Pharmaceutical-grade cellulose ethers, such as hydroxypropyl methylcellulose (HPMC) and hydroxypropyl cellulose (HPC), are widely used as excipients due to their effects on viscosity, mechanical strength, hydration behavior, and drug release performance. In formulation development, excipient selection is commonly based on nominal viscosity grades; however, steady shear viscosity alone does not adequately describe the time dependent and deformation sensitive behavior that governs critical quality attributes and in-process performance. A comprehensive rheological evaluation of multiple HPMC and HPC grades obtained from different manufacturers was performed by preparing different concentrations of polymeric dispersions using steady shear and oscillatory rheometry. Flow sweep, amplitude sweeps, and frequency sweep analysis were conducted to characterize viscosity, viscoelastic behavior, and structural response under varying deformation conditions. All cellulose ether samples exhibited non-Newtonian, shear thinning behavior. Distinct differences in viscoelastic properties were observed between polymer types and grades. HPMC demonstrated molecular weight and concentration dependent transitions toward elastic dominated, gel like behavior, while HPC remained predominantly viscous dominated under comparable experimental conditions. The observed differences in rheological behavior highlight the limitations of relying solely on nominal viscosity grades for excipient selection. Advanced rheological parameters, particularly viscoelastic characteristics, provide deeper insight into polymer structure property relationships and their impact on formulation performance. Cellulose acetate phthalate and hydroxymethyl cellulose phthalate are also used for enteric coating of tablets. Targeting of drugs to the colon following oral administration has also been accomplished by using polysaccharides such as hydroxypropylmethyl cellulose and hydroxypropyl cellulose in hydrated form; also they act as binders that swell when hydrated by gastric media and delay absorption. This paper assembles the current knowledge on the structure and chemistry of cellulose, and in the development of innovative cellulose esters and ethers for pharmaceuticals.
Shafaat* et al. (Wed,) studied this question.