1. Introduction
Composites with vegetal fibres have received a lot of attention due to the need to develop materials that have new properties and that are more respectful to the environment at the same time. During the last years, vegetal fibre demand has increased in the industrial sector, where its use is a trend thanks to its low density, low cost, low energy consumption, and biodegradability [
1,
2], among other factors, compared to synthetic fibres. However, they have some disadvantages, such as variable properties, matrix incompatibility, and moisture sensitivity [
3,
4,
5,
6]. Since the 1990s, vegetal-fibre-reinforced polymer composites have been used as an alternative to synthetic fibre reinforcements in different applications, mainly in the automotive [
7,
8] and construction sectors [
9,
10,
11]. Nevertheless, new vegetal fibres with lower cost and greater availability than the commonly studied and used ones need to be evaluated [
12].
In this sense,
Opuntia, also known as barbary fig, cactus pear, and prickly pear [
13], is presented as a candidate to consider. It is a species belonging to the
Cactaceae family that has historically been used as food [
14]. Its main body is formed by articulated stems called cladodes, pads, or “
nopalitos” (the youngest) with a flattened padded growth form [
15]. Spines and mucilage are characteristic elements of this plant [
16]. This genus has an asynchronous reproduction and Crassulacean Acid Metabolism (CAM), a photosynthetic adaptation to environmental stress that allows it to grow with a high level of efficiency under limited water conditions, adapting to arid areas and adverse environments easily [
17].
Inside the
Opuntia cladodes, there is a fibre network similar to a skeleton, with the same shape as the pad that it supports. Different processes have been evaluated to extract this fibre network, such as water retting [
18] or burial [
19,
20] methods. On the other hand, this skeleton can be collected directly from nature: when cladodes finish their life cycle they tend to dry out and lose part of their greenish elements, leaving only the fibre network structure, which can be directly exploited [
21,
22].
Opuntia is considered an invasive species in different parts of the world, so frequent pruning is required to contain its rapid growth and expansion [
23]. Moreover, during fruit processing large amounts of waste and by-products are generated [
24,
25]. Consequently, a lot of waste is obtained that can be used and evaluated as reinforcement of composites.
Mannai et al., (2018) have valued the
Opuntia fibre network extraction, and also its structure and properties, showing good mechanical responses [
18]. Other studies have analysed the use of the
Opuntia ground cladodes directly as reinforcement [
26,
27], and also the
Opuntia fibre [
28,
29]. Matrices such as polylactic acid [
21,
28,
30], polypropylene [
26,
31], high density polyethylene [
27], and polyester [
19] have been reinforced with
Opuntia, mainly by compression moulding, offering good results in energy absorption tests and increasing the tensile elastic modulus (for example, from 800 MPa for net polyester to 1480 MPa for the composite [
19]). However, no studies focusing on the previous chemical characterisation of the fibres and their treatment to improve compatibility with polymeric matrices have been found. Chemical treatments can improve the interfacial adhesion and enhance the composite mechanical properties [
32,
33].
The research to date has tended to focus on
Opuntia cladodes composition rather than
Opuntia fibre composition. It is due to the multiple uses of
Opuntia pads, which have been evaluated, such as raw materials for bioethanol production [
17] or nutritional supplements, showing functional properties like antidiabetic, antihyperglycemic, and hypoglycemic effects [
34,
35]. Moreover, the treatment of cladodes with water, ethanol, and lemon juice [
36] or with NaOH and KOH [
37] have been reported, but information about
Opuntia fibre treatment is not found in the literature.
Considering that vegetal fibres’ composition affects their properties as reinforcement [
7], a chemical characterisation of the
Opuntia fibre is needed to enhance its use as reinforcement of polymeric matrices. This constitutes the main novelty of this study due to the lack of information on the issue. Vegetal fibres are composed of structural components (cellulose, hemicellulose, and lignin) and non-structural components (pectin, fats, waxes, etc.). This composition depends on various factors, such as time of harvesting, climatic history, soil characteristics, vegetative state or plant age, fibre extraction process, and fibre characterisation method [
38,
39,
40]. Considering this variability, the aim of this study is to determine the chemical composition of
Opuntia cladodes, on the one hand, and
Opuntia fibre, on the other, analysing how different treatments influence them.
2. Materials and Methods
2.1. Plant Samples and Fibre Obtaining
There are two species of Opuntia considered invasive in the Canary Islands according to the Spanish Catalogue of Invasive Alien Species: Opuntia maxima (OM) and Opuntia dillenii (OD). OM plants can reach up to 6 m tall, and their pads may have thorns (thin and whitish) or not. On the other hand, OD plants have many ramifications, they do not exceed 3 m in height, and their pads have more thorns (thick and yellowish).
In this study, samples of 8 different wild plants (4 OM plants and 4 OD plants) have been collected in the island of Gran Canaria (Spain), using the letters A, B, C, and D to differentiate one from the other; each plant will be referred in this paper with the abbreviation of the specific species (OM or OD) and the specimen at the end (for example, OM.A refers to specimen A in O. maxima).
From plants A, B, and C of both species, three young green cladodes were collected (18 young green cladodes in total). Only three old brown cladodes from plants A, B, and C of OM were collected. Old cladodes were not collected from OD plants because of the difficulty of accessing them.
Figure 1 shows the different types of pads collected. The thorns were manually removed from the cladodes for easy handling. D plants were used to obtain
Opuntia fibre directly from nature (
Figure 2a,b). In
Table 1, the different samples obtained are shown.
To obtain
Opuntia fibres under controlled conditions (apart from the fibre obtained from D plants), young and old cladodes from OM.A, OM.B, and OM.C plants were collected and immersed in water for 15–40 days in a closed container. After this time, which depended on the size of the cladode and its maturity, it was possible to obtain the fibre network manually from the interior of the cladodes. In the case of mature cladodes, it is formed by several layers, as can be seen in
Figure 2d.
Considering that OD cladodes are smaller than OM, that their thorns make processing difficult, and that their old cladodes are inaccessible, OD fibres were not obtained by the water extraction process.
As shown in
Figure 2a,b, fibres collected directly from nature (D plants) seem more deteriorated than those obtained after immersion in water, whose whitish colour suggests that they are in a better state.
2.2. Cladode Treatments
Young cladodes collected from both species were cut into cubes and subjected to the following treatments with the aim of removing part of the non-structural components:
Stirring with water at room temperature for 2 h, avoiding the use of chemical agents and trying to simplify the process;
Stirring with acetic acid (10% vol) for 2 h, considering that the acidic medium facilitates the separation of the fibres and the removal of part of the lignin present in the fibre surface.
After the treatments, samples were washed, filtered, dried (at room temperature and then at 60 °C in an oven) and finally ground (Retsch Ultra Centrifugal Mill ZM 200, Verder Scientific, Haan, Germany).
Table 1 shows the different treated cladodes samples obtained.
The cladode treatments’ yields were calculated as the ratio of the weight of the dry cladodes’ cubes after the treatments and the weight of the fresh cladodes.
2.3. Fibre Treatments
It is well known that a good matrix–reinforcement interface bond is required to achieve an optimal reinforcement [
41]. In this way, chemical treatment is one of the best ways to increase fibre–matrix compatibility [
1]. Alkaline treatment with NaOH is a very common method in the literature: it removes the amorphous content, hemicellulose and lignin, which leads to the fibre surface becoming rough [
42]. Moreover, it reduces the fibre’s hydrophilic character and water absorption [
43]. On the other hand, sodium chlorite bleaching can remove lignin, and this delignification can provide an increase in the reinforcing power of the fibres, increasing the cellulose content and favouring the transfer of stress [
44].
Ground fibre samples (with a size smaller than 500 µm) obtained by the water extraction process from old cladodes were subjected to the following treatments during 1 h (1:25 ratio):
Stirring with NaOH 1 M at room temperature and at 70 °C;
Stirring with 1% wt sodium chlorite at room temperature and at 70 °C (adjusting the pH to 4.5 with acetic acid);
Stirring with 1% wt sodium chlorite at 70 °C (adjusting the pH to 4.5 with acetic acid) for 1 h, filtered and washed with water, and then re-immersed in a NaOH 1M solution at room temperature (for 1 h also).
These treatments have focused on fibre obtained from old cladodes because they show a higher fibre content than the young cladodes [
37]. A maximum yield of 9% (dry weight) was obtained for young cladodes fibre extraction, while from old cladodes, a maximum yield of 34% (dry weight) was achieved. These results are consistent with those reported by Mannai et al., (2018): yields for young cladodes (1.6%) are lower than for old cladodes (30%) [
18].
Fibre samples obtained directly from nature were only subjected to NaOH 1M treatment at room temperature.
After the treatments, samples were washed, filtered, and dried (at room temperature and then at 60 °C in an oven).
The fibre treatments yields were calculated as the ratio of the weight of the dry ground fibres after the treatments and the weight of the dry ground fibres before the treatment.
Identification of all samples obtained (46 treated and untreated samples) is summarised in
Table 1.
2.4. Plant and Fibre Characterisation
The biometric parameters (length and width) of 30 cladodes from OM were measured using a ruler (
Figure 3a). The width of the fibre’s bundles obtained by the water retting process was measured using an optical microscope (Olympus BX51), as can be seen in
Figure 3b. A total of 600 measurements were made. The
Opuntia fibre bundle structure was also inspected using scanning electron microscopy (SEM, Hitachi TM3030).
Moisture, ash, and crude protein (CP) were determined according to standard methods as described in the AOAC (2000) (methods 930.15, 942.05, and 976.05, respectively).
An extraction method using a Soxhlet apparatus (with petroleum ether) during 6 h was performed to determine the ethereal extract (fats, vegetable pigments, waxes, etc.).
The structural components were estimated according to neutral detergent fibre (NDF), acid detergent fibre (ADF), and acid detergent lignin (ADL), known as Van Soest methods [
45], using an ANKOM 220 Fibre Analyzer apparatus (ANKOM Technology, Macedon, New York, United States). Hemicellulose was calculated according to Equation (1), and cellulose in accordance with Equation (2):
Lignin content is equivalent to ADL content.
Surface characteristics of the samples were also analysed by Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were obtained in a Perkin Elmer Spectrum Two apparatus (PerkinElmer, Waltham, Massachusetts, United States), equipped with an attenuated total reflectance (ATR) device, from 4000 to 500 cm−1, at a resolution of 4 cm−1, obtaining each spectrum as the average of 50 scans.
4. Discussion
Biometric parameters obtained are similar to those reported in previous studies (34.9 cm length and 17.6 cm width) [
47].
Results obtained for the chemical composition of
Opuntia cladodes are between the values found in the literature: a variable protein content (3–18%), a high ash content (15–33%), a low lignin content (1–4%), and similar values of cellulose (6–27%) and hemicellulose (15–27%) [
17,
35,
36,
61,
62]. Higher levels of hemicellulose have been obtained in this study, reaching up to 50%. This difference might be due to abiotic factors (soil, climate, etc.), characterisation methods used, post-harvest handling, vegetative state, or plant age, among others factors [
18,
34,
38,
39,
40].
According to other studies, OM cladodes have a higher ash and structural components content and a lower moisture content than OD cladodes [
47]. In this study, these results depend more on the plant than on the species analysed. For this reason, the comparative results between both species are not considered generalisable.
Considering the cladodes treatments, acetic acid was used because acidic medium can facilitate the
Opuntia fibres separation, obtaining a uniform structure and removing part of the lignin which covers the fibres’ outer surface [
36]. In this study, it was not possible to remove lignin from the surface (FTIR spectra do not reflect modifications), and an increase in all structural components is only observed in OD cladodes after the acid treatment. It is possible that acid facilitates the removal of pectins from the pads.
There is a difference between the composition of young and old cladodes, showing for the latter a higher content in structural components, in line with the results reported by Ventura-Aguilar et al. (2017) [
34]. However, the chemical composition and FTIR analysis results do not reveal differences between the fibre extracted by water retting process from young and old cladodes, although morphologically, they show different geometric parameters. Fibre from old cladodes is thicker, and according to the previous studies, it can be assumed that it presents better mechanical properties due to a higher fibre density and a smaller area of the mesh pores [
18,
19].
In general, fibres with a higher cellulose content have better mechanical properties [
63], because cellulose provides strength, stiffness, and dimensional stability to the fibre [
64]. In accordance with this study,
Opuntia fibre has a cellulose content between 50% and 67%. This cellulose content is greater than bamboo (26–43%) and coir (32–43%) fibres, and is closer to those of kenaf (31–72%) and abaca (56–63%) fibres [
6]. Lignin can protect fibre from thermal and biological degradation [
65]. In this study,
Opuntia fibre show a lignin content between 6% and 13%. Only one study has been found that studied the
Opuntia fibre’s composition; it reported a lower lignin content (4.8%) and similar hemicellulose (10.9%) and cellulose (53.6%) contents [
48]. Therefore, these results can promote the use of
Opuntia fibre as reinforcement for polymeric matrices.
The alkaline fibre treatment under optimal conditions can improve its mechanical properties as a reinforcement for polymer matrices, increasing the tensile and flexural strength [
66,
67]. Analysing the fibre treatment effect, the modifications made by the alkaline treatment are clearly observed in the FTIR spectra, and they are of greater importance than those obtained with the sodium chlorite treatment. Hemicelluloses and pectins are removed from the fibre surface after NaOH treatment, increasing the cellulose content. These findings are consistent with other studies: pineapple crown fibres increased their cellulose content from 17.4% to 53.3% [
68] after alkaline treatment, and
Agave americana fibres from 68.54% to 78.65% [
69]. Alkaline treatment can also reduce the lignin content from vegetal fibres [
70,
71], but the compositional results indicate an increase in lignin (at room temperature and at 70 °C). It seems possible that these findings are due to the short treatment period and the method used to determine the structural components. A lignin reduction is only achieved with the SC treatment at 70 °C.
Kenaf fibres have been treated under similar conditions, increasing the cellulose content from 56.81% to 65.24% after the alkaline treatment and decreasing the hemicellulose and lignin content from 13.59% to 10.42% and from 18.27% to 15.93%, respectively. A greater increase in the cellulose content is achieved (from 56.81% to 75.92%) by combining the alkaline treatment with an SC treatment, due to a greater removal of hemicellulose (from 13.59% to 9.03%) and lignin (from 18.27% to 8.03%). The alkaline treatment also improves the thermal stability of the fibres, whereas the alkaline and SC treatments together have a greater effect on increasing the tensile strength than the alkaline treatment alone [
58]. The alkaline treatment of jute fibres increased the tensile strength of a natural rubber from 10.52 MPa (with untreated jute fibres) to 14.21 MPa (with treated jute fibres) [
72]. On the other hand, bleaching treatments with SC increased the tensile strength of sisal fibres and polyester composites reinforced with it [
71], and poly(methyl) methacrylate reinforced with birch veneer [
44]. Therefore, further work is required to establish if it is better to use an alkaline treatment or a combination of alkaline treatment with SC.
Finally, no references have been found comparing Opuntia fibre directly obtained from nature and Opuntia fibre obtained by water retting process. The first method reduces the time needed to obtain the raw material (as natural processes happening in the plant itself are degrading the non-fibrous part of the pad). On the other hand, the use of the water retting process, even if taking some weeks to allow fibre extraction, leads to more homogeneous fibres, with higher cellulose content and with more controlled properties. Further research might explore what conditions are better to obtain the fibre and use it as reinforcement in terms of economic and environmental viability.
The chemical nature of fibre as reinforcement has an important influence on the properties of the polymeric composites [
73]. Therefore, the results obtained in this study are expected to enhance the knowledge about the composition of untreated and treated
Opuntia fibre in order to exploit it as a reinforcement for polymeric matrices. The untreated and treated samples will be combined with different polymeric matrices using different process such as injection (short fibres bundles) or compression (short fibres bundles or fibre network). Considering the composition results, it is expected that the treatment with NaOH or SC70 can improve the fibre–matrix compatibility and consequently the mechanical properties of the polymeric matrices, showing better results than those reported up to now.
5. Conclusions
Despite the variability of wild plant materials, this paper has investigated the composition of Opuntia cladodes and fibre. It can be considered one of the few studies focusing on Opuntia fibre composition to date. Fibre structural components results (50–66% cellulose, 8–13% hemicellulose, and 6-14% lignin) suggest that it can be used for obtaining composites.
Opuntia fibre can be extracted from OM cladodes without the need for chemicals or a laborious process, and it can also be collected directly from nature. In this study, their chemical composition has been compared, although future research to compare their mechanical properties as reinforcement is needed.
Obtaining the fibre from OM is easier than from OD. However, OD cladodes can be evaluated as reinforcement. Considering that vegetal fibres’ mechanical properties depend on their chemical composition, OD cladodes properties can be improved by acetic acid treatment, which allows one to remove non-structural components.
Chemical composition and FTIR results of untreated and treated fibre suggest that NaOH treatment at room temperature can improve the fibre–matrix compatibility due to the removal of hemicelluloses and pectin from the fibre surface. Moreover, sodium chlorite treatment at 70 °C reduces the lignin content.
Future research will focus on analysing the effect of the treatments on the thermal stability of the fibres and obtaining composites that combine untreated and treated Opuntia fibres and polymer matrices. Mechanical properties of these composites will be analysed in order to assess if the treatments have a positive influence on the fibre properties as reinforcement, which is expected considering the composition results.