Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-08-28
2001-05-29
Niland, Patrick D. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S839000, C524S840000
Reexamination Certificate
active
06239213
ABSTRACT:
FIELD OF INVENTION
This invention relates to a process for the preparation of stable aqueous polyurethane dispersions. More particularly it relates to a process for the preparation of aqueous polyurethane dispersions by reacting a polyester polyol, preferably, branched and more preferably, a polyester polyol containing a long chain triol, having an average molecular weight of about 300-3000 and containing
i. an ionizable group of about 2-100 milliequivalents per 100 gram of prepolymer;
ii. optionally a polyether polyol, preferably poly(propyleneglycol) of molecular weight 500-5000, and
iii. aliphatic or aromatic isocyanate having functionality ≧2.
The isocyanate terminated and carboxyl group containing prepolymer prepared as described above, upon neutralisation with amines, preferably, tertiary amines, gives stable aqueous dispersions of polyurethanes.
BACKGROUND OF THE INVENTION
Polyurethanes are used for a wide range of commercial applications such as adhesives, coatings on different substrates including textiles, plastics, wood, glass fibers, metals and the like due to their extremely good chemical and abrasion resistance, touglness, flexibility and durability. Conventionally, these polyurethane coating products are diluted with organic solvents to get desirable consistency before application. As the coating dries, these organic solvent vapors escape into the atmosphere. This is both economically and environmentally disadvantageous on account of the higher cost of organic solvents, and the pollution and health hazards caused by such fugitive emissions. More importantly, these organic vapors pollute the atmosphere and create serious health hazards.
Many approaches are reported in prior art to form polyurethane coatings from the dispersion of these polymers in water. The use of water is economically advantageous as well as it does not pollute the atmosphere. However, polyurethane polymers are not compatible with water i.e they do not form stable dispersions in water until special process and/or special monomer(s) are used in their preparation.
One such approach in the prior art is the use of external emulsifiers to disperse and stabilise the polymers in water (U.S. Pat. No. 2,968,575). Though these emulsifiers stabilise the polymer in water, they posses the disadvantage that such coatings are themselves sensitive to water, and hence give poor coating performance in terms of hydrolytic stability. To overcome the above draw back, ionic groups are incorporated into the polyurethane backbone during polymerisation Upon neuralising these ionic groups the respective salts formed act as an internal emulsifier. One such approach (Neth. Pat. Appln. 6,410,928) discloses the use of cationic ionomers for preparing polyurethane latexes. In another approach (Ger. Offen. 2,744,544) polyurethane production was disclosed using anionic ionomers. The incorporation of nonionic emulsifiers in the polyurethane back bone is disclosed in U.S. Pat. No. 4,190,566. Further improvements in properties of coating derived from polyurethane dispersion are obtained by combining both ionic and non-ionic group in the polymer back bone. This has been disclosed in U.S. Pat. No. 4,238,378.
These prior art patents are related to the production of linear polyurethane dispersions. Coatings derived from these linear polyurethane dispersions possess limited water resistance and do not have good solvent resistance.
Ger Pat. 4,237,965 discloses the production of aqueous polyurethane binders from dimer diol and trimethylol propane as isocyanate reactive group for stoving lacquers. Herein, branching in the diol is expected to increase water resistance along with other important properties. Nevertheless, the externally added short chain triol increases the prepolymer viscosity, resulting in the reduction of the solid content of the dispersion to about 25%. Jap.Pat. 06,329,744 and Jap.Pat. 06,93,068 disclose the production of water dispersible polyurethane resin using trimethylol propane along with other isocyanate reactive components. The externally added short chain triol contributes more to the hard segment of the polymer. Further, crosslinking with aminoplast resins results in coatings with diminished flexibility and more prone to cracking.
Polyurethane dispersions disclosed in the prior art are normally derived from linear diols as one of the isocyanate reactive component and some quantity of short chain triols to impart a certain degree of branching/crosslinking . The main disadvantage of using short chain triols externally are a) undesirable increase in viscosity during prepolymer production b) insufficient branching/crosslinking c) short chain triols contribute more to hard segment and cracking of film is observed d) poor hydrolytic stability.
OBJECTS OF THE INVENTION
Therefore, the object of the present invention is to provide a process for the preparation of stable aqueous polyurethane dispersions which are stable over a wide range of temperature and under high shear force.
Another object of the present invention is to obtain coating with good water and chemical resistance particularly when crosslinked with aminoplast resins.
Yet another object of the present invention is to provide a coating with superior flexibility.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, the present invention provides a process for the preparation of aqueous polyurethane dispersions which comprises of preparing an isocyanate terminated carboxyl group containing prepolymer, by preparing a solution of a polyol in an organic solvent, adding an ionizable group containing compound to this solution, heating this mixture to a temperature ranging between 40° C. and 120° C., adding an isocyanate in this reaction mixture, either in the presence or absence of a catalyst depending upon the isocyanate, and stirring the mixture for a period ranging between 1 and 16 hours, neutralizing this mixture with a base, cooling the mixture to ambient temperature and dispersing the mixture in water to obtain the product.
In an embodiment of the present invention, the polyol used may be a linear polyether and/or polyester diol which is normally used in polyurethane synthesis and, more particularly, branched polyester polyol of molecular weight in the range of 300 to 5000 more preferably in the range of 300 to 3000.
In another embodiment the polyether/polyester diols may contain three or more hydroxyl groups, and have a hydroxyl number between 50 and 100 mg KOH/gm and acid number preferably less than 2 mg. KOH/gm.
In another embodiment the organic solvent used for preparing the solution of the polyol, may be selected from substantially non-reactive organic solvents to the isocyanate polyaddition reaction containing carbon and hydrogen with or without other elements such as oxygen or nitrogen exemplified by dimethyl formamide, esters, ethers, ketoesters, ketones (e.g. acetone and butan-2-one), glycol ether esters, chlorinated hydrocarbons, aliphatic and alicyclic hydrocarbons, pyrrolidones (e.g. N-methyl-2-pyrrolidone) hydrogenated furans and aromatic hydrocarbons or mixtures thereof.
In yet another embodiment the ionizable group used may be an organic compound, containing, at least, one active hydrogen and, at least one group, capable of salt formation and may be selected from compounds having the general formula,
(HO)
m
R(COOH)
n
.
Wherein R represents a straight or branched hydrocarbon containing 1 to 12 carbon atoms and m, and n represent values between 1 and 3, more preferably the acids of the aforementioned formula wherein m=2, n=1 and R=—CH
3
, exemplified by œ,œ′-dimethylol propionic acid, and other acids including hydroxy, amino hydroxy, amines and mercapto carboxylic acids, sulphonic acids, hydroxy and amino sulphonic acids such as dimethylol propionic acid, oxaluric acid, anilido acetic acid, dihydroxy tartaric acid, 2,6-dihydroxy benzoic acid, glycolic acid, thio glycolic acid, glycine, alpha alanine, 6-amino caproic acid, 2-hydroxy ethane sulphonic acid, 4,6-diamino benzene, 1,3-disulphonic acid, and 2-amino diphenyl
Ramanathan Lalgudi Srinivasan
Raut Kundalik Ganpat
Sivaram Swaminathan
Srinivasan Subagiri Ramaswamyiyengar
Council of Scientific & Industrial Research
Ladas & Parry
Niland Patrick D.
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