Relevance and Performance of LDH Platelets in Coatings

International audienceThe present article presents a review devoted to Layered Double Hydroxide (LDH) as filler in coatings from the patent domain and the applicative literature. Other than bulk properties generally encountered in polymer nanocomposite aspect, LDHs filler relevance for coatings concerns surface properties as well as requiring diffusion barrier into limited width by isolating or healing substrate from aggressive medium. Enhancement of the substrate is performed with rather low pigment/filler concentration underlining the importance of nanometric dispersion to create large interfacial exposure between filler and polymer as well as being crucial for substrate adhesion. Largely exemplified with corrosion coating protection, LDHs is reported here as a corrosion inhibitor nano-container (C.I.NC.), and the self-healing protective effect is occurring from the delivery of inhibitor on demand, the guest interleaved species possess corrosion inhibition ability triggered when it is spontaneously released from the host matrix into aqueous environments in contact with corroding metal substrates. From the academic and patent literature, the review enlists the strategy to employ LDHs as efficient filler for bare metal substrate, mostly aluminum, magnesium and steel, as well as for polymer coatings and conversion films to prevent corrosion, as well as other properties: anti-UV, anti-abrasion and impact resistance, bactericide and antifouling. LDHs platelets dispersion either exfoliated or intercalated is found to be a key to provide great properties such as barrier properties and energy dissipation through nano-spring effect, respectively. The accent is also underlined on the concerns relative to ecological and economic considerations for coatings in an industrial setting and the chemicals regulation. Finally perspectives point out the topical relevance of LDHs nanofiller to supply adaptive and multifunctional properties to coatings

Unraveling EDTA corrosion inhibition when interleaved into Layered. Double Hydroxide epoxy filler system coated onto aluminum AA 2024.

International audienceTo entrap a corrosion inhibitor agent into a host matrix and avoid its possible weakening/plasticizing toward an organic coating as well as in enabling its progressive release under stimuli, Layered Double Hydroxide (LDH) framework is here selected as its role as a coating filler to provide a barrier effect due to its associated aspect ratio. To protect aluminum alloy AA 2024 ethylenediaminetetraacetic acid (EDTA) was chosen because of its interaction with copper rich intermetallic phases. Characterization of the inorganic-organic (I/O) hybrid phase confirms the formation of LDH-type sheets with an unusual high charge density. LDH/EDTA is evaluated as corrosion inhibitor agent in the presence of aggressive chloride anions and compared to chromate, carbonate and chloride interleaved LDH anions. The mechanism of inhibitor release from the inorganic LDH container is studied by X-ray diffraction and its efficiency when embarked into an epoxy-based coating is characterized by Electrochemical Impedance Spectroscopy (EIS) and DC-Polarization experiments using Tafel plot refinements. The possible corrosion mechanisms are discussed as a function of the EDTA/LDH action under such particular conditions and the associated protection is scrutinized evolving diadochy, buffering and possible complexing reaction against electrolyte salt concentration versus exposure time. Counter intuitively the deleterious role of released EDTA in inhibiting the corrosion process is here demonstrated, however underlining the benefit of the LDH platelets

Organo-modified layered double hydroxide in coating formulation to protect AA2024 from corrosion.

International audienceSuggested as efficient filler for impact resistance when dispersed into a coating formulation, layered double hydroxide (LDH) materials organo-modified by aniline and benzene derivatives, 3- and 4-aminobenzensulfonic acid (ABSA) and 3,4-dihydroxybenzoic acid (HHBA), are further investigated here to protect from corrosion aluminium alloy 2024. The concomitant release of interleaved organic molecule as well as the uptake of chloride anion was studied by X-ray diffraction to demonstrate the LDH framework efficiency to act as chloride nanotrap. HHBA was found to be immobilised while the two other molecules are released. DC-polarization experiments demonstrate the efficiency of all three hybrid LDH materials in lowering the corrosion current suggesting that the corrosion inhibition mechanism should either proceed by an anion exchange and/or by a complete dissolution of inorganic framework inducing both the anions release but in the latter case by an additional leaching of the LDH sheet cations. Electrochemical impedance spectroscopy experiments indicate two opposite phenomena: an increase in the barrier properties of the coating in the presence of LDH pigments but as well as in the water uptake process that subsequently causes the protective polymer film to blister. Such delamination allows the electrolyte to migrate but also the corrosion inhibitor agents to flow onto the damaged area. Elemental analyses mapping around damaged area using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy confirm the release of interleaved anticorrosive species that is occurring by anion exchange and/or platelets dissolution when LDH fillers are dispersed into the polymer coating formulation

Highly Ordered Graphene Oxide and Reduced Graphene Oxide Based Polymer Nanocomposites: Promise and Limits for Dynamic Impacts Demonstrated in Model Organic Coatings

Graphene oxide (GO) dispersed in water has been combined with a mixture of aqueous polymer dispersions and melamine formaldehyde resin (MF). Stable low viscous fluids with no obvious signs of mesoscale ordering at 0.3 wt % yield transparent films with GO loadings up to one weight percent in the form of homogeneously aligned double strands, each comprising few individual layers of the carbon allotrope. While baking of the films at 160 °C results in minor thermal reduction of GO, in situ reduction with excess hydroxylamine (HA) in the presence of the polymer colloids yields stable dispersions in which amphiphilic graphene like flakes temporarily encapsulate gaseous reaction products. Depending on the parameters in the time–temperature domain, the hollow spheres may be transferred into solid material or disassemble during film formation, the latter case providing black, smooth, and transparent films with up to eight magnitudes increased electrical conductivity and an oxygen permeability 30-fold higher compared to the neat polymer matrix. In contrast, GO reduces oxygen permeability by that factor, while water permeability stays unchanged. Thermo-mechanical measurements reveal matrix stiffening by the platelets as well as by HA, the latter via modifying the MF reactivity. Excellent stone chip resistance and ballistic impact tests demonstrate efficient energy dissipation and crack deflection provided by the laminate like morphology of GO based composite. On the contrary, the same material only provides moderate substrate protection in rain erosion tests

In situ generation of layered single- or double-hydroxide inorganic platelets (LSH and LDH) assisted by bola amphiphiles

International audienceA novel preparation route for layered hydroxide based hybrid phases has been investigated combining the polyol route with the in situ generation of inorganic platelets, in the presence of an amphiphilic polymer as well as bola-amphiphiles. The polyol route consists of hydrolysis in an alcoholic medium containing acetate metal cations as precursor(s), to yield an LDH Zn2Al cation composition or LSH-Zn (layered single hydroxide). A bola amphiphile is described as hydrophobic polymer segment-telechelic-chains terminated by two anionic hydrophilic end groups, using volatile ammonium cations as counter ions. The impact of both process conditions as well as the chosen system with regard to the metal hydroxide framework on the morphology of the obtained hybrid phases is scrutinized by X-ray diffraction (XRD), small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), and compared to that of the corresponding physical mixture. For the ex situ approach, the diffusion of large cumbersome polymers or amphiphilic bolas between the inorganic platelets was found to be efficient, mostly driven by an anion exchange reaction between interleaved acetate anions and carboxylate functions of the molecular backbones, and keeping intact the inner sheet integrity through a topotactic process. In particular with LSH-Zn, a multi-stratified assembly has been observed combining an acetate pristine structure and a partly bola diffused structure, leading to a biphasic structure, aggregated and intercalated. To the best of our knowledge, observation of LSH-Zn single platelets has never been reported, making the combined process polyol/in situ an interesting new route to reach exfoliation. Indeed, it leads to the generation of either LSH or LDH platelets of lateral size ranging from 10 up to 200 nm. However, the platelets were found to be porous; it is considered as a drawback for barrier properties. It is our belief that such porosity may open new insights in “tectonic” architecture by intertwining 2D and 1D-type fillers. Rather counterintuitively, the ex situ approach based on a topotactic exchange reaction matches the in situ templating reaction in many cases as a function of the dispersion state regardless of the polymer or bolas as well as the platelet cation composition

In situ platelets formation into aqueous polymer colloids: The topochemical transformation from single to double layered hydroxide (LSH–LDH) uncovered

International audienceLayered Single Hydroxide (LSH) of chemical composition Zn5(OH)8(acetate)2·nH2O is synthesized under in situ condition in an aqueous dispersion of an amphiphilic, carboxylate bearing polyester via a modified polyol route. The one-pot LSH generation yields agglomerates of well intercalated platelets, 9–10 nm separated from each other. However the corresponding Layered Double Hydroxide (LDH) of formal composition Zn2Al(OH)6 (acetate)·nH2O is found to proceed via the formation of crystallized, similarly spaced LSH sheets in the neighborhood of amorphous Al rich domains as evidenced by X-ray diffraction and transmission electron micrographs. The initial phase segregation effaces over time while LSH platelets convert into the LDH phase. Fingerprinted by the change of in-plane cation accommodation, the associated topochemical reaction of the edge-sharing octahedral LSH platelets involves the transformation of metal lacunae, adjacently covered by one tetrahedral coordinated cation on each side to balance the negative surcharge, into fully occupied and monolayered platelets of edge-sharing octahedral LDH, the former voids being occupied by trivalent cations. This replenishing process of empty sites, coupled with the dissolution of tetrahedral sites is likely to be observed for the first time due to the presence of well separated, polymer intercalated platelets. TEM pictures vision crystal growth arising from the zone of the LSH edge-slab and by using solid state kinetics formalism the associated high activation energy of the first-order reaction agrees well with a plausible dissolution re-precipitation mechanism. The conversion of LSH into LDH platelets may be extended to others cations as Co2+, Cu2+, as well as the aluminum source (AlCl3) and the water-soluble polymer (NVP), thus indicating it is a new prevalent facet of LDH

Inhibition of steel corrosion and alkaline zinc oxide dissolution by dicarboxylate bola-amphiphiles: self-assembly supersedes host-guest conception.

International audienceFor many decorative applications like industrial and architectural paints, prevention of metal substrates from corrosion is a primary function of organic coatings. Triggered release of inhibitor species is generally accepted as a remedy for starting corrosion in case of coatings damage. A polyurethane based coating, doped with bola-amphiphiles of varying molecular weight but with a common head group motif that stems from ring-opened alkenyl succinic anhydride, enables passivation of the defect and mitigates cathodic delamination, if applied on cold rolled steel. An antagonistic effect results from the intercalation of the bola-amphiphiles into layered double hydroxide Zn2Al(OH)6 and subsequent incorporation of the hybrid phase into the organic matrix. In particular higher molecular weight bola-amphiphiles get immobilized through alkaline degradation of the layered framework in the basic milieu at the cathode. By means of sediments from colloidal states it is demonstrated that in-situ formed zinc oxide encapsulates the hybrid phase, evidenced by impeded dissolution of the ZnO based shell into caustic soda. While inhibition of steel corrosion results from a Donnan barrier layer, impeded zinc oxide dissolution is rooted in zinc catalyzed bola-amphiphile hydrolysis and layered deposition of the crystalline spacer diol hydrogenated bisphenol-A

Giant zirconium-bisphosphonate nano-ribbons and their liquid crystalline phase behaviour in water

In decimolar aqueous solutions, zirconium oxychloride octahydrate forms several micrometer long and approximately 15 nm wide thin ribbons through the reaction with excess amounts of the sodium salt of 1-hydroxyethane-1,1-diphosphonic acid (HEDP, known as etidronic acid). Primarily deduced from SAXS, TEM, EXAFS and solid-state NMR analyses, a consistent structural model enables congruous explanations for the colloidal behaviour of the purified ribbons as well as of their reaction products with ammonia and amines, respectively. Properties of the lyotropic, liquid crystalline phases are discussed in the light of potential applications in aqueous coatings

A First Wide‐Open LDH Structure Hosting InP/ZnS QDs: A New Route Toward Efficient and Photostable Red‐Emitting Phosphor

International audienceThe architecture of Zn-Al layered double hydroxides (LDHs), organo-modified with bola-amphiphiles molecules, is matching its interlayer space to the size of narrow-band red-emitting InP/ZnS core–shell quantum dots (QDs) to form original high-performance functional organic–inorganic QD-bola-LDH hybrids. The success of size-matching interlayer space (SMIS) approach is confirmed by X-ray diffraction, small angle X-ray scattering (SAXS), TEM, STEM-HAADF, and photoluminescence investigations. The QD-Bola-LDH hybrid exhibits a photoluminescence quantum yield three times higher than that of pristine InP/ZnS QDs and provides an easy dispersion into silicone-based resins, what makes the SMIS approach a change of paradigm compared to intercalation chemistry using common host structures. Moreover, this novel hybrid presents low QD–QD energy transfer comparable to that obtained for QDs in suspension. Composite silicone films incorporating InP/ZnS (0.27 wt%) QD-bola-LDH hybrids further show remarkable improved photostability relative to pristine QDs. An LED overlay consisting of a blue LED chip and silicone films loaded with QD-bola-LDH hybrids and YAG:Ce phosphors exhibits a color rendering index close to 94