Companies close in on substitutes for 6PPD in tyres under California SCP scheme

A collaboration of tyre corporations has found several possible substitutes for the anti-degradant 6PPD under California’s Safer Consumer Products (SCP) programme, according to a preliminary alternatives analysis (AA) report. 

Spearheaded by the US Tire Manufacturers Association (USTMA), the 29 March submission marks the first time industry has determined safer choices could exist for a priority product under the decade-old green chemistry scheme.

Meredith Williams, director of the state’s Department of Toxic Substances Control (DTSC), told Chemical Watch News & Insight the early results represent "a crucial milestone in our efforts to compel manufacturers to look for safer [6PPD] alternatives" that preserve both road safety and imperiled salmon. The USTMA’s work paves the way for businesses to "integrate safer alternatives across all tyres", she said. 

The ‘stage 1’ AA report stems from the industry-supported designation of 6PPD-containing tyres as an SCP priority product last year. The document responds to the ensuing mandate for tyre manufacturers to examine potential replacements for the compound, which has recently faced regulatory scrutiny by the US EPA and others because of the ecological effects of its transformation byproduct 6PPD-quinone (see box). 

Finding substitutes 

The USTMA alliance scored the viability of over 60 candidate 6PPD replacements selected through a literature review – including other phenylene diamines (PPDs) that are "possibly easiest to implement" – through a new screening technique that factored in breakdown substances, the trade organisation said. This narrowed the options to 40. 

After evaluating the potential hazards, physical-chemical characteristics and available performance information for these 40 compounds, the group zeroed in on five for further assessment in the next stage of the AA process: 

• n-(1,4-dimethylpentyl)-n′-phenyl-p-phenylenediamine;  
• n-isopropyl-n′-phenyl-p-phenylenediamine;   
• n,n’-bis(1,4-dimethylpentyl)-p-phenylenediamine;   
• n,n'-dicyclohexyl-p-phenylenediamine; and  
• specialised graphene, also known as a graphene nano-platelet. 

For each of these choices, "the consortium considered data suggesting that the chemical and its transformation products have reduced adverse impacts to salmonids or that this could be easily explored using in vitro methods," the USTMA said. 

Dr Tom Lewandowski, project lead with Gradient, which assisted with the AA, emphasised "the large and diverse number of candidate alternatives identified and screened [as well as] the robust approach to considering potential impacts of possible alternatives across the entire product lifecycle". 

Stage 2 and beyond 

The initial document will feed into the tyre sector’s continued study of the five candidate 6PPD substitutes in the second phase of the AA.   

That is when the 30-member consortium must "confirm the ‘short list’ of possible alternatives" by evaluating any new data and assessing the compounds’ potential impacts, the USTMA said. This will involve a more in-depth inspection of the substances’ potential hazards and exposure-related properties.  

An ultimate stage 2 AA report is due one year after the DTSC okays the stage 1 version (see box), with the possibility of extensions. 

"USTMA is optimistic that the consortium will have identified one or more possible alternatives that hold promise to replace or materially reduce 6PPD in motor vehicle tyres, subject to future performance testing to ensure comparable tyre safety," it said. "Additional toxicity testing may need to be performed to satisfy regulatory requirements and to fill important data gaps."  

Field analyses to verify performance over a product’s lifespan will take at least four years, according to the organisation. 

The DTSC received 40 preliminary AA submissions by the 29 March filing deadline, including a handful from entities outside the USTMA that "may be investigating different potential alternatives which could prove to be safer and feasible", the agency said. 

The agency, which has 60 days to examine all the materials, said it plans to recommend improvements. Companies will then have 60 days to incorporate those changes and can start the final AA upon agency approval.