Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Shaping by extrusion
Reexamination Certificate
1999-09-09
2001-01-16
Boykin, Terressa M. (Department: 1711)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Shaping by extrusion
C528S271000, C528S272000, C528S485000
Reexamination Certificate
active
06174480
ABSTRACT:
The present invention relates to a new process for removing polymerization inhibitors from monomer mixtures, especially from ethylenically unsaturated monomers.
In the industry of polymerization of a large number of ethylenic monomers an important problem has to do with the storage and/or transport of these monomers. This is because an uncontrolled spontaneous polymerization of these monomers in the course of time, starting from free radicals, can be observed. These ethylenically unstable monomers are especially those which have a second unsaturation such as a COOH, C═O, C≡N, C═C, C═S or C═N functional group; they may be, for example, the following monomers: styrene, butadiene, isoprene, (meth)acrylic esters, acrylonitrile, acrolein, chloroprene, vinyl acetate, etc.
To avoid this degradation of the monomers it is known to stabilize them by means of inhibiting substances which prevent polymerization from taking place.
These substances, more generally known as “polymerization inhibitors” can be chosen from picric acid, nitroaromatics, quinone derivatives (hydroquinone, benzoquinone), naphthols, amines (p-phenylenediamine, phenothiazine), phosphites, p-methoxyphenol, p-tert-butylcatechol, etc.
When is it desired to employ inhibited monomers in order to polymerize them or to use them in chemical reactions, it is often necessary to remove the polymerization inhibitors. A number of means are employed for this purpose:
it is possible to add a large quantity of initiator to the reactor to combat the effect of the inhibitor; however, this technique is not suitable in every case,
the temperature can be raised considerably to produce the thermolysis of the inhibitor, but the monomer must have a high thermal stability,
the charge of monomers and of inhibitor can be distilled, but the monomer must exhibit good thermal stability; in addition, such an operation is difficult to carry out on an industrial scale, and the boiling point of the inhibitor is in many cases higher than that of the monomer,
the inhibitor can be removed by adding a dilute solution, for example of sodium hydroxide, the monomer charge being subsequently washed with water in order to remove all traces of caustic compounds; however, the treatment of the liquid effluents also presents industrial problems,
lastly, the inhibitor can be adsorbed with the aid of a compound such as alumina, silica gel, activated carbon, calcium oxide, aluminium silicate, talc, calcium sulphates, magnesium sulphates, copper sulphates, magnesium silicate clays, a resin etc.
Adsorption is one of the most advantageous methods because it does not exhibit any of the abovementioned disadvantages. Among the adsorbents employed, activated alumina is preferred.
The objective of the present invention is to propose a new alumina for the adsorption of polymerization inhibitors from ethylenically unsaturated monomers, exhibiting adsorption capacities which are improved in relation to the aluminas of the prior art.
To this end the invention relates to a process for adsorption of polymerization inhibitors from ethylenically unsaturated monomers, in which these inhibitors are placed in contact with an alumina, the said alumina being produced by forming by drop coagulation or by extrusion-blending.
The principle of the invention therefore relates to the manner in which the alumina has been prepared, very particularly concerning its forming. The purification process according to the invention thus uses an alumina which can be obtained in accordance with a number of forming methods.
According to a first method the alumina employed may be in the form of alumina beads. These beads are produced by forming using drop (or oil-drop) coagulation. This forming consists in introducing drops of an aqueous solution based on an aluminium compound into a water-immiscible liquid (oil, kerosene, etc.) in such a way that the drops form substantially spherical particles and these particles are coagulated simultaneously with and/or after the spheroidal forming, by a gelling agent. The beads are subsequently recovered and then dried and calcined.
Beads of this type can be prepared, for example, according to the process described in patent EP-A-097 539, or by coagulation as drops of a suspension or of an aqueous dispersion of alumina or of a solution of a basic aluminium salt which is in the form of an emulsion consisting of an organic phase, an aqueous phase and a surface agent or an emulsifier. The said organic phase may in particular be a hydrocarbon; the surface-active or emulsifying agent is, for example, Galoryl EM 10®.
These beads can also be prepared according to the process described in patent EP-A-015 801 by mixing, at a pH lower than 7.5, an ultrafine boehmite sol and spheroidal alumina particles, then drop coagulation of this mixture as indicated above and, finally, drying and calcining.
According to this first method using beads, the latter must not originate from a process of forming alumina using a rotational technique. A rotational technique is intended to mean any equipment in which the agglomeration is performed by bringing into contact and rotating the product to be granulated about itself. Equipment of this type which may be mentioned is the rotary coating pan and the rotating drum.
It has been noted that these alumina beads exhibit inferior properties for the adsorption of corrosion inhibitors, compared with the alumina beads employed in the present invention.
According to a second method of the invention, alumina extrudates may also be involved. These are generally obtained by blending and then extruding an alumina-based material and, lastly, calcining. The starting material may be of very varied nature: it may be produced by partial and rapid dehydration of hydrargillite, according to the teaching of application FR-A-1,108,011, or by the precipitation of boehmite, pseudoboehmite, bayerite alumina or of a mixture of these aluminas. During the blending the alumina may be mixed with additives such as pore-formers. To give an example, the extrudates may be prepared by the process of preparation described in U.S. Pat. No. 3,856,708.
In general, it is preferred to employ alumina extrudates rather than beads produced by drop coagulation.
These extrudates may exhibit all kinds of shapes: solid or hollow cylinders, multilobes, etc.
The alumina employed in the process according to the invention preferably has a total pore volume (TPV) of at least 0.2 ml/g, preferably of at least 0.3 ml/g.
This total pore volume (TPV) is measured as follows. The value of the grain density and of the absolute density is determined, the grain (Dg) and absolute (Da) densities being measured by the pycnometry method with mercury and with helium respectively. The TPV is then given by the formula:
1
Dg
-
1
Da
In general, aluminas of particle size of between 0.8 and 10 mm, preferably between 1 and 5 mm, are used. In the case of forming by drop coagulation the particle size corresponds to the bead diameter and, in the case of extrudates, to the diameter of their cross-section.
The alumina preferably has a specific surface of at least 10 m
2
/g, or even of at least 50 m
2
/g.
This specific surface is a surface measured by the BET method.
A surface measured by the BET method is intended to mean the specific surface determined by nitrogen adsorption in accordance with ASTM Standard D 3663-78, established from the Brunauer—Emmett—Teller method described in the periodical “The Journal of the American Chemical Society”, 60, 309 (1938).
The process according to the invention preferably uses an alumina including at least one compound of an element chosen from the alkali metals, the rare earths and the alkaline-earth metals.
This compound may be an oxide, a hydroxide, a salt or a mixture of these. For example, in addition to hydroxides, the sulphates, nitrates, halides, acetates, formates, carbonates and carboxylic acid salts may be mentioned.
Elements chosen from sodium, potassium, lithium, lanthanum and cerium are preferably employed.
The content of alkali metal, alkaline-eart
Boykin Terressa M.
Burns Doane Swecker & Mathis L.L.P.
Rhodia Chimie
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