U.S. patent application number 13/697668 was filed with the patent office on 2013-03-14 for biodegradable pellets foamed by irradiation.
This patent application is currently assigned to Novamont S.p.A.. The applicant listed for this patent is Catia Bastioli, Roberto Lombi, Matteo Nicolini, Daniele Turati. Invention is credited to Catia Bastioli, Roberto Lombi, Matteo Nicolini, Daniele Turati.
Application Number | 20130065055 13/697668 |
Document ID | / |
Family ID | 43332744 |
Filed Date | 2013-03-14 |
United States Patent
Application |
20130065055 |
Kind Code |
A1 |
Bastioli; Catia ; et
al. |
March 14, 2013 |
BIODEGRADABLE PELLETS FOAMED BY IRRADIATION
Abstract
This invention relates to biodegradable starch-based pellets
which foamable by irradiation, which are particularly suitable for
the manufacture of foam articles, characterised in that they have a
porous structure with a low porous external skin. This invention
also relates to foam articles obtained from these.
Inventors: |
Bastioli; Catia; (Novara,
IT) ; Lombi; Roberto; (Novara, IT) ; Nicolini;
Matteo; (Borgomanero (NO), IT) ; Turati; Daniele;
(Buscate (MI), IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bastioli; Catia
Lombi; Roberto
Nicolini; Matteo
Turati; Daniele |
Novara
Novara
Borgomanero (NO)
Buscate (MI) |
|
IT
IT
IT
IT |
|
|
Assignee: |
Novamont S.p.A.
Novara
IT
|
Family ID: |
43332744 |
Appl. No.: |
13/697668 |
Filed: |
May 13, 2011 |
PCT Filed: |
May 13, 2011 |
PCT NO: |
PCT/EP2011/057802 |
371 Date: |
November 13, 2012 |
Current U.S.
Class: |
428/407 ;
264/142; 428/403 |
Current CPC
Class: |
C08J 2303/02 20130101;
C08J 7/123 20130101; C08L 3/02 20130101; Y10T 428/2991 20150115;
C08J 2201/03 20130101; C08L 3/02 20130101; C08J 9/34 20130101; C08L
29/04 20130101; C08L 2666/04 20130101; C08J 9/16 20130101; C08J
2300/16 20130101; C08L 2666/02 20130101; C08J 2205/044 20130101;
C08L 3/02 20130101; Y10T 428/2998 20150115 |
Class at
Publication: |
428/407 ;
428/403; 264/142 |
International
Class: |
B32B 5/16 20060101
B32B005/16; B29B 9/06 20060101 B29B009/06 |
Foreign Application Data
Date |
Code |
Application Number |
May 14, 2010 |
IT |
MI2010A000865 |
Claims
1. Biodegradable low density, self-sealing pellets foamable by
irradiation particularly suitable for the production of foamed
articles, said pellets comprising starch essentially free of native
crystallinity which does not have endothermic gelatinisation peaks
associated with a .DELTA.H of more than 0.4 J/g of dry starch when
analysed by differential scanning calorimetry in a hermetic capsule
with a water/dry starch ratio of 4, said pellets being
characterised by: a porous internal structure having void area
greater than 15% with respect to the area of the cross-section of
the pellet; a mean equivalent diameter of pores less than 100
microns and an external skin with pores in number less than 80%
with respect to the internal structure having a mean equivalent
diameter lower or equal to the mean equivalent diameter of the
pores of the internal structure.
2. Biodegradable pellets according to claim 1, wherein the fact of
comprising at least a further polymer of synthetic or natural
origin.
3. Biodegradable pellets according to claim 2, wherein the fact of
comprising: 10-99.5% by weight of starch essentially devoid of
native crystallinity, 0.5-90% by weight of at least a further
polymer of synthetic or natural origin, 0.1-60% by weight of water,
with respect to the total weight of the dry pellet.
4. Biodegradable pellets according to claim 2, wherein said further
polymer of natural origin is selected from cellulose, lignin,
proteins, phospholipids, casein, polysaccharides, natural gums,
rosinic acid, dextrins, their mixtures and derivatives thereof.
5. Biodegradable pellets according to claim 2, wherein said further
polymer of synthetic origin is selected from: thermoplastic
polymers comprising homopolymers and copolymers of linear or
branched aliphatic hydroxyacid having C2-C24 main chain, their
lactons and lactides as well as their copolymers with aliphatic
polyesters of the diacid/diol type; vinyl polymers comprising
polyvinyl alcohol with various degrees of hydrolysis, possibly
modified with acrylates or methacrylates, polyvinyl
alcohol-co-vinylacetate block and random copolymers with degree of
hydrolysis >70%, preferably higher than 80 and even preferably
higher than 85 mol %, polyvinyl alcohol plasticized or modified for
the purpose of lowering its melting point, polyvinylacetate,
copolymers of vinylacetate at various degrees of hydrolysis with
vinylpyrrolidone and/or styrene, polyethyloxazoline or
polyvinylpiridine; polycarbonates; ethylene-vinyl alcohol
copolymers, preferably with up to 50% by weight of ethylenic units,
copolymers of ethylene with vinyl acetate or unsaturated acids,
polyamides 6, 6-6, 6-9, 6-10, 9, 9-9, 10, 10-10, 11,11-11, 12,
12-12 and their mixtures, aliphatic polyurethanes, random and block
copolymers polyurethane-polyamide, polyurethane-polyether,
polyurethane-polyesters, polyamide-polyether, polyamide-polyester,
polyester-polyether and epoxy resins; random and block aliphatic
and aliphatic-aromatic polyesters and copolyesters of the
diacid/diol type; synthetic rubbers; non biodegradable polymers
such as polyolefins, aromatic polyesters, polyvinylchloride,
polystyrene, acrylonitrile-butadiene-styrene copolymers;
polyoxyalkylenes with molecular weight >50000.
6. Biodegradable pellets according to claims 5, wherein said
further polymer of synthetic origin is a polyvinyl
alcohol-co-vinylacetate copolymer with degree of hydrolysis >70
mol %.
7. Biodegradable pellets according to claim 1, wherein said pores
are characterized by a mean equivalent diameter of less than 50
.mu.m,
8. Biodegradable pellets according to claim 1, wherein the fact of
showing, when conditioned to a water content of 22.+-.2% by weight
with respect to the total weight of the dry composition, a density
<1.1 g/cm3 and higher than 0.5 g/cm3.
9. Biodegradable pellets according to claim 1, wherein said starch
shows an intrinsic viscosity higher than 1.5 dl/g and lower than 3
dl/g.
10. Process for the preparation of biodegradable low density,
self-sealing pellets foamable by irradiation wherein: a porous
internal structure having void area greater than 15% with respect
to the area of the cross-section of the pellet; a mean equivalent
diameter of pores less than 100 microns and an external skin with
void area lower than 15%, with respect to the area of the external
skin of the pellet. said process being characterized by the fact of
comprising the steps of: (a) feeding a composition comprising
starch and water to an extruder, (b) extruding said composition to
form a melt and at an extrusion rate, residence time and shear rate
at the outlet suitable to destroy the native crystallinity of the
starch and to produce a swelling followed by collapsing of the
extrudate on leaving the nozzle, (c) cutting the collapsed
extrudate into the form of pellets soon after the exit from the die
in such a way that the extrudate is cut when it is not yet
completely solidified to seal possible open pores and reconstitute
a skin at the cut surface, (d) conditioning said pellets in order
to adjust the moisture content at level between 10 and 45%, with
respect to the total dry weight of a pellet by exposure to air at
room temperature or higher.
11. Foamed articles obtained by irradiation of the biodegradable
pellets according to claim 1, wherein the fact of showing an
intrinsic viscosity in the range 1.9-3 dl/g.
12. Foamed articles according to claim 11, wherein the fact of
showing density of less than 80 kg/m.sup.3.
13. Use of the foamed articles according to claim 11, for producing
protective packaging for home appliances, products for the
electronic industry, furnitures, food packaging, agricultural
packaging.
14. Biodegradable pellets according to claim 3, wherein said
further polymer of natural origin is selected from cellulose,
lignin, proteins, phospholipids, casein, polysaccharides, natural
gums, rosinic acid, dextrins, their mixtures and derivatives
thereof.
15. Biodegradable pellets according to claim 3, wherein said
further polymer of synthetic origin is selected from: thermoplastic
polymers comprising homopolymers and copolymers of linear or
branched aliphatic hydroxyacid having C2-C24 main chain, their
lactons and lactides as well as their copolymers with aliphatic
polyesters of the diacid/diol type; vinyl polymers comprising
polyvinyl alcohol with various degrees of hydrolysis, possibly
modified with acrylates or methacrylates, polyvinyl
alcohol-co-vinylacetate block and random copolymers with degree of
hydrolysis >70%, preferably higher than 80 and even preferably
higher than 85 mol %, polyvinyl alcohol plasticized or modified for
the purpose of lowering its melting point, polyvinylacetate,
copolymers of vinylacetate at various degrees of hydrolysis with
vinylpyrrolidone and/or styrene, polyethyloxazoline or
polyvinylpiridine; polycarbonates; ethylene-vinyl alcohol
copolymers, preferably with up to 50% by weight of ethylenic units,
copolymers of ethylene with vinyl acetate or unsaturated acids,
polyamides 6, 6-6, 6-9, 6-10, 9, 9-9, 10, 10-10, 11,11-11, 12,
12-12 and their mixtures, aliphatic polyurethanes, random and block
copolymers polyurethane-polyamide, polyurethane-polyether,
polyurethane-polyesters, polyamide-polyether, polyamide-polyester,
polyester-polyether and epoxy resins; random and block aliphatic
and aliphatic-aromatic polyesters and copolyesters of the
diacid/diol type; synthetic rubbers; non biodegradable polymers
such as polyolefins, aromatic polyesters, polyvinylchloride,
polystyrene, acrylonitrile-butadiene-styrene copolymers;
polyoxyalkylenes with molecular weight >50000.
16. Biodegradable pellets according to claim 4, wherein said
further polymer of synthetic origin is selected from: thermoplastic
polymers comprising homopolymers and copolymers of linear or
branched aliphatic hydroxyacid having C2-C24 main chain, their
lactons and lactides as well as their copolymers with aliphatic
polyesters of the diacid/diol type; vinyl polymers comprising
polyvinyl alcohol with various degrees of hydrolysis, possibly
modified with acrylates or methacrylates, polyvinyl
alcohol-co-vinylacetate block and random copolymers with degree of
hydrolysis >70%, preferably higher than 80 and even preferably
higher than 85 mol %, polyvinyl alcohol plasticized or modified for
the purpose of lowering its melting point, polyvinylacetate,
copolymers of vinylacetate at various degrees of hydrolysis with
vinylpyrrolidone and/or styrene, polyethyloxazoline or
polyvinylpiridine; polycarbonates; ethylene-vinyl alcohol
copolymers, preferably with up to 50% by weight of ethylenic units,
copolymers of ethylene with vinyl acetate or unsaturated acids,
polyamides 6, 6-6, 6-9, 6-10, 9, 9-9, 10, 10-10, 11,11-11, 12,
12-12 and their mixtures, aliphatic polyurethanes, random and block
copolymers polyurethane-polyamide, polyurethane-polyether,
polyurethane-polyesters, polyamide-polyether, polyamide-polyester,
polyester-polyether and epoxy resins; random and block aliphatic
and aliphatic-aromatic polyesters and copolyesters of the
diacid/diol type; synthetic rubbers; non biodegradable polymers
such as polyolefins, aromatic polyesters, polyvinylchloride,
polystyrene, acrylonitrile-butadiene-styrene copolymers;
polyoxyalkylenes with molecular weight >50000.
17. Biodegradable pellets according to claim 2, wherein said pores
are characterized by a mean equivalent diameter of less than 50
.mu.m,
18. Biodegradable pellets according to claim 3, wherein said pores
are characterized by a mean equivalent diameter of less than 50
.mu.m,
19. Biodegradable pellets according to claim 4, wherein said pores
are characterized by a mean equivalent diameter of less than 50
.mu.m,
20. Biodegradable pellets according to claim 5, wherein said pores
are characterized by a mean equivalent diameter of less than 50
.mu.m,
Description
[0001] This invention relates to biodegradable low density,
self-sealing pellets foamable through irradiation particularly
suitable for the production of high cushioning foamed articles,
comprising high viscosity starch which is essentially free of
native crystallinity characterised in that they have an internal
porous structure with a low porous external skin.
[0002] This invention also relates to foam articles obtained from
these pellets characterized by thin-walled cells and dynamic shock
cushioning characteristics comparable or even better than expanded
polystyrene (EPS) with density of 26 kg/m3. Because of their
cushioning characteristics the foamed articles according to the
invention can be used at equal or lower thickness than EPS
foams
[0003] Among the foam materials conventionally used as protective
packaging, EPS is undoubtedly the most widely used because of its
mechanical properties, for example its impact strength,
compressibility, cushioning properties, low density, heat
insulating characteristics and low cost.
[0004] The use of EPS is particularly suitable for packaging of a
wide range of goods such as, CD-DVD players (fragility factor
expressed in G's of 40-60), Hi-Fi, TV's (G's of 60-85),
electric-home-appliances (G's of 85-115) or industrial equipments
(G's>115).
[0005] However, the widespread use of EPS in the field of
protective packaging, has created problems associated with the
accumulation and disposal of this material. In addition to this,
the synthetic origin of the monomer of which it is composed limits
the ability of this material to significantly reduce the
consumption of resources (feedstocks) originating from
non-renewable carbon.
[0006] The plastics industry has therefore focused its activities
in researching and developing new materials having properties
similar to those of conventional foam materials which at the same
time help to solve the environmental problems associated with the
accumulation of these materials and their disposal at the end of
their life cycle, as well as the consumption of resources
originating from non-renewable carbon.
[0007] In this respect attempts to produce foam materials based on
starch, which being biodegradable and from a renewable source offer
a first attempt at solution of the abovementioned problems, are
known in the state of the art.
[0008] Starch-based foam materials may for example be prepared by
means of extrusion processes of starch-based compositions conducted
in the presence of large quantities of water, such that expansion
takes place directly at the outlet from the extruder as a result of
rapid evaporation of the water leaving the extruder. In such a case
intrinsic viscosities between 0.5 and 1.5 dl/g have to be achieved
in order to obtain low density foams at the exit of the die. It
means it very high molecular weight reduction in comparison with
the starting material resulting in a resiliency far less than EPS.
Moreover, in order to obtain three-dimensional protective packaging
structures (so-called foam blocks), it is necessary adding
adhesives due to the non self-sealing skin of said starch-based
materials. As for as the foamed materials obtained therefrom is
concerned, they show high sensitivity to humidity.
[0009] An alternative method for preparing starch-based foam
materials comprises compression/decompression treatment of
non-foamed pellets at high temperature. These pellets are subjected
to a higher than atmospheric pressure in the presence of water
vapour, and then rapid decompression. During the pressurising
treatment water penetrates within the pellets. The sudden sharp
fall in pressure causes sudden evaporation of the water present in
the material and its consequent expansion.
[0010] Yet another method of preparing a foam material comprises
subjecting the unfoamed pellets of the type described above to a
treatment of irradiating them with microwaves. In particular,
because of the possibility of imparting high energy densities to
the surface and interior of the pellets the latter type of process
allows the water contained within them to be heated and quickly
evaporated causing expansion of the pellets.
[0011] These irradiation expansion processes have significant
difficulties. It is in fact difficult to ensure that the pellet
expands in a regular way in the course of irradiation at atmosphere
pressure, with good self-sealing and creating a uniform
distribution of cells at low cell wall thickness, moreover
achieving sufficiently low density values and resiliency to render
the foamed materials so produced competitive on the market.
[0012] The problem underlying this invention is therefore that of
finding a biodegradable material in the form of low density
self-sealing pellets foamable by irradiation allowing the
production of expanded articles having a regular homogeneous
structure, mechanical properties, dimensions, cell distribution,
cell wall and density so as to enable them to be effectively used
as a replacement for the foam materials conventionally used as
protective packaging.
[0013] Starting from this problem it has now been surprisingly
found that it is possible to obtain pellets foamable by irradiation
even at atmospheric pressure that are particularly suitable for the
production of high cushioning foam articles characterized by
thin-walled cells, said pellets comprising high viscosity starch
which is essentially free of native crystallinity and characterised
in that they have a porous internal structure with a low porous
external skin. In particular the present invention refers to
biodegradable low density, self-sealing pellets foamable by
irradiation particularly suitable for the production of foamed
articles said pellets comprising starch essentially free of native
crystallinity and being characterised by [0014] a porous internal
structure having void area greater than 15% with respect to the
area of the cross-section of the pellet; [0015] a mean equivalent
diameter of pores less than 100 microns and [0016] an external skin
with pores in number less than 80% with respect to the internal
structure having a mean equivalent diameter lower or equal to the
mean equivalent diameter of the pores of the internal
structure.
[0017] The characteristics and advantages of the pellets according
to the present invention and the foamed articles obtained from them
in comparison with the known art will be obvious from the following
description.
[0018] By pellets are here meant discrete portions of plastic
material, preferably obtained by extrusion processes, regardless of
their shape and size.
[0019] Preferably, the pellets according to this invention are of a
substantially spherical, helicoidally, disk-shaped, cylindrical or
toroidal shape, or are 8, S or U ring-shaped or of any other shape
which can be obtained by passage through a nozzle. In case of
pellets, the dimensions of the pellets according to this invention
preferably lie between 0.1 and 10 cm, preferably between 0.3 and 3
cm and more preferably between 0.5 and 2.5 cm measured along the
largest dimension. It is also possible to produce ribbons, plates,
profiles or even shaped parts which can be foamed through
irradiation.
[0020] The pellets according to the present invention are defined
as "low density" with this expression being meant that said
pellets, when conditioned to a water content of 22.+-.2% by weight
with respect to the total weight of the dry composition, have a
density of <1.1 g/cm3 and higher than 0.5 g/cm3 and preferably
<1 g/cm3 and higher than 0.6 g/cm3. The density of the pellets
is lower than the density of their polymeric components and
specifically of the amorphous starch density (1.4 g/cm3) because of
the presence of the porous structure. Preferably the density of the
"low density" pellets has to he comprised between 80 and 35% and
more preferably between 70 and 45% of the density of the non porous
material.
[0021] Said porous structure preferably show a uniform distribution
of pores throughout the entire pellet structure (i.e. a
highly-dispersed system of pores).
[0022] Said porous structure shows pores which can be of regular
and/or irregular shape. It means that said pores, when seen in
cross-section, may be of a circular, elliptical shape or of
whatever other shape.
[0023] Said porous structure is advantageously characterized in
that the total area of the pores detectable in a cross-section of
the pellet (so-called void area) is higher than 15%, preferably
higher than 20% with respect to the total area of said
cross-section and that said pores show a mean equivalent diameter
of less than 100 .mu.m, preferably less than 50 .mu.m, more
preferably less than 30 .mu.m and even more preferably less than 20
.mu.m.
[0024] In particular, void area and mean equivalent diameter of the
pores may be determined on a portion of pellet whose internal
structure has been rendered accessible for examination under
scanning electron microscope (SEM) For this purpose the pellet
which is to be examined is immersed in liquid nitrogen and
subsequently fractured so as to obtain a fracture surface along a
cross-section of the pellet. The portion of the pellet which is to
be examined is then dried and a thin layer of metal is deposited
thereupon, for example a mixture of gold/palladium, using a
"sputter coater". Finally the surface of the fracture is examined
under SEM and void area and equivalent diameter of the pores are
measured. By equivalent diameter is meant here the diameter of the
circular pore that exhibits the same area to that of the
investigated pore.
[0025] The area of the pores can be measured either by using image
analysis softwares or by manual methods.
[0026] The void area can be calculated according to the following
equation:
void area ( % ) = i = 1 N A i D * 100 ##EQU00001##
where: [0027] A is the area of pores which can be seen in the SEM
inside the cross-section examined expressed in micron.sup.2, [0028]
N is the number of pores which can be seen in the SEM inside the
fracture surface area examined, [0029] D is the area of the
cross-section of the pellet examined at the SEM expressed in
micron.sup.2.
[0030] The mean equivalent diameter can be calculated according to
the following equation:
mean equivalent diameter = i = 1 N d i N ##EQU00002##
where: [0031] N is the number of pores which can be seen in the SEM
inside the fracture surface area examined, [0032] d.sub.i is the
equivalent diameter of the individual pores.
[0033] The pores preferably show a closed cells structure with less
than 30% of pores interconnected each other.
[0034] The external skin of pellet according to the present
invention is defined as "low porous", by this term it being meant
that said skin shows pores resulting in a void area lower than the
void area of the pellet structure. Preferably the number of the
pores on the external skin are less than 80%, more preferably less
than 50%, of the pores per unit of area detectable in the internal
structure of the pellet. Said pores are preferably isolated from
each other. Preferably, the external skin pores show a mean
equivalent diameter of less than 100 .mu.m, preferably less than 50
.mu.m, more preferably less than 30 .mu.m and even more preferably
less than 20 .mu.m. In any case it is preferred that the dimension
of the pores at the surface is comparable with the dimension of the
pores of the internal structure of the pellet.
[0035] More specifically the low porous external skin, allows
vaporization and expansion of the water contained in the low
density self-sealing pellets without significant escaping through a
path of pores interconnected to the surface.
[0036] The skin can be easily inspected by SEM without fracturing
the pellets.
[0037] Number and dimension of pores of the skin can be determined
by SEM analysis inspecting micrographs at similar magnification of
those obtained to inspect internal structure also if skin
structures presents corrugation of the surface.
[0038] The low density self-sealing pellets comprise starch with no
more cristallinity of type A, B or C and as less as possible
residual granular structure (no maltese crosses under polarized
light) as well as intrinsic viscosity (measured at 25.degree. C.
using as a solvent DMSO containing 0.5 wt % of LiCl) higher than
1.5 dl/g and lower than 3 dl/g, and far more preferably comprised
between 1.9 and 2.8 dl/g. Because of these viscosity values, the
starch of the pellets according to the present invention is defined
"high viscosity". Thanks to the above properties, the pellets
according to the present invention show viscoelastic behaviour
suitable to produce during foaming a resilient foamed article.
Particularly, in case of too low viscosity the pellets will be too
brittle and not elastic enough to permit expansion.
[0039] The pellets according to the present invention are defined
as "self sealing" with this expression being meant that said
pellets, when foamed irreversibly adhere each other.
[0040] Thanks to their specific structural characteristics the
biodegradable pellets according to the present invention are
capable of expanding if subjected to irradiation by means of
electromagnetic waves such as microwaves, radio waves or infrared
radiation. Of these microwaves are preferred. The said irradiation
may be accompanied by treatment to pressurise the pellets, but the
properties of the pellets according to the present invention allows
expansion also in conditions of atmospheric pressure. This aspect
is of key importance and can permit a simple and safe scale-up of
the process at industrial level.
[0041] In particular the porous internal structure combined with
the low-porous external skin of the pellets according to the
present invention allows the pellets to expand regularly, creating
a material having a uniform distribution of cells within it.
[0042] As a result of this the expanded material achieves density
values which are sufficiently low to enable it to be effectively
used as a replacement for the foam materials conventionally used as
protective packaging.
[0043] The pellets according to this invention comprise starch
essentially free of native crystallinity. Preferably they comprise
starch and at least one other polymer of synthetic or natural
origin. The preferred range of compositions comprises: [0044]
10-95.5% by weight of starch essentially free of native
crystallinity, [0045] 0.5-90% by weight of at least another polymer
of synthetic or natural origin, [0046] 0.1-60% by weight of water,
with respect to the total thy weight of a pellet.
[0047] More preferably, the pellets according to the present
inventions comprise: [0048] 50-98% by weight of starch essentially
free of native crystallinity, [0049] 2-50% by weight of at least
another polymer of synthetic or natural origin, [0050] 5-45% by
weight of water, with respect to the total dry weight of a
pellet.
[0051] In the context of this application, by the expression
"starch essentially free of native crystallinity" is meant a starch
in which its native crystallinity has been completely disrupted or
is in any event present in a quantity such as not to adversely
affect the properties indicated below. In particular, starch
essentially free from native crystallinity according to this
invention does not have endothermic gelatinisation peaks associated
with a .DELTA.H of more than 0.4 J/g of dry starch when analysed by
differential scanning calorimetry (DSC) in a hermetic capsule with
a water/dry starch ratio of 4.
[0052] Preferably the starch essentially free of native
crystallinity according to this invention has lost its native
granular structure. As far as the native granular structure is
concerned, this can be advantageously identified by optical phase
contrast microscopy. The presence of residual granular structures
can be also detected by SEM analysis.
[0053] Preferably the starch which can be used to prepare the
pellets may be a refined starch, a corn starch (so-called grits)
containing starch and cellulose materials and/or lignin.
Particularly suitable according to the invention are potato starch,
wheat starch, rice starch, pea starch, starch from legumes,
sorghum, tapioca and yucca as well as starches having on content of
amyloses, preferably in excess of 30% by weight, and "waxy"
starches. it is also possible to use mixtures of starches. Potato
starch, tapioca starch, and binary mixtures of these are
particularly preferred and tapioca starch is even more preferred.
Physically and chemically modified starch, for example starch
ethoxylate, oxypropylate, acetate, butyrate, propionate, with a
degree of substitution between 0.1 and 2.5, cationic starch,
oxidised starch, cross-linked starch, gelatinised starch, partly or
completely destructured starch, complex starch or mixtures thereof
may also be used in the process for preparing pellets.
[0054] Starch is present in the pellets in a quantity of between 10
and 99.5%, preferably between 20 and 99%, more preferably between
40 and 98.5% and even more preferably between 50-98% by weight with
respect to the total dry weight of a pellet.
[0055] In addition to starch the pellets preferably contain at
least another polymer of synthetic or natural origin.
[0056] This is preferably present in a quantity between 0.5 and 90%
by weigin, preferably between 1 and 80% and more preferably between
1.5 and 60% and even more preferably between 2-50% by weight with
respect to the total dry weight of a pellet.
[0057] In the case of natural polymers these are preferably
selected from cellulose, lignin, proteins such as gluten, zein,
casein, collagen, gelatin, phospholipids, caseins, polysaccharides
such as pullulanes, alginates, chitin, chitosanes, natural rubbers,
rosinic acid, dextrin, their mixtures and their derivatives such as
for example esters or ethers. Cellulose may also be modified and in
this respect mention may for example be made of esters of cellulose
having a degree of substitution between 0.2 and 2.5. Thermoplastic
lignin may also be used.
[0058] As far as synthetic polymers are concerned, these may also
be obtained by fermentation and are advantageously selected from:
[0059] a. thermoplastic polymers comprising homopolymers and
copolymers of straight or branched chain aliphatic hydroxy acids
having a main C.sub.2-C.sub.24 chain, their lactones and lactides,
as well as their copolymers with aliphatic polyesters of
diacids/diols. Among these poly-L-lactic acid, poly-D-lactic acid,
poly-L-lactic-poly-D-lactic stereo complex and copolymers of
L-lactic acid and D-lactic acid, poly-.epsilon.-caprolactones, poly
glycolic acids, long and short chain polyhydroxy alkanoates of the
polyhydroxy butyrates type and their copolymers with C5-C18
hydroxyalkanoates such as for example polyhydroxybutyrate valerate,
polyhydroxybutyrate pentanoate, polyhydroxybutyrate hexanoate,
polyhydroxybutyrate decanoate, polyhydroxybutyrate dodecanoate,
polyhydroxybutyrate hexadecanoate, and polyhydroxybutyrate
octadecanoate are preferred, [0060] b. vinyl polymers comprising
polyvinyl alcohol with different degrees of hydrolysis which may
also be modified with acrylates or methacrylates, polyvinyl
alcohol-co-vinylacetate either block and random copolymer with
degree of hydrolysis >70%, preferably higher than 80 and even
preferably higher than 85 mol %, polyvinyl alcohol plasticised or
modified in order to lower its melting point, polyvinyl acetate,
copolymers of vinyl acetate having various degrees of hydrolysis
with vinyl pyrrolidone and/or styrene, polyethyloxazoline or
polyvinyl pyrridine, [0061] c. polycarbonates, for example of the
polyalkylene carbonate type, [0062] d. ethylene-vinyl alcohol
copolymers, preferably containing up to 50% and more preferably
10-44% by weight of ethylene units, ethylene copolymers with vinyl
acetate or unsaturated acids, 6, 6-6, 6-9, 6-10, 9, 9-9, 10, 10-10,
11, 11-11, 12, 12-12 polyamides and their mixtures, aliphatic
polyurethanes, random and block copolymer polyurethane-polyamides,
polyurethane-polyethers, polyurethane-polyesters,
polyamide-polyesters, polyester-polyethers and epoxy resins, [0063]
e. polyesters and copolyesters of aliphatic and/or
aliphatic-aromatic diacids-diols, both random and block. As far as
the aliphatic polyesters and copolyesters are concerned, these
comprise C.sub.2-C.sub.22 aliphatic diacids and aliphatic diols.
The aliphatic-aromatic polyesters and copolyesters instead have an
aromatic part comprising mainly at least one aromatic acid having
multiple functional groups, such as terephthalic acid and
2,5-furandicarboxylic acid, or mixtures thereof, the aliphatic part
comprising C.sub.2-C.sub.22 aliphatic diacids and aliphatic diols,
[0064] f. synthetic rubbers such as for example polybutadiene,
nitrile rubber, for example BUNA-S and BUNA-N, chlorobutadiene
(Neoprene), polyisoprene, butadiene-ethylene-propylene terpolymers,
[0065] g. non-biodegradable polymers such as for example
polyolefins, for example polypropylene and polyethylene, aromatic
polyesters, for example polyethylene terephthalate, polybutylene
terephthalate, polytrimethylene terephthalate, polyvinyl chloride,
polystyrene, acrylonitrile-butadiene-styrene copolymers, [0066] h.
Polyoxyalkylenes having a molecular weight >50,000 and more
preferably >100,000.
[0067] Mixtures of the synthetic polymers mentioned in point a. to
h. are also preferred, and mixtures of synthetic and natural
polymers are particularly preferred.
[0068] The water content of the pellets according to this invention
preferably lies between 0.1 and 60%, preferably between 5 and 45%,
and even more preferably between 15 and 40% with respect to the
total dry weight of a pellets.
[0069] The water content encourages adhesion between the pellets
after expansion and makes it possible for example to produce
three-dimensional structures which are particularly suitable for
the manufacture of protective packaging (so-called foam blocks). In
particular, if caused to expand within a mould the pellets
according to this invention make it possible to manufacture
three-dimensional blocks of foam material even without the addition
of specific additives or surface treatments on the foamable
pellets.
[0070] According to requirements, for example transport or storage,
it is also possible to modulate the water content so as to avoid
problems associated with undesired adhesion between the as yet
unexpanded pellets and/or the development of moulds. In this case
it is sufficient to rehydrate the pellets in order to be able to
use them for the production of foamed articles.
[0071] The biodegradable pellets according to this invention
preferably comprise one or more plasticisers. When used these are
present in quantities of between 0.1 and 20%, preferably between
0.5 and 5% by weight with respect to the total dry weight of the
composition.
[0072] Plasticisers may comprise all the compounds known for the
purpose to those skilled in the art such as for example glycerine,
polyglycerol, sorbitol, mannitol, erythritol, low molecular weight
polyvinyl alcohol, as well as oxyethylate or oxypropylate derivates
of the aforementioned compounds. Of these, glycerine is
preferred.
[0073] The pellets according to this invention may also comprise
one or more nucleating agents. The quantity of these agents is
preferably between 0.005 and 5%, more preferably between 0.0.5 and
3% and even more preferably between 0.2 and 2% by weight with
respect to the total dry weight of a pellet.
[0074] Usable nucleating agents are, for example, inorganic
compounds such as talc (magnesium silicate), calcium carbonate,
nano particles such as montmorillonites and hydrotalcites. These
agents may possibly receive surface treatment with adhesion
promoters such as silanes, titanates, etc. Organic fillers such as
the husks of yeasts originating from the processing of beet, dried,
ground and powdered beet pulp, sawdust, cellulose powder, lignin
and its derivatives, etc., may be used.
[0075] The nucleating agent may be added in pure form or as an
alternative in the form of masterbatch. In this case the
masterbatch may contain quantities of between 10 and 50% of one or
more nucleating agents.
[0076] The pellets according to this invention may also include
other additives such as for example flame retardants, antimoulding
agents, pigments, colouring agents, rodent repellants, lubricants,
dispersants, surfactants, physical or chemical expansion agents,
mineral and natural fillers, fibres and microfibres.
[0077] Particularly, the pellets according to this invention may
comprise one or more antimoulding agents, such as for example
organic compounds such as sorbic acid and its salts and primaricin,
in quantity preferably of 0.005-5%, more preferably of 0.05-3% and
even more preferabiy of 0.2-2% by weight with respect to the total
dry weight of a pellet. Among antimoulding agents, potassium
sorbate is particularly preferred.
[0078] The antimoulding agent may be added in pure form or in the
form of masterbatch.
[0079] Retardants such as peroxides, mono-, di- and poly-expoxides,
acrylate polyepoxides and their copolymers with styrene, aliphatic,
aromatic or aliphatic-aromatic oligomer and polymer carbodiimides,
isocyanates, isocyanurates and their combinations, and hydrides and
polyanhydrides which are compatible with starch and other synthetic
or natural polymers may also be added.
[0080] Preferably the pellets according to this invention have a
density <1.1 g/cm3 and higher than 0.5 g/cm3 and more preferably
less than 1 g/cm3 and higher than 0.6 g/cm3
[0081] As far as measurement of the density of the pellets
according to this invention is concerned, this may be carried out
as follows: a number of pellets indicatively between 60 and 70 is
weighed and their weight is recorded (P.sub.g).
[0082] A volume of silica V.sub.1, for example glass beads,
unwashed--150-212 .mu.m (Sigma Aldrich), such as to completely fill
a graduated cylinder (for example 100 mL) is weighed on an
analytical balance (accuracy 0.01 g) and its weight P.sub.1 is
recorded. The density of the silica is thus determined using the
following formula:
d Silica = P 1 V 1 ##EQU00003##
[0083] A number N of pellets selected in such a way that they
occupy approximately 60-70% of its volume are placed in the same
graduated cylinder which has previously been emptied. Subsequently
it is made up to volume using the silica previously measured,
taking care to compact it carefully. The remaining silica is
weighed (P.sub.2) on an analytical balance (sensitive to 0.01 g)
and the volume of the individual pellets is calculated using the
following formula:
Mean density of the pellets = ( d silica ) ( P 2 ) P g
##EQU00004##
[0084] The same procedure may be used to measure the density of the
pellets after expansion. The pellets according to the invention
preferably have an intrinsic viscosity (measured at 25.degree. C.
using as a solvent DMSO containing 0.5 wt % of LiCl) higher than
1.5 dl/g and lower than 3 dl/g, preferably comprised between 1.9
and 2.8 dl/g and more preferably comprised between 2 and 2.7
dl/g.
[0085] As far as the intrinsic viscosity measurement is concerned,
it can he performed directly on the pellets material when the
pellet polymeric component are soluble in DMSO. Otherwise the
intrinsic viscosity measurement is performed on the DMSO soluble
fraction of the pellet. Separation of the DMSO soluble fraction of
the pellet can be performed by means of filtration or
centrifugation.
[0086] The pellets according to this invention are biodegradable
according to standard EN 13432.
[0087] The pellets according to this invention are preferably
prepared through a process characterised in that it comprises the
following stages: [0088] (a) feeding a composition comprising
starch and water to an extruder, [0089] (b) extruding said
composition to form a melt and at an extrusion rate, residence
time, and shear rate at the outlet of the nozzle, suitable to
destroy the native crystallinity of the starch and to produce a
swelling followed by collapsing of the extrudate on leaving the
nozzle, [0090] (c) cutting the collapsed extrudate into the form of
pellets soon after the exit from the die to seal possible open
pores and reconstitute a skin at the cut surface, [0091] (d)
conditioning said pellet in order to adjust the moisture content at
level between 10 and 45%, preferably between 15 and 40 and even
more preferably between 20 and 30% with respect to the total dry
weight of a pellet by exposure to air at room temperature or
higher.
[0092] As far as stage (a) in the process is concerned, the
composition fed to the extruder preferably comprises at least one
other polymer. In stage (b) of the process either a single-screw or
a twin-screw extruder may be used.
[0093] As far as stage (b) is concerned, when the composition
comprises polymeric components which, in the presence of water
content of the formulation, show a melting point below 100.degree.
C. preferred extrusion temperatures are comprised between 30 and
120.degree. C., more preferably between 40 and 100.degree. C.
[0094] Preferably stage (b) of the process is conducted imposing a
shear rate at the outlet: from the nozzle of between 20 and 700
s.sup.-1, more preferably between 100 and 600 s.sup.-1, and even
more preferably between 120 and 400 s.sup.-1,
[0095] On leaving the nozzle, the extrudate of stage (b), swell to
a diameter preferably from 3 to 8 times greater than the diameter
of the extruder die. After having been swollen, the extrudate
collapses preferably reaching a final diameter which lies between 2
and 3 times the diameter of the extruder die.
[0096] Stage (c) of the process is advantageously performed using a
blade cutting device, a so-called head cutter, directly on the
extruder nozzle, in such a way that the extrudate is cut when it is
not yet completely solidified.
[0097] Because of their specific structural characteristics the
biodegradable pellets according to this invention are capable of
expanding if subjected to irradiation by means of electromagnetic
waves such as microwaves, radio waves or infrared radiation. Of
these microwaves are preferred. The said irradiation may also be
advantageously accompanied by treatment to pressurise the
pellets.
[0098] Examples of equipment and processes for the production of
expanded articles by radiation are for example described in WO
02/14043, WO 03/037598 and WO2005/051628.
[0099] This invention also relates to foamed articles obtained from
pellets according to this invention. In fact because of their
specific structural characteristics the pellets according to this
invention may be used for the production of foamed articles having
mechanical properties, dimensions and a cell distribution and
density such as to enable them to be effectively used in the
protective packaging sector.
[0100] The foamed articles obtained from pellets according to this
invention preferably comprise starch essentially free from native
crystallisation in the meaning of this invention.
[0101] Preferably the foamed articles of the invention can reach a
density lower than 80, preferably lower than 60, more preferably
lower than 50 and even more preferably lower than 45 Kg/m3. Said
foamed materials show a cellular structure with walls having mean
thickness lower than 5 microns, preferably lower than 3 microns and
more preferably lower than 1.5 microns
[0102] As far as the wall thickness measurement is concerned, it
can be performed on a portion of foamed material whose internal
structure has been rendered accessible for examination. For this
purpose the foamed material which is to be examined is immersed in
liquid nitrogen and subsequently fractured so as to obtain a
fracture surface along a cross-section of the material. The portion
of the teamed material which is to be examined is then dried and a
thin layer of metal is deposited thereupon, for example a mixture
of gold/palladium, using a "sputter coater". Finally the surface of
the fracture is examined under a scanning electron microscope (SEM)
and thickness of the visible walls are measured.
[0103] By the term walls is meant here the non expanded material
interposed between two and no more than two cells.
[0104] The thickness of the walls is measured considering a
magnification that permits to observe in a single picture a number
of cells between 5 and 12. The thickness is measured (in
perpendicular direction) taking into consideration more than 10
points for every cell. The measure is repeated on 10 pictures if
the cells' distribution is not homogeneous. The mean values is
expressed as:
mean wall thickness = i = 1 N M i N ##EQU00005##
where: [0105] N is the number of measures which can be seen in the
SEM inside the fracture surface area examined, [0106] m.sub.1 is
the single measure on the will thickness.
[0107] The foamed articles according to the present invention show
a good homogeneous and compact surface as well as an intrinsic
viscosity of the foamed articles material in the range of 1.5-3
dl/g preferably of 1.9-2.8 dl/g. Such intrinsic viscosity values
being measured with the same method used for the pellets.
[0108] In particular the foamed articles obtained from pellets
according to this invention are particularly suitable for use as
protective packaging for domestic electrical articles, electronics
products, furniture, food packaging, agricultural packaging, for
example in the form of: [0109] foamed blocks and/or sheets, [0110]
foamed beads, [0111] sheet moulded packaging, [0112] multi-layer
cardboard packaging with foamed beads, blocks and/or sheets
according to the invention within it, [0113] combinations of foamed
blocks on cardboard, board, plastic sheet, etc. substrates, [0114]
loose fillers.
[0115] The foamed blocks and sheets may be used as such or further
shaped through cutting for example using hot wire, blades or
punches. They preferably have a density of less than 80 kg/m.sup.3,
more preferably less than 60 kg/m.sup.3, more preferably of 50
kg/m.sup.3 and even more preferably less than 45 kg/m.sup.3.
[0116] Multi-layer packaging is preferably a sandwich structure. In
this case the sandwich structure may be formed directly in the
process of expanding the pellets according to the invention.
[0117] As for as the loose fillers obtained according to this
invention are concerned, these may be used as such or as
agglomerates. In the case of agglomerates, they may be prepared
from loose fillers as such through the effect of adhesives and/or
humidity, including during the stage of packaging itself.
[0118] FIG. 1 shows the internal structure of the pellet according
to Example 1.
[0119] FIG. 2 shows the external skin of the pellet according to
Example 1.
[0120] FIG. 3 shows the foamed cylinder according to Example 2
(foamed at 55 kg/m.sup.3).
[0121] FIG. 4 shows the cell walls of the foamed block according to
Example 2 (foamed at 55 kg/m.sup.3).
[0122] FIG. 5 shows the foamed cylinder according to Comparative
Example 2 (foamed at 55 kg/m.sup.3).
[0123] FIG. 6 shows the internal structure of the pellet, according
to Comparative Example 3.
[0124] FIG. 7 shows the external skin of the pellet according to
Comparative Example 3.
[0125] FIG. 8 shows the pellets after irradiation according to
Comparative Example 4.
EXAMPLE 1
[0126] 30 kg/hour of a polymer composition comprising 61.8% by
weight of tapioca starch, 8.5% by weight of poly(vinyl alcohol)
having a level of hydrolysis of 87%, 0.2% by weight of tale as
nucleating agent and 29.5% by weight of water are fed to a
single-screw extruder having the following characteristics: [0127]
D=51 mm, [0128] L/D=8 [0129] RPM=350 [0130] Die diameter: 6 mm
[0131] Shear stress at the outlet 314 s.sup.-1, [0132] temperature
profile: 40-45-50-50.degree. C., [0133] T on the extruder
head=78.degree. C.
[0134] The extrudate, showing a swelling ratio of 5 (i.e. the ratio
between the extrudate diameter on leaving the nozzle and the
diameter of the extruder die), are then cut in form of pellets
using a blade cutting device directly on the extruder nozzle. The
pellets (5 minutes after the production) show a collapsing ratio
(i.e. the ratio between the extrudate diameter after collapsing and
the diameter of the extruder die) of 2.5 and a water content of
28.8%.
[0135] The pellets have been dried in oven at 50.degree. C. in
order to adjust the moisture content to 23.7% by weight and showed
the following properties: [0136] Density: 0.89 g/cm.sup.3 [0137]
Intrinsic viscosity: 2.43 g/dL [0138] An internal porous structure
with pore frequency (pore number/surface measured on SEM picture):
990/mm.sup.2, covering a surface of 28% (void area) and having a
mean equivalent diameter of 17 .mu.m. [0139] An external skin
showing few isolated pores with a mean equivalent diameter of 10
.mu.m
[0140] Pellets have been also analysed by differential scanning
calorimetry (DSC).
[0141] Approximately 2 grams of pellets were dried to a water
content of 8% (.+-.1%) in a ventilated stove at 50.degree. C. and
then ground at ambient temperature to a particle size of less than
200 microns.
[0142] 2159 mg of water (water/dry starch ratio=4) were added to
700 mg of the ground material and homogenised. Approximately 22 mg
of this mixture were then placed in a DSC analysis capsule
hermetically sealed.
[0143] The calorimeter (Perkin Elmer DSC Diamond) was set to
perform a single scan between 20 and 90.degree. C. with a
temperature gradient of 5.degree. C. min. The graph was not showing
detectable gelatinisation peaks.
EXAMPLE 2
Expansion by Irradiation in a Mould
[0144] 152 g of pellets (useful for getting a density of 55
kg/m.sup.3 in the foamed cylinder) prepared according to Example 1
were placed in a microwaves transparent mould (ULTEM.RTM.) with
internal volume of 2240 cm.sup.3 and provided with holes for steam
degassing. The mould with the pellets within was subsequently
placed in a microwave oven at atmospheric pressure having a power
of 16 kW and irradiated for approximately 20 seconds.
[0145] The foamed cylinder obtained, presented the following
properties: [0146] It fills completely the mould; [0147] The
pellets showed self-adhesion property (all the pellets adhere each
other); [0148] It has an intrinsic viscosity of 2.35 g/dL [0149] It
has a wall mean thickness of 0.7 .mu.m;
[0150] The article presents a three-dimensional structure that
fills completely the mould. These characteristics enable it to be
effectively used in the protective packaging sector.
EXAMPLE 3
Expansion by Irradiation in a Mould
[0151] Using the same conditions of Example 2, 22.1 g of pellets
obtained in Example 1 in order to obtain a foamed cylinder having
density of 80 kg/m.sup.3
EXAMPLE 4
Foam Characterization
[0152] The foams obtained in Examples 2 and 3 were characterized in
term of Dynamic Shock Cushioning Characteristic (measured according
to ASTM D1596) in comparison to commercial EPS having density of 26
kg/m3.
[0153] The comparison was performed at fragility factor of 100 G's.
in the following operative conditions: [0154] Drop Height: 80 cm;
[0155] Sample diameter: 105 mm [0156] Thickness: 50 mm and 75 mm
[0157] Temperature 23.degree. C., [0158] Relative Humidity 50%
[0159] The following results were obtained:
TABLE-US-00001 Thickness 50 mm 75 mm Material Static Stress Static
Stress (average 2.sup.nd 5.sup.th impact) (kg/cm.sup.2)
(kg/cm.sup.2) EPS--density 26 kg/m.sup.3 0.07 0.18 Example 2 0.10
0.22 Example 3 0.22 0.45
COMPARATIVE EXAMPLE 1
Starch is Not Essentially Free of Native Crystallinity
[0160] 30 kg/hour of a polymer composition comprising 61.8% by
weight of tapioca starch, 8.5% by weight of poly(vinyl alcohol)
having a level of hydrolysis of 87%, 0.2% by weight of talc as
nucleating agent and 29.5% by weight of water were fed to a
single-screw extruder having the following characteristics: [0161]
D=51 mm, [0162] L/D=8, [0163] RPM=250 [0164] Die diameter: 8.5 mm
[0165] Shear stress as the outlet 111 s.sup.-1, [0166] temperature
profile: 40-45-50-50.degree. C. [0167] T on the extruder
head=78.degree. C.
[0168] The extrudate so obtained, that showed a swelling ratio of
2.35, was then cut into the form of pellets using a blade cutting
device, a so-called head cutter, directly on the extruder
nozzle.
[0169] The pellets (after 5 minutes after the production) had a
collapsing ratio of 2 and a water content of 27%.
[0170] The pellets were dried in oven at 50.degree. C. in order to
adjust the moisture content to 22.7% by weight.
[0171] The pellets so obtained had the following properties: [0172]
Density of 0.9 g/cm.sup.3 [0173] Intrinsic viscosity: 2.25 g/dL
[0174] A porous structure with density of pores (measured on SEM
picture): 1375/mm.sup.2, covering a surface of 26% and having a
mean diameter of 13 .mu.m. [0175] A skin having isolated pores with
a mean diameter of 9 .mu.m
[0176] These were also analysed by differential scanning
calorimetry (DSC). Approximately 2 grams of pellets were dried to a
water content of 8% (.+-.1%) in a ventilated stove at 50.degree. C.
and then ground at ambient temperature to a particle size of less
than 200 microns.
[0177] For the DSC analysis 2159 mg of water (water/dry starch
ratio=4) were added to 700 mg of the around material and all was
homogenised. Approximately 22 mg of this mixture were then placed
in a DSC analysis capsule which was then hermetically sealed.
[0178] The graph obtained shows an endothermic peak with a .DELTA.H
of 2.1 J/g of dry starch confirming a significant presence of
residual native crystallinity in the starch.
COMPARATIVE EXAMPLE 2
Expansion by Irradiation in a Mould
[0179] 152 g of the pellets (needed to reach a density of 55
kg/m.sup.3 on the foam cylinder) obtained in Example comparative 1
were placed in a mould produced from a polymer which is transparent
to microwaves (ULTEM.RTM.) having internal dimensions and an
internal volume of 2240 cm.sup.3 and provided with holes for the
degassing of steam. The mould with the particles within was
subsequently placed in a microwave oven at atmospheric pressure
having a power of 16 kW and irradiated for approximately 20
seconds.
[0180] The foam cylinder obtained, presented the following
properties: [0181] It fills not completely the mould [0182] It
presents a limited adhesion with some pellets detached from the
cylinder [0183] It has an intrinsic viscosity of 2.13 g/dL
COMPARATIVE EXAMPLE 3
Not Porous Internal Structure is Present
[0184] 30 kg/hour of a polymer composition comprising 61.8% by
weight of corn starch, 8.5% by weight of poly(vinyi alcohol) having
a level of hydrolysis of 87%, 0.2% by weight of talc as nucleating
agent and 29.5 % by weight of water were fed to a single-serew
extruder having the following characteristics: [0185] D=51 mm,
[0186] L/D=8, [0187] RPM=350 [0188] Die diameter: 6 mm [0189]
temperature profile: 40-45-50-50.degree. C., [0190] T on the
extruder head=82.degree. C.,
[0191] The extrudate so obtained, that showed a swelling ratio of
2.0, was then cut into the form of pellets using a blade cutting
device, a so-called head cutter, directly on the extruder
nozzle.
[0192] The pellets (after 5 minutes after the production) had a
collapsing ratio of 2 and a water content of 29.8%.
[0193] The pellets were dried in oven at 50.degree. C. in order to
adjust the moisture content to 22.3% by weight.
[0194] The pellets so obtained had the following properties: [0195]
Density of 1.08 g/cm.sup.3 [0196] Intrinsic viscosity: 1.7 g/dL
[0197] A porous structure with density of pores (measured on SEM
picture): 954/mm.sup.2, covering a surface of 10% and having a mean
diameter of 10 .mu.m. [0198] A lacerated skin having several not
isolated and interconnected pores with a mean diameter of 36.7
.mu.m.
[0199] These were also analysed by differential scanning
calorimetry (DSC). Approximately 2 grams of pellets were dried to a
water content of 8% (.+-.1%) in a ventilated stove at 50.degree. C.
and then ground at ambient temperature to a particle size of less
than 200 microns.
[0200] For the DSC analysis 2159 mg of water (water/dry starch
ratio=4) were added to 700 mg of ground material and all was
homogenised. Approximately 22 mg of this mixture were then placed
in a DSC analysis capsule which was then hermetically sealed.
[0201] The graph obtained did not show any detectable
gelatinisation peak
COMPARATIVE EXAMPLE 4
Expansion by Irradiation in a Mould
[0202] 152 g of the pellets (needed to reach a density of 55
kg/m.sup.3 on the foam cylinder) obtained in Example comparative 3
were placed in a mould produced from a polymer which is transparent
to microwaves (ULTEM.RTM.) having internal dimensions and an
internal volume of 2240 cm.sup.3 and provided with holes for the
degassing of steam. The mould with the particles within was
subsequently placed in a microwave oven at atmospheric pressure
having a power of 16 kW and irradiated for approximately 20
seconds.
[0203] The product after irradiation, presented the following
properties: [0204] It isn't foamed [0205] Pellets do not adhere
each other [0206] It has an intrinsic viscosity of 1.7 g/dL
* * * * *