Hybrid dispersion Coalescent

The properties of hybrid dispersions prepared using different methods (la, 2 and 3 - see Section 6.3.2) and different levels of coalescent (NMP) as well as of films and coatings made from these dispersions are presented in Tables 6.15 to 6.17. In all dispersions the chemical structure of the polyurethane-urea and acrylic/styrene polymer component was the same (see relevant Tables in Section 6.5.2). All dispersions were prepared using a water-soluble initiator. 

Table 6.15 Properties of hybrid dispersions of the same chemical composition differing only in the concentration of coalescent (NMP) ...

In Tables 6.18 to 6.20 the properties of two hybrid dispersions prepared using the same method la (see Section 3.2) and having the same structure of the acrylic/styrene part of the hybrid but based on different polyols, as well as of films and coatings made from them are presented. Neither dispersion contained coalescent (NMP)... 

The properties of hybrid dispersion prepared according to two different methods (la and 3 - see Section 3.2) and having the same chemical composition and differing only in the chemical structure of the acrylic/styrene polymer component are presented in Tables 6.24 to 6.26. All dispersions were synthesised using a water-soluble initiator. Dispersions prepared according to method la did not contain any coalescent while dispersions prepared according to method 3 contained 11.6% of NMP. 

The morphology of particles of hybrid dispersion synthesised according to method 2 using water-soluble and redox initiators is presented in Figures 6.32 and 6.33, respectively. Both pictures show both the single particles and the coalesced particles to demonstrate what happens to the particle morphology in the process of film formation. White colour represents the polyurethane-urea part of the hybrid. 

Figure 6.32 Morphology of particles of hybrid polyurethane-urea-acrylic/styrene hybrid dispersion prepared according to method 2 (See Section 6.3.2) using water-soluble initiator (MDPUR-ASD 97). Micrograph was taken using TEM. Both single particle and coalesced particles are shown. Reproduced with permission from Professor A. E. Czalych, Institure of Chemical Physics of the Russian Academy of Sciences, Moscow.

The effect of coalescent on the properties of films made from hybrid dispersions seems to be not quite significant (see Table 6.16) which is strange when taking into account the important role of coalescent in the process of film formation. The only observation that can be made here is that, as has already been stated in Section 6.6.1.2.1, high levels of coalescent cannot be recommended in preparation of hybrid dispersions. 

Here, the effect of coalescent is clearly visible (see Table 6.17), but only if the coalescent content is high (MDPUR 243). Then, drying time increases and coating hardness decreases. However, no differences can be observed between the properties of coatings made from hybrid dispersions which do not contain any coalescent and those which contain only small amounts of coalescent. 

Here, the effect of chemical structure of the polyurethane-urea part of the hybrid is, of course, substantial (see Table 6.19). For hybrid dispersion synthesised without coalescent using polyesterdiol as a starting material for the prepolymer-ionomer, the Tg is so high that films cannot he obtained. On the other hand, if polyetherdiol is applied as a starting material in the synthesis of the same dispersion, films of very good mechanical properties are obtained. This was the reason for using polyetherdiol rather than polyesterdiol as the starting material for synthesis of dispersions in this study. 

The chemical structure of the acrylic/styrene part of the hybrid has a substantial effect on the properties of films made from hybrid dispersions (see Table 6.24). When the monomers that form polymers of high Tg (styrene or MM) are used and the synthesis is carried out according to method la, no film is obtained. For the same monomers but a different method of dispersion synthesis (method 3), a film was obtained only for styrene and only when a high level of coalescent was applied. In this case, the mechanical properties, water and solvent resistance of the film were quite good, but the film was not transparent, which... 

One can also make combined acrylic/urethane (hybrid) aqueous dispersions (33). Acrylic monomers are emulsion polymerized in the presence of an aqueous dispersion of a hydroxy-terminated polyurethane. The polyurethane stabilizes the aqueous dispersion, minimizing need for surfactant. Coalescence requires balance of the Tg of both the urethane and acrylic parts of the system. Compositions based on an IPDl/polypropylene glycol/DMPA urethane with styrene/methyl methacry-late/butyl acrylate are reported to form films at low temperatures. 

The sectional quadrature method of moments (SQMOM) represents a hybrid between the sectional method and the quadrature method of moments (QMOM) and was proposed by Attarakih et al. [10] in order to solve the PBE for poly-dispersed systems. The novel idea is based on the concept of primary and secondary particles, where the former is responsible for distribution reconstruction while the latter is responsible for different particle interactions such as breakage and coalescence. 

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