MXPA98010821A - Flexible polyester foam - Google Patents

Abstract

Un material celular espumado que se puede obtener a partir de resinas de poliéster aromáticas espumadas con una densidad de masa de 50 a 700 kg/m3 por calentamiento en vacío a temperaturas que son más altas que la Tv del material e inferiores a su punto de fusión. El material espumado, generalmente en la forma de una lámina o panel, tiene características de alto nivel en términos de flexibilidad y termoestabilidad dimesional dependiendo del grado de cristalinidad después del tratamiento bajo vacío.

Description

FLEXIBLE POLYESTER FOAMS DESCRIPTIVE MEMORY The present invention relates to foamed cellular materials (foams) derived from polyester resin, consisting of materials having high flexibility and satisfactory elastic recovery as well as thermosetting and flexible materials and to their method of preparation. Conventional foamed polyester materials have valuable mechanical properties but little flexibility. The rigidity of the material excludes them from applications where flexibility is an essential requirement. US-A-5 110 844 discloses foamed polyester materials having the characteristics of synthetic leather and are obtained by subjecting a polyester sheet partly in the form of foam to further foaming and then compressing it to a temperature below the point Tv of the material. EP-A-0 442 759 discloses thermosetting but rigid foamed polyester materials obtained from partially foamed polyester materials which are cooled at the outlet of the extruder at a temperature below the Tv of the material, so as to maintain the crystallinity at relatively low values, lower 15%, subsequently subjecting them to additional foaming in an aqueous environment at temperatures above the Tv of the material and then heating it to temperatures above 100 ° in a non-aqueous environment. The treatment with water causes the absorption of water, which then expands to a temperature above 100 °, thus producing additional foaming of the material. US-A-4 284 596 discloses a process for preparing polyester foams by starting from polyester resins with the addition of a polyepoxy, in which the resin, at the outlet of the extruder and while still in the molten state, is It passes through a chamber under reduced pressure and is then solidified. The reduced pressure (from 200 to 300 millibars) applied to the still melted resin allows obtaining low density foamed materials with cells having uniform shape and volume which are uniformly distributed within the mass of the foamed material. The resulting foam is not flexible. A method has now been unexpectedly discovered allows obtaining a broad scale of foamed cellular materials from aromatic polyester resins having high flexibility and elastic recovery characteristics or combining dimensional thermostability and flexibility. The method according to the invention comprises the following steps: a) extrusion / foaming of a foamable aromatic polyester resin to obtain a foamed material with a mass density of between 50 and 700 kg / nr; b) cooling of the foamed material at the exit of the extruder at temperatures and with cooling speed do not allow the material to reach a degree of crystallinity higher 15%; c) heating the material to a temperature above its Tv but below its melting point, if it is not already at such a temperature, with heating rates to prevent the material from reaching crystallinity values above 15%; d) vacuum treatment of the heated foamed material as in part c), maintaining it at a temperature which is higher its Tv but lower the melting point of the material for a sufficient time to determine a decrease in mass density of the material of at least 30% with respect to the density after the passage of part (a); e) returning the material to atmospheric pressure, preferably after cooling it to room temperature while still under vacuum. The material after step e) generally has a mass density of less 500 kg / m3, preferable less 100 kg / m3. The cooling of the material at the outlet of the extruder is preferably carried out with water at cooling speeds which maintain the crystallinity of the material between 5 and 12%. It is also possible to cool the extruded material, for example, in the form of a panel with a thickness of 10 mm or greater, bringing it to a temperature such that in the core of the panel the temperature corresponds to which the material is going to be subjected in the vacuum treatment (for example 180 ° C), and to directly introduce the material thus cooled inside the vacuum chamber. The temperature above the Tv to which the material is brought for vacuum treatment is, for example, between 80 ° and 180 ° C. Working at temperatures between approximately 80 ° and 130 ° C it is possible to obtain even considerable decreases in density without significantly increasing the crystallinity of the material. The highly flexible materials, which have good elastic recovery, are thus obtained. Working at higher temperatures, for example 170-180 ° C, a considerable decrease in mass density is still achieved along with a significant increase in crystallinity, which can reach 30-40% or more; At these values one obtains a material that is still flexible and has high characteristics of dimensional thermostability. The heating of the material to bring it to the temperature of the vacuum treatment can be carried out in an air oven or with pressurized steam or other means. The duration of the vacuum treatment is such as to decrease the density of the mass by at least 30% with reference to the density of the material after the passage of part b). The times are generally between 2 and 20 minutes, preferably 15 to 20 minutes. For example, a time of 15 minutes produces decreases in mass density of 70-80% or more starting from sheets of 2-4 mm thick, either operating at temperatures of 90-130 ° C or at higher temperatures (170-180). ° C). In the case of treatment at high temperatures (170-180 ° C), if the treatment is continued for more than 15-20 minutes, for example 60 minutes, the material collapses and the density of the mass increases considerably. Working at lower temperatures (80 ° C) and increasing the duration of the treatment (60 minutes) the density of the mass remains practically constant. The vacuum to which the material is subjected is, by way of indication, 20-40 mbar; harder voids and less extreme voids can also be used. The harder the vacuum is, the greater the effect on the decrease in density, other conditions remaining the same. Preferably, the material is cooled to room temperature while still under vacuum; this produces a greater decrease in density than with the material cooled to atmospheric pressure. The preparation of the foamed cellular material by extrusion-foaming processes of foamable polyester resins is carried out according to conventional methods, for example by extruding the polyester resin in the presence of a polyfunctional compound, such as a dianhydride of a tetracarboxylic acid. Pyromellitic dianhydride (PMDA) is a representative and preferred compound. Methods of this type are described in US-A-5 000 991 and US-A-5 288 764, the description of which is included by reference. As an alternative, and as a preferred method, the polyester resin is substituted in the solid state in the presence of a tetracarboxylic aromatic dianhydride (PMDA is the preferred compound) under conditions which make it possible to obtain a resin with an intrinsic viscosity of more than 0.8 dl / g, a foundry viscosity higher than 2500 PA's and a casting strength of more than 8 cN. The blowing agents which can be used are of a known type: they can be easily volatile liquid hydrocarbons, such as for example n-pentane, or inert gases, such as nitrogen and carbon dioxide, or chemical blowing compounds. Blowing agents are generally used in amounts between 1 and 10% by weight of the resin. The foamed material is generally extruded in the form of a sheet with a thickness of a few millimeters, by way of example 2-4 mm, or as a panel with a thickness of approximately 20-50 mm. By "foamable polyester resin" is meant herein a resin having the rheological characteristics described above which make it foamable or a resin which is capable of developing these characteristics during extrusion. The aromatic polyester resins to which the process of the invention is applied are obtained by polycondensation of a diol with 2-10 carbon atoms with an aromatic dicarboxylic acid, such as for example terephthalic acid or lower alkyl diesters thereof. Copolymers of polyethylene terephthalate and alkylene terephthalates in which up to 20 mol% of terephthalic acid units are replaced with isophthalic acid units and / or naphthalene dicarboxylic acids are preferred resins. The polyester resins, preferably polyethylene terephthalate and copolyethylene terephthalates, can be used in blends with other polymers such as polyamides, polycarbonates, polycarbonate and polyethylene glycol used in amounts preferably up to 40% by weight of the mixture. The polymer is extruded with the polyester resin in the presence of pyromellitic dianhydride or a similar anhydride in an amount between 0.1 and 2% by weight of the mixture and the resulting alloy is then improved in the solid state at temperatures between 160 ° and 220 ° C. An example of method mode is as follows. The foamed material, once it has left an annular extrusion head, is fitted on a calibration mandrel cooled by water and then cut. The resulting sheet is then pulled and rolled to form rolls of which the sheet is continuously drawn into a heating furnace, in order to bring the temperature of the material to the chosen value, and is then introduced into a vacuum chamber. from which it is passed to a water bath while it is still under vacuum and is then returned to atmospheric pressure. The characteristics of flexibility and dimensional thermostability of the material obtained with the method according to the invention depend on the degree of crystallinity and the mass density of the material. The material offers flexibility and good elastic recovery when its crystallinity is below 15-20% and is more rigid, but provided with good dimensional thermostability, when the degree of crystallinity is around 30-35%. Foamed cellular material that can be obtained with the method according to the present invention from foamed material with a mass density of 50 to 700 kg / m3 by heating under vacuum at temperatures above the Tv of the material and below of its melting point and subsequent cooling has the following characteristics when subjected to compression cycles of constant tension (constant stress). The characteristics, with reference to a sheet of polyethylene terephthalate or copolyethylene terephthalates with 1-20% isophthalic acid units, with a crystallinity of less than 15% and a density of less than 100 kg / m are: - maximum deformation by constant effort: between 10 and 60%; - residual deformation after constant effort (after 120 minutes): 10 to 30%; - Elastic recovery: between 40 and 80%. The characteristics of a sheet with a density between 200 and 300 kg / m3 and with a crystallinity of less than 15% are: - maximum deformation by constant effort: between 5 and 15%; - residual deformation after constant effort (after 120 minutes): 1 to 5%; - Elastic recovery: between 75 and 90%. The characteristics of the material with a crystallinity of more than 30%, particularly between 35 and 40%, are as follows, with reference to a sheet with a density of less than 100 kg / m: - maximum temperature of dimensional stability (stretched to <5% at 30 MPa): up to 150 ° C; - Maximum deformation of residual constant stress: 6-20% - residual deformation after constant effort for 120 minutes: 2-10%; - Elastic recovery: 50-80%. In the case of a polyethylene terephthalate material with 10% isophthalic acid, the maximum temperature of dimensional stability is 148 ° C. In the case of a material with a density of 200 to • or 300 kg / m the maximum temperature of dimensional stability can reach 165 ° C, while the other properties remain similar to the material having a density of less than 200 kg / m3. The measurements under constant tension were made with the following method. The samples tested were circular (discs with a diameter of approximately 20 mm). A dynamic-mechanical DMA Perkin-Elmer analyzer operating in elio (40 cc / min) was used in a configuration with parallel sample plates having a diameter of 10 mm. The samples were then subjected to series of constant force stresses (constant stress) with a load of 2600 mN, as explained hereafter. The sample was placed between the two plates and compressed with a virtually zero charge (1 mN).

The test started after approximately 5 minutes of stabilization and consisted of applying a load of 2600 mN for 5 minutes (constant effort). After this period, the load was removed instantaneously, allowing the sample to recover for 5 minutes. The procedure was repeated 12 times for 120 minutes on the same sample, such as to produce a constant effort recovery sequence. The footprint of the deformations suffered by the sample as a consequence of the individual recovery steps of constant effort was thus recorded. During the constant effort the sample suffered an elastic-plastic deformation that was (partially) recovered during the recovery step. The recovered part was considered as being an elastic deformation, while the unrecovered part remained as a permanent deformation (footprint). It was discovered that after almost 120 minutes of constant-recovery stress sequences the situation stabilized, producing constant values for elastic and permanent deformation. The degree of crystallinity of the material was determined by DSC from the enthalpy of melting of the material minus the enthalpy of crystallization of the material and was compared with the enthalpy of the perfectly crystalline material (117 kJ / mol in the case of PET); in the case of crystallized material, the crystallization enthalpy is equal to O J / g. The rheological measurements were conducted at temperatures between 260 and 300 ° C according to the type of polyester resin and the rheological characteristics thereof, using a Geottferd capillary rheometer (reference should be made to US-A-5 362 763 for a description more detailed of the method). For example, when the polyester resin was a polyethylene terephthalate homopolymer, the melt strength measurements were made at 280 ° C; they were instead made at 260 ° C when the resin was a copolyethylene terephthalate containing 10% isophthalic acid units. The melt viscosity was determined at 300 ° C for PET and at 280 ° C for the copolyester. The intrinsic viscosity was determined by means of 0.5 g resin solutions in 100 ml of a 60/40 weight mixture of phenol and tetrachloroethane at 25 ° C, working according to ASTM 4063-86. The density of mass was determined by the ratio between the weight and the volume of the foamed material. The following examples are given to illustrate but not to limit the invention.

EXAMPLE 1 PRODUCTION OF PET SHEET MADE FOAM 90 kg / h of polyethylene terephthalate homopolymer material having a casting strength of 100-150 cN, a casting viscosity of 1800 Pa's at 300 ° C and 10 rad / sec and an intrinsic viscosity of 1.25 dl / g, obtained for improving the polymer at 210 ° C in the presence of 0.4% by weight of pyromellitic dianhydride (COBITECH), they were continuously fed to a twin screw extruder with a screw diameter of 90 mm. A static mixer was placed behind the screws to improve the homogenization of the various components of the mixture. The temperatures set in the extruder were 280 ° C in the casting region, 280 ° C in the compression region, 270 ° C in the mixing region and 265 ° C in the extrusion head. The extruder screws rotated at 18 rpm. 1.8% by weight of n-pentane (blowing agent) were added to the PET in the region of the extruder located behind the melting of the polymer and mixed intensely with the polymer matrix. The PET / n-pentane composition, once mixed, was extruded through an annular head having a diameter of 90 mm and an extrusion aperture of 0.23 mm. A calibration mandrel with a diameter of 350 mm and a length of 750 mm, cooled with water at 20 ° C, was disposed on the extrusion head. The foamed material, once it left the extrusion head, was adjusted over the mandrel and cut. The resulting sheet was pulled and rolled to produce rolls. The resulting sheet had the following characteristics: - density 0.145 g / cm3 - weight 290 g / m2 - thickness 2 mm - average cell diameter 300 μm - degree of crystallization 8% EXAMPLE 2 PRODUCTION OF FLEXIBLE PET SHEET MADE FOAM The sheet produced as described in Example 1 was subjected to a treatment as described hereinafter. The sheet was continuously stretched in a heating oven which brought the sheet to a temperature of about 115 ° C in about 5 minutes after which the sheet was introduced into a vacuum calibration device, where the residual pressure was about 30 minutes. mbar The retention time of the sheet within the vacuum chamber was approximately 5 minutes: the sheet thus treated was then passed through a water bath maintained at 25 ° C and then returned to atmospheric pressure. The characteristics of the resulting sheet are as follows: - density 0.029 g / cm3 - weight 290 g / m2 - thickness 10 mm - degree of crystallization 10% The sheet produced according to this treatment is called "flexible sheet" and was subjected to copression measurement cycles in order to evaluate its compression resistance and its elastic recovery. All tests were performed in parallel with the sheet produced during the first step, which is called "base sheet". Table 1 lists the values discovered during these characterizations.

TABLE 1 FLEXIBLE SHEET BASE SHEET Maximum constant stress strain (%) 6.4 39.6 Residual deformity after constant stress (after 120 minutes) (%) 4.1 22.4 Permanent deformation (%) 64.1 56.6 Elastic recovery (%) 35.9 43.4 These measurements were made by means of a thermomechanical analyzer, submitting the samples to 12 consecutive compression and decompression cycles.

EXAMPLE 3 PRODUCTION OF FLEXIBLE SEAMLESS THERMOPHABLE FOAM PET SHEET The sheet produced in Example 1 was subjected to a treatment as described hereinafter. The sheet was continuously pulled in a heating oven, which brought the sheet to a temperature of about 125 ° in about 5 minutes; After this, the sheet was introduced into a calibration device under vacuum, in which the residual pressure was about 30 mbar. The retention time of the sheet within the vacuum chamber was approximately 8 minutes; the sheet was maintained at a temperature of 180 ° C. Before leaving the chamber under vacuum, the sheet thus treated was passed through a water bath maintained at 25 ° C and then returned to atmospheric pressure. The characteristics of the resulting sheet are as follows: density 0.033 g / cm3 weight 290 g / m2 thickness 8.8 mm degree of crystallization 35% The sheet produced in accordance with this treatment, which was called "thermostable flexible sheet", was subjected to compression measurement cycles to evaluate both the compression strength and the elastic recovery as well as the temperature dependent deformation. All tests were conducted in parallel with the sheet produced during the first step, which was called "base sheet". Table 2 lists the values discovered during these characterizations.

TABLE 2 THERMOSTABLE FLEXIBLE THREAD BASE SHEET Maximum temperature < 90 ° C < 150 ° C dimensional stability (tension <5%) at 30000 Pa Maximum deformation of 6.4 11.6 constant effort (%) Residual deformation after constant effort (after 120 min) (%) 4.1 3.9 Permanent deformation (%) 64.1 33.6 Elastic recovery (%) 35.9 66.4 These measurements were made by means of a thermomechanical analyzer.

EXAMPLE 4 PRODUCTION OF A THERMOSTABLE FLEXIBLE FOAMED PET SHEET: WATER AT 125 ° C The sheet produced as described in Example 1 was subjected to a treatment as described hereinafter. The sheet was continuously pulled and heated by means of water at 125 ° C for 5 minutes, after which the sheet was introduced into a calibrator under vacuum, in which the residual pressure was about 30 mbar. The retention time of the sheet within the chamber under vacuum was approximately 8 minutes. The sheet was maintained at a temperature of 180 ° C before leaving the chamber under vacuum and was then passed through a water bath maintained at 25 ° C and then returned to atmospheric pressure. The characteristics of the resulting sheet were: density 0.038 g / cm3 weight 290 g / m2 thickness 7.6 mm degree of crystallization 38% The sheet produced in accordance with this treatment, called "thermostable flexible sheet", was subjected to compression measurement cycles in order to evaluate both the compression strength and the elastic recovery as well as the temperature dependent deformation. All the tests were conducted in parallel with the sheet produced during the first step, which was called "base sheet". Table 3 lists the values observed during these characterizations.

TABLE 3 THERMOSTABLE FLEXIBLE THREAD BASE SHEET Maximum temperature < 90 ° C < 160 ° C dimensional stability (tension <5%) at 30000 Pa Maximum deformation of 6.4 10 constant effort (%) Residual deformation after constant stress (after 120 min) (%) 4.1 3.7 Permanent deformation (%) 64.1 37 Elastic recovery (%) 35.9 63 These measurements were carried out with a thermomechanical analyzer.

EXAMPLE 5 PRODUCTION OF A FOAMED PET PANEL 90 Kg / h of copolyethylene terephthalate material containing 10% by weight of isophthalic acid with a casting strength of 100-150 cN, intrinsic viscosity of 1.25 dl / g and melt viscosity of 1800 Pa "s at 280 ° C (obtained by improving the polymer at 280 ° C in the presence of 0.4% by weight of pyromellitic dianhydride (COBITECH ™)) were fed continuously into a twin screw extruder with a screw diameter of 90 mm.A static mixer was disposed downstream of the screws In order to improve the homogenization of the various components of the mixture, the temperatures set in the extruder were 260 ° C in the casting region, 250 ° C in the compression region, 240 ° C in the mixing region and 225 ° C in the extrusion region The screws of the extruder rotated at 18 rpm 2.4% by weight of the blowing agent 134a (1,1,1,2 tetrafluoroethane) were added to the PET in the extruder region located behind the casting of the polymer and mixed intensely with the polymer matrix. The PET / 134a composition, once mixed, was extruded through a flat head. The resulting panel had the following characteristics: density 0.115 g / cm ~ thickness 22 mm average diameter of the cell 280 μm degree of crystallization 8% EXAMPLE 6 PRODUCTION OF A FLEXIBLE FOAMED PET PANEL The panel produced as described in Example 5 was subjected to a treatment carried out a few seconds after extrusion as described hereinafter. The extruded panel was cooled in the calibration region, and once a temperature of 180 ° C was reached in the core of the panel, said panel was inserted into a calibrator under vacuum, where the residual pressure was about 30 mbar. The residence time of the panel inside the chamber under vacuum was approximately 5 minutes. The panel was maintained at a temperature of approximately 120 ° C before leaving the chamber under vacuum and then passed through a water bath maintained at 25 ° C and then returned to atmospheric pressure. The characteristics of the resulting panel were: density 0.030 g / cm3 thickness 55 mm - degree of crystallization 10% The resulting panel (called "flexible panel") was subjected to compression measurement cycles in order to evaluate compression strength and elastic recovery. All tests were conducted in parallel on the panel produced during the first step (base panel). Table 4 lists the measured values: TABLE 4 FLEXIBLE PANEL BASE PANEL Maximum deformation 2.4 24 constant effort (%) Residual deformity 1.6 5.7 after constant effort (after 120 min) (%) Permanent deformation (%) 66 23.7 Elastic recovery (%) 34 76.3 EXAMPLE 7 PRODUCTION OF A THERMOSTABLE FLEXIBLE FOAMED PET PANEL The panel produced as described in Example 5 was subjected to a treatment carried out a few seconds after extrusion, as described hereinafter. The extruded panel was cooled in the calibration region and once a temperature of 180 ° C was reached in the core of the panel it was introduced into a vacuum calibrator device, where the residual pressure was about 30 mbar. The residence time of the panel inside the chamber under vacuum was approximately 10 minutes. The panel was maintained at a temperature of 180 ° C and before leaving the chamber under vacuum was passed through a water bath maintained at 25 ° C and then returned to atmospheric pressure. The characteristics of the resulting panel were as follows: density 0.038 g / cpp thickness 52 mm degree of crystallization 36% The panel produced in accordance with this treatment (referred to as "thermostable flexible panel") was subjected to compression measurement cycles to evaluate both compression strength and elastic recovery as well as temperature dependent deformation. All tests were conducted in parallel on the panel produced during the first step (base panel). Table 5 lists the measured values.

TABLE 5 BASE PANEL FLEXIBLE THERMOSTABLE PANEL Maximum temperature < 80 ° C < 148 ° C dimensional stability (tension <5%) and at 30000 Pa Maximum deformation of 2.4 16 constant effort (%) Residual deformation 1.6 5.1 after constant effort (after 120 min) (%) Permanent deformation (%) 66 31.9 Elastic recovery (%) 34 68.1 EXAMPLE 8 PRODUCTION OF A FOAMED PET SHEET 90 kg / h of polyethylene terephthalate homopolymer (COBITECH ™) used in example 1, were continuously fed to a twin screw extruder with a screw diameter of 90 mm. A static mixer was placed downstream of the screws in order to improve the homogenization of the various components of the mixture. The temperatures set in the extruder were 280 ° C in the casting region, 280 ° C in the compression region, 270 ° C in the mixing region and 265 ° C in the extrusion head. The screws on the extruder rotated at 15 rpm. 2.5% by weight of nitrogen (blowing agent) were added to the PET in the region of the extruder located after the melting of the polymer and they were mixed intensely with the polymeric matrix. The PET / N2 composition, once mixed, was extruded through an annular head having a diameter of 120 mm and an extrusion aperture of 0.14 mm. A caliper mandrel with a diameter of 350 mm and a length of 750 mm, cooled with water at 20 ° C, was placed on the extrusion head.

The foamed material, after leaving the extrusion head, was fixed on the mandrel and cut. The resulting sheet was pulled and rolled to produce rolls. The resulting sheet had the following characteristics: - density 0.400 g / cpr - weight 500 g / m2 - thickness 1.25 mm average cell diameter 130 μm - crystallization degree 10% EXAMPLE 9 PRODUCTION OF A FLEXIBLE FOAMED PET SHEET The sheet produced as described in Example 8 was subjected to a treatment as described hereinafter. The sheet was continuously pulled in a heating furnace which brought the sheet to a temperature of about 115 ° C in about 3 minutes, after which the sheet was placed in a calibrating device under vacuum, in which the residual pressure was approximately 30 m bar. The residence time of the sheet was approximately 5 minutes and the temperature was maintained at 115 ° C. Before leaving the chamber under vacuum, the sheet thus treated was passed through a water bath maintained at 25 ° C and returned to atmospheric pressure. The characteristics of the resulting sheet were as follows: density 0.260 g / cm3 - weight 500 g / m2 - thickness 1.95 mm - degree of crystallization 11% The sheet produced in accordance with this treatment (which was called "flexible sheet N2" ) was subjected to compression measurement cycles in order to evaluate both compression strength and elastic recovery. All tests were conducted in parallel on the sheet produced during the first stage (base sheet of N2). Table 6 lists the values discovered during these characterizations.

TABLE 6 N2 BASE SHEET FLEXIBLE SHEET N2 Deformation 2.9 8.5 maximum shear stress (%) Residual deformation 0.8 1.2 after constant stress (after 120 minutes) (%) Deformation 27.6 14.1 permanent (%) Recovery 72.4 85.9 elastic (%) Measurements were performed by means of a thermomechanical analyzer, subjecting the samples to 12 consecutive compression and decompression cycles.

EXAMPLE 10 PRODUCTION OF A THERMOSTABLE FLEXIBLE FOAMED PET FILM The sheet produced in Example 8 was subjected to a treatment as described hereinafter. The sheet was continuously pulled into a heating furnace which brought the sheet to a temperature of 115 ° C in about 3 minutes, after which the sheet was introduced into a vacuum gauging device, where the residual pressure was about 30 mbar . The residence time of the sheet within the chamber under vacuum was approximately 5 minutes; the sheet was maintained at a temperature of 180 ° C. Before leaving the chamber under vacuum, the sheet was passed through a water bath maintained at 25 ° C and then returned to atmospheric pressure. The characteristics of the resulting sheet were: density 0.243 g / cm3 - weight 500 g / m - thickness 2.05 mm - degree of crystallization 37% The sheet produced in accordance with this treatment (which was called "thermo stable flexible sheet of N2") was subjected to compression measurement cycles in order to evaluate the compressive strength and the elastic recovery as well as for the temperature dependent deformation. All tests were conducted in parallel on the sheet produced during the first step (base sheet). Table 7 lists the values discovered during these characterizations.

TABLE 7 BASE SHEET N2 THERMOSTABLE FLEXIBLE SHEET N2 Maximum temperature < 90 ° C < 165 ° C of differential stability (tension <5%) at 30,000 Pa Maximum deformation 2.9 7.4 constant effort (%) Deformation resi0.8 1.7 dual after constant effort (after 120 min) (%) Deformation 27.8 24 permanent (%) Recovery 72.4 76 elastic (%) These measurements were taken with a thermomechanical analyzer.

EXAMPLE OF COMPARISON 1 A sheet produced as described in Example 1 of US-A-5 110 844 was subjected to thermomechanical characterization and compared to the sheet of Example 4. The results of these characterizations are listed in Table 8.

TABLE 8 FLEXIBLE LAMINA BASE SHEET THERMOSTABLE THREAD AGREEMENT WITH EXAMPLE 1 US-A-5 110 884 Temperature < 90 ° C < 160 ° C < 90 ° C maximum dimensional stability (tension <5% to 30000 Pa) Deformation 6.4 10 6.1 maximum constant effort (%) Deformation 4.1 3.7 residual after constant effort (after 120 minutes) Deformation 64.1 37 65.6 permanent (%) Recovery 35.9 63 34.4 elastic (%) The measurements were taken with a temomechanical analyzer.

EXAMPLE OF COMPARISON 2 A sheet produced as described in Example 1 of US-A-4 284 596 was subjected to thermomechanical characterization and compared to the sheet of Example 4. The results of these characterizations are listed in Table 9.

TABLE 9 FLEXIBLE SHEET BASE SHEET THERMOSTABLE THREAD AGREEMENT SHEET TO EXAMPLE 1 US-A- 4 284 596 Temperature < 90 ° C < 160 ° C < 90 ° C maximum dimentional stability (tension <5%) at 30000 Pa Deformation 6.4 10 2.2 maximum constant effort (%) Deformation 4.1 3.7 residual after constant effort (after 120 minutes) (%) Deformation 64.1 37 91 permanent Recovery 35.9 63 elastic (%) The measurements were taken with a thermomechanical analyzer.

Claims (24)

NOVELTY OF THE INVENTION CLAIMS

1. - A foamed cellular material derived from polyester aromatic resins, obtainable from foamed cellular material of aromatic polyester having a mass density of 50 to 700 kg / m by heating under vacuum at temperatures higher than the Tv of the material and lower than the melting point thereof, for a sufficient time to achieve a decrease in mass density of at least 30%.

2. - A foamed cellular material according to claim 1, having a degree of crystallinity of less than 15%.

3. - A foamed cellular material according to claim 1, having a degree of crystallinity of more than 30%.

4. - A foamed cellular material according to claim 2, having a degree of crystallinity between 5 and 12%.

5. A foamed cellular material according to claim 3, having a degree of crystallinity between 30 and 40%.

6. - A foamed cellular material according to any of the preceding claims, having a mass density of less than 100 kg / m3.

7. - A foamed cellular material according to claim 6, having a degree of crystallinity of less than 15%.

8. - A foamed cellular material according to claim 6, having a degree of crystallinity of more than 30%.

9. - A foamed cellular material according to any of claims 1 to 5, having a mass density of 100 to 500 kg / m.

10. A foamed cellular material according to any of the preceding claims, obtained from polyester resins selected from polyethylene terephthalate and copolyethylene terephthalates containing up to 20% units derived from isophthalic acid.

11. A foamed cellular material according to any of claims 1 to 9, obtained from polyester resins in the form of an alloy with polymers chosen from polyamides, polycarbonates and polycaprolactone, obtained by extruding the resin and the polymer in the presence of pyromellitic dianhydride and then improving the alloy at temperatures between 160 ° C and 220 ° C.

12. - A foamed cellular material according to claim 11, further characterized in that the polyester resin is polyethylene terephthalate or copolyethylene terephthalate containing up to 20% units derived from isophthalic acid in the form of an alloy with a chosen polymer between polyamides, polycarbonates and polycaprolactone used in an amount of up to 40% by weight of the total.

13. - A foamed cellular material derived from aromatic polyester resins having a mass density of less than 100 kg / m3, a degree of crystallinity of less than 15% and the following tensile properties: - maximum deformation by constant effort between 10 and 60%; Residual deformation after constant effort for 120 minutes between 10 and 30%; - Elastic recovery between 40 and 90%.

14. A foamed cellular material derived from aromatic polyester resins having a mass density of 200 to 300 kg / m, a degree of crystallinity of less than 15% and the following tensile properties: - maximum strain of constant effort between 5 and 15%; - residual deformation after constant effort for 120 minutes between 1 and 5%; - Elastic recovery between 75 and 90%.

15. - A foamed cellular material of aromatic polyester resins having a mass density of less than 100 kg / m, a degree of crystallinity of more than 30% and the following properties of dimensional thermostability and tensile: - maximum temperature of permanence Shape: up to 150 ° C; - maximum deformation of constant effort: between 6 and 20%; - residual deformation after constant effort for 120 minutes: between 2 and 10%; - Elastic recovery: between 50 and 80%.

16. A foamed cellular material according to claim 13, which has a degree of crystallinity of 35 to 40% and a maximum temperature of dimensional stability of up to 165 ° C.

17. - A foamed cellular material derived from aromatic polyester resins having a mass density of 100 to 300 kg / m, a degree of crystallinity of 35 to 40% and a maximum temperature of dimensional stability of up to 165 ° C .

18. - A foamed cellular material according to any of claims 11 to 15, obtained from polyester resins selected from polyethylene terephthalate and copolyethylene terephthalate containing up to 20% isophthalic acid units.

19. - A foamed cellular material according to any of claims 1 to 16, in the form of a sheet with a thickness of 1 to 3 mm or a panel with a thickness of 10 to 50 mm.

20. A foamed cellular material according to any of claims 1 to 17, obtained from aromatic polyester resins having an intrinsic viscosity of more than 0.8 dl / g, a melt viscosity of more than 2500 Pa "s and a casting strength of more than 8 cN

21. A method for preparing foamed cellular materials according to any of the claims 1 to 18, which consists of the following steps: a) extrusion-foaming of a foamable aromatic polyester resin; b) cooling the foamed material at the outlet of the extruder at a temperature and with cooling speeds such that the material does not reach a degree of crystallinity of more than 15%; c) heating of the material, if it is not already at such temperature at the exit of the extruder after the passage of subsection b), at temperatures higher than the Tv of the material but lower than its melting point, with heating speeds such that the crystallinity of the material remains below 15%; d) vacuum treatment of the material thus heated, keeping it under vacuum at a temperature which is higher than the Tv of the material but lower than the melting point for a time which is sufficient to achieve a decrease in the mass density of the material. material of at least 30% with respect to the density of the material after the passage of part b); e) Return of the material to atmospheric pressure.

22. A method according to claim 21, further characterized in that the material, after the vacuum treatment, is cooled to room temperature and kept under vacuum.

23. - A method according to any of claims 21 and 22, further characterized in that the vacuum is between 10 and 50 mbar and the temperature of the vacuum heating is 90 to 180 ° C.

24. - A method according to any of claims 21 to 23, further characterized in that the material is obtained from a polyester resin chosen between polyethylene terephthalate and copolyethylene terephthalate containing up to 20% units derived from isophthalic acid.