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THE USE OF NANOFIBRE STRUCTURES IN FIBRE-REINFORCED COMPOSITES: CHALLENGES AND OPPORTUNITIES

K. De Clerck1*, S. Van der Heijden1, B. De Schoenmaker1, I. De Baere2, W. Van

Paepegem2 and H. Rahier3 1Department of Textiles, Ghent University

Technologiepark-Zwijnaarde 907, 9052 Zwijnaarde, Belgium 2Department of Materials Science, Ghent University

Technologiepark-Zwijnaarde 903, 9052 Zwijnaarde, Belgium 3Vrije Universiteit Brussel, Department Materials and Chemistry, Pleinlaan 2, B-1050 Brussels,

Belgium karen.declerck@ugent.be

Abstract: Owing to their light weight and high stiffness and strength, fibre-reinforced epoxy resin composites are widely used in industry. However, an epoxy matrix is a brittle material, so an improvement of the crack resistance of the interlaminar space would be interesting. Therefore, secondary (sub-)micron reinforcements are often incorporated in the matrix. Since it is difficult to obtain a homogeneous dispersion of these nanoparticles with common techniques, the mechanical improvement of the composites is only moderate. Thermoplastic nanofibrous structures can tackle this dispersion issue, as they can be inserted by other means. Therefore, this study investigated the effect of polyamide 6 nanofibrous structures on the mechanical properties of a glass fibre/epoxy composite. In addition to the mechanical properties the effect of the nanofibrous structures on the epoxy matrix is looked at through a curing study. The nanofibres are produced using a multi-nozzle electrospinning set-up. Keywords: Nanofibres, fibre-reinforced composites, mechanical properties. 1. Introduction Fibre reinforced epoxy resin composites are widely used in industry, due to their light weight and high stiffness and strength [1,2]. However these composites still face a serious problem, the epoxy matrix is a brittle material, which could lead to unexpected failure of the composite. A fair amount of research has been done to improve the mechanical properties of the epoxy matrix, one possible solution consists of adding nanoparticles, like nanoclays and carbon nanotubes (CNT) to the matrix. In CNT nanocomposites, one aims to improve the resin and fibre/resin interface properties, by mixing in carbon nanotubes with a theoretically exceptionally high stiffness and strength [3-5]. However the overall improvement in mechanical properties (stiffness, fracture toughness) of the epoxy matrix is only moderate in most cases [6-8]. Furthermore the toughness of the matrix can also be improved by incorporating rubber particles or embedding thermoplastic inclusions [9,10]. At present both techniques are still facing problems. An important issue in mixing (nano)particles or a thermoplastic in the epoxy matrix is the need for special mixing methods to obtain a homogeneous dispersion in the resin [3-5]. Also the high production cost of CNT and even more importantly the potential health hazard involved with the use of nanoparticles need to be tackled [11]. Thermoplastic nanofibres have the potential to provide a solution for these problems. Nanofibrous webs can be readily embedded in the resin, they have the large benefit of their inherent nanoscale distribution which may improve the traditional limitations in (nano)particle dispersion. Owing to their macro scale length, no health hazards are involved in the use of electrospun nanofibres. Recent literature indicates that nanofibres may contribute substantially to the ductility and fracture toughness of the composite [12-15]. The present paper describes the results of a running research project in which the potentials of nanofibrous materials are looked at to improve the mechanical performance of glass fibre reinforced epoxy composites. Focus is given to the toughening of the epoxy matrix as the envisaged effect is similar to toughening of epoxies through the addition of thermoplastics.

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Both the electrospinning of the nanofibres as well as their effect on the epoxy matrix behaviour is to be discussed. The effect on the epoxy matrix will be looked at through a detailed curing study and their toughening potential. 2. Materials and Methods The PA6 nanofibres are electrospun from a 1:1 formic acid/acetic acid solution on a multi-nozzle set-up [16]. The tip-to-collector distance and flow rate are set at 7 cm and 2 mL/h, respectively. Furthermore, the voltage is adjusted between 25 and 30 kV, in order to obtain steady state conditions [17]. A Jeol Quanta 200F Field emission gun Scanning electron microscope (FE-SEM) is used to examine the fibre diameter of the electrospun nanofibres and the composite structure. Prior to SEM analysis, the samples are coated by a gold/platinum alloy using a sputter coater (Balzers Union SKD 030). The average fibre diameter and their standard deviations are based on 50 measurements, using CellD software from Olympus. An optical microscope, a Olympus BX51 with a Olympus UC30 camera, is used to visualize crack propagation on the polished edges of the composites. The (MT)DSC measurements are performed using a TA Instruments Q2000 Tzero™ DSC, purged with a constant nitrogen flow of 50 mL/min. The instrument is calibrated using sapphire (Tzero calibration), indium (heat flow rate and temperature), and tin (temperature). The modulation amplitude for the MTDSC measurements is chosen at 0.5°C with a period of 60 seconds. For the curing experiments Epikote resin 828 LVEL (diglycidyl ether of bisphenol A, DGEBA, difunctional) from Hexion is used as epoxy. It is combined with the tetrafunctional methylenedianiline (MDA) from Sigma-Aldrich. Both products are used as received. The matrix of the larger scale composite plates was EPIKOTE resin MGS RIMR 135 with EPIKURE curing agent MGS RIMH 137, both purchased from Hexion (Currently Momentive). The composite plates were reinforced with unidirectional E-glass fabric (Roviglas R17/475). In the fibre direction the reinforcement was 475 g/m², while in the perpendicular direction the reinforcement was 17 g/m². The composite plates were manufactured by vacuum assisted resin transfer moulding (VARTM) using a closed steel mould. The epoxy is first cured at room temperature for 24 hours and, thereafter, post cured for 15 hours at 80 °C, as described by the supplier. For all composite plates the stacking sequence was [0°,90°]2s. The samples were cut to dimensions on a water-cooled diamond saw. All specimens had a thickness of 3.0 mm and a nominal width of 30 mm, as described in the ASTM D3039/D3039M-00. End tabs were used to avoid failure at the clamps. The nanofibres were incorporated in the glass fibre/epoxy composites in two different ways. On one hand, they were inserted in between the various glass fibre mats as a stand-alone structure. On the other hand they were directly deposited on one side of the glass fibre reinforcement. Of course, also a reference composite without nanofibres was manufactured, to be able to investigate the improvement of the nanofibre addition. The tensile tests on the composites were performed on an electromechanical Instron 5800R machine with a load cell of 100 kN following the ASTM D3039/D3039M-00 standard. The tests were displacement controlled with a speed of 2 mm/min and both displacement and load were recorded. All specimens were instrumented with two strain gauges to measure the longitudinal and transversal strain, εxx and εyy respectively. Not all samples were loaded till failure, since needed for other experiments. 3. Results 3.1 Curing Experiments Small composite samples were produced by adding resin to nanofibrous structures in the DSC crucibles. SEM analysis confirmed the sample preparation was optimised to ensure the resin is totally impregnated in between the fibres. If it would remain on top of the nanofibrous structure, mainly the reaction of the bulk would be measured and investigated. A non-isothermal study of the curing behaviour of pure resin and resin impregnated within nanofibres was performed [18]. This already revealed an earlier onset of curing on the addition of nanofibres. It seems from that the nanofibres have a catalytic effect on the curing reaction of DGEBA/MDA. Therefore the activation

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energy (Ea) is analysed using the Friedman method [19]. The activation energy indeed shows to be lower when nanofibres are added to the resin, which confirmes that the nanofibres have a catalytic effect on the curing reaction. Moreover, this Ea is not only lower in the beginning, at lower conversion degrees, but for all the different degrees of conversion. Next to non-isothermal experiments, a curing reaction can also be investigated with quasi-isothermal MTDSC experiments. These also revealed the curing of the resin accelerates when PA 6 nanofibres are added to the resin system. The influence of different nanofibre fractions was examined by comparing the curing behaviour of samples containing 0, 5, 15, 27 and 42 wt% PA 6 nanofibres, Figure 1 [18]. Incorporating higher nanofibre fractions, the reaction accelerates: the initial rate is higher and the maximum of the heat flow rate curve shifts to shorter times.

Figure 1. Influence of the PA 6 nanofibre fraction on the quasi-isothermal curing of the DGEBA-MDA

system: heat flow as a function of time (Tcure=80°C) [18].

In addition to nanofibres also the effect of microfibers is looked at. Despite the huge difference in concentration of amide groups at the surface, the curing kinetics of the DGEBA-MDA system did not change very explicit by adding similar weight fractions of macrofibres instead of nanofibers. This suggests that the extra amide groups available on the nanofibres do not influence significantly the curing kinetics. It has to be noticed that the moisture absorption of both nonwoven structures is similar, which suggests that especially water molecules present inside the polyamide nanofibre and released during cure at more elevated temperature (80 °C) may cause the catalytic effect. 3.2 Mechanical Behaviour For studying the mechanical behaviour of glass fibre-epoxy composites upon the addition of a secondary nanofibrous reinforcement, larger scale composite samples were produced [20]. Again the nanofibres are well dispersed within the resin-rich region, Figure 2. Further the SEM-images show the epoxy/nanofibre regions are to be considered as extra separated plies.

Figure 2. Cross section of the secondary nanofibre reinforced glass fibre composites (A.) and a zoom of the

resin rich region filled with nanofibres between two plies with different fibre orientation (B.)[20] . Table 1 summarizes the tensile properties of different composite plates. Glass fibre composite plates with a secondary reinforcement of interlayered and deposited nanofibres are compared to samples without a

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secondary reinforcement. No substantial increase in Young’s modulus is observed. Indeed in the 0°-direction the mechanical properties of the glass fibres are dominant compared to the ones of the thermoplastic nanofibres. Glass fibres have a lot higher strength and stiffness compared to the used PA 6 nanofibres. Furthermore, the stiffness of thermoplastic nanofibres is around 1 to 2 GPa [21], while the applied epoxy matrix has a stiffness of 3.27 ± 0.07 GPa. On the other hand adding nanofibres in between the glass fibre plies increased the stress at failure. Interlayered nanofibres increased the stress at failure from 550 MPa to 581 MPa. When the nanofibres are directly deposited onto the glass fibres the stress at failure increases further to 611 MPa. Thus, even though the mechanical properties of the nanofibres are inferior compared to the properties of the glass fibres, they improve pronouncedly the failure stress. It seems that the deposited nanofibres even further facilitate the transfer of the load to the primary reinforcement. This can be understood by taking into account that there is a direct contact between the deposited nanofibres and the glass fibres, which improves the load transfer to the glass fibres. This is further confirmed through optical microscopy. By testing the composites under 45° the mechanical properties of the glass fibres are less dominant compared to the other components. So, the advantages of nanofibres should be more pronounced. This is confirmed in Table 1 illustrating the shear modulus.

Table 1. Tensile properties of the different composite plates [20].

Stress at failure

[MPa]

Young’s modulus

[GPa]

Shear modulus

[GPa]

Glass fibre composite 550 ± 16 26.0 ± 0.6 4.0 ± 0.4

Interlayered nanofibres 581 ± 13 26.0 ± 0.2 4.7 ± 0.3

Deposited nanofibres 611 ± 18 27.2 ± 0.6 4.7 ± 0.2

4. Conclusions Addition of polyamide nanofibres into the resin resulted in a catalytic effect on the DGEBA-MDA cure. This was proven by the lower activation energy when PA 6 nanofibres were added to the resin. However, as the difference in catalytic behaviour between conventional and nanofibres of PA 6 was very small, the amide groups at the fibre surface are not thought to be responsible for the acceleration. As the moisture absorption of nanofibres and microfibres was similar, and as the acceleration could be correlated with the moisture content of the fibres, the catalysing effect can most probably be attributed to water diffusing from the nanofibres into the polymer matrix. This will be further elaborated in future work. The incorporation of polyamide 6 nanofibrous structures in the [0°,90°]2s-composite increases the stress at failure, deposited nanofibres are slightly better than the interlayered nanofibrous structures. It is found that nanofibres prevent or minimize the formation of delamination cracks between two glass fibre plies. The tensile experiments under 45° also demonstrate that the deposited nanofibres facilitate the load transfer to the glass fibres. Thus, it can be concluded that deposited nanofibres improve significantly some mechanical properties of a glass fibre composite.

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