Microcapsule size distribution

Using the so-called two-step process [15, 16], polymer nanoparticles are first synthesized via emulsion polymerization. The size of the resulting nanoparticles can be tuned by a simple process parameter and covers a range of about 30-400 nm. In a second step these nanoparticles are used to coat microbubbles in a controlled bubble formation process. The nanoparticles migrate to the surface of the bubbles (this is related to the interface activity of hydrophobic nanoparticles in general) and build a monolayer around the bubbles. Consequently, the size of the nanoparticles determines the shell thickness of the final microcapsules. Additionally, a carefully chosen nanoparticle concentration regime results in a certain microcapsule size distribution. In principle, particle sizes in the range of 0.5-10 jum can be adjusted and the microcapsule size distributions are ex-... 

Kentepozidou A, Kiparissides C. Production of water-containing polymer microcapsules by the complex emulsion/solvent evaporation technique. Effect of process variables on the microcapsule size distribution. Journal of Microencapsulation. November-December 1995 12(6) 627—638. PubMed PMID 8558385. 

Microcapsules can have a wide range of geometries and stmctures. Figure 1 illustrates three possible capsule stmctures. Parameters used to characteri2e microcapsules include particle size, size distribution, geometry, actives content, storage stabiHty, and core material release rate. 

Particle size distribution, on a population basis, presented a predominantly unimodal distribution, with a mean size of 26.53 pm for 1 1 ratio microcapsules and 50.29 pm for 2 1 ratio systems. On a population basis the number of aggregates is small, although some of those produced from the 2 1 core wall systems were 200-300 pm. 

Guang Hui Ma et al. [83] prepared microcapsules with narrow size distribution, in which hexadecane (HD) was used as the oily core and poly(styrene-co-dimethyla-mino-ethyl metahcrylate) [P(st-DMAEMA] as the wall. The emulsion was first prepared using SPG membranes and a subsequent suspension polymerization process was performed to complete the microcapsule formation. Experimental and simulated results confirmed that high monomer conversion, high HD fraction, and addition of DMAEMA hydrophilic monomer were three main factors for the complete encapsulation of HD. The droplets were polymerized at 70 °C and the obtained microcapsules have a diameter ranging from 6 to 10 pm, six times larger than the membrane pore size of 1.4 p.m. 

Nevertheless, monolithic microspheres can be made to release drug at an approximately constant rate. " The core loading of these microspheres may be increased to create structures similar to those of reservoir microcapsules. An optimum combination of particle sizes (a size distribution), may be prepared to achieve a constant rate of drug release. Preparing microspheres with an erodible polymer in such a way that maximum erosion occurs in conjunction with minimum diffusion may establish a constant release rate. Although the principles described here appear simple, they are difficult to utilize because of their dependence on a number of factors, each of which can complicate the process. 

The properties of the microcapsule in relation to use, size distribution, porosity and permeability of the wall... 

Nuyken, O. Dauth, J. Pekruhn, W. Thermosensitive microcapsules. II. New azomonomers, homo- and cocondensation, microencapsulation, release measurements, size distribution and thermo-printing. Angewandte Makromolekulare Chemie (1991), 190, 81-98. 

Li, Z. Chen, S. Zhou, S. Factors affecting the particle size and size distribution of polyurea microcapsules by interfacial polymerization of polyisocyanates. International Journal of Polymeric Materials (2004), 53(1), 21-31. 

In most cases, anionic water-soluble polymers such as poly(styrene-maleic anhydride), polyacrylic acid, etc., are apphed. These kinds of emulsifiers can influence the microcapsule preparation, mean particle size, and particle size distribution. By emulsification, an electric double layer generates on the dispersed phase. Then the electrostatic interactions between the protonated amino resin prepolymer and the negatively charged orgaific phase can act as a driving force, which enable the wall material polycondensate on the surface of the oil droplets but not throughout the whole water phase. ... 

FIGURE 19.6 Mean size and size distribution of polysulfone/vanillin microcapsules. 

Delivery devices do not necessarily have a spherical shape, as illustrated in Figure 32.2e. A great variety of shapes can be obtained when a solid core material is encapsulated by a shell. Particle size is an important characteristic of these structures because it is one of the many parameters that can be tailored to control release rates of encapsulated ingredients. However, the production of microcapsules often gives a certain particle size polydispersity. The active ingredient release kinetics depends on the particle size distribution. It is thus necessary to determine both the mean particle size and the size distribution for the targeted delivery. 

Size and size distribution (narrow) of dispersed oil drops (microcapsules/nanocapsules) (100-0.1 pm)... 

Special etherified melamine derivates meet the requirements for microcapsule manufacture. They are used to maintain the size distribution of the created droplets until the resin system fully coats them. 

The microencapsulation process includes two main steps the emulsification step and the formation of the capsules. The emulsification step determines the size, and the size distribution of the microcapsules may be influenced at once by physical parameters such as the apparatus configuration, the stirring rate, the temperature, and the volume ratio of the two phases, and also, by the physicochemical properties such as the interfacial tension, the viscosities, the densities, and the chemical compositions of the two phases. 

Much work has been devoted in recent years to preparing micropartieles of narrow size distribution, with different biodegradable polymers. The sizes of common microcapsules are 40 to 50 urn (70, 71, 76-78). 

To produce microgels that have a very large pore structure with a high internal fluid content, one can utilize cross-junctions in the microfluidics to encapsulate quantum dot metal cores into a hydrogel microcapsule with size distributions typically within 5 %. Using such techniques, it is possible to obtain microgel particles in the size range of 10-1,000 pm [3]. 

To prepare microcapsules with entrapped protein (or DNA), CaCOa porous spherical microparticles (an inorganic core) with narrow bead-size distribution (mean size of 5 p.m) have been elaborated (Figure 30.5). 

Poncelet, B., Poncelet, D., and Neufeld, R. J., Control and mean diameter and size distribution during formulation of microcapsules with cellulose nitrate membranes. Enzyme Microb. TechnoL, 11,29-37, 1989. 

Good results in terms of recovery, shape, size distribution were obtained in the case of blank microcapsules (Table 1). The presence of oil causes deficiencies of the microcapsule recovery. In order to facilitate the encapsulation of oil, this should have a density comparable to that of the aqueous external phase and complete immiscibility with the external phase [6,9]. This could explain the increased yield micnoenc ulation of Rosemary oil (density 0.91 g/ml) compart to limonene (density 0.84 g/ml). 

Capsule diameter (pm) Figure 5.5 Size distributions of microcapsules containing insecticide [28]. 

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