Drying Time Determination: Simultaneous Verification of Surface Drying Test Method, Through-Drying Test Method, and Instrumental Test Method

Drying time is a key parameter for evaluating the process performance of coatings, inks, adhesives, and building materials, which directly affects the construction efficiency and final quality. Traditionally, the drying process is divided into two stages, surface dry and solid dry, corresponding to surface curing and complete curing, respectively. With the advancement of technology, instrumental measurement methods are becoming increasingly popular because they can provide objective and continuous quantitative data. The purpose of this paper is to explore the principles of surface stem assay, solid stem assay and instrumental assay, and analyze their correlation and applicability through simultaneous verification, so as to provide a reference for quality control in related fields.

Surface stem determination

The surface dry assay is used to evaluate the time of initial curing of the surface of the material. Common methods include finger touch, cotton ball and small glass ball. The finger touch method lightly touches the surface with the fingers, and if there is no adhesion, it is judged to be the surface stem; The cotton ball method lightly places the cotton ball on the surface, and after removal, there is no cotton residue to the surface dryness; The small glass ball rule dumps the glass ball of the specified diameter on the surface, and most of the spheres fall off after reversing. These methods are easy to operate, but rely on subjective judgment and are susceptible to environmental and human factors.

Practical measurement method

The dry assay is used to confirm that the material is completely cured internally and is usually performed after the surface has dried. Filter press paper, blade method and thick layer drying method are common methods. The filter paper press method places the filter paper on the surface of the specimen and applies a standard weight, and after removal, there is no trace or loss of stickiness. The blade method uses a blade to scrape off the coating to the substrate, and if there is no sticky substance remaining, it is judged to be dry. These methods can better reflect the actual state of use, but they are time-consuming and may cause damage to the specimen.

Instrumental measurement method

The instrumental method achieves objective measurement by monitoring the changes of physical parameters during the drying process, which is mainly divided into gravimetric method, mechanical method and optical method. The gravimetric method uses a thermogravimetric analyzer to record the mass loss rate, and when the loss rate is close to zero, the drying end point can be determined, and the drying rate can be approximately expressed as:
ΔW/Δt ≈ k· (W₀ - W_e)
where ΔW/Δt is the mass change per unit time, k is the drying constant, W₀and W_eThey are the initial and equilibrium masses. The mechanical method uses a drying time recorder to judge the curing stage by the resistance change of the scribe needle in the movement of the coating. The optical law uses reflectance or transmittance to monitor surface formation. Instrumental data is accurate and traceable, but the equipment cost is high.

In order to verify the correlation of the three methods, a simultaneous experiment was designed: the same batch of water-based acrylic coating was selected and the samples were prepared under a standard environment (temperature 23±2°C, relative humidity 50±5%). The surface dry and solid dry measurement is carried out in accordance with relevant national standards, and the instrument is monitored in parallel with the thermogravimetric analyzer and the drying recorder. The time points of each method were recorded, and the data discreteness was compared.

Analysis of results

The synchronous verification data showed that the instrument measurement results were in good agreement with the traditional method, with a difference of less than 10% in surface drying time and about 5% in actual drying time. The instrumental method can clearly record the drying curve, revealing that the drying process can be divided into acceleration period, constant speed period and deceleration period, while the traditional method only provides the end point judgment. In the constant speed period, the drying rate is mainly controlled by surface evaporation. After entering the deceleration period, internal diffusion becomes dominant, and the corresponding practical stage begins. The traditional method determines the surface stem near the end of the constant speed period and the solid dry near the end of the deceleration period, which is consistent with the inflection point position in the instrument data. However, for thick-coated or composite systems, traditional methods may increase deviations due to human error, and instrumentation can provide more reliable continuous monitoring.

Conclusion

The surface dry method, the solid dry measurement method and the instrumental measurement method have their own advantages: the traditional method is low cost and convenient for rapid on-site evaluation; The instrument method has high accuracy and is suitable for in-depth analysis of R&D and quality control. Simultaneous verification shows that the results of the two can be mutually corroborated, and it is recommended to choose or combine them according to the needs in practical applications. For routine inspections, the traditional methods in the standard can be used; For new material development or dispute arbitration, priority is given to instrumental measurement methods to obtain objective data. Future work can further explore the relevance of various methods under different environmental conditions and promote the standardization of instrumental methods.