Drying time is the general term for the time it takes for liquid coating materials such as coatings and inks to change from the fluid state to a solid paint film after application. This parameter is not only the core indicator in the product technical specification, but also the key basis for guiding actual production, construction and quality control. In industrial production and coating applications, the drying of coating materials is by no means a simple "drying", but a complex process covering physical volatilization and chemical cross-linking, and its completion degree directly affects the final performance of the coating, construction efficiency and the rationality of subsequent processes.
Based on relevant standards and technical data at home and abroad, this paper systematically analyzes the technical connotation, measurement method and key influencing factors of drying time.
1. Technical definition and classification of drying time
In the professional field, drying time is not a single concept, but is precisely defined as several sub-stages according to the different stages of coating curing and film formation and practical needs. This grading is crucial for guiding construction with precision.
Surface drying time- It refers to the time it takes for the surface of the wet paint film to change from liquid to solid and form an extremely thin skin under the specified drying conditions. At this time, the surface of the coating loses its viscosity, and it is non-sticky, which can block the fall of dust, but the inside of the paint film may still be liquid, and the mechanical strength is extremely low.
Practical time- It refers to the time it takes from the beginning of application to the complete transformation of the entire paint film into a solid state to achieve its expected physical and chemical properties. The coating has a stable internal structure that can withstand operations such as light pressing, stacking, or the next overcoat.
For specific materials such as printing inks, there are more detailed functional descriptions of their drying status, such as "dry enough to be reprinted", "dry enough to be cut", or "dry enough to be transported". These descriptions directly link the drying state to the feasibility of the downstream process, reflecting the strong application orientation of this parameter. The international standard ASTM D1640 also clearly states that the terms "drying", "curing" and "film forming" are often used interchangeably to describe this process.
2. Standardized measurement methods
In order to ensure the reliability and comparability of measurement results, domestic and foreign standardization organizations have developed detailed test method standards. These standards strictly define environmental conditions, instruments and operating procedures, and the core purpose is to provide a uniform yardstick.
1. Strict control of environmental conditions
All standard methods emphasize that testing must be performed under stable conditions of constant temperature and humidity to exclude environmental disturbances. For example, early ink drying inspection standards our country required testing to be performed at temperatures of 25℃ ±1℃ and relative humidity of 60%–70%. The American Society for Testing and Materials (ASTM) standard method also covers the different stages of the drying process for determination under controlled laboratory conditions, including low, ambient, and/or high-humidity environments. This ensures that data can be effectively compared between different batches and laboratories.
2. Overview of the main assay methods
According to different principles and application scenarios, the drying time measurement methods can be divided into the following categories:
Finger touch method:The most intuitive empirical method is to judge whether there is stickiness or fingerprints by touching the surface of the coating with your fingers, and is mainly used to roughly judge the dry state of the surface. This method is highly subjective, but simple and fast.
Filter pressing paper/cotton ball method: Qualitative filter paper or cotton ball is placed on the surface of the paint film, and the specified weight is applied, and after the specified time, check whether the filter paper adheres or whether the cotton ball can be blown away, so as to judge the dry state. This is one of the core methods of classical paint film drying time measurement methods (such as GB/T 1728).
Instrumentation:Use specialized automation equipment for objective, repeatable measurements. For example, the principle of an automatic drying tester for ink inspection is to cover the sulfuric acid paper on the ink scrape, apply constant pressure through a mechanical device and periodically automatically check whether the sulfuric acid paper is stained, so as to accurately record the drying time. This type of instrument effectively eliminates human error.
Scratch method:Using a specific needle or blade tool, the degree of dryness is determined by scratching the surface of the paint film at a specified angle and speed, observing whether the paint film is scratched or only scratches are left.
In recent years, the relevant standards have been continuously updated to reflect technological advancements. For example, the newly released GB/T 13217.5-2023 "Ink Drying Test Method" was implemented on April 1, 2024, integrating and replacing the previous inspection method standards for liquid ink initial drying and offset ink conjunctival drying. At the same time, the classic "Determination of Drying Time of Paint Film and Putty Film" (GB/T 1728) is also in the process of revision due to its long marking age, aiming to improve its standard structure and scope of application.
3. Analysis of key factors affecting drying time
The drying process is constrained by both the material's own characteristics and environmental factors, and understanding these factors is a prerequisite for achieving precise process control.
1. Material composition and formulation
This is an internal factor that determines the drying properties. The type of resin of the coating (such as alkyd, acrylic, epoxy), the volatilization rate of the solvent, the volume concentration of pigment (PVC), and whether additives such as drying agents and curing agents are added together form the basis of its drying kinetics. For example, the addition of standard white drying oil can significantly speed up the drying of oxidative polymeric inks.
2. Environmental parameters
Temperature and humidity are the most critical external variables. In general, increasing the temperature can accelerate the volatilization and chemical reactions of solvents, thereby significantly shortening the drying time. The high temperature environment slows down the volatilization of solvents and prolongs the drying process. ASTM D1640 is designed for Method B and Method C to evaluate the effects of low temperature or high temperature and high humidity conditions on coating drying and curing, respectively.
3. Construction and base conditions
The thickness of the wet film of the coating directly affects the length of the path of solvent escape or oxygen infiltration, and the thicker the layer, the slower the drying. The nature of the substrate is also crucial, as porous substrates (e.g., wood, paper) can absorb liquid components from the coating, which may accelerate surface drying but may affect the overall film formation; Drying behavior on non-absorbent substrates (e.g., metal, glass) is completely different. In addition, the air flow rate in the dry environment can also affect the drying rate by dispersing the surface solvent vapor.
4. The value of drying time in industrial practice
In the actual coating and printing production, the precise control of the drying time runs through the whole process.
1. Optimize the production process
In the printing shop, the drying speed of the ink must be precisely matched to the speed of the press, the characteristics of the substrate, and the subsequent folding, cutting, and binding processes. Drying too fast may cause the ink to form on the ink roller and affect the transfer. Drying too slowly will cause the back to rub and delay delivery. Similarly, in the painting line of automobiles and furniture, the drying time (surface dry vs. solid dry) of each coat (e.g. primer, color paint, varnish) determines the minimum cycle time and maximum production capacity of the production line.
2. Guarantee the performance of the final coating
Drying is not exactly the root cause of many coating defects. If the coating is put into use or over-coated before it reaches the dry level, it may lead to the softness of the paint film, insufficient hardness, poor adhesion, and even serious problems such as bottom biting and peeling between coatings. Adequate drying is the chemical basis for coatings to obtain the design anti-corrosion, wear resistance, weather resistance and other properties.
3. Guide on-site construction and maintenance
For on-site construction in construction, shipbuilding, and other fields, the environmental conditions are variable. The construction party needs to reasonably plan the interval of each process according to the drying time data marked by the product, combined with the actual temperature and humidity of the construction site, and formulate necessary maintenance measures (such as shading, ventilation, heating) to ensure the quality of the project.
In summary, drying time is a comprehensive technical parameter that integrates materials science, physical chemistry and process engineering. From rigorous laboratory standard testing to complex and changeable industrial field applications, an in-depth and accurate understanding and application of coating materials is an indispensable part of ensuring better performance and efficient and stable production. With the continuous development of new materials and processes, the monitoring and measurement technology of drying behavior will continue to evolve.