Emulsion polymerization process

A method for preparing a silicone emulsion, more particularly a method for preparing a silicone emulsion with a low cyclic siloxane level and particle size control.

TECHNICAL FIELD
 The present invention is directed to an emulsion polymerization process,
 more particularly, to an emulsion polymerization process that allows for
 control of the level of cyclic siloxanes and the particle size.
 BACKGROUND
 Current emulsion polymerization processes have cyclic siloxane levels
 greater than one percent in the final emulsion. With a lower cyclic
 siloxane level, final product formulations can be better controlled.
 Cyclic siloxane contaminants are undesirable from the perspective of health
 and safety, and there are regulations on the allowed levels in products.
 Additionally, cyclic siloxanes have a tendency to lower the viscosity of
 shampoos.
 Emulsion polymerization processes have been previously reported, but prior
 art processes utilized either cyclic siloxanes or mixtures of cyclic and
 linear siloxanes, and these processes cannot produce emulsions with low
 cyclic siloxane levels. See, for example, U.S. Pat. No. 5,504,149, which
 utilizes cyclic siloxanes, surfactant and an initiator that is a
 silanolate or an organosilanolate; U.S. Pat. No. 4,066,594, which uses
 cyclic siloxanes and benzene sulfonic acid and optionally a platinum
 catalyst; EPA 874017, which uses a metal containing catalyst for a chain
 extension reaction; and JP 9278626, which uses a catalyst and heat for the
 emulsion polymerization. When cyclic siloxanes are used, the emulsion must
 be heated to allow polymerization and then cooled for condensation, and
 the final polymer viscosity is a function of the condensation temperature.
 A process is needed in which the particle size of the emulsion can be
 controlled, as well as the viscosity and the level of cyclic siloxanes in
 the emulsion. It has been discovered that by using a neutral surfactant
 system instead of an acid catalyst surfactant during homogenization,
 particle size, viscosity and cyclic siloxanes can be controlled. A benefit
 is that with the lower cyclic siloxane level, a safer product can be
 produced.
 SUMMARY OF THE INVENTION
 In a first aspect, the present invention is directed to a process for
 making a silicone emulsion comprising:
 a) preparing a neutral aqueous surfactant by adding to water an anionic
 surfactant and a base, and mixing to form a uniform neutral aqueous
 surfactant having a pH of about 7;
 b) forming a mixture of approximately equal portions of the surfactant and
 a polysiloxane;
 c) homogenizing the mixture to form an emulsion, wherein the emulsion is
 homogenized until a desired particle size is achieved;
 d) cooling the homogenized emulsion;
 e) adding an acid to the cooled emulsion and condensing the emulsion to
 form a polymer, wherein the polymer is condensed until the viscosity
 reaches the desired level; and
 f) neutralizing the condensed emulsion with an effective amount of a
 neutralizing agent
 wherein the level of cyclic siloxanes in the emulsion is less than one
 percent by weight.
 In a second aspect, the present invention is directed to an emulsion that
 is used in personal care applications comprising the silicone produced by
 the process of the present invention.
 The process of the present invention is effective in controlling the
 particle size and the viscosity of the emulsion as well as controlling the
 level of cyclic siloxanes at a level of less than one percent. Another
 advantage of the present invention is that the emulsion does not need to
 be heated for polymerization to take place.
 DETAILED DESCRIPTION OF THE INVENTION
 In a preferred embodiment, from about 0.3 to 5.0, more preferably from
 about 0.8 to about 3.0, even more preferably from about 0.9 to about 1.8
 parts by weight ("pbw"), of an anionic surfactant and from about 0.06 to
 about 1.1, more preferably from about 0.2 to about 0.7, even more
 preferably from about 0.2 to about 0.35 pbw of a base are added to a
 vessel containing from about 30 to about 90, more preferably from about 35
 to about 70, even more preferably from about 40 to about 55 pbw deionized
 water to form a mixture. In a preferred embodiment, this mixture is then
 metered with approximately from 8 to about 65, more preferably from about
 25 to about 65, even more preferably from about 45 to about 55 pbw of a
 polysiloxane. In a preferred embodiment, the mixture is then blended by
 passing the mixture through a paste disperser, such as for example,
 Premier, and then by milling the mixture, by for example, a colloid mill
 such as IKA or Gaulin. After blending, the mixture is then fed to a
 homogenizer, such as a Gaulin, Bran & Lubbe, or a microfluidizer by
 Microfluidics, to form an emulsion. In a preferred embodiment, the mixture
 is homogenized, preferably without the addition of heat, at a pressure of
 from about 4,000 to 12,000 psig, more preferably at a pressure of from
 about 5,000 to about 8,000, even more preferably at a pressure of about
 6000 psig, and a recycle loop is utilized to set the desired particle size
 and to ensure stability of the emulsion. In a preferred embodiment, the
 particle size ranges from about 0.1 to about 2.0, more preferably from
 about 0.2 to about 1.0, even more preferably from about 0.3 to about 0.6
 microns, as measured by a Malvern Mastercizer or a Nicomp 170. Once
 homogenized, the emulsion is fed to a condensation vessel where it is
 cooled to about 15 to 50.degree. C., more preferably, from about 20 to
 35.degree. C., even more preferably to about 20 to 25.degree. C., and from
 about 0.09 to about 1.5, more preferably from about 0.25 to about 0.9,
 even more preferably from about 0.29 to about 0.45 pbw of a strong acid is
 added to bring the pH of the emulsion to about 2, more preferably less
 than 2. Once the emulsion is acidified, a controlled condensation begins.
 Once the condensation is complete and the desired polymer viscosity has
 been achieved, preferably between 100,000 centipoise (cps) and 6,000,000
 cps, more preferably between 500,000 and 3,000,000 cps, even more
 preferably between 1,000,000 and 2,000,000 cps, the emulsion is
 neutralized to prevent further condensation. In a preferred embodiment,
 the pH is from about 6 to about 8.5, more preferably from about 6.5 to
 about 8, even more preferably from about 7 to about 7.5 with a
 neutralizing agent.
 Anionic surfactants suitable for use in the present invention are those
 that emulsify and provide good condensation. Examples of suitable
 surfactants include alkylbenzenesulfonic acids and salts such as
 hexylbenzenesulfonic acid, octylbenzenesulfonic acid, decylbenzenesulfonic
 acid, dodecylbenzenesulfonic acid, cetylbenzenesulfonic acid and
 myristylbenzenesulfonic acid, more preferably dodecylbenzenesulfonic acid.
 Additionally, co-surfactants, such as sodium lauryl sulfate, may also be
 used.
 In a preferred embodiment, the base of the present invention is one that
 will neutralize the anionic surfactant. Examples of bases suitable for use
 in the present include, but are not limited to, sodium carbonate, sodium
 hydroxide, potassium hydroxide, calcium carbonate. Preferably, the base is
 sodium carbonate.
 Compounds suitable as the polysiloxane intermediate of the present
 invention are those that are hydrolyzates. Examples of polysiloxane
 intermediates that may be used in the present invention include, but are
 not limited to, linear silanol-stopped polyorganosiloxanes. Preferably,
 the polysiloxane intermediate of the present invention is a low viscosity
 silanol-stopped polymer of the formula:
EQU HO--(R.sub.2 SiO).sub.x -- H
 wherein each R is independently a monovalent hydrocarbon radical;
 and x is an integer, wherein x is chosen such that the viscosity is from
 about 15 to about 1000 centistokes, more preferably from about 50 to about
 500 centistokes, and even more preferably from about 70 to about 150
 centistokes.
 Suitable monovalent hydrocarbon groups include acyclic hydrocarbon
 radicals, monovalent alicyclic hydrocarbon radicals, monovalent and
 aromatic hydrocarbon radicals. Preferred monovalent hydrocarbon radicals
 are monovalent (C.sub.1 -C.sub.6)alkyl radicals, monovalent aryl radicals
 and monovalent aralkyl radicals.
 As used herein, the term "(C.sub.1 -C.sub.6)alkyl" means a linear or
 branched alkyl group containing from 1 to 6 carbons per group, such as,
 for example, methyl, ethyl, propyl, iso-propyl, n-butyl, iso-butyl,
 sec-butyl, tert-butyl, pentyl, and hexyl, preferably methyl.
 In a preferred embodiment, the monovalent hydrocarbon radical is a
 monovalent (C.sub.1 -C.sub.6)alkyl radical, most preferably, methyl.
 As used herein, the term "aryl" means a monovalent unsaturated hydrocarbon
 ring system containing one or more aromatic rings per group, which may
 optionally be substituted on the one or more aromatic rings, preferably
 with one or more (C.sub.1 -C.sub.6)alkyl groups and which, in the case of
 two or more rings, may be fused rings, including for example, phenyl,
 2,4,6-trimethylphenyl, 2-isopropylmethylphenyl, 1-pentalenyl, naphthyl,
 and anthryl, preferably phenyl.
 As used herein, the term "aralkyl" means an aryl derivative of an alkyl
 group, preferably a (C.sub.2 -C.sub.6)alkyl group, wherein the alkyl
 portion of the aryl derivative may, optionally, be interrupted by an
 oxygen atom, such as, for example, phenylethyl, phenylpropyl,
 2-(1-naphthyl)ethyl, preferably phenylpropyl, phenyoxypropyl, and
 biphenyloxypropyl.
 Suitable intermediate polysiloxanes are made by known methods, such as, for
 example, hydrolysis, and are commercially available.
 In a preferred embodiment, the acid of the present invention is one that is
 a strong acid that will neutralize the base to form a salt and free up the
 alkylbenzenesulfonic acid. Examples of acids suitable for use in the
 present invention include, but are not limited to, nitric acid,
 hydrochloric acid, formic acid, phosphoric acid and sulfuric acid.
 Preferably the acid is sulfuric acid.
 In a preferred embodiment, the neutralizing agent of the present invention
 is any base that will raise the final pH to about 6 to about 8.5,
 preferably from about 6.5 to about 8, more preferably from about 7 to
 about 7.5. Examples of neutralizing agents suitable for use in the present
 invention include, but are not limited to, alkanolamines of the formula:
EQU (HOR.sup.1).sub.3 N
 wherein R.sup.1 is a divalent (C.sub.1 -C.sub.8)alkyl radical. Preferably,
 the neutralizing agent is triethanolamine.
 The process of the present invention may optionally contain additional
 components, such as, for example, biocides, antifoam agents, fragrances,
 colorants, preservatives and any other additives known in the art.
 In a preferred embodiment, the process of the present invention further
 comprises adding minor amount, preferably, less than 1% by weight of the
 composition, and, more preferably, less than 0.5% by weight of the
 composition, a biocide. Suitable biocides include microbiocides such as
 Proxel GXL microbiocide, commercial available from Zeneca, and Kathon CG
 +IP II microbiocide, commercially available from Rohm & Haas. Preferably,
 the biocide is Kathon CG+IP II.
 The emulsions are useful in personal care applications such as shampoos,
 conditioners, lotions, cosmetics, etc. The emulsion is generally
 incorporated into a personal care product in an amount of about 0.01 to
 about 50 weight percent, more preferably 0.1 to 20 weight percent, of the
 total personal care composition. The personal care composition of the
 present invention may, optionally, further contain such known components
 as, for example, emollients, moisturizers, humectants, pigments,
 colorants, fragrances, biocides, preservatives, exfoliants, hormones,
 enzymes, medicinal compounds, anti-microbial agents, anti-fungal agents,
 vitamins, electrolytes, alcohols, polyols, surfactants, emulsifiers,
 silicone oils, organic oils, waxes, suspending agents, thickening agents
 and water.
 The following example illustrates the process of the present invention. It
 is illustrative and the claims are not to be construed as limited to the
 example.