Reliably Test Process Parameters and Pigment Dispersion
The demands of the plastics processing industry are constantly increasing; films are becoming thinner and thinner, and fibers are becoming finer and finer. At the same time, the demands on the dispersion of the pigments and fillers used are increasing. The market for easily dispersible pigments is large, but there is often a need to use pigments that are difficult to disperse. Consequently, methods for determining dispersion quality are becoming increasingly important. Standard methods range from visual inspection of injection-molded pellets or film samples under a light or scanning electron microscope to the pressure filter test (DFT).
With the purchase of a new testing device from Collin Lab & Pilot Solutions in the fall of 2022, ROWA Masterbatch launched an intensive, six-month testing phase, which was primarily planned and implemented by a master’s student under the supervision of Dr. Susann Neubert, Head of Quality Assurance: The test involved an E25E-MP-IS filter test unit with a melt pump and an integrated screen changer. This testing system standardizes the entire testing process and allows for filter changes without interrupting operation, thereby reducing the actual measurement and evaluation time depending on the test specifications. A laboratory extruder with a 25 mm screw diameter (25 D) was used to process the melt. The system can be used to test all common engineering plastics up to 300°C. Testing is conducted in accordance with DIN EN ISO 23900-5. The pressure filter test provides an objective measurement parameter that can be used to determine the quality of the dispersion of pigments and fillers in masterbatches.
Method and Implementation:
In the first step, the natural material is extruded, and then the masterbatch is added. The material to be tested is melted in a single-screw extruder, homogenized, and conveyed through a screen via a melt pump at a defined and constant volumetric flow rate. Particles above a certain size clog the screen, thereby reducing its free flow area. This causes a higher pressure to build up in front of the screen, which is detected by a sensor (see figure).
The pressure increase can be used to draw conclusions about the dispersion of the pigments and about impurities or fillers. To do this, the measured pressure increase is compared to the amount of pigment used. The determined filter pressure value (FPV) allows conclusions to be drawn about the dispersion. The following applies: The lower the filter pressure value, the better the pigment is dispersed and the higher the product quality. A built-in melt feeder allows the test sieve to be changed while melt processing continues uninterrupted—there is no need to shut down or stop the measuring device.
As part of this master’s thesis, the influence of process parameters—such as different screw speeds, throughput, and split-feed methods—on various polymers (LD-PE, PA6, and PMMA) combined with various pigments that are difficult to disperse (Pigment Green 7, Pigment Black 7, and Pigment Red 122), as well as the use of different dispersing additives. In total, the project team conducted 115 different experiments on a ZSK26 pilot plant. In addition to the pressure filter value, the dispersion quality was also assessed on 50 µm-thick flat films using a camera system, and the color intensity was evaluated on white-brightened injection-molded pellets (see diagram: Effect of different dispersing additives on color intensity).
Results and Conclusion:
It was demonstrated that the dispersion of, for example, Pigment Green 7 in PA6 was significantly improved with all of the tested dispersing additives under consistent process parameters. It was found, however, that contrary to the manufacturers’ specifications, not all dispersing additives are equally effective. The pressure filter values (FPV) were determined using a 10 µm sieve. It was also confirmed that the improved dispersion of the pigments used simultaneously led to increased color intensity. To this end, the masterbatch was lightened with a white pigment, as otherwise the color would be too saturated and color differences could not be detected.
During the production of 50 µm flat films (2% masterbatch in PA6), an improvement in dispersion quality of up to 99.6% was achieved. For comparison, flat films of the carrier polymers were produced beforehand to quantify the defects caused by bubbles and gel bodies. As expected, the addition of dispersing agents led to a significant reduction in agglomerates, which was consistent with the decrease in pressure filter values. (See diagram: Effect of different dispersing additives on the FPV). The produced flat films were examined using a light microscope. The diagram shows that the flat films produced with dispersing additives exhibited a lower number and smaller size of agglomerates, which illustrates better dispersion of the pigments.
The project team—which included Dr. Neubert and master’s student Mani Zendedel Haghighi, as well as Dorit Krienke, Head of Color Masterbatch Development, and Dr. Natalia Olichwer, Head of Additive Masterbatch Development, has concluded that determining the pressure filter value can complement the visual inspection of pigment dispersion in thin flat films. The Collin filter test is now an integral part of process development and optimization at ROWA Masterbatch and makes a significant contribution to meeting quality standards.
