Jaime Grunlan, Texas A&M University, USA, presented polyelectrolytes as a mode of PIN flame retardant surface application to materials such as textiles or polymers. Two different chemicals are dissolved in water. A pH change causes the two to react together and to bind to the substrate. Coatings are typically < 500 nm thick, so conserving substrate mechanical performance. An example presented used polyethenylenimine with sodium hexametaphosphate, or with borate, which generate N-P or N-borate PIN FR coatings on wood which are water resistant (because the pH change results in a chemical bonding reaction). See Kolibaba et al. Materials Chem. Frontiers 2022, in press). These polyelectrolyte reaction products can also be precipitated as a solid, then blended with polymer. For example, these polyelectrolyte products were successfully tested at 25% loading in PLA (poly lactic acid) for 3D printing (Kolibaba et al. ACS Materials Letters 2020, 2,2 15-19, DOI).
Fabienne Samyn, UMET Lille University, France, presented tests of polyethyleneimine with (bio-based) sodium phytate to generate a water-resistant PIN flame retardant treatment of wood. The components were applied together as a polyelectrolyte solution (see explanation in J. Grunlan above) by vacuum impregnation, with citric acid used for pH change to cause the polyelectrolyte reaction. The wood was first impregnated (40 seconds, 55 bars) then with citric acid (same conditions). Higher phosphorus loading (in the phytate) was correlated to improved fire performance (peak heat release rate reduced by nearly 25% compared to untreated wood). Impregnation resulting in 2% weight increase significantly improved fire performance and char formation, without increasing water uptake.