Why EVOH Films Are More Suitable for Printing Than PP/PE Films

In flexible packaging production, printing issues with PP and PE films, such as ink shrinkage and poor adhesion, are common. These challenges are primarily caused by the low surface energy of these materials. On the other hand, EVOH films are often recognized for their excellent printability and high bonding strength. But what exactly makes Evoh Films more suitable for printing? And how does their surface energy compare to that of PP and PE films?

1.Surface Energy: The Hidden Factor Influencing Printing and Lamination Quality

Surface energy essentially measures how much a material "attracts" external substances like inks and adhesives. For packaging films, surface energy directly affects both the adhesion of printed designs and the peel strength of laminated structures.

To ensure good adhesion, the surface energy of a film must be higher than that of the ink or adhesive being used. Typically, the critical threshold for surface energy falls between 38-42 mN/m. If the surface energy is lower, problems like poor wetting and weak adhesion can arise. The surface energy of PP and PE films typically ranges from 30-34 mN/m, well below the required level. As a result, these films often need additional post-processing, such as corona treatment, to improve surface activation.

2.Molecular Structure: How Polar Differences Affect Surface Energy

The surface energy of a material is largely determined by its molecular structure and polarity.

2.1 PP/PE: Non-Polar "Inert" Surfaces

PP and PE are made up of pure hydrocarbons, with their molecular chains consisting of carbon-carbon and carbon-hydrogen bonds. The distribution of electrons in these bonds is highly symmetrical, meaning they don’t form permanent dipoles. This non-polar characteristic leads to weak intermolecular forces (mainly van der Waals forces), resulting in low surface energy and chemical inertness. As a result, PP and PE films show little affinity for inks or adhesives.

2.2 EVOH: An Ideal Balance of Polar and Non-Polar Properties

EVOH, a copolymer of ethylene and vinyl alcohol, combines the best of both worlds. Its molecular structure retains the non-polar ethylene segments for ease of processing while introducing highly polar vinyl alcohol units. The hydroxyl (-OH) groups in EVOH polymer are strong polar functional groups, capable of forming hydrogen bonds, which significantly increase the material’s cohesive strength and surface polarity. These hydroxyl groups enable EVOH to interact strongly with polar inks and adhesives, giving it a much higher surface energy than PP or PE.

3.Data Comparison: EVOH Outperforms PP and PE in Surface Energy

To put theory into practice, data confirms the difference in surface energy:

Standard PP/PE Films: Surface energy is around 30-34 mN/m (untreated).

Typical EVOH Films: Surface energy typically exceeds 44 mN/m, with some grades reaching over 48 mN/m.

This data shows that EVOH films, in their as-produced state, already have surface energy well above the critical threshold needed for most inks and adhesives. This means that EVOH film can be printed and laminated without the need for additional corona treatment, simplifying production processes and eliminating the risk of quality degradation due to fading corona treatment effects over time.

4. Advantages of EVOH: Core Benefits of High Surface Energy

4.1 Streamlined Process, Reduced Costs, and Increased Efficiency

By removing the need for corona treatment, EVOH material reduces the need for additional equipment, cuts down on energy consumption, and lowers maintenance costs. It also shortens production timelines, ultimately improving overall efficiency.

4.2 Stable Quality and Higher Yield

The effectiveness of corona treatment diminishes over time. If treated films are not used promptly, it can result in quality issues in the final product. EVOH’s high surface energy is an inherent property, providing long-lasting performance stability and offering a wider processing window for more reliable quality control.

4.3 Enabling Sustainable Packaging Design

As the trend toward "mono-material recyclable packaging" grows, EVOH often serves as a barrier layer in composite materials with PP/PE. Its high surface energy ensures excellent printability on outer layers of polyolefin materials, maintaining both the aesthetic appeal and recyclability of the packaging. EVOH plastic is thus an ideal core material for achieving high-performance and sustainable packaging solutions.

5.Frequently Asked Questions

Q: Does EVOH’s high surface energy also mean it has good heat-sealing properties?

A: Actually, the opposite is true. Heat-sealing performance depends on the ability of molecular chains to move and diffuse when heated. EVOH's strong hydrogen bonding between molecules restricts the movement of its chains, resulting in poor heat-sealing properties. Therefore, in packaging structures, EVOH resin is typically used as a middle barrier layer, laminated with heat-sealable materials like PP or PE to complement each other’s properties.

Q: Does EVOH ever need corona treatment?

A: In most cases, no. However, for specialized inks (such as metallic inks) or extreme lamination requirements, light corona treatment may be applied to achieve the best possible performance.

EVOH films offer significantly higher surface energy than PP and PE films, thanks to their molecular structure. This inherent advantage means EVOH barrier films do not require the same "active treatments" as lower surface energy materials. As the packaging industry increasingly shifts towards higher quality and more sustainable solutions, understanding and taking full advantage of EVOH’s surface energy properties opens the door to process improvements and product innovations.