Microcapsules suspensions

FIG. 1 Normalized intensity-intensity time correlation function of poly (urethane-urea) microcapsule suspension at c = 5 x 10-5 g/cm measured at 60 and at 30 C. r(l/l ) is the characteristic decay time. (Reprinted with permission from the paper entitled An experimental investigation on the structure of microcapsules, by T. Dobashi. F. Yeh, Q. Ying, K. Ichikawa, and B. Chu. Langmuir // 4278. Copyright 1995 American Chemical Society.)... 

A unique means to release the active species from hard silica spheres, for example, is the drying of the microcapsules. The simple desiccation of the microcapsule suspension destabilizes the capsules, thanks to the Laplace pressure, that is, the pressure difference across a curved interface, that occurs between the microcapsules upon the evaporation of the solvent phase (Equation 18.4). For a typical microcapsule size of 3 pm, considering an amorphous... 

Finally, adding to a good solvent for the active can result in capsule burst. In colloidal systems at equilibrium such as microcapsule suspensions, chemical potentials tend to become equal. The largely different chemical composition difference between each side of the microcapsule shells implies that the overall chemical potential must be compensated by the osmotic pressure. The latter can be stronger than the mechanical resistance of the shell, breaking the microcapsules. 

Formulation l pes Emulsifiable concentrate wettable powder, granules aerosol dry seed treatment capsule suspension, smoke tablet eoating agerrt microcapsule suspension. 

Microencapsulation. Microencapsulation (qv) has now been commercially practiced for more than 30 years, following the first application of the technology to carbonless copying paper. Pesticide formulations based on microcapsules appeared in 1974 with the product Penncap-M containing the insecticide methyl parathion (4). Since then many microcapsule suspension formulations have been introduced and form the major group of CRF. 

All spectrofluorimetric studies of SNARF-1 dextran and SNARF-1 dextran/urease were carried out with the spectrofluorimeter Varian Cary Eclipse, at constant agitation, thermostatic control at 20°C, X =540 nm, slit width excitation at 10 nm and emission at 20 run. The microcapsule suspensions were used at concentration 2 10 capsules/mL, which was estimated with the cytometer chamber. All solutions were prepared on bi-distillated water. 

Figure 24. Comparison of dispersion properties of microcapsules suspension and corresponding emulsions. The bars Indicate standard deviations.

In addition to a block copolymer, a microcapsule was made from suspension interfacial polycondensation between diacid chloride having aromatic-aliphatic azo group and aliphatic triamine [70,71]. The capsule was covered with a crosslinked structure having an azo group that was thermally stable but sensitive to light so as to be applicable to color photoprinting materials. 

The US Air Force has studied the encapsulation of liq monoproplnts such as alkyl nitrates in polymer films to form small spheres much like ball powder for use as gun proplnts (Refs 6, 8 9). Suspension coating techniques were used, and microcapsules were made with gelatin,... 

Thus, spray-dried xylan/ESlOO microparticles were produced at different polymer weight ratios dissolved in alkaline and neutral solutions, separately. More precisely, xylan and ESIOO were dissolved in 1 1 and 1 3 weight ratios in 0.6 N NaOH and phosphate buffer (pH 7.4). Then, the suspensions were spray-diied at the feed rate of 1.2 mL/min (inlet temperature of 120°C) using a Biichi Model 191 laboratory spray-dryer with a 0.7 mm nozzle, separately. Cross-linked xylan microcapsules were also coated by ESIOO after spraydrying at the same conditions. 

Processes based on fluidized bed coating have been developed (49). In this process, the bioactive agent is dissolved in an organic solvent along with the polymer. This solution is then processed through a Wurster air suspension coater apparatus to form the final microcapsule product. A solvent partition technique based on continuous injection of a polymer-drug solution into flowing mineral oil has been reported (50). 

Lidocaine (112), xyloceiine, and dibucaine (113) have been formulated in homo- and copolymers of lactide and glycolide. The goal of these studies has been relatively short-term (24-hr) controlled release of the anesthetic. Injectable microcapsules of lidocaine hydrochloride were produced by an air suspension coating technique and administered i.m. to rabbits (112). Serum levels of Udocaine indicated an initial rise over the first 2 hr and then a gradual decline with clearance after about 8-10 hr. 

Sjovall J, Sjoqvist R, Huitfeldt B, Nyqvist H. Correlation between the bioavailability of microencapsulated bacampicillin hydrochloride in suspension and in vitro microcapsule dissolution. J Pharm Sci 1984 73 141—145. 

The transport of disulfoton from water to air can occur due to volatilization. Compounds with a Henry s law constant (H) of <10 atm-m /mol volatilize slowly from water (Thomas 1990). Therefore, disulfoton, with an H value of 2.17x10" atm-m /mol (Domine et al. 1992), will volatilize slowly from water. The rate of volatilization increases as the water temperature and ambient air flow rate increases and decreases as the rate of adsorption on sediment and suspended solids increases (Dragan and Carpov 1987). The estimated gas- exchange half-life for disulfoton volatilization from the Rhine River at an average depth of 5 meters at 11 °C was 900 days (Wanner et al. ] 989). The estimated volatilization half-life of an aqueous suspension of microcapsules containing disulfoton at 20 °C with still air was >90 days (Dragan and Carpov 1987). 

Dragan D, Carpov A. 1987. Microencapsulated organophosphorous insecticides. III. Some aspects on the volatilization of malathion and disulfoton from aqueous suspensions of microcapsules. Microencapsul 4 97-105. 

K. Walter, G. WeiB, A. Laicher, F. Stanislaus, Pharmacokinetics of ibuprofen following single administration of a suspension containing enteric-coated microcapsules, Drug Res 45 886-890 (1995). 

Guang Hui Ma et al. [83] prepared microcapsules with narrow size distribution, in which hexadecane (HD) was used as the oily core and poly(styrene-co-dimethyla-mino-ethyl metahcrylate) [P(st-DMAEMA] as the wall. The emulsion was first prepared using SPG membranes and a subsequent suspension polymerization process was performed to complete the microcapsule formation. Experimental and simulated results confirmed that high monomer conversion, high HD fraction, and addition of DMAEMA hydrophilic monomer were three main factors for the complete encapsulation of HD. The droplets were polymerized at 70 °C and the obtained microcapsules have a diameter ranging from 6 to 10 pm, six times larger than the membrane pore size of 1.4 p.m. 

Still another dosage form is microcapsules, which are tiny droplets of an active agent coated with a permselective barrier polymer, all in the form of a suspension. Microcapsules are conveniently formulated for injection. In such cases the barrier polymer is often chosen to be degradable into innocuous products in the body after the active agent has been dispensed. 

The first plot received 500 g.a.i./h of displarlure as NCR gelatin-walled microcapsules containing 2% ai. The formulation, applied as an aqueous suspension, also contained 1 of sticker to aid adhesion of the formulation to foliage. The second plot received 500 g./h. as Herculite Corporation sprayable laminate flakes containing 9.1 ai. The flakes consisted of two layers of vinyl, each 0.08 mm thick on both sides of a central porous layer containing the disparlure the surface area of the flakes was between 7 and 35 mm2 per side. The same sticker as that in the microcapsules was used. The third plot received 330 g.a.i./h as "Conrel" controlled release hollow fibers containing nominally 11.5% ai. a suitable sticker was also incorporated in the formulation. (Note that the use of trade or proprietary names here or elsewhere does not constitute an endorsement by the USDA). 

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