Micromechanical Characterization of Continuous Fiber Date Palm Composites

ABSTRACT

Date palm fiber composites with aligned, continuous fibers and epoxy matrix were prepared and mechanically characterized using micromechanical models. A hydrothermal process with microwave heating was utilized to extract the date palm frond (midrib) fibers, making the process eco-friendly as opposed to the commonly reported chemical extraction method. Single fibers were then characterized and found to have an average elastic modulus and ultimate tensile strength of 17.1 ± 1.5 GPa and 333 ± 27.4 MPa, respectively. Interfacial shear strengths ranging between 6 and 12 MPa were determined using a single-fiber pull-out test. Fibers were aligned to make uniaxial tensile samples with volume fraction ranging from 11% to 56%, elastic modulus ranging from 1.72 GPa to 9.52 GPa and ultimate tensile strengths ranging from 16.9 MPa to 43.8 MPa. All the results were found to obey “Rule of Mixtures” theory. The composite’s mechanical properties are similar to or exceeding those of the commercially available wood products such as medium density fiberboard, oriented strand board, thick particle board, hard fiber board, and plywood, making the hydrothermal approach with microwave heating as a promising approach for preparation of date palm composites and further opens up similar process pathway for other natural fibers.

摘要

制备了具有排列、连续纤维和环氧树脂基体的椰枣纤维复合材料，并使用微观力学模型对其进行了力学表征. 利用微波加热的水热工艺提取椰枣叶（中肋骨）纤维，与通常报道的化学提取方法相比，该工艺具有环保性. 然后对单纤维进行了表征，发现其平均弹性模量和极限拉伸强度分别为17.1 ± 1.5GPa和333 ± 27.4MPa. 界面剪切强度范围在6和12MPa之间，使用单纤维拉拔试验测定. 将纤维排列以制备体积分数为11–56%、弹性模量为1.72 GPa至9.52 GPa、极限拉伸强度为16.9 MPa至43.8 MPa的单轴拉伸样品. 所有结果均符合“混合物规则”理论. 该复合材料的机械性能类似于或超过可商购的木材产品，如中密度纤维板、定向刨花板、厚刨花板、硬纤维板和胶合板，使微波加热水热法成为制备椰枣复合材料的一种很有前途的方法，并进一步为其他天然纤维开辟了类似的工艺途径.

KEYWORDS:

• Date palm
• Natural fibers composite
• Epoxy
• Pullout
• Micromechanics
• Interfacial shear strength
• Aligned fiber

关键词:

• 椰枣
• 天然纤维复合材料
• 环氧树脂
• 拉出
• 微观力学
• 界面剪切强度
• 对齐光纤

Introduction

Concerns about using synthetic chemicals in consumer products (Bhalla and Singh 2017; Bhat et al. 2022; Hendy and Bakr 2020; Suran 2018; Thakker and Sun 2021) have led to considerable research into substituting petrochemical-based materials with bio-renewable materials (Bhardwaj et al. 2020). An important example is the use of natural fibers in producing composite materials that are renewable, nontoxic, and biodegradable. Plant fibers are the most common type of natural fibers and are often referred to as cellulosic fibers due to their high cellulose content that gives the fiber its desired properties. In addition to cellulose, plant fibers contain hemicellulose and lignin that act as a “glue” to bond and support cellulose structures (Al-Oqla and Salit 2017; Azman et al. 2021; Jaafar et al. 2019; Karimah et al. 2021). This work follows this approach by utilizing date palm fibers (DPF) from agricultural crop residues along with nontoxic chemicals to create composites with improved mechanical properties.

Most agricultural plants produce crop residue, during the annual pruning process, that is considered “waste” and goes unused or are burned, which can negatively impact the environment (Midani, Othman, and Naheed 2020). These crop residues are clean sustainable sources of a variety of materials that can be used to manufacture clean products. Additionally, proper pruning of fruiting plants encourages them to grow more rapidly (Al-Khayri, Mohan Jain, and Johnson 2015; Midani, Othman, and Naheed 2020). Extracting fibers from crop residue is an economical and ecofriendly approach to creating value-added products and to benefit the agricultural field itself (Espinach 2021).

Date palms are perennial fruiting plants with life spans typically over 100 years (Al-Khayri, Mohan Jain, and Johnson 2015; El-Mously and Darwish 2020) and produce high nutritional value fruits. Date palms also generate crop residue during their annual pruning. Date palm cultivation is primarily concentrated in the Middle East. The largest producers of dates are Egypt, Saudi Arabia, Iran, Algeria, Iraq (United Nations - FAOSTAT 2020). For example, Saudi Arabia has about 31 million date palm trees with an annual pruning process that produces about 300,000 tons/year of crop residue (Saudi General Authority for Statistics 2019; Al Fedan and Abu Ayana 2016. In years past, a small portion of these crop residues was used in building, decoration, handicrafts, or animal feeding, while the majority was burned as fuel (El-Mously and Darwish 2020). Nowadays, utilization is limited to animal feeding and compost making (Al Fedan and Abu Ayana 2016), but the bulk portion is considered unusable waste (Al-Oqla et al. 2014; Awad et al. 2021; Boumediri et al. 2019; Saba et al. 2019). A large amount of underutilized date palm crop residue has a high potential for creating new value-added products. One way of using these residues effectively is by extracting the natural cellulosic fibers from their crop residues and then using them in composite materials. Recently, there has been considerable interest in date palm fiber composites (Alarifi 2020; Maou et al. 2019, 2021, 2023; Sadik et al. 2021).

Fronds and mesh (see Figure 1(Insert at end of paragraph)) represent a high part of the residue from the date palm pruning process and are available throughout the year. In addition, the midrib of the fronds and the mesh have high cellulosic contents that range between 41.5% and 69.3% based on the cultivar (Elseify and Midani 2020). In Saudi Arabia, the Khalas cultivar represents 25% of the total number of date palm trees under cultivation (Saudi General Authority for Statistics 2019). In the annual pruning process, the midrib of fronds represents the most abundant quantity of the pruning residues (El-Mously and Darwish 2020). However, little can be found in the literature about the use of midribs compared to the use of mesh since the mesh is already in fiber form and does not require a lengthy extraction process. Given the ample availability and underutilization of midrib residue, this work used the midribs of Khalas fronds midrib as the raw material source to extract fibers and make composites.

Figure 1. Date palm tree parts.

There are many different approaches for fiber extraction, including chemical, physical, and mechanical extraction methods. Chemical extraction involves alkaline solutions such as sodium hydroxide (NaOH), and potassium hydroxide (KOH), silane etc. The most common extraction process in the literature is the chemical extraction using sodium hydroxide (Abdal-Hay et al. 2012; Abdellah, Abo-El Hagag, and Marzok 2019; Ahmad, Hamed, and Al-Kaabi 2009; Alsaeed, Yousif, and Ku 2013; Bezazi et al. 2022; Elseify et al. 2020; Nassar et al. 2023; Sbiai et al. 2010, F. S. A. Hassani et al. 2020; Shalwan and Yousif 2014; Wazzan 2005). Physical extraction includes hydrothermal, pressure explosion, ultrasound-based, and microwave-based processes for fiber extraction (Nair 2017; Nair, Rho, and Vijaya Raghavan 2013; Zulaikha, Zaki Hassan, and Ismail 2022). Mechanical extraction may be performed manually using knives or hammers or with machines such as a decorticator, hammermill, or roller (F.-Z. S. A. Hassani et al. 2020; Sabarish et al. 2020). This work aims to reduce the use of harsh alkaline chemicals by combining physical-mechanical processes to produce natural fibers that are facilely incorporated into a composite material.

Composite materials

Composite materials are a heterogeneous combination of two or more different materials mixed at the macro-scale in which each material keeps its integrity. The overall goal of composite material architecture is to combine advantages of each individual component such as high strength and light weight, enabling a single material that has a high strength-to-weight ratio (Callister and Rethwisch 2009; Clyne and Hull 2019; Kaw 2006; Nijssen 2015). The earliest known examples of composite materials were straw-reinforced clay bricks used as building materials circa 1500 BCE. From that time until the early 20^th century, composite materials were not classified as a separate class of materials. Following the commercial introduction of fiberglass in the 1930s, plastics were reinforced with glass fibers to produce light weight and high strength composites for a wide variety of applications. Since then, composite materials have been classified as a distinct type of material and further developments in the field led to a rapid increase in the number of diversity of composites (Nagavally 2017).

Epoxy resins are thermoset polymers and are one of the most often composite matrices reported in the literature. In this work, an epoxy resin was used as the matrix to ease comparison of the results of this study with previously reported values (Abdal-Hay et al. 2012; Abdellah, Abo-El Hagag, and Marzok 2019; Alsaeed, Yousif, and Ku 2013; Alshammari et al. 2019; Naveen et al. 2016; Oushabi et al. 2018; Rajeshkumar et al. 2021; Saba et al. 2019; Santos et al. 2018; Shalwan and Yousif 2014). The advantages of epoxy resin over the other polymer matrices are high modulus of elasticity, good adhesion to almost all materials, flexibility, good insulation properties, and dimensional stability (Pradhan et al. 2016). One approach for improving the sustainability of polymer composites is to combine readily available polymer matrices, including epoxy resins, with bio-renewable fillers such as natural fibers (Li et al. 2021; Paluvai, Mohanty, and Nayak 2014; Pappa et al. 2022; Zhang et al. 2021). Such an approach of utilizing natural fibers as reinforcements into the epoxy matrix achieves a two-fold benefit. Firstly, the overall environmental impact can be minimized by reducing the consumption of epoxy, a non-recyclable material. Secondly, the mechanical properties of the composite can be improved through the synergistic effects of the natural fibers, thus potentially providing a more sustainable alternative to conventional epoxy-based composites. This is the approach we use in our current work as well.

On the macro scale, each natural fiber consists of a bundle of many vascular fibers referred to as technical fibers. Then on the micro level, the wall structure of a technical fiber consists of tiny fibrils. Each fibril consists of many cellulosic chains, see (Al-Oqla and Salit 2017) for detailed structure. The cross section of natural fibers and hence the values reported in the literature for diameter vary from micrometers to millimeters based on the extraction process (Elseify and Midani 2020). In this work, using a micrometer, the diameter of the fibers was found to be ranging from 0.13 mm to 0.8 mm with an average diameter of 0.40 ± 0.02 mm.

Fiber lengths can be classified as either continuous or discontinuous. Continuous fibers have a length equal to or greater than 15 times the critical length of the fiber regardless of the composite size (Callister and Rethwisch 2009). Continuous fibers give better properties in composite materials due to better load transfer and distribution, but they make the production process more complicated. Discontinuous fibers are easier to extract and process into composites, but they give lower mechanical properties compared with continuous fibers (Callister and Rethwisch 2009; Clyne and Hull 2019; Kaw 2006).

In this work, hydrothermal and microwave processes were combined by boiling the midribs with water under atmospheric pressure using microwave as a heating source to extract the fibers with shorter time and without the need of any further chemical treatments. The extracted fibers were used along with an epoxy matrix to produce composite materials of different weight percentages (wt%) of fibers. Composite test specimens were prepared according to ASTM D-3039 (ASTM International 2017). Results are used to evaluate the properties of the date palm composites compared with the commercial wood products. The hydrothermal process made the extraction of the fibers easier by weakening the bonds between cellulose and lignin. In addition, it removed all the water-soluble content which improves the bond with the matrix. It further eliminates the need of chemical treatments making the process more economically effective and eco-friendlier. Using the microwave reduces the heating and the extraction time, resulting in faster production and less energy consumption. The extracted fibers showed mechanical properties that are competitive with the chemical treated fibers. These economical and bio-friendly advantages make this combination of hydrothermal process with microwave heating a promising natural fibers extraction process.

Theoretical background

The primary goal of this work was to create a continuous date palm fiber composite to realize the maximum strength possible, i.e., maximum fiber effectiveness in the matrix, with this combination of cultivar and matrix. The micromechanical description for the critical length of a fiber, l_c, is given by the equation:

(1) $l_c=\frac{{\sigma }_f^{\ast }d}{2{\tau }_c}$(1)

where σ_f^* is the failure strength of the fiber, d is the diameter of the fiber and τ_c is the interfacial shear strength between the fiber and the matrix or the shear strength of the matrix adjacent to the fiber whichever is smaller (Callister and Rethwisch 2009). These three quantities dictate the l_c of fibers and need to be determined using independent means. The diameter, d, is determined using a micrometer, after the fibers have been extracted. σ_f^* is found using a single fiber tensile test. Finally, τ_c is found using a single fiber pullout test.

The remainder of this article contains four sections and the conclusions. The first three sections contain discussions of how to determine the three parameters necessary to calculate l_c. The fourth section discusses the creation of the composites with fiber of length > l_c and their results.

Materials and methods

Materials

Date palm fiber

The core fibers of date palm midribs were used in this work. The Khalas cultivar of date palm was chosen the source of fibers due to its abundance in the region; it makes up approximately 25% of all date palms in Saudi Arabia. Frond leaflets were cut out and only the midribs were used and cut to pieces of 10 cm long for the extraction process as explained below in the methodology.

Epoxy matrix

In this study, a commercially available epoxy resin was used to facilitate the comparison of the results of this work with literature values. ENVIROTEX LITE^Ⓡ resin (Environmental Technology, Fields Landing, CA, USA) was used as is, without modification and per the manufacturer’s instructions. The resin properties according to the manufacturer are 21.4 MPa tensile strength and 981 tensile moduli with 1:1 mixing ratio by volume.

Methods

Fibers extraction process

Hydrothermal treatment, which is a type of physical extraction process, involves boiling the natural fiber source in an aqueous medium to weaken the bonds between cellulosic fibers, hemicellulose, and lignin. The process extracts water-soluble components that can detrimentally affect the fiber-matrix interfacial strength. The effects of boiling water treatment on fiber extraction was previously studied (Nair 2017; Nair, Rho, and Vijaya Raghavan 2013) and they concluded that the hydrothermal process helps in removing the water-soluble materials and improve the fibers properties. In the current study, the fibers were placed into a container with water and boiled (100°C) at atmospheric pressure. The exposure time of the fibers to the boiling water varied between 30 and 45 min based on the size of the midribs. After the boiling water treatment, the midribs were hammered to remove the lignin and extract the fibers Figure 2a (Insert at end of paragraph), with special care not to damage the fibers by severe hammering force that may break the fibers to shorter length.

Figure 2. (A) extraction levels of the fibers and the extraction residues; (b) composite board in mold (left) and cut (right) with specimens’ dimensions (right).

Extracted fiber lengths were cut to a nominal length of 100 ± 1.5 mm with a mean diameter of 0.40 ± 0.02 mm. The Anderson-Darling Normality Test was also applied to the averages and were found to be Gaussian, which was an expected result for a random distribution of natural fibers.

Composite creation

Four sets of composite samples, S1, S2, S3, and S4, were made with the following average weight percentages (wt%) of natural fibers in epoxy − 10%, 15%, 17%, and 24%, respectively. Fibers were laid by hand in the mold and mixed with the resin. The mold was then closed and compressed (0.5 kPa for S1, S2, S3; 2 kPa for S4) for 72 h at room temperature until the resin cured per the resin manufacturer’s instructions. Finally, DPF composite samples were prepared for tensile test by cutting the large composite board into 10 rectangular specimens according to ASTM D-3039 (ASTM International 2017). The prepared samples were 250 mm in length and 15 mm in width (see Figure 2b), per ASTM D-3039. Specimen thickness required is “as needed” as per the standard. In this work, the specimen thickness was 4.5 ± 0.04 mm.

Single fiber tensile testing

Single fiber tensile tests were performed on ten fibers to determine σ_f^* according to ASTM D3822 (ASTM International 2020), as shown in Figure 3 (Insert at end of paragraph). The displacement rate was set to 0.5 mm/min and the preload was 0 N. Average fiber diameter was 0.40 ± 0.02 mm.

Figure 3. Single fiber tensile test specimen per ASTM D3822.

Mechanical testing

A 3367 INSTRON^Ⓡ universal testing machine (Instron, Norwood, MA, USA) was used to perform all mechanical tests. The load cell capacity was 30 kN with resolution equal to 0.002 of the load cell capacity. A dynamic (2620–601) extensometer (Instron, Norwood, MA, USA) with a gauge length of 12.5 mm and travel of ±5 mm was used for accurate strain measurement for the composite samples.

Results & discussion

Single fiber tensile test results

Results of the single fiber tensile tests on six fibers are shown in Figure 4(Insert at end of paragraph). Note that some of the fibers showed a decrease in the tensile strength before failure, which was attributed to partial fracture of the fibers that was observed during the experiment (not pictured). This is possible since they are technical fibers, i.e., fibers in a bundle (groups of small fibrils). The modulus of elasticity of the single fibers ranged from 8.6 GPa to 22 GPa with mean value of 17.1 ± 1.5 GPa. The mean value of the ultimate tensile strength (UTS) is 333 ± 27.4 MPa. Elongation of the fibers ranges from 1.69% to 2.91%. It is reported in literature that the variation in the single fiber tensile results for UTS is typically quite high (Abdel-Rahman et al. 1988; Boumediri et al. 2019; Elseify et al. 2019, 2020; Shalwan and Yousif 2014), due to the fiber type, surface defects, and treatments. Figure 5(Insert at end of paragraph) shows a comparison between this work and previously reported results. The literature comparison was limited mainly to studies using date palm midrib fibers and studies using date palm mesh.

Figure 4. Representative samples of the single fiber tensile test results.

Figure 5. Comparison of single fiber properties with other works: (a) tensile strength; (b) modulus of elasticity. This work (green), midrib fibers untreated (blue), midrib fibers treated (patterned blue), mesh fibers untreated (brown), mesh fibers treated (patterned brown).

By comparing the values of untreated midrib fibers with the values of chemically treated midrib fibers reported in literature, it is seen that the untreated values are competitive with the chemical treated DPF. In the current work, UTS values ranged from 183 to 435 MPa, which is a consistent with the ranges previously reported (164–453 MPa) for treated fibers (Elseify et al. 2020) suggesting hydrothermal treatment with microwave heating as a promising green alternative to the commonly used alkaline chemical treatment of the midrib fibers.

Interfacial strength test results

Interfacial strength tests were performed to find the interfacial shear strength, τ_s, between the fiber and the matrix, which is a measure of how well the fibers and matrix are bonded together. There are different tests to evaluate the interfacial shear strength between the fiber and the matrix such as fiber pull-out test, fiber fragmentation test, and single fiber micro-droplet debond test (Bedi, Kaur Billing, and Agnihotri 2019; Oushabi et al. 2018; Shalwan and Yousif 2014). Fiber pull-out tests were used in this work since it is common method in the literature (Graupner et al. 2014; Liu et al. 2019; Oushabi et al. 2018; Pervaiz, Sain, and Ghosh 2006; Rajeshkumar et al. 2021; Santos et al. 2018). In this test, a part of the fiber was embedded in the matrix, and then it was pulled in tension until the fiber debonded from the matrix and pulled out of the matrix (Figure 6(Insert at end of paragraph)). To verify that the fiber pulled out of the matrix, black paint was applied to the fiber where it touched the matrix. Upon pull-out, a clear gap was seen between the black paint and the matrix, cf. Figures 6(c, d). The maximum force before pull-out was used to calculate the interfacial strength by the following equation:

(2) $\tau =\frac{F_{max}}{\pi DL}$(2)

Figure 6. Fiber pull-out test specimen. (a) schematic illustration of the pull-out test, (b) image of the test specimen with scale bar in mm, (c) the specimen between the jaws showing pull-out of the fiber from matrix, cf. separation of black paint, (d) thickness of the matrix.

where F_max is the maximum tensile force, D is the diameter of fiber, and L is the length of the embedded fiber.

A total of ten samples were prepared for the fiber pull-out test. Three of the samples broke outside of the matrix before pull-out. All three samples attained stress values falling into or exceeding the failure range for single fibers, cf. Figure 4. This indicated that the fibers reached their ultimate tensile strengths before interfacial failure. In the other seven specimens, the fibers were pulled out of the matrix. The results are summarized in Table 1:

Table 1. Interfacial strength test results. Samples 3, 4, and 8 failed in tension before fiber pull-out.

	Force at Maximum Load	Diameter	Embedded Fiber Length	Interfacial strength
	(N)	(mm)	(mm)	(MPa)
1	41.5	0.489	2.58	1.5
2	56.6	0.440	4.39	9.33
5	43.1	0.464	2.93	1.1
6	42.4	0.506	4.04	6.61
7	31.8	0.417	2.81	8.65
9	16.8	0.360	2.48	5.99
10	54.1	0.313	4.24	13.0
Min	16.8	0.313	2.48	5.99
Mean	40.9	0.427	3.35	9.16 ± .9
Max	56.6	0.506	4.39	13.0
SD	13.5	0.070	0.833	2.38

The critical length of the DPF may be calculated with the critical length equation (Eq.1) from the UTS of the fiber (σ_f^*) from the single fiber test, and the interfacial strength from the fiber pullout test. This calculation assumes that the shear strength of the pure matrix is higher than the interfacial strength between the fiber and matrix, which is justified as there was no matrix adhering to the surface of pulled out fibers. Taking the mean value of the fiber tensile strength and the diameter of the fiber as 0.40 mm, the critical length was found:

(3) $l_c=\frac{{\sigma }_f^{\ast }d}{2{\tau }_c}=\frac{333\times 0.40}{2\times 9.16}\approx 7.3\hspace{0.17em}mm$(3)

In this work, the fiber length was 100 mm which is about 14 times the critical length. The fibers were considered as continuous fibers because the load distribution through each fiber is approximately the same as if the fiber length were as long as the specimen. Continuous fibers give the best load transfer characteristics for a composite. Shown below in Figure 7(Insert at end of paragraph) (Alsaeed, Yousif, and Ku 2013; Oushabi et al. 2018; Rajeshkumar et al. 2021; Shalwan and Yousif 2014; Wazzan 2005) is a comparison of the interfacial strength of this work with other values reported in literature:

Figure 7. Comparison of interfacial strength test results with other works. This work (green), midrib fibers untreated (blue), midrib fibers treated (patterned blue), mesh fibers untreated (brown), mesh fibers treated (patterned brown).

These literature results show that chemical treatment improves the interfacial strength compared with untreated fibers. The improved performance of chemically treated fibers is attributed to the removal of impurities and wax layers on the fiber surfaces that can reduce the adhesion between the fiber and the matrix. Also, the wide variation in the literature results can be caused by other factors such as the type of fibers, impurities in the fiber and in the matrix, or the extraction method used. All these factors may change the surface morphology of the fibers and thereby affect the fibers strength and adhesion to the matrix.

Composite tensile test results

After investigating the properties of single fibers, fiber composites were prepared and tested to investigate the mechanical properties of DPF composites with an epoxy matrix. Tensile tests were conducted with preload of 1N and load rate of 2 mm/min, and extensometer removal was at 0.4% strain.

There was a wide variation in the tensile properties of the samples. The modulus of elasticity (MOE) ranged from 1.72 GPa to 9.52 GPa, and the UTS range was from 16.9 MPa to 43.8 MPa. The elongation of the composite specimens was ranging from 0.3% to 1.85%. Figure 8(Insert at end of paragraph) shows three representative graphs of each sample, S1 – S4. These graphs show the high variation in the mechanical properties within the sample. Due to this variation, it was found that within a sample, S1 – S4, there was a large difference in the wt% between each specimen. This variation in the wt% is attributed to the low amount of fibers and low pressure applied that cause uneven distribution of the fibers in the matrix. To investigate this difference in wt%, the volume fraction (vol%) of each individual specimen was evaluated using an image processing method. Image processing techniques are preferred for the analysis of composite materials containing bio-based fibers due to their ease of use. Furthermore, other techniques that involve matrix burning or digestion are not compatible with natural fibers. However, they suffer from increased chance of operator bias, which is normally reduced by the use of thresholding. Thresholding is a technique wherein a contrast (threshold) level between the matrix and fibers is determined for a sample and then applied to the larger set of samples without changing the threshold. Using ImageJ software, we performed image processing on the pictures of each specimen’s failed cross-section, to to find the fiber area fraction which is related to the fibers’ volume fraction (Figure 9) (Insert at end of paragraph). Using thresholding on S1 sample had specimens with ranges of vol% from 11.53% to 24.5%, S2 had vol% of 17.02% to 38.40%, S3 had vol% of 21.02% to 55.95%, and S4 had vol% of 36.91% to 53.81%. For all samples, the maximum vol% was 55.95%, which showed the highest MOE. On the other hand, its low UTS is attributable to the presence of internal defects. In general, we notice an increase in the mechanical properties as the wt% increases up to the highest reached wt%.

Figure 8. Representative graphs of each composite sample.

Figure 9. ImageJ analysis of a specimen. (a) the specimen cross-section image, (b) the image after the thresholding process.

To evaluate the data obtained from the composite tensile tests, the rule of mixtures (ROM) was applied with the MOE and the result is shown in Figure 10(Insert at end of paragraph).

Figure 10. Samples’ moduli of elasticity data with the rule of mixture boundaries.

The ROM upper limit equation is given by:

(4) $E_{cl}=E_m{V}_m+E_f{V}_f$(4)

And the ROM lower limit equation is given by:

(5) $E_{ct}=\frac{E_m{E}_f}{V_m{E}_f+V_f{E}_m}$(5)

Where $E_{cl}$ is the composite MOE in the longitudinal direction and $E_{ct}$ is the composite MOE in the transverse direction. $E_m$and $V_m$ are the MOE and the vol% of the matrix, respectively. $E_f$and $V_f$are the MOE and the vol% of the fiber, respectively.

The composite MOE should fall between upper and lower boundaries based on the direction of the applied load. If the applied load is in the direction of the fibers, then the fiber reinforcement will be highly effective, and the MOE value will tend toward the upper bound. In contrast, if the load is perpendicular to the direction of the fibers, then the fiber reinforcement will be less effective leading to a lower modulus. Experimentally, the MOE values should be enclosed within these two theoretical limits since there are no perfectly aligned fibers and or materials free of defects.

As shown in Figure 10, the data points follow the lower bound of the ROM as the volume fraction of the sample increases. The observed trend can be attributed to the difficulties of avoiding the defects associated with the increasing of the sample’s volume fraction in the preparation stage. These defects are voids that act as stress concentrators and risers. These stress risers increase the applied stress locally at the void’s sides, and when the local load reaches the UTS of the matrix, a crack starts to grow through the specimen which leads to a failure at lower tensile strength.

Theoretically, the composite fails when it reaches the ultimate failure strain of the fibers because the load is held by the fibers. In other words, when the fibers fracture the matrix will fail immediately since its strength is lower than the fibers strength. By comparing this work’s ultimate failure strain for the composite specimens with the ultimate failure strain of the fibers, the composite failure strain is lower than the lower range of the ultimate failure strain of the fibers (Figure 11(Insert at end of paragraph)). Also, it was seen from Figure 11 that the S1 sample is the closest to the ultimate failure strain of the fibers while the S4 is the furthest one. Since all the samples were made with the same treatment and procedure, the fibers debonding effect due to low interfacial strength can be excluded, and it is attributed to the defects in the samples that associated with the higher wt% samples. These defects are voids that reduce the mechanical properties of the sample. These stress risers increase the applied stress locally at the void’s sides, and when the local load reaches the UTS of the matrix, a crack starts to grow through the specimen which leads to a failure at lower tensile strength.

Figure 11. Comparison between failure strain of the specimen and failure strain of the fibers; inset is the theoretical failure point of composites.

This assumption that the main reason for this is the voids and not the interfacial strength is supported by the comparison of the load required for fiber fracture and the load needed for fiber debonding. Also, to account for the worst-case scenario, the lowest interfacial strength (τ = 5.99 MPα) and the highest tensile strength (σ = 435 MPα) values are used for comparison. Fiber fracture load (F=σπr²) with r = 0.2 mm, the load needed for fiber’s fracture is 54.7 MPa. Fiber debonding load (F = 2ττrl_c) with l_c = 7.3 mm, the load needed for fiber’s debonding is 54.9 MPa. Even considering the worst scenario, the fibers should fracture before debonding from the matrix. Postmortem samples demonstrated that few fibers in the samples were pulled out of the matrix.

Comparison with other literature or with commercial wood products, such as medium density fiberboard (MDF), oriented strand board (OSB), thick particle board (TPB), hard fiber board (HFB), and plywood, shows that date palm fibers treated with water have a promising future as composite reinforcement. In the literature, there is a remarkably high variation in the results and that is because of variation in the type, treatments, and testing of the date palm fibers. Reports that used epoxy as the matrix and fronds as the fibers source, as much as available, was considered for more consistent comparison. These conditions make the available literature limited, so natural fibers from other sources of the date palm tree with the epoxy will be used for comparison. Figure 12(Insert at end of paragraph) shows the available literatures results (Abdal-Hay et al. 2012; Abdellah, Abo-El Hagag, and Marzok 2019; Alshammari et al. 2019; Naveen et al. 2016; Rajeshkumar et al. 2021; Saba et al. 2019). This comparison shows the competitiveness of this work with the treated fibers, and this can be attributed to the removal of water-soluble materials such as wax and pectin by the hydrothermal process.

Figure 12. Comparison of the epoxy-DPF composite properties with other works: (a) tensile strength; (b) modulus of elasticity. This work (green), midrib fibers untreated (blue), midrib fibers treated (patterned blue), mesh fibers untreated (brown), mesh fibers treated (patterned brown).

This work is compared with commercial products in Figure 13(Insert at end of paragraph) to find if DPF composites could be a substitute for these products. In all cases, for tensile strength and elastic modulus, DPF composites match or exceed mechanical properties of the commercial products (OSB, TPB, plywood, HPB, and MDF). From this comparison, restricted to the available epoxy-DPF from the front and the mesh fibers literatures only, fibers extracted and treated by boiling water have competitive results with the chemically treated fibers from the literature and with the commercial products.

Figure 13. Comparing this work (green) with the commercial wood products (blue): (a) tensile/bending strength; (b) modulus of elasticity.

Conclusion

Natural fibers are an effective substitution for corresponding synthetics in many applications since these fibers are abundant, sustainable, renewable, and eco-friendly. Furthermore, in the Middle East region, and especially in Saudi Arabia, there is an abundance of underutilized date palm fibers that is often disposed as waste. While there have been several previous reports on date palm fibers extracted by chemical processes that exhibited good material properties, there is still a need for continued research into hydrothermal and mechanical techniques that are more environmentally friendly.

In this work, the extraction of date palm fibers using a hydrothermal extraction process followed by mechanical separation was investigated as guided by micromechanical models. The extracted fibers showed competitive mechanical properties compared with the chemical-treated fibers. Single fiber properties and its interfacial strengths with epoxy resin were tested where the critical length of the fibers was calculated to be 7.3 mm from these tests results. However, composite samples were made using epoxy resin as a matrix with continuous fibers reinforcement of 100 mm which is much longer than the critical length of the fibers to improve the load transfer between the fibers and the matrix. Different wt% of the fibers were used in the composite samples which are then tested in tension. The test results ranged from 1.72 GPa to 9.52 GPa for elastic modulus and from 16.9 MPa to 43.8 MPa for ultimate tensile strengths. These results are promising, comparable with the literature for the commonly used chemical treatment method, and meet the standards of the commercial wood products in the market. In conclusion, date palm fibers can be used as a substitute for synthetic fibers and other more expensive wood products with low cost, good mechanical properties, and eco-friendly impact in the Middle East region.

Highlights

• Environmentally friendly hydrothermal extraction process used to reduce the use of chemicals

• Micromechanical models used to predict composite behaviors; experiments (single fiber, fiber pull-out and tensile) used to validate predictions

• Cellulosic fibers extracted from core of date palms fronds (midrib) demonstrated an elastic modulus and tensile strength of 17 GPa and 333 MPa, respectively

• Interfacial strengths between date palm fibers and epoxy matrix measured to be between 6 and 12 MPa

• Aligned fiber, uniaxial tensile samples with volume fraction between 11 and 56% characterized with ranges of elastic modulus from 1.72 GPa to 9.52 GPa and ultimate tensile strengths from 16.9 MPa to 43.8 MPa

Acknowledgments

The authors acknowledge support provided by the Deanship of Research Oversight and Coordinator at King Fahd University of Petroleum & Minerals for funding this work through Project INAM2209.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the King Fahd University of Petroleum and Minerals [INAM2209].