Facile microencapsulation of HDI for self-healing anticorrosion coatings

Abstract

Polyurethane (PU) microcapsules containing hexamethylene diisocyanate (HDI) as core materials are facilely manufactured via interfacial polymerization reaction of commercial methylene diphenyl diisocyanate (MDI) prepolymer and 1,4-butanediol in an oil-in-water emulsion. The resultant capsules have diameters of 5-350 mm and shell thickness of 1-15 mm, both linearly related to the agitation rate in the double logarithm coordinates. The typical core fraction of microcapsules and the yield of synthesis are around 60 wt% and 70 wt%, respectively, while varying with reaction conditions. The effects of parameters including reaction duration and temperature, surfactant concentration, agitation rate, and environmental factors on the formation of microcapsules were systematically investigated and optimized. Quality assessments of each batch of microcapsules were performed using thermogravimetric analysis and scanning electron microscopy. Anticorrosion coatings mixed with synthesized microcapsules were prepared on a steel substrate. Preliminary results indicated significant corrosion retardancy happened in the self-healing coatings under an accelerated corrosion process, showing the great potential of this facile microencapsulation technique in development of catalyst-free, one-part self-healing coatings for corrosion control.

Self-healing coatings in anti-corrosion applications

“…The effect of the rate of agitation during the synthesis of the containers has been described in the literature. An increase in the agitation rate results in a reduction in capsule size [40,55]. The diameter of capsules significantly decreases also as the concentration of the surfactant (gum arabic) increases, which is ascribed to the influence of surfactant concentration (before the critical micelle concentration is reached) on the interfacial tension of the emulsion media [40].…”

“…Figure 6 schematically shows how a capsule is created using the particular methods [29]. As the building material, the following are used: poly(urea-formaldehyde) (PUF) [32][33][34][35], melamine-urea-formaldehyde (MUF) [35,36], phenol-formaldehyde [37], epoxy resin [38,39], methylene diphenyl diisocyanate (MDI) [40], poly(methyl methacrylate) (PMMA) [41,42], polystyrene [32,41], poly(allylamine) [43], polyvinyl alcohol (PVA) [43,44], polyurethane [40], polyphenol, amphiphilic block copolymers (polypyrrolidones, poly(ethylene oxide) [31], poly(caprolactone)) [31,32], cerium molybdate [45], zinc and aluminium nitrate [46,47], silica [27,32,[48][49][50], silver [42], gold [51], copper(II) sulphide [51], c-AlO(OH) [51], tin(IV) oxide [51] and titania oxides [42,52,53] and CaCO 3 microbeads [54]. The capsule material and the way of preparing capsules affect their shape and size.…”

“…Microscopic techniques are also used to assess the self-healing properties of coatings. For this purpose, the coating is cut, immersed in a corrosive solution and the place of the cut is examined at fixed time intervals using optical microscopy, scanning electron microscope (SEM), atomic force microscopy (AFM) or scanning Kelvin probe force microscopy (SKPFM) [32,37,38,40,102,115]. The scanning electrochemical microscope (SECM) is used a tool enabling one to evaluate the self-repairing properties of coatings [44].…”

“…Halloysite nanotubes and boehmite nanoparticles are employed as reservoirs for corrosion-inhibiting compounds [57][58][59]. Cerium cations [58,[60][61][62], 8-hydroxyquinoline [48,57], benzotriazole [45,48,49,63,64], mercaptobenzimidazole (MBI) [63], dicyclopentadiene (DCPD), alkoxysilanes [65], silyl ester [24], linseed oil [37,44,66], hexamethylene diisocyanate (HDI) [40], triethanolamine (TEA) [41], monomers and catalysts for ROMP [67], magnesium ions [68], chromium(III) and cerium(IV) oxides [34] and cerium(III) chloride [60,69] are used as corrosion inhibitors. The action mechanisms connected with the particular compounds are presented in Table 1.…”

“…(PPy) or polyalinine (PANI), have oxidizing properties causing steel passivation. In the case of polypyrrole having ion-exchange properties, when it is modified with PMo 12 O 40 3-and HPO 4 2-ions, self-healing is additionally achieved owing to the reaction of tetraoxomolybdate ions (MoO 4 2-) with iron. The forming iron molybdate significantly limits substrate digestion.…”