FI88172B - FOER FARING FOR FRAMSTAELLNING AV ETT OMDISPERGERBART PLASTMJOEL - Google Patents

Description

1 88172

Method of manufacture of redispersible plastic powders. -For use for the manufacture of plastic products.

The present invention relates to a process for preparing a redispersible plastic powder by drying a plastic dispersion of a carboxylic acid-containing acrylic resin having a lowest film-forming temperature MFT of less than 60 ° C and containing a dispersed copolymer having a dynamic solidification temperature max.

DE-C 25 12 238 describes the preparation of a powdered acrylic resin rich in carboxyl groups by spray-drying an aqueous dispersion. Here, it is considered essential that the MFT temperature is not exceeded in the spray-drying process when it is desired that the latex particles of the dispersion do not sinter together into glassy particles, but are desired to remain as loose aggregates. In particular, the temperature of the particles, as long as the dispersion droplets still contain a lot of water, is well below the air temperature, in which case this may initially be higher than the MFT temperature, but as the droplets dry out, the air temperature should drop below the MFT temperature.

DE-A 32 08 791 emphasizes that the redispersibility of the powdered polymer requires a presence of unglazed, loosely aggregated latex particles. Because, for the reasons mentioned above, such a structure was considered achievable only when the MFT temperature was not exceeded in the spray-drying process, this was an obstacle to the powder preparation process from carboxylic acid-containing dispersions when the MFT temperature of the underlying dispersion was low. Such dispersions, which have a considerable amount of acrylic acid esters in their structure and have a low MFT temperature due to the high softness of polymerisate 2 88172, are thus converted into a redispersible powder according to this publication by an expensive packaging drying method.

DE-A 22 22 033 describes the preparation of a mist-dispersable powder from a plastic dispersion with a low MFT temperature. Redispersibility could only be obtained by using a special polymer coating composition of vinyl ester, ethylene and acrylamide, although the air temperature at the feed point of the spray dryer was well above the MFT temperature. This information is not applicable to other polymer compositions.

According to DE-A 34 05 651, bimodal dispersions are spray-dried into powders in which the latex particle granules can be loosely aggregated or more or less co-sintered or melted, depending on the drying conditions. The inlet temperature of the air blown for spray drying is much higher than the glass transition temperature of the polymer, for example between 110 and 250 ° C, in particular between 130 and 200 ° C. The powder particles reach the temperature of the air stream at the outlet of the dryer. At final temperatures close to, but not substantially above, the glass transition temperature, spaces between loose aggregation and complete glass transition can be reached within the individual powder granules. In practice, the air temperature of the dryer from the inlet to the outlet generally drops from 130 to 200 ° C to 65 to 80 ° C. If, under such conditions, a dispersion of a carboxyl-rich plastic with a dynamic clotting temperature t (Lambda max) of less than 150 ° C and an MFT temperature of less than 60 ° C were spray-dried, a thick layer of nitrided polymer would form on the wall in the dryer.

It is an object of the present invention to be able to convert 1,88172 carboxyl-rich aqueous dispersions into redispersible powders by a simple and inexpensive spray-drying process even when their MFT temperature is below 60 ° C. Such powders are of great importance, for example, in the coating of drugs, because redispersion can be carried out at reasonable temperatures. In spray drying, it is essential to avoid thick layers of nitrated polymer precipitate on the wall which are difficult to remove.

These objectives have been found to be achievable when the plastic dispersion is spray-dried at an inlet temperature TE below the dynamic solidification temperature but still higher than the MFT temperature and at an outlet temperature TA below 65 ° C but still higher than the MFT temperature. temperature. The drying air and the sprayed dispersion are preferably passed downstream through the dryer.

Inlet temperature T-E means the temperature of the air stream in the spray dryer at which the latex droplets enter the air stream. Correspondingly, the outlet temperature T-A is the temperature at which they are separated from this air stream.

The 'dynamic solidification temperature T (lambda-max) is the maximum evaporation temperature according to DIN 53 445: in the torsional vibration test. It depends on the composition of the polymer. The lowest film-forming temperature MFT is the lowest temperature according to DIN 53 787 at which the thin layer of plastic dispersion still dries to a uniform film. It depends on e.g. the softness of the dispersed polymer, in the case of the carboxyl group-containing polymer used according to the invention, it should be noted that the latex particles dispersed in the aqueous phase are softer than the dried polymer film due to the swelling caused by water. In typical cases, the MFT is 50 ° C or more below the dynamic solidification temperature, while this difference is much smaller for dispersions with or without low carboxyl groups.

Compared to conventional spray-drying methods, the method according to the invention takes place at a lower inlet temperature. The temperature difference between the inlet and outlet temperatures is smaller than in the known methods. This manifests itself in increased air flow calculated per volatile amount of water. The amount of water evaporated is determined by the energy supply, which remains unchanged. It is unusual to perform the entire drying process at temperatures higher than the MFT temperature, which has heretofore been considered the upper limit of the outlet temperature.

One consequence of the relatively high outlet temperature is that the powder particles are generally vitrified because the outlet exceeds the MFT-1 temperature1a. In a microscope, they appear translucent and, when disintegrated, have a typical response to plastic. Despite the glass transition, the particles formed according to the invention can surprisingly be easily and completely redispersed in an aqueous alkaline solution.

dispersion

Dispersions used for spray drying can be prepared by known methods. It is characterized by the composition given in claim 1 and together with it the MFT temperature is below 60 ° C and the dynamic solidification temperature is between 60 and 150 ° C. The range considered particularly good is 100 to 140 ° C. Suitable lower alkyl esters of acrylic or methacrylic acid are those having 1 to 8, preferably 1 to 4 carbon atoms in the alkyl group, most preferably methyl, ethyl and n-butyl ester. At least one alkyl ester of acrylic acid is best present in the structure of the polymer catalyst. Some of the acrylic or methacarylic acid esters may be replaced by styrene, but preferably not more than half. The copolymer generally does not contain other monomer units, but such units may be present up to 10% by weight. All crosslinkable comonomers containing two or more radically polymerizable double bonds, such as acrylic or methacrylic groups, are suitable for this purpose. Examples of units which may be mentioned are acrylic or methacrylonitrile, acrylic or methacrylamide and their N-alkyl derivatives, acrylic and methacrylic acid alkyl esters having more than 8 carbon atoms in the alkyl group, or hydroxyalkyl esters of unsaturated polymerizable carboxylic acids.

The more acrylic or methacrylic acid units in the structure of the copolymer, the greater - otherwise similar comonomers - the temperature difference between the lowest film formation temperature and the dynamic solidification temperature. As the number of these acid units increases, both temperatures rise, but the dynamic stagnation temperature is stronger than the lowest film-forming temperature. The invention is of particular interest: for spray-drying dispersions in which the difference between said temperatures is greater than 0 ° C. Dispersions with an MFT temperature below 50 ° C, especially below 40 ° C, are best treated. Even dispersions with an MFT temperature below 35 ° C can be dried. The preparation of plastic dispersions rich in carboxyl groups is known from dE-C 21 35 073 and EP-A 73 296.

In the process according to the invention, dispersions with the highest possible solids content are expediently used. It may be between 10 and 60% by weight, most preferably between 20% and 40% by weight. In order for the dispersions to be sprayed into fine droplets, their viscosity must be less than 4000 mPa s. The viscosity is preferably less than 1000, in particular less than 100 mPa s.

The dispersion generally contains an anionic emulsifier, for example 0.01 to 3% by weight of the aqueous phase. In addition, for example, up to 5% by weight of non-ionic emulsifiers may be present, which may also be present as the sole emulsifiers.

Spray-drying

So-called spray towers are used to dry aqueous plastic dispersions. Their structure and use are detailed in K. Masters "Spray Drying Handbook" (4th edition, 1985, Verlag George Godwin Ltd.).

They are usually used downstream and especially preferably from top to bottom. In this case, hot air and dispersion are led to the top of the spray tower. The air temperature drops from top to bottom due to the energy used by the evaporation of water. At the top of the spray tower, the dispersion is broken up into fine droplets; this uses spray nozzles or, in most cases, rapidly rotating perforated plates to which the dispersion is allowed to flow. The formed powder particles accumulate in the lower part of the spray tower in a so-called cone or they can be separated from the air flow by leading to a separation cyclone.

In order to achieve a high drying efficiency, the spray tower usually uses the highest possible air flow-through. Before being fed to the spray tower, the air is heated to an inlet temperature T-E lower than the dynamic solidification temperature T <lambda-max) of the polymer to be dried, but still higher than the MFT temperature of the polymer. The difference between these temperatures can be small, for example 5 to 20 ° C. However, it is often more advantageous to use a larger difference, for example 20-50 ° C. The most advantageous I 88172 inlet temperature T-E can most often only be determined first in the spray tower, since its structure affects the optimum position. If the T-E is set too high, there is a risk that the polymer will adhere as a shell to the wall of the spray tower. However, this danger is surprisingly avoided, even if the air temperature in the entire spray dryer is kept above the MFT temperature.

The dynamic solidification temperature T (lambda-max) of the emulsion polymer used in the process of the invention is between 60 and 150 ° C, preferably between 100 and 140 ° C. The MFT is below 60 ° C, for example between 5 and 50 ° C. These temperatures define the narrow range in which the process of the invention can be performed. Typically, T-E is 60 to 100 ° C, T-A is 30 to 60 ° C, and the temperature difference between T-E and T-A is no more than 65 ° C. This difference is preferably between 30 and 50 ° C, and should therefore be smaller the lower the T-E.

While the air flow rate remains the same, the outlet temperature T-A can be controlled without changing the inlet temperature T-E to the dispersion by the addition rate. As the amount of dispersion fed per unit time is increased, more heat is removed from the air stream to evaporate the water contained in the dispersion and thus the T-A decreases. Conversely, the outlet temperature T-A can be increased by reducing the feed rate. While the flow of air and dispersion remains the same, ΤΑ rises and falls to the same extent as T-E changes.

Under these conditions, either no deposit on the wall or only a small deposit that forms can be easily removed on the spray tower.

Polymerisaattijauhe

The residual moisture of the polymer polymer powder after separation from the air stream is most often 0.1 to 7, preferably 0.3 to β 88172 5% by weight. The average size of the powder particles is 10-500, preferably 20-200 microns. They are hard and non-sticky. In the microscopic image, they appear at a magnification of 100 times as sharp-edged and translucent white-dark or clear particles, which can be explained by the more or less intense vitrification or melting of the latex particles.

Until now, vitrified powder particles have been considered unsuitable for latex redispersion. However, the dried powders according to the invention can surprisingly be redispersed by mixing with an aqueous-alkaline medium. The amounts of alkali in the aqueous medium must remain less than the amounts of alkali required for complete neutralization of the carboxyl groups, otherwise the polymer precipitate would dissolve or the dispersion would strongly thicken. Crosslinked polymers do not dissolve and therefore often tolerate a higher amount of alkali than non-crosslinked ones. Once the carboxyl groups have been partially neutralized before spray drying, the powder can also be redispersed in pure water or neutral aqueous media. Redispersion results in a film-forming latex in which the polymer is largely present again as the original latex particles. It can be used in a known manner for the preparation of drug coatings. Alkali-soluble coatings can also be prepared from these secondary dispersions for other applications. In addition, the powders prepared according to the invention are suitable as thickeners for alkali-aqueous media.

EXAMPLES

Preparation of the dispersion In a A. Witt vessel equipped with a reflux condenser, a stirrer and a feed vessel, 56 g of ammonium peroxide sulfate and 840 g of the sodium salt of an adduct of 840 g of triisobutylphenol and 7 moles of ethylene oxide were dissolved in 56 g of water at 80 ° C. . A monomer mixture prepared in advance from 12 kg of acrylic acid ethyl ester, 12 kg of methacrylic acid and 48 g of thioglycolic acid 2-ethylhexyl ester was added dropwise to the solution with stirring over 4 hours at 80 ° C. After the feed was stopped, the batch was kept at 80 ° C for an additional 2 hours, cooled to room temperature, and filtered through a fine-mesh filter cloth made of stainless steel.

A low viscosity, finely divided dispersion was obtained. MFT: 29 ° C; T: 129 ° C.

• ymax B. The procedure was the same as in Example A except that after cooling to room temperature, 6.4 kg of 5% NaOH solution was mixed into the dispersion and then the dispersion was filtered.

Drying the dispersion

A spray dryer equipped with a rapidly rotating spreading disc (20,000 rpm) through which hot air flowed 7 * downstream was used to dry the dispersion. The dry air flow rate was 400 m 2 / h.

The ratio of dispersion to air was adjusted so that the spray product exited the device at the desired air outlet temperature as a dry powder. Dry air inlet and outlet temperatures were varied; the results obtained in this case are summarized in a table.

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Redispersion of the powder In a 1.5-2 liter vessel, 600 g of water were each placed and 300 g of the powders obtained in Examples 1-4 were added portionwise with stirring. After a stirring time of about 5 to 10 minutes, 100 ml of 0.4% sodium hydroxide was added dropwise over 5 minutes with stirring and stirred for another 60 minutes. In all cases, stable fine, film-forming dispersions formed.

300 g of the powder obtained from dispersion B in Example 5 were added with stirring to 700 g of demineralized water and stirred for a further 30 minutes. The result was a stable, finely divided, film-forming dispersion.

Claims (8)

A process for preparing a redispersible plastic powder by drying a plastic dispersion having a lowest film formation temperature MFT below 60 ° C and containing a dispersible copolymer having a dynamic solidification temperature T (lambda-max) of 60 to 150 ° C, 20 to 60% by weight of acrylic and / or methacrylic acid units, 40 to 80% by weight of lower alkyl ester units of acrylic and / or methacrylic acid or a mixture thereof with styrene, and optionally up to 10% by weight of other unsaturated, radically polymerizable comonomers units, characterized in that the plastic dispersion is spray-dried at an inlet temperature TE lower than the dynamic solidification temperature 1a but higher than the MFT and at an outlet temperature TA lower than 65 ° C but higher than the MFT. Process according to Claim 1, characterized in that a plastic dispersion is used in which the carboxylic acid units are partly in the form of a salt. Process according to Claim 1 or 2, characterized in that the dry air and the sprayed dispersion are introduced downstream. Method according to one or more of Claims 1 to 3, characterized in that the proportion of dry air introduced into the spray-dried dispersion is adjusted so that the difference between the inlet and outlet temperatures (T-E to T-A) is at most 65 ° C. Process according to one or more of Claims 1 to 4, characterized in that a plastic dispersion is used whose lowest film-forming temperature (MFT) is at least 50 ° C below the dynamic solidification temperature. i3 881 72 Process according to one or more of Claims 1 to 5, characterized in that a plastic dispersion with a minimum film-forming temperature (MFT) of less than 50 ° C is used. Process according to Claim 6, characterized in that a plastic dispersion with a minimum film-forming temperature (MFT) of less than 40 ° C is used. Process according to Claim 7, characterized in that a plastic dispersion with a minimum film-forming temperature (MFT) of less than 35 ° C is used. i4 881 72