Drying of coagulated water-containing rubber particles

Details Drying of coagulated water-containing rubber particles Drying of coagulated water-containing rubber particles

2001-08-15

2003-02-25

Boykin, Terressa M. (Department: 1711)

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

Synthetic resins

Treating polymer containing material or treating a solid...

C528S487000, C528S50200C

Reexamination Certificate

active

06525172

ABSTRACT:

The invention relates to a process for the drying of coagulated water-containing rubber particles, in particular graft rubber particles.
Rubbers, or graft rubbers, are frequently prepared by homo- or copolymerization of the appropriate monomers in a liquid medium, using emulsion polymerization, miniemulsion polymerization, or microsuspension polymerization. The form in which the rubber is produced here is mostly that of an aqueous solids dispersion, from which the rubber has to be isolated if the dispersion is not intended for use as such. The isolation of the rubber from the dispersion usually takes place by coagulation. Many different processes are known for this purpose. A common feature of these processes is that the residual water then has to be removed from the coagulated particles. This mostly takes place in two or more steps. A first step separates the coagulated particles from the surface water by centrifuging, or removal by way of pressing, squeezing or filtration. The residual water included between the coagulated particles remains, and often represents more than 50% in the case of coagulated rubbers. Removal of this residual water can be achieved only by increasing the use of energy, and there is therefore mostly a subsequent drying step using heat, if desired combined with reduced pressure. This drying step is energy-intensive and time-consuming, since the particles have to be dried from the outside to the inside.
It is an object of the present invention, therefore, to provide a process which can dry water-containing rubber particles coagulated from the appropriate dispersions, and which can reduce the internal water content present between the particles and resulting from the coagulation process, without any major use of energy or time.
We have found that this object is achieved by a process in which the rubber particles are treated with an aqueous solution of any desired substances in monodisperse solution in water in such a way that, for at least some period, the surface of the rubber particles is entirely covered by the aqueous solution.
DE-A-196 47 599 has disclosed a process in which included residual water from, for example, suspension-polymerized styrene particles is removed by treating the styrene particles with an aqueous salt solution. This process can be carried out at from 20 to 105° C. It differs fundamentally from the process claimed here, since it removes internal or residual water from the styrene particles themselves, whereas this specification concerns residual water included between the rubber particles during their coagulation. The styrene particles, furthermore, are hard thermoplastics with a Tg of about 100° C., with internal water contents much lower than those of coagulated rubber particles. The residual water contents within the styrene particles are generally about 2% by weight, or, if they are very high, in the range somewhat above 5% by weight. In contrast, the residual water contents of the elastomeric rubber particles are above 20% by weight and may be up to 50% by weight or more.
Surprisingly, during development of the process of the invention in the field of rubber particles, it has been found that besides using treatment with aqueous salt solutions to reduce the residual water content it is also possible to use aqueous solutions of any desired substances, with the proviso that the substances are present in monodisperse solution in the aqueous solution. For the purposes of the present invention, substances present in monodisperse solution in water are substances which give a “genuine solution” with water.
Use is preferably made of an aqueous solution of substances selected from the group consisting of salts, alcohols, acids and sugars. Suitable salts here are those of inorganic acids, and salts derived from metals of the 1st to 3rd main group, of the 4th main group, except C and Si, of the 5th main group, except N and P, Ne and Ar of the noble gases, or else of the 1st to 7th transition group, i.e. halides, sulfates, nitrates, phosphates, or carbonates, in particular NaCl, MgSO
4
, (NH
4
),SO
4
, Al
2
(SO
4
)
3
, FeCl
3
, CaCl
2
, CoCl
2
, MgCl
2
, H
2
SO
4
, BaCl
2
, HCl, LiCl, K
2
SO
4
, HNO
3
, K
2
CrO
4
, NaBr, KOH, Na,
5
SO
4
, NH
4
Cl, NaCl, NaOH, H
3
PO
4
, sucrose, KCl, KBr, CsCl, KHCO
3
, isopropanol, CH
2
OHCH
2
OH, ethanol, D-glucose, Pb(NO
3
)
2
, D-fructose, CdCl
2
, MnSO
4
, CdSO
4
, CH
3
OH, NH
3
/NH
4
OH, NiSO
4
, CH
3
COCH
3
, CH
3
COOH, CuSO
4
, NH
2
CONH
2
and HCOOH. Particularly suitable salts are NaCl, MgSO
4
, (NH
4
)
2
SO
4
, Al
2
(SO
4
)
3
, FeCl
3
and KCl.
The concentration of the aqueous solution may be from 5% by weight to saturation, but preferably from 15 to 40% by weight, based in each case on the finished solution. At concentrations below 5% by weight, the time for drying has been found to be too long, while at excessively high concentrations there is a risk of salt precipitation if salt solutions are used. A general rule is that the drying of the rubber particles proceeds more rapidly as the concentration of the dissolved substance rises and as the temperature increases. According to the invention, the temperature here may be from about 20 to 200° C. Especially at elevated temperatures above 50° C., it should be noted that operations should be carried out at elevated pressure, and therefore in sealed vessels. The pressure here may rise to about 20 bar or higher. However, operations at low temperatures and lower pressures may be carried out using open vessels.
One preferred embodiment of the process of the invention begins by sintering the rubber particles after they have been coagulated. This agglomerates the coagulated particles and further enlarges them. The treatment with the aqueous solution is then carried out after this sintering.
The process of the invention may be carried out either continuously or batchwise, in various ways. An example of a batch procedure is one using stirred vessels. However, vessels with other mixing methods can also be used, the coagulated particles being introduced into these and mixed with the aqueous solution. It is necessary here that the polymer particles and the aqueous solution of the invention are kept in constant motion.
Particularly if the process is carried out continuously, an example of a form in which the particles are used is that of a solid packing introduced into a vertical tube. The aqueous solution of the invention is fed, for example applied by spraying, within the upper region of the tube. This solution then moves downward under gravity, or by pumping or pressure, wetting the particles. It can be collected at a lower point and, where appropriate, be reused for spraying after its concentration has been raised.
In another version of the execution of the process of the invention, both the rubber particles and the aqueous solution pass through the tube. Gravity alone may be sufficient for this purpose. The rubber particles may then be removed continuously from the lower region of the tube, for example of a tube in the form of a column. However, it is also possible for the rubber particles and the aqueous solution to be conducted in countercurrent to one another.
It is also possible for the rubber particles and the aqueous solution to be introduced into an essentially horizontal tube and to be mixed by way of at least one conveying screw, and transported through the tube. The single screw channel of the conveying screw can prevent blocking of the apparatus and at the same time provide narrow residence time distribution for the particles, giving uniform drying. Since the aqueous solution of the invention floods all of the screw channels in an arrangement of this type, there is certain to be wetting of all of the rubber particles, and this, too, is important for uniform drying.
Once the rubber particles have been treated with the aqueous solution of the invention, the particles may be washed with water until there are no longer any detectable residues of the substances dissolved in the aqueous solution.
The composition of water-containing rubber particle

Affiliated with

Also associated with

Rating

Drying of coagulated water-containing rubber particles does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Drying of coagulated water-containing rubber particles, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Drying of coagulated water-containing rubber particles will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3178397