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Background
A manufacturer using regrind polyethylene material experienced difficulties with contamination of screen plates within an extruder. The problem was thought to be caused by contamination of the regrind feedstock. ipolytech were requested to:
- Identify the contaminant
- Verify the source of contamination
- Determine how widespread the problem was
Inspection of Contamination
Debris from the screen plates was examined under an optical microscope. The contamination was found to be particulate in nature and had a glossy polymeric appearance.
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Figure 1. Contamination found on extruder screen plate.
Infrared Spectroscopy
A particle of the contaminating material was removed and analysed using fourier transform infrared spectroscopy (FT-IR Spectroscopy). The particle was compressed against the diamond window of an ATR (Attenuation Total Reflectance) accessory and a reflectance infrared spectrum recorded. The resultant spectrum was typical of a polyethylene terephthalate (PET) material.
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Figure 2. Infrared spectrum from contaminant material.
Differential Scanning Calorimetry
Confirmation of the contaminating material was gained by determining the melt characteristics of one of the particle suing differential scanning calorimetry (DSC). The resulting DSC trace shows a melt endotherm at 250°C. This is a characteristic melt temperature for PET and confirms the identification from FT-IR analysis.
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Figure 3. DSC Trace from contaminant material.
Conclusions to Analysis of Contamination
The contaminating material was positively identified as PET. Due to the high melting point of PET (250°C) it remained solid at the HDPE processing temperatures thus blocking the extruder screen plates. Subsequent analysis focused on identifying the source of the contaminant.
Analysis of Silo Batches
Material from two silos were analysed. The granules were separated simply according to density. Only one material from one of the silos was found to contain dense particles.
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Figure 4. Fragment of dense material found within HDPE regrind material
The fragment of dense material found within the HDPE regrind material in the suspect silo was analysed using DSC. As with the contaminant from the screen plates, this particle was found to have a melting point of 250°C. This confirmed the suspect silo to contain the contaminating PET material.
Source: ipolytech
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