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    Dyes and Pigments

    Dyeing of cotton with the natural dye extracted from waste leaves of green tea (Camellia sinensis var. assamica)

    Due to an increase in the production of green tea, the amount of leaf waste has increased enormously, causing serious environmental problems. With regard to environmental awareness, the possibility of reusing the waste leaves of green tea as a low‐cost and abundantly available source of natural dye for dyeing cotton fibres was investigated. Natural dye powder from the waste leaves of green tea (NDPT) was successfully applied to dye cotton fibres without mordant by batch experiments. NDPT was obtained as a dark brown powder with a yield of 2.7 ± 0.5% w/w from dried waste leaves of green tea. The optimal conditions for dyeing NDPT onto cotton fibres were: pH of dye solution, 3; material to liquor ratio, 100:1; dyeing time, 180 min; concentration of dye solution, 3.0 mg/ml; and dyeing temperature, 100 °C. The colour of cotton fibres dyed with NDPT was observed to be dark brown. The adsorption data of NDPT on cotton fibres was best fitted with a Langmuir adsorption isotherm model with a correlation coefficient (R
    2) of 0.997. It is clear that there is a strong possibility of reusing the waste leaves of green tea as a low‐cost and abundantly available source of natural dye for dyeing cotton fibres.

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    Source:: Coloration Technology

          

    In situ fabric coloration with indigo synthesised in flow

    Indigo (CI Vat Blue 1) is a water‐insoluble pigment exhibiting no affinity for fibres and must be chemically reduced in basic solution to form the water‐soluble, alkaline leucoindigo (CI Reduced Vat Blue 1), in order to exhibit substantivity for fibres. Typical vat dyeing processes are time‐ and resource‐intensive, and hazardous by‐products are formed, primarily through the use of reducing agents. We describe a method for synthesising indigo in a flow reactor that allows for application of dye precursors to fibres, milliseconds before the reaction completes. The soluble precursors soak into the cotton fabric just prior to the precipitation of the insoluble indigo, effectively providing in situ coloration, without the need for a traditional redox dyebath. The reaction may be coupled with a propellant, an adaptation that allows for a sprayable form of indigo. In situ coloration with Tyrian purple (6,6′‐dibromoindigo; CI Natural Violet 1) was also demonstrated using the flow chemistry method. This research provides compelling proof of concept, but we acknowledge that the process is in its infancy and needs further development to reach a stage where it can compete commercially with current technology.

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    Source:: Coloration Technology

          

    Comparison of madder (Rubia tinctorum L.) and weld (Reseda luteola L.) total extracts and their individual dye compounds with regard to their dyeing behaviour, colour, and stability towards light

    This paper reports on the colorimetric properties of natural dyes from madder and weld plants. For both crops, a comparison is made among several individual dye compounds and the total plant extracts. Dyeing properties, colour properties and colour stability have been studied. Insight has been gained into the role of the major dye compounds present in the plant. For madder as well as for weld, the use of the total plant extract is considered to be advantageous over the use of individual dye compounds in terms of solubility, colour tone and stability towards light, and non‐coloured components present in the plant extract are considered to be responsible for increasing the solubility and the stability of the dye compounds.

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    Source:: Coloration Technology

          

    The effect of ultraviolet‐ curable water‐borne polyurethane acrylate binder concentration on the printing performance of synthetic leather

    In this paper, synthetic leather samples were screen printed with pigmented pastes including two types of photoinitiators and three different concentrations of ultraviolet (UV)‐curable water‐borne polyurethane acrylate binder. The curing process was conducted under different combinations of lamps (gallium, mercury, gallium/mercury and gallium/gallium/mercury) at three power levels. Abrasion resistance, crock fastness, hardness and colour strength were investigated. Chemical changes in the clear and pigmented film structures because of UV curing were analysed by Fourier Transform‐infrared spectroscopy measurements. In hardness measurements, the highest hardness values were obtained with clear and pigmented formulations which have the highest solid content (57%). In colour measurements, higher K/S values were obtained in samples printed with the formulation having a binder concentration of 46%. Wet crock fastness values improved as the energy level increased during curing, and the highest values were obtained with a formulation which had a binder concentration of 57%. Greater amounts of binder in the formulations and increased amounts of energy applied to the surface during curing increased the hardness value of the prints, thus better abrasion resistance was obtained. Overall results suggested that the highest hardness, crock fastness and abrasion resistance values were obtained with the formulation with a binder concentration of 57%. However, for ease of application, printing efficiency and colour strength, the formulation with a binder concentration of 46% is recommended for printing, and curing under consecutive passes with gallium and mercury lamps at 120 W/cm is proposed in terms of energy efficiency and printing performance.

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    Source:: Coloration Technology

          

    Preparation of melamine‐formaldehyde encapsulated fluorescent dye dispersion and its application to cotton fabric printing

    In this study, CI Solvent Yellow 43 was encapsulated by melamine‐formaldehyde (MF) resin via in situ polymerisation to prepare the core‐shell structured fluorescent pigment. Fourier Transform‐infrared spectroscopy, transmission electron microscopy, scanning electron microscopy, thermogravimetric analysis and differential scanning calorimetry were used to characterise the encapsulated CI Solvent Yellow 43, indicating that MF successfully encapsulated CI Solvent Yellow 43 and that a core‐shell structure was formed. The prepared MF encapsulated fluorescent dye dispersion was also applied to flat screen printing of cotton fabrics, and the colour properties and fastness properties (washing and rubbing fastness) of printed fabrics were studied. The results showed that the encapsulated CI Solvent Yellow 43 printed cotton fabric exhibited a higher chroma and fluorescence intensity than that printed with unencapsulated CI Solvent Yellow 43. Moreover, the washing and rubbing fastness of the encapsulated CI Solvent Yellow 43 printed fabric was improved.

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    Source:: Coloration Technology

          

    A sultone‐based reversible dark red‐yellow conversion thermochromic colorant with adjustable switching temperature

    A reversible, adjustable thermochromic colorant with bichromatic conversion from dark red (cool state) to yellow (heat state) was prepared using bromocresol purple (BCP) as colour former. The thermochromic behaviour of this colorant was investigated during the heating‐cooling cycle, including the colour depth, colour difference and switching temperature. The mechanism of thermochromic colorant switching between dark red and yellow originated from the transformation of the conjugated structures of BCP, which was confirmed by infrared spectrometry. The switching temperature can be flexibility adjusted between 13–46 °C by mixing solvents according to Schröder’s equation, which was demonstrated by differential scanning calorimetry measurement. The thermochromic colorant presented good reversible thermochromic performance with stable changes in colour parameters for 50 heating‐cooling cycles. This sultone‐based reversible dark red‐yellow conversion thermochromic colorant is suitable for application as thermal‐indicating material under various conditions.

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    Source:: Coloration Technology

          

    Extraction of polyphenolic substances from bark as natural colorants for wool dyeing

    In Europe, considerable amounts of bark are available from wood‐processing industries such as forestry and timber production. Polyphenolic components can be collected by hot water extraction. The extracted compounds can then be applied as colorants in textile dyeing operations. In this study, a comparative assessment of four different tree species with regard to their colouristic potential for wool dyeing was performed. Aqueous extracts from alder, ash tree, spruce and oak bark were prepared and analysed for their total phenolic content and ultraviolet (UV) absorption at 360–370 nm. The extracts were used for meta‐mordant dyeing by adding iron sulphate mordant (FeSO4 × 7H2O). For comparison, iron salt‐based dye lakes were prepared and used in dyeing experiments. For each tree species, a specific correlation between the total phenolic content of the dyebath and the colour depth in terms of K/S and CIELab coordinates was observed, both for the aqueous extracts and the dye lakes. Based on this relationship, standardisation and quality control of raw materials and dye lakes can be installed as important stages in the industrialisation of natural colorants from bark. The preparation of concentrated dye lakes permits formation of a concentrated colorant as dye product, which then can be standardised and delivered to textile dyehouses, similar to synthetic dyes. The preparation of dye lakes offers a relevant route towards achieving the commercialisation of bark extracts as natural colorants.

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    Source:: Coloration Technology

          

    Fungal colorants in applications – focus on Cortinarius species

    Secondary metabolites in fungi offer an interesting source of bio‐based compounds that could be used as colorants in many applications. From a historical point of view, fungal natural dyes have been used more rarely than plant‐based dyes. This paper investigates the potential of fungal colorants, using Cortinarius species as examples. In our research, fruiting bodies of the fungi Cortinarius sanguineus and Cortinarius semisanguineus were used as sources of anthraquinone dyestuffs. From 10 kg of fresh fruiting bodies, 60 g of anthraquinone powder was obtained, 6% of the dry weight content. The most abundant compounds were emodin, dermocybin and their glucosides, which formed over 90% of the total dyestuff amount. Pure emodin and dermocybin, as well as the crude water extract, were used for the dyeing and printing of natural and synthetic fibres. Conventional mordant techniques and high‐temperature (HT) disperse dye techniques were applied, and light and washing fastness were tested according to International Organization for Standardization standards. Our experiments show that the yields of dye powders extracted from fungi are reasonable compared with the yields of, for example, madder (Rubia tinctorum). Natural anthraquinones produce strong and bright colours on several types of fibres. In particular, for HT disperse dyed polyester, the light and washing fastness properties were excellent. Anthraquinones are common in nature and there are many fungal species which produce them, so there are a variety of possibilities for growing fungi. The use of large‐scale cultures is an interesting perspective for future biocolorant production.

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    Source:: Coloration Technology

          

    The colour properties of polyester/cotton knitted fabrics coated with poly(N‐vinyl‐2‐pyrrolidone)

    The objective of this research was to investigate the use of crosslinked poly(N‐vinyl‐2‐pyrrolidone) (PVP) to coat polyester/cotton knitted fabric, without adversely affecting its dyeing properties. Before dyeing, the knitted fabrics were tested for bursting strength to assess the influence of the coating on their resistance. The dyeing parameters were evaluated as the exhaustion (%), K/S value, colour difference (ΔE), relative strength (RS %) and colour fastness to washing. Bursting strengths were 9.4 for coated and 9.7 kgf cm−2 for uncoated knitted fabric samples, confirming an insignificant loss in resistance. In the evaluation of K/S, ΔE and RS%, the values for the samples with the highest concentration of PVP were the most different to those for the standard sample. The colour fastness showed satisfactory results indicating that neutralisation and washing after dyeing were effective. These results could lead to increased quality in the textile industry, adding value to products.

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    Source:: Coloration Technology

          

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