absence of color in cured polymers, and (6) easy adaptability to a variety of curing conditions [1,2]. 

Savia, V., Amino resin adhesives (Chapter 25). Handbook of Adhesives (I. Skeist, ed.), 3rd ed.. Van Nostrand Reinhold, New York, 1990. 

Paraformaldehyde is available as a solid in the form of flakes or powder. It is an unstable polymer and can easily release formaldehyde when dissolved in water. Formaldehyde also exists in the form of a cyclic trimer, trioxane. It is a fairly stable compound and is not used in the manufacture of amino resin adhesives. 

Fiberglass insulation that is bonded with a phenolic resin is made more fire resistant by adding an amino resin. An amino resin adhesive may be used to bond a fire retardant to flammable draperies. 

Amino resin adhesives may also be used to improve bonding between other materials. For example, tire cord is normally treated with a rubber latex to improve adhesion to the vulcanized rubber. The latex dip solution may also contain a resorcinol-formaldehyde resin or an amino resin to improve the bonding. Both urea and melamine resins are mentioned in patents, but melamine-formaldehyde is said to be most effective. 

Savia V. Amino resin adhesives (chapter 25). In Skeist I, ed. Handbook of Adhesives, 3rd ed. New York, NY Van Nostrand Reinhold 1990. 

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