Urethane additives: getting in sync with 'CFC-free.' (chlorofluorocarbons) (Additives '92: Formulations in Flux) (Cover Story)
Change one component in a urethane foam formulation, and there's a good chance it will affect the delicate balance of other elements in the system. That is turning out to be the case with elimination of CFC blowing agents, being replaced by HCFCs, HFCs and water-blown systems. The Polyurethane Foam Association (PFA) in Wayne, N.J., says its members have already reduced CFC use in their flexible foams by more than 98% from 1986 levels. This rapid shift in the marketplace has suppliers of catalysts and surfactants scrambling to come up with optimized products compatible with the new blowing systems.
"In any foam formulation, regardless of what you're blowing with, catalysts and surfactants are the most important components," says Bob Santo, market manager for rigid foam additives at Air Products & Chemicals. "They add true performance to the foam." Air Products recently teamed up with Allied-Signal Inc. to develop catalysts that will minimize decomposition of HCFCs 141b and 123 after Allied-Signal found that these blowing agents decompose during the rigid foaming process. The problem, Allied-Signal found, was most acute in polyisocyanurate foams, possibly posing health risks and compromising foam properties.
Suppliers and users agree that proper choice of additives is critical to the quality of both flexible and rigid foams. But making that choice becomes even more complicated, foam makers say, when a change in blowing agent prompts a switch to a different polyol from that normally used. The switch to "ozone-friendly" foam systems is accelerating the pace of polyol development; and polyols can have an even greater effect on catalyst and surfactant selection than do alternative blowing agents.
PROBLEMS IN MDI FLEXIBLES
Possibly the systems experiencing the most acute problems because of CFC replacement and new polyol development are water-blown MDI-based flexible foams. With water and its byproduct C|O.sub.2~ becoming one of the most often used blowing approaches, systems such as MDI-based high-resilience (HR) foams used in automobile seats have much greater reactivity and initial viscosity than their CFC-blown counterparts. The consequence is decreased foam flowability, resulting in voids. Compounding the problem is the tendency of water-blown foams to contain higher polyurea content, leading to increased crosslink density and a higher percentage of closed cells and shrinkage. The way to counter this, additive suppliers say, is through the proper selection of catalysts and/or surfactants, as well as the possible use of one of a host of new polyols.
Air Products, for instance, developed two amine catalysts for all-water-blown MDI cold-cure seating. Dabco XF-2002 (for systems with water levels of 3.0 phr or less) and Dabco XF-2003 (for higher water levels) reportedly provide a smoother reaction profile than previous catalysts, translating into better system flowability and shorter demold times. By allowing higher water levels to be used, XF-2 can reduce the minimum molded density by 10-15%, Air Products says.
The problems seen in MDI-based flexible systems are not limited to just those foams, suppliers say. But flexible TDI-based systems blown with HFCs and HCFCs have not experienced the severity of problems seen in water-blown foams, and many of the catalysts and surfactants that worked well with CFCs are also quite compatible with non-CFC flexible systems.
Nonetheless, Air Products' European branch reported at the recent Utech '92 conference in The Hague, Netherlands, on two experimental silicone surfactants, XF-G25-19 and XF-G25-18, for TDI hot-cure molded flexibles that reportedly provide substantial increases in air-flow values over many other surfactants in non-CFC foams (see PT, June '92, p. 48). Foams produced with the new surfactants are said to be split-free and to have heights and recession values similar to CFC-blown systems. Currently, about 0.8 phr surfactant are used in a system blown with 3.5-4.5 phr of water. The company continues to evaluate these experimental surfactants in formulations with higher water levels and will examine their compatibility with some experimental softening agents.
LOTS OF WORK ON RIGIDS
Even if some TDI flexible foams may be able to get away without formulation changes, "It's a little different story on the rigid side," says Dave Naessens, polyurethane chemicals group manager for Dow Plastics. In rigid polyurethane, the incorrect matching of blowing agent, polyol and additives can result in, among other things, foams that blow too hot and are prone to sintering. Unstable foams with large cell size, high K-factors, poor flowability and inferior dimensional stability can also result if the components in a urethane system are not carefully selected for their compatibility.
Even flammability can be affected. At last fall's Polyurethanes World Conference in Nice, France, Dow Benelex reported that surfactant type also has a great influence on flame spread in all reduced-CFC and CFC-free rigid systems.
Formulating rigid foam with HCFCs 123 and 141b as the sole blowing agent usually results in products with larger cell sizes and lower insulation values than achieved with CFCs. To counter this, additive suppliers like Goldschmidt Chemical, Union Carbide, and Air Products and have begun developing a variety of new silicone surfactants.
Late last year, Air Products commercialized four silicone surfactants for rigid urethane foams blown with HCFC and HCFC/water systems. Two of the new products, DC2-5274 and DC2-5454, reportedly can produce finer-celled foams with better thermal insulation capabilities than foams manufactured with previous surfactants. Depending on the formulation, these surfactants significantly compensate for the loss in thermal insulation values that typically occurs in HCFC-blown foams, the company says. In narrow- and large-cavity pour-in-place formulations and laminate foams, the new surfactants are said to improve flowability.
The other two silicone surfactants, DC2-5357 and DC2-5367, were developed to minimize the detrimental effects of water co-blowing on foam flow and insulating properties (PT, Dec. '91, p. 10).
While looking to achieve the same result, Goldschmidt has taken a slightly different approach. According to Paul Semach, North American business manager, Goldschmidt can tailor its silicone surfactants to be compatible with a particular blowing agent. That is important, Goldschmidt says, because depending on what blowing agent is used, the surfactant chemistry must be different. Flexibility is needed while the technical and market picture is still in flux: "There's considerable debate in rigid foam over just what blowing agent should be used," Semach says.
In a technical paper presented at the Utech conference, Goldschmidt's Dr. George Burkhart pointed to solubility of various blowing agents and isocyanates as one reason why his company has decided to custom blend its surfactants. Burkhart said HCFC-123 is quite soluble and requires one type of surfactant while a host of HFCs are almost insoluble and therefore require different surfactants as well as special emulsifiers to ensure that foaming occurs.
"A simple product recommendation for any rigid foam system which is blown with an alternative blowing agent is difficult," Burkhart said. "Chemical suppliers and rigid foam manufacturers must continue to work together with the additive suppliers to meet the challenge of CFC-substitution."
Union Carbide recently unveiled Y-10733, a new surfactant for rigid foams blown with HCFCs 123 and 141b. The surfactant reportedly improves thermal insulation and physical properties.
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|Article Type:||Cover Story|
|Date:||Jul 1, 1992|
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