What is industrial painting and how paint is applied(1)

1. Painting

-Definition: Painting is a general term for operations performed to form a coating film using paint for the purpose of covering the surface of an object for protection and aesthetics, etc.

-Purpose: The purpose of painting is not only for aesthetics, but also for protection and, consequently, improvement of product quality.

1) Protection: Most of the main materials constituting automobiles are steel plates, and when a vehicle is made with a steel plate as a covering, it reacts with moisture or oxygen in the air to generate rust. The greatest purpose of painting is to protect the object by preventing such rust (rust).

2) Aesthetic: The shape of a car has several types of surfaces and lines such as three-dimensional surfaces, flat surfaces, curved surfaces, straight lines, and curves. By painting such a complex shape object, it shows a sense of color that matches the shape of the car and improves the aesthetics of the car at the same time.

3) Improvement of marketability: Currently, there are various types of automobiles on the market, but among them, when comparing vehicles with a unified shape and the same function, for example, the one with two-tone paint looks better. value increases as In this way, it is also one of the objectives to try to improve the value of the product by painting. In addition, the durability of the exterior of automobiles is required due to recent rapid environmental changes. For example, the demand for functional paints that prevent damage to the coating film caused by acid rain and deterioration of initial glossiness caused by automatic car wash brushes is increasing, thereby improving marketability. Automatic painting and manual painting are both used depending on the coating quality requirements.

2. Composition of paint: Composition of paint The paint is a viscous liquid in which the three components of pigment, resin, and solvent are uniformly mixed (dispersed).


- Pigment: A colored powder that does not dissolve in solvents or water. The difference from dyes is that they are dispersed as particles without being soluble in water or solvents. The particle size ranges from several micrometers to several tens of micrometers. Moreover, there exist various shapes, such as a circular shape, a stick shape, a needle shape, and a flaky shape. It is a powder (powder) that gives color (coloring power) and hiding power (the ability to cover and hide the surface of an object by being opaque) to the coating film, and there are two types: inorganic and organic. Pigment), polishing, and extender pigments are used to improve the feeling of land. Colorless and transparent paints called clear among paints, when pigments are excluded from the components constituting the paints,

It is used to give the coating film more luster.

1) Function of pigment

* Color pigments: imparting color, hiding power

go. Inorganic pigments: These are mainly natural pigments such as white, yellow, and reddish brown. They are metal compounds such as zinc, titanium, lead iron, copper, etc. In general, they have excellent weather resistance and heat resistance hiding properties, but in terms of color vividness, they are not as good as organic pigments. As a paint for automobiles, an inorganic pigment alone is not used. Furthermore, from the viewpoint of preventing environmental pollution, pigments containing harmful heavy metals such as cadmium and chromium are not currently used.

you. Organic pigment: It is manufactured by organic synthesis by periodic chemical reaction, and it is a substance made of a metal compound or as it is in nature. In general, the hiding property is not very good, but since a clear color is obtained, it is widely used for the vivid painting of solid color, metallic color, and mica color as a paint for the exterior of automobiles.

* Anti-rust pigment: prevention of rust

* Extender Pigment: A hard coating film can be obtained, preventing decomposition of the coating film and improving durability.

- Resin: A transparent liquid that connects the pigment and the pigment and gives gloss, hardness, and adhesion to the coating film. Another name is called a binder. Drying properties and durability of the coating film greatly depend on the properties of the resin.

1) Natural resin: It is mainly extracted or secreted from plants and is used for paints such as oil-based varnish, varnish, and lacquer.

2) Synthetic resin: It is a generic term for those synthesized through chemical reactions from various chemical raw materials. It is an organic compound with a very large molecular weight compared to natural resins. In addition, synthetic resins are divided into thermoplastic resins (softens and melts when heated) and thermosetting resins (hardens by chemical reaction by applying heat, and does not soften and melt even when heated again after cooling).


- Solvent: It is a transparent liquid that melts the resin so that the pigment and the resin are easily mixed. After painting, it evaporates like a thinner and does not remain on the coating film.

Car painting

1. Overview and Definition of Paints: From the perspective of imparting ‘rust prevention (anti-rust)’ and ‘beauty properties’, automotive paints have played a role in improving the marketability of automobiles by applying the latest technologies of the time. In the following quality items, paints and coating systems are designed to achieve these coating qualities most economically.


Paints are generally flowable and have the property of being coated on the surface of the object to be coated and forming a continuous film (coating film) through drying and curing processes. According to the physical and chemical properties of the coating film formed in this way, ‘rust prevention’ and ‘plasty’ are imparted to the object to be coated.

2. Automotive painting process: In order to obtain the target car’s coating quality in the most economical way, the coating process and coating specifications are set, and each important quality is assigned to the coating film obtained in each process. In addition, since the characteristics of the coating film depend on the good and bad process workability, the paint used in each process is designed so that the assigned main function can be maximized in consideration of the process conditions. The application is strictly controlled in the paint shop.


The above process is a 3-coat or 4-coat coating system most commonly employed for the coating of automobile exterior panels, and the coating film formed in each process exhibits the functions to be described later and establishes the coating quality of automobiles as a comprehensive coating system. In trucks and light vehicles, there are cases where a two-coat coating system in which an intermediate step is omitted from the coating step is employed. Also, in high-end cars, it is possible to achieve better quality by applying the intermediate or top coat twice.

Also, recently, a process for reducing the coating cost by integrating the middle and top coating processes has been studied and applied.

- Surface treatment process: It improves rust prevention by suppressing corrosion reaction of metal and strengthening the adhesion between the undercoat (electrodeposition film) and the material (substrate). Currently, zinc phosphate is the main component of the film, and the dipping treatment method is mainstream so that it can sufficiently treat parts with complex structures. In particular, for cationic electrodeposition, metals such as Fe, Ni, and Mn other than Zn are blended into the coating to further improve the corrosion resistance.


- Electrodeposition coating (Cathion type electrodeposition primer): Undercoating mainly shares the rust prevention function. In addition to excellent anti-rust properties, cationic electrodeposition paint based on epoxy resin has the following advantages in automotive undercoating. ① There is no elution of zinc phosphate treated film during electrodeposition coating. ② Inhibitory effect of corrosion reaction due to basicity in resin structure ③ Excellent anti-rust property due to the effect of maintaining adhesion due to high alkali resistance of epoxy resin.

1) Advantages of cationic electrodeposition

* Even complex shapes can be coated with a uniform film thickness

* Excellent internal penetration into complex parts and joints.

* Automatic painting

* Easy maintenance and management of the line.

* Good painting workability.

* UF closed-loop water washing system can be applied (less loss of paint and less contamination of wastewater)

* Low solvent content and low air pollution.

* It is a water-based paint, and there is little risk of fire.

2) Cationic electrodeposition paint: In general, it is a polyamino resin obtained by adding primary to quaternary amines to an epoxy resin. It is neutralized with acid (acetic acid) to make it water-soluble. In addition, the curing method of the coating film is a urethane crosslinking reaction type using Blocked Isocyanate as a curing agent.

3) Improving the function of electrodeposition paint: It is spread all over the world as an automobile undercoat, but research and development continues to improve not only the anti-corrosive quality of the entire automobile but also the quality of the plastering.

* Rust prevention function/protective layer

go. Absolutely coating property, penetration resistance of joints, chipping resistance

you. Anti-rust steel sheet aptitude (water-resistant adhesion, spin-resistance)

do. Low-temperature hardening (Improved rust resistance of rubber-attached parts, etc.)

* Cosmetic function/decorative

go. Coating properties of steel plate roughness (contributes to improvement of smoothness and glossiness, etc.)

you. Yellowing resistance (inhibition of yellowing of white topcoat)

- Intermediate coat: Intermediate coat plays an auxiliary role to maximize the rust prevention function of the undercoat (electrodeposition) and the plastering function of the top coat, and has the function of improving the paint quality of the entire painting system. In addition, the intermediate coating process is contributing to lowering the coating defects because it covers the unavoidable defects of the undercoat (scratches, dust adhesion, etc.) to some extent in the actual painting line.

The intermediate paint is a type that uses oil-free polyester resin as the basic resin and heat-cures it by introducing melamine resin and recently urethane (Bl). Recently, in order to improve chipping resistance, a chipping primer is sometimes coated with wet on wet in the middle pre-process.


1) Durability of the intermediate coat

* Water resistance: low absorbency and suppresses the occurrence of blisters

* Chipping resistance: Absorbs the impact energy when the stone is thrown and reduces the damage to the coating film leading to the sound and suppresses the occurrence of scab corrosion.

* Weather resistance: Less deterioration due to UV rays, and suppresses outdoor exposure peeling of top coat.

2) Plastering function of intermediate coat

* Undercoating property: Contributes to smoothing of the finished exterior by covering the surface roughness of the electrodeposition coating

* Solvent resistance: By suppressing the swelling and dissolution of the intermediate coat with respect to the solvent of the top coat, high-contrast appearance quality is obtained.

* Color adjustment: The middle coat is usually gray, but recently it is possible to apply a top coat with low hiding properties by coloring it (color sealer).

3) Intermediate paint

*Quality required for intermediate coat: chipping resistance, base hiding property, adhesion to electrodeposition film, smoothness, no loss of light, adhesion to top coat, light deterioration resistance

- Topcoat: The greatest function of topcoat is to provide cosmetic properties and to protect and maintain it. There are quality items such as color, surface smoothness, glossiness, and image quality (ability to clearly illuminate the image of an object in the coating film). In addition, the ability to protect and maintain the aesthetics of such automobiles for a long period of time is required for the top coat.

- Topcoat: The greatest function of topcoat is to provide cosmetic properties and to protect and maintain it. There are quality items such as color, surface smoothness, glossiness, and image quality (ability to clearly illuminate the image of an object in the coating film). In addition, the ability to protect and maintain the aesthetics of such automobiles for a long period of time is required for the top coat.


1) Top coat: Colors are classified according to the pigment base applied to the paint, and it is largely divided into mica color, metallic color and solid color depending on whether flake pigments such as flakes of aluminum powder are used.

* Appearance quality: smoothness, gloss, vividness, land feeling

* Durability: gloss maintenance and protection, color change, fading

* Adhesion : Recoat adhesion, 2 tone adhesion, adhesion with medium

* Solvent resistance

* Chemical resistance

* Functional quality: car wash resistance, acid rain resistance, chipping resistance

2) Environmentally friendly paint

   * High Solid: This is a high-solids paint that responds to VOC (Volatile Organic Compounds) regulations, and is a type that reduces the amount of organic solvent used. It is characterized by excellent feeling of land and using low-molecular-weight resin.

* Water Bome Type (water-based paint): This is a paint that minimizes the amount of organic solvent used and uses water (pure water) as a paint thinner thinner. As a characteristic, a preheating facility (IR_Preheat) that can evaporate water is required in the painting process, so facility remodeling is required, and the sprayer also requires an electrode method for water-based paint.

3) Functional paint

* CCS (Complex Crosslinking System, complex crosslinking type paint): It is a type of urethane (isocyanate) or silane resin in which a part of the melamine resin, which is vulnerable to acid rain in the acrylic/melamine resin system, is replaced, and the acid resistance and scratch resistance are improved.

* NCS (New Crosslinking System, New Crosslinking Type Paint): Non-melamine-based paint made by acid-epoxy curing on acrylic resin. It has excellent acid resistance, scratch resistance, and stain resistance.

- Coating workability of top coat: In order to economically obtain good reproducibility of the target top coat, good paint workability (atomization, flowability, pinhole, smoothness, etc.) is essential. For this, it is important to adjust the viscosity behavior in the multi-film formation process from painting to baking and hardening. The painting environment conditions such as temperature, humidity, and wind speed of the painting booth are also important factors.

1) Viscosity of resin: molecular weight, compatibility (solubility parameter: SP value)

2) Pigment: oil absorption, pigment concentration (PWC), dispersed particle size

3) Additives: viscous agent, leveling agent, defoaming agent, color separation inhibitor, etc.

4) Curing speed: concentration of functional groups in the base resin, reactivity of crosslinking agent

In addition, the thickness of the coating film has a great influence on the finished appearance of the top coat. Recently, a structural viscous agent such as microgel makes it possible to achieve both flowability and leveling properties, and the finished appearance is improved by thick film coating.

- Weather resistance of top coating: Although automobiles are exposed in various environments, the top coating receives the action of light, water, oxygen, heat, etc. As a result, a number of unfavorable phenomena occur that impair the aesthetics.

1) Optical phenomena

* Degradation of gloss: The smoothness of the surface of the coating film is damaged, and the diffuse reflection of light from the surface increases. The composition of the resin is important, but there is also the effect of the pigment.

* Discoloration: The color tone of the initial coating changes according to the aging of the pigment or resin in the coating film. For automotive applications, the most weather-resistant pigment should be selected.

2) mechanical phenomena

* Cracks: Cracks occur in the coating film surface layer or the entire coating film due to changes in the physical properties of the coating film due to photooxidation or hydrolysis (decreased elongation, adhesion, etc.) and internal stress. In particular, it tends to occur in a metallic clear coating film, and in addition to adjustment of the coating film physical properties of the composition of the acrylic resin and adjustment of the coating film physical properties, the application of an ultraviolet absorber and an antioxidant is effective.

* Peeling: The coating film is partially peeled off due to a decrease in the adhesion of the coating film or a decrease in rheological properties, and the action of external forces such as splashing or vibration of stones.

3) chemical phenomenon

* Stain contamination: If soot, insect corpses, or acid rain adhere to the surface of the coating film, the part becomes stained and discolors into spots. It is necessary to apply a scratch-resistant, alkali-resistant pigment and resin. One of the reasons why the clear coat is applied to the metallic color is to protect the aluminum powder.

- Future challenges of top coat: The aesthetics and design are becoming more and more important in improving the commercial properties of automobiles. While responding to the diversification of demands and changes in materials such as plastics, it is necessary to respond to social demands such as deterioration of the automobile exposure environment and reduction of air pollution. Under these circumstances, various topcoats for the next automobile are being considered.


Let’s take a closer look at the typical automotive painting processes and see where heat and mass transfer are important applications. The general painting process for automobiles is as follows.

① Pretreatment

② Electrodeposition (undercoat)

③ Sealant painting

④ Under Coating

⑤ wax painting

⑥ Anti-Chip Primer

⑦ Primer

⑧ Top Coat

⑨ Defect removal and polishing

The automobile manufacturing process takes about 20 hours, of which 10 hours, which is half, the process listed above takes about 10 hours. Among them, the most significant and important processes are pretreatment, electrodeposition coating (undercoat coating), primer coating, and top coating. Let’s focus on these processes.

Post time: Nov-08-2022