What's the Factors Affect the Heat Seal Strength of Asymmetric Co-Extruded Films?

In flexible packaging, asymmetric co-extruded films are widely used because they combine the strengths of multiple materials within a single structure. A typical design uses PA or PET as the outer layer to provide puncture resistance and temperature stability, while PE or EVA is used as the inner sealant layer to ensure flexibility and reliable heat sealing performance.

This “tough outside, sealable inside” structure makes asymmetric co-extruded films an ideal choice for demanding applications such as bone-in meat packaging, retort pouches, liquid stand-up pouches, and other high-performance packaging solutions.

At the same time, heat seal strength remains one of the most critical indicators of package integrity. Compared with symmetric film structures, asymmetric films present additional challenges because each layer differs in thermal conductivity, melting behavior, and crystallization characteristics. As a result, heat transfer and interfacial bonding during sealing become more complex and require careful material and process design.

1. Molecular Properties of Sealant Resins: The Basis of Heat Seal Strength

The heat sealing performance of an asymmetric co-extruded film largely depends on the resin used in the innermost sealant layer and its molecular structure.

Common sealant materials include LDPE, LLDPE, mLLDPE, EVA, and propylene-based elastomers. Each material has different melting temperatures and molecular diffusion capabilities, which directly affect sealing efficiency and final seal strength.

For low-temperature sealing applications—especially on high-speed automatic packaging lines—mLLDPE and EVA are frequently preferred.

These characteristics allow effective sealing at lower temperatures and shorter dwell times.

In EVA, increasing the vinyl acetate (VA) content lowers the melting point and improves low-temperature sealability. However, excessive VA content may reduce heat resistance and final seal strength. In most packaging applications, VA content is typically controlled between 5% and 18% to balance sealing performance and durability.

Molecular weight distribution is another important factor. A broader distribution often contains more short molecular chains, which diffuse quickly during the initial sealing stage to form early bonding. Longer chains then create stronger molecular entanglement, improving final seal strength.

However, an excessively broad molecular weight distribution can lead to uneven melting and unstable sealing performance.

In asymmetric film structures, outer layers may slow heat transfer to the sealant layer. For this reason, sealant resins with strong low-temperature melting performance become especially important because they help compensate for delayed heat conduction and ensure sufficient melting within limited sealing time.

2. Crystallization Behavior and Its Impact on the Heat Seal Window

Heat sealing is essentially a process in which polymer chains from two film surfaces diffuse across the interface and recrystallize together.

For semi-crystalline polymers such as polyethylene and polypropylene, crystallization speed and crystallinity strongly influence heat seal strength.

During sealing, the film is heated above its melting temperature, allowing molecular chains to move freely and diffuse across the sealing interface. As cooling begins, crystallization restricts further chain movement.

If crystallization occurs too quickly, molecular chains may become “locked” before sufficient entanglement develops, resulting in weak interfacial bonding and lower seal strength.

On the other hand, extending the molten diffusion time—either by increasing sealing temperature or extending dwell time—allows more molecular entanglement before crystallization occurs, significantly improving seal strength.

Even small increases in sealing temperature can noticeably improve molecular interpenetration and interface bonding.

In asymmetric co extruded film, cooling behavior can become uneven because the outer and inner layers have different thermal conductivities. Rapid or uneven cooling may prevent polymer chains from fully relaxing, creating internal stress that affects long-term sealing consistency.

For this reason, stable cooling conditions are just as important as the heating stage when optimizing heat sealing performance.

3. Heat Sealing Parameters: Temperature, Pressure, and Time

The three core heat sealing parameters—temperature, pressure, and sealing time—work together to determine the final seal quality of asymmetric co-extruded films.

3.1 Heat Sealing Temperature

Heat sealing temperature is usually the most influential parameter.

In asymmetric structures, outer materials such as PET often have lower thermal conductivity than PE sealant layers. As a result, heat transfer from the sealing bar to the inner sealant layer takes longer.

Differences in layer thickness can further affect thermal transfer efficiency.

Therefore, compared with symmetric films, asymmetric co extruded films may require:

• Higher sealing temperatures
• Longer dwell times
• Optimized heating profiles

to achieve the same sealing performance.

When developing customized film structures, packaging equipment parameters should always be shared with the film supplier to ensure proper formulation matching.

3.2 Heat Sealing Pressure

The purpose of sealing pressure is to ensure close contact between the two film surfaces so molecular diffusion can occur effectively.

If pressure is too low, air gaps may remain between the interfaces, preventing proper bonding.

If pressure is too high, molten resin may be squeezed out of the seal area, reducing sealant layer thickness and weakening the seal.

Optimal pressure should create full surface contact without excessive material displacement.

3.3 Heat Sealing Time

Sealing time determines how long polymer chains have to diffuse and form molecular entanglement.

On modern high-speed packaging lines, sealing time may be reduced to 0.5 seconds or less. Under these conditions, the sealant layer must melt and diffuse very quickly.

If the sealant layer is too thick, heat may not fully penetrate the interface during the available sealing time, resulting in weak seals.

For this reason, sealant layer thickness in asymmetric co-extruded films is commonly controlled between 10 and 30 microns to balance sealing speed and seal strength.

4. Heat Transfer Challenges Unique to Asymmetric Co-Extruded Films

One of the key differences between asymmetric and symmetric film structures is heat transfer behavior.

In asymmetric films, each layer may have different thicknesses and thermal conductivity values. For example:

• PET thermal conductivity: approximately 0.15–0.20 W/m·K
• PE thermal conductivity: approximately 0.33 W/m·K

A thicker PET outer layer can create a significant insulation effect, slowing heat transfer to the sealant layer.

In addition, high-melting-point materials such as PET do not melt during sealing, but they still absorb and block heat transfer. This creates a narrow processing window:

• Temperature must be high enough to activate the sealant layer
• But not so high that the outer layer deforms

To improve sealing efficiency, some advanced packaging machines use:

• Pulse heat sealing
• Double-sided heating
• Controlled thermal profiles

These methods help deliver heat more directly to the sealant layer.

When customizing co extruded multilayer film, close communication between packaging converters and film suppliers is essential to optimize layer design, sealant formulation, and sealing performance.

5. Frequently Asked Questions

Q1: Is the heat seal strength of asymmetric co-extruded films higher or lower than symmetric films?

Not necessarily. Under the same sealant material and thickness conditions, asymmetric structures may require higher temperatures or longer sealing times because outer layers can reduce heat transfer efficiency.

However, through proper material selection and process optimization—such as using low-seal-initiation-temperature mLLDPE, increasing dwell time, or applying double-sided heating—asymmetric films can achieve equal or even higher seal strength than symmetric structures.

In many cases, asymmetric films also provide better overall packaging performance because they combine multiple functional properties in one structure.

Small-scale production trials are recommended to determine the best sealing parameters.

Q2: Do slip additives always reduce heat seal strength?

Traditional slip additives such as erucamide can reduce heat seal strength to some extent, especially if additive migration creates a weak surface layer.

However, the reduction is usually manageable and often falls within an acceptable range, commonly around 10–20%.

For applications requiring very low coefficient of friction (COF), such as high-speed automatic packaging lines, a slight reduction in seal strength may be acceptable.

Polymeric slip additives are another option because they generally have a smaller impact on heat sealing performance, often reducing seal strength by less than 5%.

Q3: During seal strength testing, is “film tearing” better than “seal opening”?

Yes. In most cases, film tearing is considered an excellent result.

If the film substrate tears while the seal remains intact, it means the seal strength exceeds the internal strength of the film itself.

In other words, the sealed area becomes the strongest part of the package.

For demanding applications such as heavy-duty bags and retort pouches, many quality standards specifically require material tearing rather than seal opening during testing.

Therefore, if tearing occurs during seal strength testing, it generally indicates that the sealing performance is highly reliable.

Conclusion

The heat seal strength of asymmetric co-extruded films is influenced by multiple interacting factors, including:

• Sealant resin molecular structure
• Crystallization behavior
• Heat sealing temperature, pressure, and dwell time
• Layer structure and heat transfer characteristics

Understanding how these factors work together helps packaging manufacturers and film suppliers develop more reliable and application-specific packaging solutions.

From selecting the right sealant resin and slip additive system to optimizing sealing parameters and layer design, every detail plays a role in achieving stable sealing performance.

Because asymmetric multilayer co extruded film have unique thermal transfer characteristics and functional layer distribution, they often require more customized sealing process design than conventional symmetric structures.