Abstract
Light is one of the most adverse factors for bamboo deterioration and causes surface degradation and discoloration. The study was carried out to initiate efficient and effective photodiscoloration to modify and enrich bamboo colors that may be applied to bamboo dyeing. Different types of bamboo samples commonly used in the industry were studied experimentally under two types of ultraviolet (UV) light. Effects of light sources, radiation time, and distances on discoloration and discoloration mechanism were systematically studied. For both UV 313 and high-pressure mercury light, the bamboo surface turned red-yellow, and color parameters including Δ E , Δ a , Δ b , and C rapidly increased first and then stabilized for long time, while Δ L showed a similar trend, except for the final decrease. Compared with UV 313, high-pressure mercury lamp light was highly efficient and took less time to induce discoloration. Heat treatments darkened the bamboo color and slowed down photodiscoloration, and the higher temperature led to more photostability. The color change of bamboo scrimber under UV radiation was less and slower than that of untreated bamboo due to high density and heat treatment, and the dark carbonized scrimber changed less than that of the light carbonized scrimber. Relatively high contents of isovanillin, syringaldehyde, β-hydroxypropiovanillone, p-hydroxybenzaldehyde, and syringic acid were identified by GC-MS, and syringaldehyde, 2,6-dimethylbenzoquinone, and 3-hydroxy-4-methoxybenzoic acid were identified by HPLC-MS from a bamboo discoloration layer after high-pressure mercury lamp radiation. These products all contained a conjugated double bond and were conjectured to be degraded from lignin or aromatic extracts. Particularly, 2,6-dimethylbenzoquinone which contained a cyclohexenedione structure without benzene rings, was more chemically stable than other conjugated double bonds with benzene rings, and was supposed to be the final product (chromogenic group) during bamboo photodegradation. Untreated or light-colored bamboo under high-energy UV light initiated highly efficient and effective photodiscoloration, and UV light is recommended for being applied to industrial bamboo dyeing. Compared with traditional chemical dyeing, the photoinduced dyeing method has no chemical addition and is easy to operate and environmental-friendly.
Highlights
Color, as one important visual property of wooden materials, is mainly determined by the reflection spectrum that comes from selective light absorption of chemical components [1]
Powder was scraped from the top surface of bamboo specimens with a sharp blade after UV radiation for gas chromatography-mass spectrometry (GC-MS), highperformance liquid chromatography (HPLC-MS), and Fourier transform infrared spectrometry (FTIR)
The peaks at 1047 and 1738 cm-1 which represent unconjugated carbonyl C=O stretching vibration of acetyl groups enhanced. These results indicate the formation of new unconjugated carbonyl groups [18]
Summary
As one important visual property of wooden materials, is mainly determined by the reflection spectrum that comes from selective light absorption of chemical components [1]. Cellulose and hemicellulose are both saturated organic polymers with single bonds and do not absorb wavelength from 200 to 1000 nm, which accounts for their white color [2]. Lignin contains many unsaturated double bonds, such as carbonyl group and vinyl group. The conjugated system links with the benzene ring to form the chromogenic group [2]. Aromatic extracts such as phenols contain conjugated double bonds and strongly absorb visible light. Lignin and extracts are the main sources of wood color [3]. Unsaturated double bonds are very sensitive to light, especially ultraviolet (UV) light. The existing studies on photoinduced discoloration are focused on wood [4,5,6], but rarely on bamboo.
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