Compatibilisation of poly(ethylene terephthalate) and polyolefins with mixtures of phenoxy and acid functionalised ethylene copolymer

Polyethylene terephthalate (PET) and polyolefins, such as high density polyethylene (HDPE), are important commodity polymers. PET is widely used for fibres and fllms but is also an attractive material for moulded products, owing to its high melting point and solvent resistance. Crystalline PET, however, is brittle and susceptible to hydrolytic degradation through water absorption. Polyolefins, on the other hand, are tough materials with low water absorption but have a relatively low melting point. Blending these two materials is expected to produce an attractive combination of properties. Blending PET with polyolefins, however, produces coarse microstructures and poor mechanical properties due to the lack of compatibility of the two polymers. Compatibility is expected to improve by reducing the interfacial tension and increasing the adhesion between the two phases; the first will reduce the size of the dispersed phase, while the latter would improve the mechanical properties. Compatibilisation of immiscible blends can be achieved by blending suitable functionalised polymers capable of reacting with the two components and/or by adding a compatibilising agent, e.g. block and graft copolymers with units along the main chains that are miscible and capable of strong interactions, such as H-bonds, with the two polymers in the blend. In the present work, polyhydroxyether of bisphenol A (phenoxy), which is known to be compatible with polyesters [1,2] and anhydride/acid functionalised ethylene copolymers, i.e. copolymers and terpolymers of ethylene containing acrylic acid units or grafted maleic anhydride, as well as the corresponding sodium ionomers, were used as the base polymer components for the production of graft copolymer type of compatibilisers by reaction processing methods. Additives such as sodium ethoxide, sodium benzoate, sodium montanate and sodium p-toluene sulphonate were evaluated as reaction controller catalysts for potential interactions between the OH groups in the phenoxy and acid and/or anhydride groups in the ethylene copolymers. The extent of interactions between the compatibiliser components were characterised by solubility tests, Fourier transform infrared spectroscopy (FITR), rheological properties and optical microscopy. The efficiency of the compatibilisers was studied on blends of PET with several grades of HDPE differing in molecular weights, by scanning electron microscopy (SEM), rheological properties, differential scanning calorimetry (DSC), and by measuring the mechanical properties. The results show that the type of graft copolymers or co-ionomers of phenoxy with acid functionalised ethylene copolymers can be made and that sodium ethoxide is an effective catalyst for the production of these types of compatibilisers. These systems were found to be a very efficient compatibiliser for PET/HDPE blends as they produce fme dispersions and improve the mechanical properties. A correlation is found between the efficiency of the compatibilisers and the melt viscosity of the final blends. Furthermore this work has revealed that the further addition of sodium ethoxide to ionomers and ionomer based compatibilisers enhances considerably the nucleation of PET in the blend.