How to deal with colorants in biobased inks, coatings and plastics?
An increasing amount of biobased and biodegradable articles is being designed and used. Most of these initiatives help to reduce the environmental impact on our planet. However, many aspects of the research are still neglected, which is the case of additives that are used in small amounts, such as colorants that are often used in concentrations lower than 1%, and have very little impact on the biobased content of the product. However, they may have serious impacts on the environment!
With this article we intend to inform product designers about their choice of colorant in biobased and biodegradable plastics.
Thinking about using a biobased dye?
Before making the switch, consider whether it’s truly more sustainable than synthetic alternatives. The extraction and purification of natural dyes can involve harmful solvents and energy-intensive processes—natural indigo is a well-known example. Additionally, cultivating dye-producing crops may require significant land and fertilizer, potentially competing with food production or biodiversity. Fortunately, innovative approaches are emerging. Some focus on using agricultural waste or multi-purpose crops, while thoroughly assessing environmental impact. One such initiative is the Curcol project, which uses curcumin as a base for creating biobased, biodegradable dyes in vibrant yellow, blue, and red hues.
LCA
LCA study shows that the biobased indigo is environmentally actually much worse than synthetic indigo. This is due to land use, extraction of dyes with solvents and energy intensive purification steps. . It is however still difficult to find independent and reliable LCA studies.
© BASF
Dyes and pigments are two totally different types of colorants
A dye is a molecule that dissolves in its medium, and doesn't form a solid particle. On the other hand a pigment is an insoluble particle that is ground very finely and dispersed in its medium. Disperse dyes for example "dissolve" into PET plastics, where their presence will give the plastic bright colors at low concentrations. But as they have no capacity of giving opacity, the plastics with dyes will remain translucent. On the other hand pigments are particles that do not dissolve in the plastic, they remain present as particle, and cannot move throughout the plastic. So, as they are more difficult to get in, they can't really migrate either.
Avoid toxic chemicals, dyes and pigments
Because of ecotoxicity reasons pigments based on heavy metals should not be included in your plastics. In fact, for biodegradable plastics, these pigments are prohibited or limited in use. However, there is still a considerable variety of pigments that are still allowed because they have little to no impact on compost, such are iron oxides (yellow and red ochre) or aluminium salts (e.g. ultramarine blue). Including these pigments in your biodegradable plastics is allowed, and will have no impact on the calculation of your biobased content, as these pigments contain no carbon. This is different for dyes, as these are nearly always based on organic molecules and are therefore considered as a "fossil" addition to your biobased plastic.
Think about the end-of-life
Take into account that both dyes and pigments remain present in your polymer even after its "end-of-life". This has serious implications notably for biodegradable plastics. For example, a plastic may degrade, but the pigments remain in the soil. This is also the case for many dyes, which may biodegrade very slowly or even not at all. But also in recycled polymers, both dyes and pigments remain and devaluate the recycled polymer. They are the reason that recycled polymer pellets are often a dull gray, resulting from many different colors coming together in a recycled item. In that context, many research projects focus on the removal of not only dyes & pigments, but of other additives from recycled plastics as well.
