Introduction to Polyurethane Foaming Agents: HCFC Foaming Agents

Among compounds that can replace CFCs, hydrochlorofluorocarbons (HCFCs) contain fewer chlorine atoms and include hydrogen in their molecular structure. This makes them chemically less stable, often decomposing in the lower atmosphere. HCFCs have significantly lower ozone depletion potential (ODP) and global warming potential (GWP) compared to CFCs, resulting in a smaller environmental impact.

HCFC-141b is considered the closest performance substitute for CFC-11 as a foaming agent. Its boiling point is slightly higher than CFC-11, and its foaming process characteristics are similar, with slightly higher foaming efficiency. Compared to gaseous HFCs (e.g., HFC-134a) or flammable liquid pentane-based foaming agents, HCFC-141b offers greater operational convenience and can be used with existing equipment designed for CFC-11 foaming. Furthermore, HCFC-141b has relatively low thermal conductivity as a gas and good compatibility with polyols and isocyanates, yielding foam performance close to that of CFC-11 foams.

From the mid-1990s to around 2002, HCFC-141b (1,1-dichloro-1-fluoroethane) was widely used as a foaming agent in North America for rigid polyurethane foam in refrigerators, freezers, building insulation, and pipelines. It was also partially adopted in some European countries and Japan. HCFC-141b is mainly used for rigid polyurethane foams in China. The thermal conductivity of rigid polyurethane foam made with HCFC-141b is slightly higher than that of CFC-11 systems, but with formulation improvements and a 10% increase in foam density, thermal insulation performance comparable to CFC-11 systems can be achieved. However, HCFC-141b can have a long-term dissolving effect on refrigerator liner materials like ABS and HIPS, requiring protective measures. Additionally, achieving the same insulation effect as CFC-11 foam requires a slight increase in foam density (around 10%).

HCFC-123 is a liquid compound with lower ODP and GWP than HCFC-141b. It has good compatibility with polyols, and its foaming process and foam thermal conductivity at the same density are similar to those of HCFC-141b. While extensively studied for its application in foam during the early phase of CFC replacement and considered an alternative foaming agent alongside HCFC-141b, its slightly higher toxicity has limited its use.

HCFC-22 (chlorodifluoromethane) is an established refrigerant that is low-cost, nonflammable, and low-toxicity, making it a cost-competitive foaming agent. Due to its smaller molecular weight, less HCFC-22 is required compared to CFC-11. At room temperature, HCFC-22 is gaseous and is typically dissolved in polyol components when used as a polyurethane foaming agent. Its high vapor pressure can cause it to volatilize from the blend, leading to gas pressure buildup, which necessitates equipment modifications such as static mixers for improved blending and pressure-resistant vessels for pre-mixing. HCFC-22 systems can be used with both high-pressure and low-pressure foaming machines, and the resulting rigid foam has thermal conductivity comparable to a 50% reduced CFC-11 system.

Disadvantages of Pure HCFC-22 Use:

1. Low boiling point leads to excessive frothing, creating voids in rigid foam.

2. Surface solidification and demolding properties are less ideal.

3. Thermal conductivity of the foam changes more rapidly during high-temperature aging.

These issues can be mitigated by mixing HCFC-22 with HCFC-142b, which has a higher boiling point and improves foaming stability.

HCFC-142b (chlorodifluoroethane) has a lower ODP than HCFC-141b but slightly higher GWP and thermal conductivity. It is commonly blended with HCFC-22 for use.

Blended foaming agents, such as HCFC-141b/HCFC-22, HCFC-123/HCFC-22, and HCFC-22/HCFC-142b, reduce gas pressure in the polyol blend and lower foaming agent costs. These mixtures use readily available industrial solvents or refrigerants with mature production technology and reasonable pricing.

Since HCFCs still contain chlorine, their ODP is above zero, meaning they still harm the ozone layer. The Copenhagen Agreement mandated phased reductions in HCFC production and use: 85% reduction by 2004, 35% by 2010, 10% by 2015, 0.5% by 2020, and complete prohibition by 2030. Developed countries have already begun restricting HCFC foaming agents. HCFC-141b, a "transitional" foaming agent in the polyurethane foam industry, will ultimately be banned. The United States prohibited HCFC foaming agents in 2003.

This article introduces HCFC foaming agents as transitional replacements for CFCs in polyurethane foams, highlighting their properties, applications, environmental impacts, and limitations, alongside global efforts to phase out HCFCs due to their ozone-depleting potential.