Drug – bio-affecting and body treating compositions – Designated organic active ingredient containing – Radical -xh acid – or anhydride – acid halide or salt thereof...
Reexamination Certificate
2000-08-16
2003-08-26
Padmanabhan, Sreeni (Department: 1619)
Drug, bio-affecting and body treating compositions
Designated organic active ingredient containing
Radical -xh acid, or anhydride, acid halide or salt thereof...
C514S558000, C514S772000, C514S772300, C514S828000, C514S844000
Reexamination Certificate
active
06610741
ABSTRACT:
FIELD OF THE INVENTION
Cosmetic compositions for human skin containing a weak carboxylic acid and a random copolymer of ethylene and propylene oxides.
BACKGROUND OF THE INVENTION
Cosmetic products which improve the appearance of skin are increasingly popular with consumers. Frequently, consumers seek to alleviate or delay the signs of aged or photoaged skin, such as fine lines and wrinkles, dry and sagging skin.
Some ingredients used in topical products are potentially irritating, especially to people with “sensitive skin.” Such irritation is commonly perceived as sting or burning.
As an example, hydroxy acids and several other weak carboxylic acids have been proven to deliver cosmetic benefits, such as improvement in the appearance of photodamaged or naturally aged skin, skin lightening, treatment of age spots, etc. Unfortunately, their use at high concentrations may occasionally be associated with skin irritation, e.g. skin redness and stinging sensation upon application. For aesthetic reasons, these actives are most often delivered as oil-in-water emulsions. Practically, the final composition pH should be higher than 3 in order to prevent deleterious effects to skin tissues and unacceptable levels of irritation. Water soluble weak acids when delivered from an oil-in-water emulsion at acidic pH often induce high levels of sting. The sting occurs immediately after application, reaches a maximum intensity usually by 5-8 minutes after application and then begins to reduce in intensity.
The irritation can be ameliorated by lowering the amount of an active ingredient in the composition or by reducing the active's penetration through the skin. A serious drawback of both approaches is that the efficacy of the active is impaired. The weak acid related irritation can be reduced by raising the composition's pH but this method yields reduced efficacy due to a decreased acid penetration through the skin. It is desirable to reduce or eliminate the irritation potential of weak acids while maintaining their efficacy.
The need exists, therefore, for a composition and method that prevents or reduces the skin irritation.
One approach to lower the sting is to formulate the acid with a strong alkali metal base. Yu et al. (U.S. Pat. No. 4,105,783) suggested the use of ammonium hydroxide or an organic base. Unfortunately, this method raises the pH of the composition and reduces the ability of the weak acid to penetrate the skin, thus lowering its efficacy (see Sah et al. in
J. Cosmet. Sci
. 49, 257-273, 1998).
A clear need exists for a cosmetic composition with a weak acid that reduces sting but does not reduce dermal delivery.
The random copolymers of ethylene oxide (EO) and propylene oxide (PO) have not been used in cosmetics. They are commonly used industrial lubricants for the metalworking industry such as described by Russo et al. in U.S. Pat. No. 4,514,325. The block copolymers (e.g. BASF Pluronic series) have been used in the cosmetic industry. But the random copolymers employed in the present invention are different structurally and have different properties from the block copolymers of EO and PO.
SUMMARY OF THE INVENTION
The present invention includes a skin cosmetic composition comprising:
(i) from about 0.1 to about 20 wt. % of a random copolymer of ethylene oxide and propylene oxide having a number average molecular weight of at least about 1,000D and containing less than 70% propylene oxide units by weight of the copolymer;
(ii) from about 0.01 to about 20 wt. % of a weak carboxylic acid having pKa of above about 2; and
(iii) a cosmetically acceptable vehicle.
The invention provides a method for reducing skin irritation that may be caused by the topical application of a weak carboxylic acid, the method comprising topically applying random EO/PO copolymer in a cosmetically acceptable vehicle.
DETAILED DESCRIPTION OF THE INVENTION
Except in the operating and comparative examples, or where otherwise explicitly indicated, all numbers in this description indicating amounts of material or conditions of reaction, physical properties of materials and/or use are to be understood as modified by the word “about.” All amounts are by weight of the composition, unless otherwise specified.
The term “skin” as used herein includes the skin on the face, neck, chest, back, arms, armpits, hands and scalp.
The terms “irritation,” “sting,” and “burn,” “inflammation,”, and “redness” as used herein are synonymous and are used interchangeably.
The molecular weight is expressed in Dalton (D). The numerical terms followed by letters “KD” denote molecular weight of a compound, to be read as the numerical term ×1,000 (e.g. 10 KD means molecular weight of 10,000 D).
EO/PO Copolymer
The EO/PO copolymers used in this invention are linear random copolymers of ethylene oxides (EO) and propylene oxides (PO). An example of such a copolymer is UCON 50-HB-5100 from Union Carbide which is a 50/50 EO/PO copolymer with a number average molecular weight of 3930 D. The ratio of EO to PO must result in the copolymer remaining substantially water soluble, so that the copolymer remains effective at preventing the sting caused by the weak acid. As the ratio of PO increases above 50%, the molecule becomes less water soluble. At the PO content of 70% or more, the copolymer is water insoluble and is not suitable for use in the present invention. Union Carbide supplies EO/PO random copolymers in 3 main groups: LB fluids contain only oxypropylene groups, are water insoluble, and are not suitable for use in the inventive compositions, 50-HB fluids which contain equal amounts by weight of oxyethylene and oxypropylene groups, and 75-H fluids which contain 75 weight percent oxyethylene and 25 weight percent oxypropylene groups. These fluids are available with number average molecular weights from about 250 D to about 12,000 D. The copolymers suitable for use in the present invention have the molecular weight of at least about 1,000 D. The preferred molecular weight range is from about 1,000 D to about 12,000 D, in order to optimize the efficacy of the copolymer.
Weak Carboxylic Acid
A weak carboxylic acid suitable for use in the inventive compositions is an acid with dissociation constant, pKa, of above about 2. Preferably, the pKa is above about 3, most preferably in the range of from about 3 to about 5.
The concept of pKa
An acid is a species having a tendency to lose a proton, while a base is a species having a tendency to accept a proton. Hence for every acid, HA, there is a conjugate base A
−
:
HA⇄H
+
+A
−
Thus, lactic acid-lactate ion is an example of a conjugate acid-base pair.
Acids so defined can only manifest their properties by reacting with bases. In aqueous solutions, acids react with water, the latter acting as a base
HA+H
2
O⇄H
3
O
+
+A
−
Quantitatively, the acid strength of HA, relative to the base strength of water is given by the equilibrium constant expression by the equation
K=[H
3
O
+
][A
−
]/[H
2
O][HA]
where parentheses denote molar concentrations.
As almost all measurements are made in dilute aqueous solution, the concentration of water remains essentially constant and its activity can be taken as unity. Letting H
+
represent the solvated proton, we have
K
a
=[H
+
][A
−
]/[HA],
where K
a
is the acidic dissociation (or ionization) constant. This equation can be written in the form
pK
a
=pH
+log [
HA]/[A
−
]
where pK
a
is the negative logarithm of K
a
, and is equal to the pH at which the concentrations of HA and A
−
are equal.
pK
a
for alpha hydroxy acids are generally between 2-4, for monocarboxylic acids between 3-5, for alpha amino acids between 2-3; for salicylic acid it is 3.0.
The pK
a
of a weak water-soluble acid is obtained by titrating it with a strong base such as sodium hydroxide (NaOH). The intercept at the midpoint of the titration, ie. the point at which 0.5 molar equivale
Habif Stephan Samuel
Mukherjee Surajit
Rick Donald
Padmanabhan Sreeni
Plotkin Ellen
Unilever Home & Personal Care USA , division of Conopco, Inc.
Willis Michael A.
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