Process for the production of grafted and cross-linked polymers

Field of the invention

The present invention relates to the area of polymers and refers to an improved process for the production of certain anionic comb polymers currently used in the building industry as super- plasticizers for concrete or plaster retarders.

Background of the invention

Polyolefinic acids, particularly polyacrylic acids, grafted with polyglycolethers and mono- methylpolyglycolethers (MPEG) and their use as superplasticizers are well known from literature. They provide high fluidity to high-strength cement and concrete compositions combined with an extremely low water-to-cement or concrete ratio. In addition they reduce the decrease in fluidity, which takes place after a certain time, and control the generation of foam during the making of the final preparations. Examples for the state of the art can be found inter alia in FR 2776285 Bl (Chryso), EP 1260536 Al (BASF) or WO 97/039037 Al (Mbt). Usually, manufacture of the grafted polymers takes place in two steps: firstly, the free-radical polymerisation is carried out in water, and then the esterification is conducted. Of course, prior to esterification the water has to be removed, which makes the process time-consuming and not very efficient.

Nevertheless, the reaction can also be conducted as a "one pot" process. For example, FR 28144465 Bl (Synthro) teaches to polymerise acrylic acid in MPEG as a solvent in the presence of azobis(isobutyronitrile) as an initiator and to start the subsequent esterification by adding p-toluene sulfonic acid as a catalyst. According to WO 99/002466 Al (Arco), a similar process is suggested which differs from the Synthro process mainly by using tert. butylper- oxybenzoate as the initiator. However, the state of the art still suffers from two disadvantages: firstly, it is desirous to save the addition of an esterification catalyst, and secondly, there is a need to reduce foam generation, since foaming of the reaction mixtures makes it difficult to control the reaction.

In this context, reference is also made to international patent application WO 06/050850 Al (Cognis) disclosing anionic polymers useful as superplasticizers for concrete, which are obtained by polymerisation of (meth)acrylic acid or their esters with dipropylendiglycolacrylate (DPGDA), tripropylenglycoldiacrylate (TPGDA), acrylamidomethyl propanesulfonic acid (AMPS) and/or acryl acetate (AA) and subsequent treatment of the intermediates with mixtures of short- and long-chain alkylpolyalkyleneglycols. Another group of concrete plasticizers obtained from the polymerisation of maleic acid (anhydride) and (meth)acrylic acid is known from EP 1319676 Al (Cognis). Polyacrylic acids grafted by polyethyleneglycols and their use as superplasticizers for concrete is also known from EP 1396506 Al (Cognis).

Therefore the problem which underlies the present invention was to develop an improved one- pot process for the manufacture of poly(meth)acrylate polymers grafted and cross-linked with MPEG which avoids the disadvantages of the state of the art as outlined above and particu- larly does not need any esterification catalyst while foam generation is reduced at the same time.

Detailed description of the invention

The present invention refers to a process for the production of grafted and cross-linked polymers comprising the following steps:

(a) Free-radical polymerisation of (meth)acrylic acid in monomethylpolyethyleneglycol (MPEG) as the solvent,

(b) Esterification of the intermediate thus obtained with said solvent and

(c) Neutralisation and solubilisation of the final polymer in water,

to obtain an aqueous polymer dispersion, which is characterised in that

(i) Polymerisation is conducted in the presence of a polymerisation initiator having also the capability to form acids as decomposition products during the reaction, and (ii) Esterification is conducted in the presence of EO/PO block polymers.

During the first step of the process in which (meth)acrylic acid is polymerised in the presence of a free-radical source, using monomethylpolyethyleneglycol (typically comprising 10 to 20 ethylene oxide units) as the solvent, small amounts of the MPEG are grafted onto the polymer chains due to a transfer of a hydrogen radical taken from the methylether group of the MPEG. These grafted polymer chains, however, still contain the terminal OH groups. Surprisingly it has been observed that the use of certain polymerisation initiators - like, for example, ammo-

nium persulfate - leads to the formation of small amounts of inorganic acids, which are sufficient to catalyse the subsequent esterification step so that it is not necessary to add any further (acidic) esterification catalysts. In order to reduce the amount of foam generated in the process, it has been found useful to add small amounts of non-ionic EO/PO block polymers which also graft on the polymer chains. Combining the two features according to the present invention allows the production of grafted and cross-linked polymers in a shorter time at a better conversion and of higher quality. In addition, the process is much easier to conduct than other processes known from the state of the art.

Polymerisation

In the first step of the process, acrylic acid, methacrylic acid or their mixtures are subjected to a free-radical polymerisation. As explained above, the reaction preferably takes place in a non-aqueous media, more precisely, the agent which later acts as the alcohol compound in the esterification process - MPEG - is used as the solvent; nevertheless small amounts of water for better solubilisation can be present. As a matter of fact, the process according to the present invention is therefore not simply limited to said special polyglycolether but would be useful in the same manner for any other reaction with a comparable OH-compound. The proc- ess is conducted in the presence of a source for free radicals to start the polymerisation. According to the present invention, initiators are chosen which have the capability to also form small amounts of (inorganic) acids, for example as the result of thermal decomposition during the exothermic polymerisation reaction. Suitable examples are persulfate salts and, in particular, ammonium persulfate, which are added in typical amounts of 1 to 10 Mol-% calculated on the amount of (meth)acrylic acid. The amounts of acids - in case of persulfates it is of course sulfuric acid - are not formed initially but later in the process, so that also esterification takes place with a temporal delay. This is important since otherwise the solvent would be consumed too early. The reaction conditions for the polymerisation are not critical and can be taken from the state of the art. Usually, the reaction is conducted at a temperature of from about 90 to about 110 °C. The molecular weight of the polymer thus obtained lies typically between 4,000 and 100,000 Dalton depending on the MPEG molecular weight (250 to 10.000).

Esterification

As is typical for a "one-pot" process, there is no defined point in time where polymerisation is finished and esterification starts. Overall the polymerisation reaction is conducted about 1 to 4 h, and preferably about 2 h, mainly depending on the degree of grafting/cross-linking and mo-

lecular weight. Usually, the esterification is conducted at a temperature between 120 and 160 °C. It is advantageous to remove the water of condensation in order to shift mass balance in favour of the desired products. Therefore, pressure is reduced to 10 to 50 mbar. The final copolymer efficiency depends on the following parameters:

o length of backbone poly(meth)acrylic acid, o length of side chain MPEG and o grafting ratio, o which is directly related to the initial molar ratio MPEG/(meth)acrylic acid.

Since the alcohol compound is also used as a solvent, there is no defined molecular ratio between (meth)acrylic acid and MPEG to be adjusted. Typically one part of (meth)acrylic acid is solved in 2 to 10 parts of MPEG, which would represent a molar ratio of about 5 to 50 mol-% based on the targeted structure. Usually the esterification is completed within 2 to 4 h.

The EO/PO block polymers are directly added to the esterification mixture and are also grafted onto the poly(meth)acrylates. Typically, these copolymers

• exhibit molecular weights of about 500 to about 10.000 Dalton, and preferably of

1.000 to 5.000 Dalton, and • comprise 2 to 10 ethylene oxide units, and 5 to 20 propylene oxide units.

Typical examples are adducts of about 2 to about 10 MoI, and preferably 4 to 5 MoI ethylene oxide, and about 8 to about 12 MoI, and preferably 10 to 11 MoI propylene oxide, to saturated or unsaturated fatty alcohols having 6 to 22 carbon atoms, and preferably 12 to 18 carbon atoms or their technical mixtures, like e.g. C] _2/14 coco fatty alcohol or Ci _6/18 tallow fatty alcohol. Preferably a Ci ₆ / ₁₈ -fatty alcohol+4EO+HPO adduct is used which is sold by Cognis France, S.A.S under the trademark Bublex ^® 250. Usually said EO/PO block polymers are added in amounts of 0.1 to 5 % b.w. - calculated on the total reaction mixture.

Neutralisation and Solubilisation

Once the esterification has come to an end - which can be seen from the fact that no further water is separated off - the polymer is neutralised by adding alkali hydroxide, for example aqueous sodium or potassium hydroxide solution, to convert the acids into their salts. Finally, the product is diluted with water to obtain a dispersion showing a solid content of typically 20 to 50 % b.w., and preferably 30 to 45 % b.w.

Industrial application

The grafted polymers obtained according to the inventive process provide high fluidity and better workability to cement, concrete and plaster compositions, and can therefore be used as so-called superplasticizers for concrete or plaster retarders.

Examples

Example 1

A reactor was loaded with 55.4 g MPEG-550 under nitrogen bubbling at room temperature and heated to 100 °C. Then 24.3 g acrylic acid and 5.06 g ammonium persulfate, both solubi- lised in about 15 g of water, were continuously added over a period of about 2 hours. After the polymerisation had been completed, the mixture was heated to about 140 ⁰ C and pressure de- creased to about 30 mbar. About 1 g of Bublex ^® 250 was added. The mixture was stirred for about 3 h under these conditions and the water of condensation was continuously removed. Finally the mixture was cooled to 80 °C, pH was adjusted to about 7 by adding aqueous sodium hydroxide solution, and water was added to obtain the polymer dispersion in the form of a brownish solution showing a solid content of 35 % b.w.