While ostensibly reactive with water, it has long been known that polyisocyanate polymers can be used to prepare aqueous polyurethane dispersions. Polyurethane dispersions are generally prepared by chain extending the reaction product of an organic diisocyanate or polyisocyanate and an organic compound having two or more active hydrogen atoms such as polyalkylene ether glycols, poly (alkylen ether-alkylene thioether) glycols, alkyd resins, polyesters and polyester amides, often using an organic solvent. The diisocyanate is used in stoichiometric excess so that the reaction product, also referred to as a polyurethane/urea/thiourea prepolymer, is isocyanate terminated. Examples of polyurethane prepolymer preparations are described in U. S. Patents Nos. 3, 178, 310, 3, 919, 173, 4, 442, 259, 4, 444, 976, and 4, 742, 095, among others.

Polyurethane dispersions are reported as being useful for preparing such diverse materials as : coatings and bonds in U. S. Patent No. 4, 292, 226 ; flexible solvent barriers in U. S. Patent No. 4, 431, 763 ; adhesives in US Patent No. 4, 433, 095 ; and films in 4, 501, 852.

Films, or rather the process of dipping to make a film, can be a part of the processes for making many articles. Examples of film applications include exam gloves, organ bags, condoms, ostomy bags, and the like. While it is known that such applications can be made with polyurethane dispersions, conventional polyurethane dispersions have sometimes been found to have insufficient physical or handling properties to make them a preferred material for such applications. Also, the use of a solvent can have adverse effects for some applications.

Polyurethanes are the reaction product of a polyalcohol and a polyisocyanate.

Typically, the polyisocyanates used to prepare polyurethane dispersions have been aliphatic isocyanates such are disclosed in US Patent No. 5, 494, 960. Aromatic polyisocyanates such as toluene diisocyanate (TDI) and methylene diphenyldiisocyanate (MDI) as well as polymethylene polyphenylisocyanate are also known to be useful.

Conventional processes of preparing films from dispersions, including polyurethane dispersions, generally include a step of coagulating the dispersion onto a substrate. It is therefore necessary that dispersions used to make films have the property that they can be

coagulated onto a substrate. At the same time, it is considered desirable in the art of making polymer dispersions that the dispersions be stable, that is that they do not settle or spontaneously coagulate during shipping on storage. Accordingly, it would be desirable in the art of preparing aqueous dispersions useful for preparing films that the dispersions be capable of being coagulated onto a substrate using conventional coagulants and coagulation technology. It would also be desirable in the art to prepare such dispersions which have enhanced shear stability. It would be particularly desirable to prepare films with such dispersions which also can be prepared in the absence of organic solvents.

In one aspect, the present invention is a process for preparing polyurethane dispersions with enhanced shear stability comprising admixing a polyurethane prepolymer with water using an external surfactant and thereafter dispersing therein a second and different external surfactant.

In another aspect, the present invention is a polyurethane film prepared by the process admixing a polyurethane prepolymer with water using an external surfactant, dispersing therein a second and different external surfactant to form a dispersion, and applying the dispersion to a substrate to form a film.

A polyurethane dispersion prepared using a two step addition of different surfactants has greater shear stability than a similar dispersion prepared using only a single surfactant or a single step addition of surfactants. The dispersions of the present invention have the advantage of providing enhanced shear stability, while still being useful for the end use application. In other words, the dispersions of the present invention can be coagulated without having negative impact on film processing or the resulting film properties. The dispersions and films of the present invention have applicability in a variety of end use applications such as, for example, gloves, condoms, angioplasty balloons, medical bags, medical tubing, or catheters.

The polyurethane prepolymer dispersions of the present invention are prepared by a two step process of dispersing a polyurethane prepolymer in water using a first external surfactant and then admixing the resulting dispersion of the first step with a second and different external surfactant. The product of the second step is a polyurethane dispersion useful for preparing films but also one having more shear stability than a dispersion prepared using a single surfactant or mixtures of surfactants in a single step."Shear stability"is defined as meaning that the dispersion maintains its particle size and usefulness

for its end use application when exposed to shear, for example, during pumping, agitation or other movement of the fluid."Enhanced shear stability"means that the dispersions of the present invention have a shear stability that is enhanced as compared to dispersions prepared using only a single surfactant or a single step addition of surfactants. For purposes of the present invention, the phrase"useful for preparing films"means that while the dispersions are sufficiently stable to be stored, they are not so stable that they cannot be electrodeposited or coagulated onto a substrate to make a film or other dispersion derived product.

The dispersions of the first step of the present invention can be prepared in any way which results in a dispersion which can be used to prepare a film having acceptable physical properties for the anticipated use of the film. The dispersions can be prepared by a batch process or by a continuous process. If prepared by a batch process, preferably the dispersion is prepared by an inverse phase process wherein a small amount of water, including a small amount of anionic surfactant, is first added to a continuous prepolymer phase and mixed and then more water is added with mixing until the phase inverts.

When dispersions of the first step of the process of the present invention are prepared by means of a continuous process, preferably they are prepared by means of a high internal phase ratio (HIPR) process. Such processes are known and are disclosed in, for Example, U. S. Patent No. 5, 539, 021 to Pate, et al., and WO 98/41552 Al to Jakubowski, et al. Other continuous dispersion processes can be used with the first step of the process of the present invention with the proviso that they result in a stable dispersion or at least a dispersion which is sufficiently dispersed to be further processed in the second step and result in a stable dispersion. For purposes of the present invention, a dispersion is stable if it does not settle or separate out too quickly to be useful for its intended purpose.

In the second step of the process of the present invention, the dispersion from the first step is admixed with a different external surfactant and admixed. The admixture of the second step may be prepared by any method which results in a stable polyurethane dispersion. The product of the second step of the process of the present invention, irrespective of admixing methods used, should have a particle size sufficient to make the dispersion stable. The dispersions of the present invention will have a particle size of from 0. 9 to 0. 05, preferably from 0. 5 to 0. 07 and even more preferably, from 0. 4 to 0. 10 microns.

The first surfactant is utilized for the purpose of stabilizing the particles in the desirable particle size range, whereas the second surfactant is used to enhance the shear

stability. Surprisingly, if the first and second surfactant are added at the same time, adequate stability is not achieved, whereas when the first and second surfactant are added in a stepwise fashion, the desired particle size is achieved, as is the enhanced shear stability.

Likewise, if only a single surfactant is used, even in greater amounts, enhanced shear stability is not achieved.

The polyurethane dispersions of the present invention are prepared from a nonionic polyurethane prepolymer. The nonionic prepolymers useful with the present invention are prepared with an aromatic diisocyanate selected from the group consisting of MDI, TDI and mixtures thereof. TDI can be generally used with any commonly available isomer distribution. The most commonly available TDI has an isomer distribution of 80 percent of the 2, 4 isomer and 20 percent of the 2, 6 isomer. For the purposes of the present invention, TDI with other isomer distributions can also be used, but such isocyanates are often available only at a significantly higher cost.

When MDI is used with the formulations of the present invention, it preferably has a P, P' isomer content of from 99 weight percent to 50 weight percent. Even more preferably, when MDI is used with the formulations of the present invention, it preferably has a P, P' isomer content of from 98 to 92 weight percent. Most preferably, when MDI is used with the formulations of the present invention, it preferably has a P, P' isomer content of about 94 weight percent. While MDI with such isomer distributions can be prepared by distillation during the MDI process, it can also be prepared by admixing commonly available products such as ISONATE 125M* and ISONATE 500P*. (*ISONATE 125M and ISONATE 500P are trade designations of The Dow Chemical Company.) When mixtures of TDI and MDI are used to prepare the prepolymers useful with the present invention, they are admixed in a ratio of MDI to TDI of from 99 weight percent MDI to 1 weight percent MDI. More preferably, when mixtures of TDI and MDI are used to prepare the prepolymers useful with the present invention, they are admixed in a ratio of MDI to TDI of from 95 weight percent MDI to 40 weight percent MDI. Most preferably, when mixtures of TDI and MDI are used to prepare the prepolymers useful with the present invention, they are admixed in a ratio of MDI to TDI of 75 weight percent MDI to 50 weight percent MDI. Preferably the prepolymers useful with the present invention are prepared with MDI or mixtures of MDI and TDI. Even more preferably, the prepolymers useful with the present invention are prepared with MDI as the only aromatic diisocyanate.

In one embodiment of the present invention, the prepolymers useful with the present invention are prepared from a formulation that includes an active hydrogen containing material. In a preferred embodiment of the present invention, the active hydrogen containing material is a mixture of diols. One component of the diol mixture is a high molecular weight polyoxypropylene diol having an ethylene oxide capping of from 0 to 25 weight percent. The other component of the diol mixture is a low molecular weight diol.

The polyether diols of the formulations of the present invention can be prepared by any method known to those of ordinary skill in the art of preparing polyether polyols to be useful for preparing such diols. Preferably, the polyether diols are prepared by the alkoxylation of a difunctional initiator in the presence of a basic catalyst. For example, a polyether useful with the present invention is a product resulting from a two step alkoxylation of ethylene glycol with first propylene oxide and then ethylene oxide, in the presence of KOH as a catalyst.

The low molecular weight diol component of some of the prepolymer formulations of the present invention can also be a product of alkoxylating a difunctional initiator.

Preferably, this component is also a polyoxypropylene diol, but it can also be a mixed ethylene oxide propylene oxide polyol, as long as at least 75 weight percent of the alkoxides used, if present, is propylene oxide. Diols such as propylene glycol, diethylene glycol, dipropylene glycol, and the like, can also be used with the formulations of the present invention. The low molecular weight diol component of the prepolymer formulations, if present, has a molecular weight of from 60 to 750, preferably from 62 to 600, and most preferably, from 125 to 500.

The prepolymers useful with the present invention can be prepared in any way known to those of ordinary skill in the art of preparing polyurethane prepolymers to useful for preparing such prepolymers. Preferably the aromatic diisocyanate and polyether diol mixture are brought together and heated under reaction conditions sufficient to prepare a polyurethane prepolymer. The stoichiometry of the prepolymer formulations of the present invention is such that the diisocyanate is present in excess. Preferably, the prepolymers useful with the present invention have an isocyanate content (also known as percent NCO) of from about 1 to about 9 weight percent, more preferably from 2 to 8 weight percent, and most preferably from 3 to 7 weight percent.

The prepolymers useful with the present invention are optionally extended, sometimes using a difunctional amine chain extender when the active hydrogen containing material of the prepolymer formulation is a mixture of a low molecular weight diol and a high molecular weight polyether diol. The difunctional amine chain extender may not be optional, but rather be required when the active hydrogen containing material of the prepolymer formulation is a high molecular weight polyether diol and does not include a low molecular weight diol. Preferably, the difunctional amine chain extender, if present, is present in the water used to make the dispersion. When used, the amine chain extender can be any isocyanate reactive diamine or amine having another isocyanate reactive group and a molecular weight of from 60 to 450, but is preferably selected from the group consisting of : an aminated polyether diols ; piperazine, aminoethylethanolamine, ethanolamine, ethylenediamine and mixtures thereof. Preferably, the amine chain extender is dissolved in the water used to make the dispersion.

The prepolymers useful with the present invention are preferably nonionic. In the preferred prepolymers, there are no ionic groups incorporated in or attached to the backbones of the prepolymers used to prepare the films of the present invention. The anionic surfactant used to prepare the dispersions of the present invention is a external stabilizer and is not incorporated into the polymer backbones of the films of the present invention.

The prepolymers useful with the present invention are, in a first step, dispersed in water which contains a first surfactant. Preferably the surfactant is an anionic surfactant. In the practice of preparing the dispersions of the present invention, the first surfactant is preferably introduced into water prior to a prepolymer being dispersed therein, but it is not outside the scope of the present invention that the first surfactant and prepolymer could be introduced into the water at the same time. Any anionic surfactant can be used as the first surfactant with the present invention, but preferably the first surfactant is selected from the group consisting of sulfonates, sulfates, phosphates, and carboxylates. More preferably, the first surfactant is sodium dodecyl benzene sulfonate, sodium dodecyl sulfonate, sodium dodecyl diphenyl oxide disulfonate, sodium n-decyl diphenyl oxide disulfonate, isopropylamine dodecylbenzenesulfonate, triethanolamine lauryl sulfate, or sodium hexyl diphenyl oxide disulfonate, and most preferably, the first anionic surfactant is sodium dodecyl benzene sulfonate.

In the practice of the process of the present invention, in a second step, a polyurethane dispersion prepared with a first external surfactant is admixed with a second and different external surfactant. Preferably, the second surfactant is an anionic surfactant.

Any anionic surfactant can be used as the second surfactant with the present invention, but preferably the second surfactant is selected from the group consisting of sulfonates, sulfates, phosphates, and carboxylates. More preferably, the second surfactant is sodium dodecyl benzene sulfonate, sodium dodecyl sulfonate, sodium dodecyl diphenyl oxide disulfonate, sodium n-decyl diphenyl oxide disulfonate, isopropylamine dodecylbenzenesulfonate, triethanolamine lauryl sulfate, or sodium hexyl diphenyl oxide disulfonate, and most preferably, the second anionic surfactant is triethanolamine lauryl sulfate. Other external surfactants can also be used in the second step of the process of the present invention with the proviso that they are different from the surfactant of the first step.

The second surfactant is post-added. In other words, the second surfactant is not -added until after the polyurethane dispersion prepared with the first external surfactant reaches the desired particle size.

The dispersions of the present invention can have a solids level of from 30 weight percent to 60 weight percent. Films will not necessarily be prepared from dispersions having this level of solids. While the dispersions themselves will be stored and shipped at as high a solids content as possible to minimize storage volume and shipping costs, the dispersions can desirably be diluted prior to final use. The thickness of the film to be prepared and the method of coagulating the polymer onto a substrate will usually dictate what solids level is needed in the dispersion. When preparing films, the dispersions of the present invention can be at a weight percent solids of from 5 to 60 percent, preferably from 10 to 40 percent, and, most preferably, from 15 to 25 weight percent when preparing examination gloves. For other applications, the film thickness and corresponding solids content of the dispersion used can vary.

The polyurethane dispersions of the present invention can be prepared by including in the prepolymer formulation other materials. For example, the polyurethane dispersions of the present invention can be prepared by including a monol in the prepolymer. When present, preferably the monol is a polyether and more preferably is a polyoxyethylene polyether monol. Preferably the monol has a molecular weight of from 500 to 1500, and more preferably from 800 to 1200. Preferably the monol is included in the prepolymer

formulation at a concentration of from 0. 1 to 0. 99 percent by weight. Too much monol in the prepolymer will result in a dispersion which can not be coagulated using conventional coagulation technology such as calcium nitrate and the like.

Other materials besides monols can also be included in the polyurethane dispersion formulations of the present invention. For example, films prepared using dispersion of the present invention can be prepared such that they are self-releasing. In the art of preparing exam gloves, this ability is also known as"powder free"in reference to the fact that such gloves are occasionally prepared and sold with a layer of talcum powder, corn starch, or the like, to keep the polymer from adhering to itself, thereby making it easier to put on the gloves. The films of the present invention can be made self releasing by inclusion of a wax in the prepolymer formulation. In one embodiment, the wax is added as an emulsion to a dispersion. Preferably the wax is carnauba wax and the like. It is preferable that the wax to be used be selected from those that are not likely to induce an allergic reaction in skin that comes in contact therewith. Therefore, food grade waxes are particularly preferred for this application. When used, the waxes are preferably included in the water used to disperse the prepolymer formulation at a concentration of from 0. 1 to 2 weight percent. Even more preferably, the waxes are added to a fully prepared dispersion.

Any additive which is known to those of ordinary skill in the art of preparing films from dispersions to be useful can be used with the process of the present invention so long as their presence does not degrade the properties of the dispersions or films prepared therewith so much that the films are no longer fit for their intended purposes. The additives can also be incorporated into the formulations or films in any way known to be useful including, but not limited to inclusion in the prepolymer formulation and inclusion in the water used to make the dispersion. For example titanium dioxide is useful for coloring films of the present invention. Other useful additives include calcium carbonate, silicon oxide, defoamers, biocides, carbon particles, and the like.

Films may be prepared by applying the dispersions of the present invention to a substrate. Methods for applying the dispersions to a substrate include dipping, thermal coagulation, casting, electrodeposition, or a combination thereof.

The following examples are for illustrative purposes only and are not intended to limit the scope of the claimed invention. Percentages are in weight percents unless otherwise stated.

EXAMPLES The following materials are used in the examples below : Polyether Polyol is a 2000 molecular weight polyoxypropylene diol having 12. 5 percent ethylene oxide end capping.

Low Molecular Weight Diol is a 425 molecular weight all polyoxypropylene diol.

Polyisocyanate I is MDI having a 4, 4' isomer content of 98 percent and an isocyanate equivalent weight of 125.

Polyisocyanate II is MDI having a 4, 4' isomer content of 50 percent and an isocyanate equivalent weight of 125.

Surfactant I is triethanolamine lauryl sulfate.

Surfactant II is sodium dodecyl benzene sulfonate.

Surfactant ni is ammonium lauryl sulfate.

Surfactant IV is sodium lauryl sulfate.

Surfactant V is triethanolamine lauryl sarcosinate.

Surfactant VI is triethanolamine dodecylbenzene sulfonate.

EXAMPLE 1 A polyurethane prepolymer is prepared by admixing 23. 4 parts of Polyether Polyol and 6. 6 parts of Low Molecular Weight Diol and then heating the admixture to 50°C. This material is then admixed with 13. 1 parts of Polyisocyanate I and 1. 9 parts Polyisocyanate lI which has also been warmed to 50°C. A small amount of benzoyl chloride is added to neutralize residual base in the polyols. The admixture is then heated at 70°C for 4 hours.

A polyurethane dispersion is prepared by admixing the prepolymer and 1. 4 parts Surfactant II using a high shear mixer running at about 2500 rpm. Water is slowly added until a phase inversion is observed. Then additional water added to bring the total water content to 53. 6 parts.

The polyurethane dispersion is then further treated by adding an additional 0. 4 parts of Surfactant I and stirring.

The polyurethane dispersion is tested for shear stability by diluting the dispersion with water to a solids content of 40 percent and then stirring the dispersion using a high shear mixer running at 2000 rpm. The dispersion is observed for 30 minutes or until the dispersion thickened and formed a solid. The observation is recorded below in Table 1.

EXAMPLES 2-5

Examples 2-5 are prepared and tested substantially identically to Example 1 except that the surfactants indicated in Table 1 are used instead of Surfactant I.

COMPARATIVE EXAMPLE 6 Comparative Example 6 is prepared and tested substantially identically to Example 1 except that no second surfactant is used.

Table 1 Example No. Second Surfactant Time to thicken Time to Used solidification 1 Surfactant I Did not thicken in Did not thicken in 1800 seconds 1800 seconds 2 Surfactant 111 650 705 3 Surfactant IV 765 830 4 Surfactant V 930 1050 5 Surfactant VI 1330 1405 Comp. 6 none 350 402

COMPARATIVE EXAMPLES 7-10 A polyurethane prepolymer is prepared according to the process described above for Example 1.

A polyurethane dispersion is prepared by admixing the prepolymer and the surfactant mixture described in Table 2 below, using a high shear mixer running at 2500 rpm. For all of these comparative examples, a stepwise addition of surfactants is not used.

Water is slowly added until a phase inversion is observed. The additional water is added to bring the total water content to 53. 6 parts.

Table 2 Example Surfactant used Particle size Distribution Stability (parts by weight) (microns) Comp. 7 1.4 Surfactant II only 0.447 Monomodal No settlin Comp. 8 1. 4 Surfactant I only 2. 079 Bimodal Rapid settling Comp. 9 1. 9 Surfactant I only 1. 837 Bimodal Rapid settling Comp. 10 1. 0 Surfactant II 1. 740 Bimodal Rapid settling 1. 0 Surfactant I (added simultaneously)