1 Introduction

The demand for minimizing the use of synthetic fibers in polymer composite is continuously increasing, which has encouraged the researchers to use environment-friendly natural fibers as a viable replacement in place of synthetic fibers for low structural applications [1]. Due to their lightweight, high strength, and effective stiffness, natural fiber-reinforced polymer composites are among the top contenders as sustainable materials. Due to the efficient use of raw materials and eventual disposal, natural fibers made from renewable resources do not have a negative impact on the environment [2]. Natural fibers offer considerable properties, such as low density, low cost, and minimal tool wear, are non-carcinogenic, possess comparable specific strength, and degradable in comparison to synthetic fiber-based polymer composites [3]. However, their widespread use is restricted by their propensity to absorb water, inadequate suitability of the fibers with polymer matrix, and poor impact performance, which results in a considerable loss of stability, rigidity, and strength [4]. One of the potential lignocellulosic materials in the agricultural sector is date palm fiber (DPF) [5]. Among other natural fibers, date palm fiber is a potential reinforcement for polymeric composites [6]. The most important tropical fruit commercially accessible worldwide, after pineapple and citrus, is the date (dactylifera) [7]. In tropical areas, date palm fiber is widely available and simple to separate from the leaves [8]. The waste products of date trees, including their fruits and leaves, are often utilized to make natural fibers [9]. The main components of date palm fiber are identical to those of other lignocellulosic fibers, which include ash (1.18%), lignin (20%), and holocellulose (60–75%) [10]. Africa and the Middle East have an abundance of date palm fronds and leaves (DPL) that offer an exciting potential to create low-cost building materials [11].

Evidently, their physical characteristics, including dimensions, microfibril, and density, are extremely effective and have an extensive variety of uses in the automobile and aerospace sectors [12]-[13]. Additionally, researchers have noted that DPF has a variety of industrial uses due to its ideal aspect ratio in comparison to other natural fibers [14]. According to research, a number of factors affect the mechanical properties of natural fibers, such as chemical makeup, arrangement, cell size, microfibrillar angle, and flaws [15, 16]. DPF is well-suited for wider use as a material for construction and building, fabrics, furnishings, and parts for automobiles in addition to everyday items since it has similar mechanical and physical characteristics as other natural fibers [17]. The automobile industry has been compelled to embrace DPF because of its exceptional qualities, including its performance, eco-friendliness, cost-effectiveness, and societal impact. DPF also increases productivity and is extremely sustainable [18]. Additionally, implementing DPF has a significant impact on the environment since it allows for effective waste management. In certain situations when stiffness and high strength are not the primary concerns, it may be argued that synthetic fillers can be replaced by composites made up of these fillers. DPFs can be used as a reinforcement in composite materials, and the developed material will be eco-friendly, economical, and recyclable [19]. Many literature surveys provided comprehensive details on how different chemical treatments impact the various properties of natural fiber-based polymer composites [20, 21]. These treatments have a significant impact on properties like mechanical and flammability [22-[23]. The modification of palm fiber with chitosan impacts various properties [24].

Therefore, in the current research work, an attempt to utilize waste date palm leaves as woven reinforcement with polypropylene matrix has been made. The date palm leaves were treated with chitosan solution to increase their thermal stability. The mechanical, thermal, and flammability behavior have been investigated and reported.

2 Materials and methods

2.1 Materials

The date palm fiber leaves were collected from forest waste of Uttarakhand, India, and tailored in the woven form to use as reinforcement in composite laminate. The woven DPL was then subjected to surface modification by subjecting it to the chitosan (2% w/v in acetic acid) solution. The solution of chitosan was prepared by adding 10 g of chitosan into 500 ml distilled water with the acetic acid and then stirred it for 30 min manually with the help of a stick and left it for 16 h, and finally, we got a transparent gel-type solution. The chitosan was grafted to the DPL surface by dip method and kept for drying at 60 °C for 4 h in an oven prior to use for composite fabrication. Polypropylene (PP) was procured from IOCL Pvt. Ltd., (Panipat, Haryana, India) having a density of 0.91 g/cm3, a melting temperature (Tm) of 170 °C, and a melt flow index (MFI) of 10.5 g/10 min. PP was grafted with coupling agent maleic anhydride by employing 2 wt.% MA. The composite laminates were fabricated at the applied pressure and temperature of 3.5 MPa and 165 °C, respectively, by utilizing 5 layers of PP with a thickness of 1.2 mm each, which were prepared in the same compression molding machine at the temperature and pressure of 170 °C and 2.5 MPa, respectively, as shown in Table 1 and Fig. 1.

Table 1 Samples with their designation
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Fig. 1
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PP-DPL composites fabricated by direct compression molding (DCM) technique

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2.2 Characterization of developed laminates

The FTIR (Fourier transform infrared) method by Perkin Elmer (Waltham, MA, USA) was used to investigate any changes in chemical structure that could be represented in the biomass samples. Most FTIR spectra are seen between 500 and 4000 cm−1 wavelengths. The tensile and flexural testing was completed employing the Universal Testing Machine (UTM) (Make: Instron; Model: 5982USA). Tensile and flexural testing was done in accordance with ASTM D3039 and ASTM D7264, respectively. A strain rate of 2 mm/min was used during the testing. For the tensile test, a gauge length of 50 mm was used. To check the impact strength of the composite, the Izod impact test was done on the Model-IT 504 Pendulum impact tester. DMA (NETZSCH, DMA 242 Instrument, Selb, Germany) is a sensitive approach that examines how a material’s properties vary in relation to temperature and oscillation frequency to define its mechanical behavior. Between 20 and 120 °C, with a heating rate of 5 °C per minute, dynamic mechanical testing was carried out using a dynamic mechanical thermal analyzer in the rectangular tension mode at 1 Hz. By measuring the change in weight of a sample over time as the temperature varies, thermogravimetric analysis (TGA) is a method for thermal examination. The developed composites were thermally characterized using a thermogravimetric analyzer (EXSTAR, TG/DTA 6300). The test was carried out at a heating rate of 10 °C per minute from ambient temperature to 700 °C in a nitrogen environment. The flammability of the developed composites has been investigated using TGA and Ul-94 Horizontal burning tester. The crystalline behavior of DPL and its composite was analyzed with X-ray diffraction analysis (Make: Rigaku Ultima IV, Japan) with Cu Kα radiation (λ = 1.5405 A°) at a diffraction angle (2θ) from 5 to 60° with 4°/min scanning rate.

3 Result and discussion

3.1 FTIR (Fourier transform infrared spectroscopy)

The FTIR scan (spectra shown in Fig. 2) was analyzed to understand the chemical interaction between DPL and chitosan. The raw DPL and modified DPL with chitosan (DPL-Cs) showed few peaks that corresponded to grafted chitosan, and similar peaks with little change in intensity were seen. Because of the existence of the -OH bond in the lignocellulosic fiber, the FTIR of DPL fiber exhibits a large peak at 3300 cm−1 [15]. The C-H stretching is responsible for the strong peak at 2900 cm−1 [16]. The DPL fiber’s peak at 1740 cm−1 may be the result of > C = O stretching, which shows that the carbonyl group has been attached to the fiber’s backbone [16]. The DPL-Cs spectrum obviously demonstrates that the peak values which emerge in the range of 1577 and 1569 cm−1 are connected to the N–H and C = O groups of chitosan, respectively. These outcomes verify the existence of chitosan over the coated DPL. The significant peak at 1031 cm−1 may be attributed to the cellulose backbone’s C–OH stretching vibration and carbohydrate backbone. Overall, the analysis suggests that there is no significant modification other than chitosan elements that occurred over the DPL in terms of chemical structure, and therefore, not detectable in FTIR spectra. The obtained results are tabulated below in Table 2.

Fig. 2
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FTIR spectra of date palm leaf

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Table 2 Groups with their corresponding peak values
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3.2 Thermal and flammability behavior

Thermogravimetric analyzer was used to examine the combustion and pyrolysis behavior of date palm biomass in a nitrogen environment at 10 °C per minute heating rate. The thermographs used in the thermogravimetric analysis (TGA) represent the thermal decomposition with varying temperatures of 25 to 700 °C as shown in Fig. 3a and b. Figure 3a shows the thermal decomposition of DPL and chitosan-modified DPL (DPL-Cs). The thermographs clearly demonstrate that significant initial weight loss may be attributed to the moisture present in the specimens which evaporated during the first stage of degradation [17], between 60 and 130 °C. At this point, 2 to 4% of the specimen weight had been lost. During the second phase of depletion, both hemicellulose and cellulose presented in the fibers were broken down, which took place from 130 to 460 °C. The weight loss in the range 290–400 °C might be allocated to the decay of chitosan and cellulose presented in modified DPL (DPL-Cs), which are clearly differentiated by two deterioration peaks in the derivative (DTG) curve as shown in Fig. 3b. The DTG curve shows the weight loss of sample in relation to time and temperature. The degradation of the lignin content is responsible for the third phase of TGA curve (500 to 700 °C) [18].

Fig. 3
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Thermal weight decomposition behavior (a) TGA and (b) DTG of date palm leaf

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Furthermore, Fig. 4 demonstrates the thermal weight breakdown behavior of neat PP and created composites. Neat PP has shown a single-step breakdown process between 400 and 450 °C. These results demonstrate that the primary chain of PP contains C–C bonds, which enable random scission to occur at weak points in the chain and cause thermal breakdown and depolymerization when the temperature is raised. However, for composites reinforced with DPL and DPL-Cs, the degradation was seen to take place in three stages. The first weight decomposition stage in the range of 60–170 °C was attributed to the moisture evaporation, the second decomposition stage observed in the range between 200 and 450 °C corresponds to the deterioration (fiber’s hemicellulose and cellulose) and chitosan’s deacetylation, and the third decomposition stage was observed in the range of 450 to 600 °C that can be allocated to the deterioration of the carbonaceous remnant developed during the second stage. The degradation rate of the PPDP-Cs composite is slightly more in the first stage due to the dehydration of the chitosan molecules and significantly reduced in the second stage and third stage compared to the PPDP composite. The PP remnant completely broke down at 450 °C due to the PP material’s continued breakdown into gaseous components at higher temperature, while composites (PPDP-Cs) showed increased char residue at the end. From DTG curves also, it was observed that composites reinforced with DPL-Cs have contributed towards reducing the rate of thermal decomposition. The chemical treatment of DPL with chitosan makes a positive impact by reducing the rate of thermal degradation due to the formation of a protective char layer which plays a role of intumescent flame retardant and reduces the further degradation of the material. The DTG curve in Fig. 4b depicts the weight loss with the temperature. The PPDP-Cs composite has two peaks in the range of 240 to 360 °C, this is because of the deterioration of chitosan in this range. In this DTG curve, the PPDP-Cs composite has the lowest peak and the PPDP composite has the highest peak in the range of 400 to 500 °C, which shows the rate of decomposition in this range, and that is higher for PPDP and lower for PPDP-Cs composite.

Fig. 4
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Thermal degradation of produced composites (a) TGA and (b) DTG

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The horizontal burning behavior of the produced composite is illustrated in Fig. 5. Neat PP burns completely with frequent dripping and showed a burning rate of about 26 mm/min, which is higher than the polypropylene and date palm leaf composites (PPDP) (25.21 mm/min) and chitosan-treated polypropylene date palm leaf composites (PPDP-Cs) (23.24 mm/min). The DPL reinforcement has not significantly reduced the burning rate of composites; however, the reduced dripping effect was observed after incorporating DPL in PP matrix. Furthermore, chitosan-modified DPL, i.e., DPL-Cs, have shown reduced burning with reduced burning rate. The lower burning rate can be attributed to the fact that the DPL-Cs have shown increased thermal stability as observed from Fig. 3 and Fig. 4. In addition, the increased char content with DPL-Cs may also contribute towards the reduced burning rate.

Fig. 5
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Flammability behavior of developed composites

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3.3 Mechanical behavior

The Universal Testing Machine (UTM) was used to perform the mechanical characterization about the developed composites’ tensile and flexural performances. A fixed strain rate of 2 mm per minute was used for both experiments. The results of the single leaf tensile test are demonstrated in Fig. 6. The mean and standard deviation values are also summarized in Fig. 6. The tensile strength and modulus values of the DPL were found as 116.33 MPa and 7.46 GPa, respectively, while 109.27 MPa and 6.35 GPa, respectively, for DPL-Cs. The reduced strength can be explained from the XRD-spectra as demonstrated in Fig. 7a. It is evident that DPL-Cs have reduced intensity and therefore reduced crystallinity which could have resulted in reduced tensile strength of the DPL after treatment.

Fig. 6
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Mechanical properties of DPL and DPL-Cs

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Fig. 7
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XRD spectra of (a) fiber and  (b) developed composites

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The tensile test outcomes of PPDP composites are displayed in Fig. 8. The tensile strength of 23.38 MPa and the tensile modulus of 858.25 MPa were recorded for the neat PP. The PPDP-reinforced composite has an average tensile strength of 22.17 MPa and a tensile modulus of 1157.21 MPa. The tensile strength was found to decrease for PPDP composites, while tensile modulus values were found to be improved. Furthermore, the PPDP-Cs composite recorded a tensile strength of 23.49 MPa and a tensile modulus of 1220.61 MPa. For PPDP-Cs, a slight improvement of about 0.81% in tensile strength was recorded, while tensile modulus value improved by 42.20%. The improved mechanical properties can be attributed to the efficient stress transfer at fiber-matrix interface under the action of load and improved crystallinity (Fig. 7b) of the composites. SEM micrographs also depict a poor interface between DPL and PP matrix (Fig. 9a and b) which could be responsible for the premature failure of the composites under the loading. DPL-Cs have shown slight improvement in interfacial characteristics and therefore resulted in slight improvement in tensile strength values. The flexural characteristics of the developed composites are demonstrated in Fig. 8. The neat PP has a flexural strength of 34.7 MPa and a flexural modulus of 1094.82 MPa. The average flexural strength and flexural modulus of PPDP reinforced composite were recorded as 31.35 MPa and 1352.2 MPa, respectively. When compared to virgin PP, the tensile and flexural modulus values of PPDP composites are higher. The nature of the component, the composites’ structure, shape, and fiber-matrix interface, and the test parameters, each of these affect how well fiber-reinforced composites function in impact situations. These tests demonstrated that the addition of date palm leaves improved the impact strength of PPDP composites. The neat PP has an impact strength of 0.94990 kJ/m2. When it was reinforced by date palm leaves, then its impact strength increased to 1.16708 kJ/m2.

Fig. 8
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Mechanical properties of developed composites

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Fig. 9
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Fracture surface morphology of failed specimens

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3.4 X-ray diffraction (XRD)

Figure 7a depicts the intensity vs. angle graph of the DPL and DPL-Cs. In this figure, from 5 to 40° angle, the DPL-Cs have a lower intensity than DPL, which means lower crystallinity of DPL-Cs, and it resulted in reduced tensile strength. So, it is evident that the treatment of DPL with chitosan reduces the tensile strength of DPL. Similarly, Fig. 7b represents that the crystallinity of neat PP is lower and higher for the PPDP composite, while the PPDP-Cs composite has moderate crystallinity in the given range.

3.5 Dynamic mechanical analysis (DMA)

When using natural fiber-reinforced thermoplastic composites, the DMA distinguishes between the material’s elastic and viscous response, which are time- and temperature-dependent. A material’s ability to carry a load is indicated by its storage modulus value. Like the stiffness of the material, it is the elastic energy held throughout one oscillation cycle. Important data regarding stiffness, matrix/fiber interfacial adhesion, and the extent to which materials are cross-linked can be inferred from the E′ vs. temperature plot [19]. More effective stress transmission between the fiber and matrix results in an increase in E′ of composites [20,21,22]. E′ is the value of energy that is left in the samples after the loading cycle is finished [23]. Glassy, transitional, and rubbery areas are the three regions that make up E′, and it elaborates on the elasticity of composite components [24]. It is observed from Fig. 10a that for all the samples, storage modulus value decreases as the temperature rises. This is a result of phase transition of composite material from a glassy to a rubbery state. Here, E′ value of PPDP is lower than the PP value up to 70 °C after it becomes almost equal to the PP value, which means there is improper fiber-matrix interfacial bonding between fiber and polymer, and it will result in inadequate load transmission between matrix and fiber. The storage modulus value of the PPDP-Cs composite is higher than neat PP and PPDP composite throughout the curve. The improved storage modulus of the PPDP-Cs composite demonstrates the improved interfacial bonding between the matrix and fiber, and it will result in efficient load transfer between matrix and fiber. Therefore, it is evident that the chitosan-modified DPL resulted in improved storage modulus values of the PPDP-Cs composite.

Fig. 10
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Dynamic mechanical behavior of developed composites

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When a viscoelastic material experiences deformation, the loss modulus (E″) curve measures the amount of energy lost as heat every cycle [24]. The loss modulus measures how much energy the material releases when it is subjected to cyclic stress [25]. The fluctuation of the loss modulus with temperature is shown in Fig. 10b. In Fig. 10b, the neat PP has the lowest loss modulus (E″) in comparison with PPDP and PPDP-Cs composites. It is because of the free movement of polymer chains. As the load is applied, it stretches easily in comparison with PPDP and PPDP-Cs composites, which means the energy dissipation is less during deformation cycle. The PPDP-Cs composite has the highest E″, which means it will dissipate more heat during deformation cycle in comparison with the PPDP composite and neat PP. Overall, the PPDP-Cs composite has shown improved storage and loss modulus than the PPDP composites and implies that the stress transmission between the fiber-matrix contact is improved.

4 Conclusions

The primary objective of the current experimental investigation was to examine the mechanical and thermal characteristics of composites made from date palm leaf fiber and polypropylene. The exploration opens a new way of utilizing date palm waste as a reinforcement to develop laminated composites for non-structural applications. The incorporation of woven date palm mats into the PP matrix results in comparable mechanical properties with neat PP. The outcomes of the mechanical characterization reveal that the chitosan-modified fibers improved the tensile strength and modulus of the PPDP-Cs composite by 0.81% and 42.20%, respectively, in comparison with the PPDP composite. Thermal and flammability behavior has also been investigated and improved thermal stability, and reduced burning rate has been recorded for composites reinforced with chitosan-modified fibers. The char residue left at 700 °C is around 39% for DPL-Cs, which is more than double of DPL (that is 17%). The rate of degradation of DPL is around 17.5 mg/min, and for DPL-Cs is 5 mg/min which is less than one-third of DPL. The char residue left for PPDP-Cs is 17% while only 11% for PPDP composite at 700 °C. And the rate of thermal degradation is 15 mg/min and 20.5 mg/min for PPDP-Cs and PPDP composite, respectively. The horizontal burning rate of PPDP-Cs composite is reduced by only 7.81%, but dripping effect is reduced significantly in comparison with the PPDP composite. Failure mechanism is attributed to the poor interfacial characteristics between fiber and polymer. However, date palm leaves and polypropylene both seem to be suitable options for making environmentally friendly products for non-structural applications where lower thermal stability is a requirement. This research gives an idea that thermal and mechanical properties of these composites can be further increased with suitable treatment methods. Research shows that conventional materials can be replaced in various industrial applications where lower thermal stability and mechanical properties are required. These materials will be cost-effective in comparison of traditional materials. Further, by combining date palm leaves with other natural fibers, a hybrid composite can be developed with improved properties and functionalities with a wide range of applications.