Flexible polyester foams

A foamed cellular material which can be obtained from foamed aromatic polyester resins with a bulk density of 50 to 700 kg/m3 by heating in vacuum to temperatures which are higher than the Tg of the material and lower than its melting point. The foamed material, generally in the form of a sheet or panel, has high-level characteristics in terms of flexibility and dimensional thermostability depending on the degree of crystallinity after the vacuum treatment.

DESCRIPTION 
The present invention relates to foamed cellular materials (foams) derived 
from polyester resin, comprising materials having high flexibility and 
satisfactory elastic recovery as well as thermostable and flexible 
materials, and to their preparation method. 
Conventional foamed polyester materials have valuable mechanical properties 
but poor flexibility. 
The rigidity of the material excludes them from applications where 
flexibility is an essential requirement. 
U.S. Pat. No. 5,110,844 describes foamed polyester materials which have the 
characteristics of synthetic leather and are obtained by subjecting a 
partially foamed polyester sheet to further foaming and then compressing 
it at a temperature below the Tg of the material. 
EP-A-0 442 759 describes thermostable but rigid foamed polyester materials 
obtained from a partially foamed polyester material which is cooled at the 
outlet of the extruder to a temperature below the Tg of the material, so 
as to maintain crystallinity at relatively low values, lower than 15%, 
subsequently subjecting it to further foaming in an aqueous environment at 
temperatures above the Tg of the material and then heating it to 
temperatures above 100.degree. C. in a non-aqueous environment. 
The treatment with water causes the absorption of water, which then expands 
at a temperature above 100.degree. C., thus producing the further foaming 
of the material. 
U.S. Pat. No. 4,284,596 describes a process for preparing polyester foams 
starting from polyester resins with the addition of a polyepoxy, in which 
the resin, at the outlet of the extruder and while it is still in the 
molten state, is made to pass through a chamber at reduced pressure and is 
then solidified. 
The reduced pressure (200-300 millibar) applied to the still-molten resin 
allows to obtain 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 found which allows to obtain a wide 
range of foamed cellular materials from aromatic polyester resins having 
high flexibility and elastic recovery characteristics or which combine 
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 bulk density between 50 and 700 kg/m.sup.3 ; 
b) cooling of the foamed material at the outlet of the extruder to 
temperatures and with cooling rates which do not allow the material to 
reach a degree of crystallinity higher than 15%; 
c) heating of the material to a temperature above its Tg but below its 
melting point, if it is not already at such a temperature, with heating 
rates such as to prevent the material from reaching crystallinity values 
above 15%; 
d) vacuum treatment of the foamed material heated as in c), keeping it at a 
temperature which is higher than its Tg but lower than the melting point 
of the material for a time sufficient to determine a decrease in the bulk 
density of the material of at least 30% with respect to the density after 
step a); 
e) return of the material to atmospheric pressure, preferably after cooling 
it to ambient temperature whilst it is still under vacuum. 
The material after step e) generally has a bulk density of less than 500 
kg/m.sup.3, preferably less than 100 kg/m.sup.3. 
The cooling of the material at the outlet of the extruder is preferably 
performed with water at cooling rates 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 more, bringing it to a temperature 
such that in the core of the panel the temperature corresponds to the one 
at which the material is to be subjected to the vacuum treatment (for 
example 180.degree. C.), and to directly introduce the thus cooled 
material into the vacuum chamber. 
The temperature above Tg to which the material is brought for the vacuum 
treatment is comprised for example between 80.degree. and 180.degree. C. 
By working at temperatures between approximately 80.degree. and 
130.degree. C. it is possible to obtain even considerable decreases in 
density without significantly increasing the crystallinity of the 
material. Highly flexible materials, having good elastic recovery, are 
thus obtained. 
By working at higher temperatures, for example 170-180.degree. C., a 
considerable decrease in bulk density is still achieved together with a 
significant increase in crystallinity, which can reach 30-40% or more; at 
these values one obtains a material which is still flexible and has high 
dimensional thermostability characteristics. 
The heating of the material to bring it to the temperature of the vacuum 
treatment can be performed in an air oven or with pressurized water vapor 
or with other means. 
The duration of the vacuum treatment is such as to decrease the bulk 
density by at least 30% with reference to the density of the material 
after step 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 bulk 
density of 70-80% or more starting from sheets 2-4 mm thick, either 
operating at temperatures of 90-130.degree. C. or at higher temperatures 
(170-180.degree. C.). 
In the case of treatment at high temperatures (170-180.degree. C.), if the 
treatment is continued for more than 15-20 minutes, for example 60 
minutes, the material collapses and bulk density increases considerably. 
By working at lower temperatures (80.degree. C.) and increasing the 
duration of the treatment (60 minutes) the bulk density remains 
practically constant. 
The vacuum to which the material is subjected is, by way of indication, 
20-40 mbar; harder vacuums and less extreme vacuums can also be used. 
The harder the vacuum, the greater the effect on the decrease in density, 
other conditions being equal. 
Preferably, the material is cooled to ambient temperature while it is still 
under vacuum; this produces a greater decrease in density than with 
material cooled at atmospheric pressure. 
The preparation of the foamed cellular material by means of 
extrusion-foaming processes of foamable polyester resins is performed 
according to conventional methods, for example by extruding the polyester 
resin in the presence of a polyfunctional compound, such as for example a 
dianhydride of a tetracarboxylic acid. 
Pyromellitic dianhydride (PMDA) is a representative and preferred compound. 
Methods of this type are described in U.S. Pat. No. 5,000,991 and U.S. Pat. 
No. 5,288,764, the description of which is included by reference. 
As an alternative, and as a preferred method, the polyester resin is 
upgraded in the solid state in the presence of a dianhydride of a 
tetracarboxylic aromatic acid (PMDA is the preferred compound) under 
conditions allowing to obtain a resin with an intrinsic viscosity of more 
than 0.8 dl/g, melt viscosity higher than 2500 PA.s and melt strength of 
more than 8 cN. 
The blowing agents that 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. 
The blowing agents are generally used in amounts between 1 and 10% by 
weight on 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 about 20-50 mm. 
For "foamable polyester resin" it is herein intended a resin which has the 
above described rheological characteristics 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 a dicarboxylic aromatic acid, such as for example terephthalic acid 
or lower alkyl diesters thereof. 
Polyethylene terephthalate and alkylene terephthalates copolymers in which 
up to 20% in moles of terephthalic acid units is replaced with units of 
isophthalic acid and/or naphthalene dicarboxylic acids are preferred 
resins. 
The polyester resins, preferably polyethylene terephthalate and 
copolyethylene terephthalates, can be used in mixtures with other polymers 
such as polyamides, polycarbonates, polycarbonate and polyethylene glycol 
used in amounts preferably up to about 40% by weight on the blend. 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 on the blend and the resulting alloy is then upgraded in 
the solid state at temperatures between 160.degree. C. and 220.degree. C. 
An example of embodiment of the method is as follows. 
The foamed material, once it has left an annular extrusion head, is fitted 
on a water-cooled sizing mandrel and then cut. 
The resulting sheet is then pulled and rolled so as to form rolls from 
which the sheet is drawn continuously into a heating oven, in order to 
bring the temperature of the material to the chosen value, and is then 
introduced in a vacuum chamber from which it passes into a water bath 
whilst 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 on the bulk 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%. 
The foamed cellular material that can be obtained with the method according 
to the present invention from foamed material with a bulk density of 50 to 
700 kg/m.sup.3 by heating under vacuum to temperatures above the Tg of the 
material and below its melting point and by subsequent cooling has the 
following characteristics when subjected to constant-stress compression 
cycles (creep). 
The characteristics, referred 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.sub.3, 
are: 
maximum creep deformation: between 10 and 60%; 
residual deformation after creep (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/m.sup.3 and with a crystallinity of less than 15% are: 
maximum creep deformation: between 5 and 15%; 
residual deformation after creep (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.sup.3 : 
maximum dimensional stability temperature (stressed at &lt;5% at 30 MPa): up 
to 150.degree. C.; 
maximum residual creep deformation: 6-20%; 
residual deformation after creep for 120 minutes: 2-10%; 
elastic recovery: 50-80%. 
In the case of a polyethylene terephthalate material with 10% isophthalic 
acid, the maximum dimensional stability temperature is 148.degree. C. 
In the case of a material with a density of 200 to 300 kg/m.sup.3, the 
maximum dimensional stability temperature can reach 165.degree. C., whilst 
the other properties remain similar to the material having a density of 
less than 200 kg/m.sup.3. 
The measurements under constant stress were performed with the following 
method. 
The tested samples were circular (disks with a diameter of approximately 20 
mm). 
A Perkin-Elmer dynamic-mechanical analyzer DMA 7 operating in helium (40 
cc/min) was used in a configuration with parallel sample plates having a 
diameter of 10 mm. 
The samples were then subjected to a series of constant-force stresses 
(creep) with a load of 2600 mN, as explained hereafter. 
The sample was placed between the two plates and compressed with a 
practically nil load (1 mN). 
The test began after approximately 5 min stabilization and consisted in 
applying a load of 2600 mN for 5 min (creep). 
After this period, the load was removed instantaneously, allowing the 
sample to recover for 5 min. 
This procedure was repeated 12 times for 120 minutes on the same sample, so 
as to produce a creep-recovery sequence. 
The trace of the deformations undergone by the sample as a consequence of 
the individual creep-recovery steps was thus recorded. 
During creep the sample underwent an elastic-plastic deformation which was 
(partially) recovered during the recovery step. The recovered part was 
considered to be an elastic deformation, whilst the unrecovered part 
remained as a permanent deformation (footprint). 
It was found that after about 120 min of creep-recovery sequence the 
situation stabilized, producing constant values for elastic and permanent 
deformation. 
The degree of crystallinity of the material was determined by DSC from the 
melting enthalpy of the material minus the crystallization enthalpy of the 
material and was compared with the enthalpy of the perfectly crystalline 
material (117 kJ/mole in the case of PET); in the case of crystallized 
material, crystallization enthalpy is equal to 0 J/g. 
Rheological measurements were conducted at temperatures between 260 and 
300.degree. C. according to the type of polyester resin and to the 
rheological characteristics thereof, using a Geottferd capillary rheometer 
(reference should be made to U.S. Pat. No. 5,362,763 for a more detailed 
description of the method). 
For example, when the polyester resin was a polyethylene terephthalate 
homopolymer, melt strength measurements were performed at 280.degree. C.; 
they were instead performed at 260.degree. C. when the resin was a 
copolyethylene terephthalate containing 10% isophthalic acid units. 
Melt viscosity was determined at 300.degree. C. for PET and at 280.degree. 
C. for the copolyester. 
Intrinsic viscosity was determined by means of solutions of 0.5 g of resin 
in 100 ml of a 60/40 mixture by weight of phenol and tetrachloroethane at 
25.degree. C., working according to ASTM 4063-86. 
Bulk density 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 limitate the 
invention.

EXAMPLE 1 (PRODUCTION OF FOAMED PET SHEET) 
90 kg/h of polyethylene terephthalate homopolymer material having a melt 
strength of 100-150 cN, melt viscosity of 1800 Pa.s at 300.degree. C. and 
10 rad/sec and intrinsic viscosity of 1.25 dl/g, obtained by upgrading the 
polymer at 210.degree. C. in the presence of 0.4% by weight of 
pyromellitic dianhydride (COBITECH.TM.), were fed continuously to a two- 
screw extruder with a screw diameter of 90 mm. 
A static mixer was placed after the screws to improve homogenization of the 
various components of the blend. 
The temperatures set on the extruder were 280.degree. C. in the melting 
region, 280.degree. C. in the compression region, 270.degree. C.in the 
mixing region and 265.degree. C. at the extrusion head. 
The screws of the extruder rotated at 18 rpm. 
1.8% by weight of n-pentane (blowing agent) was added to the PET in the 
region of the extruder located after the melting of the polymer and 
thoroughly mixed with the polymeric matrix. 
The PET/n-pentane composition, once mixed, was extruded through an annular 
head having a diameter of 90 mm and an extrusion opening of 0.23 mm. A 
sizing mandrel with a diameter of 350 mm and a length of 750 mm, cooled 
with water at 20.degree. C., was arranged on the extrusion head. 
The foamed material, once it had left the extrusion head, was fitted on 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/cm.sup.3 
weight 290 g/m.sup.2 
thickness 2 mm 
average cell diameter 300 .mu.m 
degree of crystallization 8% 
______________________________________ 
EXAMPLE 2 (PRODUCTION OF FLEXIBLE FOAMED PET SHEET) 
The sheet produced as described in example 1 was subjected to a treatment 
as described hereafter. 
The sheet was drawn continuously in a heating oven which brought the sheet 
to a temperature of approximately 115.degree. C. in approximately 5 
minutes after which the sheet was introduced in a vacuum sizing device, 
where the residual pressure was approximately 30 mbar. 
The retention time of the sheet inside the vacuum chamber was approximately 
5 minutes: the thus treated sheet was then passed through a water bath 
kept at 25.degree. C. and then returned to atmospheric pressure. 
The characteristics of the resulting sheet were as follows: 
______________________________________ 
density 0.029 g/cm.sup.3 
weight 290 g/m.sup.2 
thickness 10 mm 
degree of crystallization 10% 
______________________________________ 
The sheet produced according to this treatment is termed "flexible sheet" 
and was subjected to compression 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 termed "base sheet". 
Table 1 lists the values found during these characterizations. 
TABLE 1 
______________________________________ 
BASE SHEET 
FLEXIBLE SHEET 
______________________________________ 
Maximum creep 6.4 39.6 
deformation (%) 
Residual 4.1 22.4 
deformation after creep 
(after 120 minutes) (%) 
permanent 64.1 56.6 
deformation (%) 
elastic 35.9 43.4 
recovery (%) 
______________________________________ 
These measurements were performed by means of a thermomechanical analyzer 
by subjecting the samples to 12 consecutive compression and decompression 
cycles. 
EXAMPLE 3 (PRODUCTION OF THERMOSTABLE FLEXIBLE FOAMED PET SHEET) 
The sheet produced in example 1 was subjected to a treatment as described 
hereafter. 
The sheet was pulled continuously in a heating oven, which brought the 
sheet to a temperature of approximately 125.degree. C. in approximately 5 
minutes; after this, the sheet was introduced in a sizing device under 
vacuum, in which the residual pressure was approximately 30 mbar. The 
retention time of the sheet inside the vacuum chamber was approximately 8 
minutes; the sheet was kept at a temperature of 180.degree. C. 
Before leaving the chamber under vacuum, the thus treated sheet was passed 
through a bath of water kept at 25.degree. C. and then returned to 
atmospheric pressure. 
The characteristics of the resulting sheet are as follows: 
______________________________________ 
density 0.033 g/cm.sup.3 
weight 290 g/m.sup.2 
thickness 8.8 mm 
degree of crystallization 35% 
______________________________________ 
The sheet produced according to this treatment, termed "thermostable 
flexible sheet", was subjected to compression measurement cycles to 
evaluate both compression resistance and elastic recovery as well as 
temperature-dependent deformation. 
All tests were conducted in parallel with the sheet produced during the 
first step, which is termed "base sheet". 
Table 2 lists the values found during these characterizations. 
TABLE 2 
______________________________________ 
THERMOSTABLE 
BASE SHEET FLEXIBLE SHEET 
______________________________________ 
Maximum dimen- 
&lt;90.degree. C. 
&lt;150.degree. C. 
sional stability 
temperature 
(stress &lt;5%) 
at 30000 Pa 
Maximum creep 6.4 11.6 
deformation (%) 
Residual 4.1 3.9 
deformation after creep 
(after 120 min) (%) 
permanent 64.1 33.6 
deformation (%) 
elastic 35.9 66.4 
recovery (%) 
______________________________________ 
These measurements were performed by means of a thermomechanical analyzer. 
EXAMPLE 4 (PRODUCTION OF A THERMOSTABLE FLEXIBLE FOAMED PET SHEET: WATER AT 
125.degree. C.) 
The sheet produced as described in example 1 was subjected to a treatment 
as described hereafter. 
The sheet was pulled continuously and heated by means of water at 
125.degree. for 5 minutes, after which the sheet was introduced in a 
sizing device under vacuum, in which the residual pressure was 
approximately 30 mbar. 
The retention time of the sheet inside the chamber under vacuum was 
approximately 8 minutes. The sheet was kept at a temperature of 
180.degree. C. before leaving the chamber under vacuum and then passed 
through a bath of water kept at 25.degree. and then returned to 
atmospheric pressure. 
The characteristics of the resulting sheet were: 
______________________________________ 
density 0.038 g/cm.sup.3 
weight 290 g/m.sup.2 
thickness 7.6 mm 
degree of crystallization 38% 
______________________________________ 
The sheet produced according to this treatment, termed "thermostable 
flexible sheet", was subjected to compression measurement cycles in order 
to evaluate both compression resistance and elastic recovery as well as 
temperature-dependent deformation. All tests were conducted in parallel 
with the sheet produced during the first step, which is termed "base 
sheet". 
Table 3 lists the values observed during these characterizations. 
TABLE 3 
______________________________________ 
THERMOSTABLE 
BASE SHEET FLEXIBLE SHEET 
______________________________________ 
Maximum dimen- 
&lt;90.degree. C. 
&lt;160.degree. C. 
sional stability 
temperature 
(stress &lt;5%) 
at 30000 Pa 
Maximum creep 6.4 10 
deformation (%) 
Residual 4.1 3.7 
deformation after creep 
(after 120 min) (%) 
permanent 64.1 37 
deformation (%) 
elastic 35.9 63 
recovery (%) 
______________________________________ 
These measurements were conducted 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 melt strength of 100-150 cN, intrinsic 
viscosity of 1.25 dl/g and melt viscosity of 1800 Pa.s at 280.degree. C. 
(obtained by upgrading the polymer at 280.degree. C. in the presence of 
0.4% by weight of pyromellitic dianhydride (COBITECH.TM.) were fed 
continuously in a twin-screw extruder with a screw diameter of 90 mm. 
A static mixer was arranged downstream of the screws in order to improve 
the homogenization of the various components of the blend. 
The temperatures set on the extruder were 260.degree. C. in the melting 
region, 250.degree. C. in the compression region, 240.degree. C. in the 
mixing region and 225.degree. C. in the extrusion region. 
The screws of the extruder rotated at 18 rpm. 
2.4% by weight of blowing agent 134a (1,1,1,2 tetrafluoroethane) was added 
to the PET in the region of the extruder located after the melting of the 
polymer and thoroughly mixed with the polymeric 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.sup.3 
thickness 22 mm 
average cell diameter 280 .mu.m 
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 
performed a few seconds after extrusion as described hereafter. 
The extruded panel was cooled in the sizing region, and once a temperature 
of 180.degree. C. had been reached in the core of the panel, said panel 
was inserted in a sizing device under vacuum, where the residual pressure 
was approximately 30 mbar. The residence time of the panel inside the 
chamber under vacuum was approximately 5 minutes. The panel was kept at a 
temperature of approximately 120.degree. C. before leaving the chamber 
under vacuum and then was made to pass through a bath of water kept at 
25.degree. C. and then returned to atmospheric pressure. 
The characteristics of the resulting panel were: 
______________________________________ 
density 0.030 g/cm.sup.3 
thickness 55 mm 
degree of crystallization 10% 
______________________________________ 
The resulting panel (termed "flexible panel") was subjected to compression 
measurement cycles in order to evaluate compression resistance 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 
______________________________________ 
BASE PANEL 
FLEXIBLE PANEL 
______________________________________ 
Maximum creep 2.4 24 
deformation (%) 
Residual 1.6 5.7 
deformation after creep 
(after 120 min) (%) 
permanent 66 23.7 
deformation (%) 
elastic 34 76.3 
recovery (%) 
______________________________________ 
EXAMPLE 7 (PRODUCTION OF A THERMOSTABLE FLEXIBLE FOAMED PET PANEL) 
The panel produced as described in example 5 was subjected to a treatment 
performed a few seconds after extrusion, as described hereafter. 
The extruded panel was cooled in the sizing region and once it had reached 
a temperature of 180.degree. C. in the core of the panel it was introduced 
in a sizing device under vacuum, where the residual pressure was 
approximately 30 mbar. The residence time of the panel inside the chamber 
under vacuum was approximately 10 minutes. The panel was kept at a 
temperature of 180.degree. C. and before leaving the chamber under vacuum 
the panel was passed through a bath of water kept at 25.degree. C. and 
then returned to atmospheric pressure. 
The characteristics of the resulting panel were as follows: 
______________________________________ 
density 0.038 g/cm.sup.3 
thickness 52 mm 
degree of crystallization 36% 
______________________________________ 
The panel produced according to this treatment (termed "thermostable 
flexible panel") was subjected to compression measurement cycles to 
evaluate both compression resistance 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 
______________________________________ 
THERMOSTABLE 
BASE PANEL FLEXIBLE PANEL 
______________________________________ 
Maximum dimen- 
&lt;80.degree. C. 
&lt;148.degree. C. 
sional stability 
temperature 
(stress &lt;5%) 
at 30000 Pa 
Maximum creep 2.4 16 
deformation (%) 
Residual 1.6 5.1 
deformation after creep 
(after 120 min) (%) 
permanent 66 31.9 
deformation (%) 
elastic 34 68.1 
recovery (%) 
______________________________________ 
EXAMPLE 8 (PRODUCTION OF FOAMED PET SHEET) 
90 kg/h of polyethylene terephthalate homopolymer (COBITECH.TM.) used in 
example 1 were fed continuously 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 blend. 
The temperatures set on the extruder were 280.degree. C. in the melting 
region, 280.degree. C. in the compression region, 270.degree. C. in the 
mixing region and 265.degree. C. on the extrusion head. 
The screws of the extruder rotated at 15 rpm. 
2.5% by weight of nitrogen (blowing agent) was added to the PET in the 
region of the extruder located after the melting of the polymer and was 
thoroughly mixed in with the polymeric matrix. 
The PET/N.sub.2 composition, once mixed, was extruded through an annular 
head having a diameter of 120 mm and an extrusion opening of 0.14 mm. 
A sizing mandrel with a diameter of 350 mm and a length of 750 mm, cooled 
with water at 20.degree. C., was placed on the extrusion head. 
The foamed material, after leaving the extrusion head, was fitted 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/cm.sup.3 
weight 500 g/m.sup.2 
thickness 1.25 mm 
average cell diameter 130 .mu.m 
degree of crystallization 10% 
______________________________________ 
EXAMPLE 9 (PRODUCTION OF A SHEET OF FLEXIBLE FOAMED PET) 
The sheet produced as described in example 8 was subjected to a treatment 
as described hereinafter. 
The sheet was pulled continuously in a heating oven which brought the sheet 
to a temperature of approximately 115.degree. C. in approximately 3 
minutes, after which the sheet was placed in a sizing device under vacuum, 
in which residual pressure was approximately 30 mbar. The residence time 
of the sheet was approximately 5 minutes and the temperature was kept at 
115.degree. C. Before leaving the chamber under vacuum, the sheet thus 
treated was passed through a water bath kept at 25.degree. C. and then 
returned to atmospheric pressure. 
The characteristics of the resulting sheet were as follows: 
______________________________________ 
density 0.260 g/cm.sup.3 
weight 500 g/m.sup.2 
thickness 1.95 mm 
degree of crystallization 11% 
______________________________________ 
The sheet produced according to this treatment (termed "N.sub.2 flexible 
sheet") was subjected to compression measurement cycles in order to 
evaluate both compression resistance and elastic recovery. All tests were 
conducted in parallel on the sheet produced during the first step (N.sub.2 
base sheet). 
Table 6 lists the values found during these characterizations. 
TABLE 6 
______________________________________ 
N.sub.2 BASE SHEET 
N.sub.2 FLEXIBLE SHEET 
______________________________________ 
Maximum creep 2.9 8.5 
deformation (%) 
Residual 0.8 1.2 
deformation after creep 
(after 120 min) (%) 
permanent 27.6 14.1 
deformation (%) 
elastic 72.4 85.9 
recovery (%) 
______________________________________ 
These measurements were performed by means of a thermomechanical analyzer, 
subjecting the samples to 12 consecutive compression and decompression 
cycles. 
EXAMPLE 10 (PRODUCTION OF A SHEET OF THERMOSTABLE FLEXIBLE FOAMED PET) 
The sheet produced in example 8 was subjected to a treatment as described 
hereafter. 
The sheet was pulled continuously in a heating oven which brought the sheet 
to a temperature of 115.degree. C. in approximately 3 minutes, after which 
the sheet was introduced in a sizing device under vacuum, where the 
residual pressure was approximately 30 mbar. The residence time of the 
sheet inside the chamber under vacuum was approximately 5 minutes; the 
sheet was kept at a temperature of 180.degree. C. 
Before leaving the chamber under vacuum, the sheet was passed through a 
water bath kept at 25.degree. C. and then returned to atmospheric 
pressure. 
The characteristics of the resulting sheet were: 
______________________________________ 
density 0.243 g/cm.sup.3 
weight 500 g/m.sup.2 
thickness 2.05 mm 
degree of crystallization 37% 
______________________________________ 
The sheet produced according to this treatment (termed "N.sub.2 
thermostable flexible sheet") was subjected to compression measurement 
cycles in order to evaluate resistance to compression and elastic recovery 
as well as temperature-dependent deformation. All tests were conducted in 
parallel on the sheet produced during the first step (base sheet). 
Table 7 lists the values found during these characterizations. 
TABLE 7 
______________________________________ 
N.sub.2 BASE 
N.sub.2 THERMOSTABLE 
SHEET FLEXIBLE SHEET 
______________________________________ 
Maximum dimen- &lt;90.degree. C. 
&lt;165.degree. C. 
sional stability 
temperature 
(stress &lt;5%) 
at 30000 Pa 
Maximum creep 2.9 7.4 
deformation (%) 
Residual 0.81.7 
deformation 
after creep 
(after 120 min) (%) 
permanent 27.8 24 
deformation (%) 
elastic 72.4 76 
recovery (%) 
______________________________________ 
These measurements were taken with a thermomechanical analyzer. 
COMISON EXAMPLE 1 
A sheet produced as described in example 1 of U.S. Pat. No. 5,110,844 was 
subjected to thermomechanical characterization and compared with the sheet 
of example 4. 
The results of these characterizations are listed in Table 8. 
TABLE 8 
______________________________________ 
THERMO- 
STABLE SHEET ACCORDING 
BASE FLEXIBLE TO EXAMPLE 1 OF 
SHEET SHEET US-A-5 110 884 
______________________________________ 
Maximum dimen- 
&lt;90.degree. C. 
&lt;160.degree. C. 
&lt;90.degree. C. 
sional stability 
temperature 
(stress &lt;5%) 
at 30000 Pa 
Maximum creep 6.4 10 6.1 
deformation (%) 
Residual 4.1 3.7 4 
deformation 
after creep 
(after 120 min) (%) 
permanent 64.1 37 65.6 
deformation 
elastic 35.9 63 34.4 
recovery (%) 
______________________________________ 
The measurements were taken with a thermomechanical analyzer. 
COMISON EXAMPLE 2 
A sheet produced as described in Example 1 of U.S. Pat. No. 4,284,596 was 
subjected to thermomechanical characterization and compared with the sheet 
of example 4. 
The results of these characterizations are listed in Table 9. 
TABLE 9 
______________________________________ 
THERMO- 
STABLE SHEET ACCORDING 
BASE FLEXIBLE TO EXAMPLE 1 OF 
SHEET SHEET US-A-4 284 596 
______________________________________ 
Maximum dimen- 
&lt;90.degree. C. 
&lt;160.degree. C. 
&lt;90.degree. C. 
sional stability 
temperuture 
(stress &lt;5%) 
at 30000 Pa 
Maximum creep 6.4 10 2.2 
deformation (%) 
Residual 4.1 3.7 2 
deformation 
after creep 
(after 120 min) (%) 
permanent 64.1 37 91 
deformation (%) 
elastic 35.9 63 9 
recovery (%) 
______________________________________ 
The measurements were taken with a thermomechanical analyzer.