Other Melamine-Formaldehyde Condensations

While carboxylated latexes are widely accepted as not needing a cure system, those described above for SBRs are applicable, as are melamine formaldehyde or other formaldehyde condensates. Multivalent metal compounds may also be used with carboxylated systems the most common of these is zinc oxide, but other materials such as zirconium ammonium carbonate may also be used. Some of these types of materials have the advantage that they are effective at room temperature. Some functional SBR latexes have their own cure system built into the polymer and are often referred to as self-crosslinking y or as heat-reactive in instances where heat is involved in the curing process. 

Phenolic and other formaldehyde condensation polymers are also important reactive adhesives. Powdered phenolic resin is mixed with abrasive grains and the mixture is compression molded to form grinding wheels. A B-stage phenolic (Chapter XX) in a solvent is used to impregnate tissue paper. The solvent is evaporated, and the dry sheets are placed between layers of wood in a heated press, where the resin first melts and then cures, bonding the wood to form plywood. Similarly, sheets of paper impregnated with a B-stage melamine-formaldehyde resin are laminated and cured to form the familiar Formica counter tops. 

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. 

The reaction of melamine with formaldehyde is a useful one, as the initial product (39) forms a resin on heating. Such condensates are very important polymers (see Section 2.20.6.3 and Chapter 1.11). Melamine and other amino-1,3,5-triazines form salts with aqueous acids. In addition, the amino-1,3,5-triazines form potassium and silver salts (58CRV131). 

The type of polymer obtained depends on factors such as the pH and temperature of reaction, the ratio of melamine to formaldehyde, and the type of catalyst employed. For decorative laminates, melamine-formaldehyde is prepared by reacting melamine in stainless steel kettles under reflux, alkaline conditions with 37% to 46% formaldehyde in aqueous solution. The reaction temperatures used vary from 80 to 100°C and are maintained until the condensation has reached the desired end point—that is, reacted sufficiently but still water-soluble. The end point is checked by measurements of viscosity, cure time, and water tolerance. Depending on the type of laminate to be produced, other constituents (surfactants, plasticizers, release and anti-foam agents) normally are added to the base resin before impregnation of the surface papers. It is common practice also at this stage to adjust the pH by adding acid catalysts. 

Other recent references to phenol—formaldehyde condensations include those of Yeddanapalli et al. [200, 201]. The complications of melamine/ formaldehyde and urea/formaldehyde reactions kinetics are analogous, and have been examined in two recent papers [202, 203]. 

Two-Package Coatings with Aminoformaldehvde Cure. One component consists of a hydroxyl-terminated urethane prepolymer while the other component is an alkyl ether (usually methyl or butyl ether) of a methylolmelamine or methylolurea derivative (melamine- or urea-formaldehyde condensation products or derivatives thereof) (140, 141). Various catalysts may be employed to accelerate the heat cure (. 125 C) and to lower the curing temperature. Curing results by splitting off of the respective alcohol, as shown ... 

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

At 70°C, condensation in neutral or acid media is rapid and irreversible [54]. In the case of the preparation of hexamethylolmelamine, where 1.0 mole of melamine is reacted with 8.0 moles of formaldehyde, essentially only one product is formed. However, the others consist of mixtures of products. For example, in the preparation of monoethylolmelamine, paper chromatography revealed that the mono product as well as the methylol products are formed within 5 min. Recently the kinetics of the hydroxymethylation of melamine with formaldehyde in dimethyl sulfoxide has been reported [55]. 

Melamine is also employed in condensation reactions with urea, thiourea, phenol, or other amino resin starting materials to give resins with particular properties [61]. The addition of acetaldehyde to melamine-formaldehyde or melamine-urea-formaldehyde resins has been shown to improve the storage... 

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

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. 

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 far too many instances trade-name polymer nomenclature conveys very little meaning regarding the structure of a polymer. Many condensation polymers, in fact, seem not to have names. Thus the polymer obtained by the step polymerization of formaldehyde and phenol is variously referred to a phenol-formaldehyde polymer, phenol-formaldehyde resin, phenolic, phenolic resin, and phenoplast. Polymers of formaldehyde or other aldehydes with urea or melamine are generally referred to as amino resins or aminoplasts without any more specific names. It is often extremely difficult to determine which aldehyde and which amino monomers have been used to synthesize a particular polymer being referred to as an amino resin. More specific nomenclature, if it can be called that, is afforded by indicating the two reactants as in names such as urea-formaldehyde resin or melamine-formaldehyde resin. 

The first synthetic plastics were the phenol-formaldehyde resins introduced by Baekeland in 1907 [1], Melamine and urea also react with formaldehyde to form intermediate methylol compounds which condense to cross-linked polymers much like phenol-formaldehyde resins. Paper, cotton fabric, wood flour or other forms of cellulose have long been used to reinforce these methylol-functional polymers. Methylol groups react with hydroxyl groups of cellulose to form stable ether linkages to bond filler to polymers. Cellulose is so compatible with these resins that no one thought of an interface between them, and the term reinforced composites was not even used to describe these reinforced systems. 

It also can be produced directly from natural gas, methane, and other aliphatic hydrocarbons, but this process yields mixtures of various oxygenated materials. Because both gaseous and liquid formaldehyde readily polymerize at room temperature, formaldehyde is not available in pure form. It is sold instead as a 37 percent solution in water, or in the polymeric form as paraformaldehyde [HO(CH20)nH], where n is between 8 and 50, or as trioxane (CH20)3. The greatest end use for formaldehyde is in the field of synthetic resins, either as a homopolymer or as a copolymer with phenol, urea, or melamine. It also is reacted with acetaldehyde to produce pentaerythritol [C(CH2OH)4], which finds use in polyester resins. Two smaller-volume uses are in urea-formaldehyde fertilizers and in hexamethylenetetramine, the latter being formed by condensation with ammonia. 

The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is the presence of a cross-linked network structure. As we have seen from the above discussion, in the case of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible, insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which are the most widely used thermosets, consist essentially of two classes of thermosets. These are the condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or with urea or melamine (aminoplastics or amino resins). 

The aminoplastics are condensation products of formaldehyde and urea, thiourea, melamine, or aniline. They are often filled with finely ground wood, stone, or other inorganic fillers and are used mainly as molded parts or laminates. All aminoplastics contain nitrogen and bound formaldehyde, which can be identified using chromotropic acid (see Section 6.1.4). 

Thermosets can be divided into several classes depending on the chemical composition of the monomers or pre-polymers (resins). Important thermosetting resins in current commercial applications are the condensation products of formaldehyde with phenol (phenolic resins), urea or melamine (amino resins). Other major classes are epoxy resins, unsaturated polyester resins, allyl resins and isocyanate resins. 

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