Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Patent
1998-06-03
2000-09-05
Yoon, Tae
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
523352, 528483, 528488, 3661821, 366340, C08J 316, B01F 1502, B01F 500
Patent
active
061144150
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for producing coagulated particles of polymer latex.
Latexes of polymer origin obtained by emulsion polymerization or by other methods are generally provided in the form of aqueous colloid dispersions. These dispersions, using water as dispersing medium, contain solid or gelatinous particles, the size of which is generally between 0.1 and 3 microns. The dispersion is generally electrically charged. The addition of a coagulant makes it possible to coagulate or to flocculate these particles by neutralizing the forces which keep them separate.
Once clumps have been created, the size of which now exceeds 10 microns, they can be recovered by filtration, by drying or by any process which makes it possible to collect a powder. The addition of coagulant units to a polymer emulsion, or else the addition of polymer to a coagulant solution, requires a mixing process. Mixing is generally carried out with a conventional paddle stirrer.
However, this way of operating exhibits the following major disadvantages: cannot be hoped to obtain spherical shapes, which may result in problems if it is desired subsequently to manipulate this powder. Fines are also created. Consequently, later in the process, it may be necessary to solve problems such as blinding of the filtration system or obstruction of pneumatic transportation lines. High risks of explosion due to the presence of this dust can also be envisaged when the product is dried or alternatively contamination of the atmosphere of the work site. Moreover, they represent a permanent problem during the handling of the powder. Finally, if these fine particles are in fact removed, they cannot be used and therefore constitute a direct loss of product.
An improvement in this type of method was provided in Patent Application EP-A2-0,217,006, which consists in inducing the formation of bigger coagulated clumps by cautiously dispersing coagulant units in an aqueous colloid solution. The polymer particles thus coagulate at the surface of the coagulant units; the suspension obtained is subsequently passed through a sieve in order to obtain a more uniform particle size grading. However, by adding another process following the coagulation, problems inherent in this new conversion are encountered, such as the blinding of or the necessity of including cleaning cycles for the sieves, for example. Moreover, it turns out that, during a continuous operation, the simple addition of polymer emulsion and of coagulant to the stirred receptacle results in the formation of large masses of coagulates, in particular on the walls of the receptacle and on the paddles of the stirrer. These undesirable clumps are also observed in the bottom of the receptacle and can cause the drainage pipes to become blocked. Consequently, it appears difficult to implement a continuous operation under these conditions.
If simply pouring the coagulant on to the surface of the polymer latex emulsion is envisaged, the coagulant units have a tendency to float, due to surface tension, thus forming large amorphous grains and reducing the reaction yield and the quality of the product.
If the reverse is carried out, namely pouring the emulsion into the coagulant solution, it is observed that the particle size grading is very broad and that the particles have entirely irregular shapes, which is not desirable for a powder.
Another solution is provided in Patent EP-B1-0,288,018, which consists in producing a latex vortex, still using a paddle system. Latex is poured into the receptacle using a system of pouring spout type and, at the same time, a coagulant is sprayed into the descending latex flow.
The resulting mixture is composed of approximately spherical latex grains with a uniform size. This size is of the order of 3 to 5 mm in diameter. If the fact that polymer latexes are mainly used as additives for other polymer powders, which are much finer than 4 mm, is taken into consideration, it is understood that this size is a disadvantage which can only be avoided by the addition of anoth
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patent: 4890929 (1990-01-01), Okada et al.
Bertelo Chris
Gervat Laurent
Laffont Jerome
Mackley Malcolm Robert
Meunier Gilles
Elf Atochem S.A.
Yoon Tae
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