Microcapsule-based self-protecting coatings
Corrosion causes enormous damage to products and infrastructure on an annual basis. The most common approach to mitigate corrosion is to apply a protective coating (layer) to the targeted “substrate” well before placing the item in service. The protective coating acts as a passive layer between the protected substrate and the corrosive environment. Generally, the protective layer includes additive(s) that are “sacrificial” with a much higher propensity to corrode than the target substrate material. For steels typical additives include zinc compounds, manganese and other alloying elements. More recently, self-protective coatings have been introduced using microencapsulated anti-corrosion chemicals and compounds that are released on demand at the site of active corrosion. Once triggered via capsule release, the passive coating later is activated thereby intervening in the corrosion reaction and mitigating its effects. Unfortunately, the best ant-corrosion coatings are environmentally hazardous and in some cases carcinogenic (e.g. chromates). Is there a viable coating system that is based on the microcapsule motif, yet retains high performance and is environmentally friendly (green chemistries)? This is the central question of this thesis. Here we introduce and develop an environmentally-friendly, anti-corrosion protective polymer coating for use with steel substrates. The anticorrosive performance as assessed in controlled laboratory conditions using industry standards is on par with the best commercially available systems at a fraction of the cost. We take advantage of the natural “anti-corrosion” properties of lawsone, an extract of the hemp plant, and use this as the core payload for our anticorrosion microcapsules. The capsules are made an internal phase separation technique with a polyurethane shell wall and a maximum core loading of 8 wt% with respect to a benign core (carrier) solvent. In some cases a water-based epoxy coating was used containing no volatile organic compounds (VOC) in order to demonstrate suitability with the next generation of green coating systems. Thin, uniformly coated (ca. 105 µm thick) steel substrates showed significant corrosion protection via optical and electron microscopy as well as 63% corrosion inhibition (via electrochemical analysis) after damage and exposure to simulated salt water for five days. Fully autonomous self-healing coatings were also developed and demonstrated based on the UV curing of damage-triggered release of a sunlight curable epoxy resin from specially engineered UV-blocking microcapsules. Microcapsules containing as little as 5 wt% carbon black were shown to be sufficiently UV-blocking to form a viable system. Microcapsules formed a Pickering emulsion of carbon black particles at the interior shell wall. PhotoDSC characterization shows as much as 65% UV blockage at 365 nm. Fluorescence signature (405 nm excitation and 410-600 nm emission) with an encapsulated fluorescent dye also showed as much as 95% blockage. UV-blocking microcapsules were embedded in a commercial epoxy coating, damaged with a scribe, and then exposed to simulated salt water for five days. The released capsule core material triggered from the damage event was verified by Raman spectroscopy as the cured epoxy resin after UV-exposure. Optical imaging after five days of salt water exposure showed good corrosion inhibition. Electrochemical analysis was performed over a range of damages sizes with the highest inhibition efficiency near 100%. UV-blocking microcapsules were also incorporated into dual-layered coating systems containing both epoxy and polyurethane coatings. A polyurethane top coat is commonly used in commercial coatings to protect the underlying (epoxy) coatings from UV exposure since UV is known to cause chalking and flaking of epoxy upon prolonged UV exposure. Only a 5 µm thick polyurethane top coat provides over 90% UV blockage across the solar spectrum. The average commercial film thickness (50-100 µm) is more than sufficient to provide protection from chalking of the epoxy, but also long term stability to the microcapsules until damage triggers release of the core contents.
self-healing, self-protecting, anti-corrosion, polymer coating, corrosion, electrochemical, microcapsules, UV curable, lawsone
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign