Fibre-reinforced plastics

Fibre-reinforced plastics have proven to be a genuine revolution in materials engineering over recent decades. They combine the advantages of classic plastics – such as low weight, corrosion resistance and good formability – with the high strength and stiffness of fibres. As a result, they are able to replace or complement metals in many fields of application. Glass fibre-reinforced plastics (GFRP) and carbon fibre-reinforced plastics (CFRP) in particular play a central role.

They have shaped the development of entire industries: from wind turbines to sports equipment to aerospace, much would not be possible without these materials. However, the two materials differ significantly in their properties, their cost structure and their processing. While GFRP stands out above all for its versatility and good value for money, CFRP stands for maximum performance in lightweight construction.

In order to make the right decision for an application, it is important to understand the differences and similarities precisely.

Fundamentals of fibre-reinforced plastics

Fibre-reinforced plastics consist of a matrix and reinforcing fibres. The matrix is usually a thermoset such as epoxy resin or a thermoplastic such as polyamide. It takes on the task of shaping the composite, encasing the fibres and protecting them from external influences. At the same time, it contributes to chemical resistance and temperature stability. The fibres, in turn, take on the main share of the mechanical load and give the material its extraordinary strength and stiffness.

The properties of a fibre-reinforced plastic depend heavily on various factors: the choice of fibre, whether glass, carbon, aramid or natural fibres, the orientation of the fibres in the material – for example unidirectional, as a fabric or braid – as well as the volume fraction of the fibres in relation to the matrix, are decisive. The type of matrix system, whether thermoplastic or thermoset, also significantly influences the behaviour of the material. By combining these parameters, tailor-made composite materials can be developed that are optimally matched to the respective requirements of an application.

Glass fibre-reinforced plastics (GFRP)

Glass fibres consist essentially of silicon dioxide, combined with other oxides that influence the melting process and the properties. In production, molten glass strands are drawn into fine filaments and processed into mats, fabrics or rovings. These can then be combined with resins.

GFRP is characterised by high chemical resistance to moisture, oils and many chemicals. In addition, glass fibre composites have very good electrical insulation capability, which makes them particularly interesting in electrotechnical applications. Mechanically, they are robust and durable, even if their stiffness is lower compared with CFRP. Temperature resistance up to about 180 °C makes GFRP versatile, and a significant advantage lies in its comparatively inexpensive production and processing.

Application examples:

• Rotor blades of wind turbines
• Boat hulls and sailing yachts
• Pipes and tanks in the chemical industry
• Body parts in automotive engineering
• Housings and covers in mechanical engineering

The disadvantages are the higher weight compared with CFRP and the lower stiffness. Nevertheless, owing to its low costs, GFRP is virtually without competition for large-area applications such as wind turbines.

Carbon fibre-reinforced plastics (CFRP)

Carbon fibres are produced from polyacrylonitrile (PAN) or pitch. In a multi-stage process, the fibres are first oxidised and then carbonised. This produces fibres that consist of more than 90 % pure carbon. These have an extremely high tensile strength and stiffness.

CFRP stands out above all for its very low weight combined with enormous stiffness. The ratio of strength to density is unsurpassed compared with most materials. Another special feature is the very low thermal expansion, which makes CFRP ideal for precision components that must remain dimensionally stable even under changing temperatures.

In addition, carbon fibre composites exhibit electrical conductivity, which is advantageous in some applications such as electromagnetic shielding, but can cause problems where insulation is required. Depending on the matrix system, CFRP components are also temperature-resistant up to over 250 °C, which extends their possible uses in demanding high-tech environments.

Application examples:

• Wings and structural components in aerospace
• Landing gear and chassis parts in motorsport
• High-performance sports equipment (e.g. racing bicycles, tennis rackets)
• Medical technology (e.g. prostheses, implants)
• Satellite and spaceflight components

CFRP offers unsurpassed lightweight construction advantages, but is significantly more expensive. In addition to material costs, elaborate processing, limited production speeds and difficult recycling are also major challenges.

Comparison of glass fibre vs. carbon fibre

Property	GFRP (glass fibre)	CFRP (carbon fibre)
Weight	higher than CFRP	significantly lower
Strength & stiffness	good	very high
Costs	low to medium	very high
Chemical resistance	excellent	good
Electrical properties	insulating	conductive
Production	comparatively simple	complex and cost-intensive
Typical applications	wind power, boats, tanks, housings	aerospace, motorsport, medical technology

Standards and quality standards

The safe and reliable use of fibre-reinforced plastics presupposes compliance with international norms and standards. These standards govern test methods, minimum requirements and quality criteria that are binding for different branches of industry. One example is ISO 14125, which describes standardised flexural tests on fibre composite materials.

ASTM standards define test methods for tensile, compressive and impact loads and are particularly important in international comparison. In aerospace, the strict approval guidelines of EASA (European Union Aviation Safety Agency) and FAA (Federal Aviation Administration) also play a decisive role, since here every component is safety-critical. In automotive engineering, medical technology and construction, too, there are specific approval procedures and quality seals.

For companies, this means that not only the material properties but also the production processes, the documentation and the traceability must comply with the specified standards. Only in this way can consistently high quality and the necessary safety be guaranteed.

Challenges in practice

In practice, designers and manufacturers face a number of hurdles:

• Recycling and disposal: fibre composite materials are difficult to separate and often recyclable only by pyrolysis or chemical processes.
• Costs: CFRP in particular causes high material and manufacturing costs.
• Processing: laminating, autoclave processes and CNC machining are elaborate.
• Inspection: internal damage is often difficult to detect and requires ultrasonic or X-ray testing.

Sustainability and future trends

Sustainability is playing an increasingly important role in fibre-reinforced plastics. A central goal is to develop and improve recycling processes so that both glass and carbon fibres can be recovered efficiently. Processes such as pyrolysis or solvolysis are already being trialled and offer opportunities for economic recovery. The development of thermoplastic matrix systems is also a promising approach, since, unlike thermoset systems, these can be melted down and processed again.

Alongside technical solutions, alternative raw materials are coming more strongly into focus. Natural fibres such as flax, hemp or basalt are gaining increasing importance, particularly in combination with bio-based resins, which enable a more sustainable choice of material. At the same time, industry and research are working on automating the production processes of CFRP in order to reduce costs and production times. Hybrid structures, in which GFRP and CFRP are combined, are also a future trend, since they offer a balanced ratio of cost and performance.

Overall, the future of fibre-reinforced plastics will depend heavily on how well ecological requirements, resource conservation and economic performance can be reconciled with one another.

In conclusion

The decision between glass fibre and carbon fibre-reinforced plastics is far more than just a technical consideration. It touches on questions of economic efficiency, safety and increasingly also sustainability. GFRP has established itself as a robust all-rounder that can be produced economically in large quantities and offers an ideal price-performance ratio in many areas. CFRP, by contrast, remains the material of choice for high-end applications in which every gram of weight counts and maximum strength is required.

With increasing demands on energy efficiency, environmental compatibility and resource conservation, both materials will continue to play an important role in the future. The decisive factor will be how successfully recycling processes can be industrialised, new matrix systems developed and production costs reduced. One thing is clear: fibre-reinforced plastics remain an engine of innovation that will significantly influence industrial development.

FAQ

Which is lighter – GFRP or CFRP? CFRP is significantly lighter than GFRP while at the same time having higher strength.

Is CFRP always the better choice? No – for many applications the performance of GFRP is entirely sufficient, especially when costs are the priority.

Can GFRP and CFRP be recycled? Both are difficult to recycle. However, there is progress with processes such as pyrolysis or chemical dissolution.

Where is GFRP used most? Above all in wind turbines, boats, tanks, pipes and housings in mechanical engineering.

What role does electrical conductivity play in CFRP? It can be advantageous (e.g. for electromagnetic shielding), but also a hindrance when insulation is necessary.

How do the production costs of GFRP and CFRP differ? GFRP is comparatively inexpensive, while CFRP costs many times more – both in material production and in processing.

What alternatives are there to GFRP and CFRP? Alongside aramid fibres (Kevlar), natural fibres and basalt fibres are gaining importance, above all in sustainable applications.