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Seal Strength and Package Integrity Testing
for the Food Packaging Industry

PDF format (898 KB)

Stephen Franks, Executive Vice President
TM Electronics, Inc. Boylston, MA 01505 USA

Introduction
The growth of barrier and retort pouch use in food packaging is a powerful driving force behind the need to define, standardize and institute critical seal strength and package integrity tests. Implementation of quantitative, repeatable package testing will help to ensure pouch quality, safety and standards.

Two Streams of Testing
The intent of packaging is to maintain the cleanliness and sterility of the product from the manufacturing plant through transport, shelf life and storage. This is a twofold objective: first, to ensure the integrity of the sealed package (and one can particularly see the importance of this with retortable packages), and second, to assure that no weaknesses in the sealed areas of the pouch permit leaks to develop with handling stresses and time.

To meet this two-fold objective, you must be sure that the pouch is able to maintain the integrity of both the seals and the materials under stress. This implies that your package testing system must include both package integrity testing and seal strength testing, two complementary but very different procedures. Package integrity may be thought of as a "leak test" of the package - is there a failure in the materials or process that allows contamination to enter? Seal strength testing, on the other hand, measures an attribute of the seal, which is designed to ensure that the seal presents a barrier to at least the same extent as the rest of the pouch. Both testing streams are important to your final pouch analysis.

Seal Strength Testing
Tensile seal strength testing measures the ability of a package seal to resist separation - the simple peel strength of the seal. Using a defined width sample of a package perimeter seal, a moving jaw pulls the sample apart at a constant speed while measuring the resistance force during the seal separation.

Inflation seal strength testing includes burst and creep testing. This test requires pressurizing the entire pouch and measuring the peak rupture pressure (burst test) or the time to failure at a constantly held pressure (creep test). The inflation test provides three different components of stress to the pouch: peel stress with horizontal and vertical components, tension due to hoop stress in the vertical direction, and lateral stress due to package expansion. If these stresses are greater than the strength of the seal at any point within the pouch, the seal will rupture.

Advantages and Disadvantages of Tensile vs. Inflation Seal Strength Testing

The tensile test is particularly suited to peel-open pouches. A significant advantage to this test is its sensitivity, and a disadvantage is that in the majority of cases a perimeter seal is sampled only at several locations and a total pouch seal strength measure is not obtained. Another disadvantage is that the effect of hoop and lateral stresses from inflation or non-perpendicular peel stress cannot be measured.

Inflation seal strength testing provides a more realistic representation of stresses to which your pouch will be subject. Another advantage to this type of testing is that it provides a whole-package minimum seal strength and also indicates the weakest seal area, and is equally applicable to peelable and non-peelable seals. Disadvantages are that this doesn't profile the entire seal area, and that there is no universal correlation with the tensile test.

Both the tensile test and inflation seal strength testing yield quantitative results that can be recorded and analyzed, although data return for process control is obtained much faster from the inflation test process.

Burst Testing
Burst testing determines the overall minimum seal strength of the pouch seals by inflating the pouch at a uniform rate until the seal separates at the point of greatest weakness. As you can see in this typical burst graph, the burst test is a peak inflation pressure test; you can see how the pressure increases to a maximum pressure at which the pressure drops to zero. This drop represents the rupture of the seal. The pressure at which the pouch bursts is a variable statistic that can be utilized to document process development and process control through the use of tools such as upper and lower control limits, such as those seen on this control chart segment.

Control of the inflation rate is important in a burst test to ensure consistent conditions for the test method. The porosity (or lack thereof) of the package material determines the inflation rate for the burst test. Because air escapes through the walls of a porous package during inflation, the flow rate must be increased to compensate for the lost air through the walls and create the back pressure in the porous package.

Applications of Burst Testing in the Food Industry
These are only a survey of possible applications of burst testing in the food industry. In each of these examples, the food containment is pressurized until the seal ruptures. As you can see, burst testing is by its very nature a destructive test.

Creep Testing
The Creep Test is a second general type of whole package inflation seal strength test. In the Creep Test, a filled, sealed package is inflated to a constant pressure, which is then held for a specified time, resulting in a pass/fail result. This provides a test for slow shear of the adhesive bond similar to a dead weight hanging on the seal. A suggested starting pressure for peelable seals is to begin evaluating your seal with a creep pressure that is about 80% of the burst value. The inflation rate is not critical, as long as the initial fill is not so fast as to shock the seal or so slow as to result in an overly long test time. The shortcomings of this test are the need for the operator to visually examine the seal at the end of the test to determine the degree of seal peel, and the lack of a variable statistic upon which to perform process control analysis.

Creep Testing for Retortable Pouches
Creep testing sealed packages before retort can identify points of possible failure when the package is subjected to heat and pressure stress, including weaknesses in stresses corners, material cracks or pinholes, and seal weld weakness or incomplete welds.

Restraining Plate Package Testing
Seal strength values are related to the package size, geometry, and materials. For example, pouches with a long side seal will generally fail on the long seal unless a heater failure has occurred on the shorter seal or chevron. Unsupported food tray lid seals may fail at points only relative to their geometry. Very flexible package materials may deform with pressurization to an extent that makes seal testing difficult. To address these problems, it may be advisable to use restraining plates for your inflation testing.

Package Geometry Effect
The geometry of the package under test affects the distribution of internal pressure forces on the package surface and seals; for example, a pouch-form package unrestrained in any axis exhibits circumferential hoop stress when internal pressure is applied. When the package is restrained, the load application is distributed directly on the seal area and, because material stretching and deformation is minimized, the test forces are more uniformly applied.

Relationship of Package Restraint to Burst Pressure
This graph illustrated the direct relationship between package restraint and burst pressures: the wider the gap between plates, the lower the average burst pressure. I need to insert a warning here - the use of restraining plates must be approached with caution. Because of pressures exerted on the plates, extreme care must be taken that the restraining plate fixture is designed to withstand the forces applied by the inflated package!

How do you obtain precise pressurization in a closed pouch?
To pressurize a closed pouch, a leak tight measuring path must be available between the package interior volume and the pressure source. An example is the TME Package-Port system in which a reusable plastic entry port is secured and then accommodates the pressurizing probe. The probe tip pierces the package, enabling pressurization, and the Package-Port reinforces the package material to eliminate any possible leakage of gas around the penetration point.

Inflation Testing Failure Modes in Welded Seals
There are several points to remember as you characterize any seal failures you encounter in your testing. When your seal is good, the base material will rupture before the seals will release in an inflation test. In a bad seal, you may encounter fracture or delamination of the laminate in the seal area. Other types of failure in welded seals include blowouts or pinholes in high-stress corners. Finding these weaknesses before retort or before transport and storage will minimize loss, clean-up and process down time.

Package Integrity Testing
As we discussed, inflation seal strength test results can be an excellent tool for process control. Seal strength testing is only half of the story. The other half, and equally important, is package integrity testing.

There are a number of commonly used physical tests for package integrity, including the visual inspection method ("eyeballing" the pouch for problems), the internal pressure method (bubble testing), the vacuum leak method (in which the package is submerged in water inside a vacuum chamber and escaping bubbles are observed), and trace gas detection, where the package is pressurized with a gas other than air and then examined for escaping gas with a gas leak detector. A primary drawback to each of these tests is that they are subjective, operator dependent, and non-quantitative, and in some cases messy and difficult to perform.

An alternative method that overcomes these objections is the pressure (or vacuum) decay test, which we will shortly discuss in some detail.

Package Integrity (Leak) Testing
Package integrity testing is a measure of the package's barrier material and seal - a "leak test" of the whole package. In addition to seal bonding failures or disrupted seals, leakage can be the result of large holes, pinholes or cracks in package materials. Either source of leakage represents the potential for product contamination - elements of the ambient atmosphere outside of the package entering the package - or for the materials inside the package to escape.

The pressure decay method is often used to perform package integrity (leak) testing on flexible pouches or other packages that have non-porous material surfaces and seals. This method is highly recommended as it is affordable, sensitive, repeatable and quantitative.

Pressure Decay Testing
The pressure decay test is accomplished by pressurizing the package to a fixed pressure, shutting off the pressure and connecting a pressure transducer. After a "settle" time, any observed changes in pressure may indicate the presence of leakage paths in the package seals or pinholes in the surfaces. This leak may be represented in decay pressure units or calculated leak rate units.

The pressure decay leak test cycle, from beginning to end, includes the time to engage the package with the measuring instrument. The "Charge" time is the period of time in which the pouch is being pressurized to the predetermined test pressure. A "Settle" time allows the volume of the pressurized package to change and then stabilize due to the stresses introduced by pressurization and adiabatic temperature changes. "Test" is the actual data-taking period in which the measurement of the decay of pressure is taken, and the process is completed with the "unloading" of the package from the instrument. Incidentally, in all of our discussions of "pressure decay" testing, please keep in mind that vacuum decay is an equally viable test that functions identically to the positive pressure decay test cycle.

Resolution and your Leak Test Instrument
The resolution of your leak test instrument determines the size of the leak that can be detected. The resolution of your test is defined as the smallest pressure decay (change in internal pressure of your package during the "test" phase of the pressure decay test cycle) that can be detected by your test instrument. For example, the TME Solution Multi-Port Leak and Flow Tester has a maximum resolution of 0.0001 psi (.01 mbar).

Non-Destructive Closed Package Testing
Pressure decay testing as discussed previously supposes a test package or pouch that can be pressurized, and is generally a destructive test. If your package is closed or sealed so it cannot be pressurized from an external source, an alternative method of pressure decay leak testing involves creating a closed space around the test item (a surrogate chamber) and pressurizing (or evacuating) it. Air entering the part through a leak (or in the case of a vacuum test, leaving the part through a leak) provides the measurement of leakage into the test part. This method is effective for non-porous pouch form packages, shaped pouches, trays, and uniquely shaped thermoformed containers with induction welded seals. As these drawings suggest, test chambers can be designed to closely fit the test item, whether a tray or tub or an induction welded bottle seal.

Non-Destructive Pressure/Vacuum Decay Chamber Testing
When a sealed package is placed in a surrogate chamber, a pressure differential can be created across the non-porous barrier package walls and seals. Once stabilized, air movement from the higher pressure to the lower will indicate the presence of a leak path, providing a quantitative measure of package integrity without disrupting the package seals. Leakage is measured by the pressure change in the vacant chamber space surrounding the package.

Designing your non-destructive leak test: know your seal strength
It only makes sense that if your chamber test pressure approaches the burst seal strength, the seals are going to fail or be impaired by the test, so it is critical that the leak test pressure is no more than 25% of the package's burst seal strength. In addition, some seals will creep under pressure. Before setting a final leak test pressure, be sure to do a creep test on your package at the pressure you wish to use in the leak test and carefully evaluate the effect on the package seals.

The underlying objective is to determine the pressure that will locate leaks and still be non-destructive to your package.

Some vocabulary useful in chamber test design
This example shows a liquid-filled vial in a test chamber. The "head space" is defined as the amount of space inside the package that does not contain product. Adequate head space is necessary for either the pressure or vacuum decay chamber test.

The "interstitial space" is the air-filled area inside the sealed chamber but outside the package. The interstitial space should be minimized to achieve the greatest test sensitivity. Because air moves in or out of the package in the presence of a leak, the air volume around the test object must be adequate to create a detectable change in the chamber pressure.

Technical issues to consider when designing your chamber test system
At the risk of stating the obvious, the first issue that must be considered is the porosity of the package you plan to test. Package component materials must be non-porous and the package must be completely sealed.

Either pressure or vacuum may be used for the chamber test. However, the decision of pressure or vacuum is related to the structural rigidity of your package or product, since pressurizing the interstitial space may damage a fragile object or the contents of a non-porous pouch.

One other consideration involves liquid-filled packages. If you are manufacturing a package in which the seal may come in contact with the liquid contents, there is a possibility that the liquid may block a potential leak path that you are trying to detect. So, for beverage containers and similar items, you have to evaluate your integrity test results with that in mind.

Sensitivity of the Method
Keep in mind that when you minimize the interstitial volume and maximize the instrument resolution (within reason), about 10-4 sccs is an achievable sensitivity.

Some typical non-destructive chamber test applications from the food industry
This is where we see examples of what we have just discussed, the minimizing of the interstitial space surrounding the test item. Proprietary methods enable chambers to very closely approximate the shape of the test item. This yields a very sensitive, repeatable and reliable test of the package's leak integrity.

Conclusion
We have seen here that food package testing consists of not one but two distinct testing streams, seal strength testing and package integrity (leak) testing. Only by combining these testing streams can the food packager be assured that the package will protect its contents through the packaging, transport, handling and storage processes that it will endure. The other important issue addressed here is the importance of obtaining and maintaining quantitative test results for the documentation of process development and packaging process control.

The importance of careful, documented testing during the design phase, material qualification, manufacturing and testing of packaging is similar in most industries. In the medical device industry, a process guideline document outlines the principle requirements for package process development and validation, concentrating on forming and sealing as the most critical package development processes. This document, ISO-11607, provides package designers and manufacturers with a framework of tests and evaluations that can be used to qualify the overall performance of the package during the rigors of handling, distribution and storage. Because many of the same issues are critical to food packaging, many of the guidelines found in ISO-11607 are included in this discussion, such as the importance of both seal strength and package integrity testing and the need for quantitative, retainable test data. I strongly recommend that anyone involved in the design and manufacture of the many new package forms emerging in the food industry review this document.

PDF format (898 KB)