Methylal: a greener alternative for foam

Methylal is one of a family of blowing agents that can be used as an alternative HFCs, HCFCs and HFOs. They are particularly suitable for rigid foam applications, but more recently have been used in a continuous flexible block foam plant. This opens up their potential usage considerably.

Many people are concerned that, like pentane, methylal is a highly flammable substance, and companies worry about the cost of transitions from non-flammable HCFCs and HFCs to non-halogenated blowing agents.

Our work shows the flammability of methylal can be significantly reduced by the presence of silicone surfactants. This development could have significant implications for the flammability of polyurethane formulations.

Methylal, or dimethoxymethane, is an easy-to-handle liquid at room temperature, with a boiling point of 42.3°C. There is no requirement to label containers in terms of toxicology – the OEL is 1000ppm – eco-toxicology or atmospheric chemistry according to UNECE GHS/CLP classifications. The ozone depletion potential of methylal is zero, and its global warming potential is much lower than CO₂. It is petrochemically derived but, potentially, could be made from bio-methanol.

Methlylal is used in rigid and spray foams, flexible slabstock and moulded foams, integral skin foams, shoe soles and one-component foams. It can be used alone, with water, or in combination with other blowing agents such as HFOs, HFCs and pentanes.CHART HERE

Although it is safe to handle, its flashpoint is below 23°C, which makes it a category 2 flammable liquid. While it is volatile at room temperature and its vapour pressure increases dramatically from 40kPa at 20°C to 82.5kPA at 37.8°C, methylal has a lower heat of combustion (25.44kJ/g) than, for example, cyclopentane (44.2kJ/g). The lower heat of combustion can contribute to lower flammability in foams.

However, mixing methylal with polyols significantly lowers its vapour pressure in the mixture, and also increases its flashpoint. But not all materials with a low flashpoint are combustible. Mixing polyols and methylal also affects the sustained combustibility, sustained burning and minimum ignition energy needed to start a flame.

Additionally, liquids with a flash point above 35°C that do not sustain combustion do not need to be considered as flammable liquids. For example, a mixture of 6% methylal in glycerine propoxylated polyol Mw450, hydroxyl number 383 and viscosity of 330mPa.s, has a flashpoint of 10.7°C and a vapour pressure of 10kPa. At the same temperature, methylal’s vapour pressure is more than 80kPa.

The chart above shows the change in vapour pressure of dimethoxymethane in different polyol mixtures. In the chart, LEL is the lower explosion limit or lower flammability limit, and a mixture of methylal and air with a concentration of methylal lower than the LEL will be non-flammable. UEL is the upper explosion limit or upper flammability limit, and a mixture of methylal and air with a concentration of methylal higher than the UEL will also be non-flammable.

Our work shows that blends of methylal with higher-viscosity polyols have higher flash points and that polyols with differing structures but similar viscosities have similar flash points.

We also examined the ability of polyol blends with methylal to sustain burning according to test L.2, recommendations of the transport of dangerous goods, manual of tests and criteria, 5th revised edition, published by the UN. Samples are tested at 60.5° and 75°C. To sustain burning, the test portion must ignite while the test flame is in the test position, maintained for 15s, and remains alight for more than 15s after the flame is removed.

For example, a sorbitol-based polyol with hydroxyl number 460-495 and viscosity of 35000 mPa.s with 8- 14 wt% added at room temperature were heated to between 60.5°C and 75°C. When the samples reached test temperature, a flame was introduced. This test showed that the maximum combustion time after the flame was removed for the 14% addition was 4s.

Adding silicone surfactants significantly increases the flashpoint of polyol/methylal mixtures. We evaluated this in two combinations. The first involved a 260Mw polyether triol with kinematic viscosity at 25°C between 830-980cSt, without the addition of a silicone surfactant or methylal the flash point is in the range 180- 214°C Adding 0.5pbw silicone will raise the flash point of a blend of 100 parts triol and 2 parts dimethoxymethane from about 37°C to more than 50°C in tests using Eraflash apparatus to ASTM D93A. The second involved adding the same silicone to a high functionality polyether polyol. The pure polyol has viscosity of 2500–3500 cSt at 25°C, and a flashpoint greater than 200°C. To this, we added up to 2pph (parts per hundred) silicone, and in two separate tests 2pph and then 3pph or 3 parts methylal.

The results in the graph above show that the flashpoint of the mixtures increased from 50°C for the polyol-2pbw methylal blend to 80° for the blend with up to 1.5pbw silicone, before falling with higher levels of silicone addition. The same trend was seen in the polyl-3pbw methylal blend. This is shown in the chart above.

This work shows that although methylal is a flammable blowing agent for flexible and rigid foam. The flammability of blends of the blowing agents in model formulations that are close to the real world are significantly lower than the blowing agent on its own.

This is an edited version of a paper presented at UTECH Las Americas, Mexico City 24-26 May 2022.

Michel Beaujean was the lead presenter. Benoit Labelle, Laurent Godefroid, Celine Guissart, from Lambiotte also contributed to this paper.