Titanium Dioxide Use in the Paint Industry

Titanium Dioxide in the Paint Industry Paints hold economic importance in the protection of materials, provide the final decorative appearance to products, and are applied in almost all sectors of modern industry and human activity. Paint coating refers to a material applied to an object's surface that forms a coating film with required properties. The main requirements for coating films are typically: protective properties (protection against decay and corrosion), special functional properties (electrical insulation, flame-retardant coatings) and color tones (aesthetic purpose, safety and warning colors). Paints, or coating compositions, consist of many components. The main component is the binder, which also determines the type of base/binder material used in the paint. Binders are most often used in solution or emulsion form in a solvent. Solvents consist of various organic substances or water to facilitate the incorporation of other components during manufacturing and subsequently ease paint application. Pigmented coatings, the second most important component, contain the actual pigment that gives the resulting paint film its color and opacity and increases the paint's resistance to the environment and weather conditions. In most pigmented coatings, compared to TiO2, generally very low opacity and coloring strength are observed, but fillers regulate other important parameters such as the paint's rheological properties. The use of fillers in particular significantly reduces the cost of finished paints. A coating system typically consists of several layers, each with its own function. Depending on the type of substrate, a primer or undercoat is selected. For porous materials such as wood, the purpose is to absorb and penetrate the substrate. For non-porous ones like metal surfaces, anti-corrosive coatings based on zinc phosphate are selected. As the next layer, a primer paint is typically applied. Like corrosion-preventive coatings, it is quite pigmented and can be applied by brushing or spraying. Its purpose is to prepare the coated object for the application of the top coat, that is, to level irregularities and ensure perfect adhesion between the two surfaces. This paint must have good flow and drying properties so that sanding/polishing can be performed before the final coat is applied. Finally, the final coat of the coating system is applied, which is a protective varnish. Due to the required flexibility of this coating, the final coat is significantly less pigmented than in some synthetic systems and has high gloss. Since it is frequently exposed to weather conditions, rutile TiO2, which has medium to high organic surface coating, can be used as the final coat. Compositions contain not only binder, solvent, fillers and TiO2 but also other necessary components such as driers, hardeners, plasticizers, etc., which are basic components of certain coating types. As a result, in most cases additional components are also added. These are anti-settling agents that extend shelf life, additives that improve leveling during application, matting agents, thixotropic agents as well as certain special additives such as biocides that protect the coating against fungal attacks. Paints can be classified according to various criteria: According to the function they play in the paint system, paints can be separated as follows: • Primers, • Undercoats, • Fillers, putties and mastics, • Top enamels (final coats). According to the binder basis on which paints can be made: • Natural origin (oil, varnish), • Synthetic resins (alkyd, epoxy, polyurethane, etc.). By drying method: • Physical, that is, by evaporation of solvents (nitrocellulose, alcohol), • Oxidative polymerization, that is, with atmospheric oxygen (oil, varnish, alkyd), • Chemical, that is, by reaction of two or more components (polyurethane, epoxy, polyester), • Stove drying, that is, by cross-linking at higher temperatures (alkyd/melamine, silicone, powder). Depending on the application to the substrate surface: • Brush, roller, • Spray (airless, electrostatic), • Special techniques (dipping, flow coating, roller coating). In everyday use, paints are generally separated according to: • Coated surfaces, e.g. wood, metal, plastic, silicate, • Products specifically designed for them (household appliances, cars, ships). Currently the most common classification divides paints into approximately two equal, large, basic groups: • Decorative paints, further divided by application and according to the amount of thinner used (solvent-based or water-based or by application location, indoor or outdoor), • Industrial coatings grouped according to typical applications - automotive, industrial, coil coating, powder coatings, etc.

Application Methods for Decorative and Industrial Coatings

The critical criterion for distinguishing between decorative paints and industrial paints is their application method. Paints are applied in layers that can be formed by the following techniques:

A) Painting with Brush or Roller

This method is of great importance during manual application of coatings such as windows, doors, etc. Coatings designed for manual work must spread easily and have good flow and rapid drying properties. The advantage of paints pigmented with titanium is the absence of hard sediments due to the low density of the pigment. Additionally, titanium dioxide initiates no chemical reaction with binders.

B) Dipping or Flow Coating

The method for producing coatings is primarily used for metal objects such as automobile and farm machinery bodies. Coatings designed for dipping or flow coating must have low viscosity.

C) Roller Application

Creating a paint film through rollers alongside the most modern paint application methods. A metal belt, which is said to pick up a paint layer, passes through a roller mat made of elastic, synthetic material. This cylinder picks up paint from a reservoir and transfers it to the belt. After picking up the layer, the metal belt passes through a drying and baking channel. The painted strips produced in this manner are typically used in packaging technology.

D) Spraying

Rapid progress in production techniques through spraying occurred only with the development of nitrocellulose lacquers and when problems began with the production of brilliant, multi-layer coatings using conventional techniques. Spraying can be done with air or airless. When spraying with air, a circular air jet from the compressor feeds the spray gun, which carries the paint with it. The principle of airless spraying is that the coating material is compressed under pressure in a thick-walled container to such an extent that after the gun is directed and released, sudden pressure drop and pressure cause the paint to atomize.

E) Electrophoretic Deposition Method

Electrophoretic production of paints belongs to the most modern paint application method. Electrophoresis describes the movement of charged particles in an electric field. During the electrophoretic coating process, finely binder-coated pigment particles form a dispersed phase and a water-soluble binder. Due to the negative charge of individual particles, particle coagulation does not occur. Under the influence of an electric field, binder-coated pigment particles are directed toward the object to be coated, which forms the anode. There they lose their charge, coagulate, and accumulate in the form of a water-insoluble film. The advantage of this application is the formation of a very compact film. Due to the relatively weak coating valves of compact coatings, the use of TiO2 is advantageous because of high capacity. Industrial paints and coating systems are typically applied in large, industrial paint shops associated with a production line (e.g., automobile or household appliance production) or with special techniques (e.g., airless, high-pressure, spraying of steel structures).

F) Typical Uses of Decorative Colors

Decorative paints, by contrast, are most often applied where they are used, to the surface of objects and other materials. This is particularly common in the construction sector, which is why decorative paints are also called construction paints, or architectural paints. Plasters, façade and decorative colors, door frames, roofs, gutters, fences belong here; industrially applied coatings are applied over radiators, furniture, refrigerators, etc. Common paint types (both decorative and industrial) are liquid (solvent water or organic substances) and are applied using conventional techniques (e.g., brush, roller, spray gun). However, there are some special paint groups that differ in their compositions or application methods, particularly industrial paints, for example: • Powder coatings - solvent-free, in fine powder form, applied with special guns in an electrostatic field and fired at temperatures between 130-200°C. • Automotive - various types of extremely complex paints; these include two very different basic groups for first-class production (OEM) and repairs (repair paints). • High-solids - contain only a small amount of solvent (or special solvent), mostly applied with a special high-pressure gun. • Coil coating - resistant to weather conditions, with high resistance to mechanical stress, applied to a metal sheet strip, baked after application. Only objects processed this way (mostly for the construction industry - environmental profile tiles or roof panels) are molded from metal plates.

General Principles Regarding the Use of Titanium Dioxide in Paint

Titanium dioxide is used in the paint industry to improve the application properties of coatings. TiO2 is added to paints because of its chemical inactivity, high covering power, coloring strength and optical properties, as mentioned in the introduction. Additionally, for rutile grades, high light fastness and weather resistance are required (inorganically, appropriately modified rutiles with coated crystal lattice surface show the highest resistance) and many other advantageous properties are needed. In short, it can be said that TiO2 is currently the best white pigment for the paint industry in almost every respect. It is not a simple matter to say that there are some general principles or to create a model regarding the use of TiO2 in paints. The main criterion in the use of the appropriate TiO2 type is also the question of in what environment the paint will be used, what protective or decorative purpose will be fulfilled, what its purpose is, and which paint system or respectively which paint system will be used. Application coating techniques will be used. Requirements related to TiO2 for base coats/primers and intermediate coats are not as strict and multifaceted as for final coats. Primary and underlying coatings serve to protect against corrosion, cover the substrate or isolate a base layer from the final coat. Generally, high weather resistance is not required, and therefore mostly cheaper TiO2 grades with satisfactory dispersibility and good optical properties are used. The requirement for final coats and consequently the TiO2 grade to be used is much higher and multifaceted. Final coats not only protect the substrate but also serve a decorative purpose. For exterior applications, resistance to weather conditions and resistance to the surrounding environment is required. (Factors such as chemical waste, salt water, heat stress, etc.) In interior final coats, if not serving any special purpose, rutile grades with low to medium surface coating, easily discardable in a particular environment, with high coloring strength and very good optical properties are used. For exterior and special applications, considering high resistance requirements against weather and environment, medium to high level organic, surface-coated, rutile titanium dioxides are used. In addition to these general requirements, low binder demand is also important. This allows for high volume. The concentration in dispersing equipment during dispersion, that is, the dispersion of large quantities of pigment in a single operation, also serves to reduce operating costs. Important requirements for dispersion and particularly electrophoretically applied baking paints are minimum soluble salt content and neutral pH of aqueous extract. Other requirements are low moisture content of the grades used, and that surface coating components (particularly organic) do not react with certain additives in the paint formulation. Due to their exceptional optical performance, excellent dispersibility and excellent weather resistance, inorganically surface-coated and aluminum-modified Pretiox RGU titanium dioxide has extensive application in various decorative and industrial paint types. Pretiox R 200 M is a special grade - an inorganically uncoated rutile grade with special organic surface coating suitable for use in different types of epoxy paints. PRETIOX R 200 M was developed as a result of our own research based on the requirements of our customers. Special organic purification and production technology reduces pigment reagglomeration and increases dispersibility in epoxy paints of this class.

Production of Pigmented Paints

A brief overview of paint production: Preparation of raw materials and intermediate products consists of the following stages: • Preparation (weighing) of the paint mixture and pre-mixing, • Adequate dispersion on some suitable equipment, • Final stages (e.g., dilution to the required dry matter content, viscosity adjustment, addition of additives, adjustment of color to the desired shade), • Product finishing (control filtration, individual and batch packaging).

Stability of Pigmented Coating Compositions During Storage

TiO2-pigmented paints are characterized by the possibility of long-term use because the resting of material does not lead to the formation of a solid precipitate. TiO2 is completely unreactive to most chemicals, including binders, and therefore cannot produce any reaction between pigment and binder or solvent that could initiate paint hardening. Another cause of paint hardening can be a change in solvent viscosity due to evaporation or additional absorption of some components in the coating formulation. During the production of chromatic paints, the separation of pigment from binder to prevent the pigments from separating from each other depends on the specific mass of the basic components. The specific mass of TiO2 is reduced by surface treatment. Surface treatment also increases the wettability of TiO2 at the bottom of the container in which the paint is stored. Perfect wetting of the pigment surface also prevents reagglomeration of the dispersed pigment in the paint. Methods are also adopted in the paint industry to prevent pigment separation. Process for Production of Solvent-Based Paints Pigments, fillers and part of the binder solution are pre-mixed using a solvent or mixer. In this production stage, the pigment must be perfectly wetted and the binder must be dispersed in the solution. The pre-mix is pumped into a storage channel and from there (usually by pumps) into dispersion equipment, typically various types of pearl mills or modified solvents. Inside the dispersion equipment, pigment in the binder solution is dispersed to the desired fineness, continuously controlled during the production process. The dispersed paste is pumped to the finishing tank. Here, the remaining binder solution, if necessary a solvent and some colored components (additives) are added, then color shade is adjusted by the addition of color-toning paste. The paint is filtered through special filters and after final approval by the quality control department, product finishing, filling into individual packages/containers and palletizing is performed. The application range of dispersion paints is very wide. A large part of it is used as decorative paint for wood, metal, silicate, cement and plasterboard materials in construction, particularly in decorative and façade paints and plasters.

Dispersion (Emulsion) Paint Production Procedure

If the solvent is water, this group (water-based) paints' typical representative is dispersion/emulsion paints. Unlike solvent-based paints, here a heterogeneous system based on binder consisting of particles dispersed in water is formed. The dispersed binder exists in the form of fine droplets (emulsion) that gradually coalesce during paint film drying to form a continuous film. Careless procedure during the preparation of pigmented dispersion coatings can cause a deterioration in stability and consequently collapse of the dispersion, that is, breakdown of the dispersion. Additionally, very high or very low temperature and significant reduction in viscosity (rapid dilution) can cause dispersion breakdown. Today, synthetic polymers are used almost exclusively as binder base, particularly polyvinyl. Since they are usually solid and their pure dispersion would have a suspension quality that would make continuous film formation after drying impossible, organic solvents or plasticizers are added to these polymers and they are allowed to form emulsions in an aqueous environment at normal temperature. The mixture of pigments and fillers is dispersed in a minimum amount of water using a solvent or mixer. For better wetting of the pigment, a wetting agent and a dispersion stabilizer (most commonly used, for example, carboxymethylcellulose) should be added. After dispersion, the pigment dispersion is added to a polymer dispersion and both are mixed and homogenized in the same dispersion equipment. Raw materials used, including TiO2, must be of quality to ensure perfect dispersion of components is already achieved in the dissolution stage. Their management is definitely high, the obtained shear information should provide good dispersion according to the light environmental speed of the solvent plate. During the dispersion process of a dispersion paint, for example, in a pearl mill, intense abrasion as is used during dispersion is not employed. Due to liquid movement beneath the mixer, only slight friction and homogeneous distribution of individual components occurs.

Powder Coatings (PC)

Powder paints are paints containing no solvent and water, their dry weight is 100%. Approximately 2/3 of its content is resin, 1/3 is pigments and fillers. Additionally, powder paints frequently contain additives to improve the leveling properties of the final film. The powder paint resin component consists of pure resins or a mixture thereof and a hardener. All powder paints are cured by baking at temperatures in the range of 130-200°C for a period of 5 to 20 minutes. During this period, resin and hardener cross-link with each other to form the coating film. Powder paints are supplied in powder form to end users, industrial users without exception, equipped with special application powder paints produced with technologies similar to those used in processed plastics. After extrusion, the paste or optionally a strip is cooled in a water bath. Cooling of the fragmented surface occurs only below approximately 40°C; this is to allow adhesive water from the surface to subsequently evaporate spontaneously. The dry surface is cut into small pieces or crushed and also dried with hot air. The crushed flakes are ground and classified in micro mills. The resulting powder paint particle size ranges between 20-30 μm. From a TiO2 consumption perspective, an quite interesting sector, approximately 60-70% of PC is produced in white or light pastel tones. There is no reason to select special TiO2 grades recommended for powder paint applications; only that TiO2 is well dispersed during the extrusion stage and also meets the requirements regarding optical properties must be ensured. Powder paint is typically applied as a single layer 60-80 μm thick. Therefore, for the suitability of TiO2, the determining factor primarily is the final application of the powder paint in interior, exterior, etc. Powder Paint Can Be Categorized According to Different Criteria: • By type of basic resin - most frequently epoxy resins, polyester, epoxy-ester (hybrid), polyurethane, etc. • By application area, weather resistance and other effects: for interiors - ready-to-use powder paints for general use, mostly with epoxy; for household appliances - (mostly hybrid powder paints); for demanding outdoor applications - (polyester, polyurethane-based powder paints). By Application Method: • A significant percentage of powder paint is applied electrostatically or electrokinetically. In current application, the object to be sprayed is positioned on a conveyor passing through a bath for degreasing and surface preparation. Powder paint is then applied in a spray booth with a special gun. PC particles are loaded into the spray gun and directed by air toward the grounded object and held there until baked in the oven. The second principle is based on the generation of static electricity in PC during transport by air flow through the specially shaped labyrinth of a plastic drum or spray gun. (Potential up to 40 KV). This spraying method is also called electrokinetic or triboelectric deposition. [caption id="attachment_159071" align="aligncenter"] RGU KEY PROPERTIES[/caption] [caption id="attachment_159072" align="aligncenter"] AV01SF KEY PROPERTIES[/caption] [caption id="attachment_159073" align="aligncenter"] R200M KEY PROPERTIES[/caption]   Source: Pretiox Titanium Dioxide; Petr Stolin (Ph. D.), Milan Laskafeld (RNDr.) and team of authors   Hale Zeynep Yüksel Sales Manager Asil Kimya