Hello and welcome to this plastic failure analysis post. Apart from polymer material selection, and preventing plastic part failure, I focus in my role as certified plastics expert witness to support the polymer engineering community in solving failed plastic part cases.
Example recycled PP pallet corner cracking in cold warehouse
Overview on the situation
- Part / material: Pallet (EUR/EPAL-Palett; 800 mm × 1.200 mm × 144 mm) made out of mechanically recycled polypropylene (rPP).
- What happened and which failure was observed (Figure 1): Corner cracks and brittle fracture of corner area after it was dropped at low temperature (below 10°C).
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| Figure 1: Example plastic failure analysis - broken corner of a palett made out of recycled PP. |
Plastic part failure analysis
Figure 2 shows the steps of a general plastic part failure analysis protocol [1] which can be followed to obtain a solid root cause and take corrective actions to prevent failure in the future. In this post I focus on the steps "material analysis, determination of failure mode and cause, and corrective actions".
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| Figure 2: Overview of the steps for performing a plastic part failure analysis. |
Root cause analysis and results:
- Identification of material by using Differential Scanning Calorimetry (DSC): DSC is a thermal analysis technique used to observe thermal transitions in polymers. This includes identifying key characteristics such as:
- Glass Transition Temperature (Tg): The temperature at which an amorphous polymer transitions from a rigid, glassy state to a more flexible, rubbery state.
- Melting Points (Tm): The temperature at which crystalline regions of a semi-crystalline polymer melt.
- Crystallization and Crystallization Rate: For semi-crystalline polymers, DSC can also reveal information about how they crystallize upon cooling.
Pellets and pallet sections were both analyzed with DSC and in both, pellets and pallet sections Polypropylene could be identified via the melt peak at 170°C (Figure 3). Apart from PP, Polyethylene (LDPE and HDPE) melting peaks could be identified and it is not unusual for recycled PP to contain LDPE and HDPE too. They are referred to as mixed polyolefins and use packaging and industrial waste as primary recycling source. Packaging waste contains often PS and PET too, which could not be found in our material samples. Also, three other polymers could be identified, which may come from the industrial waste stream: Polyoxymethylene (POM), Polyamide 6 (PA 6), and Polytetrafluorethylene (PTFE). Having altogether five polymers in a PP base polymer system has impact on the material and final part properties.
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| Figure 3: DSC result of pellets and pallet part - apart from PP, five other polymers were found. |
- Property variability: Contamination with LDPE/HDPE/POM/PA 6/PTFE, and unknown additives lead to a variability in mechanical and thermal properties. Also, differences in melt viscosity (via MFR) could be shown.
- Degradation: Oxidative degradation from multiple heat histories due to processing resulted in a lower molecular weight and reduced toughness.
- Impact modification: Insufficient impact modification for low-temperature use.
- Part design and processing: Poor weld line strength due to contamination and poor flow during filling phase in injection molding.
Corrective action proposals
To address and prevent plastic part failure in the future, the following corrective measures should be considered:
- Improve feedstock control: Implementation of tighter incoming quality checks, including MFR, DSC, ash content, and FTIR screening to detect contamination.
- Add stabilization: Usage of a combination of hindered phenolic and phosphite antioxidants, keeping in mind any odor constraints.
- Enhance impact resistance: Incorporation of impact modifiers (such as EPR/EPDM) and/or blend with virgin PP to maintain stable performance.
- Optimize processing: Lower shear rates, reduce residence time, and improve venting and filtration (e.g., use of melt filters) during processing.
Prevention tips for part failure:
To enhance part reliability and prevent failures, the following best practices should be considered:
- Design considerations: Account for the variability of recycled materials by incorporating optimized corners and radii, and by avoiding thin snap features in your designs.
- Quality assurance: Implement lot-based mechanical testing, such as notched Izod or Charpy impact tests at the intended service temperature, to ensure consistent performance.
Applying these measures will help improve the durability and quality of our products.
If the application is cold-impact critical, rPP should only for non-critical components considered or require certified PCR grades.
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Greetings,
Herwig
Literature:
[1] Jeffrey A. Jansen: Characterization of Plastics in Failure Analysis, Stork Technimet Inc / The Madison Group
