Crystal Structure and Mechanical Properties of Uniaxially Stretched PA612/SiO2 Films

Stretching has a significant effect on the microstructure and ultimate performance of semi-crystalline polymers. To investigate the effect of stretching on structure and mechanical properties of uniaxial stretched PA612/SiO₂, PA612 and PA612/SiO2 films were prepared at four temperatures close to the glass transition temperature at various strain. The samples were characterized by a transmission electron microscope (TEM), wide-angle X-ray diffractometer (WAXD), Two-dimensional wide-angle X-ray Scattering (2D-WAXS), differential scanning calorimeter (DSC), dynamic mechanical analyzer (DMA), and stretching tests. The results showed that the α phase was the dominant phase in PA612 casting film, no obvious γ phase was observed, while both stretching and the presence of SiO₂ can induce the generation of α phase and improve the crystallinity of PA612. Crystals were oriented along the stretching direction and the b axis was parallel to the equatorial direction after stretching. The interplanar spacing of (010/110) decreased with the increasing stretching temperature and expanded with the increasing strain, while stretching temperature and strain present negligible effect on the interplanar spacing of (100). The grain size increased with the stretching temperature while decreased with strain. The presence of SiO₂ led to reduce the yield stress and the stress drop beyond yielding of the composite. Uniaxial stretching gave rise to a significant improvement in the fracture stress and the glass transition temperature.

Development of a biaxial stretching test machine and its applications

The evaluation method for a biaxially oriented film was developed using in-situ measurement during the stretching process. It can obtain basic data such as stress-strain curves, birefringence, light scattering, three dimensional refractive indexes and birefringence distribution. Stress and strain as functions of stretching speed and stretching temperature, as well as the deformation of spherulite of semi-crystalline polymer can be obtained by measuring the birefringence and light scattering during the biaxial stretching process with a small piece of polymer sample. The experimental results show the stress, retardation and three dimensional molecular orientations behavior during the simultaneous biaxial stretching and the sequential biaxial stretching process. Stretchability, thickness uniformity and spherulite size can be obtained simultaneously. In this paper, advantages and details of the newly developed system will be discussed with some experimental data.

Bi-axially Oriented Blown Film Technology

A set of commercial polyethylenes (three LDPE and three LLDPE) was rheological characterized and processed in a laboratory double bubble extrusion line with the purpose of investigating: i) whether their relative processability could be predicted in the laboratory and ii) the effect of operating and machine variables on processability. Oscillatory shear and extensional measurements were able to differentiate the various materials, who also behaved quite differently upon processing. Those with a stronger shear thinning behaviour, higher elasticity and higher melt strength were the most suited to the process. Bubble stability and thickness uniformity were assumed as the main requirements for processability. The former was estimated by monitoring the time required to stabilize the bubble and examining any variation/oscillation in size/shape. Bubble stability depends on the interaction between various parameters, the most relevant being those related with the film cooling stage, namely the axial position of the air rings, the air flow rate and air velocity. The study was preceded by an assessment of the performance of the re-heating oven and die/calibration/centring, as these parameters could also narrow the practical operating window.