Melt flow and surface stability effects on polymer flammability : A study on polystyrene, flexible polyurethane foam and polyolefin composites

Sammanfattning: Our ability to accurately determine the flammability properties of polymers is hampered bythe erratic effects of deformation and liquid flow that occur during polymer combustion.This study aims to improve the understanding of such effects at several levels:The often obvious impact of melt flow on standardized flammability test methods forelectrical appliances is described to identify the need and to motivate others to worktowards an improved material characterization.The production of a burning degraded melt results from both, melting anddecomposition of thermoplastic polymers. The degradation of the polymer chains yieldsboth combustible volatiles that feed the flame and smaller molecules with a low viscositythat might spread to a burning pool. The molar mass is the property of the material that linksboth processes. Changes in molar mass at the surface of specimens exposed to fire?like heatfluxes were analyzed, using polystyrene as a model thermoplastic material. Analyticalexperiments were combined with bench?scale gasification and gasification/melt flowexperiments, in order to determine the extent of viscosity reduction by melting and bypolymer decomposition.A strong volume contraction to a low viscous liquid is characteristic for the combustionof flexible polyurethane foam, a low density cellular material used for cushions in furnitureand mattresses. Foam decomposition is initiated at low temperature (< 300 °C), yieldingignitable gases and leading to a fast flame spread that results in the formation of a burningliquid pool. The foam morphology changes rapidly during combustion and the impact ofthese changes on the heat release rate was analyzed. The distribution of flame heat transferled to strong variations in the burning rate, which shaped the specimen surface. The shapeof the surface affected in turn the distribution of flame heat flux, leading to an interlockingprocess. A custom?made vertical test arrangement gave the ability to study the effect of heatfeedback from the burning liquid. Foam composites with carbon nanofibers and organicallymodified clay were prepared with the aim to eliminate liquefaction. Incorporation of anetwork of carbon nanofibers in the foam struts is demonstrated to prevent foamliquefaction and collapse.An approach to prevent melt?dripping is the use of high filling levels of inorganicparticles in polyolefin cable materials and to promote reactions between the filler and thepolymer. The transition from a polymer melt to an immobile inorganic residue was studiedfor blends of acrylic copolymers of ethylene with chalk and silicone. The mechanical stabilityof the inorganic surface layer that formed during combustion was decisive for the materialsflammability properties. In?situ thermocouple measurements and ex?situ analysis of partiallyburned specimen surfaces were used to explain the coupling between the degradationmechanism of the polymer and the heat shield effect of the inorganic residue layer.