Polyurethane-polyurea microcapsules

Microcapsules with desired release rates according to the application involved can be prepared by incorporating a selected fluid (solvent), while keeping the other capsule parameters constant. These fluids include dearomatized and isoparaffinic hydrocarbons, aromatic hydrocarbons, acetate derivatives, and blends of these [51], In the preparation of polyurethane-polyurea microcapsules containing hep-tenophos (liquid pesticide, b.p. 64 °C), it has been reported that the porosity of microcapsules can be controlled by the addition of 2-ethoxy ethyl acetate or ethyl acetate in the organic phase containing isocyanate prepolymer and pesticide [52]. 

Microencapsulation by interfacial polycondensation is a usefiil method to microencapsulate a liquid core material. Especially, polyurea and polyurethane microcapsules have been extensively investigated in various industries [209]. For example, aliphatic hexamethylene diisocyanate (HMDI) and aliphatic ethylene diamine (EDA) have been used to prepare polyurea microcapsules containing insecticide called diazinon [210]. A urea linkage is formed immediately by the reaction between an amine and an isocyanate group (see Figure 4.31), and a polyurea is synthesized by the reaction between an amine with two or more amine groups and an isocyanate with two or more isocyanate groups. 

Both interfacial polycondensation and polyaddition involve two reactants dissolved in a pair of immiscible liquids, one of which is preferably water, which is normally the continuous phase, and the other one is the dispersed phase, which is normally called the oil phase. The polymerization takes place at the interface and controlled by reactant diffusion. Researches indicate that the polymer film occurs and grows toward the organic phase, and this was visually observed by Yuan et al. In most cases, oil-in-water systems are employed to make microcapsules, but water-in-oil systems are also common for the encapsulation of hydrophilic compounds. Even oil-in-oil systems were applied to prepare polyurethane and polyurea microcapsules. ... 

Heinrich, R. Erensch, H. Albrecht, K. Pressure-resistant microcapsules with a polyamide outer shell and an inner composition structured by polyurethane-polyurea and their use. DE 3020781, 1981. 

Lu, S. Xing, J. Zhang, Z. Jia, G. Preparation and characterization of polyurea/polyurethane doubleshell microcapsules containing butyl stearate through interfacial polymerization. Journal of Applied Polymer Science (2011), 121(6), 3377-3383. 

Encapsulation. Immobilization of enzymes by encapsulation within semipermeable structures dates back to the 1970s. There are three fundamental variations of this approach. In coacervation, aqueous microdroplets containing the enzyme are suspended in a water-immiscible solvent containing a polymer, such as cellulose nitrate, polyvinylacetate, or polyethylene. A solid film of polymer can be induced to form at the interface between the two phases, thereby producing a microcapsule containing the enzyme. A second approach involves interfacial polymerization in which an aqueous solution of the enzyme and a monomer are dispersed in an immiscible solvent with the aid of a surfactant. A second (hydrophobic) monomer is then added to the solvent and condensation polymerization is allowed to proceed. This approach has been used extensively with nylons, but is also applicable to polyurethanes, other polyesters, and polyureas. 

In order to prepare the microcapsule with expected structure, the chosen polymer wall material should match with the core material. The affinity interaction between the polymer wall and the core material can determine the structure of the microcapsule. Highly cross-linked polyurea and polyurethane can form so-called compact capsules with 2-methylbenzothiazole homogeneously distributed in the polymer matrix, while in the case of polyamide core-shell structure capsules were... 

Aliphatic or aromatic structure, as weU as liner or branched structure of the reactants, can give the microcapsule shell different porosity and permeability, which can greatly inflnence the release performances. Multifunctional reactants can help to achieve more thermal mechanical stable microcapsules since the wall is a three-dimensional cross-linked polymer network. Experiments have shown that dichlorides with less than eight carbon atoms do not prodnce qnahty polyamide microcapsules. The reason behind this is the competition between interfacial condensation and the hydrolysis reaction of dichlorides. More hydrophobic dichlorides can favor the polymerization and slow the hydrolysis. Similarly, for polyurethane and polyurea type microcapsules, polymeric isocy-nates are preferred because they might favor the formation of less permeable miCTocapsnles for the hydrolysis of isocynate groups are limited, which consequently reduced the COj release that contribute to the porosity increase of the polymer wall." ... 

Many efforts have been devoted to the microcapsules with tailored structure and the fabrication methods thereof. Examples like double-shell microcapsule of polyurea/polyurethane show improved thermal mechanical property and ethanol resistance, poly(acrylonitrile-divinylbenzene-styrene)/polyamide two-layer microcapsule was prepared to encapsulate water, and self-bursting microcapsules " may have potential application in agricultural field because of its unique release profile. Additionally, monodispersed microcapsules based on miCTofludic processes like SPG (Shirasu... 

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