Melamine-Formaldehyde Condensation

Crosslinking (hardening) of these pre-condensates can be carried out exactly as for the urea-formaldehyde resins, best at a pH value of 3.5-5. Melamine-formaldehyde condensates crosslink most quickly if prepared using a 2.8-3-fold excess of formaldehyde. 

Urea- and melamine-formaldehyde resins are used as moldings, lacquers, and adhesives (for wood), also as textile additives (increased crease resistance) and paper additives (improved wet strength). 

Sflfefyprecflof/o/75 Before this experiment is carried out. Sect. 2.2.5 must be read as 

Couf/on Because of the formation of gaseous formaldehyde all reactions must be carried out in a closed hood. 

The experimental arrangement consists of a 250 ml three-necked flask equipped with stirrer, double-necked head with reflux condenser and thermometer, and a rubber stopper through which passes a glass rod. 63 g (0.5 mol) of melamine and 150 g of a 40% aqueous formaldehyde solution (2.0 mol) are placed in the flask the aqueous suspension is stirred and adjusted to pH 8.5 by adding a few drops of 20% NaOH.The mixture is heated on a water bath to 80 °C within 5-10 min, with continuous stirring. Complete solution is reached at 70 to 80 °C. During this warm-up period the decrease of pH of the solution must be continually compensated by dropwise addition of 20% NaOH. The stirred solution is now heated to 80 °C at constant pH of 8.5 until the precipitation ratio (see below) reaches 2 2. Next the solution is cooled and filtered from small amounts of insoluble material. 

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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-formaldehyde condensates are also useful in textile finishing. For example, they are useful agents for permanent glazing, rot proofing, wool shrinkage control and, in conjunction with phosphorus compounds, flameproofing. 

There is also growing interest in multi-phase systems in which hard phase materials are dispersed in softer polyether diols. Such hard phase materials include polyureas, rigid polyurethanes and urea melamine formaldehyde condensates. Some of these materials yield high-resilience foams with load deflection characteristics claimed to be more satisfactory for cushioning as well as in some cases improving heat resistance and flame retardancy. 

Amino Resins and Plastics. The commercially important amino resins are the urea-formaldehyde and the melamine-formaldehyde condensates. Of lesser importance are the sulfonamide, aniline and thiourea resins... 

Fig. 11.3 Repeating units of the structures of supcrplaslicizcr anions (A) naphthalene formaldehyde condensate (B) melamine formaldehyde condensate.

Melurac [Cytec]. TM for urea-melamine-formaldehyde condensation products used mainly as adhesives for bonding of veneers for the production of exterior-grade plywood. 

Basocoll CM. [BASF AG] Anionic melamine-formaldehyde condensate-based sizing assistant for papermaking. 

Sulphonated melamine formaldehyde condensates Sulphonated napthalene formaldehyde condensates Modified lignosulphonates. 

M. Chiavarini, N. Del Fanti, R. Bigatto "Compositive Characterization of Melamine-Formaldehyde Condensates by NMR Spectroscopy, Angew. Makromol. Cliem. 56(1976) no. 818, 15-25. 

Other reactions can also crosslink resins with pendant carboxylic acid groups. For instance, one can add a melamine formaldehyde condensate ... 

Urea- and Melamine-Formaldehyde Condensation Products 4.1.4.1 Urea-Formaldehyde Resins... 

The flow properties of the slurry may be improved by adding suitable dispersing agents, such as sulforrated rraphthalene formaldehyde condensate (SNFC), sulfonated melamine formaldehyde condensate (SMFC), or lignin-based dispersants. 

Water reduction Salts of sulfonated naphthalene and melamine formaldehyde condensates, lignosulfonates... 

Pyrovatex CP (Ciba Geigy), the A-methylol derivative of dimethylphosphonopropionamide reacted with a melamine/formaldehyde condensate used with an acid catalyst such as phosphoric acid ... 

Fig. 2.41, Thermal curing of prepolymers obtained from melamine-formaldehyde condensation reaction...

The incorporation of s-triazine ring structures into thermoset resins has been practiced for a number of years by incorporation of melamine formaldehyde condensates into the formulations. Curing occurs through a further condensation reaction with the release of volatile material. Aryl cyanate esters cyclotrimerize to S triazine structures upon heating above 150°C without the release of volatiles. 

TABLE XIV Other Melamine-Formaldehyde Condensations/Resins... 

A typical example [84] involves the preparation of an alkyd resin from phthalic anhydride (712 gm), trimethylolpropane (399 gm), isodecyl alcohol (277 gm), and xylene (22 gm) and heating (190°-280°C) to remove water azeotropically and give a resin of acid value 23. The product is diluted with a 4 1 xylene butyl alcohol mixture to 75% solids. A lacquer is prepared by mixing 16 gm of the above alkyd resin solution with 3 gm of a methylated melamine-formaldehyde condensate along with 7.5 gm of titanium oxide and 0.225 gm of /7-toluenesulfonic acid monohydrate. Sufficient xylene butanol (4 1) is added to give a 55% solids solution. A film of this solution is dried for 30 min at 120°C. The film is hard and stable to 10% sodium hydroxide. 

Castor oil fatty acids have one double bond per molecule, whereas hydrogenated castor oil fatty acids are saturated consequently, the oils, or alkyds derived from them, are not film forming under autooxidation conditions. However, alkyds prepared from these oils are widely used as polymeric plasticizers for other filmforming resins the two most important types are cellulose nitrate and melamine-formaldehyde condensates. 

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