Removal of volatile organic compounds from polymer dispersions

Details Removal of volatile organic compounds from polymer dispersions Removal of volatile organic compounds from polymer dispersions

2002-09-10

2004-05-25

Reddick, Judy M. (Department: 1713)

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

Synthetic resins

Processes of preparing a desired or intentional composition...

C523S326000, C524S560000, C524S561000, C524S562000, C524S564000, C528S500000

Reexamination Certificate

active

06740691

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to the removal of volatile organic compounds from latices/dispersions of synthetic polymers.
The terms latices/dispersions as used herein are also meant to cover emulsions and suspensions.
Latices/dispersions of synthetic polymers are usually prepared by polymerizing unsaturated monomeric materials in a liquid medium, usually water, under the influence of free radicals and at the end of the polymerization stage the latices/dispersions typically contain volatile organic compounds which result from incomplete conversion of monomers, impurities of raw materials and undesirable by-products formed during the polymerization reaction. The presence of these volatile organic compounds (VOC) i.e. having a boiling point below 213.5° C. (boiling point of 2-ethylhexyl acrylate) at atmospheric pressure (101.325 kPa) in latices/dispersions of synthetic polymers is undesirable for various reasons and therefore various processes have been developed to remove these compounds. There is still a need for an efficient process to minimize the volatile organic compound (VOC) content of latices/dispersions of synthetic polymers. Especially for such a process which is friendly from an environmental point of view by increasing the yield of polymeric materials to a maximum and decreasing the amount of residual monomer and other volatiles in combination with low energy consumption.
The prior art discloses several methods.
A. Venting the reaction vessel in which the latex or dispersion has been manufactured. This process helps especially in cases where very volatile monomeric materials like e.g. ethylene are polymerized. Cf e.g. U.S. Pat. No. 3,534,009, in the Example.
B. Post-polymerization i.e. adding fresh radical generating materials after the main polymerization to polymerize residual monomeric material further. Cf e.g. U.S. Pat. No. 3,534,009 and EP Appl. 158,523.
C. Sparging air and/or water vapour through the latex/dispersion often called steam stripping is also a well known process, but when carried out in a conventional reactor it is rather time consuming and lowers the throughput of the equipment. Attempts have been made to make steam stripping more efficient e.g. by carrying out the process in special equipment like a degassing column and passing through gas and/or water vapour upwards and the latex/dispersion downwards in countercurrent. Cf e.g. U.S. Pat. No. 4,062,662, EP Appl. 584,458 and WO 97/45,184.
D. Chemical methods like hydrolysing and/or oxidising the residual monomeric materials. Cf e.g. EP Appl. 505,959.
As it is desirable to decrease the VOC level of several percents immediately after polymerization to a few or a fraction of a ppm (part per million) combinations of two or more of the above methods have been developed and disclosed.
One such combination is disclosed in EP Appl. 563,726 and combines chemical oxidation with subsequently distilling off of the residual vinylester and acetaldehyde formed. Another combination is disclosed in EP Appl. 650,977 and involves post polymerization treatment with a free radical generator under polymerization conditions followed by steam stripping under vacuum and results in polymer latices/dispersions containing 5 to 500 ppm of residual monomer and other volatiles. WO 99/14,248 (BASF) discloses the reduction of residual monomer content of e.g. latex by post-polymerization in a reactor having a mixing time which is as short as possible by dosing slowly at least one the components of the redoxinitiator in such a manner that the dosing time is about 10 to 250 times the mixing time of the liquid system in the reactor. It is preferred that the dosing of the oxidizing- and reducing components of the initiator system to the liquid in the reactor is carried out at separate points of addition (“räumlich getrennte Zugabe”) such as the reactor lid, reactor bottom or reactor side wall and that the components are dosed simultaneously or one after the other. There are 12 Examples which specify that the oxidizing agent (tert.-butylhydroperoxide) was dosed first and the reducing agent (e.g. sodium disulphite) was dosed subsequently. There is, however, one exception viz. Example 5b which specifies the simultaneous addition of both components. The tenor is that the sequence order of dosing oxidizing agent and reducing agent does not matter, although the examples seem to indicate that there is some preference for first dosing the oxidizing agent and then the reducing agent.
SUMMARY OF THE INVENTION
The present invention relates to an improvement of the process of EP appl. 650,977 and secures one or more of the following advantages: a lower VOC contents in the endproduct and/or a higher throughput than with the current processes and/or a lower energy consumption and/or a better endproduct and/or a higher economy of chemicals used.
DETAILED DESCRIPTION OF THE INVENTION
In a first embodiment the invention therefore provides a process for treating an aqueous polymer latex/dispersion prepared by polymerizing one or more monomeric materials for at least 98%, comprising the steps of:
A. adding to the polymer latex/dispersion in a sufficient amount of at least one reducing agent in one or more portions followed by adding at least one free radical generator in such a way that at least the bulk of the reducing agent has been added before addition of free radical generator is started and maintaining the mixture at a suitable temperature for a period of time which is sufficient to reduce the VOC-level of the latex/dispersion to decrease to below 1,100 ppm, preferably to below 500 ppm after which
B. water vapour and/or gas is/are fed into the latex/dispersion whilst the temperature of the latex/dispersion is maintained at a suitable temperature for a period of time which is sufficient to decrease the VOC-level to below 200 ppm, preferably to below 50 ppm, more preferably to below 15 ppm. Typically to 0.5-10 ppm.
In step A, only monomeric VOC's will be reduced by polymerization and formation of polymer, while in step B, all VOC's that are present will be removed, including non-polymerisable VOC's that were present in the monomers or other raw materials, or that were produced during the polymerisation. Therefore the combination of steps A and B is particularly advantageous. This combination has both environmental and economical advantages because step A improves the yield of polymer and reduces the amount of volatile material to be removed in step B by a relatively short thermal treatment under mild conditions resulting in a better endproduct with less volatiles and less polymer degradation.
The aqueous polymer latex/dispersion treated according to the present invention is prepared by polymerizing a mixture of one or more unsaturated monomeric materials in an aqueous medium under the influence of a free radical initiator. Emulsion and suspension polymerization techniques are well-known in the art and are e.g. disclosed in “Emulsion polymerization”, D. C. Blackley, Applied Science Publishers, 1974 and “Textbook of Polymer Science”, F. W. Billmeyer. Wiley, 1975, Chapter 12, which are incorporated by reference. In these polymerizations preferably a suitable free radical generator system is used. For example, free radical generating compounds, ultraviolet light or radiation can be used. The choice of the free radical generating chemical compound to be used depends on the desired polymerization rate and final polymer properties. Some representative examples of free radical initiators which are commonly used include the various ammonium and alkali metal salts of persulphuric or peracetic acid, such as e.g. ammonium persulphate, sodium persulphate, potassium persulphate, sodium peracetate, peroxides such as e.g. hydrogen peroxide, benzoyl peroxide, tertiarybutyl hydroperoxide, tertiaryamyl hydroperoxide, cumene hydroperoxide, paramenthane hydroperoxide, diisopropylbenzene hydroperoxide or percarbonates such as e.g. bis(4-tertiarybutylcyclohexyl)peroxydicarbonate, tertiarybutyl peroctoate and tertiarybutyl perpivalate and

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