PAPER

Accepted for
SPI Polyurethanes Conference
Boston, MA, October 10-12, 1994

New Foam Blowing Agents Containing Fluoroiodocarbons

Jon Nimitz, Ph.D., President
The Ikon Corporation
3300 Mountain Road NE
Albuquerque, NM 87106-1920
Phone: (505) 256-1463; Fax: (505) 256-1003

Introduction

This paper describes a new group of high-performance foam blowing agents that are environmentally safe, nonflammable, and appear to have low toxicity. These agents contain fluoroiodocarbons (FICs) either neat or in near-azeotropic blends with conventional flammable blowing agents such as isobutane, dimethyl ether, 1,1-difluoroethane, pentane, and cyclopentane.

FICs are chemicals containing carbon, fluorine, and iodine. The FICs of greatest interest for foam blowing include trifluoromethyl iodide (CF₃I, boiling point -22.5°C) and heptafluoro-1-iodopropane (CF₃CF₂CF₂I, abbreviated 1-C₃F₇I, boiling point 4O°C). The rigid foams that can be blown with FICs include polyurethane, polyisocyanurate, polystyrene, and polyethylene. FICs (especially 1-C₃F₇I) appear to provide the best alternative blowing agents at this time for integral (self-skinning) foams for aerospace and automotive applications.

FICs show outstanding promise not only as foam blowing agents but as general CFC and halon replacements for refrigeration, solvent cleaning, aerosol propulsion, and firefighting. FIC-based agents are undergoing tests in several of these applications.

This paper discusses the physical, thermodynamic, toxicological, and environmental properties of FIC-based foam blowing agents, as well as availability and cost.

Table 1. Potential FIC Foam Blowing Agents

IUPAC NAME	OTHER NAME(S)	FORMULA	CONDENSED	CAS NO.	MW	BP °C
trifluoroiodomethane	trifluoromethyl iodide iodotrifluoromethane	CF3I	CF3I	2314-97-8	195.9	-22.5
pentafluoroiodoethane	iodopentafluoromethane iodoperfluoroethane perfluoroethyl iodide	CF3CF2I	C2F5I	354-64-3	245.9	12
chlorofluoroiodomethane		CHClFI	CHClFI		194	15
difluoroiodomethane		CHF2I	CHF2I	1493-03-4	177.9	21.6
chlorodifluoroiodomethane		CClF2I	CClF2I	420-49-5	212.4	33
1,1,1,2,3,3,3-heptafluoro-2-iodopropane	perfluoroisopropyl iodide 2-iodoperfluoropropane	CF3CFICF3	2-C3F7I	677-69-0	295.9	39
1,1,2,2-tetrafluoro-1-iodoethane	iodo-1,1,2,2-tetrafluoroethane	CF2ICHF2	C2HF4I	3831-49-0	227.9	40
1,1,2,2,3,3,3-heptafluoro-1-iodopropane	heptafluoro-1-iodopropane perfluoropropyl iodide 1-iodoperfluoropropane	CF3CF2 CF2I	C3F7I	754-34-7	295.9	41
pentafluoroiodopropane		C3H2F5I	C3H2F5I		187.4	52
fluoroiodomethane		CH2FI	CH2FI	373-53-5	159.9	53.4

Properties of FICs

FICs appear to solve the problem of providing high-performance, nonflammable, nontoxic, environmentally safe blowing agents to replace CFC-11 and HCFC-141b for rigid foams.

FICs have attractive physical properties (similar to CFCs) and, because they undergo photolysis within two days when released into the atmosphere, they never reach the stratosphere and thus have negligible ozone-depletion potential. This short lifetime also gives them very low global warming potentials. The high molecular weights are expected to make these compounds outstanding insulating gases and to hinder permeation, thus improving retention of performance on aging. Calculations show that these blowing agents will increase the insulating performance of rigid foams by 10% to 40% over CFCs.

Structures and properties of several FICs of potential interest for foam blowing are given in Table 1. The FICs of greatest interest for foam blowing at this time appear to be trifluoromethyl iodide (CF₃I, bp -22.5°C) and heptafluoro-1-iodopropane (CF₃CF₂CF₂I, abbreviated 1-C₃F₇I, bp 40°C) because of their- desirable physical properties, demonstrated low toxicities, relatively low costs, and high availabilities compared to other FICs.

The ideal replacement foam blowing agent should have the following properties:

1. high insulating ability,
2. nonflammability,
3. low toxicity,
4. zero ODP,
5. very low GWP,
6. non-VOC, and
7. low cost.

FICs appear to provide foam blowing agents that meet all of these criteria at this time except low cost. However, as described below, FICs are only available as research chemicals at this time and costs are expected to drop sharply with bulk production.

Insulating Ability

The data on FICs indicate that they will provide substantially better insulation than CFC-11 or any of the currently available alternatives (Refs. 1- 12).

Thermal conductivity is inversely proportional to the square root of the molecular weight. Thus, the ratio of thermal conductivity Of 1-C₃F₇I to CFC-11 is given by Equation [1].

          (137.36/295.93)1/2 = 0.681                              [1]

The relative insulating value is inversely proportional to the thermal conductivity, and is given for 1-C₃F₇I relative to CFC-11 by Equation [2].

          1/0.681 = 1.468                                         [2]

Thus 1-C₃F₇I is intrinsically a better insulator than CFC-11 by about 47%. Similar comparisons made between common CFC or HCFCs and the FICs CF₃I and CF₃CF₂CF₂I are shown in Table 2.

Table 2. Predicted Improvements in Insulating Abilities and Diffusion Rates by FICs

CFC or HCFC	MW of CFC or HCFC	FIC	MW of FIC	Predicted improvement in insulating ability and diffusion rate of gas, %	Predicted improvement in insulating ability of foam, %
11	137.35	CF3I	195.9	19	10
11	137.35	CF3CF2CF2I	295.93	47	23
12	120.91	CF3I	195.9	27	14
12	120.91	CF3CF2CF2I	295.93	56	28
22	86.47	CF3I	195.9	51	25
22	86.47	CF3CF2CF2I	295.93	85	42
141b	116.95	CF3I	195.9	29	15
141b	116.95	CF3I	195.9	59	30

A foam can be considered to have two heat flow paths in series: one for the polymer matrix and one for the gas. If the polymer matrix does not change, this portion of the heat flow will not change. However, if the gas is changed, that portion of the heat flow changes. Approximately 50% of the insulating ability of the foam depends on the gas (Ref. 13). Thus, for example, if 50% of the insulating ability depends on the gas, then the improvement on substituting CF₃CF₂CF₂I for CFC-11 will be about 23% (0.50 x 46% = 23%).

The data in Table 2 show that replacement of CFCs or HCFCs with FICs is expected to lead to an improvement in the insulating ability and diffusion rate of the gas of 19% to 85%, and an increase in the insulating ability of the foam of 10% to 42%. The R-value of rigid polyisocyanurate foam per inch could therefore be raised from about 7.0 (with R-11) to somewhere between 7.7 (with CF₃I) and 8.6 (with 1-C₃F₇I). Polystyrene board could be raised from about 5.0 per inch to somewhere between 5.5 (with CF₃I) and 6.2 (with 1-C₃F₇I). Rigid polyurethane could be raised from 7.5 to between 8.2 (with CF₃I) and 10.7 (with 1-C₃F₇I).

Based on the ratio of molecular weights Of 1-C₃F₇I to CFC-11, the thermal conductivity of 1-C₃F₇I is predicted to be as shown in Equation [3].

          (0.681)(0.0084) = 0.0057 W/m-K = 0.0033 Btu/ft-°F             [3]

Typical polyurethane foams blown with CFC-11 have thermal conductivities of about 0.0250 W/m-K. The thermal conductivity of a similar foam blown with 1-C₃F₇I is predicted to be 23% less, corresponding to 0.0192 W/m-K or 0.0111 Btu/ft-°F.

Potential Energy Savings

The energy impact of FIC-based foams in the U.S. is a potential savings of about 20% of all the energy currently lost through rigid foam insulation (Refs. 14- 16). For residential refrigeration and freezing, this annual energy savings amounts to 32 billion kWh (109 trillion Btu), $2.6 billion, and an average of $18 per unit. For commercial construction, the potential annual savings is about 350 billion kWh (1.2 quadrillion Btu) and $14 billion. The potential annual savings in residential construction is 470 billion kWh (1.6 quadrillion Btu) and $17 billion.

Payback Period for Refrigeration

A typical refrigerator or freezer has an exterior surface area of approximately 70 square feet, and the insulation averages about 1.25 inches thick. Thus it requires an average of about 7.3 cubic feet of insulation. Assuming an ultimate bulk price of $5/lb for 1-C₃F₇I, the increased cost of polyurethane comes to $3.00 per cubic foot, and the added cost per unit is therefore about $22. If the energy savings is 17%, the energy cost savings is about $18/yr and the payback time is 15 months, as shown in Equation [4].

          $22 invested /$18 savings/year = 1.2 years = 15 months            [4]

A similar analysis for construction insulation yields a payback time of about 6 years.

Rate of Permeation

Insulation performance degrades with aging as the blowing agent permeates into the polymer matrix, air diffuses into the foam, and carbon dioxide (if present) diffuses out. According to Graham's Law, the rate of permeation (or diffusion or effusion) of a gas is inversely proportional to the square root of the molecular weight (Ref. 17). Thus the relative rate of permeation of the FICs CF₃I and 1-C₃F₇I can be compared with those of CFCs 11 and 12 and HCFCs 22 and 141b as shown in Table 2. The lower rate of permeation by the FICs will provide improved stability of properties on aging.

Flammability

FICs such as CF₃I and CF₃CF₂CF₂I are outstanding fire extinguishants, comparable to halons. They have been shown to prevent combustion of hydrocarbon fuels when the FIC is present in about 3% concentration by volume in air (Ref. 18). Thus they will not contribute to the flammability of foams. In a fire environment the cells in most foams rupture long before the polymer starts to burn. This rupture is caused by softening of the polymer and thermally induced pressure. The gas in the cell is released and carried away. Polyurethane polymer matrixes are often highly flammable (although flame retardants can be added to decrease this flammability); polyisocyanurates are more flame-resistant. Although FICs are not expected to make otherwise flammable foams nonflammable, at least the blowing agent will not contribute to flammability of the foam.

Toxicity and Thermal Stability

The information available regarding the acute toxicity of the candidate FICs is highly encouraging. Recent results of acute toxicity studies of CF₃I at Wright-Patterson Air Force Base show that it has very low acute toxicity (Ref. 19). Mice exposed continuously to 6% CF₃I for 3 days showed no lethalities. The lethal concentration by inhalation for 50% of a mouse population in 15 minutes (mice 15-minute LC₅₀) has been measured at 27.4%. These are reassuringly high values, indicating very low acute toxicity. Additional older toxicity information on FICs has been reviewed (Refs. 18 and 20). In one study a dog was exposed to a 910,000 ppm concentration of CF₃I by weight (91% by weight or 50% by volume) for 30 seconds. It survived and exhibited no signs of anesthesia. In another report, no lethality was observed during a 2-hour exposure of mice to 250,000 ppm by weight of CF₃CF₂CF₂I. Acute toxicity studies of CF₃CF₂CF₂I are underway at Armstrong Laboratories at Wright-Patterson AFB. Pentafluoroiodoethane (CF₃CF₂I) is judged a less attractive candidate because one unpublished report describes it as a strong cardiac sensitizer (Ref 20). Blending FICs with low-toxicity agents would further decrease any toxic effects that might eventually be found.

Although iodinated compounds are in general more reactive, more toxic, and less stable chemically than brominated compounds, the presence of fluorine atoms bonded to the carbon atom attached to the iodine atom provides substantial additional stability and greatly decreased toxicity. The presence of strongly- bonded, highly electron-withdrawing fluorine atoms on the carbon atom bonded to iodine prevents both common mechanisms of reaction for iodocarbons from occurring. Steric hindrance (physical blocking by the fluorine atoms) prevents back-side attack by nucleophiles (S_N2 or substitution, nucleophilic, bimolecular mechanism) and the increased C-I bond strength prevents unimolecular bond-breaking (S_Nl or substitution, nucleophilic, unimolecular mechanism). For toxicity and stability considerations, FICs containing a terminal iododifluoro group (-CF₂I) are preferred over other molecular arrangements. If the iodine atom is on a primary carbon atom (a carbon atom attached to only one other carbon atom) the C-I bond is stronger than if the iodine atom is on a secondary or tertiary carbon. By having two (or, in the case of CF₃I, three) fluorine atoms on the same carbon atom as the iodine the C-I bond is further strengthened and reactions are inhibited.

Published studies on thermal decomposition of CF₃I indicates that the compound is very stable (Ref. 18). In one study it is reported that CF₃I is stable in contact with metals up to 170°C (340°F). Recent work indicates that it is stable when heated to 180°F for at least 60 days (Ref. 2 1). The author is currently conducting tests of thermal stability Of 1-C₃F₇I with and without possible stabilizers. An additional indication of stability is that many fluoroiodocarbons can be purchased from vendors and stored without unusual precautions such as refrigeration.

Environmental Properties

In order to achieve an ODP of zero or nearly zero, three approaches can be taken. An alternative chemical must either

1. contain no ozone-depleting elements;
2. contain at least one weakly-bonded hydrogen atom in the molecule; or
3. absorb ultraviolet radiation in the wavelengths found in the troposphere and undergo rapid photolytic destruction.

FICs fall under category (3): they undergo very rapid photolytic destruction in the troposphere and thus never reach the stratosphere. In this respect they are similar to table salt or the chlorine used in swimming pools, which would destroy ozone if they reached the stratosphere but pose no threat because they never contact it.

The best-studied FIC to date is CF₃I, because of its promise as a replacement for Halon 1301. Other FICs are expected to have very similar environmental properties. The temperature-dependent photolytic cross-section of CF₃I has been measured and the resulting data have been included in atmospheric models (Ref. 22). The best current estimate of the atmospheric lifetime of CF₃I is 1.15 days (Ref. 23). The fraction of CF3I released at ground level that would reach the stratosphere can be estimated, assuming an upper limit of the photolytic lifetime of 1.15 days and a conservatively low average transit time of 2 months (60 days) to reach the stratosphere. The fraction surviving is e^-60/1.15,which is about 2 xl0^-23(an immeasurably small amount). Other FICs such as 1-C₃F₇I should have very similar atmospheric lifetimes, ODPs, and GWTs to CF₃I. Thus any reaction of iodine from FICs released at ground level with stratospheric ozone will be negligible. This calculation does not account for the small amount of rapid mixing that occurs between the troposphere and stratosphere by tropical storms; this issue is being addressed by atmospheric scientists at the National Oceanographic and Atmospheric Administration (NOAA) and Lawrence Livermore National Labs (LLNL). Although there is evidence that iodine that contacts ozone reacts with it, iodine from FICs released at ground level are not expected to reach the stratosphere in any significant quantities.

Tropospheric (ground-level) ozone is one of the most dangerous components of smog. Addition of iodine to smog at levels near 0.1ppm has been shown to actually decrease levels of undesirable tropospheric ozone (Ref. 18). A series of eye tests showed that the iodized atmospheres were less irritating to eyes than the control atmospheres. Thus it appears likely that FICs are "anti-VOCs" and that their release will improve air quality in urban areas. In fact, the possibility arises that FICs could be used to remediate high tropospheric ozone levels. FICs therefore appear to provide the best of all possible worlds: they will destroy "bad" tropospheric ozone while leaving the "good" stratospheric ozone intact.

Cost and Availability

In early 1992, only a few hundred pounds of CF₃I were available annually worldwide, at a cost of about $600/lb, and most of the production was in Russia. As of May 1994 CF₃I was available from three U.S. sources in amounts of up to 1000 lbs/day total at a price of about $130/lb. Thus the availability has improved dramatically and the cost has dropped by over 80% in one year. Further increases in availability and decreases in cost are expected. It has been predicted by the author and others that costs of FICs in bulk will be around $5-10/lb.

The world supply of iodine is sufficient to provide FICs as replacements for CFCs, HCFCs, and halons for many years. Proven worldwide reserves of iodine recoverable at less than $15/kg are about 14 billion pounds (Ref. 24). In addition, the oceans contain another estimated 76 billion pounds of iodine, which could be recovered directly during extraction of chlorine, bromine, or magnesium from seawater or by collecting and processing kelp. Seaweeds of the Laminaria family can extract and accumulate up to 0.45% iodine on a dry weight basis; before 1959 seaweed represented a major source of iodine.

FIC-Containing Blends

FICs can be blended with other flammable agents such as isobutane, dimethyl ether, 1,1-difluoroethane, cyclopentane, or pentane, to form nonflammable mixtures. Azeotropic blends are particularly attractive, because they have no tendency to separate on evaporation. The authors have developed a proprietary computer program (called AZEO) to predict properties of blends and to predict the likelihood of azeotrope formation. According to our model, it appears possible that CF₃I may form azeotropes with isobutane, dimethyl ether, and 1,1-difluoroethane. Heptafluoro-1-iodopropane may form azeotropes with cyclopentane and pentane. All such modeling must be confirmed by laboratory work.

Status of Testing and Commercialization

Pure CF₃I and a blend of CF₃I with isobutane have been successfully demonstrated by Peter Rand at Sandia National Labs as blowing agents for a specialty single-container foam. Additional work required includes:

1. Computer modeling to optimize agent blends and to calculate physical and transport properties
2. Laboratory testing of azeotrope formation
3. Manufacturing test quantities of polyurethane, polyisocyanurate, and polyethylene foams with FIC-based blowing agents,
4. Measurement of properties of test foams. These properties will include flammability, density, compressive strength, tensile strength, flexural strength, shear strength, compression modulus, flexural modulus, shear modulus, thermal conductivity, coefficient of linear expansion, maximum service temperature, specific heat, dielectric constant, dissipation factor, water absorption, moisture vapor transmission.
5. Improvements in the synthesis of FICs to make them less expensive.

The authors have U.S. and international patents pending on a wide range of foam blowing agents containing FICS. Industrial partners are sought to license the technology, assist with completion of the required testing, and bring these products to market.

Conclusions

A new group of nonflammable, nontoxic, environmentally safe foam blowing agents with insulating value superior to CFCs and other alternatives has been identified. These agents have attractive physical properties, essentially zero ozone-depletion potential (ODP), very low global warming potentials (GWPs), and are not VOCs. They appear to solve the problems of providing highly effective, environmentally safe, nonflammable insulating materials. The technical advantages include rendering foams less flammable, providing higher insulating abilities than traditional or current alternative foam blowing agents, and a slower permeation rate so the insulating gas is retained better on aging. A significant amount of additional validation work is still required. FICs are not yet available in bulk, and as research chemicals are quite expensive. It is anticipated that costs will drop dramatically as larger quantities are produced.

References

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9. Environmental Protection Agency, Handbook for Reducing and Eliminating Chlorofluorocarbons in Flexible Polyurethane Foams, Report 21A-4002, April 1991.
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12. "Foam Insulation" from Conservation and Renewable Energy Inquiry and Referral Service (CARIERS), Merrifield, VA, 1994.
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15. Household Energy Consumption and Expenditures 1990, Energy Information Administration, Washington, DC.
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17. Hein, M., "Foundations of College Chemistry", 6th ed., Brooks/Cole, Berkeley, CA, 1986, p. 252.
18. Nimitz, J. S., High-Performance, Environmentally Sound Replacements for Halon 1301, Prepared for McClellan AFB under contract F04699-93-C-0004, December 1993, ETEC 93-3.
19. Personal communication from Major Gary Jepson, Wright-Patterson AFB, OH, to Dr. Jon Nimitz, ETEC, Albuquerque, NM, 1994.
20. Skaggs, S. R., Dierdorf, D. S., and Tapscott, R. E., "Update on Iodides as Fire Extinguishing Agents," Proceedings of the 1993 CFC and Halon Alternatiues Conference, Washington, DC, October 20-22, 1993, pp. 800-809.
21. Personal communication from Dr. Douglas Dierdorf, New Mexico Engineering Research Institute, Albuquerque, NM to Dr. Jon Nimitz, ETEC, Albuquerque, NM, 1994.
22. Fahr, A., Nayak, A. K., and Huie, R. E., "Temperature Dependence of the Ultraviolet Absorption Cross-Section of CF₃I," Proceedings of the Halon Options Technical Working Conference, University of New Mexico, Albuquerque, NM, May 3-5, 1994.
23. Personal communication from Dr. Susan Solomon, NOAA, Boulder, CO, to Dr. Jon Nimitz, ETEC, Albuquerque, NM, 1994.
24. Mineral Commodity Summaries 1993, U.S. Department of the Interior, Bureau of Mines, Washington, DC, 1993, pp. 86-87.

For more information regarding this subject, please contact Dr. Nimitz at mailto:jnimitz@etec-nm.com