The influence of the reprocessing cycles on the color of the PLA pieces was carried out in a Konica CM-3600d COlorflex-DIFF2, from Hunter Associates Laboratory, Inc. (Reston, VA, USA). The color coordinates L*, a*, and b* were collected. L* represents the luminance (black to white), a* indicates the change between green and red, and b* represents the change from blue to yellow. The colorimeter was calibrated with a white standard tile and a mirror device for the black (no light reflection). The white color standard with coordinate values L*a*b* were obtained on five different samples and the corresponding average values and standard deviations were calculated. From this coordinate, it was possible to determine the color difference associated with this space. The distance metric, ΔE*ab, was obtained by following Equation (3) and compared with the color coordinates of the neat PLA (PLA0) piece. By following Equation (4), the yellowness index (YI) was obtained, where X, Y, and Z are the CIE color space values. To this end, the ASTM E313 method was used by considering an illuminant D65 and a standard observer function of 10°.
Color change was evaluated with the following assessment: Unnoticeable (ΔE*ab < 1), only an experienced observer can notice the difference (ΔE*ab ≥ 1 and < 2), an unexperienced observer notices the difference (ΔE*ab ≥ 2 and < 3.5), clear noticeable difference (ΔE*ab ≥ 3.5 and < 5), and the observer notices different colors (ΔE*ab ≥ 5) [61 (link),62 ].
Color change was evaluated with the following assessment: Unnoticeable (ΔE*ab < 1), only an experienced observer can notice the difference (ΔE*ab ≥ 1 and < 2), an unexperienced observer notices the difference (ΔE*ab ≥ 2 and < 3.5), clear noticeable difference (ΔE*ab ≥ 3.5 and < 5), and the observer notices different colors (ΔE*ab ≥ 5) [61 (link),62 ].
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