Polyurethane foam: industry debates the next step towards 'cleaner' blowing agents.

In just four years, rigid PUR foamers will have to stop using HCFCs, which can damage the ozone. As HFC alternatives raise doubts, pentanes attract new interest in North America.

As the 2003 deadline nears for phasing out use of ozone-depleting HCFCs, polyurethane foamers face rising urgency to answer thorny technical and economic questions about the potential alternatives. A key focus at SPI's Polyurethanes Expo '98 in Dallas last month was the flurry of R&D activity aimed at accomplishing "zero-ODP" (ozone-depletion potential) in foam manufacturing.

Liquid HFCs (designated 245fa and 365mfc) have appeared to be the logical heirs to succeed HCFC-141b, the major blowing agent used in rigid insulation foams in North America. It will be banned from use as of Jan. 1, 2003, as mandated by the 1987 Montreal Protocol. Small commercial quantities of HFCs are expected by 2000, but uncertainty about their cost and availability has led some major PUR chemical suppliers and processors to devote increasing attention to hydrocarbons - cyclopentane, isopentane, and n-pentane. These are already accepted as zero-ODP blowing agents in Europe and elsewhere overseas.

While North America is taking a closer look at hydrocarbons, some European makers of hydrocarbon-blown rigid foams are now field testing HFCs. That's because current hydrocarbon-blown foams are generally 8% higher in thermal conductivity than HFC-blown foams. It remains to be seen whether tinkering with formulations and/or equipment can shrink that differential.

Hydrocarbons for construction

North American interest in hydrocarbons is mainly for rigid foam laminations, both boardstock and sandwich panels, although evaluations in appliance foams are also under way. Earlier this year, Atlas Roofing Corp. of Atlanta, a major rigid foam insulation producer, was the first to announce its conversion from HCFC-141b to a pentane-based formulation. Atlas claims its new AC Ultra product performs as well as or better than standard foam insulation in all key characteristics.

In Dallas, ICI Polyurethanes reported on its most recent study of hydrocarbons, which shows that blends of pentanes can produce rigid polyisocyanurate foams with fire performance similar to foams made with HCFC-141b. According to ICI, such blends potentially can achieve comparable insulation performance, too. To achieve Class I fire performance in the ASTM E84 tunnel test, ICI's hydrocarbon-blown PIR foams required slightly higher isocyanate index (over 275, vs. 250-270 with HCFC-141b) and the addition of a halogen-phosphorus flame retardant. A 325-index foam also passed the Factory Mutual Calorimeter (FM 4450) test.

ICI says the optimum blend of hydrocarbons depends on the performance criteria for the boardstock laminate. In this study, initial and short-term aged K-factors were best for a 80:20 cyclopentane/isopentane blend (see graph). A 60:40 blend came in a close second, though it showed potential for slower aging. ICI will study long-term aging to test that potential.

Hennecke Machinery reported in Dallas that its pentane-based PUR sandwich-panel technology will be employed for the first time in the U.S. this year. It will be used for making metal-faced panels for industrial plants, cold storage, refrigeration depots, refrigerated trucks, and moistureproof installations.

Hennecke's Pentane Process Technology uses either the Pentamat batch-blending system or a high-pressure inline injection system together with an HK-P high-pressure reaction-casting machine. The equipment is completely encapsulated, aerated, and outfitted with exhaust devices, pentane gas sensors, and numerous other pentane-specific safety devices.

Where HFCs look good

If HFCs turn out to have a place, HFC-245fa, with its low boiling point of 60 F, appears to be a better option for refrigerators and freezers, while HFC-365mfc (boiling point is 104 F) looks better for building insulation.

The higher vapor pressure of HFC-245fa raises concern among spray foamers that its direct substitution into commercial HCFC-141b polyol premixes could result in vapor pressure above the drums' operating range. In Dallas, AlliedSignal reported on its efforts to come up with ways to meet this challenge. (AlliedSignal intends to supply HFC-245fa.) One solution it found is to add a small amount of water (0.5 to 2%) to a polyol premix, which significantly lowers its vapor pressure.

AlliedSignal also found that the right choice of surfactant can improve miscibility of the blowing agent with the polyol, which in turn helps reduce the vapor pressure of the system. (The company also reported that Tegostab B8470 from Goldschmidt Chemical provides lower vapor pressure than other surfactants in HFC-245fa spray foams and also yields better compressive strength and dimensional stability.)

Another possible solution is to use different drums that can withstand elevated pressure. According to AlliedSignal, a high-pressure drum from Greif Bros., equipped with a valve assembly from Micro Matic USA, meets pressure, shipping, and reusability requirements with HFC-245fa in some formulations. It is now being tested with spray foam systems.

Solvay of Belgium reported in Dallas that it has decided to a build a pilot plant for HFC-365mfc, which will come on stream in mid-1999. The company is testing building insulation made from non-flammable blends of HFC-365mfc with other HFCs (245fa or 134a) and from flammable blends with pentane.

While acknowledging that HFCs will cost more than pentane, Solvay notes that cost also depends on density. HFCs may provide 1020% lower density than foams made with hydrocarbons. The two blowing agents can also work together, Solvay reports. Blending HFC-365mfc with hydrocarbons can improve pentane-blown foams by allowing for lower water content, which can result in significantly decreased thermal conductivity as well as lower density.

HFC-245fa in appliances

AlliedSignal reported that refrigerators containing HFC-245fa foam insulation show lower initial energy consumption and slower aging (in terms of changing energy use over time) than units made with either HFC-134a or cyclopentane. AlliedSignal presented a cost-benefit model showing that initially, HFC-245fa would be 21% more expensive to use than HCFC-141b, while HFC-134a would be comparable in cost to HCFC-141b, and cyclopentane would be about 19% more costly. However, once new regulations requiring up to 30% energy improvement of refrigerator-freezers and freezers take effect on July 1, 2001, HFC-245fa will be significantly less expensive to use than both HFC-134a and cyclopentane, but still 12% higher in cost than HCFC-141b.

Studies and small field trials have shown that HFC-245fa appliance foams can have thermal conductivity comparable to HCFC-141b foams. A joint study by Bayer and Whirlpool Corp., Evansville, Ind., examined whether the low boiling point (59 F) of this HFC will affect the foam processing window. Results showed that the processing window of such foams did not shrink radically in comparison with HCFC-141b foams.

More important, the joint research dismissed suspicions that polyol masterbatch temperature could greatly affect foam properties. A major question about HFC-245fa has been whether its low boiling point would require lower processing temperatures than those generally used today. This research indicates that conversion to HFC-245fa foams may require less capital investment than originally thought, as existing foam equipment can be used.

BLOWING AGENT COST COMPARISON FOR SANDWICH PANELS

Blowing Agent           Cost, $/Lb

n-Pentane                0.25
i-Pentane                0.40
c-Pentane                0.85
HCFC 141b                1.00
HCFC 245fa               3.00
HFC 134a                 2.00
HCFC 22                  1.00

Source: Hennecke

RELATED ARTICLE: Additives Enhance Flexible Foams

New additives designed to improve performance of flexible PUR foams were discussed by Air Products & Chemicals and Goldschmidt, while new catalysts for all-water blown PUR and PIR spray foams were unveiled by Tosoh USA.

Air Products developed additives to overcome problems with molded high-resilience automotive seating. The company says that changes in polyol technology designed to improve seat comfort have diminished foam openness and dimensional stability. Dimensional stability is also said to suffer from the inconsistency of mechanical crushing methods intended to prevent shrinkage.

The company says its new tertiary amine blowing catalyst (Dabco BL-53) and two new silicone copolymer surfactants (X-N1586 and X-N1587) provide improved dimensional stability and an extended TPR (time-pressure-release) window for the crusher. These additives reportedly reduce the force necessary to crush freshly demolded foam without adversely affecting the physical properties.

Goldschmidt recently commercialized two silicone surfactants specifically designed to work with liquid C[O.sub.2] in flexible slabstock. Both work well on all three major commercial machinery systems, the company says. One of the new entries, Tegostab B 8232, is said to provide better FR properties than other surfactants. New "universal" surfactant Tegostab B 8240 is said to perform even better than existing universal surfactant B 8220. It gives a very good combination of universal FR performance and nucleation activity.

Two novel tertiary amine catalysts for all-water blown PUR and PIR spray foams are designed to overcome problems such as delay of initial blow, foam cracking, and high density. Toyocat-FB20 and FB30 from Tosoh USA reportedly provide fast initial blowing activity, comparable to HCFC-141b systems. They also prevent "hanging" of the foam and produce low density without sacrificing FR performance and moldability. Low odor, low friability, and improved adhesive strength are also cited.