Introduction to Polyurethane Foaming Agents – Alkane-Based Foaming Agents

Alkane compounds have gained attention due to their zero ODP (Ozone Depletion Potential), minimal greenhouse effect, non-toxicity, and negligible environmental impact. Pentane foaming technology has already been adopted by manufacturers in regions like Europe and Asia. Alkanes with zero ODP meet the requirements of the Montreal Protocol for phasing out ozone-depleting substances, though their flammability is a limitation.

In the HC (Hydrocarbon) pathway, pentane-based compounds are the most practically valuable for foaming processes. The main alkane-based foaming agents in practical use include liquid cyclopentane, n-pentane, and isopentane, collectively referred to as "pentane foaming agents."

Compared with CFC-11 and HCFC-141b, using pentane compounds as polyurethane foam blowing agents requires addressing several issues:

1. Flammability: Alkanes are volatile organic compounds (VOCs) and highly flammable, necessitating stringent safety measures during production.

2. Solubility: Pentane has relatively poor solubility in polyether polyols. The solubility of cyclopentane in general rigid foam polyether polyols ranges from 10% to 20%, while n-pentane and isopentane have solubility below 7%. Special or modified polyols and additives are typically needed to achieve the desired foaming agent concentration.

3. Thermal Conductivity: Alkanes have relatively high gas-phase thermal conductivity, resulting in rigid foams with lower insulation performance compared to HCFC-141b-based foams.

4. Diffusion Coefficient: Pentane's diffusion coefficient in polyurethane foams is higher than that of CFC-11, necessitating protective measures for the rigid foam to maintain its properties.

5. Condensation Issues: Cyclopentane, with a higher boiling point, is prone to condensation, reducing the amount of cyclopentane in foam cells while increasing the amount of CO₂ and air entering the cells. This results in increased thermal conductivity of the foam. Condensation generally occurs at temperatures near 25°C, leading to internal pressure drops and impacting the dimensional stability of the foam. To mitigate this, foam density (e.g., 38 kg/m³) is often increased, or raw materials and formulations are improved to enhance crosslink density and cell structure, thereby strengthening the foam matrix and improving stability. While n-pentane may also condense, its higher vapor pressure makes condensation less severe.

Advancements in Formulations and Technology

Initially, single-component cyclopentane or cyclopentane-water co-blowing methods were commonly used. Subsequently, mixtures of cyclopentane with lower boiling point alkanes such as isopentane or butane have been introduced to address issues caused by cyclopentane's high boiling point. Improvements in raw materials and formulations have yielded foams with excellent performance characteristics.

Among alkane-based foaming agents, cyclopentane has the lowest gas-phase thermal conductivity and relatively high solubility in polyether polyols, making it the most widely used alkane foaming agent. Cyclopentane-based foaming technology has become one of the global alternatives to CFC foaming agents, particularly in rigid foam applications for refrigerator insulation.

Cyclopentane offers several advantages:

●Cost-Effectiveness: It is the most economical alternative among chlorocarbon substitutes.

●Environmental Benefits: Cyclopentane has zero ODP.

●Safety Improvements: Safety issues associated with cyclopentane usage have been effectively addressed.

In China, cyclopentane has been chosen as a replacement for chlorocarbon foaming agents, supported by the China Household Electrical Appliances Association. Cyclopentane has broad market potential and abundant availability. It can be obtained through the separation and processing of C5 fractions, a by-product of ethylene cracking.

Industry Trends

With the ban on HCFC-141b in developed countries and the high cost of HFC-based foaming agents, many companies have adopted pentane foaming systems as a substitute for HCFC-141b in producing rigid polyurethane foams for refrigerator insulation and other applications.