New research from Ace Laboratories, Inc. at this year's Tire Tech Expo in Hanover in March has found that 6PPD-based tyre antidegradants migrate from silica-reinforced rubbers at a significantly higher rate than carbon black-reinforced rubbers.

In his presentation report, Erick Sharp, president and CEO of Ace Laboratories, Ravenna, Ohio, suggests that 6PPD can migrate from silica tread tyre wear particles at twice the rate of the average carbon black tread TWP.

6PPD is a rubber chemical used to protect tyre components from ozone and oxygen. In recent years, researchers have raised environmental concerns about the 6PPD transformation product, 6PPD quinone (6PPDQ), which affects aquatic organisms, especially salmon.Sharp's study comes in light of a sudden spike in salmon mortality in streams over the past decade.

He points out that this increase coincides with the increased use of silica as a rubber filler to improve rolling resistance, traction and tear strength of tyres. That's where we wanted to look deeper, to evaluate some of the chemicals associated with silica technology.

Commenting on the findings, Sharp said there is a theory to explain the differences associated with higher mobility in contact with water surfaces due to the TWP morphology.

Under SEM (scanning electron microscopy) analysis, the study found significant morphological differences between silica TWP and carbon black TWP. This structural difference may result in more interaction between the TWP surface and water.

Another theory may involve the role of silica in providing a "water highway" for water to move more easily in and out of the TWP, thereby accelerating the leaching of 6PPD.

The ability of carbon black to have a "carbon capture effect" on polar materials may also explain the lower mobility in carbon black, the speaker said.

Overall, according to him, the combination of these three theories may contribute to the higher mobility of antioxidants/anti-ozonants in silica tread adhesives.

Another important finding of the study is that increased migration is not unique to 6PPD. Similar patterns were observed in other tyre additives such as zinc and sulphur, as well as other chemicals not measured in the preliminary study.

As a follow-up, the Ace Labs team is conducting further studies on ozone oxidation of TWP to provide more comparative data on 6PPD-Q generation.

In addition, the company is investigating the effect of different carbon black/silica ratios to develop a correlation between the behaviour of the two fillers.

Sharp concludes that in-depth studies are currently underway to evaluate the effectiveness of fillers and additives as silica wet highway "blockers".