Powder coating is a finely ground mixture of pigments, binders, fillers, and additives. In surface finishing, it is used for metal coating. During application, the powder is electrostatically charged and attracted to metal surfaces. There, it adheres until heat treatment fuses and cross-links it into a durable, tightly bonded layer on the surface. One advantage of powder coating is the absence of solvents, which would otherwise contribute to air pollution.

The Functions of Powder Coating Components

Pigments Provide Color

Pigments are responsible for the color in powder coating. Inorganic pigments best withstand the high cross-linking temperatures. These include metal oxides, either naturally occurring or synthetically produced. These colorful minerals have long been used in painting and glassmaking. For example, powder coating owes its brilliant white to titanium oxide. Iron oxides produce yellow, red, or black. Cobalt oxides are known for intense blues, while chromium oxide gives a green coloration.

Alongside inorganic pigments, organic dyes are also used in powder coatings. These originate from natural sources as well. Examples include indigo from the woad plant or purple, once derived from certain snails. In surface technology, temperature-resistant synthetic organic dyes are used to achieve bright, glossy colors.

Fillers Add Volume and Improve Corrosion Resistance

Fillers such as chalk, talc, or barium sulfate add volume to the coating layer and optimize particle size distribution in the powder. This ensures maximum packing density: smaller pigment particles distribute evenly in the spaces between larger filler particles. This layer structure enhances the corrosion protection of the powder coating.

Additives Ensure Quality and Surface Texture

Additives optimize processes such as surface pretreatments, create gloss or intentional texture effects, and provide the required hardness of the coating. Flow agents and degassing additives are especially important. Flow agents lower surface tension, producing a smooth, defect-free layer. Degassing additives channel gaseous reaction products to the surface, preventing pinholes in the coating. Special structuring agents can be used to achieve velvety, porous, or wavy effects.

Binders Hold It All Together

Binders give powder coatings their strength. They provide resistance to mechanical, chemical, and weathering stresses. Their function develops during the final curing stage in the oven, where they form cross-linked three-dimensional structures that lock in all other components.

Binders that are often used include epoxies or polyester resins, frequently combined in hybrid systems. Polyurethane, PVC, polyamide, or acrylic binders are also possible. Epoxy and cured polyester resins create thermosetting plastics with excellent mechanical properties, high chemical resistance, and strong weatherability.

Polyurethanes, thanks to their strong adhesion, are well suited for primers. Their widespread use as clear coats and topcoats is due to their resistance to chemicals, solvents, and weathering.

The Powder Coating Process in Surface Technology

Powder coating involves three steps: surface pretreatment, powder application, and curing.

Pretreatment

Surfaces are cleaned and usually given a conversion coating. The conversion coating is formed through a chemical reaction between the metal and an acid. Steel and iron metals often receive phosphate layers. The surface of galvanized steel and aluminum is typically prepared for powder coating by chromating or anodizing. The conversion coating improves corrosion protection and provides a firm adhesion base for the powder coating. The pretreatment ends with thorough drying of the parts.

Powder Application

For application (also called deposition), the powder particles are electrostatically charged, either by friction (triboelectric charging) or by ionization (corona application). In tribo systems, Teflon-coated spray guns use compressed air to charge the particles by friction. The spray gun atomizes the particles into a powder cloud, which is then attracted to the grounded workpiece. Multiple layers can be applied evenly in this way, and the process is highly suitable for automation.

In corona application, the particles pass by an electrode that ionizes the surrounding air at 30–100 kV. The resulting glow, known as the corona effect, gives the process its name. The charged air forms a cloud together with the powder. This method consumes less air and, therefore, causes less wear on the spray guns. It is predominantly used for effect powders that cannot be triboelectrically charged.

Curing

Curing—also called baking or drying in surface technology—takes place at 230–480 °F (110–250 °C). The powder melts and forms a solid layer. Temperature control is crucial to coating quality. First, a short heat-up period (depending on layer thickness) brings the coating to curing temperature, followed by a holding time of 5–30 minutes, depending on the binder used.

Applications of Powder Coating

Surface finishing by powder coating is used wherever metals are exposed to wear, weathering, or chemicals. Thanks to the wide range of binders, coatings can be selected for specialized uses such as anti-graffiti protection.

Some powder coatings have increased heat resistance. In addition to their protective role, the ability to create vivid colors and striking textures is a major advantage. The main application area is general metal coating.

Construction Industry

In construction, the weather resistance of powder coatings is especially valued. Facades, windows, doors, gates, fences, and railings are given durable protection with these coatings.

Commercial and Agricultural Vehicles and Machinery

These vehicles and machines face harsh operating conditions in addition to weather exposure. Powder-coated components resist wear from water, sand, and dust for extended periods, helping extend equipment service life.

Household Appliances and Furniture

Household appliances such as washing machines, refrigerators, and radiators are designed to withstand frequent cleaning, heavy use, and even occasional impact. Powder coating helps maintain their durability, aesthetic appeal, and long-standing reputation as “white goods”—a term referring to large household appliances, traditionally white in color, like refrigerators and washing machines, known for their reliability and long-term use. Furniture, especially outdoor furniture exposed to the elements, also greatly benefits from long-lasting corrosion protection, preserving both its functionality and appearance.

Automotive Industry

In automotive manufacturing, powder coating is widely used for corrosion protection and finishing of smaller parts, such as wheels, providing effective defense against rust.

Der Beitrag What Is Powder Coating? erschien zuerst auf Kluthe Magazine.

Primary Use of Thinning Agents for Paint

Most paints consist primarily of binders and solvents. The binder determines the final characteristics of the coating. The solvent ensures proper mixing of the paint components and allows the coating to be applied. As the applied layer dries, the solvent evaporates.

The time required depends on the volatility of the solvent. Highly volatile organic solvents evaporate very quickly and are commonly referred to as solvents. The proportion of solvent determines the flow properties of the paint. Commercially available paints typically contain only the minimum amount of solvent.

Shortly before processing, enough thinner is added to achieve the required flow behavior.

This allows the paint to be adapted to different coating methods such as brushing, rolling, spraying and to the requirements of each of the following layers  primer, intermediate coat, and topcoat. The choice of thinning agent also influences drying time and the appearance of the coating.

Selecting Thinning Agents

Thinning agents for paints and coatings must always be appropriate for the overall system. Anyone choosing the “right” thinner from the wide range of options should consult the manufacturer or supplier. For thinning oil- and alkyd resin paints, turpentine oil is suitable. This high-quality substance is obtained from the resin of coniferous trees — hence the common name “pure turpentine.” A more affordable alternative with similar composition is mineral turpentine, derived from petroleum. Nitro-combination paints can be thinned with acetone. Another commonly used thinning agent for paints is denatured alcohol. This alcohol has been treated to make it unsuitable for consumption and exempt  from the taxes levied on alcoholic beverages.

Acetone and denatured alcohol evaporate very quickly. To slow evaporation — and therefore improve leveling and gloss — chemically similar substances with lower volatility are blended into universal thinners. These solvent blends are engineered for a specific evaporation profile. Universal thinners are available in fast-, medium-, and slow-evaporating versions. If a very fast-drying coating is required, a fast-evaporating thinner is used.

Slow-evaporating thinners produce a high-gloss surface. Kluthe offers these products under the brand name “Lösin.” These solvent blends consist primarily of acetone, straight-chain and cyclic hydrocarbons, alcohols, esters, and aromatic ring compounds derived from benzene. The latter are collectively referred to as aromatics.

For paints that contain no aromatic solvents, Kluthe’s product range includes a specifically formulated aromatic-free universal thinner.

Straight-chain hydrocarbons are also the main components of a solvent commonly known as ‘white spirit’— a term derived from its volatility (‘spirit’) and water-clear appearance (‘white’). These molecules consist of a chain of carbon atoms with attached hydrogen atoms. In cyclic hydrocarbons, the carbon atoms form a closed ring. If one hydrogen atom is replaced by an OH group, the result is an alcohol; if two OH groups are present, it is glycol. Organic acids such as acetic or formic acid readily attach to OH groups to form esters or ethers.

Use of Thinning Agents for Cleaning

Equipment used for coating must be cleaned immediately after finishing the work to remove paint and coating residues. Thinners for the respective paints are also suitable for cleaning agents. They are also used to remove paint stains from floors or furnishings. Compatibility with the affected surfaces must always be considered. Most cleaning thinners also dissolve fats and oils well, which is why they are frequently used for degreasing surfaces — even outside coating applications.

Familiar cleaners include not only specialty paint thinners but also ammonium hydroxide solutions and denatured alcohol. Turpentine oil is highly effective for cleaning wooden floors and wooden objects.

Handling Thinning Agents and Cleaning Thinners

Most thinning agents and cleaning thinners are flammable liquids. Fast- and medium-evaporating substances generate enough vapor at normal room temperatures to form a flammable atmosphere when mixed with air. In such cases, even a small spark can trigger an explosion.

These vapors are generally harmful to one’s health. After inhalation, they quickly accumulate in the fatty tissue surrounding the nerves, which may cause drowsiness or dizziness. Adequate ventilation is therefore essential when using thinners, and all open flames must be avoided.

Because thinners dissolve fats, repeated or prolonged contact affects the skin, making it dry and cracked. This can be prevented by wearing protective gloves and using skin-care products. Technical data sheets and safety data sheets provided by manufacturers contain detailed information about ingredients, safe handling, and the correct disposal of leftover quantities.

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What are VOCs?

Paints used for surface coating have historically contained and in some cases, still contain solvents. These ensure an even distribution of the various paint components, reduce the viscosity of the paint, and improve wetting behavior. They play an essential role in the coating process, with the more volatile solvents facilitating  the drying process after painting.

These volatile organic compounds are known as VOCs. According to the Environmental Protection Agency (EPA), they are categorized based on their boiling range into very volatile, volatile, and semi-volatile substances. All volatile substances share the common characteristic that they evaporate very quickly due to their low boiling point or high vapor pressure. Very volatile organic substances, abbreviated as VVOCs, boil at temperatures around 32°F to 212°F. Adhesives or colored pencils may contain such substances, often recognizable by their characteristic smell. For example, the typical smell of glue is due to ethyl acetate, and the smell of colored pencils is due to toluene. Semi-volatile organic substances, known as SVOCs, have a very high boiling range from 464°F to 752°F. In between are the Volatile Organic Compounds, VOCs, which start to evaporate around 122°F. Substances with a boiling point of up to 500°F are counted among the VOCs.

Typical VOCs include aromatic compounds, aliphatic hydrocarbons, or alcohols. Many of these volatile compounds pose risks to the environment and health, which is why their reduction has been less in demand since the late 1980s. The EPA has regulated VOCs under the Clean Air Act since 1970, with additional amendments introduced  in 1990 to strengthen these regulations. These regulations primarily govern emissions and set limits for industrial processing and stationary painting facilities, with additional requirements added for large painting operations and industry.

Why VOC Reduction Makes Sense …

Whether from natural or anthropogenic sources a high VOC level becomes noticeable, especially in summer Nitrogen oxides, for example, released by traffic, react with VOCs under the influence of UV light to form ground-level ozone. This leads to health impairments, such as respiratory irritation and breathing difficulties, coughing, or headaches.  Regardless, these solvents are subject to strict limits. These organic compounds can cause various health damages, including skin and eye irritations, kidney  issues, respiratory problems, with  some being  classified as carcinogenic. It is not without reason that in the private sector, it is advised to ventilate rooms adequately when working with solvent-based paints or varnishes. Stricter values apply in the industrial sector. Thus, VOC emissions must constitute only one percent of the solvents used, and for smaller facilities that handle less than about 2,000 pounds per year, it is 2.5%.

… and What Has Been Done in Recent History?

In 1980, VOC total emissions in the US were over 6 billion pounds per year; in 2023, according to the EPA, their value had dropped to just under 2.2 billion pounds. This VOC reduction is largely the result of  decreased emissions in the transport sector.

New Technologies Contribute to VOC Reduction up to Their Complete Avoidance

The “classic” solvent-based paint, recognizable by its smell, has been replaced over the past years and decades by ever-new products that contain fewer or no solvents at all. The introduction of special paint thinners, powder coatings, emission paints, and water-based paints are just a few examples.

Today, a modern car is usually coated with a water-based paint. While it still contains solvents, this is considered low at a maximum of 18%. Some automakers also use powder coatings for metal and plastic coating, which are completely solvent-free. With this method,  there is no need to worry about a high VOC level. At the same time, almost all the powder paint is applied to the bodywork, resulting in virtually no waste. This type of surface coating will undoubtedly belong to the future in today’s age, where both manufacturers and consumers pay attention to resource efficiency and sustainability.

Similar advancements include high-solid and ultra-high-solid paints. These are based on resins, such as alkyd or epoxy resins. Typically, these coating materials have a solid content of over 65%, meaning the proportion of liquid and volatile components is low. The VOC level is at most 30%. Electrodeposition painting also works based on water-soluble components and uses only a small amount of organic solvents. In this process, paint binders and pigments deposit on the workpiece and form a continuous coating on the surface.

Since the 1980s, when VOC emissions were significantly higher compared to today, a lot has happened in the field of surface coatings and paints, with many new technologies and products having been introduced. Whether a complete replacement of all solvents is possible, only the future can show.

Der Beitrag VOC Reduction – How Coating Technologies are Getting Cleaner erschien zuerst auf Kluthe Magazine.

Where Painted Aluminum Is Used in Modern Industry

Aluminum components are used  across a wide range of industrial fields. Due to its lightweight properties, the metal is commonly used in automotive, shipbuilding, and aircraft construction. In architecture, it serves both structural and aesthetic purposes — one high-profile example being the Empire State Building in New York, where aluminum was used extensively for the tower structure, spire, entrances, elevator doors, ornamental trim, and over 6,000 window spandrels.

Aluminum oxidizes naturally in the air, forming a stable layer of aluminum oxide. In many applications, especially those with lower technical requirements, this layer offers sufficient protection. However, the oxide layer is not uniform or precisely defined in thickness, which can be problematic for industrial applications with stringent quality standards. In some industries, specific colors or finishes are desired. Aluminum is particularly sought after in the automotive sector, where weight savings allow for smaller engines and  improved fuel efficiency.

From Raw Material to Finished Coated Product: A Multi-Step Process

Painting aluminum involves a sequence of coordinated steps. The surface must first be properly pretreated to ensure it is smooth and ready for coating. A suitable aluminum primer is essential and any contaminants from prior processing—such as oil films or lubricant residue—must be thoroughly removed. The painting procedure builds upon this foundation, which  ensures that the entire part is evenly coated. Strong adhesion between the aluminum and the paint is crucial, and issues like residual moisture must be avoided. The painting process itself consists of several stages.

Surface Pretreatment Is Key

To paint aluminum successfully, the metal’s surface must be pretreated. Two conditions must be met for proper paint adhesion: First, the natural oxide layer must be modified or properly prepared. Second, the surface must be  meticulously cleaned. Residues such as dust, metal shavings, grease, or oil from earlier stages can interfere with adhesion, resulting in uneven paint coverage and poor bonding between the metal and the coating.  These contaminants can lead to defects and even corrosion, especially for parts exposed to the elements. For US industrial applications, adherence to ASTM B117 salt spray testing and AAMA 2605 performance standards is often required to ensure coating durability and quality. In such cases, surface flaws can significantly shorten the component’s lifespan.

Depending on the size and application of the part, aluminum pretreatment can be conducted in different ways.

Mechanical Surface Treatment

Mechanical processing removes unwanted surface deposits such as scale, stubborn dirt, soot, and the naturally occurring aluminum oxide layer. Common methods include sanding, brushing, or abrasive blasting (e.g., sandblasting). Tools can range from manual sandpaper to orbital sanders and fixed grinding machines. Sandblasting is often used in industrial settings to clean off oxide residues. These mechanical treatments roughen the surface slightly, improving paint adhesion. Typically, mechanical cleaning is followed by a wash. Water-based cleaners remove loosely bound substances, while organic solvents eliminate oils and greases. The goal is a surface free of grease, which isa critical condition for proper paint adhesion. Ultrasonic cleaning is another option for removing more stubborn contaminants.

Chemical Surface Treatment

After degreasing, chemical processing typically follows. Acidic or alkaline pickling agents are used to dissolve the oxide layer while also roughening the surface—similar to the mechanical approach—to improve paint adhesion. Depending on whether the component is a flat aluminum sheet or a more complex part, the treatment can be applied using either a spray system or an immersion bath. Immersion is preferred for parts with holes or cavities, ensuring thorough coverage in hard-to-reach areas. Between each step, parts should be rinsed thoroughly with purified water, ideally in several successive baths, to remove all chemical residues and contaminants. Afterward, the surface must be dried completely before moving on to the painting stage.

Two Common Methods for Painting Aluminum in Industrial Settings

Depending on the part and performance requirements, different painting methods can be used.

Powder Coating: An Overview

Powder coating involves spraying a fine powder onto the aluminum part,  where electrostatic forces come into play: the powder particles carry a negative charge, while the aluminum is positively charged, causing the powder to adhere to the surface. TElectrostatic forces temporarily bind the powder particles to the surface. The coated part is then placed in an oven and heated to around 390°F to 400°F (approx. 200°C). The powder coating contains pigments, a resin binder (such as acrylic or epoxy), and a hardener. As the temperature rises, the hardener and binder react to cure the coating, forming a solid, durable bond with the metal. Powder coatings are typically 2.4 to 4.7 mils (60–120 µm) thick, but thickness can vary.

Powder coating is often the method of choice when aluminum parts will be exposed to environmental stresses. In the US, these coating processes must comply with EPA National Emission Standards for Hazardous Air Pollutants (NESHAP) for surface coating of metal parts, which regulate volatile organic compounds (VOCs) and hazardous air pollutants. It is highly resistant to moisture, salt, and chemicals. Compared to wet painting, it is also more eco-friendly, since it  eliminates solvent-based chemicals that evaporate during drying, using only the powder itself.

Wet Painting for Specialized Applications

For ultra-smooth finishes, powder coating is not ideal to utilize, as it results in a slightly textured surface. Alternatively,  wet painting  provides a smooth, uniform finish. The paint is applied as a liquid solution and then cures as the solvent evaporates.

Dip Coating, Curtain Coating, and Roller Coating

The are several other methods of paint application that are available. The paint can be applied using an immersion bath. In dip coating, the entire workpiece is submerged, fully wetted, and then removed. When using this method, it is important to ensure that no air bubbles form during immersion and that a uniform coating is achieved when the part is withdrawn. In curtain coating, the parts to be painted are transported on conveyor belts during the coating process. The transport speed can be adjusted while the paint is dispensed via a curtain coater—a vessel spanning the full width of the conveyor. This creates a “curtain” of paint that flows vertically down onto the workpiece. This method is ideal for flat objects—such as aluminum sheets.

Finally, the paint can also be applied to an aluminum part using rollers. A distinction is made between the application roller and the metering roller, which controls the amount of paint applied. This method can also be used to coat flat items, aluminum sheets, or even tin cans. Unlike powder coating, wet painting is not restricted by part size and is well-suited for large components such as aluminum sheets or coils.

Today, aluminum is often coated in color finishes that conceal its natural metallic appearance. This lightweight metal is widely used in industry, and when color is required, specific steps in the painting process must be followed to ensure a flawless and durable result.

Der Beitrag Painting Aluminum in Industrial Applications erschien zuerst auf Kluthe Magazine.

Painting plastic parts is challenging because plastics repel most liquids and are difficult to wet. In the automotive industry, more frequently, parts are being made from plastic materials. Exterior parts are exposed to weather conditions, while interior parts must withstand frequent handling. How does the automotive industry meet these demands?

Advantages of Plastics in Automotive Manufacturing

Plastics are lighter than metals, reducing the mass of a car  which leads to lower fuel consumption,  making this the key reason for manufacturing as many car parts as possible from plastics. Another advantage is the variety of shapes that can be created with these materials. Finally, their elasticity reduces costs from minor damage.

Parts such as bumpers, hubcaps, trim strips, and mirror housings have long been made from plastics. Modern composite materials, also known as glass fiber-reinforced plastics (GFRP), are currently used in hoods, fenders, and doors.

Paint as corrosion protection and aesthetic enhancement of plastic parts

While  it is well known that metal requires corrosion protection, plastics also experience aging and degradation over time. Sunlight’s UV rays, moisture absorption, temperature fluctuations, as well as dust, insects, and rain all take a toll on plastic materials.

Painting plastic parts not only protects them from environmental influences but also offers an opportunity to customize the vehicle’s appearance. Depending on personal taste, parts can be highlighted with special colors, or the car can be given a uniform appearance.

Special Requirements for Paint Systems on Plastic

Liquids tend to form the smallest surface area possible due to the attraction forces between liquid molecules, a phenomenon known as surface tension. The relationship between the surface tension of the liquid and the solid surface determines whether drops form or whether the liquid spreads evenly. To create a smooth film, the surface tension of the solid must be greater than that of the liquid.

Therefore, painting plastic parts requires paints with the lowest possible surface tension.

Since plastics are elastic and car parts can be easily deformed, the paint must also have sufficient elasticity to prevent cracking. It must expand and contract with the substrate.

To achieve successful painting of plastic parts, additives are required to increase the paint’s elasticity.

The automotive industry uses a wide variety of plastics for different applications, each with distinct properties. Therefore, specialized paints and paint systems are needed, tailored to the specific requirements of each material.

The paint must be compatible with the plastic material.

Painting of new parts

In large-scale production, automotive manufacturers and suppliers have equipment that allows parts to   move through various necessary stages, including cleaning, priming, and painting plastic parts. Thorough drying is required between each stage.

Cleaning

For optimal paint adhesion, parts must be clean. New parts often have residues of release agents which are used for removing parts from molds during the production process. These residues may repel paint and must be completely removed. This also applies to greasy contaminants, such as fingerprints.

Cleaning methods include the powerwash process and CO² snow blasting. The powerwash process uses heated alkaline cleaning solutions, which are sprayed onto the plastic parts under high pressure. After washing, the parts are rinsed and dried. In CO² snow blasting, the surface is blasted with solid carbon dioxide.

Residual release agents and moisture are eliminated by the process of tempering. During this process, parts are heated until the water and solvents absorbed by the plastic evaporate.

Priming

Priming promotes adhesion. The plastic, primer, and paint must be coordinated as a system. Before proceeding, the primer must be completely dry.

Painting

When painting plastic parts, several thin layers of paint are applied. Each layer must dry before the next one is applied. Drying can be accelerated by applying heat. The data sheets and instructions from the paint manufacturers provide information on optimal temperatures and required drying times.

Painting Older Parts

Damage or personal design preferences are common reasons for painting older plastic parts. This restores or enhances the car’s value in the eyes of the owner. The paint system components must be tailored to the existing material.

Many process chemical manufacturers, such as Kluthe, offer comprehensive consulting services and employee training to assess the material, choose the appropriate paint, and guide the customer through the optimal approach. For new paint jobs, thorough surface preparation is essential. Scratches, cracks, or other damage are repaired with fillers or putty. The surface is sanded with fine sandpaper and then lightly roughened again to improve primer adhesion.

An intact existing paint job can be painted over without re-priming. However, all dirt, cleaning, and care products must be thoroughly removed first. Adjacent areas that should remain unpainted need to be carefully masked with tape.  After this, the same steps are followed as those used when painting new parts.

Der Beitrag Painting plastic parts in the automotive industry erschien zuerst auf Kluthe Magazine.

Paint coagulation is a process within the wastewater treatment of paint shop systems. This process enables paint particles and water to be separated, allowing the water to be returned to the system. The remaining paint sludge is disposed of. But how exactly does this process work?

What Role Does Water Play in a Paint Shop?

In paint shops, coatings are finely atomized and sprayed onto the surfaces of parts. A portion of the paint never reaches the parts and remains suspended in the air or lands on surrounding walls. This portion is referred to as overspray. Overspray would quickly cause operational disruptions in a paint shop. To prevent this, it is partially extracted and partially bound with water. Before the overspray can be separated from the water again, paint coagulation is required.

How Does the Paint Enter the Water?

Coating operations in metal and plastic finishing are conducted in spray booths or at manual workstations. In both cases, paint mist is conveyed by exhaust systems to downstream air treatment.

Manual Workstations

Manual coating is often performed in front of a water curtain. Excess paint is captured by a downward-flowing water stream and carried away. This process can be supported by drawing the paint mist toward the water curtain. The water is recirculated within the system. To enable reuse, it is cleaned using the paint coagulation process.

Spray Booths

In spray booths, water curtains also protect the walls from contact with paint particles. The liquid collects in a floor pan and is pumped from there to the treatment system, where paint coagulation begins.

Paint Removal from Exhaust Air

Paint can be removed from exhaust air using filters. However, this increases waste volumes due to the spent filter materials. For this reason, exhaust air is generally treated with scrubbers. When water and exhaust air  come into contact, the paint remains in the water. The cleaned air can then be discharged into the environment.

How Is the Paint Separated from the Water?

The fine atomization that ensures optimal coating results makes water treatment more challenging. Paint droplets with diameters of only a few nanometers remain suspended in the water, forming colloidal solutions.

The method used to separate these solutions depends on whether the paint is to be recovered or disposed of. Water-based paints can be reclaimed using physical processes. Solvent-based paints are separated from water using chemical processes. Paint coagulation is part of the chemical treatment of paint-containing wastewater.

Recirculation of Water-Based Paints

Processes available for reclaiming water-based paints include ultrafiltration, vacuum evaporation, and electrophoresis.

During ultrafiltration, special membranes retain the larger paint molecules while allowing smaller water molecules to pass through. Vacuum evaporation makes it possible to concentrate the solution at low temperatures. The lower the pressure, the lower the boiling point. Electrophoresis takes advantage of the electrical charge of paint particles. In an electric field, they migrate toward the electrode with the opposite charge, where they are captured and removed from the circulation system.

Chemical Separation

Solvent-based paints are not suitable for recovery. In many cases, it is cost effective to dispose of water-based paints as well.

To separate the suspended particles, they must agglomerate in the water tank. Once they reach a certain size, they are heavy enough to sink or large enough to float. Sinking is referred to as sedimentation; floating is referred to as flotation. The agglomeration process itself is known as coagulation.  The purpose of paint coagulation is to initiate the agglomeration process.

How Is Paint Coagulation Achieved?

Coagulants

For paint coagulation, coagulants are mixed into the wastewater. These agents neutralize the electrical charge of similarly charged paint particles. As a result, the particles no longer repel one another and can aggregate. Certain components within the coagulants further promote agglomeration by encouraging particles to attach to them. This process is known as adsorption.

Flocculants

Through coagulation, larger particles form from the paint components. Flocculants then cause these particles to grow further and form flocs. Depending on their density, the flocs either rise or sink. The selection of coagulant and flocculant determines which outcome occurs.

Defoamers

Paint coagulation requires thorough mixing of all substances. If air is introduced during mixing, foam formation can occur and quickly lead to operational problems. Special defoamers prevent foam formation and cause existing foam to collapse.

What Happens to the Mixture of Wastewater and Additives?

During coagulation and flocculation, wet paint sludge forms and is removed either from the surface of the liquid or from the bottom of the tank.

To reduce waste volumes and improve suitability for energy recovery, the sludge must be dewatered as effectively as possible. This is achieved using filter baskets or dewatering containers.

Filter Baskets

Filter baskets are wire frames fitted with filter cloths or filter bags. The filter material allows water to pass through while retaining solids. Filter presses, which apply additional pressure, increase efficiency. When pressure is generated using compressed air, the equipment is referred to as a pressure filter (known in European practice as a “Nutsche”). The water collected downstream of the filter is returned to the circulation system.

Dewatering Containers

Dewatering containers are perforated tanks that allow water to drain away. Their base is either inclined or shaped like a horizontal half-cylinder. This creates a low point where the liquid collects. In the lower section of a dewatering container, a collection tank equipped with a pump returns the water to the circulation system.

Anti-Tack Effect of Additives

The wet paint sludge is conveyed by pumps, pipelines, and fittings to the dewatering equipment. All system components are at risk of sludge buildup.

To prevent pipelines, perforations in dewatering containers, and filter materials from clogging, the agents used for paint coagulation must provide an anti-tack effect. High-quality chemicals contain additives that reliably prevent components in the paint shop from sticking and allow for extended maintenance intervals.

How Can Disruptions in the Paint Shop Be Prevented?

Use of Suitable Chemicals

The substances used for coagulation must be matched to the paint system, the coating line, and the sludge removal system, and must provide a high separation efficiency. Coagulation chemicals are also available for mixed wastewater streams containing both solvent-based and water-based paints.

Compliance with Specified Concentrations

Manufacturers provide the recommended concentrations  in their technical data sheets and instructions for use.. These specifications must be followed precisely to ensure that paint coagulation is both cost-effective and reliable. Overdosing increases procurement costs and disposal costs due to higher waste volumes. Underdosing inevitably reduces the effectiveness of the coagulants.

Maintaining Water Quality

Water recirculation leads to water losses through evaporation. As a result, hardness-forming minerals become concentrated in the water. These minerals can interfere with paint coagulation.

The hardness level must be checked regularly. If it exceeds the limit specified by the chemical manufacturer, a portion of the water must be discharged as blowdown and replaced with fresh water.

Selection of Proper Dosing Points

The choice of dosing points has a decisive impact on mixing and, consequently, on paint coagulation.

 Ideally, the coagulant should be added into the pressure line leading to the water curtain or spray booth. The flocculant performs best when introduced into the discharge pump pressure line shortly before the separation tank. The defoamer is injected upstream of water curtains and scrubbers.

Der Beitrag Paint Coagulation in Industry erschien zuerst auf Kluthe Magazine.