The polyurethane foam industry was radically shaken up by the discovery, in the mid 1980s, that certain chlorofluorocarbons (CFCs) used as blowing agents can damage the environment. Hydrochlorofluorocarbons (HCFCs) were developed as replacements, but they are now scheduled to be phased out as they also have ozone depleting potential.
Global agreements have been introduced such as the Montreal Protocol and the Kyoto Protocol, which severely limit the use of many blowing agents. Global warming, ozone depleting potential, atmospheric lifetime and volatile organic compounds are the primary environmental issues with any blowing agent, and there are other factors to consider such as long-term breakdown products, halogen-free nature and acidification potential.
Blowing agents must also satisfy the requirements of the marketplace including cost, flammability, compatibility with materials of construction, and safe and economic manufacturing processes.
Each application for foams has its own specifications, for example, low flammability, low toxicity, load bearing capability and cushioning effects. The long-term stability of the foam structure and the insulating properties of the foam are also key. The blowing agent used in polyurethane has a critical effect on these attributes. Insulation is affected by the gas phase thermal conductivity of the blowing agent. Stability is affected by several properties, such as the solubility of the agent in the polymer and the diffusion rate compared with air.
This review discusses the legal requirements and property specifications for blowing agents in different applications. It highlights the effects of changing blowing agents including the need for reformulation. Many new polyols, isocyanates and surfactants are being developed to overcome problems. Similarly, new equipment is being produced, for example, to cope with the flammability issues surrounding the use of hydrocarbon blowing agents, such as pentane.
Each type of blowing agent is described. Key environmental and physical properties are listed, together with advantages and limitations. Foams are described by types and by applications. The review also describes, briefly, the current state of the market and which new blowing agents are likely to be used in each sector. The developments by many different companies are outlined.
1.1 Blowing Agents up to the mid 1980s
1.2 The Montreal Protocol and Other Regulations
2 Blowing Agents - Considerations Since the mid 1980s
2.1 Environmental Considerations
2.2 Feasibility Considerations
2.3 Performance Considerations
3 Alternative Blowing Agents
3.1 Hydrochloroflurocarbons (HCFCs)
3.2 Hydrofluorocarbons (HFCs)
3.3 Hydrocarbons (HCs)
3.4 Other Physical Blowing Agents
3.5 Chemical Blowing Agents
4 Blowing Agents for Low Density Rigid Foam
4.1 Specific Performance Criteria
4.2 General Developments
4.2.1 Reducing Consumption of CFC-11
4.2.2 Liquid HCFCs
4.2.3 Liquid HFCs
4.2.4 Low Boiling Blowing Agents (LBBA)
4.2.5 Degradation Products of HCFCs and HFCs
4.2.6 Liquid Hydrocarbons
4.2.7 Blends of Blowing Agents
4.2.8 All CO2 Blown
4.2.9 Partially Open Cell Foam
4.2.10 Thermal Conductivity Improvement Technology
4.2.11 Thermal Conductivity Ageing of Foam
4.2.12 Dimensional Stability of Foam
4.3 Blowing Agent Technology by End Use Market
4.3.1 Household Refrigerators and Freezers
4.3.2 Water Heaters and Other Appliances
4.3.3 Flexible Faced Laminates (Boardstock)
4.3.4 Rigid Faced Laminates
4.3.5 Entry and Garage Doors
4.3.8 Pipe Insulation
4.3.9 One Component Foam (OCF)
4.3.10 Marine Flotation
4.3.11 Miscellaneous Applications
5 Blowing Agents for Low Density Flexible Foam
5.1 Specific Performance Criteria
5.2 Blowing Agent by Manufacturing Process
5.2.1 Continuous Slabstock
5.2.2 Discontinuous Slabstock
5.2.3 Moulded Foam
6 High Density Foams and Elastomers
6.1 Specific Performance Criteria
6.2 Flexible Integral Skin Foam
6.2.1 Low ODP Technology
6.2.2 Zero ODP Technology
6.3 Rigid Integral Skin Foam
6.4 Semi-Rigid Foam
6.5 Microcellular Elastomers
Dr Singh is a leading expert on blowing agents, formulation and long-term stability of polyurethane foams, and has a collection of publications including patents. He is currently a Development Associate with Huntsman Polyurethanes (formerly ICI Polyurethanes).