What's the Key Technical Considerations for Blown Film Production of EVOH Films?

As one of the core processes in co-extruded film manufacturing, the blown film method has demonstrated distinct advantages in producing EVOH high-barrier films. With the global demand for advanced barrier packaging continuing to grow—particularly in food, medical devices, and automotive components—mastering the blown film process for EVOH materials has become a key competency for industry players.

 

1.EVOH Material Characteristics and Process Compatibility

 

1.1 Processing Sensitivity

 

Ethylene-vinyl alcohol copolymer (EVOH) is widely used in high-barrier packaging due to its excellent oxygen barrier properties. However, it is highly sensitive to processing conditions. EVOH resins exhibit a narrow melt temperature window (typically 190–220°C) and are susceptible to moisture. In blown film processes, special attention must be paid to thermal stability and rheological behavior.

 

Experimental data indicates that EVOH melt viscosity is highly temperature-sensitive—rising 10°C can reduce viscosity by 15–20%. Therefore, temperature control must maintain a precision of ±1°C.

 

1.2 Raw Material Preparation

 

Pre-drying is critical to processing quality. As per FDA and EU 10/2011 requirements, EVOH resin should be dried to <0.1% moisture at 80–90°C for no less than 4 hours. Additionally, due to the oxidation sensitivity of EVOH, nitrogen blanketing is recommended in hoppers to keep oxygen levels below 50 ppm.

 

2.Core Process Parameters and Control Points

2.1 Temperature Control

Precise temperature control is essential. Temperature variation within zones must remain under ±1°C, and die head zones should not differ by more than 2°C. PID-based intelligent temperature controllers are commonly used. For a typical PE/EVOH/PE 3-layer structure:

 

Outer layer: 190–200°C

 

Barrier layer: 200–210°C

 

Inner layer: 180–190°C

 

Processing temperatures must not exceed 230°C to prevent EVOH degradation.

 

2.2 Blow-Up Ratio (BUR) Management

 

BUR significantly influences film barrier and mechanical properties. A recommended range is 2.0–2.8, calculated as:

 

BUR = (Bubble Diameter – Die Diameter) / Die Diameter

 

Each 0.5 increment in BUR can enhance oxygen barrier by 8–12%, though excessive BUR may cause layer distribution imbalance. The EVOH layer should account for 5–10% of the total structure to ensure performance and cost efficiency.

 

2.3 Cooling System Optimization

 

Crystallization in EVOH polymer is sensitive to cooling rates. For dual air-ring systems:

 

Primary ring temperature: 20–25°C

 

Secondary ring temperature: 15–20°C

 

Air pressure: 0.3–0.6 MPa

 

Air volume ratio: 1:3

 

Cooling should not be too rapid to avoid internal stress. Per ISO 15378, the optimal cooling rate is 50–80°C/min for uniform crystallization.

 

3.Common Issues and Technical Solutions

 

3.1 Thickness Uniformity

 

Variations in barrier layer thickness critically impact performance. Solutions include:

 

Use of coat-hanger die heads for optimized melt distribution

 

Maintaining ±1°C die lip temperature

 

Regular die lip cleaning to prevent buildup

 

Using β-ray online thickness gauges enables real-time monitoring. A European client reduced thickness deviation from ±8% to ±3%, improving oxygen barrier performance by 30%.

 

3.2 Bubble Stability

 

Bubble instability can lead to non-uniform EVOH distribution. Countermeasures include:

 

Balanced airflow via adjustable air rings

 

Pulling speed variation within ±0.5%

 

Use of servo-driven nip systems with digital control

 

Installing a CCD-based vision system allows for continuous monitoring and automatic adjustments.

 

3.3 Surface Defects

 

Fish-eyes and gel spots often stem from contaminated or degraded material. Preventive steps:

 

Ensure raw material purity

 

Regular die and filter cleaning

 

Incorporate compliant antioxidants to prevent oxidation

 

Case data shows that such measures reduced surface defect rates from 3% to 0.5%.

 

4.Frequently Asked Questions

 

Q: How is interlayer adhesion controlled in EVOH films?

A: Use of functional tie-layer resins, processing temperatures between 190–210°C, and optimized die design ensures sufficient fusion. Target peel strength is 4–6 N/15 mm.

 

Q: How can gel spots be prevented during blown film production?

A: Gel spots arise from raw material contamination or thermal degradation. Recommendations:

 

Enhanced material screening and drying

 

Avoid local overheating

 

Clean dies and filters regularly

 

Laser-based online inspection systems can detect and reject defective rolls in real time.

 

Q: How can EVOH film surfaces be treated for better printability?

A: Corona treatment is preferred, targeting surface tension of 42–48 dyn/cm. Power levels should be 10–15 W·min/m², and printing should occur within 24 hours. For advanced applications, plasma treatment can increase surface energy to >50 dyn/cm.

 

 

The blown film method for EVOH high-barrier films is a complex, precision-driven process. Mastery of material behavior, equipment tuning, and environmental controls enables consistent production of high-quality films. As digitalization and intelligent control systems evolve, the technical capabilities of blown film EVOH production will continue to advance—offering new possibilities in barrier packaging innovation.