Process for the preparation of polymer powders by spray drying

A process for the preparation of polymer powders from polymer dispersions by feeding a polymer dispersion continuously into an atomization apparatus and separately therefrom but at the same time also introducing a liquid agent, the dispersion and the liquid agent being completely mixed within at the most one second and the mixture being immediately thereafter dried with evaporation of the volatile constituents to form a powder composed of particles having an average diameter of from 0.05 to 1 mm, and the polymer powders prepared by this process.

The present invention relates to a process for the preparation of polymer 
powders by atomization drying, in which liquid agents are mixed with the 
dispersion of polymer immediately before atomization and the mixture is 
dried immediately thereafter, and to polymer powders obtained by this 
process. 
Polymers may be obtained in powder form from dispersions (the term is 
intended to include emulsions) of polymers serving as industrial plastics, 
in particular thermoplastic moulding compounds, by spray drying. The 
powder obtained may then be processed to form mouldings, for example by 
conventional thermoplastic processes. 
With this method of processing polymer dispersions, purification of the 
polymer is not possible and consequently auxiliary substances and 
impurities (from the polymerisation) remain in the end product. 
Dispersions of polymers are frequently alkaline or acidic and the basicity 
or acidity remains in the end product. Corrective additives such as acids, 
bases, salts, solvents or reagents cannot be introduced into the polymer 
dispersion prior to spray drying because they would alter the dispersion 
to such an extent (e.g. by coagulation, gelling, viscosity changes or 
creaming up) that spray drying would become virtually impossible. 
A way has now been found of adding liquid agents to these polymer 
dispersions and subsequently subjecting the dispersions to atomization 
drying. 
The polymer powders so obtained have considerably improved physical 
properties, in particular those important in thermoplastic moulding 
compounds. 
A process for the preparation of polymer powders from polymer dispersions 
has thus been found, wherein a polymer dispersion is continuously fed into 
an atomization apparatus and the liquid agent is introduced simultaneously 
but separately, the dispersion and the liquid agent are completely mixed 
within at the most one second, and immediately thereafter the mixture is 
dried with evaporation of the volatile constituents to form a powder 
composed of particles having an average diameter of from 0.05 to 1 mm. 
The invention further relates to the polymer powders thus obtained. 
The polymer dispersions may be aqueous or organic dispersions, in 
particular dispersions in alcohols, esters, ketones or aromatic 
hydrocarbons. Aqueous dispersions are preferred. The liquid agents are 
preferably inorganic or organic acids such as sulphuric acid, hydrochloric 
acid, phosphoric acids, acetic acid, propionic acids, carboxylic acids of 
benzene, stearic acids, palmitic acid, maleic acid and formic acid: or 
inorganic or organic salts such as halides, sulphates, acetates, formates, 
benzoates, nitrates, nitriles or stearates of alkali metal or alkaline 
earth metals, and alcohols, epoxides, anhydrides or amides. If these 
agents are liquid, they can be introduced as such or in the form of 
solutions, dispersions or suspensions in suitable media; solid agents can 
of course only be used as solutions, dispersions or suspensions. 
The liquid agents are preferably mixtures of salts and mineral acids and/or 
C.sub.1 -C.sub.8 carboxylic acids, preferably C.sub.1 -C.sub.3 carboxylic 
acids, especially if they are pH buffer mixtures. Agents which normally 
coagulate or cream up the polymer dispersions are particularly preferred. 
Polymers of olefinically unsaturated monomers which are thermoplastic are 
particularly suitable for the purpose of this invention. 
Preferred polymers are those of styrene, p-methylstyrene, 
.alpha.-methylstyrene, halogenated styrenes, acrylonitrile, 
methacrylonitrile, vinyl chloride, vinyl acetate, C.sub.1 -C.sub.8 
-methacrylic acid esters, maleic acid anhydride, maleic acid esters, 
maleic acid imides, C.sub.1 -C.sub.8 -acrylic acid esters, ethylene, 
propylene, butadiene, butene-1, isoprene and chloroprene and copolymers 
and graft copolymers of these monomers. 
The following polymers are preferred: Resinous polymers, graft polymers on 
rubbers, in particular partially cross-linked, particulate graft rubbers, 
and mixtures thereof with particulate cross-linked rubbers. 
Graft polymers in aqueous emulsion are particularly preferred. These graft 
polymers consist of a rubber component, preferably diene or acrylate 
rubber, and resin polymers of styrene. .alpha.-methylstyrene, methyl 
methacrylate, acrylonitrile or vinyl acetate grafted on the rubber. 
Particularly preferred graft polymers have rubber contents of from 5 to 
80% by weight, in particular from 10 to 80% by weight. Emulsions of these 
graft polymers used for the process according to the invention contain 
particles having an average diameter of from 0.06 to 3 .mu.m, in 
particular from 0.08 to 0.9 .mu.m, determined by light scattering 
measurements. 
Preferred emulsions are distinctly alkaline or distinctly acid and are 
"neutralized" in the process according to the invention by acids or bases 
used as agents or they are changed in their pH or their conductivity (e.g. 
by the addition of electrolyte). 
Emulsions with pH values from 8 to 12 treated with carboxylic acid and 
optionally additional mineral acid salts in the process according to the 
invention to reduce the pH of the polymer powder to below 7 are 
particularly suitable. 
Suitable atomization apparatus for the process according to the invention 
include, for example, rotary atomization discs or plates, single material 
and multimaterial nozzles with or without auxiliary supply of energy (e.g. 
mechanical vibration) and mixing nozzles with internal or external mixing. 
Mixing nozzles designed for intensive mixing of the liquid components are 
particularly suitable. Such intensive mixing may be achieved if a rapidly 
rotating liquid film of one component is encountered by a rotating liquid 
film or jet of the second component inside a nozzle and the mixture is 
discharged through a nozzle bore immediately after mixing and is broken up 
into fine droplets as a result of the rotation. In nozzles designed for 
external mixing, the participating liquids do not meet in the form of 
sprays until they have left the nozzle, and the then undergo mixing. 
Another method of mixing the liquids consists of first mixing them in 
known apparatus (e.g. rotary mixing chambers) and immediatetly thereafter 
breaking them up in known atomization apparatus such as rotation, 
ultra-sound, pressure twisting or pneumatic atomizers. Processes using 
atomization apparatus are well known (see K. Masters: Spray-Drying; 
Leonard Hill, London, 1972). Feeding means supplying a stream of liquid 
into the atomization apparatus described above by free inflow from a 
pressurized container or by means of suitable pumps by way of tubes or 
pipes. 
The use of mixing nozzles enables average dwell times of less than 0.1 sec 
to be achieved but higher dwell times may occur in mixing apparatus used 
before the atomization apparatus. Dwell times of at the most one sec are 
achieved in the process according to the invention by using a mixing 
nozzle. 
After atomization, the polymer is dried by vaporization in a hot gas stream 
or evaporation in a stream of super-heated steam. A preferred method is 
described below: 
A polymer in the form of an aqueous emulsion is supplied to a mixing nozzle 
with internal mixing device. The liquid agent consists of a solution of an 
inorganic salt and/or an organic acid which is added to the emulsion in 
the mixture nozzle and mixed with the emulsion. Both liquids are 
introduced into the nozzle by means of pumps in which pressures of from 10 
to 30 bar occur upstream of the nozzle. After intensive mixing, the 
mixture leaves the nozzle, e.g. in the form of a hollow spray cone which 
enters the stream of hot gas or steam of a spray drying tower. At inlet 
temperatures of the hot gas of from 50.degree. to 400.degree. C., in 
particular from 100.degree. to 200.degree. C., and outlet temperature of 
from 50.degree. to 200.degree. C., in particular from 70.degree. to 
150.degree. C., the polymer dries to a powder which has particularly 
advantageous properties for technical application without requiring 
further purification.

The polymers prepared according to the invention are improved in their 
thermoplastic processing characteristics and thermostability compared with 
conventional powders prepared by atomization drying. The advantageous 
properties are particularly effective when the improved powders are used 
for the modification of vinyl chloride polymers. 
EXAMPLES 
(I) Polymer emulsions put into the process 
Polymer emulsion A 
A styrene/butadiene rubber latex (SBR latex) containing a copolymer of 35% 
by weight of styrene and 65% by weight of butadiene is used as graft 
stock. The latex has a polymer content of 32.4% by weight and an average 
latex particle diameter (d.sub.50 value) of 120 nm. The copolymer has a 
gel content of 88% by weight. The latex was prepared by radical emulsion 
polymerisation at 60.degree. C., using potassium peroxy disulphate as 
initiator and the sodium salt of disproportionated abietic acid at pH 
values of from 11 to 12. 
The graft polymer is prepared in accordance with the following Table. 
TABLE 1 
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Parts by weight 
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Solution 1: 
SBR latex 1,723 
Water 1,140 
Solution 2: 
Potassium peroxy disulphate 
10 
Water 300 
Solution 3: 
.alpha.-Methylstyrene 
773 
Methylmethacrylate 1,243 
Acrylonitrile 224 
tert.-Dodecylmercaptan 
-- 
Solution 4: 
Water 3,000 
Sodium salt of disproportionated 
45 
abietic acid 
1N sodium hydroxide solution 
40 
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Solution 1 is introduced into a reactor. After it has been heated to 
70.degree. C. and initiated with Solution 2, Solutions 3 and 4 are added 
within 5 hours. After-polymerisation is then carried out for 4 hours at 
70.degree. C. After cooling to 20.degree. C., the latex is stabilized with 
1.2 parts by weight of phenolic antioxidants, based on 100 parts of graft 
polymer. 
Polymer Emulsion B 
Aqueous emulsion of a graft polymer of 30% by weight of acrylonitrile and 
70% by weight of styrene on a polybutadiene rubber (average particle 
diameter (d.sub.50) 0.38 .mu.m; gel content 88% by weight). The graft 
polymer contains 30% by weight of rubber and 70% by weight of 
styrene/acrylonitrile resin. The emulsion was prepared by radical emulsion 
polymerisation (initiated by means of potassium peroxy disulphate) of a 
styrene-acrylonitrile mixture on the polybutadiene at a pH of from 10 to 
12, using sodium salts of disproportionated abietic acids as emulsifiers. 
Polymerisation temperature: 60.degree. to 70.degree. C. 
(II) Working up 
General description of process 
The polymer emulsion is fed from a dosing pump into a mixing nozzle 
designed for internal mixing. The liquid agent consists of an aqueous 
solution of an inorganic salt and/or an organic acid. It is also 
introduced into the nozzle by a dosing pump at a fixed quantitative ratio 
to the polymer emulsion. The resulting mixture leaves the nozzle at the 
top of a spray drying tower in the form of a hollow spray cone composed of 
droplets measuring from 10 to 500 .mu.m in diameter. The drying medium 
used is air with reduced oxygen content (less tha 5%). This air is 
recirculated with the addition of nitrogen and enters the spray drier at 
temperatures T.sub.in of from 150.degree. to 200.degree. C. and leaves the 
drier at temperatures of T.sub.out of 50.degree. to 100.degree. C. In the 
spray drying tower, the polymer is dried from its initial water content of 
from 50 to 80% to a residual water content of from 0.05 to 2%, and during 
this process agglomeration and/or a chemical reaction may take place in 
the flying droplets to improve the physical properties of the polymer 
powder. 
(II1) Working up of polymer emulsion A by the process described above 
Air temperatures: T.sub.in =170.degree. C..+-.5.degree. C., T.sub.out 
=80.degree. C..+-.30.degree. C. The quantity of air is then approximately 
25 to 30 times the quantity of water to be evaporated. Emulsion A is 
supplied as main mass stream to an internally mixing nozzle (manufactured 
by Schlick, Model 770/1) by means of an eccentric screw pump while the 
bias pressure in the nozzle is about 15 bar. A solution of 30 g of 
magnesium sulphate per 5 g of acetic acid in 250 g of water is supplied as 
smaller liquid stream per kg of solids content of emulsion A by means of a 
correspondingly smaller dosing pump. On leaving the spray drier and 
separation from the air stream, the dried polymer powder has residual 
moisture contents of from 0.2 to 0.8%. 
(II2) Working up of emulsion B by the process described above 
Air temperatures, T.sub.in =160.degree. to 170.degree. C.; T.sub.out 
=80.degree. C..+-.3.degree. C. The quantity of air is approximately equal 
to 25 to 30 times the quantity of water to be evarporated. Emulsion B is 
supplied as main stream to an internally mixing nozzle of Schlick, Model 
770/1 by means of an eccentric screw pump, the bias pressure in the nozzle 
being approximatly 10 to 12 bar. 100 g of a 4% by weight solution of 
magnesium sulphate (MgSo.sub.4) in water per 1 kg of solids content of 
emulsion B are supplied to the mixing nozzle from a dosing pump. On 
leaving the spray drier and separation from the stream of moisture, the 
dried polymer powder has a residual moisture content of about 0.5%. 
(II3) Working up of emulsion B 
By a method analogous to that of process (II2) using 100 g of a 3% by 
weight solution of calcium formate per 1 kg of solids content in emulsion 
B. On leaving the spray drier and separation from the air stream, the 
dried powder has a residual moisture content of about 0.5% by weight. 
(II4) Process for working up emulsion B 
The process is analogous to that of II3, using an aqueous solution of 
aluminium sulphate. The residual moisture contents of the powder were in 
the range of from 0.3 to 0.5% by weight. 
(III) Comparison Examples 
Spray drying by known processes (for details see K. Masters, "Spray 
Drying", Leonard Hill, London, 1972) without the addition of electrolyte. 
The polymer emulsion is delivered to a pressure nozzle (one-material 
nozzle) or a pneumatic nozzle (atomization with compressed air) by means 
of a pump. The spray of droplets produced dries in a stream of hot gas in 
a spray drier and the stream of hot gas is recirculated with the addition 
of nitrogen. The droplet size is from 5 to 500 .mu.m. The air inlet 
temperature T.sub.in is from 150.degree. to 200.degree. C., and the 
temperature of the discharged air is from 50.degree. to 100.degree. C. The 
nozzle bias pressure is from 10 to 30 bar in pressure nozzles and from 1.5 
to 10 bar in pneumatic nozzles. 
(III1) Comparison of polymer emulsion A to (II1) by this process 
Polymer emulsion A is atomized in a pressure nozzle of DELAVAN, MODEL SDX, 
using a nozzle bias pressure of 25 bar. At T.sub.in .about.170.degree. C. 
and T.sub.out .about.80.degree. C., the quantity of air is about 25 to 30 
times the quantity of water to be evaporated. On leaving the spray drier 
and separation from the air stream, the dried polymer powder has residual 
moisture contents of from 0.2 to 0.8%. 
(III2) Comparison of polymer emulsion B to (II2) to (II4) by this process 
Polymer emulsion B is atomized with a pressure nozzle of DELAVAN, MODEL 
SDX, at a nozzle pressure of 18 bar. At T.sub.in .about.165.degree. C. and 
T.sub.out .about.80.degree. C., the quantity of air is about 25 to 30 
times the quantity of water to be evaporated. On leaving the spray drier 
and separation from the air stream, the dried polymer powder has residual 
moisture contents of about 0.5%. 
(IV) Properties of polymers as modifiers for polyvinyl chloride 
Products (II1) and (III1) were worked up into thermoplastic moulding 
compounds either directly or in admixture with PVC. 
For the preparation of PVC moulding compounds, polyvinyl chloride (PVC) 
(K-value 70) is used. 2% by weight of Ba/Cd laurate (solid) are added as 
stabilizer and lubricant and 0.3% by weight of sterically hindered 
phenolic anti-oxidant (solid and 0.2% by weight ester wax) are also added. 
The PVC moulding compounds are homogenized in admixture with (II1) and 
(III1) on mixing rollers for 10 minutes (at 180.degree. C.) and pressed to 
form moulded products at 190.degree. C. The composition of the moulding 
compounds is shown in Table 3. 
For comparison with product (III1), products (II1) are worked up into 
moulded bodies at 240.degree. C. The products having the following 
properties: 
TABLE 2 
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Product II1 
Product III1 
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Notched impact strength(kJ/m.sup.2) 
7.9 6 
Dimensional stability Vicat B(.degree.C.) 
111 110 
Cloudiness (%) 8 20 
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TABLE 3 
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Moulding compounds containing PVC: Mixtures of 60 parts by weight of PVC 
and 
40 parts by weight of graft polymer II1 and III1 
Blend with II1 with III1 
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Impact strength (kg/m.sup.2) 
unbroken unbroken 
Notched impact strength (kg/m.sup.2) 
6.5 6 
Dimensional stability (Vicat B, .degree.C.) 
95 93 
Cloudiness (%) 9 25 
Light transmission (%) 
70 55 
Raw tone colourless to pale yellow 
yellow to brown 
Processing of mixture 
moulding compound shows 
moulding compound 
no unwanted sticking to 
sticks to rollers 
rollers during processing 
during homogenization 
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Notes to Tables 2 and 3: 
Impact strength and notched impact strength determined according to DIN 5 
453 
Dimensional stability determined according to DIN 53 460 
Cloudiness and light transmission determined according to DIN 5 036 
TABLE 4 
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Measurement of "Window Fogging" of the powders 
(according to DIN 75 201) at 90.degree. C. 
Products Gloss measurement (%) 
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II2 88 
II3 87 
II4 78 
II2 0 
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