Borate gels

Schultz, R.K. Myers, R.R. "The Chemorheology of Poly(vinyl Alcohol)-Borate Gels", Macromolecules 1969, 2. 

Schultz, R. K. and Myers, R. R. 1969. The chemorheology of poly(vinyl alcohol)-borate gels. Macromolecules 2 281-285. 

Metal-Ion Cross-Linker Chemistry. The most commonly used metal-ion cross-linkers are titanates, zirconates, and borates. Titanate and zirconate gels display significantly different properties than borate gels, and these differences indicate the different nature of the cross-linking for the two types of ions. For example, gels produced by transition metal ions are more thermally stable than borate gels and do not recover viscosity after exposure to shear. 

Figure 1 shows the dynamic moduli as a function of frequency for an HPG-borate gel (0.48 wt % HPG, 3 wt % sodium tetraborate, 2 wt % KGl) at ambient temperature. The moduli show solidlike behavior the storage modulus is independent of frequency and higher than the loss modulus. At 65 G, in Figure 2, the moduli show fiuidlike behavior, and both moduli decrease at low frequency. This thermal melting is characteristic of borate gels and limits their application in high-temperature environments. 

In 1949, Deuel and Neukom [131] sug sted that the origin of cro links in aqueous poly(vinyl alcohol)/borate gels is the complexation shown in Fig. 48. This concept of didiol type complexation was adopted by authors in more recent times [130,184-193]. Schultz and Myers [185] measured the dynamic moduli G and G" of polyfvinyl alcohol)/borate gels (at small deformation amplitudes in order not to disturb the gel [185]). At low frequendes, the gels showed Uquidlike behaviour this means that the borate crosslinks are dynamic in nature. Such results were also reported by Beltman [7]. He measu the dynamic moduli of a gel formed in a 4 wt% PVA98.5 solution with 2.5 wt% of borax (=0.0069 mol/1, which is equivalent to 0.0276 mol/1 of borate) at various temperatures (15-75 °C). Results are shown in Fig. 49. 

Fig. 47. Possible complex formation in aqueous potyfvinyl alcohol)/borate gels...

Fig. 50. Logarithm of weight average crosslinking index calculated from the rubber storage modulus of PYA/borate gels, plotted vs reciprocal temperature O measurements by Beltman (PVA concentration 4 wt% borate concentration 0.275 mr [7]) A OKasurements Sdinltz aid M rs (PVA concentration 4.4 wt% borate concentration 8.7 x 10 md/l) [185]...

Fig. 52. Dependence of rubber storage modulus of PVA/borate gels on added sodium hydroxide temperature 25 °C PVA concentration 28.3 kg/m initial borate concentrations in mol/m 83.2 O 69.3 A 55.9. Reproduced from data presented in [195].

According to Maerker and Sinton [197], lii r vis elastic behaviour of poly(vinyl alcohol)/borate gels is obtained at small drformation amplitudes only in that case the complex viscosity is five times as high as the zero sl r viscosity, whereas the complex viscosity measured at drformation amplitudes of 100% is equal to the zero shear viscosity. 

Fig-53. ln(Cp,Pb) of PVA/borate gels of various compositions plotted vs reciprocal gel melting temperature, calculated from data presented by Shibayama et aL [193]... 

Siddiqui, M. A., Nasr-El-Din, H. A., Al-Anazi, M. S., and Bartko, K. M. Formation Damage in Gas Sandstone Formations by High-temperature Borate Gels Due to Long Term Shut-in Periods, SPE/DOE Symposium on Improved Oil Recovery, April 17-21, 2004, Tulsa, Oklahoma (SPE 89476). 

Fig. 12 Self-correlation function of polylvinyl alcohoD/sodium borate gel [37],...

Alkoxide-derived borate gels establishes criteria for gel formation from metal alkoxlde precursors in nonaqueous solvents. 

The preceding observations prove that oxygens bridging trigonal borons residing in separate primary units (e.g., tetraborates or triborates) are unstable in aqueous (and presumably alcoholic) environments. Thus, any reasonable structural model proposed to explain borate gel networks formed in mixed alcohol-water solutions must exclude =B-0-B= bonds between... 

Alignment, shear-induced, 824, 825 Alkali borate gel, 226 Alkaline earth silicate crystallization, 777... 

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