Innovative black colourants open up new markets for recycling packaging waste

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Materials technology that enables the mechanical recycling of black plastic packaging currently destined for landfill or energy recovery has been developed through a project commissioned by WRAP. Innovations resulting from the project, undertaken by Nextek Ltd, have the potential to create new business for plastics colourant and material suppliers, packing manufacturers and recycling companies while delivering benefits over existing waste management options. Tony Gaukroger, managing director of Colour Tone Masterbatch, summarises the developments and implications of the project.

New black colourants allow Near Infrared (NIR) spectroscopy to sort black plastic packaging waste, such as pots, tubs and trays, by polymer type so they can be recycled into high quality materials. The colourants work in APET, CPET, PP, HDPE, PS, and PVC materials.

Colourants now available for packaging and other sectors, including EEE and automotive, allow the automatic detection and recovery of materials so that they can be used in new applications as substitutes for virgin plastics. Additionally they offer improvements in recycling revenues as well as savings in carbon emissions and landfill volumes.

The project report ‘Development of NIR Detectable Black Plastic Packaging’ says there are approximately one million tonnes of rigid mixed plastic packaging in the UK waste stream and black plastic packaging could represent between 3-6% of this volume. Conservative industry estimates indicate that this could be around 26,000 - 30,000 tonnes per annum of black plastic packaging. Estimates from other industry sources suggest that the figure could be as high as 60,000 tonnes per annum.

Sorting the problem
Carbon black is widely used as a colorant in food contact packaging because it provides a contrasting background and allows the colours of food to stand out. Additionally it is low cost, has good dispersion and masking properties which allows off-cuts of other colours to be mixed together and manufactured into black items. However, carbon black absorbs NIR and does not return a signal, effectively blocking identification of the polymer type by the automatic NIR sorting systems widely used by material recovery facilities.  

This project explored four possible approaches to enable automated sorting of black plastic packaging: alternative spectroscopic techniques, physical sorting methods, addition of detectable markers and the development of alternative colourants.

The report concluded that only the alternative colorant technique allows sorting of black packaging with existing NIR based mixed plastics sorting facilities.

Identification of colorants
The key principles utilised in identifying suitable black colourants were based on either of two characteristics; One: High levels of absorption of visible light and little absorption in the NIR spectrum, or two: Pigments which absorb in the visible spectrum but reflect in the NIR spectrum.

Pigments from both categories were identified from a number of suppliers and evaluated. The range was reduced to a small number in order to evaluate the key principles. The results are shown in Table 1.

Caption to Table 1: Summary of trials with alternative detectable black colourants and carbon black tested using NIR spectroscopic techniques. The recognition rate shows which pigments allow black articles to be sorted into polymer streams. Carbon black coloured items could not be sorted.

The following colourants were found to enable polymers to be readily detectable by NIR:

PP/PE - Black 95491 (pigment) – Colour Tone Masterbatch Ltd
PP/PE - Black 95492 (pigment) – Colour Tone Masterbatch Ltd
PP/PE - Black 95493 (pigment) – Colour Tone Masterbatch Ltd
PP/PS – Lumogen AK (pigment) – BASF
PP/PS – Sicopal K0095 (pigment) – BASF
APET - Dye Black-5 (liquid colorant) – Colour Matrix Group Inc.

Plastic packaging for large volume items, such as food trays, requires pigments and colourants that are low cost and the report identified the following colourants to be potentially economically viable:

PP/PE – Black 95493 (solid colorant) from Colour Tone Masterbatch Ltd
PP/PS – Sicopal K0095 (solid colorant) from BASF
APET - Dye Black-5 (liquid colorant) from Colour Matrix Group Inc.

The colorant systems were optimised through experimentation to improve tint strength and the colourants that were finally selected for large scale APET, CPET and PP trials were: Colour Tone IRR Black 95530 for PP and CPET; and Colour Matrix Dye Black 5 for APET and CPET.

Manufacturing trials
In large scale manufacturing trials at Sharpak, LINPAC Packaging Ltd and Faerch Plast, food contact trays were manufactured with PP, APET and CPET, using either the Colour Tone or the Colour Matrix colourants.

Generally the sheet extrusion was successful and thermoforming operations ran well using standard settings. Initial trials used a 2% addition rate.

During trials with CPET it became clear that the colourants need to be matched to the polymers to eradicate colour bleed and achieve a satisfactory depth of colour. Since publication of the report Colour Tone has developed Black 95591, a polymer specific product with good jetness and coverage in PET, APET and cPET.
Repeated recycling of PET and PP materials coloured with both the Colour Matrix Dye Black-5 and the Colour Tone IRR95530 did not impact the quality of colour.

Laboratory trials of filling and sealing tray samples were also successful and NIR detectable colourants were found to make no significant difference to the heat required achieving seals.

Industrial scale sorting trials
In large scale sorting trials at Jayplas Plastics virtually all the PP and PET trays (>99%) were correctly identified by the NIR detectors and were successfully sorted.

TITECH, S+S, RTT Unicorn and Pollen, all manufacturers of NIR sorting machines, were also sent samples of trays made with detectable black colourants and found that they were able to sort them by polymer type and colour. The only adjustment necessary was tweaking of the operating software to ensure that a black tray was not misinterpreted as a clear tray against the background of a black conveyor belt.

Costs and benefits
WRAP carried out a cost assessment to provide a preliminary evaluation of financial viability of the alternative black pigments compared with carbon black pigments. Its estimates were based on pricing information provided by the colorant manufacturers in April 2011. Indicative costs at various addition rates for the novel pigments were calculated and compared to conventional carbon black masterbatch for a lightweight PP tray and a heavier CPET ‘Ready meal’ type tray.

Detectable black colourants are more expensive than conventional carbon black pigments with an indicative price range similar to other specialized organic pigments of the order of £9 to £16 per kg.

The indicative increases in cost of using these alternative pigments in place of carbon black is in the region of £1.80 to £3.50 per 1000 PET trays, at a 1% addition rate. For thinner PP trays this would be in the region of £0.75 to £1.50 per 1000 trays at a 1% addition rate - around 0.075 pence to 0.35 pence per tray at a 1% addition rate and implies an incremental cost per tonne of trays manufactured in the region of £70-£140.

Cost differential is of course influenced by the final addition rate.