Special Issue Article
Self-healing thermally conductive adhesives
Journal of Intelligent Material Systems and Structures 2014, Vol 25(1) 67–74 Ó The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1045389X13498314 jim.sagepub.com
Ugo Lafont1,2, Christian Moreno-Belle2, Henk van Zeijl3 and Sybrand van der Zwaag2
Abstract Thermally conductive composites with a temperature-triggered self-healing response were produced by dispersing boron nitride or graphite particles into two types of polysulphide-based thermoset matrices. The composites produced exhibit recovery of both cohesion and adhesion properties upon thermally activated healing. Using a mild healing temperature (65°C), the materials show full recovery of their initial adhesive strength during multiple healing cycles. The composites behave differently regarding the cohesion recovery: 20%–100% recovery is achieved depending on the filler type, filler loading and the type of matrix. The thermal conductivity of the composites increases with the amount of filler. Values of 1 and 2 W/m K can be achieved for the boron nitride and graphite-based composite, respectively. The results presented in this work clearly show that multifunctional materials with different functionalities and mechanical self-healing responses can be designed using this strategy. Keywords Self-healing, composites, adhesive, cohesion recovery, thermal conduction
Introduction The need for materials, which are very reliable and durable, has been the main motivation for the development of self-healing materials (Van der Zwaag, 2007). For a decade, the newly emerging self-healing concepts in material science have been mainly applied for structural materials, which have to carry mechanical loads (Blaiszik et al., 2010) or materials, which have a protective function (Cho et al., 2009; Garcı´ a et al., 2011). For these applications, the dimensions of the damage to be healed lie in the millimetre to sub-millimetre range. Downscaling the self-healing concept to heal damage with typical dimensions in the micrometre to submicrometre range will be of great interest for systems where the main failure mode differs from structural failures. For example, restoration of interfacial properties (adhesion) and electronic, ionic or thermal conductivity could be of great interest for microelectronic and optoelectronic applications (Lafont et al., 2012a). There are basically two well-established ways to develop self-healing concepts in materials. The first one is based on the integration of discrete mono-, bi- or tridimensional containers (layers, capsule, fibres or vascular network) (Blaiszik et al., 2009) loaded with active ingredients, which are embedded in the matrix material prior to consolidation. In this approach, there is a
physical separation between the material responsible for the healing action and the material responsible for the intended functionality. The second approach relates to the development of the so-called intrinsically selfhealing materials, that is, materials containing dynamic bonds, which can restore their chemical or physical bonds under the influence of a non-disruptive external stimulus (Bergman and Wudl, 2008). In this approach, the ‘matrix’ material combines both the healing and the functional role. Among the multiple examples of intrinsic self-healing materials, some of the most elegant routes are based on Diels–Alder and retro Diels–Alder reactions (Chen et al., 2003), hydrogen bonding in supramolecular network (Cordier et al., 2008), disulphide chemistries (Amamoto et al., 2011; Canadell et al., 2011; Lafont et al., 2012b) and 1
Materials Innovation Institute, Delft, The Netherlands Novel Aerospace Materials, Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands 3 DIMES, Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology, Delft, The Netherlands 2
Corresponding author: Ugo Lafont, Novel Aerospace Materials, Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS, Delft, The Netherlands. Email:
[email protected]
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Figure 1. Molecular structure of the aliphatic (EPS25) and aromatic (EPS70) epoxidized polysulphide used. EPS: epoxidized polysulphide.
trans-esterification reaction (Montarnal et al., 2011). The use of such intrinsic self-healing material as matrix in the development of composite will be of great interest to produce self-healing composites, as such composites have the inbuilt capability of multiple healing at specific locations. This approach is expected to enhance and increase the reliability of composite functionality as a function of use time. This concept has been recently used in the development of electrically conductive self-healing composite (Tee et al., 2012). In the present work, we investigate the synthesis and characterization of a new type of thermally conductive self-healing composite using a polysulphide rubber thermoset as matrix and either hexagonal boron nitride (hBN) or graphite particles as fillers. The evolution of the thermal conductive property and the ability of the material to recover both its adhesive and cohesive properties as a function of a mild thermal stimulus are described in this article.
Experimental Material synthesis The composites were produced by mixing an uncured thermoset rubber with sub-micrometre graphite flakes or 5 mm hBN particles as filler material. The amount of fillers in the composite was varied from 10 to 40 vol%. The synthesis of the polymeric matrix is based on a basic catalysed condensation of epoxy functions with thiols. In a typical procedure, a stoichiometric amount of epoxidized polysulphide Thioplastä EPS25 (aliphatic) (640 g/eq) or EPS70 (aromatic) (310 g/eq) provided by AkzoNobel BV is mixed with pentaerythritol tetrakis(3-mercaptopropionate) (;122 g/eq) from Aldrich. To this slurry, 1 wt% of 4-dimethylaminopyridine (Aldrich) was added as the catalyst. Finally, the composites were prepared by adding the filler quantities to this slurry. After mixing for 2 min at 2500 r/min using a speed mixer, the obtained pastes were poured into a 1-mm-thick mould and cured at 65°C for 2 h. The molecular structures of the monomers used in this study are presented in Figure 1.
Characterization techniques The fillers’ size and morphology were investigated using a Tecnai TF20 transmission electron microscope operated at 200 kV. The thermal conductivity of the materials was measured at room temperature using the modified transient source technique on a C-Therm TCi apparatus (Cha et al., 2012). The adhesive behaviour of the composites has been investigated by performing single-lap shear tests using two aluminium 6082-T6 plates using a Zwick/Roell 250 tensile tester. For this purpose, thin slice of already cured resin (12.5 3 25 mm, thickness of 1 mm) was sandwiched between two identical aluminium plates (L 3 W 3 T = 100 3 25 3 2 mm) with an overlap length of ~12.5 mm. The adhesion was promoted by a 2 h thermal treatment at 65°C. During the thermal treatment, the composite and the two parts of the sample were kept in mutual contact using a paper clip. In order to monitor the adhesion recovery upon thermal healing cycles, after complete failure of the lap joint during mechanical testing, the sample ends were repositioned carefully, and the thermal treatment was repeated. This complete procedure (adhesive failure and thermally triggered re-adhesion) is named here as ‘healing cycle’. To investigate the cohesive healing ability (i.e. the ability of the sample to recover its integrity), the samples were cut into four pieces using a sharp razor blade. The pieces of material were put back together until visual contact and were placed between two glass slides. The initial cut width and area have been recorded under an optical microscope (Leica). The samples were placed into an oven at the desire healing temperature. The quantification as function of the healing time of the cohesive healing ability has been investigated at 65°C and 100°C. For this purpose, the samples were taken out from the oven during the healing procedure at different time intervals, and the evolution of the dimension of the initial cut width and area were monitored using a Leica optical microscope. The healing efficiency quantification is thus calculated looking at the ratio of the reconnected cut area over the initial cut area. Young’s moduli of the composites were derived from indentation curves
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Figure 2. TEM micrographs of (a) graphite and (b) hBN particles used as fillers. TEM: transmission electron microscopy; hBN: hexagonal boron nitride.
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using a CSM microindenter and a Vickers indenter. The glass transition temperature of the material, Tg, has been investigated using a PerkinElmer Sapphire Differential Scanning Calorimeter using 5–10 mg per sample and a heating rate of 10 K/min from 260°C to 25°C applying a protective N2 atmosphere.
Materials Figure 2 shows the transmission electron microscopy (TEM) images of the two filler used. The graphite powder exhibits clusters of nanometre size platelets. The planar nature of the graphite particles is well distinguished. The BN particles are more spherical by nature, and the particle size spans from 100 nm to 2 mm at maximum.
Young’s modulus of the composites depends on the polymeric matrix and the filler content and is plotted in Figure 3. At filler concentrations up to 20%, Young’s modulus remains more or less constant and independent of the filler type of polymer matrix and has a value of 20–25 MPa. From 30 vol% of fillers, Young’s modulus of the composites increases with the filler content. For the more aromatic modification of the polymer (using EPS70), the modulus increase is larger than for the aliphatic modification (using EPS25). The filler type and content affect the modulus of the composite prepared using the EPS25 polymeric matrix. At 40 vol% loading, Young’s modulus is 70 and 95 MPa for the EPS25–graphite- and BN-based composites, respectively, whereas the variation with the filler content of the modulus of the EPS70-based composites is
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Figure 4. Adhesion recovery results at 65°C from lap shear tests for (a) EPS25–graphite, (b) EPS70–graphite, (c) EPS25–BN and (d) EPS70–BN as function of the healing event and filler content. EPS: epoxidized polysulphide; BN: boron nitride.
independent of the filler type. The incorporation of fillers is known to increase the hardness and toughness of epoxies, and the mechanism involved is related to better stress dissipation and more distributed load transfer (Hsieh et al., 2010). Moreover, for filled adhesives, it has been shown that there is an optimum relation between the intrinsic elastic modulus and the resulting adhesive strength (Watanabe et al., 2004).
Composite properties Adhesion recovery The adhesion recovery is related to the ability of the composites produced to recover their strong interfacial contact with the aluminium strips. The results of the adhesion recovery for the composites are presented in Figure 4. Whatever the material composition, all composites recovered their initial adhesive strength after failure by thermal healing. The adhesion recovery of the polymeric matrix without any fillers as reported in a previous work (Lafont et al., 2012b) shows exactly the same trend with adhesive strength values of 0.2 and 0.3 MPa for EPS25 and EPS70 polymers, respectively. It is
clear that the addition of graphite has a positive effect on the adhesive properties of the aliphatic polysulphide (EPS25) polymer–based composite irrespective of the filler loading. The maximum shear strength varies between 0.25 and 0.40 MPa and showed a maximum at 30 vol% loading. When the aromatic polysulphide (EPS70) polymer is used, the maximum adhesive strength for the graphite-loaded composites is increased to 0.5–0.6 MPa for 10 and 20 vol% loading, respectively. At 30 vol% loading, the EPS70-based composites exhibit a decrease in its adhesive strength to 0.2 MPa. This value is even below that of the pristine polymer. At 40 vol% loading, the EPS70–graphite composites do not exhibit any significant adhesion. For hBN filler material, the composites demonstrate the same general behaviour. However, a maximum of adhesion is reached for 30 vol% loading, and values of 0.5 and 0.6 MPa can be achieved for the EPS25- and EPS70-based materials, respectively. It is clear from Figure 3 that the adhesive response depends on the filler type and loading. The increase of adhesive properties by adding a filler is a combined effect related to the increase of the surface-free energy of the adhesive and to the efficiency of the particle–matrix interactions (Baldan, 2004;
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Figure 5. Average healing efficiency over seven healing cycles for all materials as function of the filler content. EPS: epoxidized polysulphide; BN: boron nitride.
Park and Lee, 2010; Renner et al., 2010). In this respect, it is interesting to mention that the composites made of hBN fillers and the aliphatic or aromatic polymers lead to the same adhesive properties. However, the graphite composites do not behave the same when an aliphatic or aromatic resin is used. This phenomenon could be related to different matrix–filler interactions due to the presence or absence of aromatic rings in the thermoset. The adhesive healing efficiency has been calculated as the recovered adhesive strength over the first time adhesion strength. The average healing efficiency calculated over the seven healing cycles is presented in Figure 5. The error bars are related to the standard deviation. In general, all materials present at least a 80% to more than 100% recovery of their initial adhesive strength over the considered healing cycles. As previously described, only the EPS70–graphite composite does not show recovery of its adhesive properties presenting an healing efficiency of only 20% over the seven healing cycles.
Cohesion recovery The cohesion recovery is related to the ability of the material to exhibit temperature-activated mending with itself. The results of the cohesion recovery performed at 65°C for the produced composite are presented in Figure 6. The EPS25–graphite composites are able to fully recover cohesive strength in 20 min for filler loadings up to 20 vol%. When EPS70 is used, the time required to reach a full cohesive healing increases to 90 min. From a filling of 30 vol%, the graphite-loaded composites did not show any significant cohesive healing irrespective of the matrix used. When hBN particles are used, the time required to reach full mending for the EPS25-based matrix is
increased to 100 and 130 min for the 10 and 20 vol%, respectively. With higher filler content, the material does not exhibit full cohesion recovery. For the EPS70– BN composites, the time needed for full healing increases even further (190 min). The EPS70–BN composite with 40 vol% of filler exhibits only a 20% cohesion recovery. This loss in cohesion recovery for sample with more than 20 vol% of filler is attributed to the loss in the macroscopic intrinsic mobility of the polymeric chains within the composite. Indeed, the recovery of the composite cohesion is promoted by the polymeric network. Due to the presence of disulphide bond in its structure, dynamic and reversible bonds can occur (Canadell et al., 2011; Lafont et al., 2012b). This behaviour is promoted at the molecular scale when two disulphide bonds are close enough to undergo a specific bond exchange. The Tg of the pristine polymeric matrix is 246°C and 23.6°C for the EPS25- and EPS70-based polymers, respectively (Lafont et al., 2012b). At 40 vol% loading, the Tg of the EPS25- and EPS70-based composites is 244°C and 24.5°C, respectively. The incorporation of fillers does not affect the Tg of the resulting composite compared to that of the pristine polymeric matrix. The healing temperature used is far above the respective glass transition temperatures, and chain mobility at the molecular level is not hampered by increasing the filler content. In this case, the decrease in the cohesive healing response as function of the filler content appears to be dependent on phenomena occurring at a larger scale. The difference that exists between the graphite and hBN-filled composites may be related to the size and dispersion of the fillers. From the TEM analysis, we know that the hBN particles are in the micrometre range, whereas the graphite exhibits nanosized platelets. When increasing the filler content, the effective contact surface between the polymer matrix is decreased and will directly affect the mending ability. In this respect, the bigger hBN particles are expected to first hamper the macroscopic chain mobility by a screening effect and second the effective mending surface compared to the graphite filler. In order to increase the cohesive healing response, one route would be to increase the density of disulphide functions in the composite, and the other one could be to promote a better macroscopic polymer flow. Providing more energy to the system by increasing the healing temperature is a simple way, to a certain extent, to increase the polymeric chain mobility. As it can be seen in Figure 7, when the healing temperature is raised from 65°C to 100°C, the composites with the lowest cohesion recovery (e.g. with 40 vol% loading) are able to increase their healing efficiency to values close to 100%. For the graphite-based composites, only the one made using an aliphatic matrix (EPS25) is able to increase its cohesive healing efficiency from 35% to 84% at 100°C. When the healing temperature is increased to 100°C, all hBN-based composites are able to restore their
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cohesion in time regardless of the filler content and polymeric matrix. This full mending recovery is also observed for all graphite-based composites made using the aliphatic polymeric matrix (EPS25). When the aromatic (EPS70) polymeric matrix is used to produce graphite-based composite, even at a high healing temperature, above 30 vol% of graphite, the material does not reach 100% healing efficiency. This behaviour can be explained by a combination of effects that will hamper the probability and kinetics of disulphide bond exchange. Indeed, the presence of a rigid polymeric matrix with a high filler content added to a potential increase in the p–p interactions between the graphite and the aromatic backbone will drastically reduce the macroscopic chain mobility and thus the mending ability. The thermal mending ability of the materials described in this work will be further investigated to
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Figure 8. Thermal conduction values as function of the filler content for BN- and graphite-based composites using EPS25- or EPS70-based matrix. EPS: epoxidized polysulphide; BN: boron nitride.
morphology and dispersion play an important role in the percolation and thus in the formation of suitable thermal path within the composite. A better control of the particle dispersion and organization within the matrix is a suitable way to increase the thermal conductivity lowering at a same time as the particle content. A lower amount of filler but organized and aligned will have a beneficial effect on the resulting thermal conductivity as well as the healing response. The system developed in this work is based on the formation of a multifunctional composite. The cohesion recovery is dependent on the dynamic bonding ability of the polymer at a molecular scale. As presented in Figure 9, the polymeric matrix self-healing properties allow the produced composites to present thermal mending and cohesion recovery, irrespective of the fillers used. The adhesion functionality is a combined effect of the polymeric matrix and fillers, whereas its recovery is a property carried by the polymeric matrix. Finally, the thermal conduction is a function of the implemented fillers. Using the approach outlined above, it was possible to create a new material that has a non-mechanical functionality (thermal conduction) in combination with two forms of mechanical self-healing. As a result, the composites are able to recover its bulk and interfacial damages and at same time are able to restore the internal thermal conduction pathway after cohesive or interfacial damage.
Conclusion Figure 9. A graphite- and BN-based thermally conductive composite before (left) and after (right) thermal mending at 65°C. BN: boron nitride.
look at its influence on the recovery of the material’s tensile strength.
Thermal conductivity The thermal conductivity of the produced material presented in Figure 8 shows a continuous increase as a function of the filler loading. The EPS70-based material exhibits a slightly higher thermal conductivity compared to the EPS25 one. The graphite-based material intrinsically presents higher thermal conductivity compared to the hBN-loaded composites. There are several parameters that explain this difference. This difference is mainly related to the intrinsic thermal conductivity of graphite (20–2000 W/m K) being higher than that of hBN (30–600 W/m K). However, the particle size,
Multifunctional self-healing composites capable of multiple healing and showing a good thermal conductivity can be created using an intrinsic self-healing rubber thermoset containing reversible disulphide bonds and loading it with inert thermally conductive fillers. The resulting composite self-healing response functions can be tuned as function of the matrix polymeric backbones. Fillers can enhance the adhesive and thermal conductivity properties of the composites, whereas the cohesive response is mainly hampered when the filler content is relatively high. Increasing the healing temperature from 65°C to 100°C leads to a good cohesion recovery even for highly loaded composites. The selfhealing composites with thermal conduction properties produced here form an interesting route to create smart materials for microsystem applications (Lafont et al., 2012a).
Acknowledgements The authors thank Dr O. Klobes (AkzoNobel Functional Chemicals) for providing the polysulphide materials as well as technical support. The authors also thank M. Hegde for his support during thermal characterization.
74 Funding This research was carried out under project number M71.9.10381 in the framework of the Research Program of the Materials innovation institute (M2i) (www.m2i.nl).
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