Aluminium based dye lakes from plant extracts for textile coloration

Production of concentrated natural dyes is a pre-requisite for a re-introduction of plant colorant based dyes into modern textile dyeing operations. Aluminium salts such as Al2(SO4)3.14-15H2O or KAl(SO4)2.12H2O can be used to precipitate extracted plant dyes from aqueous extracts at pH 5.0–5.5. Onion peel, Canadian Goldenrod and pomegranate peel were studied as representative sources for dye extraction. As an average 5% w/w of the extracted dry plant material could be collected as precipitate. After dissolving these residues in diluted oxalic acid, the quality of the dye lake was characterised by photometric analysis of the total phenol content in the dry using the Folin-Ciocalteau method, determination of the aluminium content and measurement of the absorbance at 400 nm. Representative values of TPH in the dry solid dyestuff range from 20 to 40% and representative values for the aluminium content were determined with 3–5% w/w. Colour strength of the dissolved lakes was determined in dyeing experiments using different substrates and mordants followed by measurement of CIELab coordinates and K/S value according Kubelka-Munk function. Compared to the direct use of plant extracts the colour strength of the lakes is lower, however chroma of the dyeings is higher, as the lake formation also represents a dye purification step.

► A solid natural dye product can be precipitated from flavonoid dye containing extracts.
► Aluminium sulphate can be used to precipitate natural colorants.
► Preparation of a concentrated natural dye is a central step for technical use.
► Dyeings that result from aluminium dye lake are lighter but more brilliant in colour.