New research indicates higher 6PPD migration from silica tire particles

Hanover, Germany – Tire antidegradants based on 6PPD migrate at a significantly higher rate from silica-reinforced rubbers than from carbon-black reinforced equivalents, new research has found.

Erick Sharp, president and CEO of Ravenna, Ohio-based Ace Laboratories unveiled the findings from a study on the leachability of tire particles, at the recent Tire Technology Expo.

In his presentation, Sharp said 6PPD can migrate out of silica tread tire wear particles (TWPs) at “twice the rate” on average of carbon black tread TWPs.

6PPD is a rubber chemical used to protect the components of tires from attack by ozone and oxygen.

Over the recent years, researchers have raised environmental concerns over a 6PPD transformation product, 6PPD-quinone (6PPDQ), which can impact aquatic life, particularly salmon.

Sharp’s study came in view of a sudden spike in fatality rate of salmon by 6PPDQ in streams over the past decade.

The uptick, he noted, coincided with an increase in the use of silica as rubber filler to enhance rolling-resistance, traction and tear strength of tires.

“And that's where we wanted to dive into… evaluating some of the chemistries related to silica technology,” explained the Ace Laboratories expert.

Commenting on the findings, Sharp said one theory that could explain the higher migration-rate related differences in surface contact with water due to TWP morphologies.

Under SEM (scanning electron microscope) analysis, the research found that silica TWP and carbon black TWP are substantially different in morphology.

This difference in structure could be allowing more interaction between TWP surfaces and water, Sharp suggested.

Another theory could involve silica’s role in providing a “moisture highway” for water to come in and out of TWP more easily – thereby speeding up the rate at which 6PPD leaches out.

Carbon black’s ability to have a 'carbon capture effect' on polar materials might also explain the lower migration rate in carbon black TWPs, said the presenter.

Overall, according to Sharp, a combination of the three theories is likely to contribute to the higher migration rate of the antioxidant/antiozonant from silica tread rubber.

Another significant finding of the research was that increased migration was not exclusive to 6PPD.

A similar pattern was observed with other tire additives such as zinc and sulphur, as well as potentially other chemicals that were not measured in the initial research.

As a follow-up, the Ace Laboratory team is conducting additional studies on ozonation of TWPs to provide more comparative data on the generation of 6PPD-Q.

Furthermore, the company is investigating the impact of varying carbon black/silica ratios, towards developing correlations between the behaviours of the two fillers.

A further study is being conducted to evaluate the effectiveness of fillers and additives as “blockers” in the silica moisture highway, concluded Sharp.