Synthesis of calcium carbonate microcapsules as self-healing containers

Abstract

Contemporary studies of self-healing polymer composites are based on microcapsules synthesized using synthetic and toxic polymers, biopolymers, etc. via methods such as in situ polymerization, electrospraying, and air atomization. Herein, we synthesized a healing agent, epoxy (EPX) encapsulated calcium carbonate (CC) microcapsules, which was used to prepare self-healing EPX composites as a protective coating for metals. The CC microcapsules were synthesized using two facile methods, namely, the soft-template method (STM) and the in situ emulsion method (EM). Microcapsules prepared using the STM (ST-CC) were synthesized using sodium dodecyl sulphate (SDS) surfactant micelles as the soft-template, while the microcapsules prepared using the EM (EM-CC) were synthesized in an oil-in-water (O/W) in situ emulsion. These prepared CC microcapsules were characterized using light microscopy (LMC), field emission scanning electron microscopy (FE-SEM), fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and thermogravimetric analysis (TGA). The synthesized ST-CC microcapsules were spherical in shape, with an average diameter of 2.5 μm and an average shell wall thickness of 650 nm, while EM-CC microcapsules had a near-spherical shape with an average diameter of 3.4 μm and an average shell wall thickness of 880 nm. The ST-CC capsules exhibited flake-like rough surfaces while EM-CC capsules showed smooth bulgy surfaces. The loading capacity of ST-CC and EM-CC microcapsules were estimated using TGA and found to be 11% and 36%, respectively. The FTIR and NMR spectra confirmed the EPX encapsulation and the unreactive nature of the loaded EPX with the inner walls of CC microcapsules. The synthesized CC microcapsules were further incorporated into an EPX matrix to prepare composite coatings with 10 (w/w%), 20 (w/w%), and 50 (w/w%) capsule loadings. The prepared EPX composite coatings were scratched and observed using FE-SEM and LMC to evaluate the release of encapsulated EPX inside the CC capsules, which is analogous to the healing behaviour. Moreover, EPX composite coatings with 20 (w/w%) and 50 (w/w%) of ST-CC showed better healing performances. Thus, it was observed that ST-CC microcapsules outperformed EM-CC. Additionally, the EPX/CC coatings showed remarkable self-healing properties by closing the gaps of the scratch surfaces. Thus, these formaldehyde-free, biocompatible, biodegradable, and non-toxic CC based EPX composite coatings hold great potential to be used as a protective coating for metal substrates. Primary results detected significant corrosion retardancy due to the self-healing coatings under an accelerated corrosion process, which was performed with a salt spray test.

Fig. 1. The schematic of ST-CC microcapsules synthesized by STM, (a) process steps, and (b) the formation mechanism.

Fig. 2. The schematic of EM-CC microcapsules synthesized by EM, (a) process steps, and (b) the formation mechanism.

Fig. 3. The FE-SEM images of ST-CC microcapsules (a and b) before heat treatment, (c) hollow microcapsules after heat treatment process, (d) after encapsulation of EPX by vacuum evacuation method, (e) EM-CC microcapsules with bulgy surface, and (f) the encapsulated EPX resin within EM-CC microcapsules after breaking it.

Fig. 4. The thermogravimetric analysis (TGA) thermograms of (a) ST-CC hollow microcapsules, (b) ST-CC filled microcapsules, (c) EM-CC hollow microcapsules, (d) EM-CC filled microcapsules, and (e) EPX resin (ARALDITE 506) polymer.

Fig. 5. The FTIR spectra of (a) EPX resin (ARALDITE 506), (b) EM-CC microcapsules, (c) EM-CC microcapsules without EPX, (d) ST-CC microcapsules, and (e) ST-CC hollow microcapsules (* denotes similar peaks corresponding to CC shell material).

Fig. 6. The ¹³C solid NMR spectra of (a) EPX encapsulated ST-CC microcapsules, and (b) EPX encapsulated EM-CC microcapsules.

Fig. 7. The FE-SEM images of the scratch test on the EPX composite coatings (a) neat [EPX], (b) EPX loaded ST-CC 50 wt% [EPX/ST-CC 50], (c) EPX loaded EM-CC 50 wt% [EPX/EM-CC 50], (d) EPX loaded ST-CC 20 wt% [EPX/ST-CC 20], and (e) EPX loaded EM-CC 20 wt% [EPX/EM-CC 20] after 24 h of scratching, carried out with the help of a sharp object.

Fig. 8. The light microscopy images show a better dispersion of (a) EM-CC microcapsules 10 wt% loaded composite thin coating and (b) ST-CC 10 wt% loaded composite thin coating before scratching.

Fig. 9. The light microscopy images of (a–d) EPX/ST-CC10 with EPX thin composite coating on the glass substrate, thereby confirming the release of the encapsulated EPX after scratching with a sharp object.

Fig. 10. The light microscopy images of (a–c) EPX/EM-CC10 after scratching with a sharp object and (d) after applying a high force on the thin coating with a hammer head (all images were taken under 20 μm resolution).

Fig. 11. The corrosion test results for metal panels coated with (a) control EPX coating, and (b) EPX coating mixed with 50 wt% of ST-CC microcapsules. 0.1 M NaCl solution was sprayed on the metal plate and kept for 48 h before photography.