Applicapability of Differential Constitutive Equations for Steady Shear Flow, Elastic Recovery and Uniaxial Elongational Flow of High Density Polyethylene

Experimental data were compared with the predictions of differential constitutive equations proposed by Larson, Giesekus and Phan-Thien and Tanner (PTT). Applicapability of these equations was investigated with steady shear flow, start up of uniaxial steady elongational flow and elastic recovery of shear and uniaxial elongation of three high density polyethylene having broad distribution of molecular weight. The following conclusions were obtained (1) Larson, Giesekus and PTT-E model (Phan-Thien and Tanner model with non-linear term of exponential type) give good fit with the transient shear viscosities and transient uniaxial elongational viscosities for three HDPE. (2) Larson model gives the best fit when non-linear parameter α equals to 0.5∼0.9 under steady shear and 0.1∼0.2 under transient uniaxial elongation. PTT-E gives the best results when α equals to 0.9 under steady shear and 0.1∼0.2 under steady uniaxial elongation. The best values of α in these two models are different for shear and elongational flow. The values of α are, however, similar in three HDPE samples which have different relaxation spectra. (3) Of three models, the Giesekus model is the best from the point of applicapability to elastic recovery in shear and elongation of three constitutive models. PTT-E is moderate, and the Larson model shows unrealistic results.