MATERIALS | COLOURS AND MASTERBATCH


Right: Holland Colours’ Tintmask addresses discolouration in rPET


functional barrier colourants.” He says another important aspect is the in-


creased emphasis on food contact safety in food packaging applications over multiple recycling process cycles. “Colourants and additives suitable for food contact applications have to meet de- manding requirements in terms of migration limits. The presence of non-intentionally added substanc- es (NIAS) over multiple melt processing cycles also poses a challenge. NIAS may be formed due to interactions between polymers, additives, colour- ants, contaminants already present in the recyclate, and oxidative and thermal degradation.” He continues: “There is a need to minimise NIAS


in recycled plastics to a level that meets the stringent regulatory requirements. Fields of research focus on two approaches: prevention of the formation of these substances, or decontamina- tion of post-consumer material. Prevention can be accomplished amongst others through better and more predictable stabilisation and improved sorting, while decontamination is achieved through advanced washing technologies and in-process purification steps, for example.” Holland Colours also highlights a number of


trends driving new developments in colours and pigments for plastics recyclates. Kalisvaart says that colourants in food packaging are under scrutiny because they are perceived to affect the value of mechanically recycled plastics negatively. Fully transparent and colourless plastics can be easily mechanically recycled and retain a higher value because of the fact that they are re-useable in many applications. When recycling coloured plastics, batch-to- batch colour consistency of recycled material will


rPET without Tintmask


rPET with Tintmask


vary. However, brand owners expect their products to meet tight colour standards. If there can be a wider acceptance of recyclate colour variations, this perceived less valuable material stream will be suitable for a wider range of applications and potentially resolve part of the material availability concern.


In addition to the NIR detection challenge in


sorting coloured plastics, mechanical recycling has additional challenges. For recycling streams for which there is low demand because of undesired colour or a too high level of contamination, chemical or advanced recycling is regarded as a solution.


When it comes to the visual quality of un-col-


oured and transparent recycled plastics, yellowing and other slight discolourations need to be dealt with. “This is predominantly caused by thermal re-processing in combination with remaining trace amounts of components like functional additives, colourants, adhesives, inks and other types of


Automated PCR colour control project


Difficulties with sorting materials by colour can have an impact when re-colouring the recyclate. Due to colour variations of the input material, the colour pigment for the target colour is usually used in a high concentration during the colouring process in order to achieve a homo- geneous, consistent colour. In a joint research project between


ColVisTec and the SKZ Plastics Center, an automated solution for a colour metering control loop has been developed to save colour pigments during the plastics recycling


36


process.. This reacts to fluctuations in the input material, as well as to colour drifts and controls the dosing of the colourant accordingly. For this purpose, the colour values are recorded spectrally at two measuring points of a recycling plant. At the first measuring point, the


fluctuations of the input material are detected and forwarded to the closed loop system in order to adjust the quantity of colour pigments accord- ingly at the subsequent colour dosing point. The second measuring point is located at the end of the extruder.


PLASTICS RECYCLING WORLD | November/December 2022


Here, the input fluctuations are already largely compensated for, and only a feedback signal about continu- ous drifts is sent to the colour meter- ing system, which then automatically adjusts the metering output. By using this measuring system, PCR plastics can be reliably coloured and colourants can be saved, say the research partners. This offers the possibility to produce high quality re-granulate at an optimised price. In addition, the colour range can be expanded without additional colour pre-sorting.


www.plasticsrecyclingworld.com


IMAGE: HOLLAND COLOURS


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