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Microcellular Recycled PET Foams for Food Packaging
Vipin Kumar1, Michael Waggoner, Lee Kroeger, Stephen M. Probert,
and Krishna Nadella Department of Mechanical Engineering
University of Washington Seattle, WA 98195, USA
Greg Branch
MicroGREEN Polymers Inc. 19421 59th Ave NE
Arlington, WA 98223, USA
1Corresponding author email: [email protected]
Abstract In this paper results on the solid-state
microcellular
processing of recycled polyethylene terephthalate (RPET) using
sub-critical CO2 are presented. It was found that RPET can be
foamed using CO2 resulting in uniform microcellular structures.
Also, similar to virgin PET, RPET crystallizes in the presence of
high levels of dissolved CO2. This phenomenon was employed to
create foams with varying levels of crystallinity, thus enabling
varying levels of service temperatures. As an example of the
potential applications for such microcellular RPET foams, a 350 mL
coffee cup with an area draw ratio of 2.41 was thermoformed from a
microcellular RPET sheet of 20% relative density (ratio of density
of foam to density of solid). The cups have smooth glossy surfaces,
an average wall thickness of 1 mm and are heat-stable to 175 C.
Introduction Microcellular foams are generally closed-celled
and
have a bubble size on the order of 10 μm with a cell density of
108 cells per cm2 [1-3]. These foams can attain relative densities
as low as 10%. The solid-state microcellular foaming process (which
is conducted with on polymers in the solid state as opposed to
liquid-state foaming) involves saturation by the solid polymer with
an inert gas in a high pressure environment for a period of time
which will produce a certain gas concentration in the polymer
matrix. A schematic of the solid-state microcellular process is
shown in Fig. 1. After saturation, the sample is removed from the
high pressure environment thereby causing supersaturated
polymer-gas solution. This super-saturation results in the creation
of a large number of bubble nucleation sites, which increase with
increasing saturation pressure and gas concentration. Following the
removal from the pressure vessel, the polymer sample is foamed by
heating close to or above the glass transition temperature (Tg) of
the neat polymer by a variety of methods such as hot water, hot
oil, infrared radiation, steam, or hot air. During the foaming
stage the nucleated bubbles grow, causing volume expansion of the
sample
and associated reduction in density. After a period of time has
passed in the heating medium such that a desired foam density has
been achieved, the sample is removed from the heat source and is
quenched immediately by introducing it into a cooling medium in
order to stop the growth of bubbles. The main factors that
influence the resulting microstructure and foam density produced by
the solid-state process are the saturation time, saturation
pressure, foaming time, and foaming temperature. In this paper, the
results of the solid-state microcellular process characterization
of the RPET-CO2 system are presented.
Experimental Material and Equipment
For this study, extruded amorphous poly(ethylene terephthalate)
(PET) film (type 12822) was obtained from Eastman in 0.76 mm
thickness. Extruded amorphous recycled PET (RPET) film with an
inherent viscosity of 0.7 was obtained from LaVergne in thicknesses
of 0.71 mm, 0.89 mm, and 1.07 mm. All the materials were processed
in the as-received condition. For the gas saturation step, the ABS
square samples were placed in a 0.28 m diameter and 0.635 m long
pressure vessel. The pressure vessel, made by Ken-Weld Co. Inc, is
rated for use up to a maximum pressure of 10.34 MPa at 0 C. The
pressure inside the vessel was regulated using an OMEGA CN8500
process controller with a resolution of ±0.01 MPa. A Mettler-Toledo
AE240 precision balance with an accuracy of 10 μg was used to
measure the gas solubility in solid PET and RPET during saturation.
The same balance was used to measure the density of microcellular
RPET samples. The microstructure characterization was conducted
using a FEI Sirion XL 30 scanning electron microscope (SEM). For
the thermoforming a three-motion, pneumatically actuated, lab-scale
Illig pressure/vacuum former with a 20 X 20 cm forming area was
used. For RPET samples that were used to characterize CO2 induced
crystallinity the foaming experiments were conducted in HAAKE
silicone oil bath (accuracy of ±2 C). For thermoforming experiments
the microcellular RPET samples were prepared by foaming between the
IR panels of the lab-scale Illig thermoformer.
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For measuring CO2 induced crystallinity in the RPET differential
scanning calorimetry (DSC) experiments were conducted using a
NETZSCH DSC instrument. Sorption
Fundamental characterization of the sorption behavior of the
material is crucial to understand the behavior of a specific
gas-polymer system. 25 mm square RPET samples of varying
thicknesses were prepared from sheet stock and saturated with C02
gas at a saturation pressure of 5.0 + 0.2 MPa and a saturation
temperature of 21 ± 3°C. The mass gain of the samples was measured
periodically by briefly removing them from the high pressure
environment. After the measurement the samples were put back into
the pressure vessel for further sorption. During this measurement
process the samples were never taken out of the pressure vessel for
more than 10 minutes. The RPET samples were exposed to high
pressure CO2 for a total of over 200 hours. Desorption
The rate at which gas diffuses out of a solid gas-saturated
polymer is important as it indicates how much gas will be present
in the matrix at the time when foaming is conducted. Desorption
experiments were performed as follows: 25 mm square RPET samples
were saturated for 50 hours under the same process conditions as in
the sorption experiments. After saturation the samples were removed
from the pressure vessel and left at atmospheric pressure to desorb
at room temperature. An initial mass recording was taken upon
removal from the vessel followed by periodic mass recordings.
Effect of Saturation Time
As the time that a sample is subjected to a high pressure
environment increases, the concentration of the dissolved gas in
the polymer matrix increases. As the gas concentration increases,
the chain mobility is increased within the matrix due to
plasticization [4]. As a result of this the mobile chains align
themselves in the thermodynamically favored crystalline state. The
crystallinity in the polymer increases the toughness and strength
of the material. When compared to amorphous polymers, this increase
in crystallinity causes a reduction in the amount of volume
expansion that can occur during foaming. The increasing
crystallinity simultaneously causes a reduction in gas solubility
of the matrix, which also affects the resulting foam density. Due
to these interactions, characterizing the effect of sorption time
on the resultant foam density is critical.
Circular samples of 38 mm diameter and 1.07 mm thickness, were
saturated at 5 MPa for different saturation times ranging between
12 and 230 hours. The specimens were foamed by dipping in a heated
silicone oil bath that was held at 70 or 100 C. The samples
remained in the oil bath for 30 seconds which was shown to be long
enough to
complete foaming. After foaming, the density of the samples was
measured by ASTM standard D792-91 [5].
In order to better understand the way in which crystallinity
depresses the void fraction of the foam, the crystallinity of the
samples foamed in the water bath were measured using a differential
scanning calorimeter (DSC). Scanning electron microscopy (SEM) was
conducted on the sample cross-section in order to study the
microstructure of the RPET foams. The analysis reveals resulting
bubble size, bubble density, and skin thickness. The SEM
micrographs were analyzed using NIH’s Image J analysis software.
Bubble sizes were calculated as an average of ten bubble width
measurements. Infrared Heating
For thermoforming experiments, samples were prepared from 1.07
mm thick RPET sheets in 15.2 X 10.2 cm sheets. These sheets were
saturated for 50 hours at 5 MPa in a comparable manner to that
described in the desorption experiment. After saturation the
samples were heated in the thermoformer’s IR heating station which
consists of an upper and lower array of eight IR heaters each. The
temperature of the heaters was regulated using a duty cycle timer
that was set to 20% for these experiments. The samples were heated
for times varying between 4 and 16 seconds thereby causing foaming.
The IR heaters surface temperature ranged from 300-350 C and the
heaters were at a distance of 75 mm from the top and bottom
surfaces of the RPET sheet. In order to ensure consistent heating
during the volume expansion, the foaming RPET sheet was held in a
spring frame that accommodates the growth in the in-plane
directions thereby keeping it relatively flat. Foam density, SEM
and DSC measurements were performed on all the samples made using
IR heating. Thermoforming
For thermoforming experiments, previously foamed RPET sheets
were heated and then formed into female cup mold. A convex bottom
plug was used to pre-stretch the hot microcellular RPET sheet.
Final stretching was done using a combination of molding air (with
pressures ranging from 60 – 120 psi) and vacuum. After
thermoforming, the heat-resistance of the cups was determined by
pouring hot silicone oil at different temperatures into the cups
and visually observing for changes in shape.
Results and Discussion Sorption
Fig. 2 shows a plot of gas concentration as a function of
saturation time for RPET samples of different thicknesses. As can
be seen shows that the CO2 concentration increases to a maximum and
then starts decreasing to some lower value. This reduction in gas
concentration is due to the phenomenon of CO2-induced
crystallization which causes reduction in CO2 solubility.
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In order to understand the effects of recycling on the sorption
behavior, the sorption curves for RPET were compared to those of
virgin PET. RPET sorption data are compared to that of PET sorption
data reported by Kumar and Stolarczuk [6]. Fig. 3 shows plots of
gas concentration versus saturation time that has been normalized
by squaring the ratio of the thicknesses for both PET and RPET.
These plots clearly show that the sorption behavior and solubility
of PET and RPET is identical for given saturation conditions. This
implies that any differences between PET and the recycled PET such
as inherent viscosity (IV) do not significantly affect CO2
solubility and diffusivity of these materials. Effect of Saturation
Time
The effect of increasing saturation time on relative density of
microcellular RPET samples foamed at 70 and 100 C is shown in Fig.
4. The shapes of the two curves are identical, but offset for
saturation times below 144 hours. The dramatic jump in relative
density just above 144 hours is due to an increase in crystallinity
at this point, as shown in Fig. 5. It is clear from Fig. 4 that in
this process, microcellular RPET foams in the relative density
range of 20-30 % can be attained at multiple saturation times. The
crystallinity curve for RPET shown in Fig 5 is very similar to the
crystallinity curve for PET reported in literature [6, 7].
In Fig. 6, micrographs of microcellular RPET samples made at
increasing saturation times are arranged side by side to aid in
studying the effects of increasing saturation time and
crystallinity on the microstructure. It is apparent that the
samples saturated at 36 and 72 hours are not fully saturated
(resulting in non-uniform gas concentration through thickness)
because of the inconsistent bubble size across the cross-section.
From Fig. 6 it is clear that the 1.07 mm thick RPET reaches highest
gas concentration at approximately 90 hours. After 90 hours
saturation time, the microstructure of the foam samples reveals
uniformly distributed micro bubbles due to uniform gas
concentration across the thickness of the sample. At 144 hours, the
sample has reached crystallinity of 24% thereby resulting in an
unfoamed matrix upon heating to 100 C. Table 1. summarizes the
crystallinity, relative density, and SEM results for microcellular
RPET foams created at a 100 C foaming temperature. Infrared
Heating
In Fig.7, the effect of IR heating time on microcellular RPET
relative density is shown. It can be seen that as the heating time
increases the relative density decreases reaching 20% at 16 s
heating time. This data shows that IR heating can be used to gain
similar densities as with bath foaming; as low at 20%. The effect
of dissolved CO2 on the crystallinity of the RPET sheet during IR
foaming is shown in Fig. 8. For IR heating times above 5 seconds
the crystallinity of saturated RPET sheet increases with
increasing heating time. In contrast, for an unsaturated RPET
sheet the crystallinity remains more or less constant with
increasing IR heating time. This difference is due to the CO2
induced plasticization of saturated RPET sheets, which enables the
polymer matrix to crystallize at lower temperatures compared to
unsaturated RPET sheet. Thermoforming
The thermoforming experiments resulted in high quality coffee
cups that had an area draw ratio of 2.41. The coffee cups had
smooth, high gloss, surfaces both on the inside and out which gave
the cup the look of a solid part (see Fig. 9). This in contrast to
either extruded or compression molded PS foam cups that have a dull
finish. Heat resistance testing of the coffee cup using hot
silicone oil showed that the cups were heat stable up to a
temperature of 175 C making thermoformed articles made from
microcellular RPET foams suitable for microwave cooking
applications.
Conclusions In this study it has been shown that,
• Solid-state microcellular process can be used to create
high-quality consistent low-density foams from recycled PET
(RPET).
• The solubility and diffusivity of CO2 gas in RPET is similar
to that seen in virgin PET.
• Similar to virgin PET, RPET shows evidence of CO2 induced
crystallinity during CO2 sorption.
• Solid-state microcellular RPET sheets of relative density as
low as 20% can be thermoformed to create deep-draw, heat resistant
shapes such as coffee cups.
Acknowledgements The authors would like to thank the
Washington
Technology Center and MicroGREEN Polymers Inc. for providing
funding for this work. SEM work was performed in the NanoTech User
Facility (NTUF) at the University of Washington, a member of
National Nanotechnology Infrastructure Network supported by
NSF.
References 1. Martini-Vvedensky J.W., Suh N.P., and Waldman
F.A., 1994, “Microcellular Closed Foams and Their Method of
Manufacture,” U.S. Patent No. 4,473,665.
2. Kumar, V., “Microcellular Polymers: Novel Meaterials for the
21st Century,” Progress in Rubber and Plastics Technology, v 9, n
1, 1993, p 977-982
3. Martini J., Waldman F.A., and Suh N.P., (1982), “The
Production and Analysis of Microcellular Foam,” SPE Tehnical
Papers: XXVII, p 674-676
4. Zhang Z., and Handa Y.P., (1998), “An In-Situ Study of
Plasticization of Polymers by High-Pressure Gases,” Journal of
Polymer Science: Part B, Polymer Physics, v 36, p977-982
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5. ASTM D-792, “Standard Test Methods for Density and Specific
Gravity of Plastics by Displacement,” ASTM International (2002),
West Conshohocken, PA.
6. Kumar, Vipin and Paul J. Stolarczuk. “Microcellular PET Foams
Produced by the Solid-State Process.” Annual Technical Conference -
ANTEC, Conference Proceedings, v 2, 1996, p 1894-1899
7. Kumar, V., and Gebizlioglu, O.S., 1992, “Thermal and
Microscopy Studies of CO2-Induced Morphology in Crystalline PET
Foams,” SPE Technical Papers, v 38, p 1536-1540
Table 1. Summary of microcellular RPET properties as a function
of saturation time. The samples were saturated at 5 MPa and foamed
at 100 C for 30 seconds in a heated silicone oil bath.
Saturation Time (hrs.)
0 50 100 150 200
CO
2 Con
cent
ratio
n (m
g/g
of P
LA)
0
20
40
60
80
100
0.71 mm0.89 mm1.07 mm
Figure 2: Plot of CO2 gas concentration as a function of
saturation time for RPET sheet of various thicknesses. Note that
upon reaching a maximum level, the CO2 gas concentration decreases
due to gas-induced crystallization of semi-crystalline
polymers.
Figure 3: A plot comparing the sorption behavior of RPET and
virgin PET sheet.
Saturation Time (hrs.)
0 50 100 150 200
Rel
ativ
e D
ensi
ty
0.0
0.2
0.4
0.6
0.8
1.0
1.2
70 C100 C
Figure 4: A plot showing the effect of saturation time on
relative density for RPET sheets foamed at various
temperatures.
Sorption Time
Percent Crystallinity
Relative Density
Skin Thickness
Small Cell Size
Large Cell Size
36 12% 0.28 20 μm 15 μm 100 μm 72 12% 0.27 20 μm 15-35 μm 70
μm
108 12% 0.33 65 μm 15 μm 25 μm 120 14% 0.33 100 μm 15 μm 15 μm
132 17% 0.43 150 μm 15 μm 15 μm 144 24% 0.99 150 μm 0 0
CO2 gas cylinder
Pressure Vessel
Stage I SATURATION OF
SPECIMEN
Saturated Sample
Heated Bath
Stage III FOAMING OF
SPECIMEN
Foamed Sample
Stage II DESORPTION OF
SPECIMEN
Supersaturated Sample
Tfoam ~ Tg
Figure 1: Schematic of the solid-state microcellular foaming
process.
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Sorption Time (hrs.)
0 50 100 150 200 250 300
Perc
ent C
ryst
allin
ity
0
5
10
15
20
25
30
Figure 5: A plot showing increasing crystallinity as sorption
time increases. Note that beyond 100 hours of saturation, the
crystallinity of RPET rapidly increases to reach a plateau at 24
percent.
Figure 6: Micrographs showing the microstructure developing in
RPET with increasing saturation time.
Heating Time (s)
0 2 4 6 8 10 12 14 16 18
Rel
ativ
e D
ensi
ty
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Figure 7: A plot showing decreasing relative density of
microcellular RPET with increasing IR heating time.
Heating Time (s)
0 2 4 6 8 10 12 14 16 18
Perc
ent C
ryst
allin
ity
0
2
4
6
8
10
12
14
16
Unsaturated SheetSaturated Sheet
Figure 8: A plot showing the effect of dissolved CO2 on the
crystallinity of saturated RPET as compared to unsaturated RPET.
Note that the dissolved CO2 lowers the temperature at which
crystallization occurs.
Figure 9: Photographs showing thermoformed deep-draw coffee cups
made from microcellular RPET sheet. Note the smooth, high-gloss
surface finish.
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Microcellular Recycled PET Foams for Food
PackagingVipin Kumar, Stephen Probert, Michael
Waggoner, Lee Kroeger, Krishna NadellaMicrocellular Plastics
Lab
University of Washington, Seattle, USA
Greg BranchMicroGREEN
Polymers Inc.Arlington, WA
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Overview
IntroductionSolid-State Microcellular Foaming ProcessSaturation
and CrystallizationFoaming and
MicrostructureThermoformingScale-upConclusion
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Introduction
Solid-State Microcellular Plastics
Thermoplastic foams characterized by cells with diameter in
1-100 micrometer range.Controllable cell sizes.Controllable
densities,
ρrel
= 0.1 to 0.99
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PVC
PC
ABSABS
PMMAPMMA
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Solid State Microcellular Process
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Process Variables
vf
D
N0
Gas Saturation Process
Foam
Generation Process
Controlled Gas
Desorption
Tsat tsat Penv tfoam
Psat
Tfoam
proc
ess
varia
bles
(in
puts
)
proc
ess
prop
ertie
s (o
utpu
ts)
process parameters
td
Cini
C
-
Foamed Core
Unfoamed Skin
Thickness, h
Density = ρ
Desired Structure of Microcellular Foam Sheets
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Background: Skin CreationC
- l /2 + l /2
Cmin
Co
X
Integral skin thickness at time t
Minimum concentration for cell nucleation
Uniform concentration at t=0
Foamed core
Concentration profile after time t
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RPET Saturation BehaviorSaturation Pressure: 5 +0.2 MPa
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Saturation - RPET Vs. PETSaturation Pressure: 5 +0.2 MPa
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PET/RPET Crystallinity vs Saturation Time
Saturation Pressure: 5 +0.2 MPa
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RPET Rel. Density vs SaturationSaturation Pressure: 5 +0.2
MPa
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Microstructure vs Saturation Time
Saturation times are listed above bars.
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IR Foaming: RPET Relative Density vs Heating Time
Saturated Pressure = 5 MPa, Heater Temperature = 300-350 C
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IR Foaming : RPET Crystallinity vs Heating Time
CO2
induced plasticization allows crystallization at lower
temperatures
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Microcellular RPET Thermoforming
Hot Beverage CupsDraw ratio of 2.41Smooth, high-gloss surface
finishHeat Resistant up to 175 C
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Commercial Process
Steps 1 & 2: Semi-continuous foaming. Developed and patented
by UW (US Patent No. 5684055). Licensed to MicroGREEN Polymers.Step
3: Gas-Impregnated Thermoforming, Developed by MicroGREEN. Patents
Pending.
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Conclusions
Solid-state microcellular process can be used to create
high-quality low density foams from recycled PET.The solubility and
diffusivity of CO2 in RPET is similar to that seen in virgin
PET.Similar to PET, RPET shows evidence of CO2induced
crystallization during CO2 sorption.Microcellular RPET sheets can
be thermoformed to create deep-draw , heat-resistant shapes such as
coffee cups.
GPEC 2007table of contentRG4 Kumar-Nadella- Probert PAPERRG4
Stephen Probert- Vipin Kumar -PRESENTATIONMicrocellular Recycled
PET Foams for Food PackagingOverviewIntroductionSlide Number 4Solid
State Microcellular ProcessProcess VariablesDesired Structure of
Microcellular Foam SheetsBackground: Skin CreationRPET Saturation
BehaviorSaturation - RPET Vs. PETPET/RPET Crystallinity vs
Saturation TimeRPET Rel. Density vs SaturationMicrostructure vs
Saturation TimeIR Foaming: RPET Relative Density vs Heating TimeIR
Foaming : RPET Crystallinity vs Heating TimeMicrocellular RPET
ThermoformingCommercial ProcessConclusions
