Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-03-17
2002-09-03
Wilson, D. R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S371000, C525S329100, C525S329200, C525S329700, C525S330200, C525S330300, C525S332900
Reexamination Certificate
active
06444760
ABSTRACT:
The invention relates to a process for reducing the residual monomer content of aqueous polymer dispersions by chemical aftertreatment with a special 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 98-99% by weight, an unwanted fraction of unbonded free monomers (residual monomers). On mainly toxicological grounds the market requires aqueous polymer systems having a low content of low molecular mass constituents with no change in processing and use properties.
Physical deodorization of a monomer-containing polymer dispersion to reduce the residual monomer content often places high demands on the process technology and on the stability of the polymer dispersions, and so is not generally applicable.
There are known methods of reducing the content of residual monomers in aqueous polymer systems by adding nucleophiles which form adducts with the monomers. U.S. Pat. No. 4,278,582, for example, discloses the addition of amino or thiol compounds to styrene-butadiene-acrylonitrile copolymer dispersions, and GB 1 272 584 the addition of hydrazine, in order to reduce the monomer content. Disadvantages with these methods are that the adduct-forming agents usually have to be employed in excess, that the agents are often electrolytes, or that the agents often remain as volatile constituents in the polymer dispersions. It is also possible in some cases (cf. eg. DE-A 29 19 096) for some of the adducts formed with the monomers to break back down into the starting materials, thereby increasing the content of residual monomers again.
It is also known to carry out an aftertreatment at polymerization temperature following addition of initiators in order to reduce the content of residual monomers, and redox initiators in particular are used for this purpose. In this case particular importance attaches to the nature of the reducing agent used in the redox initiator system. EP-A 379 892 and DE-A 38 34 734 disclose the use of Rongalit C (the Na salt of hydroxy-methanesulfinic acid) as a reduction component in the redox initiator system for such an aftertreatment. A disadvantage is that formaldehyde is readily formed in the course of the postpolymerization. When acetone bisulfite is used as the reducing agent for this, as described in U.S. Pat. No. 4,360,632 and DE-A 44 19 518, acetone may be released again during the reaction. For the postpolymerization of polymer dispersions, DE-A 44 35 422 recommends redox initiators with formamidine-sulfinic acid as reducing component. Disadvantages with this, however, are its limited solubility in water and the instability of its aqueous solutions in contact with air, which hinders its industrial handling. The use of reducing sugars such as glucose, of ascorbic acid or isoascorbic acid as component in the redox initiators can lead to instances of discoloration.
It is an object of the present invention to reduce the residual monomer content in aqueous polymer systems without forming further volatile or odoriferous constituents or microcoagulum. The intention is also that the reduction of the residual monomer content should be easy to utilize industrially, being suitable for use even in concentrated systems, for example.
We have found that this object is achieved and thus that the content of residual monomers in the aqueous polymer system can be effectively reduced if the aftertreatment of the aqueous polymer dispersions comprising residual monomers is conducted with heating and addition of a redox initiator system essentially comprising.
a) from 0.01 to 5, preferably from 0.05 to 2, % by weight, based on the overall amount of monomer used to prepare the polymer dispersion, of
a1) a compound R
1
—OO—R
2
,
where R
1
and R
2
are hydrogen, a C
1
-C
8
-alkyl or a C
6
-C
12
-aryl and are identical or different, with the proviso that at least one of R
1
and R
2
is hydrogen, and/or
a2) a compound which in aqueous medium releases hydrogen peroxide, and
b) from 0.01 to 30, preferably from 0.05 to 5,% by weight, based on the overall amount of monomer used to prepare the polymer dispersion, of an organic &agr;-hydroxy carboxylic acid,
where the hydroxyl is preferably part of a hydroxymethine group (CH(OH)) and the acid contains no polymerizable, olefinically unsaturated C—C double bond, or of a salt thereof,
c) from 0 to 1000, preferably ≦100 and, in particular, from 10 to 100 ppm, based on the overall amount of monomer used to prepare the polymer dispersion, of a multivalent metal ion which is able to change its valence state,
wherein said initiator system contains no vanadium ions.
Particularly suitable &agr;-hydroxy carboxylic acids (reducing component) are aliphatic hydroxy carboxylic acids of preferably 2 to 8 carbons, such as glycolic acid (hydroxyacetic acid), glyoxylic 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). Preference is given to the use of tartaric acid.
Both optical isomers and the racemate are equally effective. The chemical deodorization system of the invention is notable for the fact that when added under suitable reaction conditions it brings about a reduction in the amount of olefinically unsaturated compounds (residual monomers) without producing further volatile compounds in the form of byproducts and without the formation of coagulum as the result of an addition of electrolyte.
The oxidizing agent of the redox initiator system (component a)) should be in a position to form free radicals. In a further embodiment it is also possible for some or all of the combination of oxidizing agent and reducing agent to be replaced by a peracid. Oxidizing agents employed in the redox system are preferably hydrogen peroxide, but also potassium peroxide, sodium peroxide, sodium perborate and other precursors which in water form hydrogen peroxide. Other suitable oxidizing agents are tert-butyl hydroperoxide, cumene hydroperoxide, ammonium, potassium or sodium persulfate, peroxodisulfuric acid and salts thereof, ammonium, potassium or sodium perphosphate or diperphosphate, potassium permanganate and other salts of peracids. An initiator system which has proven particularly effective is the combination of tartaric acid and hydrogen peroxide with iron(II) sulfate as catalyst.
The amount of redox initiator system added is preferably in the range from 0.01 to 10% by weight, in particular from 0.01 to 5% by weight, based on monomers employed.
In general, the redox initiator system used for the aftertreatment of the invention is different from the initiator system used for the main polymerization of the monomers. The components of the initiator system used in accordance with the invention for the aftertreatment are judiciously metered in gradually, simultaneously or in succession (the oxidizing agent preferably being added first in the latter case) over a period of ½ to 5 hours with stirring and at atmospheric or superatmospheric pressure, to the polymer dispersion which is heated at about 20 to 150 and, in particular, 60 to 120° C. The initiator components can be added, for example, from above, from below or through the side of the reactor. Preferred stirrers in this context are anchor stirrers and blade stirrers. The dispersed polymers preferably have a particle size of from 50 to 400 nm and a monomodal or broad distribution.
As indicated, the initiator systems added in the aftertreatment of the polymer dispersions may also include multivalent metal ions which are able to change their valence state. These ions have a catalytic effect and assist electron transfer reactions. Very suitable for this purpose are iron, copper, manganese, nickel, cerium, chromium, platinum and palla
Bauer Gerhard
Dobbelaar Johannes
Heibel Claudia
Lawrenz Sven
Rupaner Robert
BASF - Aktiengesellschaft
Wilson D. R.
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