Nitrogen resins

A series of compounded flame retardants, based on finely divided insoluble ammonium polyphosphate together with char-forming nitrogenous resins, has been developed for thermoplastics (52—58). These compounds are particularly useful as iatumescent flame-retardant additives for polyolefins, ethylene—vinyl acetate, and urethane elastomers (qv). The char-forming resin can be, for example, an ethyleneurea—formaldehyde condensation polymer, a hydroxyethylisocyanurate, or a piperazine—triazine resin. 

Figure 10. Formaldehyde loss ratios at 20 percent relative humidity for various materials. (Formaldehyde removed from a material divided by that removed from urea-formaldehyde particleboard. Board elution by nitrogen. Resin liberation by weighing bottle test. PF = phenol-formaldehyde) (ML85 5437)...

The methylol group in phenolic resins can be butylated, as in nitrogen resins (see Chapter 13), and this improves compatibility with epoxy resins. 

Very few stoving finishes contain alkyd alone chiefly the finish is hardened by blending the alkyds with nitrogen resins (Chapter 13). Short-medium oil length alkyds are used, probably with some proportion of aromatic solvent present. 

Thermosetting alkyd, polyester and acrylic paints based on nitrogen resins... 

What follows must be qualified by saying that the chemistry of the nitrogen resins has not been established beyond doubt. The account presented here is therefore a simplified outline, but one which nevertheless helps the paint and resin formulator to understand the processes involved. 

The two long-established nitrogen resins have the following features in common ... 

Such features have been assembled in other monomers to give useful nitrogen resins. Cost considerations have prevented most of these from commanding wide usage. The same features and cross-linking mechanism can be built into acrylic polymers. The polymer may be a copolymer of any suitable acrylic or vinyl monomers, but must contain copolymerized amide (e.g. acrylamide). The amide group is thus attached to the polymer chain. 

There are other ways of producing cross-linked acrylic resins (see below and Chapters 14,15 and 16). This route is described here because it makes use of nitrogen resin chemistry. 

In all cases acidity, even in the resin, catalyses the cross-linking process and combination occurs by a condensation reaction between methylol groups in the nitrogen resins and hydroxyl groups in the plasticizing resin ... 

Unlike U/F and M/F resins, acrylic nitrogen resins need not be blended with another component to obtain flexibility this can be built into the acrylic copolymer by appropriate choice of monomers (see examples on p. 171). Thus resins with ester linkages in their main polymer chains (e.g. alkyds, polyesters) can be avoided. A cross-linked polymer network can be built up which is resistant to hydrolysis by alkalis in detergents. Coatings can therefore be formulated which are suitable for domestic appliances, being additionally hard but flexible. 

Like other acrylic copolymers, these self-crosslinking acrylic nitrogen resins may be produced as latices or in water-soluble form and formulated into water-based paints. 

Typical formulations based on nitrogen resins are given below ... 

Short-medium oil epoxy-esters can be used in stoving finishes, with or without nitrogen resins, in place of the corresponding alkyds. They make excellent primers and surfacers, especially for use under motorcar finishes. 

Cross-linkers for epoxy powders can be nitrogen resins or polyamides (see p. 184), but two more popular types require special mention. 

The resin, as has been stated, may be any resin containing hydroxyl groups. This includes castor oil, alkyds, nitrogen resins, epoxy resins, cellulose derivatives and so on, provided always that, if a mixture is used, then the ingredients must be compatible with one another. However, the most popular resins used are saturated polyesters acrylic resins and, to a lesser extent, polyethers. 

Saturated polyester-resins have been defined in Chapter 12 and their use with nitrogen resins was described in Chapter 13. The resins used in polyurethane finishes might contain a selection from the ingredients listed on p. 161 and, amongst others ... 

Alkyds are used in stoving and air-drying finishes, alone and with other resins. Their scope is so great that it will be necessary to confine the following brief discussion on paints to those containing alkyds alone. Their use with nitrogen resins in stoving finishes is covered in Chapter 13. 

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