Urea-formaldehyde resins synthesis

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. 

Urea-formaldehyde resin synthesis consists basically of two steps. In the first step, urea reacts with aqueous formaldehyde under slightly alkaline conditions to produce methylol derivatives of urea (12,13). 

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. 

The synthesis of urea-formaldehyde resin takes place in two stages. In the first stage, urea is hydroxymethylolated by the addition of formaldehyde to the amino groups of urea (Figure 19.1). This reaction is in reality a series of reactions that lead to the formation of mono-, di-, and trimethy-lolureas. Tetramethylolurea does not appear to be produced, at least not in a detectable quantity. The addition of formaldehyde to urea takes place over the entire pH range, but the reaction rate is dependent on the pH. 

Rammon, R.M. 1984. The Influence of Synthesis Parameters on the Structure of Urea-Formaldehyde Resins. Ph.D. Thesis, Washington State University, Pullman, Washington. 

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]. 

Before the advent of oxygenate ethers like MTBE, formaldehyde production was the largest single application of methanol, with at least 16 manufacturers and consuming over 30% of the methanol produced. Formaldehyde is used in the manufacture of urea-formaldehyde resins, phenol-formaldehyde resins, melamine-formaldehyde resins, acetal resins, acetylenic chemicals, etc. The reactions involved in formaldehyde synthesis are ... 

N.N-Dimethylformamide [68-12-2] (DMF) [14.276] is miscible with water and organic solvents except aliphatic hydrocarbons. It is a good high-boiling solvent for cellulose esters, cellulose ethers, poly(vinyl chloride), vinyl chloride copolymers, poly(vinyl acetate), polyacrylonitrile, polystyrene, chlorinated rubber, polyacrylates, ketone resins, and phenolic resins. Alkyd resins and resin esters are partially soluble. Dimethylformamide does not dissolve polyethylene, polypropylene, urea-formaldehyde resins, rubber, and polyamides. It is used as a solvent in printing inks, for polyacrylonitrile spinning solutions [14.277], and as a solvent in the synthesis of acetylene. 

Optimization of the Synthesis of Urea-Formaldehyde Resins using Response Surface Methodology... 

In the near future, companies will face the need to produce low formaldehyde emission resins, i.e., not above the emission level of natural wood. However, for producing this new generation of urea-formaldehyde resins (UP), it is necessary to optimize the synthesis process. 

R. M. Rammon, The influence of synthesis parameters on the structure of urea-formaldehyde resins, PhD Thesis, Washington State University, Pullman, WA, USA (1984). 

A. T. Mercer, NMR analysis of strength and emission of melamine and melamine-urea-formaldehyde resins for synthesis optimisation, PhD thesis. University of the Witwatersrand, Johannesburg, South Africa (1996). 

Popova, T. A., Synthesis of Urea Formaldehyde Resins, NIITEHIM, 10, 54, 1980 (Russian). 

Urea-formaldehyde resins, MA condensations, 515 Ureidosuccinic acid, synthesis route, 48 UV radiation, polymerization initiation, 239, 241, 248... 

Disinfection by-product) production of urea-formaldehyde, phenolic, melamine, pentaerythritol and polyacetal resins industrial synthesis of a number of organic compounds cosmetics fungicides textiles embalming fluids 3210,322, 351, 3529,354... 

It was not until 1939 that melamine-formaldehyde resins were prepared and the commercial importance of melamine was realized. This led to extensive exploration of the commercial synthesis of melamine. Today, melamine is almost exclusively produced on an industrial scale from urea and dicyanodiamide (vide infra). 

An introduction to the typical resin synthesis of a UF resin used as an adhesive for wood products and in industrial applications is given below. It constitutes a handy formulation for those who want to work in this field. It is not a low-formaldehyde-emission formulation. To 1000 parts by mass of 42% formaldehyde solution (methanol < 1%) are added 22% NaOH solution to pH 8.3 to 8.5,497 parts by mass of 99% urea, and the temperature raised in 50 min from ambient to 90°C while maintaining pH in the range 7.3 to 7.6 by small additions of 22% NaOH. The temperature is maintained at 90 to 91°C until the turbidity point is reached (generally another 15 to 20 min). The pH is then corrected to 4.8 to 5.1 by addition of 30% formic acid, and the temperature is raised to 98°C. The water tolerance point is reached in 18 min and the pH is then adjusted to 8.7. Vacuum distillation of the reaction water with concomitant cooling is then initiated. After distillation of the wanted amount of water to reach a resin content of 60 to 65%, the resin is cooled to 40°C, 169 parts by mass of second urea is added, the pH is adjusted to 8.5 to 8.7, and the resin is allowed to mature at 30°C for 24 to 48 h resin characteristics solids content, 60% density, 1.268 g/cm free HCHO, 0.4% viscosity, 200cP pH, 8. 

This work describes an optimization procedure for UP resin synthesis, following tin alktiline-acid process, focusing on the conditions of the condensation step. A design of experiments methodology was employed to optimize the 3 selected factors (number of urea additions, time span between urea additions, and condensation pH), in order to produce particleboards with maximum intemtil bond strength and minimum formaldehyde release. 

Figure 3 shows the distrihution of the urea, monomethylolurea and dimethylolurea present in soluhon for the two resins stored at 25°C for 5 days after synthesis. UF-R2 has a much larger fraction of unreacted urea than resin UF-R5. The last urea was added in order to react with the free formaldehyde present, hut as the added amount was large, most of the urea remained unreacted in the final resin. However, this unreacted urea may play another role, since it may form a solvation... 

Secondary-butanol is used as an intermediate in the synthesis of methyl ethyl ketone (MEK) and sec-butyl acetate. Both are good solvents used for coating formulations. sec-Butanol is an active solvent for ester gum, and in natural and synthetic resins such as alkyds and urea or melamine-formaldehyde resins. It is also a useftil latent solvent for nitrocellulose lacquers. It is a useful coupling agent in cleaning formulations, oil field chemical blends, and in emulsion breakers. 

The main raw materials used for the synthesis of ureo-formaldehyde resins are urea and formaldehyde. Urea is a solid, crystalline, water-soluble substance. It has a weak basic character. Formaldehyde is used in solution form at 30-55% concentration. It is of advantage to use either high-concentration formaldehyde solutions or precondensates. Both processes provide a satisfactory economic efficiency and diminish residual water content. Precondensates can be obtained through the reaction between urea and formaldehyde (molar ratio 1 1.1-6), at T = 45 C and pH = 6.8-8.0 [19]. 

Amino resins are comprised mainly of urea-formaldehyde (UF), melamine-formaldehyde (MF) and guanamine-formaldehyde resins, usually prepared by hydroxymethylation of urea, melamine or benzoguanamine respectively. Details of the synthesis of these resins and the various parameters which affect their compositions and structures have recently been described elsewhere. ... 

The carbon dioxide removed in synthesis gas preparation can be reacted with ammonia, to lonn urea CO(NH2)2- This is an excellent fertilizer, highly concentrated in nitrogen (46.6%) and also useful as an additive in animal feed to provide the nitrogen for formation of meat protein. Urea is also an important source of resins and plastics by reacting it with formaldehyde from methanol. 

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