Syntheses characterization, thermal de-blocking studies of blocked isocyanates and covalent cross-linking of collodosomes using diisocyanates

Abstract

Isocyanates, the building blocks of polyurethanes have high reactivity and toxicity that creates some rigorous problems during polyurethane production. The high reactivity and toxicity of isocyanates do not allow their storage or use in one-component systems. A solution to overcome these problems, particularly used in the coating and paint indush-ies, is the use of blocked isocyanates. Encapsulation and release of active ingredients such as dmgs, proteins, vitamins, flavours, or even living cells is another emerging research area. Among the highly sophisticated techniques being developed to meet the demanding requirements of these applications, fabrication of colloidosomes is considered the most attractive technique. However, one of the key problems related to .colloidosomes fabrication is the mechanical robustness and stability, especially when they are removed from the environment in which they were formed. The strength of the supracolloidal struchlres can be enhanced by crosslinking the individual latex building blocks together. The present work deals with these two diverse areas and is divided in to two parts: syntheses, characterization and thermal deblocking studies of blocked isocyanates (part A) and covalent crosslinking of colloidosomes (part B) to impart mechanical sh'ength and stability. The first part (part A) deals with synthesis, characterization and thelmal de-blocking studies of blocked isocyanates. Using the literature procedures, blocked isocyanates were synthesized using alcohols, amines, phenols and oximes as blocking agents. The structures of synthesized compounds were confirmed by FTIR, IH NMR, ElMS and elemental analysis. Differential scalming calorimetry, thermal gravimetric analysis, carbon dioxide evolution, FTIR and IH NMR were used to detem1ine the minimum de-blocking temperatures. De-blocking temperatures determined by DSC, TGA and carbon dioxide evolution were in the following order: de-blocking temperature determined by DSC > TGA > carbon dioxide evolution method. The effect of different metal catalysts on thelmal de-blocking reaction of the blocked isocyanates was studied. It was found that iron (III) oxide has the maximum catalytic activity on de-blocking reaction. Studies on the storage stability of the blocked isocyante adducts revealed that blocked isocyante adducts are stable for more than three'~ months at 50·C. Studies on the solubility of blocked isocyanate adducts showed that blocked aliphatic isocyanate adducts give better solubility as compared to blocked aromatic isocyanate adducts. Finally, Synthesized blocked isocyanates were used in different polyurethane compositions. Isocyanate-telminated prepolymers were prepared by using blocked isocyanates adducts and ii ; I different macrodiols. Their gelation time was in the order IPDI-based prepolymer > HDIbased prepolymer > TDI- based prepolymer > MDI- based prepolymer. Stable aqueous anionic blocked polyurethane dispersions were prepared and characterized. The particle size and the viscosity of aromatic isocyanate based dispersions were higher compared to aliphatic isocyanate based dispersions. Tensile strength of cast polyurethane dispersion films decreases and elongation-at-break increases with increase of molecular weight of macrodiols. These applications reveal that blocking of the isocyanate functionality allows making onecomponent systems with required stability in aqueous medium. The second part (part B) describes the evaluation of macromonomers and block copolymers as steric stabilizers for polystyrene latexes synthesis via dispersion polymerization. It was observed that macromonomers offer better stabilizer performance for the dispersion polymerization of styrene as compared to diblock copolymers. Colloidally stable stericallystabilized latexes with minimal coagulum were produced using 5 % (v/v) methanol/water mixtures at 70De. Stabilizer concentration was shown to have a pronounced effect on latex diameter. Higher stabilizer concentrations led to an appreciable reduction in latex diameter. Some control experiments indicated that chemical grafting of the stabilizer is desirable for effective steric stabilization. Morphologies of the latex particles were examined using transmission electron microscopy and scanning electron microscopy. The presence of the macromonomer stabilizer in the polystyrene latex was confirmed by FT-IR and IH NMR spectroscopy. The Pickering emulsifier behaviour of block copolymer/Iilacromonomer-PS latexes was evaluated by emulsification with six model oils. The resulting emulsions were characterized as being oil-in-water or water-in-oil type by both conductivity measurements and also by the drop test method. The fabrication of colloidosome microcapsules using sterically-stabilised polystyrene latexes was demonstrated. The mechanical robustness and stability of fabricated colloidosomes was successfully achieved by chemical cross-linking confining oil-soluble diisocyanate cross-linkers within the droplet phase. This approach led to single-layered shells, and allowed cross-linking to be conducted at high solids. Using this technique, well-constructed spherical microcapsules have been prepared with walls comprising of close-packed latex monolayers. The interstices between the particles and microcapsule permeability can be controlled by the degree of cross-linking, which depends in tum on the cross-linker concentration and the surface density of stabilizer chains grafted. This approach led to more robust microcapsules.