Environmentally friendly plasticized PVC by means of nanotechnology
Polyvinyl chloride (PVC) is used in a wide range of industrial areas. Manufacturing of PVC products requires use of additives, such as plasticizers, heat stabilizers and flame retardants. With the incorporation of organoclays (OMMT) consisting of very thin plate-shaped impermeable particles it is possible to develop plasticized PVC nanocomposites with improved barrier properties, and thereby reduce possible migration of additives. Besides reducing permeation there are also many examples in literature proposing that nanoclays have flame retarding qualities, such as reducing the peak heat release rate (PHRR). One major challenge for producing PVC/OMMT nanocomposites is the organo-modification of natural montmorillonite (MMT) to make it suitable for incorporation in plasticized PVC. Commercially available OMMTs are usually modified by exchanging existing cations with quaternary ammonium cations (“quats”). However, when quats degrade during melt processing, they promote dehydrochlorination of PVC and for this reason it is necessary to find an alternative organo-modifier. In this work we investigated the influence of various parameters on the organomodification when different types of MMT were organically modified with alkyl citrates. The selection of modifiers to be tested was facilitated by applying Hansen Solubility Parameters (HSP) to predict the miscibility between potential organomodifiers and PVC. OMMTs with increased d-spacing were successfully prepared.
Furthermore, MMT was organically modified with tributyl citrate (TBC) as the organic modifier (OM) and then processed with diisononyl phthalate (DINP)-plasticized PVC to form polymer nanocomposites. The produced composite materials showed a contradictory change in properties to that expected of a layered silicate nanocomposite, with a decreased E-modulus and increased gas permeability compared with a material without OMMT. In a series of experiments, it was shown that the TBC modifier was extracted from the OMMT and was dispersed in the PVC/DINP matrix, whereupon the OMMT collapsed and formed micrometre-sized agglomerates. A PVC nanocomposite with the TBC acting both as the OM for MMT and as the primary plasticizer was produced. This material showed a significantly increased E-modulus as well as a decrease in gas permeability, confirming that it is possible to develop a nanocomposite based on plasticized PVC, if both the organo-modification of the MMT and the formulation of the matrix are carefully selected.
Hansen Solubility Parameters