PROCESS CONTROL | INLINE MEASUREMENT
IMAGE: COLVISTEC/SKZ
for colouring with masterbatch. With the InSpectro X2, residence time distributions in the extruder were determined by adding a tracer pigment at a precise time and with a specific dosing. The colour fluctuations of the
material within a recycling batch are high, usually up to two units on each colour axis. In some industries a much smaller colour difference may be required, for example dE* of less
Figure 1: Schematic showing the arrangement of the SKZ cascade recycling system and location of the ColVisTec measurement probes
second extruder coloured the polymer melt. At the die outlet of each extruder a colour measurement probe was installed in contact with the polymer melt (Figure 1). The probes were connected to an InSpectro X2
unit from ColVisTec. “This is a high-resolution UV/ Vis inline spectrometer for process monitoring that can monitor two measuring channels in parallel,” says Fuat Eker, Director of Sales, Marketing and Customisation at ColVisTec. “It is designed for 24/7 use in production environments containing, for example, dust, splash water, temperature changes and vibrations. The measuring probes used have been specially developed for the operation in extruders and are suitable for melt temperatures of up to 400°C and 250 bar melt pressure.” To develop the control loop, various PP regrinds (not pre-sorted in terms of colour) were presented
Figure 2: Test results from the SKZ/ColVisTec research project show variation in L* within
individual batches and at batch changes (Figure 2, left) and the results achieved using the automatic colour control system (Figure 2, right)
than 1. Current methods add excess colorant to achieve a uniform colour, but an investiga- tion at SKZ has shown that often this excess is far beyond the concentration of colorant at which the colour would already be saturated. So, if the fluctuation of the base material is measured and the residence time of the extruder is known, variation can be compensated for through a controlled variable addition of colorant. The research team says that software has been developed that allows the specification of a desired setpoint for any colour axis. The basis for this is the colour variation of the recycled material detected at the die outlet of the first extruder determined with probe one. The addition of masterbatch for colouring is varied based on these colour values, with the result being a constant colour value for the recyclate end product. In an example trial run, PP regrind was used in light and dark blends with around 12 units of variation in L* (lightness). For the trial run, a value of 62 was specified as the target value for L*. The first extruder was fed with the light PP compound at the beginning, followed by the dark PP com- pound (after approximately 700s), then switched
Image: ColVisTec/SKZ
60 COMPOUNDING WORLD | November 2022
www.compoundingworld.com
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