Stable rinse aid formulation and method of forming same

The present invention is concerned with rinse aid formulations useful in machine dishwashers for promoting wetting of the rinse water on substrates and thereby reducing spotting of the dried substrate surface. More particularly, the invention is directed to the use of an alkali neutralized high molecular weight polymer to compatibilize a low foam nonionic surfactant with a low molecular weight neutralized (meth)acrylic acid polymer to form a stable, non-phase separating aqueous rinse aid dispersion.

Rinse aids are invariably used in commercial and institutional machine dishwashers and frequently in household dishwashers. A final rinse of fresh water serves to displace pre-final rinse water and its attendant detergent and soil residues. In large commercial machines the final rinse water is normally introduced at a temperature of 82.2°C (180°F) or above; the high temperature being used to sanitize as well as to promote rapid flash drying of tableware, glassware or plasticsware (herein collectively referred to as "substrate") surfaces as they exit the machine. In some "low energy" commercial dishwashers the final rinse water may be used at a lower temperature of about 60°C (140°F), and may contain about 50 parts per million (ppm) of a conventional chlorine releasing agent for sanitization. "Sanitization" in household dishwashers is achieved by effective batchwise cleaning with multiple wash and rinse cycles using fresh water.

Rinse aid formulations are aqueous solutions containing a low foam nonionic surfactant. The rinse aid is injected into the final fresh water rinse at a concentration of about 50 to about 100 ppm. The surfactant in the rinse water lowers the surface tension of the rinse water and improves the wetting action of the rinse water on the somewhat hydrophobic substrate surfaces. Improved wetting reduces the tendency of the rinse water to form drops containing dissolved solids on the substrate surface which give rise to spots upon drying. Accordingly, the functions of the surfactant in the rinse aid are to effectively reduce the surface tension during the draining period and to be low foaming so as to avoid traces of foam on the rinsed substrate which result in a residue upon evaporation.

In commercial dishwashers, the final rinse water will mix with and become the pre-final rinse water. The rinse water may also be fed back into the wash water or be used directly as the wash water for the next cycle. Consequently when a rinse aid is formulated the surfactant and any other additive should be chosen based on their effect in the wash bath as well as in the rinse water. An additional important aspect of a rinse aid, therefore, is the ability of the rinse aid to defoam food soils in the alkaline wash bath. Proteinaceous food soils are particularly prone to foam in agitated alkaline wash baths. Foam, or more specifically entrapped air in the wash spray, will reduce the mechanical efficiency of the spray and interfere with maximum soil removal. Many low foam surfactants are effective soil defoamers. However, other additives may interfere with soil defoaming.

While low foam surfactants have improved the wetting of rinse water on substrates, they have not completely eliminated spotting and streaking problems. It is known that the addition of a low molecular weight neutralized polyelectrolyte, such as polyacrylic acid, to the rinse water can further reduce spotting and filming or streaking. It is believed that these low molecular weight water-soluble polymers can adsorb onto slightly soiled substrates and make the surface more hydrophilic. A more hydrophilic surface can be more readily wetted by the surfactant-containing rinse water. (Meth)acrylic polymers are especially useful because they do not contribute to foam formation and do not interfere with the soil defoaming activity of the low foam surfactants. A major obstacle, however, exists to the use of low molecular weight (meth)acrylic acid polymers in rinse aids. This obstacle is due to the incompatibility of these polymers in aqueous rinse aid formulations containing low foam surfactants. Combining such polymers and surfactants in water results in phase separation. Upon standing for a short period of time the water containing these polymers and surfactants will form two or more layers of different compositions. This phase separation is obviously unsatisfactory since non-uniform addition of the desired components will occur as the formulation is injected into the machine. For example, the formulation may contain too little surfactant to provide adequate wetting or too much leading to excessive foam.

Hydrotropes such as sodium xylene sulfonate, cumene sulfonate and short chain alkyl sulfates have been used to raise the cloud point of low foam surfactants to permit the formulation of stable aqueous concentrates. (See, for example, US-A-3,563,901 and US-A-4,443,270). These hydrotropes have little effect, however, on compatibilizing low foam surfactants with low molecular weight polymers in aqueous concentrates. Moreover, even in cases where hydrotropes give limited compatibility, they suffer from the major disadvantage of interfering with the food soil defoaming activity of the surfactants.

Water-miscible solvents, such as isopropanol and propylene glycol, and hydrogen bond-breaking compounds, such as urea, have also been proposed for use in formulating rinse aids containing low foam nonionic surfactants. However, they have been found to have little or no effect on compatibilizing acrylic acid polymers with low foam surfactants. Combinations of a hydrotrope and such solvents offer some improvement over the use of either compound alone, but the combinations still result in rinse aids having limited compatibility and adversely affect food soil defoaming activity.

Low molecular weight polyelectrolytes have been combined with low foam surfactants in detergent formulations. US-A-4,203,858 discloses a low foaming, phosphate-free, dishwashing composition comprising an alkali metal or ammonium carbonate, such as sodium carbonate, a water-soluble salt of a polyelectrolyte having a molecular weight of from about 500 to 4,000, and optionally up to 10 weight percent of a foam-suppressing nonionic surfactant. The weight ratio of polyelectrolyte to carbonate ranges from 5:95 to 20:80. Typical of the polyelectrolytes are acrylic, methacrylic, maleic and itaconic acid polymers. Homopolymers and copolymers of acrylic and methacrylic acid having a molecular weight ranging from 504 to 1291 are preferred. US-A-4,203,858 discloses that the major differences between this composition and prior polyelectrolyte-built dishwashing compositions are the low concentration of polyelectrolyte and the poor metal ion sequestering capability of these polyelectrolytes. Other references disclosing (meth)acrylic add polymers and their salts in detergent and cleaning applications include US-A-3,671,440; 3,853,981; 3,950,260; 3,933,673; 3,922,230 and 4,521,332. None of these references, however, address or offer solutions for combining polyelectrolytes with low foam surfactants in rinse aid concentrates.

The present invention enables the compatibilization of an aqueous rinse aid comprising low foam nonionic surfactant and low molecular weight neutralized (meth)acrylic acid homopolymer or copolymer without the use of a hydrotrope, and enables the provision of a stable aqueous rinse aid dispersion which reduces spotting and filming while improving sheeting action and drainage in comparison to known conventional rinse aids.

We have found that high molecular weight, substantially alkali neutralized methacrylic acid copolymer is useful as a compatibilizer or stabilizer for low foam nonionic surfactnats and low molecular weight neutralized (meth)acrylic acid polymers in water, and that a stable aqueous dispersion comprising from 0.5 to 20 weight percent low molecular weight (meth)acrylic acid homopolymer or copolymer, from 5 to 60 weight percent low foam nonionic surfactant, and from 0.3 to 5.0 weight percent of said high molecular weight substantially neutralized methacrylic acid compatibilizer copolymer is useful as a rinse aid formulation.

According to the present invention there is provided a stable aqueous rinse aid dispersion, which comprises: <ul id="ul0001" list-style="none"><li>1) 0.5 to 20 weight percent (meth)acrylic acid homopolymer or copolymer having a weight average molecular weight of from greater than 1,000 to less than 50,000;</li><li>2) from 5 to 60 weight percent nonionic surfactant;</li><li>3) from 0.3 to 5.0 weight percent of alkali soluble or swellable compatibilizer polymer <ul id="ul0002" list-style="none"><li>a) formed from monomers comprising (i) from 25 to 70 weight percent methacrytic add and (ii) from 75 to 30 weight percent of one or more copolymerizable monomers selected from lower alkyl (C₁ to C₆) acrylates and methacrylates, the percentages of (i) and (ii) being based on the weight of (i) plus (ii);</li><li>b) having a weight average molecular weight of from 500,000 to 8 million; and</li><li>c) wherein at least 50 percent of the add groups in the polymer are neutralized with alkali to form a rinse aid dispersion stable at a pH of from 7.5 to 10.3 (herein often referred to as "substantially neutralized" compatibilizer polymer); and</li></ul></li><li>4) water.</li></ul>

The present invention also provides a method of stabilizing an aqueous rinse aid formulation comprising a mixture of 5 to 60 weight percent of low foam nonionic surfactant and from 0.5 to 20 weight percent of low molecular weight (meth)acrylic acid homopolymer or copolymer having a weight average molecular weight of from greater than 1,000 to less than 50,000, the method comprising adding to said mixture from 0.3 to 5.0 weight percent of high molecular weight, alkali soluble or swellable compatibilizer polymer formed from 25 to 70 weight percent methacrylic acid and from 75 to 30 weight percent of one or more copolymerizable monomer selected from lower alkyl (C₁ to C₆) acrylates and methacrylates, said high molecular weight polymer having a molecular weight of from 500,000 to 8 million, and adding to said mixture a sufficient amount of alkali to neutralize at least 50 percent of the acid groups on the backbone of said high molecular weight polymer.

The rinse aid formulation of the invention is a stable homogeneous dispersion which does not phase separate into layers upon storage or use and which provides improved wetting and spot reduction, when compared to known conventional rinse aid formulations, without adversely increasing foaming. In preparing the rinse aid dispersion, the pH of the dispersion can be adjusted to from pH 7.5 to pH 10.3 by the addition of alkali, for example, to neutralize at least 85 percent of the acid groups in the compatibilizing polymer.

The rinse aid formulation of the invention may contain conventional low foam nonionic surfactant, conventional low molecular weight neutralized (meth)acrylic acid polymer, and high molecular weight substantially neutralized compatibilizing methacrylic acid copolymer.

Nonionic surfactants useful in the rinse aid formulation may be any known low foam nonionic surfactant used in machine dishwashing applications. Typical suitable nonionic surfactants include the following commercially available materials: Triton CF-10 (an alkylaryl polyether) and Triton DF-16 (a modified polyalkoxylated alcohol) manufactured by Rohm and Haas Company; Pluronic L-62 (a polyoxyethylene-polyoxypropylene block copolymer) manufactured by BASF Wyandotte Corporation; and Antarox BL-330 (a modified linear aliphatic alcohol polyethoxylated and chloro-terminated compound) manufactured by GAF Corporation. The rinse aid formulation of the invention may contain one or a mixture of such low foaming nonionic surfactants.

The rinse aid formulation also contains low molecular weight neutralized (meth)acrylic acid polymer. This polymer may be a homopolymer of acrylic or methacrylic acid or a copolymer formed from at least 50 weight percent acrylic acid and 50 weight percent or less of suitable copolymerizable comonomer. Suitable comonomers include lower alkyl (C₂ to C₄) acrylates; methacrylic acid; and lower alkyl (C₂ to C₄) methacrylates and amides, such as acrylamido sulfonic acids including 2-acrylamido-2 methylpropane sulfonic acid (AMPS). The (meth)acrylic acid polymer may be a low molecular weight polymer, e.g. in the form of its alkali metal or ammonium salt, having a weight average molecular weight ranging from 2,000 to 40,000.

The high molecular weight compatibilizer polymer is a methacrylic acid copolymer formed from monomers comprising (i) 25 to 70 weight percent, preferably at least 50 weight percent, methacrylic acid and (ii) from 75 to 30 weight percent of one or more copolymerizable lower alkyl (C₁ to C₆) acrylate or methacrylates, the percentages being based on the weight of (i) plus (ii). The monomers from which the compatibilizer polymer is formed may also include a minor amount, up to about one weight percent, based on the weight of monomers, of crosslinking monomer, for example, diallylphthalate, dimethacrylate of butane diol, allyl methacrylate, or ethylene glycol dimethacrylate. The weight average molecular weight of the compatibilizer polymer is at least 500,000 and may be as high as about eight million. The copolymer must be alkali-soluble or swellable and capable of being substantially neutralized by the addition of base at pH ranging from pH 7.5 to pH 10.3. Preferably, at least 85 percent of the acid groups in the compatibilizing copolymer are neutralized with alkali.

When the high molecular weight compatibilizing copolymer is substantially neutralized by the addition of an alkaline material which reacts with at least 50 and up to 100 weight percent of available acid sites on the polymer backbone, the dispersion which results is homogeneous and remains stable at pH of from pH 7.5 to pH 10.3. The use of the high molecular compatibilizer solves the incompatibility or phase separation problem associated with aqueous systems containing low foam surfactant and low molecular weight neutralized (meth)acrylic acid polymer additive. The stable dispersion does not phase separate upon storage for a reasonable time or under actual use conditions and permits the components thereof to effectively reduce spotting and filming and improve sheeting action without the need for a hydrotrope and without interfering with the foaming and defoaming action of the surfactant.

A preferred rinse aid formulation of the invention is an aqueous dispersion at a pH of 8 to 8.5 and containing 15.0 weight percent nonionic surfactant, 2.0 weight percent low molecular weight (meth)acrylic acid homopolymer or copolymer, and 1.0 weight percent of the compatibilizing copolymer. It is preferably prepared by stirring the desired amount of the surfactant into an aqueous solution of the low molecular weight (meth)acrylic acid polymer followed by the gradual addition of the compatibilizing polymer and substantial neutralization to the desired pH range.

The formulation may also contain other additives including sequestants such as nitrilo triacetic acid (NTA), ethylene diamine tetraacetic acid (EDTA), or sodium citrate, and water-miscible solvents such as isopropanol or propylene glycol.

We have found the rinse aid dispersions remain stable up to at least two (2) weeks at 50°C, are room temperature stable for at least 90 days, and pass five (5) freeze/thaw cycles.

The present invention will now be further illustrated by way of the following examples which are for illustrative purposes only and are not to be construed as imposing any limitation on the scope of the invention. In the examples, the compatibilizer copolymers used (i.e. compatibilizers A to D) have a weight average molecular weight of at least 500,000.

Example 1 - Preparation of Rinse Aid Formulations

The following procedure was used to prepare a rinse aid formulation of the invention containing 15.0 weight percent low foam nonionic surfactant, 2.0 weight percent low molecular weight (meth)acrylic acid polymer, and 1.0 weight percent high molecular weight methacrylic acid copolymer compatibilizer.

In a first beaker 15.0 grams of first low foam nonionic surfactant Triton CF10 (100 wt % active material) was admixed with 15.8 grams of a second low foam nonionic surfactant Triton CF32 (95.0 wt % active material) (an alkylamine alkoxylate manufactured by Rohm and Haas Company) and 8.2 grams of Acrysol LMW45 (48 wt % active material) (a polyacrylic acid having a weight average molecular weight of about 4500 and manufactured by Rohm and Haas Company), for about 5 minutes with stirring at 600 rpm. In a second beaker 10.0 grams of a (20 wt % active material) high molecular weight methacrylic acid copolymer compatibilizer (Compatibilizer A), formed by a standard aqueous free radical initiated emulsion polymerization of 34.97 wt percent ethyl acrylate, 65 weight percent methacrylic acid and 0.03 wt % dimethacrylate of butane diol as a crosslinking monomer, was diluted with 147.0 grams of deionized water. This diluted polymer was then slowly added to the mixture being stirred in the first beaker to form a dispersion. Four grams of sodium hydroxide (50 wt % active material) was then gradually added to the first beaker to form 200.0 grams of the rinse aid formulation (Formulation A). The viscosity of the dispersion was about 0.45 Pa.s (450 cps) as measured using a Brookfield viscometer at 12 rpm, spindle number 3. The dispersion had a pH of about pH 8.5 indicating that the high molecular weight compatibilizer was essentially neutralized (95.0 to 100%).

The stability of the formulation was then examined. The formulation remained stable - no phase separation - at the end of 2 weeks at 50°C and after 5 freeze/thaw cycles. The formulation remained stable at the end of 90 days after storage at room temperature.

Similar rinse aid formulations, employing different surfactants and surfactant mixtures, acrylic acid polymers and compatibilizer copolymers, were prepared using the above procedure and were used in the following examples.

In the following examples, references to compatibilizers "A", "B", "C" or "D" are references to the compatibilizer used in Example 1 (compatibilizer A) or to the compatibilizers defined in Table 1 below (compatibilizers B to D). All the compatibilizer polymers were prepared by standard free radial initiated aqueous emulsion polymerization reactions. <tables id="tabl0001" num="0001"><img file="EP0245987B1_D0001.tif" /></tables>

Example 2 - Foaming and Defoaming

This example illustrates that the addition of the low molecular weight polyacrylic acid and the high molecular weight compatibilizer copolymer do not interfere with the performance of the surfactant in the rinse aid formulation (see Table 2). Table 2 also presents comparative examples which illustrate the adverse effect of the addition of a hydrotrope (sodium xylene sulfonate (SXS)) when used in combination with a low foam surfactant and polyacrylic acid. <tables id="tabl0002" num="0002"><img file="EP0245987B1_D0002.tif" /></tables>

Table 2 illustrates that Formulations 2 and 3 are low foam and are effective defoamers, and that Formulations 2 and 3 do not adversely affect the low foam performance exhibited by the surfactant alone (Formulation 1). Formulations 4 to 6 illustrate the adverse effect of the addition of a hydrotrope on food soil defoaming regardless of the presence of the polyacrylic acid. Note that the defoaming height is more than doubled when the hydrotrope is employed. Defoaming of food soils is dependent on the conditions employed and on the choice of surfactant, but as a general rule, a defoam height less than 2.0 cm is excellent.

Example 3 - Dishwasher Performance

The following rinse aid formulations, prepared according to the procedure of Example 1, were evaluated for their ability to reduce spotting on glassware in a Hobart AM 14 dishwasher. Two glasses were put through four wash/rinse cycles. A detergent and dry milk solids were introduced into the wash cycle and each rinse aid formulation was added to the rinse cycle. The detergent was employed at a concentration of 0.25 wt percent based on the weight of the water in the wash cycle. The detergent contained 25 wt % chlorinated trisodium phosphate, 25 wt % sodium tripolyphosphate, 25 wt % sodium hydroxide and 25 wt % sodium metasilicate. The dry milk solids were added at a concentration of 0.10 wt % based on the weight of the water in the wash cycle. Each of the formulations identified below were added to the rinse cycle at a concentration of 0.005 wt % based on the weight of the water in the rinse cycle. The results of the test are shown in Table 3. <tables id="tabl0003" num="0003"><img file="EP0245987B1_D0003.tif" /></tables>

The results using Formulation 10 demonstrate the reduction in spotting resulting from the combined effect of the surfactant, low molecular weight polyacrylic acid and high molecular weight (meth)acrylic copolymer compatibilizer as compared with comparative formulations 7, 8 and 9.

Example 4 - Effect of pH on Dispersion Stability

Table 4 illustrates the effect of pH, and the resulting percent neutralization of the compatibilizing polymer, on the stability of a rinse aid formulation of the invention. The formulation used for this experiment was identical to Formulation 10 of Example 3 containing 15.0 wt % low foam nonionic surfactant (Triton CF-10), 2.0 wt % of Acrysol LMW 45 (polyacrylic acid) and 1.0 wt % of Compatibilizer A, the percentages being weight percentages of active ingredient based on the weight of the formulation. The only change in the formulation procedure described in Example 1 was the variation in the amount of sodium hydroxide added to the aqueous dispersion. <tables id="tabl0004" num="0004"><img file="EP0245987B1_D0004.tif" /></tables>

The results illustrate that the compatibilizer of this example should be at least about 87% neutralized by alkali to yield a stable dispersion, but that an excess of about 20% alkali over that needed to completely neutralize the compatibilizer will result in an unstable dispersion.

Table 4 also illustrates that surprisingly there is no direct correlation between the viscosity and the stability of the dispersion at the lower pH values, i.e. the stability of the dispersion cannot be totally explained by the viscosity of the dispersion.

Example 5 - Compatibilizer Concentration

This experiment illustrates the compatibilizing effect of the high molecular weight (meth)acrylic acid copolymer compatibilizer on the stability of the rinse aid formulation as a function of the concentration of the compatibilizer. The same rinse aid formulation described in Example 4 and Example 1 (Formulation A) was used with the only variation being in the choice of compatibilizer and concentration of the compatibilizer, which varies from 0.25 wt % to 5.0 wt % of active material based on the weight of the formulation. The results are shown in Table 5. <tables id="tabl0005" num="0005"><img file="EP0245987B1_D0005.tif" /></tables>

The results indicate that, when the compatibilizer is employed at 0.25 wt %, the viscosity of the dispersion was very low and a stable rinse aid dispersion was not achieved.

Example 6 - Surfactant Variations

This experiment illustrates that a number of conventional low foam nonionic surfactants may be employed in the rinse aid formulation of the invention without adversely affecting the stability of the dispersion. The formulation used was the same as in Examples 1, 4 and 5 (Formulation A) only the surfactants being varied. The results are shown in Table 6. <tables id="tabl0006" num="0006"><img file="EP0245987B1_D0006.tif" /></tables>

Example 7 - (Meth)acrylic Acid Polymer Variations

This experiment demonstrates the stability of the rinse aid formulation (Formulation A) when the composition of the low molecular weight (meth)acrylic acid polymer component is varied. The variations to the low molecular weight (meth)acrylic acid polymer component included varying the molecular weight of (meth)acrylic acid homopolymer, as well as the use of other copolymerizable comonomers in various amounts to form copolymers of various weight average molecular weight. These homopolymers and copolymers were prepared by a standard aqueous solution polymerization reaction utilizing conventional free radical initiators and chain transfer agents. The wetting data was obtained in the absence of surfactant and high molecular weight compatibilizer in order to identify the contribution of the (meth)acrylic acid polymers to wetting. The ability to wet a surface was determined by the reduction in the contact angle of a droplet of water on a soiled glass surface as compared with the reduction in the contact angle of a droplet of water containing the (meth)acrylic acid polymer.

The results indicate that the compatibilizer is effective to compatibilize a rinse aid formulation containing one or more conventional nonionic surfactants. <tables id="tabl0007" num="0007"><img file="EP0245987B1_D0007.tif" /></tables>

<tables id="tabl0008" num="0008"><img file="EP0245987B1_D0008.tif" /></tables>

The results indicate that (meth)acrylic homopolymers having a Mw (weight average molecular weight) of 1000 or less or a Mw of 50,000 or higher do not provide effective wetting. It also demonstrates that copolymers formed from about 5 to 50 wt % copolymerizable comonomers contribute effectively to wetting.

Example 8 - High Molecular Compatibilizer Variations

This experiment demonstrates the effect of variations to the composition of the high molecular weight compatibiliser on the viscosity and stability of the rinse aid formulation. The formulation of Example 1 (Formulation A) was used with variations to the compatibilizer polymer component. Conventional thickeners were also examined for comparative purposes. All the high molecular weight compatibilizers were prepared by aqueous emulsion polymerisation utilizing conventional emulsifiers and free radical initiators. The dispersions were alkali neutralized as in Example 1. The results are shown in Table 8. <tables id="tabl0009" num="0009"><img file="EP0245987B1_D0009.tif" /></tables>

Example 9 - Concentration of Substantially Neutralized Low Molecular Weight (Meth)Acrylic Acid Polymer and Low Foam Surfactant in Rinse Aid

Table 9 illustrates that stable rinse aid formulations can be formulated to contain at least about 20 weight percent (active material) substantially neutralized low molecular weight (meth)acrylic acid polymer. As the concentration of the substantially neutralized low molecular weight (meth)acrylic acid polymer in the formulation is lowered to below about 0.3 weight percent active, the formulation remains stable, but the contribution to improved wetting by the rinse water formulation becomes negligible. Table 9 also illustrates that the rinse aid formulation can contain at least about 60 weight percent (active material) of low foam surfactant without adversely affecting the stability of the formulation. As with the substantially neutralized low molecular weight (meth)acrylic acid polymer component, the lower concentration limitation on the surfactant in the rinse aid formulation is not the stability of the formulation. At surfactant concentrations below about 5 weight percent active, the formulation becomes too diluted to be practical with regard to the costs of packaging and shipping the formulation. <tables id="tabl0010" num="0010"><img file="EP0245987B1_D0010.tif" /></tables>

The terms "Triton", "Acrysol", "Pluronic" and "Antarox" referred to herein are trademarks, which may be registered in some or all of the designated states.