An inner liner is a thin layer, often only a millimetre, whose job is to keep air in. Its compound is built around low permeability, not strength.
Identify the polymer family
- IIR (butyl) and its halogenated forms, BIIR (bromobutyl) and CIIR (chlorobutyl). IR shows the characteristic gem-dimethyl doublet near 1365 and 1390 cm⁻¹. The glass transition is about −65 °C.
- Halogen content separates BIIR and CIIR: bromine or chlorine by elemental analysis, typically about 1 to 2 % halogen.
- Many liners are blends of halobutyl with NR or other rubbers for adhesion and tack. Blend ratio from IR and DSC.
Fillers and resins
- Platy mineral fillers such as clay or talc improve barrier properties by lengthening the path of gas molecules.
- Carbon black at moderate levels, often a mid-structure grade.
- Hydrocarbon tackifier resins for building tack, found in the extract.
Cure
Halobutyl cures with zinc oxide and stearic acid, with sulfur and accelerators if desired. Zinc oxide levels are readily found by ash analysis.
Properties to match
Air permeability (not part of a normal rubber test list and needs a gas permeation instrument), tack and green strength, adhesion to the adjacent layer and fatigue behaviour. Chemistry that matches but permeability that does not is a failed reconstruction.
Lesson for seals
The same logic applies to seals that must block a gas, such as in high-pressure gas service: barrier and decompression behaviour are properties to measure directly, not infer.
About this article. It is original writing prepared from public sources, standards and general industry practice. It is not a copy of any book or standard and it is not engineering advice. Confirm values against the current edition of the standard and test in your own application. Only analyse material you own or are entitled to analyse.
