Moisture induced crack filling in barrier coatings containing montmorillonite as an expandable phase
Introduction
The durability of current man-made materials and the products made thereof is often limited by the absence of built-in ‘self healing’ mechanisms. In general terms, self healing is defined as the capability of a material to repair a deterioration of functionality – damage – spontaneously or by means of an external trigger Currently, degradation mechanisms tend to be irreversible leading finally to catastrophic failure over time, so it is desirable to develop active protection mechanisms that continuously repair damage and therefore prevent material failure.
In an ideal world, these self-healing mechanisms would lead to a complete and full restoration of the physical and chemical functionality of the system (attributive repair). However, such perfection is difficult to achieve, although partial re-establishment of the original properties of the material (functional repair) might still provide significant and important benefits.
Coatings are commonly used as thin films for protective, esthetical and mechanical reasons. Incorporating self-healing processes in coatings would provide obvious advantages but also presents significant challenges that need to be addressed. First of all, there is the requirement for the full physical restoration of the protective barrier function of the coating after the self-sealing of cracks. Compared to bulk materials, the limited thickness of the film creates difficulties in relation to the addition and dispersion of healing agents along the coating film (e.g. micro-encapsulation). For a material to heal, the transport of the healing agent to the damaged area is essential which means that the chosen transport mechanism must be able to function under the conditions imposed by the physical and chemical properties of coatings as well as external conditions, e.g. temperature and moisture.
The role of the healing agent is to restore the functionality of the damage zone and in the best circumstances to fill the damage zone. To this aim White et al. [1] have developed and extended their well known micro-encapsulation route in which a liquid healing agent is released from an inert capsule and made to react in the crack tip of a coating or bulk epoxy. A major and intrinsic drawback of this technique is the limited supply of healing agent. The micro-encapsulation technique therefore yields its best results for situations in which the restoration (blunting) of the crack tip is important and complete crack filling is not required. For the case of substantial crack filling extensive healing agent supply involving long range mass transport is required. This route has been explored successfully by Bond et al. [2], [3], [4] using filled hollow glass fibers for damage healing in fiber reinforced composites and by Toohey et al. [5] using a 3-dimensional microvascular substrate system for healing agent supply.
An alternative approach to solve the supply issue and taking into account the intrinsic nature of coatings is to make use of an external source to supply a crucial ingredient for the healing to occur. Typical examples of this route are the healing of high temperature healing of ceramics [6] or thermal barrier coatings on turbines [7] in which local oxidation is used to fill the crack with a load bearing deposit. A complete overview of the principal healing mechanisms for coatings and bulk materials can be found elsewhere [8]. In this work we report on an alternative healing mechanism that uses moisture supply through the gas phase to induce the expansion of montmorillonite and to fill cracks.
Clays or phyllosilicates are common constituents of soil and are naturally occurring materials. Due to their highly versatile properties, clays are commonly used for many different applications ranging from additions to packaging foils to increase their barrier properties [9] to making nanocomposite materials [10]. Clay minerals consist of strongly bounded sheets of silica tetrahedra and alumina octahedra bearing a negative charge at their surface that is compensated for by cationic species in between the layers [11]. Under increasing relative humidity, the cationic species hydrate, increasing the spacing in between layers (d-spacing) and so to macroscopic volume expansion of the clay. The mechanism of swelling has been extensively studied and it is generally accepted to occur in a step-wise manner during which water is absorbed in between clay layers as a monlayer/monohydrated, bilayer/bihydrated, and so forth [12].
The volumetric expansion of the montmorillonite as a function of the moisture content is shown in Fig. 1 (adapted from [13]). Results clearly indicate that at about 100% R.H. the expansion of the clay is about 70% in the perpendicular direction to the basal plane. No data on the two orthogonal directions was available but, given the nature of the expansion process, this was considered to be zero.
In a recent work Hikasa et al. [14] used the volume expansion of clays to create self healing ceramic-type coatings where they prepared multi layer coating systems made of alternating layers of hectorite and silica. In their work they studied the properties in relation to corrosion protection and found that the clay-containing multi layer coatings were, to some extent, effective towards corrosion protection even though no direct evidence for the expansion of the clay layers was presented.
Here, we describe a unique extension of this latter principle. We have investigated a two layer coating system made of a montmorillonite [12], [13], [14], [15] layer deposited on a glass substrate and covered by an engineering polysiloxane coating containing well defined scratches.
Section snippets
Materials
Natural sodium montmorillonite (EXM 757) with a CEC of about 95 mequ/100 g was supplied by industries Südchemie AG, Germany. Tetraethoxy orthosilicate (TEOS) 99+%, obtained from Sigma-Aldrich, was used as a 20 wt.% ethanolic solution to improve intercoating adhesion. A suitable primer was prepared by dissolving the appropriate amount of TEOS in a mixture of water/pure ethanol (2:98). The pH of the resulting colorless solution was adjusted to a value of approximately 2 by addition of HNO3 and
Initial scratch profile vs. load
It was observed that the severity of the damage imparted to the coating surface increased as the weight of the load increased. The width of the damage was estimated using roughness profile measurements and the values are graphically illustrated in Fig. 2.
These results show that the extent of the damage, as measured by the width of the scratch is clearly related to the applied load with a maximum width achieved when a force of around 0.3 N or higher is applied to the coating. This complements
Conclusions
The multi layer clay-containing coatings described in this paper show effective crack-filling properties when treated under moist air conditions at 50 °C highlighting the potential of clay to be used for restoration of barrier coatings. Scratch widths and damage area were obtained from roughness analysis and their evolution in time throughout the healing process was used to quantify the healing of the coatings. The actual nature of the crack filling was found to be Na-montmorillonate by an
Acknowledgements
PPG Industries Netherlands B.V. (former Ameron B.V.) is kindly acknowledged for supplying the siloxane top coat. Thanks go to Mr. Eric Peekstok and Mr. Frans Oostrum for experimental support. Alexander Schmets from DCMat is acknowledged for the fruitful discussions. Thanks go also to Dr. G. Song for the EDX measurements. This work was sponsored by the Delft Center for Materials (DCMat).
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