Urea-formaldehyde reaction products

Urea—Formaldehyde Reaction Products. Urea—formaldehyde (UF) reaction products represent one of the older controlled release nitrogen technologies. An early disclosure of the reaction products of urea [57-13-6] and formaldehyde [50-00-0] was made in 1936 (1) (Amino resins and plastics). In 1948, the USDA reported that urea (qv) and formaldehyde (qv) could react to produce a controlled release fertilizer at urea to formaldehyde mole ratios (UF ratio) greater than one (2). 

Urea—formaldehyde reaction products represented the first synthetically produced form of controlled release nitrogen and were commercialized in 1955 under the trade names Uramite (DuPont) and Nitroform (Nitroform Corp.). 

Liquid Compositions. Urea—formaldehyde reaction products also are available commercially as Hquids that can be categorized into two classes, ie, water suspensions and water solutions. 

The nitrogen content of granular urea—formaldehyde reaction products typicahy ranges from 35 to 42% depending on the methylene urea polymer distribution. 

Once in the soil solution, urea—formaldehyde reaction products are converted to plant available nitrogen through either microbial decomposition or hydrolysis. Microbial decomposition is the primary mechanism. The carbon in the methylene urea polymers is the site of microbial activity. Environmental factors that affect soil microbial activity also affect the nitrogen availabiUty of UF products. These factors include soil temperature, moisture, pH, and aeration or oxygen availabiUty. 

The detection limits for triazines are 300 ng [7] and for urea formaldehyde reaction products they are 1 to 5 pg substance per chromatogram zone [1]. 

UREAFORM. A urea-formaldehyde reaction product that contains more than one molecule of urea per molecule of formaldehyde. It can be used as a fertilizer because of its high nitrogen content, its insolubility in water,... 

Urea-formaldehyde reaction products, usually called ureaform, are produced by about six manufacturers in the United States and several other countries. Unlike IBDU and CDU, ureaform is not a definite chemical compound. It contains methylene ureas of different chain lengths the solubility increases with decrease in chain length. It usually contains about 38 percent nitrogen. 

Amino resin chemistry goes back to the 1880s. The first mention of a urea-formaldehyde reaction product was by Tollens (9) in 1884 in a report of work done by an associate named Holzer. Very few references relating to amino resins can be found in the chemical literature prior to 1900. Ludy (10) in 1889 and Carl Goldschmidt (11) in 1896 mentioned products that were similar to the material... 

Urea-formaldehyde reaction products were described as early as 1908, but the first useful commercial product, a molding compound invented in England by Edmond C. Rossiter, did not arrive until almost 20 years later. It was a fairly complex formulation using purified cellulose fiber as reinforcement. The amino resin contained equimolar amounts of urea and thiourea. The new product could be supplied in light translucent colors. The molded products had a hard, stain resistant surface, and there was no objectionable phenolic odor. In short, the product was unique for its time. 

Furthermore, some cross-linking reactions were studied. The gum is easily cross-linked in the solid state with formaldehyde, when hydrochloric acid is used as catalyst. The acid causes some hydrolysis of the glycosidic linkages. The products obtained are completely insoluble in water. It is believed that, especially between primary hydroxyl groups of different molecules, methylene bridges are formed. Cross-linking with urea-formaldehyde condensation products was useful in preparing water-resistant films. 

The kinetics of the formation and condensation of mono- and dimethylolureas and of simple UF condensation products has been studied extensively. The formation of mono-methylolurea in weak acid or alkaline aqueous solutions is characterized by an initial fast phase followed by a slow bimolecular reaction [4,5]. The first reaction is reversible and is an equilibrium which proceeds to products due to the uptake of the products, the methylolureas, by the second reaction. The rate of reaction varies according to the pH with a minimum rate of reaction in the pH range 5 to 8 for a urea/formaldehyde molar ratio of 1 1 and a pH of 6.5 for a 1 2 molar ratio [6] (Fig. 1). The 1 2 urea/formaldehyde reaction has been proved to be three times slower than the 1 1 molar ratio reaction [7]. 

AMIDE-FORMALDEHYDE-REACTION-PRODUCTS 3.4.3. N-hydroxymethyl-ureas... 

The reaction of urea with formaldehyde yields the following products, which are used as monomers in the preparation of urea formaldehyde resin. 

Phosphoric Acid-Based Systems for Cellulosics. Semidurable flame-retardant treatments for cotton (qv) or wood (qv) can be attained by phosphorylation of cellulose, preferably in the presence of a nitrogenous compound. Commercial leach-resistant flame-retardant treatments for wood have been developed based on a reaction product of phosphoric acid with urea—formaldehyde and dicyandiamide resins (59,60). 

Urea and melamine adhesives represent products of very mature and overaged technologies. Essentially, they are simple reaction products of urea or melamine with formaldehyde they may be Hquids or powders. Liquids are converted to dry powders by "spray drying." Melamine-urea combinations generally are spray-dried powders of co-reacted Hquid melamine and area-formaldehyde resias. 

Formaldehyde may react with the active hydrogens on both the urea and amine groups and therefore the polymer is probably highly branched. The amount of formaldehyde (2—4 mol per 1 mol urea), the amount and kind of polyamine (10—15%), and resin concentration are variable and hundreds of patents have been issued throughout the world. Generally, the urea, formaldehyde, polyamine, and water react at 80—100°C. The reaction may be carried out in two steps with an initial methylolation at alkaline pH, followed by condensation to the desired degree at acidic pH, or the entire reaction may be carried out under acidic conditions (63). The product is generally a symp with 25—35% soHds and is stable for up to three months. 

Amino Resins. Amino resins (qv) include both urea- and melamine—formaldehyde condensation products. They are thermosets prepared similarly by the reaction of the amino groups in urea [57-13-6] or melamine [108-78-1] with formaldehyde to form the corresponding methylol derivatives, which are soluble in water or ethanol. To form plywood, particle board, and other wood products for adhesive or bonding purposes, a Hquid resin is mixed with some acid catalyst and sprayed on the boards or granules, then cured and cross-linked under heat and pressure. 

UF solutions are clear water solutions. They contain only very low molecular-weight, water-soluble UF reaction products plus unreacted urea. Various combinations of UF solutions are found. They contain a maximum of 55% unreacted urea with the remainder as one or more of methylolureas, methylolurea ethers, MDU, DMTU, or triazone, a cycHcal oligomer. AAPFCO has defined this class of compounds as urea—formaldehyde products (water- s oluble). 

Various processes can be employed to manufacture urea—formaldehyde products. They are generally categorized into two types, ie, dilute solution processes and concentrated solution processes. Table 3 Hsts select U.S. manufacturers of UF reaction products and their products. 

Urea-formaldehyde resins are usually prepared by a two-stage reaction. The first stage involves the reaction of urea and formaldehyde under neutral or mildly alkaline conditions, leading to the production of mono and dimethylol ureas (Figure 24.1). The ratio of mono to dimethylol compounds will depend on the urea-to-formaldehyde ratio and it is important that there should be enough formaldehyde to allow some dimethylol urea formation. 

In typical manufacturing processes the freshly prepared urea-formaldehyde initial reaction product is mixed with the filler (usually with a dry weight... 

By far the preponderance of the 3400 kt of current worldwide phenolic resin production is in the form of phenol-formaldehyde (PF) reaction products. Phenol and formaldehyde are currently two of the most available monomers on earth. About 6000 kt of phenol and 10,000 kt of formaldehyde (100% basis) were produced in 1998 [55,56]. The organic raw materials for synthesis of phenol and formaldehyde are cumene (derived from benzene and propylene) and methanol, respectively. These materials are, in turn, obtained from petroleum and natural gas at relatively low cost ([57], pp. 10-26 [58], pp. 1-30). Cost is one of the most important advantages of phenolics in most applications. It is critical to the acceptance of phenolics for wood panel manufacture. With the exception of urea-formaldehyde resins, PF resins are the lowest cost thermosetting resins available. In addition to its synthesis from low cost monomers, phenolic resin costs are often further reduced by extension with fillers such as clays, chalk, rags, wood flours, nutshell flours, grain flours, starches, lignins, tannins, and various other low eost materials. Often these fillers and extenders improve the performance of the phenolic for a particular use while reducing cost. 

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. 

Aluminum Trichloride. In an analogous way, an AICI3 containing waste of isopropylbenzene production [189] can be used as an acid hardener for urea-formaldehyde resins. The waste from the production of isopropylbenzene (via a Friedel-Crafts reaction) contains approximately 200 to 300 g/liter of AICI3. 

The major disadvantage associated with urea-formaldehyde adhesives as compared with the other thermosetting wood adhesives, such as phenol-formaldehyde and polymeric diisocyanates, is their lack of resistance to moist conditions, especially in combination with heat. These conditions lead to a reversal of the bond-forming reactions and the release of formaldehyde, so these resins are usually used for the manufacture of products intended for interior use only. However, even when used for interior purposes, the slow release of formaldehyde (a suspected carcinogen) from products bonded with urea-formaldehyde adhesives is observed. 

The difference between the pH profiles of the two stages of urea-formaldehyde resin synthesis is taken advantage of in the production of these resins (Figure 19.2). In general, the commercial production of urea-formaldehyde adhesive resins is carried out in two major steps. The first step consists of the formation of methylolureas under basic conditions (pH 8 to 9), to allow the methylo-lation reactions to proceed in the absence of reactions involving the condensation of the methylolureas. 

Hexamethylolmelamine can further condense in the presence of an acid catalyst ether linkages can also form (see Urea Formaldehyde ). A wide variety of resins can be obtained by careful selection of pH, reaction temperature, reactant ratio, amino monomer, and extent of condensation. Liquid coating resins are prepared by reacting methanol or butanol with the initial methylolated products. These can be used to produce hard, solvent-resistant coatings by heating with a variety of hydroxy, carboxyl, and amide functional polymers to produce a cross-linked film. 

Urea-formaldehyde resins are generally prepared by condensation in aqueous basic medium. Depending on the intended application, a 50-100% excess of formaldehyde is used. All bases are suitable as catalysts provided they are partially soluble in water. The most commonly used catalysts are the alkali hydroxides. The pH value of the alkaline solution should not exceed 8-9, on account of the possible Cannizzaro reaction of formaldehyde. Since the alkalinity of the solution drops in the course of the reaction, it is necessary either to use a buffer solution or to keep the pH constant by repeated additions of aqueous alkali hydroxide. Under these conditions the reaction time is about 10-20 min at 50-60 C. The course of the condensation can be monitored by titration of the unused formaldehyde with sodium hydrogen sulfite or hydroxylamine hydrochloride. These determinations must, however, be carried out quickly and at as low temperature as possible (10-15 °C), otherwise elimination of formaldehyde from the hydroxymethyl compounds already formed can falsify the analysis. The isolation of the soluble condensation products is not possible without special precautions, on account of the facile back-reaction it can be done by pumping off the water in vacuum below 60 °C imder weakly alkaline conditions, or better by careful freeze-drying. However, the further condensation to crosslinked products is nearly always performed with the original aqueous solution. 

Properties of composites obtained by template poly condensation of urea and formaldehyde in the presence of poly(acrylic acid) were described by Papisov et al. Products of template polycondensation obtained for 1 1 ratio of template to monomers are typical glasses, but elastic deformation up to 50% at 90°C is quite remarkable. This behavior is quite different from composites polyacrylic acid-urea-formaldehyde polymer obtained by conventional methods. Introduction of polyacrylic acid to the reacting system of urea-formaldehyde, even in a very small quantity (2-5%) leads to fibrilization of the product structure. Materials obtained have a high compressive strength (30-100 kg/cm ). Further polycondensation of the excess of urea and formaldehyde results in fibrillar structure composites. Structure and properties of such composites can be widely varied by changes in initial composition and reaction conditions. 

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