Process for producing polyurethane foam having a modified property

Abstract: Polyurethane foam is modified by a post-treatment process comprising the steps of contacting the polyurethane foam with a swelling agent for the polyurethane foam and interpenetrating the swollen foam with a chemical additive, then shrinking the interpenetrated foam by removing the swelling agent under conditions where the chemical additive remains trapped in the polymeric structure of the foam.

Description: This invention relates to a modified polyurethane foam and a post-treatment process for producing the same. It has long been known that it is not possible to generate an explosion of any fuel/air mixture in the presence of reticulated polyurethane foam structures. Because of this characteristic of reticulated polyurethane foam and the additional benefits of light weight, minimal reduction in a fuel tank's usable space and attenuation of violent surging of fuel, reticulated polyurethane foam structures have found extensive use in military aircraft and land vehicles. These properties reduce the vulnerability of these vehicles by providing protection against explosive ballistic hits, explosions following fuel tank rupture, explosive static discharge and explosions caused by lightning. Unfortunately, the use of foam can give rise to a different danger. Since the foam has a low electrical conductivity, (or as as more conveniently measured, a high electrical resistivity) then there can be a buildup of electrostatic charges and the electrostatic charges can result in sparking, leading to a fire or explosion within the non-foam containing areas of the fuel tank. The build up of electrostatic charges can also adversely effect sensitive aircraft instrumentation. Consequently, it is an object of this invention to provide a foam for fuel tanks which minimizes the possibility of fuel explosion from gunfire, electric ignition and lightning. It is a further object of this invention to provide a foam which has a higher electrical conductivity needed to minimize the danger of sparking caused by the buildup of electrostatic charges. It is a further object of this invention to provide fuel tanks which minimize the release of fuel in the event of rupture and which resist sparking due to the buildup of electrostatic charge. It is also an object of this invention to provide a process for the modification of the properties of a polyurethane foam. One process for the modification of polyurethane foam is treating the foam with a chemical additive so that the chemical additive coats the surface of a foam. Such processes typically involve spraying, padding or melting material onto polyurethane foam. The disdavantages of adapting such a process to the formation of foam for use in fuel tanks include the tendency of the chemical additive to either flake off the foam into the fuel, thus providing particles which can lead to fuel line blockage or an increased chance of a fire or to leach out of the foam and become dissolved in the fuel. A second process for the modification of polyurethane foam is incorporating a chemical additive into the polymer itself by the so-called in situ technique. Incorporation can be achieved " in situ " by mixing a chemical additive with polyurethane foam-forming reactants. The chemical additive is mixed with the foam-forming reactants during the reaction, and consequently the chemical additive becomes intimately mixed with the subsequently formed polymer. The disadvantages of in situ incorporation include the possibility that the desired chemical additive, or a carrier for the chemical additive, will react with the polyurethane foam-forming reactants to adversely effect either the chemical additive itself, the properties of the subsequently formed foam, or the processing of the foam. This generally limits both the nature and the amount of chemical additive that can be incorporated into the polyurethane foam. Another disadvantage is that in situ impregnation is not useful with a volatile or heat-sensitive chemical additive, for the heat of the urethane polymerization reaction will degrade the additive or will volatilize and drive the additive out of the foam-forming mixture. According to the present invention there is provided a process for producing a conductive polyurethane foam in which the foam is contacted with an electrical conductivity increasing chemical additive in a manner such that the additive is inter-penetrated in the foam and the additive comprises a quaternary ammonium salt, characterized by the fact that the additive is mixed with a swelling agent and the contacting of the foam with the swelling agent is for a time sufficient to swell the foam to at least a ten percent increase in its original volume to allow the chemical additive to interpenetrate the foam, and thereafter shrinking the foam by removing the swelling agent under conditions whereby at least 1%, by weight of the foam, of the chemical additive remains trapped in the polymeric structure of the foam. Also provided according to the present invention is a reticulated polyurethane foam whenever produced by the process according to the present invention. Further, the present invention provides a reticulated polyurethane foam having an electrical resistivity of 1 x 10¹³ ohm-cm or less comprising a polyurethane foam and at least about 1%, by weight of the polyurethane foam, of an electrical conductivity increasing chemical additive trapped in the polymeric structure of the polyurethane foam, where the chemical additive is soluble or miscible in a swelling agent for the polyurethane foam. The present invention also provides an improved fuel tank comprising an outer shell to confine the fuel and having at least 35% of its volume filled with a polyurethane foam prepared by a process according to the present invention, said foam having an electrical resistivity less than 1 x 10¹³ ohm-cm. Further provided in accordance with the present invention is an improved fuel tank comprising an outer shell to confine fuel and having at least 35% of its volume filled with a reticulated polyurethane foam, having an electrical resistivity less than 1 x 10¹³ ohm-cm. FR-A-2 239 489 discloses preparation of cushioning and mattress foams which are non-reticulated products. The publication is concerned with the problem of improving the anti-static properties of polyurethane foams by introducing quaternary ammonium salts as anti-static agents. According to the preferred method of the citation, the quaternary ammonium salts are added to the reaction forming the polyurethane foam, but there is disclosure of the possibility of adding the quaternary ammonium salts to the foam after it has been formed, and that the foam can be contacted by the quaternary ammonium salts dissolved in water or alcohol. The publication states that when the quaternary ammonium salt is added to the formed foam, the foam should be pressed by a suitable means and then allowed to expand to its original volume to absorb the quaternary ammonium salt. This differs from the present treatment of the foam with a swelling agent to increase the foam's original volume by at least 10%. The publication is concerned with the effect of an antistatic additive on rebound and cushioning properties. Applicant's invention is concerned with the problem of producing a polyurethane foam, preferably reticulated foam, having improved conductivity. The improvement is achieved by interpenetrating the foam after it has been formed, by contacting the foam with a mixture of an electrical conductivity increasing chemical additive and a swelling agent for a time sufficient to swell the foam to at least a 10% increase of its original volume. The swelling of the foam by the swelling agent is critical to obtaining interpenetration of the foam by the chemical agent to obtain the desired conductivity as is demonstrated by the examples of this application. GB-A-2 038 739 discloses the formation of a fuel tank with a reticulated polyurethane foam by feeding and then foaming in the fuel tank a mixture containing a polyurethane forming composition. The British patent uses an in-situ reticulation; i.e. the foam is formulated to produce a highly permeable foam. However, additives, such as antistatic agents, used during the formation of the foam may interfere with the reaction or the properties of the foam or with obtaining it in a reticulated form. Applicant has discovered that post-treatment of a foam with a swelling agent and an additive in accordance with the invention produces a foam having an electrical resistivity less than 1 x 10¹³ ohm-cm. To reduce fuel retention by the foam, very low fuel flow resistance, obtainable only by reticulation, is required. Applicant's fuel tank can employ a foam reticulated after the foam formation. Consequently, it is an object of this invention to provide a process for the modification of polyurethane foam by interpenetrating the foam with a quaternary ammonium salt after it has been formed, to increase the electrical conductivity of the polyurethane foam by interpenetrating the foam with said chemical additive. The present invention relates to a process for post-treating polyurethane foam to impart thereto the desired electrical conductivity and the modified foam so formed. The post-treatment process comprises the steps of contacting the polyurethane foam with a swelling agent for the polymer and interpenetrating the swollen foam with a chemical additive selected to impart the desired electrical conductivity to the foam, then shrinking the interpenetrated foam by removing the swelling agent under conditions where the chemical additive remains trapped in the polymeric structure of the foam. The method of this invention is generally useful with conventional reticulated and non-reticulated polyurethane foam. Conventional polyurethane foams include all foams formed from polymers containing a polyurethane linkage. Reticulated foams are preferred. Representative polymers include polyether based polyurethanes, polyester based polyurethanes, copolymer polyol based polyurethanes, and mixtures thereof. Polyether based polyurethanes are preferred in those embodiments where hydrolytic stability is required. Additionally, in preferred embodiments, the polyurethane foam has a void volume greater than 80%, most preferably a void volume greater than 90%. The interpenetrated polyurethane foams of this invention are produced by treating the foam with a swelling agent which serves to both swell the foam and in some embodiments to transport an interpenetrating quaternary ammonium salt into the foam. Upon removal of the swelling agent, said chemical additive remains, trapped in the polymeric structure of the foam and imparts desired increased electrical conductivity to the foam. A wide variety of swelling agents can be employed in this invention. Some requirements of the agents are that the swelling agent must totally or partially dissolve the chemical additive, must swell the foam and must have no adverse effect on the properties of the polyurethane foam or the chemical additive. For example, the swelling agent must swell the foam without dissolving it. Further, the swelling agent should be easily removable from the foam after the swelling and impregnation steps, so that upon removal, the chemical additive remains trapped in the polymer structure of the foam. Agents which swell polymers are known. Representative swelling agents include: aliphatic, aromatic and halogenated hydrocarbons, ketones, aldehydes, alcohols, esters, ethers having from one to twenty-five carbon atoms and mixtures thereof. Preferred swelling agents include acetone, aniline, benzene, carbon tetrachloride, cyclohexane, ethyl acetate, ethyl chloride, methanol, ethanol, isopropanol, methylene chloride, chloroform, perchloroethylene, methyl ethyl ketone, nitrobenzene, xylene, trichloroethylene, vinyl acetate, acetaldehyde, diethyl ether, freon and mixtures thereof. Additional swelling agents include sulfur solvents such as dimethylsulfoxide, thioethers and mercaptans; amides such as dimethylformadide; isocyanates; and ethylene glycol monomethyl ether. Most preferred swelling agents include methanol, methylene chloride and trichloroethylene. Moreover, the swelling agent can be mixed with other fluids, such as water, so long as the resulting mixture has the ability to swell the foam. Without being limited to the following theory concerning the effect of the swelling agent, it is believed that the swelling agent does more than provide a medium for contacting the quaternary ammonium salt to the exterior surface of the foam or the cellular voids therein. It is believed that the swelling agent swells the polymeric strands which comprise the foam, substantially throughout the entirety of the foam, thereby permitting said chemical additive to become both more intimately mixed in the polymer and intimately mixed with more of the foam than is possible when the chemical additive is sprayed, padded or melted onto polymeric foam. The quaternary ammonium salt is selected to impart electrical conductivity to the foam. It is a compound which does not exert an adverse effect on the foam and is soluble in or miscible in the swelling agent for a sufficient time to be interpenetrated in the polymer. It will provide a long-lasting or permanent modification of the foam. It should be compatible with the polymer, i.e. it must not leach out over time or conditions of use. The additives according to this invention are quaternary salts and include quaternary ammonium compounds, such as monoalkyl trimethyl quaternaries, dialkyl dimethyl quaternaries, trialkyl monomethyl quaternaries, methyldialkoxy alkyl quaternaries, hydroxy alkylene quaternaries, dimethyl alkyl benzyl quaternaries, complex diquaternaries and tetra alkyl ammonium ethyl sulfate; imidazolinium quaternaries; and quaternary ammonium based copolymers. Where the expression alkyl, alkoxy or alkylene appears, it should be understood that the hydrocarbon chain contains 1-25 carbon atoms. Representative quaternary ammonium compounds include: methylpolyoxyethylene(15) cocoammonium chloride, methylpolyoxyethylene(15) decylammonium chloride, ethyl bis(polyhydroxyethyl) alkyl ammonium ethyl sulfate, [(3-lauramidopropyl) trimethyl-ammonium] methyl sulfate, stearamidopropyldimethyl-B-hydroxyethyl ammonium nitrate. Representative imidazolinium quaternaries include methyl-1-oleyl amido ethyl-2-oleyl imidazolinium-methyl sulfate and monoalky imidazolium ethyl sulfate. Representative quaternary ammonium based copolymers include dimethyl dialkyl ammonium chloride diacetone acrylamide copolymer. The preferred chemical additives are quaternary ammonium compounds such as ethyl bis(polyhydroxyethyl) alkyl ammonium ethyl sulfate and hydroxy alkylene quaternary ammonium salts; alkoxylated diamines such as N,N,N'-polyoxyethyl(15)-N'-tallow-1,3-propanediamine; and quaternary ammonium based copolymers such as dimethyl dialkyl ammonium chloride diacetone acrylamide. According to this invention the swelling agent is premixed with the chemical additive so that the swelling and the interpenetration are carried out contemporaneously. In general, the swelling agent contains from 0.1 to about 100% and preferably from about 1% to about 100% chemical additive, based on the weight of the mixture. The minimum level of the chemical agent is determined by the minimum amount required to produce the desired increased electrical conductivity of the polyurethane foam properties. The maximum level is the amount that is soluble in, miscible in, the swelling agent for a sufficient time to be interpenetrated into the polymeric structure of the polymer. In other embodiments, a polyurethane foam is swelled with a first swelling agent, then under conditions where there is a transfer of fluid, the first swelling agent is replaced with a second chemical additive containing swelling agent. Finally, the interpenetrated polyurethane foam is shrunk by removing the swelling agent under conditions such that the quaternary ammonium salt remains trapped in the polymeric structure of the foam. Representative methods of removal include evaporation, liquid extraction and chemical modification. Chemical modification is a method whereby a second agent is caused to penetrate the polyurethane foam and to react with the first swelling agent thereby causing the polyurethane foam to shrink. In preferred embodiments, said chemical additive is chosen so that it is less volatile than the swelling agent and the removal of the swelling agent is accomplished by evaporation. 1 to 50 %, preferably from about 3 to about 20 % chemical additive, based on the weight of the polymer, remains trapped in the polymeric structure of the polyurethane foam. The modified foams of the invention are reticulated or non-reticulated polyurethane foams interpenetrated with a chemical additive. The preferred foams also have a void volume greater than 80%, most preferably greater than 90%. The modified foams having high electrical conductivity, e.g. on electrical resistivity of 1 x 10¹³ ohm-cm or less, contain at least about 1%, preferably about 1 to about 50%, most preferably from about 3 to about 20%, chemical additive, by weight of the foam, trapped therein; the additive is a quaternary ammonium compound, such as ethyl bis(polyhydroxyethyl) alkyl ammonium ethyl sulfate and hydroxy alkylene quaternary ammonium salt; alkoxylated diamines such as N;N,N'-polyoxyethyl(15)-N'-tallow-1,3-propanediamine; and quaternary ammonium based copolymer such as dimethyl dialkyl ammonium chloride diacetone acrylamide and mixtures thereof. Other chemical additives include: inorganic salts, Werner complexes, such as chrome complex with methacrylate, coordination complexes, such as nickel chloride hexahydrate, ethoxylated alkylamines, fire retardants, such as chlorinated phosphate esters, organo titanates, polar organic compounds, anionic and non-ionic surfactants, such as ethoxylated alkylphenols and alkoxylated diamines. The improved, explosion-resistant fuel tank of the invention is produced from any conventional fuel tank by filling at least from 35% to 100% of the fuel tank interior with a polyurethane foam having a resistivity of 1 x 10¹³ ohm-cm or less. The most preferred foam for fuel tanks is a reticulated polyether based polyurethane. Such foams are easily produced by the process of this invention using a quaternary ammonium salt as the chemical additive. The fuel tank may be advantageously employed in any vehicle, including land, air and marine vehicular fuel tanks. In preferred embodiments, the fuel tank is an airplane fuel tank. Having generally outlined the details of the invention, the following, nonlimiting examples provide more specific details to the invention. All amounts are based on parts by weight unless otherwise clearly indicated. Example 1 A 20% solution of CP7091 additive, a dimethyl diallyl ammonium chloride diacetone acrylamide copolymer from Calgon Corporation, in methanol was prepared. A sample of reticulated, 15 pores per 25.4 mm (per inch), 20.8 kg/m³ (1.3 pounds per cubic foot) polyether polyurethane foam was immersed in the solution, allowed to reach its maximum swell of about 56%, and removed. The foam was passed through nip rollers to remove excess solution and to control the amount of CP7091 retained by the foam. The foam was then dried in on oven at 66° C to evaporate the methanol. Retention level of the chemical additive and resistivity of the treated foam were measure. With a 9.8% additive retention level (by weight of foam) a resistivity of 3.7 x 10⁹ ohm-cm was observed. This is compared to an untreated foam resistivity of 1 x 10¹⁵ ohm-cm. Example 2 Example 1 was repeated except a solution of 20% CP7091, 75% water and 5% methanol was used as the chemical additive and swelling agent. The foam swelled less than 25%. A 11.6% retention level resulted in a resistivity of 7.6 x 10¹³ ohm-cm. Example 3 (Comparative Example) Example 1 was repeated except water, a non-swelling agent, was used instead of a swelling agent. The foam did not swell. While the process resulted in a 9.9% retention level, a resistivity of only 1.9 x 10¹⁵ ohm-cm was obtained. This resistivity is in the same range as the resistivity of an untreated foam. Example 4 Example 1 was repeated except the foam used was a reticulated, 25 pores per 25.4 mm (per inch), 20.8 kg/m³ (1.3 pound per cubic foot), polyether, polyurethane foam. A 9.8% retention level resulted in a resistivity of 1.6 x 10⁹ ohm-cm. Example 5 Example 1 was repeated except the solution was 3.5% Ethoduomeen T/25, a N,N,N'-polyoxyethyl (15)-N'-tallow-1,3-propanediamine supplied by Armak Chemicals, in methylene chloride. The foam was dried at 40° C. A 12.8% retention level resulted in a resistivity of 2.7 x 10¹¹ ohm-cm. Example 6 Example 1 was repeated except a 4.8% solution of Varstat 66, a ethylbis (polyhydroxyethyl) alkyl ammonium ethyl sulfate from Sherex Chemical Company, in methylene chloride was used. The foam was dried at 40° C. a 12.8% retention level resulted in a resistivity of 2.7 x 10¹¹ ohm-cm. Example 7 Example 1 was repeated except a 6.2% solution of Varstat 55, a mono alkyl imidazolium ethyl sulfate from Sherex Chemical Company, in methylene chloride was used. A 17.8% retention level resulted in a resistivity of 6.7 x 10¹⁰ ohm-cm.

Claims: A process for producing a conductive polyurethane foam in which the foam is contacted with an electrical conductivity increasing chemical additive in a manner such that the additive is inter-penetrated in the foam and the additive comprises a quaternary ammonium salt, characterized by the fact that the additive is mixed with a swelling agent and the contacting of the foam with the swelling agent is for a time sufficient to swell the foam to at least a ten percent increase in its original volume to allow the chemical additive to interpenetrate the foam, and thereafter shrinking the foam by removing the swelling agent under conditions whereby at least 1%, by weight of the foam, of the chemical additive remains trapped in the polymeric structure of the foam. A process according to claim 1, characterized by the fact that the swelling agent is an aromatic hydrocarbon, a halogenated hydrocarbon, a ketone, an aldehyde, an alcohol, an ester, an ether or mixtures thereof where the swelling agent contains from one to twenty-five carbon atoms. A process according to claim 1 or 2, characterized by the fact that the swelling agent is acetone, aniline, benzene, carbon tetrachloride, cyclohexane, ethyl acetate, ethyl chloride, methanol, ethanol, isopropanol, methylene chloride, chloroform, perchlorethylene, methyl ethyl ketone, nitrobenzene, xylene, trichloroethylene, vinyl acetate, acetaldehyde, diethyl ether, freon or mixtures thereof. A process according to any one of claims 1 to 3, characterized by the fact that the quaternary ammonium salt is a quaternary ammonium compound, an imidazolinium quaternary salt, a quaternary ammonium based copolymer, or mixtures thereof. A process according to any one of the preceding claims, characterized by the fact that the swelled polyurethane foam is shrunk after swelling by evaporation, liquid extraction or chemical modification of the swelling agent. A process according to any one of the preceding claims, characterized by the fact that the polyurethane foam is a polyether-based, polyester-based, copolymer-polyol-based polyurethane foam or mixtures thereof. A process according to any one of the preceding claims, characterized by the fact that the polyurethane foam is reticulated. A reticulated polyurethane foam whenever produced by the process according to any one of the preceding claims. A reticulated polyurethane foam having an electrical resistivity of 1 x 10¹³ ohm-cm or less comprising a polyurethane foam and at least 1%, by weight of the polyurethane foam, of an electrical conductivity increasing chemical additive trapped in the polymeric structure of the polyurethane foam, where the chemical additive is soluble or miscible in a swelling agent for the polyurethane foam. A polyurethane foam according to claim 9, characterized by having a void volume of at least 90%. An improved fuel tank comprising an outer shell to confine the fuel and having at least 35% of its volume filled with a polyurethane foam prepared by a process according to any one of claims 1 to 7, said foam having an electrical resistivity less than 1 x 10¹³ ohm-cm. An improved fuel tank comprising an outer shell to confine fuel and having at least 35% of its volume filled with a reticulated polyurethane foam, having an electrical resistivity less than 1 x 10¹³ ohm-cm.

Inventors: VOLZ ROBERT A, VOLZ, ROBERT A.
Applicants: FOAMEX LP, FOAMEX L.P.
Application year: 1985
Main classification: C08J9
IPC codes: C08G18, H01B1, C08J9