Encapsulation types

Figure 187. Types of ice storage tanks, Left Ice on coil type, Center Slurry type, Right Encapsulated type...

Fig. 13.3.2 Schematic preparation processes of ordered units and composite or encapsulated-type particles.

Encapsulation type The type of encapsulation technique (e.g., auger, vacuum, dosator) required for the formulation needs to be determined and justified. Examples are... 

The type of encapsulation system, together with encapsulating material properties, significantly affect the release properties of the payload. In Figure 37.3, a schanatic representation of the release profiles of the main classes of encapsulation types is reported. 

Encapsulated type. Discrete particles of the colorant are trapped inside zircon crystals, e.g., the iron coral pink where very fine particles of red Fe203 are incorporated in zircon crystals. 

Data sheet values. Table 2.27 contains a list of property values for a wide range of epoxy molding compounds— both for the low-pressure encapsulation types and the high-pressure types. 

Cables should, preferably, be connected only at apparatus terminals. If intermediate joints are made, they should be of the encapsulated type, or a terminal box sealed with compound should be used. 

Lattice type. The dopant ion replaces a Zr ion in the zircon lattice, e.g., vanadium blue (V replaces Zr ) and praseodymium yellow replaces Zr ). Encapsulated type. Discrete particles of the colorant are trapped inside zircon crystals, e.g., the iron coral pink where very fine particles of red Fe203 are incorporated in zircon crystals. 

Metallofullerenes are commonly found witli [74], [76], [80] and [82] fullerene and span composites tliat have a single (Mf2 Cg2), two or even tliree metal atoms encapsulated. The first type of... 

Classification of the many different encapsulation processes is usehil. Previous schemes employing the categories chemical or physical are unsatisfactory because many so-called chemical processes involve exclusively physical phenomena, whereas so-called physical processes can utilize chemical phenomena. An alternative approach is to classify all encapsulation processes as either Type A or Type B processes. Type A processes are defined as those in which capsule formation occurs entirely in a Hquid-filled stirred tank or tubular reactor. Emulsion and dispersion stabiUty play a key role in determining the success of such processes. Type B processes are processes in which capsule formation occurs because a coating is sprayed or deposited in some manner onto the surface of a Hquid or soHd core material dispersed in a gas phase or vacuum. This category also includes processes in which Hquid droplets containing core material are sprayed into a gas phase and subsequentiy solidified to produce microcapsules. Emulsion and dispersion stabilization can play a key role in the success of Type B processes also. 

Interfacial Polymerization. Many types of polymerization reactions can be made to occur at interfaces or produce polymers that concentrate at interfaces thereby producing microcapsules. Accordingly, this approach to encapsulation has steadily developed into a versatile family of encapsulation processes. Figure 4 schematically illustrates five types of encapsulation processes that utilize these types of reactions. 

Figure 5 illustrates the type of encapsulation process shown in Figure 4a when the core material is a water-immiscible Hquid. Reactant X, a multihmctional acid chloride, isocyanate, or combination of these reactants, is dissolved in the core material. The resulting mixture is emulsified in an aqueous phase that contains an emulsifier such as partially hydroly2ed poly(vinyl alcohol) or a lignosulfonate. Reactant Y, a multihmctional amine or combination of amines such as ethylenediamine, hexamethylenediamine, or triethylenetetramine, is added to the aqueous phase thereby initiating interfacial polymerisation and formation of a capsule shell. If reactant X is an acid chloride, base is added to the aqueous phase in order to act as an acid scavenger. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

Figure 4c also describes the spontaneous polymerisation ofpara- s.yX en.e diradicals on the surface of soHd particles dispersed in a gas phase that contains this reactive monomer (16) (see XylylenePOLYMERS). The poly -xylylene) polymer produced forms a continuous capsule sheU that is highly impermeable to transport of many penetrants including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. This process is a Type B encapsulation process, but is included here for the sake of completeness. 

Another biomedical appHcation of mictocapsules is the encapsulation of Hve mammalian ceUs for transplantation into humans. The purpose of encapsulation is to protect the transplanted ceUs or organisms from rejection by the host. The capsule sheU must prevent entrance of harmful agents into the capsule, aUow free transport of nutrients necessary for ceU functioning into the capsule, and aUow desirable ceUular products to freely escape from the capsule. This type of encapsulation has been carried out with a number of different types of Hve ceUs, but studies with encapsulated pancreatic islets or islets of Langerhans ate most common. The alginate—poly(L-lysine) encapsulation process originally developed in 1981 (54) catalyzed much of the ceU encapsulation work carried out since. A discussion of the obstacles to the appHcation of microencapsulation in islet transplantation reviewed much of the mote recent work done in this area (55). Animal ceU encapsulation has also been researched (56). 

For prolonged action therapy, granular-sized encapsulated particles, ie, beads, are used and can be both uncoated or coated. The uncoated beads provide the initial dose the others are made to dissolve at various rates depending on the coating type and thickness. 

Joints are stmcturaHy unique. They permit bodily movement and are bound together by fibrous tissues known as ligaments. Most larger joints are encapsulated in a bursa sac and surrounded by synovial fluid which lubricates the joint continuously to reduce friction. The skeleton is constmcted of various types of moveable joints. Some joints allow for no movement, such as those connecting the bones of the skull. Other joints permit only limited movement. For example, the joints of the spine allow limited movement in several directions. Most joints have a greater range of motion than the joints of the skull and spine. 

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