Solvent-free polyurethane dispersion

Details Solvent-free polyurethane dispersion Solvent-free polyurethane dispersion

2000-10-30

2003-10-21

Niland, Patrick D. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C524S591000, C524S839000, C524S840000

Reexamination Certificate

active

06635723

ABSTRACT:

The present invention relates to a solvent-free polyurethane dispersion having a high solids content of polyurethane polymer or fillers, a process for preparing such polyurethane dispersions and their use in the building sector for single-component, isocyanate-free and solvent-free coatings, sealants, adhesives or membranes.
For reasons of environmental protection and with a view to adherence to existing emission guidelines, considerable efforts have been made in recent years to develop water-based polyurethane dispersions having a very low content of volatile organic compounds (VOC). These low-solvent (low VOC) or solvent-free (zero VOC) products offer both ecological and economic advantages and already largely correspond in terms of their performance to solvent-containing products. The excellent material properties of the polyurethane dispersions make many applications possible for these binders.
Polyurethane dispersions are also gaining ever greater importance in building applications such as paints and varnishes, coatings, sealants and adhesives. In building applications, solvent-free polyurethane dispersions having a high solids content of polyurethane polymer or fillers, which can be made available by means of efficient and at the same time universal production processes, are particularly sought.
The preparation of aqueous polyurethane dispersions has been known for many years and is described in detail in a great number of publications, e.g. Houben-Weyl, Methoden der organischen Chemie, volume E 20, part I, pp. 1659-1681; D. Dieterich, Prog. Org. Coat. 1981, 9, 281-330; J. W. Rosthauser, K. Nachtkamp, Journal of Coated Fabrics 1986, 16, 39-79; R. Arnoldus, Surf. Coat. 1990, 3 (Waterborne Coat.), 179-98.
Aqueous polyurethane dispersions comprise polyurethane polymers or polyurethane-polyurea polymers which contain both urethane groups and urea groups and are obtainable by polyaddition reactions of polyols, polyisocyanates and polyamines. The polyols and the polyisocyanates are firstly reacted to produce polyurethane prepolymers which are then dispersed in the aqueous phase and chain-extended using polyamines to form the polyurethane-polyurea polymers. Moreover, the polyurethane polymers contain a sufficient number of hydrophilic groups which ensures stabilization in the aqueous phase. These hydrophilic groups can be anionic, cationic and/or nonionic groups. The charge density of the hydrophilic groups on the polyurethane polymer depends on the characteristics and the stoichiometry of the formative components used. Polyurethane dispersions are two-phase systems made up of micelles comprising polyurethane polymers and an aqueous phase. When the polyurethane dispersions are dried, the micelles coalesce or melt together to form a film of the polyurethane polymers.
For the preparation of polyurethane dispersions in the industry, the prepolymer mixing process and the solvent process have achieved the greatest importance.
However, these conventional processes for preparing polyurethane dispersions suffer from various problems.
In the prepolymer mixing process, significant amounts of high-boiling and water-soluble solvents, e.g. N-methylpyrrolidone, are added to reduce the viscosity of the polyurethane prepolymers. These solvents remain in the polyurethane dispersion after the production process. When the polyurethane dispersions or the products produced therefrom are dried, these solvents are given off into the environment. It is not only the solvent content but also the low solids contents, the usually moderate material properties and the large amounts of hydrophilic groups necessary to stabilize the polyurethane dispersions which are disadvantageous. However, the prepolymer mixing process represents a simple and effective production process with a large synthetic bandwidth, which is of great advantage in many cases.
In the solvent process or acetone process, the complete formation of the polyurethane polymers is carried out in the presence of large amounts of low-boiling and water-soluble solvents, for example acetone or methyl ethyl ketone. After the preparation of the polyurethane dispersion, the solvents have to be removed again by costly redistillation, so that the resulting polyurethane dispersions are largely solvent-free. The freedom from solvents and also the high solids contents, the excellent material properties and the small amounts of hydrophilic groups required for stabilizing the polyurethane dispersions are advantageous. However, the solvent process is a complicated and not economically optimal production process giving a low space-time yield, which is a great disadvantage especially with a view to building applications.
In addition, there are also various combinations of prepolymer mixing process and solvent process, but these have similar problems. Thus, a mixture of low- and high-boiling solvents can be used in the prepolymer mixing process, or the complete formation of the polyurethane dispersion can be shifted into the aqueous phase in the solvent process. However, the abovementioned problems cannot be eliminated even by these methods.
More recently, there have been increasing efforts on the part of manufacturers of polyurethane dispersions to replace solvents such as N-methylpyrrolidone by ecologically acceptable glycol ethers which are not subject to labeling laws, for example dipropylene glycol dimethyl ether (Proglyde DMM® from Dow). However, such a change leads to an increase in costs in the prepolymer mixing process.
The patent applications EP-A-0 712 877 and EP-A-0 808 859 describe solvent-free polyurethane dispersions having high solids contents which are produced by a modified solvent process. These polyurethane dispersions are used for producing sealing compounds and sprayable sealers. However, the process used for preparing the polyurethane dispersions is restricted to polypropylene glycols and cannot be applied to further polymeric polyols. In the preparation of these polyurethane dispersions, simultaneous dispersion and chain extension is carried out in the aqueous phase, and the solvent present is subsequently removed again by distillation. The examples described involve only polyurethane dispersions which are chain-extended by means of water. However, it is known from the chemistry and technology of polyurethane dispersions that such a procedure suffers from serious problems. In the case of polyurethane dispersions based on polypropylene glycols, the redistillation of solvents such as acetone or methyl ethyl ketone is made considerably more difficult by polar interactions. Chain extension using water is known to lead to products having a poor shelf life and reproducibility. The strong gas evolution from the isocyanate/water reaction during a chain extension makes the redistillation of the solvents even more difficult.
EP-A-0 741 152 discloses polyurethane dispersions based on various polypropylene glycols having a comparatively low degree of unsaturation or a low content of monools. These polyurethane dispersions are prepared by the prepolymer mixing process, comprise N-methylpyrrolidone as solvent and have only low solids contents.
DE-A 40 11 455 discloses solvent-free polyurethane dispersions based on polyalkylene glycol mixtures. These polyurethane dispersions are prepared using relatively low NCO/OH equivalent ratios and accordingly have low solids contents and high viscosities.
Polyurethane dispersions having a low VOC content are also described in EP-A-0 553 714. These polyurethane dispersions are based on various polymeric polyols and 1,3-bis(1-isocyanato-1-methylethyl)benzene (m-TMXDI) and may further comprise N-methylpyrrolidone. They are prepared by the prepolymer mixing process using low NCO/OH equivalent ratios. The solids content of these systems is relatively low.
Finally, U.S. Pat. No. 5,656,701 discloses low-solvent or solvent-free anionic and cationic polyurethane dispersions based on various polymeric polyols, which are prepared by means of the prepolymer mixing process or the solvent process using low NCO/OH equivalent ra

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