Structure allophanate

Modes of origin of allophane have been discussed by Fieldes [1962, 1966]. The latter paper suggests that structural randomness is the principal intrinsic property of allophane. Allophane is frequently produced by weathering of glasses and other disordered aluminosilicates or by processes leading to fast precipitation or very fine subdivision (Fieldes and Furkert [1966]) which are likely to result in random structures. Allophane accumulates in soils where soil conditions favor the persistence of random structures. Effects of this nature on the formation of allophane in rocks and soils are summarized briefly below. 

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

Lastly, of course, the main reaction of interest is the formation of urethane groups by reaction of isocyanate groups and hydroxy-groups of the polyester or polyether. Even these reactions do not exhaust the possibilities available to the highly reactive isocyanate group. It will then go on to react with the urethane links to form a structure known as an allophanate (see Reaction 4.13). 

This addition reaction proceeds readily and quantitatively. Side reactions can give amide, urea, biuret, allophanate, and isocyanurate groupings, so that the structure of the product can deviate from that above such side reactions are sometimes desired (see Sect. 4.2.1.2). 

It follows from the structure of cured KL-3 that secondary reactions are possible in the formation of the network manifested in the formation of allophanate and biuret... 

Differential thermal (Belyankin and Ivanova, 1936) and infrared (Adler et al., 1950) studies prove that allophane is not a fine mechanical mixture of alumina and silica but that these are chemically combined as in co-precipitated silica alumina gels. X-ray patterns usually show one or more diffuse bands, which White (1953) interpreted to mean that the structure was more ordered than glass. 

Allophane, in any abundance, is most commonly formed from volcanic material although it can presumably form from any alumino-silicate minerals and indeed is probably present as a transitory stage in the alteration of any material to a clay mineral if any significant structural re-arrangement is required. In its most pure form it occurs as veins and incrustations. Most analyses (Table LXXII) are of samples from this type, of deposit. In volcanic soils (Japan, Australia, N.W.U.S., etc.) where allophane is abundant it is usually intimately mixed with halloysite and collection of... 

The hitherto unknown 3-thioallophanate anion has been trapped in the host lattice of [( -C4H9)4N]+[H2NCSNHC02] -(NH2)2CS. The cyclic structure and molecular dimensions of the allophanate and 3-thioallophanate ions are compared in Fig. 20.4.18. As expected, the C-Obond involved in intramolecular hydrogen bonding in the 3-thioallophanate anion is longer than that in the allophanate anion. 

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

The allophanates represent a hybrid of the urea and urethane structures, and their hydrogen-bonding capability lies between those of the parents. 

Structure 4.1) [7,8,42] as well as absence of unstable biuret and allophanate units [9] seem to be responsible for increased thermal stability and chemical resistance to nonpolar solvents. The reactivity of amine is shown in Scheme 4.2 [42],... 

The oxazolidone formed is not an interesting component in PU chemistry because it does not have reactive groups such as hydroxyl or amino groups to enter into the PU structure (just the low reactivity -NH-COO- urethane group which leads to an allophanate). 

As soils weather and Si, Ca, Mg, Na, and K are leached away, the soil s colloidal fraction becomes enriched with Al, Fe, Mn, and Ti oxides and hydroxyoxides. The structural organization of these hydroxyoxides ranges from amorphous to crystalline. These Al, Fe, and Ti oxides and allophane are prominent nonlayer silicate minerals in most soils and then- content in soils increases with increased weathering. 

Aluminium, on the other hand, accumulates in the clay mineral fraction because it forms insoluble aluminosilicates and hydroxyoxides. The AI remains behind in the soil as other ions leach away. Iron also accumulates in soils but this is not apparent from Table 7.3 because the silicate clay minerals, with the exception of hydrous mica, are low in Fe. Iron precipitates in soils only as hydroxyoxides. Hydrous mica is altered parent material and is not reconstituted from the soil solution as are kaolinite, montmorillonite, and allophane. The <105° C water in Table 7.3 is, roughly speaking, adsorbed water the >105° C water is hydroxyl ions and water within crystal structures. 

Ildefonse Ph, Kirkpatrick RJ, Montez B, Calas G, Flank A-M, Lagarde P (1994) 27Al MAS NMR and aluminum X-ray absorption near edge structure study of imogolite and allophanes. Clays Clay Minerals 42 276-287... 

---