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.
Isotropic Biaxial Stretch
Meaning
Simultaneous or sequential deformation of a polymer film occurs equally in both the machine and transverse directions. This process of isotropic biaxial stretch produces films with balanced mechanical, optical, and thermal properties along both axes. The stretch is carried out at temperatures between the glass transition and the melting point of the polymer.
It does not apply to uniaxial stretching processes or to processes where the stretch ratio is highly unbalanced.
Film Properties
Stretching the polymer film equally in both directions aligns the molecular chains in a balanced two-dimensional plane. This molecular structure results in equal tensile strength, elongation, and tear resistance in both the longitudinal and transverse directions. Balanced properties are necessary for packaging films that must withstand handling and shipping without tearing.
In addition, this process improves the clarity and gloss of the film by reducing the size of light-scattering spherulites.
Process Execution
In a tenter frame, the cast polymer sheet is gripped by clips on both edges and heated in a multi-zone oven. The clips are driven by a mechanism that speeds up to stretch the film longitudinally, while the rails diverge to stretch it transversely. Precise control of the temperature and stretch rate in each zone is necessary to prevent film breakage.
If the temperature is too low, the film will tear, whereas if it is too high, the film will crystallize before stretching is complete. This process must also account for the thermal contraction that occurs during the subsequent annealing step, where the film is heat-set to lock in its dimensional stability and minimize future shrinkage.
Industrial Application
This technique is used to manufacture high-performance packaging films, such as biaxially oriented polyethylene terephthalate and biaxially oriented polypropylene. These films are utilized in food packaging, electrical insulation, and capacitor films. The balanced orientation ensures that the film remains flat and dimensionally stable during subsequent printing, lamination, and metallization steps.
Unbalanced films would warp or shrink unevenly when exposed to the high temperatures of curing ovens or lamination rollers.