Efficient encapsulation of water soluble inorganic and organic actives in melamine formaldehyde based microcapsules for control release into an aqueous environment

Highlights

• Water soluble actives encapsulated for long-term storage in aqueous environment.

• Novel melamine formaldehyde-based microcapsules were fabricated and characterized.

• Superhydrophobic OTS in the shell has generated an outstanding barrier property.

• No active leakage from the microcapsules to water at least 1 month is remarkable.

Abstract

Melamine formaldehyde (MF) is a versatile and widely used chemical cross-linking agent in microcapsule shells for the encapsulation of oil phases in an aqueous continuum. However, it has not been possible to form MF microcapsules with water soluble actives, to achieve sustained/no release in aqueous environment. Herein, 5 types of MF based microcapsules were designed and synthesised with 4 polymers, including neutral and polar poly(acrylamide-acrylic acid), anionically charged biocompatible shellac and ion exchange resin polystyrene sulfonate as well as a superhydrophobic polymer precursor octadecyltrichlorosilane, via an in situ polymerisation method. Potassium chloride and allura red (dye, M_w = 496.42 g·mol⁻¹) were used as models of inorganic and organic water soluble actives with low molar mass, respectively, to study their size distributions, encapsulation efficiencies, payloads, morphologies and release rates in aqueous environment. The release rates ranged from complete release within 15 min to no release up to 1 month in aqueous environment.

Introduction

Microencapsulation is an important technology to segregate and stabilise active ingredients from other ingredients in a formulation and/or control the active release, in a range of industrial sectors including healthcare, household care, cosmetics and agrochemicals (Ciriminna and Pagliaro, 2013, Ciriminna et al., 2011, Mitragotri et al., 2014). Various techniques including interfacial polymerisation, in situ polymerisation, solvent evaporation and coacervation, have been developed to encapsulate different types of active ingredients, such as oil-soluble, water-soluble, amphiphilic, powders or cells (Casanova and Santos, 2016, Jyothi et al., 2010, Kim et al., 2014, Lee et al., 2016, Windbergs et al., 2013). Encapsulation of water-soluble active ingredients is usually achieved by forming a water in oil (W/O) emulsion, initially stabilised by the self-assembly of molecular and/or polymolecular surfactants at the liquid/liquid interface, followed by further consolidation through chemical cross-linking of the surfactants at the interface, leading to robust isolatable capsules (Ciriminna et al., 2011, Zhang et al., 2012). The actives that have been encapsulated range from biomacromolecules, such as proteins, (Mitragotri et al., 2014) polysaccharides (Sui et al., 2013) and enzymes (Abdekhodaie et al., 2015, Keen et al., 2014) to small molecules, such as doxorubicin, (Sui et al., 2013, Zhao et al., 2015) peroxide, (Dogan et al., 2017) polyphenols (Elabbadi et al., 2011) and inorganic salts (Bhaumik et al., 2015, Sui et al., 2017). Regarding the encapsulation of potassium chloride (KCl) in ethylcellulose (Wu et al., 2003a) and Eudragit (Wu et al., 2003b) microspheres with a size distribution of 180 to 830 μm in diameter, as well as the saturated polyglycolyed glycerides matrices (Wu et al., 2002) with unknown size range, they showed a sustained release of KCl for 6 h in water. The novel encapsulation of potassium chloride (KCl) in PSS-silica microspheres achieved a sustained release (>48 h) in water, but its limitation is the exploitation of anion exchanged PSS resin only effecting on the cationically charged actives (Sui et al., 2017).

Melamine formaldehyde (MF) is a versatile chemical cross-linking agent and has been broadly used in a number of encapsulation applications, due to its polycondensates providing excellent cross-linking in microcapsule shells, leading to low leakage, and thermal and mechanical stability, as well as acid/alkaline resistance of the microcapsules (Chen et al., 2016, Ge et al., 2016, Ruan et al., 2014). The amidogen and hydroxyl groups of the MF precondensate offer plenty of functional sites to integrate with various molecules, displaying different properties. For example, MF polycondensates have been used as shell material to encapsulate oils in carbonless copy paper (White, 1998), fragrant oil (Ghaemi et al., 2016, Mercade-Prieto et al., 2012) and herbicide/insecticide delivery (Pretzl et al., 2012). There are many other examples which utilise MF as a shell cross-linking material to encapsulate oil phase/hydrophobic ingredients via the formation of O/W emulsion (Ghaemi et al., 2016, Long et al., 2009, Mercade-Prieto et al., 2012). Long et al. (2010) prepared MF based microcapsules which exhibited a sustained release of only 2% of the encapsulated core oil over 24 h (Long et al., 2010). It is comparatively easy to disperse the MF precondensate shell materials in the aqueous continuous phase forming the MF shell at the interface of the O/W emulsion via in situ polymerisation process. However, there are few examples utilising MF to form microcapsules to encapsulate water soluble ingredients, since the continuous phase is oil. It is still a challenge to encapsulate small water soluble ingredients exhibiting a long-term release, or no release, in an aqueous continuum environment. Herein, a novel way to synthesise MF microcapsules was developed, as well as MF microcapsules fabricated in combination with poly(acrylamide-acrylic acid (PAA), shellac, polystyrene sulfonate (PSS) or octadecyltrichlorosilane (OTS) to encapsulate water soluble ingredients, including potassium chloride (KCl) and allura red (dye, M_w = 496.42 g·mol⁻¹) as models of inorganic salt and organic molecule, respectively. These microcapsules were formed via an in situ polymerisation method, and the different microcapsule shells provided varying releases rates of KCl and allura red. The success in encapsulation of water-soluble active ingredients using MF based microcapsules can significantly broaden their application scope and make them even more versatile.