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Polymer matrices serve as bonding agents to fibers. They bond the fibers together and help in load transfer to the fibers. Also, the polymer matrices allow for good‐quality finish of composite surfaces as well as protection of the reinforcing fibers from chemical attacks. Two common classifications of polymer matrices are thermosetting and thermoplastic resins. They are subsequently elucidated.

 Thermosetting resins: Curing process (chemical reaction) occurs with this type, thus linking polymer chains and connecting the whole matrix in a three‐dimensional (3D) network. It should be noted that once curing occurs, re‐melting or reforming becomes impossible. These resins are highly stable in dimension, resist high temperature as well as offer good resistance to solvents, due to their cross‐linked 3D structure [4]. Some thermosetting resins that are used frequently in composites are vinylesters, polyesters, phenolics, epoxies, bismaleimides (BMIs), and polyamides (PAs).

 Thermoplastic resins: These resins differ from thermosetting resins, because their thermoplastic molecules are not cross‐linked and can be melted when heated and made into solids and then cooled, thus allowing for reforming and reshaping repeatedly. Apart from being generally ductile, thermoplastic resins have more toughness than their thermosetting counterparts. They are broadly used for nonstructural applications without fillers and reinforcements. Their mechanical properties, which are factors of attraction, include good fatigue and compression strength, excellent tensile strength, excellent stiffness, high dimensional stability, excellent damage tolerance, and excellent durability. Furthermore, their flame‐retardant as well as wear‐resistant features broaden their applications and make them relevant, especially in an aerospace sector [4]. Common examples of thermoplastic resins include, but are not limited to, polyvinylidene fluoride (PVDF), polypropylene (PP), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polymethyl methacrylate (PMMA, also called acrylic), polyetherketoneketone (PEKK), and polyetherimide (PEI).

Figure 1.1 Descriptive molecular structure of both (a) thermoplastic and (b) thermoset polymers.

Source: Bergstrom [7]. © 2015, Elsevier

Figure 1.1a,b depicts the molecular structure of thermoplastic and thermosetting resins, respectively. The cross‐links in the molecular structure of the thermosetting resins (shaded molecules) are depicted in Figure 1.1b.

There are different categories that exist for the manufacturing process of polymer matrix composites (PMCs). These include squeeze flow methods, short‐fiber suspension methods, and porous media methods [4]. Table 1.3 depicts some partial and complete natural and synthetic hybrid FRP composites, their resins/matrices, and manufacturing methods.

It is well known that there is no single engineering material that can be all‐encompassing in terms of its applicability to operations and processes. Therefore, natural FRP composites have some limitations, despite their outstanding benefits. Table 1.4 presents some of the benefits as well as disadvantages of natural FRP composites.

The key elements that affect the mechanical response of natural FRP hybrid composites are subsequently identified [5]:

 Fiber selection, which includes the type, method of extraction, time of harvest, natural fiber aspect ratio, content, as well as its treatment

 Interfacial strength

 Matrix choice

 Fiber distribution

 Composite manufacturing process

 Fiber arrangement [9]

 Void presence/porosity, among others.

Table 1.3 Manufacturing processes of some hybrid (mainly natural) FRP composites.

Source: Sathishkumar et al. [8]. © 2014, SAGE Publications.

Hybrid fiber	Resin	Curing agent Catalyst	Accelerator	Manufacturing methods
Pineapple/sisal/glass	Polyester	MEKP	Cobalt napthenate	Hydraulic press
Sisal/silk	Polyester			Hand lay‐up technique
Kenaf/glass	Polyester			Hand lay‐up and cold press
Woven jute/glass	Polyester			Hand lay‐up
Banana/Kenaf	Polyester			Hydraulic compression molding process
Banana/sisal	Polyester			Hand lay‐up method followed by compression molding
Glass/palmyra	Polyester			Hydraulic compression molding process
Jute/glass	Polyester			Hand lay‐up
Roselle/sisal	Polyester			Hand lay/up technique
Silk/sisal	Polyester			Hand lay‐up technique
Banana/sisal	Epoxy			Hydraulic compression molding process
Glass/glass	Epoxy	HY95 I hardener		Hand lay‐up technique
Carbon/glass	Epoxy	HY225 Hardener		Hand lay‐up technique
Oil palm/jute	Epoxy	Hardener		Compression molding process
Chicken feather/glass	Epoxy	n‐tert‐Butyl peroxybenzoate		Hot press
Basalt/Hemp	Polypropylene			Hot pressing
Flax, Hemp, and jute	Polypropylene			Hydraulic press
Flax/wood fiber	HDPE			Twin screw extrusion
Banana/glass	Polypropylene			Twin screw extrusion
Cork/coconut	HDPE			Screw extrusion and compression molding
Kenaf/pineapple	HDPE			Mixing and compression molding
Bamboo/glass	Polypropylene			Injection molding
Cordenka/jute	Polypropylene			Injection molding
Bamboo/cellulose	Poly lactic acid			Injection molding
OPEFB/glass	Vinyl ester			Resin transfer molding
Aramid/sisal	Phenolic			Stirring, drying, compression

HDPE, high‐density polyethylene; MEKP, methyl ethyl ketone peroxide; and OPEFB, oil palm empty fruit punch.

Table 1.4 Benefits and drawbacks of natural FRP hybrid composites.

Source: Modified from Pickering et al. [5]. © 2014, SAGE Publications.

Benefits

Drawbacks

Renewable source of fibers/matrices and sustainabilityLow danger/risk during manufacturing processesLow density, stiffness, and high specific strengthLow process/production energy and environmental friendlinessLower production cost when compared with synthetic fibers, such as carbon and glassLow release of harmful fumes when heating and during end of life process (incineration)Lower abrasive attack on processing tools, when compared with synthetic FRP compositesPossibility of predicting better balanced mechanical behaviors, such as toughness

Lower responses, especially impact strength in comparison with the synthetic FRP compositesHigher variability of behaviors, due to discrepancies in sources and qualitiesLower durability in comparison with synthetic FRP composites. However, it can be enhanced significantly using treatmentsPoor fiber orientation and/or layer stacking sequence, causing weak fiber–matrix interfacial adhesionHigh water/moisture absorption, consequently causes swelling effectLower processing parameters, such as degradability temperatures. Hence, it causes limiting matrix and fiber options and structural applications