4.3 Materials for encapsulation of self-healing and anti-corrosion agents
4.3.1 Polymer capsules
Microencapsulation technologies have been optimised and capsules are used in several branches of industry such as pharmacy, food processing, carbonless paper and pesticides (BRACE GmbH, 2012 ) (Stenzel & Rehfeld, 2012 ). Several commercial companies have specialised in the production of polymer capsules (BRACE GmbH, 2012 ; Follmann & Co. GmbH & Co.
KG, 2012 ), the diameters of which typically range between 1 and 5000 μ m (Stenzel & Rehfeld, 2012 ).
During the encapsulation process, different chemicals are used as a basis for the formation of the polymer capsule. Approaches based on (poly)urea- formaldehyde (PUF) have been investigated by several scientists (Brown et al., 2003 ; Kumar et al., 2006 ; Yuan et al., 2006 ). White et al. (2001) described the application of fi lled PUF-microcapsules for self-healing materials. Their idea was to produce a composite material with fi lled, brittle microcapsules which break when a crack occurs. The contents then leach into the crack and polymerise in contact with a suitable catalyst positioned in the matrix, thus sealing the damaged spot. Figure 4.3 shows a scanning
4.2 Scheme of a fi lled microcapsule (left) and a fi lled porous nanoparticle (right) (Fraunhofer IFAM).
Host Guest
electron microscopy (SEM) image of a fracture surface of a composite with incorporated PUF-microcapsules. The illustrated capsule was ruptured during the sample preparation and shows an optimal adhesion to the coating.
In order to realise this self-healing concept, several requirements regarding the capsule are essential, as pointed out by Kessler (2002) . The capsule material must prevent the active species entrapped in the core from diffusion, the capsules must adhere to the polymer matrix and the capsule must provide a rupture strength that allows handling without damage but also allows the capsule to break in the composite when a crack occurs. The PUF-capsules with their rough surface in combination with a highly crosslinked network that gives the brittle structure allows these requirements to be fulfi lled (Brown et al., 2003 ). Typical PUF-capsules prepared in our laboratory as illustrated in Fig. 4.4 show a favourable surface roughness which facilitates the bonding to the coating matrix. Typical capsule diameters are in the range from approximately 10 to 100 μ m.
White et al. encapsulated dicyclopentadiene (DCPD) which polymerises in contact with Grubbs’ catalyst via ring-opening metathesis polymerisation (ROMP). In their work, the fi lled PUF-microcapsules and the Grubbs’
catalyst were incorporated into an epoxy-based composite (White et al., 2001 ). In further investigations in our laboratory, DCPD-fi lled microcapsules were added to a model formulation representing a typical polyurethane topcoat (Mock et al., 2007 ). Besides DCPD as healing agent, research
4.3 SEM picture of a broken urea-formaldehyde microcapsule in a composite (Fraunhofer IFAM).
groups encapsulated several substances with PUF and explored their use as self-healing and anti-corrosive agents in composite materials.
Using polydimethylsiloxane (PDMS) as healing agent (Cho et al., 2006 ), the catalyst di- n -butyltin dilaurate (DBTL) was encapsulated while the functionalised PDMS was phase-separated in a vinyl ester matrix. In 2007 this interesting approach was amended by encapsulation of high molecular weight vinyl functionalised PDMS with a platinum catalyst on the one hand and encapsulation of hydrosiloxane copolymer on the other hand (Keller et al., 2007 ). Both types of microcapsules were incorporated into an elastomeric PDMS matrix and analysed with respect to their self-healing properties. Also, Yin et al. (2007) investigated a two-component system as self-healing agent by embedding PUF-encapsulated diglycidylether of bisphenol-A into an epoxy matrix while a complex of CuBr 2 and 2-methylimidazole as latent hardener was uniformly dissolved in the matrix itself.
As an alternative to two-component systems that require intimate contact between the resin and the catalyst or hardener respectively, scientists have concentrated on the development and investigation of one-component systems. Encapsulated linseed oil added to an epoxy coating (Suryanarayana et al., 2008 ) may harden based on an oxidative polymerisation mechanism.
A more detailed overview of the microencapsulation approaches and also other technologies for the development of self-healing materials has been published (Murphy & Wudl, 2010 ).
4.4 SEM picture of fi lled PUF-capsules (Fraunhofer IFAM).
A special type of polymer capsule can be based on naturally occurring microparticles. As plant cells in particular are known to be surrounded by mechanically, medially and thermally stable cell walls, they constitute attractive capsule materials. Flavour-fi lled yeast cells can be considered a commercially viable (Dardelle et al., 2007 ) host/guest system of such a capsule type. Processes for immobilising a wide diversity of active agents inside natural microparticles and for incorporating the fi lled capsules into curable polymer systems have been developed in the authors’ laboratory (MultiMat, 2011 ). Demonstrating the functional principle of a mechanically stimulated release of an active agent encapsulated in a natural microcapsule embedded in a coating system, Fig. 4.5 shows an electron micrograph of a fracture surface obtained after manually scratching a functional coating.
Traces of released guest species have been detected even on the fracture surface where the matrix has lost the contact to the capsule as a consequence of the mechanical impact.
A special structural constellation included in this paragraph is a low-viscosity or high-viscosity active agent encapsulated directly in the polymer matrix, and this can be achieved by benefi ting from the phase separation that occurs during the curing of the composite. As known from adhesive bonding technology, matrices containing polymer particles can be
4.5 Electron microscopical image of a fracture surface obtained after manually scratching a functional coating system (Fraunhofer IFAM).
The active agent released from a natural capsule mechanically broken within the region of the scratch is shown in bright colours due to OsO 4 staining contrasting its C = C bonds.
applied as reinforcing coatings for sheet components (Schoenfeld &
Schumann, 2005 ).