Yellowness index is used to characterize the yellowing degree of white samples, which can be transparent or opaque. The yellowness index is in good agreement with the degree of yellowness observed under sunlight, so it can evaluate the quality and aging degree of plastics. Calculation formula of yellowness index: Yi = 100 (1.28x-1.06z) / y. Where, x, y and Z are the three stimulus values representing the color of the material measured with a colorimeter, color difference meter or spectrophotometer under the standard C light source. The CHNSpec spectrophotometer series (as shown in Figure 1) CS-580, CS-600, CS-610, CS-650 and CS-660 can test the degree of yellowing.

Figure 1 - CHNSpec spectrophotometer series

At the request of a transparent plastic plate manufacturer, the customer sent a plastic plate sample and asked the CHNSpec instrument to test the yellowness of the sample. The following describes how to test the yellowness of transparent plastic plates.
Measuring instrument: CS-600 spectrophotometer of Hangzhou Caipu Technology Co., Ltd
Measurement conditions: standard C light source: the color temperature is 6774k. It is composed of a light source (gas filled spiral tungsten filament lamp with color temperature of 2856K, and its light color is yellow) and a C-type D-G liquid filter. The light color is equivalent to cloudy sky light.
Observer angle: 2 °
Determination range: it is applicable to the test method of yellowness index of colorless, transparent, translucent and nearly white opaque plastics. It is suitable for plate, sheet, film, powder and granular samples, but not for plastics containing fluorescent substances.
Test steps:
Step 1: prepare three samples of the instrument and the transparent plastic plate to be tested (as shown in Figure 2). Samples 1, 2 and 3 are respectively from left to right. Sample 1 is the plastic plate before aging, sample 2 is after aging for 12 hours, and sample 3 is after aging for 24 hours;

Figure 2 - instrument and sample to be tested

Step 2: calibrate the instrument and calibrate the instrument in black and white (as shown in Figure 3 and Figure 4);

Figure 3 - Black calibration

Figure 4 - white calibration

Step 3: after the black-and-white calibration is completed, select through the up and down selection keys, and press the "enter" key to enter the "measurement setting" page. Select Yi in the display setting, i.e. yellowness (as shown in Figure 5), select C light source in the light source selection (as shown in Figure 6), then select "Observer" through the up and down keys, press "enter" to enter the observer selection page, and select 2 ° observer angle (as shown in Figure 7);

Figure 5 - display yellowness

Figure 6 - select C light source

Figure 7-2 ° observer angle

Step 4: after the measurement conditions are set, the test can be started. When testing the yellowness of transparent plastic plate, it is necessary to pick up the sample and put it in the air. There is no need to pad an object at the bottom of the sample. After picking up the sample, directly align the test hole of the instrument with the sample to be tested, press the "print" key, first locate the measurement area by image, then press the "test" key, and then complete the measurement after "dripping" sound, and view the test results (as shown in Figure 8);

Figure 8 - yellowness index of sample 1

Step 5: repeat the measurement steps, measure the other two samples respectively, and the yellowness can be measured directly (as shown in Fig. 9 and FIG. 10).

Figure 9 - yellowness of sample 2

Figure 10 - yellowness of sample 3

After the measurement is completed, △ Yi represents the difference of yellowness index. Through the measurement results, it can be seen intuitively that the longer the aging time, the higher the aging degree of the sample, and the greater the value of yellowness. The spectrophotometer of CHNSpec can quickly and conveniently measure the yellowness index and yellowness of transparent plastic plates through simple manual operation, which can fully meet the requirements of manufacturers for product quality inspection.