Shell materials

Microencapsulation is the coating of small solid particles, liquid droplets, or gas bubbles with a thin film of coating or shell material. In this article, the term microcapsule is used to describe particles with diameters between 1 and 1000 p.m. Particles smaller than 1 p.m are called nanoparticles particles greater than 1000 p.m can be called microgranules or macrocapsules. 

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. 

Table 1 Hsts representative examples of capsule shell materials used to produce commercial microcapsules along with preferred appHcations. The gelatin—gum arabic complex coacervate treated with glutaraldehyde is specified as nonedible for the intended appHcation, ie, carbonless copy paper, but it has been approved for limited consumption as a shell material for the encapsulation of selected food flavors. Shell material costs vary greatly. The cheapest acceptable shell materials capable of providing desired performance are favored, however, defining the optimal shell material for a given appHcation is not an easy task.

Table 1. Shell Materials Used to Produce Commercially Significant Microcapsules...

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). 

Spray Drying. Spray-dry encapsulation processes (Fig. 7) consist of spraying an intimate mixture of core and shell material into a heated chamber where rapid desolvation occurs to thereby produce microcapsules (24,25). The first step in such processes is to form a concentrated solution of the carrier or shell material in the solvent from which spray drying is to be done. Any water- or solvent-soluble film-forming shell material can, in principle, be used. Water-soluble polymers such as gum arable, modified starch, and hydrolyzed gelatin are used most often. Solutions of these shell materials at 50 wt % soHds have sufficiently low viscosities that they stiU can be atomized without difficulty. It is not unusual to blend gum arable and modified starch with maltodextrins, sucrose, or sorbitol. 

The second step is to disperse the core material being encapsulated in the solution of shell material. The core material usually is a hydrophobic or water-knmiscible oil, although soHd powders have been encapsulated. A suitable emulsifier is used to aid formation of the dispersion or emulsion. In the case of oil core materials, the oil phase is typically reduced to a drop size of 1—3 p.m. Once a suitable dispersion or emulsion has been prepared, it is sprayed into a heated chamber. The small droplets produced have a high surface area and are rapidly converted by desolvation in the chamber to a fine powder. Residence time in the spray-drying chamber is 30 s or less. Inlet and outlet air temperatures are important process parameters as is relative humidity of the inlet air stream. 

Liquid food ingredients encapsulated are typically oil-soluble flavors, spices (see Flavors and spices), and vitamins (qv). Even food oils and fats are encapsulated (63). These core materials normally are encapsulated with a water-soluble shell material appHed by spray drying from water, but fat shell formulations are used occasionally. Preferred water-soluble shell materials are gum arabic, modified starch, or blends of these polymers with maltodextrins. Vitamins are encapsulated with 2ero bloom strength gelatin by spray drying. 

D, the tank diameter and the shell material allowable stresses. This 1-ft (0.305-m) equation is slightly conservative. For tanks over 200 ft (61 m) in... 

Tube material includes any that can be formed into a coil, but usually copper, copper alloys, and stainless steel are most common. The casing or shell material can be cast iron, cast steel, cast bronze, fabri-catea steel, stainless, and other high-alloy materials. Units are available with pressure vessel code conformance. 

This design has a strut-supported porous shell (Figure 9-19). The shell attached to the strut is of wire from porous material. Cooling air flows up the central plenum of the strut, which is hollow with various-size metered holes on the strut surface. The metered air then passes through the porous shell. The shell material is cooled by a combination of convection and film cooling. This process is effective due to the infinite number of pores on the blade surface. The temperature distribution is shown in Figure 9-20. 

The measured mass susceptibility values for bucky-bundle (both xb aid xu). Qo. the gray-shell material, the polycrystalline graphite anode, and the... 

Fig. 5. Temperature dependence of the magnetic suseepli-bilities measured in a magnetic field of 2 T (a) Qo powder, (b) polycrystalline graphite anode, (c) gray-shell material, (d) buckybundle axis perpendicular to H, and (e) buckybundle axis parallel to H.

By putting the corrodent on the tubeside and the hot vapour on the shellside provided the vapour is compatible with the shell material this is the normal configuration for steam-heated exchangers. 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

Fig. 6 shows PL spectra of CdS nanoparticles and CdS-ZnS core-shell nanoparticles. In PL spectrum of CdS nanoparticles, the emission band is seen at around 400nm. The emission band of CdS-ZnS core-shell nanoparticles is higher dian that of CdS ones at around 400nm. The PL enhancement of CdS-ZnS core-shell nanoparticl is due to passivation which means that surface atoms are bonded to the shell material of similar lattice constant and much larger band gap [9], Althou the sur ce treatment conditions are different, the ranission band of CdS-ZnS core-shell nanoparticles is same in PL spectra of Fig. 6(b). This indicates that interfacial state between CdS core and shell material was unchan d by different surfaKs treatment. 

CdS and CdS-ZnS core-shell nanoparticles were synthesized by inverse micelle method. Crystallinity of CdS nanoparticles was hexagonal structure under the same molar ratio of CM and S precursor. However it was changed easily to cubic structure under the condition of sonication or higher concentration of Cd than S precursor. The interfacial state betwran CdS core and shell material was unchanged by different surface treatment. 

Design type Tub-a, shell material Design pressure Design temperature... 

A similar strategy was used to prepare BaFe12019-Ti02 core-shell materials, with the core of BaFei20i9 used to magnetically recover the catalyst and the shell ofTi02 that promoted degradation of dyes. The activity was dependent on the shell thick-... 

Equation (9.50) is the primitive quasichemical approximation that was used to obtain the results of Figs. 9.1 and 9.2. Primitive emphasizes that the equilibrium constants are obtained with initial neglect of the effects of the outer-shell material, as (9.50)... 

The first point from this development and example is that, although the quasichemical approach is directed towards treating strong attractive - chemical - interactions at short range, it can describe traditional packing problems accurately. The second point is that this molecular-field idea permits us to go beyond the primitive quality noted above of the primitive quasichemical approximation, and specifically to account approximately for the influence of the outer-shell material on the equilibrium ratios Km required by the general theory. This might help with cases of delicate structures noted above with anion hydrates. 

How the latter application of QCT can be formulated has been discussed in some detail [10]. That discussion nearly closes a logical circle PDT —> QCT —> pdt. The final pdt is, however, approximate, as is natural when utilizing a molecular-field description of the influence of the outer-shell material. Specifically,... 

Shell materials. See also Microencapsulation biodegradable, 76 441-442 types of, 76 439t water-soluble, 76 456-457 Shell Middle Distillate Synthesis (SMDS) process, 6 778, 829... 

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