Thermal and Mechanical Characteristics of Okra (Abelmoschus esculentus) Fibers Obtained via Water- and Dew-Retting ^†

Abstract

In this research, fibers were extracted from different parts of the okra plant (Abelmoschus esculentus) via water- and dew-retting methods. The fibers were subjected to physical and thermal analyses. The fibers obtained from the upper part of the okra plant showed higher breaking strength and lower linear density. Fibers obtained via water-retting exhibited higher breaking strength, higher elongation at break rates, and lower linear density values. The paper also presents the results of thermogravimetric analysis of the okra fibers. Tests were carried out in oxygen and inert gas atmospheres. Slight differences were found in the thermal resistance of the tested fibers, which was confirmed by an analysis using the α_s-α_r methodology. The calculated activation energy showed a widespread range of values.

1. Introduction

Sustainable development (SD) is defined by the World Commission on Environment and Development as: “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”, in the report “Our Common Future”, more famously known as the Brundtland Report [1]. One way to implement the principles of sustainable development is to reduce the use of non-renewable resources on a scale that allows them to be gradually replaced by appropriate substitutes with renewable origins. Environmental protection is enforced by ever-tightening EU legal regulations that limit and even eliminate the use of many existing technologies and materials in conventional industrial production, and by an increase in societal environmental awareness. That is why the search to improve eco-friendly production methods and invent novel ones, and the use of new material solutions, especially materials based on renewable sources, has become one of the most important tasks for science and technology today [2,3].

Currently, as the world is struggling with growing waste problems, new legal regulations are being introduced to guarantee the production of more environmentally-friendly materials. They promote new products that can be recycled or are naturally biodegradable. Biocomposites, i.e., materials in which at least one component is of natural origin are an important option because they can easily be utilized for energy recovery [4,5].

The development of contemporary environment-friendly composite materials involves the constant search for and development of technologies for obtaining the composites themselves, and the use of natural plant fibers. The increased interest in natural fillers as additives in composite materials is also related to current industry trends, which include recycling plastics, reducing manufacturing costs and controlling the product life cycle through the use of environmentally-friendly materials [6]. Several methods are used to obtain natural plant fibers, including water-retting, dew-retting, chemical-retting, and mechanical decortication [3]. Water-retting and dew-retting are traditional methods that have been widely applied, in fact, dew-retting is thought to be the oldest retting technique. In this technique, plant stems are laid on the ground under a thin layer of soil and kept there for three to four weeks. Plant stems are immersed in water tanks for the water-retting method. The properties of dew-retted fibers have been reported to be inferior and show higher variability compared to water-retted fibers. However, dew-retting still has some advantages, such as ease, low cost, and less environmental impact compared to water-retting [7].

Plant fibers from cotton [8], flax [9], hemp [10], jute [11], and sisal [12] have been the focus of research and commercial interest for their use in composites over the last few decades. However, several other plant varieties that are planted mainly to supply the food industry can also be used as alternative sources of fibers [13]. Unused parts of different edible plants, including okra stems and branches [14], wheat straw [15], nettle stems [16], corn husks [17], and banana pseudo-stems [18] can be used as sources of fiber for composite reinforcement applications. The utilization of agricultural residues for fiber production not only saves the environment because biodegradable materials based on renewable sources are used, but also, developing rural communities prosper as their crops receive added value. Furthermore, more arable land may be allotted for food production to feed our growing world population [19].

Okra (Abelmoschus esculentus) is a plant grown in tropical and subtropical regions, mainly for its eatable fruits and its gum, to serve the pharmaceutical industry [14,20]. The first written mention of okra was recorded in Egypt and dates from 1216. The domesticated form of the species Abelmoschus esculentus spread further to the regions of the Red Sea and the Arabian Peninsula, and then to Southern Europe. In the eighteenth century, okra also appeared in the English dictionary, and slaves brought it from Africa to the United States. Today, apart from Africa, the okra plant is successfully grown in the southern part of the USA, the Caribbean, South America, the Middle East, Greece, and Turkey. However, the okra plant is mainly produced in underdeveloped countries across the globe. According to the Factfish database, total okra production was almost 10 million tons in 2017. That year, the top three okra producers were India, Nigeria, and Sudan. More than half of the ten biggest okra producers were African countries [21].

The main application of okra is food; however, the remaining parts of the plant can be used as a structural element in composite materials. Okra bast fibers are considered an alternative to the most commonly used fibers, such as jute, flax, or hemp, which is in line with the general attempt to broaden the number of botanical species from which fibers are extracted [22,23].

An additional advantage of using okra bast fibers is that the inedible parts are used, which are normally recyclable waste. Obtaining okra stalk fibers is, in fact, removing food production waste from the fields. Therefore, okra fiber production is not in competition with food production as both processes take place in parallel in the same area [24].

Bark from the okra plant can be utilized as a source of natural fibers. Similar to other conventional plant fibers, various methods can be used to obtain the fibers from okra plant bark, including water-retting, dew-retting, chemical-retting and mechanical extraction by a number of degumming techniques based on the biological activity of microorganisms, the effects of chemical agents or mechanical means [3,24].

Unlike cotton, which has single-cell elementary fibers, okra bast fibers are technical fibers that are multicellular and lignocellulosic, which is similar to jute and hemp. Okra bast fibers are partially separated bundles of cellulosic fibers attached by hemicellulose, lignin, and pectin [19,25].

The literature on okra bast fibers is limited to a few studies. The influence of some chemical treatments on the physical characteristics of okra bast fibers was investigated by Khan et al. [26]. The mechanical, chemical, morphological, and thermal performance of okra bast fibers and the effects of some chemical treatments, including scouring, bleaching, and acetylation, on these were studied by De Rosa et al. [22,27]. Yilmaz et al. [14] investigated the influence of some chemical and enzymatic treatments on the physical, mechanical, and chemical characteristics of okra bast fibers. They also dyed the okra fibers with an industrial biowaste colorant (green mate tea waste) and studied the ability of okra fibers to remove heavy metal ions from aqueous solutions.

Alam and Khan [28] have reported that the chemical composition of okra bast fibers is cellulose (60–74%), hemicelluloses (15–20%), lignin (5–10%), and pectin and waxes.

Even though the literature contains some studies on okra bast fibers, there are none available on dew-retting okra bast fibers, as all of the mentioned studies have investigated water-retted okra bast fibers. Furthermore, there are no systematic studies of the thermal properties of okra bast fibers.

The aim of this work is to investigate and compare the properties of okra fibers obtained from various parts of the plant via dew-retting and water-retting. Special emphasis has been given to the thermal performance of okra bast fibers as they are processed under various conditions, and as part of composite systems operating at elevated temperatures.

2. Materials and Methods

2.1. Materials

Okra (Abelmoschus esculentus) stems were collected from local agricultural farms in Denizli Province, Turkey. The approximate diameter of the fibers was between 17.5 and 19.5 µm. The images from an electron microscope of selected fibres are presented in Supplementary materials (Figures S1 and S2).

2.2. Methods

2.2.1. Fiber Preparation Methodology

Before starting any degumming procedures, the okra plant stems were divided into three parts: the top, middle, and bottom portions were separately subjected to retting processes. The view of an okra stem is presented below (Figure 1):

2.2.2. Water-Retting Method

Water-retting was carried out at room temperature (20 ± 3 °C) for around 30 days by immersing okra stems in closed beakers filled with tap water.

2.2.3. Dew-Retting Method

Dew-retting was carried out at room temperature (average 20 °C) for around 15 days by burying okra stems in a tray filled with soil.

2.2.4. Linear Density Measurements

All experiments were carried out under specified climate conditions, as described in the EN ISO 139:2006 standard and annex A:2011. The standardized conditions were as follows: air temperature, 20 ± 2 °C and humidity, 65 ± 4%. Linear density was measured as described in the PN-EN 13392:2002. Fifty specimens of each sample were measured.

2.2.5. Mechanical Properties Measurements

Samples were tested using a Zwick ZW2.5/TNIP apparatus (current: 2A, frequency: 50–60 Hz), according to the conditions described in the PN-EN ISO 2062:2010 standard. Briefly, these were fiber length: 50 mm, speed: 50 mm/min, and pressure: 0.5 ± 0.1 cN/tex. Fifty specimens of each sample were measured. The results of the linear density and mechanical measurements were statistically investigated by applying analysis of variance with α at the 0.05 significance level. The apparatus is shown in Figure 2 (below).

2.2.6. Thermogravimetric Analysis

The thermal stability of the okra fibers was investigated by thermogravimetric (TG) analysis using a Perkin Elmer TGA 7 instrument. The measurements were conducted at a 10 °C/min heating rate (platinum pan) in dynamic air and nitrogen atmospheres (the gas flow rate for both gases was 20 cm³/min). The weight of each sample was about 7 mg. All measurements were repeated at least four times.

2.2.7. Differential Scanning Calorimetry

To characterize their differential scanning calorimetry (DSC), okra bast fibers were investigated using a Q2000 TA Instrument Inc. calorimeter. Temperature was increased stepwise at 10 °C/min. The material was cyclically heated to 300 °C.

3. Results

The okra fibers were obtained from various parts of the plant via water-retting and dew-retting procedures. Depending on the origin of the fiber and the way it was processed, the samples were identified by the following symbols (

Table 1. Sample codes.

Sample Source	Sample Code
Water-retted bottom okra bast fiber	BBW
Water-retted middle okra bast fiber	MBW
Water-retted upper okra bast fiber	UBW
Dew-retted bottom okra bast fiber	BBD
Dew-retted middle okra bast fiber	MBD
Dew-retted upper okra bast fiber	UBD

):

3.1. Fiber Yield

Fiber yield is the weight of fibers obtained per unit weight of okra stem as a percentage. The fiber yield rates varied between 10.85% and 14.73% for the studied okra bast fibers. No significant effect of the okra plant portion or retting method was detected (p values 0.63 and 0.76, respectively) (

Table 2. The effect of extracted plant part and retting method on the physical and mechanical properties of okra bast fibers.

Okra Bast Fiber	Fiber Yield (%)	Linear Density (tex)		Breaking Strength (cN/tex)		Elongation at Break (%)
		µ	σ	µ	σ	µ	σ
BBW	14.47	11.90	4.30	42.01	17.40	3.33	1.07
MBW	14.33	10.25	2.66	45.76	16.29	3.40	0.92
UBW	12.65	9.45	2.79	51.80	18.30	3.48	1.30
BBD	14.73	17.09	6.95	33.93	18.99	2.75	0.98
MBD	10.85	13.13	3.68	32.50	14.09	2.83	0.87
UBD	14.25	11.17	3.07	37.12	16.71	2.90	0.89

).

3.2. Linear Density of Fibers

The linear density values of okra bast fibers are presented in Table 2 and Figure 3. As can be seen, the results show a clear, decreasing trend from the bottom > middle > top (p-value 3.69 × 10⁻¹¹). This trend is valid for both water-retted and dew-retted fibers. Similar findings were reported by Yilmaz et al. [14] for okra bast fibers and by Bacci et al. [16] for nettle fibers. These findings suggest that okra bast fibers, which occur throughout the okra stem, have a tapered shape. However, it should also be noted that the measured fibers are not single fibers; they are a bunch of fibers in basic form. So, the efficiency of the fiber separation and of the removal of extra-cellulosic substances also play an important role in the final linear density of the fibers. The dew-retted fibers generally exhibit higher linear density values compared to the water-retted fibers (p-value 7.29 × 10⁻¹¹). Dew-retting for longer durations could be tested to determine if it leads to lower linear density values. Histograms of different samples are shown in Figure 4. The broadest diameter distribution belongs to fibers extracted from the bottom portion of the okra stems, for both water- and dew-retting.

The linear density values of the obtained okra bast fibers are given in Figure 3.

3.3. Mechanical Properties

The most important role of the fibrous component in composite materials is to transfer mechanical load, thus, it is necessary to determine the basic mechanical properties of the obtained fibers. Table 2 and Figure 5 and Figure 6 show the results of measuring the breaking strength and elongation at break for the tested fibers.

Both the plant fraction and the retting method were found to affect the breaking strength (p values 0.02 and 3.24 × 10⁻⁹, respectively). The highest breaking strength (51.80 cN/tex) was obtained from the fibers extracted from the upper part of the okra stem via water-retting. The breaking strength values varied between 42.01–51.80 cN/tex for the water-retted fibers and 32.50–37.12 cN/tex for the dew-retted fibers. In terms of the rate of elongation at break, the extraction fraction had no significant effect, whereas the retting method had a significant effect (p values 0.57 and 1.24 × 10⁻⁶, respectively). The water-retted fibers exhibited higher elongation at break rates (3.33–3.48%) compared to the dew-retted ones (2.75–2.90%). The obtained results agree with the literature, which reports the inferior performance of dew-retted fibers compared to water-retted fibers [7]. The comparison of mechanical properties of okra and other types of fibers are shown in Supplementary materials (Table S1).

3.4. Thermal Properties

At present, composites reinforced with okra fibers are not among the materials used in high-temperature applications. However, determining their full thermal degradation characteristics will enable better understanding of this process, and will assist in establishing the boundary conditions that determine the practical application of these fibers and composites.

3.4.1. Thermo-Oxidation

Understanding the dependence between thermal resistance and the chemical composition of compounds used in high temperatures is essential, therefore, it is important to determine the thermal decomposition parameters, such as activation energy (E), reaction order (n), or frequency coefficient (A). These parameters are vital for determining the polymer degradation mechanism [4,5,29,30] and its thermal stability [6].

There are a number of methods used to determine the kinetic pyrolysis parameters. They vary according to the kind of data analysis and assumed hypotheses, and also in the method of mathematical elaboration. However, even the most modern methods using complicated calculation schemes make use of the original basic theories [31,32,33,34,35].

In this work, two methods were selected for kinetic analysis, which differed in their theoretical approach in terms of assumptions and simplifications. The most popular approximations to the Arrhenius integral in polymer science are those of van Krevelen [36] and Coats and Redfern [37].

Figure 7 and

Table 3. Characteristic temperatures (average values) of main stage thermal degradation of analyzed samples.

Sample	Tintit	T50%	Tfinal
BBW	291.4	355.0	382.8
MBW	287.4	366.1	398.0
UBW	275.8	342.4	375.5
BBD	281.6	354.3	383.5
MBD	298.4	370.2	397.9
UBD	298.6	353.7	397.7

show examples of thermogravimetric curves for the distribution of samples tested under thermo-oxidation conditions and characteristic temperatures for thermal decomposition, respectively.

As can be seen from the data presented in Figure 7 and Table 3, the thermal distribution curves of the samples have two-stages. The first stage is related to depolymerization and thermal degradation of the cellulose macromolecules. This stage has also been determined in several studies [38,39,40]. In the second stage, the afterburning of low-molecular products (oxidation of the charred residues) of the first stage takes place [41,42]. The characteristic temperatures of the first main stage of decomposition are shown in

Table 4. Activation energy of thermo-oxidation process of okra fibers (standard deviation 5%).

Sample	Activation Energy (kJ/mol)
BBW	84.1
MBW	69.9
UBW	41.9
BBD	46.6
MBD	36.4
UBD	39.3

. The distribution of the fibers from different parts of the plant are similar in nature and occur in similar temperature ranges. For the okra bast fibers, the temperature range for each of the thermal characteristics is about 20 °C: the start of decomposition is 275–298 °C, T50% is 342–371 °C and the end of the process is 375–399 °C. The thermal degradation of fibers obtained by the water-retting method occurs slightly earlier (T50% = 354.5 °C) than those obtained by the dew-retting technique (T50% = 359.4 °C).

The processes of the thermal degradation of okra fibers were compared using the αs–αr evaluation method [43,44], which compares the thermal reactivity of different substances to a reference base. The data obtained from the TG of water-retted bottom okra bast fiber (BBW) were used as the reference base and denoted by αr, whereas the other five were denoted by α_s. The α coefficient was determined using the following equation:

α = (w_i − w)/(w_i − w_f)

(1)

where:

w is the mass fraction of a substance at a given temperature,

w_i is the mass fraction of the substance at the initial temperature,

w_f is the mass fraction of the substance at the final temperature.

The plots of α_s vs. α_r (Figure 8) were prepared for the main transformation range of a TG run. As one can see in Figure 8, dew-retted middle okra bast fiber (MBD) has slightly higher thermal stability than the reference sample, whereas the other samples are less stable. The UBW sample (water-retted upper okra bast fiber) is the least stable. A similar conclusion can be drawn from the αs coefficient temperature plot (Figure 9). TG measurements show that with regard to their thermal stability, the investigated fibers can be arranged as follows: MBD > MBW > UBD > BBD > BBW > UBW. As can be seen in Figure 8 and Figure 9, this order does not change throughout the whole course of the main thermal degradation stage, which leads to the conclusion that fibers coming from the middle part of the okra bast plant are very slightly better in thermal stability than those coming from other parts.

The main stage of the thermal decomposition under thermo-oxidation conditions was subjected to kinetic analysis. The activation energy values of the decomposition process were calculated using the Coats-Redfern method [37]. The results of the analysis are shown in Table 4.

Based on the data shown in Table 4, the activation energy values at the given conditions (kinetic parameters significantly depend on the heating rate, as well as on the assumptions adopted in a given calculation method [45]), are at the 36–84 kJ/mol level. The large range of spread suggests that the assumptions associated with this method [37] cause a moderate adaptability of this method to the tested samples. However, while elaborating on the results of thermal tests in air, a coefficient of determination at the 98–99% level was found with the Coats-Redfern method. The authors of that method used an integration procedure. They obtained the following equation:

$$\ln (\frac{\alpha }{{\mathrm{T}}^2})=\ln \frac{\mathrm{AR}}{\beta \mathrm{E}}(1-\frac{2\mathrm{RT}}{\mathrm{E}})-\frac{\mathrm{E}}{\mathrm{RT}}$$

(2)

where:

A—Pre-exponential factor (min⁻¹)

α—Degree of conversion or fractional mass loss

β—Heating rate (K min⁻¹)

E—Apparent activation energy (kJ mol⁻¹)

R—Gas constant 8.3136 (J mol⁻¹K⁻¹)

T—Temperature (K)

By plotting ln (α/T²) = f(1/T), E value can be calculated. It must be remembered that the equation is only true for zero reaction order, which results from the former simplifications. The results obtained by this method are true for low α, but they can be generalized for the whole process assuming that the reaction mechanism does not change during the reaction. From a practical point of view, this method is moderately laborious. It requires taking the α values from the thermogram and doing the necessary calculations to obtain the plot. Researchers have used this method, among others, to investigate poly (tetrafluoroethylene) [37] and poly (3-dimethyloacryloyloxyethyl) ammonium propanosulfate [46], and the results correspond to the results obtained from other calculation methods.

In comparing the values of the activation energy obtained for samples processed by the water- and dew-retting methods, we found an interesting relationship between the average EA results, which were 65.5 and 40.8 kJ/mol, respectively.

The results of the thermal decomposition analysis of okra fiber samples from different parts of the plant in the presence of oxygen have not yet been published. From the results presented in this study, it can be concluded that regardless of the origin of the material, okra fibers can be used in composites up to a temperature of 290 °C. The activation energy of the decomposition process varies depending on how the fiber is pretreated.

3.4.2. Pyrolysis

The next stage of the thermal research was thermogravimetric analysis in an inert atmosphere. Pyrolysis is a type of thermolysis observed when heating organic materials in an oxygen-free atmosphere. The mechanism for the chemical changes occurring during pyrolysis is often very complex, and it can be difficult to study them thoroughly due to the variability in the composition of the raw materials subjected to pyrolysis (for example, biomass pyrolysis). As a result of this process, only gaseous products can be formed; however, the process almost always proceeds with the formation of a solid residue.

It has been known for some time that the type of atmospheric gas can strongly affect the position of TGA curves on the weight axis. One of the most attractive features of using TGA in an inert atmosphere as a method of thermal stability analysis for polymers, is that it is almost always possible to glean some information from the data record. The representative thermograms of the okra samples prepared at ambient conditions are presented in Figure 10, and the characteristic degradation temperatures can be found in

Table 5. Characteristic temperatures (average values) of main stage thermal degradation of the analyzed samples.

Sample	Tintit	T50%	Tfinal
BBW	300.0	374.8	406.2
MBW	303.5	389.1	426.3
UBW	299.9	372.4	403.6
BBD	300.0	377.6	406.6
MBD	300.0	382.2	410.9
UBD	299.9	389.7	412.4

.

According to the data shown in Figure 10 and Table 5, the thermal distribution curves of the samples have two-stages, similar to the thermo-oxidation process. Most likely, the first stage involves depolymerization and the thermal degradation of cellulose macromolecules and the second stage is the afterburning of low-molecular breakdown products. The degradation of the fibers from different parts of the plant has a very similar character and occurs in similar temperature ranges; the differences between individual samples are smaller than for the decomposition process in air. The temperature of half decomposition for bast fibers obtained by the water-retting method is 378.8 °C, while for the fibers obtained by dew-retting it is 383.2 °C, which shows the same trend as the thermo-oxidation process.

The main stage of thermal decomposition under neutral conditions was subjected to kinetic analysis. The activation energy values of the decomposition process were calculated using the Coats-Redfern method [37]. The results of the analysis are shown in

Table 6. Activation energy of the pyrolysis process of okra fibers (standard deviation 4.5%) calculated by the Coats-Redfern method.

Sample	Activation Energy (kJ/mol)
BBW	60.4
MBW	50.9
UBW	50.4
BBD	42.8
MBD	54.3
UBD	62.0

.

The data from Table 6 indicate that the EA values under given conditions (in an inert atmosphere) range from 42.8–73.5 kJ/mol, which is a much narrower range. The coefficient of determination values are above 99%. Since the decomposition process in an inert atmosphere occurs at a higher temperature than for thermo-oxidation, it is understandable that the average value of activation energy for nitrogen is slightly more than for air (56.6 and 53.0 kJ/mol, respectively).

Studies in the literature that show thermogravimetric curves for okra fiber (without taking into account the part of the plant from which the fiber originated) in an inert atmosphere, show a 50% distribution temperature at about 350 °C [22] and in the 329.8–349.9 °C range (depending on the treatment technique) [23].

The main thermal degradation step is presented in the form of α_s = f(α_r) plots (Figure 11) and shows very small differences between the TG run of each okra sample, which confirms the similar thermal stability of the analyzed samples, as was concluded from the comparison of thermogravimetric curves. However, some minor differences can be seen in Figure 11. In ambient conditions, the UBD (dew-retted upper okra bast fiber) sample has slightly higher thermal stability than the reference sample. Taking the αs = f(αr) plot into account, fibers can be arranged as follows: UBD > MBD > MBW > UBW > BBW > BBD. As can be seen in Figure 11, this order does not change throughout the whole course of the main thermal degradation stage in nitrogen conditions, which leads to a similar conclusion as for investigation in air, that fibers coming from the middle part of okra bast plant are very slightly better in regards to thermal stability than those coming from other parts.

3.4.3. DSC Analysis

Because thermogravimetry techniques only show the occurrence of those processes that are associated with changes in sample mass, differential scanning calorimetry analysis was also performed to obtain a full description of the thermal decomposition process. This allowed us to observe the endo- and exo-energetic changes occurring in tested samples. Figure 12 presents the DSC thermograms for the tested samples.

As can be seen in Figure 12, the DSC thermograms look very similar; and there is an exothermic signal at 100 °C, which is related to water evaporation. Thus, in the −70–300 °C temperature range, there are no other changes in the samples of the tested materials.

4. Conclusions

The paper describes the results of thermal and physical analyses of okra fiber samples from various parts of the plant’s stem. The fibers also differed according to how they were processed to remove the non-fibrous components, that is, by water-retting and dew-retting. The physical and mechanical properties of the fibers can be summarized as linear density, breaking strength, and elongation at break range and the results for these were between 9.45–17.09 tex, 32.50–51.80 cN/tex, and 2.75–3.48%, respectively. Fibers from the upper part of the okra stem show higher breaking strength and lower linear density, whereas water-retting resulted in higher breaking strength, higher rates of elongation at break, and lower linear density values. The thermal characteristics of all samples were similar, and the differences between the types of fibers were rather small. The main stage of thermal decomposition took place in the 275–400 °C temperature range for thermo-oxidation and 300–425 °C for pyrolysis. Activation energy values were determined for both processes, and the results showed a large dispersion of values. The DSC curves of the samples tested show only one exothermic signal, which was attributed to the evaporation of water. In conclusion, the maximum safe temperature for the use of okra fibers as composites is around 275 °C in the presence of oxygen.