WO2011099949A1

WO2011099949A1 - Process for producing aqueous polymer dispersions having a high solids content

Info

Publication number: WO2011099949A1
Application number: PCT/TR2011/000042
Authority: WO
Inventors: Guia Kaslowski; Sibel Altinok; Guillermo Perez Lorenzo
Original assignee: Organik Kimya Sanayi Ve Ticaret Anonim Sirketi
Priority date: 2010-02-12
Filing date: 2011-02-09
Publication date: 2011-08-18
Also published as: TR201001098A2

Abstract

The invention relates to a process for producing aqueous polymer dispersions having an active content of more than 60% by polymerizing one or more ethylenically unsaturated monomers by means of free -radical initiated aqueous emulsion polymerization in the presence of a variety of surfactants, and in the presence of a radical - generating initiator, wherein the initial step of polymerization is carried out at a distinct flow/temperature pattern then the remainder of components are metered in after that initial step in a conventional way.

Description

PROCESS FOR PRODUCING AQUEOUS POLYMER DISPERSIONS

HAVING A HIGH SOLIDS CONTENT

Description

Field of the Invention

The invention relates to a process for producing aqueous polymer dispersions having an active content of more than 60% by polymerizing one or more ethylenically unsaturated monomers by means of free-radical initiated aqueous emulsion polymerization in the presence of surfactants and initiators.

Background of the Invention

Aqueous polymer dispersions are used in a large number of applications: For example, as base materials for paints, coatings, adhesives, as impregnating and or coating media for paper, as construction and building materials such as caulks and sealants, waterproofing roof coatings, cement additives, and the like.

Typical polymer dispersions found in the market have solids content of from 45 to 60% by weight. As it is well known in the art, when higher solids content is targeted, the viscosity rises sharply as the solids content increases. High viscosity products are not only difficult to handle, they are also risky to produce, tending to generate gels and grit during processing, if not resulting in partial or total coagulation of the product.

From prior art, a number of processes can be found for the preparation of high solids polymer dispersions. Most of these are either based on very narrow and specific processing conditions, or combinations of seed latexes that have to be produced and carefully controlled beforehand.

U.S. Pat. No 4,371 ,659 describes a composition to produce polymer dispersions with high solids content based on the simultaneous presence in the reacting medium of two incompatible monomers, vinyl acetate and styrene, together with a significant amount of polymerization inhibitor. In most polymerization plants the lines transporting these two monomers are separated and never fed the same reactors to avoid the undesired effect of reaction inhibition that these two monomers induce in each other.

The new compositions and processes proposed in this patent application do not require and specifically avoid this uncomfortable combination of incompatible monomers. U.S. Pat. No. 4,424,298 describes a composition for polymer dispersions with high solids in which a very precise combination of surfactants is prescribed (at least one sulfosuccinate based surfactant together with a fatty alcohol ethoxylate and sulphated one).

The new compositions and processes proposed in this patent application do not require such narrow and specific surfactant combinations.

EP-A 784060 relates to a process for preparing polymer dispersions having a high solids content of more than 67%, in which carboxyl-functional monomers are polymerized with further ethylenically unsaturated monomers in the presence of an surfactant and where further surfactant is added at a monomer conversion of from 40% to 60%.

The new compositions and processes proposed in this patent application do not require such specific in-process surfactant changes; neither the use of carboxyl functional monomers is strictly needed.

U.S. Pat. No. 4,456,726 prescribes the use of two polymer latexes of different particle size to be included in the initial charge and the monomers are polymerized subsequently.

The new compositions and processes proposed in this patent application do not require the use of any seed latexes beforehand prepared.

In EP-A 554832, the procedure used to prepare highly concentrated polymer dispersions involves preparing the monomers in the presence of a hydrophobic polymer and in the presence of a copolymerizable surfactant.

The new compositions and processes proposed in this patent application do not require /the use of neither the hydrophobic polymer, nor a copolymerizable surfactant, nor combinations thereof.

Patents, U.S. Pat. Nos. 5,496,882, 5,498,655, 5,442,006 and 5,340,859 rely on to very complex processes for preparing high solids content polymeric dispersions. They prescribe the use of extremely finely divided latex included at least in part of the process and subsequently the monomers are polymerized under very complex process conditions, or they depend upon a specific seed latex mixture comprising latex particles of up to 400 nm in size and latex particles of up to 100 nm in size, both made beforehand, quality controlled and carefully stored before use, or even a combination of very coarsely particulate and very finely particulate one are included in the initial charge and/or in monomer mixture to be metered in.

A common feature across the different compositions and processes proposed in prior art as exemplified by the patents here referred to is that either rather inflexible narrow compositions and/or complex or risky processes are proposed in order to obtain high active content polymer dispersions.

Disclosure of the Invention

The objective of this invention is to provide a relatively simple process, safe and easy to implement in state-of-the art automated polymer emulsion plants and applicable to a wide variety of compositions, including those where significant amounts of vinyl acetate are present, to obtain polymer dispersions with active content above 60%, exhibiting low viscosity and virtually free from gels and grit. Active content in this invention means, the actual amount of film forming polymer generated by letting or forcing the water to evaporate.

The mention of vinyl acetate monomer (VAM) in this invention is not without importance. Vinyl acetate is a low cost monomer when compared to acrylate and methacrylate monomers, available worldwide. It is, therefore, desirable that the proposed processes and compositions to obtain high active content polymer dispersions contemplate its use in significant amounts.

However, its relatively slow reactivity with other monomers from the (meth) acrylate family and low boiling point poses specific process design challenges and more so when high solids content is sought.

Most compositions proposed in prior art either do not include VAM or, if they do, it is only in limited amounts, typically less than 10%. The processes and compositions contemplated in this invention do not strictly require the use of VAM, but when the use of that monomer is desired, it is capable to handle it in amounts well above 10%.

The invention provides a process for preparing aqueous polymer dispersions having an active content of more than 60% by polymerizing one or more ethylenically unsaturated monomers by means of free-radical initiated aqueous emulsion polymerization in the presence of from 0.1 to 5.0% by weight of surfactant, based on the overall weight of the monomers, and in the presence of initiator, wherein the surfactant is not present in the initial reactor charge, or if so it is very low as in an amount of from 0.001 % to 0.01% by weight, based on the overall weight of monomers. Only buffering salts are present in the initial reactor charge in amounts ranging from 0.01 to 1.0% based on the overall weight of monomers. By buffering salts we mean salts that are soluble in water at 50 °C at concentrations equal or higher than 5% by weight, are preferably those who also provide a convenient buffered media for the polymerization reaction, and more so when vinyl acetate monomer is present such as ammonium, sodium and potassium salts of acetic acid, carbonic acid, phosphoric and poly-phosphoric acids, ethylene-diamino-tetraacetic acid, citric acid, tartaric acid, and the like. If a very small amount of the surfactant were to be present at the initial charge, the amount of buffering salts has to be at least two times higher than that of the surfactant present in the initial charge.

All or a fraction of the initiator is then added to the initial charge equilibrated at a temperature distinctly lower than the temperature at which most of the monomers will be metered in, at least 5 °C, preferably 10 °C or more, then monomer and remaining catalyst, if any, are metered in at a distinctly lower feed rate, preferably between 1/4 and 1/2 of than that at which most of the monomers shall be metered in after the slow feed initial stage and for a time sufficient to meter from 1% to 15% of the total monomer. After this initial stage of low temperature/slow flow (LT/SF), temperature is allowed to rise to the desired reaction temperature while increasing monomer(s) feed rate to that allowing the total reaction of monomer for a time sufficient for the reactor equipment to cope with the heat generated by the exothermal reaction.

After all monomer(s) are metered in, temperature is either increased or maintained constant for some time as to allow any unreacted monomer to react. Further on, reactor is cooled down, optionally a redox system is dosed to further reduce the level of unreacted monomer(s), pH is increased if desired with a suitable alkaline solution (e.g. ammonia) and the final product is filtered and checked for solids content, viscosity, pH and gel or grit.

Suitable monomers are one or more from the group of the vinyl esters of branched or unbranched carboxylic acids having from 1 to 12 carbon atoms, the esters of acrylic acid and methacrylic acid with branched or unbranched alcohols having 1 to 12 carbon atoms, vinyl aromatics, vinyl halides, olefins, and dienes.

Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2- ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of a- branched monocarboxylic acids having 9 to 11 carbon atoms, an example being VeoVa9® or VeoValO® and vinyl acetate is particularly preferred.

Preferred methacrylic esters or acrylic esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n- butyl methacrylate, and 2-ethylhexyl acrylate. Particular preference is given to methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate.

Preferred vinyl aromatics are styrene, methyl styrene, and vinyl toluene. The preferred vinyl halide is vinyl chloride. The preferred olefins are ethylene and propylene and the preferred dienes are 1 , 3-butadiene and isoprene.

If desired it is also possible to copolymerize from 0 to 10% by weight of auxiliary monomers, based on the overall weight of the monomer mixture. Examples of auxiliary monomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid and maleic acid; ethylenically unsaturated carboxamides and carbonitriles, preferably acrylamide and acrylonitrile; monoesters and diesters of fumaric acid and maleic acid such as the diethyl and diisopropyl esters, and also maleic anhydride, ethylenically unsaturated sulfonic acids and salts thereof, preferably vinylsulfonic acid or 2-acrylamido-2-methylpropane sulfonic acid.

Further examples are crosslinking and other functional comonomers such as ethylenically polyunsaturated comonomers, examples being divinyl adipate, diallyl maleate, allyl methacrylate and triallyl cyanurate, or postcrosslinking comonomers, examples being acrylamidoglycolic acid (AGA), methylacrylamidoglycolic acid methyl ester (MAGME), N- methylolacrylamide (NMA), N-methylolmethacrylamide, N-methylolallylcarbamate, alkyl ethers such as the isobutoxy ether or esters of N-methylolacrylamide, of N- methylolmethacrylamide and of N-methylolallylcarbamate. Also suitable are epoxy- functional comonomers such as glycidyl methacrylate and glycidyl acrylate. Further examples are silicon-functional comonomers, such as acryloxypropyl-tri(alkoxy)- and methacryloxypropyltri(alkoxy)- silanes, vinyl trialkoxysilanes and vinyl methyldialkoxysilanes, possible examples of alkoxy groups present being ethoxy and ethoxypropylene glycol ether radicals. Mention may also be made of monomers having hydroxyl or carbonyl groups, examples being hydroxyalkyl acrylates and methacrylates such as hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate and also compounds such as diacetone acrylamide and acetylacetoxyethyl acrylate or methacrylate, typically from 0 to 5%. The most preferred auxiliary monomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid and maleic acid or other unsaturated non carboxylic acids, such as vinylsulfonic acid or 2-acrylamido-2- methylpropane sulfonic acid and their salts; ethylenically unsaturated carboxamides, preferably acrylamide and methacrylamide; and ethylenically unsaturated monomers having hydroxyl groups, preferably hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate.

In the copolymers described later on as examples, the amounts in percent by weight add up in each case to 100% by weight. In general, the monomers and/or the weight fractions of the comonomers are/is selected so as to give a glass transition temperature (Tg) of from -70 °C to +100 °C, preferably from -65 °C to +50 °C. The glass transition temperature (Tg) of the polymers can be determined in a known manner by means of differential scanning calorimetry (DSC). The Tg can also be calculated approximately in advance by means of the Fox equation. According to Fox T. G., Bull. Am. Physics Soc. 1 , 3, page 123 (1956) the following is a valid approximation:

Where, x_n represents the mass fraction (% by weight/100) of the monomer n and Tg_n is the glass transition temperature, in Kelvin Degrees, of the homopolymer of the monomer n. Tg values for homopolymers are listed in the literature, as for instance in Polymer Handbook 2^nd Edition, J. Wiley & Sons, New York (1975).

Particular preference is given to monomers and monomer mixtures which lead to homopolymers or copolymers listed below, the amounts in percent by weight, together with the auxiliary monomer fraction if appropriate, adding up to 100% by weight.

From the group of (meth)acrylate polymers, polymers of n-butyl acrylate or 2- ethylhexylacrylate and blends thereof; copolymers of methyl methacrylate with n-butyl acrylate and/or 2-ethylhexyl acrylate; copolymers of methyl methacrylate with n- butylacrylate and/or ethyl acrylate, copolymers of methyl methacrylate with ethyl or methyl acrylate also with either butyl acrylate and 2-ethylhexyl acrylate or both.

From the group of styrene polymers, styrene-acrylate copolymers such as styrene-n-butyl acrylate or styrene-2-ethylhexyl acrylate and copolymers of styrene with both butyl acrylate and 2-ethylhexyl acrylate with a styrene content of in each case from 5 to 70% by weight. Also copolymers as above, with total or partial substitution of butyl acrylate or 2- ethylhexyl acrylate by ethyl or methyl acrylate.

Copolymers as in the two also mentioned groups, in which partial substitution of methyl methacrylate by styrene and vice versa are performed.

From the group of vinyl esters, copolymers of vinyl acetate with butyl acrylate, vinyl acetate with 2-ethylhexyl acrylate or vinyl acetate with both acrylates are preferred, as well as copolymers of vinyl acetate with VeoVa9® and/or VeoValO® in which the amount of vinyl acetate monomer represents from 5 to 80% by weight of the total monomer. Also copolymers where the amount of vinyl acetate monomer is partially substituted by a methacrylate monomer (e.g. methyl methacrylate), as well as copolymers of vinyl acetate as above where part of the VeoVa monomer is substituted by one or both of the acrylates already mentioned. In this group of vinyl acetate copolymers, the vinyl aromatic monomers (e.g. styrene) are excluded.

Maximum preference is given to polymerizing the monomers or monomer mixtures just mentioned in the presence of from 0.1 to 5% by weight of one or more functional monomers from the groups containing carboxylic groups( acrylic acid, methacrylic acid, itaconic acid, and the like), amido groups (acrylamide, methacrylamide,) and hydroxyl groups (such as hydroxyethyl, hydroxypropyl and hydroxybutyl acrylate), and sulfonic groups (such like in sodium vinyl sulfonate and sodium acryl amidopropylsulfonate).

When performing emulsion polymerization accordingly with the present invention, polymerization temperatures after the initial low temperature/ slow flow (LT/SF) stage are generally between 60 °C to 100 °C, more preferably from 70 °C to 90 °C. Accordingly, temperature during the initial LT/SF varies from 50 °C to 90 °C, more preferably from 60 °C to 80 °C as to maintain a difference from 5 °C to 30 °C, more preferably from 10 °C to 20 °C with the polymerization temperature of the main and longer part of the polymerization process.

The total amount of water contained in the recipes for the high active content polymer emulsions object of this invention is partially in the reactor pre-charge, where only the buffering salts are dissolved, with the make-up to 100% water amount is shared between a monomer emulsion and a metered-in catalyst. When no monomer emulsion is prepared to run the process (monomix scenario), the make-up water can be used to dilute the accompanying surfactant, which is dosed simultaneously and in parallel with the monomix. Any in-between scenario is also possible, where part of the monomers are emulsified with part of the water and surfactant, and the remaining monomer which is not pre-emulsified is metered-in as monomix. Whenever both pre-emulsified monomers and monomix are present, the monomix can be total or partially metered in during the LT/SF initial stage, alongside with the monomer emulsion or after the monomer emulsion.

All these variants are valid in accordance with the present invention and give enough flexibility to the polymer chemist to tailor its process for the monomer combinations of choice as well as for the particular installation deemed to run the process.

The polymerization reaction is initiated by means of the initiators or initiator combinations which are common for emulsion polymerization. Examples of suitable organic initiators are hydroperoxides such as tert-butyl hydroperoxide, tert-butyl peroxopivalate, cumene hydroperoxide, isopropylbenzene monohydroperoxide or azo compounds such as azobisisobutyronitrile and its water soluble variants. Suitable inorganic initiators are the sodium, potassium and ammonium salts of peroxodisulfuric acid. These initiators are used generally in an amount of from 0.05 to 5% by weight, based on the overall weight of the monomers.

Redox initiators are used as combinations of the above mentioned initiators with reducing agents. Suitable reducing agents are the sulphites and bisulphites of alkali metals and of ammonium, an example being sodium sulphite, the derivatives of sulfoxylic acid such as zinc or alkali metal formaldehyde-sulfoxylates, an example being sodium hydroxymethanesulfinate, and ascorbic acid. The amount of reducing agent is preferably from 0.01 to 5.0% by weight, based on the overall weight of the monomers.

In order to control the molecular weight of the final polymer, it is possible to use polymer chain length regulators (also known as chain transfer agents) during the polymerization. They are used commonly in amounts of from 0.01 to 5.0% by weight, based on the total amount of monomers to be polymerized, and are metered in separately or else as a premix with reaction components (monomers, catalyst, surfactant, etc). Examples of such regulators are n-dodecyl mercaptan, tert-dodecyl mercaptan, mercaptopropionic acid, methyl mercaptopropionate, isopropanol and acetaldehyde.

The polymerization mixture is stabilized by means of surfactants and/or protective colloids. Preference is given to stabilization by means of surfactants in order to obtain a low dispersion viscosity. The overall amount of surfactant is preferably from 0.1 to 5% by weight based on total amount of monomers, depending of the chemical nature of the surfactant in place. The surfactant(s) may not be present in the reactor pre-charge, and if it is so, it will only be in tiny amounts and always in concentrations at least three times less than that of the above mentioned buffering salts.

Suitable surfactants are anionic or nonionic surfactants or mixtures thereof, examples being: 1 ) Alkyl sulphates, especially those having a chain length of 8 to 18 carbon atoms, alkyl and alkyl aryl ether sulphates having 8 to 18 carbon atoms in the hydrophobic radical and from 1 to 50 ethylene oxide (EO) units. 2) Sulfonates, especially alkyl sulfonates having 8 to 18 carbon atoms, alkylaryl sulfonates having 8 to 18 carbon atoms, di-esters and monoesters of sulfosuccinic acid with monofunctional alcohols or alkyl phenols having 5 to 20 carbon atoms in the alkyl radical; if desired, these alcohols or alkyl phenols may also be ethoxylated with from 1 to 100 ethylene oxide units. 3) Phosphoric acid partial esters and their alkali metal and ammonium salts, especially alkyl and alkyl aryl phosphates having 8 to 20 carbon atoms in the organic radical, alkyl ether and alkyl aryl ether phosphates having 8 to 20 carbon atoms in the alkyl or alkyl aryl radical and from 1 to 50 EO units. 4) Alkyl polyglycol ethers, preferably having from 8 to 40 EO units and alkyl radicals having 8 to 20 carbon atoms. 5) Alkylaryl polyglycol ethers, preferably having from 8 to 40 EO units and 8 to 20 carbon atoms in the alkyl and aryl radicals. 6) Ethylene oxide/propylene oxide (EO/PO) block copolymers, preferably having from 8 to 40 EO and/or PO units.

There is not a preferred or specified surfactant or surfactant blend. The examples here below will contain just one as a matter of simplicity. Most surfactants can be used according to the present invention, only the amount relative to monomers and the way of meter them in, and/or blending with complementary ones, have to be adapted to a specific surfactant, if that surfactant is the main choice due to some applications properties desired of the polymer obtained.

If desired, the surfactants can also be used in a mixture with protective colloids. Examples of these are one or more protective colloids from the group consisting of partially hydrolyzed polyvinyl acetates, polyvinylpyrolidones, carboxymethyl-, methyl, hydroxyethyl- and hydroxypropyl-cellulose, starches, proteins, poly(meth)acrylic acid, poly(meth)acrylamide, polyvinylsulfonic acids, melamine-formaldehyde sulfonates, naphthalene-formaldehyde sulfonates, styrene-maleic acid copolymers and vinyl ether- maleic acid copolymers. If protective colloids are used, they are used preferably in an amount of from 0.05 to 5.0% by weight, based on the overall amount of the monomers. The protective colloids can be included in the initial charge prior to the beginning of polymerization, or can be metered in along with the pre-emulsified monomer or together with the catalyst solution, the surfactant solution or anywhere in-between.

In a preferred embodiment, no monomers and no seed latexes may be present at the initial reactor pre-charge prior the start of the LT/SF initial phase.

For initiating the polymerization, either some of the initiator may be included in the initial charge and some metered in alongside with the monomix or the monomer emulsion, or it can all be metered in. Preferably, the polymerization is started by heating the mixture to the pre-established polymerization temperature of the initial LT/SF stage and metering in all or part of the initiator, preferably in aqueous solution. The metered additions of surfactant and monomers can be conducted separately or in the form of a mixture. In the case of the metered addition of mixtures of surfactant and monomer, the procedure is to premix surfactant and monomer in a mixer upstream of the polymerization reactor with a fraction of the water available in the recipe (known as the monomer emulsion). Preferably, the remainder of surfactant and the remainder of monomix not included in the monomer emulsion are metered in separately from one another before, simultaneously with, or after the monomer emulsion has already been metered in.

As stated before, during the initial LT/SF phase, from 1 to 10% of the total monomer will be metered in, and, thereafter, both feed rate and polymerization temperature will be raised to the main polymerization process conditions. There's no impediment to continue the whole metering-in process at either the initial slow flow or the initial low temperature, however these conditions are not needed and the less desired from the productivity standpoint.

When the initial LT/SF has been passed and after normal flow/temperature metering-in polymerization is at an end, residual monomer can be removed using known methods of post polymerization, by means, for example, of post polymerization initiated with redox catalyst. Volatile residual monomers can also be removed by means of distillation, preferably under reduced pressure, and with or without inert entraining gases such as air, nitrogen or steam being passed through or passed over the mixture.

The aqueous dispersions obtainable with the process and compositions according to the invention have a solids content of more than 60% by weight, preferably 65% and above, and exhibiting viscosities way below 3000 mPa.s The aqueous dispersions of high solids content obtainable according to this invention are suitable for use as textile binders, paper & pulp binders, coating compositions, adhesive compositions, spackling compounds, caulks and sealants, cement modifiers, gypsum modifiers, high solids / low viscous latexes for highly productive spray dried latexes and related applications. Preference is given to their use in caulks & sealants, adhesive compositions, with particular preference as pressure-sensitive adhesives, and modifiers.

The examples below serve to illustrate the invention further. It is obvious that the practice and final use of the products according to this invention is by no means restricted to the examples given below, these examples being given as just a way to illustrate and make the invention clearer.

Comparative EXAMPLE 1 (failed example)

No slow flow applied in the initial LT/SF stage:

Just one surfactant and vinyl acetate monomer are present at about 30% of the total monomer mixture.

A 2 liter three-necked flask equipped with a reflux condenser and anchor stirrer was charged with 122.3 g of demineralised water (DW), 1,35 g of Sodium-Ethylene-diamino tetraacetate (EDTA) and 1.35 g of sodium hexametaphosphate. The content of this reactor initial charge is kept under gentle agitation.

In separate dosing funnels, a delayed feed containing in total 145 g of DW, 30 g of Disponil FES77 30% strength (fatty alcohol ethoxylatedsulfated from Cognis AG), 18.0 g of sodium vinyl sulfonate (25% strength), 10.35 g of methacrylic acid, 270 g of vinyl acetate, 210 g of butyl acrylate, and 420 g of 2-ethylhexyl acrylate.

In another separate dosing funnel, a reaction catalyst solution of 2.70 g of sodium peroxo disulphate in 47.3 g of water is prepared.

With the content of the reactor at 75 °C, a solution of 2.70 g of sodium peroxo-disulphate in 12.3 g of water is added and a constant flow of all remaining components and catalysts is started as calculated for 3 hours. 75 °C are kept constant for the initial stage of 30 min. After 30 min, the temperature is raised to 85°C. Intense reflux is observed in the condenser and before the feeds are finished, the product in the reactor coagulates completely and the reaction has to be abruptly stopped. Comparative EXAMPLE 2 (failed example)

No Low Temperature applied in the initial LT/SF stage:

Just one surfactant and vinyl acetate monomer is present at about 30% of the total monomer mixture.

Same as in Example 1 , but the initial stage is started at 85 °C. A slow flow equivalent to 1/3 of the flow calculated for a 3 hour feed time for all delayed feeds is applied. Again and intense reflux is observed that only goes worse when, after 30 min, the flow rate is increased to normal rate. The increasing reflux made the process just uncontrollable and the reaction has to be interrupted before the delay feeds have been metered in.

Comparative EXAMPLE 3 (in accordance with the invention)

Low Temperature/Slow flow applied in the initial LT/SF stage:

Same as in Example 1 , but the initial stage is started at 72 °C. A slow flow equivalent to 1/3 of the flow calculated for a 3 hour 30 minutes total feed time for all delayed feeds is applied.

After the 30 minutes of the LT/SF stage have passed, temperature is allowed to rise to 82 °C (10 °C temperature difference) and normal flow (3 times higher than initial slow flow) is applied.

The whole metering process goes smoothly, no reflux observed. After all remaining monomers, water, surfactants and catalysts are in, the reaction continues for 30 minutes more, and then the contents of the reactor are cooled down to 60 °C when a customary monomer reduction step is performed by adding 1 g of sodium metabisulphite dissolved in 19 g of water, followed by 1g of ter-butyl hydroperoxyde dissolved in 11 g of water.

After partial neutralisation with 4 g of ammonia 25% strength and further cooling, the contents of reactor are discharged through a 150 micron filter.

The final product has 67.5% solids, pH 5.8, Brookfield viscosity of 480 mPa.s at 60 rpm, and the latex is virtually free from gels or grit. Comparative EXAMPLE 4 (in accordance with the invention)

Low Temperature/Slow flow applied in the initial LT/SF stage:

Same as Example 3, only the surfactant is Disponil FES993 (RTM of Coghis, lower degree of ethoxylation) instead of Disponil FES77.

Process is carried out the same way as in Example 3, no reflux observed, easy to control reaction.

Final product has 67% solids, pH 5.4, and viscosity was 600 mPa.s

EXAMPLE 5 (in accordance with the invention)

Low Temperature/Slow flow applied in the initial LT/SF stage:

Just one surfactant, no vinyl acetate monomer, pure acrylic composition

A 3 liter three-necked flask equipped with a reflux condenser and anchor stirrer was charged with 300 g of demineralised water (DW), 1.5 g of Na4-Ethylene- diaminotetraacetate and 1.5 g of Sodium Hexametaphosphate. The content of this reactor initial charge is kept under gentle agitation.

In a separated dosing funnel, a monomer emulsion containing 180 g of DW, 42 g of Disponil FES77 30% (strength) (Cognis AG), 12,8 g of acrylic acid, 25.5 g of methacrylic acid, 128 g of methyl methacrylate, 510 g of butyl acrylate, 638 g of 2-ethylhexyl acrylate and 0.75 g of n-dodecyl mercaptane is kept under stirring.

In a separate dosing funnel, a reaction catalyst solution of 3.75 g of Potassium Peroxo- disulphate in 71.3 g of water is prepared.

With the content of the reactor at 75 °C, a solution of 2.55 g of potassium peroxodisulphate in 52.5 g of water is added and a slow monomer emulsion flow is started, together with the reaction catalyst solution.

The reaction medium is kept constant at 75 °C for the first 30 minutes (LT/SF initial stage) by means of cooling, after which reaction temperature is allowed to rise to 85 °C and the metering in flow rate is increased to double as to feed all the remaining monomer emulsion and catalyst solution in about 3 hours. The reaction temperature is kept constant at 85 °C by means of cooling until all monomer and catalyst are dosed in.

After all feeds are in, the reactor is kept at 85 °C for another 30 minutes, then cooled down to 70 °C when monomer reduction is applied as in previous examples, followed by pH adjustment with ammonia solution.

The resulting dispersion is virtually free from gel or grit by direct observation through filtration, and has a solids content of 66.2% by weight, a pH of 5.6 and a Brookfield (LVT) viscosity of 280 mPas at 60 rpm.

Claims

1. The invention provides a process for preparing aqueous polymer dispersions having an active content of more than 60% by polymerizing one or more ethylenically unsaturated monomers by means of free-radical initiated aqueous emulsion polymerization in the presence of a variety of surfactants, and in the presence of a radical-generating initiator, wherein the initial step of polymerization is carried out at a distinct flow/temperature pattern than the remainder of components, and the remainder of components are metered in after that initial step in a conventional way.

2. The process in claim 1 , wherein, at the start of polymerization there is no seed latex, none or very small amount of surfactant, none or very small amount of monomers is present.

3. The process of claim 2, wherein at the start of polymerization, only water and from 0.01 to 1 % based on the total amount of monomers of water soluble salts are present.

4. The process of claim 1 , wherein from 1.0% to 15% of the total amount of monomers are metered in at the initial stage of polymerization in a distinct flow/temperature pattern than the remaining amount of monomers and components.

5. The process of claim 3, wherein from 1 % to 100% of the total amount of the radical- generating catalyst is present at the start of polymerization together with the water soluble salts and the remaining amount of catalyst, if any, is metered in at any time or during the rest of polymerization process.

6. The process of claim 4, wherein the distinct flow at the initial step is from as low as 1/100 up to 2/3 of the steady flow applied for the remainder of monomers, more preferably from 1/4 to 1/2 of the steady flow applied to the remainder of monomers and that initial distinct slow flow is maintained constant during the initial step.

7. The process of claim 4, wherein the initial distinct temperature during the starting phase is just sufficient for the kind of radical-generating catalyst to decompose and provide sufficient amount of radicals for the reaction to take place, typically from 40 °C to 80 °C, more preferably from 65 °C to 75 °C, as to maintain a difference from 5 °C to 30 °C, more preferably from 10 °C to 20 °C with the polymerization temperature of the main and longer part of the polymerization process, and that temperature is maintained constant for the initial polymerization step along with the slow flow of claim 6.

8. The process of claim 1 , wherein the monomers comprise at least one monomer selected from the group, consisting of vinyl esters of branched or unbranched carboxylic acids having from 1 to 12 carbon atoms, esters of acrylic acid and methacrylic acid with branched or unbranched alcohols having 1 to 12 carbon atoms.

9. The process of claim 1 , wherein a monomer chosen from the vinyl ester family (of the vinyl acetate family) is present as co-monomer in ratios equal to or above 10% based on the total amount of monomers, in which case, vinyl aromatic (of the styrene family) monomers are excluded from the composition.

10. The process as claimed in claim 1 , wherein polymerization is carried out in the presence of from 0 to 10% by weight, based on the overall weight of the monomer mixture, of at least one auxiliary monomer selected from the group consisting of ethylenically unsaturated acids, or copolymerizable monomers containing a moiety susceptible of forming ionisable salts, such as ethylenically unsaturated mono- and di- carboxylic acids, ethylenically unsaturated sulphonic or phosphonic acids and the like.

11. The process as claimed in claim 1 , wherein polymerization is carried out in the presence of from 0 to 10% by weight, based on the overall weight of the monomer mixture, of at least one auxiliary monomer selected from the group consisting of ethylenically unsaturated carboxamides, and/or ethylenically unsaturated monomers containing hydroxyl groups.

12. The process as claimed in claim 1 , wherein polymerization is carried out in the presence of from 0 to 5% by weight, based on the overall weight of the monomer mixture, of further auxiliary comonomers such as divinyl adipate, diallyl maleate, allyl methacrylate and triallyl cyanurate, or postcrosslinking comonomers, examples being acrylamidoglycolic acid (AGA), methylacrylamidoglycolic acid methyl ester (MAGME), N-methylolacrylamide (NMA), N-methylolmethacrylamide, N-methylolallylcarbamate, alkyl ethers such as the isobutoxy ether or esters of N-methylolacrylamide, of N- methylolmethacrylamide and of N-methylolallylcarbamate, epoxy-functional comonomers such as glycidyl methacrylate and glycidyl acrylate, silicon-functional comonomers, such as acryloxypropyltri(alkoxy)- and methacryloxypropyltri(alkoxy)- silanes, vinyl tri-alkoxysilanes and vinyl methyldialkoxysilanes, ethoxy and ethoxypropylene glycol ether radicals, monomers having hydroxyl or carbonyl groups, examples being hydroxyalkyl acrylates and methacrylates such as hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate and also compounds such as diacetone acrylamide and acetylacetoxyethyl acrylate or methacrylate.

13. The process as claimed in claim 1 , wherein the monomers and their weight fractions are selected so as to provide copolymers with a glass transition temperature (known as Tg) of from -70 °C to +100 °C.

14. The process as claimed in claim 1 , wherein the water soluble salts can be chosen from all salts that are soluble in water at 50 °C at concentrations equal or higher than 5% by weight, and preferably those who also provide a convenient buffered media for the polymerization reaction, and more so when vinyl acetate monomer is present such as ammonium, sodium and potassium salts of acetic acid, carbonic acid, phosphoric and poly-phosphoric acids, ethylene-diamino-tetraacetic acid, citric acid, tartaric acid, and the like.

15. The process of claim 1 , wherein the surfactant comprises at least one member selected from the group consisting of anionic surfactants, and non-ionic surfactants.