Polymers for Innerliner Applications – New Developments

Polyisobutylene and its copolymers are notable for their low permeability to small-molecule diffusants as a result of their efficient intermolecular packing.  The first use for butyl rubber (IIR, poly[isobutylene-co-isoprene]) was in innertubes, whose air-retention characteristics contributed significantly to the safety and convenience of tires.  Halogenation of butyl rubber greatly extended the usefulness of IIR by providing much higher vulcanization rates and improving the compatibility with highly unsaturated elastomers, enabling the production of tubeless tires. 
Chlorinated butyl (CIIR) and brominated butyl (BIIR) rubbers are currently the polymers of choice for a tire innerliner since these tires have low inflation pressure retention (IPR) loss rates and low intracarcass pressure (ICP) values that impact the tire’s rolling resistance, treadwear, handling and durability, leading to improved vehicle performance and passenger safety.  Experiments compare innerliner compounds made with varying ratios of BIIR and NR to the performance of tires made with these compounds.  Results show that use of a 100-phr BIIR innerliner compound gives tires with the desirably lowest IPR loss rate and lowest ICP values, and the highest tire durability as measured in hours to failure during indoor roadwheel testing.  Analysis of failed tires by dissection into internal components and their characterization demonstrates the effectiveness of tires with the lowest IPR values in protecting other internal components of the tire. 
Poly[isobutylene-co-p-methylstyrene] (BIMSM) is the next generation isobutylene-based elastomer for tire innerliner applications.  It is being studied as a dynamically vulcanized alloy (DVA) when optimally mixed with nylon by reactive extrusion.  This process gives an Exxcore^TM DVA resin that can be blown into a film which can be used as a tire innerliner.  The tire with this latter innerliner can have a factor of five reduction in air permeability compared to a tire made with a comparably thick 100-phr BIIR cured innerliner sheet.  Properties of the DVA resin, the blown film, and tires made with this DVA innerliner material are reported.