Correlating Peel and Burst Tests for Sterile Medical Device Packages

The ability to mathematically relate peel and burst test data could allow results to be taken more rapidly to production and quality control personnel.

Peel and burst tests are used by both users and suppliers of pouches to perform incoming and outgoing inspections. They are also used to validate the sealing parameters of a package and as a process control tool. Users and suppliers alike have an interest in clarifying the relationship between peel and burst tests.

Pouch users typically have developed specifications for tensile peel seal strength that are used when testing medical packaging seals. With a model for correlation between peel and burst tests, the faster burst test could be used to update such specifications without requiring an extensive revalidation program. This would make sampling and testing less expensive by reducing labor and material usage costs.

Because pouch suppliers' customers would prefer to use only one package-testing method, suppliers need to demonstrate that the method they use will correlate with the customer's method. Additionally, if the burst and peel tests can be mathematically related, pouch suppliers can use the faster burst test technique to provide peel test values directly to production and quality control personnel. In this way, corrective action can be taken almost immediately to prevent the production of pouches that are out of specification.¹ This will increase productivity and, at the same time, reduce labor and material costs.

The authors have developed three mathematical models and a procedure for using them. With these tools, peel seal strength can be calculated from burst data or burst strength from tensile peel strength measurements. In a study of the models, measured strengths were in the range of 1–5% of calculated values. Thus, the authors recommend the procedure as a way to correlate peel tests and burst tests.

The peel and burst tests are the most commonly used methods for measuring the seal strength of peelable flexible packages. The peel test measures the force required to peel apart a 1-in.-wide sample strip cut from the seal material. This test has some disadvantages. It is time-consuming because many strips have to be cut from a package in order to get a true measure of the seal strength. The sample strips may not contain a weak spot that would be present in the pouch. If only a small number of strips from a given package are tested, some substandard seal surface may be overlooked.²

There is some controversy over whether to use peak or average force as a value for seal strength in the peel test. Peak force is the maximum force required to separate the sealed webs, and thus the maximum force created during peeling. Average force is calculated by dividing the energy, which is the area under the force-deformation curve, by the total length of peel. Barcan believes that the average force is a better measure, providing a more comprehensive picture of seal strength, because it considers the entire sample strip of seal and not just an instant during peel.³

There is also some debate regarding how to conduct the test. Testing can be performed using a supported tail or an unsupported tail. According to Earl Hackett, in order to obtain good test repeatability, the peel angle of the two substrates must be maintained throughout the measurement.⁴ An unsupported peel test, is preferred by others who suggest that it more closely emulates real-world conditions.

The burst test consists of pressurizing a package until it breaks. In pouches with peelable seals, the break typically occurs as a separation of the seal rather than as a rupture in the material. The pressure required to break the package is recorded and interpreted as an indication of the seal strength. The burst test can be performed with or without the use of restraining plates. When plates are used, the deformation of the pressurized package is minimized.

The burst test has gained acceptance in the medical device and medical packaging industries because it is easier to perform and can be completed in less time than the peel test. It also provides an evaluation of the entire package system, not only the seal. By pressurizing the package with air, burst testing subjects the entire sterile package system to typical stresses that the package encounters in the manufacturing, distribution, and use environments.⁵

Theory

Theoretical equations, based on force diagrams, have been developed in order to explain a pouch's behavior during a restrained burst test. In Figure 1, D represents the plate separation, and L is the length of the pouch under test. This dimension is shorter than the restraining plates. The restraining plates apply pressure to the pouch over the contact area (length [L] times width of the pouch [W]).

To find a correlation between peel and burst tests, one must equate a force acting on a surface (burst test) with a force acting on a seal line (peel test). Good agreement was found by Wachala and Yam for paper/plastic pouches and meals-ready-to-eat (MRE) pouches, respectively.^2,6 Yam reported that good correlation was obtained for MRE pouches when tensile peel rate was controlled to ensure that it was the same as the rupture (or peel) rate in burst tests.

Yam's theoretical formula (P = 2S/D where P = burst pressure, S = strength, and D = plate separation ) was studied by Feliú-Báez for Tyvek/plastic pouches.^7,8 The tensile peeling times were controlled within a range of 1–8% of the burst times. The results showed that the formula P = 2S/D overestimated the actual burst pressure; the overestimation increased at smaller gaps. The overestimation of burst pressure was 22–49% at 0.50-in. plate separation.⁷

The formula P = 2S/D does not account for package size. Feliú-Báez found that package size has a significant effect on the restrained burst pressure.⁸ For that reason, it was hypothesized that the model accuracy could be improved by accounting for the package's dimensions. A force diagram analysis was performed on the entire pouch in order to develop a new model that accounts for package size. Figure 2 (a) shows the original length and width dimensions (L_O and W_O) of a flat pouch. Figure 2 (b) shows the pouch after it is pressurized in a restrained burst test. The dashed rectangular patch in Figure 2 (b) represents the flat area that is in contact with the restraining plates. The dotted intersecting lines represent the original length, L_O, and original width, W_O, of the pouch.

Figure 3 illustrates a force diagram of a pressurized pouch. It shows the original width of the pouch, W_O; the width of the rectangular patch in contact with the restraining plate, W_C; and the width of the pressurized pouch, W_P. It is assumed that the unsupported outer border of the pouch deforms into a segment of a circle and that the material does not stretch much.

The original dimensions of the flat pouch are L_O and W_O. The length and the width of the rectangular patch in contact with the restraining plates are the following:

L_C = L_O – 2/4(2pD/2) = L_O – (pD/2) = L_O – 1.571D (1)

W_C = W_O – 2/4(2pD/2) = W_O – (pD/2) = W_O – 1.571D (2)

The length and the width of the pouch under pressure, with gap D, are the following:

L_P= [L_O – (pD/2)] + 2(D/2) = L_O – (p/2 – 1) D = L_O – 0.571D(3)

W_P= [W_O – (pD/2)] + 2(D/2) = W_O – (p/2 – 1) D = W_O – 0.571D(4)

The plate balances the pressure acting on the rectangular patch, and the seal balances the pressure acting on the unsupported perimeter, such that

S [perimeter] = P [working area]

Where working area = total pressurized area – contact area,

S [2(L_P + W_P)] = P [(L_P * W_P) – (L_C * W_C)]

Using Equations 3 and 4 for total pressurized area and perimeter and Equations 1 and 2 for contact area,

P = 2S/D * [(L_P + W_P) / ( L_O + W_O c 2.142D)]

P = 2S/D * [(L_O + W_O – 1.142D) / (L_O + W_O – 2.142D)] (5)

The term within the square brackets in Equation 5, (L_O + W_O – 1.142D) / (L_O + W_O – 2.142D), will be referred to as the correction factor (CF).

The correlation obtained with Equation 5 will not be better than P = 2S/D. Because the CF is a quantity greater than 1, it will predict a higher burst pressure. Thus, it will overestimate even more than P = 2S/D. The fact that the correlation is not better even when the package size is considered suggests that other factors not included in the model can affect it. For example, the model does not account for wrinkles formed in the seal area and the amount of stretch that occurs in pouch materials during a restrained burst test (sometimes called ripple effect). In addition, if the actual geometry of the working area when deformed during a restrained burst test is not semicircular, as assumed in the model, deviations will occur between the actual and predicted burst pressures. All of these factors are difficult to measure and derive with the force diagram analysis.

Because the new model described in Equation 5 did not improve the predictive accuracy, it was decided to use an empirical approach. Multiple-regression analysis was used to fit actual data to a power law model. The following paragraphs provide an explanation of the methodology used and the results obtained.⁹

Experiment Design

The materials tested in the experiments included a number of peelable pouches from two manufacturers. These were divided in three categories:

The equipment used in the experiments included the following:

Testing Procedures. The pouches were conditioned at ASTM standard test conditions for 48 hours prior to testing. The following procedures were then performed.

A restrained burst test using an open package fixture was performed according to ASTM F1140-96 and ASTM F2054-00.^10,11 Parameters under study were the flow index (1, 5, and 9) and plate separation (0.5 and 1.0 in.). The sample size used for each flow-index/plate-separation combination was 10 (n = 10). Therefore, a total of 60 pouches of each material combination were burst tested (3 flow values x 2 plate separations x 10 samples per flow-index/plate-separation combination). Power calculations were used to determine the sensitivity of the experiments when using a sample size of 10.⁹

An unsupported-tail peel test was performed according to ASTM F88-99.¹² As shown in Figure 4, 1-in.-wide specimens were cut from the region where the pouch failed during the burst test. The parameters under study were crosshead speed (10 and 12 in./min) and grip separation (1.0 and 2.0 in.). The sample size used for each crosshead-speed/grip-separation combination was 10 (n = 10). Therefore, a total of 40 pouches of each material combination were peel tested (2 crosshead speeds x 2 grip separations x 10 samples per crosshead-speed/grip-separation combination). Power calculations were used to determine the sensitivity of the experiments when using a sample size of 10.⁹ Peak and average forces were recorded.

Data analysis included both 2² factorial analyses and model validation.

2² Factorial Analyses. As shown in Table I, the factorial experiments had the purpose of determining the effect of flow index and plate separation in the burst test. They also demonstrated the effect of crosshead speed and grip separation in the peel test. All possible combinations of the factors under study were considered.^13,14

Model Validation. Two approaches were used for model validation—theoretical and empirical. In the theoretical approach, a force diagram analysis produced two theoretical models:^8,9

P = 2S/D and

P = 2S/D * [(L_O + W_O – 1.142D) / (L_O +W_O – 2.142D)]

The second model is Equation 5. The analysis of these theoretical models consisted of calculating the predicted burst pressure (P) with the two formulas above, and comparing the result with the actual burst values obtained experimentally. A calculation of the percent error also was performed in order to get a numerical comparison.

In the empirical approach, multiple-regression analysis was performed.⁹ The method was used to fit experimental data to different power law models. The regression parameters, coefficient of determination (R²) and the correlation coefficient (R) were obtained using statistical software. The adequacy of these models was determined by comparing actual values with the predicted ones and by calculating percent error.

Results 2² Factorial Analysis
The analyses of variance performed with the burst test results showed that plate separation had a significant effect on the burst pressure at all the package sizes for each of the material combinations tested. All p values for the analyses equaled 0.000. Additionally, the analyses demonstrated that flow index had a significant effect on the burst pressure when the extreme values, 1 and 9, were used. This was true for all package sizes and each of the material combinations tested. When comparing flow indexes, flow index 5 produced a higher burst pressure in larger packages than did flow index 1. When the comparison was made between flow indexes 5 and 9, the burst pressure tended to show no significant difference.⁹

The analyses of variance performed with the peel test results showed that the crosshead speed and grip separation did not have a significant effect on either the peak or average peel test results. All p values equaled > 0.050.

Model Validation
Predicting Burst Pressure. Two theoretical models and three empirical models were used to predict the burst pressure from the seal strength values (Table II).

The empirical models in Table II are based on the theoretical formulas. The independent variables (X₁, X₂, and X₃) are equal to terms that were derived theoretically with the use of force diagram analysis ((S/D) and CF).⁹

Regression analysis was performed for the three empirical models. The following four statements summarize the findings:

A summary of the results for all models that predict burst pressure using peak and average seal strength values for supplier A's 1073B Tyvek/polyester poly laminate pouches is shown in Table III. Similar results were obtained for the other two material combinations under study.⁹

Results for the three empirical models using average values are shown in Table IV, which includes all material combinations tested.⁹

Predicting Seal Strength. The models discussed predict burst pressure (P) when the seal strength (S), the plate separation (D), and the package dimensions (L_O and W_O) are known. There might be situations in which seal strength (S) must be predicted from burst pressure (P), plate separation (D), and package dimensions (L_O and W_O). Analysis indicated that this can be done. The two theoretical models were solved for seal strength (S). Two approaches were used for the empirical models: solving for (S) from the burst pressure (P) models, and running a multiple-regression analysis to predict S/D.⁹ Both methods work.

Data analysis was the same as that used for burst pressure prediction models, and the results were similar. Lower average percent errors were obtained when average forces were used, and the empirical models tended to have a lower average percent error than the theoretical models.

Conclusion

The experiments described revealed that empirical models explained the relationship between peel and burst test results better than theoretical models. The reasons for the disagreement between the actual values and those predicted by the theoretical models are believed to be the following:

The validation results for the empirical models were good. The correlation coefficients were all 0.96 or higher and the average percent errors between actual and predicted values ranged from 1 to 7% for predictions of burst test value from seal strength and vice versa. It should be noted that these results are applicable for the material combinations tested within the range of package sizes tested. Nevertheless, because the independent variables for the empirical models are based on theoretical principles, and the models were run for three different Tyvek/plastic types of pouches, the model should work for other types of Tyvek/plastic combinations. A change in the regression parameters when the materials under study are changed should be expected.

The results obtained when using average values tended to be slightly better than the ones obtained when using peak values. This was anticipated because the average peel force results were less variable than the peak peel force results.

This study resulted in a procedure that can help industry perform a simple experiment for the purpose of estimating the regression coefficients for any particular material combination.⁹ The recommended procedure is outlined in Figure 5.

When using this procedure to obtain a mathematical model for the correlation between peel and burst tests, the following facts should be kept in mind:

REFERENCES

1. DS Barcan and SH Franks, "Comparing Tensile and Inflation Seal-Strength Tests for Medical Pouches," Medical Device & Diagnostic Industry 21, no. 10 (1999): 60–67.

2. TP Wachala, "Correlating Tensile and Burst Test in Pouches," Medical Device & Diagnostic Industry 13, no. 2 (1991): 12–15.

3. DS Barcan, "Using a Seal Matrix to Optimize Package Sealing Variables," Medical Device & Diagnostic Industry 16, no. 9 (1995): 112–122.

4. ET Hackett, "Automated Peel Test for Process Control Eliminates Variables," Packaging Technology and Engineering 7, no. 2 (1998): 44–49.

5. D Bohn, "Using Burst Testing to Evaluate Sterile Blister Packaging," Medical Plastics and Biomaterials 1, no. 1 (1994): 14–20.

6. KL Yam, J Rossen, and X Wu, "Relationship between Seal Strength and Burst Pressure for Pouches," Packaging Technology and Science 6 (1993): 239–244.

7. R Feliú-Baéz, HE Lockhart, and G Burgess, "Correlation of Peel and Burst Test for Pouches," Packaging Technology and Science 14 (2001): 63–69.

8. R Feliú-Baéz, "Analysis and Evaluation of Burst Test Methods Using Restraining Fixtures." Master's thesis, Michigan State University, 1998, 11–20, 37–41, 109–121, and 138–164.

9. R Feliú-Baéz, "Analysis of the Relationship Between Peel and Burst Test for Peelable Flexible Packages." PhD diss., Michigan State University, 2001.

10. ASTM Committee F-2 on Flexible Barrier Materials, ASTM F1140-96: Standard Test Methods for Failure Resistance of Unrestrained and Nonrigid Packages for Medical Applications, (West Conshohocken, PA: American Society for Testing and Materials, 2000), 1176–1179.

11. ASTM Committee F-2 on Flexible Barrier Materials, Draft Proposal ASTM Standard: Standard Test Method for Burst Test Seal Strength Testing of Flexible Packages using Internal Air Pressurization within Restraining Plates, (West Conshohocken, PA: American Society for Testing and Materials, 1997).

12. ASTM Committee F-2 on Flexible Barrier Materials, ASTM F88-99: Standard Test Method for Seal Strength of Flexible Barrier Materials, (West Conshohocken, PA: American Society for Testing and Materials 2000), 989–993.

13. DC Montgomery, Introduction to Statistical Quality Control, 3rd ed. (Hoboken, NJ: Wiley, 1997), 490–491.

14. DC Montgomery, Design and Analysis of Experiments, 3rd ed. (Hoboken, NJ): Wiley, 1991), 216–217 and 607.

Rosamari Feliú-Báez, PhD, is the packaging engineering manager at Glaxo Smith Kline (Cidro, Puerto Rico). Hugh E. Lockhart, PhD, and Gary Burgess, PhD, are professors at the Michigan State University School of Packaging (East Lansing, MI).

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