The wet cup method is established to test the water vapor pass rate of the coating film

1 Introduction:
different applications, there are different requirements for the water vapor through the coating. Such as cultural relics protection paint, in the process of use, not only requires a better protection and decorative role, but also has a higher requirement for its vapor permeability, so that the moisture of the substrate evaporates out at a faster rate, to avoid water vapor retention caused by paint film foaming, shedding. There are very few reports on the test method of coating water vapor through, and there are no requirements for coating by national standards, so it is necessary to establish a test method for this indicator
The
. There are two ways to choose from the water vapor pass rate of the test material, namely, the dry cup method and the wet cup method. Mattel and others have introduced the dry cup method, taking into account that the external wall coating contact with the atmospheric ambient humidity is often relatively large, it is necessary to establish a wet cup method to determine the water vapor through the coating. This method was established in this study with reference to U.S. data.
2 Experimental Method
2.1 Experimental Principle
Add a certain amount of water to the test container, seal the opening of the test container with the test coating film (the sealant must not affect the water vapor through or water vapor pressure, and will not gain or lose weight), under the prescribed test conditions, weighing once every 24h, do weight) time map, the water vapor pass rate is calculated by the slope. and make a membrane thick) water vapor pass rate map.
2.2 Materials and Instruments
Coatings (homemade), Coaters (15Lm, 25Lm, 80Lm, 100Lm), flat glass bottles 3, beeswax, pine, analytical balance, glass plate, PVC film, plastic sheet, tin foil.
2.3 Preparation of free coatings
the homemade cultural relics protection coatings evenly. Take the appropriate amount of paint evenly placed at one end of the substrate, hands uniform force, continuous one-time completion of the scraping coating, after the scraping cool 4h, and then scrape the second coating, and then at constant temperature and humidity, that is, temperature (23 taxi 0.6) oc, relative humidity (50 s 2) % of the conditions of maintenance for two days, the coating film from the substrate peeling, and then in constant temperature and humidity conditions for 7 days; Carefully examine the coating surface, remove the diaphragm with holes, folds, stains and other defects, and light transmission to check the uniformity of the film thickness;
2.4 wet container
contain distilled water in a dry, clean container, 6mm above the mouth of the bottle. Seal the spare free coating film evenly and tightly at the mouth of the wet container, and then seal it with a molten mixture of beeswax and pine, so that the water vapor does not leak from the edge of the bottle.
2.5 Experimental permeable area and conditions
The permeable area of the experiment was: 2015.45mm2, 2009.88mm2, 2351.37mm2, 2379.99mm2, 3155.35mm2, That is, the diameter corresponds to: 50.67mm, 50.60mm, 54.73mm, 55.06mm, 63.40mm. The experiments were conducted at temperature (23 taxis 0.6) and relative humidity (50 taxis 2)%.
2.6 sample weighing and measurement cycle
using an analytical balance weighing permeable cup with an accuracy of 0.0001g, recording the weight and weighing time of the permeable cup, each weighing cycle is 24h, and each weighing time is controlled within 10min. After 7 days of the experiment, compare the quality changes of the permeable cup, do the time) mass map to find the slope.
2.7 The results of the experiment are represented and calculated
using the water vapor pass rate (WVT) to represent the experimental results:
WVT s $m/ (A-t)
: WVT is the water vapor pass rate, g/ ( h- m2 $m); A for experimental permeable area, m2; t is the measurement time, h.
3 Results and discussion
3.1 paint film substrate selection
for a variety of film substrate selection test, think that the glass plate as the substrate, the film layer uniform continuous smooth, easy to peel, and low price of glass, reusable, more suitable.
3.2 Coating film thickness experiment
according to the requirements of different paint samples, the use of a certain specification coater, in a clean and smooth glass plate to prepare the sample coating film, to determine its thickness. The experimental results show that the coating thickness of the coating device is smaller than the size of its corresponding specifications, and with the decrease of the size of the coater, the error is gradually reduced. The 80Lm coater has a coating thickness error of 25% and the 25Lm coater has a coating thickness error of 10%. Therefore, the thickness of the dry coating film must be accurately measured with a membrane thickness meter.
3.3 Water vapor pass rate experiment
3.3.1 water vapor transmission rate
with different film thickness free coating film at temperature (23 taxi 0.6) oc, relative humidity (50 s 2)% conditions, every 24h test the weight of the permeable cup. Do quality) time diagram, data can be found in Table 1.
the flow rate of water vapor through different membranes
with time as the horizontal axis, mass as the vertical axis, for the return of the line equation, as shown in Figure 1. As can be seen from Figure 1, the quality of the permeable cup is evenly reduced over time. Explain that for a certain thickness of free coating film, its vapor permeability is a constant value, that is, equivalent to the straight line slope in the figure.
Water vapor transmission rate and time relationship
3.3.2 film thickness on the effect of vapor permeability

the relationship between film thickness and permeable
the effect of film thickness on permeable gas is shown in Figure 2. In addition, the unearrality of the coating film thickness also affects the vapor permeability of the coating film, which directly affects the quality of the coating film protection function.
effect of film thickness on vapor permeability
Conclusion
(1) This method is simple and practical, the test data is accurate, the error is small, suitable for laboratory, factory quality inspection department to test the vapor permeability of the coating film.
(2) During the preparation of free film, attention should be paid to the separation of the membrane from the substrate. Must ensure that the film to be continuous, uniform thickness, smooth surface without bubbles, pinholes, fish eyes.
(3) from the experimental results can be seen that the protection of cultural relics paint vapor transmission performance is good. The permeability of the coating film is very relevant to the thickness of the film, the vapor permeability decreases with the increase of the thickness of the film, and the permeable area of the coating film has little effect on the permeable rate.