Why does polyester film achieve high rigidity after biaxial stretching?
Publish Time: 2025-10-15
As a high-performance polymer material, polyester film is widely used in a variety of high-tech fields, including electrical insulation, food packaging, new energy, magnetic recording, and optical displays. Its excellent overall performance, particularly its combination of high rigidity, high hardness, and excellent toughness, makes it stand out among other film materials. The core source of these exceptional mechanical properties lies in its key manufacturing process: biaxial stretching. So, how does this process transform a formerly soft polyester sheet into a remarkably rigid film? The materials science behind this process deserves a deeper analysis.1. Oriented Molecular Structure: The Microscopic Basis for Enhanced RigidityPolyester film is made from polyethylene terephthalate, a linear polymer. In an unstretched extruded sheet, the PET molecular chains are randomly coiled and entangled, resulting in a loose structure and overall softness and low strength. The biaxial stretching process, however, applies tensile forces to the sheet in both the longitudinal and transverse directions, forcing the previously chaotic molecular chains into highly oriented alignment along the stretching direction. This ordered arrangement significantly enhances the van der Waals and cohesive forces between the molecular chains, resulting in a material with greater macroscopic rigidity and hardness. Just as a loose bundle of straw is difficult to bear, a neatly bundled bundle can withstand significant pressure. The oriented alignment of the molecular chains is the fundamental reason for this increased rigidity.2. Increased Crystallinity: Enhanced Structural Stability and Mechanical StrengthBiaxial stretching is typically performed within a specific temperature range. This process not only promotes molecular orientation but also induces strain-induced crystallization in the PET material. Under the action of tensile force, the molecular chains in some amorphous regions rearrange to form tiny crystalline regions. These crystalline structures act as "physical crosslinks," further locking in the movement of the molecular chains and improving the material's crystallinity. Increased crystallinity directly enhances the film's modulus, tensile strength, and dimensional stability. Highly crystallinity PET films are less likely to deform under stress, effectively resisting external impact and meeting the demands of high-strength applications.3. Biaxial Synergistic Effect: Achieving Isotropic Mechanical PropertiesBiaxial stretching is key. If stretched only in one direction, the molecular chains are highly oriented in only one direction, resulting in significant anisotropy—high longitudinal strength but prone to tearing in the transverse direction. Biaxial stretching, however, precisely controls the longitudinal and transverse stretch ratios to uniformly orient the molecular chains within a plane, forming a two-dimensional network structure. This structure not only enhances overall rigidity but also imparts excellent isotropic mechanical properties to the film, meaning it possesses high strength and toughness in all directions. This makes polyester film less susceptible to deflection or tearing in applications such as packaging, laminating, and wrapping, resulting in more stable and reliable performance.4. Film Densification: Improving Hardness and Barrier PropertiesDuring the stretching process, the tighter arrangement of the molecular chains increases the overall density of the film, reducing internal voids and creating a denser structure. This densification effect not only improves surface hardness, making it scratch and abrasion resistant, but also significantly enhances the film's airtightness and aroma retention. For example, in food packaging, highly rigid and dense PET film effectively blocks the permeation of oxygen, water vapor, and aroma molecules, extending shelf life. At the same time, the dense structure also improves the film's puncture resistance, making it less susceptible to punctures by sharp objects during transportation and use.5. Heat Setting: Locking in Properties and Preventing ShrinkageAfter biaxial stretching, the film undergoes a heat setting treatment, maintaining tension at high temperatures for a period of time to stabilize the molecular chain orientation and crystal structure. This crucial step eliminates internal stresses, preventing the film from shrinking and deforming due to heat during subsequent use, ensuring long-term high rigidity and dimensional stability.The high rigidity of polyester film after biaxial stretching is the result of a synergistic effect of multiple physical changes: molecular orientation, increased crystallinity, structural densification, and heat setting. This process transforms the originally flexible polymer material into a high-performance film that combines rigidity, toughness, and stability, providing key material support for technological advancements in electrical insulation, high-end packaging, and new energy.
