CA1163033A

CA1163033A - Preparation of copolymer dispersions having a narrow particle size distribution, and exhibiting dilatant flow over a broad range of concentrations

Info

Publication number: CA1163033A
Application number: CA000381041A
Authority: CA
Inventors: Dieter Distler; Hans Wolf; Gerhard Welzel
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1980-07-05
Filing date: 1981-07-03
Publication date: 1984-02-28
Also published as: EP0043464A2; US4371636A; DE3163371D1; DE3025562A1; ATE7302T1; EP0043464B1; JPH036166B2; EP0043464A3; JPS5744614A

Abstract

PREPARATION OF COPOLYMER DISPERSIONS HAVING A
NARROW PARTICLE SIZE DISTRIBUTION, AND EXHIBITING
DILATANT FLOW OVER A BROAD RANGE OF CONCENTRATIONS
Abstract of the Disclosure A process for the preparation of copolymer dispersions having a narrow particle size distribution, and exhibiting dilatant flow over a broad range of concentra-tions, by emulsion copolymerization of from 1 to 10 percent by weight, based on total monomers, of .alpha.,.beta.-mono-olefinically unsaturated monocarboxylic acids and/or dicarboxylic acids with other mono-olefinically unsaturated monomers and with from 0 to 5 percent by weight of polyolefinically unsaturated monomers in the presence of conventional emulsifiers, dispersants and polymerization initiators, and in the presence or absence of regulators, wherein the monomers are copolymerized in aqueous emulsion, with simultaneous agglomeration but only partial fusion of the initially formed polymer particles, at a temperature near the glass transition temperature of the copolymer formed, and thereafter A) the copolymer content of the aqueous copolymer dispersion is brought to 35 to 55 percent by weight an the pH
to 3 to 5, or B) the water contained in the aqueous copolymer dispersion is entirely or partially replaced by a water-miscible liquid which does not dissolve or swell the copolymer.

Description

~ ~ 63033 PREPARATION OF COPOLYMER DISPERSIONS HAVING A
NARROW PARTICLE SIZE DI~TR~BUTION, AND EXHIBITING
DILAT~NT FLOW OVER A BR_AD RANGE_OF CON~ENTRATIONS
Background of the Invention 1. Field of the Inven-tion Copolymer dispersions are described as exhibiting dilatant flow if they thicken under the action of shearing forces, i.e , if their viscosity increases with increasing rate of shear, unaccompanied by a measurable time-dependence. This thickening under shear is reversible and isothermal. In a dispersion exhibiting dilatant -flow, a particular shearing force corresponds to a particular viscosity. This thickening under shear can be accompanied by an increase in volume, and it is this which is the origin of the description "dilatant", but such an increase need not necessarily occur.

2. Description of the P ior Art W.K. Bauer and F.A. Collins, in "Rheology" (Acad.
Press, Vol. 4, page 724 et seq.) describe many examples of dilatant systems and mention, as causes for their dilatancy, reasons which are in part contradictory. For example, they give, as reasons, both particularly high stability of the systems and particularly low stability of the systems, and both aggregation under shear and localized drying-out. The dilatant systems of the prior art are either dilatant only over a vèry narrow range of solids content, for example, from 59.5 to 61 percent by weight, or have inadequate shelf life, for example in the case of certain pigment disper-~ 7~30~

.
sions. Furthermore, in many cases, thickening only occursat very high shearing rates.
British Patent 1,217,446 discloses a dilatant latex which is obtained by emulsion polymerization. The polymerization must be carried out in the presence oE a polyhydric alcohol which is water-soluble but is insoluble or virtually insoluble, in the latex polymer product. The latex is intended to be used for coating agents. It is stated that the latex is prepared from mono-olefinically unsaturated esters of 4 to 7 carbon atoms together with mono-olefinically unsaturated esters of 5 to 13 carbon atoms and from 1 to 5 percent by weight, based on total monomers, of an al~phatic mono-olefinically unsaturated carboxylic acid of 3 or 4 carbon atoms. However, the latices obtained by this process contain coagulant and, after filtration, fail to show dilatant behavior up to a shearing rate of 500 sec~l _u~mary of the _nvent _n We have found that copolymer dispersions having a narrow particle size distribution and exhibiting dilatant flow over a broad range of concentrations can advantageously be prepared by emulsion copolymerization of from 1 to 10 percent by weight, based on total monomers, of ~,~-mono oleEinically unsaturated monocarboxylic acids and/or dicarboxylic acids with other mono-olefinically unsaturated monomers and with from 0 to 5 percent by weight of poly-olefinically unsaturated monomers in the presence of ~;~63~3 conventional emulsifiers, dispersants and polymerization initiators, and in the presence or absence of regulators, if the monomers are copolymerized in aqueous emulsion, with simultaneous agglomeration but only partial fusion of the initially for~ed polymer particles, which have a particle diameter of from 5 to 200 nm, at a temperature near the glass transition temperature of the copolymer formed, and thereafter A) the copolymer content of the aqueous copolymer disperison formed is brought to 35 to 55 percent by weight and the pH to 3 to 5, unless these values already obtain, or B) the water contained in the aqueous copolymer dispersion formed is entirely or partially replaced by a water-miscible liquid which does not dissolve or swell the copolymer.
In a preferred embodiment, the emulsion copolymer-izatin of the process mentioned above is carried out with (a) from ~ to 10 percent by weight of the ~
mono-olefinically unsaturated monocarboxylic acid and/or dicarboxylic acid, (b) from 0 to 98 percent by weight of acrylic acid esters and/or methacrylic acid esters of alkanols of 1 to 4 carbon atoms, (c) from 0 to 98 percent by weight o styrene and~or methyl methacrylate, and ~ ~ ~3Q33 (d) from 0 to 3 percent by weight of polyolefini-cally unsaturated monomers, the amount of the monomers (b) and (c) together being from 90 to 98 percent by weight, at 60 to 95C, in the presence of from 0 to 0.5 percent by weight of an anionic emulsifier and from 0 to 1 percent by weight of a non-ionic emulsifier, the percentages being based on the amount of monomers.
Description of the Preferred Embodiment In the novel process, the ,~-mono-olefinically unsaturated monocarboxylic acids and/or dicarboxylic acids are preferably employed in amounts of from 2 to 10 percent by weight based on total monomers. Suitable carboxylic acids are especially monocarboxylic acids and dicarboxylic acids of 3 or 4 carbon atoms, e.g., acrylic acid, meth-acrylic acid, maleic acid, itaconic acid and fumaric acid as well as monoesters of such dicarboxylic acids with alkanols, generally of 1 to 4 carbon atoms, e.g., monomethyl maleate, monoethyl fumarate and n-butyl itaconate. The monocarboxylic acids and dicarboxylic acids (including the half-esters) of the stated type in general are of 3 to 9 carbon atoms. Acrylic acid, methacrylic acid and maleic acid are of particular interest. The other mono-olefini-cally unsaturated monomers to be used can be, quite generally, those which give water-insoluble homopolymers.
Monomers of this type are, in particular, mono-olefinically unsaturated carboxylic acid esters of 1 to 4 carbon atoms, especially esters of acrylic acid and methacrylic acid with 9 ~ 63033 alkanols of 1 to 10, especially 1 to 4 carbon atoms, as well as vinyl esters of straight-chain or branched aliphatic carboxylic acids the acids generally being of 2 to 12 carbon atoms. Examples of such olefinically unsaturated esters are, in particular, the esters of acrylic acid and methacrylic acid with methanol, ethanol, n-propanol, n-butanol, isobutanol, tert.-butanol and 2-ethylhexyl alcohol, especially ethyl acrylate, ethyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate and methyl methacrylate. Suitable vinylesters are, in particular, vinyl acetate, vinyl propionate and vinyl n-butyrate. Styrene is also of great interest as a mono-olefinically unsaturated monomer. Acrylonitrile can also be used as a comonomer, in amounts of up to 10 percent by weight, based on total monomers. Other suitable mono-olefinically unsaturated monomers are vinyl halides, e.g., vinyl chloride and vinylidene chloride. By way of example, a monomer mixture of particular interest for the preparation of the dilatant dispersions comprises (a) from 2 to 10 percent by weight of the ~,~-mono-olefinically unsaturated monocarboxylic acid and/or dicarboxylic acid (b) from 0 to 98 percent by weight of acrylic acid estes or methacrylic acid esters o~ alkanols of 1 to 4 carbon atoms, (c~ from 0 to 98 percent by weight of styrene and/or methyl meth-acrylate and (d) from 0 to 3 percent by weight of poly-olefinically unsaturated monomers, the amount of the monomers (b) and (c) together being 90 to 98 percent hy ~ ~ ~3033 weight. Such a mixture can advantageously be copolymeriæed at Erom 60 to 95C in aqueous emulsion in the presence of ~rom 0 to 0.5 percent by weight of anionic emulsifiers and from 0 to 1 percent by weight of non-ionic emulsifiers, the percentages being based on the amount of monomers.
The conventional emulsifiers and dispersants can generally be used for the process, for example, alkali metal salts and ammonium salts of C12-C18 alkylsulfonates and of C12-C18 alkyl-sulfates, oxyalkylated,e specially oxy-ethylated and/or oxypropylated r alkylphenols where alkyl isof 8 to 12 carbon atoms, or oxyalkylated fatty alcohols of 12 to 18 carbon atoms, the products containing from 5 to 50 alkylene oxide radicals per molecule. The polymerization initiators employed are those conventionally used for emulsion polymerizations; examples of suitable initiators are ammonium salts and alkali metal salts of peroxydi-sulfuric acid, hydrogen peroxide and cumene hydroperoxide.
In general, it is not necessary to employ regulators in carrying out the process, but in some cases the use of small amounts of conventional regulators, for example, tert.-dodecylmercaptan, can be advantageous.
The novel emulsion copolymerization process is intended to be carried out in such a way that the primary polymer particles first formed partially fuse with one another when they have a particle diameter of from 5 to 200 nm. This is a special type of agglomeration of the copolymer particles first formed,and is made possible by

3 0 3 ~

keeping the polymeriæation batch, during this s~age, near the glass transition temperature of the copolymer Eormed.
In general, the temperature can, for this purpose, be up to about 10C above or below the glass transition temperature, i.e., the polymerization temperature range is about +10C
around the glass transition temperature of the copolymer formed. The degree of permitted deviation from the latter temperature depends on the viscosity of the particular copolymer particles and can be the greater, the higher the viscosity of the copolymer particles first formed. The choice of temperature is intended to ensure that complete fusion of the copolymer particles first formed does not take place. (The glass transition temperature of the copolymer system can easily be calculated from teh known glass transition temperatures of the homopolymers, with due allowance ~or the fact that the glass transition temperature of the polymer particles formed are lowered by the monomer present. Glass transition temperatures are to be found, for examples, in the Polymer Handbook by Brandrup and Immergut (Interscience Publ., New ~ork, N.Y., 1967). Accordingly, the emulsion copolymerization carried out in accordance with the invention have a very uneven irregular surface. This shape of the particles remains preserved even iE further monomers are polymerized in a dilatant polymer dispersion prepared according to the invention.
The conditions for the agglomeration of the copolymer particles first formed are known per se. In the ~ ~ 63~33 process according to the invention, the content of mono-olefinically unsaturated monocarboxylic acids and/or dicarboxylic acids is one of the factors. As an example of an additional measure, the amount of emulsiEier used in carrying out the emulsion copolymerization can be kept relatively low. It is also possible to add, during the copolymerization, additives which cause agglomeration, for example, copolymers of ethylene oxide and propylene oxide, having cloud points near the polymerization temperature, or high molecular weight oxidized polyethylene oxides.
If the emulsion copolymerization of the monomers is initiated in a conventional manner, particles of diameter from 5 to 200 nm are first formed in the course of 50 to 80 percent of the total polymerization time or, if an emulsion feed process is employed~ after the addition of 50 to 80 percent of the monomer emulsion to the aqueous phase. A
particularly marked dilatant behavior is exhibited by polymer disperions~ prepared by the novel process, in which the copolymer particles first formed have a particle diameter of 50 to 100 nm when they are about to undergo partial fusion. The liyht transmission (light scattering) measured during the copolymerization can serve as a measure of the particle diameter of the primary copolymer particles.
~ The aqueous copolymer emulsions prepared in the manner described above are only dilatant, per se, if their copolymer content is 35 to 55 percent by weight, based on `~ ~ 63033 the dispersion, and their pH is less than 5. If, on the other hand, the copolymer content or the pH are outside these ranges, the content must be adjusted by adding water or by concentrating, and the pH must be adjusted by addition oE dilute alkalis, if a dilatant aqueous copolymer dispersion is to be obtained.
If, however, it is preferred - and this is advantageous for certain types of application - to prepare a non-aqueous copolymer dispersion exhibiting dilatant flow, it is not necessary to adjust the concentration range and pH
range and instead the water contained in the aqueous copolymer dispersion can be partially or completely replaced by a water-miscible liquid which does not dissolve or swell the copolymer. Advantageous liquids of this type include formamide, ethylene glycol, diethylene glycol, triethylene glycol and glycerol. Amongst these, ethylene glycol and diethylene glycol are preferred. ~fter having added the amount of such liquid required to adjust the solids content, the water can be completely or partially removed by evapora-tion, leaving a dispersion which exhibits dilatant flow.
Finally, it is also possible to remove the water,by freeze drying or spray drying, from the aqueous copolymer dispersion first prepared and then to disperse the resulting copolymer powder in a liquid which does not dissolve or swell the copolymer. This, however, is only possible if the glass transition temperature of the copolymer is substantially above room temperature and above the drying temperature used.

_g_ ~ ~ B3033 Using the novel process, dilatant copolymer dispersions having a narrow particle size distribution are obtained in these dispersions, the ratio of the weight-average of the particles to the number-average should in general be from 1 to 1.2, especially from 1 to 1.1. The copolymer content of the novel dispersion should preferably be from 40 to 55 percent by weight. For most applications, the monomer composition is chosen to give a coolymer having a glass transition temperature of above 50C. By addition-ally using polyolefinically unsaturated monomers (d) -suitable monomers are, in particular, ethylene glycol diacrylate and dimethacrylate, butane-1,4-diol diacrylate and dimethacrylate, trimethylolpropane triacrylate and trimethacrylate and divinylbenzene - coalescence of the agglomerating particles is made more difficult, since crosslinks are introduced into the polymer particles.
Instead of the polyole~inically unsaturated monomers of the stated type, it is also possible, in some cases, to employ monomers which additionally to an olefinic double bond contain another reactive group. Examples of such comonomers are N-methylolacrylamide, N-methylolmethacrylamide, glycidyl acrylate and glycidyl methacrylate.
The degree to which a copolymer dispersion prepared by the process according to the invention can be thickened by shearing depends on its solids content. As the solids content increases, the critical shearing rate, i.e., the minimum shearing rate at which the viscosity of the 3 ~ 3 3 dispersion increases under shear, decreases. In general, the range of critical shearing rates of the copolymer dispersions obtained by the novel process is from 0.5 to 500 sec~l The copolymer dispersions prepared by the novel process are very stable, have a good shelf life, and exhibit highly dilatant flow. They can be used, for example, in the production of sports shoe pads, especially ski boot pads, hydraulic clutches shock-absorber fillings and speed governors, as well as for armoring of vehicles and for filling orthopedic cushions.
In the Examples which follow, parts and percentages are by weight.

~ ~ 63033 Example 1 420 parts of water are mixed in a receiver with 140 parts of feed I and 18 parts of feed II and the mixture is heated, with stirring, to 85C, at which temperature the polymerization starts. The remainder of feeds I and II is then run in uniformly in the course of 3 hours, and polymerization is subsequently continued at ~5C.
Feed I: 450 parts of styrene, 500 parts of tert.-butyl acrylate, 20 parts of maleic acid, 30 parts of methacryl~
amide, 3~8 parts of water, 15 parts of sodium lauryl-sulfonate (20 percent strength aqueous solution) and 25 parts of a conventional 25:1 adduct of ethylene oxide with isooctylphenol (20 percent strength aqueous solution).
Feed II: 7 parts of potassium peroxydisulfate and 180 parts of water.
The resulting dispersion, while still hot, is - diluted to a solids content of about 40 percent and brought to a pH of from 3 to 4.5 by adding ammonia.
Instead of diluting with water and adjusting the pH, it is also possible to dilute with 1,000 parts of ethylene glycol and then to distill off all or part of the water, if appropriate, under reduced pressure.
In both cases, a copolymer dispersion having a narrow particle size distribution and exhibiting highly dilatant flow is obtained.
At room temperature, the critical shearing rate D it f the dispersion depends on the pH of the g ~ 63033 latter, the values for a 48 percent strength aqueous dispersion being as follows:

Dcrit (sec ) p~
-3.8 3.85 180 4.32 320 4.46 500 4.5 The critical shearing rate of the 48 percent strength aqueous dispersion, at pH 4.24, increases with temperature as follows:

Dcrit (sec ) TC

220 ` 25 Example 2 A mixture of 24 parts of water, 7 parts of feed I

and 1 part of feed II is heated to 80C in a receiver, and kept at this temperature for 15 minutes (to start the polymerization). The remainder of feeds (I) and (II) is then ad~ed in the course of 2-1/2 hours, after which polymerization is continued for a further hour at 80C.

Feed I: 40 parts of styrene, 6 parts of ethyl acrylate, 1.5 parts of acrylic acid, 1.5 parts of methacrylamide, 1 part -13~

of maleic acid and 0.375 part of sodiumlauryl-sulfate (40 percent strength aqueous solution).
Feed II: 8 parts of water and 0.35 part of potassium peroxydisulfate.
After completion of the polymerization, the procedure described in Example 1 is followed.
The 50 percent strength diethylene glycol-contain-ing dispersion obtained after replacing the water by diethylene glycol by the second procedure of Example 1 has a low viscosity; its critical shearing rate is about 5 sec~
at room temperature and 100 sec~l at 50C~
Example 3 The procedure described in Example 2 i5 followed, except that 0.5 part of butane-1,4-diol diacrylate is added to feed I when half of this feed has already been added to the polymerization batch. Thereafter, the procedure described earlier is followed, and a dilatant copolymer disperisons is obtained.
The diethylene glycol-containing dispersion obtained, at a polymer content of 45 percent, has a critical shearing rate of 15 sec~l at room temperature, 200 sec~l at 40C and 480 sec~l at 50C.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the preparation of copolymer dispersions having a narrow particle size distribution, and exhibiting dilatant flow over a broad range of concentra-tions, by emulsion copolymerization of from 1 to 10 percent by weight, based on total monomers, of .alpha. .beta.-mono-olefinically unsaturated monocarboxylic acids or dicarboxylic acids or a mixture thereof, with other mono-olefinically unsaturated monomers and with from 0 to 5 percent by weight of polyolefinic-ally unsaturated monomers in the presence of conventional emulsifiers, dispersants and polymerization initiators, and in the presence or absence of regulators, wherein the monomers are copolymerized in aqueous emulsion, with simultaneous agglomeration but only partial fusion of the initially formed polymer particles, which have a diameter of from 5 to 200 nm, at a temperature near the glass transition temperature of the copolymer formed, and thereafter A) the copolymer content of the aqueous copolymer dispersion formed is brought to 35 to 55 percent by weight and the pH to 3 to 5, unless these values already obtain, or B) the water contained in the aqueous copolymer dispersion formed is entirely or partially replaced by a water-miscible liquid which does not dissolve or swell the copolymer.

2. A process as claimed in claim 1, wherein the emulsion copolymerization is carried out with (a) from 2 to 10 percent by weight of the .alpha.,.beta.-mono-olefinically unsaturated monocarboxylic acid or dicar-boxylic acid or a mixture thereof, (b) from 0 to 98 percent by weight of acrylic acid esters or methacrylic acid esters of alkanols of 1 to 4 carbon atoms or a mixture thereof, (c) from 0 to 98 percent by weight of styrene or methyl methacrylate or a mixture thereof, and (d) from 0 to 3 percent by weight of poly-olefinically unsaturated monomers, the amount of the monomers (b) and (c) together being from 90 to 98 percent by weight, at 60 to 95°C, in the presence of from 0 to 0.5 percent by weight of an anionic emulsifier and from 0 to 1 percent by weight of a non-ionic emulsifier, the percentages being based on the amount of monomers.