MXPA01007813A - Open-celled polyurethane foams containing graphite which exhibit low thermal conductivity - Google Patents

Abstract

A process is disclosed for preparing an open-celled rigid polyurethane foam by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent, a cell opening agent and exfoliating graphite, wherein the thermal conductivity of the foam is from 28 to 35 mw/mk. Such foams are applicable in the construction industry where it is desired to have a foam having flame retardation properties and a low thermal conductivity.

Description

OPEN CELLULAR POLYURETHANE FOAMS CONTAINING GRAPHITE SHOWING LOW THERMAL CONDUCTIVITY The present invention relates to the preparation of rigid open-cell polyurethane foams having improved burn properties based on the inclusion of exfoliating graphite in the foams. There are numerous approaches in the matter to increase the slow burning properties of polymeric foams. A common approach for rigid polyurethane foams is to include phosphorus-containing or halogenated compounds in the composition. Another approach is the use of melamine as a flame retardant either alone or in combination with other flame retardants. Other approaches include changes in the molecular structure of the polymer, for example, polyisocyanurate formation or higher concentrations of aromatic units. Such approaches generally require relatively large amounts of the particular flame retardant. For example, U.S. Patent 4,221,875 describes the use of 20 to 100 parts of melamine powder per one hundred parts of the polyhydroxyl compound. Another flame retardant that has been reported to give slow burning properties, particularly in the area of flexible foam is expandable graphite (exfoliating), see for example, US Patents. 4,698, 369 and 5,023,280. Despite the abundance of processes described to obtain slow burning foams, it continues to be a need to improve the slow burning properties of the foams. Accordingly, it is an object of the present invention to provide a process for preparing a rigid slow-burning open cell foam which is capable of passing the B2 test (German standard DIN-4102 Teil 1, May 1998, baustoffklase B2) . It is another object of the present invention to produce such foams using exfoliating graphite as the sole slow burning agent. It is still another object of the present invention to provide foams that meet the fire test B2. A further objective of the present invention is to provide a process for preparing a rigid open cell slow burning foam free from the use of halogenated chlorofluorocarbons, chlorofluorocarbons or volatile organic compounds as blowing agents. Such foams are particularly useful in application where it is desired to use a low density foam having thermal insulation properties and can provide structural stability. The present invention is a process for preparing an open cell rigid polyurethane foam by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent, an exfoliating graphite cell opening agent. The present invention is also for a polyurethane foam having a density of 10 to 45 kg / m3 and a thermal conductivity of 28 to 35 mw / mk where the foam contains more than 50 percent open cells and contains 2 per cent. one hundred or more by weight of exfoliating graphite.

The present invention further provides a process for producing such foams wherein the blowing agent is substantially water. The foams prepared by the process of the present invention can pass the B2 test without the need for additional slow burning agents, such as halogenated compounds or phosphate ester. The foams are thus produced without the need for a flame retardant volatile. Due to the fine cellular structure, the foams have a low thermal conductivity while maintaining their compressive strength. Foams that have low thermal conductivity and compressive strength are ideally suited for insulating construction applications. The addition of other slow burning agents to the foams further increases the slow burning properties of the foam. It has unexpectedly been found that open cell foams they can be produced with exfoliating graphite as the only flame retardant wherein the foams have improved slow burn properties and still maintain a high compressive strength without thermal conductivity relaxation as compared to standard closed cell foams. This unexpected result is obtained when the foam mixture contains cell opening agents so that the size of the cell is 300 μm or less. The use of exfoliating graphite as the only flame retardant also allows the production of slow burning foams that are free or have reduced amounts of volatile compounds.

Polyisocyanates useful for making polyurethane foams for use in the present invention include aliphatic and cycloaliphatic and preferably aromatic polyisocyanates or combinations thereof, advantageously having an average of from 2 to 3.5, and preferably from 2 to 3.2 isocyanate groups per molecule . A crude polyisocyanate can also be used in the practice of this invention, such as crude toluene diisocyanate obtained by the phosgenation of a diamine mixture of toluene or the crude diphenylmethane diisocyanate obtained by the phosgenation of crude methylene diphenylamine. Preferred polyisocyanates are aromatic polyisocyanates such as are described in the patent of US Pat. No. 3,215,652. Especially preferred polyisocyanates for use in the present invention are polymethylene polyphenylene polyisocyanates (MDI). As used herein, MDI refers to polyisocyanates selected from isomers of diphenylmethane diisocyanate, polyphenylene polymethylene polyisocyanates and derivatives thereof which carry at least two isocyanate groups. In addition to the isocyanate groups, such compounds may contain carbodiimide groups, uretonymine groups, isocyanurate groups, urethane groups, allophanate groups, urea groups or biuret groups. MDI is obtained by condensing aniline with formaldehyde, followed by phosgenation, which processes productions called raw MDI. By fractionation of crude MDI, pure and polymeric MDI can be obtained. The pyro or polymeric, crude MDI can be reacted with polyols or polyamines to produce modified DI. The MDI advantageously has an average of from 2 to 3.5, and preferably from 2.0 to 3.2, isocyanate groups per molecule. Especially preferred are methylene bridge polyphenyl polyisocyanates and mixtures thereof with crude diphenylmethane diisocyanate, because of their ability to degrade polyurethane. The total amount of polyisocyanate used to prepare the polyurethane foam should be sufficient to provide an isocyanate reaction rate of typically from 60 to 300. Preferably the index is greater than 70. More preferably the index is greater than 80. Preferably the index it is not greater than 250. More preferably the index is not greater than 220. An isocyanate reaction number of 100 corresponds to an isocyanate group per hydrogen atom reactive of isocyanate present in the water and the polyol composition. Polyols which are useful in the preparation of cellular foams based on polyisocyanate include those materials having two or more groups containing an active hydrogen atom capable of undergoing reaction with an isocyanate. Preferred among such compounds are materials having at least two hydroxyl groups, primary or secondary amine, carboxylic acid or thiol per molecule. Compounds having at least two hydroxyl groups per molecule are especially preferred because of their desirable reactivity with polyisocyanates. Generally, polyols typically suitable for preparing polyurethanes include those having an average molecular weight of from 1,000 to 1,000,000. Such polyols also advantageously have a functionality of at least 2, preferably 3, and up to 6, preferably up to 8, active hydrogen atoms. For the production of a rigid foam, it is preferred that the polyol or polyol mixture have an average molecular weight of 100 to 2,000 and an average functionality of 2 or more; generally in the range of 2 to 8. More preferred are polyols or polyol blends having an average molecular weight of 150 to 1, 100. Representative polyols include polyether polyols, polyester polyols, acetal resins terminated in polyhydroxy, amines finished in hydroxyl and polyamines. Examples of these and other suitable isocyanate reactive materials are described more fully in the U.S. Patent 4., 394,491. Preferred are the polyols prepared by adding an alkylene oxide, such as ethylene oxide, propylene oxide, butylene oxide or a combination thereof, to an initiator having from 2 to 6, preferably 3 to 4, active hydrogen atoms. . Due to the slow burning properties associated with aromatic initiator polyols, it is advantageous to use an aromatic initiated polyether polyol such as the polyol or part of a polyol mixture. In addition to the polyols described above, the amine initiated polyols can be used. Advantageously, the aromatic initiated polyether polyol is an alkylene oxide adduct of a phenol / formaldehyde resin, often referred to as a "novolac" polyol, as described in US Patents. 3,470, 1 1 8 and 4,046,721 or an alkylene oxide adduct of phene / formaldehyde / alkanolamine resin, often called "Mannich" polyol as described in the E. OR . 4,883, 826; 4,939, 182 and 5, 120,815. The slow-burning material used in the foams of the present invention is exfoliating (expandable) graphite. Exfoliating graphite is graphite that contains one or more exfoliating agents so that considerable expansion occurs in exposure to heat. Exfoliating graphite will be prepared by methods known in the art. Generally the graphite is modified first with oxidants, such as nitrates, chromates, peroxides or by electrolysis to open the crystal layer and then nitrates and sulfates are intercalated within the graphite. The amount of exfoliating agent used in the foams to give the desired physical properties is generally less than 50 percent by weight of the final foam. Preferably, the amount of graphite is 40 percent or less by weight of the final foam. More preferred is 30 percent or less by weight of graphite in the final foam. More preferred are foams containing 20 percent or less by weight of graphite. To satisfy the B2 test, the foams generally contain 2 percent or more by weight of graphite. More preferred are foams containing 3 percent or more by weight of graphite. More preferred are foams containing 3 to 10 weight percent graphite in the foam. In accordance with this invention, the walls of the individual cells in the foams are broken during the foaming process. The rupture of the cell walls is carried out by the inclusion of a liquid or solid cell opening agent. Such cell opening agents are known in the art and are generally surface active substances such as surfactants, fatty acid polyols or castor oil and modifications thereof and materials having a critical surface free energy of less than 23. mJ / m2 as described in the US Patent 5,312,846. A combination of these cell opening agents can also be used. Examples of surface active substances include compounds that support the homogenization of the starting materials and are optionally suitable for regulating the cellular structure. Examples include emulsifiers such as sodium salts of fatty acids as well as salts of fatty acids with amines, for example, dietanonalin oleate, diethanolamine stearate, diethanolamine ricinoleate, sulfonic acid salts, for example, ammonium or alkali salts. of dodecylbenzenesulfonic acid or dinaphthylmethane-disulfonic acid and ricinoleic acid; foam stabilizers such as siloxane-oxalkylene copolymers or polymers and other organopolysiloxanes, oxethylated alkylphenols, oxethylated fatty alcohols, paraffin oils, castor oil and ricinoleic acid esters, turkey red oil or peanut oil; as well as cell regulators such as paraffins, fatty alcohols and dimethyl polysiloxanes. In addition, oligomeric acrylates with fluoroalkane or polyoxyalkylene secondary groups are also suitable for improving the emulsifying effect, the cellular structure and / or stabilizing the foam. These surface active substances are generally used in an amount of 0.01 to 6 parts by weight based on 1000 parts by weight of the polyol. Such materials are commercially available, for example, TEGOSTAB B8466, TEGOSTAB B8919, TEGOSTAB 8450, and ORTEGOL 501 from Th. Goldschmidt AG, and Surfactant 6164 from OSI Specialties-Witco. Examples of solid materials described in the U.S. Patent 5,312,846 include fluorinated polymers such as poly (hexafluoropropylene), poly (1,1-dihydro-perfluorooctylmethacrylate) and poly (tetrafluoroethylene). Such materials are available from ICI under the trade name FLUOROGLIDE including FL1710 and FL1200, and from Dupont under the trade name TEFLON including TEFLON MP 1 100, TEFLON MP 1200, TEFLON MP 1300 and TEFLON MP 1500. Suitable liquid agents are also disclosed. as fluorinated organic compounds sold by 3M under the trade name FLUORINERT including substances identified as FC-104, FC-75, FC-40, FC-43, FC-70, FC-5312 and FC-71 and substances sold by Rhone-Poulence under the trade name FLUTEC including substances identified as PP3, PP6, PP7, PP10, PP1 1 , PP24 and PP25. It is preferred that the blowing agent consist essentially of water as the substantially unique blowing agent. The water reacts with isocyanate in the reaction mixture to form carbon dioxide gas, thereby blowing the foam formulation. The amount of water added is generally in the range of 4 to 10 parts by weight per 100 parts by weight of polyol. Preferably water is added in the range of 4 to 8 parts, and more preferably 5 to 7 parts per 1 00 parts of polyol. If necessary, a volatile liquid such as a halogenated hydrocarbon or a low boiling hydrocarbon (boiling point from -10 ° C to + 70 ° C) at normal pressure), such as pentane and / or isomers thereof may be used. . In addition to the above critical components, it is often desirable to employ certain other ingredients to prepare cellular polymers. Among these additional ingredients are catalysts, surfactants, preservatives, dyes, antioxidants, reinforcing agents, crosslinkers, chain extenders, stabilizers and fillers. To make polyurethane foam, it is generally highly preferred to employ a minor amount of a surfactant to stabilize the foaming reaction mixture until it hardens. Such surfactants advantageously comprise a solid or liquid organosilicone surfactant. Others, less preferred surfactants include polyethylene glycol ethers of long chain alcohols, tertiary amine or alkanolamine salts of long chain alkyl sulfate esters, alkyl sulfonic esters and alkyl arylsulfonic acids. Such surfactants are used in amounts sufficient to stabilize the foaming reaction mixture against collapse and the formation of large uneven cells. Typically, 0.2 to 5 parts of the surfactant per 100 parts by weight of polyol are sufficient for this purpose. One or more catalysts for the reaction of the polyol (and water, if present) with the polyisocyanate are advantageously used. Any suitable urethane catalyst can be used, including tertiary amine compounds and organometallic compounds. Exemplary tertiary amine compounds include triethylene diamine, N-methylmorpholine, N, N-dimethylcyclohexylamine, pentamethyldiethylenetriamine, tetramethylethylenediamine, 1-methyl-4-dimethylaminoethylpiperazine, 3-methoxy-N-dimethylpropylamine, N-ethylmorpholine, diethylethanolamine, N-co-morpholine, N , N-dimethyl-N ', N'-dimethyl isopropylpropylenediamine, N, N-diethyl-3-diethylaminopropylamine and dimethylbenzylamine. Exemplary organometallic catalysts include organomercury, organoplomo, organoferric and organotin catalysts, with organotin catalysts being preferred among these. Suitable tin catalysts include stannous chloride, tin salts of carboxylic acids such as dibutyltin di-2-ethyl hexanoate, as well as other organometallic compounds such as are described in US Patent No. 2,846,408. A catalyst for the trimerization of polyisocyanates, which results in a polyisocyanurate, such as an alkali metal alkoxide may also be optionally employed herein. Such catalysts are used in an amount that measurably increases the rate of polyisocyanurate or polyurethane formation. Typical amounts are 0.001 to 5 parts of catalyst per 1000 parts by weight of polyol. Preferred catalysts are those that contain one or more reactive hydrogen atoms. Alternatively, other slow burning ingredients, known per se, can be used in addition to the graphite. Examples of such ingredients include phosphorus and / or halogen-containing compounds, antimony oxides, compounds containing boron, or hydrated aluminas. Generally, when the supplemental flame retardant is present, an amount of from 5 to 20 weight percent of the final foam will be added. The addition of a supplemental flame retardant will influence the amount of graphite that must be added to satisfy the B2 flame test. The foams of the present invention generally have a density of 10 to 45 kg / m 3. Preferably, the foams have a density of 15 to 35 kg / m 3. To make the polyurethane foam, the polyol (s), polyisocyanate, perforating agent and other components, including exfoliating graphite are contacted, mixed thoroughly and allowed to expand and harden into a cellular polymer. It is often convenient, but not necessary, to pre-mix certain raw materials before reacting the polyisocyanate and the components containing active hydrogen. For example, it is often useful to mix the polyol (s), blowing agent, surfactants, catalysts, drilling agent, exfoliating graphite and other components except polyisocyanates, and then contacting this mixture with the polyisocyanate. In a preferred embodiment, the exfoliating graphite is dispersed homogeneously in the polyol component. Alternatively, all the components can be introduced individually into the mixing zone where the polyisocyanate and polyclon (s) are contacted. In such a process, the dispersion of exfoliating graphite in polyol can be added as a concentrate in the polyol by a separate line in the mixing zone.

It is also possible to pre-react all or a portion of the polyol (s), in the absence of water, with the polyisocyanate to form a prepolymer. The foams produced by the process of the present invention can be used each time it is desired to use an insulating foam. The foams are particularly applicable as thermal insulation materials. The following examples are given to illustrate the invention and should not be construed as limiting in any way. Unless stated otherwise, all parts and percentages are given by weight.

EXAMPLES A description of the raw materials used in the examples is as follows: Polyol A is a 90: 10 mixture of a polyether polyol of propylene oxide initiated in sugar having a molecular weight of 614 and a hydroxyl number of 410 and a polyol. of propylene oxide initiated in glycol having a molecular weight of 1 01 1. Polyol B is an aromatic initiated propylene oxide polyether polyol having a hydroxyl number of 196 and a molecular weight of 945. IXOL B251 is a halogenated polyether polyol available from Soivay. Saytex RB 79 is a diether diol of tetrabromoephthalate anhydride available from Albe Marie.

RA 640 in a propylene oxide polyol initiated in ethylene diamine having a molecular weight of 350 and a hydroxyl number of 640, available from The Dow Chemical Company. RN 482 is a propylene oxide polyol initiated in sorbitol having a molecular weight of 700 and a hydroxyl number of 480, available from The Dow Chemical Company. B 8466 is a silicone-based surfactant available from Th. Goldschmidt Chemical Corporation. TEFLON MP 1 100 is a poly (tetrafluoroethylene) available from E. l. Du Pont DeNemours and Company. DMMP is the internal combustion dimethyl methylphosphonate available from Aibright & Wiison Ltd. TCPP is tris (1-chloro-2-propyl) phosphate internal combustion additive available from Albright & Wilson Ltd. TEP is the internal combustion triethylphosphate available from Bayer Ag. Graphite exfoliating graphite used in the examples was S 1 5- PU 120 obtained from Ajay Metachem, India. Desomrapid DB is a benzylamine dimethyl catalyst available from Bayer Ag. POLYCAT 5 is a pentamethi-diethylene triamine catalyst available from Air Products and Chemicals, Inc. M229 is an MDI available from The Dow Chemical Company.

A mixture of base polyol was prepared by mixing the following, given in parts by weight: 13 polyol A; 24.4 polyol B; 9.75 RA 649; 4.14 RN482; 6.5 glycerin; 1 .58 B8466; and 1.86 MP1 100C. The polyol base was added to a beaker, and then any flame retardant. Water and catalyst were then added to the above mixture and gently stirred. The isocyanate was added and the mixture was stirred for 10 seconds at 3000 rpm and then poured into a box mold of 50 by 35 by 15 cm. The characteristic of the foam produced with variable components is given in Table 1. To pass the B2 flame test, as measured by the standard German, DIN-41 02 Teil 1, May 1998, baustoffklase B2, the flame must be less than 1 5 cm. TABLE 1 The results show (Example 6) that the addition of graphite as the single flame retardant, at a level of 8 percent by weight of the foam, was so effective in reducing the flame produced during the B2 test as a reference foam. which contains retardants to standard flames. The use of graphite with additional flame retardants was also effective in reducing the flame as measured by the B2 test. It is within the skill in the art to practice this invention in numerous modifications and variations in light of the above teachings. Therefore, it is to be understood that various embodiments of this invention described herein may be altered without departing from the spirit and scope of this invention as defined by the appended claims.

Claims (13)

1. CLAIMS 1. A process for preparing an open cell rigid polyurethane foam by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent, a cell opening agent and an effective amount of exfoliating graphite.
2. 2. The process according to claim 1, characterized in that the exfoliating graphite is present in an amount from 2 to 40 weight percent of the foam.
3. 3. The process according to claim 2, characterized in that the exfoliating graphite is present in an amount from 3 to 20 weight percent of the foam.
4. 4. The process according to claim 1, characterized in that the foam has a thermal conductivity of 28 to 35 mw / mk.
5. 5. The process according to claim 1, characterized in that the cells are 300 μm or less.
6. The process according to claim 1, characterized in that the blowing agent is water, an organic blowing agent, a hydrocarbon or a combination thereof.
7. The process according to claim 6, characterized in that the blowing agent is substantially water.
8. 8. The process according to claim 1, characterized in that the foam has a density of 10 to 45 kg / m3.
9. 9. The process according to claim 8, characterized in that the foam has a density of 15 to 35 kg / m3.
10. The process according to claim 1, characterized in that the polyol has an average molecular weight of 100 to 2,000. eleven .
11. The process according to claim 10, characterized in that the polyol has an average molecular weight of 1 50 to 1, 100.
12. 12. The process according to claim 10, characterized in that the polyol has a functionality of 2 to 8 ..
13. 13. A foam polyurethane having a density of 10 to 35 kg / m3 and a thermal conductivity of 28 to 35 mw / mk where the foam contains more than 50 percent of open cells that have a diameter of less than 300 μm and contains 2 per one hundred or more by weight of exfoliating graphite.