Self-healing materials – from concepts to market

Resume : It has been well established that combining self-healing bonds with permanent bonds, can significantly increase the fracture toughness of soft hydrogels. Yet the details of how this actually occurs and in particular the link between bond breaking and healing dynamics and fracture properties, is not yet known. In this work we investigate in detail the rheology in small and large strain and the fracture behavior of a model hydrogel based on polyvinyl alcohol crosslinked both chemically with glutaraldehyde, and physically with a borax solution. We carried out a series of crack propagation experiments on prenotched gel samples over a wide range of loading rates and measured both the critical energy release rate where the crack propagates, and the average crack propagation velocity for each case. Using a recently proposed 3D mechanical model to describe the breaking and healing of the reversible bonds we can simulate the energy stored and dissipated ahead of the crack and we attempt to explain quantitatively how the physical bonds affect the fracture process of the hybrid hydrogels.

Resume : The concept of an autonomic self-healing material, where initiation of repair is integrated to the material, is now being considered for engineering applications (LG released in 2014 a mobile phone using a self-healing finish (LG G Flex)) and is a hot topic in the literature (SciFinder: 2090 references were found containing the concept "self-healing material"). Among several concepts/techniques, two are most interesting: i) Capsules: Integration of microcapsules in or at the surface of coatings or fibre-like structures has recently gained much attention. Upon damage-induced cracking, the microcapsules are broken by the propagating crack fronts resulting in release of an active chemical (healing agent) by capillary action, subsequently repairing and avoiding further crack growth. ii) Self-healing polymers: Interestingly, the introduction of dynamic covalent bonds into polymer networks has also recently been used as a powerful approach towards the design of various intrinsically self-healing polymer systems. The idea behind this is to reconnect the chemical crosslinks which are broken when a material fractures, restoring the integrity of the material and thereby prolonging its lifetime. We propose here to integrate both self-healing concepts (capsules, self-healing polymers) in electrospun fibres and coatings. Different capsule preparation approaches have been investigated in SINTEF. The most advanced method to produce capsules is based on emulsification to create a water-in-oil emulsion before polymerization. The healing agent is a polyurethane-based dispersion that was encapsulated in shell materials consisting of urea-benzaldehyde resins. Results showed successful preparation of microcapsules and release of the agent when capsules break. Since capsules are produced in water-in-oil systems we mainly investigated organic solvent based coatings while a major challenge resides in the incorporation of capsules into water-based coatings. We also focused on developing more robust microcapsules to prevent premature rupture of the capsules. The capsules have been characterized in terms of size, and encapsulation and release might be visualized by incorporating fluorescent dyes and examine the capsules by microscopy techniques. Alternatively, Electrospinning is an innovative technique that has attracted enormous attention due to unique properties of the produced nano-to-micro fibers, ease of fabrication and functionalization, and versatility in controlling parameters. Especially roll-to-roll electrospinning is a unique method which has been used in industry to produce nanofibers continuously. Electrospun nanofibers can usually reach a diameter down to 100 nm, depending on the polymer used, which is of interest for the concept with self-healing polymer systems. In this work we proved the feasibility of fabrication of POSS-based (POSS: polyhedral oligomeric silsesquioxanes, tradename FunzioNano™) nanofibers via electrospinning. Two different formulations based on aqueous or organic solvents have shown nanofibres with diameter between 200 – 450nm with low defects. The addition of FunzioNano™ in the polymer blend also showed enhanced properties in term of wettability, promising for e.g. membrane technology. The self-healing polymer systems developed are here POSS-based materials synthesized to develop dynamic soft brushes.

Resume : CaCO3 precipitation is one of the main mechanisms to heal cracks in bacterial based self-healing concrete or self-healing concrete incorporating superabsorbent polymers (SAPs), but also in traditional concrete, cracks can be healed autogenously due to CaCO3 precipitation. To study the mechanical properties of the CaCO3 precipitated in the cracks of these three types of concrete, the indentation technique was used. A scanning electron microscope (SEM) equipped with a special stage to combine the indentation technique with SEM/EDS (energy dispersive X-ray spectroscopy), was used in this study. The advantage of this system is that the location of the indentation test can be accurately determined and it is known for certain for which phase the mechanical properties are measured and calculated. The results show that there is no significant difference in the Martens hardness of the calcite formed in autogenously healed specimens and specimens incorporating bacteria or SAPs. Furthermore, the mechanical properties of the cement paste were also measured and the influence of the strength class of the cement (CEM I 42.5 N vs. CEM I 52.5 N) and presence of SAPs was investigated. This work was supported by the SHeMat project "Training Network for Self-Healing Materials: from Concepts to Market" within the scope of the Seventh Framework Programme [FP7/2007-2013] under grant agreement no 290308 by the European Commission's Marie Curie programme.

Resume : Recently, self-healing concrete has been introduced as a novel construction material which can result in a considerable reduction in annual maintenance costs of concrete structures such as bridges, tunnels and retaining walls. Although many research works have been done for optimizing properties of self-healing concrete, the bottleneck for its valorisation is the encapsulation method of the healing agent because the capsules have to demonstrate multifunctional properties. They (i) have to protect the healing agent over long time, (ii) have to release the healing agent upon crack formation and (iii) should not affect the fresh concrete workability and the early and long term mechanical properties. Moreover, it is essential that capsules can easily be mixed in concrete and/or can survive during concrete casting. Consequently, the concrete production process is not remarkably affected and the processing costs will almost not increase. In addition, the capsules should provide good barriers against both oxygen and water to protect the healing agent. These requirements may cause difficulties in finding a suitable encapsulation material because the capsules should not be broken during concrete preparation whereas they should break immediately upon crack formation. Although no commercial product seems suitable for these purposes, within the CAPDESIGN project it is aimed to develop, optimize and test novel types of capsules for application in self-healing concrete. Hybrid nanoparticles developed at SINTEF will be used to improve both mechanical properties and barrier properties of the capsules. Within this project, it is also the aim to develop capsules which can be applied to the concrete via an innovative placement technique. The advantage of capsules which survive during mixing/operation are (i) decreasing the cost of self-healing concrete, (ii) facilitating production of self-healing concrete by companies, (iii) open the possibility to valorise self-healing concrete. Consequently, mass production of self-healing concrete structures which do not require maintenance costs will result into a lot of economic, environmental and social advantages such as less traffic jams or higher safety levels.

Resume : Self-healing concrete aims at the autonomous healing of small cracks, with widths in the order of a few hundred micrometers. This way, the service life of reinforced concrete structures can be extended and their watertightness guaranteed for longer periods, while costly and potentially complex repair works can be avoided. As with other emerging concrete technologies, this novel repair technique poses new challenges regarding the conception of new test methods that are able to assess its performance in realistic conditions and able to continuously monitor it. Thus, this paper presents results of applying the transmission of ultrasonic shear waves for continuous monitoring of small scale cement mortar specimens cracked and healed with encapsulated precursors of flexible polymers. Preliminary results on larger concrete specimens showed already that this technique successfully detects the continuous hardening of the precursors after rupture of the capsules and further filling of the crack, corresponding to an increase of the amplitude of shear waves transmitted through the healed crack. Additionally, the reverse was observed when failure occurred, due to rupture of the polymer’s matrix or to its detachment from the crack walls. In this study, the transmission of ultrasonic shear waves is used to assess the limits of the elongation of the polymers bridging the cracks and their resistance to fatigue while healing a moving crack induced by cyclic loads. The polymer precursors used include moisture curing polyurethanes forming solid films or foams with closed-cell structure. ACKNOWLEDGEMENTS The research has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 309451 (HEALCON).

Resume : Utilization of expansive agents and mineral admixtures as an approach for self-healing of cementitious composites has been the subject of extensive research over the last two decades. However, it has been consistently shown that the main challenges of this approach are the significant loss in workability of fresh concrete and the occurrence of early reactions with cementitious materials, which would decrease the self-healing efficiency. Therefore, this work aims to study the influence of incorporating coated pellets with different cargoes of expansive minerals into the cementitious composites on the crack self-healing performance with the elapse of time. Four different types of pellets containing (1) Blend of high reactive Magnesium Oxide (MgO) and Bentonite, (2) Blend of MgO and Silica fume, (3) Low reactive MgO and (4) Medium reactive MgO were used. The first two types were developed in the laboratory while the latter two are commercial pellets. All of them are with (1-2) mm diameter and they were coated by a film layer of polyvinyl alcohol (PVA) to control the time of autonomic healing. These coated pellets were added to mortar mixes as partial sand replacement. Controlled width cracks of 300 µm were induced in three prism specimens from each mix after 7 days of curing in water. Then monitoring of cracks closure using optical microscope images was applied every 7 days for 28 days. Thereafter, the specimens were re-cracked by three point bending test to examine their flexural strength recovery. The results indicate that the mortar samples containing the coated pellets possessed promising crack self-healing performance compared to the control samples with superiority to those developed in laboratory.