Urea-Formaldehyde Resin

Urea-formaldehyde, also known as urea-methanal, so named for its common synthesis pathway and overall structure,is a non-transparent thermosetting resin or polymer. It is produced from urea and formaldehyde. These resins are used in adhesives, finishes, particle board, MDF, and molded objects.It was first synthesized in 1884 by Hölzer, who was working with Bernhard Tollens. In 1919, Hanns John (1891–1942) of Prague, Czechoslovakia obtained the first patent for urea-formaldehyde resin.

UF and related amino resins are a class of thermosetting resins of which urea-formaldehyde resins make up 80% produced globally. Examples of amino resins use include in automobile tires to improve the bonding of rubber to tire cord, in paper for improving tear strength, in molding electrical devices, jar caps, etc

Urea-formaldehyde resin's attributes include high tensile strength, flexural modulus, and a high heat distortion temperature, low water absorption, mould shrinkage, high surface hardness, elongation at break, and volume resistance. Index of Refraction = 1.55.

The chemical structure of UF polymer consists of [(O)CNCH₂]_n repeat units. In contrast melanine-formaldehyde resins feature NCH₂OCH₂N repeat units. Depending on the polymerization conditions, some branching can occur. Early stages in the reaction of formaldehyde and urea produce bis(hydroxymethyl)urea.

Approximately 1 million metric tons of urea-formaldehyde are produced annually. Over 70% of this production is then put into use by the forest products industry for bonding particleboard (61%), medium density fiberboard (27%), hardwood plywood (5%), and laminating adhesive (7%).

Urea-formaldehyde is pervasive. Examples include decorative laminates, textiles, paper, foundry sand molds, wrinkle resistant fabrics, cotton blends, rayon, corduroy, etc. It is also used to glue wood together. Urea formaldehyde was commonly used when producing electrical appliances casing (e.g. desk lamps). Foams have been used as artificial snow in movies.

Urea formaldehyde is also used in agriculture as a controlled release source of nitrogen fertilizer. 

Urea formaldehyde’s rate of decomposition into CO2 and NH3 is determined by the action of microbes found naturally in most soils[4]. The activity of these microbes, and, therefore, the rate of nitrogen release, is temperature dependent. The optimum temperature for microbe activity is approximately 70-90 °F (approx 20-30 °C).

Urea-formaldehyde foam insulation (UFFI) dates to the 1930s and made a synthetic insulation with R-values near 5.0 per inch. It is a foam, like shaving cream, that is easily injected or pumped into walls. It is made by using a pump set and hose with a mixing gun to mix the foaming agent, resin and compressed air. The fully expanded foam is pumped into areas in need of insulation. It becomes firm within minutes but cures within a week. UFFI is generally found in homes built before the 1970s, often in basements, crawl spaces, attics, and unfinished attics. Visually it looks like oozing liquid that has been hardened. Over time, it tends to vary in shades of butterscotch but new UFFI is a light yellow color. Early forms of UFFI tended to shrink significantly. Modern UF insulation with updated catalysts and foaming technology have reduced shrinkage to minimal levels (between 2-4%). The foam dries with a dull matte color with no shine. When cured, it often has a dry and crumbly texture.

Health effects occur when urea-formaldehyde based materials and products release formaldehyde into the air. Generally there are no observable health effects from formaldehyde when air concentrations are below 1.0 ppm. The onset of respiratory irritation and other health effects, and even increased cancer risk begins when air concentrations exceed 3.0-5.0 ppm. This triggers watery eyes, nose irritations, wheezing and coughing, fatigue, skin rash, severe allergic reactions, burning sensations in the eyes and throat, nausea, and difficulty in breathing in some humans (usually > 1.0 ppm). 

Occupants of UFFI insulated homes with elevated formaldehyde levels experienced systemic symptoms such as headache, malaise, insomnia, anorexia, and loss of libido.[citation needed] Irritation of the mucous membranes (specifically the eyes, nose, and throat) was a common upper respiratory tract symptom related to formaldehyde exposure. However, when compared to control groups, the frequency of symptoms did not exceed the controls except when it came to wheezing, difficult breathing, and a burning skin sensation. Controlled studies have suggested that tolerance to formaldehyde's odor and irritating effects can occur over a prolonged exposure.

In industrial production, urea resins are made by the condensation of formaldehyde and urea in an aqueous solution, using ammonia as an alkaline catalyst. The condensation reaction gives a colourless, syrupy solution that can be spray-dried to a powder for later use in coatings or adhesives; it can also be mixed with cellulose filler to produce powders for molding into solid objects. Under the influence of heat and pressure, the resin, at this point made up largely of low-molecular-weight intermediate polymers or prepolymers, is cured to its final state, which consists of a three-dimensional network of interlinked polymers.

Urea formaldehyde began to be replaced in molded articles in the 1950s by melamine-formaldehyde resin and by new thermoplastic resins such as polystyrene. Like phenolic and melamine resins, urea-formaldehyde polymers are now employed primarily as wood adhesives. They are less durable than the other two resins, however, and do not have sufficient weather resistance to be used in exterior applications. Because urea-formaldehyde resins are lighter in colour than phenol-formaldehyde resins, they are traditionally reserved for interior plywood and decorative paneling, but concerns over the release of formaldehyde into the air have led to substitution even here by phenolics.

Urea-formaldehyde polymers are used to treat textile fibres in order to improve wrinkle and shrink resistance, and they are blended with alkyd paints in order to improve the surface hardness of the coating.