Project funded by the European Commission under grant agreement n°696656

Graphene Core 1

Graphene-Based Disruptive Technologies

H2020 RIA

WP15: PRODUCTION

Deliverable 15.5

“Leading edge manufacturing trials. Process set up”

Main Author(s):

Asunción Butragueño, Tamara Blanco. Airbus Operations SL

Ana Reguero, Santiago Cuellar. Internacional de Composites, SA

César Merino, Santiago Blanco, Mª Pilar Merino. Grupo Antolin Ingeniería, SA

Due date of deliverable: M16

Actual submission date: M16

Dissemination level: PU

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LIST OF CONTRIBUTORS

Partner Acronym Partner Name Name of the contact

35 AIRBUS Airbus Operations, SL Asunción Butragueño

35 AIRBUS Airbus Operations, SL Tamara Blanco

145 AERNNOVA Internacional de Composites, SA Santiago Cuellar

145 AERNNOVA Internacional de Composites, SA Ana Reguero

67 ANTOLIN Grupo Antolin Ingeniería, SA César Merino

67 ANTOLIN Grupo Antolin Ingeniería, SA Mª Pilar Merino

67 ANTOLIN Grupo Antolin Ingeniería, SA Santiago Blanco

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TABLE OF CONTENTS

Contents Deliverable Summary ............................................................................................................................... 4

Introduction and materials ........................................................................................................................ 5

Dispersion of graphene in the aeronautical resin considered .................................................................. 7

Aerostructural composite laminates manufacturing. Process set up. ...................................................... 8

Coupons definition and results ............................................................................................................... 10

Conclusions ............................................................................................................................................ 14

References ............................................................................................................................................. 15

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Deliverable Summary

In deliverable D15.5 “Leading edge manufacturing trials. Process set up.” it is described the

manufacturing process and the developments implemented for the fabrication of an aeronautical

composite prototype by means of resin transfer moulding (RTM) technology, using a thermoset epoxy

resin doped with graphene related materials (GRM) in order to improve the impact strength of the

structure. This challenge entails the possibility to improve current aerostructure performances without

significant changes either in design or in manufacturing process.

In the previous deliverable, D15.4 “Aerostructure leading edge technical specifications and

requirements”, the considered prototype part was described and a general description of present

manufacturing process. The chosen part is a piece of the leading edge of the horizontal tail plain (LE-

HTP) from the AIRBUS A350, which is already being manufactured in series at AERNNOVA´s plant in

Toledo (Spain).

RTM technology consists of a closed mould made of metal with an inner cavity with the final part

shape. A preform made of dry carbon fibre is placed in the cavity of the metal mould. Then, the mould

is closed and sealed, and liquid resin is injected into the tooling under pressure and heating conditions.

With an estimated fibre volume close to 60%, GRMs dispersed into the resin should have a good

distribution among the fibres. This fact is really challenging, since the distance between fibres in a

single yarn is just a few micrometres. Therefore, lateral dimensions of GRMs should be mainly

submicrometric in order to avoid the so called “filtering effect” of graphenic particles by the reinforcing

fabric. Besides significant increase of resin viscosity due to its doping with GRM must be avoided to

achieve a proper filling of the RTM mould.

Highly graphitic helical ribbon carbon nanofibres (CNFs) are used as raw material for obtaining GRMs

instead of graphite in order to achieve graphenic particles with lateral dimensions low enough. Three

different exfoliation procedures of CNFs have been tested and a dispersion technique based on wet

milling and high shearing of GRM into the epoxy resin. Temperature control during the dispersion

process is key, since the resin selected by AIRBUS for this application is monocomponent (the

hardener is incorporated into the resin), highly exothermic during curing and its viscosity varies quickly

by increasing resin temperature.

It has been observed that the incorporation of the GRMs considered into HexFlow® RTM 6 epoxy resin

at loadings below 1% weight does not significantly modify the processing characteristics of the resin

selected for manufacturing the aerostructure. Therefore, it has been possible to ensure that the

production process of the LE-HTP could be achieved without the necessity of major changes in the

manufacturing process. Manufacturing trials and the process set up are described in this deliverable.

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Introduction and materials

Graphene obtained from top down procedure is mainly produced from graphite although other graphitic

precursors have also been employed [1]. The major disadvantage of graphite, as starting material, is

the low efficiency of the exfoliation processes due to the high number of stacked layers present in its

structure and the large lateral size of the obtained graphene sheets for certain applications. As an

alternative route, ANTOLIN has developed physicochemical processes to obtain few layer sheets of

graphene and graphene oxide by using their GANF (Grupo Antolin NanoFibers) as starting material.

GANF present a singular helical ribbon graphitic structure, composed by a graphitic ribbon of

approximately five graphene layers rolled along the fibre axis (Figure 1) [2]. This structure makes them

very attractive as starting material for graphene production [3-4]. The low number of stacked graphene

layers in GANF allows the achievement of a highly effective exfoliation of graphene sheets with very

low lateral size as it can be seen in the Figure 2.

Fig. 1: Helical ribbon Grupo Antolin carbon NanoFibre (GANF) used as graphitic precursor for obtaining

graphene related materials

Fig.2: SEM and AFM images of a droplet of graphene suspension obtained from GANF deposited by spray-

coating on a silicon dioxide substrate

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Currently continuous carbon fibres are used as the baseline reinforcements for demanding structural

applications. But the complexity of the existing multifunctional requirements make it necessary to tailor

a multimaterial laminate, where glass fibres are also frequently used. Dry continuous fibre fabrics with

different fibre orientation and thick aerostructural laminates are considered in this Task 15.4 (Figure 3)

to evaluate the graphene doped resin homogenization and impregnation to be processed by RTM

moulding technology.

Considering the LE-HTP structure is a curved shape structure, the main baseline material for the

design and manufacturing process shall be a satin fabric as the drapeability of the fabric laminate plays

an important role in the adaptation to the small radii of the leading edge while avoiding spring-back

forces. Additionally other reinforcement fabrics can be added in order to improve other characteristics

in specific areas (drilling area, ribs, stringers….). A full scale demonstrator is expected to be

manufactured at the end of Task 15.4.

Fig. 3: SEM micrographs of aerostructural composite laminates considered in Task 15.4

Three different GRMs produced by ANTOLIN have been tested at three concentrations for every case,

0.2%, 0.4% and 0.8% resin weight:

GRMi: GANF helical ribbon carbon nanofibres milled into the resin and partially

exfoliated using high energy milling and shearing processes.

GRMii: Single and few layer graphene oxide produced from GANF by the so-called

Hummers-Offerman method [3].

GRMiii: Single and few layer graphene obtained from GANF following a

mechanochemical process using melamine as intercalant for exfoliation [4].

Lateral size of GRMs used in hierarchical thermoset resin composites should be low enough to avoid

the filtering effect reported by Zhang et al. [5] in the PolyGraph project. This is the main advantage of

GRAPHENE D15.5 10 August 2017ºº 7 / 15

using GANF carbon nanofibres instead of graphite for producing GRMs for this application, since

lateral size of the products obtained are mainly submicrometric as it can be seen in Figure 2.

Composition of the laminate: Carbon fibre fabric 5H Satin weave of 370 g/m2, HR 6K [6]

Woven S-2 Glass fibre fabrics 8H Satin, 295 g/m2 [7]

HexFlow® RTM 6 monocomponent epoxy resin

Fibre volume fraction: 57%

Dispersion of graphene in the aeronautical resin considered

The epoxy resin used for producing this part is supplied by Hexcel, its trade name is HexFlow® RTM 6.

It is a resin specifically designed for RTM processes and to fulfil the requirements of the aerospace

industry. GRMs are dispersed in the resin at a temperature above 40ºC in order to avoid to work with

high resin viscosities during dispersion. The resin is preheated at 80ºC before injection and the mould

is maintained at 120ºC. One of the main problems faced in the project is that shelf life of this resin at

room temperature is 15 days maximum. Therefore, Grupo Antolin and Aernnova have to schedule very

well the dispersion of GRMs into the resin in Burgos and the resin injection runs in Toledo.

Since the curing of RTM 6 is highly exothermic and it is a monocomponent resin, eventual overheating

points during the dispersion of graphene particles must be avoided. On the other hand, very high

energy procedures, due to the use of hard ceramic balls with diameter lower than 1 mm, and long

processing time are necessary in order to achieve proper dispersions. Therefore, the equipment

selected for these studies has been a basket mill with internal refrigeration capability, Figure 4.

Besides, this technique allows high throughput and it could be readily up scaled.

Fig. 4: High energy milling and shearing equipment used for graphene dispersion

Alternative dispersion techniques like for example rotor stator high shear mixers or ultrasound

equipment do not allow a proper temperature control of the resin during the process. On the other hand

GRAPHENE D15.5 10 August 2017ºº 8 / 15

horizontal ball mills are less appropriate than basket mills for long processing times of tempered resins.

Finally, three roll mills are characterized by a very low throughput and they cannot be used for low

viscosity resins.

Aerostructural composite laminates manufacturing. Process set up.

In figure 5 it is described the complete process for manufacturing a LE-HTP by RTM. After the curing

cycle and cooling, the mould is opened and the final part is removed from it, ready for final operations

like ultrasonic inspection, trimming, drilling and painting [8-10].

Fig. 5: Manufacturing process of LE-HTP by Resin Transfer Moulding technology

Temperature, vacuum and pressure conditions are controlled and recorded from start to end point of

the injection cycle by means of measurement devices (thermocouples, vacuometer and control unit).

The control of these parameters on each step is essential for successful manufacturing. After positive

result of the vacuum test, tooling is heated up to 120ºC, temperature at the injection stage.

Simultaneously, the resin is heated for degassing and maintained at 80ºC in order to be injected. Then,

the resin tempered in the dar is injected into the tool cavity and pressure is applied in order to ensure

resin distribution along the entire fabrics preform. The complete resin transfer during this step is

fundamental to avoid porosity and obtain a homogeneous distribution of resin in the final part. Figure 6

illustrates the full equipment involved in the process:

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Figure 6. RTM full equipment and control devices sketch

In order to evaluate the mechanical properties of the RTM aerostructural composite laminates, a mould

has been designed and manufactured by Aernnova incorporating resin injection points at the sides of

the mould cavity and a vent in the middle (Figure 7). The specimen injection tool has been designed to

manufacture 600x400 mm laminates with adjustable thickness by using removable inner steel plates

during the process.

Figure 7: Mould and equipment designed for the fabrication of RTM aerostructural composite laminates

standardized for evaluating their physicochemical properties.

ThermocoupleMould

Carbon fibre laminate

Thermocouple

TEMPERATURE

CONTROL

DEVICE

PARAMETER

RECORDER

Resin inner hose

VACUUM AND

PRESSURE

CONTROL

DEVICE

Resin outer hose

DAR

Silicone vacuum hose

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Coupons definition and results

Before HTP Leading Edge manufacturing, several specimens are going to be injected in order to define

the most suitable GRM and its concentration. Therefore, the project has been scheduled in three

phases as it can be seen in Figure 8.

Figure 8: Description of the phases scheduled for Task 15.4 activities.

Phase 1: In a first stage lamination trials have been performed with Glass Fibre fabric instead of

Carbon Fibre in order to have the possibility of optically evaluate any possible filtering effect of the

GRM doped resin by the white glass fabric. In Table 1 are described the different specimens that have

been injected. All of them have been visually inspected after demoulding to discard significant signals

of filter effect. Moreover, ultrasonic inspection has been performed to confirm the absence of porosity.

Firstly, three specimens with 0.2% weight graphene concentration have been manufactured using

different type of GRMs. Then, the graphenic material that show filter effect by the glass fibre fabric has

been discarded for further project phases. Later, a fourth specimen has been injected with 0,8%

graphene concentration to check whether high loading could result in a negative effect of the graphene

distribution. Results from visual inspection after demoulding are summarized in the Table below:

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Specimen Type of

GRM GRM production process

% w

loading

Filter

effect

Viscosity

increase

1 Gi Milling + partial exfoliation into the resin 0.2 No No

2 Giii Mechanochemical treatment with melamine 0.2 No No

3 Gii Hummers-Offerman chemical oxidation 0.2 Yes No

4 Gi Milling + partial exfoliation into the resin 0.8 No No

Table 1. Results for glass fibre fabric reinforced polymer (GFRP) specimens manufactured in Phase 1

These trials allowed to discard single and few layer graphene oxide produced from GANF for the

following stages of the project, since evidence of dispersion problems were readily noticeable (Figure

9). No evidence of filtering problems was noticed for the case of GRMi and GRMiii.

(a) (b)

Figure 9: GRMs in glass fibre fabric laminates: (a) GRMii, presence of aggregates (blue circles),

(b) GRMi and GRMiii, homogeneous dispersion.

It has been observed that the incorporation of the GRMs considered into HexFlow® RTM 6 epoxy resin

at the graphene concentrations considered does not increase resin viscosity. This fact is crucial for the

success of the project (Figure 10).

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Figure 10: Viscosity of neat RTM6 epoxy resin and formulations incorporating GRMs

In the second stage laminates using satin carbon fibre fabrics were manufactured injecting RTM6 resin

doped with GRMi and GRMiii, Table 2. No evidence of problems related to mould cavity filling, resin

viscosity or curing were observed, neither of filtering effect for any of the three graphene

concentrations considered (0.2%, 0.4% and 0.8% weight), as it can be seen in Figure 11. The GRM

loaded laminates were black, it was not possible to differentiate them from the non-graphene loaded

laminates and one sample from another with different GRM concentration.

Specimen Type of

GRM GRM production process % w loading

1 None None 0.0

2

GRMi Milling + partial exfoliation into the resin

0.2

3 0.4

4 0.8

5

GRMiii Mechanochemical treatment with melamine

0.2

6 0.4

7 0.8

Table 2. Carbon fibre fabric reinforced polymer (CFRP) laminates manufactured in Phase 1

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(a) Non loaded RTM6 (b) RTM6/ GRMi (c) RTM6/ GRMiii

Figure 11: Carbon fibre laminates manufactured by RTM with epoxy resin loaded with GRMi and GRMiii.

All laminates have been tested by Airbus and Aernnova standard quality control based on ultrasonic

inspection, a well-established technique for locating and documenting internal defects in the composite

[11 - 13]. Ultrasound maps obtained do not show any manufacturing problem (Fig. 12a). Besides, X-ray

computed tomography studies of the laminates have been carried out (Fig. 12b). This technique is

based on the computer-assisted reconstruction of three-dimensional bodies based on X-ray

radiographies taken from various viewing angles. No evidence of important defects were noticed in the

laminates, only very small pores and few tiny aggregates of GRMs.

(a) Ultrasonic inspection (b) X-ray computed tomography

Figure 11: Non-destructive evaluation performed on carbon fibre laminates manufactured by RTM with

epoxy resin loaded with GRMi and GRMiii.

Other key characteristics of the epoxy resin selected for manufacturing the aerostructure, such as

curing properties and glass transition temperature must not be modified by the incorporation of GRM.

Preliminary results ensure that the production process of the LE-HTP undergoes no unintended

modification.

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Conclusions

Resin Transfer Moulding aerostructural composite laminates manufacturing technique is characterized

by the use of very low viscosity epoxy resins and production equipment designed for non-continuous

processes, in opposite to aerostructural prepreg formulations for autoclave manufacturing of composite

laminates. Therefore, this technique has been selected for the manufacture of an aerostructural

composite demonstrator in WP15 of Core 1 phase of the Graphene Flagship.

Low viscosity is key to attain a proper dispersion of graphene particles into the epoxy resin considered.

Besides, filtering effect of graphenic particles by the reinforcing carbon and/or glass fibre fabric

laminate preforms located in the mould cavity must be avoided during RTM resin injection runs. Both

requirements could be fulfilled by mainly submicrometric lateral size graphene. Due to these reasons

GANF carbon nanofibres have been used in Task 15.4 as graphene related material precursor instead

of standard graphite. Three different GRMs have been considered at concentrations below 1% weight

loading: GRMi - GANF milled and partially exfoliated into the resin using high energy milling and

shearing processes, GRMii - Graphene oxide produced from GANF, GRMiii - Graphene obtained from

GANF following a mechanochemical process using melamine as intercalant for exfoliation.

No resin viscosity increase has been noticed for any of these GRM at the concentrations considered.

Therefore, it has not been necessary to modify the RTM manufacturing parameters or process

equipment. Graphene dispersion problems in the aerostructural laminates have been noticed for GRMii

by using GFRP. In the case of GRMi and GRMiii no filtering effect was observed and the inspections of

laminates following standards from Airbus were successful. Additionally, no changes in the glass

transition temperature and curing properties of the composite have been preliminary noticed. GRMii is

then not considered for next steps of the project. CFRP Laminates manufactured with GRMi and

GRMiii at 0.2%, 0.4% and 0.8% weight concentration are being tested in order to determine the most

proper resin formulation for manufacturing the Horizontal Tail Plane Leading Edge section

demonstrator.

The wide use of composite laminates in primary aerostructures is challenging, since they could suffer

impacts during its service life, being the HTP Leading Edge an area driven by bird impact. In this type

of events, the use of laminated composite materials with their inherent advantages becomes a

disadvantage because of its low impact resistance. Usually this type of loading events causes internal

composite damages like delamination or matrix crushing, which reduce significantly its strength.

Standardized resin dominated mechanical tests related to delamination issues are being considered for

selecting the most proper GRM and its concentration in the epoxy resin: Interlaminar Shear Strength

(ILSS) and Mode I Interlaminar Fracture Toughness (G1c).

Once the most proper graphene loaded resin formulation is selected for phase 3 of the project, which

involves the production of the final innovative HTP-LE demonstrator, additional laminates will be

GRAPHENE D15.5 10 August 2017ºº 15 / 15

manufactured in phase 2 in order to assess a set of relevant mechanical properties to evaluate the

performance of the graphene doped material versus the reference material.

References

1. Cristina Botas, Noel Díez, Julio Gómez, Cesar Merino, Kostas Kostarellos, Vincenzo Palermo,

Elvira Villaro; Raul Arenal, Martin Lohe, Reinhard Berger, Xinliang Feng, Xiaodong Zhuang,

Steve Hodge, Andrea Ferrari, Jonathan Coleman, Claudia Backes, Teófilo Rojo, Daniel

Carriazo, Ester Vazquez, Maurizio Prato, Cristina Martín, Paolo Samori; Artur Ciesielski, “Top-

down production of graphene and inorganic 2D-materials in the liquid phase”, Review paper in

preparation within the Graphene Flagship.

2. Vera-Agullo J, Varela-Rizo H, Conesa JA, Almansa C, Merino C, Martin-Gullon I. “Evidence for

growth mechanism and helix spiral cone structure of stacked-cup carbon nanofibers”, Carbon

45 (14), 2007, pp 2751-8.

3. B. Lobato, C. Merino, V. Barranco, T.A. Centeno. “Large scale conversion of helical-ribbon

carbon nanofibers to a variety of graphene-related material”, RCS Advances 6 (2016) pp.

57514.

4. Antonio Esaú Del Rio-Castillo, César Merino, Enrique Díez-Barra, Ester Vázquez. “Selective

suspension of single layer graphene mechanochemically exfoliated from carbon nanofibres”,

Nano Research, July 2014, Volume 7, Issue 7, pp 963-972.

5. Han Zhang, Yi Liu, Shanshan Huo, Joe Briscoe, Wei Tu, Olivier T. Picot, Amir Rezai, Emiliano

Bilotti, Ton Peijs. “Filtration effects of graphene nanoplatelets in resin infusion processes:

Problems and possible solutions”, Composites Science and Technology 139 (2017) 138-145.

6. AIMS05-04-009 - Airbus Material Specification - Standard Modulus Carbon Fibre Fabric (with

or without Binder) – 5H Satin, 370 g/m2 – Liquid epoxy resin 180ºC curing - Material

Specification.

7. AIMS05-31-001 - Airbus Material Specification - Woven S-2 Glass fibre fabrics (with or without

Binder) – 8H Satin, 295 g/m2 – Liquid epoxy resin 180°C curing – Material Specification.

8. AIPS03-07-021 - Airbus Process Specification – Preparation of dry fiber preforms for injection

and infusion techniques.

9. AIPS03-02-022 – Airbus Process Specification – Manufacture of Monolithic and Sandwich

Structural Components by Impregnation of Dry Reinforcement Materials with Thermoset Resin.

10. AIPS03-07-002 – Airbus Process Specification – Machining of (non-)continuous fibre

reinforced and un-reinforced plastic components.

11. AITM6-4005 - Airbus Test Method - For Inspection Processes - Ultrasonic pulse-echo

inspection of carbon fibre plastics.

12. AITM6-4011 - Airbus Test Method - For Inspection Processes - Ultrasonic inspection.

13. AITM6-4012 - Airbus Test Method - For Inspection Processes - Ultrasonic pulse-echo

inspection of glass fibre reinforced plastics.