Formulation Work

After the decision for the desired application form has been made the formulator has to decide on the vehicle system for the antiperspirant active. It is the intent of this section to summarize some of the current knowledge concerning influence of actives with the formula, efficacy of different delivery systems, and the function of the ingredients used in antiperspirants.

Antiperspirant actives like ACH or ZAG complexes are soluble in water. Application of a concentrated aqueous solution of an antiperspirant active gives a rather tacky feeling [36]. Reduction of tackiness can be best achieved by silicone oils (cyclomethi-cones) or ester oils like Di-(2 ethylhexyl) adipate [27]. The acidic pH value (pH 4.0-4.2) has to be taken into account by selecting additional components for the desired drug delivery system. Loss of viscosity and problems of a final formula with color stability are often a hint to change the gellant and/or perfume. Aluminum powders in anhydrous systems (aerosols, suspension sticks) often leave visible white residues on skin or clothing. Liquid emollients, like PPG-14 butylether or the aforementioned adipate ester, minimize these residues. Another approach is to use the solid emollient isosorbide monolaurate (ICI, Arla-mol ISML) [37]. In anhydrous aerosol formulations the ACH powder settles down and

forms a hard to redisperse cake at the bottom of the aerosol can. Suspending aids like Quaternium-18 Hectorite or Quaternium-18 Bentonite prevents settling of the active and additionly thickens the cyclomethicone oil phase. Usage of fine powders of ACH is another approach to overcome natures law of gravity.

The reader should be aware that hydrophobic ingredients like emollients have an influence on the effectiveness of an antiperspirant active because a cosmetic oil phase or wax can cover the pores of the eccrine duct. The efficacy of an antiperspirant active like ACH is higher in water-containing systems compared with anhydrous formulations. The following rules concerning efficacy might be helpful:

1. Efficacy: Aqueous solution > Anhydrous suspension

2. Since diffusion of an active in the vehicle and from the vehicle to the skin after application has to considered one can further differentiate the expected efficacy trends.

Efficacy: Aqueous solution > Sprayable O/W emulsion > O/W-emulsion rollon > O/W-emulsion cream

3. It is accepted that antiperspirant actives in the outer phase of an emulsion have a higher efficacy than in dispersed phase.

Efficacy: O/W-emulsion > W/O-emulsion

4. In water-free systems the viscosity of the drug delivery system might be of relevance. Suspended ACH in anhydrous vehicles needs to be solubilized after application to the axilla by sweat (water). The effectiveness of suspension sticks depends on the rapidity of active solubilization. The usage of ultrafine powders of ACH is expected to boost efficacy compared with fine powders. Efficacy: low viscous suspension > suspension stick

The interested reader is referred to the literature concerning vehicle effects on antiperspi-rant activity [7,38,39].

Not only lipophilic ingredients might have an influence on the efficacy of a product because it is known that the water-soluble propylene glycol can form complexes or hydrogen bonds with aluminum polycationic species thereby altering the efficacy of the salt

[40]. Additionally propylene glycol in high concentrations may result in skin irritations

[41]. Successful formulation work aims at finding the right viscosity for the product in the desired application form, a lower viscosity during flow into the underarm pit and a higher viscosity after application so that the product stays where it was applied. Conventional shear shinning flow curves are characteristic for antiperspirant products. The reader is referred to the literature concerning rheology aspects of cosmetic products [42].

Deodorant/Antiperspirant Sticks

It is at present not easy to give the reader an overview about sticks because nowadays there exists many technologies to develop this solid delivery system. In Figure 1 an attempt |

was made to summarize this area. In the following section only systems of major importance are discussed.

Sticks can be divided into different classes like suspension sticks, gel sticks, and emulsion sticks. Soft sticks have some properties of all three categories (Fig. 1). jb

Suspension Sticks 2

Dry deodorants, or antiperspirant solids, are synonyms for an application form where the s active in the form of a powder is suspended in a silicone oil phase. Stearyl alcohol is

Figure 1 Overview of cosmetic Deo/AP-sticks.

usually used as the hardening agent. The molten mass crystallizes into a matrix of stearyl alcohol saturated with the silicone oil and suspended particles [43,44]. Settling of the actives can be reduced by Quaternium-18 Hectorite. Cyclomethicones give the stick a dry, silky feel, and nonvolatile oils like PPG-14 butylether minimize white residues on skin [43]. Low-residue sticks can be obtained by using a combination of high-melting and low-melting waxes and a volatile and nonvolatile silicone-oil combination [45].

Suspension stick

Wt%

Stearyl alcohol

20.0

Cyclomethicone

54.0

PPG-14 butylether

2.0

Hydrog. castor oil

1.0

Talc

2.0

Antiperspirant

20.0

Fragrance

This classes can be subdevided into the groups white anhydrous gel sticks, clear anhydrous gel sticks, clear water-based soapgel sticks. The last mentioned is discussed in the deodorant chapter.

White Anhydrous Gel Sticks. Shear solids or ultra-clear solids are synonyms for sticks with improved wash-out performance compared with the classic suspension sticks. They contain N-acyl aminoacid amides (N-lauroyl-L-glutamic acid dibutylamide) and 12-hydroxyacid as gelling agents for an oil-phase mixture (e.g., silicone oil/mineral oil). The wash-out agent is an ethoxylated solubilizer like Ceteareth-20. These white sticks turn clear after application to the skin (no-residue stick) [46].

Clear Anhydrous Gel Sticks. They are quite popular in the United States because clarity is associated by the consumer with a lack of white residue on skin, no dangerous ingredients, and high efficacy. A typical gelling agent is dibenzylidene sorbitol (dibenzyaldehyd monosorbitol acetal, DBMSA). This acetale is not stable in an acidic aqueous environment [47]. The sticks usually contain a high level of alcohol and/or polyols. At high polyol concentration the active is regarded to be solubilized instead of suspended in the gel matrix [48]. An alternative gelling agent is a polyamide [49].

White anhydrous gel sticks

Wt%

Clear anhydrous gel sticks

Wt%

N-Lauroyl-glutamic acid dibutyl amide

5.0

Dibenzylidene sorbitol

2.0

12-Hydroxystearic acid

5.0

Dimethicone copolyol

2.0

Cyclomethicone

40.0

Diisopropyl sebacate

2.0

Hydrog. polyisobutene

15.0

Glycine

1.0

Diisopropyl myristate

15.0

Dipropyleneglycol

10.0

Antiperspirant powder

20.0

Propyleneglycol

33.0

Antiperspirant powder

Emulsion Sticks:

They can be grouped into clear o/w emulsions, white w/o emulsions, and clear w/s: emulsion gels. The last mentioned is will be discussed shortly.

Clear O/W Emulsions. They contain a high surfactant combination with the active solubilized in the external water phase. The high concentration of surfactants is a disadvantage; no products based on this technology are known to the author [47].

W/O-Emulsion Sticks.

The water phase containing the active is solubilized by a surfactant like Polyglycerol-4 Isostearate. A typical example for an oil/wax-phase combination is a mixture of silicone oil/stearylalkohol [50].

W/O Emulsion Stick

Wt%

Stearyl alcohol

19.0

Volatile silicone

26.0

Mineral oil

1.0

2-Methyl-2,4 pentandiol

2.0

Polyglyceryl-4 isost.

2.0

ACH solution (50%)

Soft Sticks (Soft Solids, Smooth-Ons)

These sticks can be differentiated into two subgroups: white, anhydrous creams (suspensions) and clear water-in-silicone emulsion gels. Both delivery systems are packed in a container that gives the impression of a stick. The suspension or gel is extruded onto the skin from holes in the top of the stick container to a wide smooth area around the holes.

White, Anhydrous Creams. These creams contain an antiperspirant active, a volatile and nonvolatile silicone oil and a thickener (N-acyl glutamic acid amide).

Clear Water-in-Silicone Emulsion Gels. These formulations can be achieved by adjusting the refractive index of the water and silicone-oil phase. Silicone formulation aids (Dow Corning 3225 C) are mixtures of cyclomethicone and dimethicone copolyol helping to solubilize the active [7,46,48,51]. Low surface tension of cyclomethicones facilitates good spreading of a product on the skin and reduces the tackiness of anti-perspirant actives.

Antiperspirant Roll-Ons

Roll-on products can be differentiated into several categories (see Fig. 2). O/W emulsion-based delivery systems are quite popular in Europe, whereas anhydrous suspension roll-ons or transparent water-in-silicone emulsions are preferred in the United States. A new trend concerning the size of the roll-on applicator has been identified. Consumers prefer the big-ball format (3.0-3.5 cm) because of the ease of applying the product to the underarm pit [52]. The popularity of roll-ons in general is due to the nongreasy and nonoily feel in the axilla and the good spreadability of the content on the underarm skin.

Clear Hydroalcoholic Roll-On

This delivery system contains a water/alcohol solution of the antiperspirant active thickened with a water-soluble polymer like hydroxyethylcellulose. The alcohol in the formula gives, compared with the clear aqueous solution-based roll-ons, a fresh sensation in the

Figure 2 Overview of cosmetic Deo/AP-roll-on types.

Figure 2 Overview of cosmetic Deo/AP-roll-on types.

axilla and faciliates drying of the product. Excellent antiperspirant efficacy is another benefit of hydroalcoholic roll-ons.

O/W Emulsion Roll-On

This delivery system uses ethoxylated surfactants like PEG-40 stearate to solubilize an oil phase like mineral oil. The active is dissolved in the outer phase, allowing the formulation of a highly effective product. In alcohol-free formulated systems microbiological stability has to be checked.

O/W emulsion roll-on

Wt%

Hydroalcoholic roll-on

Wt%

PEG-40 stearate

5.0

Antiperspirant active

20.0

Cetyl alcohol

3.0

PPG-5 ceteth 20

2.0

Mineral oil

2.0

Water

35.4

Polysorbate-80

1.0

Ethanol

42.1

Glycerin

1.5

Hydroxyethylcellulose

.5

Mg-aluminum silicate

.8

Antiperspirant active

20.0

Water

66.7

They are weaker in efficacy because the actives are encapsulated and the external oil phase often gives a sticky feeling.

W/Si Emulsion Roll-On

Silicone oils allow to formulate products based on a ''W/O-technology'' because the skin feeling is not comparable to traditional oily components like ester oils or triglycerides. The concentration of the thickener is reduced compared with sticks based on this type. The technology is discussed under soft sticks (see p. 696).

O/W Microemulsion Gel

An alternative approach to transparent products uses the PIT technology. A suitable mixture of surfactants, oils, and water is heated to 60 to 90°C to give a w/o emulsion above the phase inversion temperature (PIT). During cooling the mixture shows phase inversion to give white or transparent o/w emulsions. o/w Microemulsion gels are obtained in the presence of hydrophobically modified water-soluble polymers [53]. The technology is explained in more detail in the deodorant chapter.

Suspension Roll-On

The antiperspirant active in powder form is suspended in cylomethicone. The roll-on can be formulated with or without ethanol. Quaternium-18 Hectorite is used as a thickener to prevent settling of the active. Consumers in the United States prefer this delivery system since it does not give a wet feeling after application and because of the easy drying [39]. Actives like ZAG-complexes give high efficacy underarm products.

Suspension roll-on Wt%

Volatile silicone 65.0

Quaternium-18 hectorite 13.5

Silica .5

Antiperspirant powder 20.0

Fragrance 1.0

Antiperspirant Aerosols

Aerosols in Europe and Asia are popular delivery systems for consumers who prefer a hygienic and easy-to-use application form. Typical ingredients for aerosols include isopro-pylmyristate, isopropylpalmitate, volatile silicone, dimethicone, silica, clays, propylene carbonate, and ethanol. Propellants include propane, butane, and isobutane.

Antiperspirant aerosol Wt%

Volatile silicone 13.4

Quaternium-18 hectorite .8

Ethanol .8

Antiperspirant powder 10.0

Propellant (butane/propane) 75.0

Because acidic aqueous ACH solutions lead to corrosion of the aerosol can, current aerosol antiperspirant products are formulated as water-free suspensions. The active is suspended as a powder in an oil phase like cyclomethicone or in a mixture of ester oils/cyclomethi-cone. Agglomeration of solid particles and settling of actives can be minimized by usage of suspending agents like fumed silica (amorphous silicon dioxide) or clays (bentonite, hectorite). The clays form a weak gel in the presence of an oil phase that can be destroyed by shaking the aerosol can before usage. The gel structure is reformed on standing, thereby holding the active in suspension. Because the organoclays are agglomerated, shear is needed to deagglomerate the platelets, and a polar activator like propylene carbonate or ethanol is used to disperse them and induce the gelation of the oil phase.

The steps involved to prepare an aerosol product can be summarized in the following sequence [7]:

1. Preparing a bentonite or hectorite clay with the emollient in the presence of the polar activator and shearing the mixture

2. Adding the antiperspirant active until a uniform agglomeration-free suspension is obtained

3. Filling the concentrate into the aerosol can and adding the propellant (pressure filling)

Efficacy studies of aerosols including comparison with other drug delivery systems have been reported in the literature [30]. ZAG-complexes (see discussion p. 691) are not allowed to be used in aerosols.

Environmental Issues

Aerosols contain volatile organic compounds (VOCs) usually in a weight ratio propellant/ concentrate of 75/25 [54]. The environmental impact of VOC like the reaction with NOx in the presence of sunlight causes formation of unwanted ozone in the lower atmosphere. U.S. antiperspirant companies especially were forced to reduce VOC emissions by reformulating and/or exchanging of hydrocarbon propellants to the fluorohydrocarbons 1,1 difluorethane (Propellant 152 a) or 1,1,2,2 tetrafluorethane (Propellant 134 a). The water-soluble dimethoxyethane (DME) is another propellant that is thought to have no impact on the damage of the ozone layer [55].

The current trends in the aerosol market can be summarized as follows:

• Higher ratio of concentrate/hydrocarbon propellant

• Higher amount of silicone oils

• Usage of 1,1 difluorethane (Propellant 152 a)

• Formulations with lower vapor pressure

• Usage of smaller aerosol cans

Aerosols containing 20 to 50% propellants with a concentrate/propellant ratio from 1.0 to 1.0 to 2.3 to 1.0 have been patented [56].

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