The shift towards fourth-generation blowing agents is well under way, with growing concerns about the global warming potential of older blowing agents on top of earlier worries about ozone depletion. Sarah Houlton looks at the current state of play.
Blowing agents are critical components of any foamed polyurethane, whether it’s flex foam, rigid boards or spray polyurethane foam insulation. While CFCs were, in many ways, ideal – non-toxic, non-flammable and good insulators – their stable carbon–chlorine bonds meant they stayed intact in the atmosphere, allowing photolytic reactions over the north and south poles to create ozone-destroying chlorine radicals, manifesting as the ozone holes.
The transition to HCFCs started in the early 1990s as the problem of ozone depletion became clear, as including hydrogen atoms shortened their lifetime by an order of magnitude. A decade later, a third generation of blowing agents without any chlorines, the HFCs, emerged, with ozone depletion potential.
But it has since become abundantly clear that ozone depletion was only part of the problem: all three classes are powerful insulators, with significant global warming potential. Carbon dioxide is assigned a global warming potential of 1; some of the HFCs have values 3000-4000 times higher than CO2, because they are better insulators and have a long lifetime in the atmosphere.
A fourth generation of blowing agents, HFOs, was designed to avoid both of these problems. ‘They have a carbon–carbon double bond in the molecule,’ said Ernest Wysong, global technology leader for the foam blowing agents business at Chemours. ‘This is like an Achilles heel that allows atmospheric hydroxyl radicals or UV light to break the mol down relatively quickly compared to HFCs. Even if it is a good insulator, it is not going to last long in the atmosphere.’
But, he said, there is a caveat. ‘When it starts to break down, you must make sure the breakdown products are not toxic and not persistent,’ he said. ‘This is why there are only a handful of molecules today that fulfil this.’
There is now a big regulatory drive to replace HFCs with less damaging alternatives. The proposed federal ban in the US faced political delays, but California led the charge to introduce a state-level ban. Yet HFCs are still used in some Midwest states, according to Roy Choudhury, global business director at Foam Supplies. ‘Obama said we should have been out of HFCs by 2021 or 2022,’ Choudhury said. ‘Then the administration changed and everything was put on hold, except for integral skin foams where the change was already mandated. Now, with the new administration, we hope things will be different, and the old plan comes back on track.’
Much of the rest of the industrialised world is further advanced with the HFC bans and elsewhere, Wysong said, the UN is trying to promote the jump from HCFCs directly to generation 4 products, skipping HFCs. ‘We have seen some projects in Latin America where they have been doing this, and also in the Middle East,’ he said.
Enter the HFOs
According to Jonathon Rhodes, EVP at distributor Maroon Group, all of the major systems houses in the US are all either using, or have a line of, HFO products. ‘I haven’t heard anyone talking about buying HFCs for more than a year,’ he said.
Chemours is one of a number of companies to offer HFO blowing agents. Its Opteon 1100 and 1150 products are the two geometric isomers of 1,1,1,4,4,4-hexafluoro-2-butene; 1100, the E-isomer, has a boiling point of 33°C and was launched first, while the Z-isomer 1150 boils at 7.5°C. ‘We are targeting 1150 more for applications that need something that volatilises more quickly,’ Wysong said. ‘This could be in low-pressure foaming applications, or as a synergist to enhance foam formation in combination with higher boilers like 1100. That was chosen for the first launch because it most closely matched HCFC 141b, which was a workhorse molecule with a boiling point of 31°C.’
There are various other HFOs on the market. For example, Honeywell’s Solstice LBA, which is chlorinated, boils at 19°C, while Solstice GBA is a three-carbon fluoroolefin that is a true gas, as it boils at –19°C. Arkema’s FBA also contains chlorine, and also boils at about 19°C. Although two of these three molecules do contain chlorine, their lifetime in the atmosphere is very short because of the double bond, making their ozone depletion potential very small.
HFOs can be used in pretty much any application where HFCs had been employed. ‘We have not found any applications where, from a functionality standpoint, you cannot,’ Wysong said. ‘From an economics perspective, it’s more of a stretch, but from the functionality of the molecules, we can pretty much play wherever HFCs have played.’ His colleague Luc Boulleau, who is global business director for the foams business, said the conversion is happening in highly industrialised places such as the US, Europe and Japan. ‘We are more focused on markets where they are looking for high performance, but we do have interest coming out of Latin America, China and Korea,’ he said. ‘If there are no regulations, people will not adopt them. But for us it’s about performance. Our limitation is going to be the cost.’
This expense means additional benefits will be key for their. ‘We have found that some of these molecules perform better, because they have better insulation properties,’ Wysong said. ‘Opteon 1100’s boiling point of 33°C, which makes it easier to handle in some operations, and also helps retention in the foams. They can have better expansion efficiency and better yields, and that translates to better cost efficiency for the customer – getting more blow for your buck!’
But there is no such thing as a drop-in replacement for HFCs, he said. ‘For solubility, you may have to make adjustments to surfactants or types of polyols to get the same performance. With the different boiling point, you may have to make adjustments in the temperatures.’
Some have a limited operational window in terms of application temperature or shelf-life, he added; those with chlorines next to the double bond are more susceptible to nucleophilic attack from the amine catalysts commonly used in PUs, shortening the shelf-life, and new compatible catalysts and surfactants are required. Many of the new HFO recipes require extensive requalification testing, Wysong said, and the
Not so pricy
HFOs are not the only options when switching away from HFCs, and there are cheaper alternatives, some of which have been in widespread use for some time. Pentanes, for example, are commonly found in boardstock insulation foam. ‘Whole plants were designed to work with pentane,’ said Ibrahim Sendijarevic, BD director at PU technology company Troy Polymers.
‘An R value of about 5 can be achieved with cyclopentane or pentane in a typical panel,’ Chemours’ Wysong said. ‘If they want R6, R7 or something even higher, you will need to blend in. Particularly in Europe, there are some requirements for high performance panels, and they are using them as blends, or even neat materials.’
But, Sendijarevic said, it cannot be used in, say, spray foam applications because it has miscibility issues, and readily phase-separates. This is not the case with another alternative, methylal, which was recently given approval under the significant new use policy (SNAP) by the US EPA for spray foam – the last category of PU foam application it had yet to achieve SNAP approval for. ‘Methylal is compatible with many other reagents used in spray foam systems,’ he said. ‘It works well with Opteon and Solstice, where it creates stable blends that should have suitable shelf life.’
Methylal is manufactured by French company Lambiotte, and marketed under the brand name Novicell, and Ohio-based Maroon Group is a distributor. ‘As people were transitioning from HFCs, they were looking for a cost-competitive blowing agent that would still give the functionality they wanted, especially in terms of heat transfer and conductivity, at a cost point in the market that made sense,’ said Jonathon Rhodes, EVP at Maroon. ‘We work side-by-side with both Opteon and Solstice, and almost all our formulation work is done with those products in the formulation as well. We can bring a better cost point to our customers, and still maintain physical parameters, such as thermal conductivity and dimensional stability.’ He reports that SPF manufacturers are already buying it. ‘We have seen upwards of 10% cost savings on the B side,’ he said.
Methylal is good from both atmospheric and environmental standpoints, with Sendijarevic citing its main downside as its relatively high flammability. ‘We are looking to use it in combination with HFOs, which are great materials apart from the cost,’ he said. ‘One part of methylal can offset two parts of HFO in the formula without negatively impacting its thermal properties. By reducing the cost of spray foam systems, it will make materials more accessible to a broader range of insulation applications.’ It could, Rhodes believes, work on its own in SPF products, but customers have already built catalyst packages and other ingredients around HFOs, so conversations with customers often start with combinations.
It is also particularly appropriate for integral skin foam, Rhodes said, because of its viscosity reducing properties. ‘With complex integral skin parts such as dashboards, it can be difficult to get the formulation to flow well into all the different parts of the cavity,’ Rhodes said. ‘Methylal is good at reducing the viscosity so that it flows in such a way that you do not go past the cream time, or impact the ultimate quality of the foam. It also forms a very compact, hard outer layer, a function of its evaporative effects.’
He is also in discussions with customers in flexible foam, such as mattress manufacturing, where it could reduce the overall cost of the blowing agent. ‘Using water can give scorching and internal temperature issues in flexible foam, but the evaporative effects of methylal make it good at helping to manage those issues,’ he said.
Another alternative blowing agent, methyl formate, was commercialised by Foam Supplies in the early 2000s. ‘When we transition to methyl formate, we optimise the system to get the best performance in the foam,’ Choudhury said. ‘With every blowing agent, there is a trade-off. An advantage is that it has a similar lambda value for the foam compared to existing blowing agents, and it is more economical so there is no big change to the end-user in price terms from the transition after optimisation. You get zero ozone depletion with methyl formate, and you do not contribute to global warming or VOCs, which is a concern when using hydrocarbons such as pentane.’
Again, it can be used in most applications, including sandwich panels, pour-in-place and spray foams. ‘On the flexible side, it’s used in integral skin foams, because the boiling point of the blowing agent is about right to get good condensation,’ he said. Here, it is often competing with water. ‘You don’t have to heat the mould as much as you need to with water,’ he said. While flex foams in seating have been water-blown ever since CFC-11 was phased out, he said, some moulded applications require a blowing agent, and Ecomate technology performs well. ‘It is also used in slabstock foams, mainly in the US, especially when you get lower density slabstock and would like to use a blowing agent alongside water. In short, you can use it across any type of foam that requires a blowing agent.’
Care must be taken by the formulator or systems house, he said, as it is flammable. ‘When formulating the system, you need to pay attention to the flammability; depending on your choice of polyols you can use a certain amount of blowing agent in that particular system, but you do not need much blowing agent to get the desired foam density compared to an HFC or an HFO in, for example, a panel foam. We have systems where we use Ecomate on its own alongside water; we have systems where it is used in conjunction with an HFC, an HFO or an HCFO. It all depends on the performance you are looking for.’
By going to a low GWP blowing agent, it is not going to contribute to global warming if it escapes into the atmosphere, Wysong said. ‘But if you have a better insulator and it’s locked up in the building for many years, the energy demand is going to be less, and less fossil fuels are being used to heat and cool the building, too. I think that’s a strong selling point – these things contribute to sustainability in multiple ways.’
Can we expect to see a fifth generation of blowing agents in the coming years? He thinks it’s unlikely. ‘If you want insulation as a function of a gas that retards heat flow, I think we’re pretty much pushing the envelope. We have something that is high mass, low flammability and low toxicity. I would say the next frontier is probably vacuum!’
Choudhury thinks that rather than a future fifth generation of blowing agents, future developments will rely on blends. ‘I don’t think we will live in a world where we rely on only one blowing agent,’ he says. ‘For the performance of the foam and tighter requirements, blends will be the norm. I guess that will be the fifth generation.’