Material Analysis How Cleaning Granules Affect the Low-Temperature Impact Resistance of TPO

Accidental contamination with cleaning granules during a color change can significantly impair the mechanical properties of TPO components. A materials analysis demonstrates how the shift in polymer composition affects impact strength at low temperatures and how differential scanning calorimetry (DSC) reveals the underlying cause.

Reduced impact strength at low temperatures was observed in a TPO component. DSC analyses revealed a shift in the polymer composition toward a higher proportion of crystalline polypropylene and a lower proportion of polyethylene. The cause was the unintentional addition of cleaning pellets during a color change. The altered composition results in a more brittle material behavior.

Initial Situation

The component under investigation was manufactured from a TPO compound based on polypropylene (PP) with EPDM and polyethylene (PE) as impact modifiers. Production alternated between black and white; cleaning pellets were used for the color change. Reduced impact strength at low temperatures occurred exclusively in white components following the material change.

Analytical Findings

DSC analysis allows for the determination of melting temperatures and crystallinity of the semi-crystalline phases PP and PE. The amorphous EPDM phase cannot be detected thermally and was not taken into account.

The measurement results show a significantly increased enthalpy of fusion in the PP range and a reduced enthalpy of fusion in the PE range. This indicates an increased proportion of crystalline polypropylene along with a reduced proportion of polyethylene. A comparison of the DSC curves for the reference sample and the defective part (see graph) illustrates this shift: While the PP peak in the defective part exhibits significantly higher enthalpy values (64 J/g versus 48 J/g), the PE content is visibly reduced (0.2 J/g versus 0.8 J/g).

Polyethylene plays a key role in the impact resistance of TPO systems, particularly at low temperatures. A reduction in this proportion directly leads to lower energy absorption. At the same time, an increased proportion of crystalline polypropylene results in greater stiffness and promotes brittle failure. The combination of these effects explains the observed differences in mechanical behavior.

Cause and Effect

The cause was identified as an unintentional contamination with the cleaning granules used during the color change. These granules consisted of pure polypropylene and do not contribute to impact modification. This led to a measurable shift in the composition and, consequently, to the observed loss of properties.

In practice, this necessitates a thorough separation of materials during color changes as well as a complete flushing of the system. In addition, the polymer base of the cleaning pellets used should be specifically taken into account. In this context, DSC proves to be an efficient method for the rapid identification of changes in composition in semi-crystalline polyolefin systems.