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Anticorrosive coating formulations containing nanocomponents showing bacteriostatic or biocidal

action

Autor: STEFAN KUBICA

• Electromechanical mechanism; corrosion reactions are prevented either by metal passivation with anticorrosion pigments or by creation of strongly adherent, stable layers

• Barrier mechanism; factors contributing to corrosion, present in the surrounding environment, have limited access to the surface; These two mechanism operate simultaneously, and it is binder and pigment type that decides which one dominates.

Mechanisms of surface protection with organic coatings

• Chromium compounds (VI)• Lead compounds,

due to their toxicity eliminated from paint formulations • Zinc phosphorate,

considered non toxic for a long time, it has been placed on the list of dangerous substances recently (Directive 2004/73/EC; R-50/53 – very toxic for water organisms)); EU member states are obliged to implement rules of that Directive by the end of October 2005.

Conclusion:It is justified to assume that modern protective coatings will play their role only due to the excellent barrier properties and anticorrosion pigments or corrosion inhibitors will be excluded from their formulations.

Effective anticorrosion pigments

• Structure of a coating influences its barrier properties and plays a crucial role in surface protection; The barrier performance of a coating is determined by:– Chemical structure of a polymer/binder,– Homogenous dispersion of solid phase (pigments and extenders),– Affinity of a coating surface to polymer matrix

• Improving coatings’ structure• lowers:

– Water, electrolyte, and gas permeability • increases:

– Adhesion and scratch resistance as well as resistance to other mechanical damage

New ideas• Addition of new components eg, nanocomposites to improve barrier

properties of coatings

Effectiveness of protective properties of organic coatings

• Hybrid organic/inorganic coatings are obtained by addition of nanocomposites to a binder either with dispersion method or with a sol-gel method. Preparation of nanocomposite structure with surfactants prior to addition facilitates incorporation of a extender to a paint, increasing its effectiveness in a coating.

• Nanocomposites used in paints are, most often,silicas, silicates, titanium dioxides, barium sulfate, aluminium or cyrconium oxides, with paritcles sizes up to some hundred nm. They can be used in acrylic, poliurethane and epoxy binders, both water and solvent borne.

Nanocomposites in coatings

Improvement of a barrier properties of coatings: – Corrosion resistance– Mechanical properties, – Combining properties of organic compounds

(elasticity, low softening point) with properties of inorganic nanoparticles ( hardness, weathering resistance)

– Possibility to obtain coatings with homogeneity comprised between organic and inorganic parts that would be controlled on molecular level.

Why nanocomposites ?

Growing demands of car users and car operating conditions are the main factor determinig type of coatings used by automotive industry. Increasing interest in coatings with nanoparticles is atributed to properties they can assure. Coatings used by automotive industry must show:• corrosion resistance; • resistance to splinter;• wet and water resistance;• scratch resistance;

• resistance to acids, solvents and chemicals.

Coatings with nanoparticles in automotive industry

• Paints with nanoparticles fulfill all demands required from automotive coatings listed previously. As nanoparticles size is comparable with visible light wave length range (400-800 nm), they disperse small amount of light and do not influence optical properties of a coating; as such they can be used in transparent top coats.

• Special automotive coatings with diamond nanoparticles - material with the highest hardness among all known substances. Coatings with diamond nanoparticles show excellent impact and scratch resistance as well as resistance to chemicals (mainly solvents); such coatings are also resistant to dirt pick-up (coating with antiadhesive properties).

• Research has been done to utilize nanoparticles in steel and aluminium alloys coatings, that would enable exchanging controversial chromium coating and anticorrosive primer with strontium chromate.

• Organic/inorganic coatings for a aviation industry, obtained with sol-gel method are expected to exchange systems that utilize chromates and allow elimination of poliurethane coating.

Nanoparticles in coatings – development directions

For many years heavy metals, either as inorganic salts or as organic compounds, have been used to destroy and inhibit growth of various microorganisms; excellent antimicrobial effectiveness is observed for minor amounts of heavy metals e.g..– Zinc organic compounds, – Lead and mercury salts

despite splendid antimicrobial and technological properties, their use is either completely banned or very strongly limited, due to the toxicity towards humans and animals Heavy metals with strong antimicrobial action but not toxic to humans and animals such as copper and silver can be used instead.

Heavy metals as factor inhibiting microorganisms growth

• Silver ions in concentrations as low as a few ppm assure effective protection against microorganisms; AgNO3 (lapis infernalis) has proved to be effective aseptic medium in medicine for more then 100 years.

• Cu (II) or Ag (I) ions in concentrations 10-6 mol/l inhibit growth of various bacteria. Cu (II) ions in concentration 10-3 mol/l are sufficient to inhibit growth of yeast and most of moulds. There are however species resistant to Ag or Cu ions e.g. Penicilinum sp. or Asp. niger can grow and proliferate in saturated CuSO4 solutions

Silver and copper ions as protection against microorganisms

• Controlled and effective release of silver ions into the surrounding environment or a product is a key factor determining its use as a antimicrobial agent.

• An example of such a controlled release is a AgCl on porous TiO2. Composition Ag/TiO2 shows effectiveness toward Escherichia coli, Staphylococcus aureus and Staphylococcus xylosus.

Conditions and ways of utilization of silver as antimicrobial agent

• Titanium dioxide nanoparticles with silver. • Silver complex compounds. It is noteworthy high activity

against wide range of bacteria e.g. Aspergillus niger, Penicillum citrinum, Aspergillus terreus, Rhisopus stolonifer, Cladiosporium caldosporioides, Penicillium islandicum).

• Silver complex compounds, with silver bound with sulphur atom also show antimicrobial properties. However lower compared to other complexes.

• Gold complex compounds also possess antimicrobial properties.

The most up-to-date solutions in bacteriostatic protection