The standards for energy efficiency have been constantly rising, and at the same time the rigid polyurethane foam industry is also facing ever increasing pressure to address climate change by eliminating global warming substances. Blowing agents are vital foam components and are responsible for the outstanding thermal performance of PU foams that are commonly used for thermal insulation in appliances and residential and commercial buildings.
To meet these challenges, Arkema is investigating a new range of blowing agents, the AFA series, designed for most PU applications including appliances, pour-in-place (PIP), spray, and polyisocyanurate (PIR) boardstock. AFA blowing agents are both liquid and gas, and have very low global warming and negligible ozone depletion potential.
The focus of this study was to evaluate solutions based on AFA-L1, which is a liquid under ambient conditions and is a candidate for replacement of the hydrofluorocarbon HFC-245fa and hydrocarbons. Using other blowing agents such as cyclopentane, HFC-134a, 245fa, and HCFC-141b (a hydrochlorofluorocarbon) as references, AFA-L1 was evaluated using an Edge Sweets high-pressure foam machine. AFA-L1 displayed a similar blowing efficiency, provided some improvement in dimensional stability, and an advantage in initial kfactor versus HFC-245fa and hydrocarbons. The solubility of AFA-L1 based agents in various polyols, and its compatibility with materials such as plastics, elastomers, and metals, was also evaluated.
Since the phase-out of chlorofluorocarbons (CFCs) in the mid 1990s as a result of the Montreal Protocol, the rigid polyurethane foam industry in North America has faced a constantly evolving period of change in the availability and use of different blowing agents.
Although the regulations have imposed a significant cost burden on system suppliers and foam manufacturers due to the need to ensure that new blowing agents perform acceptably and that products conform to building code requirements, one aspect of the enforced changes is that the industry has developed a much greater understanding of the properties and performance attributes of different blowing agents. This has led to a greater degree of sophistication in use of combinations of liquid and low-boiling blowing agents. For example, liquid blowing agents are used in a wide range of applications including spray foams.
CFC-11 was a remarkably versatile blowing agent but was phased out due to its very high ozone-depletion potential (ODP). The hydrofluorochlorocarbon HCFC-141b was a good replacement, but its higher boiling point and its greater solubility in the polymer matrix led to some problems with dimensional stability requiring an increase of the foam density. The inclusion of the low-boiling blowing agent, HCFC-22, gave improvements in dimensional stability and a reduction in overall density.
However, the high ODP of these HCFCs also led to their eventual phase-out.
To fill the void created by these phase-outs, third-generation blowing agents, such as the hydrofluorocarbons HFC-34a and HFC-245fa were developed, commercialised, and have become widely used in many rigid PU foam uses in appliances, pour-in-place and spray.
However, the chemical industry is now facing ever-increasing pressure to address climate changes by focusing on carbon footprint and more specifically on the Global Warming Potential (GWP) of blowing agents. In Europe, the European Parliament has committed to adoption of the Kyoto protocol whose purpose is to reduce emissions of greenhouse gas by 8 percent, compared to the 1990 level, from 2008-2019.
Because of their high GWP values (1300 for 134a and 1020 for 245fa), HFCs are becoming regulated for some applications. For example, the European Mobile Air Conditioning directive prohibits use of HFC-134a in MAC systems for new platforms in 2011 and all new cars in 2017.
To address future market needs, Arkema is therefore contemplating the development of fourth-generation blowing agents to replace HFCs such as 245fa and 134a. To be fully environmentally acceptable, this next generation must have a zero ODP and a very low GWP while maintaining excellent general foam properties, including insulation performance.
Properties Arkema is investigating a range of new blowing agents, called the AFA series. Designed for most PU applications including appliances, PIP (pourin- place), spray, and PIR (polyisocyanurate) boardstock, these molecules possess very low GWP and negligible ODP. Among them, AFA-L1, which is a liquid under ambient conditions, is a potential candidate for replacement of HFC- 245fa and pentanes.
Table 1 summarises the properties of AFA-L1 and other blowing agents such as cyclopentane (cC5), isopentane (iC5), normal pentane (nC5), HFC-245fa, HFC-365mfc and HCFC-141b.
Physical properties â Besides its low GWP, AFAL1 did not exhibit either a flashpoint (FP) or flammability limit under ambient conditions.
|Table 1Properties of AFA-L1 and references|
|*Flash Point **At ambient temperature|
|Table 2: PU formulations|
|Total blow ml/g||20||20||20||20|
|% physical blowing||60||60||60||60|
|% water blowing||40||40||40||40|
In that earlier work1, physical test samples were made in 6x6x6â (15x15x15 cm) open-box pours by conventional hand mixing. One box each was made for k-factor testing and dimensional stability at 70Â°C/97 percent Relative Humidity (RH). Due to the nature of the free rise foams, k-factor samples were cut such that the foam rise was parallel to the test face, in order to minimise the effect of any defects running right through the sample thickness. Also, since the k-factor samples were undersized, 5x5â, each test piece was surrounded by like material in order to test a full 12x12x1â sample. Dimensional stability test samples were cut such that the foam rise was perpendicular with the 4x4â test face. Material compatibility Table 3 lists the elastomers and plastics tested for compatibility with AFA-L1. Three dog-bone shaped samples of typical dimensions 75 mm x 4 mm x 2 mm were prepared from each material. Each piece was measured then introduced into a test tube filled with AFA-L1.
|Table 3: Elastomers and plastics|
|EPDM (ethylene propylene diene M-class rubber)|
|Hypalon (chlorosulphonated polyethylene)|
|SBR (styrene-butadiene rubber)|
|NBR (nitrile rubber)|
|PBT (polybutylene terephthalate)|
The tube was sealed and placed in a water bath kept at slightly above the boiling point of AFAL1 for 5 minutes, 24 or 100 hours. Then the polymer was removed from the test tube and any change in dimensions measured. The samples were then subjected to a tensile test with crosshead speed of 50 mm/min and the distance between grips set at 30 mm. Solubility/miscibility experimental details One of the first considerations in using a new blowing agent in PU and PIR foams is to evaluate its solubility in polyols and isocyanates. This parameter for AFA-L1 was determined by two methods: 1) a simple miscibility experiment, where the amount of blowing agent in the polyol was steadily increased until incompatibility was reached, as determined visually; and 2) a more involved determination from measuring the vapour pressure of the blend at various levels of AFA-L1 addition. Solubility experiments The setup consists of a pressure vessel with magnetic stirring, a pressure and a temperature transducer. Temperature inside the vessel is controlled to within 0.1Â°C and the pressure to within 0.1 percent. In the vessel (volume about 100ml), 50g of polyol was loaded. The vessel was then placed under vacuum to remove air. The change of pressure in the metal cylinder was monitored to ensure that there was no leak. The blowing agent was introduced in the vessel with a specially designed gas syringe. The amount of blowing agent loaded was verified by measuring the weight of the syringe before and after introduction. The temperature of the vessel was maintained at 50Â°C (above the boiling point of the different blowing agents) and the speed was maintained at 300 rpm. The vapour pressure of the blowing agent was recorded as a function of time. It was important to allow time for the system to reach equilibrium. After reaching equilibrium, the amount of blowing agent dissolved in the polyol was the difference between the added blowing agent present in the polyol and the blowing agent present in the gas phase of the vessel. Then another small amount of blowing agent was added. The procedure was repeated several times until the pressure in the vessel was equal to the liquid-gas equilibrium vapour pressure of the blowing agent at this temperature (maximum attainable pressure at temperatures below the critical temperature of the blowing agent). Miscibility experiments Table 4 lists the various polyols and isocyanates examined for miscibility with AFA-L1. Blends were prepared by adding a predetermined weight of polyol to a 125ml (~4 oz.) clear Boston Round bottle with Taperseal lined cap.
|Table 4: Miscibility study|
|Glycerine based polyether polyols|
|Amine based polyether polyols|
|Sucrose based polyether polyols|
|Mannich based polyether polyols|
|Sorbitol based polyether polyols||polyols|
|Aromatic polyester polyols|
|Table 6 :11 Initial k-factor for liquid blowing agents (BTU.in/ft2.h.Â°F)|
|Mean Temp (F)||HCFC-141b||HFC-245fa||Cyclo/isopentane (80/20)||AFA-L1|
|Table 6 :11 Month aged k-factor for liquid blowing agents (BTU.in/ft2.h.Â°F)|
|Mean Temp (F)||HCFC-141b||HFC-245fa||Cyclo/isopentane (80/20)||AFA-L1|