In the production of PC optical film, the biaxial stretching process significantly influences the molecular chain orientation by precisely controlling the temperature, stretching rate, and direction, thereby optimizing the film's optical, mechanical, and thermal properties. The core of this process involves heating the polycarbonate (PC) resin to a temperature range above its glass transition temperature but below its melting point, rendering it highly elastic. Then, longitudinal and transverse stretching forces are applied sequentially through a longitudinal stretching machine and a transverse stretching machine, forcing the molecular chains to align in two perpendicular directions. This process not only changes the original disordered state of the molecular chains but also fixes the orientation structure through a heat-setting step, resulting in anisotropic microstructures.
During the longitudinal stretching stage, the molecular chains are first straightened in the machine direction (MD), gradually stretching the previously curled segments and aligning them parallel to each other. This process significantly improves the film's tensile strength and modulus in the MD while reducing its elongation at break. As the stretch ratio increases, the degree of molecular chain orientation gradually increases. However, if the stretching rate is too high or the temperature is too low, the molecular chains may break due to insufficient mobility, resulting in cracks in the film or deteriorating performance. Therefore, precise control of process parameters is key to ensuring uniform longitudinal orientation.
The transverse stretching stage further alters the alignment of the molecular chains. Under the action of the transverse stretching force, the already longitudinally oriented molecular chains are forced to twist and rearrange, forming an oriented structure in the transverse direction (TD). This process not only enhances the film's mechanical properties in the TD but also improves the film's tear strength by cross-aligning the molecular chains. It is important to note that transverse stretching is typically performed after longitudinal stretching, and the stretching temperature should be slightly higher than that during the longitudinal stretching stage to reduce the resistance to molecular chain relaxation and avoid deformation or warping of the film due to internal stress concentration.
The effect of the biaxial stretching process on molecular orientation is also reflected in the film's optical properties. The aligned molecular chains are more tightly packed and orderly, reducing light scattering and refractive index inhomogeneities, thereby significantly improving the film's transparency and gloss. Furthermore, the oriented structure reduces the film's birefringence, resulting in more stable optical properties under polarized light. This is particularly important for high-precision applications such as liquid crystal displays and optical lenses.
Regarding thermal properties, the biaxial stretching process improves the film's thermal stability through molecular orientation. The oriented molecular chains must overcome a higher energy barrier to relax upon heating, significantly reducing the film's thermal shrinkage. Furthermore, the oriented structure enhances the film's resistance to thermal deformation, maintaining dimensional stability even in high-temperature environments. This property is valuable in high-temperature applications such as automotive lighting and aerospace.
However, biaxial stretching also presents challenges. For example, mismatched stretch ratios in the longitudinal and transverse directions can lead to significant differences in film anisotropy, potentially causing warping, delamination, or uneven optical properties. Furthermore, internal stresses generated during stretching must be fully released during the heat setting step; otherwise, the film may deform due to stress relaxation during use. Therefore, process optimization requires comprehensive consideration of material properties, equipment precision, and environmental factors to achieve precise control of molecular orientation.
From a material modification perspective, biaxial stretching offers the potential for functionalizing PC optical films. By adjusting stretching parameters or introducing co-modifiers, the molecular orientation structure can be further optimized, imparting specialized properties such as enhanced transmittance, stiffening, and antistatic properties to the film. For example, by adding nanoparticles or functional coatings to PC resin and combining it with a biaxial stretching process, an optical film with high transparency and excellent surface hardness can be produced to meet the needs of the high-end display field.
