Measuring and simulating morphology gradients in injection‐molded talc‐reinforced isotactic polypropylene

Abstract

Injection molded polymer parts are known to exhibit structural gradients of crystallinity, crystallite phases and crystallite orientations. The structural variations depend on the geometry, the material properties, and the processing conditions, and affect the mechanical properties of the molded part. We explore the use of raster‐scanning small‐ and wide‐angle X‐ray scattering (SAXS, WAXS) for mapping the microstructure in dogbone specimens of an isotactic polypropylene (PP) homopolymer and a talc‐reinforced isotactic PP compound. The specimens were injection molded with different mold temperatures and injection speeds, and the mapping approach revealed systematic structural heterogeneities and asymmetries. Accompanying numerical simulations of the injection molding process yielded predictions of the flow pattern, including the shear rate distribution and the resulting orientation of the flake‐shaped talc particles. We found a clear correspondence between the experimentally observed data and the simulations, in particular regarding the asymmetry of the orientation distributions relative to the center of the dogbone cross section, caused by asymmetric flow through the entrance of the mold. Furthermore, the shear rate distribution correlated with the occurrence of α‐ and β‐phases. Subtle differences in the crystallized structures along the long axis of the dogbones suggest an explanation to the observation that the specimens studied always tended to break at the same position in tensile tests. The results clearly demonstrate the potential of mapping experiments which combine lateral resolution on macroscopic length scales with the molecular‐scale resolution from scattering. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1157–1167

Predicting the location of weld line in microinjection‐molded polyethylene via molecular orientation distribution

“…Healy et al utilized SAXS and WAXD to examine the residual chain orientation in microinjection‐molded linear polyethylene, and unraveled that it is the oriented crystal growth instead of residual chain orientation that contributes to the majority of the orientation. Granlund et al mapped the two‐dimensional (2D) chain orientation distribution in the cross‐section parallel and perpendicular to the flow direction in injection‐molded talc‐reinforced isotactic polypropylene using the raster scanning SAXS and WAXD, and indicated the structural heterogeneities and asymmetries as a result of lateral temperature gradient across the mold. Ulcer et al applied matrixing microbeam XRD and transmission optical photomicrographs to analyze the structure at the weld region of poly(arylene ether ketone), and revealed that the sample exhibits a multilayer structure at the weld line region.…”

Retrieving the spatially resolved preferred orientation of embedded anisotropic particles by small-angle X-ray scattering tomography

“…The sample used in this study was a specimen of injectionmoulded isotactic polypropylene (iPP) containing talc particles, chosen because we have previously performed raster scanning wide-and small-angle X-ray scattering of physically cut specimens of the same material (Granlund et al, 2013(Granlund et al, , 2014. The fact that the spatially resolved preferred orientation of the embedded talc flakes is known in advance makes it possible to compare the results from the SAXS-CT technique with the true orientation distribution.…”

“…For the reference raster scan experiment, a cross section of 1 mm thickness was physically cut, and the X-ray beam, being in this case parallel to the long axis of the original sample, was raster scanned in two dimensions along the x-and z-axis directions with a 15 mm step length, recording the scattering pattern at each position of the beam on the sample. Further experimental details regarding the sample material and preparation can be found in the paper by Granlund et al (2014).…”

“…the orientation varies slowly with position, wjr'j<< 1, where w is the voxel size in the reconstruction), we can take the particles within each voxel to have a uniform orientation, whereby the orientation of voxel ij is characterized by a single director n ij . Motivated by our prior knowledge about the injection shear alignment of the talc particles, all directors {n ij } are taken to be confined perpendicular to the flow direction z and thus confined to the xy plane (Granlund et al, 2013(Granlund et al, , 2014. In other words, the z 0 component of n ij is set to zero.…”

“…Another material thoroughly studied by scanning SAXS is bone, owing to its relevance for medicine and industry (Rinnerthaler et al, 1999;Jaschouz et al, 2003;Giannini et al, 2012). Granlund et al (2013Granlund et al ( , 2014 have recently published detailed studies of the spatially resolved orientation of talc particles in extruded isotactic polypropylene, also demonstrating that modern area detectors make it feasible to collect the necessary data in a practical amount of time ( hours) even in home laboratories.…”