Hybrid dispersion Synthesis

Method of Hybrid Dispersion Synthesis and Kind of Initiating System... 

The results of investigations of the effect of method of hybrid dispersion synthesis (la, lb, 2 or 3 - see Section 3.2) on the properties of dispersions as well as of films and coatings made from them are presented in Tables 6.9 to 6.11 (dispersions prepared using water-soluble initiator) and in Tables 6.12 to 6.14 (dispersions prepared using redox initiating system). 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 the dispersions contained a similar low level (2-3.6%) of NMP. 

If a comparison were made between hybrid dispersion systems taking into account the kind of dispersion particles they contain, it would appear that the best properties should generally be observed for systems obtained by synthesis for which the particles have a uniform structure (they may be called true hybrid particles) and the worst - for simple blends of the two dispersions [14] (see Figure 6.1). 

Figure 6.1 Comparison of hybrid dispersion systems obtained by (a) blending, (b) and (c) by synthesis, taking into account particle type.

Figure 6.2 Examples of different particle morphologies of hybrid dispersions obtained by synthesis (such morphologies may be considered intermediate between core-shell and true hybrid morphology shown in Figure 6.1).

Based on the literature survey (see Section 6.2) and the anthor s preliminary experiments the following methods of synthesis of hybrid dispersions (after this point in the chapter they will be called modified DPUR (MDPUR)) were selected ... 

Hybrid dispersions obtained by polymerisation, i.e., according to methods la, lb and 2, will be designated as MDPUR-ASD and hybrid dispersions obtained by synthesis of PUR in ASD, i.e., according to method 3, will be designated as MDPUR. 

Method of Synthesis Designation of hybrid dispersion Properties of dispersions ... 

Properties of hybrid dispersions prepared according to the different methods (la, lb and 2 - see Section 6.3.2) and based on the same polyol (PTMG 2000), but differing in the presence or absence of double bonds in the polyurethane-urea part of the hybrid, as well as of films and coatings made of them, are presented in Tables 6.21 and 6.22. All dispersions have a similar low level (2.0-3.3%) of coalescent and have the same structure of the acrylic/styrene part of the hybrid. Redox initiator was used in the synthesis of dispersions according to the method 2, and in all other dispersions presented in these tables a water-soluble initiator was applied. 

Method of synthesis Designation of Hybrid Dispersion Double Bonds in polyurethaneurea % Properties of coatings ... 

Designation of hybrid dispersion Designation of starting dispersion Method of hybrid synthesis a max % Soluble fraction, S, % Crosslinking density, j Fraction of active chains. Vs... 

S.4.4 Investigation of the Effect of the Method of Synthesis of Hybrid Dispersions on their Degree of Crosslinking... 

From a practical point of view it is very important that, regardless of the type of initiator and method of synthesis, the films made from hybrid dispersions are transparent (this proves that their structure is quite uniform), strong, elastic and resistant to water (very good resistance) and organic solvents (good resistance). 

As can be seen from Table 6.18, the chemical structure of the polyurethane-urea part of the hybrid does not affect the properties of hybrid dispersions, obviously not counting the MFFT which is much higher if polyesterdiol has been used as a starting material for the prepolymer-ionomer synthesis (compare MDPUR-ASD 300 and MDPUR-ASD 24). 

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... 

When BA or a mixture of monomers (BA/MM/S) was used for synthesis of hybrid dispersions, transparent films of very good mechanical properties and water resistance were obtained (the best properties were obtained when the dispersions were prepared according to method 3). Solvent resistance was good only for films made from the dispersion prepared according to method 3. 

Table 6X7 shows that the properties of hybrid dispersions prepared with additional crosslinking of acrylic/styrene polymer do not differ from the properties of dispersions of the same composition prepared without additional crosslinking. Also the properties of films made from both dispersions were very similar (see Table 6.28). However, lack of positive effects of additional crosslinking of the acrylic/styrene part of the hybrid may result from improper selection of starting materials for hybrid synthesis. Therefore, we intend to investigate this effect further in another study along with further studies on the effect of introducing double bonds to the polyurethane-urea part of the hybrid.

Investigations of swelling of DPUR particles with monomers indicate that although all monomer being fed into the system may theoretically enter DPUR particles, since the maximum degree of swelling is more than twice that of the monomer/ DPUR solids w/w ratio used commercially in synthesis of hybrid dispersions, in practice there will always be a natural tendency of the system to reach equilibrium between monomer droplets, non-swollen DPUR particles and swollen DPUR particles. Therefore, when hybrid dispersions are prepared according to method la, lb or 2 (see Section 6.3.2) the result will always be a mixture of particles of hybrid structure with DPUR and ASD particles, as has been explained in Section 6.3.2.1). 

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