MELAMINE CYANURATE

Flame retardants designated for nylon include halogenated organic compounds, phosphorous derivatives, and melamine cyanurate (160—163). Generally, flame retardants are difficult to spin in nylon because of the high loading required for effectiveness and their adverse effects on melt viscosity and fiber physical properties. 

The ideal recommended cyanuric acid concentration is 30—50 ppm (Table 2). Although this range can be readily maintained when using hypochlorite sanitizers, it cannot be maintained when using chloroisocyanurates since they increase the cyanuric acid concentration. The NSPI recommends a maximum of 150 ppm cyanuric acid. Many health departments limit cyanuric acid to 100 ppm. No significant increase in stabilization occurs beyond 50—100 ppm, and since high levels of cyanuric acid slow down the rate of disinfection, the pool water should be partially drained and replaced with fresh water to reduce the cyanuric acid to below recommended maximum levels. Cyanuric acid is determined turbidimetricaHy after precipitation as melamine cyanurate. 

Although much weaker than the parent compound isocyanic acid (pK = 3.7), CA is sufftciendy acidic to form salts. Many inorganic and organic salts of CA have been reported (11). Lead and 2iuc cyanurates are usehil as corrosion inhibitors (20). The 1 1 adduct, melamine cyanurate [37640-57-6] has achieved commercial importance. 

Melamine cyanurate is useful in preparation of flame retardant polyamide resins and compositions (133). It also is useful as a soHd lubricant (134). 

Hydrolysis of polyamide-based formulations with 6 N HC1 followed by TLC allows differentiation between a-aminocaproic acid (ACA) and hexamethylenedi-amine (HMD) (hydrolysis products of PA6 and PA6.6, respectively), even at low levels. The monomer composition (PA6/PA6.6 ratio) can be derived after chromatographic determination of the adipic acid (AA) content. Extraction of the hydrolysate with ether and derivatisa-tion allow the quantitative determination of fatty acids (from lubricants) by means of GC (Figure 3.27). Further HC1/HF treatment of the hydrolysis residue, which is composed of mineral fillers, CB and nonhydrolysable polymers (e.g. impact modifiers) permits determination of total IM and CB contents CB is measured quantitatively by means of TGA [157]. Acid hydrolysis of flame retarded polyamides allows to determine the adipic acid content (indicative of PA6.6) by means of HPLC, HCN content (indicative of melamine cyanurate) and fatty acid (indicative of a stearate) by means of GC [640]. Determination of ethylene oxide-based antistatic agents... 

Above its melting point (360 0 about 90% of melamine cyanurate is converted to volatile products (360-450 0, 2nd step) in which free melamine and melamine cyanurate were recognised by IR. This indicates that a competition takes place between evaporation of the unalterated salt and its thermal dissociation to melamine and cyanuric acid. Melamine behaves then as described above while cyanuric acid which, heated alone in TG volatilises completely above 300 0, is known to decompose to cyanic acid (8). 

Figure 4. IR of residue of dimelamine cyanurate heated at 350 C melamine cyanurate (by comparison with reference compound).

Generally, flame retardants for engineering PET compositions are based on bromine-containing compounds (such as brominated polycarbonate, decabro-modiphenyl oxide, brominated acrylic, brominated polystyrene, etc.). Such compounds are available commercially (such as from the Ethyl Chemical Corporation, Great Lakes Chemical Corporation, Dead Sea Bromine Company, etc.) In addition, the flame-retardant package generally contains a synergist, typically sodium antimonate. PET may also be flame-retarded with diarylphosphonate, melamine cyanurate or red phosphorus. 

The 1,3,5-triazines are amongst the oldest known organic molecules. Originally they were called the symmetric triazines, usually abbreviated to s- or sym- triazines. The numbering follows the usual convention of beginning at the heteroatom as shown for the parent compound 1,3,5-triazine (1). Rather non-systematic nomenclature is prevalent even in the current literature, because some of the compounds have been known for at least 150 years. The non-systematic names of some of the more important 1,3,5-triazines are listed in Table 1. The terms melamine, cyanuric acid and cyan uric chloride will be used throughout this chapter, and the term triazine will refer to 1,3,5-triazines only. In addition to the above names, 2,4,6-trialkoxy-l,3,5-triazines (2) are called cyanurates. Similarly, 1,3,5-trialkyl-1,3,5-triazines (3) are called isocyanurates. 

Figure 10.51 Melamine-cyanuric acid derivatives arranged according to their relative stability as measured by the HBI(N — 1) parameter. (Reprinted with permission from Section Key Reference 1995 American Chemical Society).

FIGURE 4.3. Molecular structures and schematic representations of tetra- hexa- and octamelamines 2, 3 and 4, respectively, and di(melamine-cyanurate) 5. The molecular structure of DEB and different isocyanuric acid derivatives (CYA) are also shown. 

One of the earliest examples of chiral melamine-cyanurate was reported by Whitesides and collaborators, who studied the hydrogen-bonded aggregate between three molecules of chiral cyanurate and a complementary achiral fr/ s-melaminc unit based on a 1,3,5 trisubstituted benzene with a melamine unit appended at each arm (Fig. 5) [20]. The fris-mclaminc unit is not chiral itself, but in the complex with cyanurate forms a tightly bonded hydrogen bond platform with C3 symmetry which is chiral at a supramolecular level due to the clockwise (P) and counterclockwise (M) conformation of the pendants (Fig. 5). 

Scheme 3.4 Tape vs. hexagonal rosette formation in Whitesides melamine-cyanurate aggregates [10].

Braun, U. and Schartel, B. 2008. Flame retardancy mechanisms of aluminium phosphinate in combination with melamine cyanurate in glass-fibre-reinforced poly (1,4-butylene terephthalate). Macromol. Mater. Eng. 293 206-217. 

J. Zhang, M. Lewin, E. Pearce, M. Zammarano, and J.W. Gilman, Flame retarding PA6 with melamine cyanurate and layered silicates, Proceedings of the 19th BCC Conference on Flame Retardancy, M. Lewin (Ed.), Business Communications Co Editions, Norwalk, CT, 2008. 

Braun U, Bahr H, Sturm H, Schartel B. Flame retardancy mechanisms of metal phosphinates and metal phosphinates in combination with melamine cyanurate in glass-fiber reinforced poly(l,4-butylene terephthalate) The influence of metal cation. Polym. Adv. Technol. 2008 19 680-692. 

We used our experience with reversible encapsulation to arrive at a rule regarding the proper filling of space, the 55% solution [87]. The filling of space probably drives other recognition phenomena, even in those synthetic receptors that do not completely surround their targets [88]. Some of the earliest, finite self-assemblies in solution based on melamine/cyanuric acid recognition [89] had no other function than to fill space. However unconventional, the departure from mainstream physical organic chemistry [90] is familiar to us and may offer rewards. 

The complex between melamine and cyanuric acid (1 1) was reported in the literature in the late 1970s, but it was only in the early 1990s that the contributions from Whitesides and the concept of self-assembly popularized these systems [45]. Whitesides and co-workers reported the formation of tapes (Fig. 11.11), crinkled tapes and cyclic hexamers (rosettes) formed between barbituric acid and N,N -bis(p-substituted phenyl)melamine [46], In this they effectively blocked one face of melamine and, by manipulating substituents at the para position, different structures were obtained. Whiteside s putative suggestion that melamine/cyanuric acid formed an extended array (Fig. 11.12) was confirmed recently by Rao et al. with the crystal structure [47]. Hamilton and coworkers reported the crystal structure of a 5-substituted isophthalic acid derivative, which forms a cyclic aggregate held together with six pairs of hydrogen bonds, which in a way resembles the trimesic acid (Fig. 11.13) [48]. 

Figure 11.12. Melamine/cyanuric acid crystal structure.

This approach mimics familiar biological self-assembly phenomena such as protein folding [ 192], protein aggregation [ 192] and nucleotide pairing [ 188]. It incorporates features described in each of the above strategies (i.e., I—III), to give specialized nanoscopic structures, that can be precisely designed, usually with excellent control over CMDPs. Recent examples include so called structure directed synthesis by Stoddart [3a] (see Chapter 1 of this book) to produce toroidal bis-bipyridinium cyclophanes that are reminiscent of a molecular abacus , melamine-cyanuric acid lattices by Whitesides [193] and unique helical structures based on coordination of bipyridyl units to copper (II) ions by Lehn [194],... 

Fig. 21 The hexagonal melamine-cyanuric acid hydrogen-bonded array [ 166]...

---