Polyurethane conference preview: foam formulators chase moving target.
Much of the activity at next month's Polyurethanes 92 conference in New Orleans (sponsored by SPI's Polyurethanes Div.) will focus on development efforts toward chlorine-free alternatives with zero ozone-depletion potential (ODP). Closely related topics such as the quest to tailor polyols, surfactants and catalysts to be compatible with the new blowing agents, and designing CFC-free foams for construction, automotive and home-furnishing uses will also be explored.
STRIVING FOR ZERO ODP
Among the blowing agents showing the most promise as long-term solutions to the ozone-depletion problem are hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs). In a paper from Allied-Signal Inc., Morristown, N.J., these gaseous blowing agents are compared with volatile liquid blowing agents such as CFC-11 and HCFC-141b. Inherent thermal insulation characteristics of HCFC-141b compare quite favorably with those of CFC-11, and a major challenge to the next generation of blowing agents will be meeting this level of insulation. HFCs and HFEs tend to exhibit higher vapor-phase thermal conductivities, Allied-Signal says, reducing their ability to provide adequate insulation. HFC gases, such as the already commercial HFC-134a, and still developmental HFC-32 and HFC-125, may hold some promise, if the insulation problem can be overcome Allied-Signal reports.
Researchers from ICI Polyurethanes in Belgium (U.S. office in West Deptford, N.J.) have also taken a look at HFC-134a, a chlorine-free alkane with reported zero ODP. This blowing agent usually produces foams with thermal conductivity inferior to those made with CFC-11. But with a unique HFC-134a-based formulation developed as a drop-in replacement for CFC-11, ICI was able to produce foams with microfine cell morphology and thermal conductivity and physical properties equivalent to foams manufactured with CFC-11 systems. The key to ICI's HFC-134a system is a small amount of insoluble liquid emulsion in the expanding polyurethane mixture. The emulsion droplets act as nucleation points, generating a microfine-celled foam, ICI says. Furthermore, the nucleating material was shown to stabilize the frothing effect, resulting in more uniform morphology and allowing higher loadings of HFC-134a.
BASF Corp., Wyandotte, Mich., will report on HFCs and HFC ethers (HFEs) as blowing agents for rigid foams. According to BASF, hydrogen atoms in the compounds provide the potential blowing agents with increased reactivity in the lower atmosphere, reducing their global warming potential. These same hydrogen atoms also enhance the agents' solubility in polyols, eliminating the need for special emulsifiers. Other benefits of HFC and HFEs include boiling points in an easily usable range, compatibility with existing catalysts and silicone surfactants and ability to produce foams with tack-free times shorter than those for foams blown with HCFC-141b, BASF says. HFCs and HFEs reportedly can be used with co-blowing agents such as water. Rigid foams blown with a combination of HCFCs and HFEs were found to have closed-cell contents greater than 90% and initial K-factors of approximately 0.14 Btu-in./hr-sq ft.
Jim Walter Research Corp. of St. Petersburg, Fla., will discuss its experiments with perfluorinated hydrocarbons--compounds mainly used as synthetic blood substitutes and soldering fluids--as additives to rigid urethane and isocyanurate insulating foams. According to the company, these compounds can dissolve large quantities of oxygen, providing rigid foams with smaller cell size and improved initial K-factors. The perfluorinated hydrocarbons apparently work with both CFCs and HCFCs 123 and 141b. Since they have boiling points that would make them acceptable as blowing agents, they could possibly perform double duty as blowing agents and K-factor enhancers. However, the perfluorinated hydrocarbons' high cost would probably exclude them from consideration as a blowing agent.
A slightly different approach toward CFC replacement is being pursued by Akzo Chemicals, Inc., Dobbs Ferry, N.Y., which will report on its attempts to use a chemical blowing agent as a drop-in replacement for CFCs in both rigid and flexible foams. Concentrating mostly on low-density flexible foams, Akzo used dialkyl dicarbonate CBAs together with specific tertiary amine catalysts. The dialkyl dicarbonate rapidly decomposes at ambient temperatures, Akzo says, giving off carbon dioxide. This reaction generates enough gas to create an efficiency comparable to CFC-11 or methylene chloride. The amount of C|O.sub.2~ can be tailored to meet a specific need, Akzo says.
HCFCs IN THE INTERIM
But for now, HCFCs seem to be the path most rigid foam makers are following until techniques for using these zero-ODP blowing agents are perfected. Given that HCFCs will continue to hang on well past the turn of the century, work is under way to better understand their behavior.
The most popular HCFC, HCFC-141b, has been found to decompose during the foaming process. Decomposition products commonly include either chlorofluoroethane (HCFC-151a) or chlorofluoroethylene (HCFC-1131a). Air Products & Chemicals Inc., Allentown, Pa., and Allied-Signal's Buffalo, N.Y., Research Laboratory analyzed various urethane and isocyanurate foams blown with HCFC-141a, finding traces of both 151a and 1131a in all the foams. While the level of 151a was usually as low as 50-100 ppm, the 1131a was usually in the several-thousand ppm range in isocyanurates and in the several-hundred ppm range for urethanes. Although the degradation of HCFC-141b produces no noticeable effect on the performance or processing of these foam systems, Allied-Signal and Air Products warn that minimizing their formation would be prudent since limited toxicity data are available for either chemical. This can be accomplished, they say, by using trimerization catalysts based on quaternary ammonium carboxylates such as Air Products' Dabco TMR and TMR 2, rather than potassium-based catalysts.
PROGRESS IN POLYOLS
Polyols are rapidly evolving to keep pace with the changes in blowing agents. According to a three-company polyol research team, one area where this is particularly evident is in rigid spray foams where increasing use of water-blown formulations is rendering obsolete many of the polyols that worked fine with CFCs. Furthermore, the increased amount of isocyanate required in such formulations presents problems for much of the spray-foaming equipment used today because of the necessary increase in the A:B ratio.
Researchers from Dow Plastics, Midland, Mich.; Foam Enterprises, Houston; and IPI, Inc., Elkton, Md., have apparently made some progress. They will report on a polyol developed for water-blown urethane spray foams that reportedly maintains the inherent reactivity and combustibility properties of existing polyols with an accompanying reduction in viscosity.
A highly functional carbamyl-methylated melamine polyol for rigid water-blown foams will be presented by the Chemical Research Div. of American Cyanamid Co., Stamford, Conn., and Polymer Technologies, Inc., Detroit. Researchers at the two companies compared this Cylink HPC polyol with an aromatic amine polyol and a sucrose-based polyol, finding the Cylink polyol provided better K-factor retention, improved thermal conductivity, and reduced combustibility.
A novel polyol containing smaller amounts of mono-ol byproduct than conventional polyols is the key to a new technology for all-water-blown, integral-skin flexible foams used in automobiles. Developed by Asahi Glass Co., Ltd., Tsukakoshi, Japan, the new process reportedly produces foams with a compact skin layer and the same ratio of skin to core as with CFC-11 blown integral-skin foams. The polyol, trade named Preminol, has a very narrow molecular-weight distribution, Asahi Glass says, giving foam makers greater control over the reaction profile between the C|O.sub.2~ foaming and polymerization. Water-blown foams are historically prone to post-expansion due to the high foaming pressure of C|O.sub.2~. However, Asahi Glass says this problem becomes moot with Preminol since it forms oligomer from the isocyanate, increasing the crosslink density in the urethane resin and forming a highly durable skin layer.
ADDITIVES KEEP PACE
A problem that always faced the flexible foam makers, even when CFCs were in vogue, was polyol oxidation. Suppliers were able to develop antioxidants that worked well with the CFC-blown formulations; but with the move away from CFCs, forcing more foam makers to switch polyols, the need has arisen for a new generation of antioxidants. Uniroyal Chemical Co., Middlebury, Conn., will provide an overview of recent developments in polyol stabilization. Among the topics Uniroyal will discuss are liquid antioxidant systems, elimination of BHT, and the use of phosphates, phenothazine and other compounds not currently employed in polyol stabilization.
Urethane processors are also trying out new catalysts to accommodate new non-CFC formulations. This is especially true in flexible foams, which have shifted largely to water-blown systems. These systems have exhibited flowability problems, as well as decreased foam softness and high closed-cell content. Primary amines released during the water/isocyanate reaction are also believed to lead to automotive windshield fogging.
With the auto industry taking a look at using more urethane foams in auto interiors, this fogging behavior is becoming an important issue. Arco Chemical Products Europe, Inc., Villers St. Paul, France (a branch of Arco Chemical Co., Newtown Square, Pa.), will report on a new antioxidant package for polyether polyols that reportedly lowers fogging values by 80-90% compared with other additives.
Several other papers will focus on catalysts and surfactants. UOP, Des Plaines, Ill., and Polymer Technologies will report on use of secondary aromatic diamines in rigid isocyanurate foams. Urea modification with these catalysts reportedly results in foams with higher compressive strengths, finer cell structure, slower diffusion of C|O.sub.2~ in water-blown foams, and higher closed-cell content. Dimensional stability of water-blown foams increases dramatically, the companies say.
Tosoh U.S.A., Inc., Atlanta, will report on newly developed amine catalysts designed specifically for automotive instrument panels. With instrument panels growing larger, wider and more complex in design, researchers are looking for ways to decrease the panels' foam density. However, Tosoh says, these low-density foams have presented moldability problems. Tosoh found that using non-fugitive amine catalysts counteracts this problem and also provides resistance to PVC discoloration and embrittlement while speeding foam cure.
While still a small part of the flexible foam market, polyester urethanes continue to play a vital role in several applications. Th. Goldschmidt AG of Essen, Germany, and Air Products & Chemicals PURA GmbH & Co. of Norderstedt, Germany, will report on new surfactants and catalysts for these foam systems.
Goldschmidt will discuss modified silicones and organic co-surfactants that can reportedly help foam makers control cell size, openness, pin-holing and flammability.
Air Products will report on a new silicone surfactant and two amine catalysts for polyester flexible slabstock. The surfactant, Dabco 5526, reportedly can provide very good foam stability while producing foams with a fine cell structure and no pinholing. New catalysts, XF-H20-38 and XF-H20-39, are aimed at replacing traditional catalysts for polyester flexible slabstock. XF-H20-38 is designed to replace NEM in TDI 80/20 formulations. XF-H20-39 is a low-odor, low-volatility products developed to replace BDMA in TDI 80/20 and 65/35 systems.
As foam formulations continue to change, so does the machinery used to process these materials. Because most of the newer blowing agents have lower boiling points than CFCs, Dow Benelux in Terneuzen, The Netherlands, has developed high-pressure dispensing and nucleating machinery for making fine-cell foams using these new blowing agents and soluble nucleating gases such as nitrogen or other noble gases and liquids.
New equipment from the Gusmer Div. of PMC, Lakewood, N.J., reportedly allows the benefits of nucleation to be used in application areas outside of RIM. Gusmer will describe its system, in which gas is first injected at 130 psi into a fluid stream that is at 100 psi. This mixture is then fed to a high-pressure pump where the pressure is increased to 1500 psi, and then to an impingement mixer where nucleation and pressure drop occur. The fluid then flows to an accumulator equipped with a pressure-switch system that maintains a pressure of 175-200 psi on the nucleated material, allowing on-demand feed to any type of machine. Nucleated material maintained in the accumulator remains stable for more than 24 hr, Gusmer says.
A production system that allows a custom molder to economically produce flexible molded foam for various customers with widely divergent specifications will also be discussed. Created by the Hennecke Machinery Div. of Miles Inc., Pittsburgh, the new machine combines fast-response, variable-speed drives on metering pumps with faster and more accurate mixhead positioners, allowing the molder to formulate different systems within the head. Fast-cure TDI systems using polyols with different levels of water and PHD fillers allow independent variation of ILD and density.
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|Title Annotation:||Technology News: Polyurethanes; polyurethane producers strive to develop chlorine-free alternatives with zero ozone-depletion potential|
|Date:||Sep 1, 1992|
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