Based on feature article published in ERJ’s November/December issue:
Simon Holmes, Saudi elastomers downstream development, explains how ExxonMobil Chemical is working with tire companies to achieve advance levels of air retention in inner liners using solutions based on the latest grades of halobutyl polymers:
The inner liner of a tire is typically based on halobutyl rubber blends and manufactured as a highly uniform sheet, in thicknesses of less than 1mm. Good control of the compound viscosity and process is required during manufacture to ensure the highest quality.
To test how well a tire holds air, the industry-recognised ASTM F1112 IPLR (inflation pressure loss rate) test method is used to measure the percentage of air loss per month.
This static test is conducted at 210˚C in a temperature-controlled room, with the tire inflated to 2.4bar or 35 psi and can be completed in as few as 42 days.
Great care has to be taken when mounting the tire and installing the valve stem. This is necessary to ensure a good seal to the rim, eliminating any possibility of air leaks.
Repeatability of these tests is very good, as long as equipment is well-maintained and calibrated and well defined testing and handling procedures are used.
A 100 phr halobutyl content can be more expensive – depending on the inner liner thickness requirements – and tends to be more difficult to process. However, some manufacturers are already adopting these materials in production tires today.
A 100phr bromobutyl content at around 1.0mm thickness is the basis for the current, best-in-class inner liners to obtain a consistent, below 1.75% IPLR. Though on-going work and testing continues, initial results of ExxonMobil’s next generation of halobutyl polymers (a para-methylstyrene version of halobutyl) have achieved improvements of over 10% in permeability with a target of increasing this to around 20%.