Biaxial Extensional Deformation Dynamics of Semi Crystalline Polymer Sheets
Biaxial extensional strain hardening controls sheet thinning, requiring precise melt strength, tool friction, and temperature tuning to set uniform container wall thickness.
Crystallite Orientation
Meaning
Crystalline domains align themselves along specific directions during polymer flow or drawing processes. Within semi-crystalline polymers, crystallite orientation determines the anisotropy of the final molded or extruded part. This alignment occurs during the cooling phase as the melt experiences shear forces or tensile elongation.
It does not apply to amorphous polymers that lack defined lattice structures, nor does it describe the alignment of added short fibers or fillers.
Flow Influence
In injection molding, the high shear rate near the cavity wall induces a high degree of crystalline alignment. This skin-core structure causes directional differences in mechanical properties such as tensile strength and shrinkage. Moulders must balance injection velocity and mold temperature to control this effect.
When the alignment is too pronounced, the part is prone to warpage and cracking along the flow lines.
Stretching Effect
Tensile stretching of extruded films above the glass transition temperature forces the crystal lattices to align parallel to the stretch direction. This processing step increases the puncture resistance and barrier properties of the film. In biaxially oriented polypropylene films, the polymer is stretched in both the machine and transverse directions to create a balanced structure.
By adjusting the draw ratio in each direction, the film manufacturer can tune the tear resistance and clarity of the packaging material.
Structural Characterization
Analysis of the crystalline alignment relies on wide-angle X-ray diffraction or polarized infrared spectroscopy. These analytical techniques measure the intensity of diffracted beams or absorbance spectra along different axes of the sample. In laboratory settings, these measurements yield an orientation parameter that ranges from zero for random dispersion to one for perfect alignment.
Molders use this parameter to validate that the extrusion or molding process has achieved the molecular structure required for demanding mechanical applications. For example, a film with high alignment along the machine direction exhibits high tensile strength in that direction but tears easily in the transverse direction. This anisotropy is evaluated to prevent premature failure of plastic strapping bands during shipping.