Methods of polymer impregnation

Methods of impregnating various polymer substrates with an impregnation additive, by simultaneously contacting the polymer substrate with an impregnation additive, carrier liquid, and supercritical fluid are provided. The impregnation additive is substantially insoluble in the supercritical fluid, and the carrier liquid is preferably substantially insoluble in the supercritical fluid.

FIELD OF THE INVENTION 
This invention relates to methods of impregnating polymeric materials with 
additives utilizing supercritical fluids. 
BACKGROUND OF THE INVENTION 
A variety of methodologies have been employed in an attempt to impregnate 
polymers, and in particular thermoplastic polymers, with various 
impregnation additives. For example, certain polymers can be impregnated 
with selected additives by immersing the polymers in a solution comprised 
of the additives for an extended period of time. In addition, it may also 
be possible to incorporate the additives into polymers during melt 
processing and/or extrusion. Furthermore, additives may be impregnated 
into polymers by dissolving the additives into various compounds, such as 
CO.sub.2, N.sub.2 O, and ethylene, maintained at or near their 
supercritical temperatures and pressures, and contacting this mixture with 
the polymer or polymers to be impregnated. Above a defined temperature and 
pressure, these pressurized compounds form supercritical fluids that serve 
both as swelling agents for the polymers to be impregnated, and as 
volatile solvents for additives to be impregnated into the polymers. 
Existing methods of impregnating polymers with additives using 
supercritical fluids are limited by the requirement that the selected 
additive or additives be soluble in the supercritical fluid, and that the 
mixture of the additive solubilized in the supercritical fluid be 
compatible with (i.e. soluble in) the polymer to be impregnated. For 
example, U.S. Pat. No. 4,598,006 (Sand) discloses a method for 
impregnating a thermoplastic polymer with an impregnation material (i.e. a 
fragrance, a pest control agent, or a pharmaceutical composition) by 
dissolving the impregnation material in a volatile swelling agent (e.g., 
CO.sub.2 maintained at or near supercritical conditions, swelling the 
thermoplastic polymer by contacting it with the supercritical or nearly 
supercritical volatile swelling agent containing the impregnation 
material, and reducing the pressure so the volatile swelling agent 
diffuses out of the thermoplastic polymer. Among other limitations, Sand 
teaches that the impregnation material must be soluble in the volatile 
swelling agent, and that the volatile swelling agent be compatible with 
(i.e. soluble in) the polymer to be impregnated. Given the lipophilic 
nature to the volatile swelling agents and polymers disclosed in Sand, the 
impregnation materials disclosed in Sand are also lipophilic. See also, 
U.S. Pat. No. 4,678,684; EPO Patent Application Nos. 0 200 197, 0 401 713, 
0 405 284; and Australian Patent Application No. 57091/86. 
Similarly, U.S. Pat. No. 4,820,752 (Berens et al.) discloses a process for 
infusing an additive into a polymer by dissolving the additive into a 
compressed normally gaseous fluid solvent (e.g., CO.sub.2) that has a 
boiling point below room temperature and a density of at least 0.01 g/cc, 
contacting the solution of the additive and normally gaseous fluid solvent 
with a polymeric material for a time sufficient to allow at least part of 
the solution to be absorbed into the polymeric material, and separating 
the normally gaseous fluid Bolvent from the polymeric material leaving the 
additive infused within the polymeric material. Importantly, Berens et al. 
discloses that the additive must have some degree of solubility in the 
compressed fluid, and the solution of the compressed fluid and additive 
must have some degree of solubility in the polymeric material. See also, 
EPO Patent Application No. 0 222 207. 
In addition, supercritical fluids have also been used as a solvent to 
re-impregnate aromatic components into a tea residue after the caffeine 
component of the tea had been extracted (U.S. Pat. No. 4,167,589; Vitzthum 
et al.), as a solvent during the preparation of substance embedded 
microspheres, by dissolving a substance and polymeric carrier, with or 
without a liquid medium, into a supercritical gas (U.S. Pat. No. 
5,043,280; Fisher et al.), and as a solvent for various monomers or 
polymers to be impregnated into porous materials, such as wood, to 
increase the strength and other properties of the porous materials (U.S. 
Pat. Nos. 4,992,308 and 5,169,687; Sunol). 
None of the previously disclosed methods can be used to successfully 
impregnate additives, and in particular hydrophilic additives, into 
polymers when the additives are incompatible with (i.e. substantially 
insoluble in) the supercritical fluid. In fact, to date, no method has 
been provided for the impregnation of additives into polymers when such 
additives are substantially insoluble in the supercritical fluid. 
SUMMARY OF THE INVENTION 
Surprisingly, it has been discovered that impregnation additives that are 
substantially insoluble in a supercritical fluid can be impregnated into 
polymer substrates by simultaneously contacting the polymer substrate with 
the impregnation additive and a carrier liquid, such as water, in the 
presence of the supercritical fluid. Even more surprisingly, such 
impregnation can be accomplished using impregnation additives that are 
incompatible with (i.e. insoluble in) the polymer substrate, and using 
carrier liquids that are substantially insoluble in the supercritical 
fluid and/or are incompatible with (i.e. insoluble in) the polymer 
substrate. 
In particular, the present invention provides a method of impregnating a 
polymeric material with an impregnation additive by simultaneously 
contacting a polymeric material with a carrier liquid and an impregnation 
additive, exposing the polymeric material, carrier liquid and impregnation 
additive to a supercritical fluid in a pressure vessel for sufficient time 
to swell the polymeric material, such that the carrier liquid and 
impregnation additive can at least partially penetrate the polymeric 
material, and releasing the pressure in the pressure vessel so that the 
carrier liquid diffuses out of the polymeric material, thereby entrapping 
an amount of the impregnation additive within the polymeric material, 
wherein the impregnation additive is substantially insoluble in 
supercritical fluid. 
In addition, the present invention also provides a method of impregnating a 
polymeric material with a hydrophilic impregnation additive by 
simultaneously contacting a polymeric material with a carrier liquid and a 
hydrophilic impregnation additive, exposing the polymeric material, 
carrier liquid and hydrophilic impregnation additive to a lipophilic 
supercritical fluid in a pressure vessel for sufficient time to swell the 
polymeric material, such that the carrier liquid and hydrophilic 
impregnation additive can at least partially penetrate the polymeric 
material, and releasing the pressure in the pressure vessel so that the 
carrier liquid diffuses out of the polymeric material, thereby entrapping 
an amount of the hydrophilic impregnation additive within the polymeric 
material. 
These and various other advantages and features of novelty which 
characterize the invention are pointed out with particularity in the 
claims annexed hereto and forming a part hereof. However, for a better 
understanding of the invention, its advantages, and objects obtained by 
its use, reference should be had to the accompanying drawings and 
descriptive matter, in which there is illustrated and described preferred 
embodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
General Method 
FIG. 1 schematically illustrates an impregnation apparatus 10 for the 
impregnation of polymers with selected impregnation additives according to 
the methods of the present invention. The major components of the 
impregnation apparatus 10, include a tank 15 that holds the material to be 
used as a supercritical fluid, a compressor 20 to pressurize and transfer 
the supercritical fluid from the tank 15 to a pressure vessel 25, a water 
or oil bath 30 in which the pressure vessel 25 is suspended, a temperature 
regulator 35 to maintain the water/oil bath 30 at a predetermined 
temperature, a pressure transducer 40 to monitor and maintain the pressure 
within the pressure vessel 25 at a predetermined level, and a vent line 
45, to be used to vent the supercritical fluid from the pressure vessel 25 
after impregnation of a polymer has been accomplished. 
In use, a polymer sample to be impregnated (not shown) is placed in a 
container 5, such as a beaker or test tube, within the pressure vessel 25. 
The polymer sample is covered with a solution of a carrier liquid and one 
or more impregnation additives (not shown), and maintained completely 
submerged in this solution via a glass wool plug or other inert material 
placed in the top of the container 5. The pressure vessel 25 is then 
sealed, and placed or maintained in water/oil bath 30. 
To start the impregnation process, a selected material, such as carbon 
dioxide, is transferred from tank 15 via line 16 to compressor 20, where 
it is pressurized to the critical pressure (P.sub.c) of the material, or 
greater. The compressed material leaves compressor 20 via line 22 and 
valve 24, and is transferred into the pressure vessel 25 containing the 
polymer sample to be impregnated, after which valve 24 is closed. 
When the pressurized material enters pressure vessel 25, it may already 
comprise a supercritical fluid, so long as the temperature of the 
pressurized material exceeds the critical temperature (T.sub.c) of the 
material. However, if the pressurized material has not yet reached or 
exceeded T.sub.c, then water/oil bath 30 can be heated using temperature 
regulator 35 to rapidly convert the pressurized material into a 
supercritical fluid capable of swelling the polymer sample according to 
the methods of the present invention. In this regard, it will be 
appreciated that both temperature regulator 35 and pressure transducer 40 
can be used to maintain pressure vessel 25, including the supercritical 
fluid, polymer sample, impregnation additive, and carrier liquid contained 
therein, at a preselected temperature and pressure above the T.sub.c and 
P.sub.c of the supercritical fluid. 
After sufficient time has passed to complete impregnation of an 
impregnation additive into the polymer sample in container 5, the 
supercritical fluid contained in pressure vessel 25 is vented from the 
pressure vessel 25 via vent line 45 by keeping valve 24 closed, and 
opening valve 46. In this regard, pressure vessel 25 should be vented in a 
controlled manner (e.g., at a slow regular rate) to prevent damage (e.g., 
fracturing and/or foaming) to the polymer samples. 
It will be appreciated that vent line 45 may be vented directly to the 
atmosphere, or may be vented into a holding container (not shown), 
re-circulated to tank 15, as need be. After the supercritical fluid has 
been vented, pressure vessel 25 can be opened, and the impregnated polymer 
sample recovered from container 5. 
While the impregnation of a polymer sample with one or more impregnation 
additives according to the methods of the present invention has been 
illustrated with respect to FIG. 1, it will be appreciated that any 
apparatus capable of containing a supercritical fluid, polymer sample, 
carrier liquid, and impregnation additive(s), such that the polymer sample 
is impregnated with the impregnation additive(s), is considered to fall 
within the scope of the present invention. In this regard, those skilled 
in the art will be readily capable of adapting the apparatus illustrated 
in FIG. 1, such as through the incorporation of a thermocouple into 
pressure vessel 25, thereby eliminating the need for water/oil bath 30, or 
in any other manner consistent with the practice of the methods of the 
present invention. 
Supercritical Fluid 
As used herein, a supercritical fluid refers to a material maintained at or 
above its critical temperature (T.sub.c) and critical pressure (P.sub.c) 
(i.e. above its critical point (C.sub.p)), so as to place the material in 
a supercritical fluid state. Typically, supercritical fluids are gases at 
ambient temperature (approximately 22.degree. C.) and pressure 
(approximately 1.01 mega Pascals (MPa)). However, when maintained at or 
above C.sub.p, the supercritical fluid displays properties of both a gas 
and a liquid. In particular, such a supercritical fluid has the solvent 
characteristics of a liquid, but the low surface tension of a gas. 
Accordingly, as with a gas, the supercritical fluid can more readily 
diffuse into a selected solute material, such as a polymer. 
FIG. 2 diagrammatically illustrates the various states of matter of a 
typical material capable of forming a supercritical fluid. At appropriate 
temperatures and pressures the selected material may take the form of a 
solid, a liquid, or a gas. However, above a defined T.sub.c and P.sub.c, 
the material takes the form of a supercritical fluid displaying the 
properties noted above. Thus, the supercritical fluid region for such a 
material is defined by the shaded region of FIG. 2, encompassing all 
temperatures and pressures beyond the C.sub.p of the material. 
Table 1 lists several nonlimiting examples or supercritical fluids, 
including their critical temperatures and pressures, that are useful in 
the methods of the present invention. 
TABLE 1 
______________________________________ 
Critical temperatures (T.sub.c) and critical pressures (P.sub.c) of 
selected supercritical fluids. 
Supercritical Fluid 
T.sub.c in .degree.C. 
P.sub.c in MPa 
______________________________________ 
carbon dioxide 31.1 7.38 
nitrous oxide 36.5 7.26 
ethylene 9.3 5.03 
ethane 32.3 4.88 
chlorotrifluoromethane 
29.9 3.92 
______________________________________ 
In addition to the supercritical fluids listed in Table 1, a large number 
of other materials are also useful in the methods of the present 
invention, including without limitation, nitrogen, propane, propylene, 
cyclohexane, isopropanol, benzene, toluene, p-xylene, ammonia, water, 
methane, trichlorofluoromethane, tetrafluoroethylene, perfluoroethane, 
tetrafluoromethane, trifluoromethane, and 1,1 difluoroethylene. The 
specific T.sub.c and P.sub.c for each of these materials, and for any 
other supercritical fluid useful in the methods of the present invention, 
are readily obtainable in a number of standard references, including the 
CRC Handbook of Chemistry and Physics, 67th ed., CRC Press Inc., Boca 
Raton, Fla., 1987, Matheson Gas bata Book, 6th ed., Matheson Co., Inc., 
Lyndhurst, N.J., 1980, Merck Index, 10th ed., Merck and Co., Rahway, N.J., 
1983, and Lange's Handbook of Chemistry, 12th ed., McGraw Hill Book Co., 
New York, N.Y., 1979, the disclosures of which are herein incorporated by 
reference. Furthermore, it is also contemplated that mixtures of two or 
more supercritical fluids could also be used in the methods of the present 
invention. 
While any of a variety of supercritical fluids are useful in the methods of 
the present invention, it is preferred that the supercritical fluid be 
substantially nonreactive and nontoxic (i.e. inert) with respect to the 
impregnation additives, carrier liquids, and polymers used in the methods 
of the present invention. In fact, the relative inertness of the 
supercritical fluid used in the methods of the present invention is 
particularly important with many of the biologically active additives 
impregnated into polymers according to the methods of the present 
invention. For example, when impregnating an active polypeptide (e.g. 
insulin) into a polymer substrate, a reactive supercritical fluid could 
inhibit or completely destroy the desired biological activity of such a 
polypeptide. Likewise, a toxic supercritical fluid may remain as a 
residual additive within the polymer substrate along with the impregnated 
polypeptide. Under either or both scenarios, the usefulness of 
impregnating such a polypeptide additive into a polymer substrate would be 
compromised or lost. 
Other factors that can influence the selection of a supercritical fluid for 
use in the methods of the present invention include cost of the 
supercritical material, solubility of the supercritical material in the 
polymer to be impregnated and the carrier liquid, as well as the practical 
working limits of the T.sub.c and P.sub.c of the supercritical fluid. In 
this regard, it is preferred that the T.sub.c of the supercritical fluid 
be as close as possible to ambient conditions (e.g. approximately 
22.degree. C.), such that the supercritical fluid can be maintained at a 
temperature of from about 0.degree. C. to about 100.degree. C., preferably 
from about 20.degree. C. to about 90.degree. C., and most preferably from 
about 30.degree. C. to about 80.degree. C. These Preferred temperature 
limits will prove particularly important with respect to biologically 
active additives, such as the active polypeptides noted above, which can 
be particularly susceptible to thermal degradation at temperatures in 
excess of about 40.degree. C. 
The preferred limits on the PC and the operating pressures of the 
supercritical fluid used in the methods of the present invention are 
practical in nature. For example, the upper limits of the operating 
pressures will be dictated, among other things, by the cost and 
availability of equipment capable of containing pressures in excess of 138 
MPa (20,000 psi), as well as the susceptibility of the impregnation 
additive and/or polymer to degradation at higher pressures. In this 
regard, it is preferred that the supercritical fluid be maintained at 
pressures from about 4 MPa to about 138 MPa, preferably from about 5 MPa 
to about 45 MPa, and most preferably from about 7 MPa to about 30 MPa. 
with the preferred temperature limitations noted above, biologically 
active additives will preferably be subjected to the minimum critical 
pressures necessary to ensure impregnation according to the methods of the 
present invention. 
With respect to the solubility of the supercritical fluid in the polymer or 
polymers to be impregnated by the methods of the present invention, it is 
preferred that the selected supercritical fluid show minimal solubility in 
the polymer to be impregnated. Thus, the supercritical fluid should have 
sufficient solubility to swell the polymer matrix, and thereby allow for 
the penetration of the carrier liquid and impregnation additive therein, 
but not provide such a degree of solubility that the polymer matrix loses 
its form and/or dissolves substantially into the supercritical fluid. 
Given the requirements outlined above, supercritical carbon dioxide 
provides a particularly preferred supercritical fluid for use in the 
methods of the present invention. Supercritical carbon dioxide is a low 
cost, inert, material displaying a T.sub.c of 31.1.degree. C. and a 
P.sub.c of 7.38 MPa. Furthermore, supercritical carbon dioxide displays 
sufficient solubility to swell a wide variety of polymeric materials, 
including both homopolymers and copolymers, such as polyethylene, 
polypropylene, polyamide, polyurethane, silicone, albumin, lactic acid 
polymers, and glycolic acid polymers, without dissolving or otherwise 
dissociating the polymer matrix. 
Impregnation additives 
The impregnation additive can comprise any element, compound or composition 
capable of being impregnated into a polymer using a supercritical fluid 
and a carrier liquid according to the present invention, so long as the 
impregnation additive is substantially insoluble in the supercritical 
fluid. As used herein, an impregnation additive is substantially insoluble 
in a supercritical fluid when none or virtually none of the impregnation 
additive will dissolve into the supercritical fluid at a predetermined 
temperature and pressure above the C.sub.p of the supercritical fluid. In 
this regard, an impregnation additive will be considered to be 
substantially insoluble in a supercritical fluid, so long as no more than 
an insignificant quantity, not readily detected by conventional means, 
dissolves into the supercritical fluid. 
Nonlimiting classes of impregnation additives useful in the methods of the 
present invention include dyes, monomers, drugs, proteins, polypeptides, 
nucleotides, and combinations thereof. Preferably, the impregnation 
additive will comprise a biologically active drug, polypeptide, or 
protein, including enzymes, hormones, antibiotics, anti-inflammatory 
agents, analgesics, calcium channel blockers, beta-blockers, 
antidepressants, antacids, antidiabetics, cerebral stimulants, sedatives, 
anti-parasitics, decongestants, muscle relaxants, anti-Parkinsonism 
agents, antiviral agents, bronchodilators, vitamins and dietary 
supplements and the like. Nonlimiting examples of suitable polypeptides 
and proteins useful as impregnation additives in the methods of the 
present invention include immunomodulators such as Thymic Humoral Factor, 
growth Factors such as Human Growth Factor and Fibroblast Growth Factor, 
antitumorals such as BCNU and epirubian, hormones such as LHRH, and 
steroidals such as medroxyprogesterone acetate and magestrol acetate. 
In an alternative embodiment of the present invention, the impregnation 
additive comprises one or more monomers, such as acrylic acid, ethylene, 
or propylene. In this embodiment, a polymer substrate undergoes a series 
of impregnations with one or more monomers followed by a polymerization of 
such monomers in situ to increase the strength, modulus, or other 
properties of the polymer substrate. 
Preferably, the impregnation additive according to the present invention is 
substantially insoluble over the entire critical temperature and pressure 
ranges of the supercritical fluid used in the methods of the present 
invention. However, it is to be understood that the impregnation additive 
need only be substantially insoluble in a supercritical fluid maintained 
at a set temperature and pressure to be considered within the scope of the 
present invention. 
One method of determining the insolubility of an impregnation additive in a 
supercritical fluid is to compare the solubility parameter of the 
impregnation additive with that of the supercritical fluid at a set 
temperature and pressure, or over a temperature and pressure range. When 
the solubility parameter for a solvent and a solute are within about 1 
(cal/cc).sup.1/2, then complete miscibility between the solvent and solute 
will generally occur. As the difference between the solubility parameters 
of the solvent and solute increase, the solute will becoming increasingly 
less soluble in the solvent, until a point is reached where the solute is 
substantially insoluble in the selected solvent. 
The solubility parameters of most materials are readily available and/or 
determinable by those skilled in the art. In this regard, such parameters 
can typically be found in a number of standard references, including 
Volume 21 of the Encyc. of Chemical Technology, 3rd ed., Wiley & Sons, New 
York, N.Y., pp. 377-401, 1984, the disclosures of which are herein 
incorporated by reference. Even when such parameters are not available in 
a standard reference, they can be estimated using standard equations for 
solubility parameters, such as Gaddings equations. For a more through 
discussion of solution chemistry and estimation of solubility parameters, 
reference should be had to Volume 21 of the Encyc. of Chem. Tech. and the 
Encyc. of Polymer Science and Engineering, Vol. 15, pp 380-402, 1989, the 
disclosures of which are herein incorporated by reference. 
Even when the solubility parameters of an impregnation additive and/or 
supercritical fluid cannot be readily obtained, the solubility, or lack 
thereof, of the impregnation additive in the supercritical fluid can be 
determined by a simple series of tests utilizing an apparatus, such as the 
impregnation apparatus illustrated in FIG. 1 herein. Specifically, in a 
first test, the impregnation additive and polymer to be impregnated can be 
placed in separate open containers 5 and 5' (not shown) inside of pressure 
vessel 25, which is then charged with the selected supercritical fluid at 
a predetermined temperature and pressure. After a predetermined period of 
time, typically an hour or more, the pressure vessel 25 is vented via vent 
line 45, and the polymer and impregnation additive removed. 
At the same or different time, in a second test, the impregnation additive 
and polymer should be simultaneously contacted with a carrier liquid in 
pressure vessel 25 using the same supercritical fluid and conditions 
according to the method of the present invention. Thereafter, the pressure 
vessel 25 is vented via vent line 45, and the polymer removed. 
Both polymer samples can then be visualized or otherwise assayed by means 
well known to those skilled in the art to determine whether or not the 
impregnation additive impregnated the polymer sample. If the polymer 
sample subjected to the method of the present invention shows 
impregnation, but the polymer sample maintained in separate containers 
does not, then it can be concluded that the impregnation additive was 
insoluble in the supercritical fluid at the selected conditions, and 
accordingly, that only the method of the present invention can provide for 
the impregnation of the polymer with the impregnation additive at the 
given temperature and pressure conditions. 
Carrier liquids 
The carrier liquid used in the method of the present invention is normally 
a liquid at atmospheric pressures and room temperature, and will typically 
remain a liquid during contact with the supercritical fluid. Preferably, 
the carrier liquid should be capable of partially or completely dissolving 
(i.e., forming a solution with) the impregnation additive to be 
impregnated into a polymer substrate according to the methods of the 
present invention. However, solubility of the impregnation additive into 
the carrier liquid is not required to practice the method of the present 
invention. Thus, in addition to true ionic or molecular solutions of the 
carrier liquid and impregnation additive, colloidal suspensions and 
two-phase dispersions of the impregnation additive and carrier liquid are 
also considered to fall within the scope of the methods of the present 
invention. 
As with the selected supercritical fluid, the carrier liquid will 
preferably be low cost, and inert (i.e. nonreactive and nontoxic) with 
respect to the impregnation additive, polymer substrate, and supercritical 
fluid. In addition, the carrier liquid will preferably be substantially 
insoluble in the polymer to be impregnated, but at the very least, the 
carrier liquid must not dissolve or otherwise dissociate the polymer 
substrate to be impregnated. Furthermore, it is also preferable that the 
carrier liquid does not have a high degree of solubility in the 
supercritical fluid under the process conditions, as the carrier liquid 
would then evaporate from the impregnation additive/carrier liquid 
solution, leaving a dry or nearly dry impregnation additive incapable of 
forming a solution with the supercritical fluid, and therefor incapable of 
impregnating the chosen polymer. 
Given the above requirements, water is the preferred carrier liquid for use 
in the methods of the present invention. Water is a low cost, inert 
liquid, that is poorly soluble or insoluble in most supercritical fluids 
and polymers to be impregnated. In addition, water is an excellent solvent 
for a wide variety of ionic compounds, and is readily capable of forming 
molecular solutions, colloidal suspensions, and various two phase 
dispersions. 
A variety of other carrier liquids may also be used in the methods of the 
present invention, including, without limitation, methanol, ethanol 
(ETOH), hexane, and combinations thereof. All of these carrier liquids 
typically suffer from one or more disadvantages (e.g. toxicity, 
reactivity, COBT, solubility in the polymer substrate, and/or solubility 
in the supercritical fluid) with respect to water, that make them less 
preferred in the methods of the present invention. However, some of these 
shortcomings may be overcome or diminished by using various mixtures of 
carrier liquids (e.g a mixture of water and ETOH) as the carrier liquid 
component in the methods of the present invention. 
For example, ETOH can serve as a useful carrier liquid in the methods of 
the present invention, particularly when the impregnated polymer is being 
employed in a nonbiological system. In addition to potential toxicity 
problems, ETOH is also somewhat soluble in typical supercritical fluids, 
such as supercritical carbon dioxide. Thus, when ETOH is employed as a 
carrier liquid, excess ETOH should be placed in the pressure vessel, such 
that a saturated solution of ETOH in the supercritical fluid is formed 
during processing. By maintaining a saturated environment within the 
pressure vessel, the polymer to be impregnated will remain immersed in the 
ETOH/impregnation additive solution throughout processing, thereby 
ensuring impregnation of the impregnation additive into the polymer 
substrate. 
Polymers 
Virtually any swellable polymeric material, including both homopolymers and 
copolymers, is useable in the methods of the present invention. 
Nonlimiting examples of polymeric materials useful in the method of the 
present invention include polyolefins, polyamides, polyimides, polyesters, 
polyurethanes, polyacrylates, polycarbonates, polyacetylenes, polyisoprene 
polymers, polystyrenes, styrenebutadiene polymers, chloroprene polymers, 
polyether-amides, vinyl chloride polymers, vinylidene chloride polymers, 
natural rubbers, butyl rubbers, nitrile rubbers, silicone, polyvinyl 
alcohol polymers, cellulobe derivative polymers, protein derivative 
polymers (e.g., albumin), lactic acid polymers, glycolic acid polymers, 
and combinations thereof. Preferred polymers include, without limitation, 
low density polyethylene, linear low density polyethylene, polypropylene, 
polyamide, polymers, albumin (e.g., BSA), and polymers prepared from 
lactic acid alone, glycolic acid alone, or lactic acid and glycolic acid 
copolymers. 
Process Parameters and Advantages 
While not being held to a theory of operation, it is believed that the 
method of the present invention functions to impregnate polymer substrates 
with an impregnation additive in a significantly different manner than 
that of the prior art. In the prior art methods, the supercritical fluid 
serves both as a solvent for the additive and as a swelling agent for the 
polymer to be impregnated. In contrast, in the methods of the present 
invention, the impregnation additive is substantially insoluble in the 
supercritical fluid. Therefore, the supercritical fluid acts only to swell 
the polymer, after which the impregnation additive solubilized in the 
carrier liquid impregnates or otherwise diffuses into the polymer 
substrate. Thus, it is believed that only the intimate simultaneous 
contact of an impregnation additive solubilized in the carrier liquid with 
the swollen polymer substrate will allow for the impregnation of the 
impregnation additive into the polymer substrate. 
After the impregnation additive and carrier liquid impregnate the swollen 
polymer substrate, the pressure in the pressure vessel containing the 
polymer and supercritical fluid is slowly reduced. It is believed that the 
gradual release of the pressure decreases the swelling of the polymer, 
thereby entrapping an amount of the impregnation additive within the 
polymer substrate. For example, when an ionic or molecular solution of a 
carrier liquid and impregnation additive (the CL/IA solution), impregnate 
a swollen polymer, the gradual shrinking of the swollen polymer by 
releasing the pressure in the pressure vessel, resulting in the 
precipitation or other deposition of the impregnation material from the 
CL/IA solution, eventually entrapping the impregnation additive within the 
polymer matrix. 
Since the carrier liquid acts to transport the impregnation additive within 
the polymer matrix, the methods of the present invention should function 
whether the impregnation additive forms an ionic or molecular solution, a 
colloidal suspension, or a two phase dispersion with the carrier liquid. 
With a colloidal suspension or two-phase dispersion, the mixture of the 
impregnation additive and carrier liquid may require pulsating or 
continuous agitation during impregnation to ensure that the additive 
remains essentially evenly dispersed or suspended in the carrier liquid. 
Absent such agitation, it is believed that an ionic or molecular solution 
of the impregnation additive in the carrier liquid will be a preferred 
manner of practicing the method of the present invention. 
A number of parameters and characteristics of the selected supercritical 
fluid, impregnation additive, carrier liquid, and polymer substrate 
influence the degree to which the impregnation additive impregnates the 
polymer substrate. For example, three important variables, temperature, 
pressure, and time, come into play during the impregnation of a polymer 
substrate by an impregnation additive. 
In general, the higher the temperature employed during impregnation, the 
greater the swelling of the polymer substrate. This in turn has the 
capacity to increase the amount of impregnation additive entering the 
polymer substrate. However, as noted above, the ability to use higher 
impregnation temperatures is tempered, among other things, by the 
susceptibility to the impregnation additive to thermal degradation. Thus, 
many biologically active materials, and in particular biologically active 
polypeptides, are readily subject to thermal degradation. Furthermore, the 
susceptibility of the polymer substrate to thermal degradation and/or 
melting will also place a practical limit on the temperatures employed in 
polymer impregnation. 
In a similar fashion, the pressures employed during polymer impregnation 
are also limited by practical considerations. While increased pressures 
generally lead to increased swelling of the polymer substrate, a point is 
reached where the surface pressure exerted by the supercritical fluid at 
very high pressures may counteract the increased swelling of the polymer 
substrate, thereby placing a practical cap on the pressures that can or 
should be employed in such a method. In addition, it is known that many 
biologically active materials, such as polypeptides, degrade or otherwise 
lose their activity when exposed to high pressure conditions, particularly 
over an extended period of time. Accordingly, such materials should be 
impregnated at as low a pressure as possible, while still staying at or 
above the P.sub.c of the supercritical fluid being used in the methods of 
the present invention. 
In general, longer periods of exposure to a supercritical fluid at 
supercritical conditions will favor deeper penetration of an impregnation 
additive into a polymer substrate. In this regard, the need to utilize 
relatively long periods of exposure will prove particularly necessary the 
larger and thicker the polymer substrate to be impregnated. For example, a 
3 mm in diameter polyethylene bead, having a relatively large surface 
area, may require approximately two hours in which to impregnate an 
aqueous dye solution (e.g. rose bengal dye) to its core, while a 2.5 
cm.times.2.5 cm.times.0.5 cm thick polyethylene film may require in excess 
of 6 hours of exposure to the impregnation conditions to achieve the same 
degree of penetration as the polyethylene bead. However, the total period 
of exposure to impregnation conditions will always have to be balanced 
against the degree of degradation of the impregnation additive and/or 
polymer substrate resulting from prolonged exposure at the chosen 
temperature and pressure conditions employed. 
Several characteristics or properties of the polymer, such as the 
crystallinity, density, orientation, and amount of crosslinking appear to 
influence the degree to which any given polymer can be impregnated with an 
impregnation additive. In general, the higher the density, crystallinity, 
orientation and crosslinking of the polymer substrate, the more rigorous 
the conditions needed to successfully impregnate an impregnation additive 
into a polymer substrate. Thus, a low density, nonoriented polymer that is 
highly amorphous and non-crosslinked in structure should be relatively 
easier to impregnate with a given additive than a similar high density, 
oriented, crystalline polymer with a relatively high degree of 
crosslinking. 
The methods of polymer impregnation of the present invention provide a 
number of advantages over presently available methods. Principal among 
these is the ability to impregnate additives which are substantially 
insoluble in the supercritical fluid swelling agent. To date, existing 
methods have required the impregnation material to be capable of forming a 
solution with the supercritical fluid in order to impregnate the 
impregnation material into a polymer substrate. Thus, only the methods of 
the present invention can provide for the impregnation of this class of 
materials into polymer substrates. 
The ability of a carrier liquid to transport an impregnation additive into 
a polymer matrix swollen by exposure to a supercritical fluid provides for 
the incorporation of incompatible additives and polymer substrates. Thus, 
hydrophilic impregnation additives, such as rose bengal dye, can be 
impregnated into hydrophobic polymers such as polyethylene, polypropylene, 
and polyurethane via the methods of the present invention. In this regard, 
such hydrophilic impregnation additives could not be impregnated using 
existing supercritical methods, since these additives are incompatible 
(i.e., insoluble) with both the lipophilic supercritical fluid (e.g., 
supercritical CO.sub.2) and the hydrophobic polymer substrates. 
Furthermore, it will also be appreciated that the preferred carrier liquid 
of the present method is water, which is likewise incompatible with 
preferred supercritical fluids and hydrophobic polymer substrates. 
Though the method of the present invention utilizes impregnation additives 
that are substantially insoluble in the supercritical fluid, it will be 
appreciated that the use of carrier liquids according to the present 
invention could enhance the impregnation of additives with a low degree of 
solubility in the supercritical fluid. In addition, the method of the 
present invention may also prove useful to impregnate additives with 
gaseous mediums, such as CO.sub.2, maintained below their C.sub.p (i.e., 
subcritical). In this regard, as long as the pressurized gaseous medium is 
capable of swelling the polymer substrate, the intimate simultaneous 
contact of the impregnation additive, carrier liquid, and swollen polymer 
substrate should allow the carrier liquid to transport the additive into 
the polymer matrix, albeit at a somewhat reduced level than would be 
accomplished using a supercritical fluid. 
Use of supercritical CO.sub.2 as the preferred supercritical fluid, and 
water as the preferred carrier liquid will also result in a number of 
advantages. Both CO.sub.2 and water are low cost, inert (e.g., nonreactive 
and nontoxic) materials, that are easy and safe to work with, and in most 
instances will have no deleterious effects on either the polymer substrate 
or impregnation additive. 
In addition, water is an excellent solvent or dispersing medium for many 
impregnation additives, but is substantially insoluble in supercritical 
CO.sub.2 over a wide range of temperature and pressure conditions. 
Furthermore, the T.sub.c of CO.sub.2 is relatively close to ambient 
conditions, such that a wide variety of biologically active additives can 
be impregnated into polymer substrates without significantly altering 
their desired biological activity. 
The fact that the impregnation additives of the present invention are 
substantially insoluble in the supercritical fluids allows for the 
impregnation of several different additives into polymer substrates at the 
same time, without unwanted cross-contamination between the various 
samples. In addition, this same property, in combination with an inert 
carrier liquid such as water, will allow for complete recovery of often 
costly unimpregnated additives after the impregnation process is complete. 
Further, the use of a carrier liquid will, in many cases, prevent the 
unwanted extraction of various components, such as plasticizers and 
tackifiers, from the polymer substrates during the impregnation process. 
The invention will be further illustrated by reference to the following 
non-limiting Examples. All parts and percentages are expressed as parts by 
weight unless otherwise indicated. 
EXAMPLES 1 AND 5-10, AND COMATIVE EXAMPLES 2-4, AND 11-13 
Five gram samples of various polymeric materials were placed in an open 
glass vial in a 300 cubic centimeter MICRO SERIES.TM. pressure vessel 
(2-9/16 inch outside diameter, 11-3/4 inch overall length; Newport 
Scientific, Inc., Jessup, Md.). The specific polymeric materials utilized 
included, ASPEN.TM. 6806 polyethylene (PE) beads (3 mm diameter; melt 
index=105.degree. C., linear low density polyethylene; Dow Chemical Co., 
Midland, Mich.), No. 3145 polypropylene (PP) beads (3 mm diameter; melt 
index=300.degree. C.; Exxon Chemical Co., Darien, Conn.), and No. 19, 
105-1 polyamide pellets (PA) (3 mm diameter; melt index=95.degree. C.; 
Aldrich Chemical Co., Milwaukee; Wis.). The polymeric materials were 
contacted with, or remained separate from, an impregnation additive 
consisting of 0.25 g of rose bengal dye (Aldrich Chemical Co.) in either a 
dry, powdered form, or in a solution or dispersion of 0.25 g of rose 
bengal dye in 15 ml of various carrier liquids, including deionized water, 
hexane, or ethyl alcohol (ETOH). A glass wool plug was used to keep the 
beads and pellets submerged in the dye/carrier liquid solutions. 
After placement of the polymeric material, and dye (with or without a 
carrier liquid) in the pressure vessel, the vessel was closed and charged 
with either CO.sub.2 or N.sub.2 gas. The enclosed system was adjusted to 
either supercritical or subcritical temperature and pressure conditions 
for the contained gases (supercritical CO.sub.2 =31.degree. C. and 7.38 
MPa; supercritical N.sub.2 =-147.degree. C. and 3.39 MPa), and maintained 
at those conditions for 2 hours, after which the pressure vessel was 
vented down to ambient conditions. The polymeric materials were recovered 
from the pressure vessel, rinsed with deionized water, allowed to dry at 
ambient conditions overnight (e.g. 12-18 hrs.), and were observed to 
determine the degree of dye impregnation into the polymeric materials, if 
any. The specific combination of polymer material, impregnation additive, 
carrier medium (if any), supercritical fluid, including temperature and 
pressures utilized, method of sample treatment, and observed color change 
(if any), for Examples 1, and 5-10, and Comparative Examples 2-4, and 
11-13 are given in Table 2 below. 
After visual observation, each of the dried polymer beads were analyzed 
using a Minolta Chroma Meter No. CR-200/Cr-231 tribtimulus color analyzer 
to determine the degree of rose bengal dye impregnation, if any. Undyed 
and untreated polymer beads and pellets were utilized as controls. The 
Chroma Meter is a compact tristimulus color analyzer for measuring 
reflective colors of surfaces. Absolute measurements were taken in L*a*b* 
(CIE 1976) in the Munsell color system. For a more in depth review of the 
measurement parameters of the Chroma Meter color analyzer, reference 
should be had to the Minolta Chroma Meter CR-200/CR231 Technical Reference 
Manual, Version 2.0, Minolta, Inc., Japan, the disclosure of which is 
herein incorporated by reference. 
Prior to measuring the reflective color of the beads, the Chroma Meter was 
color corrected on a standard white plate. Thereafter, control and treated 
polymer beads were placed in a small plastic weigh boat on the standard 
white plate in such a way that a layer, two beads thick, could be 
maintained. The Chroma Meter was placed in intimate contact with the beads 
and three readings were taken per sample in different locations on the 
beads. Mean values of L, a and b color space were obtained using the 
internal statistic capability of the unit. 
Color difference between the control and treated beads was calculated as 
.DELTA.E*.sub.ab using the following formula: 
EQU .DELTA.E*.sub.ab = (.DELTA.L*).sup.2 +(.DELTA.a*).sup.2 +(.DELTA.b*).sup.2 
where, L* is the lightness factor (L=0 is black; L=100 is white), and a* 
and b* are chromaticity coordinates (+a=red; -a=green; +b=yellow; and 
-b=blue). Total color difference .DELTA.E is defined by the geometric mean 
of the differences in the L, a, and b color coordinates between the 
control and treated polymer beads. The color chromatic coordinates (L, a 
and b) for the control beads and sample beads of Examples 1, and 5-10, and 
Comparative Examples 2-4, and 11-13, as well as the color difference 
(.DELTA.E) between the control and sample beads are shown below in Table 
3. 
A further analysis was performed on the rose bengal dye-impregnated PE 
beads of Example 9 to determine the amount of dye incorporated therein. 
Several grams of these beads were placed in 50 ml deionized water and 
stirred for 48 hours to extract the dye from the PE beads. A spectroscopic 
analysis was performed on the dye solution using a Perkin-Elmer, Lambda 
4B, uv-vis spectrometer (Perkin-Elmer, Wilton, Conn.) at a wavelength of 
356 nm. The results were compared to a standard curve for rose bengal dye. 
Spectroscopic analysis of the solution confirmed that approximately 0.04 
mg of dye per gram of the polyethylene beads had been incorporated into 
the polyethylene beads during the impregnation process. 
TABLE 2 
__________________________________________________________________________ 
Selected combinations of polymer material, impregnation additive, carrier 
medium (if any), supercritical fluid, including 
temperature and pressures utilized, method of sample treatment, and 
observed color change (if any), for Examples 1 and 5-10, 
and Comparative Examples 2-4, and 11-13. 
Polymer 
Impreg. 
Carrier 
SC Temp 
Press. 
Ex. No. 
Material 
Additive 
Medium 
Fluid 
(.degree.C.) 
(MPa) 
Sample Treatment Observation 
__________________________________________________________________________ 
Ex. 1 
PE rose water 
CO.sub.2 
60 13.8 
PE beads immersed in solution 
PE beads dyed dark 
beads 
bengal dye of rose bengal dye and 
pink throughout 
C. Ex. 2 
PE rose none CO.sub.2 
60 13.8 
PE beads in first vial, dry 
PE beads remain 
beads 
bengal dye bengal in second vial 
milky white in color 
C. Ex. 3 
PE rose water 
CO.sub.2 
60 13.8 
PE beads in first vial, sol. 
PE beads remain 
beads 
bengal dye rose bengal and water in second 
milky white in color 
C. Ex. 4 
PE rose none CO.sub.2 
60 13.8 
PE beads in vial with dry 
PE beads remain 
beads 
bengal dye bengal dye milky white in color 
Ex. 5 
PP rose water 
CO.sub.2 
60 13.8 
PP beads immersed in solution 
PP beads dyed pink 
beads 
bengal dye of rose bengal and water 
throughout 
Ex. 6 
PA rose water 
CO.sub.2 
60 13.8 
PA pellets immersed in 
PE beads dyed light 
pellets 
bengal dye of rose bengal and water 
pink throughout 
Ex. 7 
PE rose ETOH CO.sub.2 
60 13.8 
PE beads immersed in solution 
PE beads light pink, 
beads 
bengal dye of rose bengal and ETOH 
not in bead center 
Ex. 8 
PE rose hexane 
CO.sub.2 
60 13.8 
PE beads immersed in 
PE beads dyed pink 
beads 
bengal dye dispersion of rose bengal and 
thoughout 
Ex. 9 
PE rose water 
CO.sub.2 
35 13.8 
PE beads immersed in 
PE beads dyed pink 
beads 
bengal dye solution of rose bengal dye and 
throughout 
Ex. 10 
PE rose water 
CO.sub.2 
60 13.8 
PE beads in vial with dry rose 
PE beads remain 
beads 
bengal dye 
vapor water in bottom of pressure 
milky white in color 
C. Ex. 11 
PE rose water 
CO.sub.2 @ 
60 1.3 PE beads immersed in 
PE beads dyed very 
beads 
bengal dye solution in rose bengal and 
light pink 
C. Ex. 12 
PE rose water 
N.sub.2 @ 
60 13.8 
PE beads immersed in 
PE beads remain 
beads 
bengal dye solution of rose bengal and 
milky white in color 
C. Ex. 13 
PE rose ETOH CO.sub.2 
60 13.8 
PE beads in first vial, sol. of 
PE beads remain 
beads 
bengal dye bengal and ETOH in second 
milky white in 
__________________________________________________________________________ 
color 
@1.3 MPa comprises subcritical pressure conditions for CO.sub.2, and 13.8 
MPa comprises subcritical pressure conditions for N.sub.2. 
TABLE 3 
______________________________________ 
Color chromatic coordinates (L, a and b) for the control beads 
and sample beads of Examples 1, and 5-10, and Comparative 
Examples 2-4, and 11-13, and color difference (.DELTA.E) between the 
control and sample beads. 
Controls and Example Nos. 
L a b .DELTA.E 
______________________________________ 
Polyethylene Control 
68.82 -0.52 +0.84 
Example 1 53.52 +24.22 -9.58 30.90 
Comparative Example 2 
75.58 +0.73 -0.97 7.10 
Comparative Example 3 
72.95 +1.66 -1.61 5.27 
Comparative Example 4 
70.28 +6.90 -2.84 8.41 
Polypropylene Control 
70.28 -1.42 +6.52 
Example 5 53.85 +18.44 -3.18 27.54 
Polyamide Control 
52.74 -1.14 +29.35 
Example 6 30.16 +32.22 +9.56 45.10 
Example 7 68.75 +3.22 -1.00 4.17 
Example 8 66.77 +14.11 -7.61 17.12 
Example 9 na na na na 
Example 10 51.69 +28.05 -10.17 35.08 
Comparative Example 11 
51.83 +12.82 -5.19 22.43 
Comparative Example 12 
63.33 +4.05 -4.25 8.18 
Comparative Example 13 
73.99 +1.81 -1.59 6.67 
______________________________________ 
Comparative examples 2-4 show that rose bengal dye, whether separate or in 
contact, dry or in solution with water separate from the PE beads, does 
not impregnate the PE beads, and thus is insoluble in supercritical 
CO.sub.2 at the disclosed conditions. Only by practicing the method of the 
present invention in Example 1, by simultaneously contacting the PE beads 
with an aqueous solution of rose bengal dye, is impregnation of the beads 
accomplished. In addition, Examples 5-9 illustrate that the same or 
similar results can be achieved with other polymers (PP and PA), other 
carrier liquids (ETOH and hexane), and at different temperature conditions 
(Example 1=60.degree. C.; Example 9=35.degree. C.). Example 10 shows that 
impregnation can be accomplished via a carrier liquid in a vapor (e.g., 
humid H.sub.2 O) rather than a liquid state. However, in such an instance 
the degree Of Visible dye impregnation is considerably lower than that 
accomplished using the carrier liquid in a liquid state (compare Examples 
1 and 10). 
Comparative Example 11 shows that some, albeit a significantly lesser 
degree of dye impregnation, can be accomplished using subcritical CO.sub.2 
(e.g., 1.3 MPa-v-13.8 MPa). However, use of subcritical N.sub.2 does not 
result in dye impregnation. See Comparative Example 12. Furthermore, 
Comparative Example 13 confirms the result of Comparative Example 3, 
except using ETOH versus water as the carrier liquid. 
The color measurements provided in Table 3 provide further quantification 
of the subjective observations of degree of dye impregnation noted in 
Table 2. Examples 1, 5, 6, and 10 using the preferred method of the 
present invention show the most pronounced darkening of the sample beads 
(i.e., reduced L value), the greatest increase in red coloration (greatest 
+a values), and largest overall color different (.DELTA.E) relative to the 
control samples. In contrast, the coloration of Comparative Examples 2-4 
show little or no significant difference in coloration from the controls. 
In this regard, the human eye can at beet detect a visible color 
difference of +5 units. In addition, the values for Examples 7 and 8 show 
that ETOH and hexane are not as effective carrier liquids as water 
(Compare to Example 1). 
EXAMPLE 14 
Four grams polyamide pellets (No. 19, 105-1, 3 mm diameter, melt 
index=95.degree. C., Aldrich Chemical Co.) were placed in each of two 
separate 12 cc glass vials. A blue dye solution of 0.0437 g indigo carmine 
dye (Aldrich Chemical Co.) in 10 ml deionized water was placed in one 
vial, and a red dye solution of 0.0755 g rose bengal dye (Aldrich Chemical 
Co.) in 10 ml deionized water was placed in the second vial. Each vial was 
covered with a plug of glass wool to maintain the polyamide pellets 
immersed in the dye solutions. The vials were stacked in a 180 cc MICRO 
SERIES.TM. pressure vessel (Newport Scientific Inc.), and the pressure 
vessel sealed. The vessel was charged with CO.sub.2, stabilized at 13.8 
MPa and 60.degree. C., and maintained at these conditions for 17 hours. 
The vessel was slowly vented over a 15 minute period and the sample 
pellets recovered. Some of the polyamide pellets in the indigo carmine 
solution were floating, and showed visible blue dye impregnation where 
they were submerged in the dye solution, and no dye impregnation where 
they had floated above the dye solution. The polyamide pellets in the rose 
bengal dye solution remained completely submerged, and showed complete 
pink dye impregnation. There was no evidence of cross-contamination 
between the vials containing the dye solutions, as the dye solutions 
retained their original blue and rose tints, which in turn demonstrated 
insolubility of the dyes and dye solutions in supercritical CO.sub.2 at 
the indicated temperature and pressure conditions. 
The unimpregnated dyes were recovered from the solutions via rotary 
evaporation (recovered rose bengal dye=0.0636 g or 84%; recovered indigo 
carmine dye=0.0380 g or 87%). Thus, the impregnated polyamide pellets 
contained approximately 16% rose bengal dye, and 13% indigo carmine dye 
respectively, assuming negligible loss of dye during impregnation. 
EXAMPLE 15 
Five grams of polyamide pellets (No. 19,105-1; 3 mm diameter; melt 
index=95.degree. C.; Aldrich Chemical Co.), five grams of polyethylene 
beads (ASPEN.TM. No. 6806; 3 mm diameter; melt index =105.degree. C.; 
linear low density polyethylene, Dow Chemical Co.) and an approximately 
2.5 cm.times.2.5 cm.times.1 mm thick polyurethane film (source unknown) 
were placed in a single glass vial containing a dye solution of 50 ml 
deionized water, 13 mg rose bengal dye (Aldrich Chemical Co.) and 2 drops 
of an aqueous solution of FD&C blue dye No. 1 (Schilling Food Color; 
McCormick & Co., Hunt Valley, Md.). A glass wool plug was used to keep the 
polymer samples submerged in the dye solution. The vial was placed in a 
180 cc MICRO SERIES.TM. pressure vessel, which was sealed, charged with 
carbon dioxide, stabilized at 50.degree. C. and 13.8 MPa, and maintained 
for 17 hours. Thereafter, the vessel was vented down over a 1 minute 
period, and the polymer samples and dye solution recovered. Each of the 
polymer samples were rinsed with two portions of deionized water and 
blotted dry. The polyurethane film was removed, and the polyamide pellets 
and polyethylene beads were separated from one another by shape 
differentiation. All polymer samples were colored bluish-rose indicating 
impregnation by both dyes therein. 
Each of the three polymer samples were placed in a beaker containing 20 
mils of a pH 9 aqueous buffer solution (Fisher Scientific Co., Fairlawn, 
N.J.), and stirred overnight. Each of the aqueous buffer solutions were 
qualitatively analyzed in a Perkin-Elmer, Lambda 4B, uv-vis spectrometer 
(Perkin-Elmer, Wilton, Conn.) at wavelengths of 514 nm and 550 nm. Both 
the buffer solutions containing the polyamide pellets and the polyethylene 
beads contained partially extracted dyes, which when analyzed chowed 
absorbencies at 514 nm and 550 nm for the FD&C blue dye No. 1, and rose 
bengal dye, respectively. The buffer solution containing the polyurethane 
film sample did not contain any extracted dyes. 
EXAMPLE 16 
Five grams polyethylene beads (ASPEN.TM. No. 6806; 3 mm diameter; melt 
index=105.degree. C.; linear low density polyethylene, Dow Chemical Co.), 
a 25 cm.times.25 cm.times.1 mm thick polyurethane (PU) and polyethylene 
terephthalite (PET) laminated film (source unknown), and an approximately 
3.5 cm.times.3.5 cm.times.1 mm thick silicone film (SILASTIC.TM. No., Dow 
Corning corp., midland, Mich.) were placed in a 25 cc glass vial 
containing 0.25 g robe bengal dye (Aldrich Chemical Co.) dissolved in 25 
mls of absolute ethyl alcohol (ETOH). The vial was placed in a 300 cc 
pressure vessel. In addition, 30 mils ETOH was also placed in the bottom 
of the pressure vessel, outside the glass vial, to create a saturated 
ETOH/supercritical CO.sub.2 solution, and thereby prevent the complete 
uptake of the ethanol f rom the sample vial, when the pressure vessel was 
pressurized. 
After placement of the vial and excess ETOH, the pressure vessel was 
closed, charged with CO.sub.2, stabilized at 20.7 MPa and 40.degree. C., 
and maintained for 4 hours. Thereafter, the pressure vessel was vented 
down over a 2 minute period, and the polymer samples recovered and rinsed 
with deionized water. The polyethylene beads were pink, and the silicone 
film was red, indicating impregnation by the rose bengal dye. In addition, 
the polyurethane side of the PU/PET film was red, indicating dye 
impregnation, but the PET layer on the opposite side of the film laminate 
remained clear. Thus, the disclosed impregnations conditions were 
sufficient to provide impregnation of all but the PET polymer. In this 
regard, the PET would probably require more rigorous temperature and 
pressure conditions to provide for impregnation of the rose bengal dye 
therein. 
COMATIVE EXAMPLE 17 
Two pieces of polyvinyl chloride (PVC) tubing (TYGON.TM. tubing; Norton 
Co., Worcester, Mass.), which contain a plasticizing agent were weighed. 
The first sample weighed 2.0732 grams and wall placed in a test tube 
without water. The second sample weighed 2.1546 grams and was placed in a 
test tube containing a solution of 0.1050 g of indigo carmine dye (Aldrich 
Chemical Co.) in 20 ml of deionized water. Both test tubes were placed in 
a pressure vessel, which was sealed, charged with CO.sub.2, stabilized at 
20.7 MPa and 50.degree. C., and maintained for 5 hours. Thereafter, the 
pressure vessel was rapidly vented down to atmospheric pressure, and the 
samples were recovered. 
The first sample of PVC tubing exposed directly to supercritical CO.sub.2 
foamed, visibly shrank in size, became very stiff, and lost 0.4307 grams, 
or approximately 20 percent of its total weight. In addition, the test 
tube containing the first sample also contained a small portion of an oily 
liquid, assumed to contain extracted plasticizer, based on the stiff 
physical characteristics of the foamed tubing. 
The second sample of PVC tubing immersed in the indigo carmine dye solution 
retained its soft, pliable character, and foamed due to the rapid venting 
of the pressure vessel. The final weight of the second sample was 2.0770 
grams, a slight increase of 3.8 mg, most likely due to absorption of 
moisture into the foamed tubing. In addition, none of the oily substance 
observed with the first sample was observed on the tubing, or in the dye 
test tube containing the dye solution. Furthermore, no impregnation of the 
indigo carmine dye was observed in the second sample of foamed tubing. 
This example shows that the method of the present invention does not lead 
to unwanted extraction of additives from the polymer to be impregnated, as 
is observed when utilizing the prior art method. In addition, while dye 
impregnation was not accomplished under the disclosed conditions, it may 
be reasonably assumed that more rigorous temperature and/or pressure 
conditions could result in the impregnation of indigo carmine dye into the 
PVC tubing. Furthermore, through more selective control of the venting of 
the pressure vessel (i.e. more slowly), foaming of the PVC tubing could be 
prevented. 
While in accordance with the patent statutes, description of the preferred 
weight fractions, and processing conditions have been provided, the scope 
of the invention is not to be limited thereto or thereby. Various 
modifications and alterations of the present invention will be apparent to 
those skilled in the art without departing from the scope and spirit of 
the present invention. 
Consequently, for an understanding of the scope of the present invention, 
reference is made to the following claims.