Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2001-08-17
2002-08-13
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C524S800000, C524S832000
Reexamination Certificate
active
06433132
ABSTRACT:
The invention relates to a process for reducing the amount of residual monomer in aqueous polymer dispersions by chemical aftertreatment with a specific redox initiator system.
Following their preparation by free-radical polymerization or copolymerization, aqueous polymer dispersions include not only a polymer solids fraction of from 30 to 75% by weight but also, owing to the incomplete polymerization of the monomers employed in the free-radical main polymerization, which is usually carried out to a monomer conversion of 95 and preferably 98 to 99% by weight, an unwanted fraction of unpolymerized free monomers (residual monomers). On mainly toxicological grounds the market requires aqueous polymer systems having a low residual monomer content with no change in processing and use properties.
In addition to the nonchemical methods, a wide variety of chemical methods is available for lowering residual monomer contents of aqueous polymer dispersions.
For reducing the residual monomer content in polyvinyl ester based aqueous dispersions, EP-B 563 726 discloses a process in which the monomeric vinyl ester units are cleaved by selective saponification into the carboxylates and acetaldehyde and, at elevated temperature, the acetaldehyde—or, preferably, unsaponified residual vinyl ester monomers and acetaldehyde simultaneously—is removed by distillation.
According to U.S. Pat. No. 4 529 753, the residual monomer contents in aqueous polymer dispersions are lowered using a redox system consisting of an oxidizing agent which forms free radicals, such as an organic peroxide, a reducing agent, such as reducing sugars or sulfur compounds, and a transition metal salt, at subatmospheric pressure.
According to EP-B 003 957, the residual styrene content in styrene latices can be reduced by means of a secondary redox catalyst consisting of an organic hydroperoxide and an activator such as diethylenetriamine, triethylenetetraamine or tetraethylenepentamine, for example.
EP-B 028 348 describes a process for lowering the amount of residual acrylonitrile in polyacrylic homo- and copolymers, in which a comonomer copolymerizable with acrylonitrile, and a redox catalyst consisting of an organic peroxide and a reductive heavy metal compound, are used for monomer depletion.
For preparing aqueous polymer dispersions with low residual monomer contents, DE-A 195 29 599 recommends an aftertreatment stage essentially comprising a redox initiator system consisting of an inorganic oxidizing agent and a thiocarboxylic acid and the alkali metal salts and ammonium salts thereof, said aftertreatment being conducted within a pH range from 6 to 10.
EP-A 764 699 discloses the aftertreatment of aquous polymer dispersions with tert-butyl hydroperoxide, isoascorbic acid, iron(II) sulfate, and the sodium salt of ethylenediaminetetraacetic acid.
Within the redox systems, hydroxymethanesulfinic acid and its salts find broad application as reducing agents.
For instance, DE-A 38 34 734 recommends lowering the residual monomer content of aqueous polymer dispersions by subjecting them, after the main polymerization reaction is at an end, to the action of a free-radical redox initiator system whose oxidizing agent comprises organic peroxides, such as acetyl peroxide or tert-butyl hydroperoxide, and whose reducing agent comprises a sulfinic acid or its alkali metal salts, such as sodium hydroxymethanesulfinate, and also iron salts and vanadium salts.
To deplete residual monomers in methyl methacrylate
-butyl acrylate dispersions, DE-A 37 18 520 proposes post-polymerization with the redox system comprising tert-butyl hydroperoxide/sodium hydroxymethanesulfinate and subsequent vacuum distillation.
According to DE-A 42 32 194, the amount of residual monomers in bead polymer systems is reduced using water-soluble redox initiator systems composed of oxidizing agents, such as hydrogen peroxide, ammonium persulfate or tert-butyl hydroperoxide, for example, reducing agents, such as disulfites, dithionites, sulfites or thiosulfates in the form of water-soluble salts, and also sodium hydroxymethanesulfinate, for example, and transition metal compounds.
For the use of carbonyl compounds and/or their reaction products in the aftertreatment of aqueous polymer dispersions it is necessary to start from the following prior art.
According to 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. Aftertreatment is performed in the presence of metal compounds that are soluble in the aqueous medium.
For the reduction of residual monomer contents, EP-A 767 180 advises a redox initiator system comprising organic hydroperoxides whose solubility in water is, at best, poor and, inter alia, adducts of aldehydes having a carbon chain of 4 to 6 atoms with bisulfites.
It is an object of the present invention to provide a new and effective process for reducing the amount of residual monomer in aqueous polymer dispersions. The intention is also that the reducing of the amount of residual monomer should be easy to apply industrially without the formation of microcoagulum.
We have found that this object is achieved and thus that the amount of residual monomers in aqueous polymer dispersions can be effectively reduced if the aftertreatment of the aqueous polymer dispersions comprising residual monomers is accompanied by the addition of 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 dispersion,
a
1
) of an oxidizing agent
R
1
OOH,
where R
1
is hydrogen or a C
1
-C
8
-alkyl or a C
6
-C
12
-aryl group, and/or
a
2
) 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 dispersion, of a reducing agent prepared from
b
1
) an aldehyde
R
2
CH═O,
where R
2
is a C
1
-C
12
-alkyl group which may contain functional groups and/or may be olefinically unsaturated, and
b
2
) an inorganic dithionite, 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 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 peracids. 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 a 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 0.5% by weight, based on the total monomer amount.
Suitable reducing agents are, in general, reaction products of aldehydes, especially aliphatic alkyl-, cycloalkyl-, alkylaryl- and aryl-aldehydes of 2 to 13 carbon atoms, examples being ethanal, propanal, butanal, methylpropanal, pentanal, 2-methylbutanal, 3-methylbutanal, hexanal, 2-methylpentanal, 3-methylpentanal, 4-methylpentanal, 2-ethylbutanal, heptanal, 2-methylhexanal, 3-methylhexanal, 4-methylhexanal, 5-methylhexanal, 2-ethylpentanal, 3-ethylpentanal, octanal, 2-methylheptanal, 3-methylheptanal, 4-methylheptanal, 5-methylheptanal, 6-methylheptanal, 2-ethylhexanal, 3-ethylhexanal, 4-ethylhexanal, 2-n-propylpentanal, 2-iso-propylpentanal, nonanal, decanal, undecanal, dodecanal, cyclopentylmethanal, cyclohexylmethanal, cyclopentylethanal, cyclohexylethanal, phenylmethanal
Baumstark Roland
Dobbelaar Johannes
Wood Claudia
BASF - Aktiengesellschaft
Boykin Terressa M.
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