01 Introduction
With the development of technology and increasing demands for film applications, forming processes are no longer limited to casting and blowing. New methods, such as casting combined with MDO (machine direction orientation), blowing combined with MDO, and biaxially oriented films (e.g., BOPET, BOPP), have been widely adopted.
Why do so many films undergo uniaxial or biaxial stretching? What impact does stretching have on film structure and performance? This paper discusses the influence of stretching on both aspects and aims to provide reference insights for film production and application.
02 Effect of Stretching on Film Structure
Stretching involves heating the film to a certain temperature (usually near its glass transition temperature) and then applying longitudinal or transverse tension. This may be achieved through roller speed ratios for machine-direction stretching or by chain clamps for transverse stretching.
During extrusion, molecular chains are released under pressure and form disordered coils. After heating and stretching, the chains are straightened, oriented, and aligned, resulting in:
- Increased molecular orientation
- Higher crystallinity
- Improved order and structural regularity
Annealing after stretching further enhances crystallinity, ensuring stable structural improvement.
03 Effect of Stretching on Film Performance
(1) Mechanical Properties Stretched films, though thinner, achieve superior strength, stiffness, and puncture resistance compared to unstretched films. The oriented and crystalline molecular chains provide higher tensile strength and modulus.
(2) Optical Properties For easily crystallized materials such as PE and PP, stretching significantly increases crystallinity (up to 80–90%). A large number of crystals scatter incident light, turning films from transparent to white or hazy.
(3) Thermal Properties In stretched and crystallized films, amorphous chain segments are divided into smaller regions by crystals. This reduces thermal shrinkage, enhances heat resistance, and improves thermal stability.
(4) Barrier Properties Gases such as water vapor and oxygen mainly permeate through amorphous regions. With higher crystallinity, the permeation pathway becomes longer and more tortuous, improving barrier performance.
04 Factors Requiring Attention During Stretching
While stretching enhances performance, it may also cause problems:
- Residual internal stress: May result in deformation or breakage during subsequent processing or use.
- Defects from uneven stretching: Such as holes, cracks, or local thickness variation.
- Optical haze: High crystallinity may compromise transparency, which could limit application in packaging requiring high clarity.
Therefore, precise control of temperature, stretching ratio, and uniformity is essential to prevent defects.
05 Practical Applications and Benefits
Stretching technology brings multiple benefits for film manufacturers and end-users:
- Enhanced performance: Higher strength, heat resistance, and barrier ability.
- Material efficiency: Produces thinner films without sacrificing performance.
- Energy saving and cost reduction: Thinner films reduce raw material use and improve application efficiency.
- Wider application range: Suitable for packaging, electronics, automotive, and construction films.
However, careful process optimization remains critical to balance improved performance with the risks of internal stress and deformation.
06 Conclusion
The performance enhancement of films mainly results from increased crystallinity, and stretching is an effective method to achieve it. Stretching improves mechanical, optical, thermal, and barrier properties while reducing material usage and improving efficiency.
Nevertheless, stretching introduces internal stress, which may cause deformation or breakage if not controlled. To ensure consistent and stable performance, manufacturers must precisely manage stretching parameters and eliminate process-related defects.
Post time: Nov-11-2025