Cleaning compositions for solid surfaces – auxiliary compositions – Cleaning compositions or processes of preparing – For cleaning a specific substrate or removing a specific...
Reexamination Certificate
2000-08-03
2002-07-30
Kopec, Mark (Department: 1751)
Cleaning compositions for solid surfaces, auxiliary compositions
Cleaning compositions or processes of preparing
For cleaning a specific substrate or removing a specific...
C510S158000, C510S159000, C510S417000, C510S426000, C510S488000, C510S491000, C510S537000
Reexamination Certificate
active
06426326
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to liquid cleansing compositions of the type typically used in skin cleansing or shower gel compositions which compositions are “structured” lamellar phase compositions.
2. Background of the Invention
The rheological behavior of all surfactant solutions, including liquid cleansing solutions, is strongly dependent on the microstructure, i.e., the shape and concentration of micelles or other self-assembled structures in solution.
When there is sufficient surfactant to form micelles (concentrations above the critical micelle concentration or CMC), for example, spherical, cylindrical (rod-like) or discoidal micelles may form. As surfactant concentration increases, ordered liquid crystalline phases such as lamellar phase, hexagonal phase or cubic phase may form. The lamellar phase, for example, consists of alternating surfactant bilayers and water layers. These layers are not generally flat but fold to form submicron spherical onion like structures called vesicles or liposomes. The hexagonal phase, on the other hand, consists of long cylindrical micelles arranged in a hexagonal lattice. In general, the microstructure of most personal care products consist of either spherical micelles; rod micelles; or a lamellar dispersion.
As noted above, micelles may be spherical or rod-like. Formulations having spherical micelles tend to have a low viscosity and exhibit Newtonian shear behavior (i.e., viscosity stays constant as a function of shear rate; thus, if easy pouring of product is desired, the solution is less viscous and, as a consequence, it doesn't suspend as well). In these systems, the viscosity increases linearly with surfactant concentration.
Rod micellar solutions are more viscous because movement of the longer micelles is restricted. At a critical shear rate, the micelles align and the solution becomes shear thinning. Addition of salts increases the size of the rod micelles thereof increasing zero shear viscosity (i.e., viscosity when sitting in bottle) which helps suspend particles but also increases critical shear rate (point at which product becomes shear thinning; higher critical shear rates means product is more difficult to pour).
Lamellar dispersions differ from both spherical and rod-like micelles because they can have high zero shear viscosity (because of the close packed arrangement of constituent lamellar droplets), yet these solutions are very shear thinning (readily dispense on pouring). That is, the solutions can become thinner than rod micellar solutions at moderate shear rates.
In formulating liquid cleansing compositions, therefore, there is the choice of using rod-micellar solutions (whose zero shear viscosity, e.g., suspending ability, is not very good and/or are not very shear thinning); or lamellar dispersions (with higher zero shear viscosity, e.g. better suspending, and yet are very shear thinning). Such lamellar compositions are characterized by high zero shear viscosity (good for suspending and/or structuring) while simultaneously being very shear thinning such that they readily dispense in pouring. Such compositions possess a “heaping”, lotion-like appearance which convey signals of enhanced moisturization.
To form such lamellar compositions, however, some compromises have to be made. First, generally higher amounts of surfactant are required to form the lamellar phase. Thus, it is often needed to add auxiliary surfactants and/or salts which are neither desirable nor needed. Second, only certain surfactants will form this phase and, therefore, the choice of surfactants is restricted.
In short, lamellar compositions are generally more desirable (especially for suspending emollient and for providing consumer aesthetics), but more expensive in that they generally require more surfactant and are more restricted in the range of surfactants that can be used.
When rod-micellar solutions are used, they also often require the use of external structurants to enhance viscosity and to suspend particles (again, because they have lower zero shear viscosity than lamellar phase solutions). For this, carbomers and clays are often used. At higher shear rates (as in product dispensing, application of product to body, or rubbing with hands), since the rod-micellar solutions are less shear thinning, the viscosity of the solution stays high and the product can be stringy and thick. Lamellar dispersion based products, having higher zero shear viscosity, can more readily suspend emollients and are typically more creamy. Again, however, they are generally more expensive to make (e.g., they are restricted as to which surfactants can be used and often require greater concentration of surfactants).
In general, lamellar phase compositions are easy to identify by their characteristic focal conic shape and oily streak texture while hexagonal phase exhibits angular fan-like texture. In contrast, micellar phases are optically isotropic.
It should be understood that lamellar phases may be formed in a wide variety of surfactant systems using a wide variety of lamellar phase “inducers” as described, for example, in U.S. Pat. No. 5,952,286 titled “Liquid Cleansing Composition Comprising Soluble, Lamellar Phase Inducing Structurant” by Sudhakar Puvvada, et al., issued Sep. 14, 1999. Generally, the transition from micelle to lamellar phase are functions of effective average area of headgroup of the surfactant, the length of the extended tail, and the volume of tail. Using branched surfactants or surfactants with smaller headgroups or bulky tails are also effective ways of inducing transitions from rod micellar to lamellar.
One way of characterizing lamellar dispersions include measuring viscosity at low shear rate (using for example a Stress Rheometer) when additional inducer (e.g., oleic acid or isostearic acid) is used. At higher amounts of inducer, the low shear viscosity will significantly increase.
Another way of measuring lamellar dispersions is using freeze fracture electron microscopy. Micrographs generally will show lamellar microstructure and close packed organization of the lamellar droplets (generally in size range of about 2 microns).
One problem with certain lamellar phase compositions is that they tend to lose their lamellar stability in colder temperatures (e.g., 0 to 45° F.). While not wishing to be bound by theory, this may be because, in cold conditions, the oil droplets become less flexible and the spherical structure characterizing the lamellar interaction breaks into lamellar sheets instead.
BRIEF DESCRIPTION OF THE INVENTION
Applicants have discovered that the use of surfactants containing low salt levels enhances freeze/thaw stability in personal wash structured liquid formulations. The ability of a structured liquid to maintain viscosity under freeze/thaw conditions is extremely desirable to demonstrate moisturization and aesthetics. Applicants have found that the combination of amphoteric and anionic surfactants, and either soluble or insoluble lamellar structurants, with oil creates a stable structured liquid with excellent freeze/thaw viscosity stability in conjunction with low overall salt content.
For example, when low salt cocamidopropyl betaine is added to a liquid formulation containing the following compounds: an anionic surfactant like sodium lauryl ether sulfate, high levels of an emollient such as sunflower seed oil, and a lamellar structurant fatty acid like lauric or isostearic acid; a structured liquid with excellent freeze/thaw viscosity is produced. The role of salt (e.g. NaCl) in the cocamidopropyl betaine to affect this stability is clearly seen by the poor freeze/thaw stability of the structured liquids containing higher salt containing cocamidopropyl betaine. Likewise, when low salt sodium lauryl ether sulfate is added to a similar liquid formulation with sodium lauroamphoacetate, the formulation exhibits improved freeze thaw stability. Furthermore, the presence of salt separately added to the lamellar formula also worsens the freeze thaw stability. Therefore, the structure
Mitra Shuman
Puvvada Sudhakar
Bornstein Alan A.
Kopec Mark
Unilever Home & Person Care USA, a division of Conopco, Inc.
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