Urea-formaldehyde microcapsules

Ultrasound tomography (UST), 205 Ultrasound transducer arrangement, 14 Urea-formaldehyde microcapsules, 67 UVP-DUO systems, 2, 7, 12-13 UVP measurements. See Ultrasonic velocity profiler (UVP) measurements... 

Cosco, S. Ambrogi, V. Musto, P Carfagna, C. Urea-formaldehyde microcapsules containing an epoxy resin Influence of reaction parameters on the encapsulation yield. Macromolecular Symposia (2006), 234 (Trends and Perspectives in Polymer Science and Technology), 184-192. 

Figure 21.6 Urea-formaldehyde microcapsules containing silicon oil. (Courtesy of Dr K. Park, NJ Institute of Technology, Newark, NJ.)...

Another example for microcapsule-based self-reporting materials involved the use of charge-transfer complexes (CTCs). " CTCs form between an electron donor and an electron acceptor. The transfer of charge results in the attraction between the two species and a change in colour. Poly(urea-formaldehyde) microcapsules were filled separately with a donor, hexa-methylbenzene (HMB), and with an acceptor, chloranil (CA) (Figure 11.21). Both dyes were dissolved in toluene. The capsules were embedded into a PDMS matrix. They ruptured and released the dyes upon mechanical stimulus. This resulted in the formation of CTCs which changed the colour of scratches and of deformed areas from yellow to red. 

Figure 6. ESEM image of (a) UF microcapsules loading in TMPTA monomer and Darocur 1173 photomitiator with a smooth surface, (b) UF microcapsules loading m TMPTA monomer and Darocur 1173 photoinitiator with a rough surface and (c) ESEM image of ruptured Urea-Formaldehyde microcapsule...

Repair of relatively large-scale delaminations in a self-healing fiber-reinforced structural polymer composite was achieved by incorporating urea-formaldehyde microcapsules containing DCPD and first-generation Grubbs ... 

A recent innovation in in-situ microencapsulation is the development of acid-triggered release of pesticide from the microcapsules [12]. Diols and aldehydes are reacted to form an acid labile acetal moiety. The acetal is then reacted with isocyanate to create a prepolymer. The prepolymer is a polyisocyanate cmitaining the acid labile moiety and suitable for in-situ shellwall polymerization. The prepolymer is dissolved into a pesticide, emulsified into water, and shellwall formed in-situ. Under alkaline or neutral pH conditions in a container, the insecticide is safely contained in the microcapsules. Acid could be added to the spray tank to rapidly release capsule contents prior to application. Alternate shellwall chemistry for in-situ microencapsulation utilizes etherified urea-formaldehyde prepolymers in the oil phase that are self-condensed with acid catalyst to produce encapsulating aminoplast shellwalls [13]. This process does not have the problem of continuing CO2 evolution. Water-soluble urea-formaldehyde and melamine-formaldehyde prepolymers can be selected to microencapsulate water or aqueous solutions [14]. 

Another innovation in in-situ microencapsulation is aminoplast shellwalls containing base-cleavable ester moiety [15]. Polyols reacted with diacids that contain thiol or hydroxy functionality produce crosslinking groups. These crosslinking groups along with urea-formaldehyde prepolymer are dissolved into the pesticide and the in-situ microencapsulation process is completed. The resultant microcapsules may contain an insecticide that is safer to handle under acidic conditions yet will rapidly release the insecticide in the alkaline gut of an insect. Alternately, base could be added to the spray tank to rapidly release capsule contents prior to application. 

Ai, Q.-S. Zhang, Q.-Y. Xing, R.-Y. Zhang, J.-P. Direct in-situ polymerization for preparing microcapsules based on reactive ethylene silicone oil encapsulated by poly(urea-formaldehyde). Zhongguo Jiaonianji (2010), 19(4), 13-17. 

Microcapsules containing a small amount of additives that will be released by crack propagation or other triggering mechanisms have been incorporated into polymeric coatings [21]. Urea/formaldehyde capsules containing silicone oil are shown in Figure 21.6. 

Other encapsulations utilize more or less similar methods for the formation of the capsule wall. Complex coacervation utilizes the reaction of an anionic water-soluble polymer with a cationic material to form the shell wall that separates from the solution. As the coacervate separates from the solution, it will tend to coat suspended particles with a protective shell. The shell wall is then hardened with a cross-linking agent. In situ polymerization is used to form urea formaldehyde or melamine formaldehyde shells by using heat to cross-link the monomers forming the shell waU. Interfacial polymerization with isocyanates via hydrolysis is another method to form a shell wall at an organic-water interface. In this case, water acts to hydrolyze some of the polyisocyanate to an amine, which cross-links to form the polyurea microcapsule waU. 

Table 5.1 Effect of crosslinking of urea-formaldehyde resin on density of matrix and release ofcarbofuran from microcapsules.

The appeal of DCPD and ENB for self-healing has led to the development of several encapsulation techniques for these monomers. Most prominent techniques consist of forming a shell material around monomer droplets by in situ polymerization of an amino resin melamine-urea-formaldehyde (MUF, Figure 10.2a) [56], urea-formaldehyde (UF, Figure 10.2b) [13-16], or melamine-formaldehyde (MF) [17, 18] walls in an oil-in-water emulsion. Comparison of microcapsules made with different shell wall materials is challenging because... 

Poly(urea-formaldehyde) (UF) microcapsules prepared by in situ polymerization of urea and formaldehyde meet the demanding criteria required for use in self-healing materials (Suiyanarayanaa, 2008). These criteria includes excellent bonding to the matrix, sequestration of the healing monomer fiom the surrounding environment, and rapture and release of the monomer into the crack plane upon matrix damage (Blaiszik B. J., 2009). 

Different models of microcapsules have been tested Poly(urea/formaldehyde),... 

Thermogravimetric Analysis - Empty UF microcapsule shows - Figure 8, a weight loss (around 10 %) until 110 °C corresponding to the evaporation of free water in microcapsules, and an important decreasing in weight (around 80 %) starting from 180 °C due to the decomposition of urea-formaldehyde microparticles. Therefore the urea formaldehyde could not be used to self-heal spatial device composites because of polymer decomposition below 300 °C (maximum temperature in space). 

Silica-oragnic microcapsules based on 50 wt. % MPTS and 50 wt. % TEOS have been selected to encapsulate the monomer trimethylol propane triacrylate TMPTA and the photoinitiator Darocui 1173. The microcapsules prepared have diameter size of 4.00 0.30 Lun in optimal synthesis conditions reaction time of 2 h and stirring rate of 450 rpm. These silica-gel macrocapsules have been chosen over urea/formaldehyde or polyurethane due to their thermal stability up to 375 °C. 

Microcapsule Induced Toughening. Brown and co-workers have demonstrated that the incorporation of microcapsules into an epoxy matrix do not adversely affect the mechanical properties of the matrix. On the contrary, the fracture toughness measurements of samples cast from an EPON 828 epoxy resin system were observed to increase with concentration up to an optimal concentration that is dependent on the average diameter of the microcapsules. Overall, the incorporation of microcapsules containing DCPD into epoxy samples yielded up to a 127% increase in fracture toughness, outperforming samples containing similar concentrations of silica microspheres, or solid urea-formaldehyde particles (31). 

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