Fibre-reinforced Plastics

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Fibre-reinforced plastics

Classification of fibre-reinforced plastics

The term ‘fibre-reinforced plastics’ (FRP) or ‘fibre-plastic composites’ is a synonym for an extremely heterogeneous group of materials, which is characterised by the type of fibre reinforcement and the fibre volume fraction, the possible matrix materials, various manufacturing processes and the set or generated fibre orientation. This group of materials is classified as fibre-reinforced composites, which themselves generally belong to the class of composite materials (see also: Composite materials testing) with any matrix and reinforcement material.

Basically, a fibre-plastic composite is a combination of any reinforcing fibres and a plastic matrix that encases the fibres and enables force transmission between the composite partners by means of adhesive or cohesive interaction. As most plastics are not suitable for structural applications due to their low strength, high elongation at break and shrinkage behaviour (see also: Processing shrinkage) as a result of their viscoelastic properties (thermoplastics) or very low elongation at break and brittle fracture behaviour (thermosets), reinforcement with fibres or filling with particles is an economical option for producing structural materials.

Due to the variability of fibres and matrix materials, the properties of the composites can be specifically adjusted within a wide range (tailor-made materials), making applications in the lightweight construction sector for the aircraft and automotive industries possible in the first place. Regardless of the type of reinforcement and the selected matrix, fibre-reinforced plastics have high specific strengths and moduli of elasticity (characteristic value in relation to density), but they generally also have high anisotropy (directional dependence) of the mechanical and thermal properties.

The basic principle for all fibre-reinforced plastics is that the fibres assume the load-bearing function in the composite, while the matrix performs a bedding and protective function, e.g. to prevent EULER buckling under normal stress on the fibres. The fibres are provided with a so-called coating, which prevents the fibres from sticking together during the production process, and an adhesion promoter (see also: fibre-matrix adhesion). The matrix must contain a coupler, e.g. maleic anhydride. The coating and the coupler form the interface between matrix and fibre and transfer the load stress in the volume via shear stresses (see: bend test – shear stress) in the boundary surface to the fibre, which then reacts with a normal stress if it is oriented approximately in the load direction. For an efficient composite material in terms of strength and modulus of elasticity, the following rules must be observed in addition to the fibre geometry (l/d ratio) and the fibre volume fraction:

$E_{F}>E_{M}\!$

(1)

$\epsilon _{BF}<\epsilon _{BM}\!$

(2)

$\sigma _{MF}<\sigma _{MM}\!$

(3)

with index

F		fibre
M		Matrix

Types of reinforcing plastics

In fibre-reinforced plastics, organic and inorganic fibres (e.g. glass fibres (GF), mineral fibres (MF), carbon fibres (CF) or natural fibres (NF)) are used as one-dimensional (e.g. fibres, rovings), two-dimensional (e.g. fabrics, scrims) or three-dimensional (laminates) reinforcing structures.

The most important inorganic amorphous fibres are mineral fibres (MF) and glass fibres (GF), with the following types being used specifically for thermoplastic and thermosetting composites:

• Glass fibres (E-glass)
• Basalt fibres
• Boron fibres or
• Ceramic fibres

Due to their mostly high orientation, organic fibres also have a high anisotropy of elastic properties and strength with limited heat resistance. These fibres are preferably used for thermoplastics but also for thermoset composites, e.g. carbon fibres (CF):

• Polyaramide fibres (aramid fibres, AF)
• Carbon fibres (CF, high-strength, high-modulus)
• Polyester fibres
• Polyamide fibres (nylon)
• Polyethylene fibres
• Poly(methyl methacrylate) fibres (acrylate or Plexiglas fibres)

Natural fibres (NF) are also increasingly being used for fibre-reinforced plastics, although their mechanical properties are comparatively low due to their low density and therefore they cannot be used as high-performance composites. They are usually only used for thermoplastics as a reinforcing or filling material (e.g. WPC-Wood Polymer Compound). The most important renewable reinforcing fibres are:

• flax fibres
• hemp fibre
• wood fibres
• sisal fibres

Regardless of the type of fibre and its geometry, the following fibre length-dependent classification features are used.

• Short fibres with 0.1 ≤ L ≤ 1 mm for thermoplastic fibre composites (injection moulding, extrusion)
• Long fibres with 1 ≤ L ≤ 50 mm for thermoplastic and thermoset fibre composites (injection moulding, fibre spraying)
• Continuous fibres with L > 50 mm for thermoset fibre composites (rovings, woven fabrics, scrims, multiaxial scrims, stitched fabrics)

Fibre-reinforces plastics

Thermoplastic fibre-reinforced plastics

Almost all amorphous and semi-crystalline plastics can be used as thermoplastic matrix materials, although the efficiency of the reinforcement can vary greatly. The advantage of most thermoplastic fibre-reinforced plastics is the variety of manufacturing and shaping processes as well as their weldability and bondability. The disadvantage of these fibre-plastic composites is that they soften and shrink when the glass temperature Tg is reached. However, the tendency of these materials to creep and shrink (see also: processing shrinkage) decreases as the fibre volume content increases, whereby anisotropic stiffness, strength and deformation behaviour occurs as a result of the orientation. Suitable thermoplastic matrix materials are e.g:

• polyamide (abbreviation: PA)
• polybutene (abbreviation: PB)
• polycarbonate (abbreviation: PC)
• poly(butylene terephthalate) (abbreviation: PBT)
• polyethylene (abbreviation: PE)
• polyoxymethylene (abbreviation: POM)
• polypropylene (abbreviation: PP)

The following thermoplastics can be used as a matrix for high-temperature applications up to a maximum of 300 °C:

• polyetheretherketone (abbreviation: PEEK)
• poly(phenylene sulfide) (abbreviation: PPS)
• polyether sulfone (abbreviation: PESU)
• polysulfone (abbreviation: PSU)
• polyetherimide (abbreviation: PEI)
• polychlorotrifluoroethylene (abbreviation: PCTFE)
• polytetrafluoroethene (abbreviation: PTFE)

Thermosets fibre-reinforced plastics

Fibre-plastic composites on a thermoset basis can be cold or hot curing systems that can no longer be formed after curing, although mechanical processing methods such as sawing or milling can be used. Pre-impregnated fibre matrix semi-finished products such as SMC (Sheet Molding Compound), BMC (Bulk Molding Compound) or prepregs (Preimpregnated Fibers) are often used for the production of complex components with defined main stress directions. The resins listed below are used as thermosetting matrix materials:

• dialkylphthalate resin (abbreviation: DAP)
• epoxy resin (abbreviation: EP)
• urea resin (abbreviation: UF)
• melamine resin (abbreviation: MF)
• phenol-formaldehyde resin (abbreviation: PF)
• polyurethane (abbreviation: PUR)
• unsaturated polyester resin (abbreviation: UP)

See also

• Material & Werkstoff
• Plastics
• Processing shrinkage
• Fibre–matrix adhesion

References

[1]	Altstädt, V.: Prüfung von Verbundwerkstoffen. In: Grellmann, W., Seidler, S. (Eds.): Kunststoffprüfung. Carl Hanser, Munich (2024), 4th Edition, pp. 527–579 (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23)
[2]	Grellmann, W., Seidler, S. (Eds.): Mechanical and Thermomechanical Properties of Polymers. Landoldt Börnstein. Volume VIII/6A2, Springer, Berlin (2014) (ISBN 978-3-642-55166-6; see AMK-Library under A 16)
[3]	Elsner, P., Eyerer, P., Hirth, T. (Eds.): Domininghaus – Kunststoffe, Eigenschaften und Anwendungen. Springer, Berlin (2012) 8th Edition, (ISBN 978-3-446-44350-1)
[4]	Schürmann, H.: Konstruieren mit Faser-Kunststoff-Verbunden. Springer, Berlin (2007) 2nd Edition (ISBN 978-3-540-72190-1)
[5]	Neitzel, M., Mitschang, P., Breuer, U.: Handbuch Verbundwerkstoffe: Werkstoffe, Verarbeitung, Anwendung. Carl Hanser, Munich (2014), (ISBN 978-3-446-43696-1), (see AMK-Library under G 12)
[6]	Ehrenstein, G. W.: Faserverbund-Kunststoffe, 2nd Edition, Carl Hanser, Munich (2006), (ISBN 3-446-22716-4) (siehe AMK-Library under G 6-2)
[7]	Erhard, G.: Konstruieren mit Kunststoffen. Carl Hanser, Munich (2008) 4th Edition (ISBN 978-3-446-41646-8)
[8]	Ehrenstein, G. W.: Mit Kunststoffen konstruieren. 3rd Edition, Carl Hanser, Munich (2007), 3rd Edition (ISBN 978-3-446-41322-1) (see AMK-Library under G 42)
[9]	Ehrenstein, G. W.: Polymerwerkstoffe – Struktur – Eigenschaften – Anwendung. Carl Hanser, Munich (2011) 3rd Edition (ISBN 978-3-446-42283-4)