Method of entraining solid particulates in carbon dioxide...

Details Method of entraining solid particulates in carbon dioxide... Method of entraining solid particulates in carbon dioxide...

1999-02-12

2001-05-01

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S636000, C210S638000, C134S001000, C134S010000, C134S013000

Reexamination Certificate

active

06224774

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of cleaning a contaminant from a substrate, and more particularly, to a method of cleaning a contaminant from a substrate using carbon dioxide and an amphiphilic species contained therein.
BACKGROUND OF THE INVENTION
In numerous industrial applications, it is desirable to sufficiently remove different contaminants from various metal, polymeric, ceramic, composite, glass, and natural material substrates. It is often required that the level of contaminant removal be sufficient such that the substrate can be subsequently used in an acceptable manner. Industrial contaminants which are typically removed include organic compounds (e.g., oil, grease, and polymers), inorganic compounds, and ionic compounds (e.g., salts).
In the past, halogenated solvents have been used to remove contaminants from various substrates and, in particular, chlorofluorocarbons have been employed. The use of such solvents, however, has been disfavored due to the associated environmental risks. Moreover, employing less volatile solvents (e.g., aqueous solvents) as a replacement to the halogenated solvents may be disadvantageous, since extensive post-cleaning drying of the cleaned substrate is often required.
As an alternative, carbon dioxide has been proposed to carry out contaminant removal, since the carbon dioxide poses reduced environmental risks. U.S. Pat. No. 5,316,591 proposes using liquefied carbon dioxide to remove contaminants such as oil and grease from various substrate surfaces. Moreover, the use of carbon dioxide in conjunction with a co-solvent has also been reported in attempt to remove materials which possess limited solubility in carbon dioxide. For example, U.S. Pat. Nos. 5,306,350 and 5,377,705 propose employing supercritical carbon dioxide with various organic co-solvents to remove primarily organic contaminants.
In spite of the increased ability to remove contaminants which have limited solubility in carbon dioxide, there remains a need for carbon dioxide to remove a wide range of organic and inorganic materials such as high molecular weight non-polar and polar compounds, along with ionic compounds. Moreover, it would be desirable to remove these materials using more environmentally-acceptable additives in conjunction with carbon dioxide.
In view of the foregoing, it is an object of the present invention to provide a process for separating a wide range of contaminants from a substrate which does not require organic solvents.
SUMMARY OF THE INVENTION
These and other objects are satisfied by the present invention, which includes a process for separating a contaminant from a substrate that carries the contaminant. Specifically, the process comprises contacting the substrate to a carbon dioxide fluid containing an amphiphilic species so that the contaminant associates with the amphiphilic species and becomes entrained in the carbon dioxide fluid. The process may further comprise separating the substrate from the carbon dioxide fluid having the contaminant entrained therein, and then separating the contaminant from the carbon dioxide fluid.
The carbon dioxide fluid may be present in the supercritical, gaseous, or liquid phase. Preferably, the amphiphilic species employed in the carbon dioxide phase comprises a “CO
2
-philic” segment which has an affinity for the CO
2
. More preferably,—the amphiphilic species further comprises a “CO
2
-phobic” segment which does not have an affinity for the CO
2
.
Various substrates may be cleaned in accordance with the invention. Exemplary substrates include polymers, metals, ceramics, glass, and composite mixtures thereof. Contaminants that may be separated from the substrate are numerous and include, for example, inorganic compounds, organic compounds, polymers, and particulate matter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention is directed to a process for separating a contaminant from a substrate that carries the contaminant. Specifically, the process comprises contacting the substrate to a carbon dioxide fluid which contains an amphiphilic species. As a result, the contaminant associates with the amphiphilic species and becomes entrained in the carbon dioxide fluid. The process also comprises separating the substrate from the carbon dioxide fluid having the contaminant entrained therein, and then separating the contaminant from the carbon dioxide fluid.
For the purposes of the invention, carbon dioxide is employed as a fluid in a liquid, gaseous, or supercritical phase. If liquid CO
2
is used, the temperature employed during the process is preferably below 31° C. If gaseous CO
2
is used, it is preferred that the phase be employed at high pressure. As used herein, the term “high pressure” generally refers to CO
2
having a pressure from about 20 to about 73 bar. In the preferred embodiment, the CO
2
is utilized in a “supercritical” phase. As used herein, “supercritical” means that a fluid medium is at a temperature that is sufficiently high that it cannot be liquefied by pressure. The thermodynamic properties of CO
2
are reported in Hyatt,
J. Org. Chem.
49: 5097-5101 (1984); therein, it is stated that the critical temperature of CO
2
is about 31° C.; thus the method of the present invention should be carried out at a temperature above 31°.
The CO
2
fluid employed in the process of the invention may be a non-aqueous fluid. The tern “non aqueous” refers to the fluid being substantially free of water, generally containing less than about 5 percent by weight/volume of water. Preferably, the non-aqueous fluid contains less than about 2 weight/volume percent, more preferably less than 1 weight/volume percent, and most preferably less than about 0.5 weight/volume percent.
Although not necessary, the CO
2
fluid can be employed in a multi-phase system with appropriate and known aqueous and organic liquid co-solvents. Such solvents may be those that are miscible or immiscible in the CO
2
fluid and include, for example, fluorinated solvents, alcohols, hydrocarbons, ethers, ketones, amines, and mixtures of the above. In such a multi-phase system, the CO
2
fluid can be used prior to, during, or after the substrate is contacted by the liquid solvent. In these instances, the CO
2
serves as a second fluid to facilitate the transport of the contaminant from the substrate.
The process of the present invention employs an amphiphilic species contained within the carbon dioxide fluid. The amphiphilic species should be one that is surface active in CO
2
and thus creates a dispersed phase of matter which would otherwise exhibit low solubility in the carbon dioxide fluid. In general, the amphiphilic species lowers interfacial tension between the contaminant and the CO
2
phase to promote the entrainment of the contaminant in the CO
2
phase. The amphiphilic species is generally present in the carbon dioxide fluid from 0.001 to 30 weight percent. It is preferred that the amphiphilic species contain a segment which has an affinity for the CO
2
phase (“CO
2
-philic”). More preferably, the amphiphilic species also contains a segment which does not have an affinity for the CO
2
-phase
The CO
2
fluid employed in the process of the invention may be a non-aqueous fluid. The term “non aqueous” refers to the fluid being substantially free of water, generally containing less than about 5 percent by weight/volume of water. Preferably, the non-aqueous fluid contains less than about 2 weight/volume percent, more preferably less than 1 weight/volume percent, and most preferably less than about 0.5 weight/volume percent. (“CO
2
-phobic”) and may be covalently joined to the CO
2
-philic segment.
Exemplary CO
2
-philic segments may include a fluorine-containing segment or a silixane-containing segment. The fluorine-containing segment is typically a “fluoropolymer”. As used herein, a “fluoropolymer” has its conventional meaning in the art and should also be understood to include low molecular weight oligomers, i.e., those which have a degree of polymerization greater than or equal to two. See

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