Melamine, condensation

Figure 4d represents in situ encapsulation processes (17,18), an example of which is presented in more detail in Figure 6 (18). The first step is to disperse a water-immiscible Hquid or soHd core material in an aqueous phase that contains urea, melamine, water-soluble urea—formaldehyde condensate, or water-soluble urea—melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule sheU (18). This is an anionic polymer or copolymer (Fig. 6). SheU formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2—4.5. The system is heated for several hours at 40—60°C.

The salts can be classified in three main classes salts which undergo thermal dissociation to acid and melamine salts of strong acids which catalyse melamine condensation salts of acids which react with melamine condensation products. Implications of the thermal behaviour of the salts in the mechanism of fire retardance is briefly discussed. 

Salts of strong acids which catalyse melamine condensation... 

Salts of acids which react with melamine condensation products. 

Salts of Strong Acids which Catalyse Melamine Condensation. 

Limited decomposition of the sulphate and extensive melamine condensation overlap in this second step with condensation of sulphuric acid residues to pyrosulphuric structures. This is shown by water evolution and appearance of typical absorptions of pyrosulphate... 

Salts of Acids which React with Melamine Condensation Products. 

Figure 12). In this step the DTG curve shows a very broad peak with a narrower maximum superimposed (550 C) indicating the occurrence of different overlapping processes. This thermal behaviour cannot be explained on the basis of that of melamine condensation products or of ultraphosphates (e.g. ammonium salt). Indeed melamine condensate undergoes complete fragmentation to volatile products below 750 C (18) while ammonium ultraphosphate does so mostly below 700 C (29). in TG at 10 C/min. The presence of P in the material obtained at 650 C is shown by the solid state 31P NMR which however gives broad complex... 

Combined condensation of melamine and of the acid residue is also shown by the phosphate and borate. In these cases however, the polymeric dehydrated compound derived from the acid is thermally stable up to above 500 C allowing chemical reactions with melamine condensation products simultaneously formed. The resulting material is stable to 950 C (phosphate, ca 30% of original salt) or to above 1100 C (borate, ca. 20% of original salt). 

Melamine, a white powder, was discovered and identified by Liebig in 1834 but commercial manufacture came only in 1939, by Cyanamid Company of America with dicyandiamide as raw material. Melamine is 2,4,6-triamino-l,3,5-triazine with a structure as shown in Figure 53. On reaction with formaldehyde in aqueous solution the melamine powder dissolves rapidly on heating to form various methylol melamines, as in Figure 54. After further heating and the elimination of water the methylol melamines condense to form resinous polymers. 

Melamine condenses with formaldehyde to give a widely used plastic with good heat resistauce, most familiar iu lamiuates for kitchen worktops and as housewares, and known as Formica. 

Acrylated melamines can also cure by condensation reactions. The condensation reactions will proceed either before or after a UV cure. Use of acrylated melamines in a dual cure process allows a unique combination of melamine condensation and acrylate free radical addition chemistries. This can result in increased hardness,... 

The combustion behavior of melamine pyrophosphate and dimelamine phosphate are different from those of melamine and the other melamine salts (Table 3.4.1). The former are ineffective at low concentrations (> 15%) and become effective at a loading of 20-30% because the intumescent char is formed on the surface of burning specimens. The mechanism of fire retardant action of both melamine pyrophosphate and dimelamine phosphate is similar to that of APP since, by analogy with ammonia melamine volatilizes, whereas the remaining phosphoric acids produce esters with nylon-6, which are precursors of the char [146]. Some part of the freed melamine condenses probably forming melem and melon [147]. 

Upon heating, melamine-based salts dissociate, and re-formed melamine volatilizes in a manner similar to pure melamine. However, in the case of melamine salts, a larger portion of melamine undergoes progressive condensation than does pure melamine therefore, the condensed-phase contribution of the salts is larger. If the anion contains phosphorus, the phosphoric acid released will phos-phorylate many polymers and produce a flame retardant effect similar to that of other typical phosphorus-based additives (see above). Melamine condensates and phosphoric acid react further at temperatures above 600° C, where triazine rings are opened and cross-linked. A (PON) type of structure known as phosphorus oxynitride is formed. Phosphorus oxynitride is very thermally stable and in some polymers can contribute to condensed-phase mechanisms. ... 

Melamine-formaldehyde colloidal solutions are produced by melamine condensation with formaldehyde in aqueous suspension at pH = 4-9 and CH2O melamine molar ratio = 3-5. The precondensate thus formed is mixed with an aqueous solution of formaldehyde (5 to 20 mol per mol of melamine). The duration of the process is much more reduced if a Mannich base is used as a catalyst [36]. 

Chem. Descrip. Fatty melamine condensate Uses Used to make wax-melamine textile water repellents X78-2 [Lubrizol Advanced Materials]... 

It is manufactured by heating dicyandiamide, H2N C(NH) NH CN, either alone or in the presence of ammonia or other alkalis, in various organic solvents. Melamine is an important material in the plastics industry. Condensed with melhanal and other substances it gives thermosetting resins that are remarkably stable to heat and light. U.S. production 1980 80 000 tonnes. 

Melamine - formaldehyde polymers. Melamine (2 4 6-triamino-1 3 5-triazine), obtained by heating dicyandiamide under pressure, condenses with formalin to give melamine - formaldehyde polymers (Beetle - Melamine), which have similar uses, but better stability to heat... 

Titanium alkoxides are used for the hardening and cross-linking of epoxy, siUcon, urea, melamine, and terephthalate resins in the manufacture of noncorrodable, high temperature lacquers in the sol-gel process as water repellents and adhesive agents (especially with foils) to improve glass surfaces as catalyst in olefin polymeri2ation, and for condensation and esterification. 

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. 

By the mid-1990s world production of aminoplastics was estimated at about 6 000 000 t.p.a. of which more than 5 000 000 t.p.a. were urea-formaldehyde resins. The bulk of the rest were melamine-formaldehyde. Such bald statistics, however, disguise the fact that a considerable amount of aminoplastics used are actually co-condensates of urea, melamine and formaldehyde. 

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. 

In a typical process a jacketed still fitted with a stirrer and reflux condenser in charged with 240 parts 37% w/w (40% w/v) formalin and the pH adjusted to 8.0-8.5 using sodium carbonate solution with the aid of a pH meter. One hundred and twenty six parts of melamine (to give a melamine formaldehyde ratio of 1 3) are charged into the still and the temperature raised to 85°C. The melamine goes into solution and forms methylol derivatives. For treatment of fabrics, paper and leather this product may be diluted and cooled for immediate use. It may also be spray dried to give a more stable product. Cooling the solution would yield crystalline trimethylolmelamine, which may be air dried but which is less soluble in water than the spray-dried product. 

Resins for this purpose generally use melamine-formaldehyde ratios of 1 2.2 to 1 3. Where electrical grade laminates are required the condensing catalyst employed is triethanolamine instead of sodium carbonate. 

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. 

Paper with enhanced wet-strength may be obtained by incorporating melamine resin acid colloid into the pulp. Melamine resin acid colloid is obtained by dissolving a lightly condensed melamine resin or trihydroxymethylmelamine, which are both normally basic in nature, in dilute hydrochloric acid. Further condensation occurs in solution and eventually a colloidal solution is formed in which the particles have a positive charge. Careful control over the constitution of the colloidal solution must be exercised in order to obtain products of maximum stability. 

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