Aqueous Dispersions of Polyurethanes

The preparation of polyurethane lattices is possible in several ways. In one instance special monomers are polymerized in an aqueous medium to produce a thermoplastic polyurethane emulsion. Or thermoplastic polyurethanes made in solution or dissolved in solvents are emulsified in water, then solvent is removed. Or an isocyanate-terminated urethane prepolymer, possibly with hydrophilic branches, is blocked with an appropriate isocyanate blocking agent, and emulsified in water together with a crosslinking agent. 

The general formula of an emulsified polyurethane or aqueous dispersion used for adhesive purposes is shown in Fig. 8.11. Aqueous polyurethane dispersions usually have 40% solids of a thermoplastic type (for aqueous inertness), aliphatic diisocyanate polymer (for light stability) and an anionic surfactant is used. However, if a polyurethane ionomer is prepared this does not contain added emulsifier, the polyurethane itself having its own emulsifier function. The incorporation of this function in the polyurethane chains as (a) sodium sulphonate or (b) sodium carboxylate groups is seen in Fig. 8.11. 

Polyurethane water dispersions contain a little solvent, acetone or toluene or others, in very low concentration as this helps the formation of 

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Aqueous dispersions of polyurethanes have also become available which may be used instead of solutions in organic solvents for such applications as leather treatment, adhesives and surface coatings. 

Aqueous Systems. Type 6 in Table 2.5. Aqueous dispersions of polyurethanes that are slightly supported by ionic groups also undergo physical film formation. The binder consists of polymer chains that are coupled via urethane and urea groups, and contain basic or acid groups. Neutralization by salt formation provides the necessary hydrophilicity if the self-emulsifying properties resulting from incorporation of hydrophilic polyether radicals are insufficient. 

Neumaier, H. H., Aqueous Dispersions of Polyurethane lonomers for Coating and Laminating, Paper at AATCC Symposium on Coated Fabrics Technology, Marriott Motor Inn, Newton, Mass. (Mar. 28-29, 1973). 

Although two-pack epoxy primers and polyurethane intermediate coats have high solids contents, they still contain significant amounts (20-30 wt%) of organic solvents. In polyurethane topcoats, the VOC is even higher. Anticorrosive, waterborne primers based on aqueous dispersions of two-pack epoxy resins and one-pack acrylic resins have been developed to decrease solvent emission. Waterborne, one-pack acrylic topcoats are also used. All of these waterborne paints contain 2 - 5 % organic cosolvents that are required for film formation. 

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. 

Wiersma et al. [284] described a method for preparation of P(Py) or P(ANi) + polyurethane dispersions which may be applied to textile fibers. Aqueous solutions of pyrrole (or anilinium sulfate) and Fe(N03)3 are added to a dispersion of polyurethane in water. After 20 hrs reaction time, a P(Py) (or P(ANi)) poly-(urethane) dispersion is obtained, which can then be used to fabricate coatings with claimed conductivities up to 10 S/cm. 

Table 1. Characteristics of the aqueous dispersions of cation-active polyurethane-semicarbazides...

The factors determining the colloid-chemical properties of the cation-active polyurethane-semicarbazides are the nature and concentration of the ionic centers, and their position in the repeat unit of the macromolecule. The aqueous dispersions of the cation-active polyurethane-semicarbazides obtained have long shelf lives and form hydrophilic films. 

Prom the prepolymers based on aromatic diisocyanates containing sulfite groups, aqueous dispersions of anion-active polyurethane-semicarbazides are obtained by phase inversion [10]. The respective repeat units XX can be represented as ... 

Table 8. Influence of aqueous dispersions of ionomeric polyurethanes on the productivity and protein content in grains of winter wheat under arid conditions...

For printing inks that require specific properties not obtainable by conventional styrene acrylic emulsions, an aqueous dispersion of an add functional polyurethane-epoxy acrylate hybrid (self crosslinking for improved chemical resistance) [9] patented by Air Products and Chemicals, Inc. or a self crossHnking styrene acrylic emulsion which reacts upon evaporation of water [10] patented by Akzo Nobel Resins BV, may be used. The Air Products novel dispersion contains a quaternary ammonium polyurethane acrylic hybrid carboxylate salt and pendant acrylate epoxide that selfcrosslink upon evaporation of water and ammonia. Akzo s novel polymer contains a diacetone acrylamide reactive monomer and a bishydrazide. The crossHnking reaction between ketone groups and a bishydrazide proceeds rapidly at room temperature, after evaporation of water from the ink. 

The coating is applied by traditional trade practices, namely, using rubber squeegees and paint rollers. The applied coating is cured by a mobile curing unit, which consists of a cart housing power supply for the UV lamp (see Figure 111)T If the concrete is porous, a seal coat should be applied (aqueous UV curable polyurethane dispersion (PUD) is recommended). 

Conducting composite polymer materials have also been prepared from the dispersed phase of concentrated emulsions. Polyurethane/polypyrrole composites [165] were obtained by blending an aqueous suspension of polypyrrole with a HIPE of a chloroform solution of polyurethane in aqueous surfactant... 

It should be pointed out that this method relies on the efficient dispersion of nanotubes in the relevant solvent. The choice of solvent is generally made based on the solubility of the polymer. However, pristine nanotubes usually cannot be well dispersed in most solvents. To get around this problem, Xia et al. (17) compared the dispersion of MWNT-graft-PU, MWNT-OH and raw MWNTs in 0.2% aqueous solution of sodium lauryl sulfate. The results showed that MWNT-graft-PU has a better dispersion stability compared to MWNTs and MWNT-OH. The incorporation of polyurethane-grafted carbon nanotubes had a better reinforcing effect compared to the raw carbon nanotubes. This should be attributed to the improved interfacial interaction between polyurethane matrix and carbon nanotubes. 

The growing commercial use of polymer colloids whose preparation involves step polymerisations, hybrid systems and/or new routes to dispersion of polymers has not yet spawned an equivalent level of activity in academic studies. The success of the Avecia ex Zeneca Resins) technology for preparation of polyurethane dispersions is a good example of the importance of such developments. There also is the attraction of using conventional solvent-borne coating vehicles e.g., alkyd resins) in aqueous media and hybrid systems... 

Encapsulation. Immobilization of enzymes by encapsulation within semipermeable structures dates back to the 1970s. There are three fundamental variations of this approach. In coacervation, aqueous microdroplets containing the enzyme are suspended in a water-immiscible solvent containing a polymer, such as cellulose nitrate, polyvinylacetate, or polyethylene. A solid film of polymer can be induced to form at the interface between the two phases, thereby producing a microcapsule containing the enzyme. A second approach involves interfacial polymerization in which an aqueous solution of the enzyme and a monomer are dispersed in an immiscible solvent with the aid of a surfactant. A second (hydrophobic) monomer is then added to the solvent and condensation polymerization is allowed to proceed. This approach has been used extensively with nylons, but is also applicable to polyurethanes, other polyesters, and polyureas. 

Ayres, E., Oreflce, R. L., Yoshida, M. 1. (2007). Phase morphology of hydrolysable polyurethane derived from aqueous dispersion, Pnlvm. L 43, 3510-3521. 

D. Dieterich, Aqueous emulsions, dispersions and solutions of polyurethanes synthesis and properties, Progr. Org. Coat. 9, 281 (1981). 

Using a mixture of castor oil and hydroxyether of bisphenol-A (HBA) as the polyol and reacting with TDI in the presence of DBTDL catalyst at 35°C, a series of polyurethanes are prepared in an air-circulating oven for 48 h. A series of poly(urethane urea)-vinyl polymer hybrid aqueous dispersions are prepared. Waterborne poly(urethane urea) is synthesised from castor oil and polyoxypropylene polyol M = 1000 g moL ), dimethylol propionic acid and isophorone diisocyanate and then neutralised with tertiary amine. The hybrid aqueous dispersion with high oil content poly(urethane urea) exhibits excellent compressive mechanical strength. Sunflower oil-modified waterborne polyurethane resin may also be prepared. The waterborne resin is synthesised from the monoglyceride of the oU, poly(propylene-ethylene) triol, tartaric acid and toluene diisocyanate, then neutralised with triethyl amine. 

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