Measurement principle
Based on spectral reflectance measurement, the spectrophotometer uses a d/8° geometric optical structure (diffuse illumination, 8° reception) to irradiate the ink sample by xenon lamp or LED light source to detect the intensity distribution of reflected light in the visible light band of 380-780 nm. The instrument has a built-in grating spectroscopy system that decomposes the reflected light into continuous spectra, and the array detector obtains the reflectance data of each wavelength point. Occlusion is defined as the ability of an ink to mask the color difference of the substrate, calculated according to the ISO 2814 standard by comparing the difference in reflectance of the ink on the black and white substrate.
Sample preparation
The ink sample was evenly coated on the black and white standard backing paper, the thickness of the wet film was controlled at 100 μm, and a flat film layer was formed after drying. Prepare 3 parallel samples per sample and place them in a standard environment (23 °C, 50% relative humidity) for 4 hours. The optical properties of black and white backing paper should be ensured that the white background reflectance is ≥ 85%, the black background reflectance is ≤5%, and the spectrum is flat and there is no fluorescence whitening interference.
Spectral measurement steps
1. The instrument is preheated for 10 minutes and calibrated using a matching standard whiteboard (traceable to national benchmarks).
2. Place the ink sample in the measuring hole, measure the white background and black background area three times in turn, and record the spectral reflectance of each wavelength point.
3. Take the average of the 3 measurements as the final reflectance data. If the reflectance deviation of any wavelength point exceeds 0.5%, the sample needs to be retested.
Data calculation model
The occlusion force value is calculated by the transformation of the Kubelka-Munk theory. First, the reflectivity R∞ (infinitely thick film reflectivity) of the ink film layer on the black and white substrate is obtained as follows:
R = (1 - ρg) × (1 - ρ0) × (1 - R∞²) / (1 - ρg × R∞)
R is the measured reflectance, ρg is the reflectance of the substrate, and ρ0 is the reflectance coefficient at the interface between the ink and the substrate (usually 0.04). The R∞ is solved by numerical iteration, and then the occlusion force ratio CR is calculated:
CR = (R_white - R_black) / (R_∞ × (R_white + R_black)) × 100%
CR value > 98% is considered to be completely occluded.
Table 1: Example of a typical ink sample coverage force calculation
| Wavelength (nm) | Coverage CR (%) |
| 400 | 78.3 |
| 500 | 92.1 |
| 600 | 97.6 |
| 700 | 99.2 |
The data in the table show that the short-wavelength light scattering is weak, and a thicker coating is required to achieve high occlusion. In actual production, it is necessary to weigh the color vividness and coverage according to the needs of the ink application.
Analysis of influencing factors
The measurement error mainly comes from:
1. For light scattering variation caused by uneven film thickness, it is recommended to use an automatic coating machine to control the accuracy of ±2 μm;
2. For the difference in the number of substrates, the standard test strips specified in ISO 2814 should be selected;
3. The linear deviation of the instrument's photovoltaic deviation needs to be verified quarterly with standard color filters.
In addition, the particle size distribution of pigments in inks has a direct impact on the light scattering efficiency, and the particle size-occlusion force correlation can be analyzed with the help of SEM (scanning electron microscopy).
Application expansion
The spectrophotometer can simultaneously obtain the Lab chromaticity value, chromatic aberration ΔE and homochromatic index of the ink. In actual production, the masking force index is used to control the thickness of the printing ink layer, evaluate the dispersion efficiency of pigments, and predict the effect of multi-layer overprinting. For example, in packaging printing, inks with low coverage power need to increase the number of printing passes, which directly affects cost and efficiency.
Cited source
1. International standard ISO 2814: Coatings and varnishes - Method for determination of covering force (comparative method).
2. ASTM D2805: Evaluating occlusion by reflectance measurement.
3. Printing Ink Technical Manual, Internal Technical Literature of the Chemical Industry (3rd Edition), 2018.