Film drying method: principle, process and application

Coating film drying is a crucial part of the coating process, which directly affects the final performance and quality of the coating. This paper will systematically introduce various methods of coating film drying, including natural drying, heat drying, radiation drying and vapor phase drying, and analyze in detail the principle, applicable conditions, process parameters and influencing factors of each method, discuss the technical characteristics, equipment requirements, safety specifications and application cases of different drying methods, and provide comprehensive technical reference for coating process design and technical personnel.

Natural drying method

Natural drying, also known as air drying or drying, refers to the process of exposing the coating film to the atmosphere and curing it through physical or chemical action at room temperature. This drying method requires no additional energy input and no special equipment, making it one of the most economical and easy ways to cure coatings. Natural drying is mainly suitable for volatile coatings, oxidative polymerization coatings and polymeric coatings with some external reinforcing agents, such as nitro paints, alkyd resin paints and some acrylic coatings.

Mechanism and type of natural drying

The film-forming mechanism of natural drying varies depending on the type of coating. The drying of volatile paints, such as nitro paint and perchloroethylene paint, is a purely physical process. As the solvent gradually evaporates from the coating film, the resin molecules in the coating move closer to each other and eventually form a continuous film. The drying rate of these coatings is mainly determined by the rate of evaporation of the solvent, which in turn is closely related to the type of solvent, ambient temperature and air flow.

Oxidative polymerization coatings, such as oil-based paints and alkyd paints, have a more complex drying process that involves both physical and chemical mechanisms. In the initial stage, solvents in the paint volatilize (physical processes); Subsequently, the unsaturated fatty acids in the paint undergo a cross-linking reaction (chemical process) under the action of oxygen in the air, forming a three-dimensional network structure. This process is usually slow and can take hours or even days to fully cure.

The drying mechanism of emulsion-based coatings (e.g. latex paints) is that after the water evaporates, the emulsion particles come close to each other, deform and finally merge into a continuous film. This process is also physical drying, but the quality of the film formation is significantly affected by environmental conditions, especially temperature and humidity.

A key factor influencing natural drying

The natural drying process is affected by a variety of environmental factors, which are not only related to the drying speed, but also directly affect the quality of the final coating film. Temperature is one of the important influencing factors, and the increase in temperature will accelerate the rate of solvent volatilization and chemical reactions. In general, the drying time can be halved for every 10°C increase in temperature. However, too high temperature may cause the surface to dry too quickly, resulting in a "crusting" phenomenon, which hinders the volatilization of internal solvents and affects the drying effect.

The effect of humidity on natural drying is also not negligible. High humidity inhibits solvent evaporation, especially for water-based coatings, where the difficulty of evaporating moisture will significantly extend the drying time. What's more, the cooling of the coating film surface caused by solvent volatilization may cause water vapor condensation, resulting in defects such as whitening and loss of light in the coating. Practice has shown that when the relative humidity exceeds 80%, the drying quality of most paints will be significantly reduced.

The air movement has a dual effect on the drying process. Moderate air flow (wind speed of about 0.5-2m/s) can accelerate the diffusion of solvent vapor and promote drying; However, excessive wind speed may lead to uneven surface drying, and even cause dust pollution. In industrial coatings, a dedicated self-drying site is usually set up to optimize the drying environment by controlling the ventilation system.

Light conditions, especially ultraviolet light, contribute to the drying of certain paints. Oxidative polymerization coatings dry faster when exposed to sunlight because UV light stimulates free radical reactions and accelerates the crosslinking process. However, too much light can also cause the surface to dry too quickly, forming a stress difference with the inside, causing cracking and other problems.

Advantages and disadvantages of natural drying and applications

The advantage of natural drying is its simplicity and economy – it requires no additional energy and equipment investment, is easy to operate, and is particularly suitable for on-site construction and painting of large components. This method does not require the heat sensitivity of the substrate and can be widely used in the coating of non-heat-resistant materials such as plastics, wood, and paper.

However, natural drying also has significant limitations. Slow drying is a major drawback, often taking hours to days to fully cure, severely impacting productivity. The strong environmental dependence also makes it difficult to control the drying quality stably, and seasonal and weather changes may lead to fluctuations in coating properties. In addition, the solvents released during the natural drying process are discharged directly into the atmosphere, which is environmentally stressful48.

In practical applications, natural drying methods are commonly used in the fields of architectural coating, corrosion protection of large steel structures, furniture manufacturing, and automotive repair. Especially for large workpieces or heat-sensitive materials that should not be heated, natural drying is often an unavailable option. In order to improve the drying efficiency, the "forced drying" method is sometimes used, that is, moderate heating is carried out below 80°C, which not only speeds up the drying speed, but also avoids thermal damage to the substrate.

Table: Typical conditions and characteristics of natural drying of different types of coatings

Heating drying method

Heat drying is a drying method that provides energy from an external heat source to accelerate the curing process of the coating film, and occupies a dominant position in the field of industrial coating. Compared to natural drying, heat drying can significantly reduce curing time and increase productivity, while also improving coating properties such as enhanced adhesion, increased hardness and improved chemical resistance. According to the different drying temperature and mechanism, heating drying can be divided into two forms: forced drying and baking drying.

Classification and temperature range of heat drying

Forced drying is a process in which a coating that would otherwise cure naturally is moderately heated (usually below 100°C) to reduce the drying time. This method is suitable for nitro paints, partial alkyd paints, etc., and shortens the drying time from hours to tens of minutes by increasing the temperature to accelerate the solvent volatilization or oxidation reaction. The key to forced drying is to control the temperature not to exceed the tolerance limits of the substrate and coating to avoid thermal damage.

Bake drying is a process designed for coatings that must be heated to cure, such as amino baking paints, epoxy powder coatings, etc. These coatings contain thermoset resins that need to reach a certain temperature to trigger the crosslinking reaction. According to the different curing temperatures, baking drying can be divided into three zones: low-temperature drying (below 100 °C), medium temperature drying (100-150 °C) and high-temperature drying (above 150 °C). The temperature selection depends on the coating chemistry and the heat resistance of the substrate.

Typical coatings suitable for different temperature ranges include: the low temperature zone (60-100°C) is commonly used for forced drying of solvent-based coatings, such as nitro paints (60-80°C) and alkyd paints (90-110°C); Medium temperature zone (100-150 °C) is suitable for amino alkyd baking paint (120-140 °C) and some acrylic resin coatings; The high temperature zone (above 150 °C) is mainly used for powder coatings (170-190 °C), electrophoretic coatings and water-based high-temperature baking paints24.

Heat transfer method of heating and drying

There are three main heat transfer methods for heating and drying in industry: convection heating, radiant heating and induction heating, each with its own characteristics and applicable scenarios.

Convection heating is a commonly used drying method that uses hot air as a medium to transfer energy to the workpiece and coating by convection heat transfer. A typical convection drying chamber consists of a heating system, a circulating fan, a temperature control system and an exhaust gas treatment device. The hot air is evenly distributed through the air distribution board to ensure the consistency of the temperature field. The advantages of convection heating are precise temperature control and uniform heating, which is suitable for workpieces with complex shapes; The disadvantages are slower temperature rise, lower thermal efficiency (typically 40-60%), and the potential for the coating surface to cure first and prevent the internal solvent from evaporating, resulting in defects such as pinholes28.

Radiant heating uses infrared or far-infrared rays (wavelength 0.76-1000 μm) as an energy carrier, which is directly absorbed by the coating and substrate and converted into heat energy. When infrared radiation penetrates the air, the loss is small, the energy utilization rate is high (up to 70-80%), and the heating rate is fast (3-5 times that of convective heating). According to the wavelength, radiative heating is divided into near-infrared (0.76-2.5 μm), mid-infrared (2.5-4 μm) and far infrared (above 4 μm), among which the far infrared is better matched with the absorption band of most coatings. Radiant heating is particularly suitable for the rapid curing of flat or simple shapes, but uneven heating may occur for complex structural parts.

Induction heating is an emerging drying technology that uses the principle of electromagnetic induction to generate eddy currents inside a metal substrate to generate heat. Heat is transferred from the inside to the outside, which is contrary to the traditional heat transfer mode from the outside to the inside, which can effectively avoid the problem of "dry outside and wet inside". The induction heating efficiency is extremely high (up to more than 90%), and the response speed is fast, which is suitable for continuous coating lines on metal substrates. However, the equipment investment is large and is only suitable for the heating of conductive materials8.

Key points of the process of heating and drying

The core parameters of the heat-drying process are the temperature profile and time control, which must be precisely set based on the technical data provided by the paint supplier. In general, the drying temperature refers to the temperature reached by the coating or substrate, rather than the set temperature of the drying environment, and there can be significant differences between the two.

A complete heating and drying process usually consists of three stages: preheating, holding and cooling. The rate of heating needs to be controlled during the warm-up phase (heating zone), especially for solvent-based coatings, where too rapid heating can lead to boiling, pinholes, or wrinkling. Experience has shown that most solvent-based coatings should be heated to within 10-20°C/min, while thick coatings or high solids coatings require a slower ramp up.

The holding phase (curing zone) is the area where the crosslinking reaction takes place predominantly, and it is important to ensure that the temperature and time are at the minimum required for the coating. Insufficient temperature or too short time will lead to incomplete curing, which will affect the coating performance; Temperatures that are too high or too long can cause overbaking, causing the coating to become embrittle or discolored. Modern Drying Ovens are typically designed with multiple temperature zones and precise temperature control (up to ±5°C) via PID control.

The cooling phase is equally important, especially for thermoplastic substrates such as plastics, where the cooling rate needs to be controlled to avoid stress cracking. Forced cooling also shortens cycle times and increases efficiency.

In addition to temperature and time, the following process details should be paid attention to in heat drying: solvent-based coatings should have enough flash drying time (usually 5-15 minutes) before baking to allow most of the solvent to evaporate; The drying furnace needs to maintain a certain air flow rate (usually 0.5-2m/s) and exhaust gas emission (solvent concentration is controlled below 25% of the lower explosion limit); The air in the furnace should be filtered and purified to avoid dust contamination of the uncured coating.

Heating drying equipment with safety

Industrial heating and drying equipment comes in a variety of forms, from simple ovens to complex continuous drying furnaces, and is designed to take into account factors such as production scale, workpiece characteristics, and paint type. Common drying equipment includes two categories: batch oven and continuous Drying Oven28.

The batch oven is suitable for small batches and multi-variety production, the workpiece enters and exits by trolley or suspension, and the heat source can be electricity, gas or steam. Continuous Drying Ovens, on the other hand, are suitable for high-volume production and are typically designed as tunnels, with the workpiece passing through the preheating, curing and cooling zones at a constant speed through a conveyor chain, resulting in high productivity but low flexibility.

The safety risks of heating and drying cannot be ignored, especially the fire and explosion hazards. The flammable vapors released by the coating during the drying process can mix with the air and can create an explosive atmosphere. Safety measures include: setting up combustible gas concentration alarm devices; The exhaust fan is interlocked with the heating system (the heat source is automatically cut off when the fan fails); The range of 3 meters around the charging door of the drying room is the explosion-proof area; Spaces where vapors tend to accumulate, such as pits, need special ventilation.

Table: Heating and drying process parameters for common paint types

Radiation curing method

Radiation curing is a drying technology that uses electromagnetic waves or particle beams to initiate the cross-linking reaction of coating films, mainly including ultraviolet (UV) curing and electron beam (EB) curing. Compared with traditional thermal curing, radiation curing has significant advantages such as low energy consumption, high efficiency, and good environmental protection, and is especially suitable for heat-sensitive substrates and high-speed production lines. UV curing technology has been quite mature since the 60s of the 20th century, while EB curing has shown great potential in specific fields due to its unique advantages.

Ultraviolet (UV) curing technology

Ultraviolet curing is a process that uses ultraviolet light with a wavelength of 300-450nm to trigger the production of free radicals or cations by photosensitizers in coatings, which in turn triggers the resin cross-linking reaction. The UV curing system is mainly composed of an ultraviolet light source, a reflection device, a cooling system and a control unit. High-pressure mercury lamps are commonly used UV light sources, and their radiation spectra have strong peaks at 365nm, 405nm and 436nm, which are well matched with the absorption bands of most photoinitiators.

The UV curing process is extremely fast, typically taking only seconds to minutes to complete, dozens of times faster than traditional thermal drying. The curing speed is affected by three main factors: UV light intensity, irradiation distance, and coating thickness. The higher the light intensity, the closer the distance, and the smaller the film thickness, the shorter the curing time. In industrial applications, the light intensity is usually controlled at 200-1000mW/cm², and the irradiation distance is 10-30cm, which is suitable for coatings with a film thickness of 5-50μm.

The chemical system of UV-curable coatings is special, mainly composed of photosensitive resins (such as unsaturated polyesters, epoxy acrylates, polyurethane acrylates, etc.), reactive diluents (such as various acrylate monomers) and photoinitiators. Photoinitiators are key ingredients that produce free radicals or cations upon UV irradiation, initiating the polymerization of resins and monomers. According to the initiation mechanism, it can be divided into two types: free radical type and cationic type, the former has a fast reaction rate but is sensitive to oxygen, and the latter is slow but not affected by oxygen inhibition polymerization8.

The advantages of UV curing technology are outstanding: the energy consumption is only 10-20% of that of thermal curing; Virtually no solvent emissions, low VOC problem; Fast curing speed, suitable for high-speed production line; It can be operated at room temperature and is suitable for heat-sensitive materials such as plastic, paper, wood, etc. But the technology also has limitations: it only works with clear or translucent coatings; Complex shapes, workpieces may have shadow areas, and incomplete curing; high cost of raw materials; Some monomers may cause skin allergies, etc.

In practical applications, UV curing is widely used in wood coating (such as furniture, flooring), plastic decoration (such as mobile phone cases, home appliance panels), printing and packaging (such as cartons, labels), and electronics industry (such as optical fiber coating, PCB ink). In recent years, UV-LED technology has emerged to offer lower energy consumption, longer life (up to 20,000-30,000 hours) and instantaneous switching compared to traditional mercury lamps, with significantly lower long-term operating costs, despite higher initial investment8.

Electron beam (EB) curing technology

Electron beam curing is a process in which high-energy electrons (usually energy at 150-300 keV) bombard the coating, directly initiate the ionization or excitation of resin molecules to produce active species, and then initiate a polymerization reaction. Compared with UV curing, EB curing does not require photoinitiators, electrons can penetrate opaque coatings, and are not affected by pigments or fillers, making it more versatile.

EB curing equipment mainly consists of an electron accelerator, a scanning system, a shielding room and a control unit. According to the electron generation method, it can be divided into two types: scanning beam and linear cathode. The scanning beam system enables the electron beam to be quickly scanned on the surface of the workpiece through electromagnetic deflection, which is suitable for large-area curing; The linear cathode generates a ribbon electron beam, which is suitable for continuous coating of web materials. To prevent air molecules from scattering electrons, EB curing is usually performed in an inert gas (e.g. nitrogen) atmosphere, and the oxygen concentration needs to be controlled below 100 ppm.

EB curing has many technical advantages: energy utilization is as high as more than 90%; Curable thick coatings (up to 300 μm) and colored systems; Extremely fast response (milliseconds); There is no photoinitiator residue problem. However, this technology also has significant drawbacks: the equipment investment is large (3-5 times that of UV systems); There is an irradiation blind area, and it is difficult to cure complex shape workpieces; Strict radiation protection is required; High operation and maintenance costs.

Due to these characteristics, EB curing is mainly used in high-end applications such as packaging materials (food contact layers), automotive components (wheel hub coatings), electronic components (insulation) and special printing (metal decoration). In recent years, the development of low-energy electron beam (80-150keV) technology has reduced equipment costs and protection requirements to a certain extent, so that EB curing has been applied in a wider range of fields.

Process control for radiation curing

Achieving a high-quality radiation-curable coating requires precise control of several process parameters. For UV curing, key parameters include UV light intensity, spectral distribution, exposure time, temperature, and oxygen concentration. Insufficient light intensity can lead to incomplete curing, which is manifested as sticky or poor abrasion resistance of the coating surface; Overexposure, on the other hand, may cause embrittlement or yellowing. Spectral matching is equally important, and it is important to ensure that the emission peaks of the light source coincide well with the absorption band of the photoinitiator.

Film thickness control is particularly critical for radiation curing, especially for UV curing. Due to the limited penetration depth of UV light, an overly thick coating may result in undercuring of the underlayer. Generally, it is recommended that the single-pass coating should not exceed 50μm, and when a thicker coating is required, a multi-layer construction method should be adopted, and each layer should be cured separately. The electron beam has a high penetration capacity and can handle thicker coatings, but the process parameters still need to be adjusted based on the electron energy.

Substrate properties can also affect radiation curing results. For non-absorbent substrates (e.g. metals), most of the radiant energy will be absorbed by the coating; Absorbent substrates (e.g., wood, some plastics) may compete to absorb radiant energy, reducing curing efficiency. Temperature also has an effect on the curing rate, with a moderate increase in temperature (40-60°C) usually speeding up the reaction, but too high a temperature may cause deformation of the substrate or coating sagging.

The safety of the radiation curing system cannot be overlooked. UV radiation can damage the eyes and skin, so protective shields and warning systems are required; EB equipment needs to be tightly shielded to prevent X-ray leakage. Ozone is a by-product of high-pressure mercury lamps and needs to be discharged or decomposed in a timely manner. In addition, some active monomers are irritating and should be handled with personal protection.

Emerging radiation curing technologies

As technology advances, some new radiation curing methods are being developed. Y-ray curing uses γ rays generated by radioactive isotopes (such as cobalt-60) to initiate polymerization, which has strong penetrating power and can be used for the overall curing of thick products or complex components, but the radiation protection requirements are extremely high, and the current application is limited.

High-frequency oscillation curing is a high-frequency electromagnetic field (MHz-GHz) that causes the polar molecules or ions in the coating to move violently and generate heat, and may directly trigger certain chemical reactions. This method combines the characteristics of thermal curing and radiation curing, and the equipment is relatively simple, but the technology is not yet fully mature.

Plasma curing uses the active particles (free radicals, ions, etc.) generated by gas discharge to initiate coating cross-linking, which can be carried out at room temperature and pressure, and is especially suitable for the preparation of ultra-thin functional coatings. This technology has potential application value in electronics, optics and other fields.

Although these emerging technologies have not yet been industrialized on a large scale, they represent an important development direction in the field of radiation curing, especially for the preparation of special application scenarios and high-end functional coatings. With the advancement of material science and equipment technology, the application scope of radiation curing will be further expanded, and it will play a greater role in green manufacturing and efficient production.

Vapor phase curing and other drying methods

Vapor phase curing is a special coating drying technique that cures the coated workpiece by exposing it to a gaseous catalyst environment, initiating a chemical reaction in the coating. Compared with traditional drying methods, vapor phase curing has unique advantages such as simple equipment, low energy consumption, and no solvent emission, and is especially suitable for some two-component coating systems. In addition to vapor phase curing, there are some special drying methods such as microwave curing, catalytic curing, etc., which play an important role in specific fields.

The principle and process of vapor phase curing

The core mechanism of gas-phase curing is to use a gaseous catalyst to activate the chemical reactive groups in the coating to achieve rapid cross-linking. At present, the mature industrial application is the isocyanate-hydroxyl system, which places the coated workpiece in an amine vapor (such as triethylamine) environment, and the amine is used as a catalyst to promote the polyurethane reaction between the isocyanate and the hydroxyl resin. The amine concentration is usually controlled at 1000-1500ppm, and the basic cure can be achieved after an exposure time of 2-3 minutes, followed by a few minutes in air for final curing.

The vapor phase curing equipment is relatively simple, mainly composed of an amine gas chamber, a vapor generation system, a circulation device and an exhaust gas treatment unit. The key is to transform a conventional Drying Oven into a confined space that can maintain a constant amine concentration and ensure even vapor distribution. Compared with thermal curing, the investment in vapor curing equipment can be reduced by 30-50%, the energy consumption can be reduced by 60-80%, and there is no need for complex temperature control systems.

Vapor phase curing coatings are usually two-component systems, mainly carbamate modified resins. Before construction, the resin components are mixed with an isocyanate-containing curing agent, which needs to be properly leveled (5-15 minutes) after painting, and then entered the amine gas chamber for curing. The advantages of this type of coating are solids separation (up to 100%), no solvent emissions; Fast curing speed and high production efficiency; The coating has excellent performance, good wear resistance and chemical resistance. The disadvantage is that the cost of raw materials is high, and the amine vapor is irritating, so it needs to be protected.

Vapor phase curing technology is currently mainly used in the field of wood coating and plastic coatings, especially for small and medium-sized batch production where quick turnaround is required. Typical cases include furniture components, electronic product shells, automotive interior parts, etc. With the increasingly stringent environmental protection requirements, this VOC-free technology is expected to be applied in more fields.

Catalytic curing technology

Catalytic curing is a method of accelerating the cross-linking reaction of a coating film by adding a catalyst or activating a catalyst in a specific environment. Unlike vapor phase curing, catalysts for catalytic curing are typically pre-mixed into the coating or substrate and can be activated by only the right conditions (e.g., humidity, temperature)8.

A common catalytic curing system is a moisture-cured polyurethane, which uses moisture in the environment to react with isocyanates to produce a cross-linked structure. These coatings are usually one-component, easy to apply, and the curing speed is greatly affected by ambient humidity. It can take more than 24 hours to cure completely in a dry environment, and can be shortened to several hours in high humidity conditions. Moisture curing technology is widely used in the fields of floor paints, anti-corrosion coatings and elastomeric sealing materials.

Another important catalytic curing system is redox initiation, which is commonly used for room-temperature curing of unsaturated polyester resins. By adding peroxides (such as MEKP) and accelerators (such as cobalt salts), free radicals are generated at room temperature, initiating the copolymerization reaction of the resin and styrene monomer. This method is widely used in the fields of FRP products, artificial stone and ship repair.

The main advantages of catalytic curing are simple equipment, low energy consumption, suitable for on-site construction and large-scale coating; The disadvantages are that the curing rate is highly dependent on environmental conditions, which is difficult to precisely control, and some catalysts can cause storage stability issues. In recent years, the development of microencapsulation catalyst technology has revitalized this field, and by encapsulating the catalyst in microcapsules, "on-demand" activation can be realized, greatly improving process controllability.

Microwave drying technology

Microwave drying is a method that uses microwaves (usually 2.45 GHz or 915 MHz) to vibrate polar molecules (such as water and solvents) in a coating or substrate at high speed to generate heat, thereby achieving rapid drying. Unlike traditional heat conduction, microwave heating is volumetric heating, in which energy is directly applied to the inside of the material, so it is highly efficient, heats up quickly, and consumes less energy.

Microwave drying is particularly suitable for water-based coatings and porous substrates (e.g. wood, textiles) because water molecules absorb microwaves strongly. Compared with convection drying, microwave drying can shorten the time by 70-90%, and the drying is more uniform, and it is not easy to produce surface crusting. However, microwaves are highly reflective to metallic materials and are only suitable for non-metallic substrates or specially designed metallic coating systems.

Industrial microwave drying equipment is mainly composed of microwave generator (magnetron), resonator, conveyor system and control system. The key challenge is to ensure that the energy is evenly distributed and that hot and cold zones are avoided. Modern systems improve uniformity with multi-magnetron configurations and mode agitators, and closed-loop control with infrared temperature measurement. Safety is also paramount, as microwave leaks must be effectively shielded to protect the health of operators8.

Microwave drying has been successfully used in the fields of wood coating, paper coating and textile finishing, especially for thick coatings or high moisture content systems. However, factors such as high equipment investment, complex maintenance, and scale constraints limit its wider application. The combination of microwave and other drying methods (e.g., hot air, infrared) is a current research hotspot, which can give full play to their respective advantages and improve the overall efficiency.

Special drying methods and options

In addition to the above-mentioned methods, there are also special drying technologies in the industry that are suitable for specific scenarios. Condensation drying uses a dehumidification system to reduce the environmental dew point and accelerate water evaporation, which is especially suitable for temperature-sensitive aqueous systems. Vacuum drying is carried out in a low-pressure environment, which can reduce the boiling point of the solvent and avoid high-temperature damage, and is used for precision electronic components and optical coatings; Ultrasonic drying uses high-frequency vibration to accelerate molecular movement and promote solvent volatilization, which is still in the experimental stage.

With so many drying methods, there are multiple factors to consider when making a reasonable choice. The type of paint is the primary factor: solvent-based paints are suitable for convection or radiant heating; Aqueous systems can be considered microwave or condensation drying; UV/EB curable coatings require a corresponding radiation source; Two-component reactive coatings, on the other hand, are available with gas-phase or catalytic curing. The properties of the substrate are equally critical: heat-sensitive materials (plastics, wood) should be cured at low temperatures or by radiation; Metal parts can withstand high temperature convection; Complex-shaped workpieces need to ensure heating/radiation uniformity.

Production requirements also directly affect the choice of drying method: high-volume continuous production is suitable for tunnel drying furnaces or UV curing lines; For small batches and large varieties, it is inclined to choose equipment with high flexibility (such as trolley oven); For ultra-high efficiency requirements, EB curing or induction heating can be considered; Stringent environmental requirements may lead to low-VOC technologies such as UV curing or vapor phase curing.

Table: Comparison of the technical and economic benefits of different drying methods

Quality control and safety specifications for coating film drying

The drying quality of the coating film directly affects the final performance and service life of the coating, and the stability and reliability of the drying process must be ensured through systematic process control and testing. At the same time, as one of the links with high energy consumption and many potential safety hazards in the coating process, the safety specifications and implementation of the drying process are directly related to the health of personnel and the safety of the factory. This section will discuss in detail the key points of quality control for coating film drying, the analysis of common defects, and the relevant safety standards and accident prevention measures.

Quality control parameters for the drying process

The temperature-time curve is the control parameter of the heating and drying core and must be precisely set according to the technical data provided by the paint supplier. In actual production, a data logger or infrared thermometer should be used to monitor the surface temperature of the workpiece to ensure that it reaches the specified value and remains for a sufficient period of time. It is important to note that there may be a significant difference between the temperature displayed in the Drying Oven and the ambient temperature and the temperature of the workpiece, and the actual measurement and calibration must be carried out.

Solvent evaporation rate is another key parameter, especially for solvent-based coatings. Too fast evaporation can lead to defects such as pinholes, orange peel, etc., while too slow can prolong the drying time. The rate of evaporation can be controlled by adjusting the temperature, humidity and air speed of the flash zone, and if necessary, the environmental safety can be monitored with a solvent vapor concentration Detector.

Film thickness uniformity has a significant impact on drying quality, and uneven film thickness can lead to differences in dryness. The film thickness should be monitored with a wet film card or in-line Thickness Gauge to ensure that it is within the recommended range of the coating. Especially for UV-curable coatings, changes in film thickness can significantly affect the depth of UV light penetration, resulting in uneven curing.

Environmental cleanliness should also not be overlooked, as dust particles in a dry environment can adhere to the surface of the uncured coating and form defects. The drying furnace or self-drying area should be maintained at positive pressure, and the inlet air should be filtered (usually requiring a filtration accuracy of ≤ 10 μm). For high-gloss coatings, cleanliness is required.

Modern coating lines are increasingly employing in-line monitoring systems that provide closed-loop control by means of infrared spectroscopy, dielectric constant measurement, or mechanical property testing to assess the degree of coating cure in real time. This proactive quality control method can significantly reduce the rate of rejects, but the equipment investment is high.

Common drying defects and their solutions

Surface defects are one of the common problems in the drying process. "Pinholes" are often caused by the solvent evaporating too quickly or by escaping gases from within the coating, and can be addressed by extending the flash drying time, reducing the initial ramp rate, or adjusting the solvent formulation. "Orange peel" is caused by uneven surface tension and can be solved by improving leveling, adjusting the drying rate or optimizing the viscosity of the coating.

"Blistering" is another serious defect that is caused by solvent retention or outgassing of the substrate. Thick coatings, heating too quickly, or high substrate porosity can all cause blistering. Preventive measures include: gradual evaporation of the solvent by segmented heating; Reduced film thickness; Pre-baked substrate removes moisture and volatiles; and the selection of suitable solvent systems to extend the opening time.

"Insufficient curing" is characterized by a sticky, low hardness, or poor chemical resistance of the coating, which can be caused by insufficient temperature, insufficient time, low radiation dose, or catalyst failure. It is necessary to check that the drying equipment meets the set parameters, that the UV/EB system is producing enough energy, and that the paint is within its shelf life. Conversely, "over-baking" can cause the coating to become embrittle, discolored, or lose its shine, requiring a lower temperature or shorter time.

"Poor adhesion" can be one of the signs of improper drying. Adhesion problems can be caused by contamination of the substrate surface, inadequate pre-treatment, or excessive stress during drying. For metal substrates, sometimes an appropriate increase in temperature can improve adhesion; Plastic substrates, on the other hand, may require precise temperature control to avoid thermal distortion affecting adhesion.

Safety specifications for the drying process

Explosions and fires are the main safety risks in the coating drying process. The flammable vapors released by the coating can mix with the air to form an explosive atmosphere, while the presence of high temperatures and ignition sources (e.g., electric sparks, static electricity, open flames) increases the probability of accidents. GB14443-2007 "Safety Technical Regulations for Coating Drying Room" clearly requires: the working area of the drying room and the surrounding range of 3 meters are explosion danger areas; Combustible gas concentration alarm devices must be set up (usually set at 25% of the lower explosion limit); The exhaust system should be interlocked with the heating system (the heat source should be automatically cut off in the event of an exhaust failure).

Exposure to hazardous substances is another significant concern. Solvent vapors released during the drying process, decomposition products, or ozone from radiation solidification can all be hazardous to workers' health. Occupational exposure limits should follow the GBZ2.1 standard, the workplace should be adequately ventilated (usually required to have a wind speed of 0.5-2m/s), and operators should be equipped with appropriate protective equipment (such as gas masks, protective glasses, etc.).

The risk of mechanical injuries should not be overlooked, especially in the conveyor system of continuous ovens. All moving parts should be provided with protective covers, and emergency stop devices should be arranged in an easily accessible position. High-temperature surfaces, such as radiant heaters, need to be protected from scald warnings and isolation measures.

Radiation safety is particularly critical for UV and EB curing systems. UV equipment needs to ensure that the shield is intact to prevent UV leakage from harming the eyes and skin; EB systems must have reliable radiation protection (usually steel or concrete shielding) and interlocks to prevent radiation leakage due to mishandling28.

Optimization and innovation of drying processes

Process optimization is an effective way to improve drying quality and efficiency. Segmented drying strategies can significantly improve the curing quality of thick coatings, such as the combination of infrared pre-drying + hot air curing, which can both quickly remove solvents and ensure complete cross-linking. In terms of parameter optimization, the Design of Experiments (DOE) method can systematically study the interaction of parameters such as temperature, time, and wind speed to find a better process window.

New drying technologies are constantly emerging. Compared with traditional mercury lamps, UV-LED curing has the advantages of low energy consumption, long life, and no ozone generation, and although the initial investment is higher, the operating cost is significantly reduced. Near-infrared (NIR) curing uses short-wave infrared (0.8-1.1μm) to rapidly heat the surface of the coating, which is especially suitable for drying aqueous systems and can reduce energy consumption by 30-50% compared to traditional hot air.

Intelligent control is the development direction of drying technology. The drying furnace monitoring system based on the Internet of Things can collect parameters such as temperature, humidity, and gas concentration in real time, and predict maintenance needs and optimize energy consumption through big data analysis. Artificial intelligence algorithms can autonomously learn the optimal drying curve and adapt to different products and environmental changes. These technological advances are driving film drying to be more efficient, more accurate, and safer.

The concept of green drying is also gaining more and more attention. Heat pump technology recovers waste heat from drying furnace exhaust, which can save energy by 20-30%; The catalytic oxidation system treats organic waste gas to achieve standard discharge; The promotion of water-based coatings and UV curing technology has significantly reduced VOC emissions. In the future, with the increasingly stringent environmental regulations, low-energy, low-emission drying technology will become the mainstream.