Almond skin,a bio-waste for green dyeing of wool fibres

Abstract

During the industrial processing of almond fruits, tons of almond skins are generated. Valorization of blanched almond skin wastes by recovering natural polyphenols using water, instead of organic solvents, for direct application of aqueous baths for dyeing of wool fabrics was studied in this work. The results showed that it was possible to recover polyphenols from almond skins and use them as a dye for wool fibres. Kinetics of extraction was modelled by using the diffusion-based Chrastil’s model and Peleg’s empirical model, obtaining excellent fit in both cases (R² > 0.96). The amount of polyphenol extracted was enough to achieve correct direct dyeing with excellent washing, rubbing and perspiration fastness although the shades obtained were dull. Nevertheless, it was possible to achieve a better coloration by using ion(II) sulfate as the mordant resulting in dyeings with reasonable good washing and rubbing fastness.

Keywords

Almond skin natural dye wool dyeing

Current research on natural colorants focuses, in most cases, on the evaluation of the properties of certain substances of natural origin (mainly plants) to provide color to fabrics. The reasons given to promote such research are usually the pollution caused by synthetic dyes or the valorization of multiple natural substances, either exotic (e.g. Terminalia arjuna) or common (e.g. red onion or eggplant).¹ However, within this general trend it is possible to find a research line mainly focused on optimizing the use of agro-industrial waste or aqueous effluents related to the usual treatment of raw materials.² Such an approach has some particularly interesting features such as the current abundance and ubiquity of wastes and the elusion of treatment technologies for said waste in case of success. The use of nonedible shells, peels or skins from dried fruits would be a paradigmatic example of this approach.

According to the Food and Agriculture Organization of the United Nations (FAO), world production of almonds with shell was 3.5 million tonnes in 2020, the United States being the major producer of almonds, followed by Spain, Iran, Italy, Syria and Morocco.³ During the industrial processing of almond fruits both the shell and the skin are usually removed, particularly to produce almond flour or almond beverages. Therefore, substantial amounts of these by-products are generated. So far, any application of such by-products has been industrially implemented and residues are usually dumped into landfill or burned to obtain energy⁴ without any further valorization that could minimize the related environmental impacts.

Nevertheless, numerous valorization attempts have been under research during the past few years.⁵ On one hand, almond shells have been proposed as adsorbents for heavy metal and dyes,^6,7 as a suitable source in preparing activated carbons⁸ or as a source of lignins, hemicellulose and cellulose.⁹ On the other hand, almond skins, which constitute 4–8% of the total shelled almond weight,¹⁰ were mainly proposed as a source of bioactive polyphenols that stand out for their antioxidant activity.¹¹

Taking the second approach, it has been demonstrated that slightly brown colored phenolic compounds can be extracted from almond shell and skins, similar to what happens with other several plant sources.^12,13 It is worth mentioning that obtaining polyphenols is possible using blanched almond skin by-products, although it is known that the industrial blanching process already removes around 70% of water-soluble available flavonoids and other polyphenols.¹⁴ Thus, blanched almond skins can still be considered as a usable source of a total of 21 flavonoids (flavanols, flavonols and flavonones), and phenolic acids, the major compounds being (+)-catechin, (–)-epicatechin, kaempferol and isorhamnetin.¹⁰

It is important to mention that most natural dyes produce soft shades as compared with synthetic dyes,¹⁵ because they do not show substantivity with textiles, particularly to cellulose fibres, so very often mordants must be applied for a successful tinction. The role of such mordants is generally focused on the creation of coordination complexes between the natural dye and the fiber.¹⁶ However, regarding polyphenolic-based products, mordants are not strictly necessary to increase fixation as it has been proved that polyphenols have the ability to absorb strongly onto protein fibres such as silk and wool.^17–20 Besides, the absorbed polyphenols have the capability to interact with some metals, forming metal complexes of dark shades. For instance, when iron-based salt is used as the mordant, dark brown shades result.²¹ The color change that occurs when the mordant is added can be related to the Fe²⁺-polyphenol oxidation described in the literature.²² These properties are very useful for dyeing applications. Consequently, it seems reasonable to explore the extraction of polyphenolic compounds from blanched almond skins and their valorization in textile dyeing processes as an alternative industrial route for this by-product, which, in case of success, could become an eco-friendly natural dye, following the concept of circular economy.

Obviously, the first step for the application of phenolic compounds as dyes is their extraction, that can be achieved by solid–liquid extraction methods. Usually, such methods are based on the use of polar organic solvents such as methanol or ethanol or, sometimes, mixtures of these solvents with water.^23,24 However, the use of alcohol-based solvent hinders its direct application to textile dyeing operations because dyeing is usually carried out in aqueous media, and an evaporation step to separate the solvent would considerably increase the overall cost of the process. For that reason, although a methanol/HCl mixture in a 1000:1 v/v ratio is, up to now, the most effective solvent for the recovery of polyphenols from almond skins,²⁵ an interesting alternative is to evaluate the possibility of extracting such compounds using water-based baths at high temperature, so as to simplify the further dyeing process. Hence, in order to quantify the polyphenol extraction process, the extraction kinetics with water were examined by application of Chrastil’s model, based in Fick’s second law of diffusion,^26,27 and by the empirical Peleg’s model,²⁸ often used for the description of sorption processes.

The aim of the present work was to explore the possibilities of the valorization of blanched almond skin wastes by recovering natural polyphenols using water, instead of organic solvents, for direct application of aqueous baths for dyeing of wool fabrics. Although some researchers have worked on dyeing with almond shell extracts,^17–19 this work reports on the possibilities of using already bleached almond skin waste as an alternative for extracting polyphenols and using them for producing several brown hues by interaction with different concentrations of iron(II) sulfate. Moreover, the application of iron(II) sulfate as the mordant for the fixation and development of the final color was researched. Color fastness tests were carried out to analyse the effectiveness of the two alternatives.

Materials and methods

Materials and reagents

Woven, scoured and bleached 2/2 twill serge wool fabric weighing 193 g/m² (warp density 29 yarns/cm, weft density 23 yarns/cm), kindly provided by the International Wool Secretariat (United Kingdom), was used in this study, without any pretreatment.

Samples of blanched almond skins were kindly provided by Joan Escoda S.A. (Reus, Tarragona, Spain). Samples were discarded solid wastes obtained after the blanching process, which consists of heating almonds in hot water for a few minutes. To avoid degradation, skins were dried for 48 h at 60°C when received.

Before the extraction process, the dried waste was ground in a cutting mill machine (Retsch SM100, Germany) until the particle size was 2 mm or less. After grinding, the particle size fractions were separated by sieving for 90 min (Retsch AS 200 digit, Germany). The content of dry matter (w_db, in %) was determined by weighing almond skin samples before and after drying at 105°C in a laboratory oven until a constant weight was reached. Equation (1) provides the calculated value of w_db:

w_{d b} = \frac{m_{2}}{m_{1}} \cdot 100

(1)

where m₁ and m₂ are the mass of almond skins before and after drying.

Analytical grade sodium carbonate (>99.5% wt.; Panreac AppliChem, Barcelona, Spain), gallic acid (>99.5% wt.; Scharlab S.L., Barcelona, Spain) and Folin-Ciocalteu phenol reagent (Scharlab S.L., Barcelona, Spain) were used for the determination of total polyphenols. Technical grade iron(II) sulfate heptahydrate (>98% wt.; Panreac AppliChem, Barcelona, Spain) was used as the mordant.

Extraction process

Extracts from previously blanched almond skins were prepared by using hot deionised water as the solvent. Solid samples were mixed with a certain volume of solvent to obtain a defined solid–liquid ratio between 1:100 and 1:20 (mass of solid in g/volume of water in ml) using stainless steel containers of 100 ml. Containers were placed in the water bath of a Linitest K1-25 machine (Original Hanau, Germany) at different temperatures (30, 60 and 90°C) rotating at a frequency of 40 rpm. Extraction kinetics were determined by monitoring the extraction process several times, from 3 to 240 min. Each extraction was performed at least twice. After extraction, suspensions were paper filtered to remove suspended solids and subsequently centrifuged for 5 min at 3500 rpm (Martin Christ, Germany).

Determination of total polyphenols in the extracts

The total polyphenolic content of the extracts was assessed by a modified Folin-Ciocalteu spectrophotometric method,²⁹ using gallic acid as the standard. Each measurement was made as follows: 2 ml of extract was mixed with 1 ml of Folin-Ciocalteau reagent, 20 ml of sodium carbonate solution (75 g/l) was added after 10 min, and the volumetric flask was filled up to 50 ml by using deionized water. Solutions were left in the dark for 1 h before measuring the absorbance at 725 nm (UV/Vis Shimadzu UV-1700 Pharmaspec Spectrophotometer) against a blank sample, prepared with all the reagents except the gallic acid standard.

The total phenolic concentration of the extracts (C) was expressed as gallic acid equivalents (GAE) in units of mg/l. Moreover, GAE per a dry basis (C in mg GAE/g_db) was calculated by using equation (2):

C = c \cdot V \cdot \frac{1}{m} \cdot \frac{100}{w_{d b}}

(2)

where c is the concentration of total polyphenols (mg GAE/l), V is the volume of extract after solid–liquid extraction (l), m is the mass of skins of almonds used (g) and w_db is the content of dry matter (in %).

Kinetic of solid–liquid extraction

Fick’s second law of diffusion is usually used to describe solid–liquid extraction processes.²⁶ However, it is difficult to describe the diffusion rates when the shape and size of the solid particles are not defined with confidence. In order to overcome this issue, Chrastil found, for diffusion-limited sorption on solid substrates, that diffusion rates can be accurately expressed by exponential binomial converging series.²⁷ Taking into account the similarity of sorption and solid–liquid extraction kinetics, it seems reasonable to obtain the extraction kinetics of polyphenols from solid almond skin by adapting the equation proposed by Chrastil (equation (3)):

\frac{C (t)}{C_{e}} = {(1 - e^{- K_{c} \cdot A_{0} \cdot t})}^{n}

(3)

where C(t) and C _e are the concentration of polyphenols at time t and at equilibrium (mg GAE/g_db), respectively; K _c is the specific rate constant which depends on the diffusion coefficient as is defined by Fick’s law (l/g min); A₀ is the concentration of the solid (g/l) and n is the heterogeneous structural diffusion resistance constant, dependent on the steric structure of the system.

Alternatively to diffusion-based models, empirical extraction kinetic models such as those of Peleg, Elovich, Weibull and Patricelli have also been proposed to study the solid–liquid extraction of phenolic compounds from natural sources.³⁰ Among them, the empirical Peleg’s model²⁸ has been demonstrated to be adequate for predicting the kinetics of these kinds of systems also offering the possibility for estimating the initial rate and the maximum extraction yield from their parameters (equation (4)):

C (t) = C_{0} + \frac{t}{K_{1} + K_{2} \cdot t}

(4)

where C(t) is the concentration of polyphenols (mg GAE/g_db) at the extraction time t (min), C₀ is the initial concentration of polyphenols (mg GAE/g_db), K₁ is the Peleg’s rate constant (min g_db/mg GAE) and K₂ is Peleg’s capacity constant (g_db/mg GAE).

Equation (5) shows the extraction rate R(t) as a time derivative of C(t):

R (t) = \frac{d C (t)}{d t} = \frac{K_{1}}{(K_{1} + K_{2} \cdot t)}

(5)

From equations (4) and (5) it can be seen that at very short reaction times the Peleg’s rate constant K₁ relates to the initial extraction rate (R₀ in mg GAE/g_db min). When t → 0:

R_{0} = \frac{1}{K_{1}}

(6)

Besides, Peleg’s capacity constant (K₂) relates to the maximum extraction yield which corresponds to the equilibrium concentration of polyphenols (C_e in mg GAE/g_db min) when time has been enough to reach the equilibrium (t → ∞):

C_{e} = \frac{1}{K_{2}}

(7)

As the rate kinetics constants of both models (equations (3) and (4)) depend on temperature, Arrhenius’ equation can be used to correlate the data (equation (8)):

k_{i} = A e^{- \frac{E_{a}}{R T}}

(8)

where k_i is the rate kinetic constant under study, A is the pre-exponential factor, T is the absolute temperature (K), R is the gas constant and E _a is the corresponding activation energy.

The above-mentioned two models were used to fit the extraction kinetic data by using nonlinear regression analysis software (Data Fit for Windows, version 8.0.32; Oakdale Engineering, PA, USA) for the estimation of their corresponding parameters and constants.

Dyeing of wool fabric

Dyeing was carried out by exhaust method using a Linitest K1-25 (Original Hanau, Germany). Polyphenolic extracts of different concentrations (25, 50, 75, 100, 125 and 159 mg GAE/l) were used to dye 1 g of wool fabric at a liquor ratio of 1:40. Dyeing was performed at 90°C either without a mordant or with meta-mordanting strategy using iron(II) sulfate. After 30 min of dyeing, unmordanted samples were removed from the bath. In the case of meta-mordanted samples, after 30 min of dyeing, wool fabrics were temporally removed from the bath and a suitable amount of iron(II) sulfate was added and dissolved to fit the corresponding concentration of 0.1, 0.2, 1, 3, 5 and 7 g/l (Figure 1). Samples were reintroduced and dyeing continued for 30 additional minutes. For all the experiments, after the afore-mentioned treatment time, samples were rinsed with warm tap water and air-dried at room temperature. A sort of blank samples was also prepared by dyeing the original wool fabric only with iron(II) sulfate.

Figure 1.

Temperature–time chart of meta-mordanting dyeing.

Color characterization of dyed fabrics

Color yield and color coordinates

To characterize the dyeing performance, the CIELab coordinates (L*, a* and b*) and K/S values of the dyed samples were measured using a Minolta M-3600d spectrophotometer (Konica Minolta, USA) under D65 illuminant at 10° observer. Cylindrical coordinates L*, C^* _ab and h_ab were calculated using equations (9) and (10):

C_{a b}^{*} = \sqrt{{(a^{*})}^{2} + {(b^{*})}^{2}}

(9)

h_{a b} = arctg (\frac{b^{*}}{a^{*}})

(10)

K/S values were calculated from the diffuse reflectance R of the samples determined at the wavelength of maximum absorption (λ_max = 400 nm) according to equation (11):

\frac{K}{S} = \frac{{(1 - R)}^{2}}{2 R}

(11)

In addition, the color difference ΔE _CMC(2:1) was determined from L*, a* and b* values according to the ISO 105-J03 standard.³¹

Color fastness

Washing fastness tests were performed according to ISO 105-C06³² selecting the A1S method (T = 40°C, 30 min) using the Linitest K1-25 machine (Original Hanau, Germany). Rubbing fastness tests were performed according to ISO 105-X12³³ using a Crockmeter (Atlas Electric Devices Co., USA). Perspiration fastness tests were performed according to ISO 105 E04³⁴ using a Perspirometer (Atlas Electric Devices Co., USA). For this research, light fastness was not tested, but it is presumed that it would not be adequate because natural colorants do not usually show high light fastness values.^35,36

Results and discussion

Solvent extraction of polyphenols

To study the effect of particle size on solid–liquid total polyphenol extraction, the skins of almonds were milled and sieved, obtaining different size fractions. Fractions were classified into six particle size classes: <0.25 mm, 0.25–0.50 mm, 0.50–0.63 mm, 0.63–0.80 mm, 0.80–1.0 mm, and 1.0–2.0 mm. Fractions were separately weighed and the mass percentage of each class, related to the whole sample, is shown in Figure 2, which provides a particle size distribution in terms of mass. To increase representativeness, determination was done in duplicate, obtaining very close values for two different batches of ground skins. From the chart it is clear to see that two main fractions of approximately 25% of the total mass were obtained, namely 0.25–0.5 mm and 0.63–0.8 mm.

Figure 2.

Particle size distribution of two different samples of milled and dried almond skins.

Samples corresponding to each size fraction were used for polyphenol extraction at 90°C for 120 min and a solid–liquid bath ratio of 1:100. Figure 3 shows the influence of particle size on solid–liquid total polyphenol extraction (C). Contrary to expectations, the extraction of polyphenols was lower for smaller particles and the extraction yield significantly increased as the particle size increased, until it reached the fraction comprising size particles between 1 and 2 mm, which corresponds to the biggest particles studied. It is noteworthy to mention that the increase was much more accentuated for the variation of smaller sizes. A higher extraction yield could be expected for smaller particles because the total available surface increased for smaller particles. The anomalous observed trend could be due to possible particle agglomeration that hinders the effective extraction and the efficient constant between the solid sample and the extracting media.

Figure 3.

Total polyphenol extraction at 90°C for 120 min and at a 1:100 bath ratio for different particle size fractions.

According to the previous results, the particle size fraction of 0.63–0.83 was selected for further experiments as it was the most abundant fraction of larger sizes. Such a fraction was used to study the influence of the solid–liquid bath ratio on extraction, maintaining some of the experimental parameters (working temperature 90°C and maximal residence time 120 min).

Figure 4 shows the following trend: the higher the bath ratio (expressed as grams of solid per millilitre of liquid), the lower the extraction of polyphenols. That might happen because decreasing the volume of the bath for the same amount of almond skin provoked a very inefficient soaking. It is important to remember that almond skins are very light in terms of density, so a very bulky amount of them is needed to fulfil the mass requirements of each experiment. Accordingly, a minimal amount is required so the bath ratio cannot reach high values. Taking these results into account, and considering a practical point of view, the ratio of 1:50 (0.02 g/ml) was selected for further experiments, because a better extraction can be envisaged with this moderate bath ratio.

Figure 4.

Total polyphenol extraction at 90°C for 120 min with almond skin powder (particle size 0.63–0.83 mm) for different solid–liquid bath ratios.

Besides, Figure 5 shows the effect of the temperature (30, 60 and 90°C) on the extraction kinetics of total polyphenols for a solid–liquid bath ratio of 1:50 and using a particle size fraction of 0.63–0.83 mm. In the figure, the symbols represent the experimental data of two different experiments, whereas the continuous or dotted curves show the estimations coming from the regression obtained from Peleg’s equation (equation (4)). Clearly, the extraction yield increased with the temperature, and it was found to be about 3.5 times higher at 90°C than at 30°C. In addition, it is possible to state that at short times extraction was significant, becoming progressively lower as time increased, reaching equilibrium at around 120 min.

Figure 5.

Solid–liquid extraction of total polyphenols from almond skin for different temperatures.

As experimental and calculated values are very close, the extraction can be accurately described by Peleg’s model, for which the main values are summarized in Table 1. In fact, the correlation coefficient (R²) was higher than 0.96 for the three studied temperatures. Thus, Peleg’s model allows a satisfactory prediction of the extraction kinetics even if the structural parameters related to the diffusion-limited systems are unknown in this particular case. Regarding the values reported in Table 1, Peleg’s rate constant (K₁) and Peleg’s capacity constant (K₂) decreased when increasing the temperature of the extraction process. Consequently, the initial extraction rate (R₀) and the maximum extraction yield (C _e in equation (7)) increased with the temperature. The Arrhenius equation was used to estimate the activation energy of the extraction process from the (1/K₁) values determined at each temperature. The obtained value was 20.8 kJ/mol.

Table 1.

Parameters determined from regression analysis of extraction kinetics at different temperatures using Peleg’s model

Temperature (°C)	K₁ (min g_db/mg GAE)	K₂ (g_db/mg GAE)	C₀ = 1/K₁ (mg GAE/min g_db)	C_e = 1/K₂ (mg GAE/g_db)	R²
30	1.5484	0.5692	0.6458	1.7568	0.9634
60	0.8193	0.3242	1.2205	3.0845	0.9661
90	0.3936	0.1577	2.5406	6.3411	0.9777

On the other hand, Chrastil’s model was also used for fitting experimental data. The two parameters of the model were estimated by nonlinear regression analysis and the results are shown in Table 2. In all cases, a good agreement with experimental data was obtained (R² > 0.989) and aspects related to the diffusion process of extraction can be deduced as the K _c rate constant is proportional to the Fick’s diffusion coefficient and n is a structural diffusion resistance coefficient. When diffusion resistance is small, n tends towards 1 (for low resistance films n = 0.9–1.0) and the kinetic is of the first order. Otherwise, if the system is strongly limited by diffusion resistance, n is small (high resistance structures n = 0.5–0.6).²⁷

Table 2.

Parameters determined from regression analysis of extraction kinetics at different temperatures using Chrastil’s model: Chrastil’s rate constant (K _c ), structural diffusion resistance coefficient (n) and correlation coefficient (R²)

Temperature (°C)	K_c (l/g min)	n	R²
30	0.00061	0.1645	0.9936
60	0.00097	0.1768	0.9889
90	0.00140	0.1999	0.9959

From the results, it was observed that the solid–liquid extraction system is strongly limited by diffusion resistance because n is significantly lower than 1, approximately n = 0. 2. Also, n is almost constant for all the temperatures corroborating that n is a structural parameter independent of the temperature and polyphenol concentration. The rate constant K _c depends on the diffusion coefficient, which it is known to increase when the temperature rises. Consequently, an increase of K _c was observed when increasing the temperature from 30°C to 90°C. Using the Arrhenius equation (equation (7)), the pseudo-activation energy of the extraction process was estimated, which was 12.6 kJ/mol.

In conclusion, both Peleg’s and Chrastil’s models were found to be appropriate for estimating the kinetic parameters of the aqueous extraction of polyphenols from blanched almond skin, even if the regression analysis of experimental data provided a slightly higher correlation coefficient when using Chrastil’s model.

Direct use of extracts for dyeing

Water-based bath extracts of different concentrations were used directly for dyeing wool fabric with and without iron(II) sulfate as the mordant. In Table 3, the CIELab coordinates and K/S values of dyeings are shown.

Table 3.

CIELab coordinates and K/S values at λ_max = 400 nm of dyeings during direct dyeing and meta-mordanting with iron(II) sulfate mordant

[Polyphenol] (mg GAE/l)	Iron(II) sulfate (g/l)	L *	a*	b*	K/S (λ_max = 400 nm)
Direct dyeing only with polyphenols
0	0	83.69	–0.53	11.97	0.59
25	0	79.93	1.48	11.48	0.70
50	0	78.50	2.48	12.83	0.80
75	0	76.77	3.43	14.09	0.95
100	0	75.04	4.30	15.05	1.10
125	0	73.34	4.94	15.41	1.22
159	0	71.34	6.02	16.96	1.47
Treatment only with mordant
0	0.1	74.16	2.63	18.53	1.53
0	0.2	68.98	5.29	22.02	2.39
0	1	56.78	15.66	30.68	6.89
0	3	53.86	19.73	33.54	9.27
0	5	56.51	18.49	35.19	8.73
0	7	54.45	20.35	38.07	11.52
Meta-mordanting dyeing with polyphenols
25	5	57.20	5.79	21.17	4.90
50	5	56.68	2.43	16.09	4.13
75	5	54.96	1.17	13.53	4.12
100	0.1	66.97	1.35	7.74	1.30
100	0.2	63.31	0.99	6.95	1.59
100	1	52.65	0.22	8.86	3.58
100	3	53.05	0.46	11.42	4.19
100	5	53.95	0.85	12.73	4.23
100	7	51.78	1.01	12.86	4.87
125	5	51.93	0.65	11.99	4.67
159	5	50.20	0.46	10.86	4.86

Dyeing wool with polyphenols without mordant (PP) was possible and resulted in light brown colors. From the results shown in Table 3 and Figure 6 (PP light grey dots), a slight decrease of the lightness accompanied by a low increase of K/S values when increasing the polyphenol concentration were observed, indicating that the coloration of wool fabric was produced. However, even for the sample dyed with the bath of maximum concentration of polyphenols, the shade obtained was not significantly dark. It was observed that all the values of hue lay in the yellow–red quadrant of the color space diagram and that the variety of shades was not so wide as the hue moved from 82.5 to 70. In this regard, the hue angle of the color decreased around 12.5° when increasing the polyphenol concentration, indicating that the color changed to a redder hue. Conversely, the chroma of the color decreased, so the color became duller (Figure 7, PP light grey dots).

Figure 6.

Lightness versus hue angle of colors obtained when dyeing wool with polyphenols with and without iron(II) sulfate mordant (B: undyed wool blank; M: treatments of wool with iron(II) sulfate at a concentration between 0.1 and 7 g/l; PP: dyeings with only polyphenols at 25, 50, 75, 100, 125 and 159 mgGAE/l without mordant; PP-M: dyeings with polyphenols and iron(II) sulfate as the mordant, a (mgGAE/l)-b (g/l of iron(II) sulfate).

Figure 7.

Lightness versus chroma of colors obtained when dyeing wool with polyphenols with and without iron(II) sulfate mordant (B: undyed wool blank; M: treatments of wool with iron(II) sulfate at a concentration between 0.1 and 7 g/l; PP: dyeings with only polyphenols at 25, 50, 75, 100, 125 and 159 mg GAE/l without mordant; PP-M: dyeings with polyphenols and iron(II) sulfate as the mordant, a (mg GAE/l)-b (g/l of iron(II) sulfate).

From Figures 6 and 7 it can also be observed that lightness of the samples, dyed and meta-mordanted with iron(II) sulfate (PP-M), was reduced significantly compared with the samples dyed only with polyphenols. For samples dyed using a bath with a concentration of polyphenols of 100 mg/l, the increase of mordant from 0.1 to 7 g/l produced a significant decrease of lightness. However, no significant differences in lightness were observed when the concentration of mordant was higher than 1 g/l that may probably be caused by an excess of mordant in relation to the number of polyphenol molecules fixed to the wool fabric. This behavior was also evident when examining the K/S values that became very similar for the concentration of mordant higher than 1 g/l. Increasing the concentration of mordant, the hue angle of the color increased from 80 to 85, indicating that the color changes to a less red hue (PP-M dark grey dots for a concentration of polyphenols of 100 mg/l). In addition, the chroma of the color also increased, so the color became more vivid (Figure 7, PP-M dark grey dots for a concentration of polyphenols of 100 mg/l).

Apart from the changes observed when increasing the concentration of mordant, the properties of the resulting color obtained from direct dyeing wool fabric with extraction baths of an increasing concentration of polyphenols using the same concentration of mordant (5 g/l) was also studied. In this case, the lightness of the samples decreased when increasing the polyphenol concentration, indicating a significant interaction between polyphenol and mordant that resulted in darker colors. In addition, the hue angle of the color increased from 75 to 87.5, indicating that the color changed to a less red hue (PP-M dark grey dots for a concentration of mordant of 5 g/l). Conversely, the chroma of the color decreased, so the color become duller (Figure 7, PP-M dark grey dots for a concentration of mordant of 5 g/l).

Figures 6 and 7 also pointed out that the interaction between polyphenols and iron(II) sulfate is crucial for the development of the final color of the wool fabric, because if mordant is used without the presence of polyphenol (M), the hue and chroma of the resulting color was totally different than that observed for the samples dyed and meta-mordanted, particularly when the concentration of mordant was higher than 1 g/l (M orange dots in Figures 6 and 7). In this case, the hue decreased to values of 60–65° (redder) and the chroma increased to values of 35–45 (more vivid) as the color of the resulting samples are a vivid orange.

When comparing, in terms of color difference ΔE _CMC(2:1) , the colors of all the samples shown in Figures 6 and 7, the result is that only four pairs of samples are indistinguishable: 125–5 and 100–3, 75–5 and 100–5, 100–5 and 100–7, 125–5 and 100–7. The color matching was produced for an intermediate concentration of polyphenol (75–125 mg/l) and a high concentration of mordant (3–7 g/l). The rest of the samples are distinguishable between them, so a variety of shades can be obtained by varying the concentration of polyphenol and mordant (Figure 8).

Figure 8.

Apparent color of dyed wool fabric with increasing the concentration of mordant and polyphenol. Sample codification X–Y where X is the concentration of polyphenol (in mg GAE/l) whereas Y is the concentration of iron(II) sulfate (in g/l).

Color fastness

Analysis of color fastness properties of dyed samples is important because one of the most important limitations of dyeing with natural compounds is its insufficient fastness to specific processes such as washing, perspiration or rubbing. Consequently, the fastness properties of wool fabric dyed with polyphenol extracts (100 mg/l) using iron(II) sulfate as the mordant (5 g/l) were determined and the results are presented in Table 4.

Table 4.

Washing, rubbing and perspiration fastness of dyed wool fabric with and without iron(II) sulfate as mordant

	Wash fastness		Rubbing fastness		Perspiration fastness
	CC	CS	Dry	Wet	Acidic		Alkali
	CC	CS	Dry	Wet	CC	CS	CC	CS
Without mordant	4–5	5	4–5	4–5	4–5	5	4	5
With mordant	3–4	5	4–5	4–5	2–3	5	3	5

CC: color change; CS: color staining: tested for di-acetate, cotton, polyamide, polyester, acrylic, wool; 5: excellent, 4: good, 3: fair, 2: poor, 1: very poor.

The unmordanted polyphenol-dyed wool fabric has been found to have good to very good wash fastness ratings of 4–5 on the grey scale (Table 4). Moreover, in this case, no staining on all types of adjacent test samples was observed with ratings of 5 on the grey scale.

The dyed wool fabric mordanted with 5 g/l of iron(II) sulfate have been found to give fairly good to good wash fastness ratings of 3–4 on the grey scale. Similar to nonmordanted samples, no staining on all types of adjacent fabric was observed having ratings of 5 on the grey scale.

It was observed that rubbing fastness was very good (values of 4–5 on the grey scale) for dyeing carried out either with or without the use of iron(II) sulfate as the mordant. Surprisingly, both wet and dry rubbing fastness measurements provided the same values (e.g. 4–5), although wet rubbing fastness values usually tend to be lower than those of dry rubbing fastness. That can be attributed to the fact that when an incomplete rinsing is performed, the wet rubbing test can remove those dye molecules that are located on the surface, very loosely bonded to the wool fibres and, therefore, which are easily removable. In our case, fabric samples underwent a thorough rinsing after dyeing and before the rubbing fastness measurements, minimizing the afore-mentioned effect. Moreover, it has been proved³⁷ that dry rubbing fastness is primarily influenced by the rubbing force rather than by the types of textile fibre or fabric. Thus, it is important to take into account that from a rigorous point of view, only samples tested in the very same conditions (such as those analysed in this study) are comparable.

Anyhow, it was demonstrated that dyeing wool fabric with polyphenols resulted in colorations with very good rubbing fastness in all cases, regardless of the use of mordant because it does not affect this property.

On the contrary, color change due to perspiration was much more significant, even worse than that attributable to washing. Indeed, color change in acid and alkali media were noticed for mordanted samples obtaining ratings of 2–3 and 3 on the grey scale, respectively. It is presumably that acid-base reactions significantly affect the stability of the complex formed between iron ions and polyphenols. For other polyamide-based materials such as silk, it has been suggested that ionization of the dyes may occur during alkaline perspiration testing or that dyes may detach from the substrate during acidic perspiration treatments.³⁸

Nonetheless, the samples showed excellent behavior against the staining of adjacent fabrics caused by perspiration: ratings of 5 on the grey scale were obtained. It is worthy of mention that perspiration and wet rubbing fastness are quite consistent, which is somehow logical as in both cases fastness is measured as the ability to stain an adjacent fabric and, in all cases, the actual transfer to the adjacent fabric is very low or imperceptible.

Conclusions

Blanched almond skin wastes were valorized by recovering natural polyphenols using water, instead of organic solvents, for the direct application of aqueous baths for dyeing of wool fabrics. According to the kinetics of extraction, up to 5–6 mg of GAE per gram of almond skin was possible to extract with water at 90°C in a time of 30 min. Moreover, the kinetics of extraction was modeled by using the diffusion-based Chrastil’s model and Peleg’s empirical model obtaining excellent fit in both cases (R² > 0.96). From the results, it was observed that the solid–liquid extraction system is strongly limited by diffusion resistance because the ‘n’ parameter of Chrastil’s equation is significantly lower than 1.

The amount of dye extracted from almond skin is enough to achieve the correct direct dyeing of wool, although the shades obtained resulted in light brown colors even using baths of the maximum concentration of polyphenols. However, it was possible to achieve a better coloration by using ion(II) sulfate as the mordant. In particular, meta-mordating wool fabrics using iron(II) sulfate in combination with extracted polyphenols resulted in distinguishable dark brown colors that changed according to the ratio of polyphenol and mordant. Consequently, a variety of shades can be obtained by varying the concentration of polyphenol and mordant. In addition, except for perspiration fastness of mordanted fabrics, color fastness to washing and rubbing was found to be very good in relation to color change and staining, with fastness ratings higher than 3–4 on the grey scale.

Footnotes

Acknowledgements

Authors express appreciation to Joan Escoda S.A. (Reus, Tarragona, Spain) for kindly providing the blanched almond skins used in this study.

Author contributions

Héctor Gómez: Investigation, writing – original draft preparation; Aïda Duran and Remedios Prieto: investigation; Jorge Macanás: formal analysis, reviewing and editing; María Dolores Álvarez: methodology, validation; Fernando Carrillo-Navarrete: supervision, funding acquisition, writing – reviewing and editing.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Fernando Carrillo-Navarrete

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