Melamine-formaldehyde microcapsules

Li, W. Wang, J. Wang, X. Wu, S. Zhang, X. Effects of ammonium chloride and heat treatment on residual formaldehyde contents of melamine-formaldehyde microcapsules. Colloid and Polymer Science (2007), 285(15), 1691-1697. 

Su, J.-R Wang, X.-Y Dong, H. Influence of temperature on the deformation behaviours of melamine-formaldehyde microcapsules containing phase change material. Mater Lett 84 (2012) 158-161. 

Son et al. (2014) efficiently attached melamine-formaldehyde microcapsules incorporating vitamin E acetate (tocopheryl acetate) to cotton knitted fabric using a pad-dry cure approach. The authors also studied the color properties of microcapsule-treated fabric previous dyed with indigo dye as well as the impact of adding softener. Supplied vitamin E acetate-loaded microcapsules of 2.36 pm in... 

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]. 

Lu, R. Wang, J. Li, Y Xu, K. Zheng, X. Influencing factors on encapsulation rate of melamine-formaldehyde resin microcapsule phase change material. Huaxue Gongcheng (2011), 39(12), 26-30, 39. 

Sumiga, B. Knez, E. Vrtacnik, M. et al. Production of melamine-formaldehyde PCM microcapsules with ammonia scavenger used for residual formaldehyde reduction. Acta Chim. Slov. (2011), 58(1), 14-25. 

Zhen, Z.-H. Chen, Z.-H. Effect of emulsifier on the encapsulation of melamine formaldehyde resin microcapsule. Zhongguo Zaozhi Xuebao (2006), 21(1), 47-51. 

Hwang, J. Kim, J. Wee, Y. et al. Preparation and characterization of melamine-formaldehyde resin microcapsules containing fragrant oil. Biotechnology and bioprocess engineering (2006), 11, 332-336. 

Frank, G. Biastoch, R. Low-formaldehyde dispersion of microcapsules of melamine-formaldehyde resins. US6224795, 2001. 

Hoffman, D. Eisermann, H. Low-viscosity melamine-formaldehyde resin microcapsule dispersions with reduced formaldehyde content. US6759931, 2004. 

All major manufacturers of microcapsules with scented oils as the core material now use different variations of melamine-formaldehyde systems. 

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. 

Figure 3.2 Encapsulation of FITC-dextran (75 kg mol" ) into (PSS/PAFI)4 microcapsules prepared on melamine-formaldehyde (MF) particles (a,b) and CdCOj crystals...

A considerable number of microcapsules intended for textile applications are fabricated using melamine-formaldehyde resin. The reason for this is its superior performance, including high hardness and mechanical robustness, excellent heat resistance, water resistance, outdoor weatherability, and unlimited colorability (Fei et al., 2015). Properties of polymer wall material should be carefully considered when opting for the best embedding method and choice of a compatible binder. Salaiin et al. (2009) investigated the adhesive properties of microcapsule wall material, melamine... 

The size of obtained microcapsules is defined by the one of colloid microparticles (cores) and can be in an interval from several tens nanometers up to tens micrometers. A wide variety of colloid microparticles of organic and inorganic nature can be used as soluble cores for reception of microcapsules. There are microparticles based on melamine formaldehyde polymers and polystyrene latex, crystals of organic dyes, particles based on inorganic compounds such as metal carbonates Si02, protein aggregates, cellular constructions. ... 

Keeping the microcapsules at raised temperature can lead to the change in their size. (PSS/PDADMAC)5 microcapsules obtained on the basis of melamine formaldehyde particles increased in diameter from 5.5 up to 7.5 micrometers at heating (40°C, 2 hours). Possible reasons of such behavior are supposed to be connected with reorganization of the polyelectrolyte shell structure caused by the change of ionic contact number in a polyelectrolyte complex under the temperature influence. The loosening of the shells leads to the change of its extension, and the diameter of a microcapsule. 

The simplest way of inclusion of inorganic nanoparticles in the microcapsule shells is their adsorption. Thus, a usual layer-by-layer assembly technique is used to embed nanoparticles in a capsule shell structure. For example, microcapsules containing various number of metal (silver or gold) nanoparticles were fabricated. Magnetite particles were included into the inner volume of polyelectrolyte capsules to obtain magnetic-driven delivery system. In this case magnetite was adsorbed on the surface of a melamine formaldehyde latex core, then polyelectrolyte layers were placed, after that the core was dissolved. Also microdrops of octane-based iron oxide nanoparticles suspension emulsificated in polyelectrolyte water solution were used as template cores. 

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... 

Fig. 4 Enzyme-catalyzed polymerization of phenols within polyelectrolyte microcapsules. MF = melamine formaldehyde PSS = poly(sodium 4-styrenesulfonate) PAH = poly(allylamine) hydrochloride...

Various formaldehyde scavengers are known. Some common ones are urea, ammonia, melamine, " ammonium chloride, and acetoacetamide. A linear correlation exists between the added ammonia amount and the reduction of residual formaldehyde in a nticrocapsule slurry. Furthermore, ammonia can improve the microcapsules waU impermeability/durability and thermal resistance. A similar finding was also reported by using melamine as the formaldehyde scavenger. ... 

The amount of literature on the field of MF microcapsules has greatly increased in recent years, with journals and patent disclosures focusing particularly on the requiranent to reduce free formaldehyde. Progress had indeed been made in using a wide variety of co-condensation systems for example, melamine-urea and melamine-latent formaldehyde donors. ... 

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