Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2002-03-08
2003-10-28
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, C525S329100, C525S329200, C525S329300, C525S329400, C525S329500, C525S329600, C525S370000, C525S371000, C525S387000, C526S919000
Reexamination Certificate
active
06639048
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for reducing the amount of residual monomer in aqueous polymer dispersions by chemical aftertreatment with an initiator system.
2. Description of the Background
Following their preparation by a free-radically initiated aqueous emulsion polymerization, aqueous addition-polymer dispersions include not only a polymer solids fraction of generally from 30 to 75% by weight but also an unwanted fraction of unpolymerized, free ethylenically unsaturated monomers (residual monomers). This fraction originates from incomplete polymerization of the ethylenically unsaturated monomers used in the free-radical main polymerization, which is usually carried out to a monomer conversion of 95% by weight and preferably from 98 to 99% by weight. On mainly toxicological grounds the market requires aqueous polymer systems having a low residual monomer content with no change in processing and performance properties.
For the use of reducing agents based on organic carbonyl compounds in the chemical 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 chemically 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 this 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 comprising, inter alia, adducts of aldehydes whose carbon chain has 4 to 6 carbon atoms with bisulfites.
The application filed under number 19839199.4 at the German Patent and Trademark Office discloses lowering the residual monomer contents in aqueous polymer dispersions by aftertreatment with a redox initiator system comprising a peroxide-containing oxidizing agent and a reaction product of aldehydes and inorganic dithionites.
DE-A 19743759 discloses reducing agents comprising derivatives of sulfinic acid which are obtainable on the basis of substituted carbonyl compounds and which are used, inter alia, as a constituent of redox initiator systems for reactions including free-radically initiated aqueous emulsion polymerization.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel effective process for reducing the amount of residual monomer in aqueous addition-polymer dispersions. Furthermore, the reduction in the amount of residual monomer should be easy to implement industrially and should avoid the formation of microcoagulum.
We have found that this object is achieved and thus that the amount of residual monomers in aqueous addition-polymer dispersions can be advantageously reduced if the chemical aftertreatment of the aqueous polymer dispersions comprising residual monomers is carried out with 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, of
a
1
) at least one oxidizing agent
R
1
OOH
in which R
1
is hydrogen or an alkyl or aryl group, and/or
a
2
) at least one 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 at least one reducing agent prepared from
b
1
) a carbonyl compound of the formula I
R
2
(C═O)R
3
(I)
in which
R
2
is a hydrogen atom or an alkyl, alkenyl, cycloalkyl or aryl group with or without one, two or three substituents which independently of one another are selected from hydroxyl, alkyl, O-alkyl, halogen and trihalomethyl, and
R
3
is —CO
2
M, —SO
3
M, —(C═O)R
4
or —CO
2
R
5
, where M is a hydrogen atom, an ammonium ion, a monovalent metal ion or one equivalent of a divalent metal ion from Groups Ia, IIa, IIb, IVa or VIIIb of the Periodic Table, R
4
is a hydrogen atom or an alkyl group and R
5
is an alkyl group,
and, if R
2
is an alkyl, alkenyl, cycloalkyl or aryl group, R
3
can also be a methyl group, and
b
2
) an inorganic dithionite.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The said at least one oxidizing agent of the redox initiator system should be in a position to form free radicals. The oxidizing agent used in the redox initiator system of the invention is preferably hydrogen peroxide. In many cases, however, use is also made of potassium peroxide, sodium peroxide, sodium perborate, and further precursors which in aqueous medium form hydrogen peroxide. It is also possible, for example, to use ammonium, potassium or sodium persulfate, peroxodisulfuric acid and its mono- and di-ammonium, -potassium or -sodium salts, and also ammonium, potassium or sodium perphosphate or diperphosphate, potassium manganate, and other salts of per acids. Likewise suitable in principle are organic hydroperoxides, such as tert-butyl hydroperoxide and cumene hydroperoxide, for example. It is also possible to use mixtures of different oxidizing agents.
The amount of the at least one oxidizing agent added in the aftertreatment in accordance with the invention is customarily within the range from 0.001 to 5% by weight, preferably from 0.002 to 3% by weight, with particular preference from 0.003 to 2% by weight, with very particular preference from 0.01 to 1% by weight, and, preferably, from 0.02 to 0.5% by weight, based on the total monomer amount used to prepare the polymer dispersion.
The at least one reducing agent used in the aftertreatment of the invention comprises reaction products of carbonyl compounds of the formula I with inorganic dithionites such as, for example, sodium dithionite, potassium dithionite, magnesium dithionite, calcium dithionite, strontium dithionite, barium dithionite and zinc dithionite or mixtures thereof. Sodium dithionite is preferably used.
Carbonyl compounds of the formula I used in accordance with the invention are compounds in which
R
2
is a hydrogen atom or an alkyl, alkenyl, cycloalkyl or aryl group with or without one, two or three substituents which independently of one another are selected from hydroxyl, alkyl, O-alkyl, halogen and trihalomethyl, and
R
3
is —CO
2
M, —SO
3
M, —(C═O)R
4
or —CO
2
R
5
, where M is a hydrogen atom, an ammonium ion, a monovalent metal ion or one equivalent of a divalent metal ion from groups Ia, IIa, IIb, IVa or VIIIb of the Periodic Table, R
4
is a hydrogen atom or an alkyl group and R
5
is an alkyl group,
and, if R
2
is an alkyl, alkenyl, cycloalkyl or aryl group, R
3
can also be a methyl group.
For the purposes of the present invention, the expressions set out below preferably have the following meanings: alkyl is straight-chain or branched alkyl groups having generally 1 to 6 and frequently 1 to 4 carbon atoms. Examples of alkyl groups are methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, n-hexyl etc. The same applies to the alkyl group within O-alkyl. Alkenyl is straight-chain or branched alkenyl groups having generally 3 to 8 and frequently 3 to 6 carbon atoms. A preferred alkenyl group is the allyl group. Cycloalkyl is, in particular, C
3
to C
6
cycloalkyl, particular preference being given to cyclopentyl or cyclohexyl. Aryl is preferably phenyl or naphthyl. If the aryl radical is a phenyl group and is substituted, it preferably has two substituents. These substituents are present in particular in positions 2 and/or 4. Halogen is fluorine, chlorine, bromine and iodine, preferably chlorine and bromine and, in the case of trihalomethyl, preferably fluorine. M is preferably an ammonium ion, an alkali metal ion or one equivalent of an alkaline earth metal ion or zinc ion. Suitable alkali metal ions are, in particular, sodium ions and potassium ions; suitable alkaline earth metal ions are, primarily, magnesium ions and calcium ions.
Preference is given to the use of a carbonyl compound of the for
Beckmann Eberhard
Schneider Reinhard
Wood Claudia
BASF - Aktiengesellschaft
Boykin Terressa M.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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