Foam, Fire and Free Volume - Chemical and Physical Aspects of Porous Polymers

Abstract: Automotive seating has been under continous development since it was described for the first time in 1885. At that time one could read about a “padded leather bench seat sitting on springs mounted directly to the frame of a horse-less carriage”. With the development of flexible polyurethane foam in 1950-1960 the seats were considerably improved and the traditional springs were eventually replaced by foams throughout the seat. Flexible polyurethane foam constitutes a large part of the interiour of our vehicles and it represents the interphase between the driver/passanger and the vehicle, therefore it needs to meet up to several requrements coupled to safety and comfort of driving. In this study several aspects of flexible polyurethane foams have been investigated. Specific focus has been on flame retardancy, mechanical properties and noise reduction. It was found that an intumescent system – a flame retardant system which swells when subjected to heat – containing ammonium polyphosphate, pentaerythritol and melamine could be incorporated to a loading of 7.5 % in a flexible polyurethane foam formulation. This novel flame retarded material showed significantly reduced values of heat release measured with cone calorimetry. There was furthermore evidence of char formation after the cone calorimetry measurement, indicating that intumescence occurred to some degree. Characterization of mechanical properties of flexible polyurethane foams are in general well developed in tensile and compression modes. In the present study we expanded the view on automovtive foams by looking at the shear behaviour of flexible polyurethane foams under uniaxial compressive deformation. The foams were found to exhibit negative apparent Poisson ratios at a certain uniaxial deformation window. A promising, alternative route to increased load bearing in flexible polyurethane foams was investigated. Small additions of a hyperbranched polymer (HBP) enhanced the load bearing properties considerably parallel to a small but significant increase of damping at 1 Hz. The addition of HBP furthermore affected the sound absorption of flexible polyurethane foam. In a region between 500 and 1600 Hz the normal incident sound absorption coefficient alfa increased systematically with HBP loading. The chemical nature of the strut material was investigated to gain knowledge in mechanism behind the sound absorption enhancement, since no morphological differences in cell structure could be detected among the foams. Solvent extraction and FTIR analysis confirmed incorporation of HBP into the polymer backbone by reaction with isocyanate. Supercritical carbon dioxide scCO2 offers an environmental friendly and inexpensive approach to solvent treatment of polymers. The supercritical state of CO2 is reached at temperatures above 31 °C and pressures above 7.4 MPa At these conditions the compound exhibits properties intermediate of a gas and a liquid, with gas-like viscosity and liquid-like density. The scCO2 plasticizes and swells the polymer, inducing crystallization and certain crystal transitions. In the present study positron annihilation lifetime spectroscopy (PALS) was utilized to investigate the scCO2 treatment effect on free volume of semi-crystalline polymers. Supercritical CO2 treatment resulted in an increased size of the free volume sites of syndiotactic polystyrene and poly(ethylene naphthalate). Among the treatment parameters time, temperature and pressure, only pressure had a significant effect on the value of free volume expansion. Treatment pressure as well as temperature affected the crystallization and crystal transitions taking place during the scCO2 treatment. Crystal transitions not feasible at ambient conditions, e.g. gamma-crystalline sPS into the beta-form, were performed in scCO2. The expanded free volume, which is coupled to the amorphous fraction of the polymer, was observed several weeks after treatment, long after the gas had desorbed from the material, indicating a long-term effect caused by the scCO2 treatment.