IMPROVING THE AIRCRAFT SAFETY BY SELF HEALING STRUCTURE AND PROTECTING NANOFILLERS
UNIVERSITA DEGLI STUDI DI SALERNO
Via Giovanni Paolo Ii 132
84084 Fisciano Sa
Higher or Secondary Education Establishments
€ 533 292
Liberata Guadagno (Prof.)
Sort by EU Contribution
EASN TECHNOLOGY INNOVATION SERVICES BVBA
€ 153 800
€ 252 400
VYZKUMNY A ZKUSEBNI LETECKY USTAV A.S.
€ 146 500
ALENIA AERMACCHI SPA
€ 107 969
CENTRO ITALIANO RICERCHE AEROSPAZIALI SCPA
€ 217 729
€ 327 604
CARBURES EUROPE SA
€ 178 900
€ 252 100
NANO 4 SA
€ 226 972
Grant agreement ID: 313978
1 September 2012
31 August 2015
€ 3 270 839
€ 2 397 266
UNIVERSITA DEGLI STUDI DI SALERNO
The elixir of life for aeroplanes
Grant agreement ID: 313978
1 September 2012
31 August 2015
€ 3 270 839
€ 2 397 266
UNIVERSITA DEGLI STUDI DI SALERNO
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Final Report Summary - IASS (IMPROVING THE AIRCRAFT SAFETY BY SELF HEALING STRUCTURE AND PROTECTING NANOFILLERS)
Main objective of the IASS Project was the Development of a New generation of Composites able to overcome some of the current limitations of aeronautic materials, such as:
• Reduced electrical conductivity;
• Poor impact damage resistance;
• Poor flame resistance.
The traditional approach to the development of structural aeronautic materials is to address the load-carrying and other functional requirements separately, resulting in suboptimal load-bearing materials with the penalty of added weight. The IASS project aimed at developing self-healing, load-bearing materials and structures with all functionalities integrated in a single material able to meet many important requirements of structural materials for primary structures in aeronautics. The main concept underpinning the project was the use of the nanotechnology strategy for the production of new, high performance structural multifunctional materials. Many of the strategies proposed in the DOW of the IASS project have been found successful to achieve the planned goals.
Using all the results of the IASS consortium, promising multifunctional resins able to increase flame resistance, electrical conductivity and regenerative ability have been developed. Multifunctional carbon fiber reinforced panels (CFRPs) have been manufactured using the multifunctional resin. CFRPs (impregnated using the resin with all functionalities integrated) have been manufactured by Resin Film Infusion (RFI) using a non-usual technique to infuse a nano-filled resin into the carbon fiber dry preform (Responsible of the Processing: CIRA - Responsible of the requirements: ALENIA). Several flat panels have been produced and tested. The manufactured panels have been tested with respect to all the integrated functionalities. The electrical conductivity was found to be about 2x104S/m in the direction parallel to the fibers, whereas a value between 3.0 and 4.0 S/m was found in the directional orthogonal to the fibers. These values are among the highest values reached until now for nanofilled resins impregnating carbon fibers. The panels also revealed enhanced flame resistance properties (LOI ~ 56, Ignition time (s) ~ 81 etc.. - for other parameters see deliverable D6.3). Furthermore, due to the autorepair ability, a significant decrease in the fatigue crack growth rate by approximately 80 % was found.
Prepreg with modified and charged epoxy resin has also been developed. The manufacture of the aeronautical prepreg demonstrators was carried out following the aeronautical standards I+D-P-233 from Airbus ("Manufacture of composite structures with carbon fiber").
Two final demonstrators have been produced: a flat panel with three T shape stringers, manufactured by infusion, and a curved panel with two omega shape stringers, manufactured by ad hoc prepreg.
In the last months of the Project a new ruthenium catalyst stable in high reactive environments has been designed and synthetized.
The new catalyst can be applied to activate self-healing mechanisms in polymeric materials or for the synthesis of many important petrochemicals by Ring-opening metathesis polymerization of cycloalkanes.
This synthetized catalyst is able to overcome critical points related to the application of ruthenium catalysts in the activation of ROMP reactions in thermosetting epoxy resins solidified using DDS (aromatic primary amines).
In the first period of the project, the members of the IASS Consortium developed new formulations for the epoxy matrix (also at low moisture content) and new components among which new hardeners and functionalized nanofillers.
Study of the effect of the nanofillers on the electrical, mechanical and flame resistance properties in the developed epoxy resins have been performed together with the identification of the best systems.
Project Context and Objectives:
The title of the IASS Project is:
Improving the aircraft safety by self-healing structure and protecting nanofillers.
As inferred from the title, main objective of the IASS Project is the “Formulation, preparation and characterization of multi-functional self-healing composites containing dispersed protective nanofillers”.
Why to use the nanofillers ?
There is today the fascinating possibility to transfer some of the very interesting nanostructure properties to specific composites. This opens up new fantastic and smart prospects and solutions, most of all in the field of multifunctional materials.
Multifunctional materials can be designed today to have integrated electrical, electromagnetic, flame resistance properties, or regenerative ability and other functionalities that work in synergy to provide advantages beyond the sum of the individual capabilities.
The improvement in the aircraft safety by self-healing structures and protecting nanofillers is a revolutionary approach that should lead to the creation of novel generation of multifunctional aircraft materials with strongly desired properties and design flexibilities. Furthermore, in recent years, the development of new nanostructured materials has enabled an evolving shift from single purpose materials to multifunctional systems that can provide greater value than the base materials alone; these materials possess attributes beyond the basic strength and stiffness that typically drive the science and engineering of the material for structural systems.
In this context, IASS Project is aimed to develop a new generation of composites able to overcome the following current limitations in the field of aeronautic materials:
1. Reduced electrical conductivity;
2. Poor impact damage resistance;
3. Poor flame resistance.
Point 1 - Some modern aircrafts are made of advanced composites which are significantly less conductive than aluminum. This fact has raised concern over the performance of the composite structure during a lightning event due to the remarkable risk that a puncture of the structural part would cause a catastrophic failure of the aircraft. Then, the composites are reinforced with conductive metal fibres or metal screens in order to dissipate lightning currents. But many of these solutions add additional weight and partially reduce composite advantage. However, in the last decade, the availability of different nanofiller or nanostructured conductive materials has sensibly contributed to the continuous improvement of the engineering properties or abilities of the composites for aeronautic industries.
By choosing the appropriate control of the matter structure as well as the specific fillers, researchers with appropriate expertise can work to overcome this critical point.
Electrical conductive lightweight resins need to be developed for applications in aeronautic field.
Point 2 - Another very important restriction in composites can arise from the effects of impact damage on the structural integrity of the material. Internal damage is difficult to detect and even more difficult to repair. Several non-destructive damage detection techniques have been developed including ultrasonic, infrared thermography, x-ray tomography, and computerized vibro-thermography. This technology has helped to detect damage but repair of this damage has been limited to reinforced patch bonding and/or bolting.
Actually, durability and reliability are still problematic in the field of these structural materials; in fact, in order to achieve the mechanical strength required for many structural applications, highly cross-linked polymeric materials are necessary. The trade off for this gain in mechanical strength is that the resulting material tends to be brittle and is therefore more prone to developing cracks through normal usage, ultimately failing.
Currently, most industrial materials rely entirely on passive protection mechanisms; However, they will always stay passive, and therefore their lifetime and functionality is limited (mechanical stresses, impact of the hail on the fuselage crown during a storm, impact of the stones on the keel during the grounding, consumption, bird impact , adverse environmental conditions etc..)
Therefore, better, and preferentially active processes for the protection/repair of damaged materials (self-repairing processes) need to be developed for applications in aeronautic field.
Point 3 – In the last decades the use of composite in the primary structures in the Aeronautics is rapidly increased. The most representative example is the Boeing 787 Dreamliner, built with more than 50% of composites. The fuselage and wings of the new Boeing 787 are constructed of composite. The composite consists of multiple, alternately directed layers of epoxy-impregnated continuous graphite fibers (Carbon fiber reinforced composites–CFRCs).
Unfortunately, the epoxy resin between the carbon fiber layers can burn under accidental aircraft fire conditions; for this reason FAA certification for Boeing 787 required to demonstrate that the level of fire safety in the B-787 was equivalent to a conventional transport (aluminum) aircraft. This regulation has been extended to any other structural aeronautic materials. It is evident that no new material can be developed in this field without considering its behavior in the flame condition.
To design a material able to resist under fire conditions, new research strategies have to be developed.
Of course the functionalities related to these drawbacks (points 1, 2 and 3) must be integrated in a composite characterized by good mechanical performance and other suitable properties such as for example suitable water transport properties.
IASS members have firstly studied the strategy to impart the single functionalities to an epoxy mixture tailored to meet specific needs of aeronautic materials, so specifically formulated for aeronautic application and able to be used as nanofilled formulation.
A very innovative aspect of this research work is that the activities related to the different functionalities have been carried out in the view to subsequently merge all functionalities in a single multifunctional load-bearing material (see Figure 1 of the attached file – file name - “Figures”).
At the only purpose to give some examples on the adopted work strategy we can make the following point.
Expert in the field of aeronautic materials know that aromatic primary amines are employed as hardeners to impart good mechanical properties to the resin used to manufacture composites for primary structures.
Among these, a common industrial hardener is the 4,4’Diaminodiphenyl-sulfone (DDS).
This hardener can be designed to have a molecular structure also able to increase flame resistance – of course preserving the chemical groups responsible of the good mechanical performance of the resins solidified using this type of hardener.
Furthermore, multifunctional materials can be designed to manifest synergic effects between the different functionalities. For ex. if we consider the material characterized by increased electrical and flame resistance properties and autorepair function, the efficiency of the autorepair function in the multifunctional material is higher than the efficiency of the same functionality in the material without other functionalities (see Figure 2).
Aim of the Project and research strategies as summarized below:
DRAWBACK: REDUCED ELECTRICAL CONDUCTIVITY RESEARC STRATEGY:
Use of conductive nanofiller into polymeric matrix to enhance mechanical and electrical properties and to improve additional properties such as flame retardant behavior and tailored EM properties.
DRAWBACK: POOR IMPACT DAMAGE RESISTANCE RESEARC STRATEGY:
Dispersion of the Grubbs catalyst in the polymeric matrix at molecular level with a substantially uniform distribution, which disregards its particular crystallographic modification and its morphological parameters and does not compromise its activity.
Immobilization of Self-healing catalysts on solid surface on nanoparticles (used to add other functionalities).
Investigation on the possibility to apply the click reactions to enable high yielding, reactivity in ambient conditions and fast reaction kinetics.
Integration of the “click” reactions into the materials
Fourth Approach (scientific part)
Evaluation of new crosslinking concepts (chemistry)
DRAWBACK: POOR FLAME RESISTANCE
Identification of reactive organophosphorus compounds that could be incorporated into epoxy formulations to provide fire-resistant structural composites without to compromise the processing condition and handling, physical and mechanical properties
Analysis on he possibility to create a sunergistic effect of carbon nanotube or graphene sheets (already used to impart conductivity) and clay for improving the flame retardancy. In literature a benefit of these nanofillers has been experimented for thermoplastic matrices
Inclusion of new molecular fillers (POSS) with reactive epoxy groups.
As described above, IASS Project considers for each functionality many different approaches, this to reduce the risk of failure and also because, part of this work, has been performed to explore alternative concepts on the possibility to develop efficient multifunctional resins prepared using chemicals not commercially available yet.
Description of the different Researc Strategies
To impart autorepair function to the material, four different approaches have been adopted. The first approach refers to a self-healing materials based on the microencapsulation concept (see Figure 3).
In this design, the components for the self-healing functionality are embedded inside the epoxy resin; they are microcapsules containing a polymerizer agent and catalyst particles for the polymerization (the polymerization is activated by Ring Opening Metathesis Polymerization (ROMP) reactions.
How does it work? When a crack forms in the matrix, it propagates, intercepts and ruptures the microcapsules which release the healing agent into the crack plane through capillary action; the healing agent contacts the catalyst, triggering polymerization that bonds the crack faces closed.
In the second approach we have considered the immobilization of the self-healing catalysts on solid surface of nanoparticles (the same nanoparticle embedded in the matrix to add other functionalities). For example on the solid surfaces of nanoparticles (CNTs, graphene sheets etc..) embedded in the epoxy mixture to increase electrical conductivity (see Figure 4, Figure 5 and Figure 6).
Other approaches (second and third approaches) are related to the possibility to develop efficient self-healing resins based on new concepts “alternative” to the “microencapsulation” such as for ex. the strengthening of attractive reversible forces acting on nano and micro-scale level.
Reduced electrical conductivity
Concerning the electrical insulating properties of the epoxy resins, as planned in DOW, the members of the IASS consortium have tried to increase the electrical conductivity of the epoxy formulation selected for this project (with the contribution of ALENIA) by dispersing conductive nanofillers inside the matrix. The members of IASS consortium have worked with different electrical conductivity nanoparticles, all based on carbon nanostructured forms, then with unidimensional shaped forms ((among which unfunctionalized and functionalized Carbon Nanotubes (CNTs) (see Figure 7) and untreated and heat-treated Carbon nanofibers (CNFs) – see Figure 8)), and with bidimensional shaped forms such as “exfoliated graphite” characterized by different degree of exfoliation and functionalities and single graphene layers embedded in the matrix (see Figure 9).
At this point, it is worth noting that, in recent years, many other research groups have explored the unique properties of nanoparticles (CNTs and graphene-based nanoparticles) dispersed in resin or introduced between lamina interfaces, to address these above described limitations. The use of carbon nanotubes (CNTs) especially, generated much excitement due their phenomenal structural and transport properties. The results to date have been highly variable and have fallen well short of expectations. This is partly due to a lack of interdisciplinary collaboration where fundamental questions, requiring input from chemists, physicists, material scientists and research engineers, were not adequately investigated. The IASS members have dealt with this subject with the highly complementary expertise of the members. Fundamental understandings of the influence of physical and chemical characteristics of different carbon-based nanoparticles on the properties of nanofilled resin and composites have been achieved. Un-expected results have been described in the different deliverables such as for example the influence of single graphene layers on the properties of the composites. In this case of this last nanofiller, many research groups are trying to obtain and disperse single graphene layers in the resin. The members of the IASS project have found that full exfoliation of graphite (graphene layers) may not be the approach to pursue to achieve very low percolation thresholds and high electrical conductivity especially for bulk resins. The IASS results highlight that perfect graphene layers (without defects) originated from full exfoliated graphite tend to reassemble during the manufacturing of the nanocomposites. In fact, strong functionalization procedures are necessary to obtain graphene in the form of single layer embedded in the polymeric matrix.
The functional groups attached to the graphene layers prevent the re-assembling of layers due to steric and energy factors. Unfortunately, due to the transition from sp2 to sp3 hybridization of carbon atoms, functionalized single layers (SL) of graphene tend to lose delocalized electrons and, therefore, the very interesting electronic properties of graphene, hence reducing the electrical conductivity of graphene-based nanocomposites. This effect is further worsened by the difficulty of single layers to form conductive paths inside the bulk polymeric matrix due to characteristic morphological features of functionalized SL graphene layers. In fact, the functional groups act as defects imparting to the layers strong tendencies to screw up which lead to severe inhomogeneities in the nanofiller dispersion also preventing the formation of conductive pathways. On the other hand, significant benefits in terms of physical properties can be achieved for ultrathin graphitic stacks which preserve a large part of sp2 hybridized carbon atoms, and hence their graphene-like electronic properties. A deep balance and control of the inherent complexity of these systems at nanoscale level may drive the changes in the nanocomposite properties towards the set goals. In light of these considerations, samples of partially exfoliated graphite have been considered both to avoid the negative effect of single graphene layers and to maximize the beneficial effects of graphene-based materials for bulk samples. Edge structures of graphitic blocks can be controlled for improving the performance of nanocomposites, as here evidenced by analyzing the properties of epoxy resins filled with very similar graphene-based materials, differing essentially for the exfoliation degree and consequently for the concentration of carboxylated groups on the nanoparticle edges. The impact of the chosen strategy on the electrical and mechanical properties is very significant. The chemistry of graphene edges strongly affects the physical properties of the resin where these nanoparticles are embedded and drive the changes in the nanocomposite properties towards the desired goals.
At the end of the IASS Project only the most promising CNTs have been used to manufacture the final multifunctional Panels. Many strategies found were successful and this forced us to make a choice!
Poor flame resistance
Also to impart enhanced flame resistance properties different approaches have been identified. In the first approach, organophosphorous compounds have been designed, synthetized and used as hardeners (see Figure 10) to partially replace the DDS.
Another successful approach has been based on the inclusion in the resin of nano-cages of Polyhedral Oligomeric Silsesquioxanes (POSS) compounds with specific functionalizations.
Project Main results:
Self-Healing functionality (first approach)
Concerning the self-healing functionality based on the “microencapsulation” concept (first approach), in the initial stage of the Project, IASS members have synthetized microcapsules (WP1 and WP2) containing healing agent able to polymerize also at very low temperature with commercial catalysts (see Figure 11 and Figure 12) [Smart Materials and Structures, 23(4), 045001 (11pp) (2014) ; Advanced Composites Materials (2014) DOI:10.1080/09243046.2014.937135; Polymer Engineering and Science, 54(4), 777-784 (2014); Polymer Composites 34 (9) , pp. 1525-1532 (2013)]
In addition to the microcapsules able to activate self-healing reaction based on ROMP reactions, microcapsules containing polymerizing agent able to activate click-chemistry reactions have been developed (see Figure 13) (see section 3.2.1,1.4 - “synthesis and encapsulation of trivalent azide and alkyne based healing agents” of WP2) (Polym. Chem., 2014,5, 992-1000 DOI: 10.1039/C3PY01151H)
Then the IASS members have selected the most promising components to be embedded in specific epoxy resins.
Following this approach self-healing systems characterized by high values in the healing efficiency have been developed (see Figure14 and Figure15) .
Different self-healing strategies and matrix compositions based on the “microencapsulation” concept have been developed and evaluated.
The experimental tests were carried out on different compositions (see Figure 16) with the intent to select the most appropriate composition of self-healing system for aeronautic vehicles.
Despite the very interesting values in the healing efficiency, it was found that the developed systems did not meet all the mechanical requirements suggested by AIRFRAME MANUFACTURERS for aeronautic materials. Of course these restrictions are due to the very high standards of aeronautic materials for primary structures.
In particular, the maximum value in the glass transition temperature was found to be between 100 °C and 115 °C with respect to the value of 180 °C suggested by ALENIA (see Figure 17).
These mechanical drawbacks were found due to:
i. The impossibility to use as hardeners aromatic primary amines (e.g. DDS) in combination with catalysts active in the ROMP. Limit: poor mechanical performance
ii. The impossibility to use curing cycles at high temperatures as those scheduled for aeronautic materials designed for primary structures. Limit: poor mechanical performance
iii. The impossibility to use the catalyst dispersed in the form of molecular complex in chemically very reactive environments, such as fluid epoxy mixtures containing reactive epoxy rings at high temperatures. Limit: cost
How to overcome these drawbacks?
Whithin the activities of feedback improvement, Mitigations Actions and Plans have been activated.
Mitigations Actions and Plans
To overcome the previous drawbacks THERE WERE only two possible choices !
First: to investigate on the possibility to design and synthetize a new highly chemically stable catalyst (of course not commercially available).
Second: to resort other planned strategies alternative to the “microencapsulation” concept (see scheme of Figure 18).
Of course the first choice had a wide appeal for members of IASS consortium, but it was also at very high level of risk.
The other strategies based on concepts alternative to the “microencapsulation” concept were already planned in the initial stage of the Project (DOW), but also in this case complex issues had to be taken into account. In fact, the real challenge to face, in this second case, is to invent and characterize novel self-healing systems developing new ideas, always keeping in mind the main challenge: to combine sufficient structural integrity with sufficient molecular dynamics. Then, the very critical issue, for these alternative mechanisms, was to apply “alternative” methods to thermosetting matrices which are characterized by a reduced mobility of the segments between the crosslinking points.
Not been sure on the way to choose, IASS members followed both roads.
Fortunately they succeeded in the design and realization of a new highly stable ruthenium catalyst able to activate self-healing reactions in presence of aromatic primary amines (as curing agents) and also after curing cycles at high temperatures (Italian Patent – Title Nuovo catalizzatore per reazioni di metatesi in ambienti ad elevata reattività - Data di deposito : 14.09.2015 Numero domanda : 102015000051271)
On the other hand, Self-healing mechanisms alternative to the “microencapsulation” concept have been developed.
In particular, these last mechanisms are based on attractive reversible Hydrogen bonding forces between the nanocages of POSS embedded in the resin (to increase flame resistance) and chemical groups of the epoxy precursors (the material has been designed, thanks to the nanotechnology strategy, with very small domains of the resin at higher chain mobility finely interpenetrated in the resin). The main results are summarized in Figure 19 and are related to the Self–Healing functionalities based on two different mechanisms.
Here below are summarized very important results of the performed activities:
1) Design based on the “Microencapsulation” Concept
The new ruthenium catalyst can be applied to activate self-healing mechanisms in polymeric materials or for the synthesis of many important petrochemicals by Ring-opening metathesis polymerization of cyclic polyolefin.
This synthetized catalyst is able to overcome critical points related to the application of ruthenium catalysts in the activation of ROMP reactions in thermosetting epoxy resins solidified with aromatic primary amines or mixture of hardeners containing the group -NH2.
A detailed description of the activities related the synthesis of the new catalyst is reported in the Periodic Report (see WP1 and in the test of the Patent).
Considerations on the performed synthesis
Synthesis and use of a new ruthenium catalyst stable in high reactive environments for self-healing applications and for the synthesis of many important petrochemicals.
To better understand this important result of the IASS activities, it is worth noting that the sensitivity of organometallic catalysts to oxygen, epoxy groups, amines, water or in general heteroatom functionalized substrates has often hampered their evolution from research laboratories to full-scale, on-line industrial processes.
In the specific efforts of the IASS members to develop ring-opening metathesis polymerization (ROMP) catalysts for self-healing mechanisms inside epoxy resins, this new stable catalyst for the ROMP reactions has been synthetized only in the last period of the Project, after many months of activities (there is no a commercial catalyst able to activate ROMP reaction in aeronautic resins for primary structures).
It rapidly polymerize cyclic olefins in highly active environments and in very drastic condition of temperature, reactivity and viscosity.
The synthetized catalyst is able to overcome critical points related to the application of ruthenium catalysts in the activation of ROMP reactions in thermosetting epoxy resins characterized by high values of glass transition temperature and storage modulus.
Furthermore, it can be used in the form of molecular complex inside epoxy mixtures allowing a strong reduction of the manufacturing costs. (To understand the reason of the higher costs of self-healing systems based on commercial catalyst see Appendix A – Section 1 of this Report)
It is stable at high temperature and in very reactive chemical environments. Its thermal stability allows curing temperatures of the epoxy mixtures at very high temperatures as those scheduled for the curing cycle of structural (aeronautical) resin (up to 180 °C). Furthermore, its chemical stability allows the use of aromatic primary amines (such as the DDS which is used as common hardener in aeronautical resins) without undergoing deactivation.
Evaluation of the catalytic activity of the new catalyst
The evaluation of the catalytic activity of this new catalyst has been reported in description of WP3 (see z-catalyst). Within WP1 the synthesis of this new catalyst has been optimized; related activities have been performed within the activities of WP3.
In particular, tests performed in this last WP, highlighted that the catalyst is active in the solid sample solidified using DDS (aromatic primary amines) and containing vessels filled with the healing agent. (The vessels were in the form of microcapsules containing a blend of ENB/DCPD)
Furthermore, the catalytic activity of this new catalyst is preserved in the resin even in the presence of microcapsules and after a curing cycle of 180°C for 3 h.
The cure degree, calculated by thermal analysis, of the sample corresponding to the self-healing resin is 94% as that of aeronautical resins. The FT/IR spectrum of the self-healing samples shows the absence of the two weak absorption bands (N-H stretching vibrations) – ( one at 3471 cm-1 and the other near 3373 cm-1). These bands represent, respectively, the “free” asymmetrical and symmetrical N-H stretching modes. The absence of these signals proves that the sample is almost completely cured as also evidenced by thermal analysis.
The new synthetized catalyst has proven to be active also inside an epoxy formulation hardened up to 200 °C.
Self-Healing functionality (other approaches)
Design based on Self-healing mechanisms alternative to the “microencapsulation” concept
Self-healing mechanisms alternative to the microencapsulation “concept” have been developed.
These last mechanisms are based on attractive reversible Hydrogen bonding forces between the nanocages of POSS embedded in the resin to increase flame resistance and groups of the epoxy precursors (see Figure 20).
These self-healing mechanisms have been integrated in multifunctional resins and Carbon Fiber Reinforced panels (CFRPs) have been manufactured using this multifunctional resin (see Figure 21).
High values in the electrical conductivity have been found together with good flame resistance properties and autorepair properties.
CFRPs (impregnated using the resin with all functionalities integrated) have been manufactured by Resin Film Infusion (RFI) using a non-usual technique to infuse a nano-filled resin into the carbon fiber dry preform (Panels – serie E) Responsible of the Processing: CIRA WP5 - Responsible of the requirements: ALENIA WP6). The processing of nanofilled carbon fiber reinforced panels is the same of those described in RSC Advances, 5(8), 6033-6042 (2015) - DOI:10.1039/C4RA12156B and Composites Part B: Engineering Volume 80, 8 June 2015, Pages 7-14 DOI:10.1016/j.compositesb.2015.05.025.
Several flat panels have been produced and tested. The manufactured panels have been tested with respect to all the integrated functionalities. The electrical conductivity was found to be about 2x104S/m in the direction parallel to the fibers, whereas a value between 3.0 and 4.0 S/m was found in the directional orthogonal to the fibers. These values are among the highest values reached until now for nanofilled resins impregnating carbon fibers. The panels also highlighted enhanced flame resistance properties (LOI ~ 56, Ignition time (s) ~ 81 etc.. - for other parameters see deliverable D6.3). Furthermore, due to the autorepair ability, a significant decrease in the fatigue crack growth rate by approximately 80 % was found (see WP6).
Synergic effect between the different functionalities have been designed and found (as expected).
It is worth nothing that the oxygen atoms of the GPOSS nanocages were expected to increase the attractive reversible forces based on hydrogen bonding interactions. This beneficial effect was expected increased (as found) in the presence of nanoparticles embedded in the epoxy matrix. In fact, in this last case, small domains of polymeric matrix at higher mobility are finely interpenetrated in the resin in the zones around the nanoparticles. The higher mobility of the resin is expected to favor the arrangement of hydrogen bonding interactions. In fact, in presence of electrical conductive functionality (formulation with 0.5 % of MWCNTs), the healing efficiency is between 45% and 50%. The value of the efficiency decreases in the resin without CNTs. In addition, the use of POSS in the CFRCs enhances the flame resistance properties.
The epoxy resin without CFs has a limiting oxygen index (LOI) measured according to standard ASTM 2863, which is 33%, if the resin comprises the silsesquioxanes compound (POSS) utilized in the multifunctional formulation (Panels –series E – for the composition - see Table 5 of Deliverable D6.2) or 40%, if the resin is obtainable by curing an epoxy precursor formulation with an hardening agent which is an aromatic organo-phosphorous compound. (Glass Transition temperature of about 250 °C and G' of about 2.5 GPa in the range 50°C - 210 °C) (see RSC Advances, 5, 10974-10986 (2015) DOI: 10.1039/C4RA11537F)
The mixture of GPOSS in the matrix (see Samples E1, E2 and E3 corresponding to 5wt% of GPOSS and 0.5wt% of MWCNTs) provides a multifunctional panels characterized by all the functionalities integrated in the panel (self-healing functionality see D6.1 high electrical conductivity and flame resistance). Electrical tests performed on this last panel (E series) also evidenced that the presence of the flame retardant (GPOSS) preserves its AC electrical stability.
Furthermore, it has been observed that the inclusion in the resin of CNTs does not lead to big differences in the curing kinetics behaviour with respect to the unfilled epoxy resin [E] (see 2nd periodic Report on WP3 - RSC Adv., 2015,5, 90437-90450 -DOI: 10.1039/C5RA14343H).
Self-Healing functionality (second approach)
Immobilization of the self-healing catalyst on the solid surfaces of the nanoparticles.
Different type nanofillers (CNT, graphene, clay, and POSS) have been functionalized by applying different synthetic methods. The newly prepared functionalized nanofillers fulfil one or more of the following requirements:
• Enable efficient dispersion of nanofillers during composite formulation.
• Exhibit specific functions and/or properties such as suitable electric conductivity, high thermal stability or auto-repair function.
• Enable further modification and attachment of the desired ruthenium- or copper- based catalysts, which are able to activate the self-healing processes either via ring opening metathesis polymerisation (ROMP) or via azide/alkyne “click” reaction.
For this purpose a series of functionalized nanofillers have been prepared and a detailed synthesis and characterization for the functionalized nanofillers has been discussed in Deliverable 1.3 1.6 2.3 and 3.4.
Examples of CNT functionalizations (Functionalization able to activate autorepair mechanisms)
Functionalization of CNTs with Ruthenium catalysts
For example, Functionalizations of Carbon nanotubes with catalysts active in the metathesis polymerization have been completed and tested in the selected formulations. In particular, Grubbs catalyst 2nd generation (G2) and Hoveyda-Grubbs catalyst 2nd generation (HG2) were covalently bonded on the wall of MWCNTs (see Figure 22).
Considering the activities also carried out during the first period, the following conclusions have been drawn: Grubbs catalysts bonded to the walls of MWCNTs (G1-MWCNTs and G2-MWCNTs) deactivate during the process of preparation of the self-healing epoxy mixtures. Hoveyda Grubbs catalysts bonded to the MWCNTS (HG1-MWCNTs and HG2-MWCNTs) are not deactivated during the process of mixture preparation at 90 °C, but they deactivate during the curing process at high temperature. They potentially may be used in other systems which can be solidified at lower temperature (less than 110) – also in nanofilled systems of PDCPD, PENB etc...
Carbon-Supported Copper Nanomaterials: Recyclable Catalysts for Huisgen [3+2] Cycloaddition Reactions
Highly disperse copper nanoparticles immobilized on carbon nanomaterials (CNMs; graphene/carbon nanotubes) were prepared and used as a recyclable and reusable catalyst to achieve CuI-catalyzed [3+2] cycloaddition click chemistry. Carbon nanomaterials with immobilized N-heterocyclic carbene (NHC)-Cu complexes prepared from an imidazolium-based carbene and CuI show excellent stability including high efficiency at low catalyst loading. The catalytic performance evaluated in solution and in bulk shows that both types of Cu-CNMs can function as an effective recyclable catalysts (more than 10cycles) for click reactions without decomposition and the use of external additives (Chemistry - A European Journal Volume 21, Issue 30, 1 July 2015, Pages 10763-10770- DOI: 10.1002/chem.201501217)
Functionalization of graphene oxide with Ruthenium catalyst
Graphene Oxide (GO) was prepared by chemical oxidation of high surface area graphite (G). GO was used to support ruthenium catalysts with the aim to activate self-healing reactions in multifunctional materials able to integrate simultaneously the healing reactions with the very interesting properties of graphene-based materials. Grubbs catalysts 1st (G1) and 2nd generation modified (G2o-tol), Hoveyda-Grubbs catalysts 1st (HG1) and 2nd generation (HG2) were covalently bonded to GO (see Figure 23) preserving the same catalytic activity of the catalysts not bonded to the graphene sheets. However, GO-G2o-tol and GO-G1 were found to deactivate during the process of preparation of the self-healing epoxy mixtures at 90°C. The self-healing activity of the various catalytic complexes was studied for both uncured and cured samples. Results showed that GO-HG1 and GO-HG2 were not deactivated and they were thus found to be able to trigger self-healing reactions based on the ROMP of 5-ethylidene-2-norbornene (ENB). This behavior is justified by the formation of the more stable formation of 16 electron Ru-complexes, as opposed to the 14 electron complexes of GO-G1 and GO-G2 catalysts ((Polymer (United Kingdom) Volume 69, 26 June 2015, Pages 330-342 DOI: 10.1016/j.polymer.2015.04.048).
Furthermore, an expected increase was detected in the storage modulus but not in the Tg. Also in this case, it was found that no aromatic amines could be used to solidify the epoxy matrix towards obtaining high mechanical performance. IASS members explored the protection of the catalytic sites (containing Ru on the graphene sheets) by polymerizing units of polymeric monomers around the ruthenium atoms, towards forming a globular shell around the catalyst sites (Proceedings of the 5th International EASN Association Workshop on Aerostructures. 2-4 september 2015, Manchester UK, pp 1-4). The system was able to allow high curing temperatures, however the open part of the shell (required to allow the catalytic activity) is low and this strongly influences the kinetics of the ROMP reactions inside cracks of the selected epoxy matrix. This is an important drawback, as structural applications can require self-healing operations to be active under extreme environmental conditions (for ex. very low temperatures) and that they do in a timely manner, so as to avoid crack propagation (when the entity of the propagation is relevant there is no healing of the material due to the significant distancing between the rigid crack faces).
ELECTRICAL CONDUCTIVITY FUNCTIONALITY (MAIN RESULTS)
As described in the previous section, the members of the IASS consortium have tried to increase the electrical conductivity of the epoxy formulation selected for this project by dispersing conductive nanofillers inside the matrix.
The effect of different carbon nanostructured fillers on the electrical, mechanical and thermal properties has been analyzed. The morphological organization of the samples (nanofilled resin and CFFRPs) was studied.
ACTIVITIES PERFORMED USING CARBON NANOFIBERS (CNFS)
Carbon nanofibers. Vapor-grown carbon nanofibers in the form of powders used in this study were produced at Applied Sciences Inc. and were from the Pyrograf III family. The pristine CNFs used in for the IASS Project are labeled as PR25XTPS1100 where XT indicates the debulked form of the PR25 family, PS indicates the grade produced by pyrolytically stripping the as-produced fiber to remove polyaromatic hydrocarbons from the fiber surface and 1100 was the temperature in the process production. The nanofibers have (a) an average bulk density of product (g cm−3) ranging from 0.0192 to 0.0480; (b) a nanofiber density (including hollow core) (g cm-3) from 1.4 to 1.6; (c) a nanofiber wall density (g cm-3) from 2.0 to 2.1; (d) an average catalyst (iron) content (ppm) < 14 000; (e) an average outer diameter (nm) from 125 to 150; (f) an average nner diameter (nm) from 50 to 70; (g) an average specific surface area, m2 g-1 from 65 to 75; (h) a total pore volume (cm3 g-1) of 0.140; (i) an average pore diameter (angstroms A° ) of 82.06 and lengths ranging from 50 to 100 µm. Sample PR25XTPS1100 was heat treated to 2500 °C to provide the best combination of mechanical and electrical properties, giving the sample the name PR25XTPS2500.
The heat treatment was performed in an atmosphere controlled batch furnace. Approximately 300 g of nanofibers were placed in a ceramic crucible for the heat treatment. The furnace was purged with nitrogen gas for 1 h prior to heating. The heating rate was 100 °C h-1 and the furnace was held at a temperature of 2500 °C for 1 h prior to cooling.
ROLE OF CARBON NANOFIBER DEFECTS ON THE ELECTRICAL AND MECHANICAL PROPERTIES OF CNF-BASED RESINS
Heat treatment of carbon nanofibers has proven to be an effective method in removing defects from carbon nanofibers, causing a strong increase in their structural perfection and thermal stability. It affects the bonding states of carbon atoms in the nanofiber structure and causes a significant transformation in the hybridization state of the bonded carbon atoms. Nanofilled resins made of heat-treated CNF show significant increases in their electrical conductivity even at low concentrations. This confirms that enhancement in the perfection of the fiber structure with consequent change in the morphological features plays a prominent role in affecting the electrical properties. Indeed heat-treated CNFs display a stiff structure and a smooth surface which tends to lower the thickness of the unavoidable insulating epoxy layer formed around the CNF which, in turn, plays a fundamental role in the electrical transport properties along the conducting clusters. This has been proven to be very beneficial in terms of electrical conductivity.
The Graphitization of carbon nanofibers also causes a significant increase in the oxidative stability enhancement (about of 200 °C). In particular, the investigation on the dc conductivity around the percolation threshold shows a significant difference between the two different nanofibers.
The lower percolation threshold and higher conductivity exhibited by the nanofilled resins based on heat-treated CNF can be justified on the basis of their stiffness and smoothness of surface graphitized CNFs which determines a lower thickness of the insulating epoxy layer around the fibers. This hypothesis has been also supported by FTIR analysis of untreated and heat-treated CNFs. FTIR data have shown that fewer chemical groups are attached on the wall of heat-treated CNFs. These groups, more numerous on the wall of as-received CNFs, are most probably responsible for covalent and/or non-covalent bonds such as intermolecular forces due to
hydrogen bonds. These stronger interactions should favor the mechanical reinforcement and, conversely, decrease the electrical conductivity. The morphological features and the chemical changes on the CNFs walls affect the electrical conductivity and the dynamic mechanical properties, causing a lower reinforcing effect in the storage modulus of the resin nanofilled with heat-treated CNFs than for the samples filled with untreated CNFs. The heat treatment at high temperature also causes an increase in the oxidative stability of the nanofillers and their
loaded resins. The oxidative stability of the reinforced resin tends to increase with increasing nanofiller percentage. A very interesting result observed for CNFs is that the value of the electrical conductivity of the resin filled with CNFs treated at 2500 °C is the higher value obtained for epoxy resins filled with a low percentage of CNTs. The values of electrical conductivity are only slightly different from those found for the same epoxy matrix filled with CNTs. However, it has to be
considered that CNFs/epoxy resins are obtained by an easier production process mainly in the step of nanofiller dispersion inside the epoxy liquid mixture, which is a very difficult step before the curing process.
The formulations loaded with CNFs show values of in the storage modulus between 6000 MPa and 2500 MPa in the temperature range between -90 °C and 80 °C. the value of the glass transition temperature is about 260°C (for the main transition).
ROLE OF CARBON NANOTUBES ON THE ELECTRICAL AND MECHANICAL PROPERTIES OF CNT-BASED RESINS
Using the most promising carbon nanotubes (MWCNTs), epoxy mixtures for application in the field of aeronautic materials have been developed. In particular, CNTs able to increase electrical conductivity of aeronautic resins at very low concentration of nanofiller have been selected between CNTs characterized by different morphological parameters and functionalization. Properties of the nanofilled resin important for the planned applications have been analyzed.
The epoxy matrix used for these activities was the formulation N°1. This formulation was prepared by mixing a tetrafunctional epoxy precursor with a reactive diluent which allows the moisture content to be reduced and facilitates the nanofiller dispersion step. The reactive diluent also proves to be beneficial for improving the curing degree of nanofilled epoxy mixtures. It increases the mobility of reactive groups resulting in a higher cure degree than the epoxy precursor alone. This effect is particularly advantageous for nanofilled resins where higher temperature treatments are needed, compared to the unfilled resin, to reach the same cure degree. As nanofiller, different carbon nanostructured fiber-shaped fillers are embedded in the epoxy matrix with the aim of improving the electrical properties of the resin. The results highlight a strong influence of the nanofiller nature on the electrical properties especially in terms of electrical percolation threshold (EPT) and electrical conductivity beyond the EPT. Among the analyzed nanofillers, the highest electrical conductivity is obtained by using multiwalled carbon nanotubes (MWCNTs) and heat-treated carbon nanofibers (CNFs). The achieved results are analyzed by considering the nanofiller morphological parameters and characteristics with respect to the impact on their dispersion effectiveness.
The presence of reactive diluent 1,4-butandioldiglycidylether and particular composition of the resin in the epoxy mixture reduces the sorption at equilibrium of liquid water (Ceq) of about 35%. This percentage is very relevant for epoxy mixtures to apply in the aeronautics because absorbed moisture reduces the matrix-dominated mechanical properties. Absorbed moisture also causes the matrix to swell. This swelling relieves locked-in thermal strains from elevated temperature curing.
These strains can be large and large panels, fixed at their edges, can buckle due to the swelling strains. In addition, during freeze–thaw cycles, the absorbed moisture expands during freezing and can crack the matrix. In addition, during thermal spikes, absorbed moisture can turn to steam. When the internal steam pressure exceeds the flatwise tensile strength of the composite, the laminate will delaminate. The reduction in the water absorption was also found for nanofilled epoxy mixtures formulated to increase the electrical conductivity. The presence of the reactive diluent allows to reach higher curing degree compared to the epoxy precursor alone providing an efficient strategy for energy-saving. The morphological feature of the nanofillers has proven to play a relevant role in determining the electrical properties of the analyzed nanofilled resins. The composites obtained with CNTs are characterized by the lowest value, among all considered systems, of the percolation threshold and by a dc conductivity of the same order of magnitude of heat treated CNFs.
The incorporation of a small concentration of MWCNTs(0.32%) in the temperature range of
-60/180 °C causes an increase in the elastic modulus value with respect to the epoxy matrix alone.
The analyzed nanofilled formulations show values of in the storage modulus between 3600 MPa and 2000 MPa in the temperature range between -60 °C and 80 °C. the value of the glass transition temperature is about 240°C (for the main transition).
ACTIVITIES PERFORMED USING GRAPHENE-BASED NANOPARTICLES
Two samples of partially exfoliated graphite (pEG) and carboxylated partially exfoliated graphite (CpEG), differing for the content of carboxylated groups, were prepared with the aim to achieve consistent comprehension about the properties of resins filled with graphene-based nanoparticles.
Graphene-based nanoparticles pEG and CpEG are characterized by two different degree of amorphous phase (Xa): 56% (pEG) and 60% (CpEG).
The elementary analysis of the graphitic samples highlighted an oxygen content of 0,4 wt% for the sample pEG and 8,5 wt% for the sample CpEG. The sample pEG and CpEG were prepared as follows: a mixture containing nitric and sulphuric acid and natural graphite was used. After 24 h of reaction, intercalation within graphene sheets took place to form intercalated graphite compound. Then the mixture was filtered, washed with water, and dried in an oven at low temperatures. The intercalated graphite compound was subjected to sudden heat treatment temperature of 900 °C and rapid expansion then occurred. The expansion ratio was as high as 300 times. Changes in the degree of exfoliation was obtained by varying the resident time in the fluidized bed as the time increases, the trapped intercalate and/or gases would have a second the chance to escape causing further expansion and exfoliation. The considered filler has a two dimensional (2D) predominant shape and it is obtained with an exfoliation procedure from natural graphite, that leads to obtain 2D conductive particles with an average diameter of 500 µm.
The epoxy matrix was prepared by mixing a tetrafunctional precursor with a reactive diluent (Formulation N° 1).
All the mixtures were cured by a two-stage curing cycles: a first isothermal stage was carried out at the lower temperature of 125°C for 1 hour and the second isothermal stage at higher temperatures up to 200°C for 3 hours.
The performed research activities highlighted that the degree of graphite exfoliation and edge-carboxylated layers can be controlled and balanced to design lightweight materials characterized by both low electrical percolation thresholds (EPT) and improved mechanical properties. So far, this challenging task was undoubtedly very hard to achieve. The results performed during the activities of the IASS project highlight that the effect of exfoliation degree and the role of edge-carboxylated graphite layers is beneficial to originate self-assembly structures embedded in the polymeric matrix. Graphene layers inside the matrix may serve as building blocks of complex systems that could outperform the host matrix. Improvements in electrical percolation and mechanical performances has been obtained by a synergic effect due to finely balancing the degree of exfoliation and the chemistry of graphene edges which favors the interfacial interaction between polymer and carbon layers. In particular, for epoxy-based resins including two partially exfoliated graphite samples, differing essentially for the content of carboxylated groups, the percolation threshold reduces from 3wt% down to 0.3 wt%, as the carboxylated group content increases up to 10 wt%. Edge-carboxylated nanosheets also increase the nanofiller/epoxy matrix interaction, determining a relevant reinforcement in the elastic modulus.
In conclusion, the surface chemistry of thin graphitic edges can be tuned at nanoscale level, to pave the way towards an effective strategy to overcome drawbacks related to the application of graphene-based materials.
A sound choice of the nanofiller nature allows to drive the changes in the nanocomposite properties towards the set goals. In this investigation, it has been found that the functionalization help in providing a better polymer filler interface. This interface had help in enhancing both electrical and mechanical properties. These tangible results were obtained because of strong interactions between nanofiller and epoxy matrix and self-assembly structures in aeronautic matrices. Work is still in progress, but a giant step towards understanding the electrical and mechanical behaviour of in-bulk resins filled with bi-dimensional shaped carbon forms was taken (RSC Adv., 2015, 5, 36969. DOI: 10.1039/c5ra04558d).
Considerations on the resins filled with graphene based nanoparticles (comparison with unidimensional filler)
During the activities performed in IASS, the current status of epoxy/graphene nanocomposites was analyzed with the aim to find effective strategies to transfer some of excellent physical properties of graphene layers to epoxy matrices. The evaluation of edge structures of very similar graphene-based materials, differing only for the concentration of carboxylated group on the edges, highlights that the effect on the electrical and mechanical properties is impressive. The chemistry of graphene edges strongly affects the physical properties of the resin where these nanoparticles are embedded. Integrate characterizations techniques (RX, Micro-Raman, FTIR, TGA, SEM) have been applied to facilitate our understanding of graphene-based materials and their development towards applications.
In particular, with samples constituted by roughly 50% of graphite with a correlation length perpendicular to the structural layers ranging between 12 nm e 9.8 nm and 50% of exfoliated graphene, for standard epoxy composites, electrical percolation threshold can be achieved with nanofiller concentration less than 0.5% by weight. This value of nanofiller concentration allows to reach dc electrical conductivity raging between 1-2 S/m. The good electrical and mechanical performance was ascribed to self-assembly mechanisms determined by attractive interactions between edge-carboxylated graphene particles. This hypothesis is reflected in the different electrical behavior of Epoxy based composites including two partially exfoliated graphite samples, differing essentially only for the content of carboxylated groups. These two samples have shown an electrical percolation threshold which decrease from 3wt% down to 0.3 wt%, as the carboxylated group content increases up to 10 wt%. A self-assembly of layers due to the attractive interactions between edge-carboxylated graphene particles was found to favor the electrical percolative paths. Edge-carboxylation also increases the nanofiller/epoxy matrix interaction determining a relevant reinforcement in the elastic modulus. This controllable feature of the nanoparticles can be an alternative parameter to design epoxy resins where high electrical conductivity is required at very low filler concentration. A very important result of this study is that the values detected for the electrical parameters were found very similar to those obtained with mono-dimensional shaped nanofillers in the same epoxy matrix, where the electrical properties found for the nanocomposites loaded with CpEG samples, are compared to those achieved for composites based on the same epoxy matrix and employing other typical conductive fillers, i.e. multi-walled nanotubes, (MWCNT), pristine (CNF1100) and heat-treated (CNF2500) carbon nanofibers as shown in the histogram.
The electrical percolation threshold for the CpEG samples is the lowest among all the different nanofillers adopted with this type of epoxy resin. In general, the electrical conductivity is very similar for heat-treated CNFs and MWCNTs. In particular, for the weight percentage of 0.32 wt%, the epoxy resin filled with CpEG presents a slightly higher electrical conductivity than that obtained with the other fillers. Furthermore, a filler concentration of only 0.5% wt of CpEG is sufficient to reach values of electrical conductivity comparable with those obtained by using nanotubes or heat-treated fibers with a higher content, i.e. 0.64% wt. In the explored range of filler loading, the higher value for the conductivity is achieved for the epoxy mixture filled by CNF2500 (at 1.30 %wt).
Modelling and Design (Main Results)
Although considerable efforts have been dedicated to the study of nanocomposites reinforced with different carbon nanoparticles, many aspects concerning the relations linking their electrical properties with their morphological, structural and physical characteristics remain to be clarified. To reach this goal, within IASS activities, interdisciplinary research activities have been carried out.
In order to achieve an effective design of the composites for aeronautic applications, the experimental electrical characterizations have been supported by a massive work of simulation activities.
In particular, it has been developed a suitable 3D model able to explore the relation between the structural and morphological properties of the composites and the conduction and polarization mechanisms. Some physical constrains like the particle impenetrability, the minimum distance between two neighbour particles greater than the Van der Waals separation (0.34 nm) and total containment in the representative cell are ensured in order to have a model as realistic as possible. The reliability of the proposed model was validated by comparison with experimental data and theoretical studies.
During the first period of the IASS project the modelling activity has been focused on 1-dimensional filler such as CNTs and CNFs in order to predict, to support and to achieve more details concerning the conduction mechanisms and the electrical properties experimentally observed for epoxy resins reinforced with the above described nanoparticles.
The adoption of the 3D model has allowed the investigation of the role of several factors, such as electron tunneling and energy barrier in determining the final performance of the composite. Furthermore, the introduction in the model of the capacitive effects exhibited by the material, usually not considered in other simulations approaches, has allowed to study the composite behaviour also in the frequency domain, a feature which is relevant for applications as EMC and EMI shielding.
As already discussed in the previous section on the mono-dimensional conductive filler, the experimental characterization has revealed that the graphitization of CNFs by heat-treatment at 2500 °C may be an effective method in order to reduce the structural defects of the fibers and to obtain filler characterized by an higher aspect ratio with respect to as-received one. As result of the different interaction with the matrix, the resulting nanocomposites are characterized by higher electrical conductivity and a lower electrical percolation threshold (EPT), thus overcoming the well known drawbacks of insulating epoxy resin commonly used in the aeronautic field. The experimental results are confirmed by simulation studies carried out with the developed model focused on the role of the filler aspect ratio on the electrical properties of the nanocomposites.
The promising perspectives offered by graphene-based nanocomposites puts new requirements on the material modelling until today devoted to composites filled by one-dimensional carbon particles and their applications. Therefore, during the IASS project, the 3D model has been improved and extended in order to reproduce morphological structures and to investigate the electrical behaviour of composites reinforced with bi-dimensional shaped forms such as graphene-based particles.
As described in the section on the electrical properties of the resin filled with graphene-based nanoparticles, experimental studies have been carried out to prepare and characterize epoxy/amine-based composites filled with two types of exfoliated graphite particles, i.e. partially exfoliated graphite (pEG) and carboxylated partially exfoliated graphite (CpEG) that differing in the exfoliation degree (56% and 60%, respectively) and hence for the content of carboxylate groups. The incorporation of CpEG results in a sharp insulator-to-conductor transition with a EPT typically achieved when using mono-dimensional carbon based fillers whereas for the pEG-based composites ranging in the wider interval. Therefore, the CpEG reinforcements are capable to create the percolation conductive pathways more easily and at lower filler amounts through the resin compared with pEG particles.
The simulation studies support the idea that this may be due to higher concentration of carboxylated groups detected at the edge of graphene sheets. These functional groups affect the compatibility filler/resin by attractive intermolecular bonding between the dispersed particles favoring the formation of the electrical networks with a sort of self-assembled structures.
INFLUENCE OF CARBON NANOFILLERS ON THE CURING KINETICS OF EPOXY-AMINE RESIN
The cure kinetics of an epoxy resin based on the tetrafunctional epoxy precursor N,N0-tetraglycidyl
methylene dianiline-(TGMDA) hardened with 4,4-diaminodiphenyl sulfone (Formulation 1) has been investigated. The influence of carbon nanofillers (carbon nanotubes, carbon nanofibers, and graphene based nanoparticles) on the cure kinetic was studied. Kinetic analysis was performed by dynamic and isothermal differential scanning calorimetry (DSC). In dynamic experiments, the activation energy was computed using an advanced isoconversional method while under isothermal conditions, the Kamal’s model of diffusion control was applied to simulate the systems throughout the curing process. The isothermal analysis showed that the introduction of the diluent decreases, particularly the activation energy of secondary amine-epoxy reaction. A similar effect was obtained by the dynamic DSC analysis that shows a decrease in the activation energy for a > 0.7 a value of conversion for which it is considered that the reaction of secondary amines is active. The inclusion in the resin of one-dimensional fillers does not lead to big differences in the curing kinetics behaviour with respect to the raw epoxy. An increase in the activation energy is found in the case of highly exfoliated graphite. This is likely due to a reduction of free molecular segments of the epoxy network trapped inside self-assembly structures (RSC Adv., 2015, 5, 90437 DOI: 10.1039/c5ra14343h).
FLAME RESISTANCE FUNCTIONALITY
The epoxy matrix T20B (Formulation N° 1 was used to prepare the flame-resistant formulation.
Four different POSS compounds were dispersed in the epoxy matrix: GPOSS, TCPOSS and ECPOSS functionalized with a different number of oxirane rings, and DPHPOSS functionalized with phenyl groups. POSS/epoxy composites were prepared with 5 wt% of POSS.
The curing agent used for the curing was 4,4’-diaminodiphenyl sulfone (DDS). This hardener agent was added at a stoichiometric concentration with respect to all the epoxy rings (TGMDA, BDE and POSS – in the case of POSS with epoxy rings).
Flame retardation of the epoxy resin containing the reactive diluent was also evaluated for the formulation solidified with BAMPO and BAPPO.
BAMPO and BAPPO were synthetized, the synthesis procedure of these two hardeners has been described in the first Periodic Report while TGMDA, BDE, DDS were obtained from Sigma-Aldrich, and POSS compounds from Hybrid Plastics Company.
Within the activities of the IASS Project, multifunctional epoxy resins characterized by improved flame resistance incorporating electro-conductive nanofillers have been developed. DC conductivity (S/m) values of the formulated resins range between 3.5×10-3 and 1.68 x 10-1 S/m. LOI (% O2) values range between 30.2 and 40 and PHRR (kW/m2) range between 293 and 629. The nanofilled samples behave as multifunctional systems to increase flame resistance and electrical conductivity.
The incorporation of 5% of POSS into T20BD epoxy resins has proven to be beneficial for improving its flame retardancy. The most promising POSS compounds are GPOSS and DPHPOSS. (GPOSS was found more suitable of DPHPOSS to manufacture panels due to its liquid consistence able to lower the viscosity of the formulation) Data on the dispersion of the analyzed POSS within the epoxy mixture T20B show that the structure of the POSS compound plays an important role on the dissolution/dispersion of these compound into the matrix. GPOSS was solubilized in the matrix using two steps: ultrasonication at 90°C and magnetic stirring in oil bath at 120°C for 1h.
The chosen procedure allows a good level of dissolution into the initial liquid epoxy mixture. This result is most probably due to the structure of GPOSS that is fully epoxidized with glycidyl groups which makes compatible the POSS molecule with epoxy precursors and reactive diluent. In addition, its structure allows the reaction and inclusion into the T20BD network formation during the curing cycle. This could explain the better fire behavior of GPOSS compared to the other analyzed POSS.
Another relevant result of the activities performed by IASS members regards the fire behavior of DPHPOSS which leads to epoxy system fire enhancement thanks to its aromatic pendant groups although it does not solubilize in the initial epoxy precursors. In fact, homogeneous dispersions of very small aggregates of DPHPOSS are achieved thanks to ultrasonication.
As described before, BAMPO and BAPPO have been synthesized by IASS members (UNISA) and used as curing agent for the epoxy system based on the TGMDA. The obtained formulations have been characterized and the fire properties have been studied. The results show that the synthesized phosphorus based hardeners are more efficient than DDS to increase epoxy system LOI. The PHRR of the epoxy system decreases when BAMPO or BAPPO are used in comparison to DDS. Moreover, BAMPO and BAPPO lead to important intumescence of the systems when compared to DDS based system.
Multifunctional epoxy resins characterized by improved flame resistance were modified by incorporating electro-conductive nanofillers (CNTs). DC conductivity (S/m) values of the multifunctional resins are found to range between 3.5×10-3 and 1.68 x 10-1. LOI (% O2) values range between 30 and 38 and PHRR values (kW/m2) range between 293 and 753. The best compromise of performance of the multifunctional composites is obtained for the nanofilled sample solidified with BAMPO. More precisely, the formulation (T20B+BAMPO+0.5%CNT(3100) exhibits a DC conductivity of 1.68 x 10-1 S/m. The lower value of PHRR (293 kW/m2) is obtained for the multifunctional nanocomposite T20BD+5%GPOSS+0.5%CNT(3100). For this last formulation the DC conductivity is 3.5 x 10-3 S/m whereas it is 8.00 x 10-13 S/m for the unfilled formulation confirming the successful obtainment of a multifunctional formulation.
CFRPs – Flame Properties
The most promising flame-resistant epoxy formulations have been used to manufacture carbon fiber reinforced panels (CFRPs).
Electrical Properties of the Panels manufactured using the flame-resistant resin are detailed described in the deliverable D6.2 and in the second Periodic Report, whereas the properties related to the flame resistance of the manufactured Panels are described in the deliverable D6.3. Concerning mass loss calorimetry testing, for TB20D based panels, the presence of carbon fibers increases the time of ignition from 38 to 44s in comparison to their non-reinforced equivalents and from 47 to 81s for TB20D system modified with 0.5% of CNT and 5% of GPOSS. This increase can be explained by the replacement of around 80% combustible material by carbon fibers as they are known to be relatively inert in pyrolysis conditions. In addition, it is also known and was already reported in the scientific literature that the heat conductivity and the heat absorption capacity of the composites are drastically changed in the presence of a large amount of fibers. This phenomenon can also contribute to the increase of time of ignition observed for the composites. It is interesting to note that the presence of fibers also allows to reducing the time of flame out of the composites in comparison to their non-reinforced equivalents. This effect can also be explained by the decrease of combustible material available in the composite. Interestingly enough, the time of flame out of the composites based on TB20D epoxy matrix modified with 0.5% of CNT and 5% of GPOSS appears to last longer than the time of flame out of the composite made out of pristine TB20D epoxy matrix. This trend is similar to the one of the corresponding non-reinforced matrices. It can be explained by the modification of heat conductivity of TB20D system brought by well-dispersed nanoparticles. In addition, the peak of heat release rate (PHRR) and the total heat release (THR) obtained using an irradiance of 50kW.m-2 are found to be strongly influenced and lowered very significantly in the presence of carbon fibers when compared to their corresponding epoxy matrices. This result can be explained by the fact that the composites contain much less combustible material than the matrix (about 5 times less).
Here below are described the main results related to the Manufacturing Processes
Multifunctional CFRP Manufacturing Process- Development and Optimization
The challenge proposed in the IASS Project was to find a practicable way to manufacture (WP4,WP5,WP6) a real CFRP (industrial methods) with different multifunctionalities (self-healing capability, flame resistance properties and high electrical conductivity) using materials selected and assessed in the other WPs (WP1,WP2 and WP3).
Based on infusion and prepreg techniques, several manufacturing experiments have been carried out to define the best manufacturing process and the related parameters.
Several flat panels have been produced and tested.
Two final demonstrators have been produced: a flat panel with three T shape stringers, manufactured by infusion, and a curved panel with two omega shape stringers, manufactured by ad hoc prepreg.
For the infusion process, the best results have been obtained by the combination of thick wet film and infusion under vacuum bag in autoclave.
This produces a sufficient quality of CFRP, with a satisfying resin content control, a low filtering effect and limited cost.
Moreover, the possibility to impregnate fibers before or during deposition/winding (wet Filament Winding) using a specific robotic device, has been investigated. A simple cylinder has been manufactured.
Mixing techniques final assessment and batch production
Two different formulations have been selected through a trade-off assessment of the best parameters, in terms of mechanical, rheological properties and costs, aiming at Infusion Process assessment and coupons production.
The first formulation selected was aimed at the improvement of electrical conductivity; the second formulation at the improvement of electrical conductivity, flame retardancy and auto-repair function (mechanisms alternative to the microencapsulation concept) in a CFRP.
Formulation N 2 (without hardener agents based on aromatic primary amines) was selected for self-healing panels based on the microencapsulation concept. This formulation was selected in the first period of the project – when the new active catalyst active in the resins cured with DDS was not yet available). The selected components were:
DGEBA Epikote 828, BDE Grilonit RV 1806 or Heloxy BD, microspheres synthetized by MLU and UNISA.
Infusion Process final assessment and coupons production and infused panels
Four trials have been performed to assess the procedure and the parameters for the production of panels and coupons.
After that, 13 panels have been manufactured using the formulation 1 (see Table 1 and Figure 24), with several dimensions and thicknesses .
The adopted procedure was based on the use of a thick wet film infusion , combined with an external pressure in autoclave.
The quality of manufacturing has been proved by microscope inspection and mechanical tests (see Figure 25 and Figure 26))
The process has been further modified to define the procedure to disperse microcapsules into a carbon fiber laminate with formulation 2 (see Figure 27).
Simple panels have been produced with and without microcapsules (see Figure 28).
Infrared spectra were performed at room temperature by using a Bruker Vertex 70 FTIR spectrophotometer with a 2 cm-1 resolution (64 scans collected) to control the activation of the self-healing functionality in the panels.
The SEM images of the microcapsules used to manufacture the Panels (UNISA, MLU) have been detailed described in the deliverables related to the self-healing functionalities (see Figure 29)
The histogram of diameter of microcapsule used for the manufactured the first self-healing panel (Panel SH1) indicates a mean diameter of 1.5 µm (see Figure 30).
Morphological investigation on HG1 catalyst was carried out through FESEM to evaluate the size of catalysts’ particles and thus the suitability of the powders in the self-healing carbon fiber reinforced composites. In order to reduce the effects of infiltration through the preform, ensuring a more uniform distribution through the panel thickness, the catalyst powders were pulverized into particles of smaller size (directly from solid state), by mechanical agitation through the use of a small magnet. After treatment, the size of the particles have been found significantly more homogeneous, and only few catalysts’ particles have larger size (see Figure 31).
The healing efficiency of the manufactured panels was evaluated using FT/IR investigation (see Figure 37 of the Appendix section). FT/IR investigation has provided evidence that the embedded catalyst is active also after processing conditions (curing cycle and pressure) of the CFRC panel. Curing cycle of the self-healing panels and pressure applied during the impregnation process is shown in in Figure 38 of the Appendix section . Optical pictures of coupons extracted from self-healing panels are shown in Figure 39.
Evaluation of self-healing efficiency by means morphological analysis (AFM)
Atomic Force Microscopy is a local and surface analysis technique which was used as an important tool to demonstrate the healing efficiency of manufactured panels.
Figure 40 shows AFM images of fracture surface of the self-healing panel where various healed micro-cracks in the form of thin strands are observable (see white arrows in the two AFM phase pictures on the right), indicating the formation of the metathesis product and therefore catalyst activity, which infers that self-healing process is successfully achieved.
This result clearly suggests that there is real potential for using self-healing epoxy resin in structural fiber reinforced composites in order to heal impact damage.
The incorporation of self-healing technology in fiber reinforced composite materials will seriously contribute to improve the reliability of these materials for structural applications by making them more damage resistant. The results shown in the present investigation are very encouraging. They constitute a solid basis for bringing this new technology to the self-healable fiber reinforced resins for aerospace applications especially using the new synthetized catalyst (Z-catalyst) which will allow to use the formulation N°1 for the resin impregnating the carbon fibers.
Prepregging Process assessment and batch production
Formulation 1 resin has been used to verify the possible fabrication of prepreg by means of an industrial setup (see Figure 32).
The production has been very difficult and limited to a small quantity, even if the quality of prepreg obtained was acceptable for the project aims.
The impregnation during the deposition of fibers was approached by using a robotic device, with an automated head for the wet winding of high viscous resin (see Figure 33).
A cylinder has been wounded (see Figure 34). Its quality is comparable with that obtained for the
A cylinder has been wounded. Its quality is comparable with that obtained for the Infused panels.
But the production has been very difficult due to very high viscosity and limited pot life of nanocharged resin (this resin has been loaded with DPHPOSS before the impregnation).
Final demonstrators production
Two final demonstrators have been produced.
A flat panel with three T shape stringers, manufactured by wet thick film infusion.
The tool used is based on a combination of a metal rigid base and an expansion rubber tool, to facilitate the deposition of resin and fiber preform (see Figure 35).
A curved panel with two Ω shape stringers, has been manufactured by ad hoc prepregs (Carbures) (see Figure 36).
Mixing techniques assessment and batch production
The proposed procedures worked properly for the batch production.
No major obstacles appear to scale up the processes toward industrial application.
Infusion Process assessment and coupons production
The proposed procedure, based on the use of a thick wet film infusion combined with an external pressure in autoclave, has demonstrated a satisfying efficacy in the production of panels with high viscosity nanocharged resins (with respect to the suggested values in literature).
A wide number of flat panels and coupons has been produced to assess the process parameters. Finally, a complex panel with, T shape stringers, has been manufactured.
The process appears viable for the production of real products. Dimensions of panel can be scaled. Several off the shelf fabrics can be applied.
As usual, to increase the Technology Readiness Level, a massive production of coupons has to be implemented to assess the allowables for each selected combination of fiber and resin. But non major obstacles appear to pave the way.
Self-healing Panels based on the “microencapsulation” concept.
For microcapsule charged resins, aimed at self-healing, the limited experience has confirmed the necessity to use alternative approach to thick wet film infusion, which seems suitable only for thin laminates (2-3 layers only). The interaction between the microcapsules, the fibers and the resin is too strong to let the resin flow.
The unique suitable strategy in the project appeared to be based on manual dispersion of capsules and resin, layer by layer. The obtained quality is not sufficient enough to proceed towards real products at this stage.
Prepregging - Process assessment and batch production
With the selected formulations the production has been very difficult and limited to a small quantity, within a very limited shelf life after the production of prepreg.
Major ostacles are still evident towards a scaled application to real products, even if the quality of prepreg obtained in the project was acceptable for the specific aims of laboratory demonstration.
Wet Winding Process assessment
The production has been very difficult with the formulation used (containing DPHPOSS), due to very high viscosity and limited pot life of nanocharged resin.
With the proposed formulation, the scale up of the process appears not feasible, due to limited pot life. Different formulation developed in IASS could lead better results.
Final demonstrators production
The expected manufacturing demonstrators have been produced, with a sufficient quality.
The infusion approach seems to be viable, through a better control of resin quantities.
The prepreg approach seems to be too difficult, at the moment.
To proceed on, the number of produced test articles has to be increased, defining a benchmarking/testing campaign.
The results on the panels characterization are described in WP6
The list of the main results obtained within the different WPs is reported below:
Development of multifunctional epoxy resin (WP1, WP2, WP3)
Self–Healing functionality, improved mechanical performance, increased electrical conductivity and improved flame resistance (with respect to current aeronautic resins) are integrated in the same multifunctional resin for structural application.
Development of multifunctional CFRPs (WP1, WP2, WP3,WP4, WP5,WP6, WP7)
Multifunctional Panels manufactured using multifunctional resins have been produced. Synergic effects between the different functionalities have been obtained.
Structural materials with increased fire resistance (WP1, WP2, WP3)
A number of multifunctional epoxy mixtures have been developed, exhibiting increased fire resistance compared to mixtures currently used in the Aeronautical.
Dispersion method (WP3,WP4)
Dispersion method used to realize the premix/masterbatch have been analyzed to manufacture the panels.
Possibility to do the up-scaling of the dispersion method.
Structural material with increased fatigue delamination resistance (WP1, WP5, WP6)
Panels manufactured with multifunctional epoxy mixture have been developed, exhibiting increased delamination fatigue resistance compared to the Panels manufactured with unfunctionalized mixtures.
Optimized dispersion in resin material (WP1, WP2, WP3)
Surface modifications of nanofillers (CNT, Graphene, POSS) enable optimized dispersion in resin material
Autonomous, room temperature based self-healing systems (WP2)
Development of autonomous, room temperature based self-healing systems using ROMP or CuAAC concepts
Stabile dispersion during process engineering (WP2)
Reinforcement of nanocontainer surfaces to enable stabile dispersion during process engineering
Encapsulation technology (WP1,WP2)
Encapsulation technology of DCPD as well as multivalent alkynes and azides
Catalytically active nanofillers (WP1,WP2)
Preparation of catalytically active nanofillers (CNTs, Graphene, POSS) for ROMP and CuAAC (WP2)
Epoxy resin with low humidity content (Patent EP2873682 (A1) ― 2015-05-20) (WP1,WP6)
Epoxy resin based on the tetrafunctional epoxy precursor TGMDA characterized by values in water sorption in the range 3,0 - 4,8 %. The resin reduces the value in Ceq from a minimum of 15% at a maximum of 30%.
Epoxy resin with low humidity content and high electrical conductivity (Patent EP2873682 (A1) ― 2015-05-20) (WP1, WP6)
Development of nanofilled epoxy formulations able to combine advantages in the water transport properties with the better mechanical and electrical performance of the nanofilled formulations. The developed formulation are characterized by values in the electrical conductivity of 1,73E-01 S/m for a concentration of 0.5 wt% of MWCNTs, 3,08E-01 S/m for a concentration of 0.5 wt% of carboxylated partially exfoliated graphite and 1,80E-01 S/m for a concentration of 0.5 wt% of CNFs. The nanofilled formulations can be tailored to be highly flame resistant.
Multifunctional Epoxy resin with enhanced flame resistance (Patent EP2883896 (A1) ― 2015-06-17)
(WP1,WP3, WP5, WP6)
Multifunctional epoxy resins with improved electrical conductivity and flame resistance. The formulation (T20B + BAMPO + 0.5% CNT) is characterized by a value of dc conductivity of 0.2S/m a limiting oxygen index (LOI) measured according to standard ASTM 2863, which is 30%, if the resin comprises at least a silsesquioxane compound (POSS), or 38%, if the resin is obtainable by curing an epoxy precursor formulation with an hardening agent which is an aromatic organo-phosphorous compound. (Glass Transition temperature of about 250 °C and G' of about 2.5 GPa in the range 50°C - 210 °C). This resin can be used to manufacture CFRPs.
Synthesis of new highly stable ruthenium catalysts for ROMP reactions (self-healing applications and in general synthesis of many important petrochemicals by Ring-opening metathesis polymerization of cycloalkenes (WP1,WP6)
The thermal stability and chemical inertness towards the epoxy groups of these synthetized catalysts allows to use the catalyst dissolved in the resin at the molecular level and to cure the resin at the temperatures required by aeronautical industries. This will enable a considerable saving on the amount of catalyst to be used in the production of composite materials in aeronautics with self-healing capacity. This result can be exploited in all fields where the Ring opening metathesis reaction (ROMP) is supposed to be carried out in strongly interactive environments and drastic conditions of temperatures (the new catalyst is stable up to 180 °C in contact to oxirane rings also at very low concentration - molecular solubility of the catalyst).
Development and scaling up of new hardener agents able to increase flame resistance of unfilled and nanofilled formulations (WP1,WP3).
Two flame retardants have been successful
synthesized: the Bis (3-aminophenyl) phenylphosphine oxide (BAPPO) and the Bis
(3-aminophenyl) methylphosphine oxide (BAMPO) and used as hardener agents. The scaling up at the intermediate level was optimized. Synergistic effects can be obtained to increase LOI and ignition time of epoxy resin using non-covalent modification of CNT functionalized with poly(dimethylsiloxane) (PDMS) and the new hardener agent Bampo.
Development of new components : new nanofillers (WP1,WP3)
A) Development of new non covalent functionalizations able to assure good levels of nanofiller dispersion preserving the electrical conductivity of the unfunctionalized nanofiller.
B) Development of graphene-based nanofillers able to enhance electical conductivity and mechanical performance of epoxy resins.
Electromagnetic modeling of 1D filled nanocomposites (WP7)
A morphological model of the nanocomposites filled with 1D conductive particles has been developed which is capable of reproducing the electromagnetic behaviour of the material both with DC and AC excitation. The model accounts for the mechanisms governing the electrical conduction in multiphase materials involving 1D nanocarbon particles and can be used for the design of tailored material characteristics.
Graphitization as improvement of the effective AR (WP7)
Simulation activity based on the developed model provides results in accordance with experimental findings. Simulations allows to study the role of the filler aspect ratio on the electromagnetic properties of the composites . For example, different values of the aspect ratio associated to the graphitization of the filler particles (due to heat treatment) can be easily considered to optimize the EM behaviour of the nanocomposites.
Multifunctional CFRP reinforced with CNT (WP4, WP5)
CFRPs of 0.5mx0.5m reinforced with multifunctional epoxy-resin and filled with CNT have been realized.
The difference on the electrical performance of carbon fiber reinforced composites (CFRCs) when two different Resin Film Infusion (RFI) manufacturing techniques are used has been investigated. For the panels obtained by bulk infusion the measured in plane and out of plane electrical conductivities were 2.0 x 104 S/m and 3.9 S/m respectively and for the panel prepared using the traditional resin film infusion the values were 1.1 x104 S/m and 1.7 S/m respectively. Morphological investigations on the sections of etched panels have highlighted that this difference in the electrical conductivity was strictly related to the different distribution of multiwall carbon nanotubes (MWCNTs) between the carbon fibers (CFs) of the plies.
High electrical conductivity in the out-of-plane direction for CFRPs reinforced with CNT (WP5, WP6)
Carbon Fiber Reinforced Panels developed inside the IASS project show the highest value of the out-of-plane electrical conductivity with respect to similar results available in literature
CFRPs as "resistive" devices in the out-of-plane direction (WP1,WP5, WP7)
CFRPs with the adopted formulation show frequency stability of the EM properties (out-of-plane direction) in the kHz range
Self-healing resins based on the microencapsulated systems (WP1,WP2).
Self-healing resin characterized by a value in the Tg between 100 °C -115 °C; and a value in the storage modulus which ranges between 2500 Mpa e 2000 Mpa (-50 °C + 80 °C);
Multifunctional self-healing resins based on the activation of hydrogen bonds (WP1, WP2, WP5, WP6).
Design and development of self-healing resins able to increase flame resistance and electrical conductivity. The resin has proven to improve fatigue resistance by approx. 80%. . The main initial stage of thermal degradation of the nanofilled samples substantially occurs in the temperature range of 380–480°C. The glass transition temperature of the multifunctional resin is 260 °C, while the storage modulus ranges between 5420 MPa and 2000 Mpa in the range of temperature between -90 °C and 90 °C
Development of a method to impregnate a carbon fiber dry preform with a nanocharged resin (WP4, WP5).
When a resin is charged with nanopaticles eg. CNT the viscosity grows up to level that usual methods to impregnate a carbon fiber laminate became unfeasible. Also is very frequent to have a filtration of nanoparticles by the dry preform. A new method based on a sort of film infusion called "wet film infusion" has been developed and parameters as temperature, time and external pressure have been optimized.
Manufacturing of a reinforced panel with 3 T-stringer using a nanocharged resin (WP5).
The method to impregnate carbon fibers with nanocharged resin has been applied on a more complex shape as flat panel reinforced with 3 cocured T-stringer.
The IASS members have conducted experiments using commercial catalysts which evidenced that when catalyst particles were solubilized at molecular level (to reduce the cost of the self-healing materials), the particles which were in contact with the oxirane rings (during the curing reactions) were deactivated, hence reducing the actual amount of the active catalyst. Additional experiments were performed on this issue in order to better understand the reason of this deactivation with regards to the most promising ROMP catalysts (catalysts active in the Ring Opening Metathesis polymerization). In the end, an equimolecular reaction between the epoxide ring and the alkylidene of the ruthenium compound was found to be responsible of the deactivation. It was therefore concluded that it is only possible to enable self-healing reactions in epoxy matrices when cured at high temperature (130°C-170°C) using solid catalyst particles; these retain an intact “heart” of catalyst which is not deactivated when in contact with the oxirane rings of the epoxy matrix. As already mentioned the usage of common industrial hardeners (primary aromatic amines) deactivates the (ROMP) catalysts. This created the need to introduce alternative hardeners, such as aliphatic tertiary amines (Ankamine K54) which are however of poor industrial interest as they cause the production of a very large fraction of homopolymers, instead of rather dense networks, thus reducing the mechanical performance which is required for structural applications.
Evaluation of self-healing efficiency by means FTIR investigation
Infrared spectroscopy provides a useful way to identify metathesis products and therefore catalyst activity in the panel after damage. FTIR spectrum of the metathesis product inside the self-healing panel (SH1) is shown in Figure 37. The coupon extracted from self-healing panels was cut by a serrated blade; the powder was collected in a mortar and two drops of ENB healing agent were added before dispersing the powder sample into the KBr disks for FTIR investigation. The highlighted peak at 966 cm-1 is attributable to ring-opened poly(ENB). This peak is assigned to the trans substituted alkenes, characteristic of the ring opened cross-linked product [poly(ENB)] providing evidence that the embedded catalyst is active also for impregnation conditions (curing cycle and pressure) of CFRC panel. Curing cycle and pressure for the impregnation process is shown in Figure 38.
The concept behind the IASS project was to utilize appropriately the potential of the materials used in the aviation sector, by further exploring their functionalities and by experimenting with new materials in order to achieve optimum results. By keeping in mind that the European taxpayer is the main financial sponsor of the EC funded results, it was really important to showcase the return on investment, by producing safer and more durable avionic materials. The project partners were very conscious of their mission within the project and were oriented towards achieving tangible and reliable outcomes.
The IASS consortium consisted of academic, research, industrial and SME partners, thus including all types of entities and benefiting from their diverse capabilities and potential. The scope of the work performed within the IASS project enabled the employment of scientists of various academic backgrounds, industrial developers and experienced researchers. The merging of their expertise allowed for producing accurate and trustworthy results. The fact that two of the participating entities are major players in the aviation industry maximizes the potential of market uptake of the project’s outcomes. The commercial exploitation of the IASS produced material by large industrial entities can have two important effects on the lives of the EU citizens.
The first significant outcome of the commercial exploitation of the IASS project material is the production of more efficient aircrafts, since the improved functionalities of the material can allow for the reduction of repair costs, due to its increased durability and the self healing agents employed. The sustainability of the project produced material means less in-flight damage, which in result ensures safer flights for the passengers of the aircrafts. Considering the importance of safe travel, since the volume of passengers flying across the globe daily is constantly increasing, it becomes evident how the IASS material impacts the everyday lives of the EU citizens.
The other important aspect of commercializing the project produced material is the economic impact on the aviation sector and on the employment rates. The establishment of mass production of the project produced materials would allow a significant reduction of the repair costs of aircrafts, thus contributing to the cost efficiency targets set by ACARE for the aviation sector. Additionally and subsequently, the production and application of the IASS technology to the European aviation can also boost the employment rates and decrease the unemployment in various personnel categories.
It should be noted, though, that for the time being, the IASS project results cannot be massively reproduced and widely employed, yet, even before the end of the project’s lifetime many partners have expressed their interest in further exploring its results and in commercially exploiting the new materials as soon as the cost for their production has been balanced.
In order to effectively communicate the aforementioned outcomes and their potential to the appropriate target groups, an effective dissemination and exploitation strategy has been implemented by the IASS consortium, predominantly aiming at ensuring:
• The effective, timely and constant dissemination of the generated knowledge and technologies; this is being primarily directed to the entire European Aeronautics Community, but focused groups related to the IASS research were also closely targeted (e.g. aircraft manufacturers, R&T Centres, establishments offering Aeronautical Engineering training, the general public, etc.)
• The exploitation of the project’s results; this contributed significantly to maintaining and reinforcing the industrial leadership, competitiveness and technological advantage over competition from outside Europe. Similar to dissemination activities, exploitation was primarily addressed at the European Aeronautics Community, while triggering interconnection and technology spill over into other industrial areas.
• The conveyance of new knowledge into the engineering education base; the foreground developed and the undermining research will be introduced in academic education, so as to meet the evolving skill needs of the European Aeronautics Industry, thus, increasing European industrial competitiveness.
• The creation of a greater public awareness; towards stimulating the public interest for research findings and achievements, and promoting knowledge sharing, transparency and education.
• The communication of the right information to the right people at the right time using the right language and taking into account the dissemination needs of the project at each stage of its lifecycle.
• That the full range of potential end users and uses of the IASS results are addressed, including research, commercial, investment, environmental, skills and educational training.
Towards achieving the aforementioned objectives, the IASS Consortium identified appropriate, relevant and cost-effective dissemination and exploitation activities which targeted specific channels and the most related scientific and industrial groups transferring knowledge and fostering new collaborations.
Dissemination is an important tool used to link the consortium members, the general public and the stakeholders of the related scientific fields to the outcomes and the activities performed throughout the project’s lifetime. Exploitation activities involved all actions related to the potential use of the foreground generated within the project. Consequently, through the effective and strategically planned dissemination and exploitation of the project’s results, greater public awareness is created and knowledge sharing, transparency and education are encouraged.
As indicated by the Eurobarometer (2014), more than two-thirds (70%) of Europeans support the idea that it is important “to increase the support for research and development policies and turn inventions into products”. Considering that EU-funded activities and projects significantly contribute to the creation of new jobs and novel technologies and to the improvement of the citizens’ quality of life, public interest for their findings and achievements is constantly growing over time. Moreover, since the largest and foremost financial contributor of such projects is the European taxpaying citizen, through the research allocated part of the European Union’s budget, it is a crucial precondition that the maximum return on investment is ensured and that full transparency about the actions financed is provided. In this light, it is important to ensure that the knowledge generated within research and innovation projects is adequately disseminated and that the tools for delivering such knowledge to the society are being effectively utilized. This can be realized through the commercial exploitation of products and services, which is the primary way of conveying the research results to the European citizens (since they are their end-users).
It was very clear to all consortium partners from the very early stages of the project that an extremely important precondition for ensuring increased exploitation, high impact and improved likelihood of uptake of the project’s results, is to attentively and effectively diffuse and communicate the appropriate information to the relevant and interested audience; in a terse, well-articulated, understandable and appealingly packaged manner. Thus, the IASS partners by communicating the right information and messages to the right people, using the right language and proper means have implemented a very successful dissemination strategy. In this light, the IASS consortium identified and classified the different target groups - audiences, the messages and information to be addressed to each of them, as well as the communication language, measures and channels which should be used to transmit the project’s outcomes to each tier of audience, therefore increasing significantly the possibility of exploitation of the project’s findings.
The main dissemination tools used during this period for providing information to the “IASS Community” include: i) the IASS public website which was regularly updated, ii) the inclusion of IASS news in the EASN Newsletter, which is released on a quarterly base, iii) the implementation of numerous IASS related dissemination activities (e.g. publications in highly ranked European / International Journals, Presentations in high-impact International Conferences, Press releases, newsletters, poster display and leaflet distribution) addressing different target audiences (e.g. scientific community, industry, general public), iv) the participation in one major aeronautics event, the Aerodays 2015 Fair and v) the organization of the final project meeting along with the ICEAF IV conference. In addition, the project activities realized within the dissemination work package were related to monitoring the performed dissemination activities in order to ensure that the disseminated materials are in line with the IPR rules and legal requirements.
Figure 41 of the of the attached file – file name - “Figures” presents an overview of the IASS website users across the period April 2013-October 2015 (the IASS website was launched in April 2013).
Based on this figure, we can easily observe that the months that coincide with the publication of progress updates at the different issues of the EASN newsletter, present peaks in the overall traffic, and in fact these values remain approximately stable. It is therefore suggested that these are effective dissemination activities which target users relevant to IASS and managed to engage them towards visiting the project website for more information. It also appears that face-to-face presentations at events significantly contribute to the engagement of users, which are then prompted to seek for more information on the project. The most outstanding activity to have affected the dissemination of the project appears to be the partners’ participation at major aeronautics events, i.e. Aerodays 2015 and the 4th International Conference of Engineering Against Failure (ICEAF IV). This reasonably suggests that the selection of the targeted audience is very important, that high-impact events extend their impact to the project and that multiple presentations provide a very strong overall appearance. Also, we can consider that exhibition stands are one of the most effective means towards attracting users. Furthermore, the referencing of the IASS website at the partners’ organizational websites directly maximizes the potential outreach of the project by introducing it to existing customers with pre-defined needs. Last but not least the figure 41 suggests that not only were the number and type of activities realized carefully chosen, but it also verifies that the aforementioned “communication channels” are recognized as being of significant impact for the researchers of their respective fields.
In regards to the performed dissemination activities, based on their analysis, most of these are scientific publications (51%) in peer reviewed journals and conference proceedings, while presentations (34%) in conferences, workshops, seminars etc., follow (Figure 42). In addition, IASS related information was included in newsletters (e.g. EASN newsletter), websites (e.g. IASS public website, partner’s websites) and press releases. Finally, project information was also disseminated through leaflets distribution and poster displays in relevant conferences and workshops. In regards to the target groups addressed by the already performed dissemination activities, the respective analysis shows that the scientific community was the main target group addressed (47%), while industry (23%), policy makers (15%), general public (12%) and media (3%) follow (Figure 43) The allocation of the addressed target audience and the focus on the scientific community is mainly attributed to the content of the IASS related dissemination activities performed. Due to the highly scientific nature of the project’s outcomes, relevant expertise is required for fully understanding the project produced knowledge, thus the dissemination activities of the second period were mostly addressed to the scientific community. Yet, since a major objective of dissemination and exploitation activities is transparency in terms of the financial resources spent and as Europe's largest public financer of research is the European taxpayer, it is an obligation to ensure full openness about actions financed. In this light, the IASS consortium took action to ensure that the project’s achievements were widely diffused to Europe’s general public through media coverage and relevant communication tools (such as the project’s public website).
As far as the geographic coverage of the performed dissemination activities is concerned (Figure 43), the majority of the related events were performed within Europe, a logical outcome given the fact that all IASS partners are concentrated in Europe. However, it is worth mentioning that events of international focus have been also performed by the IASS partners in the project’s second period. Also, based on Google Analytics (Figure 44) , we can estimate the current geographical impact achieved through the project website. Based on Figure 43 and Figure 44, IASS has managed to generate a strong international impact with followers in Europe, USA and Asia.
This Final Report describes the dissemination activities that took place during the course of the project, as well as the ones that are planned for a later date. The report is mainly based on the input provided by the IASS partners during the latest update of the Plan for the Use and Dissemination of Knowledge (August 2015) which can be found in section 4.2 of this report.
List of Websites:
Prof. Liberata Guadagno email: firstname.lastname@example.org
Dr. Michael Papadopoulos email: email@example.com
Grant agreement ID: 313978
1 September 2012
31 August 2015
€ 3 270 839
€ 2 397 266
UNIVERSITA DEGLI STUDI DI SALERNO
Deliverables not available
Grant agreement ID: 313978
1 September 2012
31 August 2015
€ 3 270 839
€ 2 397 266
UNIVERSITA DEGLI STUDI DI SALERNO
Grant agreement ID: 313978
1 September 2012
31 August 2015
€ 3 270 839
€ 2 397 266
UNIVERSITA DEGLI STUDI DI SALERNO