Polymerizing vinyl chloride in carbon dioxide

Details Polymerizing vinyl chloride in carbon dioxide Polymerizing vinyl chloride in carbon dioxide

1999-06-09

2001-07-10

Acquah, Samuel A. (Department: 1711)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S088000, C526S089000, C526S227000, C526S229000, C526S230500, C526S291000, C526S292700, C526S318200, C526S344000, C526S344100, C528S392000, C528S397000, C522S005000, C522S006000, C522S060000, C504S145000, C504S145000

Reexamination Certificate

active

06258910

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a method of polymerizing vinyl chloride and vinylidene chloride. In particular, it relates to the use of supercritical, subcritical, or liquid carbon dioxide as a solvent in which vinyl chloride or vinylidene chloride is polymerized.
Polyvinylchloride (PVC) is made by polymerizing vinyl chloride monomer (VCM) in an aqueous suspension, an aqueous dispersion, or bulk (i.e., without a solvent). While these processes are effective, there are some problems associated with them. For example, in the suspension and dispersion processes energy is needed to evaporate solvent. In all three processes, unreacted VCM must be removed from the PVC because it is a carcinogen, and its removal to acceptable levels is difficult. Also, those processes are subject to reactor fouling and the formation of fish eyes in the PVC.
SUMMARY OF THE INVENTION
We have discovered that VCM and vinylidene chloride will polymerize in supercitical, subcritical, or liquid CO
2
. We have found that VCM is soluble in that CO
2
, but that PVC is not soluble in it. Thus, as the PVC forms, it separates from the VCM.
Surprisingly, PVC made according to this invention is more porous than PVC made by other methods. As a result, it is easier to remove unreacted VCM from the PVC.
We have found that there is a relationship between the density (i.e., pressure) of the CO
2
and the molecular weight of the PVC in that higher molecular weight PVC is made when the density of the CO
2
increases. We also found an inverse relationship between the ratio of CO
2
to VCM and the molecular weight of the PVC. That is, if that ratio is higher, the molecular weight of the PVC is lower.
The use of CO
2
offers several advantages over other methods of making PVC. There is less reactor fouling and fewer fish eyes occur when CO
2
is used as the solvent. Little or no energy is required to evaporate the solvent because the CO
2
evaporates at room temperature. CO
2
is inexpensive, non-toxic, non-flammable, relatively environmentally innocuous, and can be easily recycled. If it is recycled, removal of unreacted VCM from it is not necessary.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process of the invention can be used to polymerize VCM or vinylidene chloride. The polymerization of VCM is preferred as it is commercially more important. Up to about 20 wt % (based on total monomer weight) of a comonomer, such as vinyl acetate, methylacrylate, methyl methacrylate, acrylonitrile, vinyl ether, vinyl fluoride, or vinylidene fluoride can be copolymerized with the VCM or vinylidene chloride. Vinylidene chloride can be a comonomer for VCM and vice versa.
Supercritical carbon dioxide is CO
2
at a temperature and pressure above its critical point. That is, its temperature is higher than 31° C. and its pressure is greater than 1066 psi (73.8 atm or 7.3 MPa). Subcritical carbon dioxide is CO
2
at a temperature above 31° C. and a pressure below 7.3 MPa. Liquid carbon dioxide is CO
2
at a temperature between −55 and 31° C. that is under sufficient pressure to be a liquid. The amount of CO
2
used (by weight) should be about 0.5 to about 5 times the amount of monomer. Less may result in reduced porosity and the presence of fish eyes and more may reduce throughput and lower the molecular weight of the product. Preferably, the amount of CO
2
is about 0.8 to about 1.2 times the amount of monomer. The CO
2
can be placed in the reactor as a solid (dry ice) or as a gas, as long as it is converted into supercritical, subcritical, or liquid CO
2
for the polymerization of the VCM or vinylidene chloride.
The polymerization requires a free radical initiator. In order to have a good reaction, the initiator must be soluble in the CO
2
or in a solvent that is miscible with the CO
2
Suitable initiators include organic compounds containing a peroxy (—OO—) group, such as diacyl peroxides (R—CO—OO—CO—R), percarbonates (RO—CO—OO—CO—OR′), and peresters (R—CO—OO—CO—OR′). Examples include diacetyl peroxide, dibenzoyl peroxide, diethyl peroxydicarbonate, di-n-propyl peroxydicarbonate, di-sec-butyl peroxydicarbonate, t-butyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and t-amyl peroxypivalate. The preferred initiators are di-n-propyl peroxydicarbonate and diethyl peroxydicarbonate as they have been found to work well. The amount of initiator used should be about 0.1 to about 1 phr (parts by weight per 100 parts by weight of monomer) as less is ineffective and more is unnecessary. Preferably, the amount of initiator is about 0.3 to about 0.9 phr.
If the initiator does not dissolve in the CO
2
, it should be dissolved in a solvent that is miscible with the CO
2
. Depending upon the particular initiator, suitable solvents can include toluene, alcohols such as methanol, ethanol, isopropanol, and hexanol, or hydrocarbons such as pentane, hexane, cyclohexane, and heptane. Partially fluorinated and perfluorinated solvents, such as 2,2,2-trifluoroethanol, &agr;,&agr;,&agr;-trifluorotoluene, hexafluorobenzene, and perfluoroalkanes are also expected to be useful due to their high solubility in CO
2
. The preferred solvents are ethanol and toluene because they are inexpensive, readily available, and easily dissolve most initiators. The amount of solvent used should be sufficient to dissolve the initiator; excess solvent should be avoided because it provides no additional benefit and presents additional processing costs.
A support is preferably used to disperse the initiator as supported initiators produce a less agglomerated product. The support can be in addition to the solvent or as an alternative to the solvent. Examples of suitable supports include amorphous or fumed silica or alumina and other metal oxides, such as titania, and silica gel. The amount of support used should be about 0.12 to about 0.6 g/g of initiator; the preferred amount of support is about 0.2 to about 0.4 g/g of initiator.
The polymerization can be performed either as a batch or as a continuous process. In a batch process, the reactor can be cooled to less than −10° C. to prevent the premature polymerization of the monomer. The reactor is charged with the monomer, which liquefies in the reactor, or liquid monomer can be used. The initiator is injected into the reactor followed by the CO
2
while stirring. The reaction mixture is heated to the reaction temperature, about 40 to about 70° C., and preferably about 45 to about 65° C. Lower temperatures require an initiator having a lower half life temperature, which is hard to handle, and higher temperatures may result in side reactions and inferior products. The polymerization reaction can be followed by observing the reactor pressure or by gas chromatography (GC). Typically, it requires about 2 to about 5 hours to achieve about 80% monomer conversion. The polymerization can be performed as a continuous process by charging the monomer, initiator, and CO
2
as liquids. The product is submicron-sized highly porous particles.


REFERENCES:
patent: 5312882 (1994-05-01), DeSimone et al.
patent: 5527865 (1996-06-01), DeSimone et al.
patent: WO9828351 (1998-07-01), None

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