Encapsulation processes, gases

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. 

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. 

The reverse is true at 45°C. The 40°C IFT data indicate that a G/GA complex coacervate phase will not preferentially wet or encapsulate lemon oil 1 at 40°C while encapsulation will occur at 45°C. This is consistent with the experimental observation that changes in wetting behavior of a G/GA complex coacervate phase occur during G/GA encapsulation processes. Additional IFT data at 35°C are needed to complete the picture. Such data should yield IFT values for the G/GA complex coacervate phase/lemon oil 1 interface below those of the G/GA supernatant phase, since the G/GA encapsulation process has been used to encapsulate lemon oil. 

The thermal process encapsulates all constituents into the desired resin. At the terminal end of the thermal encapsulation process, the resin/additive mixture is forced or extruded through a set of hot dies where it is drawn into long, thin strands. As the resin/additive mixture passes through the hot dies, minute quantities of the petroleum wax lubricants and emulsifiers contained in the product are volatilized. This emission, commonly called die gas, is an opaque smoke comprised of paraf-... 

To obtain quantitative information about guest encapsulation processes with cyclodextrins, Xe NMR spectroscopy was later employed (34). This gas is highly polarizable, but inert and hydrophobic. Two isotopes are easily accessible... 

The pressure in panel A is well below the 10 Pa target value as compared to panel C whose pressure is one order of magnitude higher. Pressure in panel B, which was just encapsulated, was slightly lower than in panel C. For comparison, the calculated pressure increase in the panel, as predicted based on the model discussed in Section 4.3, is also shown. This result confirms the role of the getter to compensate for the encapsulation process and to ensure VIP longevity and also supports the gas load model previously described. 

The proposed mechanism is based on the charred layer acting as a physical barrier which slows down heat and mass transfer between the gas and the condensed phases (Figure 2). More, the layer inhibits the evolution of volatile fuels via an encapsulation process. Finally, it limits the diffusion of oxygen to the polymer bulk. 

A final category of encapsulating materials consists of reaction products of the nucleus material and a reagent. For example, pellets of nitronium perchlorate have been encapsulated in shells of the less reactive amm perchlorate (AP) by exposing the pellets to ammonia gas. The fragile AP shells were usually further protected by a top-coating of A1 or a polymer film (Ref 2). The most familiar example of this process is the natural one wherein A1 powders (or articles) become coated with a protective coating of A1 oxide thru exposure to atmospheric air... 

As an example of the latter technique, Volkman et al. demonstrated the feasibility of using spin-cast zinc oxide nanoparticles encapsulated in 1-dodecanethiol to fabricate a functional transistor.44 The zinc oxide was deposited on a thermally grown silicon dioxide layer on a conventional silicon wafer, with thermally evaporated gold source and drain electrodes. As reported, the process requires very small particles (3nm or less) and a 400 °C forming gas anneal. A similar approach was also reported by Petrat, demonstrating n-channel thin-film transistor operation using a nanoparticle solution of zinc oxide dispersed onto a thermally grown silicon dioxide layer on a conventional... 

Red phosphorus has been used as an effective PBT FR, is non-halogen-based, and very high in active ingredient [55, 56], However, red phosphorus melt blending requires some special considerations. The potential generation of phosphine gas and acidic decomposition products under incorrect melt processing conditions is a concern. Recently, encapsulated grades of red phosphorus have minimized some of these potential issues. Red P blends are also limited in color capability. 

Modification of zeolites, based on chemisorption of silane or diborane and subsequent hydrolysis of the chemisorbed hydride groups can also be applied for encapsulating gas molecules in zeolites. For example, krypton and xenon can be encapsulated in mordenite combining the modification process with a physical adsorption of the noble gases at moderate pressures and temperatures (e.g. 100 kPa, 300 K). The encapsulates are homogeneous and stable towards acids, mechanical grinding and y-irradiation. By controlling the pore size reduction however, the thermal stability can be controlled. 

Sasol in South Africa produces a porous, prilled ammonium nitrate (PPAN) that finds its widest application in a mix with fuel oil. This mixture is used as an explosive and is commonly known as ANFO (Ammonium Nitrate Fuel Oil). Standard PPAN contains randomly distributed closed pores of an uncontrolled variable size and quantity. Sasol also makes EXPAN by using a patented process where polymeric microspheres are entrained uniformly in individual prills. Surfactants are added prior to the prilling process to ensure that the microspheres are evenly distributed in the prill. The addition of these microspheres (or encapsulated gas bubbles) reduces and controls prill density to desired levels. This improves the sensitivity and performance of the explosive while retaining the desirable properties of the standard prills (mechanical strength, oil absorption and free-flowability)106. 

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