Novel synthesis of low VOC polymeric dispersions and their application in waterborne coatings

B. Vogt-Birnbrich
Herb&s GmbH, D-42271 Wuppertul, Germany
Received 24 July 1995; revised 3 November 1995; accepted 12 March 1996
Abstract
A new approach for the synthesis of reactive solvent-free polyurethane dispersions is discussed in detail. A powerful tool is introduced by incorporating /?-ketoester groups in the polymer chain. P-Ketoesters have two different reaction sites. On the one hand the keto groups can react with nucleophiles like polyamines. On the other hand the carbon atoms of the activated CH, groups are centers for the attack of electrophiles like aldehydes and isoyanates. Polymers functionalized in this way are interesting new intermediates in chain extension reactions for aqueous polyurethane dispersions. Another advantage of the functionalized polyurethane dispersions is the possibility of additional crosslinking with melamine resins or blocked isocyanates. Keywords; Solvent-free polyurethane dispersions; Chain extension; Acetoacetate; Waterborne coatings
0 1996 Elsevier Science S.A. All rights reserved
32
B. Vogt-Birnbrich/
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in Organic
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29ity. The synthesis of solvent-free polyurethanes via the prepolymer mixing route is not possible, because the synthesis of the isocyanate prepolymer leads to gelation in the melt. An alternative way to obtain a solvent-free polyurethane dispersion is the synthesis of blocked polyurethanes. The socalled hot melt process [ 1,2] uses the capping of isocyanate terminated prepolymers with urea at temperatures above 130 “C to form a biuret. The capped polyurethane is dispersed with hot water (100 “C) to minimize the reaction viscosity. Chain extension is carried out with formaldehyde in the presence of water. In this way methyl01 groups are formed which self-condense to give a high molecular weight product. This paper presents the results of investigations for a new synthesis of aqueous polyurethane dispersions where no organic solvent is needed during the whole process. A novel approach for chain extension is introduced. Acetoacetate groups are used as reactive sites in the polymer backbone.
1. Introduction High requirements concerning environmental protection are the reason for an increased interest in low VOC paint technologies. One strategic route is the development of waterborne coating systems. Aqueous polyurethane (PU) dispersions are of special interest. They offer a good balance between mechanical properties like abrasion resistance, hardness or tensile strength and the film forming temperature. The properties of polyurethane dispersions can be modified in a wide range by varying the composition of the typical building blocks, such aspolyols and polyisocyanates or the chain extension chemistry. High molecular weight products are desired with respect to coating applications. Low molecular weight resins have only limited stability in the humidity test and poor chemical resistance is observed. It has been shown that dispersions with small particles are helpful in terms of film coalescence. Polyurethane ionomers lead to dispersions with very low particle size ( 10-100 nm). The ionic groups in the polymer backbone also play an important role controlling the morphology and the properties of the system. Many contributions to this field have been made by Dieterich [ 11, Rosthauser and Nachtkamp [ 21, Eisenberg [ 31, and Visser and Cooper [ 41. The techniques for preparing aqueous polyurethane dispersions are reviewed by Dieterich [ 11.
PROGRESS INORGANIC COATINGS
ELSEVIER
Progress in Organic Coatings 29 ( 1996) 3 1-38
Novel synthesis of low VOC polymeric dispersions and their application in waterborne coatings
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Modern synthetic processes have two things in common. In all cases the first step of the synthesis is the formation of a medium molecular weight isocyanate terminated prepolymer by the reaction of suitable polyols and polyisocyanates. The second step is the dispersion process and the chain extension reaction. The main routes differ in the chemistry or the process of chain extension. They are summarized in the following. In the so-called solution process [ l] the isocyanate prepolymer is chain extended in solution in order to prevent high viscosity. The preferred solvent is acetone and therefore this process is often called the acetone process. This process is limited to uncrosslinked, acetone soluble polymers. Solventfree aqueous polyurethane dispersions can only be yielded after the removal of the solvent by distillation. A drawback is the waste of organic solvent and the long process time as a result of foaming during the early stage of the distillation. Another way to synthesize polyurethane dispersions is the so-called prepolymer mixing process [ 1,2]. A hydrophilitally modified isocyanate prepolymer, commonly dissolved in a small amount of water-miscible solvent, N-methylpyrrolidone for example, is chain extended with diamine or polyamine during the aqueous dispersion process. The reaction is possible because of the preferential reactivity of the amine group to isocyanates rather than that to water at lower temperatures. The quality of these dispersions is very high. They have small particle size distribution and good storage