**4. Hybrid composites**

To improve on the properties of natural fiber composites and/or overcome some of their limitations such as moisture absorption, thermal stability, brittleness and surface quality, the concept of hybrid composite was developed. The idea is to combine natural fibers with other fibers or particulate reinforcements, which can be of natural or synthetic origin such as glass fibers or rubber particles [15, 51, 63, 106–109]. The main purpose of blending different rein‐ forcements is to obtain a material with better properties than using a single reinforcement. Assuming there is no chemical/physical interaction between each type of fibers, the resulting properties of hybrid composites (*PH*) should follow the rule of hybrid mixtures (RoHM) given as [106, 110, 111]:

**Matrix Fiber**

Post consumer PP

73–76].

29, 73, 75, 85, 91, 92].

**4. Hybrid composites**

**source**

Oil palm

BagasseCompression molding

Wood Compression molding

**Processing Fiber**

280 Composites from Renewable and Sustainable Materials

**content (%)**

**Fiber surface treatment**

Wood Extrusion – – MAPP– 450–

copolymer; CAPE: carboxylated polyethylene; TDM: titanium‐derived mixture.

Extrusion – – MAPP– 340–

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

**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,

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,

To improve on the properties of natural fiber composites and/or overcome some of their limitations such as moisture absorption, thermal stability, brittleness and surface quality, the concept of hybrid composite was developed. The idea is to combine natural fibers with other fibers or particulate reinforcements, which can be of natural or synthetic origin such as glass fibers or rubber particles [15, 51, 63, 106–109]. The main purpose of blending different rein‐ forcements is to obtain a material with better properties than using a single reinforcement. Assuming there is no chemical/physical interaction between each type of fibers, the resulting

30 – MAPE

**Additive Mechanical**

50, 60 – MAPE– – 9–18 – – 20–

**CA BA E**

TDM

CAPE TDM

**properties**

36.1

27.3– 29.8

18.7– 19

2230– 2940

1870– 2150

30.1– 33.8

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

– – 22.3–

490

380

**TD (°C)**

353.3

499

– 268– 495

– [100]

[60]

[101]

[101]

– – – 348.5–

35

43–51 – 285–

**References**

$$P\_H = P\_{C1}V\_{C1} + P\_{C2}V\_{C2} \tag{1}$$

where *PC*1 and *PC*2 are the properties of composite *C*1 and *C*2, respectively, while *VC*1 and *VC*<sup>2</sup> are their respective volume fractions such that:

$$V\_{C1} + V\_{C2} = 1\tag{2}$$

Naturally, the model can be generalized for more than two types of reinforcement.

Natural and synthetic reinforcements combination has showed to improve several composite characteristics such as thermal stability [106, 112–114], impact strength [63, 115–117] and water uptake [70, 112–114, 118, 119]. But the combination of two different types of lignocellulosic fibers was shown to control water absorption [53, 103, 110] and increased impact strength [103, 120], especially when using coupling agents.

The final properties of hybrid composites depend are function of different factors [53, 74, 104, 120], and **Table 4** summarizes some of the most important mechanical and thermal properties of hybrid composites based on thermoset matrices. The effect of fiber and matrix type, as well as fiber surface treatment is reported with their mechanical properties and thermal degrada‐ tion temperature. Similarly, **Table 5** reports the corresponding information for hybrid com‐ posites based on thermoplastic matrices. In general, it is observed that combining natural fibers with inorganic reinforcements leads to improved thermal stability and impact strength, as well as higher flexural and tensile moduli. Moreover, **Table 6** shows that water uptake decreases by combining two natural fibers from different sources, or using natural fibers with inorganic reinforcements in hybrid composites based on thermoplastics matrices.




**Matrix Fibers Manufacturing**

282 Composites from Renewable and Sustainable Materials

Oil palm/ glass

Banana/ kenaf

Ramie/ cotton

Sisal/ roselle

Sisal/ glass

Sisal/jute/ glass

Hemp/ glass fibers

jute

Banana/ sisal

Jute/ bagasse

Jute/ coir

Banana/ silica

Sisal/ silica

wool

wool

Coconut/ sisal/ glass

Jute/ ramie

Polyurethane Hemp/

Vinyl ester Hemp/

Epoxy Banana/

**process**

Compression molding

Compression molding

Hand lay‐out + compression molding

Hand lay‐up + compression molding

**Fiber treatment**

NaOH SLS

RTM – – 30.1–

Hand lay‐up – – ∼78–

Hand lay‐up – – 111.2–

– 0.6–

0.7

0.7

0.3– 0.7

∼0.3– 0.7

9.1

6.1

6.8

Molding – – – – – 1993–

NaOH solution

Hand lay‐up – 0.6–

HCl solution

Hand lay‐up – 6.5–

Hand lay‐up – 4.7–

VARTM – 6.7–

Cyclohexane/ ethanol Furfuryl alcohol

Hand lay‐up NaOH

Hand lay‐up NaOH

Hand lay‐up Solutions of:

**E (GPa)** 

5.5

– – 24.2–

– 45–

– ∼2.5–

**TS (MPa)** 

∼20– 75

139

118

58.7

95

232.1

16.6– 19

16.1– 18.6

0.6– 1.7

∼8.5– 35

Pultrusion – 18.91 122.66 ∼142 ∼12 – – [18]

Pultrusion – 15.27 112.54 ∼143 ∼13 – – [18]

6.2– 6.7–

**Mechanical properties TD**

**FM (GPa)** 

∼1.5– 8

27.4

∼2.1– 11

– – – – – 345 [107]

8.9– 9.1

8.9– 9.3

0.6– 1.7

∼0.5– 1.5

– – – – – [111]

– – – – – [111]

– –– 18–19 – [131]

16373

– [117]

**IS (kJ/ m2 )** 

∼7– 16

– ∼15– 28

– 1.39– 1.41

> ∼66– 88

13.44– 18.23

13.2– 17.9

6.9– 15.9

– – – [118]

376.5– 380

438.2– 475.9

– – [130]

[108]

[109]

– [129]

**FS (MPa)** 

∼30– 138

75– 172.2

48.4– 63.5

∼70– 265

214.1– 308.6

57.2– 59.8

57.3– 62

6.9– 15.9

∼39– 37

– 6.3–

**(°C)**

– [124]

– [125]

– [127]

– [128]

– [126]

**References**

E: tensile modulus, TS: tensile strength, FS: flexural strength, FM: flexural modulus, IS: impact strength, TD: thermal degradation.

**Table 4.** Mechanical and thermal properties of natural fiber hybrid composites based on thermoset matrices.




**Manufacturing** 

**Composite Coupling agent**

284 Composites from Renewable and Sustainable Materials

MAPP (5%)

MAPE (2%, 4%)

MAPP (3%, 5%)

N/A (3.5%)

MAPP (3%)

MAPP (5%)

MAPP PP‐g‐MA POE‐g‐MA

LDPE‐ banana/coir fibers

HDPE‐ coir/Oil palm fibers

HDPE‐kenaf/ pineapple leaf fibers (PALF)

PS‐banana/ glass fibers

PP‐SBR rubber/ birch wood

glass fiber

PP‐sisal/ glass fibers

RPP‐date palm wood/ glass fiber

PP‐hemp/ glass fibers

PP‐wood flour/glass fiber

**Filler content (%)**

**Filler surface treatment**

15 Solutions of: NaOH Acetylation bleaching with H2SO4

40 Hot water and soap

– 20 Solutions of:

10, 20, 30

NaOH Benzoyl chloride PSMA

Boiled in methanol and benzene mixture and with NaOH solution

30 – 29.2–

– 30 – 19.5–21 1100–

40 – 52.5–59 3800–

40 – 28–45.4 – 39.7–

31.6

0–40 – 10.5–25 520–

**Mechanical properties**

> **TS (MPa)**

36.2–50 29.5–

29–38.8 1462.2– 1558.3

1560

2330– 2430

1300

4300

**FS (MPa)** 

52.4

8–13.5 550–630 17–27 1570–

**FM (GPa)** 

2380

2100

– – – – [51]

7.9–11.3 489.7– 698.8

– – – – 100 190–

66.7– 68.8

62.8

4.03– 4.14

97–101 5000– 5400

> 2680– 3497

– – – 361.8–

16.7– 20

49– 55.4

**IS (J/m)**

9.3– 13.6

**E (MPa)** 

– 40 – 27–30 550–680 23–28 1700–

**TD (°C)**

473 [135]

– – [120]

– – [110]

– – [136]

230

331.3– 464.7

479.4

360– 474

– 345– 363

[112]

[113]

[114]

[57]

[137]

**References**

**process**

Injection + compression

Injection moldingPP‐sisal/


MAPP: maleic anhydride‐grafted PP; MAPE: maleic anhydride‐grafted PE; GTR: ground tire rubber; LDPE: low density polyethylene; HDPE: high density polyethylene; PS: polystyrene; SBR: styrene butadiene rubber; RPP: recycled polypropylene; PP‐g‐GMA: glycidyl methacrylate‐grafted PP; POE‐g‐MA: maleic anhydride‐grafted ethylene‐octene copolymer; SEBS‐g‐MA: maleic anhydride‐grafted hydrogenated styrene‐butadiene‐styrene; PLA: polylactic acid.

**Table 5.** Mechanical and thermal properties of hybrid composites based on thermoplastic matrices.


PP: polypropylene; HDPE: high density polyethylene; MAPE: maleic anhydride‐grafted polyethylene; GTR: ground tire rubber; MMT: montmorillonite.

**Table 6.** Water uptake in hybrid composites using thermoplastic matrices.

#### **5. Auto‐hybrid composites**

Composites reinforced with two sizes of the same type of reinforcement are referred to as auto‐ hybrid composites. As these composites only have a single type of reinforcement, they are easier to recycle. But most importantly, these materials were shown to exhibit a positive deviation from the RoHM depending on fiber concentration, weight ratio, size and type [64, 102, 147]. Nevertheless, the auto‐hybridization effect seems to be more influenced by the total fiber content than coupling agent addition [64, 147]. However, coupling agent addition is always important to improve tensile strength [102]. As total fiber content, fiber type and coupling agent content, all affect the level of deviation from the RoHM, and optimization of these parameters is a new challenging field of research to develop better composite perform‐ ances. **Table 7** summarizes the limited amount of work on auto‐hybrid composites using natural fibers as reinforcement.

**Manufacturing** 

Extrusion HDPE‐

**Composite Coupling agent**

286 Composites from Renewable and Sustainable Materials

glass microspheres

wood/bast fibers

HDPE‐ wood/Kevlar

Wood/SiO2 Wood/CaCO3 Milled glass fibers

tire rubber; MMT: montmorillonite.

**5. Auto‐hybrid composites**

**Table 6.** Water uptake in hybrid composites using thermoplastic matrices.

**Filler content (%)**

– 60 Vinyl

– 60 Allyl and 3–

PP‐jute/glass – 20, 30, 40 – 42–63 4660–

**Table 5.** Mechanical and thermal properties of hybrid composites based on thermoplastic matrices.

**Filler surface treatment**

triethoxysilane

trimethoxy silyl‐propyl

MAPP: maleic anhydride‐grafted PP; MAPE: maleic anhydride‐grafted PE; GTR: ground tire rubber; LDPE: low density polyethylene; HDPE: high density polyethylene; PS: polystyrene; SBR: styrene butadiene rubber; RPP: recycled polypropylene; PP‐g‐GMA: glycidyl methacrylate‐grafted PP; POE‐g‐MA: maleic anhydride‐grafted ethylene‐octene copolymer; SEBS‐g‐MA: maleic anhydride‐grafted hydrogenated styrene‐butadiene‐styrene; PLA: polylactic acid.

**Matrix Reinforcements Observations References** MAPE GTR rubber/hemp fiber GTR decreases water uptake [63] PP Kenaf/coir/MMT Water uptake is reduced by hybridization [132]

Hemp/glass fibers Glass fiber reduced water uptake [57] Wood/glass fibers Increasing fiber glass weight ratio, water uptake was reduced. [146]

Coir/agave fibers Coir reduced water uptake in hybrid composites [103]

HDPE Pine/agave fibers Pine fiber decreased water uptake in hybrid composites [53]

PP: polypropylene; HDPE: high density polyethylene; MAPE: maleic anhydride‐grafted polyethylene; GTR: ground

Composites reinforced with two sizes of the same type of reinforcement are referred to as auto‐ hybrid composites. As these composites only have a single type of reinforcement, they are

**Mechanical properties**

> **TS (MPa)**

**FS (MPa)** 

42–44 650–700 73–77 4900–

24.5– 3600

72.8– 102.5

3050– 4100

7170

SiO2, CaCO3 and milled grass decreased water uptake [133]

**FM (GPa)** 

5250

2200– 3400

3550– 5950

**IS (J/m)**

**E (MPa)** 

13.8– 19.8

**TD (°C)**

– – [144]

– – [145]

– – [69]

**References**

**process**

Extrusion calendering



\* MAPP was not used in auto‐hybrid composites.

**Table 7.** Overview of the different investigations on auto‐hybrid composites based on natural fibers.
