A review of leaf fiber reinforced polymer composites

The utilization of natural fiber-reinforced polymer composite has received greater attention in various fields due to its recyclability; inexpensive, nonabrasive, specific properties; light-weight, naturally decomposed, abundant availability; etc. Natural fibers are generally lignocellulosic and multicellular, a better alternate to the synthetic materials. Among the natural fibers, leaf fibers are hard fibers, used in the making of filaments, threads, ropes, mats, fabrics, etc. PALF, sisal, henequen, cantala, fique, alfa, and sansevieria family are the examples of the leaf fibers. The present comprehensive review aims to provide different types of leaf fibers, their properties, and their reinforced composites. The effect of various factors like fiber volume fraction, fiber aspect ratio (length/diameter), fiber orientation, packing arrangement, matrix content and coupling agents, and processing techniques towards the mechanical properties of leaf fiber-reinforced polymer composites, is discussed. The surface modification of fiber such as alkaline, silane, KMnO 4 , and their effects on the mechanical properties is given. Scanning electron microscopy (SEM) and water absorption (WA) characteristics are also discussed.


Introduction
Natural fibers have been gaining much attention from the researchers, engineers, and industrialists and have been considered as a viable alternative to the synthetic counterparts.These fibers have many advantages such as light weight, available in nature, pollution-free, economical and eco-friendly, design flexibility, low pressure, and temperature requirement during manufacturing.
Natural fibers are referred to as vegetable fibers extracted from the plants and can be used as a filler or reinforcement in polymer matrices, and it is mainly consists of cellulose.Based on the strength, stiffness, and location of extraction, these fibers have been categorized into the following: (i) leaf fibers are hard fibers, obtained from the leaves or leaf stalks of various perennial, monocotyledonous plants, and the fiber lumen is larger in relation to the cell wall; (ii) stem or bast fibers occur in the phloem, typically low in elongation and recovery from stretch (jute, ramie, flax, banana, kenaf, hemp); (iii) seed and fruit fibers are attached to hairs or in the form of bundle or encased in a husk (coconut, cotton, kapok); (iv) grass fibers occur in stems and leaves (bagasse, elephant grass, bamboo); (v) wood fiber is extracted from trees, having high level of porosity used in the manufacture of hardboard and paperboard (Eucalyptus, pine, beech, birch); and (vi) straw fibers, an agricultural by-product, obtained from the stalks of cereal plants after the grains were removed (barley, wheat, oat, rice, corn).Based on the main sources, bast, leaf, seed, wood, and grass, the plant fibers which are categorized further into different types are presented in Fig. 1.These abundantly available plant fibers consist various properties such as reliable quality, inexpensive, light weight, and nontoxic which make these fibers more popular among the researchers, scientists, and industrialists for the purpose of enhancing their applicability.
The studies undertaken by various authors on the leaf fiber-reinforced composites are presented in this review.Different molding processes such as hand layup, compression, and injection twin-screw extruder and different leaf fibers and their reinforced composites have been studied by several authors.The most common matrix materials used in this review are thermoplastic and thermosets, i.e., epoxy, polyester, polypropylene (PP), and polyethylene (PE).Also, their mechanical properties were determined and compared to the properties of synthetic fibers.

Leaf fiber as reinforcing material
The contribution of plastics, problems at their end of lifetime, and the depletion of nonbiodegradable resources have emphasized the utilization of eco-friendly materials.Compared to synthetic fibers, there is a wide variety of natural fibers available throughout the world, which act as a sustainable and suitable reinforcing agent in the biodegradable composites.At the time of manufacturing, synthetic fibers such as glass and carbon could create different environmental and health hazard issues for the employees.With the help of nonhazardous plant fibers, these problems can be solved by combining, strengthening, and shaping composites in polymer matrix.The characteristic features of the plant fibers over the synthetic fibers are reduced tool wear, inexpensive, contribution of greener environment to the society, non-toxicity, low density and it is very easy to dispose at their end of life cycle, etc. Conversely, it has disadvantages too, i.e., low thermal and water resistance, lower durability, and hydrophilic nature of the plant fibers lead poor fiber-matrix interfacial bonding.Many attempts are made by researchers and technologists [1][2][3][4] to utilize leaf fiber by mixing it with suitable matrix for the preparation of composite material.The surface modification of fiber can enhance fiber-matrix interfacial bonding, decrease the absorption of moisture, and could improve the mechanical property of the polymer composites.

Constituents and properties of leaf fiber
Leaf fibers are versatile materials, and their characteristics may vary with chemical constituents and physical structure.The physico-mechanical properties of leaf fibers are mainly influenced by the fiber extraction method, age of the plant, climatic conditions, moisture content, technical process involved during harvesting, retting, and decortication.Major components present in the fibers are as follows: (i) cellulose -a linear homopolymer made up of glucose units; (ii) hemicellulose -strengthen the cell wall of the fibers, hydrophilic in nature, and have bonding with the cellulose; (iii) lignin -a phenolic compound which conferred rigidness to plant cell wall; (iv) pectin -glues the elementary fibers to form bundles and found in the primary cell wall; and (v) wax -protects the primary wall, which is composed of cellulose crystalline fibrils.The mechanical properties of cellulose-based fibers are highly dependent upon the fibrillar angle and cellulose content present in the fibers.Table 1 displayed the chemical constituents of leaf fibers in which they significantly differed from leaf fiber types and origins.Generally, the primary composition, cellulose, is at 43 − 80%, hemicellulose 10 − 39%, lignin 3-15%, and the remaining parameters are given in Table 1.The leaf fibers exhibited a wide range of tensile strength ranging from 230 to 1627 MPa, Young's modulus 0.2 to 22 GPa, and density 0.8 to 1.4 g/cm 3 (Table 2).The TS values are generally lower than those of synthetic fibers such as E-glass fiber which has a TS of approximately 2000-3500 MPa [5].

Leaf fiber-reinforced polymer composites
There has been a growing interest in recent years to replace traditional synthetic fiber with leaf fiber to reinforce polymer resins as it eliminates the environmental issues and fossil fuel depletion.These leaf fibers have been gaining importance from materialists and scientists due to its ecological and economical attribution.Researchers extracted the fibers from leaves for their studies, and the typical view of different types of leaf with their respective fibers is shown in Fig. 2A-J.Approximately, 30 million tons of plant fibers are annually generated and utilized as a constituent in various applications includes, automobile, construction, sports equipment, packaging and research industries.In the following section, the leaf fibers, such as alfa, cantala, fique, henequen, PALF, sisal, Sansevieria cylindrica Sansevieria ehrenbergii, Sansevieria roxburghiana, and Sansevieria trifasciata, reinforced polymer composites which are discussed.

Alfa fiber-reinforced polymer composite
Alfa belongs to the Gramineae family and is the esparto grass also called tussock grass, extracted from the leaves of Stipa tenacissima L. grass.The fiber bundles are characterized by a mean diameter of 113 µm (ranging from 90 to 120 µm) and a density of 0.89 g/ cm 3 [17].Mansour et al. [18] studied the impact of alkali (NaOH)-treated composites from alfa fiber included with polyester matrix.The treated fiber in 1%, 5%, and 10% NaOH solution for both periods of 24 h and 48 h was taken for the study.The flexural strength (FS) of 57 MPa was observed for the 10% NaOH-treated (for 24 h) fibers, which was nearly 60% greater than the untreated fiber composites.A similar pattern is observed in flexural modulus (FM) as that of FS, i.e., ≈62% increased for the 10% NaOH-treated fibers compared to untreated one.The same procedure is adopted for alfa fiber (20 wt% as fixed)reinforced polypropylene (PP) composites.Arrakhiz et al [19] evaluated the influence of alkali, ethrification and esterification on the mechanical properties of the composites using hot pressing molding techniques.A significant enhancement in Young's modulus (1405 MPa) of alfa palm/polypropylene composites was noted, which is 35% greater than untreated fiber and two times greater than plain PP.Then significant improvement in thermal stability was noted for the etherification-treated alfa fibers with gains in the temperature up to 80 °C.
Polypropylene incorporated with three natural fibers such as alfa, coir and bagasse composites with the effect of alkali-treated fiber on the mechanical properties [20].Different fiber loadings at 5, 10, 15, 20, 25, and 30 wt% were taken for the studies.Compared to plain PP, the Young's modulus of all the three fiber composites have greater value, and tensile strength (TS) was lower value with increase in fiber loadings (at 30 wt%).The FM for alfa coir and bagasse composites were 2077.5 MPa, 2088.5 MPa, and 1841 MPa, respectively, at 30 wt% of fiber loadings.According to FS, it remains constant for all the three composites from 5 to 30 wt% of fiber.A considerable enhancement in torsion modulus (in power law model) were noted with increasing frequency and fiber loadings; thus, the authors concluded that the prepared material behaved like an elastic solid.
Hybrid polymer composites based on PP reinforced with two fillers alfa fiber and clay particles were fabricated [21] by using injection molding techniques.The incorporation of clay in the PP composites improved the Young's modulus of 3120 MPa, i.e., an increase of 300% than plain PP.Contrarily, the TS had a greater value for the alfa fiber composite rather than clay-filled composites.
Sami Ben and Ridha Ben [22] prepared the alfa fiber incorporated with PE composites and investigated the mechanical properties with the effect of fiber orientation and fiber fraction of the composite.The longitudinal and transverse Young's modulus of alfa/PE composite were 12.3 GPa and 5 GPa, respectively, at 45% fiber loadings.The effect of fiber orientation on the composite mechanical properties denoted that the longitudinal Young's modulus and stress at break were decreased with increasing angle (0 to 90°).But Poisson's ratio increases with increasing angle up to 10° and then decreases considerably.
Using extrusion and injection molding process, El-Abbassi et al. [23] evaluated the impact of alkali-treated fiber on mechanical and water aging properties of alfa fiber blended polypropylene (AFRP) with respect to different fiber weight fractions (0%, 10%, 20%, & 30 wt%).After applying the alkali treatment on fibers, the Young's modulus and Fig. 2 a Alfa plant and its fiber.b Cantala plant and its fiber.c Fique plant and its fiber.d Henequen plant and its fiber.e PALF plant and its fiber.f Sisal plant and its fiber.g Sansevieria cylindrica plant and its fiber.h Sansevieria ehrenbergii plant.i Sansevieria roxburghiana plant.j Sansevieria trifasciata plant and its fiber tensile strength were enhanced by 23% and 16%, respectively, and a significant reduction in WA properties was also noted.
Mechakra et al. [1] investigated the outcomes of optimizing parameters of alkaline treatment (24 and 48 h) and fiber volume fractions (0, 10,20,30) for the preparation of short alfa fiber blended with PP resin.In their studies, the authors found that the Young's modulus, TS, and breaking stress had a greater value for the PP charged with 30% alfa fiber treated at 24 and 48 h.As the volume of fiber increases, the mechanical property increases.Also, the treatment of alkaline indicated a significant raise in strength and decreases the breaking strain.
Med Amin et al. [24] observed that the wool-alfa-reinforced hybrid polyester composites exhibited a medium hydrophilicity through water contact angle (62 ± 2)° measurement.During the second heating run by the DSC analysis, the observed glass transition temperature (T g ) of the composite was 69.2 °C.From the TGA, the material is less thermally stable at 400 °C, and a 2.6% of mass loss at 84 °C was obtained due to the moist structure of the natural fibers.
Sair et al. [25] modified the alfa fiber surface by alkali treatment with different conc. of 0%, 5%, 7.%, 10%, and 12%.The effect of various volumes of fibers 5, 10, 15, 20, 25, and 30 wt% on the thermal, mechanical, and acoustical characteristics of the alfa fiber PU composites was determined.After applying the alkali treatment, the tensile test results showed that the tensile strength of treated fiber composites was enhanced, but for plain PU and untreated fiber, composite showed the reversed effects.The 20% of alkali-treated fiber was the optimum parameters for the composites where the Young's modulus and TS raised from 2.7 to 4 GPa and 14.3 to 24.9 MPa, i.e., an enhancement of 48.14% and 74.12%, respectively.A weak resistance between the fiber and matrix at 30% wt was noted for the composites too.

Cantala fiber (CF)-reinforced polymer composites
Cantala is a member of the Agave family (Agavaceae), which grows in a moist, humid soil.The fiber is lighter in color than other agaves, and its strength depends on its preparation.Wijang et al. [26] described the mechanical properties of cantala fiber and short cantala recycled HDPE composite with reference to the influence of treatments such as alkali, silane, and combination of both.Alkali-treated fiber exhibited superior FS (increased by 16%) than untreated composite; conversely, the TS of alkali-silane-treated fiber composite was lower than alkali-treated fibers.The highest surface energy of 45.37 mN/m was observed for the alkali treated (2 wt% NaOH), and for alkali-silane treatment (with 0.75 wt%), it showed the greater thermal stability (up to 507.1 °C) and interfacial shear strength (IFSS) value of 3.6 MPa.
Tipu Sultan et al. [2] modified the cantala leaf fiber with NaOH or sodium chlorite (NaClO 2 ) and fabricated the treated and untreated fiber PP composites using hot press molding process.The elastic modulus and thermal resistance of alkali-treated CF have higher value than the untreated fiber composites.
Ilham et al. [27] analyzed the effect on FS of the brake pad composite from CF (0, 4, 8, 12) reinforced with PP using the cold press cum hot press method.Increasing the volume fraction of CF increased the flexural properties of specimens.Fiber pullout occurs due to the lower interfacial bonding between the fiber, and the matrix was observed by SEM.
Wijang et al. [7] evaluated the influence of soaking time in alkali solution on the IFSS of cantala fiber recycled HDPE composites.The surface modification of CF by alkali was carried out on 2% conc. of NaOH, for a period of 0, 4, 8, 12, 16, 20, and 24 h.The IFSS (determined by the single fibre pullout test method) values of the modified fibre after different soaking hours were observed to be 2.44 MPa, 2.15 MPa, 2.93 MPa, 2.63 MPa, 2.80 MPa, and 3.42 MPa.Extending the duration of immersion would result in an elevation of the Interfacial Shear Strength (IFSS) value of the composites.

Fique fiber-reinforced polymer composites
Fique fibers or cabuya belongs to the Asparagaceae family, extracted from the Furcraea andina plant.It is used for making ropes, sacks, and handicrafts.An increase in the modulus of rigidity was noted from 2.5 to 7.2 GPa when alfa fiber treated with 5 wt% NaOH solution followed by starch-based polymer of 35 wt% [28].Miguel et al. [29] studied the mechanical and thermal properties of biocomposites from linear LDPE-nonwoven industrial fique fiber (LLDPE-fique) and epoxy fique composites using resin film infusion process.Compared to neat LLDPE, LLDPE-fique has the higher tensile modulus (TM) of 1370 MPa, TS of 19.6 MPa, FM of 686 MPa, and FS of 16.2 MPa.Similar observation is followed for the epoxy fique than the plain epoxy.From the DSC analysis, a decrease in enthalpy from 144 to 118 J/g for LLDPE and LDPE-fique was noticed.
Catalina and Analía [30] prepared the unidirectional epoxy/fique composites with the treated (NaOH at 18 w/v%) and untreated fiber by pultrusion method.The parameters such as flexural properties were determined after 20 days of aging of composites which was subjected to various environments (in distilled water pH = 6.0, alkaline pH = 12.0, cement mortar).After the surface modification of fiber, the flexural properties were raised than matrix modification.Treated fiber epoxy has the lowest diffusion coefficient, and then the enhancement at the composite interface reduced the water and calcium hydroxide absorption by six times.The FM of epoxy fique composite was better than conventional wood such as oriented strand board (OHB); thus, the authors suggested that this material opened the new possibility to replace the conventional wood used in construction purposes.
Sandra and Diego [31] manufactured the natural rubber (NR)/butadiene styrene rubber (SBR)/polybutadiene (BR) matrix-reinforced fique fiber composites and evaluated its characteristics.A weight loss of 78% was observed from the TGA thermograms.As fiber loading increased from 0 to 40% of 10-mm length, the tensile loads, compressive loads, TS, and hardness were increased, while scratch time, cure time, elongation at break, and wear resistance were decreased for the composites.
Michelle et al. [32] conducted a study on dynamic mechanical properties of fique fabric-reinforced epoxy of different fique content of 15, 30, 40, and 50 vol%.A considerable increase in the storage modulus (5073 MPa) with increasing volume (at 50%) of fique fabric at the initial temperature of − 50 °C was noted.Loss modulus assigned to the T g tend to be shifted to higher temperature of 83-89 °C, and it was higher for fique epoxy compared to polyester composite (28-52 °C).
Sergio et al. [33] gave a comparative study on the mechanical and dynamic vibratory properties of fique epoxy and glass fiber epoxy laminates.The TS and elastic modulus for E-glass epoxy were 153.5 MPa and 4290 MPa, whereas for fique epoxy it was 36.2MPa and 1272.98MPa.A slight increase in natural frequency (Hz) for E-glass composites was noted than the fique fiber composites.The poor bonding between the fique and the resin was evidenced by SEM, which was also reflected in the properties of fique epoxy compared to E-glass epoxy.
Michelle [34] studied the notch toughness evaluation of epoxy matrix composites reinforced with various fiber loadings (15, 30, 40, and 50 vol%).Composite enhanced with 40% of fique is good to notch toughness, whereas brittle fracture with poor impact energies for 15 and 30% of fiqu, were observed.At 50% fique, Izod and impact energy suffered a small decrease according to the Roger and Plumtree model.

Henequen fiber (HF)-reinforced polymer composites
Henequen fibers (Agave fourcroydes) are leaf fibers, used for the manufacture of twines and ropes.The leaves grows of 1.2 to 1.8 m long with a thick stem (reach 5 ft) and a terminal spine of 2-3 cm long.Surface treatments by steam explosion technology with the inclusion of polyethylene glycol (PEG) was given to the raw henequen fiber.The treated fiber-/PLA-reinforced composite was prepared by thermo-compression molding technology [35].Compared to plain PLA, the degree of crystallinity is higher for PLA composite due to that this henequen fibers induced nucleating.Contrarily, the thermal degradation temperature was lower for fiber treated with PEG composite.An increase in tensile properties was observed for 90% PLA with 10% fiber without PEG.Lower flexural properties were noted for all composite than plain PLA.
According to TAPPI standard T257, the chemical composition of henequen fibers was analyzed, and the values are 68.1% cellulose, 18.2% hemicellulose, 8.7% lignin, 1.3% ash, and 3.7% extractives, respectively [36].The fiber strands were more 1 m long with diameter of 220.8 ± 106.45 µm.The TS of fiber was measured as 442 MPa and Weibull shape factor as 2.60 with a gauge length of 6.35 mm.Theoretical TS value of henequen fiber was 450.7 MPa with a gauge length of 485.04 µm.The critical fiber length from the IFSS by von Mises and Tresca was 360 and 414.7 µm, respectively.Also, the tensile characteristics of fiber PP composites were noticed with 4 wt% of coupling agent.
Biocomposites-reinforced henequen and silk fiber with the influence of poly(butylene succinate) were prepared using a compression molding method [37].For the hygrothermal effect, the composite was placed in the chamber at 60 °C and 85 °C relative humidity for about 1000 h.In WA properties, the weight increase of biocomposites of both silk and henequen was noted and also absorb maximum amount of water within 50 h.The storage modulus of henequen composite was 4GPa and decreased with the duration (more than 500 h) resulting the half value of 2 GPa.The tan δ peak has been shifted to high temperature, and intensity was decreased for the henequen fiber, because the lesser polymer chains were participated in the transitions.
The fibers from henequen have been used as reinforcement for epoxy resin composites by compression molding process, and their mechanical properties were investigated [38].The untreated fiber/epoxy showed a TS as 234 ± 11.3 MPa, FS as 197.32 ± 7.64 MPa, and impact strength as 116.04 ± 14.65 kJ/m 2 , whereas for treated, it was 233 ± 11.98 MPa, 199.52 ± 7.42 MPa, and 90.81 ± 18.57 kJ/m 2 .As shown, the experimental results for both forms (treated and untreated) of the composite are identical, and it can be inferred that their mechanical properties have not been enhanced by chemical treatment.
The density value of the alkali and alkali + heat-treated henequen fibers was larger than the raw fibers [39].The alkali-treated fiber epoxy composite has higher TS of 49.04 MPa, whereas it was 18.16 MPa and 11.7568 MPa for untreated epoxy and plain epoxy composite respectively.For the fractured samples (SEM analysis), the presence of matrix to the fiber surface and uncoiling of microfibrils has been observed for the alkali-treated samples, whereas there is no trace of matrix to the fiber surface in case of untreated fiber composite.The TM (2.189 GPa) and FS (54.482MPa) were higher for heat-treated fiber and alkali + heat-treated fiber epoxy composite respectively.By DTG analysis, the composites have high degree of crystallinity and thermally stable at 366 °C, 371 °C, 388 °C, and 369 °C for untreated, alkali, heat, and alkali + heat-treated fiber epoxy composites.

Pineapple leaf fiber (PALF)-reinforced polymer composites
PALF is obtained from the leaves of pineapple plant Ananas comosus, and a perennial herbaceous plant holds 80 leaves in its lifetime.The postharvest waste PALF is multicellular, lignocellulosic fiber in nature and can reduce negative environmental impacts in the preparation of composite work.
A comparative study on mechanical evaluation of coir fiber epoxy and PALF epoxy composites were given [40].Compared to coir (30%wt) fiber epoxy composite strength (28.7 MPa), 30%wt PALF fiber epoxy (86.4 MPa) yielded greater strength.The impact strength (946 J/m), elastic modulus (7.97 GPa), and elongation (1.3%) were greater for PALF fiber composites.For multilayer armor system (MAS), coir fiber has the highest depth of penetration (DOP of 31.6 mm) with impact energy of 3.52 kJ, whereas PALF has DOP of 18.2 mm with 3.48-kJ impact energy.
Gabriel et al. [41] used the lignocellulosic fiber (PALF) to study the tensile properties of fiber-reinforced polyester (PE) composites.The fiber content of 10%, 20%, and 30% and diameters from 0.09 to 0.30 mm were taken for the composite preparation.An observation was made that the inclusion of 30wt% of fibre composites resulted in an augmentation of tensile strength (103.25 MPa), elastic modulus (1.99 GPa), and deformation (5.14%).The introduction of PALF would improve the tensile qualities, including TS, elastic modulus (EM), and elongation.
`The determination of FS by means of 3-point bend tests and Weibull statistical analysis for epoxy composites incorporated with continuous and aligned PALF fibers was analyzed [42].The determination of Weibull parameters such as characteristic strength (θ is 101.5 MPa for 30 wt%), modulus (β-3.38 for 20 wt%), and precision adjustment (R 2 = 0.9635 for 10 wt%) was noted for the PALF-reinforced epoxy composites.Also, the rupture mechanisms associated with reinforcing were analyzed by the SEM.
Ridzuan et al. [43] studied the influence of PALF, napier, and hemp fibers with different fiber weight of 5, 7.5, and 10% on the scratch resistance of epoxy composites.Compared to PALF and hemp fiber-filled epoxy composites, the highest peak loads at 28.6 N (5%wt), 30.2 N (7.5 wt%), and 36.4N (10 wt%) were obtained for napier epoxy composites.The coefficient of friction (COF 0.703), fracture toughness (4.24), and scratch hardness were also higher for napier composites.Conversely, the density and porosity have the lowest value for napier fibers.Hence, the lower porosity might be the reason for obtaining the higher scratching resistance.
Indra et al. [44] conducted an experiment study on the mechanical characteristics of hybrid (jute, PALF, and glass) fiber-reinforced epoxy polymer composites.The ratio of each fiber is 1:1:1, with 0.18 to 0.42 wt% of fibers and 1.5% content of resins which were used for the preparation of composites by hand lay-up process.The TS (71.66 MPa), FS (239.37 MPa), and TM (> 800 MPa), respectively, were noticed for the maximum fiber content of 0.42%.As the fiber content raised in the epoxy composites, the mechanical properties also increased.
Using compression molding technique, the PALF-reinforced PE composites were fabricated, and the samples were tested [45].The obtained results showed the crosshead, speed of 5 mm/min, and gauge length of 50 mm yielded highest TS (33.13 MPa), TM (1.553 GPa), and elongation at break (4.11%) respectively for 45wt% fiber loading.At the same time, the flexural strength (82.97 MPa) and modulus (6.37 GPa) showed an increasing trend, with an increase in fiber content of the PALF composites.
Pujari et al. [46] suggested that PALF-reinforced natural rubber matrix composites could be successfully used to transformer applications.The specimens containing a different volume fraction of fibers, i.e., 5, 10, 15, 20, 25, and 30 wt%, have been selected for the thermal, physical, and dielectric studies.The WA coefficient (%), oil absorption coefficient (%), and dielectric strength (kV/mm) of the composites were increased, whereas the thermal conductivity (W/m˚k) value decreased, as the volume fraction of the fiber increases.
Parameswara et al. [47] revealed that the influence of fiber orientation (0°, 30°, 45°, 60°, and 90°) on dynamic mechanical properties of hybrid PALF reinforced with basalt epoxy composites.The orientation of the fiber played a major role, because it gave the effect on storage modulus and loss tangent along with mechanical characteristics.At frequencies, 0.1 Hz, 1 Hz, and 10 Hz, the storage modulus were ̴ 3.86 GPa, ̴ 4.26 GPa, and 4.23 GPa and lost tangent as 0.16, 0.12, and 0.09, respectively.The composite with 0° fiber orientation of thickness 2.87 mm has good damping properties; also, the similar resulting pattern was followed for mechanical properties including Young's modulus and flexural modulus.
Ayu et al. [48] developed the PALF-reinforced polypropylene (PP) composites on different fiber volume ratio of 30, 40, 50, 60, and 70 wt%.Using compression molding technique with random orientation, the composites were prepared for the treated (alkaline) and untreated fibers.At 30 wt% of fiber, the tensile strength was increased with 12.9% with increase of fibers and decreased drastically by − 76.4% with the inclusion of fiber fraction (up to 70 wt%).It was found that the higher tensile strength (16.71MPa), hardness (62.8 shore-D), and density (0.93 g/cm 3 ) at 30% fiber weight were noted, and this is the optimum parameters (30 wt%) for the preparation of composites.
Krutibash et al. [55] studied the experimental analysis on the impact of fiber loading (0%, 10%, 15%, 30 wt%) and surface treatment of natural fibers (jute and sisal for NaOH at 2 h) on the mechanical and WA properties of glass/jute/sisal (GF/JF/SF) fiber PP composites.The hybrid fibers (each fibers 10% and PP 70%) specimens has the highest TS (33.18 MPa), TM (3282.09MPa), hardness (100.1 R/scale), and impact strength (44.155J/m), whereas the other fiber loading composites treated has lowest mechanical properties.The flexural strength (61.39 MPa) and modulus (3453.15MPa) were obtained for the hybrid (glass-15%, jute-15%, PP-70%) fiber composites.WA were found to be lower for all polymer composite, and hybrid samples have below 0.3% (by weight).
Pramod et al. [56] concentrated his work on the compression and WA characteristics of banana and sisal hybrid fiber epoxy composites using hand lay-up process.The fiber was modified by alkali with 5% for 24 h and various fiber loading taken as banana: sisal were 25:15, 15:25, 20:20, and 10:30.The compressive strength of 430 MPa was observed for 10:30 (% of banana & sisal) treated fiber epoxy, whereas for untreated, it was 322 MPa.WA properties of the above said samples were minimum than the treated and other proportion fiber samples.
Changes in the mechanical properties of sisal fiber and human hair-based hybrid epoxy were noticed [57].By hand lay-up process, the composites were prepared with various percentage of fiber (5, 10, 15%) and epoxy resin (95, 90, 85%).From the experimental results, it was clearly seen that, as the percentage of fiber increased, the quality of mechanical properties was also increased.The ultimate tensile strength (27.7 N/mm 2 ), impact energy (46.182J/m), FM (963.86 MPa), and flexural stress at maximum flexural load (38.158MPa) were higher for 15% of fiber composites, whereas hardness (82 HRB) was higher for 5% fiber samples.
Sandeep et al. [58] focused the influence of mustard cakes and pink needles on TS, impact energy, and abrasion of sisal fiber-based hybrid polyester composites.Among the various proportion of fillers, particulates, and fibers, the (55% PE, 40% sisal, and 5% pine needles) hybrid PE composites have the highest value of TS (41.45 MPa), void fraction, impact energy (8 J), and lowest specific wear rate (3.019 × 10 −7 mm 3 N/m).
Composites from sisal(S), waste tea fibers (T), and glass fibers (GF) reinforced with epoxy-based hybrid composites were prepared and studied their acoustical, mechanical, and chemical properties [59].The TS (75.6 MPa), impact energy (95 kJ/m 2 ), and modulus (5.82 GPa) were higher for hybrid (2%S and 10%T) epoxy, and the specimens with high sisal and glass fiber exhibits more flexural strength.Regarding acoustic behavior, the weak sound (for 20 wt.% of sisal fibers and 5 wt.% of tea fiber) and high sound (20 wt.% of tea fiber and 5 wt.% of sisal) absorption were measured.By varying the frequencies range from 63 to 6300 Hz, the sound absorption coefficient (α) value varies between 0.03 and 0.27.Surface treatment and hybrid effect enhanced more adhesion between the fiber, and the resin used in the specimens was analyzed by SEM.
Laminated composites were developed with the help of sisal, banana fibers, and polyester resin using compression molding techniques [60].The effects of fiber weight (%) and fiber surface treatment on flexural properties and damping factor for 50 wt% of fiber were analyzed.The increased FM (N/mm 2 ) were noticed for 50 wt% of banana, sisal, and hybrid fibers.The natural frequency and damping factor were higher for the treated fiber composites.
Physico-mechanical and micrographs were examined under different situations such as effect of silica microparticles, volume fraction of sisal, and maleic anhydride of unidirectional sisal fiber epoxy hybrid composites [61].It was revealed that, at low weight fraction of fiber, the composites provided better tensile properties, but maleic anhydride treatment affected the flexural properties, WA, and apparent porosity.The authors also suggested that these composites opened an alternative material for engineering applications.
In order to improve the adhesion between the fiber and matrix, alkaline treatment was given, then the static and dynamic properties of three different fiber composites that are sisal, jute, and sisal/jute reinforced epoxy composites [62].From the three different samples, the hybrid (sisal/jute) epoxy has the highest TS, FS, and damping factor.Successive resonance sets and accelerance level confirmed the good dynamic properties of hybrid composites.
Sivakandhan et al. [64] conducted a study on mechanical and morphological analysis of sisal and jute fiber hybrid sandwich epoxy composites.The coaxial TS and FS were 22.53 N/mm 2 and 56.31 N/mm 2 , respectively, for the hybrid epoxy increased with increasing jute fiber content.The transaxial TS and impact strength were increased by 17.99 N/mm 2 and 0.8 5 J for the hybrid epoxy with increase of sisal fiber.
Senthil et al. [65] studied the effect of different stacking sequence of hemp and sisal fiber-reinforced hybrid epoxy composites on the mechanical properties.The pure hemp fiber epoxy has the TS of 31.997MPa, modulus of 1158.95MPa, ILSS of 4.68 ± 0.33, and compressive strength of 41.088 MPa, which was higher than the pure sisal epoxy and hybrid fiber epoxy composites.A poor compatibility between the two fibers and poor adhesion between the sisal epoxy-hemp hybrid reinforced composites could be the reason for lower mechanical properties of hybrid composites.
Physical and morphological studies were determined [51] for the composites, prepared from sisal fiber as a filler and epoxy as a matrix.The mechanical properties such as TS, FS, IS, and WA were increased with the increasing fiber content.Three types of immersing agents (ordinary, sea, and distilled water) were used to study the WA test, and the result revealed that the composites absorb more in the ordinary and distilled water compared to seawater.
Arun et al. [52] conducted a comparative studies of the mechanical characteristics of epoxy composites made from sisal and jute.The experimental research revealed that sisal epoxy exhibited a much higher impact of energy (7.02 kJ/m ), ultimate TS (35.52 MPa), and FS (69.41 MPa).On the other hand, both sisal and jute epoxies had a greater hardness value (95 MPa).The SEM analysis revealed the presence of fibre pullout and cracks in the matrix.The prepared material offers a superior alternative to NFPC.
Athith et al. [66] examined the mechanical and tribological studies of composites from jute/sisal/E-glass fabric blended with matrix such as NR and epoxy.The different proportion of fibers and matrix filled with different proportion of tungsten carbide were taken for the preparation of composites.From their findings, it was noticed that the filler loading could increase the mechanical properties especially in the glass fiber epoxy at the same time the wear rate was decreased with increase in abrading distance.
The improvement of PP/sisal fiber bonding was done [53] with the aid of chemical treatments by using polymeric diphenylmethane di-isocyanate (PMDI) and gamma aminopropyl triethoxysilane (silane A1100).Yield strength more than 50% and T g up to 6.8 °C increased for the PP-sisal composite with PMDI treatment.A good agreement between the theoretical model and experimental results of treated and untreated sisal fiber-PP samples was proved by Halpi-Tsai and Nielson mathematical model.
Senthil et al. [67] investigated the mechanical and free vibration properties with possible trilayering sequence of sisal (S) and coconut sheath (CS) hybrid PE composites.The influence of alkali treated (ATC) and trichlorovinyl silane treated (STC) on the composite was studied.Among the various stacking sequence of fibers, the CS/ sisal/CS hybrid stack had the better performance of mechanical and damping factor.Also, the fracture morphology of the fiber and PE resin was analyzed by SEM.
Phiri et al. [3] addressed the mechanical and thermal properties of sisal fiber-kenaf fiber (SF-KF)-reinforced injection molded composites.The addition of fiber content increases the impact strength for SFC than KF composite.Increase in TS and Young's modulus and decrease in strain at break were noticed.The incorporation of water glass (WG) showed higher T g of KFC and has a positive influence on the flammability.In the same way, WG gave a negative on the mechanical properties.
Priyadharshini and Ramakrishna [68] used two parameters of rheological analysis such as flow rate and cohesion (by vane shear rate).The effect of water/cement ratio, polymer volume, and fiber content with and without treatment on sisal fiber-reinforced cement mortar composites was performed.From the studies, it was noted that the increase of fiber content decreases the flow value but increases the cohesion of the composite and vice versa for the increase of polymer dosage.Compared to treated composite, untreated fiber exhibited larger flow rate and lower cohesion.
Various fibers, such as sisal, jute, and glass reinforced polyester composites, were prepared by hand lay-up process and studied its properties [55].The study demonstrated that the jute fiber PE has the maximum TS of 229.54 MPa.The hybrid (glass + sisal + jute) composite had maximum FS with a displacement of 14.2 mm and 3.00-kN load, whereas the sisal PE has the highest impact values of 18.67 J. Incomplete distribution of fiber and matrix, void formed in the fracture surface, was analyzed by SEM.

Sansevieria cylindrica fiber (SCF)-reinforced polymer composites
SCF belong to the family Asparagaceae, a stemless, rigid, cylindrical snake plant which can yield 100-150 leaves before flowering.Sansevieria cylindrica leaf yields a strong white elastic fiber (5%), cuticle (1%), dry matter (10%), and water (84%) and can be used as reinforcement in cement and polymer matrix [69].The fibers were identified and extracted from the leaves [70].The hierarchical cell structure of the fiber was analyzed through polarized light microscopy and SEM.Microstructural analysis of leaves exhibited the presence of two types of fibers (structural and arch).The cross-sectional area, porosity fraction, density, fineness, TS, and TM were 0.0245 mm 2 , 37%, 0.915 g/cm 3 , 9 Tex, 658 MPa, and 7 GPa for fibers with elongation at break between 10 and 12%.Also, the presence of I β with a crystallinity index of 60% was analyzed by X-ray diffraction method.
A study was conducted to analyse the erosion characteristics of both treated (alkali) and untreated SCF vinyl ester composite (SCFVEC) [71].Fibre lengths of 30 and 40 mm were used, along with fibre concentrations of 30%, 40%, and 50% wt, for both treated and untreated SCFVEC samples.These samples were then subjected to an erosion test using an abrasive air jet erosion rig.Using the Taguchi analysis, the optimized erosion parameters of fiber length 30 mm, fiber content 40 wt%, impingement angle 90, impact velocity 41 m/s, erodent discharge 4 g/min, and exposure time of 15 min were noted for prepared SCEVEC.
Sreenivasan et al. [72] experimented the mechanical characterization of Sansevieria cylindrica fiber-reinforced polyester composites (SCFRPC) by compression molding technique.The tensile strength (ASTM D 3039) was 76 MPa, modulus as 1.1 GPa, the flexural strength (ASTM 790-86) was 84 MPa and modulus as 3 GPa, elongation at break was lied between 7% and 8.3%, and the impact test (ASTM 256-98) was 9.5 J/ cm 2 , respectively.Compared with the theoretical projections, the experimental tensile strengths were found to be in perfect agreement with Hirsch's model.An X-ray diffraction (XRD) analysis possessed the presence of cellulose IV (2θ = 22.5°) with a crystallinity index of 60% and large crystallite size of 68 nm.
Ramachandra et al. [73] conducted a study on tensile and flexural behavior of epoxy tamarind fruit (TF) fiber and S. cylindrica fiber (SCF) hybrid composite with different fiber ratios (as 0:20, 5:15, 10:10, 15:5, 20:0).A considerable increase in flexural strength, modulus, and dielectric strength were noted for epoxy filled with TF fiber and SCF composite.The optimum strength enhanced with the composition 10 wt% of TF, and 10 wt% of SC was observed for the filled epoxy hybrid composite.
The effect of layering (three) sequence of alkali and silane-treated SCF/coconut sheath (CS)/PE composite on the mechanical and vibration behavior was investigated [74].For the untreated coconut, sheath (three layers)/PE has better TS and impact strength than the SCF (3 layers) composite, whereas alkali treated reverse effect was observed.The dynamic behavior and mechanical strength of SCF/CSF/PE were found to be significantly influenced by layering sequence and chemical treatment (alkali and silane).

Sansevieria ehrenbergii fiber (SEF)-reinforced polymer composites
SEF belongs to the family of Dracaenaceae, a snake grass plant, traditionally used for antiseptic and for making baskets, roofs, and clothes.Each leaf consists of 100-200 fibrils approximately fiber (8%), cuticle (1%), dry matter (12%), and water (81%).Identification and characterization of new cellulose Sansevieria ehrenbergii (SE) fibers for polymer composites were studied [75].Using optical microscopy, the diameter of longitudinal and transverse section of the raw fiber was around 25-250 μm and 20-240 μm, whereas it was 40-165 μm for SEM analysis.The cross-sectional area and density were 0.0215 mm 2 and 0.887 g/cm 3 respectively for the raw fibers.The presence of I β cellulose and semicrystalline nature of fiber were analyzed by X-ray diffraction.
Sathishkumar [76] prepared the Sansevieria ehrenbergii fiber (SEF) with PE composites.The static, dynamic, thermal, and mechanical properties on the alkali treated, KMnO 4 treated and untreated fiber PE composite by using hand layup followed by compression molding process.The tensile, flexural, storage modulus, and impact test of KMnO 4 -treated composite were higher than paperboard, plywood, and hardboard sheet.According to tan δ, peak width was maximum, and WA is lower for KMnO 4 -treated composite.The author was concluded that this SEF samples could be replaced the woodbased composites imperial applications.
Sathishkumar et al. [77] analyzed that alkali, benzoyl peroxide (BP), benzoyl chloride (BC), KMnO 4 , and stearic acid (SA) treatments increase the physico-mechanical properties of SEF/isophthalic PE composites.According to their work, mechanical properties have a maximum value, and WA has lower value for the chemically treated composite than the untreated composite.SEM evaluated that a rough surface was formed on the fiber when it was chemically treated, and this was attributed to the removal of lignocellulose part of the fiber.The same author continued his work [78] with the randomly and longitudinally arranged SEF/PE composites with and without WA (swelling time variation at 4, 8, 12, 16, 20, & 24 h).The percentage of WA increased, and TS was decreased with respect to water swelling time.The chemically treated composite has the possibility to utilize as automotive and household applications.

Sansevieria roxburghiana fiber (SRF)-reinforced polymer composites
SRF (Agaveceae) is a rigid, stemless, perennial, medicinal plant and used for making bowstrings, cordage, and mats.Mangesh and Akshay [79] studied the chemical composition and physical and structural properties of SRF (untreated and treated with 2, 5, 10, 15, 20% NaOH).The hemicellulose, ash, and moisture content was found to be decreased from 30 to 17%, 2 to 0.5%, and 9.0 to 6.5%, respectively.The cellulose and lignin content were increased from 54 to 65% and 12 to 17%, respectively.Then the crystallinity index and TS were increased from 72 to 76% and 260 to 311 MPa, from untreated to treated (2 to 15% NaOH) fibers.
Among the various varieties and abundant availability of plant fiber, Ramanaiah et al. [10] was selected the SRF to reinforced PE unidirectional composites.It was inferred that the TS (92.6 MPa), impact strength (206 J/m), and specific heat capacity (1464.83J/kg) were maximum for the SRF/PE composites.The lowest thermal diffusivity of 0.9948E-07 m 2 /s was noted.The result also indicated that as the volume of fiber increases, thermal conductivity decreases, and the prepared composites could be used in construction and automobile industries.
Using hand lay-up process, the composites were prepared [80] by SRF and Calotropis procera (PCF) treated it with and without lignite fly ash (LFA).The tensile strength (13.92 MPa), compressive strength (48.13 MPa), FS (44.71 MPa), and hardness (98 RHN) were higher for SRF/epoxy, CPF + LFA/epoxy, CPF/epoxy, and SRF + LFA/epoxy composites, respectively.Compared to the other composites, SRF has less wear rate and frictional force due to the effect of fly ash.The presence of voids, lignite fly ash, and fiber breakage in the composites was evidenced by SEM analysis.

Sansevieria trifasciata fiber (STF)-reinforced polymer composites
STF belongs to the family Asparagaceae, commonly called the snake plant.Mature leaves are dark green with light gray-green cross-banding and usually range between 70-90 cm in length and 5-6 cm in width.Tensile test and TGA analysis were used to measure the raw and alkali treated (1, 3, & 5 wt% for 2 h).It was inferred that the increase in TS for raw STF as 647 MPa and 902 MPa for 5% NaOH treated one.Similarly, TGA showed the increase in thermal resistance as 288 °C for raw STF and 307 °C for 5% NaOH fiber.The experiment also proved that the chemical treatment affects the tensile and thermal properties of STF.
Thanesh et al. [81] obtained fibres from Sansevieria trifasciata Laurentii plants (STF) and fabricated composites by combining these fibres with PE resin using a manual layup procedure.The tensile, flexural, and impact properties were evaluated by altering the weight percentage of fibres (10, 20, and 30%) and the fibre length (10, 20, 30 & 40 mm).The authors discovered that a fibre size of 40 mm and a fibre fraction of 20 wt% yielded superior properties and were deemed to be the optimal size among the four sizes selected.
Rangga et al. [12] investigated the mechanical properties of STF vinyl ester composites.In order to enhance the quality of composites, first it was given by alkali (NaOH 3% conc.for 2 h) treatment followed by maleic anhydride (for 2 h).The samples were prepared by solution casting process with the different fiber fraction of 0, 2.5, 5, 7.5, and 10%.The study showed that the addition of fiber fraction (from 2.5 to 10%) decreases the volume fraction of void (7.9, 6.87, 3.49, and 2.55%).The actual density (1173 kg/m 3 ), TS (57.45 MPa), and modulus of elasticity (3472.5 MPa) were achieved higher for 10 wt% fraction of composites.Also, the surface treatment has improved the interfacial bonding between STF and vinyl ester matrix of the composites.
Samson et al. [83] reported the extraction method (STF fiber), fabrication, and properties of STF-banana pseudostem fiber (BF) epoxy resin composite.The prepared BF epoxy has better property than STF epoxy composite.Three parameters like storage modulus, loss modulus, and damping factor (tan δ ) were determined by DMA.The results indicated that compared to STF epoxy composites, the higher storage modulus of 5.4 GPa and T g as 120 °C was noted for banana woven epoxy.Similarly, the loss modulus was higher for banana epoxy composites.According to damping factor, the STF composites had a better interfacial bonding between the matrix and fiber, and the value is tan δ = 0.35.
Raghava et al. [84] manufactured and explained the mechanical, thermal, and morphological properties of randomly oriented STF-carbon fiber (CF)-reinforced hybrid vinyl ester composites using hand lay-up method.The composites were prepared with different proportions of clay filled at 0, 1, 3, and 5 wt%.At 3% wt of clay content, the maximum tensile strength and thermal stability at 352-356 °C (at 5%wt) were found for the composites.
Nurzam et al. [85] studied the mechanical, morphological, and thermal properties of STF/natural rubber (NR)/HDPE composites by hot pressing process.The specimens were prepared, using the following parameters that is fiber loadings of 10-40% and fiber sizes of 1 mm, 500 µm, 250 µm, and 125 µm.From their findings, it was observed that the overall performance of the specimen was strongly influenced by the fiber size.STF at 125 µm gave the highest TS and TM.Thermal analysis was not affected much, and no crystallinity peak in DSC was observed by varying the fiber size.The SEM micrograph was utilised to analyse the fractured samples that exhibited a strong interaction between the STF and matrix.

Evaluation of mechanical behavior of leaf fiber-reinforced polymer composites
Several researchers have evaluated the mechanical properties of various leaf-based matrix composites.From Table 3, it is inferred that the authors have analyzed the influence of fiber treatment, fiber type, fiber length, fiber loading, fiber orientations, resin types, processing techniques, etc.It was highlighted that the surface treatment of fibers and fiber loading, significantly increased the mechanical properties of leaf fiberreinforced polymer composites (Table 3).Most of the research works are based on, especially, the fiber content and fiber surface modifications.Sakuri et al. (2020) showed that 6% soaking time of alkali treatment has to be found, enhancing the mechanical characteristics of the composites.A greatest TS is noticed for fique/LDPE-AI composites, due to the pre-impregnation treatments as well as the fiber contents, which is illustrated in Fig. 3.In the same way, the 30% wt of Furcraea foetida with epoxy combination has the highest strength of 170.47 MPa.The effect of fiber orientation is determined for warp and weft direction with woven PALF layers (2,3,4) of the composites.It was revealed that three-layer woven PALF with warp direction has greater mechanical properties than the weft direction (Hadi et al. 2022).In another research work, the effect of surface modification on mechanical properties of Sansevieria ehrenbergii/polyester composites is studied (Sathishkumar et al. 2014).It is observed that among the various treatments, KMnO 4 -treated fiber composites depicted superior mechanical characteristics.The figure indicates that there is a significant body of studies focused on leaf fibers and leaf fiber-based hybrid composite materials.These materials have a wide range of applications across many manufacturing industries.

Conclusions
Leaf fibers obtained from agricultural waste can be turned into new composite products through proper technology.Fiber-reinforced polymer composites have replaced synthetic materials to a greater extent because of its biodegradability, low density, ease to dispose, and less expensive.The favorable properties of this composites are affording positive benefits to the environment too, i.e., harmless to health during manufacturing and CO 2 neutral balance.The usage of leaf fibre composite has been prevalent in several industries such as automotive (for door panels, roof and dashboard), construction (for fences, park benches and indoor ornamental boards), and even in packaging and household appliances, which are increasingly being recognised as potential applications in the field of composites [106].The utilisation of leaf fibre as a reinforcing material in polymer composites has demonstrated a beneficial impact on the mechanical characteristics.This review article explored the following findings: i) The various types of leaf fibers used as a reinforcing material are discussed.ii) The major components present and their physico-mechanical properties of leaf fibers are presented through table and figures.iii) Various researchers prepared the composites by adopting various processing techniques, variety of leaf fibers, and fiber parameters (length, orientation) which are discussed in the leaf fiber-reinforced polymer composite section.iv) Several studies have focused on the mechanical properties (such as TS, YM, FS, FM, and IS), the effects of surface modifications, different matrices used in composite preparation, and an overview of the performance of composites reinforced with leaf fibres.v) The effect of various leaf fibers and their hybrid composites is also the focused in this review.
Finally, as world is moving towards usage of environmental friendly materials, conduct of research for the increased utilization of leaf fiber, which is abundantly available, is of timely needed one.This review of research over the period of one decade shows positive sign towards increasing the utility of leaf fibers in various industries, household appliances, construction, etc.The researches on influence of surface modification and fiber loadings on mechanical performance of leaf fiber-reinforced polymer composites show favorable signs in the composite world.

Fig. 1
Fig. 1 Different type of plant fibers

Fig. 3
Fig. 3 Histogram of various leaf fiber/matrix composites and its mechanical properties

Table 1
Chemical compositions of leaf fibers

Table 2
Physical and mechanical properties of leaf fibers

Table 3
Mechanical properties of various leaf fiber-reinforced polymer composites