Characterisation and optimisation of flame-retarding palm oil-based polyurethane/montmorilloniteammonium polyphosphate foam

Abstract

The organic chemical composition in polyurethane (PU) foam has adversely affected the flammability of the foam that causes high susceptibility to fire. Many researchers and industrialists have invested vast efforts to counter this issue, including incorporation of synthetically-produced flame retardants and fillers, and chemical modification on the constituting materials of PU foam. All these methods are not environmentally-friendly. Montmorillonite (MMT) has been known to have flameretarding properties in numerous polymer composite systems, however, its effects in PU foam application has not been investigated. The aim of this project was to develop and optimise palm oil-based PU foam reinforced with ammonium polyphosphate (APP) and MMT in the form of blended (PU/MMT/APP) and hybrid (PU/APP-MMT) systems. Palm oil-based PU foam was chosen as a matrix due to its renewable resources and sustainability. Two types of multi filler systems were prepared: PU/MMT/APP blend and PU/APP-MMT hybrid. The APP-MMT hybrid filler was prepared through an ion-exchanged surface-treatment method. PU foams were then fabricated at different MMT/APP and APP-MMT loadings, and were characterized for their compressive properties, fire retardancies, thermal stabilities, and morphologies. Hybrid PU/APP-MMT system had improved the fire retardancy where the limiting oxygen index (LOI) had reached 24.19 at 10 wt. % filler loading as opposed to 23.85 showed by blended PU/MMT/APP filler system. This probably indicates the synergistic effect between APP and MMT in fire-retarding mechanism where a more stable alumino-phosphate species was formed during the combustion, which enhanced the thermal-insulating and fire protection properties. However, the compressive modulus of hybrid APP-MMT showed the highest value (2.857 MPa) at 6 wt. %, where further filler inclusion beyond this point had deteriorated the strengths of the foams. Hindered intermolecular hydrogen-bonding was thought as the main contributor for the reduction. Response Surface Methodology was then used to find the optimised filler formulation for both multi-filler systems with compressive modulus and LOI as the responses. The optimisation yielded filler combination of 0.9 wt. % Na-MMT and 4.9 wt. % APP was the best for blended system, whereby the optimised hybrid APPMMT value was computed as 9.2 wt. %. The characterization of both optimised filler formulations portrayed some improvements, when compared to its pristine counterpart. The LOI values were improved up to 44 %, with compromising only 5 % of compressive modulus. From the results obtained, bio-based PU foam has a great potential to be used in structural panel-related applications that requires moderate loadbearing and flame-retardancy capabilities.