Modification of a commercial poly (VDF-co-HFP) copolymer latex

Abstract

Fluorinated polymers are niche macromolecules that play an essential role in modern life. The special properties of fluorine, including among others, a large electronegativity (ca 3.98), low polarisability, small van der Waal’s radius (135 pm) and the strong C-F bond (ca 485 kJ · mol−1), impart unique properties to organofluorine compounds. Flu-oropolymers exhibit a combination of desirable traits, including high thermal stability, low coefficient of friction, chemical inertness, oleo- and hydrophobicity, and low surface tension. Among the fluoropolymers, polyvinylidene fluoride (PVDF), and copolymers of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), have found applications in the coatings industry as the binder in exterior coatings. The chemical inertness of poly(VDF-co-HFP) copolymer, however, prevents disper-sion of pigments into the coating and also inhibits adhesion of the coating onto substrates. An acrylic modifier polymer is typically added to the poly(VDF-co-HFP) copolymer to improve the dispersion of pigments and the adhesion of the coating. This acrylic copoly-mer is physically blended with the poly(VDF-co-HFP) copolymer on a macromolecular scale (i.e. it forms a thermodynamically miscible blend). The loading of acrylic copolymer in commercial PVDF coatings is often in the range of 20 to 30 % by weight of polymer solids. Typically, copolymers of methyl methacrylate, ethyl acrylate and methacrylic esters are employed. Alternative strategies to overcome the adhesion problem include, among others, chem-ical modification of the surface of the fluoropolymer film. This can be achieved by graft copolymerisation or core shell emulsion polymerisation. These methods are used to funcionalise the polymer chains, while maintaining the desirable properties of the parent polymer. Due to environmental regulations, industry focus has shifted towards develop-ing coatings with a low volatile organic compound (VOC) content. Aqueous, low VOC, air-drying coatings can be formulated directly from the acrylic modified fluoropolymer (AMF) latex and have superior properties to solvent based, high VOC, air-dry coatings. Their advantages include low viscosities, reduced flammability, reduced odour and easy application using conventional equipment. A large portion of the aqueous coatings are sold into the architectural market with over 70 % of architectural paints used in the United States being classified as aqueous. Arkema Inc. has developed a commercial aqueous fluoropolymer latex using the method of seeded emulsion polymerisation. VDF and HFP monomers are randomly copolymerised via emulsion polymerisation. This poly(VDF-co-HFP) copolymer may be used as the seed material in a core-shell polymerisation using acrylic monomers. Kato et al. [49] discloses the preparation of an AMF formulation for poly(VDF-co-HFP) copoly-mer. Preliminary testing of membrane textiles coated with such formulations showed that the AMF coatings degrade under UV irradiation more rapidly than is is expected for poly(VDF-co-HFP) copolymer. The patent indicates that the nature of the product formed by the emulsion polymerisation is not well understood and the product my be either a graft copolymer of a core-shell system. The aim of this research reported in this dissertation was to shed light on the nature of the final product, and to verify the claims made in the above-mentioned patent. Various acrylic monomers were copolymerised via seeded emulsion polymerisation us-ing commercial poly(VDF-co-HFP) copolymer as the seed material. The concentration and the ratios of the monomers were varied according to the formulation guidelines in Kato et al.[49]. ATR-FTIR spectroscopy and19F NMR spectroscopy was used to de-termine the microstructure of the resultant latexes. ATR-FTIR spectra confirmed the presence of C=C and C=O bonds in latexes. This indicates that unreacted acrylic com-ponents are present. The ATR-FTIR spectra of the films indicated the disappearance of the C=C bonds from the latex, which indicates that the monomers are evaporated easily from the latexes during film formation. The 19F NMR spectra confirmed that no modi-fication of the poly(VDF-co-HFP) copolymer backbone took place during the reactions. The particle size distribution graphs showed an increase particle sizes and this suggested that some self polymerisation of the monomer occurred. The viscosity of the latexes were lower compared to the due to the experiments being conducted under dilution. The flow characteristics of the poly(VDF-co-HFP) copolymer was also influenced with some reactions yielding shear thickening latexes as compared to the shear thinning poly(VDF-co-HFP) copolymerc. The reactions also yielded latexes which displayed lower and higher surface tensions than the poly(VDF-co-HFP) copolymer. Therefore, the conclusion may be drawn from this work that core-shell formation occurred during the emulsion copolymerisation, as opposed to grafting of the monomer onto the poly(VDF-co-HFP) copolymer backbone. The claims made in the literature could not be substantiated; in particluar, the reported improvements in film forming ability were not realised. No commercially useful advantage exists for the emulsion copolymerisation of poly(VDF-co-HFP) copolymer with acrylic monomers over the solution blending of poly(VDF-co-HFP) copolymer with acrylic copolymers.