U.S. patent application number 13/178300 was filed with the patent office on 2012-01-12 for method of producing compostable or biobased foams.
This patent application is currently assigned to Lifoam Industries. Invention is credited to Jeffrey J. Cernohous, Kent Kaske, Adam R. Pawloski.
Application Number | 20120007267 13/178300 |
Document ID | / |
Family ID | 45438021 |
Filed Date | 2012-01-12 |
United States Patent
Application |
20120007267 |
Kind Code |
A1 |
Pawloski; Adam R. ; et
al. |
January 12, 2012 |
Method of Producing Compostable or Biobased Foams
Abstract
The present invention describes compostable or biobased foams
that are useful for fabricating foamed articles. The foams are
produced using a compound comprising a compostable or biobased
polyester and a blowing agent. Additives including plasticizers and
chain extenders are optionally included in the compostable or
biobased composition. These foams can be produced using
conventional melt processing techniques, such as single and
twin-screw extrusion processes. In one embodiment, foamed strand
profiles are cooled and cut using conventional strand pelletizing
equipment. In another embodiment, foamed beads are produced by
cutting the foamed strand at the face of the extrusion die and the
foamed bead or strand is subsequently cooled. The resulting
compostable or biobased foamed bead has a specific gravity less
than 0.15 g/cm.sup.3 and the foam is compostable, as determined by
ASTM D6400.
Inventors: |
Pawloski; Adam R.; (Lake
Elmo, MN) ; Cernohous; Jeffrey J.; (Hudson, WI)
; Kaske; Kent; (Woodbury, MN) |
Assignee: |
Lifoam Industries
Hunt Valley
MD
|
Family ID: |
45438021 |
Appl. No.: |
13/178300 |
Filed: |
July 7, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61362009 |
Jul 7, 2010 |
|
|
|
Current U.S.
Class: |
264/55 ;
521/60 |
Current CPC
Class: |
B29B 9/06 20130101; C08J
9/16 20130101; C08J 2203/06 20130101; B29K 2105/0044 20130101; B29K
2105/0032 20130101; C08J 2367/02 20130101; Y10T 428/2982 20150115;
B29K 2067/04 20130101; C08J 9/122 20130101; C08J 2300/16 20130101;
C08J 2367/04 20130101; B29C 44/348 20130101; B29K 2105/0005
20130101; C08J 2201/03 20130101; B29C 44/3461 20130101; B29K
2995/006 20130101; C08J 2203/08 20130101; B29K 2105/0038
20130101 |
Class at
Publication: |
264/55 ;
521/60 |
International
Class: |
B29C 44/08 20060101
B29C044/08; C08J 9/18 20060101 C08J009/18 |
Claims
1. A method comprising: blending a melt processable composition
comprising a compostable or biobased polymer with a blowing agent;
processing the composition in an extruder to form an extrudate; and
pelletizing the extrudate to form a bead of a substantially closed
cell structure.
2. The method of claim 1, said melt processable composition further
comprising a nucleating agent.
3. The method of claim 1, further comprising: extruding the
extrudate through a nozzle die attached to an end of the
extruder.
4. The method of claim 3, the wherein the pelletizing occurs at the
face of the die of the extruder.
5. The method of claim 3, further comprising: cutting the extrudate
with a rotary blade in contact with the front end surface of the
nozzle die while allowing the extrudate to foam to produce foamed
beads.
6. The method of claim 5, wherein pelletization of the extrudate at
the face of the extrusion die occurs prior to complete expansion of
the extrudate foam.
7. The method of claim 5, wherein said foamed beads have a density
less than 0.3 g/cm.sup.3.
8. The method of claim 1, wherein said blowing agent comprises a
physical blowing agent.
9. The method of claim 1, wherein said blowing agent comprises
super critical CO.sub.2.
10. The method of claim 1, said compostable polymer comprising a
polymer of polylactic acid.
11. The method of claim 1, further comprising: pressurizing the
bead with a liquid or gaseous blowing agent.
12. The method of claim 11, wherein said gas is selected from the
group consisting of: air; CO.sub.2; N.sub.2; and hydrocarbon.
13. The method of claim 5, wherein foaming of the bead occurs after
pelletization of the extrudate mixture of polymer and blowing
agent.
14. The method of claim 1, further comprising: moving the beads
into a mold; and further expanding and fusing the beads in the mold
by application of heat.
15. The method of claim 1, wherein the bead is capable of holding
an internal pressure inside the closed cell structure providing
volumetric expansion of the foamed bead during heating.
16. A method for producing a foamed molded product, comprising the
steps of: creating foamed beads according to the method of claim 5;
bringing the foamed beads under temperature and pressure conditions
so that a foamed molded product is obtained.
17. The method of claim 16, wherein the method uses heated gas to
promote fusion of the foamed beads.
18. The method of claim 17, wherein the method uses steam.
19. The method of claim 17, wherein the method uses air or a
mixture of air and steam.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims benefit of
copending and co-owned U.S. Provisional Patent Application Ser. No.
61/362,009 entitled "Biodegradable Foams", filed with the U.S.
Patent and Trademark Office on Jul. 7, 2010 by the inventors
herein, the specification of which is incorporated herein by
reference.
BACKGROUND
[0002] 1. Field of the Invention
[0003] This invention relates generally to compostable or biobased
material compositions and to novel methods for producing
lightweight, compostable or biobased foams and, in particular, to
methods for producing foams using melt processing techniques to
blend compostable or biobased materials and blowing agents that do
not contain any volatile organic components (VOCs) such as pentane.
The compositions and processes are useful for the production of a
variety of products.
[0004] 2. Description of the Background
[0005] Polymeric foams include a plurality of voids, also called
cells, in a polymer matrix. By replacing solid plastic with voids,
polymeric foams use fewer raw materials than solid plastics for a
given volume. Thus, by using polymeric foams instead of solid
plastics, material costs can be reduced in many applications.
Additionally, foams are very good insulators that can seal building
structures from air and moisture intrusion, save on utility bills,
and add strength to the building.
[0006] Microcellular foams have smaller cell sizes and higher cell
densities than conventional polymeric foams. Foam processes, in
some cases, incorporate nucleating agents, some of which are
inorganic solid particles, into the polymer melt during processing.
These agents can be of a variety of compositions, such as talc and
calcium carbonate, and are incorporated into the polymer melt
typically to promote cell nucleation. The dispersion of nucleating
agents within the polymer mixture is often times critical in
forming a uniform cell structure.
[0007] The material used for expandable polystyrene (EPS) is
typically an amorphous polymer that exhibits a glass transition
temperature of about 95.degree. C. and a melting temperature of
about 240.degree. C. The process of converting EPS resins into
expanded polystyrene foam articles requires three main stages:
pre-expansion, maturation, and molding. Expandable beads produced
from polystyrene and a blowing agent are made, and then expanded by
steam in a pre-expander. The purpose of pre-expansion is to produce
foam particles of the desired density for a specific application.
During pre-expansion, the EPS beads are fed to a pre-expander
vessel containing an agitator and controlled steam and air
supplies. The introduction of steam into the pre-expander yields
two effects: the EPS beads soften and the blowing agent that is
dispersed within the EPS beads heats to a temperature above its
boiling point. These two conditions cause the EPS beads to expand
in volume. The diameter of the particles increases while the
density of the resin decreases. The density of pre-expanded
granules is about 1000 kg/m.sup.3, and that of expanded beads lies
in the range of 20 to 200 kg/m.sup.3; depending on the process, a 5
to 50 times reduction in density may be achieved.
[0008] Maturation serves several purposes. It allows the vacuum
that was created within the cells of the foam particles during
pre-expansion to reach equilibrium with the surrounding atmospheric
pressure. It permits residual moisture on the surface of the foam
particles to evaporate. And, it provides for the dissipation of
excess residual blowing agent. Maturation time depends on numerous
factors, including blowing agent content of the original resin,
pre-expanded density, and environmental factors. Pre-expanded beads
that are not properly matured are sensitive to physical and thermal
shock. Molding of such beads before maturation may cause the cells
within the particles to rupture, thereby producing an undesirable
molded foam part.
[0009] Once the pre-expanded beads have matured, they are
transferred to a molding machine containing one or more cavities
that are shaped like the desired molded foam article(s). The
purpose of molding is to fuse the foam particles together into a
single foam part. Molding of EPS may follow a simple sequence:
first, fill the mold cavity with pre-expanded beads; heat the mold
by introducing steam; cool the molded foam article within the mold
cavity; and eject the finished part from the mold cavity. The steam
that is introduced to the molding machine causes the beads to
soften and expand even further. The combination of these two
effects in an enclosed cavity allows the individual particles to
fuse together into a single solid foam part.
[0010] There is an increasing demand for many plastic products used
in packaging to be biodegradable, for example trays in cookie and
candy packages. Starch films have been proposed as biodegradable
alternatives for some time. U.S. Pat. No. 3,949,145 describes a
starch/polyvinyl alcohol/glycerol composition for use as a
biodegradable agricultural mulch sheet.
[0011] A common approach to creating biodegradable products is to
combine polylactic acid (PLA) with starch to create a
hydrolytically degradable composition. Difficulties have been
encountered in producing starch based polymers particularly by hot
melt extrusion. The molecular structure of the starch is adversely
affected by the shear stresses and temperature conditions needed to
plasticize the starch and pass it through an extrusion die.
[0012] Blowing agents typically are introduced into polymeric
material to make polymer foams in one of two ways. According to one
technique, a chemical blowing agent is mixed with a polymer. The
chemical blowing agent undergoes a chemical reaction in the
polymeric material, typically under conditions in which the polymer
is molten, causing formation of a gas. Chemical blowing agents
generally are low molecular weight organic compounds that decompose
at a particular temperature and release a gas such as nitrogen,
carbon dioxide, or carbon monoxide. According to another technique
a physical blowing agent, i.e., a fluid that is a gas under ambient
conditions, is injected into a molten polymeric stream to form a
mixture. The mixture is subjected to a pressure drop, causing the
blowing agent to expand and form bubbles (cells) in the polymer.
Several patents and patent publications describe aspects of
microcellular materials and microcellular processes.
[0013] U.S. Pat. No. 6,593,384 to Anderson et al. describes
expandable particles produced using broad polymer materials and a
physical blowing agent. U.S. Pat. No. 7,226,615 to Yuksel et al.
describes an expandable foam based on broad disclosure of
biomaterials combined with a bicarbonate blowing agent. U.S.
Published Patent Application No. 2006/0167122 by Haraguchi et al.
describes expandable particles derived from the combination of PLA,
a blowing agent, and a polyolefin wax. U.S. Published Patent
Application No. 2010/0029793 by Witt et al. describes a method of
producing PLA foam by impregnating resin beads with carbon dioxide
(CO.sub.2).
[0014] U.S. Pat. No. 4,473,665 to Martini-Vvedensky et al.
describes a process for making a foamed polymer having cells less
than about 100 microns in diameter. In the described technique, a
material precursor is saturated with a blowing agent, the material
is placed under high pressure, and the pressure is rapidly dropped
to nucleate the blowing agent and to allow the formation of cells.
The material then is frozen rapidly to maintain a desired
distribution of microcells.
[0015] U.S. Pat. No. 5,158,986 to Cha et al. describes formation of
microcellular polymeric material using a supercritical fluid as a
blowing agent. Using a batch process, the patent describes various
processes to create nucleation sites.
[0016] U.S. Pat. No. 5,866,053 to Park et al. describes a
continuous process for forming microcellular foam. The pressure on
a single-phase solution of blowing agent and polymer is rapidly
dropped to nucleate the material. The nucleation rate is high
enough to form a microcellular structure in the final product.
[0017] International patent publication no. WO 98/08667 by Burnham
et al. provides methods and systems for producing microcellular
material, and microcellular articles. In one method, a fluid,
single-phase solution of a precursor of foamed polymeric material
and a blowing agent is continuously nucleated by dividing the
stream into separate portions and separately nucleating each of the
separate portions, then recombining the streams. The recombined
stream may be shaped into a desired form, for example by a shaping
die.
[0018] It is generally accepted in the field that to create enough
nucleation sites to form microcellular foams, one must use a
combination of sufficient blowing agent to create a driving force
for nucleation, and a high enough pressure drop rate to prevent
cell growth from dominating the nucleation event. As blowing agent
levels are lowered, the driving force for nucleation decreases.
Yet, while higher blowing agent levels can lead to smaller cells (a
generally desirable result in the field of microcellular foams),
according to conventional thought, higher blowing agent levels also
can cause cell interconnection (which by definition increases cell
size and can compromise structural and other material properties)
and less-than-optimal surface properties (compromised surface
properties at higher gas levels can result from the natural
tendency of the blowing agent to diffuse out of the material).
[0019] In other words, it is generally accepted that there is a
trade off between small cell size and optimal material properties
as blowing agent levels in microcellular polymeric material are
altered.
SUMMARY
[0020] Accordingly, it is an object of the present invention to
provide a compostable or biobased foam that avoids the
disadvantages of the prior art.
[0021] It is another object of the present invention to provide a
method for producing compostable or biobased foams using melt
processing techniques. A related object of the present invention is
to provide a method for producing compostable or biobased foams
using blowing agents that do not contain volatile organic
components. A further related object of the present invention is to
provide a method for producing compostable or biobased foams using
blowing agents that do not contain pentane.
[0022] It is another object of the present invention to provide a
compostable or biobased, foamed bead that can be processed using
conventional molding equipment.
[0023] Another object of the present invention is to provide a
foamed bead that is capable of chemically degrading into lower
molecular weight materials by the process of composting.
[0024] A further object of the invention is to provide a
compostable or biobased, foamed bead that can be fabricated into a
three-dimensional shape.
[0025] These and other objects of the present invention are
accomplished by providing a composition and process for producing
foamed beads from a compostable or biobased polymer and for using
such beads in producing a variety of items. In one embodiment,
lightweight beads are produced by melt processing a compostable or
biobased polymer and a blowing agent. In another embodiment, the
melt processable composition includes additional additives that
improve the rheological characteristics of the compostable or
biobased polymer, making it more amenable for producing
lightweight, foamed beads. The foamed beads of this invention can
be further processed using conventional molding equipment to
provide a lightweight, compostable or biobased, foamed article.
Articles of this invention have utility in applications where
conventional expandable polystyrene (EPS) is utilized today,
including those applications relating to protective packaging,
sound dampening, and thermal insulation.
[0026] Polymer compositions are widely utilized in numerous
applications, including automotive, home construction, electronic
and consumer good products. The polymers may be composed of either
biobased polymers or petroleum-based polymers. Compostable or
biobased polymers are preferred to address environmental concerns
associated with disposal of the materials once they are no longer
useful for their intended purpose and minimizing the use of
petroleum. However, the polymers must meet certain physical and
chemical characteristics in order for them to be suitable for the
intended application. In expandable foams, the polymer composition
must be able to be fabricated into a three dimensional shape that
is lightweight and provides impact, sound, and thermal resistance
or protection. The invention described herein discloses compostable
or biobased foams having attributes that are required to form
products that posses these attributes.
[0027] For purposes of the present invention, the following terms
used in this application are defined as follows:
[0028] "Biodegradable Polymer" means a polymeric material or resin
that is capable of chemically degrading into lower molecular weight
materials.
[0029] "Nucleating agent" means a material that is added to a
polymer melt that provide sites for crystal formation. For example,
a higher degree of crystallinity and more uniform crystalline
structure may be obtained by adding a nucleating agent.
[0030] "Chain Extender" means a material that when melt processed
with a polymer, increases the molecular weight by reactively
coupling chain ends.
[0031] "Melt Processable Composition" means a formulation that is
melt processed, typically at elevated temperatures, by means of a
conventional polymer processing technique such as extrusion or
injection molding as an example.
[0032] "Melt Processing Techniques" means extrusion, injection
molding, blow molding, rotomolding, or batch mixing.
[0033] "Extrudate" is the semisolid material that has been extruded
and shaped into a continuous form by forcing the material through a
die opening.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] The above and other features, aspects, and advantages of the
present invention are considered in more detail in relation to the
following description of embodiments thereof shown in the
accompanying drawings, in which:
[0035] FIG. 1 shows a general process schematic for foamed bead
production by extrusion foaming process according to the present
invention.
[0036] FIG. 2 shows a cross-section of a foamed bead produced by an
exemplary process according to one embodiment of the present
invention.
[0037] FIG. 3 shows a summary flow chart illustrating the process
flow for producing foamed articles according to the present
invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0038] The invention summarized above and defined by the enumerated
claims may be better understood by referring to the following
description. This description of an embodiment, set out below to
enable one to build and use an implementation of the invention, is
not intended to limit the invention, but to serve as a particular
example thereof. Those skilled in the art should appreciate that
they may readily use the conception and specific embodiments
disclosed as a basis for modifying or designing other methods and
systems for carrying out the same purposes of the present
invention. Those skilled in the art should also realize that such
equivalent assemblies do not depart from the spirit and scope of
the invention in its broadest form.
[0039] The present invention is directed toward a variety of
products that are made of compostable or biobased materials. The
compostable or biobased materials can include either or both of an
externally or an internally modified polymer composition, as those
terms are described below.
[0040] Degradability
[0041] Biodegradability refers to a compound that is subject to
enzymatic decomposition, such as by microorganisms, or a compound,
portions of which are subject to enzymatic decomposition, such as
by microorganisms. In one instance, for example, a polymer such as
polylactic acid can be degraded by hydrolysis to individual lactic
acid molecules that are subject to enzymatic decomposition by a
wide variety of microorganisms. Microorganisms typically can
consume carboxylic acid-containing oligomers with molecular weights
of up to about 1000 daltons, and preferably up to about 600
daltons, depending on the chemical and physical characteristics of
the oligomer.
[0042] Biobased means materials that are synthesized from
biological sources and refers to ingredients that reduce the use of
non-renewable resources by integrating renewable ingredients as a
replacement for at least a portion of the materials in a product.
For example, replacement of petroleum used in making EPS. Biobased
ingredients can be used in many products without hindering their
performance.
[0043] Composting is the biological process of breaking down
organic waste into a useful substance by various microorganisms in
the presence of oxygen.
[0044] Preferably, the polymer in the present materials breaks down
by composting. The degradation characteristics of the polymer in
the present materials depend in large part on the type of material
being made with the polymer. Thus, the polymer needs to have
suitable degradation characteristics so that when processed and
produced into a final material, the material does not undergo
significant degradation until after the useful life of the
material.
[0045] The polymer of the present materials is further
characterized as being compostable within a time frame in which
products made from the materials break down after use. The
materials of this invention degrade in a time period of a few weeks
to a few years, whereas similar mass-produced, nondegradable
products typically require decades to centuries to break down
naturally.
[0046] The present invention describes compostable or biobased foam
beads that are useful for fabricating foamed articles. The foams of
this invention are produced using a compound comprising a
compostable or biobased thermoplastic polymer and a blowing agent.
Such compostable thermoplastic polymer material may be used to
replace expandable polystyrene (EPS) with a foamed bead produced
from the compostable or biobased polymer resin in the construction
of foamed articles. Ideally, one would substitute polystyrene with
a compostable or biobased polymer of the same chemical and physical
properties.
[0047] Additives including plasticizers and chain extenders are
optionally included in the compostable or biobased composition.
Preferably, the polymer is greater than 50% biobased content, most
preferably greater than 80% biobased. These foams can be produced
using conventional melt processing techniques, such as single and
twin-screw extrusion processes. In one embodiment, foamed beads are
produced by cutting extrudate at the face of the extrusion die. The
foamed bead is subsequently optionally cooled by contacting with
water, water vapor, air, carbon dioxide, or nitrogen gas. After the
bead is cut at the face of the die, the bead continues to foam,
thus forming a closed cell foam structure with a continuous surface
skin, i.e. there is no open cell structure at the surface of the
bead. In one embodiment, the resulting compostable or biobased,
foamed bead has a specific gravity less than 0.15 g/cm.sup.3. In
another embodiment, the compostable or biobased, foamed bead has a
specific gravity of preferably less than 0.075 g/cm.sup.3, and most
preferably less than 0.05 g/cm.sup.3. In another embodiment, more
than 50 wt % of the foam is produced from compostable materials, as
determined by ASTM D6400. In a preferred embodiment, more than 80%
of the foam is produced from compostable materials. In a most
preferred embodiment, greater than 95% of the foam is produced from
compostable materials.
[0048] The compostable or biobased polymers of this invention are
produced by melt processing compostable or biobased polymers with a
blowing agent and, optionally, additives that modify the rheology
of the compostable or biobased polymer, including chain extenders
and plasticizers. The compostable or biobased polymers may include
those polymers generally recognized by one of ordinary skill in the
art to decompose into compounds having lower molecular weights.
Non-limiting examples of compostable or biobased polymers suitable
for practicing the present invention include polysaccharides,
peptides, polyesters, polyamino acids, polyvinyl alcohol,
polyamides, polyalkylene glycols, and copolymers thereof.
[0049] In one aspect, the compostable or biobased polymer is a
polyester. Non-limiting examples of polyesters include polylactic
acids, poly-L-lactic acid (PLLA), poly-D-lactic acid (PDLA) and
random or stereoregular copolymers of L-lactic acid and D-lactic
acid, and derivatives thereof. Other non-limiting examples of
polyesters include polycaprolactone, polyhydroxybutyric acid,
polyhydroxyvaleric acid, polyethylene succinate, polybutylene
succinate, polybutylene adipate, polymalic acid, polyglycolic acid,
polysuccinate, polyoxalate, polybutylene diglycolate, and
polydioxanone.
[0050] Preferred polymer resins for this invention include known
compostable materials derived from biological sources (e.g.
compostable biopolymer resins), but synthetic polymers capable of
being composted are also acceptable. The biopolymer polylactic acid
(PLA) is the most preferred example due to its known compostability
and its biobased origins from agricultural (e.g. corn) feedstocks.
Both amorphous and semi-crystalline PLA polymers can be used.
Examples of compostable or biobased polymers include Ingeo 2002D
and Ingeo 4060D grade plastics and Ingeo 8051D grade foam from
NatureWorks, LLC, and Cereplast Compostable 5001.
[0051] Blowing agents are materials that can be incorporated into
the melt processable composition (e.g., the premix of the
additives, polymeric matrix, and/or optional fillers, either in
melt or solid form) to produce cells through the release of a gas
at the appropriate time during processing. The amount and types of
blowing agents influence the density of the finished product by its
cell structure. Any suitable blowing agent may be used to produce
the foamed material.
[0052] There are two major types of blowing agents: physical and
chemical. Physical blowing agents tend to be volatile liquids or
compressed gases that change state during melt processing to form a
cellular structure. In a preferred embodiment, the physical blowing
agent is carbon dioxide. In the most preferred embodiment, the
physical blowing agent of carbon dioxide in its supercritical state
is mixed with the polymer melt. Chemical blowing agents tend to be
solids that decompose (e.g., thermally, reaction with other
products, and so forth) to form gaseous decomposition products. The
gases produced are finely distributed in the melt processable
composition to provide a cellular structure.
[0053] Blowing agents can be divided into two major
classifications: organic and inorganic. Organic blowing agents are
available in a wide range of different chemistries, physical forms
and modification, such as, for example, azodicarbonamide. Inorganic
blowing agents tend to be more limited. An inorganic blowing agent
may include one or more carbonate salts such as Sodium, Calcium,
Potassium, and/or Magnesium carbonate salts. Preferably, sodium
bicarbonate is used because it is inexpensive and readily
decomposes to form carbon dioxide gas. Sodium bicarbonate gradually
decomposes when heated above about 120.degree. C., with significant
decomposition occurring between approximately 150.degree. C. and
200.degree. C. In general, the higher the temperature, the more
quickly the sodium bicarbonate decomposes. An acid, such as citric
acid, may also be included in the foaming additive, or added
separately to the melt processable composition, to facilitate
decomposition of the blowing agent. Chemical blowing agents are
usually supplied in powder form or pellet form. The specific choice
of the blowing agent will be related to the cost, desired cell
development and gas yield and the desired properties of the foamed
material.
[0054] Suitable examples of blowing agents include water, carbonate
salts and other carbon dioxide releasing materials, diazo compounds
and other nitrogen producing materials, carbon dioxide, decomposing
polymeric materials such as poly (t-butylmethacrylate) and
polyacrylic acid, alkane and cycloalkane gases such as pentane and
butane, inert gases such as nitrogen, and the like. The blowing
agent may be hydrophilic or hydrophobic. In one embodiment, the
blowing agent may be a solid blowing agent. In another embodiment,
the blowing agent may include one or more carbonate salts such as
sodium, potassium, calcium, and/or magnesium carbonate salts. In
yet another embodiment, the blowing agent may be inorganic. The
blowing agent may also include sodium carbonate and sodium
bicarbonate, or, alternatively, sodium bicarbonate alone.
[0055] Although the blowing agent composition may include only the
blowing agent, a more typical situation is where the blowing agent
includes a polymeric carrier that is used to carry or hold the
blowing agent. This blowing agent concentrate may be dispersed in
the polymeric carrier for transport and/or handling purposes. The
polymeric carrier may also be used to hold or carry any of the
other materials or additives that are desired to be added to the
melt processable composition.
[0056] The inclusion levels of the blowing agent in the concentrate
may vary widely. In some embodiments, the foaming additive includes
at least about 2.5 wt % of blowing agent, at least about 5 wt % of
blowing agent, or, suitably, at least about 10 wt % of blowing
agent. In other embodiments, the foaming additive may include about
10 to 60 wt % of blowing agent, about 15 to 50 wt % of blowing
agent, or, suitably, about 20 to 45 wt % of blowing agent. In yet
further embodiments, the foaming additive may include about 0.05 to
90 wt % of blowing agent, about 0.1 to 50 wt % of blowing agent, or
about 1 to 26 wt % of blowing agent.
[0057] As mentioned previously, the blowing agent concentrate may
also include a polymeric carrier or material that is used to hold
the other additives to form a single additive. The polymeric
carrier or polymeric component may be any suitable polymeric
material such as hydrocarbon or non-hydrocarbon polymers. The
polymeric carrier should be capable of being melted or melt
processed at temperatures below the activation temperature of the
blowing agent. In some instances, however, a polymeric component
having a melting point above the activation temperature of the
blowing agent may be used as long as it is processed quickly enough
so that a suitable amount of active blowing agent remains. In one
embodiment, the polymeric carrier has a melting point of no more
than about 150.degree. C., no more than about 125.degree. C., no
more than about 100.degree. C., or, suitably, no more than about
80.degree. C. In a preferred embodiment, the blowing agent
concentrate contains a compostable or biobased polymer.
[0058] In another embodiment, a plasticizer may be added or
incorporated into the composition to address desired physical
characteristics of the melt processable composition. Non-limiting
examples of plasticizers include polyaklylene glycols and
functionalized naturally occurring oils. Non-limiting examples of
polyalkylene glycols include polyethylene glycols sold under the
Carbowax trade name (Dow Chemical Co., Midland, Mich.).
Non-limiting examples of functionalized naturally occurring oils
include malinated or epoxidized soybean, linseed, or sunflower
oils, which are commercially available from Cargill Inc.
[0059] In another embodiment, the compostable or biobased
composition may include a chain extender to increase the molecular
weight of the compostable or biobased polymer during melt
processing. This also has the effect of increasing melt viscosity
and strength, which can improve the foamability of the compostable
or biobased polymer. An example of chain extenders useful in this
invention include those marketed under the CESA-extend trade name
from Clariant, and those marketed under the Johncryl trade name
from BASF.
[0060] In the composition of the present invention, moldability can
be improved by adding a nucleating agent. The dispersion of a
nucleating agent within the polymer mixture helps in forming a
uniform cell structure. Examples of nucleating agents include
inorganic powder such as talc, kaolin, mica, silica, calcium
carbonate, barium sulfate, titanium oxide, aluminum oxide, clay,
bentonite, and diatomaceous earth, and known chemical blowing
agents such as azodicarbodiamide. Among them, talc is preferred
because it facilitates control of the cell diameter. The content of
the nucleating agent varies depending on the type of the nucleating
agent and the intended cell diameter.
[0061] In another aspect of the invention, the compostable or
biobased, melt processable composition may contain other additives.
Non-limiting examples of additives include antioxidants, light
stabilizers, fibers, blowing agents, foaming additives,
antiblocking agents, heat stabilizers, impact modifiers, biocides,
compatibilizers, tackifiers, colorants, coupling agents, antistatic
agents, electrically conductive fillers, and pigments. The
additives may be incorporated into the melt processable composition
in the form of powders, pellets, granules, or in any other
extrudable form. The amount and type of additives in the melt
processable composition may vary depending upon the polymeric
matrix and the desired physical properties of the finished
composition. Those skilled in the art of melt processing are
capable of selecting appropriate amounts and types of additives to
match with a specific polymeric matrix in order to achieve desired
physical properties of the finished material.
[0062] The amount of components in the melt processable,
compostable or biobased foam composition may vary depending upon
the intended end use application. The compostable or biobased
polymer may comprise from about 40 to about 99 percent by weight of
the final composition. The blowing agent may be included at a level
of up to 20 percent by weight. The compostable or biobased
plasticizer may comprise from about 1 to 50 percent by weight of
the final composition. The chain extender may comprise about 0.1 to
10 percent by weight of the final composition. Nucleating agents
(such as talc) can be included up to about 5% by weight, more
preferably less than 1% by weight, most preferably 0.5% by
weight.
[0063] The physical blowing agent, such as supercritical CO.sub.2,
is combined with the melt early in the extruder mixing process.
Then, as the mixture exits the extruder and is cut, the
supercritical CO.sub.2 expands to form the foamed beads.
Optionally, heating of the beads during a secondary expansion
process allows for expansion of the material to lower density.
[0064] In some embodiments, the foamed beads may optionally be
pressurized with a gas that will allow for additional expansion of
the bead in the molding operation for the desired end product. The
optional pressurization is used to make the internal pressure of
the cells within the foam greater than the atmospheric pressure.
The fact that the foam has a closed cell structure allows the bead
to maintain an internal pressure greater than atmospheric pressure
after the impregnation step. When the beads are heated during
molding, this internal pressure allows for further expansion of the
foamed bead. Such pressurization of the foamed beads will typically
be done with a gas such as air, CO.sub.2, N.sub.2, hydrocarbon,
etc. Then, the beads are put into a mold to form a selected
product.
[0065] In the extrusion foaming process, the temperature profile of
the extruder must be carefully controlled to allow for melting and
mixing of the solids, reaction with the chain extension agent
(optional), mixing with blowing agent, (for example supercritical
CO.sub.2), and cooling of the melt mixture prior to extrusion
through the die. The temperatures of the initial barrel sections
allows for melting and mixing of the solids, including the
dispersion of nucleating agent within the melt. At the same time,
the optional chain extension agent reacts with the chain ends of
the polymer, increasing branching and molecular weight, which
increases viscosity of the melt and improves the melt strength of
the plastic. Prior to injection of the blowing agent, a melt seal
is created within the extruder by careful design of internal screw
elements to prevent the flow of the blowing agent from exiting the
feed throat. The melt seal maintains pressure within the extruder
allowing the blowing agent to remain soluble within the melted
plastic. After injection of the blowing agent, mixing elements are
used to mix the blowing agent with the melt. Soluble blowing agent
within the melt plasticizes the melt dramatically, greatly reducing
its viscosity. The plasticization effect allows for the cooling of
the melt to below the normal melting temperature of the compostable
of biobased polymer in the final sections of the extruder. The
cooling is necessary to increase the viscosity of the plasticized
melt, allowing for retention of a closed cell structure during
foaming at the die.
[0066] Nucleating agents serve as nucleation sites for blowing
agent evolution during foaming. When depressurization occurs at the
die, the blowing agent dissolved in the plastic melt comes out of
solution into the gas phase. By entering the gas phase, the volume
occupied by the blowing agent increases dramatically, producing a
foamed structure. By dispersion of the nucleating agent in the
melt, the blowing agent will evenly evolve from its soluble state
within the melt to its gaseous form during depressurization, thus
producing a fine cellular foam. Without properly dispersed
nucleation sites, the foaming can be uneven, producing large voids
or open cell structure where cell walls are fractured and
interconnected. Large voids and open cell structure creates a
harder, more brittle foam. Very low density foams with closed cell
structure can be described as spongy, having a good elastic
recovery after significant compression.
[0067] As extrudate exits the die and is foamed, rotating knives of
the pelletizer cut the bead at the face of the die. When cut, the
foam is not completely established. The foaming process continues
to shape the structure of the bead after it has been cut. The
blowing agent continues to evolve, expanding the particle. The
outer skin of the particle remains rubbery while cut, allowing the
surface of the foamed bead to flow and reform a smooth, solid
surface.
[0068] The melt processable, compostable or biobased foam
composition of the invention can be prepared by any of a variety of
ways. For example, the compostable or biobased polymer, blowing
agent, nucleating agent, and optional additives can be combined
together by any of the blending means usually employed in the
plastics industry, such as with a mixing extruder. The materials
may, for example, be used in the form of a powder, a pellet, or a
granular product. The mixing operation is most conveniently carried
out at a temperature above the melting point or softening point of
the polymer. The resulting melt-blended mixture can be processed
into foamed beads by cutting the extrudate mixture of polymer and
blowing agent at the face of the extrusion die. By cutting the
extrudate at the face of the extrusion die, a bead is formed before
complete expansion of the foam has occurred. After pelletization, a
foamed bead is formed from expansion of the extrudate by the
blowing agent. The foamed bead cools by the release of blowing
agent, but subsequent cooling can be applied by contacting with
water, water vapor, air, carbon dioxide, or nitrogen gas. The
resulting foamed beads can be molded into a three-dimensional part
using conventional equipment utilized in molding expandable
polystyrene. In one embodiment, the foamed beads contain residual
blowing agent and can be post expanded in the molding process. In
another embodiment, the foamed beads are pressurized with a gas,
such as air or carbon dioxide, before molding to allow for
expansion during molding.
[0069] Melt processing typically is performed at a temperature from
about 80.degree. to 300.degree. C., although optimum operating
temperatures are selected depending upon the melting point, melt
viscosity, and thermal stability of the composition. Different
types of melt processing equipment, such as extruders, may be used
to process the melt processable compositions of this invention.
Extruders suitable for use with the present invention are
described, for example, by Rauwendaal, C., "Polymer Extrusion,"
Hansen Publishers, p. 11-33, 2001.
[0070] In one embodiment, the resulting compostable or biobased,
foamed bead has a specific gravity less than 0.15 g/cm.sup.3. In
another embodiment, the compostable or biobased, foamed bead has a
specific gravity of preferably less than 0.075 g/cm.sup.3, and most
preferably less than 0.05 g/cm.sup.3.
[0071] Preferably, the polymer for making the foamed bead is
greater than 50% biobased content, most preferably greater than 80%
biobased. In one embodiment, more than 50 wt % of the foam is
compostable, as determined by ASTM D6400. In a preferred
embodiment, more than 80% of the foam is compostable. In a most
preferred embodiment, greater than 95% of the foam is
compostable.
[0072] The first three examples below utilize a single type of PLA
resin. It is known, however, that the degree of crystallinity in
PLA is controlled by two general aspects, first composition, and
second by process. The PLA polymer is composed of lactic acid
monomers, but there are two types of lactic acid monomers. Although
composed of the same elements, functional groups, and chemical
bonds, the stereochemistry of the monomers is different. The two
isomers of lactic acid, the so-called l and d-isomers, have a
different three-dimensional `handedness.` The result is that the
type of isomer can affect the position of the pendant methyl groups
along the backbone of the PLA polymer chain. PLA chains that are
100% composed of either l or d-isomers will be highly crystalline
because the polymer chains can pack tightly against each other. By
introducing small concentrations of the other isomer, the
crystallinity begins to decrease because the position of the
pendant methyl groups begins to disrupt the higher order structure
of crystallinity. PLA with nearly 50/50 mixtures of l and d-isomers
results in a completely amorphous polymer. The l-isomer of lactic
acid is the predominant natural form of lactic acid, so most
semi-crystalline PLAs are predominantly composed of l-isomer with
random impurities of the d-isomer. It is very difficult to produce
PLAs from either 100% l or d-isomer, so all semi-crystalline
materials available in bulk quantities will contain a small
d-isomer content. The 8051D resin has a d-isomer content of about
3.7 to 4.6%, whereas the 4032D resin has a d-isomer content less
than 2% (between 1.2 and 1.6%).
[0073] A second aspect of thermal stability in PLA is the process
and thermal history of the plastic. PLA is slow to crystallize.
Although the d-isomer content may be within an appropriate range to
support crystallinity, this does not necessarily happen if the
material is cooled too quickly. All crystallinity is lost when the
plastic is heated above its melting point, and a slow thermal
annealing is required to induce crystallization. Fillers, such as
high performance talcs are often used to promote a more rapid
crystallization, yet most extrusion applications that are hoping to
take advantage of high crystallinity for thermal stability will
require an annealing step between 100.degree. and 130.degree. C.,
to sufficiently crystallize the PLA. However, in the extrusion foam
application, there is sufficient shear and elongation during
generation of the foam to induce crystallinity within the very thin
films of plastic separating the closed cells of the foam. In
addition, nucleating agents used to promote dispersion and
nucleation of CO.sub.2 dissolved into the melt during foam
processing, also improve crystallization kinetics. Therefore, the
extrusion foam process induces rapid crystallization of PLA. From
the perspective of thermal stability, this is fortuitous because no
annealing step is required.
[0074] FIG. 1 shows a process schematic for bead production by an
extrusion foaming process. The extruder used for the mixing process
in the examples below was a Leistritz ZSE 27 MAXX co-rotating
twin-screw extruder having ten stages in the barrel. The barrel of
the extruder was equipped with an injection port to supply
supercritical carbon dioxide (CO.sub.2) into the plastic melt in
the fourth barrel section. CO.sub.2 in the supercritical state was
produced by pressurizing liquid CO.sub.2 from a pressurized
cylinder with a TharSFC P-50 high-pressure pump to a pressure of
27.6 MPa (4000 psi). All pressurized tubing was jacketed for
cooling with an ethylene glycol--water mixture at a set point of
2.degree. C. (35.degree. F.).
Compounding Procedure
[0075] Compostable or biobased polymer compositions were prepared
using the following protocol.
Example #1
[0076] A dry mix blend of plastics was produced consisting of
approximately 97% by weight of NatureWorks Ingeo 8051D polylactic
acid (PLA), approximately 2% by weight of Clariant CESA-extend
OMAN698498 styrene-acrylic multifunctional oligomeric reactant, and
approximately 1% by weight of Cereplast ECA-023 talc masterbatch.
The dry mix of pellets was fed gravimetrically into the feed throat
section of the twin-screw extruder. The feed rate for the solids
was set to 3.5 kg/hr (7.7 lbs/hr), and the screws were rotating at
40 rpm. Supercritical carbon dioxide (CO.sub.2) was injected into
the plastic melt in the fourth barrel section at 10 g/min. A single
strand die with a 3 mm opening was bolted to the end of the
extruder.
[0077] Initially a flat temperature profile at 210.degree. C. was
used. Upon start up, the extrudate was hotter than 200.degree. C.;
however, at this high temperature, the extrudate was poorly foamed,
exhibited low melt strength, and lacked the viscosity to hold onto
the blowing agent. The cell structure collapsed quickly from
rapidly escaping CO.sub.2 leaving an open cell structure with only
a minor density reduction. The temperature profile over the ten
barrel sections from feed to exit was systematically adjusted to
achieve 210.degree. C., 199.degree. C., 177.degree. C., 155.degree.
C., 122.degree. C., 111.degree. C., 100.degree. C., 102.degree. C.,
101.degree. C., and 85.degree. C. across the extruder. At these
conditions, the melt pressure at the die was 11.7 MPa (1700 psi).
The extrudate was foamed to a density less than 0.04 g/cm.sup.3
(2.5 lb/ft.sup.3) with a closed cell structure. The surface
temperature of the strand extrudate was less than 40.degree. C.
Example #2
[0078] The process described in Example #1 was followed and
improved to include a pelletizing operation at the die face. An
off-axis, two-blade pelletizer was mounted to the extruder and die
assembly. Foamed beads were cut at the face of the die with a
pelletizer operating at 1500 rpm. The foamed beads were free
flowing and did not stick together. The surface of the foamed beads
was complete and did not exhibit open or broken cells. The density
of the foamed beads was less than 0.04 g/cm.sup.3 (2.5
lb/ft.sup.3), and the bead diameter was approximately 10 mm.
Example #3
[0079] The process described in Example #1 was modified to replace
the 3 mm single strand die, with an eight-hole die having 0.8 mm
die openings. The new die included an adapter section that added
one heating zone before the die. The pelletizing system was changed
to an on-axis, two-blade cutting system, operating at 2500 rpm. The
feed rate of the dry blend of resin, chain extender, and talc
masterbatch was decreased to 2.3 kg/hr (5 lbs/hr). The final
process temperature profile during production of low density foam
was adjusted to 210.degree. C., 199.degree. C., 177.degree. C.,
155.degree. C., 115.degree. C., 115.degree. C., 115.degree. C.,
115.degree. C., 115.degree. C., 130.degree. C., and 135.degree. C.
across the extruder and die. The extruder screws operated at 25
rpm. The feed rate of supercritical CO.sub.2 was 7.0 g/min at a
pressure of about 10.3 MPa (1500 psi). The melt pressure during
operation of the extruder was about 15.8 MPa (2300 psi) behind the
die. The foamed beads produced had a diameter in the range of 2 mm
to 5 mm with a density less than 0.045 g/cm.sup.3 (2.8
lb/ft.sup.3). FIG. 2 displays a micrograph taken by scanning
electro-microscopy of a wedge-shaped cross-section of a foamed
bead, showing a closed cell structure with cell size in the range
of 50 to 150 .mu.m.
Example #4
[0080] The process described in Example #3 was modified to produce
foamed beads with a smaller bead diameter and from a different
composition. The die was replaced with a twelve-hole die having 0.6
mm die openings. The feed composition was pre-compounded on a 38 mm
SHJ-38 co-rotating twin-screw extruder from Lantai Plastics
Machinery Company with a flat temperature profile of 180.degree. C.
For this operation, a dry blend mix was prepared from approximately
87% by weight NatureWorks Ingeo 8051D PLA, approximately 10% by
weight of NatureWorks Ingeo 4032D PLA, approximately 2% by weight
of Clariant CESA-extend OMAN698498 styrene-acrylic multifunctional
oligomeric reactant, and approximately 1% by weight of Cereplast
ECA-023 talc masterbatch. The compounded formulation was
subsequently fed into the feed throat of the Leistritz ZSE 27 MAXX
extruder at 2.3 kg/hr (5.0 lbs/hr) with a screw speed of 25 rpm.
The feed rate of supercritical CO.sub.2 was 7 g/min, and the
temperature profile followed 210.degree. C., 199.degree. C.,
177.degree. C., 155.degree. C., 115.degree. C., 115.degree. C.,
115.degree. C., 115.degree. C., 115.degree. C., 150.degree. C., and
150.degree. C. The pelletizer operated at 1920 rpm, cutting the
extrudate at the face of the extrusion die. The melt pressure
behind the die was about 15.2 MPa (2200 psi). The foamed beads
produced had a diameter in the range of 1 mm to 4 mm with a density
less than 0.045 g/cm.sup.3 (2.8 lb/ft.sup.3). The foamed beads
produced in this process were compared for relative heat stability
to the foamed beads produced in Example #3. Placed side-by-side on
a hot plate and heated with an increasing temperature ramp, the
foamed beads softened at a higher temperature than the foamed beads
from Example #3.
Example #5
[0081] The foamed beads from Example #4 were pressurized in a
sealed vessel at 0.45 MPa (65 psi) for less than 30 minutes. A
rapid depressurization of the vessel was performed to remove the
beads. The surface of the beads was taut from the internal pressure
exceeding atmospheric pressure. The beads were vacuum fed into the
cavity of a steam chest molding press (Hirsch HS 1400 D) within 1
minute of removal from the pressure vessel. The initial mold cavity
temperature during fill was about 25.degree. C. A conventional
aluminum mold for expandable polystyrene (EPS) was used in the
shape of a box. A four-step process was used for molding of a final
product. The purge cycle was set for 1 second at 0.55 bar steam
pressure and a 30% valve opening. The first cross steam process was
set for 20 seconds at 0.55 bar steam pressure and a 90% valve
opening. A second cross steam process, reversing the direction of
steam flow, was used for 20 seconds at a steam pressure of 0.65 bar
and a 90% valve opening. Cooling water was applied for 15 seconds
on both sides of the mold, followed by 30 seconds of cooling air at
4 bar pressure. After cooling air, 5 seconds of vacuum was applied.
The molded box was removed from the press. The shapes of the beads
after molding clearly demonstrated secondary expansion of the
foamed beads within the mold. Surface depressions and textures from
the mold cavity were replicated into the surface of the article.
Based on weight and geometry of the box, the density of the molded
article was less than 0.03 g/cm.sup.3 (2.0 lb/ft.sup.3).
[0082] The invention described herein allows for the conversion of
an existing EPS manufacturing plant to produce a foamed article
based on a compostable or biobased polymer. FIG. 3 shows a summary
of the steps for creating a finished article using the composition
and process described in the above examples. First, the raw
materials of PLA polymers, nucleating agent, and other additives
are compounded. In some embodiments, such as described in Example
#4, the raw materials may be compounded in a separate extruder.
Next, a blowing agent, preferably supercritical CO.sub.2, is added
to the admixture. Small, lightweight, foamed beads are produced by
hot face pelletization of extruded foamed strands at the extruder
die face. In some embodiments, the foamed beads may be cooled using
a water bath or other appropriate method. The foamed beads are then
pressurized to promote secondary expansion in the molder for the
desired end product. Such pressurization of the foamed beads will
typically be done with a gas such as air, CO.sub.2, N.sub.2,
hydrocarbon, etc. Then, the beads are put into a mold to form a
selected product. As described in Example #5, a steam press may be
used for molding. The beads are expanded in the mold to create a
finished product.
[0083] The invention has been described with references to specific
embodiments. While particular values, relationships, materials and
steps have been set forth for purposes of describing concepts of
the invention, it will be appreciated by persons skilled in the art
that numerous variations and/or modifications may be made to the
invention as shown in the disclosed embodiments without departing
from the spirit or scope of the basic concepts and operating
principles of the invention as broadly described. It should be
recognized that, in the light of the above teachings, those skilled
in the art could modify those specifics without departing from the
invention taught herein. Having now fully set forth certain
embodiments and modifications of the concept underlying the present
invention, various other embodiments as well as potential
variations and modifications of the embodiments shown and described
herein will obviously occur to those skilled in the art upon
becoming familiar with such underlying concept. It is intended to
include all such modifications, alternatives and other embodiments
insofar as they come within the scope of the invention. It should
be understood, therefore, that the invention might be practiced
otherwise than as specifically set forth herein. Consequently, the
present embodiments are to be considered in all respects as
illustrative and not restrictive.
* * * * *