The physical or chemical change process of liquid paint applied on the surface of an object from a fluid layer to a solid paint film is commonly known as the drying of the paint. The drying process is generally divided into three stages: surface drying, actual drying and complete drying. From the perspective of construction, it is better to hope that the paint film dries quickly, which can shorten the time and cost of painting; but the faster the better, the quality of the paint film needs to be the premise. The drying speed also needs to be compatible with the leveling property of the coating, which should be considered comprehensively. Since the paint requires a long time to dry completely, it is generally common to measure both surface dryness and actual dryness.
1) Determination of surface drying time
The methods for measuring surface drying time include: blowing cotton ball method, finger touch method and small glass ball method. It is specified in CiB6753.2-86.
According to the provisions of GB1728-89, the measurement conditions and methods for the dryness of the coating film surface are as follows:
(1) Materials and equipment
Tin plate: 50xl20x(0.2~0.3)mm;
65X150X(0.2~0.3)mm;
Copper sheet: T2, hard, 50X100x(0.1~0.3)mm;
Aluminum plate: LY12,50X120xlmm;
Absorbent cotton ball: lcm3 loose cotton ball;
Qualitative paper, standard weight 75g/m2, l5x5cm;
Drying Tester: weight 200g, area 1cm2;
Stopwatch: division is 0.2s;
Safety blade and electric blast box.
(2) Measurement method
First, according to GB1727-79 "General Preparation Method of Paint Film", prepare a paint film on a tin plate and a copper sheet, and then dry it under the conditions specified by the product.
At intervals or when the product standard is reached, within a range of not less than 1cm from the edge of the film surface, use the measurement method specified in GB1728-89 or (] B6753.2-86 to test the drying time of the coating film surface.
① Blowing cotton ball method Gently place a absorbent cotton ball on the coating film, use your mouth to keep the cotton ball 10~15cm away, blow lightly along the horizontal direction, if it can be blown away and there is no cotton thread left on the film surface, the surface is considered to be dry .
②Finger touch method Lightly touch the surface of the paint film with your fingers. If you feel a little sticky, but no paint sticks to your fingers, the surface is considered to be dry or called finger dry.
③Small glass ball method Drop a small glass ball weighing about 0.5g with a diameter of f125~f250mm from a height of 50~150mm onto the surface of the paint film. When the small glass balls on the paint film can be brushed off gently by the brush without damaging the surface of the paint film, the surface is considered to be dry. Record its time and compare it with the corresponding relevant standards, and judge whether it is qualified according to the product regulations.
2) Determination of actual drying time
The actual drying time is usually measured by filter paper method, cotton ball method, blade method and thick layer drying method. There are detailed regulations in the national standard GB1728-79 (confirmed in 1989). In the IS091l7-1990 standard, there is a method of applying a load to the coating to determine the degree of complete drying.
① Pressing paper method: Press a piece of qualitative filter paper (smooth surface in contact with the coating film) on the paint film with a drying Tester (see Figure 8-2), remove the Tester after 30 seconds, and turn the sample down, if the filter paper If it can fall freely or tap a few times with your fingers on the back of the model, the filter paper can fall freely without sticking to the filter paper fibers, which means that the coating film has reached the actual dryness.
② Cotton ball pressing method: Same as the filter paper pressing method, use a drying Tester to press a degreasing cotton ball on the surface of the coating film for 30 seconds, then remove the Tester and the degreasing cotton ball. Although there are 1~2 cotton threads left, they can be gently flicked off with a cotton ball, which means that the coating film has reached the actual dryness.
③Blade method: Use a safety blade to cut and scrape the coating film on the coating film sample, and observe that there is no adhesion between the bottom layer of the paint film and the inside of the film, that is, the paint film has reached the actual dryness. This method is suitable for thick coating film and putty film. The thick layer drying method is mainly used for insulating varnishes.
The drying time of the paint film is affected by factors such as the temperature, humidity, ventilation, and light of the surrounding environment. Therefore, when measuring, it needs to be carried out in a constant temperature and humidity room under certain environmental and equipment conditions. (Related instruments: constant temperature and humidity Test Chamber)
Due to the drying of the coating film and the formation of the coating film is a relatively slow and continuous process. In order to observe the entire change during the drying process, an automatic drying time Tester can be used for measurement, and the entire process of the formation of the paint film and the drying can be recorded.
