Christian G'SEll

Blends of low‐density polyethylene (LDPE) and polylactide (PLA) were prepared by melt coextrusion. The plastic behavior of the LDPE/PLA blends was investigated at room temperature under uniaxial tension by means of a video‐controlled system. The constitutive behavior was analyzed in terms of the variations of true stress vs. true strain at constant true strain rate. With increasing concentrations of PLA, the blend show: (i) higher Young's modulus, (ii) stiffer viscoelastic response, (iii) increase of elastic limit stress, and (iv) earlier fracture. Particular attention was paid to the evolution of the volume strain with the applied strain. While dilatation begins very lately for the neat LDPE, the LDPE/PLA blends show increasing deformation damage as the PLA content is increased. Scanning electron microscopy of deformed specimens shows that cavitation occurs preferentially at the poles of the PLA particles whose adhesion to the LDPE matrix seems very weak despite the partial grafting of polyethylene chains with maleic anhydride. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.

The behavior of low‐density polyethylene (LDPE) and two blends prepared with polylactide (PLA) was determined by means of a novel video‐controlled testing method under stretching at constant true strain rate, under creep at constant true stress, and under creep at constant nominal stress. Most tests were performed at 23°C and 50°C. In this second part, the experimental data are modeled with the G'Sell‐Jonas phenomenological law expressing the axial true stress versus axial true strain and axial true strain rate. This model describes correctly the various deformation stages: (i) initial viscoelasticity, (ii) plastic yielding, and (iii) strain hardening up to rupture. It shows clearly the reinforcing effect of the PLA particles that increases the yield stress in stretching experiments and slows down the deformation kinetics under creep. It is shown how the local stress/strain behavior is related to the standard force/extension curves. Consequently, it is proposed that tensile tests at constant true strain rates should be systematically preferred to creep tests for the characterization of constitutive relations because they take much less time to be performed. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.

-Processus de la variation volumique du polyéthylène à haute densité au cours d'essais de traction et de fluage-Des éprouvettes de polyéthylène à haute densité ont été soumises à des essais de traction et des tests de fluage à l'aide d'un dispositif d'essais mécaniques à pilotage vidéométrique (VidéoTraction ®). L'évolution du volume spécifique de ce polymère semi-cristallin est caractérisée en fonction de la déformation vraie. Dans le stade élastique, on mesure une expansion hydrostatique puis, dans le stade plastique, on note une compétition entre un effet de compaction et un phénomène de dilatation. La compaction, sans doute surestimée dans les expériences, est toutefois un mécanisme important dû à l'orientation des chaînes amorphes au cours de l'étirage. Ce phénomène est attesté par des mesures de diffraction des rayons X qui montrent une réduction de la distance moyenne entre les chaînes amorphes. Le processus de dilatation s'explique par la diminution du taux de cristallinité et par la formation, la croissance et la coalescence de craquelures au sein et entre les sphérolites. L'observation d'échantillons déformés par microscopie électronique révèle ces défauts. La compétition compaction/dilatation est dictée par la mobilité de la phase amorphe influencée elle-même par la température et le temps.

Different blends of high‐density polyethylene (HDPE) with calcium carbonate (CaCO₃) were mechanically tested under uniaxial tension with or without poly(ethylene‐co‐octene) elastomer grafted with maleic anhydride (POEg), as an impact modifier. In some materials, the surface of the CaCO₃ was treated with an amino acid and in others the mineral particles were left untreated. The stress–strain behavior were determined at constant true strain rate by using the VidéoTraction^© system. Also, the volume changes upon stretching was assessed by means of the video extensometer and correlated with X‐ray densitometry measurements. The dependence of modulus, yield stress, and cavitation is shown to depend on the relative percentage of the three constituents. In particular, the cavitation rate increases markedly with the CaCO₃ content and decreases with the POEg content. By contrast, the surface pretreatment of the CaCO₃ particles appear to be of much lesser importance. POLYM. ENG. SCI., 46:1512–1522, 2006. © 2006 Society of Plastics Engineers