Allophane properties

Allophane and Imogolite. AUophane is an amorphous clay that is essentially an amorphous soHd solution of sUica, alumina, and water (82). In allophane less than one-half of the aluminum is held in tetrahedral coordinations and the Si02 to AI2O2 ratio typically varies between 1.3 and 2.0, but values as low as 0.83 have been reported. The typical morphology of allophane is cylindrical (37). AUophane may be associated with haUoysite, smectite minerals, or it may occur as a homogeneous mixture with evansite, an amorphous soHd solution of phosphoms, alumina, and water. Its composition, hydration, and properties vary. Chemical analyses of two allophane samples are given in Table 5. 

Amines produce polyurethanes with better mechanical properties than when diols are used for curing. Amines produce polyurethanes with a lower temperature resistance than when diols are used. The use of catalysts has been found to direct the cross-linking reactions away from the biuret to the allophanate reactions. 

Using controlled reaction conditions such as the temperature profile and rate and time of the addition of polyols, more uniform materials can be produced. The correct spacing of the hard segments required to produce the physical properties can be obtained. The controlled conditions will also help prevent the formation of undesirable side products such as allophanate, biuret, and trimers. These reactions will give branching of the polymer chains. 

However, another factor that could contribute to the Initial improvement In physical properties of the polyurethanes Is that the allophanate and biuret crosslinks, which are highly susceptible to hydrolysis, may be early victims of the degradation process. 

Table 7-4 Selected properties of phyllosilicates, allophane, and imogolite"... 

As intended by the adhesive formulation, no parasitic side reactions such as formation of urea, new uretdione, isocyanurate, biuret, or allophanate are detected for the bulk and for thick films on Al, Cu, and Au (dpu>l pm cf Figs. 6.3, 6.7). Even ultrathin PU films on Au correspond to bulk properties to a very good approximation (Fig. 6.8), which is a recommendation for films on Au as references with respect to effects in the interphase on different metals. 

The <2 mm fraction of an Andisol (a 2C horizon of a Typic Hapludand) containing 40% of allophanic material, from Roccamonfina Volcano (Italy), was used in this work. The general descriptions and selected chemical, physicochemical, and mineralogical properties of this soil are given hy Vacca et al. (2003). 

The concentrations of the allophanate links varies with the time of cure. Also, if the crosslinking reactions are conducted in inert nitrogen atmospheres, veiy little scission of crosslinks takes place and a network structure forms during the cure. In open air, however, the scissions of crosslinks are extensive and the products have poorer physical properties. 

Thus, socydimitgroups exhibit different functionalities according to the reaction conditions they behave bifunctionally in addition polymerization [Equation (15-4)], monofunctionally in condensation polymerization [Equation (15-3)], and semifunctionally with respect to initially present hydroxyl groups in allophanate formation [Equation (15-5)]. Thus, the functionality is not an absolute property, but is a relative parameter which also depends on the reaction partner and reaction conditions. 

Since 2-9% excess isocyanate was used to prepare the polymers in this study, the polymers undoubtedly contained some allophanate groups. The effect of excess -NCO content compared to hydroxyl number of the polybutadiene on tensile properties of the polyurethane are shown in Figure 1. As the -NCO content increased, the ultimate tensile strength, a f passed through a maximum at about 6% excess -NCO and then stayed constant. As shown in Table 3, Shore A Hardness increased continuously with % free-NCO content. 

Besides urethane bonds, other chemical functionalities are present due to reaction of isocyanate groups with water and other compounds. In fact, urea and isocyanurate bonds are more abundant than the urethane linkages in some commercial products that are designated as polyurethanes. Allophanate, biuret, urea, carbodiimide, and isocyanurate linkages may be found. These bonds certainly affect the properties of the final polymer, but unfortunately it is almost impossible to determine them quantitatively (Figure 6). 

When the difunctional precursors (diisocyanate, polyol and extender) are allowed to react in a stoichiometric amount, a thermoplastic PU is formed. Thermosetting PU are made by using excess diisocyanate (excess diisocyanate reacts with a urethane structure to form allophanate bonds) or by using a trifunctional extender like glycerin or trimethylol propane [92-94]. The unique feature of PU resin is that the change in UPE between crosslink offers a wide change in properties, especially the strain (which reflects flexibility). For example, a PU system with a molecular weight between two... 

Increased temperature also has an important effect on the properties of the cured polymer since both the allophanate and biuret linkages tend to dissociate at higher temperatures giving a more linear polymer. Some results are given in Table 4.5. 

The network structure of linear and branched TPUs is obviously different. The branched thermoplastics are capable of forming allophanate and/or possibly biuret crosslinks under suitable conditions. These conditions are partly met by the processing temperatures but usually a subsequent hot-air cure is necessary to achieve the optimum set properties. Inevitably, therefore, the branched TPUs have considerably lower compression and tension set properties than the truly all-linear thermoplastic TPUs, and in this respect approximate to the castable polyurethanes. 

The basic polymer structure, which results from the formation of urethane groups and may be linear or crosslinked, largely determines the polymer s properties. However, other than these primary reactions, secondary reactions such as biuret, allophanate, dimer and trimer formation also have an effect on the properties of the polymer and their presence can also be detected by IR techniques. 

The fractal geometry can help to derive porous and structural properties of the allophane aggregates. In the case of a fractal aggregate it is known that the evolution of the local bulk density follows ... 

---