U.S. patent application number 11/442844 was filed with the patent office on 2007-05-31 for microcellular foam of thermoplastic resin prepared with die having improved cooling property and method for preparing the same.
Invention is credited to Ki-Deog Choi, Bong-Keun Lee, Kyung-Jip Min, Kyung-Gu Nam, Jong-Sung Park, Seon-Mo Son.
Application Number | 20070123598 11/442844 |
Document ID | / |
Family ID | 38088377 |
Filed Date | 2007-05-31 |
United States Patent
Application |
20070123598 |
Kind Code |
A1 |
Nam; Kyung-Gu ; et
al. |
May 31, 2007 |
Microcellular foam of thermoplastic resin prepared with die having
improved cooling property and method for preparing the same
Abstract
The present invention relates to a microcellular foam of a
thermoplastic resin and a method for preparing the same, and more
particularly to a microcellular foam comprising a skin layer having
a porosity of below 5% and a core layer having a porosity of at
least 5%, wherein the thickness of the skin layer accounts for 5 to
50% of the entire foam, and a method for preparing the same. The
microcellular foam of the present invention is advantageous in that
it has a thicker skin layer and smaller and uniform micropores in
the core layer, compared with conventional microcellular foams,
while having mechanical properties comparable to those of
conventional non-foamed sheets.
Inventors: |
Nam; Kyung-Gu;
(Daejeon-city, KR) ; Choi; Ki-Deog; (Daejeon-city,
KR) ; Park; Jong-Sung; (Daejeon-city, KR) ;
Son; Seon-Mo; (Daejeon-city, KR) ; Lee;
Bong-Keun; (Daejeon-city, KR) ; Min; Kyung-Jip;
(Seoul, KR) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Family ID: |
38088377 |
Appl. No.: |
11/442844 |
Filed: |
May 30, 2006 |
Current U.S.
Class: |
521/79 |
Current CPC
Class: |
B29C 48/82 20190201;
B29C 48/845 20190201; B29C 48/87 20190201; B29C 48/08 20190201;
B29K 2105/04 20130101; B29C 48/40 20190201; B29C 48/875 20190201;
B29C 48/525 20190201; B29C 48/06 20190201; B29C 48/12 20190201;
B29C 48/022 20190201; C08J 2201/03 20130101; B29C 48/865 20190201;
B29C 48/404 20190201; B29C 48/86 20190201; B29C 44/3419 20130101;
C08J 9/34 20130101; B29C 48/395 20190201 |
Class at
Publication: |
521/079 |
International
Class: |
C08J 9/00 20060101
C08J009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2005 |
KR |
10-2005-0115637 |
Nov 30, 2005 |
KR |
10-2005-0115638 |
Claims
1. Microcellular foam comprising a skin layer having porosity,
which is defined in Equation 1 below, of below 5%, and a core layer
having porosity of at least 5%, wherein the thickness of the skin
layer accounts for 5 to 50% of the entire thickness of the foam:
Porosity(%)=(.rho..sub.N-.rho..sub.F)/.rho..sub.N.times.100
[Equation 1] where .rho..sub.N is the density of a non-foamed
portion and .rho..sub.F is the density of a foamed portion.
2. The microcellular foam of claim 1, wherein the foam is a sheet,
a ""-shaped cross-sectional body, or a chassis having a chamber
inside thereof.
3. The microcellular foam of claim 2, wherein the cross-sectional
thickness of the foam is from 0.5 to 5 mm.
4. The microcellular foam of claim 1, wherein the skin layer has an
average thickness of 50 to 500 .mu.m.
5. The microcellular foam of claim 1, wherein the overall porosity
of the foam is from 5 to 80%.
6. The microcellular foam of claim 1, wherein the overall porosity
of the foam is from 15 to 30%, and the impact absorption energy of
the foam measured by a rheometric drop test in accordance with ASTM
D4226 is at least 70% of that of a non-foamed counterpart prepared
under comparable conditions.
7. The microcellular foam of claim 6, wherein the overall porosity
of the foam is from 15 to 30%, and the impact absorption energy of
the foam measured by a rheometric drop test in accordance with ASTM
D4226 is 90 to 150% of that of a non-foamed counterpart prepared
under comparable conditions.
8. The microcellular foam of claim 1, wherein the core layer has
pores having an average diameter of 0.1 to 50 .mu.m.
9. The microcellular foam of claim 1, wherein the overall porosity
of the foam is from 15 to 30%, and the elongation of the foam
measured in accordance with ASTM D638 is at least 70% of that of a
non-foamed counterpart prepared under comparable conditions.
10. The microcellular foam of claim 9, wherein the overall porosity
of the foam is from 15 to 30%, and the elongation of the foam
measured in accordance with ASTM D638 is 90 to 150% of that of a
non-foamed counterpart prepared under comparable conditions.
11. The microcellular foam of claim 1, wherein the overall porosity
of the foam is from 15 to 30%, and the tensile strength of the foam
measured in accordance with ASTM D638 is at least 70% of that of a
non-foamed counterpart prepared under comparable conditions.
12. The microcellular foam of claim 11, wherein the overall
porosity of the foam is from 15 to 30%, and the tensile strength of
the foam measured in accordance with ASTM D638 is from 90 to 150%
of that of a non-foamed counterpart prepared under comparable
conditions.
13. The microcellular foam of claim 1, which comprises at least one
polymer selected from the group consisting of an
acrylonitrile-butadiene-styrene (ABS) copolymer, polycarbonate
(PC), polyvinyl chloride (PVC), polystyrene (PS), polymethyl
methacrylate (PMMA), polyester, polypropylene (PP), and nylon.
14. The microcellular foam of claim 1, which is an
interior/exterior construction material.
15. The microcellular foam of claim 1, which is prepared by a
process comprising the steps of: a) mixing a plasticized
thermoplastic polymer resin with a foaming agent using an extruder;
b) forming micropores by passing the plasticized mixture through a
pressure drop region; and c) cooling the melted mixture in which
the micropores have been formed by passing it through a cooling
region, by using an extrusion die comprising both the pressure drop
region and the cooling region, wherein a temperature difference at
the end of the pressure drop region and the beginning of the
cooling region is 30 to 200.degree. C.
16. The microcellular foam of claim 15, wherein the extrusion die
further comprises a temperature change region between the pressure
drop region and the cooling region, wherein a temperature change
rate in the temperature change region defined by Equation 2 below
is 2 to 40.degree. C./mm: T.sub.L=(T.sub.h-T.sub.c)/L [Equation 2]
where T.sub.L is the temperature change rate, T.sub.h is the
temperature at the end of the pressure drop region, T.sub.c is the
temperature at the beginning of the cooling region, and L is the
length of the temperature change region.
17. A preparation method of a microcellular foam comprising the
steps of: a) mixing a plasticized thermoplastic polymer resin with
a foaming agent using an extruder; b) forming micropores by passing
the plasticized mixture through a pressure drop region; and c)
cooling the melted mixture in which the micropores have been formed
by passing it through a cooling region, wherein a temperature
difference at the end of the pressure drop region and the beginning
of the cooling region is 30 to 200.degree. C.
18. The preparation method of claim 17, wherein the extrusion die
comprises a heating means at the end of the pressure drop region
for preventing a temperature decrease.
19. The preparation method of claim 17, wherein the extrusion die
comprises a cooling means at the beginning of the cooling region
for preventing a temperature increase.
20. The preparation method of claim 17, wherein the temperature at
the end of the pressure drop region is 150 to 250.degree. C.
21. The preparation method of claim 17, wherein the temperature at
the beginning of the cooling region is 40 to 150.degree. C.
22. The preparation method of claim 17, wherein the temperature
change of the pressure drop region and the cooling region is
maintained to be within .+-.5.degree. C.
23. The preparation method of claim 17, wherein the transfer rate
of the thermoplastic polymer resin is 0.5 to 20 m/min.
24. The preparation method of claim 17, wherein a temperature
change region is present between the pressure drop region and the
cooling region, and the temperature change rate defined by Equation
2 below at the temperature change region is 2 to 40.degree. C/mm:
T.sub.L=(T.sub.h-T.sub.c)/L [Equation 2] where T.sub.L is the
temperature change rate, T.sub.h is the temperature at the end of
the pressure drop region, T.sub.c is the temperature at the
beginning of the cooling region, and L is the length of the
temperature change region.
25. The preparation method of claim 24, wherein the length of the
temperature change region is 1 to 150 mm.
26. The preparation method of claim 17, wherein the thermoplastic
polymer resin comprises at least one polymer selected from the
group consisting of an acrylonitrile-butadiene-styrene (ABS)
copolymer, polycarbonate (PC), polyvinyl chloride (PVC),
polystyrene (PS), polymethyl methacrylate (PMMA), polyester,
polypropylene, and nylon.
Description
CROSS REFERENCES TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application Nos. 10-2005-0115637, and
10-2005-0115638, both filed in the Korean Industrial Property
Office on Nov. 30, 2005, and both of which are hereby incorporated
by references for all purpose as if fully set forth herein.
FIELD OF THE INVENTION
[0002] The present invention relates to a microcellular foam of a
thermoplastic resin and a method for preparing the same, and more
particularly, to a microcellular foam of a thermoplastic resin
having a low specific gravity but mechanical properties comparable
to those of conventional non-foamed sheets and a method for
preparing the same.
BACKGROUND OF THE INVENTION
[0003] Foams used for thermal insulation, sound absorption,
buoyancy, elasticity, weight reduction, soundproofing, etc. in
soundproofing materials, heat insulating materials, construction
materials, lightweight structural frames, packing materials,
insulating materials, cushions, vibration-proof materials, shoes,
etc., are produced by using physical or chemical foaming
agents.
[0004] Examples of physical foaming agents are carbon dioxide,
nitrogen, hydrofluorocarbon, etc., and examples of chemical foaming
agents are gas-producing organic materials like
azodicarbonamide.
[0005] According to U.S. Pat. No. 6,225,365, superior foams can be
obtained with physical foaming agents, with no residue at all,
whereas chemical foaming agents leave residues in the foam after
their decomposition. However, the resultant foams tend to have poor
mechanical strength and toughness because of their large pore size
(about 100 .mu.m or larger) and high porosity (about 50% or
higher).
[0006] In order to solve this problem, microcellular foams having
large pore density and small pore size were developed as disclosed
in U.S. Pat. No. 4,473,665.
[0007] Many other methods for continuously producing foams having
microstructure have been proposed. U.S. Pat. No. 5,866,053
discloses a continuous process for producing microcellular foams,
characterized in that a nucleated stream is created by rapidly
lowering the pressure of a single-phase solution comprising a
foaming agent and a polymer, and in which the rate of nucleation is
maintained sufficiently high to obtain a microcellular structure in
the final product.
[0008] Korean Patent Publication No. 2004-34975 discloses a method
of producing microporous fibers characterized by the steps of
preparing a single-phase polymer melt-gas solution with a uniform
concentration by melting a fiber-forming polymer in an extruder and
feeding a supercritical gas into the extruder, preparing
microporous materials through a rapid pressure drop, rapidly
cooling the microporous materials with a coolant, and rolling the
resultant fiber at a rate of 10 to 6,000 m/min, so that the
spinning draft becomes 2 to 300.
[0009] Japanese Patent No. 3,555,986 discloses a method of
producing thermoplastic resin foams having fine and uniform
micropores comprising the steps of impregnating an inert gas or a
foaming agent into a thermoplastic resin which has been melted by a
first extruder and a mixer attached to it, cooling the melted resin
while maintaining the applied pressure using a second extruder,
forming many pore nuclei through a rapid pressure drop, and
controlling the pore diameter uniformly.
[0010] Japanese Laid-Open Patent Publication No. 2004-322341
discloses a method of producing microcellular foams comprising the
steps of melting a molding material comprising a crystalline
thermoplastic resin, mixing the melted molding material with an
inert fluid, and extruding the mixture of the inert fluid and the
molding material at a temperature that is 0.5 to 5.degree. C.
higher than the crystallization temperature.
[0011] Japanese Laid-Open Patent Publication No. 2004-338396
discloses an extrusion foaming method of producing microcellular
foams comprising the steps of melting a molding material comprising
a thermoplastic resin, mixing the melted molding material with an
inert fluid, extruding the mixture of the inert fluid and the
molding material at a temperature that is higher than the setting
temperature so that foam is not practically formed or it is formed
in a small amount at the instant of extrusion, and applying an
external force to the extruded molding material.
[0012] However, all the products produced from the above-mentioned
patents have mechanical properties that are poorer than those of
non-foamed counterparts.
SUMMARY OF THE INVENTION
[0013] The present invention was made to solve the above problems,
and an object of the present invention is to provide a
microcellular foam having a low specific gravity and mechanical
properties that are comparable to those of a non-foamed counterpart
with a thick skin layer and a controlled size and distribution of
micropores in the core layer.
[0014] Another object of the present invention is to provide a
method of preparing such a microcellular foam by rapidly changing
the temperature when a pressure drop is finished and cooling begins
to increase the thickness of the skin layer and to control the size
and distribution of micropores in the core layer.
[0015] To attain the objects, the present invention provides a
microcellular foam comprising a skin layer having a porosity of
below 5% and a core layer having a porosity of at least 5%, wherein
the thickness of the skin layer accounts for 5 to 50% of the total
thickness.
[0016] The present invention also provides a method of preparing a
microcellular foam comprising the steps of a) mixing a plasticized
thermoplastic polymer resin with a foaming agent using an extruder,
b) forming micropores by passing the plasticized mixture through a
pressure drop region of an extrusion die, and c) cooling the melted
mixture by passing it through a cooling region of an extrusion die,
wherein a temperature difference at the end of the pressure drop
region and the beginning of the cooling region is 30 to 200.degree.
C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] A more complete appreciation of the invention, and many of
the attendant advantages thereof, will be readily apparent as the
same becomes better understood by reference to the following
detailed description when considered in conjunction with the
accompanying drawings, wherein:
[0018] FIG. 1 is a cross-sectional view of an exemplary extrusion
die comprising a pressure drop region, a temperature change region,
and a cooling region.
[0019] FIG. 2 is a cross-sectional view of an exemplary extrusion
die comprising a pressure drop region, a temperature change region,
and a cooling region along with a plurality of cooling means and
heating means.
[0020] FIG. 3 is a construction diagram of an extruding apparatus
for preparing the microcellular foam of the present invention.
[0021] FIG. 4 is a cross-sectional view of the extrusion die used
to prepare the microcellular foams of Comparative Examples 1 and
2.
[0022] FIG. 5 is a cross-sectional view of the extrusion die used
to prepare the microcellular foam of Comparative Example 3.
[0023] FIG. 6 is a scanning electron micrograph showing the
cross-section of the microcellular foam sheet of Example 1.
[0024] FIG. 7 is a scanning electron micrograph showing the
cross-section of the foam sheet of Comparative Example 3.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The microcellular foam of the present invention comprises a
skin layer, which is thicker than conventional microcellular foams,
and a core layer in which micropores are formed. In the present
invention, the porosity is calculated by Equation 1 below. The
"skin layer" is defined as a portion having a porosity of below 5%
and the "core layer" is defined as a portion having a porosity of
at least 5%. Preferably, the core layer has a porosity of 5 to 90%
in order to ensure superior mechanical properties.
Porosity(%)=(.rho..sub.N-.rho..sub.F)/.rho..sub.N.times.100
Equation 1 where .rho..sub.N is the density of a non-foamed portion
and .rho..sub.F is the density of a foamed portion.
[0026] Preferably, in the microcellular foam of the present
invention, the thickness of the skin layer accounts for 5 to 50%,
more preferably 10 to 40%, of the total thickness of the
microcellular foam. If the thickness of the skin layer is less than
5% of the total thickness of the microcellular foam, such
mechanical properties as elongation may be poor. In contrast, if it
exceeds 50%, it is difficult to obtain a desirable decrease in
specific gravity.
[0027] The shape or configuration of the microcellular foam of the
present invention is not particularly limited, but it is preferable
that the foam is a sheet, a ""-shaped cross-sectional body, or a
chassis having a chamber inside thereof. Since the microcellular
foam can be prepared into a suitable thickness depending on the
purpose, the thickness of the cross-section of the microcellular
foam is not particularly limited, but a thickness of 0.5 to 5 mm is
preferable.
[0028] Also, in the microcellular foam of the present invention,
the skin layer preferably has an average thickness of 50 to 500
.mu.m. If the thickness of the skin layer is smaller than 50 .mu.m,
mechanical properties may be not good. In contrast, if it exceeds
500 .mu.m, it is difficult to obtain a desirable decrease in
specific gravity.
[0029] Preferably, the microcellular foam of the present invention
has an average porosity of 5 to 80% and more preferably 10 to 70%,
and particularly preferable is a range from 15 to 30%. If the
average porosity of the microcellular foam is below 5%, the foam
cannot normally function as microcellular foam. In contrast, if it
exceeds 80%, the excessive porosity may worsen physical properties
of the microcellular foam.
[0030] When the average porosity of the microcellular foam ranges
from 15 to 30%, it is preferable that the impact energy absorption
measured by rheometric drop test according to ASTM D4226 is at
least 70%, and more preferably 90 to 150%, of that of the
non-foamed counterpart. Although, the higher the impact energy
absorption the better, it is practically difficult to obtain an
impact energy absorption higher than 150% of that of the non-foamed
counterpart.
[0031] Preferably, the pores formed in the core layer of the
microcellular foam have an average diameter of 0.1 to 50 .mu.m, and
more preferably 1 to 30 .mu.m. The smaller the pore size, the more
improved the physical properties of the microcellular foam.
However, it is difficult to form micropores having a diameter
smaller than 0.1 .mu.m. If the average diameter of the pores
exceeds 50 .mu.m, the mechanical properties tend to be poor.
[0032] The microcellular foam of the present invention comprises a
thermoplastic resin that is capable of forming foam, and it is
preferably at least one polymer selected from the group consisting
of an acrylonitrile-butadiene-styrene (ABS) copolymer,
polycarbonate (PC), polyvinyl chloride (PVC), polystyrene (PS),
polymethyl methacrylate (PMMA), polyester, polypropylene, and
nylon, and it is more preferably at least one polymer selected from
the group consisting of an acrylonitrile-butadiene-styrene (ABS)
copolymer, polycarbonate (PC), polyvinyl chloride (PVC), and
polystyrene (PS).
[0033] The microcellular foam of the present invention preferably
has an elongation measured in accordance with ASTM D638 of at least
70%, and more preferably 90 to 150%, of that of the non-foamed
counterpart. The larger the elongation of the foam the better, but
it is practically difficult to obtain an elongation exceeding 150%
of that of the non-foamed counterpart. If the elongation is less
than 70% of that of the non-foamed counterpart, the foam cannot be
utilized.
[0034] And, preferably, the microcellular foam of the present
invention has a tensile strength measured in accordance with ASTM
D638 of at least 70%, and more preferably 90 to 150%, of that of
the non-foamed counterpart produced in the comparable condition.
The higher the tensile strength of the microcellular foam the
better, but it is practically difficult to obtain a tensile
strength exceeding 150% of that of the non-foamed counterpart. If
the tensile strength falls short of 70% of that of the non-foamed
counterpart, the foam cannot be utilized because of poor physical
properties.
[0035] The microcellular foam of the present invention may be
utilized as an interior/exterior construction material, an optical
reflection plate of a display device, etc. It is suitable for use
as an interior/exterior construction material, particularly as a
soundproofing material, a heat insulating material, a construction
material, a light structural material, a packing material, an
insulating material, a cushioning material, a vibration-proof
material, etc.
[0036] The method of preparing a microcellular foam in accordance
with the present invention comprises the steps of a) mixing a
plasticized thermoplastic polymer resin with a foaming agent using
an extruder, b) forming micropores by passing the plasticized
mixture through a pressure drop region of an extrusion die, and c)
cooling the melted mixture in which the micropores are formed by
passing it through a cooling region of an extrusion die.
[0037] Preferably, the temperature difference between the end of
the pressure drop region and the beginning of the cooling region is
maintained at 30 to 200.degree. C., and more preferably at 50 to
150.degree. C. If the temperature difference is smaller than
30.degree. C., the micropores formed in the pressure drop region
continue to grow and it is difficult to obtain a skin layer that is
thick enough for a foam. In contrast, if the temperature difference
exceeds 200.degree. C., rapid solidification interferes with a
smooth preparation process.
[0038] The pressure drop region and the cooling region may be
present in a single extrusion die or may be present in separate
block-type extrusion dies. Preferably, the regions are present in a
single extrusion die for efficient control of the micropores and
formation of the skin layer. When the regions are present in
separate extrusion dies, it is preferable that they are strongly
connected, so that the pressure at the end of the pressure drop
region is maintained at the cooling region.
[0039] The extrusion die may further comprise a heating means to
prevent a temperature decrease at the end of the pressure drop
region. The heating means may be present inside of the pressure
drop region of the extrusion die or at both the inside and outside
of the pressure drop region of the extrusion die.
[0040] The heating means may be a common electric heater, but is
not particularly limited in the present invention.
[0041] The extrusion die may comprise a cooling means to prevent a
temperature increase at the beginning of the cooling region. Like
the heating means, the cooling means is also preferably present
inside the cooling region of the extrusion die, but it may also be
present at both the inside and outside of the cooling region of the
extrusion die.
[0042] The cooling means may be a pipe in which a coolant flows,
but is not particularly limited in the present invention.
[0043] FIG. 1 is a cross-sectional view of an exemplary extrusion
die 10 comprising a pressure drop region 11, a temperature change
region 12, and a cooling region 13. A nozzle 14 is present inside
of the extrusion die 10 along the extrusion direction. The actual
pressure drop occurs at the end of the nozzle.
[0044] The extrusion die comprises a heating means 15 for
maintaining the pressure drop region at a specific temperature and
a cooling means 17 for maintaining the temperature of the cooling
region. However, the construction of the extrusion die used in the
present invention is not limited to that shown in FIG. 1.
[0045] In the preparation method in accordance with the present
invention, the temperature at the end of the pressure drop region
may be adjusted depending on the particular thermoplastic resin
used, but a temperature of 150 to 250.degree. C. is preferable. If
the temperature at the end of the pressure drop region is below
150.degree. C., not enough micropores may be formed. In contrast,
if it exceeds 250.degree. C., deterioration of the thermoplastic
resin or over-foaming may occur.
[0046] In addition, the temperature at the beginning of the cooling
region may also be adjusted depending on the particular
thermoplastic resin used. A temperature that is slightly higher
than the melting point or softening point of the thermoplastic
resin is preferable, and a temperature of 40 to 150.degree. C. is
more preferable. If the temperature at the beginning of the cooling
region is below 40.degree. C., rapid solidification may hinder a
smooth preparation process. In contrast, if it exceeds 150.degree.
C., the micropores formed in the pressure drop region continue to
grow in the cooling region, making it difficult to obtain a
sufficiently thick skin layer.
[0047] It is particularly preferable that the temperature change of
the pressure drop region and the cooling region is maintained
within .+-.5.degree. C., and more preferably within .+-.2.degree.
C. If the temperature change of the pressure drop region and the
cooling region exceeds .+-.5.degree. C., uniform extrusion becomes
difficult, and thus it is difficult to attain good mechanical
properties.
[0048] The transfer rate of the thermoplastic polymer resin in the
pressure drop region and the cooling region is not particularly
limited as long as normal processing is possible, but a rate of 0.5
to 20 m/min is preferable.
[0049] A temperature change region may be present between the
pressure drop region and the cooling region, and a rapid
temperature change occurs in the temperature change region while
heat exchange between the pressure drop region and the cooling
region is prevented. It is preferable that the temperature change
rate in the temperature change region, which is defined in Equation
2 below, is at least 2.degree. C./mm, more preferably from 3 to
40.degree. C./mm. The higher the temperature change rate, the
better. If the temperature change rate is below 2.degree. C./mm,
the effect of control of the micropores in the cooling region
becomes only slight. T.sub.L=(T.sub.h-T.sub.c)/L Equation 2
[0050] where T.sub.L is the temperature change rate, T.sub.h is the
temperature at the end of the pressure drop region, T.sub.c is the
temperature at the beginning of the cooling region, and L is the
length of the temperature change region.
[0051] A shorter length of the temperature change region is favored
since more abrupt temperature change is possible, but a length of 1
to 150 mm is preferable. If the length of the temperature change
region exceeds 150 mm, temperature change between the pressure drop
region and the cooling region becomes gradual and it is not good
for preparation of the microcellular foam.
[0052] Preferably, the pressure drop region, the temperature change
region, and the cooling region are present in a single extrusion
die. In particular, the extrusion die preferably comprises a
heating means at the end of the pressure drop region in order to
prevent a temperature decrease and a cooling means at the beginning
of the cooling region in order to prevent a temperature increase.
Details of the heating means and the cooling means are the same as
described above. The temperature change region may be defined as
the region between the heating means and the cooling means.
[0053] FIG. 2 is a cross-sectional view of an extrusion die 20 in
which a plurality of heating means 25, 26 and cooling means 27, 28
have been added to enhance the effect of the heating means and the
cooling means. It is also preferable that the pressure drop region
21, the temperature change region 22, and the cooling region 23 are
present in a single die. However, separate block-type extrusion
dies may be used as long as the internal pressure is maintained. A
nozzle 24 is located inside of the extrusion die 20 along the
extrusion direction.
[0054] The heating means and the cooling means may be added as
required. The construction of the extrusion die used in the
preparation method in accordance with the present invention is not
limited to that shown in FIG. 2.
[0055] The thermoplastic polymer resin may be any thermoplastic
resin that is capable of forming foam. Preferably, it comprises at
least one polymer selected from the group consisting of an
acrylonitrile-butadiene-styrene (ABS) copolymer, polycarbonate
(PC), polyvinyl chloride (PVC), polystyrene (PS), polymethyl
methacrylate (PMMA), polyester, polypropylene (PP), and nylon. More
preferably, it comprises at least one polymer selected from the
group consisting of an acrylonitrile-butadiene-styrene (ABS)
copolymer, polycarbonate (PC), polyvinyl chloride (PVC), and
polystyrene (PS).
[0056] Preferably, the foaming agent used in the present invention
is an inert gas, and is more preferably carbon dioxide, nitrogen,
or a mixture thereof. Also preferably, the mixing proportion of the
foaming agent to the thermoplastic resin is 3-0.1 to 97-99.9 based
on weight. If the content of the foaming agent falls short of 0.1
part by weight, sufficient foaming does not occur in the pressure
drop region, and thus micropores are not formed. In contrast, if it
exceeds 3 parts by weight, the foam is not melted in the resin and
thus becomes useless.
[0057] Preferably, the foaming agent is mixed in a supercritical
state. In a supercritical state, the foaming agent has better
compatibility with the polymer resin and enables formation of
uniform pores inside the resin, thereby reducing pore size and
increasing pore density. The foaming agent may be fed in the
supercritical state or may be transformed to the supercritical
state after being fed to the extruder.
[0058] For example, carbon dioxide has a critical pressure of 75.3
kgf/cm.sup.2 and a critical temperature of 31.35.degree. C.
Nitrogen has a critical pressure of 34.6 kgf/cm.sup.2 and a
critical temperature of -147.degree. C. In general, the transition
of the gas inside the extruder to the supercritical state
preferably takes place at a pressure of 70 to 400 kgf/cm.sup.2 and
a temperature of 100 to 400.degree. C.
[0059] The condition for transition of nitrogen to the
supercritical state can be adjusted depending on the particular
foaming agent used, and is not particularly limited in the present
invention.
[0060] Hereinafter, the present invention is described in further
detail through examples. However, the following examples are only
for the understanding of the present invention and the present
invention is not limited to or by them.
EXAMPLES
Example 1
[0061] An extrusion apparatus 30 was prepared by attaching an
extrusion die 34 that is capable of temperature control, which
comprises a pressure drop region 31, a temperature change region
32, and a cooling region 33, and an adapter 35, to a twin screw
extruder 36 (Gottfert Extrusiometer 350), as in FIG. 3. The lengths
of the pressure drop region 31, the temperature change region 32,
and the cooling region 33 of the extrusion die were 125 mm, 27 mm,
and 40 mm, respectively.
[0062] 98 parts by weight of a rigid polyvinyl chloride (PVC)
compound (LG Chem) used for interior/exterior housing and
construction materials was added to the extruder. After the PVC was
completely plasticized, 2 parts by weight of nitrogen was added to
the barrel 4 of the extruder using a high-pressure pump. The
resultant single-phase mixture was foamed to obtain a microcellular
foam sheet 2 mm thick and 100 mm wide.
[0063] The temperature of the barrel 1 was maintained at
190.degree. C., that of barrels 2 to 4 at 180.degree. C., and that
of barrel 5 at 175.degree. C. The temperature of the adapter was
maintained at 135.degree. C.
[0064] The temperatures of the pressure drop region, the
temperature change region, and the cooling region were maintained
as given in Table 1 below.
Examples 2 and 3
[0065] A microcellular foam sheet was prepared in the same manner
as in Example 1 except that the temperatures of the pressure drop
region, the temperature change region, and the cooling region were
changed as given in Table 1.
Example 4
[0066] A microcellular foam sheet was prepared in the same manner
as in Example 1 except that the temperatures of the pressure drop
region, the temperature change region, and the cooling region were
changed as given in Table 1, and a die that produces a 1 mm-thick
sheet was used.
Comparative Example 1
[0067] A foam sheet was prepared in the same manner as in Example
1, except that a foaming agent was not used, and an extrusion die
40 comprising only a pressure drop region with a nozzle 44 and
heating means 45, 46, without a temperature change region or a
cooling region, was used, as shown in FIG. 4.
Comparative Example 2
[0068] A foam sheet was prepared in the same manner as in Example
1, except that an extrusion die 40 comprising only a pressure drop
region with a nozzle 44 and heating means 45, 46, without a
temperature change region or a cooling region, was used, as shown
in FIG. 4.
Comparative Example 3
[0069] A foam sheet was prepared in the same manner as in Example
1, except that a foaming agent was not used, and an extrusion die
50 comprising a pressure drop region 51, a temperature change
region 52, and a cooling region 53, wherein a nozzle 54 is located
inside the pressure drop region, a heating means 55 is located
outside of the pressure drop region, and cooling means 57, 58 are
located inside of the cooling region, was used, as shown in FIG. 5.
TABLE-US-00001 TABLE 1 Temperature Pressure drop Temperature
Cooling (.degree. C.) region change region region Position
Beginning End Beginning End Beginning End Example 1 165 165 165 52
52 50 Example 2 175 175 175 45 45 43 Example 3 160 160 160 50 50 50
Example 4 177 177 177 77 77 74 Comparative 180 180 -- -- -- --
Example 1 Comparative 180 180 -- -- -- -- Example 2 Comparative 170
155 145 100 91 75 Example 3
Testing Example
[0070] Physical properties of the sheets prepared in Examples 1 to
4 and Comparative Examples 1 to 3 were tested as follows. The
result is given in Table 2 below.
[0071] 1. Specific gravity: Specific gravity of the entire sheet
was measured in accordance with ASTM D792.
[0072] 2. Porosity, pore size, and thickness of skin layer:
Measured using a scanning electron microscope (SEM) along the
cross-section of the sheet.
[0073] FIG. 6 is a scanning electron micrograph showing the
cross-section of the microcellular foam sheet of Example 1, and
FIG. 7 is a scanning electron micrograph showing the cross-section
of the foam sheet of Comparative Example 3.
[0074] 3. Tensile strength and elongation: Measured in accordance
with ASTM D638.
[0075] 4. Impact resistance: Impact absorption energy was measured
by the rheometric drop test.(RDT) in accordance with ASTM D4226.
TABLE-US-00002 TABLE 2 Comp. Comp. Comp. Example 1 Example 2
Example 3 Example 4 Example 1 Example 2 Example 3 Specific 1.2 1.14
1.2 1.15 1.4 1.0 1.0 gravity Thickness 300 300 200 150 -- <50
<50 of skin layer (.mu.m) Average 30 20 25 30 -- 126 60 pore
size (.mu.m) Elongation 136 150 136 112 130 24 42 (%) Tensile 40 43
41 44 44 23.9 36 strength (N/mm.sup.2) Impact 12 13 11 8 15.2 1.2
3.3 absorption energy (J)
[0076] As seen in Table 2, the microcellular foams prepared in
accordance with the present invention have a fine and uniform pore
size, as seen in FIG. 6. Also, since they have a thick skin layer,
they show physical properties comparable to those of non-foamed
sheets, in spite of a low specific gravity. In contrast, the foamed
sheets without a cooling region or produced through a smooth
cooling treatment have a large pore size, as seen in FIG. 7, and a
thin skin layer.
[0077] As is apparent from the above description, the microcellular
foam of the present invention is advantageous in that it has a
thicker skin layer and smaller and uniform micropores, compared
with conventional microcellular foams, while having mechanical
properties comparable to those of non-foamed counterparts.
[0078] Although the present invention has been described in detail
with reference to the preferred embodiments, those skilled in the
art will appreciate that various modifications and substitutions
can be made thereto without departing from the spirit and scope of
the present invention as set forth in the appended claims.
* * * * *