Cationic Pretreatment of Cotton and Dyeing with Fallopia Japonica Leaves Kationska predobdelava bombaža in barvanje z listi japonskega dresnika (Fallopia Japonica)

This work examines the possibility of using leaves from the invasive plant species Fallopia japonica (Japa-nese knotweed) as a source of dye for the natural dyeing of cotton. To achieve a higher uptake of extracted dye, a cationic agent instead of a classical mordant was used to treat the cotton prior to dyeing. Distilled water and 0.5 M NaOH were used as extraction mediums to produce natural dyebaths with diﬀ erent concentrations (10, 20 and 50 g/L) of Fallopia japonica leaves. The colorimetric measurements revealed that a higher concentration of extract, the extraction of leaves in NaOH and a cationic pretreatment of cotton yield a dark-brown-coloured cotton with good wash stability.


Introduction
Invasive alien plant species (IAPS) cause damage to European ecosystems and economies. Th e "AP-PLAUSE" project aims to use IAPS as a resource for the development of new products (e.g., dyes for textiles). Th e rhizome and leaves of Fallopia japonica (F. japonica) have the potential to be used as dyes for textiles. To date, no research has been published on the subject, except for a study on the dyeing of plasma-treated cotton and bamboo rayon with F. japonica rhizome extract [1], where it was found that plasma-treated textile samples had a higher adsorption of dye and, consequently, better antibacterial properties. Th e traditional pretreatment of textiles, when dyeing with natural dyes, is mordanting. Mordants are metal salts that enable a higher adsorption of dye onto textiles, and can be used to achieve different colours with the same natural dye [2]. Alternatives to the classical mordants are plasma, chitosan, and cationic agent pretreatments [3][4][5]. Plasma is a partially ionised gas that enables the formation of new functional groups on the fi bres' surface and/or their micro-to nano-etching [6]. It is an ecologically benign form of textile modifi cation. Natural dyes extracted from curcumin and green tea were shown to have a higher adsorption onto cotton fabric if the fabric was pretreated with ammonia plasma [3]. In contrast, the adsorption of natural dyes was shown to decrease if the cotton samples were pretreated with oxygen plasma. It was found that plasma pretreatment did not infl uence the change in the bathochromic or hypsochromic shift of the absorption spectra of dyed cotton. Th e pretreatment of wool with oxygen plasma was shown to enable a better adsorption of dendrimer with amine end groups, and consequently a better adsorption of cochineal dye [7]. A great amount of dye was absorbed due to the electrostatic attraction between the negatively charged dye molecules and positively charged dendrimers as well as the positively charged amine groups of wool fi bres. Chitosan is poly-cationic amino polysaccharide and has acquired great importance as a new functional material for textile applications mainly because of its biocompatibility and non-toxicity [8]. When it is used to pretreat textiles for dyeing with natural dyes, it serves as a "biological" mordant that enhances the adsorption of the dye, e.g., between wool and tea [9], wool and henna [10] and cotton and black tea [11]. Th e purpose of this research was to examine the potential of F. japonica leaves as a source of textile dye and to use a cationic agent instead of metal mordants to pretreat cotton.

Material
For the research bleached 100% cotton fabric (Tekstina d. o. o., Ajdovščina) was used. Th e Japanese knotweed leaves were collected and delivered by SNAGA d. o. o., washed with water to remove the dirt, air dried at room temperature and grounded into powder.

Preparation of extracts
Th e extracts were prepared in two extraction media, i.e., deionised water (H 2 O) and 0.5 M NaOH. Th e weighted leaves powder (at concentrations of 10, 20 and 50 g/L) was put in cold extraction medium and heated to boiling. Th e extraction of the dye took 60 minutes. Aft erwards, the mixture was fi ltered. Th e extract was further used as a dyeing bath.

Cationic pretreatment
Prior to dyeing, cotton samples were treated with 5% Denimcol FIX-OS (CHT, Switzerland) as a cationic agent. Th e cationic bath included 2.5 mL/L of 32% NaOH. Th e cationic pretreatment was performed at a liquor-to-goods ratio of 10:1, at 50 °C, for 20 min. Th e cotton fabric was neutralised aft erwards with 5 mL/L of 30% CH 3 COOH at room temperature for 5 min.

Dyeing
Dyeing of untreated and cationic pretreated samples was performed in a GyroWash laboratory machine (James Heal, Great Britain), which simulates exhaustion dyeing. Dyeing was performed in the prepared extracts at the liquor-to-goods ratio of 20:1, at 60 °C, for 60 min. Aft er dyeing, the samples were washed with water and air-dried at room temperature.

Wash test
A wash fastness test of dyed samples was performed in a GyroWash laboratory machine according to the ISO 105-C06:2010 standard method. Th e samples were washed 10 times at 40 °C for 45 min. Th e washing solution contained 4 g/L of Standards Development Committee (SDC) standard detergent. Aft er washing, the samples were rinsed twice in distilled water at 40 °C and then air-dried at room temperature.

Colour measurements
Th e colour (CIELAB values) of dyed samples was measured on a refl ectance spectrophotometer (Datacolor Spectrafl ash SF 600 PLUS-CT). All measurements were performed using four layers of fabric with a 9 mm aperture, wherein the specular component was under D65 illumination and a 10° standard observer. An average of 10 measurements was recorded for each sample.

Results and discussion
Th e results of the colour measurements (Table 1) show that the colour is dependent on three parameters, i.e., the initial concentration of F. japonica leaves for extract preparation, the extraction medium (H 2 O or NaOH), and the cationic pretreatment of cotton.  Increasing the concentration of F. japonica leaves for extract preparation caused the dyed sample to become darker (CIE L* decreases), redder (CIE a* increases) and yellower (CIE b* increases), regardless of the medium used for extract preparation or cationic pretreatment. Th e samples that were dyed in extract prepared in NaOH medium were darker and redder. Th e value on the yellow-green axis differed according to the concentration of the used extract; i.e., at a lower concentration (10 g/L) the samples were less yellow (CIE b* decreases) and at higher concentrations (20 and 50 g/L) the samples were more yellow. In the case where the samples were pretreated, the samples that were dyed in NaOH extract were darker, redder and yellower for all used extract concentrations. Th e cationic pretreatment of cotton infl uenced the colour of dyed samples mostly on the lightness axis (CIE L*), as the pretreated samples were much darker than the untreated samples. Th e eff ect was even more pronounced for those samples that were dyed in extract prepared in NaOH medium. Th e dye that is present in F. japonica is emodin [1]. Figure 1 presents the reaction of emodin with NaOH. Th e newly formed compound is red in colour, water soluble and more substantive to cellulose. Th e dye was ionized to yield a negatively charged anion dye with positively charged sodium cations [12]. Th e negative potential of the cellulose (cotton fi bers) can repulse the anions in the dye; however, the large amount of sodium ion content ensures that the negative potential of the fi bers is overcome, so it is not necessary to put any additional common salt in the dyeing bath [12]. Th e cationic pretreatment (which is a reactive polyammonium compound) of cotton ensures that the fi bers have cationic sites onto which anionic dyes can be more easily adsorbed; therefore, the dyeability of the cotton is increased [5].

Conclusion
Th e results of this study show that extract from F. japonica leaves has great potential to be used for the dyeing of cotton in diff erent brown shades (from light to very dark), depending on the amount of leaves used for the extraction and cationic pretreatment of cotton. Th e cationic pretreatment, which rendered good wash stability, can be used in the natural dyeing of cotton instead of traditional mordants. Th e samples that were not pretreated with the cationic agent were found to have very poor wash stability.