Auto-deposition coating

Excerpt:

General Industrial Group - Technology Division PPG Asian Paints Pvt. Ltd.

ABSTRACT

Auto-deposition is faster and more environmentally responsible than conventional coating processes; however, a typical seven-stage auto-deposition process still requires several cleaning stages. Recent advances are pushing auto-deposition technology past this boundary to offer manufacturers and coaters a coating solution that lowers the environmental impact even further, while also reducing the overall manufacturing production footprint and increasing efficiency in the assembly process.

Introduction

AUTO-DEPOSITION COATINGS are thin, highly corrosion resistant organic coatings that are deposited in a chemical reaction with a metal surface. Because the auto-deposition process deposits a coating only on metallic surfaces, coating of just the metal portion of metal-plastic or metal-rubber assemblies is possible. The overall auto-deposition process includes stages of clean, water rinse, coat, reaction rinse, and cure. The use of a reaction rinse after the auto-deposition stage is unique among coating processes and allows new properties to be introduced to the coating before curing.

This report will briefly review the general chemistry of auto-deposition, and then focus on how corrosion performance and physical characteristics of a recently developed epoxy- acrylic auto-deposition coating are designed into the product and how the auto-deposition process controls and these properties. The role of the organic polymer and the reaction rinse in all the properties will be discussed. In general, physical properties, such as flexibility, hardness, UV durability, and chemical resistance are affected by the polymer design. On the other hand, corrosion properties are largely conferred by the chemistry of the reaction rinse.

Auto-deposition is a method of applying a layer of anti-corrosive paint to metal using a chemical reaction. The auto-deposition process has been in commercial use since 1975, and has grown to include more than 130 commercial installations in 22 countries. Since its inception, this simple and reliable industrial coating process has coated billions of square feet of surfaces for a wide variety of applications. There are as many similarities to electro-less plating and conventional painting techniques. Electro-less plating provides many unique features that enable the coating of components not possible by other systems.

Auto-deposition has been described as "electroless electrodeposition since the process appears similar to electrodeposition in some regards, although no electric current is introduced into the coating bath. However, bath compositions, mechanisms and controls are quite different. In contrast, electrodeposition uses an applied current to move electrically charged paint materials to an oppositely charged electrode (the workpiece). Auto-deposition is the controlled destabilization of an anionically or negatively charged latex polymer on ferrous surfaces by chemical means. Auto-deposition and electrodeposition are similar in that:

  • The coating thickness obtained is proportional to the amount of time the metal surface is immersed in the coating bath

  • A “levelling” action occurs due to better deposition through thin films relative to thick films.

  • Deposited films are resistant to water (or permeate) rinsing prior to cure.

  • The coating thickness is uniform.

Auto-deposition is a waterborne, anticorrosive paint layer applied to metal by means of a chemical reaction rather than by electrical or contact methods. The auto-deposition process is simple in comparison to other processes used for applying corrosion-resistant coatings.

Following conventional cleaning and rinsing stages, metal parts are immersed in a coating chemical bath, where the pigment and resin particles are deposited. The growth in film thickness is time dependent.

A rapid deposition rate is followed by a decrease in growth (limited by the diffusion of ions on the metal), which results in a uniform and “self-limiting” coating thickness. A protective barrier film is formed wherever the coating bath makes contact with the metal work piece. This process is faster and more environmentally responsible than conventional coating processes; however, a typical seven-stage auto-deposition process still requires several cleaning stages.

Recent advances are pushing auto-deposition technology past this boundary to offer manufacturers and coaters a coating solution that lowers the environmental impact even further, while also reducing the overall manufacturing production footprint and increasing efficiency in the assembly process.

Comparison With ED Coat

In plating, there are two basic kinds of processes: electrolytic and electroless. The terms are pretty self explanatory— electrolytic plating processes apply a current through the parts to attract the metal being plated, while in electroless processes plating occurs as a result of chemical processes only. An analogous situation exists in organic coating with electrocoating and auto-deposition paint processes. In Ecoat, a current applied through the parts being coated attracts particles of paint with an opposite charge. In auto-deposition, paint is deposited as the result of chemical reactions only. In Ecoat, paint deposition stops when the current stops. In auto-deposition, deposition stops when parts move to a rinse tank. Film thickness is self limiting, ensuring uniform coating thickness on any part surface that can be wetted by the paint material.

Both processes are waterborne and use no organic solvents, so they require no air pollution controls. Both are effective for steel parts ranging from fasteners to auto bodies.

Now, advances in auto-deposition coating materials and processes are making the process more competitive for a variety of part materials and applications, according to developer Henkel Corp. (Madison Height, MI). The company says its Aquence brand of auto-deposition coating processes are now displacing conventional metal pretreatment and the electrocoat process for an entire automotive vehicle body.

According to Henkel, the process can allow customers to reduce capital expenditures and paint shop complexity, decrease energy consumption, eliminate heavy metal sludge, and improve inside out corrosion performance compared with Ecoat. Because it uses fewer stages than ecoat, the process also has a significantly smaller footprint, says Kevin Woock, global director, Aquence Coatings.

“Ecoat systems normally include 12 or more stages to properly clean, rinse and coat a metal part,” Woock says. “The equivalent clean, rinse, and coat auto-deposition coating process is seven stages—a 40% reduction in the number of steps needed to properly coat a part, and in the corresponding footprint of the process.”

A safer, more efficient process

The newest development in auto-deposition coatings allows manufacturers to eliminate several layers of cost and complexity. It also has the ability to coat large volumes of densely racked or complex shaped metal parts uniformly, giving finishing professionals the choice to coat their parts as an assembly or as individual pieces.

Compared to the seven stages required in a conventional coating process, the advanced auto-deposition process has only four process stages where chemicals are used. The phosphate pretreatment stages are eliminated. Instead, the advanced auto-deposition process adds cleaning and rinsing stages within the four processes. The use of a reaction rinse after the auto-deposition stage allows new properties (such as higher corrosion and chemical resistance performance) to be introduced to the coating before curing. The coating bath uses a dispersion of a paint emulsion at low solids, along with acidified ferric fluoride. This technique frees ferrous ions that destabilize the emulsion locally at the metal surface, causing the paint layer to be deposited. The process provides a hard yet flexible coating that cannot be achieved with conventional paints.

The advanced auto-deposition process is a simple, reliable, industrial-capable finishing system that is employee-friendly. The coating materials are water-based and have extremely low levels of solvents, which mean that no flammable or explosive products are present in the process. Thus, there are fewer limitations on the process location, and workers can remain nearby to monitor the process or take part in other manufacturing functions. The auto-deposition process also eliminates high-voltage electrical fields in or around the coating tank because there is no need for electricity to deposit the coating in the process.

The advanced auto-deposition process coats only metallic surfaces in a metal-plastic or metal rubber assembly. This selectivity allows manufacturers to reduce capital expenditures and paint shop complexities.

Higher performance with less environmental impact

The advanced auto-deposition coating conforms to the shape of the metal surface, yet it is not affected by variances in electrical energy at high and low areas of complex parts. This feature of the process means that the coatings can provide better edge protection than other paint processes while also improving inside-out corrosion performance. Additionally, the coating thickness is uniform throughout the outside and inside of the component. This uniformity allows design engineers to reconsider the need for extra metal structure thickness in areas where conventional coatings will not penetrate.

Since the autodeposition process deposits a coating only on metallic surfaces, the coating of metal in a metal-plastic or metal-rubber assembly is possible. This capability allows manufacturers to reduce capital expenditures and paint shop complexities. Autodeposition coatings are used on automotive seat tracks, drawer slides and some applications normally served by electroplating because of the high hardness and wear resistance of the coating.

The advanced auto-deposition coating is also an organic coating technique that decreases energy consumption because no electricity is needed to drive the deposition of the coating. Beyond saving time and energy, other environmentally sustainable initiatives are used in the chemistry of the coating process. Because the water-based coatings have low or zero volatile organic compounds (VOCs), they meet and, in some cases, exceed current clean air legislation. The coatings also contain no heavy-metal anticorrosive pigment, and they do not require toxic heavy-metal based treatments.

Continued advances

Many industrial and automotive component manufacturers are investing in lean or modular manufacturing systems. The key to all of these systems is the need to make and paint more part variations (large, small, with thick or thin metal masses) to reduce in-process inventories and respond more quickly to changes in market demands for different models or components. Advanced autodeposition coatings allow for the opportunity to coat many or a few parts in a single coating bath without requiring adjustments to process conditions or equipment. The coatings also offer the opportunity to design combined primer-topcoat systems that reduce the effective paint line footprint by up to 50% compared to conventional coating systems.

Topcoat-primer auto-deposition coating systems have been designed and launched with only one curing oven needed in what is commonly known as a “co-cure” process. This concept reduces process steps and equipment length to give coaters and manufacturers the option to source higher-performance, more affordable topcoat-primer painting operations in areas that previously would allow only lower-performance powder coating systems.

Another advance is a new gray color epoxy-acrylic urethane auto-deposition coating that provides all the performance advantages of previous auto-deposited coatings with a lighter color basecoat for topcoating with powder and liquid paints. Applications include any ferrou smetal part or structure requiring superior cyclic corrosion protection, thermal stability and good topcoat characteristics.

The ability of the coating to provide a lighter color primer base and precise film thickness provides the opportunity for lower paint consumption and better topcoat control.

Yet another new development is a coating designed to enhance corrosion performance for demanding applications such as vehicle frames and chassis components. This new coating has achieved 1,000 hours of performance during neutral salt spray testing and has performed similar to cathodic e-coat and zinc phosphate pretreatment on automotive manufacturer cyclic corrosion tests. The epoxy-acrylic urethane coating has excellent thermal stability, topcoat-ability and flexibility. Its small environmental footprint is further reduced with even lower VOCs and the elimination of hazardous air pollutants.

These and other advances are allowing autodeposition coating processes to reduce environmental impact, improve the functional performance of the part and lower total manufacturing costs in automotive paint shops and other applications. The technology already is being used in automotive manufacturing facilities to displace conventional metal pretreatment and electrocoat processes for coating entire vehicle frames. Global research in the automotive industry and at universities is expanding the opportunities for autodeposition coatings even further. Improvements under investigation include reducing cure time/temperature through polymer research, enabling multimetal substrate coating applications, and enhancing surface appearance on all substrates.

Chemical concepts to back up Auto-Deposition

It is known that when an article of steel is immersed in an aqueous dispersion of a film- forming polymer, the thickness of the resulting coating depends on such factors as total solids, specific gravity and viscosity. Time of immersion is not a determinative factor. For a given aqueous dispersion, the thickness obtained after ten minutes immersion is not appreciably different from the thickness obtained after five seconds immersion. Further, when the article is withdrawn from the aqueous polymeric dispersion, it cannot be rinsed without removing virtually all of the polymer from the surface, thus demonstrating a lack of adherence to the substrate. Another shortcoming of this method of applying a coating is that when the article is withdrawn from the coating bath, little or no coating is formed on the edges of the article.

Objectives behind invention

  • It is the prime object of this invention to produce polymeric coatings on metallic substrates of controllable thickness or coating weight, the coating weight being a function of coating time.

  • To provide a method and composition for applying coatings to metallic surfaces from aqueous polymeric dispersions, which coatings can be made appreciably thicker than those obtainable heretofore by single stage operations.

  • The provision of a method and composition for applying polymeric coatings to metallic surfaces which render it unnecessary to resort to multiple stage coating operations to attain a coating of the desired weight and properties.

  • To form coatings on metallic surfaces from aqueous polymeric dispersions, which coatings display appreciably improved corrosion resistance and adhesion properties.

  • Production of coatings from aqueous polymeric dispersions on metallic surfaces which coatings are initially adherent, and thus capable of being rinsed before baking or drying without removing more than a superficial layer of the coating, to thereby provide increased flexibility in processing and handling.

  • To provide a resinous coating composition and method for applying a coating to the edges of a metallic surface.

  • To provide metallic surfaces with coatings which are continuous and free of pinholes and holidays when the coating is applied by immersing the surface in an aqueous dispersion of resin.

It has been found that objects set forth above can be realized by immersing or dipping a metallic surface in an acidic aqueous composition comprising water, hydrogen ion, fluoride ion, an oxidizing agent selected from the class consisting of hydrogen peroxide and dichromate and particles of resin dispersed in the composition, and withdrawing the surface from the composition. A preferred composition for use in the process of this invention comprises the aforementioned ingredients wherein the source of the resin dispersion is a latex of the resin, wherein the hydrogen and fluoride ions are added to the composition in the form of hydrofluoric acid, and wherein the pH of the composition is within the range of about 1.6 to about 3.8.

In formulating the composition for the coating fulfilling the above said objectives it is preferred that the non-resinous ingredients of the composition be added to a latex, that is a dispersion of insoluble resin particles in water. Latices, which are the source of the dispersed resin particles, are readily available and those sold commercially can of course be utilized. In addition to having dispersed therein resin solids, lattices usually contain other ingredients including for example emulsifiers and protective colloids. The other ingredients used in the composition of this invention are preferably added to the latices in solution form, such as for example a 70% solution of hydrofluoric acid, a 30% solution of hydrogen peroxide or a solution of a water soluble chromate or dichromate compound. Upon addition of these other ingredients to a latex, there is obtained a composition which can be characterized as an acidic aqueous solution of fluoride and oxidizing agent having dispersed there in resin particles.

Lattice systems that can be incorporated:

  • Styrene- butadiene copolymer

  • Acrylic co polymer

  • Polyvinyl chloride

  • Polyethylene

  • Terfluoroethylene

  • Epoxy based lattices

Autodeposition process flow

The process consists of four basic steps:

  • Cleaning of metal

  • Coating application

  • Rinsing off the foreign unwanted contaminants

  • Drying

The coating step is the coating bath itself, where a dispersion of a paint emulsion at low solids (usually around 4-6 % w/w) is made. A starter solution of acidified ferric (Fe3+) fluoride is added to the bath. The coating emulsion is stable in the presence of ferric ions, but unstable in the presence of ferrous ions (Fe2+). Therefore, if ferrous ions are produced at the surface, local paint deposition will occur on the metal surface, which is then baked to produce a coating.

If you immerse a component made from ferrous metal (shown as the grey oblong above) into the bath, there is surface attack from both the ferric fluoride and the acid. This results in the liberation of ferrous ions that will destabilize the emulsion locally at the metal surface, causing the paint layer to be deposited.

This figure illustrates iron trapped in the deposited paint film so that the deposited coating is made up of latex (emulsion), pigment and iron. This unique combination is the reason autodeposition coatings provide an exceptionally hard, yet flexible, coating. The deposited iron gives a pencil hardness of 5-6H, yet the coating will not crack even when folded and crimped to 0-T bend.

Auto-Deposition Connotes a process for depositing a paint film on metallic surfaces by means of a chemical reaction between the metal and the components of the paint bath. Although this technology has been employed commercially since the mid 1970's, the advent of new resin chemistry a decade later led to significantly improved performance. Additionally, the increased environmental constraints placed on the paint industry make auto-deposition an appealing finishing process. These factors have resulted in a large expansion of the number of installed auto-deposition lines, as well as the markets where this technology is used.

Deposition mechanism

The coating mechanism for the auto-deposition process is the chemical reaction between the metallic surface and the inorganic constituents of the coating bath. This process can be contrasted to the electrodeposition of paint, where the coating mechanism involves the electrolysis of water due to an applied external current imposed on the work. The mechanism of the auto-deposition reaction involves the controlled destabilization of an aqueous polymer latex dispersion, which is negatively charged, by the positive ions generated at the surface of the metal by a chemical reaction.

The cleaning stages required before the coating bath are the same as encountered in a phosphating line. They typically include a spray rinse stage and an immersion cleaning stage. After cleaning, the parts are rinsed well, including a deionized water rinsing stage, and immersed in the auto-deposition coating stage. The components of an auto-deposition coating bath include a weak acid (hydrofluoric acid, HF), an anionically stabilized latex and pigment dispersion (latex/pigment), and chelated ferric ion in solution (FeF3). The total solids content of the bath is less than 10%, and the coating solution has the viscosity of water. The chemical reactions that result in an auto-deposition coating can be shown as:

Iron Dissolution:

Major contributor

Minor Contributor

Deposition:

Fe2++(latex/pigment) Fe (latex/pigment)

All components can be simply analyzed and controlled in the paint shop and adjusted to the recommended levels. As the film builds, the diffusion of reactants to the surface is slowed, and the rate of film deposition decreases. This self limiting mechanism results in a final coating that is extremely uniform and conformant to the underlaying surface. All areas exposed to the coating bath become coated. This feature of the auto-deposition process is important since even enclosed areas will be protected against corrosion, as long as the solution has wet the surface. Typical coating thicknesses are about 15 – 30 pm (0.6 - 1.2 mils).

There are, currently, two auto-deposited coating polymer bases in commercial use. Auto-deposition I is an acrylic based system which offers very good corrosion performance, hardness, solvent resistance, heat stability and UV resistance. The Auto-deposition II system is based on PVDC co-polymers and offers excellent corrosion performance, low cure temperature (100 °C), hardness and flexibility, solvent resistance and the environmental advantages of no VOC or heavy metals.

The wet film that emerges from the coating solution has a cohesive strength sufficient to withstand the subsequent immersion rinse stages. The rinsing includes a water rinsing stage to remove supernatant coating bath and a chemical seal stage that contributes to the overall performance of the coating. After rinsing, the coatings are then cured in an oven. The Auto-deposition I coatings are cured in a two stage oven where the first stage is a drying stage set at 120°C (250°F) and the second stage is set at 175°C (350°F). The Auto-deposition II coatings are cured in a single-stage oven set at 105°C (220°F).

Total curing time is approximately 20 to 30 minutes. It is possible to use different curing schemes, e. g. there are currently commercial lines where infrared curing is used resulting in a total curing time of about 5 minutes.

Auto-deposition sequence

  • Alkaline Clean (heated)

  • Water Rinse

  • Auto-deposition (room temperature)

  • Water rinse (room temperature)

  • Sealing rinse

  • Cure

While the cleaning considerations for auto-deposition are similar to those for phosphate and conventional coating, the remainder of the coating process is very different and much simpler. After cleaning and rinsing, the work is treated in the acidic auto-deposition bath.

An auto-deposition bath contains:

  • A latex polymer (usually pigmented) dispersed in water

  • A mineral acid (0.2 to 0.3 weight percent hydrofluoric acid)

  • An iron salt (to aid in acceleration of the coating process)

When a metal object is immersed in the coating bath, a mild dissolution of the surface occurs. This results in the formation of positively charged metal ions at the surface which destabilize the negatively charged latex on the surface producing deposition.

The latest generation polymer Currently employed for commercial auto-deposition is a high molecular-weight polyvinylidene chloride material which provides superior tensile strength, impact strength and corrosion resistance. The viscosity of a coating bath made up with this material is low (< 5 cps) which enables application by simple immersion (conveyor line or indexing hoist) and by flow coat. Control of the bath is accomplished simply by maintenance of the polymer solids, acid level (via conductivity) and a redox potential measurement, which specifies the relative amount of iron in the system.

This series of polymers employed in auto-deposition require zero organic solvents (or other volatile additives); hence, the atmospheric emissions (other than water) are essentially zero. Although bath solids can be varied over a wide range, current commercial operations range from 3 to 8 percent non-volatiles.

This, again, represents a dramatic departure from typical paint applications where solids levels of 20 to 60 percent are common. It is preferable to use deionized water to make up an auto-deposition bath as well as to replace water lost by drag-out and by evaporation. Although there is no immediate harm in using local or "plant water, constant evaporation of losses with water containing soluble salts will result in a build-up of those salts far in excess of those in the feed water supply. Increasingly higher conductivities can interfere with the auto-deposition mechanism due to the presence of soluble ions which "protect" the latex polymer from reaction with metal cations.

The auto-deposition reaction is quite mild. It is observed that even when a bath is being used to process metal at a high rate, there is no measurable temperature increase due to chemical exothermic reactions. However, bath temperature increase resulting from heated parts entering the bath as well as ambient temperature can produce irregularities in the auto-deposited paint film. Bath temperatures in excess of 80°F can result in the formation of small voids in the coating as a result of increased chemical reaction. Conversely, temperatures below 60°F will produce a lower film growth rate than that produced at the recommended operating range of 68 to 72°F. Heat exchanger coils should be used during steady operation to maintain close coating control but, unlike other coating systems which consume energy, there is no need to dissipate heat generated by the coating mechanism.

Where it wets- it coats

Few paint systems can be successfully applied to recessed areas of complicated parts. Conventional waterborne and solvent borne immersion paint systems will wet most areas but suffer from the problem of solvent wash (i.e., during the curing of these materials, the water and solvent volatilized from the coating refluxes within any recessed section and washes most of the coating away). Electropaint will resist wash during the solvent evaporation step; however, electropaint cannot apply coating very far into an enclosed space due to electrical "caging" effects.

Attempts to coat deeply recessed areas by increasing the applied voltage could lead to rupture of previously deposited exterior coatings.

Since auto-deposition depends upon chemical rather than electrical energy, the bath will coat any area it can wet. For example, one can coat the interior of narrow tubular-style workpieces of extended length at a coating thickness nearly equal to that on the exterior surface.

Final sealing rinse

When a coated surface is removed from an autodeposition bath and water rinsed, the coating is still permeable (i.e., the workpiece can be returned to the bath and coating growth will resume). This is believed due to the presence of iron (presumably as FeF,) which is not bound to the negatively-charged latex particles. Films are processed through a final rinse tank containing a neutral salt which is capable of precipitating the iron and sealing the film. This greatly enhances the adhesion and corrosion resistance of the surface after cure.

Curing

The cure schedule for 800-Series coatings is of low temperature and duration relative to conventional industrial finishes. Typical production convection oven parameters vary from 20 to 40 minutes at 200 to 230°F. Medium-intensity infrared ovens have also been commercially employed. Since the mechanism of film coalescence is simply water removal, these units are effective in rapidly heating the wet film and forcing evaporation. Commercial units vary in exposure times of 4 to 8 minutes and dramatically reduce line space. It is also possible, with very thick work pieces (all sections 114 inch or greater) to utilize a technique known as "hot-water cure (HWC)" wherein the part is immersed in an aqueous solution at 190 to 210°F for 90 to 120 seconds and then withdrawn. The term “WWC" is misleading in that the immersion serves only to quickly heat the piece.

Coating properties of typical Auto-deposition systems

A greatly reduced frequency of rack stripping is a bonus for users of auto-deposition. In the case of racks constructed of "inert" materials, such as plastic (which will not react with the coating bath), the small amount of coating solution dragged out by surface tension is rinsed away by the first plant water stage.

Steel racks will be coated (like the parts) but, after curing, the coating becomes resistant to the hot alkaline cleaners as well as the coating bath.

Since the rack is no longer susceptible to chemical attack, combined with the hydrophobic nature of the auto-deposition coating, any clinging bath composition (which has a viscosity similar to water) drains rapidly back into the coating tank when the rack is raised. Any remaining material has a low solids content (i-e., 4 to 7%) and thus contributes very little to a loss of solids from the system. The racks show no measurable build-up of coating thickness, and most commercial auto-deposition operations clean the racks at a rate of about once every three years. This results in savings of energy, chemicals, labor, and waste disposal costs usually involved in such an operation.

Equipment considerations

Mild steel is the normal material of construction for all tanks, piping, and related accessories for any auto-deposition line. The coating tank is usually lined with any elastomeric material having resistance to mild acid as well as mechanical strength to prevent accidental damage from displaced parts or racks. It is also recommended that any rinse stages made up with deionized water be lined or constructed of corrosion-resistent material (e.g., plastics or stainless steel).

Coating bath control parameters

  • Total solid

  • Acid level

  • Ratio of Fe3+ to Fe2+ in bath

Formulating principles

The typical manufacturing process for the auto-deposition paints involves preparation of a mini emulsion of epoxy acrylic system mainly out of emulsion polymer mechanisms. Here as in our project we will try to cover each and every aspect of manufacturing process:

Introduction

Hybrid mesoscale materials have become one of the most favored research topics lately because they offer novel properties or properties combinations. Especially inorganic–organic hybrid nanoparticles and defined pigment-polymer aggregates have gained increased interest due to influential new applications in the fields of optics, electronics, medicine, cosmetics and last but not least in functional coatings. Especially for the latter application, water-based dispersions are preferred or required due to safety, health and environmental (SHE) regulations. In fact the concept of inorganic–organic hybrid particles can be used for advanced water-based dispersions which allow the substitution of traditional solvent-based coating applications without losing performance, e.g. in terms of chemical or mechanical resistance, and is therefore expected to gain further importance and industrial relevance. For this purpose the inorganic components typically have to be homogeneously distributed throughout the coatings film, which is almost impossible to achieve following just mechanical pathways like blending, milling, or mixing. More successfully in this respect are chemical or a physical-chemical synthetic approaches with sol-gel chemistry being the most popular one.

For example PPG Industries and BASF have successfully implemented commercial products with increased scratch resistance based on silica-polymer hybrid materials. Many other examples following the sol-gel approach can be found from different companies and for various applications. Another successful, but different physical chemical concept towards water-based inorganic-polymer coatings, where an increased attractive interaction between latex particles from emulsion polymerization and preformed inorganic pigments (usually in the micrometer regime) is utilized to obtain functional coatings with high homogeneity and to avoid pigment aggregation. In general, synthesis through emulsification is among the most important synthesis routes for water-based hybrid materials. In this relationship the miniemulsion process gained significant importance in the past decade. In a miniemulsion system the individual droplets can be considered as so-called “nanoreactors”. The nanoreactor concept allows new reactions and material combinations in quasi-aqueous media which cannot be achieved by traditional methods. Typical of emulsion systems, miniemulsions are made up of a water continuous phase and oil dispersed phase (oil-in-water, o/w, direct miniemulsions) or vice versa (water-in-oil, w/o, inverse miniemulsions). For the formation of miniemulsions, a high shear force is required. On a laboratory scale, ultrasonication is used most often, while on industrial scale, high pressure homogenizers are applied. As a result of the high shear, fission and fusion of the droplets occur till a steady state is reached. The special features of miniemulsions lie in the mode used to stabilize the droplets against collision, as well as to suppress Ostwald ripening of the droplets. The amount of surfactant used for stabilizing miniemulsions against collision is efficiently used resulting in an incomplete coverage of the droplets and are thus dubbed critically stabilized systems.

Moreover, in a miniemulsion system, no free micelles exist. To synthesize hybrid inorganic-organic particles in miniemulsion, there are mainly two paths, with respect to the inorganic material, that can be carried out: either to use preformed inorganic materials or to synthesize them in-situ. In contrast to the standard macroemulsion polymerization preformed, inorganic pigments can become encapsulated inside of individual miniemulsion droplets provided they are small enough in size. For this purpose a compatibilization step can be required between the inorganic and the organic constituents of the emulsion. This can be done via hydrophobization, treatment with surfactant, creation of an electrostatic attraction between both components, or even through hydrogen bonding interactions. If the compatibilizer is not to take part in the polymerization, the disadvantage of this system, besides having to find a suitable compatibilization method, is that with higher loadings of the inorganic material, problems with homogeneity can arise.

Alternatively, inorganic materials can be synthesized in-situ, mainly via sol-gel process. However, the focus of our project report is based on a versatile in-situ pigment preparation process which was extended to prepare water-based inorganic-polymer hybrid particles. At first the co-homogenization of inverse precursor miniemulsions is used for the preparation of inorganic pigments within a polymerizable continuous phase. Afterwards the system is converted to a direct miniemulsion by addition of an aqueous surfactant solution followed by homogenization. Subsequent polymerization of the polymerizable phase, which is now in the dispersed droplet state and still contains the inorganic material from the previous in-situ synthesis, leads to the desired water-based inorganic-polymer hybrid material dispersion. Functional coatings were the focus of the application of this system. For this reason, three pigments were synthesized via this process: zinc phosphate, calcium carbonate, and barium sulfate, all of which are widely used in coatings. As for the polymer matrix, a rather complex epoxy-acrylic-styrene polymer composition was chosen as the constituting system.

In present scene iron corrosion plays a vital role in designing a coating system. Corrosion has a great impact on our history, economy, and our everyday life. In history, for example, the downfall of the Roman Empire was said to be partially due to corrosion reasons. In modern history there are also catastrophic events that have occurred due to corrosion reasons such as the downfall of the Silver Bridge in Ohio in 1967. On the economy, the impact, thus, cannot be undervalued. On the other hand, the attention given to corrosion in the automotive industry is justified: cars are required to perform well under different environments. In the same location, there is minimal temperature difference of 20 °C between the different seasons. That is besides extreme temperature regions, such as near-polar regions, desert areas, or humid areas and coastal locations

Car owners' expectations have also increased, which pressures the automotive manufacturers to meet their expectations technologically and financially. It is said, for example, that corrosion protection and durability of a car's color and gloss have doubled in comparison to how it was 25 years ago. Clearly any possibility to cut costs while keeping the system environmentally friendly, with no toxic components as well as reduced or no VOC (volatile organic compounds) components, as well as superior performance, is highly valued.

Radical polymerization

In 1929, Carothers had classified polymerization reactions to addition and condensation reactions. Alternatively, another classification divides the reaction to chain and step growth polymerizations. Chain growth polymerizations are characterized by high degree of polymerization already at an early stage of the reaction. This polymerization proceeds through reactive centers, which further categorize the polymerization reactions into: radical, cationic, anionic and coordination. Radical polymerization was used in this work and would thus be described here, especially in heterophase, emulsion systems. Radical initiators are used to generate radicals, either thermally, via exposure to light or high energy or via redox reactions. Polymerization reactions then proceed through the well established initiation, propagation and termination steps.

Formulation basics

Primarily for auto-deposition systems we incorporate two types of coating systems which can be differentiated on the basis of pH of the coating system; among the two one is Acidic in nature while the other is slightly alkaline in nature.

Acidic Compositions (Aqueous):

It is an aqueous auto-deposition coating composition in the form of an acidic coating composition (pH approximately 1.6 to 5) that contains a water dispersible or water-soluble organic film-forming resin fluoride ions or fluoride ions and complex fluoride ions; one or more ions selected from ions of the following metals: zinc, cobalt, manganese, nickel, iron, and aluminum; and tungstate ion and/or rnolybdate ion. The aqueous auto-deposition coating composition is capable of forming a highly corrosion-resistant, strongly adherent resin film on metal surfaces when brought into contact with the surface of a metal, for example, a ferriferous metal, zinciferous metal, aluminiferous metal, magnesium-based metal, and the like.

These aqueous autodeposition coating composition has a pH of about 1.6 to about 5 and contains

  • Water-dispersible or water-soluble organic film-forming resin;

  • At least one of fluoride ions and flouride ions and complex fluoride ions;

  • Ions of at least one metal selected from zinc, cobalt. manganese, nickel, iron, and aluminum; and

  • Tungstate ion and/or molybdate ion.

Organic film forming material as mentioned above could comprise of urethane resins, epoxy resins, polyester resins, and polymer resins composed of one or more monomers selected from methyl acrylate, ethyl acrylate, n-butyl acrylate,2-hydroxyethyl acrylate, 2- hydroxypropyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethylmethacrylate, n- butyl methacrylate, 2-hydroxyethylmethacrylate, 2-hydroxypropyl methacrylate, glycidylacrylate, glycidyl methacrylate, acrylamide, methacrylamide, acrylonitrile, ethylene, styrene, vinylchloride, vinylidene chloride, vinyl acetate, acrylic acid, methacrylic acid, and the like.

The organic film-forming resin useful in the practice may be anionic, cationic, nonionic, or amphoteric and is not specifically restricted in this regard. The content of resin solids in the coating composition preferably falls within the range of 5 to 550 g/L and more preferably falls within the range of 50 to 100 g/L.

Sources for fluoride ions and complex fluoride ions may be among zirconium hydrogen fluoride, titanium hydrogen fluoride, silicon hydrogen fluoride, boron hydrogen fluoride, hydrofluoric acid, and the ammonium, lithium, sodium, and potassium salts of the preceding acids. The content of fluoride or fluoride ion and complex fluoride ion in the coating composition preferably falls within the range of 0.1 to 5 g/L as fluorine and more preferably falls within the range of 0.5 to 3 g/L as fluorine.

The pH of the coating composition should be maintained within the range of about 1.6 to about 5. Formation of the resin film becomes problematic when the pH is substantially outside this range. The pH of the coating composition may be regulated using one or more acids selected from inorganic acids such as the acids listed above as sources of fluoride and complex fluoride ion and their salts, as well as nitric acid, phosphoric acid, and boric acid; and organic acids selected from phytic acid and tannic acid. The addition of nitric acid, phosphoric acid, boric acid, phytic acid, or tannic acid has the effect of improving film adherence to the substrate.

Alkaline compositions:

The aqueous composition for coating the metal can contain a polymer dispersion. For example, one feature of an auto-depositable coating can be that the dispersed material is stabilized by functional groups on the polymer and/ or provided by surface active agents Which are sensitive to multivalent ions entering the aqueous phase. Deposition can occur by interaction between the multivalent ions and the functional groups, causing the dispersion to precipitate on the surface . When sufficient concentration of multivalent ions occurs at the metal surface. The multivalent ions can also crosslink the dispersion particles via reaction With particle surface carboxyl groups and/ or With other functional surface groups and with the metal substrate.

The alkaline coating compositions includes a latex and an amount of base sufficient to raise the pH of the coating composition to an alkaline pH. The process includes immersing at least a portion of the metal substrate surface in the coating composition, where the coating auto-deposits on the metal substrate surface, as metal ions from the metal substrate surface react with and destabilize the alkaline coating composition. In some embodiments, the deposition of the latex can continue until the coating has a thickness of at least about 1A inches (0.635 centimeters).

The pH can be in a range of about 7.1 to about 12, preferably about 9.5 to about 11.5.

The auto-deposition rate can be dependent on the pH of the coating composition. Also, the base used to raise the pH of the coating composition can be selected from a group including ammonia, sodium hydroxide, potassium hydroxide, barium hydroxide, cesium hydroxide, calcium hydroxide, lithium hydroxide, tetramethyl ammonium hydroxide, tetraethylammonium hydroxide, an amine, and any mixture thereof.

Copolymers for preparation of latex can include 2-ethylhexyl acrylate, octylacrylate and isooctyl acrylate, n-decyl acrylate, isodecyl acrylate, tert-butyl acrylate, methyl methacrylate, butyl methacrylate, hexyl methacrylate, isobutyl methacrylate, isopropyl methacrylate as Well as 2-hydroxyethyl acrylate and acrylamide. The preferred (meth)acrylates are methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, isooctyl acrylate, methyl methacrylate, and butyl methacrylate.

Other suitable monomers include Lower alkyl acrylates and methacrylates including acrylic and methacrylic ester monomers: methyl acrylate, ethyl acrylate, n-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, isobomyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl meth acrylate, isobutyl methacrylate, sec-butyl methacrylate, cyclohexyl methacrylate, isodecyl methacrylate, isobornyl methacrylate, t-butylaminoethyl methacrylate, stearyl methacrylate, glycidyl methacrylate, dicyclopentenyl methacrylate, and phenyl methacrylate.

These alkaline compositions are most suitable for the non steel surfaces Typical Compositions

Auto-deposition composition and process with acrylic terpolymer coating resin An auto-depositing aqueous coating composition has a pH of 1.6 to 5.0 and comprises resin solids, acid, oxidising agent, water, anionic surfactant either as a free component or as a copolymerized component in the said resin, and other optional components, with the resin solids being dispersed in the water and being polymer resin solids principally composed of specified proportions of the following monomers: (A) methyl methacrylate and/or acrylonitrile, (B) ethyl acrylate and/or butyl acrylate, and (C) acrylic acid and/or methacrylic acid and with the anionic surfactant being present in a specified proportion based on the total amount of the said components (A), (B), and (C). Such an auto-depositing composition produces on active metal surfaces contacted with it a high corrosion-resistant coating, even without any post-treatment with an aqueous hexavalent chromium solution.

This composition relates to an acrylic resin-based auto-depositing aqueous coating composition that upon contact with a metal surface forms a resin coating thereon. Auto-depositing aqueous coating compositions, which are acidic compositions that contain an organic film-forming resin, are able to form a resin coating on metal surfaces when brought into contact with such surfaces. A characteristic feature of auto-depositing aqueous coating compositions is their ability to form resin films whose thickness or weight increases with immersion time when a clean metal surface is immersed in the coating composition. Moreover, film formation is itself achieved through the chemical activity of the coating composition overlying the metal surface (metal ions are eluted from the metal surface by etching and act on the resin particles, leading to the deposition of both resin and metal ions on the surface). As a result of this chemically-induced deposition, a resin coating can be efficiently formed on the metal surface without utilizing an external source of electromotive force, as is used in electro- deposition.

The post-treatment is run after immersion of the substrate in the coating composition and before curing of the resin film (i.e., before the initially wet resin film formed on the substrate while it is in contact with the coating com- position is caused to dry, whether by simple exposure to air or by heating) and consists of a rinse with an aqueous solution containing a hexavalent chromium- containing compound (hereinafter called an "aqueous hexavalent chromium solution"). However, environmental considerations make it undesirable to use aqueous hexavalent chromium solutions, and in the last several years this has led to increased demands to avoid the use of hexavalent chromium compounds during the coating of metals and other substrates.

At the same time, auto-depositing aqueous coating compositions based on conventional acrylic resin components suffer from a major drawback: in the absence of post-treatment with an aqueous hexavalent chromium solution, these compositions are unable to generate resin coatings that can fully satisfy various property requirements, particularly as concerns corrosion resistance.

The disperse resin comprising the organic coating-forming component in this composition is a polymer comprising:

  • 2 to 10 weight % of carboxyl-containing ethylenically unsaturated monomer, for example, acrylic acid monomer, and

  • 90 to 98 weight % of ethylenically unsaturated monomer other than (1), for example, acrylate ester monomer.

This composition also contains nonionic surfactant in an amount not exceeding 12 weight % of the weight of the resin. Even with the use of this composition, however, it has been desirable to implement a post-treatment with an aqueous hexavalent chromium solution in order to form a coating with an acceptable corrosion resistance in severely corrosive environments.

It has been discovered that a highly corrosion-resistant resin coating is produced by an auto-depositing aqueous coating composition that contains resin solids that are polymers of at least three particular monomer types and that are dispersed and stabilized by amounts of anionic surfactant within a specified range.

More specifically, in one of its embodiments the present composition is an autodepositing aqueous coating composition having a pH of 1.6 to 5.0 and comprising, preferably consisting essentially of, or more preferably consisting of, resin solids, acid, oxidizing agent, water, and anionic surfactant either as a separate component (called an '"external surfactant") and/or as a copolymerized component (called an "internal surfactant") in the said resin, and, optionally, one or more of nonionic surfactant molecules, metal ions, and film-forming auxiliary molecules, wherein the said resin solids are dispersed in the water and are polymers of a mixture of monomers that consists principally of the below-described components (A), (B), and (C), in each case expressed on the basis of the total amount of components (A), (B), and (C) .

  • 30 to 80 weight % of molecules selected from the group consisting of methyl methacrylate and acrylonitrile,

  • 10 to 60 weight % of molecules selected from the group consisting of ethyl acrylate and butyl acrylate, and

  • 2 to 10 weight % of molecules selected from the group consisting of acrylic acid and methacrylic acid, and wherein total anionic surfactant, present as a separate component and/or as a copolymerized component in the said resin is present in the autodeposition composition in an amount that is from 0.2 to 5.0 weight %, based on the total weight of the residues of above-noted components (A), (B), and (C) in the poly-meric resin solids present in the composition.

The term "resin solids" in the acrylic resin-based auto-depositing aqueous coating cpmposition according to the present composition denotes the resin solids in the bath of the polymer resin afforded by the copolymerization of components (A), (B), and (C). In the present description, the term "resin solids" does not refer simply to the total solids in this polymer resin bath, but rather denotes only the solids that constitute the resin itself. Accordingly, "resin solids" excludes such substances as polymerization initiator, pigment, filler, surfactant, and the like, and excludes even any anionic surfactant that is copolymerized into the resin itself.

The polymerization technique for preparing the subject polymer resin is not crucial as long as the composition meets the particular proportions specified for components (A), (B), and (C). However, emulsion polymerization is preferred. Also, there are no narrow restrictions on the polymerization conditions when the copolymer resin bath is prepared by emulsion polymerization, and conventional techniques can be used for this purpose. To provide a specific example, the copolymer resin bath can be prepared by subjecting a mixture of at least water, anionic surfactant, polymerization initiator, and components (A), (B), and (C) to a copolymerization reaction at one or more temperatures in the range form 20 °C to 90 °C for a time in the range from 1 to 10 hours. The polymerization initiator can be those polymerization initiators ordinarily employed for the polymerization reactions of acrylic monomers, for example, ammonium persulfate, potassium persulfate, tert-butyl hydroperoxide.

The anionic surfactant used in the acrylic resin-based auto-depositing aqueous coating composition according to the present composition is thought to support the maintenance of the mechanical stability of the dispersed resin. The following are well-suited for use as the anionic surfactant under consideration: o salts of higher fatty acids, e.g., sodium laurate, sodium oleate, and the like; salts of sulfate esters of higher alcohols, e.g., sodium lauryl sulfate, sodium oleyl sul- fate, and the like; and higher alkylarylsulfonates, e.g., sodium dodecylbenzene- sulfonate, sodium dodecyldiphenyl ether disulfonate.

Nonionic surfactant is an optional component for the acrylic resin-based auto-depositing aqueous coating composition according to the present composition. The nonionic surfactant is thought to stabilize the dispersed resin against salts through the formation of a hydration layer by oxyethylene moieties present in the surfactant molecules. Suitable nonionic surfactants are exemplified by polyoxy- ethylene.nonylphenyl ethers, polyoxyethylene lauryl ethers, polyoxyethylene stearyl ethers, polyoxyethylene oleyl ethers, polyethylene glycol monolaurates, polyethylene glycol monostearates, polyethylene glycol distearates, and the like. In those cases where nonionic surfactant is used, it is ordinarily added during the preparation of the dispersed resin, but may be added after preparation of the dispersed resin or during preparation of the coating composition. No narrow limitations apply to the acid and oxidizing agent required in a composition according to this composition, but the acid is preferably hydrofluoric acid and the oxidizing agent usually is preferably hydrogen peroxide. The acrylic resin-based auto-depositing aqueous coating composition according to the present composition may contain metal ions as provided by a metal ion compound, and in particular may contain ferric ions as provided by a ferric compound such as ferric fluoride, e.g. It is thought that metal ions — and particularly ferric ions — function to accelerate dissolution of the metal ions, particularly those of iron, from the metal surface by the acid, particularly when it is hydro-fluoric acid.

A film-forming aid for the purpose of promoting fusion or coalescence of the deposited resin particles by lowering the minimum film-forming temperature. The film-forming aid is exemplified by tri- alkylpentanediol isobutyrate, alkyl carbitols, and the like. The presence of a film- forming aid (also called "coalescing agent" in the art) in a composition according to this composition is generally preferred.

An acrylic resin-based auto-depositing aqueous coating composition according to the present composition may also contain other optional components as follows: plasticizer for imparting flexibility to the produced coating: conventional plasticizers such as, for example, dibutyl phthalate, can be used.

Pigments, for example, carbon black, phthalocyanine blue, phthalocyanine green, quinacridone red, Hansa yellow, and benzidine yellow.

The polymerization initiator used for resin preparation may be present in whatever form it occurs.

Synthesis of a water based titanium dioxide polymer particles via mini-emulsion

Introduction

The aim of this project was to synthesize also hybrid inorganic-organic particles, namely water based titanium dioxide-polymeric particles, via a single miniemulsion process. This miniemulsion should serve as a white pigment containing polymeric miniemulsion that would act directly as a coating, without any need for further pigmentation.

Designing the synthetic route

The idea was to synthesize titanium dioxide in-situ via the following process: a titanium dioxide precursor, miscible with the monomeric dispersed phase, was used. This titanium dioxide precursor should remain stable within the dispersed phase constituents as well as during mini-emulsion formation. If this condition was not met, titanium dioxide particles will form before the mini-emulsion droplet formation, i.e. before creation of our nanoreactors, disrupting as such the main advantages of having a mini emulsion with a possible min -emulsion polymerization. Once the direct mini-emulsion was formed containing the titanium dioxide precursor and vinylic monomers in its dispersed phase, either titanium dioxide formation will take place spontaneously with time via a sol-gel process due to excess water in the continuous phase, or can be induced via pH changes after polymerization.

Finding a stable precursor system

In order to dissolve the titanium dioxide precursor in the monomeric dispersed phase, the use of an organic precursor is inevitable, which must be stable, as previously mentioned, till the direct miniemulsion is formed. On a lab scale, a minimum of 15 min are sufficient, while on an industrial scale, 8 – 9 h of stability are necessary.

Since in our system, the emulsion environment will be acidic, so while choosing a precursor emphasis must be given to the stability of organic precursor molecule under acidic ambience.

Of all the tested precursors, isopropoxide precursor showed the highest stability till a pH < 1, for a time span of ~ 1 week, followed by ethoxide and butoxide where they both showed a stability only at a pH = 0.3 for a time span of ~ 5 and 3 days respectively. Only the 2-ethylhexyl oxide precursor did not show sufficient stability for a miniemulsion to be formed. The two factors which gretly influence the instability of precursor molecule under aqueous conditions are:

  • Branching of chain

  • The ease of removal of alkoxy group of the precursor molecule

When an alkoxide is mixed with an alcohol, an exchange reaction is expected between the alkoxy group and the alcohol group, given by:

This exchange reaction will be highly dependent on the electronic structure of both the alcohol being used and the alkoxy group, favoring the use of a high Lewis acidity of the titanium or the high Lewis basicity of the alcohol. Considering the Lewis acidity of the titanium metal in titanium (IV) 2-ethylhexyl oxide in this case would be the lowest in Lewis acidity, affecting the exchange reaction, while the ethoxide precursor would be the highest in terms of Lewis acidity (though lowest in terms of steric hindrance). In this case, it seems that both the extent of the Lewis acidity (electronic effects) and the steric hindrance effects given by the branching play a mutual role and give the optimum stability of the isopropoxide (in this case, highest at pH = 1).

Carboxylic acids as legands: Oleic acid

Several works have shown that the use of oleic acid, in specific or the carboxylic acids in general will significantly retard the reactivity of titanium alkoxy precursors, where the carboxylic acid would form a complex with the precursor, and therefore, less alkoxy groups would be exposed to the water phase for hydrolysis, affecting the usual vigorous hydrolysis and condensation.

It can be shown with the following reaction scheme:

Acrylic and methacrylic acid

Both, acrylic acid and methacrylic acid are bidentate ligands, coordinating with both oxygens in the carboxylic acid group. Each was added to titanium (IV) isopropoxide with a molar ratio of 1 : 10 and 1 : 31.5 titanium (IV) isopropoxide : acrylic acid or methacrylic acid and were tested for stability in excess water in the pH range between 1 and 7. Both showed the same results: instantaneous precipitate was formed upon addition of excess water. One possible explanation for this instability could be the miscibility of acrylic acid and methacrylic acid with water; it does not seem to give enough protection or enough hydrophobizing effect to the precursor in the presence of excess water, as was the case with the highly hydrophobic oleic acid.

General remark on the hydrolytic stability of alkoxide precursor molecule

The problem with hydrolytic stability of precursors has been carried out in a limited amount of water while in order to form a direct miniemulsion, excess water is present in our system as the continuous phase. Several works on sol-gel chemistry confirm that the amount of water available is influential.

Hoebbel et al. have expressed the amount of water used for hydrolysis in molar ratio to the – OR groups available in the alkoxide – “h = H2O:OR”. Similarly, Blanchard et al. have introduced the variation of amount of water in molar reference to titanium – “H = [H2O]/[Ti]”. In general, the hydrolytic stability decreases with the increase of either “h” or “H”, i.e. the water content.

Hydrolytic stability within vinylic monomer mixture

Oleic acid has shown so far the highest hydrolytic stability obtained, oleic acid was added to a mixture of three vinylic monomers that are mainly used in coatings application: butyl acrylate, methyl methacrylate and styrene, with compositions given in Table.

A molar ratio of 1 : 10 of titanium (IV) isopropoxide to oleic acid was used which showed a high hydrolytic stability as was the case with pure oleic acid. The molar ratio was then reduced to 1 : 4 and the same stability was also observed. Hence, with this formulation, a miniemulsion was ready to be carried out and eventually followed by polymerization.

Formation of miniemulsion using oleic acid along with vinylic monomer system

First results

Two miniemulsions were performed using the monomer / oleic acid mixture shown in Table, each with a different Ti to oleic acid molar ratio, namely 1 : 10 and 1 : 4. Even though the previous hydrolytic stability tests were performed using titanium (IV) isopropoxide, those miniemulsions were prepared with titanium (IV) 2-ethylhexyl oxide based on the results obtained from oleic acids miniemulsions, as above mentioned. Investigation of the miniemulsions with DLS measurements showed bimodal droplet size distributions for both samples. Since no significant difference was available, it was decided that polymerization would be pursued with samples using 1 : 4 Ti to oleic acid molar ratios to avoid an unnecessary presence of oleic acid. Polymerization was then carried out using KPS and AIBN. Only with the latter did polymerization proceed to completion. To check if TiO2 can be induced, a quick test was performed where the miniemulsion was treated with ethylene diamine, diethylamine or triethylamine. In all three cases, the miniemulsion phase separated while titanium dioxide precipitated. As a concept check, it was successful, but modification of the process was still required so that miniemulsion would remain stable.

Modification using increased Tio2 loading

Assuming a full condensation of the TiO2-precursor to TiO2, we would have only a TiO2 loading of 1.6 wt% in the miniemulsion, as above prepared. Therefore, one main target is to increase the loading of TiO2 to about 5 wt% of the miniemulsion. To do this, one should increase the amount of the TiO2-precursor in the formulation which consequently means increasing the oleic acid content in the formulation.

First changes were made to achieve this higher loading by changing the alkoxide precursor from titanium (IV) 2-ethylhexyl oxide to titanium (IV) n-butoxide. This was done for a couple of reasons: with an n-butoxide based precursor, we have lower weight in the dispersed phase to induce the same amount of TiO2; also when oleic acid caps the titanium (IV) n- butoxide, n-butanol is then released which has more water solubility and can thus be released from the droplet or even act as phase compatibilizer. The result of several trials showed that the lowest oleic acid content used to stabilize the higher loading was found to be a Ti to oleic acid molar ratio of 7 : 10. Table the modified monomer mixture with increased oleic acid content.

Another variable that was changed was the ALS-33 content. So far, in all experiments, ALS- 33 concentration of 2.5 wt% to dispersed phase has been used. The target was to reduce the ALS-33 concentration so that the mean droplet diameter would reach about 350 nm, so that titanium dioxide particles that would eventually be generated inside the particles would not be too small, i.e. so that white pigment properties would remain. As a result of several experiments, it was found that a mean droplet diameter of about 340 nm before polymerization was obtained with an ALS-33 amount of 0.25 wt% at a solid content of 20 wt%

Polymerization process

Initiators: oil soluble azo based thermal initiators (mixture of V70 and AIBN) with ten hour half life time of 30 °C and 65°C in toluene, respectively. The miniemulsion was first heated to 30 °C for about fourteen hours bringing the solid content to 13 wt%, followed by increasing the temperature to 65°C for six hours brought the polymerization to completion without any coagulates.

There were two main remarks at this point: 1) the color of the miniemulsion had changed to white (from pink) during the process, which was a first indication that the Ti-oleic acid complex has been disrupted. 2) DLS measurements, shown in Figure demonstrate the formation of bimodal particle size distribution but after polymerization with V-70 which persists with post polymerization using AIBN.

Drawbacks associated

There were two main drawbacks for this system, which will be stated hereunder. One is related to the miniemulsion properties, namely that the miniemulsion always ended up with a bimodal distribution after polymerization. Since the miniemulsions were prepared using n- hexadecane, Ostwald ripening is thus repressed. Also, since the miniemulsions were prepared with relatively hydrophobic monomers, no secondary nucleation is expected during polymerization. This was attributed to a budding effect where additional stabilization was created within the growing particles during polymerization as a result of formation of ionic polymers. Eventually, those fractions of the particles which are additionally stabilized split. Our hypothesis is that this same budding effect does take part as well, also due to the presence of additional stabilization that takes place via oleic acid. If oleic acid is slowly consumed within the mini-emulsion for the stabilization of new particles, this might also lead to a release of titanium species leading to hydrolysis and a subsequent TiO2 formation, which actually also fits to the change in color that occurred during polymerization. The second main drawback, the size of TiO2 particles and the film properties. TiO2 formed particles are about 10 nm or smaller. At this size, typically sizes from 2 – 50 nm of titanium dioxide particles, become transparent as visible light is in this case transmitted. Thus they are used mainly as UV blockers as they lose any white pigment properties. Another important property observed for this film was that it was formed at room temperature instead of usually at about 60°C. This indicated that the minimum film formation temperature was greatly reduced due to the presence of oleic acid, which has obviously acted in the system as a softener or plasticizer in certain cases.

Conclusion

The auto-deposition coating process is a simple and reliable industrial finishing system. It has a long and global record of providing uniform protective coatings to the automotive, metal furniture, agricultural equipment and appliance industries. Auto-phoretic coatings provide remarkably uniform coatings that provide unique advantages, including coating complex metal and non-metal assemblies and conducting post forming operations on coated parts. Auto-phoretic coatings are exceptionally hard yet flexible films that compete in functional performance with baking enamels, electro-plating, electro-coating and powder-coating technologies. Auto-phoretic coating products provide unique energy, environmental and worker-friendly performance.

References

  • Japanese Patent Publication Number Sho 47-17630 [17,630/ 1972],

  • Japanese Patent Publication Number Sho 48- 14412 [14,412] 1973].

  • Japanese Patent Publication Number Sho 52-21006 [21,006/1977]

  • N. M. Holyk, T.C. Jones, Proc. Automotive Corrosion and Prevention Conf., SAE, 1986, Paper 862020.

  • Organic coatings: Science and technology By Zeno W. Wicks, Jr., Frank N. Ones, S. Peter Pappas, Douglas A. Icks

  • C. G. Weller, B. D. Bammel, and W. E. Fristad, SAE Technical Paper Series, 2002-01-0270.

  • Patent US8871309

  • Patent US3592699

  • Patent US3709743

  • T.C. Jones, Finishing '91, SME Technical Paper, 1991, FC91-371.

  • O. E. Roberto and R. G. Hart, SAE Technical Paper Series, 1991, 192291.

  • T. C. Jones and W. E. Fristad, SAE Technical Paper Series, 1997, 970729.

Acknowledgement

Many thanks to PPG Asian Paints management team for supporting this review work , The author also thanks colleagues, family and friends for their support and guidance.