Process for separating organic monomers or auxiliaries

Details Process for separating organic monomers or auxiliaries Process for separating organic monomers or auxiliaries

1998-10-20

2001-01-30

Teskin, Fred (Department: 1713)

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

Synthetic resins

Treating polymer containing material or treating a solid...

C528S490000, C528S503000, C528S903000

Reexamination Certificate

active

06180755

ABSTRACT:

The invention relates to a process for separating organic monomers or auxiliaries which are used in synthesizing organic polymers or take part in the polymerization reaction, whereby the monomers or auxiliaries are extracted from the prepolymer obtained by the synthesis by means of compressed carbon dioxide as solvent and whereby the carbon dioxide is used in its thermo-dynamic state above its critical pressure and above its critical temperature.
Such a process, which serves to remove the monomers used in the synthesis, which are present in excess or are thermally unconverted, as well as the additives, such as solvents, diluents, stabilizers, starters or the like, used in the synthesis as auxiliaries, from the prepolymer, is disclosed in U.S. Pat. No. 4,871,460. There, the carbon dioxide is introduced in a moist and supercritical state. The monomers or auxiliaries separated in this way are recovered with a relatively high proportion of moisture and therefore are unsuitable for further use, for example for return to the production process.
German Patents DE 3,836,093, DE 4,136,490 and DE 4,232,015, as well as German Letters of Disclosure DE 2,414,391 A1 and European Patents 0,464,483 and 0,340,584, likewise disclose a variety of processes for separating monomers and/or polymers. In these known processes additional auxiliaries, which for example react with the monomers to be neutralized in the prepolymer, are added to the prepolymer for neutralizing. Consequently, neutralizing involves a chemical process in which the reaction product of monomers, [with] the auxiliaries added for neutralizing, remains in the prepolymer. Another common disadvantage of the known processes is that the separated monomers are impure and hence unsuitable for purposes of further synthesis and therefore must be disposed of at high cost.
The object of the invention is to refine the process of the type mentioned at the beginning in such a way that the monomers or auxiliaries are recovered essentially moisture-free, so that further use of the monomers, in particular return of the monomers directly to the production process, is possible.
According to the invention, this object is accomplished by a process for separating organic monomers or auxiliaries which are used in synthesizing organic polymers or take part in the polymerization reaction, whereby the monomers or auxiliaries are extracted from the prepolymer obtained by the synthesis by means of compressed carbon dioxide as solvent, and whereby the carbon dioxide is used in its thermodynamic state above its critical pressure and above its critical temperature. The process according to the invention is characterized in that the carbon dioxide is dried to a moisture content under 20 ppm before it is brought together with the prepolymer.
The process according to the invention allows the residual monomer content in the purified prepolymer to be reduced to values under 0.1%, so that in further processing of the purified prepolymers, for example, no special protective measures are necessary on account of the monomers contained. In addition, because of the use of dry CO
2
the separated monomers accumulate with a high purity of up to 99.8%, for example, hence can be used for the synthesis again and need not be disposed of in onerous fashion. The dry carbon dioxide may alternatively be returned for further extraction. This permits a closed extraction circuit in which no emissions are released. Nor need any additional chemicals be used. Added to this is the fact that the protective-gas effect of carbon dioxide may also be utilized in, in particular, containerizing the purified prepolymer. Deterioration of the prepolymer due to oxygen or moisture may be prevented in simple fashion. Overall, considerably improved product quality is obtained, with simultaneous and complete recycling of the raw materials used, monomers or auxiliaries.
For extraction, the compressed and dried carbon dioxide is brought into contact with the prepolymer. Then, the monomers or auxiliaries are dissolved out of the prepolymer by the carbon dioxide and dissolve in the carbon dioxide. In this way, the content of monomers or auxiliaries in the prepolymer is reduced.
The process of high-pressure carbon dioxide extraction for recovering extracts of natural products is already well known (German patent Nos. DE 2,127,618, DE 2,127,611 and DE 4,335,321). However, on the basis of the experience gained in these applications of high-pressure carbon dioxide extraction, the process for separating organic monomers or auxiliaries, which are used in synthesizing organic polymers or take part in the polymerization reaction, from the prepolymer seemed unsuitable for synthesis. Namely, the previous applications showed that carbon dioxide is a suitable solvent for lipophilic substances, while hydrophilic polar substances or substance classes are insoluble in carbon dioxide.
This is clearly apparent in the example of hops extraction: The lipophilic constituents are recovered as total extract by means of high-pressure carbon dioxide extraction, while the hydrophilic polar substances (cellulose, sugar, starch) remain as residue. The extract or mixture thus obtained, consisting of a multiplicity of lipophilic substances/substance classes (e.g., a-acids, b-acids, hops oils, aromatic substances, etc.), cannot be further broken down into the individual substances or components or separated into fractions by means of high-pressure carbon dioxide extraction, because of the similar solution behavior of these substances.
As with natural products, the polymer mixtures examined likewise exhibit pronounced lipophilic behavior, given their good solubility in hexane or, in the case of some polymers, even complete miscibility with hexane. A person skilled in the art would therefore have to assume that mixtures of substances which consist predominantly of lipophilic components and have lipophilic properties (are soluble in hexane, for example) cannot be further separated into their individual constituents or components by means of high-pressure carbon dioxide extraction.
Surprisingly, however, separation of monomers or auxiliaries from the prepolymer by means of high-pressure carbon dioxide extraction has been found to be possible.
Tests have shown that, for example, monomers, acrylates and methacrylates, aldehydes, dioxanes and low-molecular weight cyclic esters, as well as diisocyanates (TDI, MDI, HDI, IPDI, H12MDI, etc.), can be separated from the prepolymerizate virtually without residue.
In addition, it has been found that even troublesome oligomeric synthesis constituents which adversely affect the physical properties of certain polymers, for example, can be removed jointly with the above-mentioned monomers.
Especially surprisingly, it has been shown that, in a process according to the dependent claims in particular, selective separation of the monomers concerned can be obtained at high purity.
The carbon dioxide is preferably used in its thermodynamic state above its critical pressure and above its critical temperature. The critical pressure of carbon dioxide is 73.8 bar, and the critical temperature is 31.06° C. Above the critical pressure and the critical temperature, the dissolving power of carbon dioxide is especially high for the monomers or auxiliaries to be extracted.
In order to take full advantage of the increased dissolving power of carbon dioxide's supercritical pressure and temperature, a process is preferred in which the prepolymer and the solvent, pure dry carbon dioxide with a moisture content under 20 ppm, are brought together at pressures between 100 bar and 320 bar. Correspondingly, a process is preferred in which the prepolymer and the solvent are brought together at temperatures between 40° C. and 80° C. The increased dissolving power of carbon dioxide already exists in the selected temperature range, but the risk of cracking of the substances contained in the prepolymer is not yet present.
Drying of the commercially available CO
2
takes place in that for drying the gaseous CO
2
is co

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