Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2002-06-28
2004-11-30
Wyrozebski, Katarzyna (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S496000, C523S215000, C525S332600
Reexamination Certificate
active
06825262
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing carbon-black-filled rubber powders using rubbers prepared in and/or present in organic solvents, and to the resultant powders and their use in the rubber-processing industry.
2. Description of the Background
U. Görl and K. H. Nordsiek in
Kautsch. Gummi Kunstst
. 51 (1998) 250; and U. Görl and H. Lauer in
Gummi, Fasern Kunstst
. 53 (2000) 261 and R. Uphus, O. Skibba, R. H. Schuster and U. Görl in
Kautsch. Gummi Kunstst
. 53 (2000) 276 disclose the reasons for, and benefits from, the use of rubber powders, and also disclose possible processes for preparation of the rubber powders. The interest in pulverulent rubbers can be explained as an inevitable consequence of the processing technology used in the rubber industry, where rubber mixtures are prepared at high costs because of the time, energy and manpower needed to produce the rubber powders. The main reason for this is that crude rubber material is procured in the form of bales and the remaining constituents of the rubber material which is subject to vulcanization have to be incorporated into the rubber phase.
Comminution of the bale and intimate mixing with fillers, mineral oil plasticizers, and vulcanization auxiliaries takes place on rolls or in internal mixers, in a number of process stages. Between the stages, the mixture is generally cooled on a batch-off system, laid out in milled sheet form on pallets, and put into intermediate storage. Downstream of the internal mixers or rolls the rubber material is subjected to appropriate extrusion or calendering processes.
The only way to avoid this very complicated method of rubber processing is to use completely new process technology. For some time, therefore, there have been discussions on the approach of using free-flowing rubber powders which allow rubber mixtures to be processed as simply and rapidly as thermoplastic powders or pellets.
Rubber products are generally produced by the rubber industry, by polymerization of various and appropriate monomers, by two fundamentally different processes:
a) Polymerization in Water (Emulsion Polymerization)
As suggested by the title of this process, this process embodiment involves monomer polymerization in water initiated by a free-radical, with the aid of suitable initiator molecules, to give polymers of high molecular weight. If emulsifiers are present, the form in which the rubber molecules are present after the polymerization is as finely divided droplets in water. The term latex or rubber emulsion is used in this context. The latex of emulsion produces the raw rubber material, in turn, for the preparation of rubber powder from the aqueous phase, the powder generally being produced by acid-catalyzed coprecipitation after addition of fillers.
U. Görl and K. H. Nordsiek in
Kautsch. Gummi Kunstst
. 51 (1998) 250; U. Görl and H. Lauer in
Gummi, Fasern Kunstst
. 53 (2000) 261; R. Uphus, O. Skibba, R. H. Schuster and U. Görl in
Kautsch. Gummi Kunstst
. 53 (2000) 276; DE 28 22 148, DE 37 23 213, DE 37 23 214, EP 99 911 7844.5, DE 198 154 53.4, DE 198 43 301.8 and DE 100 56 636.0 are all relevant literature showing developments in this area.
It should merely be stated that the best known rubbers which are prepared in, or are present in, water include natural rubber (NR), emulsion styrene-butadiene rubber (ESBR), nitrile rubber (NBR), and chloroprene rubber (CR). All of these types of rubber may be modified with fillers, e.g. industrial carbon blacks or precipitated silicatic fillers, to give rubber powders.
b) Polymerization in an Organic Solvent
The second large group of rubbers is that of products which are generally polymerized anionically in an organic solvent and therefore are also present in this solvent after the polymerization. Important rubbers prepared in this way include styrene-butadiene rubber (LSBR), butadiene rubber (BR), butyl and halobutyl rubbers, and also ethylene-propylene rubbers with (EPDM) or without (EPM) a copolymerized tercomponent.
The process for preparing a rubber powder from solution polymers, which are dissolved in organic solvent, has to take account of this completely different type of rubber starting material. In contrast to the aqueous rubber emulsion, the following new problems arise:
Direct addition of the filler, in particular of the carbon black, to the rubber solution is problematic, because the filler (lipophilic) absorbs all of the solvent and the rubber/filler mixture therefore clumps, making it difficult to prepare a free-flowing rubber powder. The high adsorptive forces generated by the carbon black, furthermore, bind the solvent, and it is therefore difficult to remove the solvent quantitatively from the product, even under conditions of distillation. A product of this type used in the rubber industry would pose a risk of continuing unacceptable emission of traces of solvent during processes.
Direct preparation of the rubber powder from the polymer solution makes it difficult to take measures to control (reduce) tackiness, this being a precondition for a free-flowing product which is capable of being conveyed and silo storage.
In the case of rubber powders made from aqueous rubber emulsions, this could be achieved by applying a separate filler layer around each grain of rubber during coagulation of the latex with certain adjustments to pH.
In an organic solvent, on the other hand, acid-catalyzed coagulation is not possible, and nor therefore is the application of an effective release layer around each grain of rubber. The only method which remains in this case is subsequent powdering of the product, with the risk that this release agent will separate from the grain during conveying and silo storage, resulting in increased tack and then serious problems during processing. Another consequence could be variations in filler level. Another risk when using release agents which are not a constituent of a rubber mixture is that of product contamination, possibly leading to unacceptable impairment of performance profile.
This means that preparation of a carbon-black-filled rubber powder based on solution polymers which are dissolved in an organic solvent with the desired product properties of flowablity, free-flowability, capable of being conveyed and silo storage, storage stablity, etc. can be achieved only if it is possible to combine the abovementioned aspects in a preparation process, and this in principle appears to be possible only by way of a two-phase system (org./aqueous).
The patent literature describes in detail the preparation of carbon-black-filled rubber powders from organic rubber solutions, using a number of processes. DE 28 22 148 describes the preparation of rubber powders both from aqueous rubber emulsions and from rubber solutions. In the latter case, a carbon black suspension treated with acid, Al
2
(SO
4
)
3
, and water glass is heated to the boiling point of the solvent, the pH is readjusted, and then the rubber solution is added. While the solvent evaporates, the rubber coagulates on the carbon black under the action of the additives. The rubber powder may be obtained after solid/liquid separation and drying, but no other measures have been taken to inhibit tack and thus ensure that, even after long storage times, the material is capable of being conveyed and being stored in a silo. In practice it has been found that unless precautionary measures are taken a product prepared in this way has at best short-term capability to remain free-flowing, the result therefore being problems with silo storage and automatic feeding of mixers.
DE 21 35 266, DE 22 14 121, DE 26 54 358 and DE 24 39 237 describe processes which first convert the organic rubber solution into an aqueous emulsion with the aid of large amounts of emulsifiers. Carbon black filler suspended in the water is added to this emulsion, and the entire emulsion is made to flow into a heated sodium water glass solution. The result is coprecipitation of rubber and filler with simultaneous removal of the solvent.
Görl Udo
Gouw Andreas
Schmitt Matthias
Stober Reinhard
PKU Pulverkautschuk Union GmbH
Wyrozebski Katarzyna
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