Dhuanne Dodrill

President of Rollprint Packaging Products

Rollprint Packaging Products

Abstract

Today’s resin technology and new chemistry options have enabled the development of peelable heat sealants that can better meet today’s performance and economic challenges. A few of the echnologies that have seen significant advancement are: peelable polyolefin sealants that provide a consistent seal strength virtually unaffected by temperature, pressure, dwell, and age; peelable polypropylene sealants for retort applications that provide improved processing and performance characteristics and; peelable polyester sealants for applications requiring chemical and/or temperature resistance.

Introduction

Consumer demands for easy open packages, concerns about safety, tamper resistance, and product efficacy, the need control to costs and maximize manufacturing efficiencies, and new regulatory requirements regarding validation, child resistance, and disposal all place increasing demands upon the performance of peelable sealants. Fortunately, advances in resin technology, new chemistry options, and improvements in heat sealant application methods have allowed the development of peelable heat sealants that are better able to meet these challenges.

Discussion

In order to understand and appreciate the advances in peelable heat sealants, it is important to be familiar with the approaches for achieving peelable heat seals; both the peel failure mechanisms that can be designed into the sealants and the methods for incorporating sealants into the composite packaging material.

Heat Seal Mechanisms
There are three basic peel mechanisms for achieving peelable heat seals:

• Adhesive Peel
An adhesive peel separates between the interface of the two webs being heat sealed together as shown in Figure 1.

Figure 1. Adhesive Peel

As a result, it provides a relatively poor seal indicator with no transfer to the opposite web. Depending upon the application this can be considered desirable (e.g. lidding for a tray for a food application) or a drawback (e.g. peelable packages for medical devices).

The seal strength of materials designed to have adhesive peels also tend to be more sensitive to sealing parameter variation. As temperature, pressure, and/or dwell fluctuate, the seal strength will change much more rapidly than with the other peel mechanisms. This is because seal strength is greatly affected by surface mechanics.

• Cohesive Peel
With a cohesive peel, the sealant splits when peeled. Some of the sealant transfers to the opposing web while some remains with the original sealant web as shown in Figure 2.

Figure 2. Cohesive Peel

As a result, sealants with cohesive peels can provide excellent seal indicators when peeled with a positive change in appearance in both the sealant web and its partner web. Again, this can be desireable or not depending upon the application.

Sealants designed to provide cohesve peels are usually created by blending a contaminant into a base resin. The contaminant is usually an incompatible resin that will not solubilize in the base resin, disrupting the resin matrix. Because the strength of the seal interface (the bond between the two webs) is stronger than the internal bond of the sealant layer, the sealant splits when peeled.

The seal strength of a cohesive peel is controlled by chemistry. By varying the amount of contaminant blended into the sealant, the seal strength can be increased or decreased. When properly designed, use of chemistry to control the seal strength can result in sealants with a very wide operating window. This yields packages that are relatively easy to process and processes that are easy to validate.

Because the sealant splits when peeled, cohesive peel sealants are more prone to stringing (sometimes called angel hair) than adhesive peel sealants. Figure 3 illustrates an extreme example of a cohesive sealant that strings when peeled.

Figure 3. Stringing

• Delamination Peel
The final peel mechanism is delamination, sometimes called interlaminar peel. Here, the bond between the seal interface (between the sealant layer and the material to which it is being sealed) and the internal bond of the sealant are greater than the bond between the sealant layer and the balance of the sealant web. Therefore the sealant delaminates when peeled. Figure 4 illustrates the delamination peel mechanism.

Figure 4. Delamination Peel

In order for a delamination peel to open, the user must break through or tear the sealant layer to initiate the peel. As a result, stringing and webbing are a much greater concern for this peel mechanism.

Types of Peelable Heat Sealants
There are a number of approaches for applying a peelable heat sealant to a packaging material. The three main categories are detailed below.

• Heat Seal Coatings
Heat seal coatings have a very long history and were the original way to achieve peelable seals. Today, there are a tremendous variety of heat seal coatings. Options are available to provide peelable seals to most substrates.

Heat seal coatings are traditionally solution applied. Resins and additives are dissolved into a solvent (an organic solvent or water) to create the coating, the coating is applied to a web, and then the coated web passes through an oven to evaporate the solvent and in some cases cure the coating. Figure 5 illustrates the typical process.

Figure 5. Heat Seal Coating Process

Solution applied heat seal coated materials are often more expensive than other types of peelable sealants. The resins and additives used to make the coating require a relatively significant amount of processing to create the coating. If organic solvents are used as the carrier for the coating, they add additional expense. Also, the efficiency of the coating process is limited by drying capacity particularly with water-based coatings. When solvent-based coatings are used, care must be taken to ensure that the solvent concentration during the evaporation (drying) process does not approach lower flammability limits (LFL). For high coating weight applications and certain solvent families this can severely restrict the operating speed of the coating equipment.

• Films
Incorporating peelable sealants into extruded films is a very effective way of achieving a peelable sealant. Generally, films are coextrusions with a bulk layer of relatively inexpensive resin(s) and a thinner peelable sealant layer. The bulk layer provides a cushioning effect when sealing, gives the film body, and depending upon the choice of resins, can enhance durability and barrier.

Figure 6. Coextruded Film

If the coextruded film can be used “as is” without further processing (e.g. high density polyethylene/polybutylene blend coextrusion), it will generally be the most cost effective approach to providing a peelable film sealant. However, in many cases, the peelable film will need to be laminated (either adhesive laminated or extrusion laminated) to provide additional functionality (e.g. thermal stability, dimensional stability, and/or barrier). Figure 7 provides a schematic of a dry-bond adhesive lamination process.

Figure 7. Adhesive Laminating Process

• Extrusion Coatings
Some of the most exciting advancements with peelable sealants are with extrusion coatings. In an extrusion coating process, resin(s) are cast directly onto a web (e.g. paper, aluminum foil, films) as illustrated in Figure 8.

Figure 8. Extrusion Coating Process

As with films, extrusion coatings can be coextruded to maximize economics and/or to provide additional functionality.

Because the extruded resin is cast onto a stable web, the extrusion coating process requires one less manufacturing step than a similar laminated film structure. In addition, less expensive raw materials (resins versus films) are used in extrusion coating. Further economic advantages are gained from the high line speeds typical of extrusion coating processes. Processing is often two to three times faster than adhesive laminations. As a result, extrusion coating can be a very cost effective approach to creating peelable structures.

Resin combinations that are not stable enough to create a blown or cast film are options for extrusion coating. These new material combinations have allowed peelable sealant technology to advance significantly and have provided solutions to many packaging challenges. This paper will focus on a few of these advancements.

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