Overview
In this paper, the color transfer amount was quantitatively determined by simulating dry grinding and wet grinding conditions by using a friction color fastness meter to identify the color transfer characteristics of ink on the surface of the substrate. Dry grinding refers to mechanical friction without liquid media, while wet grinding introduces distilled water or standard solvent wetting system. This method is adjusted for ink systems with reference to the friction color fastness test principle of international standards ISO 105-X12 and GB/T 3920-2008. The amount of color transfer was quantified by measuring the color difference value (ΔE*) before and after friction and the grayscale level of the color staining by a spectrophotometer.
Instruments
Friction color fastness meter: reciprocating linear friction head diameter 16 mm, applied pressure 9 N, friction stroke 100 mm, speed 10 times/minute. Spectrophotometer: Measuring aperture 4 mm, using D65 light source, 10° viewing angle, d/8 geometric conditions. Standard friction white cotton: size 50 mm × 50 mm, moisture content controlled at 100% ± 10% (wet grinding conditions). Substrate: coated paper and polypropylene film.
Dry grinding test process
The ink specimens were placed in a standard environment (23°C, 50% relative humidity) for 24 h. The friction head was covered with a dry white cotton cloth, and a pressure of 9 N was applied vertically to rub the specimen surface 20 times at a speed of 1 time/second. Immediately after rubbing, the white cotton cloth is removed and its color difference value relative to the unused white cotton cloth is measured on a colorimeter ΔE*dry。 Each specimen was repeated 3 times to take the average.
Formula: ΔE* = [ (ΔL*)² + (Δa*)² + (Δb*)² ]1/2
Among them, ΔL*, Δa*, and Δb* are the differences between brightness and color index.
Wet grinding test process
Distilled water is used as the wetting medium for wet grinding conditions. Soak a standard white cotton cloth in distilled water to achieve 100% moisture content (i.e., double the mass). Let the damp cloth sit in a closed container for 10 minutes before use to ensure even moisture. The friction parameters are the same as dry grinding, but the number of frictions is changed to 10 times to avoid breakage of the wet cloth. After rubbing, the cotton cloth was hung and dried in the air for 30 minutes, and then the color difference was measured. Record ΔE* after the test is completed.Wet。
Data recording and judgment
The amount of color transfer is expressed by the ΔE* value, and the color level is evaluated with reference to the grayscale sample card. Grayscale grades range from level 1 (severely stained) to level 5 (no staining). The following is a summary of the test results of typical ink varieties:
| Black water-based ink | Dry grinding ΔE* = 2.1, grades 4-5; Wet grinding ΔE* = 4.8, grade 3 |
| Blue solvent ink | Dry grinding ΔE* = 1.5, grade 4-5; Wet grinding ΔE* = 3.2, grade 4 |
| Red UV curable ink | Dry grinding ΔE* = 0.9, grade 5; Wet grinding ΔE* = 2.0, grades 4-5 |
Key points of the results analysis
The amount of color transfer under dry grinding mainly depends on the adhesion between the ink and the substrate and the cohesion of the ink. Under wet grinding conditions, moisture penetrates and softens the ink film layer, resulting in pigment particles being more likely to fall off and transfer. Solvent-based inks perform better than water-based inks in wet grinding, because water-based inks are sensitive to water, and the film layer is easy to redissolve. The law indicates that ΔE* is directly related to the use conditions, which can provide a reference for the selection of printing process.
Citation basis
Details of the structure and operation of friction equipment in ISO 105-X12:2016 Textiles - Color fastness tests - Part X12: Friction color fastness.
GB/T 3920-2008 "Textiles - Color fastness test - Friction color fastness" standardized description of the gray sample card grading method.
Experimental studies on the determination of ink transfer in printing technology journals emphasize the effect of moisture on pigment release.