When evaluating polyurethane (PU) systems for household appliance applications, short demold times have typically forced PU processors to choose between strong insulation performance and higher productivity levels.

Fig. 1b. This image shows the cell size of Voratec SD 308 foam taken from a refrigerator.

Now, driven by the twin global needs for improved energy efficiency and high performance, the Dow Chemical Co. has developed new foam systems using various blowing agents – with a strong focus on hydrocarbons (HCs) – that allow refrigerator manufacturers to achieve both enhanced insulation performance and high productivity. These new systems also address the specific technical issues of the refrigeration industry in different regions around the world.

The greening of appliances

Enlarge this picture Table 1. Influence of Polyol-X on the physical properties of Voratec SD 302.

The Montreal protocol of 1987 required that chlorofluorocarbons (CFCs) be replaced in order to preserve the stratospheric ozone layer. As a result, the global appliance industry has been striving to eliminate CFC-11 and select alternative blowing agents.

In the early 1990s, the European appliance industry moved away from halogenated blowing agents for the expansion of rigid polyurethane foams used to insulate  refrigerators and freezers and turned to hydrocarbon blowing agents. The use of hydrocarbons as a blowing agent for appliance insulation has rapidly expanded in other areas of the globe as well, making hydrocarbons the blowing agent of choice in Europe, the Pacific and Latin America.

Over time, the global appliance industry has learned how to process hydrocarbon blown foams and what to expect in terms of foam performance and properties. Initially, cyclo-pentane was chosen due to its lower thermal conductivity, compared with other hydrocarbons molecules. However, further optimization has led to the introduction of other pentane isomers as sole blowing agents, as well as a co-blower of cyclo-pentane.  This was done to reduce the cost penalties of increased applied densities and process disadvantages, such as poorer flow and longer demolding times.

In the U.S., the primary approach focused on HCFC-141b as a first alternative for CFC-11. However, this was known to be only an interim solution while other alternative molecules were investigated, such as chlorine-free hydrofluorocarbons (HFCs), which have zero ozone depletion potential (ODP). After HCFC-141b was discontinued on December 31, 2002, both HFC-134a and HFC-245fa were chosen as blowing agents for  polyurethane-foam insulation in refrigerators and freezers.

Each blowing agent has its own specific characteristics, resulting in different processes and different final foam performances. Cyclo-pentane is an environmentally acceptable alternative to CFCs, HCFCs, and HFCs. However, it has a higher gas thermal conductivity, as well as higher boiling point than CFC-11, leading to significant performance and cost penalties.

Research of other hydrocarbons with a lower boiling point – like alternative pentane and butane isomers – led to the development of PU systems showing better flowability, faster demolding times, and increased cell gas pressure, enabling lower applied densities. However, the insulation property gap versus CFC-11 could not be improved significantly.

To meet the increasingly stringent energy efficiency requirements for refrigeration appliances, several options are available for OEMs, each with its own advantages and disadvantages. OEMs have been requesting their PU raw material suppliers to improve the Lambda values of their foams, but also have been forced to evaluate new cabinet designs, including increasing the wall thickness of the cabinets, and compressor types and sizes. (Lambda values describe a foam’s thermal conductivity, expressed in mW/mK. A lower Lambda means higher insulation properties.)

Improving insulation

In light of these challenges, Dow Europe GmbH initiated an extensive study to improve the insulation properties of hydrocarbon blown foam systems without compromising on foam process (demolding time) and costs (density). This study was completed within the framework of a joint development program with Bosch und Siemens Hausgeräte GmbH (BSHG), and resulted in the development of new polyurethane systems.

These new systems allow refrigerator manufacturers to achieve both energy efficiency and high productivity, reducing costs and specifying the best product solution for any given geographic region. They provide the lowest thermal conductivity and highest energy efficiency using hydrocarbons as a blowing agent in a conventional single shot injection foaming process.

When the same cabinet models were injected with a HFC-245fa blown foam system as benchmark, the energy data confirmed that the gap in favor of HFC-245fa can be significantly reduced using these newly developed HC-blown foam systems, while reducing blowing agent and raw material costs. Even if HFC-245fa remains the leading technology for low Lambda, its very high cost/performance ratio, as well as its unfavorable environmental characteristics, in terms of global warming potential (GWP), reduces the attractiveness of HFC-containing foam systems for the European appliance industry significantly.

The outstanding performance offered by these new PU systems, expanded with cyclo-pentane have been fully validated and commercialized at BSHG. They allow best-in- class energy efficient hydrocarbon-blown foam processing while using a single-shot injection technology.

Improved polymer

Voratec SD 302 and the Voratec SD 308 from Dow are well-balanced for flow, voids, demold and thermal conductivity. However, in order to address the need of the appliance industry to improve energy efficiency, further optimization of the insulation performance of these systems is required. To enable the improvement of the Lambda values of the foam without compromising on other foam process properties such as demold, or flow, Dow developed a new base polyol for rigid appliance foam-systems called Polyol X.

When modifying Voratec SD 302 with Polyol X to understand what this new raw material brings in performance terms, an improvement of the foam properties in the desired direction was obtained, as can be seen in Table 1.

An improved Lambda value was achieved. The level of improvement ranged between 2.5 percent to 3 percent. No deterioration in other properties was observed. The curing properties of the system also demonstrated improvement without increasing the average cross-link density of the system.

These results have led to further developments with the objective of significantly improving Lambda without deterioration of other foam parameters. The new system design was aimed at maximizing the advantages of using Polyol X in the formulation in terms of thermal insulation properties, and was further fine-tuned so that other critical foam process parameters like flow and productivity (demold) were not compromised.

Low Lambda, fast demold

The formulation development resulted in the launch of a new PU foam system, System A, which exhibits the requested improved performance of significantly lower Lambda, while maintaining the fast curing properties (and therefore short demold times) and the flow-properties of the reference.

The evaluations carried out in the Dow laboratories showed a significant improvement in Lambda value of around 6 percent, compared with the reference Voratec SD 308 polyol. This can be explained by a decrease in the average cell size of the foam. With System A, around 15 percent smaller average cell size is found compared to the reference (See Fig. 1). This corresponds quite well with the theory that follows.

It is generally accepted that the thermal conductivity of a foam system can be described by the following general equation:

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The  λc, which is the heat transferred by convection, can normally be neglected because of the very fine cell structure of the polyurethane foam and is therefore not included in the equation. The radiation heat transfer contributes for around 15 percent to 20 percent to the final thermal conductivity of a rigid polyurethane foam for the appliance industry relevant density range of 30 kg/m3 to -40 kg/m3. It is directly related to the average cell size, which shows a linear relation based on the following simplified equation as derived by L. D. Booth:

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As such, small cell size will have a positive impact on the overall thermal conductivity. The observed Lambda improvement by a decrease in average cell size of foams made with System A can be described by the previous equation.

Summary

A new hydrocarbon blown foam system has been developed for the domestic appliance industry showing low Lambda and fast curing properties. This system, blown with pure cyclo-pentane, shows good processing characteristics and is giving, on average, a 6 percent improvement in Lambda performance compared with the well-known reference Voratec SD 308 Polyol, while maintaining the good demold characteristics.

The insulation efficiency of the cabinet could be improved by more than 3 percent on average, as proven in various models, bringing the performance of hydrocarbon blown foams closer to that of HFC-245fa blown foams. This new technology has been fully validated at industrial scale by BSHG, allowing it to process best-in-class energy efficient foam, using hydrocarbons as a blowing agent, in a conventional single-shot injection foaming process.