Polyurethane/urea dispersions

In formulating polyurethane/urea systems using ultra-low monol PPG polyols, it was found that polyol MWD has a major effect on polymer properties. A broadened MWD is required to achieve a property profile similar to a PTMEG-based system. The MW-distribution effect is illustrated using TDI prepolymers cured with MBOCA, TDI moisture-cured prepolymers and aqueous polyurethane/urea dispersion coatings. 

Figure 9.15 Stress/strain curves of aqueous polyurethane/urea dispersion films...

Laboratory Preparation of Aqueous Polyurethane/Urea Dispersions using the Prepolymer Mixing Process... 

Immediately add the chain extender (ethylene diamine), which has been diluted in water slowly to the aqueous polyurethane/urea dispersion. Typically chain extend only 80-90% of the actual free NCO groups because some will be lost due to reaction with water. As an alternative, the amine extender may be added to the water prior to dispersing the neutralised prepolymer. 

The polyurethane-urea dispersions which are obtained employing these shear-force dispersant methods generally have particle sizes from 0-7 to 3 //m and, from crosslinked particles, films are formed only at temperatures exceeding 100 C. Small amounts of solvent may be added to the prepolymer to improve dispersion and remain in the dispersion, since their removal would impair stability. Best results are obtained with diamine-extended polyether-isocyanate prepolymers. The choice of suitable dispersants is of considerable importance. 

Tawa, T Ito, S. The role of hard segments of aqueous polyurethane-urea dispersion in determining the colloidal characteristics and physical properties. Polymer Journal,... 

Synthesis and Characterisation of Aqueous Hybrid Polyurethane-Urea-Acrylic/Styrene Polymer Dispersions... 

Hybrid polyurethane-urea-acrylic/styrene polymer dispersions were prepared according to methods la , lb , 2 and 3 described in Section 6.3.2. Dispersions designated as MDPUR-ASD were made by polymerisation of monomers in DPU according to the methods la, lb and 2 while dispersions designated as MDPUR were made by synthesis of DPUR in ASD according to method 3. In all syntheses the ratio of polyurethane-urea to acrylic/styrene polymer in the hybrid was 2 1. 

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. 

Figure 6.17 Particle size distribution for a typical polyurethane-urea-acrylic/styrene hybrid dispersion synthesised in this study (MDPUR-ASD 97 from Table 6.7 prepared according to method 2 using a water-soluble initiator).

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. 

Table 6.18 Properties of hybrid dispersions differing only in the structure of the polyurethane-urea part of the hybrid ...

Method of synthesis Designation of hybrid dispersion Double bonds in polyurethane- urea Properties of dispersions ... 

Method of Synthesis Designation of Hybrid Dispersion Double bonds in polyurethane-urea Properties of films ... 

Figure 6.23 Particles of hybrid polyurethane-urea-acrylic/styrene dispersion prepared according to method la using less hydrophobic monomer and water-soluble initiator (MDPUR-ASD 22). Photograph was taken using TEM.

This experiment was aimed at clarifying what is the maximum amount of monomer that would he able to swell polyurethane-urea, i.e., would be able to take part in the formation of hybrid dispersion particles, assuming that enough time is allowed to achieve equilibrium. Films made from DPUR 25 8 were used in this experiment. The following resnlts were obtained ... 

The morphology of dispersion particles was investigated using the method described in Section 6.4.1. Examples of different morphologies of particles of hybrid dispersions synthesised in this study according to methods la, lb and 3 are presented in Figure 6.31 in comparison with particles of the starting dispersion of BA/MM/S copolymer. The contrast was selected so that in pictures d and c white colour represents the polyurethane-urea part of the hybrid and in picture b the same colour represents the acrylic/styrene part of the hybrid. 

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. 

The surface free energy of films made from hybrid dispersions was determined in order to look at the values of its non-polar and polar component which could provide some information on whether polyurethane-urea or acrylic/styrene polymer is on the film surface (see Table 6.30). 

Figure 6.31 Examples of different morphologies of particles of hybrid polyurethane-urea-acrylic/styrene dispersions synthesised in this study. Micrographs were taken...

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.

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. 

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