Isobutane (CAS 75-28-5, UN 1969) is supplied as a liquefied petroleum gas under product designations ISB-R600a, ISB-AP, and ISB-F for refrigerant, aerosol-propellant, and feedstock service. The compound has a molar mass of 58.12 g/mol, a normal boiling point of -11.7 °C per ISO 17584:2005, an ASHRAE 34 safety classification of A3, a global warming potential of 3, and an ozone depletion potential of 0. Routine composition is determined by gas chromatography per ASTM D2163-14. Refrigerant-grade material is dried with molecular sieves and controlled for sulfur and olefins because moisture, hydrogen sulfide, and reactive C4 unsaturates can produce ice blockage, copper corrosion, and compressor varnish respectively. Bulk supply is provided in nitrogen-purged ISO tank containers, 45 kg returnable steel cylinders, and bulk railcars equipped with vapour-balance connections.

What Separates R600a Refrigerant Grade from Aerosol-Propellant and Feedstock Streams?

Refrigerant-grade isobutane is controlled for low-molecular-weight impurities that alter evaporator temperature and compressor discharge pressure, while aerosol-propellant grade is specified primarily for reproducible can pressure, and feedstock-grade may contain C3 and C4 saturates and olefins that are consumed in alkylation. The following release limits are typical for the three product models.

The saturated vapour pressure of isobutane at 21 °C is approximately 310 kPa absolute. That pressure is higher than n-butane and lower than propane or dimethyl ether, which determines component pressure ratings in aerosols and evaporation temperatures in refrigeration. These comparative values are shown in the following table.

In hermetic reciprocating compressor systems for 200–400 L household cabinets, the R600a charge mass is commonly 40–80 g; maximum charge limits are set by IEC 60335-2-24. Liquid density at 25 °C is approximately 0.55 kg/L, and the refrigerant operates with mineral oil or alkylbenzene lubricant rather than hygroscopic POE oil. The lower volumetric capacity of R600a requires roughly 1.6–2.0 times the swept volume of R134a for equivalent cooling capacity in low-back-pressure compressors. An evaporator temperature of -25 °C corresponds to a saturated absolute pressure near 58 kPa, which remains above atmospheric pressure and limits air ingress but makes capillary tube sizing sensitive to non-condensables. Moisture is controlled at 10 mg/kg or below because water dropout at the expansion device can form ice crystals when the refrigerant flashes below -25 °C. On production compressor test stands, residual moisture above 10 mg/kg has been associated with intermittent capillary blockage and elevated discharge temperature. Seal materials in R600a service are selected from HNBR or polychloroprene; EPDM is excluded from high-side locations due to high hydrocarbon permeation rates. In comparison with R290 propane, R600a has lower vapour pressure and lower discharge pressure but requires larger displacement. In comparison with n-butane, R600a sustains lower evaporator temperatures without vacuum operation. Relative to R134a, the product has a GWP of 3 versus 1430 for R134a, but the A3 flammability classification limits charge size and requires ventilation and leak detection in machine rooms per EN 378.

In systems converted from R134a, the larger swept volume and lower mass flow rate alter suction line gas velocity; oil return can become insufficient if the suction line inside diameter remains sized for R134a. Suction line velocities below 2.5 m/s at minimum load may cause oil accumulation in horizontal suction runs. High-side pressure at 45 °C condensing temperature is approximately 680 kPa absolute, which is lower than R134a and permits use of lighter compressor shells. However, the A3 flammability classification prohibits installation in occupied spaces above charge limits set by EN 378 and requires fixed leak detection for larger systems.

When Alkylation Feedstock Purity Falls Below 95 mol%, Acid-Soluble Oil Yield Increases

Sulfuric acid alkylation of isobutane with C3–C5 olefins operates at an isobutane-to-olefin volume ratio of 8:1 to 14:1 and a reaction temperature of 4–13 °C. At lower isobutane concentration, polymerization of olefins competes with alkylate production and acid-soluble oil formation rises. Diolefin content above 0.2 wt% increases acid consumption and shortens catalyst cycle life. Free water is coalesced to below 30 mg/kg before reactor feed because water dilutes sulfuric acid strength and promotes emulsion instability. Hydrofluoric acid alkylation runs warmer at 27–38 °C but requires the same high isobutane-to-olefin ratio and low diolefin content. Feedstock-grade isobutane may contain propane and n-butane; propane above 5 mol% increases refrigeration compressor load in autorefrigerated reactors, while n-butane dilutes the reactive isobutane pool. Sulfuric acid strength is maintained between 88–93 wt% H₂SO₄; below this range, acid-soluble oil carryover increases and phase separation in the acid settler becomes difficult. The product is distinctly different from solvent or fuel-grade butane because alkylation requires high isobutane concentration and low sulfur, whereas fuel blending tolerates far higher levels of C3 and C4 saturates.

Rotary pressure-filling lines for three-piece tinplate aerosol cans receive liquefied isobutane at 1.0–1.4 MPa and inject it through the valve stem or under the cup. At 21 °C, the saturated vapour pressure of isobutane is approximately 310 kPa absolute, which is low enough for tinplate containers rated at 1.2 MPa at 55 °C and high enough to maintain spray pressure as the liquid propellant depletes. Spray delivery rate is measured by container mass loss over a fixed spray interval per ASTM D3069. Spray pattern diameter is determined per ASTM D4041-17, and flame extension is measured per ASTM D3065-01 for household and personal care aerosols. Isobutane is blended with n-butane to reduce pressure or with propane to raise pressure; 25–45 wt% propellant concentration is typical in solvent-borne formulations. The low polarity and low water solubility of isobutane make it less corrosive to tinplate than DME-water systems, but the concentrate must be dried to below 500 mg/kg water to prevent valve corrosion and phase separation. Valve orifice diameters from 0.25 mm to 0.51 mm are used to adjust delivery rate, with larger orifices increasing particle size and reducing plume collapse. Dimethyl ether has greater water miscibility and stronger solvency, which can swell nitrile valve seals, whereas isobutane generally shows lower elastomer swelling and permits lower-cost seal materials. In high-speed rotary gassing, static discharge is controlled by maintaining conductive container contact and grounding all metal parts; aerosol bath testing at 55 °C is performed to verify that can pressure remains below the container rating.

In water-based aerosol formulations, isobutane cannot replace DME as a sole propellant because water is nearly insoluble in the hydrocarbon phase; if water levels exceed the solubility limit, the can internal headspace becomes propellant-rich and pressure drops during spray. Manufacturers using water-based concentrates therefore co-blind isobutane with DME or select mechanical breakup actuators to maintain droplet size.

Storage, Transfer Grounding, and Permeation-Resistant Seal Selection

Isobutane is stored in horizontal ASME pressure vessels or portable DOT 2P/2Q cylinders. Lower and upper flammability limits in air are 1.8 vol% and 8.4 vol% per ISO 10156; area classification is performed per IEC 60079-10-1. Liquid transfer uses conductive hoses with end-to-end resistance below 10⁶ Ω and bonding to the receiving vessel. Relief valve settings for fixed storage tanks are commonly 2.5 MPa. The product is non-corrosive to steel, copper, and aluminum when moisture remains below 10 mg/kg; wet isobutane may promote corrosion and ice formation at expansion points. Mixing with chlorine, fluorine, or strong oxidizers is prohibited because radical initiation can be violent. Published data for long-term seal performance in high-vibration compressor service is limited; therefore, seal material qualification under EN 378 should be completed for each compressor platform. In aerosol warehouses, ventilation rates should maintain concentration below 25% of the lower flammability limit, and continuous fixed-point gas detectors are installed near floor-mounted charging lines because the vapour density of isobutane is approximately 2.0 relative to air.

During filling of returnable cylinders, residual air must be purged with nitrogen until oxygen is below 5 vol% to prevent flammable mixtures in the vapour space. Cylinder tare weight is verified on electronic scales with 0.02 kg resolution, and overfill protection interlocks stop the filling manifold if weight exceeds the rated fill mass. Records are retained for each batch and linked to the certificate of analysis.

In liquefied petroleum gas motor fuel blending, isobutane is used to raise octane and vapour pressure. Its Reid vapour pressure at 37.8 °C measured by ASTM D323 is approximately 4.9×10² kPa absolute, which is lower than propane but higher than n-butane. Warm-weather autogas blends often cap propane at 5.0 mol% because excess propane produces vapour lock in automotive fuel rails; n-butane is used for seasonally lower vapour pressure. The product differs from oxygenates such as ethanol or MTBE by its low water miscibility and high volatility, and is handled under EN 589 for autogas quality.