AU2005205795A1 - Process for infusing an alkali metal nitrite into a synthetic resinous material - Google Patents

Description

P/00/011I Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "PROCESS FOR INFUSING AN ALKALi METAL NITRITE INTO A SYNTHETIC RESINOUS MATERIAL" The following statement is a full description of this invention, including the best method of performing it known to me/us: 101 NITRITEEL ONT AY THE REINE OMTRA The prsn5neto eae oapoesfrtasern atclrcaso 0atnml nakl ea at rmsprrtclcro ixd "CC2)i whc h ati nslto, ietyit ytei einu usrtfreape Ith p esentainvtion relnates oamproces fimor tasfaerd pamriculariclassitof cmiainoaniogncsaltadapeo, namely an alkali metal satnfoitprritiea carbo dixda"C-0 1 i filmdr shedol artic inrwiche oy oher wayn mofe inortporate saltis phalyeixin ith cminton aplye, t enr weltoesingrte mixtueolre. uiorl dsrbtdtruhuthboyotharil.The roblemi atclrysroswe Inatil is mdertom impregtnaplymricnflmaret wates-hape polyei artcedt hindered phenol which rieadaed beenth oyfni molded udrtme at codtin temperature above about 121 0 IC (250 9 as evidenced by the polymer tu rning yellow. It is also desirable that crystalline particles of the ingredients infused into the polymer, be invisible to the human eye. Such invisibility results when the particles are smaller than about 10 gmu (micrometers) equivalent diameter. By "equivalent diameter" is meant the diameter of a spherical particle of equal volume. All references herein to particle size refer to the equivalcot diameter, At present, the sodium nitrite is incorporated into the polymer as particles in the size range from about 10 jim to"4 pm, all of which pass o 2 through a 325 mesh screen or 45 pmn (Standard Test Sieves), as smaller particles do not get uniformly distributed when an article is molded, or film is extruded, with polymer containing the smaller particles. Further, non-uniformly distributed particles, whether large or small in the range from about 10 44 pmn, causes the polymer's melt-processed tn6 surface to be noticeably rough to the touch, compared to a smooth suz-6ce of film of the same polymecr without any sodium nitrite particles. In film, or molded articles with a o cross-section of less than 50 pirm (0.002" or 2 mul), particles smaller than 10 AMn are not visible to the human eye; larger particles are. The (imitation of concentration is o necessary because a concentration of particles in an amount of 2% by weight, or more, in the polymer substrate becomes visible to the human eye, even if the particles are smaller than 10 pmn. Though the emphasis is on the temperature sensitivity of a protective antioxidant or iuv light stabilizer, a biodegradable polymer may itself be equally sensitive.

BACKGROUND OF THE INVNION The conventional way of introducing a solid ingredient lato a polymer is by melt- procesing a mixture-of the ingredient and polymer, either in an extruder, or by plasticizing the polymer sufficiently so as to be able to mix the ingredient into the plasticized mass. Such a procedure, may be applicable if an ingredient can withstand a temperature at which the polymer is plasticizabl;, in the case of polyvinyl chloride (PVC), this temperature is in the range from about 180'C to about 200 0

C.

When the properties of an ingredient are to be preserved when it is incorporated into a polymer, melt-processing an ingredient which is degradable at a temperature required for melt-processing is thus negated.

In the prior art, SC-CO 2 has been used mainly to extract an organic ingredient from a substrate in which it is distributed, or to separate one organic compound from another. Each of the foregoing relies upon the known higher solvent power of a fluid as its density increases, A supercritical fluid is sufficiently dense to swell a polymer even if the polymer is essentially insoluble in the supercriticaA fluid, by virtue of tbrcing molecules of the suipercriticaj fluid into the pores of the polymer, as taught by U.S.

Patent No. 5,340,614 to Perman et al., provided a carrier liquid was also used to "cany" the additive into the pores of the polymeric substrate. However, Perman et ai were unconcerned with either uniformity of distribution of infuised particles, or their size, and make no mention of either.

Unlike the foregoing, U.S. Patent No. 4,820,752 teaches "infusing into a rubber or plastic polymeric material" (sic) any additive, liquid or solid, which has a "degree of o solubility in said polymeric material when said polymeric material is in a swollen ci stae", using a normally gaseous fluid which could be compressed to supercritical o conditions. Clearly, solubility of the additive in the polymer is required, the degree of solubility being at least 0. 1 percent, and under the high pressure conditions described, any microporous polymner substrate will necessarily become swollen'.

Moreover, it is not clear whether the fluid is required to dissolve either the additive or the polymer, or both, or simply swell the polymer, such equivocation being stated as follows: "A fluid may have sufficient solvent or swelling power to be useful in practicing this invention if sufficiently compressed at temperatures above, equal to, or below the critical temperature of the fluid" (see '752 patent, eel 4, l ines 45-49).

Evidently the only requirement of the fluid to provide all the necessary properties to infuse a polymer with any additive is that the fluid be derived by compressing a normally gaseous fluid. Such evidence is provided in Examples I and 2 teaching the use of carbon dioxide at 22TC and pressure of 59.6 x 10 5 Pa. (or 5960 14%, and in Examples 3 and 4 at 22TC and pressure of 65.1 x 103 Pa (or 6510 kPa), under which conditions carbon dioxide is not supercritical. The supercritical conditions fbr carbon dioxide are 31 .4 0 C and 73.4 atm (7435.42 kPa). Only Example 6 deals with SC-CO 2 which was used to impregnate a polyurethane sheet with progesterone, the solubility of which, in

SC-CC

2 or lack thereoC was not stated.

Still ftuther, the '752 patent states: "In accordance with the present invention, an additive desired to be included in a rubber or plastic composition is dissolved in a compressed normally gaseous fluid." ('752, col 2, tines 16-17) but requires only that the additive have a "degree of solubility" stated as follows: 'The fluid and additive are chosen so that the additive has a degree of solubility in the polymer into which it is to be infused and so that the solution of fluid and additive has a degree of solubility in the polymer and is capable of swelling the polymer." ('752, col 2, lines 29-3 if the CA "degree of solubility" included essentially complete solubility, then the polymer too would be dissolved in the "the solution of fluid and additive".

As evidenced by illustrative example 1, "'other sample chips were exposed in a pressure vessel to carbon dioxide in the presence of solid naphthalene at room temnperature and a pressure of 59.6 x 10 pascals for 92 hours;" indicating that all the o naphthalene was not in solution. This may be attibutable to the fact that the carbon dioxide was not under supercritical conditions. The only example where SC-CO 2 was o used (Example 6) states that a sheet of polyurethane "was exposed to progesterone in the presence of carbon dioxide at a temperature of 45 0 C and a pressure of 151.6 pascals for 4.5 he' indicating not all the progesterone was in solution. Thus, Berens et al we= unaware that solid particles of relatively large size could be transported if fully dissolved in SC-CO 2 to form a solids-free solution, and the solids redeposited in a polymer substrate as micronized crystas.

Persons of ordinary skill in the art are well aware that the solubility of an additive in a supercritical fluid, and in SC-CO 2 in particular, cannot be predicted. This is particularly true for inorganic salts. For example, as shown below, sodium nitrite is soluble in SC-C0 2 but sodium chloride is not. In view of such unpredictability it should now be evident that a teaching that any normally solid additive, whether organic or inorganic, may be transferred, from a liquid under pressure into a swellable polymer, as taught in the '752 patent, is at best overly broad.

U.S. Patent No. 4,290,912 issued to Boerwinkle et al, about two decades ago, disclosed that an inorganic nitrite, and in particular, an alkali metal nitrite, e.g.

potassium nitrite, soditum nitrite and calcium nitrite, in combination with a 2,4,6trisubstituted phenol provided an effective volatile corrosion inhibitor (VCI) when distributed in a lower (C 2 C3) polyolefin (PO) polymer, A specific '91 2 combination comprised about equal parts (lASS5 pint each) by weight of sodium nitrite and a 2,4,6tni-substituted phenol containing 9 to 24 carbon atoms, specifically 2,6-di-tert-butyl-4methyl phenol, along with small amounts of one or more inert ingredients such as finned silica and oleyl alcohol which are known to possess no anti-corrosive properties.

Effectiveness. of the '912 film was unconcerned with the primary particle size of the sodium nitrite because the '912 patent did not address the problems of uniformity of distribution, (Hi) of particle size, or (iii) of maintaining transparency of extruded film or Cl molded articles having smooth surfaces.

SUMMARY-OF THE INVENTION ON It has been discovered that an alkali metal nitrite which is substantially insoluble in liquid carbon dioxide which is not under supercritical conditions, becs highly soluble when the conditions become supercritical. The expectation that all alkali metal salts are highly soluble in supercritical carbon dioxide "SC-CC 2 is negated by the substantial insolubility of sodiuni chloride in SC-CO 2 Moreover,-one may dissolve crystals of an alkali metal nitrite larger than 10 Pan in SC-CO 2 and by impregnating a polymer substrate with the solution, deposit crystals smaller than 10 PM in the substrate. The most preferred alkali metal nitrite is sodium nitrite.

A process for rapidly infusing a synthetic resinous substrate with an alkali metal nitrite, comprise essentially completely dissolving the alkali metal nitrite in SC-

CO

2 to form a solids-free solution having from about 1 to 15% by weight of the alkali metal nitrite; contacting the synthetic resinous substrate with the solution for a time sufficient to transfer at least a portion of the alkali metal nitrite into the synthetic resinous substrate while maintaining the carbon dioxide under supercritieal conditions; and, decreasing pressure or temperature, or both, on the synthetic resinous material sufficiently to evolve carbon dioxide and leave micronized solid alkali metal nitrite crystals in an amount less than 2% by weight, preferably from about 0. 1% to about essentially uniformly distributed in the synthetic resinous substrate. If desired, up to about 15% by weight of sodium nitrite or potassium nitrite may be transported and deposited in the substrate if transparency of the substrate is irrelevant, because the substrate tends to become opaque.

By "micronized"-czystals is meant that the major portion by weight of the crystals have an average particle site smaller than 10 Pin. By "rapidly" is meant that from about 0. 1 to less than 2% of the alkali metal nitrite crystals go into solution in the

SC-CO

2 in less than 30 minutes. Such speed is essential for the impregnation stage of a.

commrercially viable two-stage process in which it is necessary to recycle and recompress to supercritical conditions the recovered carbon dioxide.

By "uniformly distributed" is meant that the uniformity of dispersed particles in (N the film may be quantified by known microscopic techniques, or by a blown film test.

0 In the blown film test the polymer containing solid powder particles is extruded through a blown film apparatus which produces a film about 0.025 mm (I mil) thick, and this film is placed over a light source of appropriate wavelength and intensity to enable one to quantify the number of particles which show up as "imperfections"; and (N the si:ze of each is also visible under appropriate magnification. No unit area of the film to have a substantially higher concentration of particles than another.

7The foregoing process is prefbrably carried out with the alkali metal nitrite in combination with an organic compound which fbrms a solids-fee solution in SC-CC) 2 in a two-stage process comprising, dissolving an alkali metal nitrite and the organic compound in carbon dioxide held in an autoclave under supereritical conditions to form a solution contaiining from about I to 15% by weight of sodium nitrite and organic compound, (ii) filtering the solution to ensure that substantially no particulate solids are present in filtered solution; (iii) contacting a polymeric substrae with the filtered solution for less than 30 minutes so as to transfer enough sodium nitrite and organic compound into the substrate so as to infutse it with less than 2% by weight of each, an alkali metal nitrite and organic compound. The impregnated substrate necessarily becomes swollen due to the high-pressure entry of SC-CO 2 molecules, but such swollen conidition reverts to normal when the carbon dioxide leaves the substrate.

In another embodiment of the process, the SC-CO 2 may be combined with a second fluid miscible (Cornning a common supercritical phase) with carbon dioxide under supercritical conditions, which second supercritical fluid, present in a minor amount by weight relative to the carbon dioxide, facilitates dissolution of an organic compound such as a hindered phenol or aromnatic amine, in the common supercritical phase. A preferred second fluid is selected from the group consisting of ethylene, ethane, nitrous oxide, cblorotrifluorometrnne and trifluoromethane which have critical temperatures and pressures in the vicinity of those for SC-CO 2 and can be readily recovered and recycled together.

The process described herein may be repeated on a polymer substrate which has o 7 been previously impregnated, and from which the impregnated solid crystals have been removed, as for example, by evolution over a long period of time, as when sodium ci nitrite crystals are used as a VCI.

DETAILED DESCRIPTION OF THE INVENTION A fluid, either gas or liquid at roomn temperature (251C) and pressure (I atm or 14.7 psia or 101.3 kPa), when subjected to the necessary combination of pressure and temperature, both of which are higher than the critical pressure and critical temperature of the fluid, produces a supercritical fluid. Above its critical temperature a gaseous fluid cannot be convented to a liquid regardless of the pressure exerted on the gas- It is essential that alkali metal nitrite crystals be essentially completely soluble in SC-CO2. When a dispersion of such crystals, smaller than 45 pgn, in SC-C 0 2 is contacted with a polyolofin article, before the crystals are dissolved, the crystals become non-unittrmly embedded in the surface of the polymer, and are readily visible to the human eye.

Example I A dispersion of sodium chloride crystals smaller than 45 pin (more than are in the range fromn 1O pmn) is essentially insoluble in SC-CO 2 as evidenced by the following experiment: 1OO g of the sodium chloride crystals are deposited in a 300 ml Micro Series pressure vessel, referred to as the "main" pressure vessel, equipped with a propeller stirrer and with a transparent glass window through which the deposited crystals are visible, The main pressure vessel is closed and charged with carbon dioxide gas from a cylinder using a compressor which pressurizes the vessel to 102.04 atm or 10.35 Mpa( 1500 psi) and the temperature of the vessel is maintained at 35 0 C so that supereritical conditions are obtained.. The outlet from the vessel is led through a fresh and uncontaminated I pm filter to a depressurizing valve through which the contents of the vessel are withdrawn into a second pressure vessel from which the carbon dioxide is to be recovered, o 8 The stirrer is started and is kept running for 10 minutes at 700 rpm. Upon stopping the stirrer, the crystals are still visible. When the vessel is depressured through the filter, the filter is removed and flushed with distilled water which is tested for the presence of NaCl by the addition of a 1 molar solution of silver nitrate. A very slight white haze develops indicating very little sodium chloride is present.

The sodium chloride crystals are removed from the vessel and weighed their weight is essentially unchanged indicating very little of the salt went into solution.

The following three additional examples IlA, l B and I C are conducted in the same pressure vessel, under supercritical conditions at slightly higher temperatures than example 1, and by continuously flowing SC-CO 2 over the NaCI crystals for 3 hr, to see if there is any change in the lack of solubility observed in example 1. At the end of each run, the crystals collected from the pressure vessel were vacuum-dried for 1 hr at 600C, then weighed.

The results are set forth in the following Table 1.

TABLE I Example Temp. 0 C Pressure, Flow rate" Wt. before Wt. after MPa (psia) Of SC-CO 2 drying drying IA 40 9.69 4.2 14,174 14.175- (1407) 1B 50 13.88 5.3 13.668 13.668 (2015) 13.78 4.5 10.001 10,003 t (2000) *standard l iters per minute From the foregoing Table it Is evident that there is no change in weight of the NaCI crystals before and after being subjected to flowing SC-C0 2 indicating that the o 9 NaCI crystals are essentially inisoluble in SC-CO 2 Cl Example 2 A dispersion of the sodium nitrite crystals smaller than 45 pmn is essentially insoluble in liquid carbon dioxide, not under supercritical conditions, as evidenced by ON the following experiment: o In a manner analogous to that described in Example I above, 100 g of the sodium nitrite crystals are deposited in the samne main pressure vessel fitted with a o freshly cleaned 5 pin filter. The vessl is then pressurized with carbon dioxide to 54.4 atmn or 30 Mpa (800 psi) and the temperature of the vessel is maintained at 256C so that liquid is visible in the vessel. The stirrer is run for 10 min at 700 rpm and then stopped.

The vessel is then depressurized through the 1 PM filter. The filtrate is collected in the second pressure vessel which is gradually depressurized so as to recover the carbon dioxide. Exam ination of the interior of the second pressure vessel shows that there are -no crystals left. As before, after the main pressure vessel is depressurized, the filter is removed and washed with distilled water and the water analyzed for sodium nitrite which is soluble in an amount of 81.5 g/ 100 ml of water at 1 5 0 C. Less than 150 ppmn of sodium nitrite is found, indicating that the sodium nitrite crystals are essentially insoluble in liquid, but not supercritical carbon dioxide, In the following examples 2A, 2B, 2C and 2D the solubility of NaCI and NaNO 2 crystas in liquid CO 2 are evaluated under two slightly higher pressures using a pressure vessel which is modified by fining a 5 prm strainer in the central vertical plane of the vessel 1 partitioning it so that pre-weighted and vacuum-dried crystals were placed on one side oly. The pressure vessel is then pressurized with liquid CO 2 under conditions which ensure that it stays in the liquid phase. The crystals were soaked in the liquid CQ for 3 hour, after which the pressure vessel was slowly depressurized.

The results are set forth in the following Table 2.

0 0 ci

C)

Co ci 0 0 ci 0 0 ci TABLE 2 Example Crystals of Temp., 0 CPressure Soaked for, M Wa (Psi a) hours 2A NaCi 28 10.335 3 (1500) 2B I NaCi 26 8.268 3 (1200) 2C NaNO 2 28 10.335 3 (1500) 2D NaNQ2 26 8.268 3 Upon visual inspection of the internals of the pressure vessel, after controlled depressurization, no cross-over of crystals from one side oft.e partitioned vessel to the other is observed, irrespective of whether the crystals are NaCI or NaNO 2 Such evidence confirms that crystals of neither salt are soluble in high pressure, nearsupereritical liquid CO 2 and therefore are unable to go through the strainer.

Example 3 Sodium nitrite crystals smaller than 45 pmn are essentially completely soluble in carbon dioxide under supercritical conditions, as evidenced by the following experiment: In a manner analogous to that described in Example 1 above, 100 g of the sodium nitrite crystals are deposited in the same main pressure vessel fitted with a freshly cleaned 5 pmn filter. The vessel is then pressurized with carbon dioxide to 102.04 atm or 10-35 Mpa (1500 psi) and the temperature of the vessel is maintained at 0 C so that supercritical conditions are obtained. The stirrer is run for 10 muin at 700 rpm and then stopped.

No crystals ar-c visible in the window of the vessel- When the vessel is depressurized and removed to be inspected, no crystals are recovered.

As before, the second pressure vessel is gradually depressurized through the N 4pm filter until all the carbon dioxide is recovered. Examination of the interior of the vessel shows deposited crystals which are recovered and weighed. Additionally, after the main vessel is depressurized, the filter is removed and washed with distilled water ON five times. All the wash water is collected and concentrated to precipitate the sodium nitrite crystals which are weighed. The combined weight of the crystals recovered from the second pressure vessel and the very small amount recovered from the filter, is more than 99 gmn, indicating that essentially all the crystals went into solution in the SC-CO,.

The solubilities of NaCI and NaNO 2 crystals are determined separately using a slight modification, to test the solubility in quiescent SC-CO 2 rather than by flowing

SC-GO

2 through the pressure vessel, as fohllows- The solubility of NaCi crystals was first tested by loading 100 g into a glass vial that was capped with coarse Whitman®V filter paper which was taped to the vial with Teflon& tape. The vial is then loaded into the pressure vessel which is pressurized, as before, to 13.78 MI~a (2000 psia) at 40 0 C. The pressure is maintained for 3 hr, after which the pressure vessel is cooled and depressurized. The crystals are recovered and dried at 601C for I hr, then weighed.

The foregoing procedure is repeated with another 100 g of NaCI crystals.

No measurable difference in the weights of the crystals, before and after being soaked in SC-C02 is observed, indicating that NaCI crystals are essentially insoluble in quiescent SC-CO 2 Next, 100 g of NaNO 2 crystals were loaded into the vial, the mouth of which is capped with filter paper and taped to the vial as before, the vial loaded into the pressure vessel which was then pressurized to 13,78 MPa (2000 psia) at 40 0 C. As before, the pressure is maintained for 3 hr, after which the pressure vessel is cooled and depressurized. The crystals are recovered and dried at 601C for I hr, then weighed.

The foregoing procedure is repeated a second time.

The avenage reduction in weighit of the crystals in the vial was 27.2 0 0 ci

C)

Co ci 0 12 From the foregoing it is evident that NaNO 2 crystals which are essentially insoluble in high-pressure, near supereritical liquid CO 2 are soluble in quiescent SC-

CO

2 to an extent of about 27%.

The supercritical properties of a variety of compounds are shown below in Table 3.

TABLE 3 Fluid Critical Temperature Critical Pressure (MPa) Carbon Dioxide 31.4 7.38 Nitrous Oxide 36.5 7.26 Ethylene 9.3 5.03 Ethane 32.3 4.88 Trifluoromethane 25.2 4. 83 Chlorotrifluoromethane 29.9 3.92 Ethanol 240.8 62.2 Acetone 234.9 46.4 As is evident from the data in the above Table, SC-ethanol and SC-acetone would require conditions very different from those required to make SC-CO;.

Polymers which lend themselves to be impregnated or may be formed from any rubber or polymer capable of being swollen by at [east about 2 percent by volume, or by o 13 ci t at least about 5 percent by volume, or even by at least about 7 percent by volume by the 9 supercritical fluid being utilized in the present invention. Such polymers include C, natural rubbers, polyisoprene polymers, styrene-butadiene polymers, butyl rubbers, 0 chloroprene polymers, polyamides, polyimides, polyesters, nitrile rubbers, polyacrylic polymers, polystyrene polymers, fluoro polymers polytetrafluoro ethylene or ON polyvinylidene fluoride), vinyl chloride polymers, vinylidene chloride polymers, t polycarbonate polymers, polyurethane polymers, polyacetylenes and polyolefins. In CI another embodiment the present invention can also be utilized to infuse one or more O additives into a precursor or resin used to form the above-listed polymer compositions.

C 10 Most preferred are polyolefins, and polyethylene (PE) and polypropylene (PP) in particular which may be required to be substantially transparent.

The process described herein also enables one to infuse one or more alkali metal nitrites and organic compounds into a biodegradable polymer, biodegradable polymer precursor or resin or a pre-formed biodegradable polymer article. Any polymer which exhibits biodegradability can be utilized in conjunction with the present invention.

Examples of suitable biodegradable polymers include, but are not limited to, biodegradable polyesters linear poly e-caprolactone biodegradable polylactic acid polymers, biodegradable polyester amide polymers, biodegradable polyester urethane polymers and biodegradable copolymers of any combination of two or more of the above.

Though any organic compound soluble in SC-CO 2 may be combined with the less than 2% by weight of alkali metal nitrite in solution, most preferred are VCIs such as are disclosed in U.S. Patent Nos. 4,290,912; 5,320,778 and 5,855,975, which are incorporated by reference thereto as if fully set forth herein; and, commonly used antioxidants such as the 2,4,6-tri-substituted phenols exemplified by BHT (2,4,6tributyl hydroxy toluene) To illustrate that a PE substrate may have even larger amounts of sodium nitrite crystals than 2% by weight, deposited in the polymer, the following experiments are set forth: Two rectangular pieces of PE film, 4.5" x 2.5" x 0.0039", and a piece of polyethylene tubing having an outside diameter of 1.75" and 0.0039" thick, are weighed O 14 and placed in the main pressure vessel described above. The vessel is then charged with Sa 20 wt solution of NaNO 2 in supercritical CO 2 at a temperature of 50°C and a Spressure of 176.9 atm (2600 psi) for about 30 minutes.

After 30 minutes, the items are removed and reweighed. The two rectangular films of PE show a weight gain which correlates to an infusion rate of 9.38 wt and 10.87 wt The PE tubing shows a weight gain which correlates to an infusion rate of I 6.64 wt SAs can be seen from Tables 4 to 6 below, as the pressure, time, or temperature 0 at which the infusion or diffusion process is conducted varies, so does the amount of 0 10 additive incorporated into the polymer. For the results listed in Tables 4 to 6, PE pellets having a diameter of about 0.2" are used with NaNO 2 in solution in CO 2 under the conditions described immediately above.

TABLE 4 Pressure (atm/psi) Weight Percent Increase 163.3 atm (2400 psi) 3.06 204.1 atm (3000 psi) 8.50 0( 0, TABLE S Time (hr) Weight Percent Increase 3.06 8.50 14.84 TABLE 6 3.06 4.46 From the foregoing it is evident that the amount of alkali metal nitrite crystals which can be deposited in a polymer substrate may be controlled by controlling the time of immersion and the particular conditions of the supercritical phase.

Claims (5)

1. 4. The process of claim 2 wherein the alkali meta nitrite is selected from the group cons isting of sodium nitrite and potassium nitrite. The process of claim 1 including introducing a second fluid miscible with 0 carbon dioxide to form a common supercritical phase. 010
2. 6. The process of clkim 2 including introducing a second fluid miscible with carbon dioxide to form a common supercritical phase.
3. 7. The process of rlam 5 wherein the second fluid is selected from the group consisting of ethylene, ethane, nitrous oxide, clilorotrifluoromethane and trifluoromethane.
4. 8. The process of claim 6 wherein the second fluid is selected from the group consisting of ethylene, ethane, nitrous oxide, chiorotritluoromethane and trifluoromethane.
5. 9. A two-sage process for infuising crystals of an alkali metal nitrite in combination with an organic compound into a polymeric substrate, comprising, dissolving the alkali metal nitrite arid the organic compound in carbon dioxide held in an autoclave under supercritical conditions to form a solution containing from about 1 to 15% by weight of the alkali metal nitrite and organic compound; (ii) filtering the solution to ensure that substantially no particulate solids are present in filtered solution; (iii) contacting the polymeric substrate with the filtered solution for less than minutes so as to transfer enough solids-free solution Into the substrate to infuse it with less than 2% by weight of the alkali metal nitrite and the organic compound; (1v) decreasing the pressure on the substrate to infuse the polymeric substrate with 8 18 crystas smnaller than 10 jim. The process of claim 9 wherein the alkali metal nitrite crystals deposited in the 0 pressure vessel are in the size range from about 10 fim to 44 pin, and infused crystals Sare present in an amount less than 2% by weight, and more than 90% of infused crystals are in a size range smaller than Cl11. The process of claim 9 including introducing a second fluid miscible with 0 carbon dioxcide to form a common supercritical. phase. 01