Plastic Resin Glue Chemistry

Urea-Formaldehyde (UF) Resins and Adhesives:

Also known as plastic resin glues, urea/formaldehyde (“UF”) adhesives are the most widely used adhesives in the wood products industry.  They are classified as thermosetting polymers and are produced by a condensation polymerization reaction between urea and formaldehyde.  These UF polymers have proven to be very good adhesives for wooden materials.  Due to their high reactivity and cost efficiency, they are the most popular binders for interior products.

UF resins and adhesives are available both in liquid and powder form with a wide range in viscosity, solids content, and molar ratio.  One shot adhesives, powder adhesives with incorporated hardener, can be delivered ready-to-use.  A wide range of resin technologies has also been developed, including a broad spectrum of hardeners, to offer the best possible solution for the different process conditions employed by a variety of customers.

Customizing involves designing the resin so the viscosity, solids content, reactivity, tack, and emission properties meet the specified process and environmental needs.  UF products are environmentally friendly and capable of meeting or exceeding the various performance criteria required by the different standards ANSI, NPA, ASTM, EN, and ISO.

Plastic Resin Glue Chemistry:

Urea-formaldehyde resins are the most common member of the amino resin classification.  As the name implies, UF resins are formed by the reaction of urea and formaldehyde.

Urea:

Urea is also called Carbamide which is the diamide of carbonic acid.  Its formula is H2NCONH2.  It is a colorless, crystalline substance that melts at 132.7° C (271° F) and decomposes before boiling.  While urea is recognized as the main nitrogenous end product of the metabolic breakdown of proteins in all mammals, its industrial derivation is actually from a reaction between liquid ammonia and liquid carbon dioxide.  These constituents react to form ammonium carbamate which subsequently decomposes at lower pressures to urea and water.

Formaldehyde:

In its pure form, formaldehyde is a gas with a strong pungent odor and is derived through oxidation of hydrocarbons such as methanol, propane or butane in the presence of a catalyst.  It is commercially offered as a 37-50% aqueous solution, called Formalin, that usually contains up to 15% methanol to inhibit polymerization.  

UF Resin Formation:

Urea and formaldehyde are united in a two-stage process in the presence of pyridine, ammonia, or certain alcohols with heat and control of pH to form intermediates (such as methylolurea or dimethylolurea) that are mixed with fillers to produce molding powders.  These are then converted to thermosetting resins by further controlled heating and pressure in the presence of catalysts.

Two-Part Urea Formaldehyde Adhesive Systems:

The most common examples of these adhesives include the venerable Weldwood (now part of DAP), Urac 185 (by Cytec), Aerolite 308 (now owned by Dynea).

These adhesives are usually supplied in powder form (Weldwood) to which water is added to activate the polymerization reaction.  However, they are also available as a two-part system with a liquid part and a powder part (Urac 185).

Regardless of the form, the urea-formaldehyde compound must be combined with an acid-cure catalyst to facilitate the polymerization.  Most wood adhesives use either ammonium chloride or ammonium sulfate as the acidic catalyst.  It is generally added to the powdered resin in powder form.  The subsequent addition of water allows the constituents to go into solution together and combine chemically.