1.2.2 Polymer Matrices
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 |