Practical examples Industry: Packaging
© DF.FotografieCracking
of returnable PET bottles
Problem: Cracking of returnable PET bottles
Reusable PET bottles typically go through twenty or more usage cycles. During washing, filling, transportation, storage and actual use by the consumer, the bottles are subject to a load spectrum that is individual to each bottle. In an increasing number of cases of damage, consumers have complained about leaks in the thread area of returnable PET bottles of a certain type.
Methodology: readjustment tests and microscopic analysis (LIM / FESEM)
- Adjustment tests for the purpose of life cycle simulation.
- The test bottles were marked and washed, filled, capped and emptied in a total of 10 cycles in running filling lines.
- Comparative microscopic examinations to detect possible crack formation depending on the filling cycles previously completed
Result: Effect overlay in the thread area
As the number of wash cycles increased, an increase in visible cracks in the thread area was observed. The cracks were initiated on the outside of the bottle. However, these cracks do not occur in every reusable bottle: A correlation between crack formation and closure design used could be established. Microscopic methods were used to detect traces of chemical material attack on the thread surface. It can be assumed that this is the effect of the washing suds used . Furthermore, processing-related orientations / stresses were found in the thread area of the bottles. The cracks are caused by a load collective of tensile / bending stresses in the thread area, which are caused by the prevailing internal bottle pressure and the closure design used. The medial load from the wash liquor multiplies the effect of the mechanical loads.
Christoph Zekorn
Head of the Microscopic Analysis Laboratory
Damage analysis on
film outer packaging for drinks bottles
Problem: Failure of the carrying device for six-pack outer packaging
Six-packs, in which six PET bottles are repackaged using a shrink film, are fitted with a glued-on carrying strap for larger containers. Sometimes the carrying device fails. The damage usually indicates either poor bonding or insufficient strength of the shrink film. In the present case, the chronological sequence of customer complaints gives the impression that only a few batches are affected, which also only have a certain percentage of defective carrying devices.
Analysis method: Tensile test and optical measurement
In order to counter these uncertainties and to be able to describe the failure behavior qualitatively and quantitatively, the deformation pattern of a bottle outer packaging during lifting and transport is determined using a tensile testing machine and a device adapted to the real load situation. Using a pinhole aperture, a dot pattern is sprayed onto the film with a non-dissolving paint. This prepared film packaging is then subjected to a practical tensile load on a universal testing machine. During the test, the deformation of the film is recorded with an optical measuring system in addition to the force. The optical measuring systems used here offer significant advantages over other methods when determining deformation.
Mechanical transducers are difficult to mount on film structures and applied strain gauges distort the measurement result due to their inherent stiffness. Optical, non-contact systems, on the other hand, ensure that disturbances to the system to be characterised are minimised. Using the test set-up, the forces acting can be related to the deformation of the film packaging. In addition to the effective force lines, a measure of the level of film stretch can also be determined approximately.
Result: Practical findings on displacement and elongation
The investigations using optical measurement provide important insights into the displacements and elongations of the film that can occur during normal handling. They also provide information about the failure process of the outer packaging. On this basis, improvement measures can be developed and specifications for the film and the bonding can be drawn up.
Tobias Conen, M. Sc.
Head of the Mechanical Testing Lab
Quality assurance
for barrier coatings on yoghurt cups
Problem: Determining the quality of SiOx barrier coatings for cups
Packaged food must often not oxidise and the water content must not change in order to keep the product edible. The penetration of oxygen and moisture also enables the growth of bacteria and mold and thus leads to spoilage of the packaged product. Plasmapoylmerisation can be used to provide plastic packaging, which otherwise only has a low barrier effect, with a coating with a high barrier effect (often SiOx layers). The manufacturer of the plastic cups has the coatings applied by a third-party company, but has no way of evaluating the quality of the applied coatings. Due to the transparency and low layer thickness of the coatings of just a few tens of nanometers, the coatings cannot be visually inspected.
Methodology: layer detection and characterisation
- Rapid test for layer detection with infrared spectroscopy.
- Coating adhesion measurements using an adhesion analyzer and comparison with empirical values from the IKV for this type of coating.
- Coating of identical cups in the IKV to form a reference.
- Comparison of the barrier improvements achieved.
Result: Sufficient quality of the supplier’s coatings confirmed
A SiOx coating can be detected on all samples supplied using the rapid test. The coating adhesion measurements showed sufficient values for the application. The barrier improvements achieved with the supplier’s coatings are almost in the same range as those that can be achieved with the coatings developed at IKV.
Dr. rer. nat. Sabine Standfuß-Holthausen
Head of the Spectroscopic Analysis Laboratory
Reprocessing
from post-consumer polypropylene
Problem: Reprocessing of recyclates
When using recyclate of uncertain origin, the purity content is often unclear. This uncertainty can be minimised, but not completely eliminated, by using a targeted method for structural characterisation of the material. During the processing of post-consumer polypropylene (PP), production stoppages occurred at short intervals. The cause was clogging of the melt filter. The random analysis of the starting material using infrared spectroscopy was unremarkable.
Methodology: Combination of morphological, spectroscopic and thermal methods
- Morphological characterisation of the added sieve (thin section): Impurities in the form of non-plasticised particles
- Infrared spectroscopic analysis of these particles: Polyethylene in polypropylene
- Astonishing result: The melting temperature of PE is far below that of PP.
- Consequently, the PE particles should be plasticizable.
- Thermal analysis of the particles using DSC: the resulting melting temperature (Tm = 135 °C) confirmed the result of the IR spectroscopy. Melting enthalpy was low.
Result: PE cross-linked and not plasticizable
The very low melt enthalpy suggests that the PE is cross-linked. As a result, it was not plasticizable and clogged the melt filter. Based on these results, the source of the contamination in the recycling chain can be specifically identified and the source of the fault eliminated.
If the material chasacterization of recyclate is carried out too one-sidedly or unsustainably, this can lead to considerable problems during processing. This example shows why a combination of spectroscopic and thermal analyses is fundamentally useful.
Dr. rer. nat. Sabine Standfuß-Holthausen
Head of the Spectroscopic Analysis Laboratory