Microcellular Recycled PET Foams for Food Packaging Vipin Kumar 1, 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 59 th Ave NE Arlington, WA 98223, USA 1 Corresponding author email: vkumar@u.washington.edu Abstract In this paper results on the solid-state microcellular processing of recycled polyethylene terephthalate (RPET) using sub-critical CO 2 are presented. It was found that RPET can be foamed using CO 2 resulting in uniform microcellular structures. Also, similar to virgin PET, RPET crystallizes in the presence of high levels of dissolved CO 2. 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 10 8 cells per cm 2 [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 (T g ) 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 solidstate 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-CO 2 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 CO 2 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.
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 C0 2 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 CO 2 for a total of over 200 hours. Desorption The rate at which gas diffuses out of a solid gassaturated 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 CO 2 concentration increases to a maximum and then starts decreasing to some lower value. This reduction in gas concentration is due to the phenomenon of CO 2 -induced crystallization which causes reduction in CO 2 solubility.
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 CO 2 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 CO 2 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 CO 2 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 CO 2 gas in RPET is similar to that seen in virgin PET. Similar to virgin PET, RPET shows evidence of CO 2 induced crystallinity during CO 2 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 21 st 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
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 CO 2 -Induced Morphology in Crystalline PET Foams, SPE Technical Papers, v 38, p 1536-1540 CO 2 Concentration (mg/g of PLA) 100 80 60 40 20 0.71 mm 0.89 mm 1.07 mm 0 0 50 100 150 200 Saturation Time (hrs.) Figure 2: Plot of CO 2 gas concentration as a function of saturation time for RPET sheet of various thicknesses. Note that upon reaching a maximum level, the CO 2 gas concentration decreases due to gas-induced crystallization of semi-crystalline polymers. 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. Sorption Percent Relative Skin Small Cell Large Time Crystallinity Density Thickness Size 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 Figure 3: A plot comparing the sorption behavior of RPET and virgin PET sheet. 1.2 Saturated Sample Supersaturated Sample Foamed Sample 1.0 CO2 gas cylinder Pressure Vessel Stage I SATURATION OF SPECIMEN Stage II DESORPTION OF SPECIMEN Heated Bath Stage III FOAMING OF SPECIMEN Tfoam ~ Tg Figure 1: Schematic of the solid-state microcellular foaming process. Relative Density 0.8 0.6 0.4 0.2 0.0 0 50 100 150 200 Saturation Time (hrs.) 70 C 100 C Figure 4: A plot showing the effect of saturation time on relative density for RPET sheets foamed at various temperatures.
30 16 25 14 Percent Crystallinity 20 15 10 5 Percent Crystallinity 12 10 8 6 4 2 Unsaturated Sheet Saturated Sheet 0 0 50 100 150 200 250 300 Sorption Time (hrs.) 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. 0 0 2 4 6 8 10 12 14 16 18 Heating Time (s) Figure 8: A plot showing the effect of dissolved CO 2 on the crystallinity of saturated RPET as compared to unsaturated RPET. Note that the dissolved CO 2 lowers the temperature at which crystallization occurs. Figure 6: Micrographs showing the microstructure developing in RPET with increasing saturation time. Figure 9: Photographs showing thermoformed deep-draw coffee cups made from microcellular RPET sheet. Note the smooth, high-gloss surface finish. 1.2 1.0 Relative Density 0.8 0.6 0.4 0.2 0.0 0 2 4 6 8 10 12 14 16 18 Heating Time (s) Figure 7: A plot showing decreasing relative density of microcellular RPET with increasing IR heating time.
Microcellular Recycled PET Foams for Food Packaging Vipin Kumar, Stephen Probert, Michael Waggoner, Lee Kroeger, Krishna Nadella Microcellular Plastics Lab University of Washington, Seattle, USA Greg Branch MicroGREEN Polymers Inc. Arlington, WA
Overview Introduction Solid-State Microcellular Foaming Process Saturation and Crystallization Foaming and Microstructure Thermoforming Scale-up Conclusion
Introduction Solid-State Microcellular Plastics Thermoplastic foams characterized by cells with diameter in 1-100 micrometer range. Controllable cell sizes. Controllable densities, ρ rel = 0.1 to 0.99
PVC ABS PC PMMA
Solid State Microcellular Process
Process Variables T sat t sat P env t foam process parameters process variables (inputs) P sat Gas Saturation Process C ini process properties (outputs) t d Controlled Gas Desorption C Foam Generation Process vf D T foam N 0
Desired Structure of Microcellular Foam Sheets Foamed Core Density = ρ Unfoamed Skin Thickness, h
Background: Skin Creation C C o Uniform concentration at t=0 Concentration profile after time t Integral skin thickness at time t C min Minimum concentration for cell nucleation - l /2 + l /2 X Foamed core
RPET Saturation Behavior Saturation Pressure: 5 +0.2 MPa
Saturation - RPET Vs. PET Saturation Pressure: 5 +0.2 MPa
PET/RPET Crystallinity vs Saturation Time Saturation Pressure: 5 +0.2 MPa
RPET Rel. Density vs Saturation Saturation Pressure: 5 +0.2 MPa
Microstructure vs Saturation Time Saturation times are listed above bars.
IR Foaming: RPET Relative Density vs Heating Time Saturated Pressure = 5 MPa, Heater Temperature = 300-350 C
IR Foaming : RPET Crystallinity vs Heating Time CO 2 induced plasticization allows crystallization at lower temperatures
Microcellular RPET Thermoforming Hot Beverage Cups Draw ratio of 2.41 Smooth, high-gloss surface finish Heat Resistant up to 175 C
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.
Conclusions Solid-state microcellular process can be used to create high-quality low density foams from recycled PET. The solubility and diffusivity of CO 2 in RPET is similar to that seen in virgin PET. Similar to PET, RPET shows evidence of CO 2 induced crystallization during CO 2 sorption. Microcellular RPET sheets can be thermoformed to create deep-draw, heat-resistant shapes such as coffee cups.