Environmentally-friendly fire retardants might be closer (to the toilet) than you think.

Polymers are ubiquitous in modern life. Some are obvious – the lunchbox that holds your food, the tyres of your bike, the clothes you wear to the gym. But these materials are increasingly being used in less visible ways; for example, in composite insulation panels for buildings. While many of these panels are safe and effective, events like the tragic Grenfell tower disaster of 2017 demonstrate that the fire performance of polymers can be deeply problematic. They display relatively short ignition times, high heat release rates and vigorous melting behaviour, which can result in severe fire hazards.

Over the years, a wide variety of flame retardants (FRs) have been successfully used to improve the fire resistance of polymer-based building materials. And more recently, research into bio-based FRs, made from compounds like keratin and starch, has made significant progress. In the latest issue of Polymer Testing [DOI: 10.1016/j.polymertesting.2023.108185], a group of Dutch and New Zealand researchers have proposed a new source for bio-based flame retardants – your local wastewater treatment plant.

They started with sludge; more specifically, extracellular polymeric substances (EPS) recovered from wastewater sludge. EPS mainly consist of polysaccharides, protein (polypeptides), nucleic acids, lipids (e.g., fatty acids, glycerol and phosphates) and humic substances (which result from the decomposition of plant and animal residues). Phosphorous-rich compounds have previously been shown to be effective FRs for polymers because they promote the formation of surface char, which acts as protective layer, inhibiting the diffusion of gases toward the flame, while also shielding the polymer from heat and oxygen. The study authors combined EPS with cellulose fibre – extracted from flax and toilet paper – which, through carbonisation, is known to enhance char formation.

The resulting EPS-cellulose FR fibres were ground and blended with polypropylene (PP) to form pellets. These pellets, in turn, were used to manufacture four composite panels: 1. PP blended with EPS and cellulose sourced from toilet paper (EPS-TP), 2.  PP, EPS and flax cellulose (EPS-F), 3. PP and toilet paper (no EPS) and 4. PP and flax (no EPS). A PP-only panel was also produced as a control. All five of the panels were then put through a series of characterisation steps, including imaging, calorimetry, and tensile analysis.

Thermal stability tests carried out between 0 and 950 °C showed that surface char failed to form on the panels that omitted EPS. In contrast, the EPS-F and EPS-TP panels saw an increase in mass – resulting from char formation – of 29.7% and 31.7%, respectively. In addition, the EPS-containing panels decomposed less quickly than those that solely used cellulose fibres (59.2% decrease for EPS-F and 71% for EPS-TP). They attributed this char formation to favourable interactions between cellulose and the phosphates in EPS.

In vertical burn tests, the PP without fibres “burnt severely with immediate and continuous flaming particles falling from the sample.” Molten PP particles also ignited a layer of cotton that sat beneath the burning sample. The EPS-F and EPS-TP panels demonstrated “slower flame spread along the sample and less severe burning behaviour.” Furthermore, the composite panels did not start dripping until later into the test. The authors say that the formation of char on the composite panels helped to suppress both the drip and burn behaviours.

Cone calorimetry showed that the presence of EPS and cellulose improved the heat- and smoke-related fire reaction properties of the panels. The EPS-F and EPS-TP panels displayed 35.4% and 37.8% lower peak heat release rate, and 23.2% and 20.7%, lower total heat release, respectively, than neat PP. Typically, char forming-polymers release more smoke and CO than standard polymers. However, the opposite was true for the composites under test. The presence of EPS and cellulose led to reduction of the total smoke release, smoke production rates and CO/CO2 yields of the composites compared to those of PP. For example, the EPS-TP panel produced 41.7% less CO2 than the PP panel.

SEM images of the char showed it to be a highly interconnected network. In addition, needle-like crystal structures, similar to hydroxyl apatite, were clearly visible. The authors write, “as an inorganic substance, HAP is highly stable at elevated temperatures up to 1300°C and is already a recognised flame retardant.”

The incorporation of EPS-cellulose fibres also significantly improved the tensile moduli of both composites; it was in the range of 83.7% higher than in PP. However, poor interfacial adhesion between then EPS-coated fibres and the PP led to a slightly reduced (-8.5%) tensile strength.

The authors suggest that more work is needed to establish “a complete understanding of influences of EPS and cellulose-based fibres on both condensed and gas phase flame-retardant mechanisms of composites.” But they say “This study opens up new possibilities for the wastewater-derived biopolymer EPS to prepare bio-inspired FRs for cellulose-based fibres and composites, and enhance sustainability of wastewater sludge treatment.”


Nam Kyeun Kim, Debes Bhattacharyya, Mark van Loosdrecht, Yuemei Lin. “Enhancement of fire resistance and mechanical performance of polypropylene composites containing cellulose fibres and extracellular biopolymers from wastewater sludge,” Polymer Testing 127 (2023) 108185. DOI: 10.1016/j.polymertesting.2023.108185