The most reliable method of determining actual drying speed is the blot method. This method is applicable not only to laboratory conditions but also to directly measuring the actual drying time of the coating film applied to the utensils during the production process. Experimenting with the finger touch method is a simple experimental method.
A paint film is considered to be practically dry when a finger is pressed against it (without applying any pressure) and the film does not become tacky or leave any finger marks on its surface.
According to the standard, the actual drying time can be determined by the following method. Put a cotton ball on the board with the coating film, and add a wooden board (area 1 cm, with a load of 200 grams) on the ball. After half a minute, remove the load, board and cotton balls and check whether there are cotton fibers or traces of cotton fibers left on the coating film. When the above conditions do not occur, it indicates the moment when the coating film is actually dry.
Leibman and Vihuang Diktova proposed a method based on the different degrees of adhesion of filter paper on the surface of the coating film. The experiment was carried out under the pressure of about 6 kg and the experiment time was 30 seconds. The pressure is from a metal die.
This method gives satisfactory results when testing most paint materials.
Yakubovich and Vorogushindo used the imprint method which had achieved very good results when experimenting with various paint materials. If this method is used, not only the actual drying time of the paint film can be measured, but also the initial stages of different drying processes can be observed.
For the determination, a copper cylinder whose bottom is in the shape of a truncated circle can be used. A soft rubber ring with a smooth surface is pasted on this bottom (1.13 cm in diameter). The diameter of the ring should be equal to the diameter of the bottom, and its thickness should be 1.5-2 millimeters. The bearing area is 1 cm. The total weight of Yuanjian is 200 grams.
When conducting experiments, ordinary typing carbon paper should be used. In order to obtain reproducible results, each experiment needs to use the same standard carbon paper. Therefore, it is better to use carbon paper that meets the requirements of FOCT 489-41.
The experiment should be carried out at a temperature of 20°(±2°)C and a relative air humidity of 60-65%. The dried paint material is pre-dried at an appropriate temperature in a thermostat.
In order to measure the drying time, place the sample with the tested coating film (the thickness of the paint film is 0.02 ~ 0.025 mm) on the surface of the horizontal position, and then place a strip of carbon paper with a width of 15 mm on the paint film, and its hanging color face down. Carefully place the circle on the carbon paper and leave it on the paper for 30 seconds.
Thereafter, the cylinder is removed, the paper strips are removed, and conclusions are drawn on the degree of dryness of the paint film from the characteristics of the impression left by the carbon paper on the paint film. These experiments need to be carried out at certain time intervals stipulated by the technical conditions.
The location of the cylinder and the location of the impression needs to be in a new and different part of the paint film in each test, but not closer than 2 cm from the edge of the board. Four consecutive stages can be marked when determining the degree of dryness.
| Characteristics of imprints | drying stage |
| Paint material stuck to carbon paper | film not yet formed |
| The carbon paper and the paint film are not only adhered together where the pressure of the heavy body is applied, but also on the entire surface where the carbon paper and the paint film are in contact | The "dust free" drying phase begins (drying process) |
| Complete impressions are left only where the weight of the body presses | Dry "clean" and turn into actual dry |
| left no mark | actually dry |
During the test, the test time should be recorded in ink as appropriate where the film is close to the impression.
The diagram shows the different drying stages of enamel. The "dust-free" drying process starts after 15 minutes, and the "dust-free" drying can be achieved after 1.5 hours, and the actual drying can be achieved after another 30 hours.
When determining drying speed according to the imprint method, the concept of actual drying can be clarified as follows. The actual drying (T) of the paint material should be understood as the time from when the paint material is applied to when the formed film (thickness is 0.020-0.025 mm) does not leave a colored imprint of the carbon paper after the action of pressure (additional The bottom area of the weighted body is 1 cm 2 , the weight is 200 grams, the time is 30 seconds, the experimental temperature is 20° (±2°) C, and the relative humidity of the air is 60-65%).
If the actual drying time of the paint film made from natural flax cooked oil is used as a comparison value (T1=24 hours), the actual drying coefficient (K) of various paint materials can be calculated:
K=T2/T1
In the formula, T1—the actual drying time of natural flax cooked oil, which is equal to 24 hours;
T2——the actual drying time of the sample.
K=1 for most oil-based paints; K>1 for most oil-based varnishes and enamels; K<1 for most volatile varnishes and paints (cellulose ester paints, vinyl chloride paints, etc.).
The actual drying coefficient of oil-based varnishes can vary within the range of 1 to 5 depending on the oil content of the varnish; the actual drying coefficient of oil-based paints can vary within the range of 1 to 2 depending on the type of cooked oil ; The actual drying coefficient of various volatile varnishes varies within the range of 0.1 to 1.
