### Zero Ozone Depleting Blowing Agents for Use in Polyurethane Based Foam Insulations

E. Blevins and J

. Sharpe Martin Marietta Manned Space Systems Marshall Space Flight Center, Alabama A. Fine and B. Kopko Global Change Division U.S. Environmental Protection Agency



### Abstract Due to environmental concerns, the polyurethane insulation industry is tasked with replacing CFC-1 1. Any replacement blowing agent must meet the dual challenge of being environmentally friendly, and retaining the properties that have made CFC-11 so desirable. These desirable physical properties include: solubility in both the isocyanate and polyol components of polyurethane based foams, a boiling point just above 23°C at ambient pressure, molecular weight around 130 grams per mole, and a low thermal conductivity. The blowing agents reported in this paper are near drop-in replacements for CFC-11. The two fluorinated ethers reported are soluble in polyurethane foam components; have boiling points of 29 to 30°C; have molecular weights similar to that of CFC-11; and produce foam -insulations with good mechanical and thermal properties. This paper reports the physical properties of these ethers, and the mechanical strength, flammability, and thermal conductivity of a rigid, polyurethane based, thermally stable, cryogenic insulation foamed with these ethers. 

### Introduction Thermal Protection Systems (TPS) of the National Aernautics and Space Administration (NASA) Space Shuttle's External Tank (ET) include polyurethane and polyisocyanurate foam insulations. These insulations serve to maintain cryogenic propellant quality, maintain structural temperature limits, and minimize the formation of ice and frost that could potentially damage the fragile insulation of the space shuttle orbiter. The TPS must pass cryogenic flexure and substrate adhesion tests at -253°C, aerothermal and radiant -heating rates of up to approximately 12 British Thermal Units per square foot per second,and thermal conductivity tests. Currently, ET foams are blown with CFC-11, and HCFC141b is scheduled for use in production by 1995. Realizing that HCFC's are an interim blowing agent, several third generation blowing agents are under evaluation for use in ET insulations. This paper reports data on ;two fluorinated ethers under investigation as blowing agents for thermal protection systems of the Space Shuttle External Tank. The fluorinated ethers E245 and E263, produced in limited quantities by Halocarbon Products of North Augusta, South Carolina, have proven to be excellent blowing agents of polyurethane based foam insulations. Data indicates that these third generation blowing agents with zero ozone depletion potential (ODP) may serve as near drop-in replacements for chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) blowing agents. Experimental Development of these blowing agents was initiated by evaluating the physical properties of the fluorinated ethers, followed by a study of their solubility in polyurethane foam components. Studies. were then performed to determine the effects of these blowing agents on the foaming reaction and the final properties of the insulations produced

### Physical Properties of the Fluorinated Ethers

Physical properties of these ethers are presented in Table 1. A review of this data indicates that these fluorinated ethers can be expected to serve as foaming agents with a blowing efficiency similar to that of CFC-1 1 because of their similar molecular weights and boiling points. The shorter atmospheric lifetime and projected lower cost of E263 are positive traits that must be weighed against its low flash point and flammability concerns. Another advantage is demonstrated in Figure 1, which shows E263 has a slightly lower vapor phase thermal conductivity than E245 at temperatures below 32°C. Both E245 and E263 were carried forward in experimentation as foam blowing agents. The initial work was to determine solubility of the ethers in the isocyanate and polyol components of 'the urethane insulation. As presented in Table 1, E245 and E263 are sufficiently soluble in these urethane components to make processing possible with existing polyurethane industry equipment.

Table 1. Properties of E245 and E263
E245 E263 CFC-11 HCFC-141b ,CH9OCHF2 CF3CH2OCH3 CFCI3 CH3CI2F "Molecular Weight 150 114 137 117 Specific Gravity 1.14 1.16 1.49 1.24 Boiling point (°C) 29 30 24 32 Ozone Depletion Potential 0 0 1 0.15 Atmospheric Lifetime 1 6 years 2 months 70 years 12 years Global Warming Potential 2 0.15 0 1 0.09 Flashpoint 3 none -16°C none none Flammability 4 non flam. 4.5-14.3% non flam

. 7.6-17.7% Isocyanate Solubility 5 13% by weight 20% 

### >50% >50% Polyol Solubility 5 infinite >26% >50% >50% Toxicity 6 limited data limited data non-toxic limited data I Initially there was a concern that these ethers may form peroxides with aging and exposure to air. The manufacturer has worked with National Institute for Standards and Technology (NIST) to evaluate the chances of this occurence in E245. The long atmospheric lifetime of this product precludes the formation of peroxides. Nevertheless, the manufacturer attempted to form peroxides by oxygenating the material, but no peroxides were detected. The degradation of E263 has not been. tested, but according to NIST representatives, the long chain ethers that typically form peroxides have an atmospheric lifetime of only several hours, where E263 has an atmospheric lifetime of 2 months. 2 Estimates only. E263 may be classified as a volatile organic compound. 3 E245 & E263 tested by ASTM D 92. CFC-11 & HCFC-141b tested by ASTM D 1310 and D 56. 4 A blend of E245 and E263 was proposed as a means to reduce the flammability of E263. By calculation using the ASTM E 681 equation to estimate Lower Flammability Limit (LFL), 79.7 molar % E245 is required to raise the LFL of an E245/E263 blend to that of HCFC-141b (7% in air). The manufacturer determined empirically that 75% E245 was required to raise the LFL according to the Cleveland Open Cup Test Method (ASTM D 92). 5 Solubility tested in proprietary foam formulation containing polymeric methylene diisocyanate and polyester polyols. 6 Limited toxicity testing has been conducted

. Four hour rate exposure tests were run on E245, and in those conditions the material was considered to be safe. A general waiver for materials used for research purposes exists. Commercial production would require EPA approval and would almost certainly require additional toxicity testing.

### Figure 1. Vapor Phase Thermal Conductivity
16

A 8 -1 4 ® CFC-11 HCFC-141b E245 E263

A 6..- 10 20 30 40 50

Temperature (°C) 

## 60

Foam Preparation Initially, 350 isocyanate index, rigid isocyanurate foams were produced with a free rise density of approximately 3.6 pounds per cubic foot (pcf). More recently, foams have been produced with a lower density of approximately 2.3 pcf. Testing of the lower density foams is currently in progress. Table 2 shows that E245 and E263 do not hinder the foaming reaction, and produce foarris with similar free rise densities when substituted on a molecular weight basis.

Table 2. Reactivity and Free Rise Density of Foams Blown with E245, E263, CFC11, and HCFC-141b
E245 E263 CFC-11 HCFC-141b High Density Gel Time (sec) 57 53 52 49 Density (pct) 3.43 3.65 3.66 3.85 Low Density Gel Time (sec) 61 48 46 Density (pcf) 2.20 2.27 2.34 2.30
Hand mix reactivity tests with 70±2°F component temperatures.

t 

### Foam Properties - Mechanical, Flammability, and Thermal Conductivity Test panels of these foam insulations were hand mixed and molded at component and mold temperatures of 25°C. In some cases, the density of the molded parts varied ±0.4 pounds per cubic foot; actual density is reported in all tables. Mechanical tests run on these molded panels included: friability (ASTM C 421), compressive strength (ASTM D 1621), bond adhesion at 21°C, 149°C, and -253°C (ASTM D 1623), cohesive strength at room temperature (ASTM D 1623) and cryogenic strain compatibility in a gradient cryoflexure test at -25-3°C (Martin Marietta Corporation in=house test). Data from mechanical tests are presented in Table 3. The data establishes foams produced with the fluorinated ethers had properties similar to the foams produced with CFC-11.Table 3. Mechanical Properties of Foams Produced with Fluorinated Ether Blowing

Agents E245 E263 CFC-11 HCFC-141b Friability (% mass loss) 33.5 29.0 27.9 31.8 Density (lb/cubic foot) 3.27 3.29 3.67 3.04 Compression (psi) 41 * . 40 40 Bond Adhesion i(psi)

21°C 20.9±4 35.8±9 39.6±10 44.1±9 149°C 16.5±3 * 20 18 -253°C 23.8±12 * 35 35 Cohesive Stength (psi) 40.4±4 * 45 45 Cryoflex, -253°C, 30 ksi 2 PASS * PASS PASS

* Data not yet available. Elected to evaluate E245, with no flashpoint, first. 1 Adhesion to epoxy primed 2219 aluminum 2 Aluminum substrate with 1 inch of insulation is mechanically stressed to 30 ksi, while a thermal stress is applied by a -253°C backface temperature.


### Limited flammability testing has been completed to date, but available-data is presented in Table 4. Since E245 and E263 are ethers, there was an initial concern that they would increase the flammability of foams produced. In a proprietary isocyanurate foam system, this was not the case. Limiting Oxygen Index (LOI - ASTM D 2863) is the percent oxygen required to sustain a flame. The foams based on E245 and E263, despite the slightly lower density, have LOI values of 28.6 and 27.7 respectively, and are comparable to foams produced with CFC and HCFC blowing agents. Flammability by NHB 8060.1B is a test developed by NASA to quantify flammability of materials in air or in oxygen rich environments. Results are reported as inches burned, and a value of 3.5 to 4.5 inches in air is excellent. Data for foam recession in a radiant heat/vacuum environment and in an aerothermal environment are not yet available. Scheduled tests will be run at heating rates from 7 to 15 British Thermal Units per square foot per second.

Table 4. Flammability of Foams Blown with Fluorinated Ethers
E245 E263 CFC-11 HCFC-141b Limiting Oxygen Index (% oxygen) 28.6 27

.7 28.9 28.4 Flammability (burn length in inches)** 3.4-4.5 * 4.0 4.1 Density (pcf) 3 .27 3.65 4.00 3.83

* Data not yet available. ** Flammability per NASA test procedure NHB 8060.1B Flammability tests are run in controlled environments and should not be construed to determine the potential hazards of these materials in fires. 

### Thermal conductivity (ASTM C 177) data is presented in Table 5. One test was run for each test temperature. Neither foam was consistently better than the other at all test temperatures. The values presented show that despite the higher thermal conductivity of the E245 blowing agent in the vapor phase, the thermal conductivity of the high density. molded foams produced with these blowing agents is comparable to foams produced with CFC-1 1. The limited solubility of these blowing agents is perhaps one reason for the lower overall foam conductivity. The lower solubility of the blowing agent makes it more. likely to have many gas nucleation sites. This greater number of nucleation sites translates into more numerous, smaller bubbles producing a foamed structure with finer cells. Figures 2 through 5 show that the cellular structure of the foams produced with E245 and E263 is finer than with CFC-11 or HCFC-141b. The foams have a wide distribution of

## Figure 2. . Cellular Structure of Polyisocyanurate Foam Produced with E245 Blowing Agent

Figure 3

. Cellular Structure of Polyisocyanurate Foam

Produced with E263 Blowing Agent

## Figure 4. Cellular Structure of Polyisocyanurate Foam Produced with CFC- 11 Blowing Agent

Figure 5. Cellular Structure of Polyisocyanurate Foam

Produced with HCFC

- 141b Blowing Agent

### cell sizes because they were hand mixed, but foams produced with the fluorinated ethers

have more small cells. This fine cellular structure plays a major role in the thermal conductivity of the foams, and compensates for the higher vapor phase thermal conductivity of the fluorinated ethers. Although data is not yet available, one would expect that the insulations produced with these fluorinated ethers would have less drift in thermal conductivity with aging. The larger, less soluble molecules should remain in the insulation longer than the smaller CFC-11 molecules.

Table 5

. Thermal Conductivity of Foams Produced with E245 and CFC-11, Thermal Conductivity (W/m°K) Test Temperature (°C) E245 CFC-11 -130 0.0220 0.0209 0 0.0249 0.0253 52 0.0326 0.0272 200 0.0502 0.0523 Density (pct) 4.86 4.45 Data for the low density foams is not yet available



### Conclusions -The data presented warrants further evaluation of these fluorinated ethers as blowing agents for urethane based foam insulations. The major issue associated with these blowing agents is the fact that these fluorinated ethers are not commercially available. Toxicity tests must still be run, and the handling of E263 with the 4.5 to 14.3% flammability limits in air must be addressed. Users and manufacturers must work together to initiate and fund the toxicity testing so these materials with zero ozone depletion potential may be made available to industry.

Acknowledgements This work was funded by the Space Shuttle External Tank Project of the National Aeronautics and Space Administration under contract NAS8-36200. Efforts were coordinated with the Global Change Division of the U.S. Environmental Protection Agency. Flammability testing of E263 was performed by N. Dean Smith of the Air and Energy Engineering Research Laboratory at EPA in Research Triangle Park, North Carolina. Vapor phase thermal conductivity testing of E245 and E263 was completed by Rich Perkins in the Thermophysics Division at NIST in Bolder, Colorado.

### References

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Medicinal Chemistry, 14, 593. Terrell, Speers, Szur, Treadwell, and Ucciardi, "General Anesthetics

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