Kinetics of thermal gelation of methylcellulose and hydroxypropylmethylcellulose in aqueous solutions

摘要

Thermal gelation properties, such as gel strength, rate of gelation, gel point, etc, of methylcellulose and hydroxypropylmethylcellulose products play key roles in many applications such as ceramics processing, extrusion of matrix food and manufacture of medicinal capsules. A thorough knowledge of the variables controlling the gelation kinetics is essential. Using a Bohlin VOR rheometer, we have developed a new technique to measure the storage modulus, G′, of the gel and the increase in the G′ values as a function of time for aqueous solutions of these cellulose ethers when heated above the gelation temperature. This technique utilizes the oscillatory shear mode and a special switching technique to raise the solution temperature as quickly as possible. The initial rate of increase in G′ is quite fast and the gel reaches about 95% of the final G′ value in about 6 min followed by slow gelation kinetics. The maximum rate of gelation (dG′/dt) and the final storage modulus of the gel (G′_f) for methylcellulose products were related to various parameters according to the following mathematical model: dG′/dt = KC~(3.3)T~(4.0) (%MeO)~(3.4) (M_w)~(0.5) G′_f = KC~(2.9) T~(0.3) (%MeO)~6 (M_w)~(0.8), where C, T, (%MeO) and (M_w) are the concentration in wt.%, the temperature in ℃, the methoxyl content, and the molecular weight, respectively. Hydroxypropylmethylcellulose polymers also exhibit similar gelation behavior with both G′_f and dG′/dt related to nearly the third power of concentration and second power of the ratio of the degree of methoxyl substitution (DS) to the molar substitution (MS) of the hydroxypropyl groups.