Detailed Explanation of Coating Drying Process and Its Measurement Method

 

The drying process of coatings is an important transition from liquid to solid state, generally divided into three main stages: surface drying, actual drying, and complete drying.

 

Surface drying is the initial stage of the coating drying process. At this stage, the surface of the coating appears to have dried, but has not yet fully cured.

Appearance features: The surface of the coating does not appear to have a wet luster, and it does not feel sticky when touched. It can be lightly touched without leaving any marks.

Testing methods: Common testing methods include cotton ball blowing method, finger touch method, and small glass ball method. Among them, the blowing cotton ball method is to place a degreasing cotton ball on the surface of the coating, blow the cotton ball with the mouth, and if the cotton ball can be blown away without leaving fibers, it is considered that the surface is dry.

 

Actual drying is the second stage of the coating drying process, also known as mid-term drying. At this stage, the coating surface has dried and is able to resist slight pressure or deformation, but has not yet fully cured.

Appearance features: The surface of the coating has a certain degree of hardness, which can be determined by simple testing methods to determine whether it is actually dry.

Testing methods: Common methods include filter paper method, cotton ball method, blade method, thick layer drying method, and scratch free test method. For example, the filter paper method uses a 200 gram drying tester to press dry filter paper or degreasing cotton balls onto a coating. After a certain period of time, the tester is removed. If the filter paper can fall freely without leaving any marks or the coating does not lose its luster, it is considered that the actual drying has been achieved.

 

Complete drying is the final stage of the coating drying process, also known as the curing stage. At this stage, the coating has fully cured and possesses all the mechanical and chemical properties designed for it.

Appearance features: The coating surface is completely dry, with final hardness, adhesion, and chemical stability.

Time requirement: Due to the long time required for complete curing, the main focus in coating production and application is usually on the surface drying and actual drying stages.

 

Blow cotton ball method

According to the GB/T 1728-79 standard, "Method for determining the drying time of paint film and putty film", by placing a degreasing cotton ball on the surface of the coating and blowing the cotton ball with the mouth, if the cotton ball can be blown away without leaving fibers, then the surface is dry. Suitable for evaluating the initial dry state of coating surfaces, easy to operate, commonly used for rapid on-site evaluation.

 

Lightly touch the surface of the coating with your fingers. If the coating is not sticky to your hands and does not feel greasy, it is considered dry. A simple on-site judgment method is usually used for rough or uneven coating surfaces.

 

Place small glass balls on the surface of the coating and observe if they can roll without sticking to determine if the surface is dry. Used for smoother coating surfaces, it can detect the surface hardening of coatings.

 

Using a 200 gram drying tester (drying weight), press a piece of filter paper or a degreasing cotton ball onto the coating. After a certain period of time, remove the tester and observe the condition of the filter paper or cotton ball to determine the actual drying. Suitable for evaluating whether the coating has dried sufficiently but has not yet fully cured during the mid-term drying stage.

 

Use a standardized scraper blade to scratch the surface of the coating, observe the formation of scratches and the reaction of the coating to determine the actual drying state. For harder coatings or laboratory measurements that require high precision, it can provide relatively accurate actual drying time.

 

Apply a thick coating on the surface of the coating and observe its drying time to determine the actual drying condition of the coating as a whole. Suitable for practical drying evaluation under specific requirements, such as thick coating coating.

 

Apply slight pressure or force on the surface of the coating and observe the reaction of the coating to determine whether it has actually dried. Used to evaluate the durability and drying state of coatings in practical use.

 

Automatic drying time testing instrument, utilizing advanced instruments and technologies such as paper tape method, wheel ring method, gear method, sanding method, scratching method, etc., can achieve precise measurement and recording of each stage of the coating drying process. Suitable for laboratory environments and applications that require high precision, it can provide detailed drying time and state analysis.

 

The drying process of coatings is a crucial part of coating application and quality control. By accurately evaluating and controlling the surface and actual drying stages of the coating, the stability and durability of the coating under different application conditions can be ensured. Various measurement methods, from simple manual testing to precise instrument measurement, provide diverse tools and technical support for the coating industry, helping manufacturers and users optimize efficiency and ensure coating quality in the production process.