Melamine-formaldehyde structure

Highly dispersed crosslinked melamine-formaldehyde structures have been prepared by Reimer [37]. In a typical open pore melamine-formaldehyde preparation, a melamine resin that has a low memelamine fonnaldehyde molar ratio (1 1.6) and that contains 15% sucrose as an additive was converted into an open pore melamine—formaldehyde oligomer by acidification with 7% phosphoric acid (based on resin solid) at 50-60°C. 

LW Hill, S Lee. Effect of melamine-formaldehyde structure on cure response of thermoset coatings. J Coahngs Technology 71(897) 127-133, 1999. 

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... 

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)... 

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 chemical structures of thermosets are generally much more diverse than the commodity thermoplastics. The most common types of thermosets are the phenol-formaldehydes (PF), urea-formaldehydes (UF), melamine-formaldehydes (MF), epoxies (EP), polyurethanes (PU), and polyimides (PI). Appendix 2 shows the chemical structure of these important thermosetting polymers. 

Both melamine—formaldehyde (MF) and resorcinol—formaldehyde (RF) followed the earlier developments of phenol—, and urea—formaldehyde. Melamine has a more complex structure than urea and is also more expensive. Melamine-base resins require heat to cure, produce colorless gluelines, and are much more water-resistant than urea resins but still are not quite waterproof. Because of melamine s similarity to urea, it is often used in fairly small amounts with urea to produce melamine—urea—formaldehyde (MUF) resins. Thus, the improved characteristics of melamine can be combined with the economy of urea to provide an improved adhesive at a moderate increase in cost. The improvement is roughly proportional to the amount of melamine used the range of addition may be from 5 to 35%, with 5—10% most common. 

By means of chemical reactions thermosetting plastics form three-dimensional structures. In the example above the nitrogen compound urea reacts with formaldehyde (methanal), in which process three molecules combine and a molecule of water is formed. In this example two H atoms react, but all other H atoms ( ) enter into the same reaction. Since urea is a three-dimensional molecule, the network will also be three-dimensional. For instance switches and sockets are made of UF. Other thermosetting plastics are polyurethane PU (insulation) and melamine-formaldehyde MF (panels). 

A structurally related macromolecular IV.lV-acetal is obtained from melamine (2,4,6-triamino-1,3,5-triazine) and formaldehyde. It is called melamine/formaldehyde resin and is likewise used as a plastic. 

Concrete Additives. Melamine-formaldehyde resins can be sulphonated and then used in fresh concrete to create a plasticizing effect. The resin makes 1) concrete more flowable, thus improving concrete s workability and 2) allowing the water content to be reduced for faster drying times. The end result is an improved, stronger material that is structurally superior. 

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

Mesoporous melamine-formaldehyde and phenolic-formaldehyde resins were synthesized in the process of polymerization in the presence of fumed silica as an inorganic template. The surface and structural characteristics of the obtained sorbents were investigated using XPS technique and sorption from gas phase. The parameters characterizing porous structure of the synthesized resins in a dry state were determined from nitrogen adsorption/desorption isotherms. The sorption processes of benzene and water vapor accompanied by simultaneous swelling of both polymers were also studied. 

Data of low-temperature nitrogen adsorption were used to evaluate the parameters characterizing the pore structure of the obtained polymeric materials in dry state. The BET specific surface area, Sbet, and the total pore volume, V, were estimated by applying the standard methods Sbet from the linear BET plots and F/ from adsorption at relative pressure p/po=0.975) [7]. The mesopore structure was characterized by the distribution function of mesopore volume calculated by the Barret-Joyner-Halenda (BJH) method [27]. In Table 2 the values of these parameters are given for both synthesized polymers. The melamine-formaldehyde resin MEA has a more developed pore structure (5 B 7=220mVg, F,=0.45cm /g) and narrower mesopores (D=7.3nm) in comparison to the phenolic-formaldehyde polymer PHD. 

The synthesis of phenolic-formaldehyde and melamine-formaldehyde resins in the presence of fumed silica allows obtaining porous organic materials with a differentiated porous structure and surface properties. The pore characteristics of the studied resins in dry state were determined from nitrogen adsorption isotherms. The differences in surface character of the synthesized polymers were estimated satisfactorily by XPS spectra showing the presence of various functional groups. The adsorption/desorption mechanism of water and benzene on the investigated porous polymers was different due to differentiated hydrophobicity of the bulk material. 

The reactions of melamine (the structure of which is shown in Scheme 1.16) are similar to those of urea except that there are three amino groups per molecule, so it is hexafunctional with respect to formaldehyde and it is possible to separate the different reaction products with formaldehyde up to hexamethylol melamine. The crosslinking reactions are identical to those of urea-formaldehyde resins. 

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