Hydrophilic polyurethanes

Details Hydrophilic polyurethanes Hydrophilic polyurethanes

1995-07-27

2002-06-04

Gorr, Rachel (Department: 1711)

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

Synthetic resins

From reactant having at least one -n=c=x group as well as...

C524S591000, C528S904000, C528S905000, C525S056000, C525S123000

Reexamination Certificate

active

06399735

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to hydrophilic, high molecular weight nonionic polyurethanes, to their production and their use as the basis for an adhesive and as a stabilizer in emulsion polymerization.
2. Discussion of Related Art
Hydrophilic polyurethanes are acquiring increasing significance in the form of aqueous dispersions. The dispersions may be stabilized by addition of emulsifiers or by the incorporation of hydrophilic groups. Typical hydrophilic groups are ionic groups, more particularly salts of carboxylic acids and alkali metals or amines. However, aqueous dispersions of polyurethanes with a nonionic internal emulsifier are also known.
Thus, EP 0 497 404 A describes an aqueous polyurethane dispersion based on tetramethyl xylene diisocyanate. The polyurethane contains as internal emulsifier a reaction product of alkoxypolyethylene glycol with an anhydride which is then further reacted with an alkylene oxide, an epoxy alcohol or a diglycidyl ether. According to Example 4, a) 133 parts by weight of a reaction product of methoxypolyethylene glycol and trimellitic anhydride, which finally was reacted with propylene oxide, b) 79 parts by weight of polypropylene glycol, c) 40 parts by weight of the ethylene glycol diether of bisphenol A and d) 146 parts by weight of isophorone diisocyanate are mixed and the resulting mixture is kept at 90° C. for 3 hours. This is followed by chain extension with water at 45 to 90° C. A dispersion containing particles 50 nm in size characterized by high resistance to water is obtained. This dispersion may be used for the production of adhesives and paints.
Aqueous polyurethane dispersions of the type in question have the disadvantage that, even with high solids concentrations, their viscosity is too low for many applications, for example as a wallpaper adhesive. In addition, the adhesive strength and tack of the correspondingly heavily diluted products are far too low or totally non-existent.
K. C. Frisch investigated the thermal and hydrolytic degradation of linear polyurethanes (Journal of Polymer Science, Vol. 11 (1973), pp. 637-648 and pp. 1683-1690). The polyurethanes were prepared from the diisocyanates toluene-2,4- and -2,6-diisocyanate (TDI), m- and p-xylene diisocyanate (XDI) and dicyclohexyl methane-4,4-diisocyanate (HMDI) and from polyoxyethylene glycol with an equivalent weight of 190, 1,485 and 2,955. In some cases, ethylene glycol was also used as chain-extending agent. Polyurethanes were prepared from these components in solvents at around 70° C. in the presence of tin octoate as catalyst. The hydrolysis was investigated using a solution of 0.75 g in 25 ml of solvent containing 10% by weight of isopropanol. The use of the polyurethanes thus produced was not discussed.
U.S. Pat. No. 4,079,028 describes a low molecular weight polyurethane as thickener for a latex and other aqueous systems. The polyurethane contains at least three hydrophobic groups with hydrophilic polyoxyalkylene groups in between. Thus, a polyurethane with an average molecular weight (Mw) of 71,000 terminated by dodecyl groups can be prepared, for example, from 200 g of polyethylene glycol, 1.7 g of dodecyl isocyanate and 1.4 g of TDI in toluene at 75° C. in the presence of dibutyl tin laurate. The molecular weight is in the range from 10,000 to 200,000. In the case of the linear polyurethanes (see Examples 1 to 102), the degree of polymerization is generally 1 to 4 and at most 18. Table 19 contains reaction products of the three components diol, diisocyanate and a monofunctional compound, such as monoisocyanate or polyhydric alcohols or amines, as a clear limitation with respect to the known linear thickeners of the two components diol and monoisocyanate on the one hand and monoalcohol and diisocyanate on the other hand. Polyethylene glycol, diisocyanates and other polyols are not mentioned. The known linear polyurethanes are used as thickeners, for example even in adhesive dispersions. Their use as adhesives is not mentioned.
The same also applies to U.S. Pat. No. 4,155,892.
EP 0 537 900 describes corresponding polyurethanes as thickeners for non-aqueous systems.
DE 41 37 247 also describes a thickener of a linear polyurethane which is similarly prepared from a difunctional isocyanate, a polyether diol and a monohydric alcohol.
SUMMARY OF THE INVENTION
Accordingly, the problem addressed by the present invention was to provide an aqueous system of polyurethanes which have a high viscosity, even at low concentrations, which could compete in price with pastes and which could be mixed with water in any ratio.
The solution provided by the invention is defined in the claims. The invention is essentially based on the provision of a hydrophilic, high molecular weight nonionic polyurethane which is soluble in water and of which the viscosity in water may be additionally increased by hydrophobic interactions.
DETAILED DESCRIPTION OF THE INVENTION
By “water-soluble” and “high molecular weight” are meant the ability of the polyurethane to form a homogeneous mixture with water at 20° C. with a considerably higher viscosity than pure water. Thus, the specific viscosity of a 1% solution is at least 0.4, more particularly at least 0.5 and preferably at least 0.7, as measured with an Ostwald viscosimeter at 20° C. The dependence of the specific viscosity on concentration is substantially linear to the usual extent. The average molecular weight Mw is at least 10,000, preferably in the range from 20,000 to 200,000 and, more preferably, in the range from 40,000 to 180,000, as determined by GPC using polystyrene as the calibration standard.
The high molecular weight derives from the high viscosity. The specific viscosity may be up to 6.0, but is at least up to 4 (1% solution at 20° C., Ostwald viscosimeter). For polyurethanes containing polyethylene glycol as sole diol component, the melt viscosity at 175° C. is greater than 5, more particularly greater than 30 and, preferably, greater than 100 Pas (Epprecht). For polyurethanes containing additional hydrophobic diols, the melt viscosity is greater than 3, preferably greater than 5 and, more particularly, greater than 10 Pas at 175° C. Products which are virtually solid at 175° C. can also occur.
3 to 20% solutions have a viscosity at 22° C. of 20 mPa·s to firm (Brookfield, spindle 7). A 2% solution has a viscosity of 20 to 24,000 mPa·s (Brookfield, spindle 7). A 40% solution has a viscosity of 700 mPa·s to firm and, more particularly, 4,000 to firm (Brookfield, spindle 7).
It may be concluded from the viscosity behavior that the solution in question is a substantially true solution in which the polyurethane is molecularly dispersed in the water. More particularly, the polyurethane is miscible with water in any ratio at 20° C. Accordingly, there is no miscibility gap. It is at least possible to prepare an aqueous solution with a polymer content of up to 70% by weight, based on the solution. The aqueous solution is surprisingly stable not only as a function of time, but also on addition of ionic compounds or on addition of organic solvents. Thus, up to 5% of sodium chloride may be added. The aqueous solution even remains stable in the event of changes in the pH value. Thus, the pH value may be varied over the range from 2 to 12.
By “nonionic” is meant that the polyurethane does not contain any ionic groups as emulsifying groups, i.e. no carboxylate, sulfonate, phosphonate or ammonium groups. Its solubility in water is attributable instead to the hydrophilic nonionic groups of the polyethylene ether —[CH
2
—CH
2
—O—]
n
— where n is a number of 8 to 500, more particularly 20 to 300 and, above all, 50 to 200. These structural units are derived in particular from the polyethylene glycols used as diols. However, polyethylene glycols in the context of the invention are not only polyaddition products of ethylene oxide with water or ethylene glycol as starter molecule, but also polyaddition products with other dihydric alcohols, for example butane diol, hexane diol, 4,4′-di

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