Herschel-Bulkley Model Describing the Rheology of Polyacrylates Superabsorbent Homopolymers

Nova Southeastern University, Davie, Florida, USA

Objective: The objective was to investigate the fitness of Newtonian and non-Newtonian rheological models in characterizing the rheological behavior of two grades of polyacrylic acid homopolymers commonly used in the preparation of semi-solid pharmaceutical dosage forms. Background: Bingham model is relevant to Newtonian flow. Herschel-Bulkley and Casson are relevant to non-Newtonian behavior, with the latter being pertinent to suspension systems. All the three models are used to describe materials with yield stress. In this study, we examined the rheological behavior of two polyacrylate homopolymers differing in their degrees of crosslinking. Methods: The samples were prepared in water at 0.1-1.0%w/w concentration range and neutralized by NaOH (pH 5.0-6.0). Cone and plate rheometer (spindle 2.4cm, 0.5mL sample) was used to generate the shear stress/shear rate and viscosity/shear rate rheograms. The confidence of fit (COF) to the three models was determined using Rheolac V3.3 software. Results: Herschel-Bulkley model fitted all concentrations of carbomer samples with COF exceeding 99%. Confidence of fit for Bingham and Casson models did not exceed 88% and 97%, respectively. Viscosity/shear rate profiles demonstrated shear thinning behavior and shear stress/shear rate profiles showed yield stress values ranging from 36-1703 D/cm2 and 188-947 D/cm2 for the highly crosslinked and lightly-crosslinked carbomers, respectively. Conclusion: Hydrophilic non-Newtonian polyacrylate homopolymers, possessing yield stress are best described by Herschel-Bulkley model, where the material is in solution, exhibiting shear thinning flow after the yield value is surpassed. Grants: This study was supported by NSU Grant 335081.