White Paper by on August 29, 2019
Advances In Peelable Sealant Technology
Advances in Peelable Sealant Technology
Rollprint Packaging Products, Inc
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 technologies 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.
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.
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:
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.
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
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.
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.
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
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.
Polybutene-based sealants have long been the standard for providing peelable seals to polyolefins. Polybutene sealants peel cohesively. The seal strength is controlled by chemistry as illustrated in Figure 9.
Figure 9. Seal Strength of Polybutylene Blends
As the concentration of polybutylene in the blend decreases from 100 percent, the seal strength drops until a plateau is reached. When the concentration of polybutylene is lowered further, the seal strength rises again. The shaded area of the graph indicates the polybutylene concentrations typically used for peelable seals. These concentrations maximize economics while providing a range of seal peelable strengths. Polybutylene-based sealants can be films or extrusion coatings.
There are some considerations when using polybutylene sealants.
Seal strength is dependent upon the thickness of the polybutlylene blend layer. As the thickness of the sealant layer increase, the seal strength increases. This relationship is nonlinear as illustrated in Figure 10.
Figure 10. Seal Strength and Sealant Thickness
The seal initiation temperature of a polybutylene sealant film that has just been extruded (non-aged) will have a lower seal initiation temperature than aged film. As a result, at a given temperature, the seal strength of non-aged film will be greater than aged film. This is illustrated in Figure 11.
The seal strength of a seal that has aged will be lower that the seal strength of a seal that has recently been made due to the crystallization of the polybutylene. Figure 11 shows the effect of aging on seal strength.
Figure 11. Effect of Aging on the Seal Strength of Polybutylene Sealants
The relationship between aging and seal strength with polybutylene-blend sealants is very repeatable and therefore predictable for any given combination of materials and equipment.
Peelable Polyolefin Sealants
The key to creating a sealant with a cohesive peel is selecting two (or more) resins that are incompatible. Generally, the greater the difference in solubility factors, the more likely the combination will achieve a cohesive peel. While polybutylene-polyolefin chemistries certainly dominate this category, much work has been done recently to explore alternate chemistries. The goal is to develop a chemistry that will provide an even more consistent peel, less prone to variations in seal strength as temperature, pressure, and dwell vary, and that is unaffected by aging. To add to the challenge, the chemistry should peel without stringing, provide a bright white seal indicator when peeled, process easily, use readily available resins, and of course, must be economical.
Figure 12 illustrates a chemistry that meets all of these objectives. The top curve shows the seal strength of a polybutylene-LDPE-EVA blend sealed to LDPE as a function of temperature. The seals have not been aged. The other three curves show an alternate chemistry (Allegro® B) also sealed to LDPE as a function of temperature. Each of the curves illustrates a different pressure/dwell combination. The graph clearly shows that this chemistry provides a very large sealing window with seal strength virtually unaffected by temperature pressure and dwell.
This alternate chemistry provides a bright white seal indicator that peels without stringing. The sealant layer can also be tinted to create colored seal indicators
As with polybutylene, the seal strength can be modified by varying the ratios of the incompatible resins. The chemistry controls the seal strength as illustrated in Figure 13.
Figure 12: Seal Strength of Alternate Chemistry
Varying Temperature, Pressure, and Dwells
Figure 13. Seal Strength is Controlled by Chemistry
Packaging for retortable and autoclaveable applications must be able to withstand 250°F (typically) in supersaturated steam and/or water environments. In addition, the peelable seal must not creep when exposed to the forces created by any pressure differential between the inside and outside of the package. Concerns regarding the stresses on the seals that result from pressure differentials cause many packagers to target high seal strengths and to often overseal their packages. However, with proper overpressure control, packages with seal strengths as low as 0.5 pounds/inch (227 grams/inch) can easily survive the retort/autoclave process.
The options available for peelable retortable/autoclaveable applications continue to grow and improve. Developments for three applications are detailed below.
Peelable to Polypropylene
Solution applied heat seal coatings have long been the standard for providing peelable sealants to polypropylene for retort and autoclave applications. Because the sealant must withstand typical retort/autoclave temperatures of 250°F and above and must seal and peel to polypropylene, the sealant chemistry is typically
polypropylene based. The heat seal coatings that have been developed generally yield strong peel strengths with a poor seal appearance and narrow operating window. More recently, polypropylene-based film options have become available.
For the first time, extrusion-coated peelable polypropylene options are being offered for retort/autoclave applications. They provide the economic advantages of extrusion coating as well as consistent, reliable performance. The polypropylene-extrusion coated sealants consist of a three-layer coextrusion of a modified polypropylene designed to promote adhesion, a bulk layer of polypropylene, and a thin peelable polypropylene sealant layer. These sealants can be coextrusion coated onto polyester, aluminum foil, and other heat stable webs.
Figure 14 shows an example of an extrusion-coated peeleable sealant sealed to polypropylene. The sealant provides a cohesive peel with a seal strength that is virtually unchanged after autoclave.
Interestingly, this same chemistry also seals and peels to high density polyethylene but with an adhesive peel. This is due to the fact that the bond between the sealant and the HDPE is less than the internal bond of the sealant as shown in Figure 14.
Figure 14. Seal Strength of Extrusion-Coated Polypropylene
Peelable to CPET, APET, PVC, and Polycarbonate
Historically, there have not been many options for sealing to CPET, APET, and polycarbonate for peelable autoclaveable applications. The ability to use resins for extrusion coating that are not stable enough to produce blown or cast films has allowed the development of high-performance, cost-effective structures. These sealants are amorphous polyester based and can only be produced on extrusion coating lines specifically designed to run polyester. In addition to CPET, APET, and polycarbonate, these amorphous polyester sealants will also seal to and peel from other polar materials such as PETG, and PVC. They can be extrusion coated onto polyester, aluminum foil and other stable webs. Figure 15 shows the seal strength of one option as a function of temperature.
Figure 15. Seal Strength of APET sealants
The in-hospital pouch and its unimpressive performance is usually the first thing that comes to mind when discussing steam sterilization. Fortunately, with advances in resin technology and manufacturing equipment, high performance options are available. Like the in-hospital pouch, the sealant chemistry is polypropylene-based. However, the new options provide a consistent seal strength with fiber-tear-free peels over a wide operating window. Constructions are available for sealing to paper in both 2D (flat) packages and 3D (formed) packages as shown in Figures 16 and 17.
Figure 16. Steam Sterilizable PP Sealants for 2D Packages
Using Tyvek® for steam sterilization provides some additional challenges. Tyvek is spunbonded HDPE and has a melt point of 265°F. However it begins to soften at 256°F. Autoclaves for steam sterilization typically operate at 250°F but often spike as high as 260°F or even higher. As Figure 18 illustrates, this means that the autoclave temperature can exceed the softening point of the Tyvek and will result in a nonfunctional package.
Figure 17. Steam Sterilizable PP Sealants for 3D Packages
Figure 18. Tyvek Softening Point as Compared to Autoclave Operating Range
To successfully use Tyvek in steam sterilization applications, the sterilizer must have excellent temperature control for the autoclave or must target lower autoclave temperature and process for a greater length of time.
Figure 19 shows the heat seal curve for one steam sterilizable sealant option for uncoated Tyvek.
Figure 19. Autoclaveable PP Sealant / Tyvek Heat Seal Curve
Selecting peelable sealants for applications needing chemical resistance is always challenging. However, many of the sealant technologies that provide high temperature resistance for retort/autoclave applications are also excellent for use when chemical resistance is needed. Peelable polypropylene and amorphous polyester (APET) sealants are excellent choices for packaging oxidizing agents, aggressive flavorants such as mint oils, and fragrances.
The APET sealants have the added advantage of being extremely clean. If extractables and/or leachables are a concern, APET is an excellent choice and is often used to hold pharmaceutical and aggressive food products.
Polyethylene blend sealants such as polybutylene or the more recent alternate chemistries can also be used in applications requiring chemical resistance if they are coextruded with or laminated to a layer of polypropylene or APET.
Figure 20 illustrates a construction used to hold iodine. Iodine is a powerful oxidizing agent. An aluminum foil barrier is needed to hold the iodine and ensure the concentration is maintained. However, iodine will oxidize the foil. A polyethylene blend sealant was desired to provide a consistent, easy-to-open peel with a bright white seal indicator. By coextruding APET with the polyethylene blend, an economical chemically resistant sealant was created.
Figure 20. Iodine Package with Polyethylene Blend Sealant
All of the technologies that have been reviewed so far involve either adhesive or cohesive peels. These peel mechanisms are the generally preferred methods to achieve consistent, repeatable, aesthetically pleasing seals over a wide operating window. However, peels via delamination still have a role.
One of the most unique ways that delamination peels are being used is for liquid fragrance applications. In these applications, the fragrance needs to be contained and preserved during distribution and storage. An aluminum foil or other high barrier layer is used to prevent fragrance evaporation. At the time of use, the fragrance still must be contained to prevent spilling and/or consumer exposure. However, the fragrance must be able to readily permeate through the containment in order to disperse.
To achieve this in an economical and efficient manner, a structure that is designed to delaminate in a controlled manner between the aluminum foil and the sealant is used. A cross section of a typical structure is shown in Figure 21.
Figure 21. Cross Section of Fragrance Package Using a Delamination Peel
Sealant selection is only one part of designing a functional package. Barrier, durability (including abrasion, puncture, and flex-crack resistance), printing requirements, appearance, thermal stability, hand, recycling requirements, packaging equipment, and volume are just a few of the things that need to be considered. Most packaging materials are composite structures consisting of many layers, each contributing a unique function.
Advances in resin technology and the availability of new chemistry options have promoted the development of many new peelable sealant technologies. The advances in peelable sealant extrusion coating technology paired with the economics that extrusion coating has to offer is particularly exciting.
Alternatives to polybutylene chemistry have resulted in polyethylene-based peelable sealants that provide a consistent seal strength virtually unaffected by temperature, pressure, dwell, and age.
New peelable sealants for retort and autoclave applications provide improved processing and performance characteristics. Options include peelable polypropylene-based sealants as well as amorphous polyester-based sealants.
When chemical resistance is needed, both the peelable polypropylene and amorphous polyester chemistries are good choices. Polyethylene-based sealants may be an option when coextruded with, or laminated to, polyester or polypropylene.
The packaging engineer has many choices to make when designing a functional and economical package. Composite structures incorporating the variety of new peelable sealant technologies can vastly simplify the process of moving from idea to commercial reality.