Bridging adhesion and barrier properties with functional dispersions : towards waterborne anti-corrosion coatings

Abstract

The successful preparation of waterborne anti-corrosion coatings based on maleic anhydride containing copolymers is described in this thesis. To obtain good anticorrosion coatings, three different properties should be present in a coating system; they should display good mechanical properties, good adhesion properties, and good barrier properties. These properties are in practice obtained by applying different layers, each with their own characteristics. In this research the aim was to combine these different properties into polymer chain. With this one stone, multiple birds strategy, a single coating layer should serve as a protective system. To combine different properties into polymer chain, different strategies can be followed. One is the use of block copolymers, each block with its own specific feature. Another approach is the random incorporation of functional groups into the polymer main chain. Both approaches were explored in this thesis. For this purpose, maleic anhydride was chosen as a monomer, since this monomer is known to be very reactive with a wide variety of different compounds. Because maleic anhydride does not homopolymerize in radical polymerizations, different comonomers were used to obtain the desired copolymers. Alternating copolymers of maleic anhydride with a-olefins were obtained by free radical polymerization. Both block-copolymers and homocopolymers based on alternating styrene and maleic anhydride (PSMA) were obtained with Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization. Block copolymers were obtained either by using different monomers or by modifying synthesized PSMA prior to synthesizing the second block. Furthermore, both PSMA obtained via RAFT polymerization and a commercially available PSMA were partially imidized to increase the hydrophobicity of the polymer as well as its stability in the water phase. Furthermore, a commercially available polybutadiene with grafted maleic anhydride (PBDMA) was studied as well. Stable, surfactant-free latexes could be prepared by partially ammonolizing the anhydrides of the different copolymers in acetone, followed by addition of this acetone solution to water and evaporation of the acetone. The obtained polymer particle diameters were in the order of 100 to 200 nm, as observed with atomic force microscopy, transmission electron microscopy and dynamic light scattering, and were stabilized by electrostatic interactions, as indicated by a negative ?-potential of -60 to -80 mV. With these latexes, films were prepared at elevated temperatures. Two different crosslinkers were compared, i.e. 1,6-diaminohexane (DAH) and adipic acid dihydrazide (ADH). The latter was found to form irreversible crosslinks at lower temperatures than DAH. Furthermore, DAH decreased the stability of the polymer latexes, whereas addition of the dihydrazide based crosslinker left the properties of the latex unchanged. Only upon evaporation of the water and heating of the sample the network formation takes place, as was shown with atomic force microscopy. The films that were formed with these latexes and ADH, proved to be hydrophobic, as was indicated by the high advancing water contact angle, varying from 95 up to 105 °, depending on the type of latex that was used. The adhesion of the coatings to different substrates was investigated with pull-off tests, and showed that the coatings strongly adhered to the aluminum substrates. Only for the hydrophobic poly(octadecene-altmaleic anhydride) (POMA) a poor adhesion was found. Furthermore, the adhesion to magnesium alloys and steel appeared to be relatively poor, compared to aluminum. On films formed with the dihydrazide as crosslinker, anti-corrosion studies were performed, which were compared to a commercially available polyester based coating applied from solution. The barrier properties, as measured with electrochemical impedance spectroscopy, indicated good corrosion protection in a corrosive medium (5 wt% NaCl aqueous solution). For longer immersion times in this medium, hydrolytic degradation was observed for the polyester, while the latex based coatings did not degrade noticeable. The results obtained in this project show that within polymer system the requirements of an anti-corrosion coating can be combined. The developed water-borne coatings show very promising properties for future anticorrosion applications.