Dissolvable cores

FIGURE 51.21 Electron micrograph of Au—Si02 (15 nm core, 10 nm shell) after exposure to 1 mM KCN at pH 10.5 in air. Hollow silica shells are obtained through complete core dissolution. Some non-dissolved cores are shown as well. 

While in the second step, a-FejO, nanoparticles were formed, in the third step, to reduce the total surface energy, the core dissolves in the presence of excess CH3COOH, while a shell of a-Fe Oj nanoparticles keeps forming on the dissolving core, resulting in hollow spheres. 

In a filament winding process the filament is applied on a mandrel. Mandrel types can be distinguished in heated mandrels, non-heated mandrels, dissolvable cores and liners. Using a heated or non-heated mandrel the quality of the mandrel surface is an important factor due to the fact that the finally wound component must be stripped (pulled or pushed) over the surface. By using a liner or a dissolvable core the surface quality is not so relevant, due to the fact that the liner remains in the product and the dissolvable core is removed after the winding process. 

Some applications are closed shapes or have a local reduction, like some pressure vessel types or rocket motor cases. If the products have no liner the core must be dissolvable. For dissolvable materials the following are usable sand (soluble, water soluble),plaster (soluble,breakout), salt (meltable, eutectic) and alloy (with a low melting temperature). All dissolvable cores are precast in an extra process step before the winding process starts. After the winding process the core is nearly complete covered with filaments. Depending on the dissolvable material used, different techniques to dissolve the core are required such as rinsing with water or heating up to a certain temperature. All dissolvable cores are non-heated. 

In some cases, especially for pressure vessels (connection quality), or the usage of slightly volatile mediums, a liner as mandrel is used. The liner can be a vital part of the product which is also considered by the requirements. The filaments are wound on the liner, which remains in the application. Materials for the liner manufacturing depend on the end-use of the application. Liners can be made, for example, of polymer, aluminum alloy or steel. Also like the dissolvable core the liner cannot be temperature controlled. 

Hollow siUca nanoparticles can be synthesized under moderate conditions using luminescent CdSe/ZnS nanoparticles, as described by Darbandi et al. [9]. The synthesis is conducted at room temperature within reverse micelles and by way of a modified water-in-oil microemulsion system, where silica is formed from the hydrolysis of TEOS. Here, the silica acts as a host for the CdSe/ZnS nanoparticles while the latter is simultaneously dissolved. The extent of this dissolution is determined by the amount of ammonia aqueous solution used and the reaction time. In a typical synthesis, cyclohexane is added to polyethylene glycol nonylphenyl ether (the surfactant), pre-prepared luminescent CdSe/ZnS nanopartides in chloroform, and TEOS, and the mixture then stirred vigorously to form the microemulsion. After 30 min, aqueous ammonia is added to initiate the encapsulation, and the reaction is then left overnight at room temperature before the nanopartides are precipitated. By increasing the amount of ammonia used, the core particles become increasingly dissolved to the point where hollow spheres can be obtained (250 (xl ammonia solution). Similarly, an increase in the duration of the reaction will yield nanopartides with increasingly dissolved cores. 

Figure 4a represents interfacial polymerisation encapsulation processes in which shell formation occurs at the core material—continuous phase interface due to reactants in each phase diffusing and rapidly reacting there to produce a capsule shell (10,11). The continuous phase normally contains a dispersing agent in order to faciUtate formation of the dispersion. The dispersed core phase encapsulated can be water, or a water-immiscible solvent. The reactant(s) and coreactant(s) in such processes generally are various multihmctional acid chlorides, isocyanates, amines, and alcohols. For water-immiscible core materials, a multihmctional acid chloride, isocyanate or a combination of these reactants, is dissolved in the core and a multihmctional amine(s) or alcohol(s) is dissolved in the aqueous phase used to disperse the core material. For water or water-miscible core materials, the multihmctional amine(s) or alcohol(s) is dissolved in the core and a multihmctional acid chloride(s) or isocyanate(s) is dissolved in the continuous phase. Both cases have been used to produce capsules. 

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. 

A key feature of encapsulation processes (Figs. 4a and 5) is that the reagents for the interfacial polymerisation reaction responsible for shell formation are present in two mutually immiscible Hquids. They must diffuse to the interface in order to react. Once reaction is initiated, the capsule shell that forms becomes a barrier to diffusion and ultimately begins to limit the rate of the interfacial polymerisation reaction. This, in turn, influences morphology and uniformity of thickness of the capsule shell. Kinetic analyses of the process have been pubHshed (12). A drawback to the technology for some apphcations is that aggressive or highly reactive molecules must be dissolved in the core material in order to produce microcapsules. Such molecules can react with sensitive core materials. 

Figure 4b represents the case where a reactant dissolved in the dispersed phase reacts with the continuous phase to produce a co-reactant. The co-reactant and any remaining unreacted original reactant left in the dispersed phase then proceed to react with each other at the dispersed phase side of the interface and produce a capsule shell. Capsule shell formation occurs entirely because of reaction of reactants present in the droplets of dispersed phase. No reactant is added to the aqueous phase. As in the case of the process described by Figure 4a, a reactive species must be dissolved in the core material in order to produce a capsule shell. 

A specific example of the process represented by Figure 4b occurs when a multihmctional isocyanate is dissolved in a Hquid, water-immiscible core material and the mixture produced is dispersed in an aqueous phase that contains a dispersing agent. The aqueous phase reacts with some of the isocyanate groups to produce primary amine functionaHties. These amino groups react with unreacted isocyanate groups to produce a polyurea capsule shell (13). 

A unique feature of in situ encapsulation technology is that polymerization occurs ia the aqueous phase thereby produciag a condensation product that deposits on the surface of the dispersed core material where polymerization continues. This ultimately produces a water-iasoluble, highly cross-linked polymer capsule shell. The polymerization chemistry occurs entirely on the aqueous phase side of the iaterface, so reactive agents do not have to be dissolved ia the core material. The process has been commercialized and produces a range of commercial capsules. 

Solvent Evaporation. This encapsulation technology involves removing a volatile solvent from either an oil-in-water, oil-in-oil, or water-in-oH-in-water emulsion (19,20). In most cases, the shell material is dissolved in a volatile solvent such as methylene chloride or ethyl acetate. The active agent to be encapsulated is either dissolved, dispersed, or emulsified into this solution. Water-soluble core materials like hormonal polypeptides are dissolved in water that contains a thickening agent before dispersion in the volatile solvent phase that contains the shell material. This dispersed aqueous phase is gelled thermally to entrap the polypeptide in the dispersed aqueous phase before solvent evaporation occurs (21). 

Chemical shim control is effected by adjusting the concentration of boric acid dissolved ia the coolant water to compensate for slowly changing reactivity caused by slow temperature changes and fuel depletion. Eixed burnable poison rods are placed ia the core to compensate for fuel depletion. 

Osmotic Pressure Controlled Oral Tablets. Alza Corp. has developed a system that is dependent on osmotic pressure developed within a tablet. The core of the tablet is the water-soluble dmg encapsulated in a hydrophobic, semipermeable membrane. Water enters the tablet through the membrane and dissolves the dmg creating a greater osmotic pressure within the tablet. The dmg solution exits at a zero-order rate through a laser drilled hole in the membrane. Should the dmg itself be unable to provide sufficient osmotic pressure to create the necessary pressure gradient, other water-soluble salts or a layer of polymer can be added to the dmg layer. The polymer swells and pushes the dmg solution through the orifice in what is known as a push-pull system (Fig. 3). The exhausted dmg unit then passes out of the body in fecal matter. 

The open hoUow fiber shape shown ia Figure 13 is made by a unique process requiring bicomponent yam technology (145). A yam is spun with a water-soluble copolyester core and nylon sheath where the core is dissolved out with an alkaH treatment ia fabric dyeiag. 

Agronomic Properties and Nutrient Release Mechanisms. The mechanism of nutrient release from SCU is by water penetration through micropores and imperfections, ie, cracks or incomplete sulfur coverage, ia the coating. This is followed by a rapid release of the dissolved urea from the core of the particle. When wax sealants are used, a dual release mechanism is created. Microbes ia the soil environment must attack the sealant to reveal the imperfections ia the sulfur coating. Because microbial populations vary with temperature, the release properties of wax-sealed SCUs are also temperature dependent. 

Chisso-Asahi uses a spouted bed process for the production of their coated materials (12). A 12,000 t/yr faciHty is located in Japan. The semicontinuous process consists of two batch fluid-bed coaters. A dilute polymer solution is prepared by dissolving 5% polymer and release controlling agent into a chlorinated hydrocarbon solvent such as trichloroethylene. The solution is metered into the spouted bed where it is appHed to the fertilizer core. Hot air, used to fluidize the granules, evaporates the solvent which is recovered and reintroduced into the process. Mineral talc, when used, is either slurried into the polymer solution or introduced into the fluidizing air. 

Scotts technology (17) uses fluid-bed (Wurster column) technology to apply polymeric coatings to a number of fertilizer substrates including urea, potassium nitrate, potassium sulfate, and monoammonium phosphate (MAP). The coating material is appHed as a water-borne latex onto the fluidized substrate. As the substrate is fluidized with warm air (40—50°C), water is driven off and the latex coalesces into a continuous film around the fertilizer particle. The particular latex compositions used have selected glass-transition and blocking temperatures, which enable quick removal of the water before the soluble fertilizer core dissolves. This obviates the need to use precoats prior to the latex appHcation. 

The simplest osmotic dosage form, ALZA Corporation s OROS elementary osmotic pump (Fig. 7), combines the dmg and sometimes an osmotic agent in a monolithic core and deflvers the dmg in solution (102). The mass dehvery rate with time dm df) of the dmg solution is described by equation 4, where is the hydrauHc permeabiUty of the membrane, a is the membrane reflection coefficient, Atz is the osmotic pressure gradient, APis the hydrostatic back pressure, A is the area of the membrane, C is the dissolved concentration of the dmg, and b is the membrane thickness. 

Osmotic Control. Several oral osmotic systems (OROS) have been developed by the Alza Corporation to allow controUed deHvery of highly water-soluble dmgs. The elementary osmotic pump (94) consists of an osmotic core containing dmg surrounded by a semi-permeable membrane having a laser-drilled deHvery orifice. The system looks like a conventional tablet, yet the outer layer allows only the diffusion of water into the core of the unit. The rate of water diffusion into the system is controUed by the membrane s permeabUity to water and by the osmotic activity of the core. Because the membrane does not expand as water is absorbed, the dmg solution must leave the interior of the tablet through the smaU orifice at the same rate that water enters by osmosis. The osmotic driving force is constant until aU of the dmg is dissolved thus, the osmotic system maintains a constant deHvery rate of dmg until the time of complete dissolution of the dmg. 

A scram causes the control rods to drop into the core, absorb neutrons and stop the chain reaction. Some rods perform both controlling and scram functions. The control rods are raised to increase the neutron flux (and power) or lowered to reduce it by magnetic jacks (W and CE) or a magnetic "clamshell" screw (B W). The chemical volume and control system (CVCS - not siiown) controls the water quality, removes radioactivity, and varies the reactivity by controUing the amount of a boron compound that is dissolved in the water - called a "poison." Thus, a PWR coiiirols reactivity two ways by the amount of poison in the water and by moving the control rods. 

The core-shell type polymer microspheres were synthesized upon the chemical crosslinking of the spherical microdomains in the microphase separated films. The block copolymers were dissolved in 1,1,2-trichloroeth-... 

The resistivity of the soil in any particular location will be a function of moisture content, soil temperature and presence of dissolved salts. At a site where climatic conditions vary considerably throughout the year, earth electrodes should be buried at a depth where such changes will not affect the resistivity. Grounding rods are generally made of copper bonded onto a steel core. The copper provides a good connection to earth and offers a high corrosion resistance, while the steel core gives the mechanical... 

Electrodes and Galvanic Cells. In connection with Fig. 9 in See. 11 we discussed the removal of a positive atomic core from a metal. The same idea may be applied to any alloy that is a metallic conductor. When, for example, some potassium has been dissolved in liquid mercury, the valence electron from each potassium atom becomes a free electron, and we may discuss the removal of a K+ core from the surface of the amalgam. The work to remove the K+ into a vacuum may be denoted by Ycr When this amalgam is in contact with a solvent, we may consider the escape of a K+ into the solvent. The work Y to remove the positive core into the solvent is much smaller than Yvac. 

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