Introduction
In the preparation and performance of coatings, the surface drying time of coating film is a key parameter to evaluate the initial curing behavior of coatings, which directly affects the subsequent processing process and the quality of the final coating. Heated automatic coating machines provide highly reproducible experimental conditions for studying the effect of temperature on the coating drying process by precisely controlling the platform temperature. In this paper, the influence mechanism of the temperature of the platform of the heated automatic coating machine on the surface drying time is systematically discussed, aiming to provide data support and theoretical reference for the optimization of coating process in related industries.
Experimental part
The experiment uses a commercial heating automatic coating machine, and the temperature control accuracy of the platform can reach ±0.5°C. The solid content of waterborne polyurethane coating samples was 40±2%. According to the general coating film preparation standard, a wet film is prepared on the substrate at a set temperature using a wire rod applicator, and the thickness of the wet film is uniformly 100 microns. The experimental ambient temperature was constant at 23±2°C, and the relative humidity was controlled at 50±5%. The surface drying time is determined by the finger touch method, that is, the time when the finger touches the surface of the coating without adhesion or traces. Each temperature condition was tested five times in parallel, taking the arithmetic mean as the final result.
Effect of platform temperature on surface drying time
Platform temperature is the main driving force to accelerate the volatilization of moisture in the coating and the initial film formation process. With the increase of platform temperature, the evaporation rate of water in waterborne polyurethane coatings is significantly accelerated, and the kinetic processes of polymer particle coalescence and chain segment diffusion are also promoted. Experimental data show that there is a significant negative correlation between surface drying time and platform temperature. In the experimental temperature range, the surface drying time showed an approximate exponential decay trend with the increase of temperature, which was in line with the general law of drying kinetics.
This process can be described by the following simplified model: The relationship between the drying rate constant k and temperature T follows the basic form of the Arrhenius formula:
k = A exp(-Ea/RT)
where A is the pre-index factor, EaIt is the apparent activation energy, and R is the general gas constant. The surface drying time t is inversely related to the drying rate constant k, that is, t ∝ 1/k. Therefore, an increase in the platform temperature will effectively reduce the time required for the coating surface to dry.
Experimental results
The surface drying time data of waterborne polyurethane coatings measured at different platform temperatures are summarized below. The data clearly show that increasing the platform temperature within a controllable range can effectively shorten the surface drying time.
| Platform temperature (°C) | Average surface drying time (min) |
| 25 | 28.5 |
| 30 | 22.0 |
| 35 | 17.2 |
| 40 | 13.8 |
| 45 | 11.1 |
| 50 | 9.0 |
Data analysis showed that when the platform temperature rose from 25°C to 50°C, the surface drying time was reduced by about 68%. It is worth noting that in the lower temperature range (e.g., 25-35°C), the surface drying time is relatively shortened for every 5°C increase in temperature. In the higher temperature range (such as 40-50°C), the time reduction caused by the same temperature rise is reduced. This may be related to the later stages of water evaporation and the polymer particle coalescence process becoming a rate control step. Excessive platform temperature may cause surface crust to form too quickly, affect the escape of underlying moisture, and even cause coating defects, so it needs to be considered comprehensively in the actual process.
Conclusion
This study confirmed that the platform temperature of the heated automatic coating machine is the key process parameter to control the drying time of waterborne polyurethane coating. Under the experimental conditions, the plateau temperature showed a negative correlation with the surface drying time, and the appropriate increase of the plateau temperature could significantly accelerate the initial drying of the coating. In actual production or research and development, it is recommended to determine the appropriate drying temperature window through systematic experiments according to the characteristics of the specific water-based polyurethane system, so as to ensure the integrity of the coating while pursuing efficiency. Future work can be further combined with the analysis of the coating microstructure to further elucidate the influence of temperature on the film formation mechanism.
References
1. Wang Jianguo, Li Xia. Research progress on drying process of water-based coatings. Coatings Industry, 2020.
2. ASTM D1640 - Standard Test Methods for Drying, Curing, or Film Formation of Organic Coatings at Room Temperature.
3. Zhang Ming, Liu Hui. Application of heated coating equipment in coating preparation. Modern Instruments and Analysis, 2021.
4. ISO 9117 - Paints and varnishes - Determination of drying time.