RECYCLING

Recycling poses a particular challenge for the polymer industry, as both the different material compositions and the color variation can be a problem.
Recycled materials must meet the same quality standards as virgin materials, despite significant variations in input composition—differences that arise across batches, regions, and even seasons. Nonetheless, consistency is expected across an entire batch. As a result, the common approach has been to add an excess of colorant to ensure uniformity in colored compounds. However, this method is inefficient and wasteful.

This is why ColVisTec, together with the SKZ (Süddeutsches Kunststoff Zentrum), has been working on automated color control (RecyColor) on the one hand and the detection and adjustment of foreign polymers (RecyQuant) on the other.

To demonstrate the viability of the system for the plastics industry, a scaled-down recycling process was simulated using a cascade of two twin-screw extruders connected via a melt line.

• • The first extruder melts the recycled material.
• • The second extruder colors the polymer melt.
• • Study impact of particle size distribution
• • At each extruder's die outlet, a color measurement probe monitors the polymer melt in real-time.

These probes are connected to ColVisTec’s InSpectro X2, a high-resolution UV/Vis inline spectrometer designed for 24/7 operation in industrial environments. The spectrometer monitors two measuring channels simultaneously, enduring harsh conditions such as temperature fluctuations, vibrations, dust, and moisture. The probes themselves are engineered to withstand melt temperatures up to 400°C and pressures up to 250 bar.

For control loop development, non-presorted PP regrinds were used and colored with masterbatch. The InSpectro X2 measured residence time distributions in the extruder by introducing a tracer pigment at a precise time and dosage. Color fluctuations within a batch were found to be significant—often up to 2 units on each color axis. In contrast, industries with stricter color requirements may demand variations below dE* < 1. To ensure color consistency, traditional methods involve excessive colorant addition, often exceeding the saturation threshold. However, by measuring base material fluctuations and understanding extruder residence time, it becomes possible to dynamically adjust colorant dosing, reducing excess while maintaining uniformity.

A developed software system allows operators to set a target color value, using real-time data from the first extruder’s die outlet. Based on these measurements, masterbatch dosing is dynamically adjusted, ensuring a stable color output in the final recyclate.
In one test case, PP regrind was processed in alternating light and dark mixtures, which varied significantly in brightness (L* value differences of approximately 12 units). The system was set to maintain a target L* value of 62, regardless of the input material.

• • Initially, light PP compound was fed into extruder 1..
• • At around 700 seconds, the feed switched to dark PP compound.
• • At 1200 seconds, it switched back to light PP compound.

Without automated control, substantial brightness fluctuations were observed both within batches and during transitions (Figure 2, left). However, the control system successfully adjusted the masterbatch dosage, maintaining a stable L* = 62 throughout the process (Figure 2, right). A similar trial for the b* color value confirmed the system’s effectiveness in managing fluctuating input materials.

This automated inline color control system has been successfully implemented for a single color axis and is adaptable to a wide range of plastic processing applications. Future developments aim to extend the system to full three-dimensional color control, requiring multiple feeder units and enhanced software capabilities. Additionally, experimental data suggest that, under certain conditions, the setup complexity could be reduced to just one measurement point, optimizing cost and efficiency.
Companies interested in exploring this technology are invited to participate in cost-neutral trials at SKZ to experience the benefits firsthand.

The RecyQuant research project has demonstrated the feasibility of monitoring and regulating foreign polymer content in recycled plastics directly within the production process. In collaboration with the Kunststoff-Zentrum SKZ, ColVisTec AG has developed an innovative solution capable of reliably detecting foreign polymers and automatically adjusting the dosing scales. This approach is based on the modeling of photometric data from the polymer melt.

Processing recycled plastics in extrusion presents significant challenges, particularly due to quality fluctuations. Post-consumer recyclates often contain foreign polymers that, when present in excessive concentrations, negatively impact material properties and reduce overall product quality.

Since 2022, SKZ and ColVisTec AG have been working on a solution to continuously measure and regulate the foreign polymer content in real time. If predefined tolerance levels are exceeded, the system automatically adjusts the dosing process to maintain a consistent material quality. The two-year project aimed to develop a market-ready technology that integrates hardware and software into a comprehensive in-line quality control system.

Extensive testing has confirmed the practical applicability of this approach. A key outcome of the project is the development of a chemometric model for accurately predicting foreign polymer content. Specifically, the concentration of polyethylene (0–10 wt.%) in a polypropylene matrix was analyzed for both virgin and recycled material. The identification of an optimized near-infrared wavelength range enables particularly high prediction accuracy.
The functionality of the system was successfully validated using a demonstrator. The developed photometric approach leverages spectroscopy and modeling to precisely detect foreign polymers. Additionally, a cost-effective broadband probe for the UV-VIS-NIR spectral range and a flexible software solution were developed to regulate the process. The software enables automatic adjustment of the dosing scales whenever predefined thresholds are exceeded.

“A key milestone of the project was successfully demonstrating the system’s measurement accuracy on realistically colored recycled materials. This proves the high sensitivity of the system, even for low-reflective materials,” explains Cosima Güttler, Scientist Spectroscopy at SKZ.
This solution significantly enhances quality control in recycled plastic processing. By reducing quality fluctuations, it not only optimizes production processes but also contributes to a more sustainable circular economy and a more efficient use of resources.