Masters Theses
Date of Award
12-2004
Degree Type
Thesis
Degree Name
Master of Science
Major
Polymer Engineering
Major Professor
Kevin M. Kit
Committee Members
Marion G. Hansen, Roberto S. Benson
Abstract
The military has recently suffered a reduction in production capacity due to the rejection of their flexible food packaging. The rejection of these polytrays was due to sealing defects and the high standards set forth for the sealing quality. These standards set by the military were likely too conservative and until a test could be developed and optimized to quantitatively measure seal integrity, the standards will remain very high. The optimized test, whether destructive or non-destructive, must show that a particular defect, which might currently cause rejection, will not affect the quality of the items contained if passed as a good package. The unavailability of such a test was the basis for the project.
Common tests, both destructive and non-destructive, were performed on the polymeric packaging to develop a basis for an optimized test. Destructive tests such as peel testing and tensile testing were performed to gain a perspective on the strength and rigidity of the seal in the packaging. Tensile testing provided materials properties, such as the elastic modulus. An average elastic modulus value of 1.4 x109 Pa and 1.8 x 109 Pa was used for the lid-stock and tray respectively and were used for the material properties input of finite element analysis.
Peel testing was also performed to gain a perspective on the strength of the both good seals and defective seals. Polytrays with no defects from Wornick Food Company proved to have the highest peel strength of all samples with a maximum and minimum peak load of 43.83 lbf and 18.27 lbf respectively. A large variation in peak load among all of the samples tested, suggested some uneven thermal sealing of the polytray during production. The first set of defective polytrays used in peel testing was from the Center for Advanced Food Technology at Rutgers University, where a good representation of “short seal” defects on the polytrays was obtained. These seals proved to have one of the lowest maximum and minimum peak loads of 24.01 lbf and 8.42 lbf respectively. Peel tests involving polytrays with artificial defects such as entrapped matter made at Stegner Food Company Trial 2 were tested as well. It was found that solid entrapped matter in the seals, such as noodles, performed poorly in peel compared to the control samples. The entrapped noodle samples proved to be the worst performing of all peel test sets with an average peak value of 15.31 lbf and the lowest recorded peak force value at 7.17 lbf.
Burst tests were performed to detect leaks in polytrays with both naturally occurring and artificial defects in the sealing area. These results were the basis for improving and implementing a PC integrated burst test system and for predicting whether or not a seal is good based on defect present. Defective seals were tested and compared to the values of good seals. Leaks were detected in channel defects as small as 50.8 mm. Leaks were also detected in seals with solid entrapped matter; the polytrays with noodles entrapped in the seal, just as in the peel test, proved to be the weakest seals. The defective seals were classified, and a basis for the rejection of certain defects in the sealing area was specified. Although these destructive tests were effective in the determination of seal quality, 100% of production packages can not be inspected by these methods. These tests only provide visual basis for which a polytray can be rejected. The results will hopefully be used increase production without affecting the design or materials selection of the polytrays currently in use.
Finite element analysis, using programs such as FEMLAB®, was used to simulate different loading conditions that the polytrays might endure while in service. These simulations provided vital information as to the way the polytrays behaved under different conditions, especially in the seal area. Pressurizations (2.9 psi through 29 psi) and corner load (10 N through 200 N) simulations on the polytrays were examined in this project. It was found that Mode I and Mode II of fracture dominate in the pressurization simulations and Mode I and Mode III of fracture dominate in the corner force loading simulations. The results, stresses produced in Mode I, II, and III of fracture at the seal, were examined within cross-sectional plots and were reported. A linear increase in maximum stresses was also observed, which was expected.
Non-destructive testing techniques, such as ultrasonic C-scan inspection, were examined as a way to provide an economic means of reducing the incidence of defective packages reaching the consumer. The ultrasonic techniques did not result in the permanent change in the medium; maintained the integrity of the food package and did not alter the mechanical properties of the polytray. The pulse-echo technique was used for this project. Polytrays were sealed specifically for ultrasonic inspection which included all four channel defects on one side of the polytray. Although the wires were detected in both ultrasonic B and C-scans, this technique was ultimately not useful. The clarity and resolution of the images captured were not of high quality even with the largest of channel defects present. Smaller defects, such as the 50.8 micron channel defect, could not be captured in the images.
Post failure characterization was carried out using FTIR/ATR spectroscopy techniques. Failed burst test specimens, lid-stock side, were utilized in an attempt to correlate the failed lid-stock layer to a particular defect. The FTIR/ATR spectroscopy provided information as to what material(s) was present on the surface of the failed burst test specimen. Through visual inspection, it was concluded that the failed burst test samples delaminated in two places; at the actual seal between the polypropylene of the lid stock and the polypropylene of the tray and between the aluminum and the polypropylene of the lid stock. ATR/FTIR spectroscopy was used to examine 119 failed sample surfaces surrounding various defects. 72 of the failed samples displayed the polypropylene spectrum; 60.5% of the total samples examined. The remaining failed samples, 47, displayed a polyester spectrum which is equivalent to 39.5%. It was determined that the defects present in the seal, whether artificial or naturally occurring, had no adverse effect on the particular failure surface of the samples from the burst test experiments.
Recommended Citation
Barlow, William James Jr., "Evaluation of Seal Integrity of Flexible Food Polytrays by Destructive and Non-Destructive Techniques. " Master's Thesis, University of Tennessee, 2004.
https://trace.tennessee.edu/utk_gradthes/1828