Microcapsule shell wall

Microencapsulation is also useful in enhancing the performance of lubricating oils when used at the application site. Pressure-ruptured microcapsules containing lubricating oil are used in powder metallurgy. The requirements for the microcapsule shell wall is that it should have a high abrasion resistance, be able to withstand... 

Rapid and facile generation of capsules from tandem assembly in aqueous media is amenable to encapsulation of water-soluble compounds. Encapsulation of ICG dye within PAH/H2PO4 aggregates was shown by Yu et al. Enzyme encapsulation and the feasibility of capsules to serve as reaction vessels was demonstrated by Rana et al. In their study, they encapsulated acid phosphatase enzyme in PLL-citrate-silica sols and suspended the spheres in a solution containing fluorescein diphosphate. Fluorescence increased in intensity within the shell walls as fluorescein was formed by enzymatic cleavage of phosphate groups. This study showed that microcapsules could serve as reaction vessels that allow enzymatic action to take place in a protective environment and allow for reactants and/or products to diffuse through permeable shell walls. 

The type of coating/shell-wall material(s). The selection of coating material decides the physical and chemical properties of microcapsules/microspheres. The polymer should be capable of forming a film that is cohesive with the core material. 

Aliphatic or aromatic structure, as weU as liner or branched structure of the reactants, can give the microcapsule shell different porosity and permeability, which can greatly inflnence the release performances. Multifunctional reactants can help to achieve more thermal mechanical stable microcapsules since the wall is a three-dimensional cross-linked polymer network. Experiments have shown that dichlorides with less than eight carbon atoms do not prodnce qnahty polyamide microcapsules. The reason behind this is the competition between interfacial condensation and the hydrolysis reaction of dichlorides. More hydrophobic dichlorides can favor the polymerization and slow the hydrolysis. Similarly, for polyurethane and polyurea type microcapsules, polymeric isocy-nates are preferred because they might favor the formation of less permeable miCTocapsnles for the hydrolysis of isocynate groups are limited, which consequently reduced the COj release that contribute to the porosity increase of the polymer wall." ... 

Other encapsulations utilize more or less similar methods for the formation of the capsule wall. Complex coacervation utilizes the reaction of an anionic water-soluble polymer with a cationic material to form the shell wall that separates from the solution. As the coacervate separates from the solution, it will tend to coat suspended particles with a protective shell. The shell wall is then hardened with a cross-linking agent. In situ polymerization is used to form urea formaldehyde or melamine formaldehyde shells by using heat to cross-link the monomers forming the shell waU. Interfacial polymerization with isocyanates via hydrolysis is another method to form a shell wall at an organic-water interface. In this case, water acts to hydrolyze some of the polyisocyanate to an amine, which cross-links to form the polyurea microcapsule waU. 

The abundance of natural and man-made polymers provides a wider scope for the choice of shell material, which may be made permeable, semi-permeable or impermeable. Permeable shells are used for release applications, while semi-permeable capsules are usually impermeable to the core material but permeable to low molecular-weight liquids. Thus, these capsules can be used to absorb substances from the environment and to release them again when brought into another medium. The impermeable shell encloses the core material and protects it from the external environment Hence, to release the content of the core material the shell must be ruptured by outside pressure, melted, dried out dissolved in solvent or degraded under the influence of light (see Chapter 7). Release of the core material through the permeable shell is mainly controlled by the thickness of the shell wall and its pore size. The dimension of a microcapsule is an important criterion for industrial applications the following section will focus on spherical core-shell types of microcapsules (Fig. 1.8). 

Although a variety of alternative microencapsulation techniques is available (for details of sol-gel techniques, see Chapter 8), no single method is suitable for encapsulating different types of core material. Ultimately, the best method will depend upon the type of core material, the required particle size, the permeability of the shell wall, and the different properties of the microcapsule, and consequently the process must be custom-tailored in order to provide a satisfactory outcome. An overview of the size of microcapsules obtained by different techniques is provided in Table 1.2. 

There is a so-called strategy "the ship in a bottle" which uses variation of microcapsule shell permeability to fill a ready-made capsule with macromolecules synthesized directly inside it. According to this approach, monomers which can get through the walls of capsules are polymerized inside the shell. As a result a synthesized polymer gets locked inside the internal volume of a microcapsule. ... 

The appeal of DCPD and ENB for self-healing has led to the development of several encapsulation techniques for these monomers. Most prominent techniques consist of forming a shell material around monomer droplets by in situ polymerization of an amino resin melamine-urea-formaldehyde (MUF, Figure 10.2a) [56], urea-formaldehyde (UF, Figure 10.2b) [13-16], or melamine-formaldehyde (MF) [17, 18] walls in an oil-in-water emulsion. Comparison of microcapsules made with different shell wall materials is challenging because... 

Scanning Electron Microscopy was performed to analyze surface morphology and shell wall thickness of the capsule. A dried microcapsule was placed on a conductive carbon tape attached to a mounting piece for imaging. Few microcapsules were mptured with a razor blade to confirm the presence of alkyd resin inside the UF shell. 

Scanning electron microscopy was performed to analyze capsule surface morphology and shell wall thickness. Figure 5 shows a resin-solvent filled microcapsule with rough exterior shell wall and size in the range of 450 pm. 

Figure 6. Shell wall morphology of a ruptured microcapsule.

Some microcapsules were ruptured with a razor blade to allow for viewing of the inner shell wall morphology. Figure 6 shows the shell wall morphology comprising of two distinct regions of thin continuous iimer shell wall and rough exterior shell wall. 

Alkyd resin along with solvent has been successfully encapsulated inside UF shell by In situ encapsulation technique. The SEM analysis shows that microcapsules were comprised of a thin continuous shell wall and a rough exterior shell wall. The FTIR spectra of original and soxhlet extracted resin proves the encapsulation of alkyd resin inside the shell. Moreover DSC and TGA analysis shows that the microcapsules contain both resin-solvent mixture as core and UF as shell material. These microcapsules incorporated in paints or coatings release the healing material when scratched which in turn heals the scratches and cracks. 

Many terms have been used to describe the contents of a microcapsule active agent, actives, core material, fill, internal phase (IP), nucleus, and payload. Many terms have also been used to describe the material from which the capsule is formed carrier, coating, membrane, shell, or wall. In this article the material being encapsulated is called the core material the material from which the capsule is formed is called the shell material. 

The release rates that are achievable from a single microcapsule are generally zero order, half order, or first order. Zero order occurs when the active ingredient/core is a pure material and releases through the wall of a mononuclear (core-shell) microcapsule as a pure material. Half-order release generally occurs with matrix particles. First-order release occurs when the core material is actually a solution. As the solute material releases from the capsule, the concentration of solute material in the solvent decreases and a first-order release is achieved. A mixture of microcapsules will include a distribution of capsules varying in size and wall thickness, and the release rate would... 

In order to prepare the microcapsule with expected structure, the chosen polymer wall material should match with the core material. The affinity interaction between the polymer wall and the core material can determine the structure of the microcapsule. Highly cross-linked polyurea and polyurethane can form so-called compact capsules with 2-methylbenzothiazole homogeneously distributed in the polymer matrix, while in the case of polyamide core-shell structure capsules were... 

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