There is need to model behaviour of time-dependent non-linear material in stress or/and strain controlled experiments. This study explores possibility to apply model in both forms developed by Schapery for viscoelastic materials. Viscoelasticity has been analysed using experimental data from creep and relaxation tests. Incremental simulation procedure, which inverts model, where strains are expressed through stresses, is used to simulate relaxation curves for bio-based polymer. Comparison of viscoelastic parameters obtained from simulated and experimental relaxation curves has been performed.