Urea-formaldehyde resins processing

Urea-formaldehyde resins can be cured with isopropylbenzene production wastes containing 200 to 300 g/liter of AICI3 as an acid hardener [189]. Isopropylbenzene is formed as an intermediate in the Hock process by a Friedel-Crafts reaction from propene and benzene. The mixture hardens in 45 to 90 minutes and develops an adhesion to rock and metal of 0.19 to 0.28 MPa for 0.2% AICI3 and 0.01 to 0.07 MPa for 0.4% AICI3, respectively. A particular advantage is the increased pot life of the formulation. 

Another of the early thermo-setting resins was the urea formaldehyde resin developed by American Cyanamid. Urea and formaldehyde gave methylol and dimethylol urea which on heating, further condensed to give the colorless resin. The technology of this process was well worked out, but the chemistry was still not completely understood. The colorless resin has been used to make many varieties of decorative materials and some essentially unbreakable dishes, among other uses. 

In the early 1930 s, a second type of resin prepared from formaldehyde was introduced to the market—namely, urea-formaldehyde resins. A few years later, melamine-formaldehyde resins also appeared. The same basic process is employed in polymerization of all these resins it consists of the catalyzed reaction of formaldehyde with the second ingredient—phenol, urea, or melamine—to evolve water and produce three-dimensional, cross-linked thermosetting polymers. 

UREA-FORMALDEHYDE RESIN. An important class of amino resin. 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 (methylolurea, dimenthylolurea) that are mixed with fillers to produce molding powders. These are converted to thermosetting resins by further controlled heating and pressure in the presence of catalysts. These were first plastics that could be made in white, pastel, and colored products. See also Amino Acids Melamine. 

American Cyanamid Bettle B W (1342) Based on urea-formaldehyde resin and cellulose filler processable by moulding and injection moulding... 

The amounts of lignins, tannins, and carbohydrates available as residues from processing of forest trees dwarf the commodity adhesive market. At the same time, the forest products industry is especially reliant on adhesives, since over 70% of all wood products are bonded, and their production consumes about 45% of all phenolic and 85% of all urea-formaldehyde resins produced in the United States. 

Application Urea-formaldehyde resins are used as adhesives in the wood-working industry and are typically used in the production of plywood and particle board. They are available as concentrated solutions or in powder form as a result of the spray-drying process. 

After cooling the resin in the reactor, the resin is pumped to the buffer tank of the connected spray dryer plant. Usually, the complete batch processing takes 4-5 h. The urea-formaldehyde resin solution can be dried in a spray dryer based on co-current flow principle. 

Formox [Formaldehyde by oxidation] A process for oxidizing methanol to formaldehyde, using a ferric molybdate catalyst. Based on the Adkins-Peterson reaction. Developed jointly by Reichold Chemicals and Perstorp (a company in the Swedish town of Perstorp). Perstorp has been making formaldehyde since the 1900s and the first Formox plant was built in 1959. The process is now owned by Perstorp Holding AB and has been licensed to more than 100 plants worldwide. Some of these use a continuous Formox process to make urea-formaldehyde resins continuously. Several other companies operate similar processes. 

The physicochemical features of the processes of formation, stabilisation and solidification of foams are best studied for a polymer foam from urea-formaldehyde resins. That is why the urea polymer foams are used here below to exemplify the principles of optimisation of the technology for production of polymer foam materials. 

The kinetics studies of solidification of urea-formaldehyde resins have shown [24] that at low solidifier (phosphoric acid) concentrations, in the range from 0.3 to 0.75%, a prolonged induction period of solidification is observed, lasting for ca. 30 min. The optimum rate of solidification is achieved at acid concentrations higher than 1-1.5%, but the process is strongly delayed at resin concentrations lower that 5-10%. 

Formaldehyde release from pressed wood products is due to latent formaldehyde. During the pressing process, hot steam from moist wood particles transfers heat, formaldehyde, and other volatiles from the surface of the mat to the core of the board where un reacted urea-formaldehyde resin components accumulate. The resulting formaldehyde concentration in the core is approximately twice that of the surface. Release of formaldehyde is diffusion-controlled and gradually decreases over time (Meyer and Hermanns 1985). Formaldehyde can also be produced by hydrolytic cleavage of unreacted hydroxymethyl groups in the formaldehyde resins. Melamine formaldehyde resins generally are more stable, and the amounts of formaldehyde emitted from them are much lower (WHO 1989). 

Straight urea-formaldehyde resins are used principally in the preparation of molding compositions and adhesives. As the name implies, the resin is made by reaction between urea and formaldehyde however, the synthesis process somewhat depends upon the end-use envisaged. The following process is for a wood adhesive resin, and illustrates the general procedures involved [10]. 

Formaline is made slightly alkaline, pH of 7.5—8.5, by adding sodium carbonate. Then, melamine is added in a 1 3 mole ratio of melamine to formaldehyde. Then the mixture is heated at 8CTC for 1—2 hr until the desired extent of reaction is attained. The resulting syrup is stabilized by borax (pH buffer) and can be used without further processing. The reaction mechanisms and pathways are also believed to be analogous to the urea-formaldehyde resin. 

Chemical reactions in sources are well known from hydrolysis of urea formaldehyde resins in chipboard resulting in the emission of formaldehyde and from the hardening processes of many sealants, glues, and varnishes (Roffael, 1993). The emission of volatile compounds from these products may in some cases be limited by formation. 

Many thermoset polymers of major commercial importance are synthesized by step-growth polymerization, as the case of unsaturated polyester, polyurethanes, melamines, phenolic and urea formaldehyde resins, epoxy resins, silicons, etc. In these systems, the crosslinking process, which leads to a polymer network formation, is usually referred to as curing. 

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