Melamine-formaldehyde polymers resin preparation

Step-growth polymerization is a very important method for the preparation of some of the most important engineering and specialty polymers. Polymers such as polyamides [7], poly(ethylene terephthalate) [8], polycarbonates [9], polyurethanes [10], polysiloxanes [11], polyimides [12], phenol polymers and resins, urea, and melamine-formaldehyde polymers can be obtained by step-growth polymerization through different types of reactions such as esterification, polyamidation, formylation, substitution, and hydrolysis. A detailed list of reaction types is shown in Table 3.2. 

When crosslinked, amino resins are very resistant to most organic solvents, though they tend to be attacked by both acids and alkalis. Urea-formaldehyde polymers are more susceptible to attack than those prepared from melamine and formaldehyde. 

These reactions form polymer melamine crosslinks (Ml and MIO), melamine-melamine crosslinks (M2, M3, M4, M5, M8, M9, and Mil) or Interconvert functional groups (M6 and M7). The Importance of the different reactions depends on the catalyst level and type, the bake conditions, and most Importantly on the structure of the melamine resin. Reaction Mil occurs only under basic conditions (used In the preparation of melamine-formaldehyde crosslinkers) and can be Ignored In coatings where acid catalysts are used. Reaction MIO Is slow compared to reaction Ml (5). The reactions Involving water probably make at most a minor contribution under normal bake conditions. The most Important reactions appear to be Ml for fully alkylated melamines and Ml and M9 for partially alkylated melamines. Reaction M4... 

In the early 1930 s, a second type of resin prepared from formaldehyde was introduced to the market—namely, urea-formaldehyde resins. A few years later, melamine-formaldehyde resins also appeared. The same basic process is employed in polymerization of all these resins it consists of the catalyzed reaction of formaldehyde with the second ingredient—phenol, urea, or melamine—to evolve water and produce three-dimensional, cross-linked thermosetting polymers. 

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. 

Currently, the bulk of the conunercial polymers that would fit into this category are urea-formaldehyde and melamine-formaldehyde resins. Over the years, however, many other materials that might fit into this group were prepared but not adopted for use for various reasons. 

Different objects may be used as the sacrificial core to prepare hollow capsules, e.g. particles of polymers e.g., melamine-formaldehyde resin, poly(sodium styrenesulfonate) (PSSS) ], inorganic compounds [e.g., CaCOg or SiOa ), metal nanoparticles, or even bio-objects such as erythrocytes or platelets. Multilayers can be also deposited directly on the nano- or microcrystals of drugs. 

Melamine-formaldehyde. The monomer used for preparing melamine formaldehyde is formed by reacting melamine with formaldehyde to yield hexamethylolmelamine. The monomer can further condense in the presence of an acid catalyst ether linkages can also form. A wide variety of resins can be obtained by careful selection of pH, reaction temperature, reactant ratio, amino monomer, and extend of condensation. Liquid coating resins are prepared by reacting methanol or butanol with the initial methylolated products. These can be used to produce extreme surface hardness, discoloration and solvent-resistant coatings by heating with a variety of hydroxy, carboxyl, and amide functional polymers to produce a cross-linked film. 

Formaldehyde-based resins were the first network polymers prepared by step polymerization to be successfully commercialized. They are prepared in two stages. The first involves the formation of a prepolymer of low molar mass which may either be liquid or solid. In the second stage the prepolymer is forced to flow under pressure to fill a heated mould in which further reaction takes place to yield a highly crosslinked, rigid polymer in the shape of the mould. Since formaldehyde is difunctional, the coreactants must have a functionality, /, eater than two and those most commonly employed are phenol (/= 3), urea (/= 4) and melamine (/= 6)... 

Amino resins are condensation thermosetting polymers of formaldehyde with either urea or melamine. Melamine is a condensation product of three urea molecules. It is also prepared from cyanamide at high pressures and temperatures ... 

Amino resins are those polymers prepared by reaction of either urea or melamine with formaldehyde. In both cases the product that results from the reaction has a well crosslinked network structure, and hence is a thermoset polymer. The structures of the two parent amino compounds are shown in Figure 1.1. 

Several copolymers and condensates of oxidized starches with polymers have been developed. For example, products of starch dialdehyde condensation with acrylamide were prepared for further copolymerization with various monomers to form resins for coatings, molding powders,585 and materials for immobilization of enzymes, for instance, alpha amylase.586 Hypochlorite-oxidized starches were also reacted with acrylonitrile.507,521 Hypochlorite-oxidized starches were allowed to react with allylated starch dialdehyde,587 polycondensates of ammonia-dimethylamine-epichlorohydrin,588 polycondensates of starch dialdehyde with melamine,589 urea433,541,590 capable of precipitation of tannin591, carboxyamides,411 urea and formaldehyde,592 proteins,524,593,594 polyfyinyl alcohol),595 alkylammonium salts,519,596 alkoxyalkylamines,597... 

Formaldehyde is employed in the production of aminoplasts and phenoplasts, which are two different but related classes of thermoset polymers. Aminoplasts are products of the condensation reaction between either urea (urea-formaldehyde or UF resins) or melamine (melamine-formaldeliyde or MF resins) with formaldehyde. Phenoplasts or phenolic (phenol-formaldehyde or PF) resins are prepared from the condensation products of phenol or resorcinol and formaldehyde. 

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