Process for modifying hydrophilic polymers

Details Process for modifying hydrophilic polymers Process for modifying hydrophilic polymers

1994-04-21

2001-10-09

Lipman, Bernard (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S328500, C525S329900, C525S330100, C525S382000, C525S384000, C525S385000

Reexamination Certificate

active

06300423

ABSTRACT:

The present invention relates to a process for modifying hydrophilic polymers by mixing with suitable modifiers.
Hydrophilic polymers can be modified with the aid of suitable additives. In particular, the surface properties of hydrophilic polymer particles can be changed, for example rendered hydrophobic or hydrophilic or compacted. It is also possible to modify the permeability, swelling behaviour, wettability, absorption capacity, etc.
DE-B 22 64 027 and DE 35 03 458 have already disclosed processes for mixing in modifying components, but these processes are unsatisfactory in mixing result and thus in the desired properties and require excessive technical complexity and excessive amounts of energy.
In one process (DE-B 22 64 027) an absorbent powder is intimately mixed with water and crosslinking agents in a twin-cylinder mixer, a so-called “ZIG-ZAG” mixer, or in a horizontal plough mixer, at liquid-addition times of 30 minutes. Agglomerates with a high degree of clumping, which are in some cases very moist, are produced and are dried in a subsequent drier and then treated thermally. It is apparent at this early stage that the high degree of clumping means that uniform distribution is not achieved. Furthermore, the thermal after-treatment causes losses in the effectiveness of the post-crosslinking agent within the agglomerates, in which excess concentrations occur; this is reflected in poor mixing of the remaining particles and an inadequate concentration of the post-crosslinking agent. Since strict requirements are made of the particle size distribution of the powder (for example 1%<100 &mgr;m and <0.5%>800 &mgr;m), the treated powder must be comminuted. The oversized particles which arise on screening are returned to the grinding step. The under-sized particles, which cannot be used further, are mixed with water in a further mixer to give granules, and the granules are dried, ground further to the required particle size distribution and screened.
The process of DE 35 03 458, in which the post-crosslinking agent is introduced into a cone-and-screw mixer and mixed in, also exhibits the crucial disadvantages of agglomerate formation and non-uniform distribution of the post-crosslinking agent on the particles.
In the cone-and-screw mixer, a vertical rotating screw which is parallel to the inclined wall rotates upward in the conical mixer housing which tapers downward, so that good mixing is ensured by the peripheral motion of the screw and the rotation of the screw on its own axis. In spite of this mixing, which is usually good, the above-described disadvantages also occur in this mixing process.
Accordingly, the object of the present invention is to provide a process which ensures homogeneous mixing of modifying components with pulverulent hydrophilic polymers in which the mixing is carried out within a very short time, so that the strong absorption behaviour of the polymer cannot have a disadvantageous effect as far as homogeneous mixing and uniform coating of the surface of the powder particles is concerned, i.e. the mixing should already have occurred before agglomerates can form due to the strong absorption capacity of the powder.
This object is achieved by a process for modifying hydrophilic polymers by mixing with a modifier, characterised in that the pulverulent polymer and the modifier are introduced continuously into a vertical, cylindrical mixer, mixed at a power of from 1,000 to 5,000 Wh/m
3
of powder by means of rotating blades at a blade rate of from 100 to 300 s
−1
at a residence time of 1-10 seconds, during which the powder is passed through zones having energy dissipation densities of 300-600 W/l of mixer volume.
The mixing is preferably carried out at a power of 1,500-3,000 Wh/m
3
of powder. The blade rate is preferably 250-300 s
−1
. The cutter heads are preferably seated on a rotating shaft which is mounted coaxially to the mixing tube, and extend with their blade ends to the tube wall, from which they are separated only by a small gap. From 1 to 3, particularly preferably 2, cutter heads, which are arranged one after the other on the shaft, are usually sufficient. Each cutter head can have a plurality of blades, preferably 6, which can be set differently on the head with respect to the flow direction. The preferred residence time is from 1 to 4 seconds. The process according to the invention thus operates at a preferred particle speed of 0.05-1 m/s during passage through the mixture.
The hydrophilic polymer to be modified by the process according to the invention is preferably fed to the mixer via a weight-metering device. It is preferred for the modifying component to likewise be metered by weight and added to the polymer just before entry into the mixer.
It is particularly advantageous if the modified component is liquid. It can be employed as the material itself, or as a solution, emulsion or dispersion. Thus, it may be expedient to carry out the process according to the invention at a temperature above the melting point of the modifying component.
In order to prevent baked deposits on the wall, the cylindrical wall of the mixer can comprise flexible material and be deformed by means of an externally applied roller cage, which is moved vertically up and down. An example of a suitable flexible material is rubber.
The liquid modifying component can be fed in via a tube, preferably via a nozzle (for example a pressure nozzle or two-substance nozzle) or an injector (for example a gas injector).
It is also possible by means of the process according to the invention to homogeneously mix small amounts of modifier with a comparatively large stream of polymer to be modified, so that the surface of the powder is uniformly coated and the powder does not need any further treatment at all with respect to the particle size distribution.
The mixed and uniformly coated powder can, if necessary, be treated in a subsequent continuous drier, in which the removal of the excess moisture and, if appropriate, a post-reaction can be carried out.
Examples of suitable driers are contact driers or fluidised bed driers. Fluidised bed driers are preferably equipped with heating panels or heating tubes in the fluidised bed.
The residence time of the modified polymer is preferably 10-60 minutes, particularly preferably 30 minutes. However, drying and any post-reaction can also be effected by high-energy radiation.
Examples of hydrophilic polymers which are suitable for the process according to the invention are homopolymers and copolymers of water-soluble monomer components, such as acrylic acid, methacrylic acid, crotonic acid, acrylamidopropanesulphonic acid, vinylphosphonic acid, maleic acid, vinylsulphonic acid and salts thereof, preferably alkali metal and ammonium salts. Further suitable monomer components are acrylamides, N-vinyl-amides, the hydroxyalkyl esters of acrylic and methacrylic acid, and basic and cationic monomer components, such as the basic esters and amides of acrylic and methacrylic acid, for example the esters of dimethylaminoethanol, or N-dimethylamino-propyl-methacrylamide, and dimethyldiallylammonium chloride or vinyl-imidazoline. If said polymers carry ionic groups, they may also have been fully or partially neutralised.
It is furthermore possible to employ graft polymers of said hydrophilic monomer components, for example on starch or polyalkylene oxides, in the process according to the invention. Examples of suitable polyalkylene oxides are compounds of the
in which R
1
and R
2
are H, alkyl, alkenyl or aryl, each of which is optionally substituted,
X is H or —CH
3
and
n is a number from 1 to 10,000.
Examples of suitable compounds are polyethylene glycols, poly-propylene glycols and copolymers and derivatives thereof, and graft matrices, as described in DE-A 39 11 433 and EP 391 108.
Preference is given to hydrophilic polymers which have been made from said monomers and have been crosslinked during the polymerisation by addition of polyolefinically unsaturated compounds. Polymers of this type are capable of absorbing aqeuous so

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