On the role of flame retardants in mechanical recycling of solid plastic waste

Flame retardants are used in a wide range of plastics to extend the time-of-escape from fires. By definition, they are designed to perform this task only in case of a fire, which is then automatically the end of the plastic's lifetime. However, not all flame retardant plastic products are eventually set on fire, which is why they are abundant in plastic waste, potentially interfering with the mechanical recycling systems in place. To date, there has been little information on the influence of flame retardant additives during the mechanical recycling of solid (thermo)plastic waste. This contribution provides a comprehensive overview of the state of the art concerning the mechanical recycling of flame retardants containing polymers and plastics. In a first part, this review discusses the effect of mechanical melt reprocessing on the flame retardant properties of different recycled thermoplastic polymers, addressing questions whether the flame retardant additives are still present and effective after recycling and whether they interfere with the mechanical recycling itself. Special attention is paid to Waste from Electrical and Electronic Equipment containing flame retardants. A second part of the review lists several upgrading strategies for common polymeric waste streams that consist of adding virgin flame retardants to recycled plastics with the purpose of bringing an additional value to the compound.