Preparation And Properties Of Fluorine-containing Polyester Powder Coating

Polyester powder coating is a new kind of thermosetting coating material besides the epoxy and epoxy-polyester powder coatings. It has received much attention due to its excellent light and weather stability, good adhesion to substrates, rich raw materials and low price. However, the cured coating of polyester is apt to be impacted by water and oil. This will lead to the deterioration of mechanical properties and contamination of the cured coating. In the practical application, the environmental factors, such as rain, humidity and dewdrop, can result in the embrittlement and cracking of the cured coating, which will cause the decline of the durability and corrosion resistance of the coating. Moreover, the presence of oil and organic solvent can also swell the cured coating of polyester. Therefore, it is vital to improve the water-and oil-resistant properties of polymer coating in the industrial applications.In this work, fluorine was introduced into the polyester chain to improve its hydrophobicity and oleophobicity. The fluorine-containing polyesters with tetrafluoropropyl pendant segments (TFPE) and fluorine-containing polyester with dodecafluoroheptyl pendant segments (DFPE) were synthesized by adding the tetrafluoropropyl glycidyl ether (TFGE) and dodecafluoroheptyl glycidyl ether (DFGE) in the phthalic anhydride/epichlorohydrin reactions system, respectively. The fluorine-containing polyester powder coatings with high hydrophobicity and oleophobicity were then prepared through blending the fluorine-containing polyester, CaCO3, curing agent and additives.TFGE was used as a fluorine-containing monomer to synthesize the fluorine-containing polyester powder coating with the tetrafluoropropyl pendant segment (TFPEPC). The fluorine-containing polyester with the tetrafluoropropyl pendant segment (TFPE) was successfully synthesized by the reaction between phthalic anhydride, epichlorohydrin, and tetrafluoropropyl glycidyl ether (TFGE), using the tetraethylammonium bromide as the catalysis and using1,4-butanediol as the initiator. The chemical structure of TFPE was characterized by1H-NMR and19F-NMR, and it was found that that the fluorine in TFGE was successfully introduced into TFPE. The fluorine content in TFPE was determined by the oxygen-flask combustion and fluoride selective electrode. It was found that fluorine content in TFPE increased with the increase of TFGE content. The wettability of water and oil on the cured coating of TFPEPC was investigated by the contact angle meter. Contact angles of water and diiodomethane on the cured coatings firstly increased and then remained constant with the increase of the fluorine content in TFPE. X-ray photoelectron spectroscopic (XPS) analysis showed that the F/C molar ratio on the coatings surface (0.313) was much higher than the overall F/C molar ratio (0.035), indicating that the fluoroalkyl groups in TFPE had enriched on the coatings surface. The F/C molar ratio on the coatings surface first increased and then kept invariable with increasing the fluorine content. This agrees with the result of surface wettability.In order to decrease the usage of fluorine in the polyester powder coating and raise the utilization rate of fluorine, the fluorine-containing polyester with dodecafluoroheptyl pendant segments (DFPE), which had longer fluorine-carbon pendant segment, was synthesized by using dodecafluoroheptyl glycidyl ether (DFGE) as the fluorine-containing monomer. The fluorine-containing polyester powder coatings with dodecafluoroheptyl pendant segments (DFPEPC) were prepared by blending DFPE, non-fluorine-containing polyester (PE), CaCO3, curing agent, and additives. The hydrophobicity and oleophobicity of DFPEPC cured coating first increased and then kept constant with increasing the fluorine content. Due to longer fluorine-carbon chain, the introduction of lower fluorine content (0.75wt%) in DFPEPC can lead to the remarkable enhancement in the hydrophobicity and oleophobicity of cured coating, compared to the case of TFPEPC (2.45wt%of fluorine content). XPS results indicated that the F/C molar ratio on the coating surface was much higher than the overall F/C molar ratio, suggesting that the fluorine-carbon chains with the lower surface free energy transferred to the coating/air interface in the curing process. The F/C molar ratio on the coatings surface first increased and then kept unchanged with increasing the fluorine content. In TFPEPC, the F/C molar ratio on the coatings surface was0.313when the fluorine content was2.45wt%. However, in the case of DFPEPC, the F/C molar ratio on the coatings surface reached0.347when the fluorine content was just0.75wt%. This indicated that the longer fluorine-carbon chain had better enrichment effects on the surface of cured coating.Introduction of low fluorine to polyester powder coating can improve the acid&alkali resistant and boiling water resistance of cured coating remarkablely, without the adhesion of cured coating to substrates decreasing.