ADAPTED PROCESSING ROUTES FOR SELF-HEALING
FIBRE-REINFORCED COMPOSITES
E. Manfredi 1 and V. Michaud 1
1
Laboratoire de technologie des composites et polymères (LTC), Ecole Polytechnique
Fédérale de Lausanne (EPFL), Station 12, CH-1015 Lausanne – e-mail:
erica.manfredi@epfl.ch; veronique.michaud@epfl.ch
Keywords: composites, impact, self-healing, X-ray tomography, compaction
ABSTRACT
Fibre-reinforced polymers development has been driven by the need for materials that combine high stiffness and low density, but composites based on thermoset matrices tend to remain sensitive to damage, that may occur early in the matrix. Improved efficiency could be gained by integrating functionalities into the structure, namely a self-healing matrix for repairing low-extent cracks (i.e. barely visible impact damage). Challenges remain to develop an adapted and effective manufacturing methodology to effectively transfer the know-how from healing polymers to self-healing composites.
Vacuum assisted resin infusion moulding (VARIM) is selected as processing technique, since it presents several advantages compared to hand lay-up, including a better quality, uniform part thickness and reproducibility compatible with low-volume industrial applications. The process is firstly optimized for production of thick glass fabric reinforced samples suitable for compression after impact tests and for fast gelation time resin systems such as EPON 862 with DETA as hardener.
Secondly, fabrics are functionalized with capsules containing EPA solvent, taking into account both geometric and capsule strength considerations. On one hand, the fabric type, the dispersion technique and the optimal capsule size are determined through X-ray tomography of fabrics and optical imaging of functionalized fabrics prior to and after processing. The optimal amount of healing agent needed to fill delaminations from small energy impacts is also determined by optical image analysis of impacted samples. In parallel, capsules are characterized for their compression behaviour and bursting strain as a function of the shell type, from pure UF shell to combined UF/PU shells with varying contents of PU. This is then related to their behaviour inside the fabric during the compaction created by the vacuum bagging prior to infusion, to ensure capsule survival during the process.
1. INTRODUCTION
Processing thick and possibly capsule-functionalized fiber reinforcement stackings via vacuum assisted resin infusion moulding (VARIM), preferred to other techniques as hand lay-up [1], is not straightforward and manufacturing parameters need to be adapted. The aim of this study is to find optimal, reproducible and reliable solutions to produce self-healing glass reinforced epoxy materials that could be further mechanically tested through compression after impact experiments.
2. MATERIALS
The composite matrix is a bisphenol F diglycidyl ether resin (Epon 862), cured with diethylentriamine in a 100:12 weight ratio for 24h at room temperature and post-cured for 24h at 35°C. With this cycle, the matrix is underpost-cured (about 71% conversion right after the post-cure treatment [2]), to allow a potential solvent-based healing to further occur. The reinforcement is a standard woven twill 2x2 E-glass with
aerial weight of 390 g/cm2. 16 layers of fabric with [(+45,-45),(0,90)]4s as stacking
sequence are used to achieve a target fiber volume fraction (Vf) of 50%.
Urea-formaldehyde shell microcapsules are produced with the established protocol developed by Caruso et al. [3] and sieved to include diameters in the range of 125-250 μm: microcapsule core is a solution of 2.5 vol% of Epon 828 in ethyl
phenylacetate (EPA). Overall capsule volume fractions (Vc) are 1.25, 2.5 or 3.75%,
corresponding to capsule volume fraction over matrix (Vc*) of 2.5, 5 and 7.5%
respectively.
3. METHODS
The superficial interstice dimensions of the selected fabric are explored by optical microscopy and X-ray micro-computed tomography. Produced capsules are characterized by optical and scanning electron microscopy, thermogravimetric analysis and mechanically tested with a micromechanical compression test. Capsules with diameters in the range of 125-250 μm are manually spread onto the fabric cloths by using a 250 μm sieve.
Prior to manufacturing, the loading-unloading properties of plain and capsule-functionalized fabrics of 100x100 mm are analyzed through a displacement controlled compression test (2 μm/s, pressure 0-8 bar) in a Universal Testing System (Walter & Bai AG, Switzerland) and compared. The pressure range 0.1-1 bar is analyzed since it corresponds to the effect of the pressure difference during VARIM.
Loading data in terms of pressure as a function of Vf are fitted to a simple power law
P=a(Vf)n and fitting parameters a and n are compared. A three zone-fitting is
considered.
Further processing of plain and capsule-containing 660x360mm panels is then performed through VARIM and specific manufacturing solutions are optimized to prevent undesired phenomena such as resin viscosity increase and wedge-shaped resin front, that would lead to a poor impregnation. Hence, post-manufacturing verifications allow us to check the composite components and porosity fractions and to validate the processing protocol in the presence and in the absence of capsules.
4. RESULTS
The 125-250 μm fraction of produced capsules (stirring rate of 400 rpm) have a number-length and volume-moment average diameters of 147 and 156 μm respectively, a shell thickness of 200 nm, typical thermal and mechanical behaviours (Figure 1), namely a gradual weight loss at about 100°C and size-dependent bursting forces ranging from 5 to 10 mN, corresponding to a Young’s modulus of 3 GPa.
0 100 200 300 400 500 600 0 10 20 30 40 50 60 70 80 90 100 Temperature (°C) M a ss p er ce nt ( %) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0 1 2 3 4 5 6 7 8 F o rc e ( m N ) Relative displacement (−) Experimental data Standard model Fitting (E=3.099 GPa)
140 160 180 200 220 240 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 Diameter ( m) Bu rs ti ng for ce ( m N )
Figure 1: Typical thermal and mechanical behaviour of produced capsules. Such a diameter range is chosen since both optical imaging and X-ray micro-computed tomography demonstrate interbundle interstices in the range 150-200 μm (width), in which such capsules can thus fit (Figure 2); the yellow area gives a visual indication of the volume –in scale- occupied of 125-250 μm capsules if laying within the interstices and/or onto the fiber bundles.
Figure 2: Optical microscopy of and X-ray micro-computed tomography images of the plain and capsule-functionalized fabric.
After manually sieving such capsules onto the reinforcement fabrics, their state of dispersion/distribution is visually checked: as expected, it is confirmed that capsules are in general prone to fall down, due to gravity, within the interstices; nevertheless, some of them lay on the top of warp/weft bundles. It is supposed that capsules fitting the interstices keep more protected and safe and do not increase significantly the overall thickness, whereas those onto the bundles would be more responsible for a thickness increase and more susceptible to breakage while the fabric stacking packs. Results of compression tests of plain and capsule-functionalized fabric (Figure 3)
showed that the presence of capsules causes an increase of the overall stack thickness, and that it constitutes an interference for fabric nesting over loading; this phenomenon becomes more significant as the amount of capsules increases. Moreover, the presence of capsules within the interstices is confirmed since the thickness increase (in unloaded conditions) is lower compared to a potential case with all capsules onto the bundles (+0.28 instead 2.35 mm for the 2.5% capsule
content). Hence, lower Vf are achievable when processing functionalized instead of
plain fabrics, for equal applied pressure. This is proved with post-manufacturing verifications of plain and capsule-containing panels, processed with equal vacuum
level (0.9 bar): preliminary results with Vc of 1.25% already showed higher average
thickness and lower Vf compared to the plain composite. However, the comparison
between expected data (from compression curves) and experimental ones is not straightforward due to the presence of an impregnating fluid in the latter case.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Th ic kn e ss d if fe re n ce w it h V c = 0 % (m m ) Stress (MPa) Vc*=2.5% Vc*=5% Vc*=7.5%
Figure 3: Compressibility of plain and capsule-functionalized fabrics (zoom 0.1-1 bar).
Table 1: Expected and experimental Vf and overall thickness at 0.9 bar pressure.
Vc* (Vc) (expected) Vf (experimental) Vf Thickness (mm) (expected) Thickness (mm) (experimental)
0 (0) 0.515 0.462 4.535 5.138
1.25 (2.5) 0.516 0.397 4.562 5.600
2.5 (5) 0.506 on going 4.654 on going
3.75 (7.5) 0.495 on going 4.764 on going
5. CONCLUSIONS
It is shown that manual sieving is an effective and quite reproducible approach to spread microcapsules onto reinforcement fabrics prior to manufacturing. The optimal capsule size is chosen depending on the fabric interstice dimensions. However, the presence of capsules modifies the fabric packing behaviour, thus different processing
conditions to obtain equivalent Vf are required.
ACKNOWLEDGEMENTS
Swiss National Science Foundation (FNRS 511482) and S. Neuser are gratefully acknowledged.
REFERENCES
[1] A. J. Patel, N. R. Sottos, E. D. Wetzel, S. R. White, Autonomic healing of low-velocity impact damage in fiber-reinforced composites, Composites Part A: Applied
Science and Manufacturing 41 (2009) 360-368.
[2] S. Neuser, V. Michaud, Effect of aging on the performance of solvent-based self-healing materials, submitted to Polymer Chemistry
[3] M. M. Caruso, D. A. Delafuente, V. Ho, N. R. Sottos, J. S. Moore, S. R. White, Solvent-promoted self-healing epoxy materials, Macromolecules 40 (2007)
