Coating anti-aging experiments commonly used methods

0 Introduction
Coating weather resistance refers to the coating in the external environment (sunlight, air, water, condensation, industrial gases, microorganisms) to maintain the original performance of the ability, the coating in the process of use due to the influence of the external environment and some irreversible damage, such as: color loss, loss of light, powdering, cracking, rusting, peeling, spot and staining. Exposed to outdoor buildings, automobiles, steel structures, etc., each year due to the external coating of color change, powdering, peeling and other aging damage caused huge losses, gradually attracted people's attention and become the focus of attention. Therefore, the weather resistance of the coating is particularly important for the quality of the coating
the
.1 The causes of coating aging
the causes of coating aging include internal and external causes, which are related to the type of film-forming material of the coating and the raw materials such as pigments and additives used, and also have a lot to do with the formulation system of the coating. The external cause of coating aging refers to the exposure of the coating to outdoor environment, affected by a variety of destructive factors, mainly sunlight, rain, condensation, temperature, oxygen, mold and industrial gases.2 coating aging experiments
the methods of coating aging experiments are divided into natural exposure experiments and laboratory artificial climate accelerated aging experiments.
2.1 Natural exposure experiment
Natural exposure experiment refers to the coating directly exposed to the natural environment, to examine the weather resistance of the coating in a specific environment, the natural environment according to the climate characteristics can be divided into dry and hot climate, wet hot climate, sub-humid hot climate, sub-humidity and thermal industrial climate, cold climate and cold temperature plateau climate. Usually the standards used in natural exposure experiments are: GB/T 9276-1996 "Coating Natural Climate Exposure Experimental Method" and ISO 2810:2004 "Color paint and varnish coating natural aging exposure and assessment." The main contents of the standard are as follows:
(1) exposure field
according to the standard settings, there are exposed test board racks, meteorological devices, etc., exposed frame is placed in the exposed field for exposure experiments bracket, made of inert materials, strong structure, exposure rack can be divided into 45 degrees south-facing, south-facing 2 degrees, vertical, horizontal and so on.
(2) Experimental model
Unless otherwise specified, the experimental model substrate shall be made using the substrate actually used, and its treatment shall be carried out in accordance with GB/T 9271-2008/requirements, and other substrates and treatment methods may also be used as agreed. The area of the model should > 0.03 m2, the edge length should be >100 mm, generally stipulated at 250 mm × 150 mm.
(3) Model preparation and coating of the back and edges
unless otherwise specified or agreed, the preparation and thickness of the coating film shall be carried out in accordance with the requirements of the various test products standards. Unless otherwise specified or agreed, after spraying the last 1 drying paint model should be maintained in a constant temperature and humidity room 1 d, since the dry type should be maintained in a constant temperature and humidity room 7 d before the experiment. A suitable coating should be selected to coat each of the 1 model edges and backs, the sealing width is generally 5 mm, and placed until the coating film is dry. It is recommended to use the sample under test or a coating with good weather resistance and corrosion resistance.
(4) Model marking method
Starting with the preparation of the model, the coating sample number applied (unique marking of each paint sample) is marked on the back of each model in the order in which it was applied, and the marking from the back of the model can be traced back to the applied sample after application.
(5) the model is
the model is placed on the exposed frame and the model is secured with a porcelain insulator.
(6) model inspection
model exposure period, may be in accordance with the requirements of product standards or agreed, can also provide for exposure model to reach a certain degree of aging damage as exposure period. The appearance of the model coating film and the physical mechanical properties required for determination should be observed and recorded before the test is cast. Use years or months as the time unit for exposure experiments. Exposed test season is generally stipulated in late spring and early summer each year, unless otherwise specified, the inspection interval is 3 months in the test, every half month to check the model, and then after 3 months to 1 a, the monthly inspection of the model, after exposure of more than 1 a, every 3 months to check the model. When the weather changes suddenly, check at any time. If the provisions in a certain exposure cycle, need to wash the model, before the inspection with cotton yarn in tap water or pure water rinse, dry after inspection; Exposed test board inspection is carried out in accordance with GB/T 1766-2008.
(7) Meteorological Data Collection
A meteorological observation instrument is provided in the exposed laboratory to collect meteorological data every 3 months, including: hours of sunlight, solar radiation, temperature, humidity, rainfall, wind and wind speed.
2.2 Artificial accelerated aging experiment
artificial accelerated aging experiment is the use of laboratory boxes to simulate natural climate effects. Compared with the natural exposure experiment under natural conditions, the artificial accelerated aging experiment only involves several limited factors such as light strength, temperature, spray, condensation and humidity, and controls and adjusts these factors, so as to achieve the purpose of accelerating aging experiment.
according to the use of light source classification, coating aging experiments are commonly used: fluorescent UV lamp type, xenon arc lamp type.
2.2.1 Fluorescent UV-type
fluorescent UV-type aging machine's light source is mainly to simulate the ultraviolet band in natural light, and through the light intensity, temperature, spray, condensation and other factors to carry out accelerated aging experiments. Common fluorescent UV lamps are UVA(UVA-340, UVA-351) and UVB (UVB-313, F40). The contrast between its spectrum and natural light can be found in Figures 1 and 2.
as can be seen from Figures 1 and 2, the emission spectrum of UVA and UVB is concentrated in the ultraviolet band 280 to 360 nm, missing the infrared light portion of natural light, where the light emitted by the UVB-313 lamp contains less than Radiation below 295 nm (natural light cut-off wavelength) may cause some deviation to the aging experiment of the material, but it is still widely used because the experimental method of fluorescent UV lamp aging can quickly examine, compare and select the weather resistance of certain special materials.
standards for artificial accelerated aging experiments with fluorescent UV lamp type are found in Table 1.
2.2.2 Xenon arc lamp type
The main components of this type of aging machine are: xenon arc light source system, filter, water spray system, cooling system, humidification system, irradiance controller, model frame, etc. The cooling mode is air-cooled and water-cooled. There are 2 types of flat and rotating structures.
From Figure 3, it can be found that the spectrum emitted by xenon lamps is very close to the spectrum of natural light, especially in the ultraviolet and visible light parts, so xenon arc lamp aging experiments can better simulate natural light to test the durability of coatings outdoors, many coating products standards using xenon lamp aging machine for artificial accelerated aging experiments.
2.2.3 Main contents of the experimental standard for artificial accelerated climate aging
after the specific method standards have been specified or agreed upon, the experimental parameters shall be determined before the artificial climate aging experiment. Such as: light source type, black label / plate temperature, box temperature, rainfall cycle, light / dark cycle, condensation, irradiation intensity, etc. Generally, there are provisions in the standards, which can also be agreed upon as needed.
(1) Test plate preparation, maintenance and testing
Generally in the relevant product standards for the test plate substrate, test coating, film thickness, maintenance time, etc. have provisions, according to the provisions of implementation, if there are special needs can also be agreed. When the maintenance period of the test coating is over, 1 standard plate is placed in the light-absorbing environment, the rest are put into experiments, and the models need to be rear-rowed regularly on the test shelf to ensure the uniformity of the factors affected by the test plate in the test box.
(2) artificial climate aging experimental time
the experimental time in accordance with the relevant product standards, special needs can also be agreed. It can also be specified to stop the experiment when certain experimental parameters (color change, loss of light, powdering, foaming, rusting, etc.) reach a certain level, and to remove the inspection test board.
(3) Inspection and rating of artificial climate aging experimental models
Test board inspection can be carried out during and at the end of the experiment, and the experimental model can be compared with the standard board during the sample inspection. To observe the abnormality, the inspection may be carried out in accordance with gb/T 1766-2008 "color paint and varnish coating anti-aging rating method", if prescribed or agreed, can also be assessed by other rating methods. 3 Acceleration and Correlation
With the wide application of coating anti-aging experiments, people are becoming more and more interested in the correlation between natural exposure experiments and artificial accelerated aging experiments, and many experiments have been carried out. The following simple correspondence is obtained through the experiment: x h (or MJ) artificial accelerated aging experiment , y month (or year) of natural exposure experiment, thus obtaining an acceleration factor to solve the actual needs. However, the acceleration factor has great limitations. Because acceleration and correlation are a pair of contradictions, acceleration is good, then correlation is poor. No laboratory exposure experiment can completely simulate exposure under actual outdoor conditions, and artificial accelerated aging experiments are no exception.
Acceleration factor is very limited and has no universal significance, because:
(1) based on artificial light source and natural light irradiation of the same amount of acceleration factors, not taking into account the temperature, humidity and other factors on the material, nor consider the performance changes of the material;
(2) acceleration factor is related to the subject material, the acceleration factor obtained by experiments with different types of materials will be very different;
(3) acceleration factor will be related to the performance of the examination, for example, the acceleration factor obtained by color change or loss of light will also be significantly different,
(4) variable factors affecting outdoor exposure and artificial acceleration aging rate will also greatly affect the calculated acceleration factor, such as temperature change can lead to the difference of acceleration factor.
The environmental factors that affect the correlation between artificial accelerated aging experiment and outdoor exposure experiment are:
(1) regional differences, different experimental sites have a great
influence on the results of natural exposure experiments;
(3) Effects caused by differences in the energy distribution of artificial light sources and natural light spectrals;
(4) effects of artificial light source settings higher than the irradiance used in practice;
(5) effects of changes in light and dark circulation set by artificial aging boxes; and
effects of higher sample temperatures in artificial aging tanks than actual temperatures used. 4 Conclusion
Coating anti-aging experiments are becoming more and more widely used in the field of coatings, coating weather resistance as an important indicator to measure the resistance of coatings to environmental impact, the method standard is indispensable in the coating industry. Due to the long period of natural exposure experiment, affected by the uncertainty factors, in recent years, many paint product standards will be developed artificial accelerated aging experiments included in order to test the weather resistance of coatings. The popularization and application of artificial accelerated aging experiments can make the industry better control the anti-aging properties of coatings, thus ensuring that coatings in the actual use of good decoration and protection performance.