How to Control the Quality of Recycled Plastic Materials

Plastic recycling presents many challenges across the manufacturing workflow. One of the biggest challenges is controlling the uncertainty of feedstock quality. Variations in quality arise from mixtures of different polymer types in the recycling process. As a consequence, many product manufacturers are hesitant to use recycled plastics as the quality variations across batches can have significant impact on maintaining quality of the end-products.

Companies need a fast and cost-effective solution

The quality control of recycled plastics is thus a crucial step in the production process for many companies. They need to be able to detect and identify impurities of recycled materials in a fast, reliable and cost-effective way.

Step 1: Verify the polymer type of the granule

The delivered polymer waste stream contained a mixture of differently colored re-granulates. Differential scanning calorimetry measurements were carried out with a NETZSCH DSC 214 Polyma at a heating rate of 10 K/min in an N₂ atmosphere.

The software Identify feature of the NETZSCH Proteus^® software compares the measurement results to entries in the database, which stores individual measurement curves, literature data and statistical classes of polymers and other materials. Figure 2 illustrates the measurement of the green polypropylene (PP) sample as well as the database entry in pink. The comparison of both curves reveals a 99.45% similarity of the green re-granule to a standard polypropylene sample. In this case, the Identify software compares significant measurement points of the sample like glass transition temperature, melting and recrystallization effects.

Step 2: Detect impurities in recycled plastic materials

Additionally, differently colored granules were also analyzed. The measurement curves of the white and black granules (see figure 3) each show an additional peak, which are melting effects, e.g., of another polymer type. Thus, impurities exist in the delivered recycled PP re-granulate.

Step 3: Identify impurities in recycled plastic materials

As shown below in figure 4, the Identify database integrated in the NETZSCH Proteus^® software identifies the source of contamination in the re-granulate. By only evaluating a user-defined section of the curve, in this case the first peak, Identify searches through the database to find a stored measurement curve that best matches the measured peak.  In figure 4, the black sample was further analyzed to identify the impurity. The software revealed that the sample of the recycled material is contaminated with linear low-density polyethylene (PE-LLD).

Step 4: Use thermogravimetric analysis for an additional insight

In cases where one method is not sufficient to distinguish between different polymer types, a second method may be applied. In the case of DSC, the use of the complementary thermogravimetric analysis (TGA) is an option. With this method, the Identify software feature can also be used to detect and identify the different polymer types using TGA curves or using a combined TGA and DSC database search.

Differential Scanning Calorimetry (DSC) is a fast, reliable and easy way to control the quality of recycled plastic materials in the manufacturing industry. The unique features of the Proteus® software additionally support a cost-efficient quality control process by automatically detecting and identifying impurities.

Corex Plastics, a leading manufacturer of twin wall profile Fluteboard® and Corflute® sheet and solid sheet extrusions, use the NETZSCH DSC 214 Polyma for quality control of recycled materials in their 24/7 manufacturing plant. Learn more about their successes with thermal analysis here.