**3. Natural fiber composites**

Natural fiber composites are materials based on a polymer matrix reinforced with natural fibers [9]. The polymer matrix can be a thermoplastic or a thermoset, the main difference being that once thermoplastics are molded they can be remelted and reprocessed by applying heat and shear, while this is not the case for thermosets [14, 15]. But thermoset matrices generally provide higher rigidity and are more chemically stable. This is why they are more difficult to recycle. The main thermoset matrices used for natural fiber composite production are poly‐ ester, vinyl ester, phenolic, amino, derived ester and epoxy resins. Thermoset composites are commonly processed via resin transfer molding (RTM), sheet molding compound (SMC), pultrusion, vacuum‐assisted resin transfer molding (VARTM) and hand lay‐up. All these manufacturing processes do not need high pressure requirements. Another advantage of thermoset matrices is that fiber loading can be higher than for thermoplastics since the resin is initially in a liquid form. So, lower viscosity improves fibers introduction and dispersion via different mixing equipment [18–22]. Fiber orientation as well as fiber content might improve mechanical properties in thermoset composites. Grass, leaf and bast fibers are more effective to increase the matrix mechanical properties, while surface treatment improves interfacial interactions. **Table 2** summarizes some work on natural fiber thermoset composites with their manufacturing process, fiber content, fiber treatments and fiber source, as well as the main results obtained from each work.

**2. Natural fibers**

274 Composites from Renewable and Sustainable Materials

Natural fibers are biosourced materials extracted from plants (lignocellulosic) or animals [7]. Lignocellulosic fibers are produced by plants for which, on a dry basis, the cell walls are mainly composed of cellulose, with hemicelluloses, lignins, pectins and extractives in lower amounts. Chemical composition and distribution mostly depend on fiber source and varies within different parts even of the same type or family [7, 8]. According to their source, lignocellulosic fibers can be classified as bast fibers, leaf fibers, fruits‐seeds fibers, grass‐reed fibers and wood

fibers [7, 9–12]. **Table 1** presents some examples of each category [13].

**Fiber type Characteristics Examples**

Bast High cellulose content, flexible, obtained from plants phloem Kenaf, hemp, flax

Seed Fibers that have grown around seeds Cotton, kapok

Fruit Obtained from fruit shells Coir, oil palm

Wood Extracted from flowering and conifers trees Maple, pine

**Table 1.** Lignocellulosic fibers classification [13].

**3. Natural fiber composites**

density (1.2–1.5 g/cm3

(2.5 g/cm3

Stalk Cereal stalks byproducts Wheat and corn straw

Grass Obtained from grass plants Bamboo, wild cane, esparto grass

Leaf Obtained by decortication of plants leaves Banana, sisal, pineapple, agave

Due to natural fibers' strength, stiffness, availability, low cost, biodegradability and lower

Natural fiber composites are materials based on a polymer matrix reinforced with natural fibers [9]. The polymer matrix can be a thermoplastic or a thermoset, the main difference being that once thermoplastics are molded they can be remelted and reprocessed by applying heat and shear, while this is not the case for thermosets [14, 15]. But thermoset matrices generally provide higher rigidity and are more chemically stable. This is why they are more difficult to recycle. The main thermoset matrices used for natural fiber composite production are poly‐ ester, vinyl ester, phenolic, amino, derived ester and epoxy resins. Thermoset composites are commonly processed via resin transfer molding (RTM), sheet molding compound (SMC), pultrusion, vacuum‐assisted resin transfer molding (VARTM) and hand lay‐up. All these manufacturing processes do not need high pressure requirements. Another advantage of thermoset matrices is that fiber loading can be higher than for thermoplastics since the resin

) compared to synthetic fillers such as talc (2.5 g/cm3

) [14–16], they can be effectively used in lightweight composites production [8, 9, 17].

) and glass fiber



E: Tensile modulus; TS: tensile strength; FM: flexural modulus; FS: flexural strength; IS: impact strength; GMA: glycidyl methacrylate; MAH: maleic anhydride; SAH: succinic anhydride; DTPA: diethylenetriaminepenta‐acetic acid; IEM: isocyanatoethyl methacrylate; DBTDL: dibutyltin dilaurate.

**Table 2.** Mechanical and thermal properties of natural fiber composites based on thermoset matrices.

The most common thermoplastic matrices used for natural fiber composites production are the different grades of polypropylene (PP) and polyethylene (PE), as well as polycarbonate (PC), nylon (PA), polysulfones (PSU), polyethylene terephthalate (PET) and polystyrene (PS). More recently, biopolymers such as polylactic acid (PLA) have gained interest to produce 100% biosourced materials [51–55]. Typical manufacturing processes for these composites are extrusion, injection, calendering, compression molding and thermoforming. Some advantages of using thermoplastic matrices are their recyclability and the production can be continuous [56–61]. Depending on the matrix, fiber and additives content, fiber treatment and manufac‐ turing process, the mechanical and thermal properties of these composites can be adjusted as presented in **Table 3**, with the main results obtained.

**Matrix Natural**

Polyurethane Kraft

**fiber source**

276 Composites from Renewable and Sustainable Materials

Elephant grass

cellulose

Phenolic Bagasse Compression

Cellulose from eucalyptus

Pineapple leaf

Rice husk Mixing and

Bamboo Mixing and

**Manufacturing process** 

compression molding

compression molding

Compression molding

molding

molding

molding

molding

Cellulose VARTM 20, 30,

Sisal RTM 10, 15,

IEM: isocyanatoethyl methacrylate; DBTDL: dibutyltin dilaurate.

Molding 1, 3, 5,

7

40, 50

20, 25, 30

Molding 20 NaOCl

Kenaf Pultrusion 40 – 9–12.5 135–145 1.6–

solution

E: Tensile modulus; TS: tensile strength; FM: flexural modulus; FS: flexural strength; IS: impact strength; GMA: glycidyl methacrylate; MAH: maleic anhydride; SAH: succinic anhydride; DTPA: diethylenetriaminepenta‐acetic acid;

**Table 2.** Mechanical and thermal properties of natural fiber composites based on thermoset matrices.

Curaua Compression

Ramie Compression

Bamboo Compression

Coir Hand lay‐up NA NaOH

Hand lay‐up 30.4,

**Fiber content (%)** 

31.3, 31.5

10, 20, 30

5, 10, 15, 20

**Fiber treatment** 

57 GMAMAHSAH solutions

> NaOH KMnO4 solutions

NA H2O2 +DTPA

+Na2O3Si +NaOH solution, IEM +DBTDL

solutions

17.6 HClO2 solution Furfuryl alcohol

17.6 HClO2 solution Furfuryl alcohol

Jute Pultrusion N/A – – 25–38 – 28–

15 – 21.2–

Vinyl ester Silk Hand lay‐up 0–15 – 0.9–1.3 40–71 – – – [47]

NaOH solution, propyl‐ trimethoxy‐ silane

**E (GPa)**  **TS (MPa)**

0.6–2.2 31.5– 118.1

– 17.9– 23.6

15.6

0.7–0.9 9.5–16.5 5.1–

40.4 – 3.3, 1.2 72.3,158 – 90–

30.1

NaOH solution 1.7–2.9 38–75 2.1–

– 3–7 – – 40–

– 0–0.2 – – – – [41]

– – 10.6–

0.4–1.6 2.5–19 0.1–

– 39–65 – 75–

**Mechanical properties References**

3–42 9.5– 40

– – – [37]

105

– 18.7– 48

– 25.9– 38.5

– – – – 17–28 [42]

– – – – 39–88 [42]

18.5– 28.0

145

63

320

160

75– 180

150– 190

68– 119

1.0

– – 210–

4.5

1.9

1.9– 3.9 [22]

– [38]

– [39]

– [43]

– [44]

– [45]

– [46]

– [20]

– [48]

– [49]

[50]

19– 105 [40]

25.6– 161.9

**FM (GPa) FS (MPa) IS (J/m)**

1.9

The main objective of adding natural fibers in polymer matrices is to increase mechanical properties regardless of polymer and fiber type [21, 26, 31, 40, 52, 54, 55, 61–68]. Since natural fibers have lower density (1.2–1.5 g/cm3 ) compared to synthetic/inorganic reinforcement such as glass fibers (2.5 g/cm3 ), lightweight composites can be produced [28, 69, 70]. Nevertheless, lignocellulosic fibers are hydrophilic and polar which causes some incompatibility with the most common polymer matrices which are hydrophobic and nonpolar. This effect leads to poor mechanical properties due to a lack of interfacial adhesion between the fibers and the matrix. Furthermore, the high amount of hydroxyl groups available on the fiber surface is increasing water absorption, even when inside a composite [65, 71, 72]. These problems can be resolved by modification of the fibers surface such as mercerization (treatment in sodium hydroxide solution to remove lignins and hemicellulose) with subsequent addition of coupling agents [22, 73–75]. There is also the possibility to combine thermomechanical refining with coupling agent addition [71, 72]. More recently, fiber treatment with a coupling agent in solution has been proposed [76].




**Matrix Fiber**

**source**

Oil palm

Argan nut shell

powder

Hemp Injection molding

Wood Injection molding

PS Agave Compression

Wood fiber

Wood flour

PP Argan nut shell

Agave Compression molding

molding

Injection molding

Flax Compression molding

Extrusion 10, 20,

Extrusion 10, 20,

UHMWPEWood

LLDPE Maple wood

Wood Injection molding

Hemp Compression molding

Hemp Compression molding

> Injection molding

Compression molding

Agave Injection molding

Compression molding

278 Composites from Renewable and Sustainable Materials

**Processing Fiber**

**content (%)**

25, 35, 45

**Fiber surface treatment**

40 Thermo‐ mechanical refining

> NaOH solution

MAPE NaOH Aldehyde Acrylic acid Methyl methacrylate Silane

30 Solutions of: NaOH MAPE

Rotomolding 0–20 – – ACA 26–

0–40 Solutions of: NaOH MAPE

0–30 – SEBS‐

– MAPP

10, 20, 30

30, 40

30, 40

10, 20, 26, 30

5, 10, 15, 20, 25

LMDPE Agave Rotomolding 5, 10, 15– – – 255–

Agave Rotomolding 15 Solutions of:

**Additive Mechanical**

**CA BA E**

– – – 1200–

0–40 – – ACA – – 1093–

MAPE MAH

– – 1136–

30, 40 – MAPP– 650–

0–20 – – ACA 225–

0–30 – – – 195–

**properties**

18.5– 27.5

1200– 2700

1634

– – – 2–2.6 – – – [71]

27.5– 43

18.8– 23

10–15 [65]

15–24 1–2.7 – – – [79]

– – – – [80]

– – – [67]

– [81]

– [76]

0.9– 7.5

83.8– 148.5

– – – – [73]

14–31 123– 260

30–62 – – – 400 [83]

94.5

– 55–68 – –

– – – 339.4–

– – – – – [86]

– – – [52]

– [75]

– – [84]

350

[85]

**(MPa) TS (MPa) FM (MPa) FS (MPa) IS (J/m)**

2000

1050

550

1795

280

440

668

184

381

4929

MAPS – – – 31–49 – 54–

– 1034– 1593

– 1000– 3200

47 – MAPP– – 30.2 – – – – [82]

– 167– 217

MAPE– 241–

– – 224–

– – ACA 3345–

MAPS – – – 31–

g‐MA

PPAA

27.2– 29.3

13– 18.8

13.1– 17.9

3–16.4119– 680

10–22 389– 1027

41.5

26.5– 30

– 650– 1260

13–18 420– 520

495– 590

12.5– 16.5

13– 17.8 **TD (°C)**

– – [59]

– – [55]

**References**


CA: coupling agent; BA: blowing agent; TD: thermal degradation; ACA: Azodicarbonamide; MAPE: Maleic anhydride‐grafted polyethylene; MAPP: maleic anhydride‐grafted polypropylene; MAH: maleic anhydride, SEBS‐g‐ MA: styrene‐(ethylene‐octene)‐styrene triblock copolymer grafted with maleic anhydride; PPAA: acrylic acid grafted polypropylene; POE: ethylene‐octene copolymer; EO‐g‐MAH: maleic anhydride grafted ethylene‐octene metallocene copolymer; CAPE: carboxylated polyethylene; TDM: titanium‐derived mixture.

**Table 3.** Mechanical and thermal properties of natural fiber composites based on thermoplastic matrices.

Coupling agents are usually copolymers containing functional groups compatible with the fibers (hydroxyl groups) and the polymer matrix [74]. These reactions (chemical or physical) are increasing interfacial adhesion leading to improved mechanical properties and water absorption reduction [22, 65, 71–73, 75, 76, 99, 102, 103]. Coupling agents can be mixed with the polymer matrix by extrusion previously to fibers addition [65, 74, 92] but can also be added during composite compounding, i.e. mixing the matrix, fiber and coupling agent all together [55, 72, 83, 90, 97–99, 102–104]. Likewise, natural fibers can be functionalized by treating them with a coupling agent in solution, to increase compatibility with the polymer matrix [22, 71, 73–76].

Since natural fibers start to degrade at lower temperature (150–275°C) than most polymer matrices (350–460°C) [60, 63, 74, 83, 105], fiber mercerization and coupling agent addition were shown to improve the thermal stability of the fibers and therefore of the final composites [24, 29, 73, 75, 85, 91, 92].
