Introduction to Polyurethane Foaming Agents – HFC Foaming Agents

HFC (Hydrofluorocarbon) compounds have an ODP (Ozone Depletion Potential) of zero and a very low GWP (Global Warming Potential), only 1.5% to 3.0% of that of CFC-11. Most HFCs are non-flammable and have low toxicity. In polyurethane rigid foam, HFCs exhibit low gas diffusion rates within foam cells, particularly in liquid HFC foams, which provide excellent thermal insulation properties.

After years of research, liquid HFCs, such as HFC-245fa (1,1,1,3,3-pentafluoropropane) and HFC-365mfc (1,1,1,3,3-pentafluorobutane), are considered strong candidates for third-generation foaming agents due to their zero ODP and low toxicity, as confirmed by detailed toxicity tests. Optimized formulations result in foams with excellent performance.

HFC-245fa

HFC-245fa, first introduced by Honeywell (formerly AlliedSignal), is non-flammable, has low toxicity, a low gas thermal conductivity, and good solubility in polyols, resulting in rigid foams with excellent properties. Its low boiling point (15°C) improves foam material flow during the foaming process. However, its high vapor pressure at room temperature requires modifications to foaming equipment. By adjusting formulations, fine, uniform foam with thermal insulation performance comparable to HCFC-141b foamed products can be achieved.

HFC-365mfc

HFC-365mfc is the only liquid HFC foaming agent with a boiling point above room temperature among the current zero-ODP products, requiring minimal equipment modifications. Promoted by Solvay Fluorochemicals, HFC-365mfc exhibits some flammability, with a flammable volume fraction in air of 3.5%–9%. Adding 5% non-flammable HFCs such as HFC-134a, HFC-227ea (heptafluoropropane, boiling point -16.5°C), or HFC-245fa can mitigate flammability. Mixtures like HFC-365mfc and HFC-245fa (95:5 by mass) or HFC-365mfc and HFC-134a (93:7 by mass) are non-flammable.

Additionally, HFC-365mfc forms azeotropic mixtures with cyclopentane (boiling point 32°C) and with normal pentane or isopentane (boiling points 27°C and 22.5°C, respectively), offering low gas-phase thermal conductivity. The resulting foams have superior properties but may still exhibit flammability.

Early HFC Alternatives

During the initial CFC replacement phase, new zero-ODP refrigerants such as HFC-134a (1,1,1,2-tetrafluoroethane) and HFC-152a (1,1-difluoroethane) were partially used as polyurethane rigid foam blowing agents.

HFC-134a has a short atmospheric lifetime, does not deplete ozone, and has minimal plasticizing effects on polyurethane foams. However, its high gas-phase thermal conductivity and low solubility in general polyols limit its application. Being a gas at room temperature, it requires pressure-resistant premix containers and immediate foaming upon mixing with components like polyether polyols, blowing agents, and polymeric MDI. Despite these challenges, fine-structured foams with improved thermal insulation and aging properties can be produced.

HFC-356mffm

Extensive research has been conducted on using HFC-356 (1,1,1,4,4,4-hexafluorobutane) as a polyurethane rigid foam blowing agent. HFC-356 has a boiling point close to that of CFC-11, with a gas-phase thermal conductivity only about 1.5 mW/(m·K) higher than CFC-11. It is the HFC with the lowest known gas-phase thermal conductivity, has low acute toxicity, and is less corrosive to refrigerator liner plastics than CFC-11 or HCFCs. However, its high molecular weight and usage requirements result in increased foam density and costs.

HFC-152a

HFC-152a has been trialed as a foaming agent. Its low boiling point and high gas-phase thermal conductivity, coupled with a wide flammable concentration range in air, present several disadvantages, leading to limited use today.