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Engineering » Energy Engineering » "Composites from Renewable and Sustainable Materials", book edited by Matheus Poletto, ISBN 978-953-51-2794-9, Print ISBN 978-953-51-2793-2, Published: November 30, 2016 under CC BY 3.0 license. © The Author(s).

Chapter 15

Production and Characterization of Hybrid Polymer Composites Based on Natural Fibers

By Wendy Rodriguez‐Castellanos and Denis Rodrigue
DOI: 10.5772/64995

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Production and Characterization of Hybrid Polymer Composites Based on Natural Fibers

Wendy Rodriguez‐Castellanos and Denis Rodrigue
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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.

Keywords: 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 CharacteristicsExamples
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.

fiber source
Mechanical propertiesReferences
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].

MatrixFiber sourceProcessingFiber
Additive Mechanical
TD (°C)References
0, 15, 30ACA 220–47014–24500–160015–2660–230[66]
MAPEACA 0.9–3.9[72]
WoodExtrusion20, 30, 40MAPE2300–29001900–3400[56]
WoodExtrusion50, 60, 70, 80MAPE3130–460011.1–30.22470–337025.0–58.8[77]
WoodInjection molding40Ethanol and toluene extraction
NaClO2 treatment
NaOH solution
WoodInjection molding25, 35, 451200–200018.5–27.51200–270027.5–43[59]
Oil palmCompression molding30, 40MAPP650–105010–15[65]
HempCompression molding0–40ACA 1093–163418.8–23[55]
AgaveInjection molding0–20ACA 225–55015–241–2.7[79]
HempCompression molding40Thermo‐mechanical refiningMAPE
Argan nut shellInjection molding5, 10, 15, 20, 25NaOH solution1136–179527.2–29.3[80]
UHMWPEWood powderCompression molding0–30195–280650–1260[67]
LMDPEAgaveRotomolding5, 10, 15255–44013–18.8495–59012.5–16.50.9–7.5[81]
AgaveRotomolding15Solutions of:
Acrylic acid
Methyl methacrylate
HempInjection molding30Solutions of:
LLDPEMaple woodRotomolding0–20ACA 26–1843–16.4119–680[52]
WoodInjection molding47MAPP30.2[82]
AgaveCompression molding0–40Solutions of:
PSAgaveCompression molding10, 20,
Wood fiberExtrusion10, 20,
30, 40
Wood flourExtrusion10, 20,
30, 40
PPArgan nut shellInjection molding0–30SEBS‐g‐MA1034–159326.5–30339.4–350 [85]
FlaxCompression molding10, 20,
26, 30
AbacaInjection molding10, 15,
20, 25
Benzene diazonium treatment, NaOH solution800–270024.5–31800–310043–5522.5–50[87]
Coir bagasseInjection molding5, 10,
15, 20,
25, 30
NaOH solution1100–170027.5–34.71400–200035–53[88]
WoodCompression molding10, 20, 30, 40MAPP600–16002100–240044–5210–17[89]
NNCCompression molding1MAPP450–66332.3–39.11809–2238[90]
SisalInjection molding10, 20,
NaOH solutionMAPP500–110023–28363.2–434.5[91]
Pine coneInjection molding5, 10, 15, 20, 25,
NaOH solutionSEBS‐g‐MA
1020–155021–27.5321–355 [92]
Wood cottonCompression molding10, 20,
PLAFlaxInjection molding15, 25,
2500–6000282–340 [54]
Maple woodInjection molding15, 25,
Maple woodInjection molding5, 10,
15, 20,
1250–189059.8–61.53650–526096.6–107 21.7–34.3250–360 [94]
WoodInjection molding20, 30,
40, 50,
55, 60,
CottonInjection molding10, 20,
30, 40,
1260–250058.1–62.63690–822097.9–106.217.5–24.3250–360 [94]
Injection molding10, 20,
Post consumer PP+HDPEWood flourCompression  molding0–40MAPP
Wood flourCompression molding0–40POE
FlaxInjection molding30MAPP
Post consumer HDPEPine woodCompression molding30MAPE
BagasseCompression molding30MAPE
WoodCompression molding50, 60MAPE9–1820–35[100]
Post consumer PPWoodExtrusionMAPP450–49027.3–29.82230–294043–51285–499 [101]
Oil palmExtrusionMAPP340–38018.7–191870–215030.1–33.8268–495 [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.

MatrixFibersManufacturing processFiber treatmentMechanical propertiesTD (°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  agentFiller
surface treatment
TD (°C)References
/flax fibers
MAPP (5%)40 (vol)522–62921.9–25.537.9–49.6[106]
MAPE‐GTR rubber
/hemp fiber
10, 30
50, 60
PP‐NNC/Maple fibersMAPP
PP‐wood/milled glass fibers
MAPP (4.5%)5032–4548–652400–3540348[133]
PP‐sisal/glass fibersMAPP
(1%, 2%, 3%)
PP‐jute/flax fibersMAPP (19.12%)25.96%PP/jute and MAPP/flax woven fabrics were treated with NaOH solution29.7–42.62437.3–2852.450.1–68.81399.7–2331.8[134]
LDPE‐banana/coir fibersMAPP
15Solutions of:
Acetylation bleaching with H2SO4
palm fibers
(2%, 4%)
40Hot water
and soap
HDPE‐kenaf/pineapple leaf fibers (PALF)4027–30550–68023–281700–2100[110]
PS‐banana/glass fibers20Solutions of:
Benzoyl chloride
Injection + compressionPP‐SBR rubber/
birch wood
(3%, 5%)
Injection moldingPP‐sisal/glass fiberN/A
10, 20,
Boiled in methanol and benzene mixture and with NaOH solution100190–230[112]
PP‐sisal/glass fibersMAPP
RPP‐date palm wood/glass fiber3019.5–211100–1300361.8–479.4[114]
PP‐hemp/glass fibersMAPP
PP‐wood flour/glass fiberMAPP
SBS‐g‐MA (3%, 6%)
PP‐wood/kenaf fibersMAPP
PLA‐kenaf/corn husk30NaOH solution
Sodium lauryl sulfate solution
Silane and potassium permanganate
20, 3020.5–26.5415–65024–32670–118037–47[53]
HDPE‐coir/agave fibersMAPE
20, 30NaOH solution19.5–25.9355–50023.3–31.9890–119042–68[103]
HDPE‐sisal/hempMA solution (10%)25, 30NaOH solution15.7–19.2[141]
shell/coir fibers
MA (8%)
Benzoyl peroxide solution
PLA‐banana/sisal fibers3057–791700–410091–1254200–5600[142]
PLA‐hemp/kenaf fibers4034.4–614920–7039
HDPE‐wood/hollow glass microspheres5026.2–313300–3600[119]
ExtrusionHDPE‐wood/bast fibers60Vinyl triethoxysilane42–44650–70073–774900–5250[144]
HDPE‐wood/Kevlar60Allyl and 3–trimethoxy
Extrusion calenderingPP‐jute/glass20, 30, 4042–634660–717072.8–102.53550–5950[69]

Table 5.

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

[i] - 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 ReinforcementsObservationsReferences
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.

ProcessingComposite Coupling  agentFiber
Crystallinity  index (%)Main results References
Injection PP‐hemp fibers MAPP
(3%, 5%)*
Powder: 45–180
20, 30Hybridization 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,
56.2–61.1Coupling 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 fiberShort fiber:  50–212
Long fiber:  300–425
10, 20, 30Hybridization 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  moldingLLDPE‐maple fibersMAPE
Short fibers:
Medium fibers:  125–250
Long fibers:  355–450
5, 10, 15, 2013–32Positive 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.

[i] - *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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