Butyl Rubber (IIR) for Custom Molded Components — Properties, Applications & Specifications
Table of Contents
Butyl rubber (IIR) achieves its near-impermeability to gases through an exceptionally low free volume in its polymer chain — the isobutylene backbone packs so tightly that gas molecules have almost no diffusion pathway through the cross-linked matrix, giving it an air/gas permeability roughly 8–10× lower than natural rubber. Without this property, pneumatic seals and diaphragms lose pressure over time through molecular diffusion, causing gradual system failure that is difficult to diagnose and often misattributed to mechanical leaks. Fenlora molds IIR components in Shore A 30–70 hardness across diaphragms, accumulator bladders, and dust-exclusion grommets for pneumatic systems, HVAC, and pharmaceutical packaging applications.
Material Properties & Specifications
| Common names / abbreviations | Butyl Rubber (IIR) |
|---|---|
| Polymer structure / monomer system | Isobutylene-isoprene |
| Tg (°C) / Brittle Point | -69 °C |
| Density (g/cm³) | 0.92 |
| Hardness range (Shore A/D) | 30 - 95 Shore A |
| Tensile strength (MPa) | 13.7 |
| Elongation at break (%) | 300 - 800 |
| 100% / 300% modulus (MPa) | 0.34 - 3.4 at 100% |
| Rebound / tanδ (temp + freq) | 30 % Rebound |
| Compression set (%) | 25.4 % |
| Continuous service temp (°C) | -40°C to 82°C |
| Environmental & Fluid Resistance | Ozone: Excellent | Oil: Not Rec. |
| ASTM D2000 Callouts | ASTM D2000 M1AA 407 |
| Electrical properties | Vol Res: 2.0 x 10^16 ohm-cm |
| Thermal properties | Thermal Cond: 0.053 - 0.09 W/mK |
| Tear Resistance | Tier 2 - Limited |
| Abrasion Resistance | Tier 5 - Excellent |
| Gas Impermeability Resistance | Tier 5 - Excellent |
| Oxygen Resistance | Tier 5 - Excellent |
| Ozone Resistance | Tier 4 - Strong |
| Weathering Resistance | Tier 5 - Excellent |
| Oil Resistance | Tier 1 - Basic |
| Acid Resistance | Tier 4 - Strong |
| Alkaline Resistance | Tier 5 - Excellent |
| Water Resistance | Tier 5 - Excellent |
| Flame Resistance | Tier 5 - Excellent |
When to Specify IIR
IIR is the correct choice when gas containment, vibration damping, or moisture vapor exclusion is the dominant design constraint — not when chemical resistance to hydrocarbons is required.
| Use IIR Butyl when… | Avoid IIR Butyl when… |
|---|---|
| The application requires minimal gas or air permeation through the elastomeric element | Contact with petroleum-based fluids, oils, fuels, or hydraulic fluid is expected |
| Sustained vibration damping with high hysteresis loss is needed (shock/NVH mounts) | Temperatures exceed 120°C continuously in service |
| Exposure to ozone, UV, and outdoor weathering is prolonged | Flexing at low temperatures below −40°C is required without compound modification |
| Chemical exposure is limited to dilute acids, alkalis, ketones, or polar solvents | High dynamic load cycling demands low heat buildup (IIR runs hot under repeated flex) |
| Moisture vapor transmission must be minimized (e.g., pharmaceutical packaging, electrical enclosures) | Contact with aromatic or aliphatic hydrocarbons is present — use NBR or Viton FKM instead |
IIR’s gas impermeability stems from the isobutylene repeat unit’s methyl side groups, which restrict segmental chain mobility and reduce diffusion coefficients for small molecules. This same steric hindrance that blocks gas movement also gives IIR its exceptional damping coefficient (tan δ up to 0.3 at room temperature), which is why it outperforms EPDM and NR in vibration isolation mounts where energy dissipation — not elasticity — is the design goal. Engineers specifying accumulator bladders should note that IIR retains internal pressure significantly longer than NR or EPDM at equivalent wall thickness, reducing recharge cycle frequency in low-duty pneumatic systems.
Custom Parts We Make in IIR
Fenlora molds IIR compounds across a range of precision components where gas retention, damping, or weather resistance is the primary functional requirement.
→ Anti-vibration and NVH mounts — IIR’s high hysteresis (damping loss factor) makes it more effective than natural rubber at dissipating vibrational energy in HVAC equipment bases, industrial motor mounts, and compressor isolators.
→ Dust and environmental grommets — IIR’s ozone and UV resistance make it a durable choice for cable and pipe pass-through grommets in outdoor electrical enclosures, where EPDM is also viable but IIR provides superior moisture vapor sealing around wire bundles.
→ Food packaging seals — IIR compounds can be formulated without sulfur-based accelerators, and its low permeability to oxygen and CO₂ makes it the standard choice for vial stoppers, syringe plungers, and blister-pack sealing elements in sterile packaging environments.
→ Roof membrane and weatherstrip profiles — Extruded and compression-molded IIR profiles resist ozone cracking and water absorption in roofing expansion joints and window perimeter seals, maintaining elasticity over long outdoor service lives.
If your drawing specifies IIR or you’re working from functional requirements around gas containment or vibration damping, contact our engineering team to discuss compound selection and hardness range.
Industries That Use IIR
Automotive & Light Vehicle NVH
Body mount bushings, suspension bump stops, and strut isolators in passenger vehicles use IIR compounds where the design priority is energy absorption rather than load-bearing stiffness, particularly in applications tuned to reduce structure-borne noise in the cabin. IIR’s stable damping characteristics over a broad temperature range (−20°C to +100°C) make it predictable for NVH tuning. (Link to automotive industry)
HVAC & Building Systems
Anti-vibration mounts for air-handling units, compressor pads, and duct expansion joints commonly specify IIR because its high damping coefficient attenuates low-frequency mechanical vibration more effectively than EPDM or NR at comparable durometer. IIR grommets in electrical panel enclosures also benefit from near-zero moisture vapor transmission, protecting sensitive electronics in humid environments. (Link to Electronics Industry)
Industrial Pneumatics & Process Equipment
Accumulator bladders, pump diaphragms, and check valve discs in pneumatic and low-pressure fluid systems rely on IIR to prevent gas migration through the elastomeric element — a failure mode that causes gradual pressure loss and unpredictable actuation timing. For systems cycling inert gases, compressed air, or steam (below 120°C), IIR provides longer service life than NR at equivalent wall thickness.
Compare Materials
SBR is the right choice in dry abrasive environments, but several specific application conditions call for a different elastomer — use this table to check your requirements before specifying.
| If your application involves… | Consider instead |
|---|---|
| Outdoor UV or ozone weathering | EPDM |
| Budget-sensitive oil resistance only | Nitrile (NBR) |
| Continuous temps above 150°C | Silicone (VMQ) |
| High dynamic flex and tear resistance | Natural Rubber (NR) |
| High abrasion and mechanical wear | Polyurethane (AU/EU) |
| Moderate oil + weather resistance | Neoprene (CR) |
| General-purpose non-oil applications | SBR |
| Extreme chemical resistance | Viton (FKM) |
Not sure which material fits your application? Send us your requirements and we’ll recommend the right compound.