Abstract:

Objective Imitating the color and spectral reflection characteristics of green vegetation contributes to the military camouflage. Bio-based polyamide 56 has many merits such as high strength, wear resistance, good chromaticity, lightweight and softness. In this research, a bio-based polyamide 56 fabric was dyed in green color to simulate spectral characteristics of vegetation by using acid dyes.

Method In this work, weak acid dye compounds were used to dye the bio-based polyamide 56 fabric to simulate the color and spectral reflection of green vegetation by adjusting the ratio of C.I.Acid Yellow 199, C.I. Acid Blue 324 and C.I.Acid Blue 185. The influence of dyeing, dyes dosage, and blending on spectral reflectance curve from 400 to 1200 nm of dyed fabric were studied. The spectral distance, angle and correlation of the spectral reflectance curves of dyed fabrics were calculated against the standard curves of green vegetation. The fabric color difference and fastness were also measured.

Results When adopting the room temperature dyeing-controlled heating method dying process, C.I. Acid yellow 199, C.I. Acid Blue 324 and C.I. Acid Blue 185 demonstrated a high dyeing rate and yield, and poor compatibility. When adopting the homo-thermal constant temperature dyeing method with 60 ℃ temperature, the compatibility of the dyes was improved. It was revealed that the higher was the dye concentration, the greater was the redshift of the spectral reflection curve. C.I. Acid Yellow 199 has a low reflectivity between 380 and 450 nm, which could be used to simulate the absorption of ultraviolet and visible light by green vegetation, and form a "green peak" at 550 nm during mixed dyeing. The "red edge" of C.I. Acid Blue 324 is at 630 nm, which shows bule shift 50 nm from the green vegetation. C.I.Acid Blue 185 has a strong absorption peak near 680 nm, and the weak absorption peaks around 880 nm and 960 nm without overlapping with water peaks of green vegetation. Therefore, C.I.Acid Blue 185 was used to simulate the "red edge" of green vegetation. The mixture of any two dyes failed to fully simulate the spectral reflectance curve of green vegetation, but that of three dyes was proved successful in simulating the green color with the reflective characteristics. In the spectral reflectance curves of three-dye mixture, a reflection peak was identified at 530 nm, and the "red edge" started from 675 nm. Moreover, the impurity peak at 630 nm was found absorbed by C.I. Acidic Blue 324 with a maximum absorption wavelength of 630 nm. The obtained spectral reflectance curve was relatively similar to that of green vegetation. By further adjusting the dye ratio, the spectral reflectance curve was found closer to that of green leaves. The spectral distance of the dyed fabric and the standard green leaf curve was less than 1.3, the spectral angle was less than 0.1 rad, and the spectral correlation coefficient was close to 1. In the visible/near-infrared range of 400-1 200 nm, the spectral characteristics of green vegetation were accurately simulated, meeting the requirements of GJB 1411—2015. The color of the dyed fabric was simulated against the color of GBJ 1082A—2021 chromatogram DG0850, MG1048 and YG1550 to meet the requirement of color difference less than 3. The color fastness of the stained samples was found satisfactory.

Conclusion The dyeing process affects the dyeing rate and compatibility. When the dyeing process starts at 60 ℃, and holds at 60 ℃ for 40 min, the dyeing rate and compatibility are good. Yellow dyes affect the "green peak", and blue dyes affect the "red edge". C.I. Acid Blue 185 has an absorption peak of 675 nm, which can simulate the "red edge". As the dye concentration increases, the reflectance value decreases, and the spectral value shows a red shift. Dye compounding produces the deep color effect, reduces the reflection value and dye dosage. The dyed fabric meets the requirements of GJB 1411—2015 and GBJ 1082A—2021, which can be applied to military combat uniforms or camouflage nets.

Key words: green vegetation imitation, bio-based polyamide 56, acid dye, spectral simulation, dye compounding, green camouflage, visible-near infrared camouflage