Expanded polystyrene (EPS), extruded polystyrene (XPS) and polyurethane (PU) are currently three organic materials that are mostly used in external wall insulation. Among them, PU is currently recognized as the best insulation material in the world, which has the lowest thermal conductivity among all insulation materials. When the density of rigid PU is 35~40 kg/m3, its thermal conductivity is only 0.018~0.023W/(m.K). The insulation effect of 25mm-thick rigid PU foam is equivalent to that of 40mm-thick EPS, 45mm-thick mineral wool, 380mm-thick concrete or 860mm-thick ordinary brick. For achieving the same insulation effect, its thickness is only about half of EPS.

A recent report pointed out that one of the reasons for the rapid spread of fire in Hangzhou Ice and Snow World was that the PU insulation materials and simulated plastic green plants applied in the buildings did not meet the requirements of non-combustibility and flame retardancy, and the smoke spread quickly after the fire. The second reason is that the fire separation measures and smoke prevention measures between Hangzhou Ice and Snow World and other areas in the building were not in place. The inner wall is made of PU sandwich panel, and the exit doors are thermal insulated doors instead of fire-rated doors, which caused the fire to spread rapidly to the entire second floor after the fire broke out.

One of the causes of casualties is that after the fire broke out, materials such as PU and plastic plants burned in a large area, producing a large amount of high-temperature toxic smoke, and the released combustible smoke kept gathering and finally caused deflagration, resulting in casualties.

All of a sudden, PU insulation materials became the target of criticism and fell into a storm of public opinion!

Pondering on this passage, the rhetoric is a bit one-sided, and there are two inadequacies.

First: PU insulation materials and simulated plastic green plants applied in the buildings did not meet the requirements of non-combustibility and flame retardancy.

According to the GB8624-1997 Classification for Burning Behavior of Building Products, B2-level polyurethane can be upgraded to B1 level after adding special flame retardants. Although PU insulation boards have the characteristics of organic materials, they can only reach the flame retardant grade of B1 under the current technical conditions. Moreover, there are still technical bottlenecks and difficulties in the development and manufacture of B1-level PU insulation boards. PU boards produced by most Chinese small and medium-sized enterprises can only reach B2 or B3 level. However, several large manufacturers in China can still achieve it. PU insulation boards are made from combined polyether and PMDI (Polymethylene polyphenyl polyisocyanate) for foaming reaction and classified as B1 flame-retardant by the standard GB8624-2012. This organic insulation material is mainly used in the fields of energy-saving building enclosures, large-scale cold storage and cold chain insulation. It can also be used for fire prevention and thermal insulation in industrial plants, ships, vehicles, water conservancy construction and many other sectors.

Second: The smoke spread quickly after the fire and PU insulation material is toxic.

There was much debate about the toxicity of polyurethane, especially when accidents such as PU materials burning occurred. At present, cured polyurethane is widely recognized as a non-toxic material, and some medical PU materials have been applied in implantable medical devices and components. But uncured polyurethane can still be toxic. Rigid PU foam is a kind of thermosetting materials. When it is burned, a carbonized layer is formed on its surface, and the carbonized layer can prevent the flame from spreading. EPS and XPS are thermoplastic materials that will melt and drip when exposed to fire, and these drips can also burn.

Fires are not caused only by insulation materials. Buildings should be considered as a system. The fire performance of an entire system is related to various elements such as construction management and daily maintenance. It is of little significance to blindly emphasize the flame retardant grade of building materials. “Actually, the material itself is fine. The key is to use it correctly and well.” As early as many years ago, Li Jianbo, deputy secretary-general of China Polyurethane Industry Association, had repeatedly emphasized similar issues in various forums and seminars. The chaotic construction site management and poor supervision of unqualified and non-compliant products are the main factors causing fires, and we should not point the finger at materials when a problem occurs. So even now, the problem still exists. Blindly identified as the problem of PU materials, the conclusion may be too one-sided.

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