Since the emergence of the problem of ozone depletion, the international community has been increasing its awareness of atmospheric environmental protection and has signed a number of international conventions aimed at protecting the earth’s atmospheric environment; the restrictions on substances that damage the atmospheric environment have also been gradually raised from mere restrictions on ozone depleting substances (ODS) to restrictions on the greenhouse effect (GWP value) and volatile organic compounds (VOC). The ideal polyurethane blowing agent is an environmentally friendly substance with zero ODP, low GWP and no VOC emissions. From an application point of view, it also needs to be safe, stable, non-corrosive, low thermal conductivity, non-toxic, easy to use and low application cost.
China’s polyurethane foam sector successfully completed the phase-out of CFC-11 at the end of 2007 ahead of schedule. Phase I of the HCFC phase-out plan for the polyurethane foam sector in China. During this phase, the complete phase-out of HCFC-141b was achieved by first adopting an environmentally friendly alternative technology using hydrocarbon and water as blowing agents (zero ODP, low GWP) in all three sub-sectors of refrigerator freezers, refrigerated containers and electric water heaters, as well as in relatively large enterprises in the sub-sectors of pipes, panels and solar water heaters.
Phase II of the PU foam sector will accelerate the phase-out of HCFCs based on Phase I. It is proposed to reduce 30% of the baseline level in 2018; 45% of the baseline level in 2020; 80% of the baseline level in 2023; and achieve complete phase-out in 2026. In addition, on 15 October 2016, the international community agreed on the historic Kigali Amendment, which puts action to limit the use of hydrofluorocarbons (HFCs), a greenhouse gas, on the agenda for most developing countries, including China, and applies to the HFC control schedule for Article 5 countries. The Kigali Amendment is another important milestone environmental document following the Paris Agreement on climate change, but the phase-out has also become more complex and challenging.
In the face of this situation, the Chinese government has focused on supporting zero ODP and low GWP blowing agent replacement technologies. In recent years, the Foreign Cooperation Centre for Environmental Protection of the Ministry of Environmental Protection has hosted a number of technical assistance projects, the completion of which is forming a scalable technical solution with some application prospects. This paper only briefly introduces the possible ODS alternative technology options that have emerged.
1, polyurethane rigid foam ODS blowing agent alternative technology solutions
From the perspective of environmentally friendly alternative technologies for polyurethane rigid foam, physical blowing agents mainly include alkane blowing agents (mainly pentane type), fluorinated olefins (HFO and HCFO), liquid CO2, methyl formate and methylal, etc.
2、Introduction of varieties
Hydrocarbons, i.e. alkanes, are mainly cyclopentane, n-pentane and isopentane. China is mainly cyclopentane, with the development of applications, the industry is also trying to use mixed pentane system, through the compound, you can get the appropriate boiling point of the mixed blowing agent system, the foaming process and foam performance has a certain optimization effect.
The biggest problem with pentane is the safety issue. However, after the first phase of HCFC-141b phase-out, a wealth of experience has been accumulated in controlling cyclopentane, and more than 80% of the completed green replacements have used cyclopentane foaming technology solutions.
In addition to safety issues, compared to HCFC-141b, pentane has a smaller molecular weight, higher thermal conductivity in the gas phase, strong non-polarity and flammability, which can bring about problems such as poor material flow, poor compatibility of the combination, poor foam pore structure, increased thermal conductivity of the foam and increased difficulty in anodizing the foam in polyurethane formulation modulation. Especially in the preparation of high flame retardant polyurethane foam, such as exterior wall insulation and cold storage panels, the polyester PIR system is usually used, or when using structural flame retardants, the pentane blowing agent system is more challenging.
(2) All-water/liquid CO2 foaming
Water is an extremely cheap and safe chemical blowing agent that reacts with isocyanates to produce CO2 and foam.
As water is a chemical blowing agent, in the formulation of using water as a blowing agent, both the chemical reaction between water and isocyanate should be considered, as well as the formation of the blowing agent CO2, the impact on the foam performance. The relative molecular mass of water is only 18, in the preparation of the same density of foam, its dosage is small, and thus does not have a physical blowing agent on the dilution of the combination of material, the combination of material viscosity is larger. Therefore, lower viscosity raw materials must be sought.
As the foaming agent for the all-water formulation system is CO2 its thermal conductivity is high and, due to its small molecular weight, it tends to exchange with the atmosphere through the polyurethane bubble wall, thus further reducing its thermal insulation properties. Therefore, the technology is somewhat limited in applications requiring high thermal insulation.