CO2 solubility in butyl rubber is relatively high compared to many other elastomers, yet butyl rubber (IIR) is still known as one of the best gas barrier materials in the rubber industry. That sounds like a contradiction until you separate two things that often get lumped together: how much CO2 a material absorbs, and how easily that gas actually moves through it. Butyl rubber can take up a fair amount of CO2 and still keep permeability low, because its dense molecular structure slows the gas down once it's inside.
This matters for anyone sourcing rubber for tire innerliners, gaskets, pharmaceutical stoppers, or any application where gas retention is part of the spec. This article breaks down why CO2 behaves the way it does in butyl rubber, what actually drives gas barrier performance, and what to check before buying.
1. What Is Butyl Rubber?
Butyl rubber, chemically known as isobutylene-isoprene rubber (IIR), is a synthetic elastomer built primarily from isobutylene with a small amount of isoprene copolymerized in for crosslinking capability. That's essentially what butyl rubber means in practical terms - a rubber made mostly of one repeating isobutylene unit, with just enough isoprene mixed in to let it cure and hold its shape.
The raw material for butyl rubber is petroleum-derived: isobutylene and isoprene are both hydrocarbons produced from refined petrochemical feedstocks, which is also where most global butyl rubber production is concentrated - major producing regions include the US, along with large manufacturing bases in China, where a number of dedicated butyl rubber plants have been built over the past two decades to serve the tire and industrial rubber markets.
What sets butyl rubber apart from general-purpose rubbers is its low gas permeability and excellent air and gas retention - properties that come directly from its molecular structure, which we'll get into below.
2. How Does CO2 Solubility Affect Butyl Rubber?
2.1 What Is CO2 Solubility, in This Context?
When a gas like CO2 contacts a polymer surface, some of it dissolves into the material itself, similar to how gas dissolves into a liquid. CO2 solubility in a rubber compound describes how much gas the material can hold once equilibrium is reached - it's a measure of gas uptake, not gas movement.
2.2 Solubility vs. Diffusivity vs. Permeability
This is the part that actually explains the "why" behind the search question, and it's worth being precise about it:
| Property | What It Measures |
|---|---|
| Solubility | How much CO2 enters and dissolves in the rubber |
| Diffusivity | How quickly CO2 moves through the rubber once dissolved |
| Permeability | The overall rate at which CO2 passes all the way through |
The relationship between them is straightforward:
Permeability = Solubility × Diffusivity
CO2 solubility tends to run relatively high in butyl rubber because CO2 is a fairly condensable, polymer-compatible gas that dissolves readily into many elastomers, butyl included. But high solubility alone doesn't translate into high permeability. What keeps butyl rubber's overall CO2 permeability low despite that solubility is diffusivity - CO2 molecules dissolve into the rubber but then move through it slowly, because the polymer's internal structure doesn't give them much room to travel.
In short: butyl rubber lets CO2 in, but doesn't let it out quickly. That's the core of why is CO2 solubility high in butyl rubber while the material is still classified as an excellent gas barrier.
3. Why Does Butyl Rubber Have Low Gas Permeability?
Three structural characteristics explain why butyl rubber has such low gas permeability compared to general-purpose rubbers:
3.1 Low Free Volume
Butyl rubber's polymer chains pack together tightly, leaving very little open space between them for gas molecules to occupy or move through.
3.2 Restricted Chain Mobility
The isobutylene backbone doesn't rotate or shift as freely as the chains in more flexible elastomers, which limits the openings gas molecules need to hop from one spot to the next.
3.3 Dense Molecular Structure
The regular, closely packed arrangement of isobutylene units creates a physical barrier at the molecular level - this is really the underlying reason butyl and natural rubber are treated as separate material categories in gas-barrier applications, even though both are elastomers. Natural rubber's more open, flexible structure allows gas to diffuse through far more readily.
4. What Factors Affect CO2 Solubility and Permeability?
Gas behavior in butyl rubber isn't fixed - it shifts with processing conditions and formulation:
| Factor | Effect on CO2 Behavior |
|---|---|
| Pressure | Higher pressure generally increases CO2 uptake into the material |
| Temperature | Affects both solubility and how fast CO2 diffuses |
| Polymer structure | Determines how strongly CO2 interacts with the rubber |
| Crosslink density | Higher crosslinking generally restricts chain mobility further |
| Fillers | Can create or block gas transport pathways depending on type and loading |
| Formulation | Additives and compounding choices affect final barrier performance |
This is why two butyl rubber compounds with the same base polymer can still perform differently under real operating conditions - formulation and processing matter as much as the raw polymer.
5. Where Butyl Rubber's Gas Barrier Properties Get Used
5.1 Tire Innerliners
This is what butyl rubber is good for more than almost anything else - its low gas permeability is exactly why it's the standard material for tire innerliners, where holding air pressure over months of use is the entire job.
5.2 Seals and Gaskets
Industrial seals and gaskets that need to maintain an airtight barrier - including CO2-specific sealing in beverage, industrial gas, and process equipment - rely on the same low-permeability behavior.
5.3 Pharmaceutical Closures
Vial stoppers and closures use butyl rubber's combined gas and moisture barrier properties to help protect sensitive formulations from oxygen and humidity ingress over shelf life.
5.4 Other Gas-Barrier Uses
Beyond these core applications, butyl rubber shows up anywhere a formulation needs to be adjusted for a specific gas, pressure, or temperature range - bladders, curing bags, and specialty industrial linings among them.
6. Butyl Rubber vs. Other Elastomers
| Material | Gas Barrier | Typical Strength |
|---|---|---|
| Butyl Rubber (IIR) | Excellent | Low gas permeability |
| EPDM | Moderate | Weather and ozone resistance |
| NBR | Moderate | Oil resistance |
| Natural Rubber | Relatively higher permeability | High elasticity |
| Silicone | High gas permeability | Temperature flexibility |
This is also the quickest way to answer what is the difference between butyl rubber and EPDM for buyers weighing the two: EPDM holds up better against weathering, ozone, and UV exposure, but butyl rubber is the clear choice when gas retention is the priority. Which one is "better" really depends on which property the application needs more.
7. What Should Buyers Check When Purchasing Butyl Rubber for CO2 Applications?
Gas permeability data on a general spec sheet isn't always enough to confirm performance under your actual operating conditions. Before ordering, it's worth confirming:
- CO2 permeability data specific to the grade, not just general gas permeability figures
- Operating temperature range the material will actually see in service
- Operating pressure the seal or barrier needs to hold
- Butyl rubber grade and isoprene content, which affect crosslinking and barrier performance
- Mooney viscosity, relevant to processing and final compound properties
- Crosslinking system and formulation used in the compound
- CO2 compatibility confirmed for your specific gas and contact conditions
- COA and TDS for batch consistency and traceability
Don't evaluate butyl rubber purely on general gas permeability figures pulled from a datasheet. Ask suppliers for CO2-specific performance data under your actual temperature and pressure conditions - general permeability numbers are usually measured under standard test conditions that may not match your process.
8. Frequently Asked Questions
Does butyl rubber absorb CO2?
Yes. CO2 solubility in butyl rubber is relatively high, meaning the material does absorb a measurable amount of gas. This is separate from how fast that gas moves through the material.
Is butyl rubber permeable to CO2?
Butyl rubber has low CO2 permeability compared to most other elastomers, even though it absorbs CO2 relatively easily. Permeability depends on both solubility and diffusivity, and butyl rubber's dense structure keeps diffusion - and therefore overall permeability - low.
Why does butyl rubber have low gas permeability?
Its isobutylene-based molecular structure has low free volume and restricted chain mobility, which limits how quickly gas molecules can move through the material once dissolved.
Does high CO2 solubility mean high CO2 permeability?
No. Permeability depends on solubility multiplied by diffusivity. A material can have relatively high solubility and still show low permeability if diffusion through the polymer is slow - which is exactly the case with butyl rubber.
Is butyl rubber suitable for CO2 sealing applications?
Generally yes, and it's a common choice for CO2-related sealing and barrier applications. Suitability for a specific application should be confirmed against the actual pressure, temperature, and gas exposure conditions with permeability data for the exact grade being considered.
Conclusion
CO2 solubility is only one part of the gas transport picture. Butyl rubber achieves excellent gas barrier performance not because it resists CO2 uptake, but because its molecular structure restricts how fast that gas can diffuse through it once absorbed - and permeability, which is what actually determines real-world barrier performance, depends on both factors together.
For buyers, the practical takeaway is to evaluate CO2 permeability alongside pressure, temperature, polymer grade, and formulation, rather than judging a material on solubility or a single general permeability figure. A supplier who can provide CO2-specific data for your actual operating conditions - not just a generic datasheet number - is generally the more reliable one to work with for gas-barrier applications.