Thermal degradation stability and fire behaviour of polyurethanes

Polyurethanes (PUR) and PUR-based products, such as foams and elastomers, are used in a
wide range of applications. One of the most common forms is PUR foam in the form of
flexible and rigid foams used for upholstery, cushioning and thermal insulation in the
construction sector. This also brings us humans very close to these materials. The Grenfell
Tower fire disaster in 2017, which took the lives of 72 innocent people, was spread by the
insulation. Therefore, it is of utmost importance to study the thermal stability, degradation,
and fire behaviour of PUR and PUR based insulation materials to gain the in-depth
knowledge. This work also has applicability in the field of PUR recycling, where it is
important to study degradation products for pyrolysis-based recycling processes.
In this thesis, PURs were synthesised with polyols of different origins (bio-based,
petrochemical-based), functionality and structure (polyether and polyester) in order to
increase the library of PURs studied. Additionally, PURs were prepared with phosphorylated
polyols and commercial flame retardants. Commercial foam samples obtained from
industrial partners were also analysed.
TGA measurements were performed on PUR according to the procedure described in ISO
11358-1:2022. The procedure suggested preconditioning the TGA for a minimum of 10
minutes. However, the measurements were found to be irreproducible. Therefore, a novel
preconditioning protocol was developed using copper oxalate as an O2 detector. This
alternative protocol uses increased flow rates and adjusted preconditioning times, leading
to a reduced presence of O2 in the TGA furnace and reproducible measurements of the
thermal degradation of PUR.
This thesis explores the thermal stability and degradation of various PURs using TGA and
applies model-free kinetics (MFK) for a kinetic analysis of the TGA data. The MFK methods
used include derivative isoconversional Friedman and integral isoconversional methods,
Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose, which gave activation energy (Ea) values
for the degradation of PUR as conversion increases. Measurements showed interesting
results regarding the differences in thermal stability and activation energies between
polyether polyol-based PURs and polyester polyol-based PURs, as well as between bio-
based and non-bio polyol-based PURs. It has been observed that network PURs and bio-
polyol-based PURs exhibit multi-step degradation, and their Ea varies significantly with
increasing conversion. However, this is not the case for linear PURs. This work also provides
a detailed insight into how, as the PURs become more complex in structure, such as network
and bio-based, it becomes difficult for the single mechanism to describe the degradation
mechanism. The work also highlights the limitations of MFK methods and suggests the use
of more advanced techniques to study selected PURs.
The combined technique of thermogravimetry coupled with mass spectrometry (TGA-MS)
was used to identify the species released during the degradation of PURs. This provided a
detailed understanding of the degradation mechanism. Regardless of the type of PUR
studied, the first step of degradation was found to be the breakage of urethane bonds.
However, some mechanisms that can occur during this breakage differed between the
studied PURs, such as bio-polyester polyol-based linear PUR and polyether polyol-based
network PUR. The thermal degradation of polyurethane is strongly dependent on the type
of polyol used in its synthesis, as shown by the TGA measurements.
To gain insight into the fire behaviour, pyrolysis combustion flow calorimetry (PCFC)
measurements were performed. The results from PCFC indicated that phosphorylated
polyols and flame retardants partially imparted fire-retardant properties to PUR. Although
commercial flame retardants perform better, bio-based materials can be equally effective
in reducing flame retardancy. Group contribution methods were applied to estimate the
heat release from the combustion of PUR, providing quite accurate values for neat PURs,
with some limitations in case the content of certain FR is too high.
Overall, this work provides insight into the thermal degradation process of PURs, how it can
be studied by combining multiple techniques, and how it may affect the fire behaviour.