In recent years, scientists have made great strides in the degradation and recycling of plastics, but not all plastics are easily recyclable. For example, foams and plastic caps made of black polystyrene are difficult to recycle due to the high concentration of color additives. Now, scientists have discovered a photolytic method that can help break down these plastics to achieve better recycling results.
Black foams and black plastic caps contain a certain proportion of carbon black, which is difficult to recycle because it is used as an additive to enhance the physical properties of the plastics. As a result, these plastics often end up in landfills or are incinerated.
Chemists from Cornell University and Princeton University in the United States have successfully used sunlight and white LED light to initiate photothermal reactions in colored polystyrene plastics. This process breaks down the plastics into shorter chains of monomers, facilitating the clean separation and recycling of the materials. The research findings were published in the American Chemical Society journal in late November this year.
The photothermal conversion method involves heating decomposition near the surface of plastic particles using visible light. The further the light source is from the plastic surface, the lower the heating intensity. Researchers can control the heating power to increase the decomposition rate and achieve higher selectivity in reactions, thereby avoiding the generation of unnecessary byproducts.
Initially, the researchers used the dye “carbon black” from commercial black plastics as the main photothermal agent and incorporated it into ordinary polystyrene plastics (PS). By exposing the photothermal agent to visible light, localized heating occurs rapidly, leading to the breakdown of the plastics into polystyrene monomers.
Upon confirming the effectiveness of this method, the researchers experimented with black foamed polystyrene, food containers, coffee cup lids, and eight common black food containers without the addition of extra carbon black or other metallic catalysts.
The results showed that these materials were successfully decomposed into styrene monomers with a yield of 53%. However, if the containers were contaminated with vegetable oil or soy sauce, the decomposition efficiency dropped to 46%. If contaminated with orange juice, the decomposition efficiency further decreased to 37%.
The researchers also found that switching from white LED light to stronger sunlight for just 5 minutes could increase the yield to as high as 80% for black polystyrene plastics.
Moreover, the study included mixing different colors of PS with black PS in varying proportions and subjecting them to white LED and sunlight exposure to observe their decomposition. The results indicated that under white LED light, the decomposition rates ranged between 44.5% and 46% for the PS mixtures, while under sunlight exposure, the average decomposition rate was 66.8%, with a high decomposition rate of 80% for all-black PS.
These experiments demonstrated that other colored polystyrene plastics can be effectively decomposed by mixing them with a small amount of black polystyrene. This suggests that in the future, when decomposing polystyrene plastics, there is no need to separate black plastics or add additional photothermal agents for efficient decomposition, enabling the effective recycling of multicolored plastics.
The researchers involved in this experiment collectively expressed to the American Chemical Society (ACS) that this simple method of visible light irradiation for recycling holds great potential for application in other commercially additive-containing plastics.