Aluminium pigment encapsulation by polymer coating for use in powder coating
Encapsulation de pigments aluminium par un revêtement polymère pour une application peinture poudre
Résumé
The high increase of powder coating market induce development of new compatible fillers, and especially
specific aluminium pigment grades to obtain high metallic effects. Therefore, polymer encapsulation of
pigments appears as a good strategy.
In this study, a new simple and envirommentally-friendly process in aqueous media is developed by the use of peroxodisulfate ions. First, these anions inhibit the aluminium corrosion by passivation, and then, they
initiate the methacrylates polymerisation. Particles agglomeration limitation and uniform polymer layer
formation, which result in optimal optic effects, are obtained by specific reaction conditions.
Concerning peroxodisulfate and aluminium interactions, an oxidation reaction has been identified. Optimal
conditions have been found on a specific aluminium pigment grade and an interaction mechanism has been
proposed to explain passivation at low rate and complete oxidation at larger rate, observed by gassing tests,
pH evolution and scanning electron microscopy. To disperse aluminium pigments in water, paste solvent (white spirit) is exchanged by water-miscible
solvent. Polymerisation rate determination, laser granulometry evolution, SEM micrographs and
chromatography analysis enabled us to distinguish different steps during the encapsulation process. The first, long, step is characterized by polymer particles formation and adsorption on the aluminium surface. Then, the polymer particles grow at the surface until their coalescence forming a film. Therefore specific reactants and conditions have been used as a specific surfactant, a sequential or continuous persulfate solution addition, a monomer mixture containing principally methyl methacrylate for its hydrosolubility and a small amount of hydrophobic monomer to increase film-forming ability.
specific aluminium pigment grades to obtain high metallic effects. Therefore, polymer encapsulation of
pigments appears as a good strategy.
In this study, a new simple and envirommentally-friendly process in aqueous media is developed by the use of peroxodisulfate ions. First, these anions inhibit the aluminium corrosion by passivation, and then, they
initiate the methacrylates polymerisation. Particles agglomeration limitation and uniform polymer layer
formation, which result in optimal optic effects, are obtained by specific reaction conditions.
Concerning peroxodisulfate and aluminium interactions, an oxidation reaction has been identified. Optimal
conditions have been found on a specific aluminium pigment grade and an interaction mechanism has been
proposed to explain passivation at low rate and complete oxidation at larger rate, observed by gassing tests,
pH evolution and scanning electron microscopy. To disperse aluminium pigments in water, paste solvent (white spirit) is exchanged by water-miscible
solvent. Polymerisation rate determination, laser granulometry evolution, SEM micrographs and
chromatography analysis enabled us to distinguish different steps during the encapsulation process. The first, long, step is characterized by polymer particles formation and adsorption on the aluminium surface. Then, the polymer particles grow at the surface until their coalescence forming a film. Therefore specific reactants and conditions have been used as a specific surfactant, a sequential or continuous persulfate solution addition, a monomer mixture containing principally methyl methacrylate for its hydrosolubility and a small amount of hydrophobic monomer to increase film-forming ability.
La forte croissance du marché des peintures en poudre nécessite le développement de nouvelles matières de charges compatibles, et notamment de grades de pigments aluminium spécifiques pour obtenir des effets métallisés intéressants. Pour ce faire, l'enrobage des pigments dans une couche polymère est une stratégie éprouvée. Cette étude a permis de montrer qu'un nouveau type de traitement simple et propre, en milieu aqueux, est possible grâce à l'utilisation d'un réactif qui joue un double rôle : les ions persulfate permettent, en effet, dans un premier temps d'inhiber la corrosion de l'aluminium par passivation, puis d'amorcer la réaction de polymérisation de monomères acryliques. La mise en oeuvre de conditions de réaction spécifiques permet alors de limiter l'agglomération des particules et de former une couche polymère homogène et uniforme garantissant des effets otiques optimaux. En ce qui concerne l'interaction entre persulfate et aluminium, nous avons mis en évidence une réaction d'oxydation par le persulfate. Son effet est croissant avec le rapport massique ions persulfates / aluminium et des conditions optimales ont été déterminées sur le grade de pigments étudié. Enfin un mécanisme a été proposé pour rendre compte des résultats obtenus lors des tests de dégazage (gassing), du suivi du ph et des observations en microscopie électronique à balayage (MEB).
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