Colloid systems and wetting agents; subcombinations thereof; pro – Compositions containing an agent for breaking ; processes of... – Continuous liquid phase colloid system and discontinuous...
Reexamination Certificate
1999-06-21
2001-02-13
Lovering, Richard D. (Department: 1712)
Colloid systems and wetting agents; subcombinations thereof; pro
Compositions containing an agent for breaking ; processes of...
Continuous liquid phase colloid system and discontinuous...
C523S335000, C528S50200C, C528S936000
Reexamination Certificate
active
06187825
ABSTRACT:
The invention relates to a process for the continuous coagulation of aqueous graft rubber dispersions in an apparatus having at least one stator/rotor combination as a shear element, and an apparatus for this purpose.
It is known that the impact strength of hard thermoplastic polymers can be increased by mixing them with graft rubbers. The resulting polymer blends, for example as ABS or ASA polymers, play an important industrial role in the production of industrial plastics articles. Such graft rubbers which serve for toughening hard thermoplastics have long been known as impact modifiers in the plastics industry. They are graft copolymers in which a particle shell (P
2
) of monomers forming hard thermoplastics is grafted onto soft particle cores of rubbers (P
1
), such as elastomeric diene polymers or elastomeric acrylate polymers. The graft shell (P
2
) is prepared by polymerization or copolymerization of monomers or monomer mixtures forming the hard graft shell (P
2
) by mass, solution, suspension or emulsion polymerization in the presence of the soft elastomer or elastomers (P
1
). It is preferable to carry out the graft polymerization, at least in the final phase, as a polymerization in aqueous emulsion, anionic emulsifiers preferably being present. The preparation of the graft rubbers thus gives a large amount of aqueous graft polymer dispersions which can be worked up to give the solid graft polymers.
As disclosed, for example, in DE-A 3149358 or EP-B 459161, the aqueous graft rubber dispersions are as a rule precipitated by the addition of coagulants, generally used coagulants being aqueous solutions of water-soluble inorganic or organic acids and/or salts thereof, such as alkali metal and alkaline earth metal chlorides, sulfates or phosphates, eg. calcium chloride or magnesium sulfate solutions. The coagulation or precipitation is generally carried out batchwise but may also be effected continuously (EP-B 459161). The precipitated coagulums which have been filtered off are then washed and dried in a known manner. A known disadvantage of precipitation by the addition of acids or salts as coagulants is that impurities often remain in the worked-up polymers and may impair the product properties.
DE-C 2917321 discloses a process for isolating polymers which have a softening range above 100° C. from an aqueous emulsion, the aqueous emulsion being coagulated in an extruder by shearing and/or heating to temperatures above the softening range of the polymers, the coagulum then being fused and being discharged in a hot state under pressure from the extruder. The water is then separated off in a subsequent process step. The process is very energy-consumptive and requires a counter-rotating intermeshing twin-screw extruder for the precipitation. Moreover, ammonium acetate is used as an assistant for accelerating the coagulation.
U.S. Pat. No. 3,821,348 describes a process in which mixtures of from 70 to 80% by weight of acrylonitrile (AN) and from 20 to 30% by weight of methyl acrylate (MA) are polymerized in emulsion with subsequent addition of 12% by weight of butadiene or in the presence of 9% by weight of butadiene/acrylonitrile rubber (based in each case on the sum of the amounts of AN+MA). The resulting acrylonitrile copolymer dispersions or graft polymer dispersions are coagulated to a paste in a Waring mixer as the shearing apparatus and then shaped in an extruder, through a fine die, the thin rods which are introduced into hot water. The product is then washed, dried and processed in a compression mold at 150° C. to give rods. The description of the process is limited to copolymers having a high acrylonitrile content and very low content of elastomeric butadiene/acrylonitrile rubber.
In U.S. Pat. No. 4,668,738, which refers to the prior art of the above-mentioned U.S. Pat. No. 3,821,348, it is stated that aqueous dispersions having a rubber content of more than 50% by weight of the solids content give readily processible products only when dispersions of styrene/acrylonitrile copolymers as a hard component, in amounts which reduce the rubber content of the copolymer blend to, preferably, less than 50% by weight (based on the total solids content), are mixed with the dispersions before their coagulation by shearing. A disadvantage of the process is that graft rubber dispersions which are prepared for toughening thermoplastics generally have a rubber content of more than 50% by weight of the solids content and can therefore be processed after the process only with the addition of dispersions of the hard component, ie. with a change of the overall composition of the polymers.
It is an object of the present invention to provide an economical process for the continuous coagulation of aqueous dispersions of graft rubbers which are suitable for toughening hard thermoplastics in the preparation of ABS and in particular ASA polymers, which process can be carried out without the addition of chemical coagulants and without the addition of other polymer dispersions, so that the polymers in the coagulated emulsions have the same overall composition as the polymers in the starting emulsions. Furthermore, in the process dispersions of graft rubbers having a rubber content of more than 50% by weight, based on the solids content, should also be capable of being coagulated to give moist free-flowing powders or moist sludge which can readily be worked up, without them having to be mixed with hard acrylonitrile copolymer dispersions prior to coagulation.
We have found that these objects are achieved in general if the graft rubber dispersions are transported through an apparatus having at least one shear element which has a circular slotted stator and a slotted rotor rotating within the stator, and are thereby subjected to the action of shearing sufficiently strong for coagulation of the dispersion by shearing.
The present invention therefore relates to a process for the continuous coagulation of aqueous dispersions of graft rubbers (P) which are suitable for toughening thermoplastics and
a) contain, as soft grafting base (P
1
), elastomeric polymers and/or copolymers of 1,3-dienes or acrylic esters having a glass transition temperature T
g
of less than −10° C. in an amount of from 30 to 85% by weight of the amount of graft rubber (P) and
b) contain a hard one-shell or multishell graft shell (P
2
) comprising monomer units which form a thermoplastic polymer or copolymer having a glass transition temperature T
g
of more than +50° C. and whose content of acrylonitrile and/or methacrylonitrile monomer units is from 0 to 45% by weight, and
c) at least in the last stage of the graft polymerization, were prepared by emulsion polymerization of the monomers for the graft shell (P
2
) in the presence of the grafting base (P
1
),
wherein the graft rubber dispersion is transported through an apparatus having at least one shear element with a stationary circular slotted stator and a slotted rotor which rotates within the stator and is mounted on a rotatably mounted drive shaft, so that the graft rubber dispersion introduced into the apparatus is passed radially from the inside to the outside as a result of the rotation of the rotor in the shear element and, during or after passage through the slots of the rotor and stator, is subjected to shearing which is so strong that coagulation of the graft rubber dispersion takes place. The present invention furthermore relates to the use of suitable apparatuses for the coagulation of graft rubber dispersions by shearing and to an apparatus which is modified by displacer elements and is intended for dispersing or coagulating dispersions.
In the novel process, the graft rubber dispersion, which is preferably introduced, in particular sucked, axially into the apparatus (
10
), is coagulated in shear elements (
13
,
14
,
15
) comprising stator/rotor combinations. Slotted rotors (
13
b,
14
b,
15
b
) are mounted on a rotatably mounted drive shaft (
12
) within the circular stators (
13
a,
14
a,
15
a
) which are generally firmly connected to th
Guntherberg Norbert
Hofmann Jurgen
BASF - Aktiengesellschaft
Keil & Weinkauf
Lovering Richard D.
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