Melamine-formaldehyde resins production

Amino and Phenolic Resins. The largest use of formaldehyde is in the manufacture of urea—formaldehyde, phenol—formaldehyde, and melamine—formaldehyde resins, accounting for over one-half (51%) of the total demand (115). These resins find use as adhesives for binding wood products that comprise particle board, fiber board, and plywood. Plywood is the largest market for phenol—formaldehyde resins particle board is the largest for urea—formaldehyde resins. Under certain conditions, urea—formaldehyde resins may release formaldehyde that has been alleged to create health or environmental problems (see Amino RESINS AND PLASTICS). 

Urea—formaldehyde resins are also used as mol ding compounds and as wet strength additives for paper products. Melamine—formaldehyde resins find use in decorative laminates, thermoset surface coatings, and mol ding compounds such as dinnerware. 

In the eady 1920s, experimentation with urea—formaldehyde resins [9011-05-6] in Germany (4) and Austria (5,6) led to the discovery that these resins might be cast into beautiful clear transparent sheets, and it was proposed that this new synthetic material might serve as an organic glass (5,6). In fact, an experimental product called PoUopas was introduced, but lack of sufficient water resistance prevented commercialization. Melamine—formaldehyde resin [9003-08-1] does have better water resistance but the market for synthetic glass was taken over by new thermoplastic materials such as polystyrene and poly(methyl methacrylate) (see Methacrylic polya rs Styrene plastics). 

N,]S7-bis(methoxymethyl)uron was first isolated and described in 1936 (41), but was commercialized only in 1960. It is manufactured (42) by the reaction of 4 mol of formaldehyde with 1 mol of urea at 60°C under highly alkaline conditions to form tetramethylolurea [2787-01-1]. After concentration under reduced pressure to remove water, excess methanol is charged and the reaction continued under acidic conditions at ambient temperatures to close the ring and methylate the hydroxymethyl groups. After filtration to remove the precipitated salts, the methanolic solution is concentrated to recover excess methanol. The product (75—85% pure) is then mixed with a methylated melamine—formaldehyde resin to reduce fabric strength losses in the presence of chlorine, and diluted with water to 50—75% soHds. Uron resins do not find significant use today due to the greater amounts of formaldehyde released from fabric treated with these resins. 

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. 

Melamine (I,3,5-triamino-2,4,6-triazine) was first prepared by Liebig in 1835. For a hundred years the material remained no more than a laboratory curiosity until Henkel patented the production of resins by condensation with formaldehyde. Today large quantities of melamine-formaldehyde resins are used in the manufacture of moulding compositions, laminates, adhesives, surface coatings and other applications. Although in many respects superior in properties to the urea-based resins they are also significantly more expensive. 

Seventy years ago, nearly all resources for the production of commodities and many technical products were materials derived from natural textiles. Textiles, ropes, canvas, and paper were made of local natural fibers, such as flax and hemp. Some of them are still used today. In 1908, the first composite materials were applied for the fabrication of big quantities of sheets, tubes, and pipes in electrotechnical usage (paper or cotton as reinforcement in sheets made of phenol- or melamine-formaldehyde resins). In 1896, for example, airplane seats and fuel tanks were made of natural fibers with a small content of polymeric binders [1]. 

Uses of Urea. The major use of urea is the fertilizer field, which accounts for approximately 80% of its production (about 16.2 billion pounds were produced during 1994 in U.S.). About 10% of urea is used for the production of adhesives and plastics (urea formaldehyde and melamine formaldehyde resins). Animal feed accounts for about 5% of the urea produced. 

The industrial production and application of reactive and non-reactive microgels in organic coatings such as binders or components of binders, e.g. together with, e.g. acrylic and/or melamine/formaldehyde resins, especially for automotive coatings, was reported in a number of publications between 1980 and... 

Melamine (cyanuramide) is obtained by heating dicyanodiamide (structure 17.20), which is obtained by heating cyanamide. Melamine, which is used for the production of melamine-formaldehyde resins, is also obtained by heating urea (structure 17.21)... 

The melamine that is made by this process has a purity of 96 wt% whereas other processes produce a product having greater than 99.8 wt% purity. Melamine-formaldehyde resin producers prefer high purity melamine. This is because the purity of melamine influences the physical properties of the resulting resins and affects processing times. However Melamine Chemicals patents claim that lower-purity melamine can be substituted in the production of melamine-formaldehyde resins if the reactor conditions (e.g., pH) are carefully controlled232. 

The natural product comprises veneers of real wood which have been sorted and joined edge-to-edge (for example, by stitching), and bonded under heat and pressure to layers of kraft paper impregnated with phenol-formaldehyde resin. A barrier layer immediately below the veneer is impregnated with melamine-formaldehyde resin and prevents upward migration of the darker phenolic resin. 

Melmac [Cytec]. TM for products molded from melamine-formaldehyde resins. 

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). 

The most widely used wood panel products are particleboard, softwood plywood, hardwood plywood, medium density fiberboard (MDF) and waferboard. The most common adhesive is urea-formaldehyde resin (UFR). Phenol-formaldehyde resins (PFR) are second in volume and melamine-formaldehyde resins (MFR) are a distant third. Recently,... 

The purpose of this study was to evaluate laboratory formaldehyde release test methods for predicting real-life formaldehyde air concentrations human exposure levels, and health risk. Three test methods were investigated the European perforator test, the gas analysis method at 60 C and 3% RH, and the gas analysis method at 23 C and 55% RH. Different types of particleboard bonded with urea-formaldehyde and urea-melamine-formaldehyde resins were tested. The results were used to rank boards as a function of test method, conditioning, short-term humidity, and temperature variations during storage. Additional experiments were conducted in small experimental houses at a Dutch research institute. Our conclusions are that relative ranking of products is influenced by the test method and by change in relative humidity. The relationship between test method and release in real-life situations is not clear. In fact, it seems impossible to use laboratory measurements to predict real-life product performance of board if the board is not fully in equilibrium with the atmosphere. 

Figure 1.6 The two important classes of amino-resins are the products of condensation reactions of urea and melamine with formaldehyde. Reactions for the formation of urea formaldehyde amino-resins (UF) are shown. Preparation of melamine-formaldehyde resins is similar.

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 ... 

Note As this table shows, the largest outlet for amino resins by far is their use as adhesives or binders for reconstituted wood products made from sawdust and wood chips. Urea-formaldehyde resin is most commonly used. Melamine-formaldehyde resin can provide improved water resistance and may be combined with the urea resin to provide an improved product. Molding compounds are about the next most important outlet for amino resins. It is approximately evenly divided between urea and melamine. The primary use for urea moldings is in the electrical field, while the most important area for molded melamine plastic is dinnerware. 

Aminoresins or aminoplastics cover a range of resinous polymers produced by reaction of amines or amides with aldehydes [14,46,47]. Two such polymers of commercial importance in the field of plastics are the urea-formaldehyde and melamine-formaldehyde resins. Formaldehyde reacts with the amino groups to form aminomethylol derivatives which undergo further condensation to form resinous products. In contras to phenolic resins, products derived from urea and melamine are colorless. 

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