The strength of the pigment's hiding power is mainly determined by the following properties: refractive index, the larger the refractive index, the stronger the hiding power; the ability to absorb light , the greater the light absorbing ability, the stronger the hiding power; The covering power of amorphous is weak ; the degree of dispersion, the greater the degree of dispersion, the stronger the covering power.
For the same weight of paint products, under the same construction conditions, products with high hiding power can coat . In order to overcome the inaccuracy of visually measuring the hiding power of black and white grid panels, the hiding power was measured with a reflectance meter, and the tested samples were coated on transparent polyhedral film with different thicknesses, and placed on black and white glass plates after drying. Measure the reflectance separately , and the ratio is the contrast ratio. When the contrast ratio is equal to 0.98, it is full coverage. The hiding power can be obtained according to the thickness of the paint film, which is suitable for white and light-colored paints. The following focuses on the measurement of hiding power by the reflectance method.
1. Scope and Description
In this method, the sample is coated on a colorless transparent polyurethane film, or on a cardboard with half and half black and white background colors, and the reflectance of the coating film on the black and white bottom surface is measured with a reflectance meter. The ratio of the reflectance on the white base is the contrast ratio , which is its corresponding opacity. International standards recommend using a reflectance meter to test the hiding power, which has high accuracy, but this method is mainly suitable for white paints and light-colored paints.
2. Apparatus and Materials
Wire Bar Coater, specification 100 (that is, the wire diameter is 100µm ); Gap paint film Applicator, 100µm; Polyurethane film, colorless and transparent, thickness 30-50µm; cardboard with black and half background colors; reflectivity It is a photoelectric instrument that can give indication readings proportional to the reflected luminosity of the tested surface, its accuracy is within 0.3%, and its spectral sensitivity is approximately equal to the relative spectral energy distribution of CIE illuminant C or D65 and that of the CIE standard observer The product of the color matching function y(in), the instrument is composed of a probe, a host, black and white standard plates (one for each), black and white top ceramic plates (two for each), etc. The probe adopts the principle of o· irradiation and diffuse reflection reception. When the reflected light of the sample acts on the surface of the selenium photocell, an electric signal is generated and input to the DC amplifier , and the reading is displayed. When measuring with a reflectance meter, the reflectance of the white standard plate should be (80 ± 2)%' and the reflectance of the black standard plate should not be greater than 1%. The instrument can be used for the determination of the hiding power of the paint film and the measurement of the reflectance.
Note: The instrument should be preheated for more than 15 minutes before use, and zero-adjusted and calibrated repeatedly. During continuous testing, the measuring head should to prevent photoelectric fatigue of the selenium photoelectric cell.
3. Measurement method
(1) Use a wire rod or a gap-type paint film preparer to evenly coat the tested paint , at a temperature of (23±2)°C and a relative humidity (50+5 )% under constant temperature and humidity conditions for 24 hours before testing.
(2) If the coating film is prepared by using the polyhalo film as the base material, drop 200# solvent gasoline spread the paint polyhalo film on it , and ensure that there is no air gap , and then use a reflectance meter to test the reflectance at least four positions, and calculate the average reflectance Rs ( on the blackboard) and Rw (on the whiteboard) .
(3) If the black and white cardboard is used to prepare the coating film, measure the scene reflectance , and calculate the average reflectance Rs (on the blackboard) and Rw (on the whiteboard) respectively. superior).
4 results show
Measure twice in parallel, if the difference between the two measurement results is not greater than 0.02, take the average of the two measurement results, otherwise re-test.
5. Reference standards
National Standard GB/T9755-2001, National Standard GB/T9756-2009, National Standard GB/T9757-2001.
