Application of Forest By-Products in the Textile Industry: Dyeing with Pine and Eucalyptus Bark Extracts ^†

Abstract

The main by-product generated in the forestry industry is bark, derived from the debarking process. Pinus and eucalyptus are nowadays two of the most important tree species exploited by the forestry industry in southwestern Europe. This work investigates the application of Maritime pine (Pinus pinaster Ait.) and Eucalyptus (Eucalyptus globulus Labill.) barks as a source of high polyphenolic content extracts, to be used as natural dyes in the textile industry. We demonstrated that it is possible to use the extracts obtained from both forest by-products as textile dyes without the need for using any metallic mordant.

1. Introduction

High water consumption, together with the use of synthetic dyes and metallic mordant agents, contribute to the high environmental impact of the textile industry. Numerous investigations have focused on the search for more sustainable raw materials and processes for this sector. One of the most promising solutions is to look towards forest by-products as a sustainable source of fibrous raw materials to substitute plastic fibers and replace partial cotton. In addition, forest by-products could also be a good source of natural dyes and textile additives, replacing synthetic ones. The main by-product generated in the forestry industry is bark, derived from the debarking process. Pinus and eucalyptus are nowadays two of the most important tree species exploited by the forestry industry in southwestern Europe. Eucalyptus globulus Labill. bark has been shown to be a valuable source of extracts with a high content of hydrolyzable tannins, and, especially, of gallotannins, similar to those present in commercial tannic acid [1,2]. Pine bark probably is one of the most investigated raw materials in obtaining extracts with high polyphenolic content, highlighting the presence of condensed tannins [3], with applicability as a component of adhesives [4], or biopolymers [5]. Recent studies have shown that tannins and tannic acid could be an interesting alternative to metallic mordant in the dyeing industry [6].

This work investigated the application of Maritime pine (Pinus pinaster Ait.) and Eucalyptus (Eucalyptus globulus Labill.) barks, as a source of high polyphenolic content extracts, to be used as natural dyes in the textile industry. Extraction was performed with water in alkali conditions. The influences of extraction conditions, in obtaining extracts to be used as natural colorants, and the dyeing conditions (pH, temperature, use of mordant), on the properties of the dyed textile samples, were evaluated. It was shown that the pH and the extraction conditions used were the variables with the greatest influence on the final properties of the dyed textile. In addition, this work also demonstrated that it was possible to use the extracts, obtained from both forest by-products, as textile dyes without the need to use any metallic mordant.

2. Material and Methods

2.1. Raw Materials

Eucalyptus globulus Labill. (EB) and Pinus pinaster Ait. (PB) bark were ground in a cutting mill (Retsch, Haan, Germany) equipped with a 4 mm sieve. The products were oven-dried at 60 °C until attaining equilibrium moisture content. Then, the PB and EB particles were subjected to a vibratory sieve shaker (Retsch, Haan, Germany). The particles between 0.5 and 2 mm were selected as the raw material to obtain the extracts. These were then used for dyeing 100% cotton knit samples. The textile samples were supplied by TINTEX-TEXTILES, S.A. Sodium hydroxide was provided by Acros Organics (Branchburg, NJ, USA), and Sodium sulfite anhydrous by PanReac AppliChem (Barcelona, Spain).

2.2. Extraction

The extractions were performed using only water as solvent, avoiding the use of organic compounds. The extraction agents were sodium hydroxide and sodium sulfite. The bark particles and water were mixed at room temperature, heated and, once the selected temperature was reached, the alkali was added. After the selected contact time, the suspension was vacuum filtered and the extracts were concentrated in a rotary evaporator (BÜCHI, Flawil, Switzerland). The tested extraction conditions are shown in

Table 1. Extraction conditions on extraction yields.

Raw Material	Extraction Agent		Temperature (°C)	S/L	Time	ID Code
	NaOH (%)	Na2SO3 (%)			(min)
Eucalyptus Bark	1	1	95	1/15	60	EBE1
Eucalyptus Bark	1	1	80	1/10	30	EBE2
Pine Bark	5	2.5	80	1/5	30	PBE1
Pine Bark	1	2	60	1/5	30	PBE2

S/L: Solid liquid relation. _E1: Extract obtained in conditions 1; _E2: Extract obtained in conditions 2.

.

The extraction yield was calculated by measuring the weight difference between the initial dry material weight and the final residue dry material (Equation (1)):

$$\mathrm{EY}(\%)=\frac{\mathrm{Raw} \mathrm{material}\left(\mathrm{g}\right)-\mathrm{Residue} \mathrm{material}\left(\mathrm{g}\right)}{\mathrm{Raw} \mathrm{material}\left(\mathrm{g}\right)}\times 100$$

(1)

2.3. Dyeing Process

The dyeing of the cotton knit samples was carried out using the concentrated extracts of pine and eucalyptus bark (at 10% concentration) as dyes, with a liquor ratio of 1/15 (mass of dry textile product/mass of dyeing solution). Samples were dyed in an ultrasonic bath (Sonorex Super RK 512 H, Bandelin, Berlin, Germany). Cotton knit samples of 5 ± 0.5 g were mixed with the dyeing liquor (2% g dry extract/g textile sample) in a closed bottle (500 mL) and placed in the ultrasonic bath at the temperature and duration time defined for each test. Once the dyeing stage was finished, the dyed textile samples were washed first with cold water, then, with water at 40 ± 5 °C and Cottoblanc STM (CHT, Tübingen, Germany) as a soap agent, and, finally, with cold water. The influences of the extraction conditions used to obtain the extracts (Table 1), and the dyeing process conditions, such as temperature (60–80 °C), the pH of the dyeing liquor (7–9) and the use of mordant with alum, were tested.

2.4. Characterization of Extracts by UV-VIS Spectroscopy

The extracts were characterized using a FLEX-STD-UV-Vis (IS) 25 μm spectrometer (Sarspec, Vila Nova de Gaia, Portugal) with light source LS-DW (Deuterium Tungsten), a transmission probe with 400 μm core diameter fibers and 200 cm stainless steel (Sarspec, Vila Nova de Gaia, Portugal). The evaluation of the color of the dyed textile samples was carried out with the same spectrometer equipment using a reflectance probe, with 400 μm core diameter fibers and 1.5 m stainless steel with a standard probe holder measured at 45 degrees. Measurements were made in quintuplicate at certain points of the textile samples.

3. Results and Discussion

3.1. Extraction

Eucalyptus and Pine bark were extracted using an alkali extraction methodology, to obtain an extract solution with potential applicability in the dyeing process of cotton textile products. Thinking of future industrial application, and from an economic point of view, the viability of the process would be conditioned by the extraction yield of the process.

Table 2. Extraction yield for different extraction conditions.

Nomenclature	Extraction Yield (%)
EBE1	10.0 ± 1.2
EBE2	5.1 ± 0.6
PBE1	13.0 ± 2.3
PBE2	7.14 ± 0.8

_E1: Extract obtained in conditions 1; _E2: Extract obtained in conditions 2.

shows the influence of extraction conditions on the extraction yield obtained.

Regarding the values obtained, the highest extraction yield values were achieved when the “harshest” extraction conditions were used, in terms of temperature or percentage of extraction agent.

3.2. Characterization of Extracts by UV-VIS Spectroscopy

The two main tannin groups, condensed and hydrolyzable tannins, could be distinguished by their UV spectra. Condensed tannins presented a single absorption maximum between 260 and 270 nm, while hydrolyzable tannins had two absorption peaks at 255 and 365 nm [7].

As shown in Figure 1, the pine bark extracts presented an absorption peak in the low wavenumber region, characteristic of condensed tannins. However, eucalyptus bark extracts showed a broad band that could be explained by the presence of a high proportion of gallotannins and ellagitannins, that had a additional absorption maxima between 350 and 450 nm.

3.3. Dyeing with Natural Extracts

The first set of experiments was carried out to understand the feasibility of both extracts to be used as a colorant in the textile industry, together with the influence of the extraction conditions used. In a second stage, the dyeing conditions were tested. Figure 2 shows the textile samples dyed with pine and eucalyptus bark extract.

Table 3. Color evaluation of the textile cotton samples dyed with EB_E and PB_E.

Samples/Dyes	L *	a *	b *
Cotton textile sample	95.4 ± 2.0	0.9 ± 0.3	−3.3 ± 1.4
EBE1	72.5 ± 9.3	16.1 ± 2.2	31.5 ± 4.0
EBE2	69.1 ± 0.9	19.4 ± 0.7	29.8 ± 1.7
PBE1	86.2 ± 4.9	19.79 ± 1.7	22.5 ± 4.6
PBE2	72.1 ± 5.8	25.3 ± 2.2	18.1 ± 4.5

Dyeing conditions: pH = unmodified, S/L = 1/15, 30 min, 60 °C. Values are presented as mean ± standard deviation (n = 6).

shows the color of the dyed textile samples, in the CIELAB color space, evaluated by UV/VIS spectroscopy, using a reflectance probe with a standard probe holder measured at 45 degrees.

As for the influence of the extraction conditions, it was greater for the PB extracts than for the EB extracts. The observed appearance of stains in the dyed textile samples was an undesirable effect. The samples dyed with the EB_E1 and PB_E2 extracts were those with the highest number of stains.

The next step was to evaluate the influence of the dyeing conditions.

Table 4. Evaluation of the dyeing conditions: influences of pH, mordant and dyeing temperature.

Samples/Dyes	L *		a *	b *
pH influence evaluation *1
EBE1	76.4 ± 2.6		11.9 ± 0.9	27.7 ± 2.0
EBE2	77.6 ± 1.8		16.2 ± 0.7	23.1 ± 2.6
PBE1	74.5 ± 6.2		21.9 ± 2.9	24.3 ± 10.7
PBE2	53.5 ± 7.7		33.0 ± 4.1	37.9 ± 11.5
Alum influence evaluation *2
EBE1	93.0 ± 8.4		6.9 ± 0.3	0.9 ± 0.3
EBE2	73.8 ± 12.9		11.6 ± 3.2	30.7 ± 18.3
PBE1	94.5 ± 1.8		11.0 ± 1.6	1.2 ± 1.9
PBE2	89.5 ± 3.2		10.7 ± 2.6	4.1 ± 4.4
Dyeing temperature influence evaluation *3
EBE2	64.1 ± 1.0		19.3 ± 1.5	23.9 ± 2.9
PBE1	59.7 ± 1.4		29.3 ± 1.7	27.2 ± 3.4

Dyeing conditions: *¹, pH = 9–10, S/L = 1/15, 30 min, 60 °C; *², pH = unmodified, S/L = 1/15, 30 min, 60 °C, Alum 3 g/L; *³, pH = unmodified, S/L = 1/15, 30 min, 80 °C. Values are presented as mean ± standard deviation (n = 6).

shows the color changes produced by variation in pH, the addition of alum as mordant, and increase in dyeing temperature, respectively.

Table 4 shows the color evaluation of the cotton knit dyed with the PB and EB extracts using NaOH as alkali agent to increase the pH of the dye bath to a pH = 9.5 ± 0.5. The solubility, viscosity and reactivity of pine and eucalyptus bark extracts were influenced by the pH [4,8,9]. Regarding this, the PB_E2 extract was the one that presented the greatest variation due to increase in pH, followed by the EB_E2 extract. However, the influence of pH on the color achieved with the PB_E1 and EB_E1 extracts was low.

With respect to the mordant influence, alum produced the precipitation of the high molecular weight polyphenols present in the extracts by complexation, due to the metallic character of the alum, and this produced a poor distribution of the dye in the textile samples.

Finally, the dyeing temperature had an important influence on the final color achieved in the textile samples when PB extract was used in the dyeing process, but not when the EB extract was used.

The influence of dyeing conditions (pH, temperature) on the final color of cotton fabrics was low when EB extracts (with high hydrolyzable tannin content) were used as dyes, which represented an important industrial advantage.

4. Conclusions

This work demonstrated that the valorization of the most important by-products generated by the forestry industry, such as eucalyptus and pine bark, is possible in the production of extracts with high applicability in the textile industry. In addition, this work also demonstrated that it is possible to use the extracts obtained from both forest by-products as textile dyes, without the need to use any metallic mordant. Polyphenolic extracts are used to protect against UV light in the development of body creams or in the treatment of wood, due to their antioxidant properties. Future work will focus on evaluating the impact of this and other properties of the extracts on the dyed textile samples.