Preparation and properties of flame retarded vermiculite composites

Abstract

Polymeric materials are increasingly used in several applications. However, their relatively high flammability presents a danger to people and property. Their use therefore requires that they are made more resistant to both the initiation and propagation of fires. Vermiculite is a fire-resistant material which can be utilised as a flame retardant (FR) additive in polymers. Industrial expansion of natural vermiculite is made at temperatures above 800 °C. However, vermiculite’s expansion onset temperature is around 450 °C and effective FR additives have their onset temperatures in the range of 200 to 350 °C. With the aim of resolving this disparity, Palabora vermiculite was modified by different procedures and methods to render it more suitable for its application in flame-retarded LLDPE, PU and PVC polymer composites. Palabora vermiculite was modified using inorganic cations, urea complexes and organic surfactants. Urea complexes and some organic surfactants reduced vermiculite’s expansion onset temperature, but did not significantly affect its maximum expansion ratio. The latter was found to be independent of the flake thickness and the nature of the modification. Application of a compressive force sufficient to compact an expanded worm-like vermiculite flake reconstituted the vermicular structure to the original dimensions. At low compressive forces the vermicular structure showed spring-like behaviour with hysteresis. The interlayer mosaic-like bonding holds the sheets together and stabilises the system via mechanical interconnects that prevent complete expansion or exfoliation. In this work the expansion onset temperature of vermiculite was successfully tuned to within the activation temperature range of intumescent systems (209  35 C) through the intercalation of metal-urea complexes. These complexes feature both urea and water molecules as blowing agents. To the best of our knowledge, this is the first time that vermiculite’s exfoliation onset temperature has been lowered to such temperatures, using ion-exchange methods and specifically urea complexes. The organic intercalation was successful and was restricted to the ion-exchange level, despite vermiculite’s pre-treatment with Na+ ions. The d-spacing values were proportional to the surfactant’s chain length. These values also increased with increasing surfactant:vermiculite ratio up to ca. 150% CEC and assumed an average value of 4.4 nm. All organo-vermiculite/LLDPE composites were translucent. Their XRD diffractograms were featureless, suggesting excellent dispersion throughout the matrix. However, the presence of mica agglomerates was detected. The organo-vermiculite improved significantly LLDPE’s mechanical and flammability properties. In PU composites the urea-vermiculite was unable to form a cohesive protective barrier layer during the cone calorimeter test. The poor compatibility between the molten PU and the exfoliated flakes also led to the consumption of the underlying PU. Nevertheless, the addition of urea-vermiculite lowered significantly the pHRR of PU composites.