Open access peer-reviewed chapter

Production and Characterization of Hybrid Polymer Composites Based on Natural Fibers

Written By

Wendy Rodriguez‐Castellanos and Denis Rodrigue

Reviewed: 22 July 2016 Published: 30 November 2016

DOI: 10.5772/64995

From the Edited Volume

Composites from Renewable and Sustainable Materials

Edited by Matheus Poletto

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In this chapter, a review is made on the processing and properties of hybrid composites based on a polymer matrix and a blend of different natural (lignocellulosic) fibers. In particular, the processing methods are described and comparisons are made between the general properties with a focus on physical, mechanical and thermal properties. A discussion is presented on the effect of the polymer and fiber types, as well as reinforcement content. Properties improvement is also discussed using fiber surface treatment or the addition of coupling agents. Finally, auto‐hybrid composites are presented with conditions leading to a positive deviation from the rule of hybrid mixture (RoHM) model.


  • hybrid composites
  • polymer matrices
  • natural fibers
  • fiber concentration
  • mechanical properties

1. Introduction

Composites are materials containing at least two constituents, each one with different chemical composition. Their combination provides a new material with better functional properties than each of the components separately [1].

The main component in the composite is the matrix, which can be a metal, ceramic or polymer, while the other part is a reinforcement which can be in particulate, laminate, short fiber or long fiber form [2]. Composite materials are widely used in construction, aerospace, aircraft, medicine, electrical and automotive industries [25]. Here, a focus is made on fiber reinforced composites made from a polymer matrix reinforced with fibers having a natural origin [6].


2. Natural fibers

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, 912]. 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 
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 
Wood  Extracted from flowering and conifers trees  Maple, pine 

Table 1.

Lignocellulosic fibers classification [13].

Due to natural fibers’ strength, stiffness, availability, low cost, biodegradability and lower density (1.2–1.5 g/cm3) compared to synthetic fillers such as talc (2.5 g/cm3) and glass fiber (2.5 g/cm3) [1416], they can be effectively used in lightweight composites production [8, 9, 17].


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 polyester, 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 [1822]. 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.

Matrix Natural
fiber source
Mechanical properties References
Epoxy  Banana  Hand lay-up  10  NaOH solution  0.6–1.4  12.1–33.6  15–34  26–69  2–12  [23]
Recycled cellulose  RTM  19, 28, 40, 46  –  –  –  0.5–5.5  60–140  5–22  [21] 
Flax  RTM  40–50  –  17.3–33.6  –  –  –  –  [19] 
Hand lay-up  50  –  8.6  –  –  –  –  [24] 
Compression molding and pultrusion  40  NaOH solution  2.7–32  50–283  8–27  0.4–4.1  –  [25] 
Oil palm  Compression molding  5, 10, 15, 20  NaOH solution  –  11–17  –  –  –  [26] 
Hemp  Hand lay-up  30  H2PO3 solution NH4OH Geniosil GF-9 Toluene solution aminosilane  3–4.8  49.1–66.5  3–5.2  69–92.8  –  [27] 
Date palm  Hand lay-up  10  NaOH solution  1.5–2.5  10–40  –  –  –  [28] 
Sansevieria cylindrical leaf  Molding  1, 5, 7, 9  NaOH solution  –  98.3–114.9  –  17–26  –  [29] 
Polyester  Jute  Hand lay-up  NA  –  –  –  –  –  3.8–4.1  [30] 
Macadamia nut shell  Hand lay-up  10, 20, 30, 40  –  –  –  4.1–4.6  26–38  –  [31] 
Flax  VARTM  20  –  15.3–20.3  188.6–230.7  2.1–2.3  16.3–17.5  –  [32] 
Curaua  RTM  0–40  –  –  –  0.1  –  20–190  [33] 
Wild cane grass  Hand lay-up  0–40  NaOH solution
KMnO4 solution 
–  –  1.8–7  –  –  [34] 
Sisal  Mixing and compression molding  10, 20, 30, 40  NaOH solution  –  –  1.49–2.68  –  –  [35] 
Typha leaf  Compression molding  7.3, 10.3, 12.6  NaOH solution Sea water  –  –  3.5–6  25–70  –  [36] 
Rice husk  Mixing and compression molding  57  GMAMAHSAH solutions   0.4–1.6  2.5–19  0.1–1.9  3–42  9.5–40  [22] 
Elephant grass  Hand lay-up  30.4, 31.3, 31.5  NaOH
KMnO4 solutions 
0.6–2.2  31.5–118.1  –  –  –  [37] 
Bamboo  Mixing and compression molding  NA  H2O2 +DTPA +Na2O3Si +NaOH solution, IEM +DBTDL  –  39–65  –  75–105  –  [38] 
Coir  Hand lay-up  NA  NaOH solutions  –  17.9–23.6  –  18.7–48  –  [39] 
10, 20, 30  –  –  10.6–15.6  –  25.9–38.5  25.6–161.9  [40] 
Polyurethane  Kraft cellulose  Compression molding  5, 10, 15, 20  –  0–0.2  –  –  –  –  [41] 
Phenolic  Bagasse  Compression molding  17.6  HClO2 solution
Furfuryl alcohol 
–  –  –  –  17–28  [42] 
Curaua  Compression molding  17.6  HClO2 solution
Furfuryl alcohol 
–  –  –  –  39–88  [42] 
Cellulose from
Molding  1, 3, 5, 7  NaOH solution, propyl-trimethoxy-silane  0.7–0.9  9.5–16.5  5.1–1.0  18.5–28.0  –  [43] 
Ramie  Compression molding  40.4  –  3.3, 1.2  72.3,158  –  90–145  –  [44] 
Jute  Pultrusion  N/A  –  –  25–38  –  28–63  –  [45] 
Bamboo  Compression molding  15  –  21.2–30.1  –  –  210–320  –  [46] 
Vinyl ester  Silk  Hand lay-up  0–15  –  0.9–1.3  40–71  –  –  –  [47] 
Cellulose  VARTM  20, 30, 40, 50  –  3–7  –  –  40–160  –  [20] 
Sisal  RTM  10, 15, 20, 25, 30  NaOH solution  1.7–2.9  38–75  2.1–4.5  75–180  –  [48] 
Kenaf  Pultrusion  40  –  9–12.5  135–145  1.6–1.9  150–190  –  [49] 
Molding  20  NaOCl solution  1.9–3.9  68–119  19–105  [50] 

Table 2.

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

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.

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 [5155]. 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 [5661]. Depending on the matrix, fiber and additives content, fiber treatment and manufacturing process, the mechanical and thermal properties of these composites can be adjusted as presented in Table 3, with the main results obtained.

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, 6168]. 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, 7375]. 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 Processing Fiber
Additive  Mechanical
TD (°C) References
HDPE Flax Injection
0, 15, 30 ACA  220–470 14–24 500–1600 15–26 60–230 [66]
Wood Compression
0–40 Thermo‐
MAPE ACA  0.9–3.9 [72]
Wood Extrusion 20, 30, 40 MAPE 2300–2900 1900–3400 [56]
Wood Extrusion 50, 60, 70, 80 MAPE 3130–4600 11.1–30.2 2470–3370 25.0–58.8 [77]
Wood Injection molding 40 Ethanol and toluene extraction
NaClO2 treatment
NaOH solution
MAPE 3570–4940 23.8–48 [78]
Wood Injection molding 25, 35, 45 1200–2000 18.5–27.5 1200–2700 27.5–43 [59]
Oil palm Compression molding 30, 40 MAPP 650–1050 10–15 [65]
Hemp Compression molding 0–40 ACA  1093–1634 18.8–23 [55]
Agave Injection molding 0–20 ACA  225–550 15–24 1–2.7 [79]
Hemp Compression molding 40 Thermo‐mechanical refining MAPE
2–2.6 [71]
Argan nut shell Injection molding 5, 10, 15, 20, 25 NaOH solution 1136–1795 27.2–29.3 [80]
UHMWPE Wood powder Compression molding 0–30 195–280 650–1260 [67]
LMDPE Agave Rotomolding 5, 10, 15 255–440 13–18.8 495–590 12.5–16.5 0.9–7.5 [81]
Agave Rotomolding 15 Solutions of:
Acrylic acid
Methyl methacrylate
167–217 13–18 420–520 13–17.8 83.8–148.5 [76]
Hemp Injection molding 30 Solutions of:
MAPE 241–668 13.1–17.9 [73]
LLDPE Maple wood Rotomolding 0–20 ACA  26–184 3–16.4 119–680 [52]
Wood Injection molding 47 MAPP 30.2 [82]
Agave Compression molding 0–40 Solutions of:
224–381 10–22 389–1027 14–31 123–260 [75]
PS Agave Compression molding 10, 20,
ACA 3345–4929 30–62 400 [83]
Wood fiber Extrusion 10, 20,
30, 40
MAPS 31–49 54–94.5 [84]
Wood flour Extrusion 10, 20,
30, 40
MAPS 31–41.5 55–68
PP Argan nut shell Injection molding 0–30 SEBS‐g‐MA 1034–1593 26.5–30 339.4–350  [85]
Flax Compression molding 10, 20,
26, 30
1000–3200 [86]
Abaca Injection molding 10, 15,
20, 25
Benzene diazonium treatment, NaOH solution 800–2700 24.5–31 800–3100 43–55 22.5–50 [87]
Coir bagasse Injection molding 5, 10,
15, 20,
25, 30
NaOH solution 1100–1700 27.5–34.7 1400–2000 35–53 [88]
Wood Compression molding 10, 20, 30, 40 MAPP 600–1600 2100–2400 44–52 10–17 [89]
NNC Compression molding 1 MAPP 450–663 32.3–39.1 1809–2238 [90]
Sisal Injection molding 10, 20,
NaOH solution MAPP 500–1100 23–28 363.2–434.5 [91]
Pine cone Injection molding 5, 10, 15, 20, 25,
NaOH solution SEBS‐g‐MA
1020–1550 21–27.5 321–355  [92]
Wood cotton Compression molding 10, 20,
MAPP 28–50 37–152 [93]
PLA Flax Injection molding 15, 25,
2500–6000 282–340  [54]
Maple wood Injection molding 15, 25,
2400–5900 282.3–342.7 [62]
Maple wood Injection molding 5, 10,
15, 20,
1250–1890 59.8–61.5 3650–5260 96.6–107  21.7–34.3 250–360  [94]
Wood Injection molding 20, 30,
40, 50,
55, 60,
5270–10300 56.8–64.6 5400–1088 77–91.8 [58]
Cotton Injection molding 10, 20,
30, 40,
1260–2500 58.1–62.6 3690–8220 97.9–106.2 17.5–24.3 250–360  [94]
Injection molding 10, 20,
1242–1865 43–60 2300–3110 55–96 30–49 [95]
Post consumer PP+HDPE Wood flour Compression  molding 0–40 MAPP
247–394 12.7–15.3 950–1889 38–65.6 [61]
Wood flour Compression molding 0–40 POE
1073–1958 16.6–22.4 [96]
Flax Injection molding 30 MAPP
608 3090 [97]
579 2921 [98]
332–608 1114–3090 [99]
Post consumer HDPE Pine wood Compression molding 30 MAPE
21.4–30.6 341.3–342.4 [60]
Bagasse Compression molding 30 MAPE
22.3–36.1 348.5–353.3 [60]
Wood Compression molding 50, 60 MAPE 9–18 20–35 [100]
Post consumer PP Wood Extrusion MAPP 450–490 27.3–29.8 2230–2940 43–51 285–499  [101]
Oil palm Extrusion MAPP 340–380 18.7–19 1870–2150 30.1–33.8 268–495  [101]

Table 3.

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

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.

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, 7173, 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, 9799, 102104]. Likewise, natural fibers can be functionalized by treating them with a coupling agent in solution, to increase compatibility with the polymer matrix [22, 71, 7376].

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].


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, 106109]. The main purpose of blending different reinforcements 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]:


where PC1 and PC2 are the properties of composite C1 and C2, respectively, while VC1 and VC2 are their respective volume fractions such that:


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, 112114], impact strength [63, 115117] and water uptake [70, 112114, 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 degradation temperature. Similarly, Table 5 reports the corresponding information for hybrid composites 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 process Fiber treatment Mechanical properties TD (°C) References
E (GPa)  TS (MPa)  FS (MPa)  FM (GPa)  IS (kJ/m2) 
Polyester  Hemp/wool  Pultrusion  –  16.84  122.12  180  11  –  [18] 
Palmyra palm
Compression molding  NaOH solution  2.3–5.1  15.3–19.3  24.7–36.4  –  [121] 
Banana/sisal  Hand lay‐up + compression molding  –  1.1–1.5  2.7–4.2  ∼16–37  –  [122] 
Coir/silk  NaOH solution  11.4–17.4  37.4–42  –  [123] 
Oil palm/glass  Compression molding  –  ∼2.5–5.5  ∼20–75  ∼30–138  ∼1.5–8  ∼7–16  –  [124] 
Banana/kenaf  Hand lay‐up  Solutions of:
–  45–139  75–172.2  –  ∼15–28  –  [125] 
Ramie/cotton  Compression molding  –  –  24.2–118  –  6.3–27.4  –  [126] 
Sisal/roselle  RTM  –  –  30.1–58.7  48.4–63.5  –  1.39–1.41  –  [127] 
Sisal/glass  Hand lay‐up  –  –  ∼78–95  ∼70–265  ∼2.1–11  ∼66–88  –  [128] 
Sisal/jute/glass  Hand lay‐up  –  –  111.2–232.1  214.1–308.6  –  –  –  [118] 
Hemp/glass fibers  Hand lay‐out + compression molding  NaOH solution  –  –  –  –  –  345  [107] 
Epoxy  Banana/jute  Hand lay‐up + compression molding  –  0.6–0.7  16.6–19  57.2–59.8  8.9–9.1  13.44–18.23  376.5–380  [108] 
Banana/sisal  Hand lay‐up  –  0.6–0.7  16.1–18.6  57.3–62  8.9–9.3  13.2–17.9  –  [129] 
Jute/bagasse  Hand lay‐up  NaOH
HCl solution 
0.3–0.7  0.6–1.7  6.9–15.9  0.6–1.7  6.9–15.9  438.2–475.9  [109] 
Jute/coir  Hand lay‐up  NaOH
∼0.3–0.7  ∼8.5–35  ∼39–37  ∼0.5–1.5  –  –  [130] 
Banana/silica  Hand lay‐up  –  6.5–9.1  –  –  –  –  –  [111] 
Sisal/silica  Hand lay‐up  –  4.7–6.1  –  –  –  –  –  [111] 
Polyurethane  Hemp/wool  Pultrusion  –  18.91  122.66  ∼142  ∼12  –  –  [18] 
Vinyl ester  Hemp/wool  Pultrusion  –  15.27  112.54  ∼143  ∼13  –  –  [18] 
Jute/ramie  VARTM  –  6.7–6.8  6.2–6.7–  –  ––  18–19  –  [131] 
Molding  –  –  –  –  –  1993–16373  –  [117] 
Vetiver/glass  Hand lay‐up  NaOH solution  1–2.4  53.2–69.8  97.3–131.9  2–3.6  –  –  [116] 
Jute/vetiver  Hand lay‐up  NaOH solution  1.7–1.9  63.3–71.7  114.8–133.1  2.9–3.6  –  –  [116] 

Table 4.

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

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

Composite  Coupling  agent Filler
surface treatment
TD (°C) References
/flax fibers
MAPP (5%) 40 (vol) 522–629 21.9–25.5 37.9–49.6 [106]
MAPE‐GTR rubber
/hemp fiber
10, 30
50, 60
120–243 9.8–14.3 363–781 139.6–239.8 294–465 [63]
PP‐Kenaf/coir/MMT MAPP
30 300–360 11–12 [132]
PP‐NNC/Maple fibers MAPP
21 444.9 25.4 1735.2 [104]
PP‐wood/milled glass fibers
MAPP (4.5%) 50 32–45 48–65 2400–3540 348 [133]
PP‐sisal/glass fibers MAPP
(1%, 2%, 3%)
30 41.75–55.1 970–1686 47.4–67.5 1900–2800 59.3–81.6 346–384 [70]
PP‐jute/flax fibers MAPP (19.12%) 25.96% PP/jute and MAPP/flax woven fabrics were treated with NaOH solution 29.7–42.6 2437.3–2852.4 50.1–68.8 1399.7–2331.8 [134]
LDPE‐banana/coir fibers MAPP
15 Solutions of:
Acetylation bleaching with H2SO4
36.2–50 29.5–52.4 9.3–13.6 473 [135]
palm fibers
(2%, 4%)
40 Hot water
and soap
8–13.5 550–630 17–27 1570–2380 [120]
HDPE‐kenaf/pineapple leaf fibers (PALF) 40 27–30 550–680 23–28 1700–2100 [110]
PS‐banana/glass fibers 20 Solutions of:
Benzoyl chloride
29–38.8 1462.2–1558.3 7.9–11.3 489.7–698.8 [136]
Injection + compression PP‐SBR rubber/
birch wood
(3%, 5%)
0–40 10.5–25 520–1560 [51]
Injection molding PP‐sisal/glass fiber N/A
10, 20,
Boiled in methanol and benzene mixture and with NaOH solution 100 190–230 [112]
PP‐sisal/glass fibers MAPP
30 29.2–31.6 2330–2430 66.7–68.8 4.03–4.14 16.7–20 331.3–464.7 [113]
RPP‐date palm wood/glass fiber 30 19.5–21 1100–1300 361.8–479.4 [114]
PP‐hemp/glass fibers MAPP
40 52.5–59 3800–4300 97–101 5000–5400 49–55.4 360–474 [57]
PP‐wood flour/glass fiber MAPP
SBS‐g‐MA (3%, 6%)
40 28–45.4 39.7–62.8 2680–3497 345–363 [137]
PP‐wood/kenaf fibers MAPP
40 39–44 2771–3008 [138]
PLA‐kenaf/corn husk 30 NaOH solution
Sodium lauryl sulfate solution
Silane and potassium permanganate
1547 [139]
33 67 4965–5577 105–108 7715–7725 119–120 295–397 [140]
20, 30 20.5–26.5 415–650 24–32 670–1180 37–47 [53]
HDPE‐coir/agave fibers MAPE
20, 30 NaOH solution 19.5–25.9 355–500 23.3–31.9 890–1190 42–68 [103]
HDPE‐sisal/hemp MA solution (10%) 25, 30 NaOH solution 15.7–19.2 [141]
shell/coir fibers
MA (8%)
20 NaOH
Benzoyl peroxide solution
26.5–29.5 1050–1300 344–349 [74]
PLA‐banana/sisal fibers 30 57–79 1700–4100 91–125 4200–5600 [142]
PLA‐hemp/lyocell 40 41.4–71.5 4643–7035 [143]
PLA‐hemp/kenaf fibers 40 34.4–61 4920–7039
HDPE‐wood/hollow glass microspheres 50 26.2–31 3300–3600 [119]
Extrusion HDPE‐wood/bast fibers 60 Vinyl triethoxysilane 42–44 650–700 73–77 4900–5250 [144]
HDPE‐wood/Kevlar 60 Allyl and 3–trimethoxy
13.8–19.8 3050–4100 24.5–3600 2200–3400 [145]
Extrusion calendering PP‐jute/glass 20, 30, 40 42–63 4660–7170 72.8–102.5 3550–5950 [69]

Table 5.

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

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] 
Milled glass fibers 
SiO2, CaCO3 and milled grass decreased water uptake  [133] 
Hemp/glass fibers  Glass fiber reduced water uptake  [57] 
Wood/glass fibers  Increasing fiber glass weight ratio, water uptake was reduced.  [146] 
HDPE  Pine/agave fibers  Pine fiber decreased water uptake in hybrid composites  [53] 
Coir/agave fibers  Coir reduced water uptake in hybrid composites  [103] 

Table 6.

Water uptake in hybrid composites using thermoplastic matrices.

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


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 performances. Table 7 summarizes the limited amount of work on auto‐hybrid composites using natural fibers as reinforcement.

Processing Composite  Coupling  agent Fiber
Crystallinity  index (%) Main results  References
Injection  PP‐hemp fibers  MAPP
(3%, 5%)*
Powder: 45–180
20, 30 Hybridization more effective at 20 wt.% reinforcement
Optimum weight ratio of 20/80 (powder/fibers)
3% of coupling agent was more efficient
Ductility and impact strength decreased with fiber content
Tensile and flexural modulus increased with fiber content
pine fibers
Short fiber:  40–105
Long fiber:  300–425
10, 20,
56.2–61.1 Coupling agent increased tensile strength, and decreased tensile modulus, flexural strength and impact strength of auto‐hybrids
Total fiber concentration affected hybridization being more effective at 20 and 30 wt.%
Higher values of mechanical properties were obtained at 30/70 (short/long) weight ratio (without coupling agent) in auto‐hybrids
Crystallinity index decreased with coupling agent addition
PP‐pine fiber Short fiber:  50–212
Long fiber:  300–425
10, 20, 30 Hybridization did not affect flexural and tensile strength
Hybridization was more effective at 30/70 (short/long) and 50/50 (short/long) weight ratio
Positive hybridization effect was higher at 20 and 30 wt.% fiber content
Impact strength was higher at 20 wt.% with a 30/70 (short/long) weight ratio
Water absorption was not affected by fiber size
PP‐agave fibers
Compression  molding LLDPE‐maple fibers MAPE
Short fibers:
Medium fibers:  125–250
Long fibers:  355–450
5, 10, 15, 20 13–32 Positive deviation of RoHM at 30/70 (smaller/longer) weight ratio, regardless of fiber size
20 wt.% showed higher RoHM positive deviation and auto‐hybridization was more effective
Positive deviation of RoHM is affected by fiber size and total fiber content
Tensile and flexural modulus increased with fiber content, but not with fiber size
Impact strength and torsion modulus of hybrid composites are affected by fiber weight ratio

Table 7.

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

*MAPP was not used in auto‐hybrid composites.


6. Conclusion

Natural fibers are now interesting alternative to replace synthetic fibers due their good specific properties (per unit weight). They have been used to develop different composites based on thermoset and thermoplastic matrices. As for any composite, their mechanical, thermal and physical properties are function of the properties of the matrix and the reinforcement, as well as fiber loading, fiber source and manufacturing process. Nevertheless, interfacial conditions are always important to optimize the general properties.

The main disadvantages of using natural fibers are water uptake, low thermal stability, as well as low mechanical properties due to fiber agglomeration and poor interfacial adhesion, especially at high concentration. The problem is usually more important in thermoplastics than thermosets due to their difference in initial resin viscosity. But most of the limitations associated to natural fiber composites can be controlled or overcome by the addition of coupling agents and/or fiber surface modifications.

Finally, another possibility to improve the properties of natural fiber composites is to add a second reinforcement to produce hybrid composites. These materials were shown to have improved mechanical and thermal properties over neat natural fiber composites as they follow the rule of hybrid mixture (RoHM) regardless of the matrix, manufacturing processing and fiber combination. Based on this concept, different class of materials was also developed such as all natural fiber hybrid composites (combination of two different natural fibers) and auto‐hybrid composites (combination of two different sizes of the same fiber). The latter is highly interesting as positive deviations from the RoHM were reported. This is usually the case around 20 wt.% of total fiber content with around 30/70 short/long fiber ratio regardless of coupling agent addition, fiber type and processing method. This opens the door to a new field of investigation as several parameters can be controlled to optimize the final properties of the materials and to design new applications for these multi‐functional composites.



The authors would like to thank the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Research centre for high performance polymer and composite systems (CREPEC), as well as Centre de recherche sur les matériaux avancés (CERMA) and Centre de recherche sur les matériaux renouvelables (CRMR) of Université Laval for technical help.


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Written By

Wendy Rodriguez‐Castellanos and Denis Rodrigue

Reviewed: 22 July 2016 Published: 30 November 2016