Comparison of natural aging and acceleration of powder coatings

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1 Instrument measurement method: Measure and calculate the total color difference (E') between the samples before and after aging in accordance with GB/T11186.
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The color difference is rated as shown in Table 1.

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2 Visual colorimetric method: When the paint film surface is uneven and the color of the paint film surface is two or more colors, etc.
, it is not suitable for the measurement by the instrument method, the visual colorimetric method should be adopted.
In accordance with the provisions of GB/T9761, the aging sample is color-compared with the aging sample (standard version), and the degree of color change before and after the aging of the paint film is graded with reference to the GB250 gray sample card.

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3 Assessment of chalking grade

The evaluation of velvet cloth powdering is carried out according to ISO4627-7, and the powdering grade of the tape method is carried out according to ISO4628-6.

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4 Evaluation of cracking grade

The degree of cracking of the paint film is expressed by the number of cracks in the paint film and the larger down.
If possible, this may indicate the depth type of cracking.
There are three main types of cracking depth: surface cracks that have not penetrated the paint film; cracks that penetrate the surface paint film but basically have no effect on the various layers of the underground paint film; indicate cracks that penetrate the entire paint film system, visible Substrate.

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5 Assessment of bubbles, rust, peeling, mildew, spots and stains

The blistering grade of the paint film is expressed by the foaming density of the paint film and the size of the blisters.
The method of expression is the difference between the bubbling density and the blistering size grade; the rust grade of the paint film is the number of rust spots (rust spots) and rust spots on the surface of the paint film.
The size is expressed by the number of rust spots and the size of the rust spots; the grade of paint film peeling is expressed by the relative area of ​​the paint film peeling off and the average size of the peeling exposed area, and the depth of peeling can be expressed according to the damage level of the paint film system .
The mildew grade of the paint film is expressed by the number of mildew on the coating and the size of the mildew; the grade of coating spots is expressed by the number of spots and the size of the spots; the stain grade is expressed by the degree of staining of the coating.

 3 Natural exposure experiment in Wuhan area

Wuhan, located in the center of China's hinterland, east of the Jianghan Plain, the middle and lower reaches of the Yangtze River, east longitude 113°41'-115°05', north latitude 29°58'-31°22', belongs to the subtropical monsoon hot and humid climate, with abundant rainfall and sunshine Sufficient, four distinct seasons, high temperature in summer and concentrated precipitation at 3.
0°C; the average temperature in July is 29.
3°C, which lasts 135 days in summer; each season is about 60 days in spring and autumn.

Taking Wuhan area as the experimental site, the entire experimental frame is facing due south and inclined at an angle of 45 degrees.
The outdoor exposure is carried out for 800 days according to the method in the national standard GB/T9276-1996 "Coating Natural Climate Exposure Test Method".

The outdoor powder coatings on the market mainly use pure polyester/TGIG system, but the pigments used are more extensive.
In this regard, we choose a system to study the color difference changes of different pigment coating films, and draw the change curve as shown in Figure 4 by collecting data on the color difference value of the coating film after exposure.
It can be seen from the figure that the color of the coating film has been slightly discolored after 800 days of exposure, especially the discoloration of light colors is significantly greater than that of dark colors.

4 Laboratory weather resistance test (artificial accelerated aging test)

The damage to the coating film in nature mainly comes from long-term oxygen activity and ultraviolet light irradiation.
The artificial accelerated aging experiment is a means to obtain the damage of the smear in a short time by simulating outdoor natural conditions in the laboratory.
It is a kind of For the rapid evaluation method of the difference in the weather resistance of the coating film, the experiment is mainly completed by an artificial accelerated aging instrument.

What we use is the QUV/spray UV accelerated aging test machine from Q-Lab of the United States, which can simulate environmental conditions such as ultraviolet light, rain, condensation, darkness, and thermal shock in natural climates.
By reproducing these conditions, they can be combined into A cycle, and can automatically execute the number of cycles to complete.
During this process, the irradiance of the lamp tube, the temperature in the test chamber, and the spray state can be automatically monitored.
Compared with other types of lamps (including xenon arc lamps), fluorescent ultraviolet lamps are more stable.
The spectral power distribution (SPD) will not change with the aging of the lamp, even after 5000 hours of use.
In this way, more repeatable test results can be obtained, while reducing lamp replacement and reducing operating costs.

 At present, domestic powder coating weather resistance testing methods mainly refer to the standard GB/T1856-2009/ISO11341:2004 in coatings, so we combine the actual situation and establish our own corporate standard operating parameters on the basis of this standard.

In the actual test process, there are still various problems.
After a large amount of data experiments, it is found that the position of the sample plate in the instrument is different, and the degree of light exposure is also slightly different.
At the same time, the weather resistance of the texture type pattern variegated powder coating test is controversial.
To deal with these problems, we only test the weather resistance for flat single-color coatings, and regularly replace the position of the sample during the test to reduce experimental errors.
Figure 5 is a graph drawn from experimental test data.

5 Comparison and conclusion of two kinds of test data

After a large number of experimental data analysis, combined with the change curves of Figure 1, Figure 4, and Figure 5, in the outdoor pure polyester/TGIC system of a single-tone flat coating film, the weather resistance and Wuhan area are determined according to the standard data of Hubei Laisi Enterprise Corresponding relationship of the tested weather resistance is:

QVB-313 tube light source irradiates for 300h, which is equivalent to 11 months of natural exposure in Wuhan;

The QVB-313 tube light source irradiates for 400h, which is equivalent to 18 months of natural exposure in Wuhan;

QVB-313 tube light source irradiates for 500h, which is equivalent to 24 months of natural exposure in Wuhan.