Control of Adhesion and Removal of Heat Sealable Tops on Polymer Containers

by Jun Li, Robert A. Shanks, Tracey Bray, Andro Lau, and Michael Hubbert
RMIT University, CSIRO Molecular Science, and Southcorp Packaging Ltd. Email: the author(s) at jun.li@rmit.edu.au

Following is an expanded summary of a complete paper available on the TAPPI web site at tappi.org. On the page, click "the PLACE" in the section designated "Journals."

Application: By reviewing peelable seals for lid-tub applications, the authors summarize basic peel and seal mechanisms and discuss variables and methods that control seal and peel properties. They offer suggestions for design to achieve optimum peelable seals.

Food packaging protects food from decay during its journey from factory to a consumer. This role is sometimes too successful by making opening of packages very difficult. The growing consumer demand for convenient, ready-to-eat food products has driven manufacturers to produce effective, user-friendly, hygienic, and safe food packaging. The solution for a user-friendly product is packages that are “easy open.”

“Easy open” refers to the ease of opening a package without using tools or utensils. A peelable seal is one solution for easy opening. Besides being easily peelable, tight sealing is necessary to ensure production line success and avoid package leakage during handling. Achieving the correct balance between tight seal and easy peel is the real challenge.

Peelable sealant materials rarely contain a single component or a single layer. A siingle component cannot meet the demands of food packaging — barrier to oxygen and moisture, balance of flexibility and rigidity, and ability to seal and peel properly. A multi-layer film can provide these combined properties.

Sealing Principles
The basic sealing process involves bonding of two polymer surfaces by forcing them into intimate contact while they are in at least a partially molten state. Failure of a heat seal bond always initiates at the weakest point. This can be adhesive failure, cohesive failure, or interlaminar failure. Figure 1 illustrates these failure mechanisms.

Many interrelated factors influence seal and peel properties:

  • Processing variables including temperature, dwell time, and pressure
  • Materials including the lidding and bottom web
  • Miscibility and morphology
  • Peel rate and angle
  • Thickness and location of layers in a multi-layer film
  • Mechanical properties of the sealant materials.

The properties of materials determine sealing temperature. A higher sealing temperature usually creates higher peel strength. The pressure and dwell time are less material-dependent and are typically 30-40 psi and 0.5 s, respectively. The dwell time must be sufficiently long to bring the interfacial temperature to a desired level. Longer times do not improve seal properties. A dwell time longer than one second will distort a sealing surface. A slight pressure forces the two rough surfaces of films into close contact and achieves good wetting. Higher pressure does not improve seal properties or seal surface appearance.

Material Selection
Material selection is critical to peelable seals because it determines seal and peel properties and processing conditions. The sealant must closely match the container materials in a structure to provide bond and a reasonable seal joint. Simultaneously, easy peeling requires a sealant to have limited similarity to container materials.

Miscibility and compatibility are crucial for making a good peelable seal. These characteristics regulate the adhesive and cohesive strength. Miscibility and compatibility have had extensive use to create and control peelable seals. Figure 2 is an example of a peelable seal using an immiscible system with polyethylene to impart cohesive failure.

The seal strength of a nonpeelable seal depends on film thickness. This is also true for peelable seals. The location of layers in a multi-layer film plays an important role in deciding film stiffness and eventually peel strength since the peeling process starts with bending. Layers in the core position contribute least to the bending stiffness of the film. Layers far from the center contribute most to film stiffness. Film stiffness and peel strength will therefore improve by exchanging inner and outer layers.

Mechanical properties of layers of lidding film contribute to peel strength. During a peel test, the sealant experiences a predominantly tensile deformation up to the point of failure. This deformation is subtle but extremely important since the mechanics of the deformation can link to that of the layers undergoing a simple tensile test. The deformation or plastic flow consumes considerable energy. This is equal to tensile energy and contributes to the peel strength since the peel strength is the sum of energy consumed by this plastic deformation and elastic process. This explains why the peel strength depends on the thickness of film. The tensile strength is proportional to the thickness of the film.

Conclusion
Cohesive and interlaminar failure are preferable because they give a tight seal that is particularly desirable for applications of hot filling and pasteurization. Cohesive failure and delamination share one common factor — decoupling of seal reliability and easy peel. With adhesive failure, the interface of peel and seal is identical. The seal strength requires compromise to allow reasonable peel. In addition, the peeling of adhesive failure is usually a bursting peel that requires much higher strength to initiate failure than to propagate. The dramatic decrease in force can cause spillage. The peel strength of adhesive failure is very sensitive to processing variables.

Cohesive failure may only require a single layer film. To achieve a cohesive failure, two basic conditions are necessary. Sealant materials must be immiscible but with low interfacial tension. The seal between lidding and bottom web should be stronger than the cohesive strength of the sealant. If the bottom web is polypropylene for example, the sealant should contain polypropylene to seal to the container. A sealant blend system is preferable since variation of the blend components can vary the peel strength. Other materials in the blends can be polymers that provide limited miscibility to the major component.

Burst strength and peel strength can be the same for cohesive failure. This creates difficulty in improving burst strength while maintaining low peel strength. Delamination failure, which requires multi-layer film, overcomes this problem. The delamination can occur adhesively and cohesively. The adhesive delamination is again a bursting peel that has risk of spillage during peeling. Cohesive delamination is therefore the best option. The requirements for cohesive delamination are similar to those for cohesive failure.


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