Amino resin systems

Most FR formulations are not resistant to leaching by water. Therefore, there have been increased efforts to develop leach-resistant chemicals that can be impregnated into wood products for use in exterior or high-humidity applications. Some of the proposed leach-resistant systems include chemical combinations that form insoluble complexes, amino-resin systems, and monomers that polymerize in the wood. A common amino-resin system for exterior use is dicyandiamide phosphoric acid formaldehyde. Guanylurea phosphate-boric acid also is a commonly used organic phosphate salt for modern commercial FR wood. 

Major Chemicals. Phosphorus. Combination salts of the phosphates have been used for retarding wood since the time of Gay-Lussac. Monoammonium and diammonium phosphates have been the most effective. The efficiency of phosphorus compounds can be increased by the presence of certain nitrogen compounds that produce a synergistic effect. The advantage of such synergism is that increased flame-spread resistance can be achieved with lower chemical loading levels. The amino-resin systems are based on this synergistic effect. 

Boron compounds have been used in several ways to achieve reduced flammability of wood products. Borax and boric acid can be incorporated into particle board chips before addition of a dicyan-diamide, phosphoric acid, amino-resin system 85). They can also be used to produce a fire-retardant hardboard. Riem and Dwars 86) added water-soluble ammonium borate to wood fibers before the board was formed. A 6-7% boron content produced a hardboard that had a flame spread of 25 or less. 

Amino-Resins. The most widely studied leach-resistant systems are the amino-resins. Goldstein and Dreher (9S) first applied these systems as fire retardants. Basically, the amino-resin systems involve the combination of a nitrogen source (i.e., urea, melamine, guanidine, or dicyandiamide) with formaldehyde to produce a methylo-lated amine. The new product is then reacted with a phosphorus compound such as phosphoric acid. Because there is a synergistic effect between the phosphorus and the nitrogen, reduced loading... 

In the past, the problems with the amino-resin systems were its limited pot life, leaching of the phosphorus, and excess formaldehyde emission. Recent research has addressed these problems. 

Researchers at the Eastern Forest Products Laboratory in Canada have evaluated the urea and melamine amino-resin systems (9, 57, 99-110). Their work demonstrates that both systems show good leach resistance and reduced flame spread. The stability of these resins is controlled by the rate of methylolation of the urea, melamine, and dicyandiamide. The optimum mole ratio for stability of these solutions is 1 3 12 4 for urea or melamine, dicyandiamide, formaldehyde, and orthophosphoric acid. However, even at the optimum mole ratios, the pot life of the melamine system is less than that of the urea system. In both systems the nitrogen is fixed to a greater degree than the phosphorus. However, the degree of fixation of the phosphorus is greater with the melamine than with the urea. The melamine structure may promote formation of compounds with phosphoric acid that are less soluble than those from urea and dicyandiamide. 

Another advantage of the amino-resin systems is their applicability to solid wood and wood-composite products. Cedar shingles were the first products treated with this type of fire-retardant system (99, 100, 113, 114). Commercially treated shingles available in the U.S. are based on these systems. Generally, these systems exhibit good durability to outdoor weathering when tested over extended periods (115-17). 

Of all the proposed leach-resistant formulations, only the amino-resin systems are used commercially. The high costs of many of the other proposed techniques limit their acceptability. 

Future Research. Improvements in leach-resistant chemicals have been a primary concern over the past decade. Advances have been made in leach-resistant systems such as the amino-resin systems however improvements still need to be made in leach-resistant compounds without increasing the cost. Other areas where research on fire retardants needs to be conducted are in coating systems, especially those that are durable to weathering and UV degradation reduction of smoke and toxic products, improvements in fire-retardant treatments for panel products and fundamental work on the mechanisms of particular formulations. 

A new class of amino resin system based on glycoluril-formaldehyde has been described by Parekh recently. This new class of resin, when compared to MF resins, has been found to cure at low temperature (for example, 120 °C in 20 min) and showed improved chemical-resistance properties, good hardness, flexibility and adhesion and low formaldehyde evolution in both water-borne and solvent-based coatings. 

Amino resins are also often used for the cure of other resins such as alkyds and reactive acryUc polymers. These polymer systems may contain 5—50% of the amino resin and are commonly used in the flexible backings found on carpets and draperies, as well as in protective surface coatings, particularly the durable baked enamels of appHances, automobiles, etc. 

Epoxy—amino resin weight ratios, 10 445 Epoxy—anhydride systems, properties of, 10 403-404... 

Both thermoset and thermoplastic resin systems are employed in the construction of composites (Table 8.3). The most common thermoset resins are polyimides, unsaturated polyesters, epoxys, PFs, and amino-formaldehydes. A wide variety of thermoplastic resins have been developed. 

Both aqueous polymer-based systems (latex), made by emulsion or dispersion polymerization, and oil-modified alkyd resin-based systems are still in wide use [781], Table 12.2 shows the composition of a typical water-based emulsion paint. There is a wide variety of coatings, ranging from broad applicability to highly specialized, including latexes, amino resins, isocyanates, epoxy resins, acrylic resins, polyester... 

Melamine is used primarily in the manufacture of amino resin. The resin formation reaction is analogous to the urea-formaldehyde system. 

Since both the amino resin and the polymer have multiple reactive sites, application of sufficient heat will yield a cross-linked structure with the degree of cure depending on the functionality of the system and on how far to the right the reaction (24) is driven. 

Nylok . [J.M. Huber] Amino-functional calcined clay filler, reinf( cement for polyamide resin systems. 

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