How Biaxial Stretching Tackles EVOH High-Barrier Film’s Humidity Sensitivity and Strength Challenges
Evoh (ethylene-vinyl alcohol copolymer) is widely used in food packaging, aseptic medical supplies, and precision electronics sealing, thanks to its oxygen barrier performance—up to 10,000 times higher than polyethylene. Yet its inherent susceptibility to humidity and limited mechanical strength have long constrained its adoption in demanding applications.
Biaxial stretching, a precision molecular-level reorientation process, is changing that. By reorganizing EVOH polymer’s internal structure, this technology not only pushes barrier performance to new levels but also dramatically improves stability in hot and humid environments. It has become a cornerstone in the next generation of high-barrier packaging materials.
1.Molecular Chain Reorientation: From Microscopic Alignment to Macroscopic Gains
In biaxial stretching, EVOH film is drawn in both the machine direction (MD) and transverse direction (TD), either simultaneously or in sequence, forcing molecular chains to align in the plane. This requires meticulous control of temperature and tension throughout the process:
Asynchronous stretching (MD 3–4:1, TD 7–9:1) helps avoid the whitening effect caused by simultaneous stretching and improves yield.
Solvent pretreatment (e.g., hydrogen-bond disruption) lowers stretching stress and prevents brittleness. A 2024 patent from Chengdu Huicheng Technology confirmed the method’s effectiveness in improving uniformity.
Typical performance improvements in a seven-layer symmetrical structure (PP/adhesive/EVOH/adhesive/PP) after biaxial stretching include:
OTR (oxygen transmission rate) ≤ 0.05 cc/(m²·24h·atm) at 23 °C, 65% RH—more than 60% lower than unstretched film.
Tensile strength exceeding 120 MPa (vs. 80 MPa for unstretched film), enabling use in heavy-duty bulk bags and high-strength silage wrap.
2.Overcoming EVOH’s Humidity Limitation
The industry-wide problem of EVOH plastic losing barrier performance in high humidity can be mitigated through three key approaches within the biaxial stretching process:
2.1 Thickness Uniformity (±3%)
High-precision co-extrusion equipment—such as domestic five-layer blown film lines with an L/D ratio of 32:1 and a 550 mm die—ensures minimal variation in EVOH layer thickness, eliminating weak spots that compromise barrier performance.
2.2 Dual Protective Layer Design
Adding EVA layers on both sides of EVOH (thickness ratio 20:(15–30):20) cushions stretching stress, improves deformation uniformity by 40%, and prevents brittleness or delamination.
2.3 Enhanced Adhesion Between Layers
KE-3000 adhesive forms molecular-level entanglement with stretched EVOH resin, retaining more than 95% of peel strength after 180 days at 135 °C—ideal for high-temperature sterilization of ready-meal packaging.
3.From Lab to Application: Biaxial Stretching in Action
4.Compliance and Sustainability Benefits
4.1 Meeting Evolving Regulations
A solvent-free co-extrusion process ensures compliance with (EU) 2025/351 limits—overall migration ≤ 10 mg/dm², phthalates ≤ 0.05 mg/kg—and zero detection of SVHCs (PAHs/DEHP) under the 2025 REACH Candidate List.
4.2 Supporting the Circular Economy
Recycled content: Stretching improves material utilization, allowing up to 30% recycled PE without loss of performance.
Bio-based EVOH: 45% bio-content (castor-oil derived) cuts carbon footprint by 30% after stretching, aligning with the EU PPWR 2030 goal of 30% bio-based packaging.
4.3 Domestic Material Innovation
As the first EVOH supplier in mainland China, Sinopec Chuanwei continues to expand performance boundaries:
EW-3201: 32% ethylene, melt temp 188 °C, OTR as low as 0.5 cc/m²·24h (20 µm) for aseptic pharma films.
EW-3801: 38% ethylene, thermoforming window 160–210 °C, excellent weather/oil resistance for fuel tanks and underfloor pipes.
5.FAQ
Q1: Why is asynchronous stretching preferred over synchronous stretching for EVOH films?
EVOH material crystallizes rapidly under heat. Synchronous stretching can cause whitening and brittleness, whereas asynchronous stretching allows controlled crystallization, improving yield—Mitsubishi Heavy Industries reports a 15% increase.
Q2: How can barrier performance be maintained in high humidity?
A dual EVA/EVOH/EVA structure absorbs moisture stress, reducing OTR fluctuation from ±25% to ±8% at 85% RH.
Q3: What metrics indicate successful biaxial stretching?
Thickness deviation within ±3%
Peel strength loss < 5% after retort
OTR tested with specified temperature/humidity (ASTM D3985)
Biaxial stretching has moved beyond a simple processing step to become a form of molecular engineering for EVOH plastic. By fine-tuning chain orientation, layer bonding, and film uniformity, it delivers exceptional gains in barrier performance, strength, and environmental stability. From aseptic food packaging to hydrogen storage tanks and flexible PV back-sheets, this technology is enabling a new generation of high-performance applications.