Preparation of W/O emulsions

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...

Reexamination Certificate

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C516S021000, C516S028000, C524S460000, C524S801000, C528S486000

Reexamination Certificate

active

06444785

ABSTRACT:

The invention relates to a process for preparing W/O emulsions comprising crosslinked water-swellable addition polymers dispersed therein, by aftertreatment with a specific redox initiator system to reduce the amount of residual monomers.
Crosslinked acrylic acid polymers are used as thickeners in textile printing and also in cosmetic and pharmaceutical formulations. These polymers can be prepared by free-radical polymerization or copolymerization in W/O emulsions. On incorporation into aqueous formulations, emulsions of this kind invert and cause thickening of the formulations.
EP 383 057 describes a crosslinked copolymer of acrylic acid and nitrile comonomers prepared in a water/paraffin emulsion. The emulsions described are used in textile printing.
DE 35 22 419 describes the inverted emulsion polymerization of acrylamide with acrylic acid in one process step.
A process for preparing water-soluble or water-swellable polymers in a W/O emulsion is claimed by EP 126 528, and comprises polymerizing the water-soluble monomers in the presence of emulsifiers with the addition of a specific dispersing system comprising alkanols.
Crosslinked acrylic acid polymers in a W/O emulsion are also described in EP 297 184 for use as water absorbents.
Following their preparation by free-radical polymerization or copolymerization, W/O emulsion polymers include an unwanted fraction of unpolymerized free monomers (residual monomers) in addition to a polymer solids fraction of from 15 to 50% by weight. The reason for this is the incomplete polymerization of the monomers employed in the free-radical main polymerization, which is normally conducted up to a monomer conversion of 95% and, preferably, from 98 to 99.8% by weight. For reasons primarily of toxicology, the market demands polymers combining a low residual monomer content with unimpaired processing and performance properties.
Methods of lowering residual monomers in aqueous polymer dispersions are well known. In addition to nonchemical methods, such as stripping with inert gas or steam, a wide variety of chemical methods is available, as described, for example, in EP-B 028 348, EP-B 563 726, EP-A 764 699, and US-A 4 529 753, to lower residual monomer contents in aqueous polymer dispersions.
In accordance with WO 95/33775, aqueous polymer dispersions can be aftertreated using redox systems whose reducing agent comprises an adduct of hydrogen sulfite anion and a ketone of 3 to 8 carbon atoms, and/or the conjugate acid of said adduct. The aftertreatment is performed in the presence of metal compounds which are soluble in the aqueous medium.
For the reduction of residual monomer contents, EP-A 767 180 recommends a redox initiator system comprising organic hydroperoxides which are soluble only very sparingly if at all in water, and, inter alia, adducts of bisulfites with aldehydes having a carbon chain of 4 to 6 carbon atoms.
The patent application DE 19840586 describes the use of oxidizing agents in combination with a redox system comprising an aldehyde for depleting residual monomers in aqueous polymer dispersions.
In accordance with DE 19741187, a system comprising an oxidizing agent and an organic &agr;-hydroxy carboxylic acid is used for chemical removal of residual monomers.
All processes relate to the treatment of aqueous polymer dispersions. No process has to date been described for lowering residual monomers in W/O emulsions of W/O emulsion polymers comprising acrylic acid. Within these W/O emulsions, the polymers are located within isolated water droplets emulsified in the oil phase. This prevents the formation, as observable in a system with a continuous aqueous phase, of a gel phase which is difficult to process.
It is an object of the present invention to provide a novel, effective process for reducing the amount of residual monomers in W/O emulsions comprising crosslinked water-swellable polymers. In such a process the performance properties of the W/O emulsion, especially the thickener effect, should remain unaffected. Furthermore, the reduction in the amount of residual monomers should be easy to utilize industrially.
We have found that the subject is achieved and that the amount of residual monomers in W/O emulsions comprising crosslinked polymers swollen in water can be effectively reduced if the W/O emulsion comprising the residualmonomers is aftertreated with the addition of a redox initiator system.
A preferred process comprises conducting the aftertreatment with a redox initiator system comprising essentially
a) from 0.001 to 5% by weight, based on the total monomer amount used to prepare the polymer,
a1) of an oxidizing agent:
R
1
OOH,
 where R
1
is hydrogen, a C
1
- to C
8
-alkyl or C
6
- to C
12
aryl group, and/or
a2) of a compound which in aqueous medium releases hydrogen peroxide, and
b) from 0.005 to 5% by weight, based on the total monomer amount used to prepare the polymer,
b1) of an &agr;-hydroxy carbonyl compound:
 where, independently at each occurrence,
R
2
is hydrogen or a C
1
-C
12
-alkyl group which if desired contains functional groups and/or can be olefinically unsaturated,
R
3
is hydrogen, OH, a C
1
-C
12
-alkyl group which if desired contains functional groups and/or can be olefinically unsaturated,
and R
2
and R
3
can form a ring structure which can include a heteroatom and/or functional groups and/or can be olefinically unsaturated, and/or
b2) a compound which in aqueous medium releases such an &agr;-hydroxy carbonyl compound, and
c) catalytic amounts of a polyvalent metal ion which is able to exist in a plurality of valence states.
The oxidizing agent of the redox initiator system should be in a position to form free radicals. Oxidizing agents employed in the redox system are preferably hydrogen peroxide but also include inorganic compounds such as potassium peroxide, sodium peroxide, sodium perborate, and other precursors which in aqueous medium form hydrogen peroxide. It is also possible, for example, to employ ammonium, potassium or sodium persulfate, peroxodisulfuric acid and its salts, ammonium, potassium or sodium perphosphate or diperphosphate, potassium permanganate, and other salts of per acids. Also suitable in principle are organic hydroperoxides, such as tert-butyl hydroperoxide and cumene hydroperoxide. It is, however, also possible to employ mixtures of different oxidizing agents.
The amount of added oxidizing agent is usually within the range from 0.001 to 5, preferably from 0.002 to 3, with particular preference from 0.003 to 2, with very particular preference from 0.01 to 1 and, most preferably, from 0.02 to 0.8% by weight, based on the total monomer amount.
The reducing agent used in the redox initiator system can be any of the agents which are usually susceptible to use.
Particularly suitable reducing agents are aliphatic &agr;-hydroxy carboxylic acids and also precursors which in aqueous solution release these &agr;-hydroxy carboxylic acids. Examples that may be mentioned of &agr;-hydroxy carboxylic acids are aliphatic hydroxy carboxylic acids of preferably 2 to 8 carbon atoms, such as glycolic acid (hydroxyacetic acid), glycoxylic acid hydrate (dihydroxyacetic acid), lactic acid (2-hydroxypropionic acid), glyceric acid (2,3-dihydroxypropionic acid), malic acid (2-hydroxysuccinic acid) or tartronic acid (2-hydroxymalonic acid). The use of tartaric acid is preferred.
Also suitable as reducing agents, however, are aliphatic &agr;-hydroxy carbonyl compounds, such as aliphatic &agr;-hydroxy aldehydes and/or aliphatic &agr;-hydroxy ketones, their isomers, and/or functional group substituted and/or olefinically unsaturated compounds and mixtures thereof, and also precursors which in aqueous solution release these &agr;-hydroxy carbonyl compounds. Examples that may be mentioned of &agr;-hydroxy carbonyl compounds are glycol aldehyde and/or its dimer, 2,5-dihydroxy-1,4-dioxane, phenylglycol aldehyde, 2-hydroxy-3-phenylpropionaldehyde, glyceraldehyde and its higher homologous compounds, such as aldotetroses, aldopentoses and aldohexoses, and also &agr;-hydroxyacetone, &agr;

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