Note: Descriptions are shown in the official language in which they were submitted.
CA 02770956 2012-03-09
PROCESS FOR ENABLING SECONDARY EXPANSION
OF EXPANDABLE BEADS
FIELD OF THE INVENTION
This invention relates generally to novel methods for producing lightweight,
foams and, in particular, to methods for producing foams using melt processing
techniques and blowing agents combining physical and chemical blowing agents
to
enable secondary expansion of the foam.
DESCRIPTION OF THE BACKGROUND
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.
The material used for expandable polystyrene (EPS) is typically an amorphous
polymer that exhibits a glass transition temperature of about 95 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
CA 02770956 2012-03-09
the blowing agent that is dispersed within the EPS beads, typically pentane,
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/m3, and that
of
expanded beads lies in the range of 20 to 200 kg/m3; depending on the process,
a 5 to 50
times reduction in density may be achieved.
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.
Once the pre-expanded beads have matured, they are transferred to a molding
machine containing one or more cavities that are shaped for 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
due to release of residual blowing agent, such as pentane. The combination of
these two
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effects in an enclosed cavity allows the individual particles to fuse together
into a single
solid foam part.
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. 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.
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 the 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.
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U.S. Patent No. 6,593,384 to Anderson et al. describes expandable particles
produced using broad polymer materials and a physical blowing agent. U.S.
Patent 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 (CO2).
U.S. Patent No. 4,473,665 to Martini-Vvedensky et at. 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.
U.S. Patent 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.
U.S. Patent 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.
International patent publication no. WO 98/08667 by Burnham et al. provides
methods and systems for producing microcellular material and microcellular
articles. In
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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.
It is generally accepted in the field that to create enough nucleation sites
to form
microcellular foams, one must use 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).
In other words, it is generally accepted that there is a tradeoff between
small cell
size and optimal material properties as blowing agent levels in microcellular
polymeric
material are altered.
SUMMARY
Accordingly, it is an object of the present invention to provide a compostable
or
biobased foam that avoids the disadvantages of the prior art.
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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 a combination of blowing agents.
It is another object of the present invention to provide a compostable or
biobased
foamed bead that can be processed using conventional molding equipment.
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.
A further object of the invention is to provide a compostable or biobased,
foamed
bead that can be fabricated into a three-dimensional shape.
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 combination of a physical blowing agent and a chemical 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,
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including those applications relating to protective packaging, sound
dampening, and
thermal insulation.
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
possess these
attributes.
For purposes of the present invention, the following terms used in this
application
are defined as follows:
"Biodegradable Polymer" means a polymeric material or resin that is capable of
chemically degrading into lower molecular weight materials.
"Compostable" means capable of undergoing biological decomposition, such that
the material is not visually distinguishable and breaks down into carbon
dioxide, water,
inorganic compounds, and biomass.
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"Biobased" means materials that are composed, in whole or in significant part,
of
biological products or renewable agricultural materials including plant,
animal, and
marine materials.
"Plasticizer" means a material that is compatible with a compostable or
biobased
polymer after melt processing. Addition of a plasticizer to a compostable or
biobased
polymer has the effect of lowering the modulus of the film composition.
"Chain Extender" means a material that when melt processed with a polymer,
increases the molecular weight by reactively coupling chain ends.
"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.
"Melt Processing Techniques" means extrusion, injection molding, blow molding,
rotomolding, or batch mixing.
"Extrudate" is the semisolid material that has been extruded and shaped into a
continuous form by forcing the material through a die opening.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
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.
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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.
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.
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.
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.
Composting is the biological process of breaking down organic waste into a
useful substance by various microorganisms in the presence of oxygen.
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
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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.
The polymer of the present materials is further characterized as being
compostable within a period in which products made from the materials break
down after
use. The materials of this invention degrade in a time period typically less
than 180 days
whereas similar mass-produced, nondegradable products typically require
decades to
centuries to break down naturally.
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
combination of blowing agents. 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.
Additives including plasticizers and chain extenders are optionally included
in the
compostable or biobased composition. Preferably, the polymer has 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
CA 02770956 2012-03-09
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/cm3. In another
embodiment, the
compostable or biobased, foamed bead has a specific gravity of preferably less
than 0.075
g/cm3, and most preferably less than 0.05 g/cm3. In some embodiments, more
than 50
wt% of the foam contains materials that are compostable, as determined by ASTM
D6400. In a preferred embodiment, more than 80 wt% of the foam contains
materials
that are compostable. In a most preferred embodiment, greater than 95 wt% of
the foam
contains materials that are compostable.
The compostable or biobased polymers of this invention are produced by melt
processing compostable or biobased polymers with blowing agents 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.
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,
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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.
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 may also be used. 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.
In this invention, a compostable or biobased polymer is melt processed with
the
blowing agents to produce a lightweight foamed bead. 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.
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
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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.
Chemical 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 C, with significant
decomposition
occurring between approximately 150 C and 200 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.
Suitable examples of blowing agents include water, carbonate and/or
bicarbonate
salts and other carbon dioxide releasing materials, diazo compounds and other
nitrogen
producing materials, carbon dioxide, decomposing polymeric materials such as
poly (t-
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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 or bicarbonate salts such as sodium, potassium, calcium, and/or
magnesium
carbonate salts. For example, the blowing agent may include sodium carbonate
and
sodium bicarbonate, or, alternatively, sodium bicarbonate alone. In yet
another
embodiment, the blowing agent may be inorganic.
This invention discloses an improvement on the production of lightweight
foamed
beads. In the improved process, both a physical blowing agent and a chemical
blowing
agent are combined during the extrusion processes for the production of
lightweight
foamed beads. The physical blowing agent, preferably supercritical CO2, is
used as the
primary source of the blowing agent during the production of lightweight beads
by
extrusion and hot face pelletization. By adding a chemical blowing agent to
the extrusion
process, such that the chemical blowing agent does not completely degrade
during
extrusion, the lightweight beads that are produced will retain some of the
chemical
blowing agent in their composition.
The secondary blowing agent may be incorporated in one of three ways. In the
first case, the secondary blowing agent may be incorporated upstream of the
primary
blowing agent. In the second case, the secondary blowing agent may be
incorporated
downstream of the primary blowing agent. And, in the third case, the secondary
blowing
agent may be incorporated simultaneously with the primary blowing agent.
Preferably,
for all cases, the primary blowing agent is a physical blowing agent like
supercritical
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CO2. This primary blowing agent is used to provide the majority of the
expansion during
extrusion to produce the foamed beads. The objective of the secondary blowing
agent is
to remain largely dormant during the extrusion and foamed bead formation so
that it can
be triggered during subsequent processing of the foamed bead in order to
enable further
expansion of the bead. The process of the present invention is carefully
designed so that
the secondary blowing agent is not completely consumed during the extrusion
foaming
process. The process of the present invention allows the secondary blowing
agent to
remain largely intact through the extrusion foam process, allowing the
secondary blowing
agent to be incorporated into the foamed bead.
It is contemplated that chemical blowing agents are most appropriate for use
as
the secondary blowing agent. For cases one and two, the chemical blowing agent
is
added into the polymer melt of the extruder before or after the primary
blowing agent is
injected into the melt. Due to the elevated temperatures of the melt, it is
possible that the
chemical blowing agent will begin to decompose and contribute gas that can
foam the
polymer. By controlling temperature of the melt and the residence time of the
polymer/blowing agent mixture in the extruder, the extent of decomposition of
the
blowing agent can be controlled. Some decomposition may occur to release gas,
but as
long as some of the blowing agent remains in the extrudate, the foamed beads
will
contain it.
For case three, the secondary blowing agent is mixed with the primary blowing
agent and injected into the polymer melt simultaneously. It is contemplated
that
supercritical CO2 is the primary blowing agent and a chemical blowing agent is
used as
the secondary blowing agent. The chemical blowing agent can be a liquid or a
solid. In a
CA 02770956 2012-03-09
preferred embodiment, supercritical CO2 may be used as a carrier phase to
dissolve the
chemical blowing agent to form a mixture. The mixture is then injected into
the barrel of
the extruder to mix with the polymer melt.
The inclusion levels of the blowing agent in the concentrate may vary widely.
In
some embodiments, a 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. The most preferred embodiment includes a concentration of
the
secondary blowing agent from 0.5 to 5% in the foamed bead.
Although the blowing agent composition may include only the blowing agents, a
more typical situation is where the blowing agent includes a polymeric carrier
that is used
to carry or hold the blowing agent. Such 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.
In another aspect of the invention, the compostable or biobased, melt
processable
composition may contain other additives. Non-limiting examples of additives
include
plasticizers, chain extenders, antioxidants, light stabilizers, fibers,
blowing agents,
foaming additives, antiblocking agents, heat stabilizers, impact modifiers,
biocides,
compatibilizers, tackifiers, colorants, coupling agents, electrically
conductive fillers, and
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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.
The amount of components in the melt processable, compostable or biobased
polymer 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 compostable or biobased plasticizer
may
comprise from about 1 to 20 percent by weight of the final composition. The
chain
extender may comprise about 0.1 to 5 percent by weight of the final
composition.
The physical blowing agent, such as supercritical CO2, is combined with the
melt
early in the extruder mixing process. In one embodiment, the chemical blowing
agent is
added after the physical blowing agent, and in a relatively cooler portion of
the extruder.
In another embodiment, the chemical blowing agent is added before the physical
blowing
agent, again in a relatively cooler portion of the extruder. Then, as the
mixture exits the
extruder and is cut, the supercritical CO2 expands to form the initial beads.
These beads
have the chemical blowing agent already impregnated in them during the
extrusion
process. The process must be carefully controlled so that the secondary
blowing agent is
not completely consumed during the extrusion foaming process. Due to the low
temperature used in the extrusion process at the point of addition of the
chemical blowing
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agent, it can remain dormant during the extrusion process. Subsequently,
heating of the
beads during a secondary expansion process, will liberate gases by thermal
decomposition of the chemical blowing agent and thus, when combined with the
right
temperature for softening the plastic, allow for expansion of the material to
lower density.
During molding for example, the beads are heated so they will melt together
and the
chemical blowing agent is activated causing a secondary expansion. That is,
the thermal
degradation of the blowing agent could be triggered during molding to enable
fusion of
the beads or a traditional pre-expansion operation to further lower density.
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. In one preferred embodiment, the
chemical
blowing agent is incorporated into the extrusion process downstream of the
injection and
mixing of the physical blowing agent. However, as described above, the
secondary
blowing agent can be incorporated in the melt upstream of the injection and
mixing of the
physical blowing agent or simultaneously with the physical blowing agent.
Typically, it
is necessary to cool the extrusion mixture before exiting the die in order to
maintain
adequate melt strength and enable good cell structure of the foam. By adding
the
chemical blowing agent in the cooler region of the extruder, there is less
thermal energy
for decomposition of the chemical blowing agent and the resonance time of the
material
in the extruder is decreased. The materials (biodegradable polymer, blowing
agent,
biodegradable plasticizer, and optional additives) may be used in the form,
for example,
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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 lightweight strands
and
subsequently cut into beads using a strand pelletizer. In another embodiment,
foamed
beads are produced by cutting the foamed strand 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 physical 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 beads can be
molded into
a three-dimensional part using conventional equipment utilized in molding
expandable
polystyrene. The objective of the secondary blowing agent is to remain largely
dormant
so that it can be triggered during subsequent processing of the foamed bead to
enable
further expansion of the bead. Preferably, the foamed beads contain residual
chemical
blowing agent and can be post expanded in the molding process.
Melt processing typically is performed at a temperature from about 80 to 300
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.
19
CA 02770956 2012-03-09
In one embodiment, the resulting compostable or biobased foamed bead has a
specific gravity less than 0.15 g/cm3. In another embodiment, the compostable
or
biobased foamed bead has a specific gravity of preferably less than 0.075
g/cm3, and
most preferably less than 0.05 g/cm3.
In one embodiment, more than 50 wt% of the foam contains materials that are
compostable, as determined by ASTM D6400. In a preferred embodiment, more than
80
wt% of the foam contains materials that are compostable. In a most preferred
embodiment, greater than 95 wt% of the foam contains materials that are
compostable.
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
CA 02770956 2012-03-09
practiced otherwise than as specifically set forth herein. Consequently, the
present
embodiments are to be considered in all respects as illustrative and. not
restrictive.
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