1. What CFRP and GFRP are (the short version)

• What they are made of: a composite in which strong fibres (carbon or glass) are bound by a polymer matrix. Hexcel's HexPly® M21 is a prepreg that supplies a toughened epoxy resin combined with carbon or glass fibreSourced
• What is impressive about them: stiffness and strength along the fibre are extremely high. Hexcel's unidirectional M21/IMA carbon fibre material has a tensile modulus along the fibre of 178 GPa, a tensile strength of 3,050 MPa and a laminate density of 1.58 g/cm³ (ASTM D3039, 23 °C, manufacturer's stated values normalised to a fibre volume fraction of 59%)Sourced
• What is hard about them: properties change greatly with direction (anisotropy), and the between-layer direction (interlaminar) and the direction across the fibre are carried by resin and interface. The same material has an in-plane shear modulus of 5.2 GPa, about 1/34 of the modulus along the fibreSourcedOur calculation (178 / 5.2 = 34)

The basics of thermosets and thermoplastics are in our explainer on thermoplastics and thermosets, epoxy resins are in our explainer on epoxy resins, and thermoplastic matrices such as PEEK are in our explainer on PEEK and super engineering plastics. This article keeps to the view of a material made by combining fibre and resin.

2. Who carries the load — fibre, resin and interface

For the same unidirectional M21/IMA material, lining up the types of load shows the values changing by an order of magnitudeSourced.

• Tension along the fibre: strength 3,050 MPa, modulus 178 GPa
• Compression along the fibre: strength 1,500 MPa, modulus 146 GPa (ASTM D695)
• Interlaminar shear (ILSS): 97 MPa (ASTM D2344)
• In-plane shear (±45° tension): strength 94 MPa, modulus 5.2 GPa (ASTM D3039)

Compressive strength is about half of tensile strength (1,500 / 3,050 = 0.49)Our calculation. In compression the resin has to stop the fibres from buckling, so it can be read that the stiffness of the resin and the interface matter (our commentary).

3. How carbon fibre is made

Hexcel's HexTow® IMA is a continuous PAN (polyacrylonitrile)-based carbon fibre supplied as a bundle (tow) of 12,000 (12K) or 24,000 (24K) filaments. The filament diameter is 5.1 µm and the carbon content is 95.0%. It is surface treated, and sizing can be applied to improve properties such as interlaminar shearSourced.

The review by Mishnaevsky (Materials, 2017) sorts carbon fibre manufacture into polymerisation of the precursor, stabilisation and carbonisation, followed by surface treatment and the application of sizing. It says surface treatment roughens the fibre surface to improve adhesion, and that the sizing amount is in the range of 0.5 to 5 wt%Sourced. The review by Frank et al. (Angewandte Chemie, 2014) lists as precursor systems PAN-based copolymers, pitch, cellulose, lignin, polyethylene and new synthetic polymersSourced.

ImageGen conceptual illustration (not a real photograph or measurement). See the original diagram for exact values, names and structure.

Show original diagram with exact annotations and values

A report from ORNL (Oak Ridge National Laboratory), in discussing how to cut the manufacturing cost of carbon fibre, introduces a survey (Kline & Company) which found that less than 50% of the precursor's mass becomes carbon fibre and that the cost of the manufacturing process accounts for more than 50% of the total cost of commercial carbon fibreSourced. It also notes that in conventional heat treatment low-temperature and high-temperature carbonisation are normally carried out step by step in entirely separate furnacesSourced.

4. Our calculation: the rule of mixtures links fibre stiffness to composite stiffness

5. A materials engineer's view (1): anisotropy — where metal intuition fails

Specific stiffness and specific strength — what dividing by density shows

In aerospace and automotive use what counts is stiffness and strength per unit density. We calculate them from the same M21 datasheet values and the laminate densitiesOur calculation.

Modulus, strength and density are Sourced (Hexcel HexPly® M21 datasheet [Reference 1]. UD per ASTM D3039 / EN 2561 B; fabrics per EN 2597 B / EN 2747). The divisions are Our calculation (units GPa·cm³/g and MPa·cm³/g). Because fibre volume fraction and test standard differ from row to row, this is a table for seeing differences of order of magnitude between materials, not a like-for-like comparison.

6. The matrix — thermoset and thermoplastic

The matrix has the jobs of binding the fibres, passing load to them, and protecting them from the environment. Matrices divide broadly into thermosets and thermoplastics.

All Sourced (M21 [Reference 1] and RTM6 [Reference 2] are Hexcel manufacturer's stated values. Tg "dry" is 24 h at 105 °C and "wet" is 14 days in 70 °C water. Thermoplastics: the review by Vaidya and Chawla [Reference 11]). We do not cover values for individual thermoplastic materials.

The review by Vaidya and Chawla (International Materials Reviews, 2008) says that the arrival of high-performance thermoplastic polymers brought a rapid increase in structural uses of thermoplastic composites, that since the 1990s the material forms and combinations of fibre-reinforced thermoplastic polymers have grown exponentially, and that applications have spread across transport, automotive, rail, marine, aerospace, military and constructionSourced. The review sorts out the types of thermoplastic polymer, material forms and impregnation methods, moulding methods using long fibre, and new moulding methods. The properties of individual thermoplastic matrices (PEEK, PPS and so on) are covered in our explainer on PEEK and super engineering plastics.

7. Moulding methods — prepreg plus autoclave, RTM, thermoplastic CFRP

(1) Prepreg plus autoclave

Prepreg is a semi-cured sheet in which fibre has been impregnated with resin in advance. For M21, curing in a press or an autoclave is said to give the best mechanical properties after cure, and Hexcel gives the following procedure as the standard cure for parts under 15 mm thickSourced.

• Pull a vacuum (1 bar) and apply an autoclave pressure of 7 bar (gauge)
• Heat to 180 °C ±5 °C so that the part's heating rate is 1 to 2 °C/min
• Hold at 180 °C ±5 °C for 120 min ±5 min
• Cool the part at 2 to 5 °C/min, and release the pressure at 60 °C or below

For parts 15 to 48 mm thick, a two-stage cure with an intermediate hold of 180 min at 150 °C is given, and M21 is described as allowing thick structures up to 48 mm to be made with a simple cure thanks to its low exotherm behaviourSourced. For sandwich structures with honeycomb or foam cores, the pressure is low at 2 to 3 bar (depending on core density), and the cure is held at 135 °C for 60 min before pressurising and then at 180 °C for 120 minSourced.

(2) RTM and infusion

In RTM (resin transfer moulding) and infusion, a stack of dry fibre (a preform) is placed in a mould, low-viscosity liquid resin is injected to impregnate it, and it is cured as it stands (our commentary). Hexcel's HexFlow® RTM6 is a one-component, aerospace-qualified epoxy for this use, with a viscosity below 100 mPa·s at process temperatureSourced. The recommended conditions are as followsSourced.

• Preheat the resin to 80 °C. Mould temperature a constant 120 to 180 °C, injection and infusion lines at 90 to 110 °C
• Vacuum infusion below 5 mbar; RTM piston pressure from atmospheric to 5 bar
• Cure for at least 90 min at 180 °C; no post-cure is needed (degree of cure α above 90%)

What matters is that the act of heating the resin to lower its viscosity is also the act of advancing the reaction. Hexcel's safety and handling guidelines point to the risk that a heated reactive resin that is not properly managed can lead to an uncontrolled exotherm, and set limits such as preheating the resin in an oven at 50 to 70 °C for at most 12 hoursSourced [Reference 7].

(3) Thermoplastic CFRP

CFRP with a thermoplastic matrix is melted and shaped by heating and solidified by cooling, so there is no waiting for a cure reaction, and there is room for joining or reshaping by reheating (our commentary). The review by Vaidya and Chawla lists as advantages of thermoplastic composites recyclability, high specific strength and stiffness, corrosion resistance, impact toughness, cost effectiveness and design freedomSourced. On the other hand, high-performance thermoplastic matrices melt at high temperatures, so the moulding temperature and equipment differ from those for thermosets (our commentary; we do not cover specific temperatures).

8. A materials engineer's view (2): curing is thermal design

9. The interface and sizing

ImageGen conceptual illustration (not a real photograph or measurement). See the original diagram for exact values, names and structure.

Show original diagram with exact annotations and values

The review by Mishnaevsky sorts out the role and makeup of sizing as followsSourced.

• Sizing for glass fibre: an aqueous suspension (3 to 10 wt% solids) is applied with rollers and dried for several hours above 100 °C to cure the sizing. It contains at least a film former and a coupling agent; a large survey found the film former at about 80 wt% of the dry sizing and the coupling agent at about 10 wt%
• The film former protects fibres from damaging one another, and is mostly a polymer close to the matrix of the final product (polyester, polyurethane, epoxy and so on). The coupling agent is mostly an organosilane, which bonds covalently to the glass fibre surface through a sol-gel reaction and, if it carries a functional group that reacts with the matrix, links fibre and matrix. The most widely used silanes carry amine or epoxy functional groups
• Sizing for carbon fibre: carbon fibre is organic and has no compatibility problem of the kind glass fibre has, but its surface is often smooth and inert, so sizing is needed. Surface treatment roughens the surface and increases surface area, improving adhesion. The sizing amount is in the range of 0.5 to 5 wt%, and plays the roles of forming the strand, reducing fuzz, improving processability and improving adhesion between fibre and matrix. It is often an emulsion of polymer components

10. A materials engineer's view (3): tests that probe the interface do not measure fibre strength

11. Uses — aerospace, automotive and wind power

(1) Aerospace

Boeing explains of the 787 that about 50% of the airframe by mass is made of composites, and that, in contrast with aluminium, composites resist corrosion and do not fatigue, which reduces the need for inspection and the time out of serviceSourced (the manufacturer's own description). In its datasheet Hexcel positions M21 as a very tough epoxy matrix for primary aerospace structures, with outstanding damage tolerance particularly against high-energy impactSourced. In primary aircraft structure, compressive strength after impact is emphasised, and the M21 datasheet gives compression after impact (CAI) of 298 MPa (IM7) at a 30.5 J impact, and 238 MPa (AS7) and 219 MPa (IMA) at 30.0 J (ASTM D7136 and D7137)Sourced.

(2) Automotive

According to BMW Group, the BMW i3 is the company's first car with a passenger cell made of carbon fibre reinforced plastic (CFRP). This Life module is formed in Leipzig from carbon fibre fabric (scrim) and joined to an aluminium chassis (the Drive module). The company says that body manufacturing and assembly in Leipzig take about half the time of conventional vehiclesSourced (the manufacturer's own description). In automotive use, cycle time and material cost are constraints of a different kind from aerospace (our commentary; we have not confirmed specific figures).

(3) Wind power

The review by Mishnaevsky (Materials, 2017) says the following of wind turbine blade materialsSourced.

• From the materials point of view, specific stiffness (stiffness / mass) matters. Turbines are expected to run for 20 to 25 years, so high-cycle fatigue of over 100 million cycles and the behaviour of bond lines and sandwich interfaces are important
• The main reinforcement is E-glass fibre, and glass/epoxy for blades contains up to 75 wt% glass. In UD composites, raising fibre volume content raises stiffness, tensile and compressive strength proportionally, but above 65% dry regions without resin appear, which can reduce fatigue strength
• Carbon fibre, with high stiffness and low density, allows thinner, stiffer, lighter blades, but has low damage tolerance, compressive strength and strain to failure, and is much more expensive than E-glass. It is sensitive to fibre waviness and misalignment, and slight disorder greatly reduces compressive and fatigue strength. Vestas and Siemens Gamesa use carbon fibre composites in the structural spar caps of large blades
• As for manufacturing, with blades growing larger vacuum-assisted resin transfer moulding (VARTM) is the most common, and prepreg (used by Vestas) is also used

12. Recycling

For a materials engineer, CFRP recycling is a question of separating fibre from resin, and deciding which value to recover.

• The review by Pimenta and Pinho (Waste Management, 2011), against the background of growing CFRP waste, sorts out the state of fibre reclamation and remanufacturing processes. It says that several recovery and remanufacturing processes are approaching maturity, with commercial-scale implementations, recycled CFRP with competitive performance for structural use, and demonstration parts already in existence, while challenges remain in establishing a recycling industry and developing a market for recycled materialSourced
• The review by Oliveux et al. (Progress in Materials Science, 2015) says that recycled glass fibre can replace a small amount of new fibre, but pyrolysis (thermolysis) and solvolysis cannot use it at concentrations high enough to be economically and environmentally viable, that carbon fibre recovered from high-tech applications cannot be returned to the same use and new uses must be developed, and that standards suited to the properties of materials using recycled fibre are neededSourced
• The review by Mishnaevsky et al. (Materials, 2021) deals with end-of-life management of wind turbine blades and argues that repair, reuse and life extension of blades are, at present, more advantageous than other approaches. It says that new-generation blades using thermoplastic or recyclable thermoset matrices have been tried in the laboratory and in small and medium blades, but technical challenges remain for use in large bladesSourced

13. What is still hard, and what we could not confirm

(1) We do not cover the conditions of each carbon fibre step

We could not confirm in the public primary sources we were able to consult the stabilisation and carbonisation temperatures, atmospheres, ramp rates, tensions and so on, so no numbers are given. The statements in the ORNL report (a heating rate above 100 °C/min in continuous carbonisation, and a two-stage furnace) come from the context of research on polyolefin precursors and are not necessarily the conditions for general PAN-based fibre.

(2) We give no comparison with metals

We could not confirm primary sources that match conditions (test method, heat treatment, shape) for comparing specific stiffness and strength with aluminium alloy, titanium and steel, so none is given. Fig. 6 stays with comparisons among materials in the same datasheet.

(3) We do not cover laminate properties (fatigue, impact, hot-wet)

Calculation of modulus and strength from the stacking sequence (classical laminate theory), fatigue, and loss of laminate strength under hot-wet conditions are not covered here. The hot-wet calculation in Section 6 is for neat resin.

(4) We do not cover specific moulding conditions or numbers for thermoplastic CFRP

Numbers such as the melting temperature of thermoplastic matrices, moulding times and the strength of joints (welding) are outside the scope of this research. We stay with introducing the statements in the review by Vaidya and Chawla.

(5) We have not used production scale or market size for automotive and aerospace

We have not used market-size or share estimates from research firms. The statements from Boeing and BMW Group are limited to each company's own description. We could not check official material for the composite share of other aircraft such as the Airbus A350.

14. Glossary

15. References (primary sources)

1. Hexcel HexPly® M21 180°C (350°F) curing epoxy matrix — Product Data Sheet — hexcel.com
2. Hexcel HexFlow® RTM6/RTM6-2 180°C Epoxy System for Resin Transfer Molding and Infusion Technologies — Product Data Sheet — hexcel.com
3. Hexcel HexTow® IMA Carbon Fiber — Product Data Sheet — hexcel.com
4. ASTM International ASTM D3039/D3039M Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials — store.astm.org
5. ASTM International ASTM D2344/D2344M Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates — store.astm.org
6. ASTM International ASTM D3171 Standard Test Methods for Constituent Content of Composite Materials — store.astm.org
7. Hexcel HexFlow® RTM6/RTM6-2 Safety & Processing Recommendations for Injection & Infusion — hexcel.com
8. Mishnaevsky et al. Materials for Wind Turbine Blades: An Overview, Materials 10, 1285 (2017) — doi.org/10.3390/ma10111285
9. Frank et al. Carbon Fibers: Precursor Systems, Processing, Structure, and Properties, Angew. Chem. Int. Ed. 53, 5262–5298 (2014) — doi.org/10.1002/anie.201306129
10. Oak Ridge National Laboratory Norris, Development and Commercialization of Alternative Carbon Fiber Precursors and Conversion Technologies, ORNL/TM-2014/239 (2014) — info.ornl.gov
11. Vaidya and Chawla Processing of fibre reinforced thermoplastic composites, Int. Mater. Rev. 53, 185–218 (2008) — doi.org/10.1179/174328008x325223
12. Boeing 787 Dreamliner — By Design — boeing.com
13. BMW Group First of its kind and innovation driver for sustainable mobility: 200 000 BMW i3 produced to date (16 October 2020) — press.bmwgroup.com
14. Pimenta and Pinho Recycling carbon fibre reinforced polymers for structural applications: Technology review and market outlook, Waste Manag. 31, 378–392 (2011) — doi.org/10.1016/j.wasman.2010.09.019
15. Oliveux et al. Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties, Prog. Mater. Sci. 72, 61–99 (2015) — doi.org/10.1016/j.pmatsci.2015.01.004
16. Mishnaevsky Sustainable End-of-Life Management of Wind Turbine Blades: Overview of Current and Coming Solutions, Materials 14, 1124 (2021) — doi.org/10.3390/ma14051124
17. ISO ISO 1172:2023 Textile-glass-reinforced plastics — Prepregs, moulding compounds and laminates — Determination of the textile-glass and mineral-filler content using calcination methods — iso.org

16. Claim-to-source audit

Last updated 2 October 2026. Sources are limited to primary material (Hexcel's public datasheets, ASTM and ISO standards pages, peer-reviewed papers, a report from a U.S. Department of Energy laboratory, and official descriptions from Boeing and BMW Group). Because the article includes structural readings and materials-design interpretations, those are marked as Commentary and kept separate from sourced fact. The conditions of each carbon fibre step, comparison with metals, laminate fatigue and hot-wet behaviour, and thermoplastic CFRP moulding conditions are not stated here because they could not be confirmed in published primary sources. Property values are manufacturers' stated values; test method, temperature and fibre volume fraction differ from value to value, and values for different grades are not lined up to rank them. We have not used market-size or share estimates from research firms. All figures are explanatory concept graphics. Figs. 1 and 3 to 8 are vector drawings; the hero image and Fig. 2 are AI-generated images, and none of them shows a real product, micrograph or equipment.