(37d) Environmentally and User-Friendly Composite Microcapsules with Superior Mechanical Properties for Potential Applications in Fast-Moving Consumer Goods

Core-shell microcapsules are used in the broader fast-moving consumer goods (FMCG) industry to encapsulate value-added fragrances, flavours, or other bioactive ingredients. These microcapsules encase functional species within protective shells, thereby enabling their controlled and efficient release. This leads to enhanced product effectiveness and prolonged shelf-life of the final FMCG, all while safeguarding costly ingredients from harsh environmental factors (temperature, pH, UV radiation) [1]. In recent years, the mechanical properties of microcapsules have garnered significant attention due to their direct influence on stability, durability, and overall performance in a plethora of end-use applications, such as detergents, foams, and everyday cosmetics. To this end, various shell materials are engineered to meet specific mechanical requirements [2]. Interestingly, synthetic shells, such as polyurethane, polyurea and polystyrene, can offer a balance of thermal, chemical and mechanical resistance, as well as providing protective barrier for active species. However, they are largely non-biodegradable and contribute to the build-up of microplastics in the environment [3]. Inorganic shells like silica can provide high thermal stability while remaining chemically inert. However, silica is inherently brittle, making silica-based shells subject to cracking or breaking under mechanical stress. Calcium carbonate (CC) is environmentally benign and has been demonstrated to economically form microcapsule shells [4]. Despite its great potential, their mechanical and barrier properties remain to be significantly improved for multifunctional applications.

Herein we present core-shell composite microcapsules with an inert model hydrophobic core (hexyl salicylate) which had been stabilised both electrostatically and sterically utilising cetyltrimethylammonium bromide (CTAB) and fumed hydrophilic silica nanoparticles (SiO₂ NPs), respectively. A seeded CC shell was formed via interfacial crystal ripening facilitated by the presence of polyacrylic acid (PAA) [5]. Subsequently, an additional coating of polydopamine (PDA) was formed via oxidative auto-polymerisation (pH 8.5) to enhance the structural stability of the CC crystals. The morphology and barrier properties of the ensuing microcapsules were characterised via fluorescence sensing/scanning (SEM)/transmission electron microscopy (TEM), energy dispersive x-ray (EDX) analysis, and UV-Vis spectrophotometry, respectively. Importantly, their mechanical properties were assessed by a micromanipulation technique based on parallel plate compression of single particles [6].

There appeared to be core-shell microcapsules (D_[3,2] = 19.7 ± 0.3 μm) with a spherical morphology. Topographical analysis revealed a continuous and relatively smooth surface (Figure 1). EDX analyses confirmed the presence of SiO₂ (1.0% wt.) due to the nanoparticles and a high content of CC (15.2% wt.). Over 24 h in a receptor medium (36% (v/v) hydro-propanol) where the solubility of hexylsalicylate is ~10.5 kg/m³ at 25 ^oC, ~50% of the core was released, with the remaining load being released within 6 days. The corresponding shell permeability/shell thickness was evaluated to be ((5.8 ± 1.3)·10^-7 m/s) via solute-diffusion modelling, which is statistically similar to that of microcapsules with a shell of melamine formaldehyde ((2.4 ± 0.9)·10^-6 m/s) [7]. TEM analysis confirmed the presence of a multi-layer composite shell, featuring a thick CC based coating of ~1 µm. Under compression, the microcapsules yielded a mean nominal rupture stress and tension of 73.5 ± 5.0 MPa and 485.6 ± 28.8 N/m, respectively, which are significantly greater than any microcapsules reported in literature. This finding confirms the extremely high mechanical strength of these microcapsules, which could be particularly appealing to many industrial sectors, including pharmaceuticals, nutraceuticals, personal care, cosmeceuticals, to name a few. Details of the study will be presented.

References

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