Prepolymer-ionomer

In practice, the structures shown in Figure 6.3 are partly or fully crosslinked. The two main methods which lead to these structures are the acetone process (historically older) [23] and the prepolymer-ionomer process [24] that is commonly used to manufacture DPUR at present. A combination of these two methods can also be used. 

In the prepolymer-ionomer process , the compound which contains the moieties which are the precursors of ionic groups is incorporated in the polymer chain already at the stage of urethane prepolymer formation. Then they are neutralised, which results in the formation of a prepolymer-ionomer which is emulsified in water and eventually crosslinked. In this process, the prepolymer-ionomer is usually dissolved in a small amount of water-miscible solvent of high boiling point, e.g., N-methylpyrolidone, which plays the role of coalescing agent in the process of film formation. It is then possible to obtain DPUR which contain either cationic DPUR with a pH of less than 7 (cationic moieties are attached to the polyurethane or polyurethane-urea chain) or anionic DPUR with a pH of greater than 7 (anionic moieties are attached to the polyurethane or polyurethane-urea chain). If non ionic hydrophilic moieties are attached to or incorporated in the polyurethane or polyurethane-urea chain, then a nonionic DPUR may be obtained. 

The basic process was first patented by Inmont, USA, in 1982 [9]. They simply claimed polymerisation of vinyl polymers in DPUR. In 1986, a patent for another method of preparation of hybrid acrylic-urethane dispersions (diluting of prepolymer-ionomer with monomers, emulsifying the resulting solution in water and polymerisation) was granted to Witco, USA [26]. 

NCO - terminated prepolymer-ionomer diluted with NMP or acetone... 

Figure 6.6 Method 2 (using the monomers as active diluents for prepolymer-ionomer)...

In this case it can be assumed that polymerisation will proceed in a similar way as described previously for Method lb. The only difference may be that both structure and size of DPUR particles formed from the prepolymer-ionomer diluted with monomers will differ from the standard case, i.e., when NMP is used as diluent for prepolymer-ionomer. Moreover, some fraction of the monomer molecules may be so closely trapped in the crosslinked polyurethane-urea structure in DPUR particles that their participation in polymerisation will be difficult. This may lead to a decrease in average MW of acrylic/ styrene polymer which will be formed inside DPUR particles. 

Method 3 Emulsifying prepolymer-ionomer in diluted acrylic/styrene dispersion... 

After addition of prepolymer-ionomer and crosslinking it with polyamine some of these particles may remain untouched, but both new hybrid core-shell particles where acrylic/ styrene polymer will constitute the core and polyurethane-urea will form the shell ... 

Table 6.1 shows the properties of DPUR used for synthesis of MDPUR (MDPUR-ASD) together with the properties of prepolymer-ionomers used as intermediates in their preparation. 

Figures 6.8 and 6.9 show the MW distribution for standard prepolymer-ionomers used in DPUR synthesis, obtained from polyetherdiol (MW = 2000) and a mixture of polyesterdiols (MW = 920 and 3190), respectively. The weight average MW (M ) and number average MW (M ) of these prepolymer-ionomers were (not counting free IPDI) ...

Table 6.1 Properties of DPUR used as starting materials for MDPUR-ASD obtained according to methods la and lb (see Section 6.3.2) and of the prepolymer-ionomers (diluted with NMP or acetone) used in their preparation. Properties of prepolymer-ionomers diluted with monomers (designated as PPUR) used as intermediates in synthesis of MDPUR-ASD...

Designation of DPUR (PPUR) Polyol NMP Content % Properties of prepolymer-ionomers Properties of DPUR ... 

Figure 6.8 MW distribution of prepolymer-ionomer synthesised from polyetherdiol (PTMG 2000, MW = 2000) as determined by GPC...

Figure 6.9 MW distribution of prepolymer-ionomer synthesised from a mixture of polyesterdiols (Bester 190 and 195, MW 920 and 3190, respectively)...

Table 6.7 Composition of MDPUR-ASD prepared according to method 2 (diluting of the prepolymer-ionomer in monomers, polymerisation and crosslinking of prepolymer-ionomer). Properties of starting prepolymer-... 

Designation of MDPUR-ASD Designation of starting prepolymer-ionomer (PPUR) Polyol Monomer Coalescent (NMP) content in MDPUR-ASD % Properties of starting prepolymer-ionomer ... 

Table 6.8 Compositions of MDPUR prepared according to method 3 (synthesis of DPUR in ASD). Properties of starting prepolymer-ionomers are also included...

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. 

Figure 6.34 Schematically presented embedded sphere morphology of the particles of hybrid dispersions prepared according to method 2 (diluting the prepolymer-ionomer with monomers, emulsifying it in water, crosslinking with polyamine and polymerisation)...

Prepolymer ionomers with isocyanate end-groups and of sufficiently low molecular weight can be mixed with water in the presence of hydrophobic organic solvents, such as methylene chloride or toluene, or, in the case of sufficiently low viscosities, even without solvent. The prepolymers form emulsions, which in turn form polymer dispersion after chain extension by reaction of the isocyanate. Such a process requires powerful high-shear mixing equipment in the presence or absence of solvents. Solutions of ionomers in hydrophilic solvents form emulsions spontaneously in the presence of water. The formation of the emulsion must be... 

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