Polyurethane microcapsules

For the analysis of experimental force-deformation data, it is necessary to use a suitable constitutive equation for the material under test. The constitutive equation relates the stresses and strains that are generated in the wall during compression, and therefore relates the tensions and stretch ratios. For example, Liu et al. (1996) used a Mooney-Rivlin constitutive equation to investigate the compression of polyurethane microcapsules and the functions f, /2 and fa are produced in... 

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. 

Frere, Y Danicher, L Gramain, P. Preparation of polyurethane microcapsules by interfacial polycondensation. European Polymer (1998), 34(2), 193-199. 

Su, J.-F., Wang, L.-X, Ren, L., Huang, Z and Meng, X.-W. (2006) Preparation and characterization of polyurethane microcapsules containing -octadecane with styrene-maleic anhydride as a surfactant by interfadal polycondensation./. Appl. Polym. Sci., 102, 4996-5006. 

Figure 5.3 Polymeric stabilizers used in the preparation of polyurethane microcapsules.

The TMPTA monomer and the Darocur 1173 photoinitiator are encapsulated in polyurethane microcapsules by in situ pol5mierization. Microcapsules are obtained by in situ pol)unerization in direct emulsion (OilAVater). At room temperature, deionized water and polyvinyl alcohol PVOH (3 wt. %) are mixed in a reactor equipped with a mechanical stirrer. Then, an organic solution of monomer (Hexamethylene diisocyanate HMDI/chloroform) containing the monomer (TMPTA) and photoinitiator (Darocur 1173) to be encapsulated is added to the aqueous solution leading to a stable QU/Water emulsion. After 5 minutes of stabilization, a hydrophilic monomer (Ethylene Diamine EDA in excess) is added to the emulsion - Figure 9. The reaction is continued until stabilization of the pH (around 5 hours). 

Infrared Analysis - The comparison between Infrared spectrums of the empty pol3mrethane microparticles - Figure 12 (b), and polyurethane microparticles loaded in TMPTA - Figure 12(c) shows the presence of new bands, in particular at 1721 and 983 cm and confirm that TMPTA monomer has been encapsulated in polyurethane microcapsules. 

Figure 12. Infrared spectrum of (a) TMPTA, (b) empty polyurethane microparticles and (c) polyurethane microcapsules loaded with TMPTA monomer...

Thermogravimetric Analysis - The thermogravimetric analysis of polyurethane microcapsules (without monomer) - Figure 13, presents a decreasing in weight (above 200°C) due to the deterioration of polymer. Therefore, this polymer is not a good candidate as well to encapsulate healed agent for aerospace applications. 

Fig. 11. Silanol condensation-based self-healing system. The mixture of HOPDMS, PDES, and solvent is either encapsulated or phase-separated. The catalyst is encapsulated in polyurethane microcapsules. Detmage throu the matrix ruptures the microcapsules and their contents mix in the crack plane initiating a polymerization that seals the crack.

Recently, many synthetic polymers such as urea/formalin resin, melamine/formalin resin, polyester, and polyurethane have been widely used as the wall material for the microcapsule, though the gelatin microcapsule is still used. Microcapsules using a synthetic polymer wall have several advantages over those using a gelatin wall (1) the preparation process is simple, (2) the size of the microcapsules is well balanced, (3) the microcapsule concentration can be increased twofold or more and (4) the microcapsules have a high resistance to water and many chemicals. Synthetic microcapsules are prepared by interfacial polymerization or in situ polymerization. 

Syntactic foamed plastics (from the Greek ovvxa C, to put together) or spheroplastics are a special kind of gas filled polymeric material. They consist of a polymer matrix, called the binder, and a filler of hollow spherical particles, called microspheres, microcapsules, or microballoons, distributed within the binder. Expoxy and phenolic resins, polyesters, silicones, polyurethanes, and several other polymers and oligomers are used as binders, while the fillers have been made of glass, carbon, metal, ceramics, polymers, and resins. The foamed plastic is formed by the microcapsular method, i.e. the gas-filled particles are inserted into the polymer binder1,2). 

Membrane-reservoir systems based on solution-diffusion mechanism have been utilized in different forms for the controlled delivery of therapeutic agents. These systems including membrane devices, microcapsules, liposomes, and hollow fibres have been applied to a number of areas ranging from birth control, transdermal delivery, to cancer therapy. Various polymeric materials including silicone rubber, ethylene vinylacetate copolymers, polyurethanes, and hydrogels have been employed in the fabrication of such membrane-reservoir systems (13). 

These additives can also be placed into microcapsules with a thin polymer skin. For example, they are filled with natural aromas and applied to the textile from a water dispersion in combination with a polyurethane or silicone binder. The ratio of micro bubbles to binder determines the efficiency and permanence of the finish. Good results after 4-12 washing cycles are reported. The encapsulated materials are released during wearing as the micro bubbles burst from rubbing caused by body movement or by diffusion through the thin layer of the capsules. A market available encapsulation in micro bubbles, built from chitosan, is described by Hampe. The incorporation and controlled release of fragrance compounds is also provided by the sol-gel nano-technique described in Section 18.4. 

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. 

Polyurethane-urea microcapsules were prepared by the interfacial polycondensation of methylene bis(phenyl isocyanate), hexamethylene diamine and anionic sodium hgnin sulphonate, as the emulsifying agent, and 2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide, as a herbicide. The morphology amd microstractuie of these microcapsules were investigated by TEM and scanning electron microscopy and the effect of caldrun chloride on the stabihty of the microcapsule stractuie examined. 22 refs. 

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. ... 

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... 

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 of galangal EO with antimicrobial activity against Staphylococcus aureus in another example of polyurethane-urea microcapsules was carried out by interfacial polymerization at oil-water interface in 0/W emulsion (Podshivalov et al., 2013). 

Podshivalov, A. V., Bronnikov, S., Zuev, V. V., Jiamrungraksa, T., and Charuchinda, S. (2013). Synthesis and characterization of polyurethane-urea microcapsules containing galangal essential oil Statistical analysis of encapsulation. Jourrwl of Microencapsulation, 30,198-203. 

Acrylate capsules produced by radical-induced polymerization. Gelatin capsules manufactured by coacervation. Polyurethane capsules produced by polyaddition of amines with diisocyanates. Aminoplast microcapsules made by polycondensation. 

Enzymes may also be immobilized by microencapsulation. In this technique, which has medical applications, enzymes are enclosed by various types of semi-permeable membrane, e.g. polyamide, polyurethane, polyphenyl esters and phospholipids. Microcapsules of phospholipids are also called liposomes. The micro-encapsulated enzymes and proteins inside the micro-capsule cannot pass the membrane envelope, but low M, substrates can pass into it, and products can leave. Such encapsulated proteins do not elicit an antigenic response, and they are not attacked by proteases outside the microcapsule. They are therefore suitable for the delivery of enzymes for therapeutic purposes. This area of application is still at an early stage of development, but positive results have been reported from animal experiments and clinical studies, e.g. treatment of inherited catalase deficiency with encapsulated catalase. There are various methods of administration intramuscular, subcutaneous or intraperito-neal injection. However, their major area of application is outside the body. For example, microencapsulated urease can be employed as an artificial kidney in hemodiffusion (Rg.2). 

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