Microcapsule particle size

Fig. 4—In vitro release profile of microcapsules (particle size ( ), 540 pm core polymer ratio, 1 2) RSPM A,S 100 n,L 100.

The composite copper-plating coating containing lube oil-microcapsules (particle size 3-8 pm) showed special regularity (curves 3,4, 5, and 6). First, the anodic corrosion current on the surface of unworn composite copper-plating coating... 

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. 

A method of the preparation of polylactic acid microcapsules of controlled particle size and drug loading, J. Microencapsul.,... 

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. 

The widely used organophosphate Insecticide methyl parathlon was the first material to be formulated as a microencapsulated pesticide. This formulation, sold under the tradename PENNCAP-M Insecticide (a registered trademark of Pennwalt Corporation), consists of nylon-type microcapsules which contain the active Ingredient. The capsules are suspended In water and typically have an average particle size of approximately 25 microns (fifty percent by weight of the capsules have a particle size of 25 microns or more). Upon application by conventional spray equipment the water evaporates, and the active Ingredient Is slowly released over an extended period of time. 

Microencapsulation is a process in which a pure active ingredient or a mixture of ingredients is coated with or entrapped within a protecting material or system (see also Chapter 24). As a result, useful and otherwise unusual properties may be conferred to the microencapsulated ingredient(s), or unusefiil properties may be eliminated from the original ingredient(s) (Shahidi and Han 1993). The particle size of microcapsules formed by either encapsulation or entrapment can vary between... 

Microcapsule Format for delivery (i.e., liquid or powder) Storage stability Stability to different process conditions Release properties Particle size Payload (bioactive core loading) Cost of production... 

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. 

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-... 

Microcapsules containing polymer and pigment were prepared in [299] by dispersing a viscous suspension of pigment and oil-soluble shell monomer forming o/w emulsions. Subsequently, a water-soluble shell monomer was added to the emulsion droplets, encapsulating them via interfacial polycondensation. These microcapsules were then heated for free radical polymerisation of the core monomers. It has been shown that polyvinyl alcohol (PVOH) used as stabiliser reacts with the oil-soluble shell monomers. The decrease of PVOH concentration as result of this interaction leads to coalescence of the particles and to the increase of their equilibrium particle size, however, methods are proposed to prevent the depletion of PVOH. 

In an early study by Lin et al., insulin-loaded polylactic acid (PLA) microcapsules were synthesized by an emulsification-solvent evaporation process originally reported by Beck et al. Several parameters in the synthesis process were modified with the intention of optimizing the insulin release profile. Such modifications included variations in types, concentrations, and viscosities of protective colloids used in the emulsification process. Polyvinyl alcohol (PVA), when used as the protective colloid in the fabrication process, was found to produce the PLA microparticles in reproducible quality. Further studies revealed that the concentration PVA directly affects the PLA particle size and the surface characteristics of the microcapsules. With higher concentrations of PVA, microparticles tended to be smaller and to have a smoother surface. When the release profiles of the microcapsules were stud-... 

The composition, mechanism of release, particle size, and final physical form of microcapsules can be changed to suit specific applications. The properties of the shell materials are extranely important for the stabilization of the core material. Importantly, it must be inert toward active ingredients. 

Most commercial processes use this type of polymerization to produce small uniform capsules in the range of 20-30 micron diameter however, the process can be tuned to produce large microcapsules. The size of these microcapsules and the properties of the wall material/polymer matrix can be altered by using different monomers, utilizing additives, and adjusting reaction conditions. The encapsulation occurs by wall formation around the dispersed core material via the rapid polymerization of monomers at the surface of the droplets or particles. The solution of a multifunctional monomer in the core material is dispersed in an aqueous phase. The polymerization is commenced at the surfaces of the core droplets forming the capsule walls, by adding a reactant to the monomer dispersed in the aqueous phase. 

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. ... 

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. 

Rourke JK. Continuous production of Emulsions and microcapsules of uniform particle size. US patent 5643506,1997. 

The lowest particle size of microparticles is 1 pm and the largest size is 1 mm. Commercial microparticles have a diameter range of 3 to 800 pm. Microparticles may be formulated as microcapsules or as microspheres that differ in morphology and internal structure. In addition other terms such as beads, microbes are also being used alternatively. 

Increasing the amount of cross-linking agent leads to an increase in the encapsulation ability. Few studies have reported that the particle size of the microcapsule was not affected by the difference of cross-linking agents. 

The particle size distribntion of microcapsules can also be measured using laser diffraction particle sizing, Mastersizer andZetasizer. For microcapsules prepared for inhalation, particle size measurement of the dry powders formnlated can be measured more accurately by the dry dispersion method. The Scirocco accessory of the Malvern Mastersizer allows particle size measurement of dry powders, with particle flow controlled by a variable feed-rate vibrating tray. 

In this paper, we report some experimental results of the studies for the permeability characteristics, particle size, and charge density of poly(L-lysine-a/r-terephthalic acid) microcapsule membrane. On the basis of such information, we shall discuss die pH- and ionic strength- induced structural change of these microcapsules. 

---