Polyimide foam

Abstract: Thermoplastic polyimide foams are provided which essentially consist of recurring units represented by the formula (I): wherein X is a bond, divalent hydrocarbon radical having from 1 to 10 carbon atoms etc. and R is an aliphatic radical having 2 or more carbon atoms, cycloaliphatic radical etc. The foams have an apparent density of from 0.02 to 0.2 g/cc, average cell diameter of 0.1 to 1 mm and a closed cell content of 50% or more.

Description: The present invention relates to a thermoplastic polyimide foam which has excellent resistance to heat, flame and abrasion, and also has good fabrication qualities. Foams which have conventionally been developed by use of macromolecular materials are roughly divided into the following two classes. (1) Foams prepared from thermoplastic resins, for example, polystyrene, polyvinyl chloride, polypropylene and polyethylene. (2) Foams prepared from thermosetting resins, for example, polyurethane resin, phenol resin, urea resin, melamine resin and imide resin. The former foams are excellent in abrasion resistance and fabrication quality and hence broadly applied to automobiles, buildings or electronic and electric fields by utilising substantial characteristics such as heat insulation property and light-weight property. However, the foams are insufficient in thermal resistance and particularly in flame resistance. For example, the foams drastically deform at high temperatures such as above 150°C and performance of foam is eliminated. When the foams are brought into contact with a flame, combustion occurs from the surface of the foams. Even though flame resistance is provided by addition of a flame retarder, the foams immediately fuse and drip from the portion in contact with the flame. Thereafter adjacent portions of the foams successively melt and finally most of the foam disappears. As a result of such disadvantage, the foams cannot be applied to fields such as buildings and aircraft parts where flame resistance is required. The latter foams which are prepared from thermoplastic resin, for example, polyurethane resin foam has also the above disadvantage against the flame. On the other hand, the foams prepared from phenol resin, urea resin and melamine resin have excellent flame resistance and do not melt in contact with the flame. However, these foams are poor in abrasion resistance, liable to crack and impossible to fabricate. These demerits have restricted fields of application. Polyimide resin foams have also been developed. One representative polyimide foam is based upon a prepolymer of International Harvester Co. which is prepared by esterifying an aromatic carboxylic acid dianhydride with an alcohol and adding a diamine whilst another is described in GB-A-1 198 484. Such foams have outstanding thermal resistance and do not deform particularly at temperatures of 200°C. Flame resistance is also excellent because the foam does not melt in contact with the flame. On the other hand, abrasion resistance of the foams is insufficient and fabrication is impossible. Once the foams have been made, they cannot be used for various applications by conducting fabrication and hence there have been restrictions imposed upon their use. Another typical polyimide foam is based upon a polyetherimide (Another typical polyimide foam is based upon a polyetherimide (ULTEM 1000™ etc., products of General Electric Co.) which is prepared from a tetracarboxylic acid dianhydride having ether bonds in the molecule. Such a foam has improved flame resistance, abrasion resistance and fabrication quality. However, its thermal resistance is still unsatisfactory. EP-A-269 319 describes a thermoplastic polyimide resin which is capable of extrusion or injection moulding and is excellent as a high temperature adhesive. It is therefore an object of the present invention to provide a novel polyimide foam having flame andthermal resistance which are merits of a foam prepared from a thermosetting resin in addition to abrasion resistance and fabrication quality which are merits of a foam prepared from a thermoplastic resin. According to the invention there is provided a thermoplastic polyimide foam having an apparent density (measured as defined herein) of from 0.02 to 0.2 g/cc, an average cell diameter (measured as defined herein) of from 0.1 to 1 mm and a closed cell content (measured as defined herein) of 50% or more of the cells, and consisting essentially of recurring units of the formula : in which X represents a bond, a divalent hydrocarbon radical having from 1 to 10 carbon atoms, a hexafluorinated isopropylidene radical, a carbonyl radical, a thio radical, or a sulphonyl radical, and R represents an aliphatic radical having two or more carbon atoms, a cycloaliphatic radical, a monoaromatic radical, a condensed polyaromatic radical, or a non-condensed polyaromatic radical, wherein the aromatic radicals are connected by a bond or a linking group. Also according to the invention there is provided a process for the production of a thermoplastic polyimide foam having an apparent density (measured as defined herein) of from 0.02 to 0.2 g/cc, an average cell diameter (measured as defined herein) of from 0.1 to 1 mm and a closed cell content (measured as defined herein) of 50% or more of the cells, comprising extrusion foaming of a thermoplastic polyimide consisting essentially of recurring units represented by the formula: in which X represents a bond, a divalent hydrocarbon radical having from 1 to 10 carbon atoms, a hexafluorinated isopropylidene radical, a carbonyl radical, a thio radical, or a sulphonyl radical, and R represents an aliphatic radical having two or more carbon atoms, a cycloaliphatic radical, a monoaromatic radical, a condensed polyaromatic radical, or a non-condensed polyaromatic radical, wherein the aromatic radicals are connected by a bond or a linking group in the presence of a foaming agent. The monoaromatic or condensed polyaromatic radical represented by R is preferably The non condensed polyaromatic radical represented by R is preferably The thermoplastic polyimides of the present invention can be obtained by reacting one or more etherdiamines represented by the formula: in which X is as defined above, with one or more of tetracarboxylic acid dianhydrides represented by the formula: in which R is as defined above. The polyimide is characterised in that a diamine having ether linkages in the molecule is used for a raw material. For example, it can be readily prepared by process such as disclosed in United States Patent Specification No. 4,847,349. Any thermoplastic polyimide described in this patent can be used in the present invention. A particularly preferred polyimide for use in the production of thermoplastic polyimide of this invention can be prepared by using as the etherdiamine one or more of compounds selected from 4,4'-bis[4-(3-aminophenoxy)phenyl] sulphide, 4,4'-bis[4-(3-aminophenoxy)phenyl] sulphone, 4,4'-bis(3-aminophenoxy)benzophenone, 4,4'-bis(3-aminophenoxy)biphenyl, 2,2-bis[4-(3-aminophenoxy)phenyl]propane and 2,2-bis[4-(3-aminophenoxy)phenyl]-1,1,1-3,3,3-hexafluoropropane, and as the tetracarboxylic acid dianhydride one or more of compounds selected from pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-diphenylethertetracarboxylic dianhydride and p-phenyleneoxydi(4-phthalic)dianhydride. The inherent viscosity of the polyimide is preferably from 0.35 to 0.65 dl/g, and more preferably from 0.40 to 0.60 dl/g, as measured at 35°C in a solution containing 0.5 g of the polyimide in 100 ml of a solvent mixture composed of p-chlorophenol/phenol in a ratio of 90/10 by weight. When the inherent viscosity of the thermoplastic polyimide is less that 0.35 dl/g, the thermoforming property extremely decreases and closed cell content also lowers. On the other hand, when the inherent viscosity exceeds 0.65 dl/g, extrusion characteristics are reduced and one cannot maintain economical productivity. The apparent density of the foam prepared from the above thermoplastic polyimide is in the range of 0.02 to 0.2 g/cc. An apparent density exceeding 0.2 g/cc gives adverse effect on the economical advantage due to weight saving and further impairs several merits such as shock-absorbing properties, heat insulation property and sound insulating characteristics of the foam. On the other hand, an apparent density less than 0.02 g/cc lowers the mechanical strengths of the foam and also leads to an unfavourable decrease in flame resistance, abrasion resistance or fabrication quality. The apparent density is measured by the following method. A foam sample is previously weighed. The foam sample is completely submerged into a prescribed volume of water placed in a measuring cylinder and volume of the foam is measured. Then the apparent density of the foam is calculated by dividing the weight with the volume. The average cell diameter and the closed cell content are calculated by the following methods. Average Cell Diameter ..... A cross-sectional photograph in a thickness direction of the foam is used. 10 each of large, medium and small cells are selected and their diameters measured. Average cell diameter is the mean value of these diameters. Closed Cell Content ..... A foam sample having prescribed dimensions (40 x 40 mm) is weighed. The sample is placed under water surface and the pressure is reduced to 400 mmHg. The foam is allowed to stand in this state for a minute and then taken out of the water and weighed. Amount of the water (V W ) absorbed into the foam is calculated from the weights of the foam before and after immersion into the water. The closed cell content is calculated by the following formula: wherein V F is volume of the foam. The thermoplastic polyimide foam of this invention can be prepared by employing a foaming extruder for use in the processing of conventional thermoplastic resins. Exemplary foaming agent which can be used in this invention includes, for example, a sublimable substance having a boiling point exceeding 200°C such as phthalic anhydride, a chemical foaming agent which decomposes in the vicinity of 200°C such as trihydrazinotriazine, barium azodicarboxylate or dinitrosopentamethylenetetramine, a low boiling point organic compound such as dichlorotrifluoroethane, monochlorotetrafluoroethane, dichlorofluoromethane, monochlorodifluoromethane or acetone, or a hydrate such as oxalic acid dihydrate which dissociates water at a temperature above 200°C. The preferred foaming agent is a sublimable substance. The amount of the foaming agent used is selected on the basis of desired apparent density of the foam to be obtained. For example, the amount can be from 0.025 to 0.3 mole of the sublimable substance per 100 g of the thermoplastic polyimide, from 1 to 10 parts by weight of the chemical foaming agent per 100 parts by weight of the polyimide, from 0.05 to 0.5 mole of the low boiling point organic compound per 100 g of the polyimide, and in the case of the hydrate, from 0.025 to 0.3 mole of dissociated water per 100 g of the polyimide. The polyimide foam of this invention can be produced through usual foaming processes by adjusting the amount of the foaming agent respectively in the above range. For example, when the foaming agent is a solid, the agent and the thermoplastic polyimide are simultaneously fed from the hopper of an extruder. Cylinder temperature of the extruder is controlled in the range of 360 to 440°C, and the polyimide and the foaming agent are uniformly kneaded in the cylinder. The temperature of the kneaded mass is gradually decreased as it moves forward towards the point of extruder. The temperature of the die is controlled in the range of 250 to 370°C. The kneaded mass is delivered through the die into the atmosphere and expands into the foam. The foam extruded from the die slit can also form a micro cell layer on the surface by cooling with air or a sizing die to a temperature range of 150 to 250°C. When the foaming agent is liquid, the foaming agent is injected into the melted thermoplastic polyimide midway of the extruder and then the foam can be produced by the same procedures as mentioned above. Additionally, the size of cells can be controlled by adding an inorganic nucleating agent such as talc, silica gel or a surface-active agent. In the cases where the foaming agent is a sublimable substance, low boiling point organic compound or a hydrate, a foam having an average cell diameter of 0.1 to 1 mm can be preferably prepared by using the above nucleating agent in an amount from 0.1 to 3 parts by weight per 100 parts by weight of the polyimide. When the foaming agent is a chemical foaming agent, it is preferred to prepare the foam by using from 0.1 to 1 part by weight of the above nucleating agent per 100 parts by weight of the polyimide. Further, the closed cell content is suitably controlled by the temperature of the kneaded mass in the extruder and cooling conditions after delivery from the die slit. The closed cell content is maintained 50% or more. No particular limitation is imposed upon combined use of the above foaming agent. Other auxiliary agents such as antistatic agents and colorants may also be added when necessary. According to the present invention, a thermoplastic polyimide foam is provided which is excellent in the resistance to heat, flame and abrasion in addition to chemical resistance and thermal insulation and also outstanding in shaping properties and fabrication quality such as lamination, adhesion, coating and other surface treatments. The invention will now be illustrated by the following Examples and Comparative Examples. Example 1 Polyimide powder obtained by reacting 4,4'-bis(3-aminophenoxy)biphenyl with pyromellitic dianhydride and having an inherent viscosity of 0.45 dl/g was extruded at a cylinder temperature of 400°C and cut into pellets. The inherent viscosity of polyimide is measured at 35°C in a solution containing 0.5 g of the polyimide in 100 ml of a solvent mixture composed of p-chlorophenol/phenol in a ratio of 90/10 by weight. A mxiture containing 0.05 mole of phthalic anhdyride per 100 g of the polyimide pellets and 0.7 part by weight of talc per 100 parts by weight of the polyimide pellets was charged to the hopper of an extruder having a diameter of 30 mm. Cylinder temperature of the extruder was maintained from 380 to 410°C. The mixture was melt-kneaded in the cylinder and the temperature of the kneaded mass was gradually lowered during forward movement toward the point of the extruder. The temperature of a die fixed on the point was controlled to 355°C. A pressure gauge mounted on the inlet of the die indicated resin pressure of 110 kg/cm². The kneaded mass was delivered from the die into a sizing die jointly with pressure reduction and thus a foam was obtained. The foam had a thickness of 5 mm, width of 100 mm, apparent density of 0.09 g/cc, average cell diameter of 0.5 mm and a closed cell content of 78%. A specimen having dimensions of 25 mm x 120 mm was cut out of the foam obtained above. The sample was horizontally placed on a pair of supporting points having a span of 100 mm. A load of 100 g was applied on the sample at the centre of the span. Under this state, the specimen was allowed to stand in a thermostatic oven at 200°C for an hour. As a result, almost no deformation was observed and excellent thermal resistance of the foam was proved. When a foam specimen having dimensions of 25 mm x 120 mm was held horizontally and brought into contact with a flame of burner for period of 10 seconds, neither combustion nor dripping was found. On the other hand, taber abrasion test was carried out in accordance with JIS K-7204 by using a specimen die-cut into a disc of 100 mm in diameter. The test was conducted under load of 250 g by using a truck wheel CS 10. Abrasion loss of the specimen after 1000 continuous rotations was 95 mg. The foam was heated in an atmosphere of 380°C for a minute and shaped in a figure U type mould having a clearance of 4 mm between male and female moulds. A good shape without cracks on the surface could be obtained by setting the mould temperature at 180°C. Example 2 Polyimide powder obtained by reacting bis[4-(3-aminophenoxy)phenyl] sulphide with pyromellitic dianhydride and having an inherent viscosity of 0.46 dl/g was pelletized by the same procedures as carried out in Example 1. A mixture containing 0.03 mole of phthalic anhydride per 100 g of the polyimide pellets and 0.3 part by weight of talc per 100 parts by weight of the polyimide pellets was extruded by the same procedures as conducted in Example 1. The foam thus obtained had an apparent density of 0.15 g/cc, average cell diameter of 0.8 mm and a closed cell content of 75%. No deformation was found in the test carried out at 200°C and excellent thermal resistance was proved. No dripping was also observed when the foam specimen was brought into contact with the flame. Loss in Taber abrasion test was 135 mg, which result was excellent. When the foam was laminated with an epoxy base adhesive, sufficient strength of the laminated board could be obtained without destruction of base materials on the interface. Comparative Example 1 A foam was obtained by carrying out the same procedures as conducted in Example 1 except that 0.1 mole of phthalic anhydride was mixed. The foam obtained had an apparent density of 0.018 g/cc, average cell diameter of 0.5 mm and a closed cell content of 66%. Abrasion loss of the foam was a large value corresponding to 2500 mg after 1000 continuous rotations in Taber abrasion test. When the foam was laminated with an epoxy base adhesive, base materials are liable to destruct on the interface and hence sufficient strength of the laminated board could not be obtained. Comparative Example 2 A foam was extruded by carrying out the same procedures as conducted in Example 1 except that the temperature of the die fixed on the point of the extruder was set at 410°C. The foam thus obtained had an apparent density of 0.11 g/cc and a closed cell content of 45%. Thermoforming of the foam was carried out by the same procedures as conducted in Example 1. However, the foam was broken at the corner portion of the figure U. Example 3 A mixture containing 100 parts by weight of the polyimide used in Example 1 and 0.7 parts by weight of talc was charged to the hopper of an extruder of 30 mm in diameter. Cylinder temperature of the extruder was set from 380 to 410°C and the polyimide was melted. Acetone was injected in the midway of the extruder in a proportion of 0.2 mole per 100 g of the polyimide and mixed with the fused polyimide. Temperature of the resulting mixture was gradually decreased during forward movement toward the point of the extruder. The temperature of a die fixed on the point was set at 320°C. The mixture was delivered from the die into a sizing die jointly with pressure reduction. The foam obtained had a thickness of 5 mm, width of 100 mm, apparent density of 0.05 g/cc, average cell diameter of 0.8 mm and closed cell content of 62%. No deformation was found at 200°C in the thermal resistance test illustrated in Example 1. No dripping was observed in the combustion test. Abrasion loss was 537 mg in Taber abrasion test. Heat shaping was carried out by the same procedures as conducted in Example 1, and a shaped article obtained was good. Comparative Example 3 A foam was obtained by carrying out the same procedures as conducted in Example 1 except that the polyether imide was prepared from 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride and m-phenylenediamine, e.g. as shown in United States Patent Specification No. 4 639 343. The foam obtained had a thickness of 5 mm, width of 100 mm, apparent density of 0.09 g/cc, average cell diameter of 0.5 mm and closed cell content of 85%. A specimen of 25 mm x 120 mm in dimensions was cut out of the foam and the thermal resistance test was carried out as conducted in Example 1. The specimen sagged at the centre of the span. Example 4 A foam was obtained by carrying out the same procedures as conducted in Example 1 except that trihydrazinotriazine was added in place of phthalic anhydride in an amount of 1.8 parts by weight per 100 parts by weight of the polyimide. The foam obtained had an apparent density of 0.15 g/cc, average cell diameter of 0.3 mm and a closed cell content of 86%. No deformation was observed in the thermal resistance test illustrated in Example 1. No dripping was found in the combustion test. Abrasion loss was 57 mg in Taber abrasion test. When the foam was laminated with an epoxy base adhesive, the laminated board had a sufficient strength without destruction of the base materials on the interface. The foam was heated in an atmosphere of 380°C for a minute and shaped in a figure U type mould having a clearance of 4 mm between male and female moulds. A good shape without cracks on the surface could be obtained by setting the mould temperature at 180°C.

Claims: A thermoplastic polyimide foam having an apparent density (measured as defined herein) of from 0.02 to 0.2 g/cc, an average cell diameter (measured as defined herein) of from 0.1 to 1 mm and a closed cell content (measured as defined herein) of 50% or more of the cells, and consisting essentially of recurring units of the formula: in which X represents a bond, a divalent hydrocarbon radical having from 1 to 10 carbon atoms, a hexafluorinated isopropylidene radical, a carbonyl radical, a thio radical, or a sulphonyl radical, and R represents an aliphatic radical having two or more carbon atoms, a cycloaliphatic radical, a monoaromatic radical, a condensed polyaromatic radical, or a non-condensed polyaromatic radical, wherein the aromatic radicals are connected by a bond or a linking group. A polyimide foam as claimed in Claim 1 in which X represents a direct bond or a thio radical. A polyimide foam as claimed in Claim 1 or Claim 2 in which R represents a benzene group. A polyimide foam as claimed in any preceding claim in which the thermoplastic polyimide is obtained by reacting one or more etherdiamines selected from the group consisting of 4,4'-bis[4-(3-aminophenoxy)phenyl] sulphide, 4,4'-bis[4-(3-aminophenoxy)phenyl] sulphone, 4,4'-bis(3-aminophenoxy)benzophenone, 4,4'-bis(3-aminophenoxy)biphenyl, 2,2-bis[4-(3-aminophenoxy)phenyl]propane and 2,2-bis[4-(3-aminophenoxy)phenyl]-1,1,1-3,3,3-hexafluoropropane, with one or more of tetracarboxylic acid dianhydrides selected from the group consisting of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-diphenylethertetracarboxylic dianhydride and p-phenyleneoxydi(4-phthalic) dianhydride. A polyimide foam as claimed in any preceding claim in which the inherent viscosity of the thermoplastic polyimide measured in a solvent mixture of p-chlorophenol/phenol at a ratio of 90/10 by weight is from 0.35 to 0.65 dl/g at 35°C in a concentration of 0.5 g/100 ml solvent. A process for the production of a thermoplastic polyimide foam having an apparent density (measured as defined herein) of from 0.02 to 0.2 g/cc, an average cell diameter (measured as defined herein) of from 0.1 to 1 mm and a closed cell content (measured as defined herein) of 50% or more of the cells, comprising extrusion foaming of a thermoplastic polyimide consisting essentially of recurring units represented by the formula: in which X represents a bond, a divalent hydrocarbon radical having from 1 to 10 carbon atoms, a hexafluorinated isopropylidene radical, a carbonyl radical, a thio radical, or a sulphonyl radical, and R represents an aliphatic radical having two or more carbon atoms, a cycloaliphatic radical, a monoaromatic radical, a condensed polyaromatic radical, or a non-condensed polyaromatic radical, wherein the aromatic radicals are connected by a bond or a linking group in the presence of a foaming agent. A process as claimed in Claim 6 in which the foaming agent is a sublimable substance and is phthalic anhydride. A process as claimed in Claim 7 in which from 0.025 to 0.3 mole of the sublimable substance per 100 g of the thermoplastic polyimide is used. A process as claimed in Claim 6 in which a chemical foaming agent is used and is trihydrazinotriazine, barium azodicarboxylate or dinitropentamethylenetetramine. A process as claimed in Claim 6 in which a low boiling point organic compound is used as the foaming agent and is dichlorotrifluoroethane, monochlorotetrafluoroethane, dichlorofluoromethane, monochlorodifluoromethane or acetone. A process as claimed in Claim 10 in which from 0.05 to 0.5 mole of the low boiling point organic compound per 100g of the thermoplastic polyimide is used. A process as claimed in Claim 6 in which a hydrate dissociating water at the temperature of 200°C or more is used as the foaming agent and is oxalic acid dihydrate. A process as claimed in Claim 12 in which from 0.025 to 0.3 mole of the dissociated water per 100 g of the thermoplastic polyimide is used. A process as claimed in any of claims 6 to 13 in which the amount of foaming agent is from 1 to 10 parts by weight of the chemical foaming agent per 100 parts by weight of the thermoplastic polyimide. A process as claimed in any of claims 6 to 14 in which an inorganic nucleating agent is present in an amount of from 0.1 to 3 parts by weight per 100 parts by weight of the thermoplastic polyimide.

Inventors: EZAWA HIROSHI, NAKAKURA TOSHIYUKI, TSUSHIMA HIROAKI, WATANABE TAKAYUKI MITSUI TOATS, EZAWA, HIROSHI, NAKAKURA, TOSHIYUKI, TSUSHIMA, HIROAKI, WATANABE, TAKAYUKI 116, MITSUI TOATSU
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1989
Main classification: C08G73
IPC codes: C08G73