ENVIRONMENTALLY BENIGN PROCESSING OF POLY(2,6-DIMETHYL-1,4- PHENYLENE OXIDE) (PPO) WITH SUPERHEATED LIQUIDS

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1 ENVIRONMENTALLY BENIGN PROCESSING OF POLY(2,6-DIMETHYL-1,4- PHENYLENE OXIDE) (PPO) WITH SUPERHEATED LIQUIDS Md Arifur Rahman, Matthew Lok, Alan J. Lesser Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA Abstract In the expanding industry of polymer processing, a prominent area of current research is to process polymers efficiently without creating any environmental hazards. Processing of intractable polymers like PPO requires high processing temperature and toxic plasticizers. Very few research works have reported the use of superheated liquids to process intractable polymers. This research work presents a systematic study to explore the advantages of processing PPO with superheated liquids composed of ethanol and water. Microcellular foams of PPO having a density range from 0.13 to 0.56 g/cm 3 can be produced with the aid of superheated ethanol, water and ethanol/water mixtures. Such foams also exhibit high specific strength. In addition, PPO can also be extruded with superheated ethanol or ethanol/water mixtures at a temperature which is 150 to 180 ⁰C below the conventional extrusion temperature for PPO. Introduction Many high-performance polymers are not easily melt processable and tend to degrade during their processing at high temperatures. These polymers are referred to as intractable polymers that include polyethersulfone (PES), polyphenylene oxide (PPO), and Polyetherimide (PEI). PPO is a high performance, high heat resistance polymer which was originally synthesized by Allan Hay when General Electric company was looking for highly flame retardant, moldable polymers [1,2]. Despite of being high heat resistance, high elastic modulus, and high heat distortion temperature, PPO could not be processed without additive as it undergoes thermos-oxidative degradation during the processing. PS based elastomers and PA6 are melt or reactively blended with PPO to process PPO at lower temperature and, to improve the overall resilience [3-5]. In addition, solvent casting of PPO usually involves the use of anhydrous solvents that contain large amounts of volatile organic compounds (VOC's) that are of great environmental concern, and are often toxic or carcinogenic. Concerning the environmental safety and energy efficient processing of polymers, it would be a huge advantage to process PPO at low temperature with environment-friendly methods. It has been reported in literatures that hydrogen bonded liquids like water or ethanol in their superheated state (below the critical point) can act as green solvents for many polymers [6-7]. When water is superheated, the hydrogen bonds break. Thus, the water becomes less polar and acts like an organic solvent such as ethanol [8]. Since superheated water or ethanol can act like organic solvents they can aid in processing of polymers, especially if the polymer has polar groups [9]. Lesser et al. recently investigated the use of superheated liquids can be used to process several polar polymers including a series of different polyamides [10]. The superheated water was observed to lower the melting point of the polyamides which depends both on amide group density in polyamides and their crystalline morphology. Interactions between superheated water and the polyamide weakened the hydrogen bonding [10,11]. Hydrogen bonding is one of the causes of a high melting temperature so weakening the bond would lower the melting temperature. Water is also known to be absorbed in polyamides increasing the distance between the chain interactions [12, 13]. Benefits to using superheated water is that it does minimal change to the properties of the polymer. Other processes add blends to the polymer, which could make the processing easier, but could also alter the desirable properties of the polymer [8]. A reaction is not needed in superheated water processing, so the process does not need to worry about product yield. Superheated water is also a better alternative to organic solvents used as plasticizers in the polymer processing system. Most organic solvents are flammable, which makes the use of organic solvents in polymer processing very hazardous. In this paper, we report first time, the use of superheated water and ethanol as additive to extrude and foam PPO. Both water and ethanol are non-toxic compounds and both FDA approved for food and medical applications. Introducing this hydrogen bonded liquids at sufficient pressures and temperatures enable them to solvate, plasticize, or otherwise enhance the processability of polymers. Moreover, these compounds can be used as both transport media as well as blowing agents to create microcellular materials that may be used for sound and thermal insulation materials, non-woven products (fabrics), and ultra-filtration materials. The key element in producing foams involves the rapid depressurization allowing the superheated liquid to vaporize and expand the media as it exits the extrusion die or the reactor is depressurized. The focus of this research has been to investigate the feasibility of using superheated water and ethanol in PPO. SPE ANTEC Anaheim 2017 / 2454

2 Experimental Materials. Poly(2,6 dimethyl-1,4-phenylene oxide) (PPO) was supplied by GE Co. Carbon-dioxide was obtained from Airgas. Water (distilled water) and Ethanol (from Sigma-Aldrich) were used in this study. Thermal Analysis. Pressurized DSC was performed with a TA Instruments Q200 DSC in stainless steel pressure capsules. Heat/cool/heat experiments from 10 C to 260 C at 5 C/min were used. Samples consisted of approximately 3 mg PPO and 10 mg water. Reported glass transition temperatures are from first heating cycle. Standard DSC was performed with aluminum hermetic pans with heat/cool/heat experiments from 0 C to 250 C at 10 C/min. Thermogravimetric analyses (TGA) were performed on neat and processed PPO at a heating rate of 10 ⁰C/min in the heating range ⁰C. The tests were conducted under nitrogen gas. Batch Extrusion and foaming of PPO. Cylindrical reactors made of stainless steel were used to expose PPO and liquids at a specific pressure and temperature for a certain period. Reactors were attached to pressure gauge and regulators through high pressure tubing to maintain isobaric condition. Carbon dioxide (CO 2) gas was used to exert pressure in the reactor. Heating was induced through resistive heating tape wrapped around the reactor and temperature was calibrated with an internal thermocouple. Compression molded thick PPO sheet (~6 mm) was used for foaming. Two important steps involved in foaming of PPO; i) rapid depressurization by opening a pressure valve of the reactor and ii) cooling by immersing the reactor in water. Figure 1a shows a generalized sketch of batch foaming setup. We also report here a pressure driven extrusion of PPO with superheated water and ethanol through a specially designed reactor. Extrusion experiments were conducted with a reactor modified with a removable plug at the base of the vessel. A diagram of the apparatus is provided in Figure 1(b). Pressurization and heating were conducted using the same methods as batch foaming experiments. Samples were heated to an elevated temperature then cooled to the target extrusion temperature and soaked for a period. Upon removal of the plug a leak path would open with two cylindrical portions at a right angle as in Figure 1b. Both cylindrical portions were 2.2 mm in diameter. The first portion was 7.5 mm in length and the second portion was 16 mm in length. Time of extrusion and mass of extrudate were recorded. Density measurement. A liquid displacement method (ASTM D , W-B) was used to measure the overall density of the foam/film samples. Scanning electron microscope. To observe the morphology of both extrudate and foams, scanning electron microscope (JEOL) was used. Cryofractured samples were coated with gold in high pressure sputter coating machine prior to testing in SEM. Compression test. Uniaxial compression tests were carried out on cylindrical foam samples having a dimension of mm at a speed of 1 mm/min. Results & Discussion Influence of superheated ethanol and water on the Tg of PPO. The processability of PPO is mainly limited due to its high T g and presence of ether group which can undergo thermo-oxidative degradation during processing. To observe the effect of superheated ethanol or water to the T g of PPO, DSC was carried out on PPO with ethanol or water in high pressure (>1 atm) stainless steel pan. Figure 2 shows the DSC thermograms of neat PPO tested in aluminum pan without any liquids and PPO tested with superheated ethanol and water. The neat PPO shows a T g at 215 ⁰C whereas T g of PPO decreases to 175 ⁰C and 110 ⁰C with water and ethanol, respectively, in superheating condition. Such depression of T g of PPO with superheated ethanol or water can be attributed to their solvation or plasticization effect on PPO. However, this requires further investigation to the change in macromolecular structure or molecular weight of PPO in superheating condition which is an interest of our future study. Extrusion of PPO with superheated ethanol, water and ethanol/water mixture. The depression of T g of PPO with superheated ethanol or water may facilitate the processing of PPO at lower temperature. Batch extrusion of PPO with superheated ethanol, water and ethanol/water mixtures was carried out in cylindrical stainless steel reactor as described earlier. PPO with superheated ethanol was extruded at 220, 200, 180 and 160 ⁰C. However, PPO could not be processed with superheated water. However, Ethanol/water (50/50, 70/30) mixtures were used successfully to process PPO. To extrude PPO with superheated liquids, a specific pressure and temperature needs to be maintained in the reactor for a specific period. Upon depressurization met flow of the PPO can be obtained. However, the PPO extrudate obtained are porous which is formed due to the SPE ANTEC Anaheim 2017 / 2455

3 evaporation of liquids and release of CO 2 gas during depressurization as the vapor pressure of liquids decreases at room temperature. Figure 3 shows the SEM images of porous extrudates. Pores in the surfaces well as at the cross-section of extrudates are evident. The size of pores at the cross-section of extrudates range from 10 to 50 µm whereas larger pores (<100 µm) can be observed on the surface due to expansion toward extrusion direction. Closed pores in extrudates can be attributed to their low density which is reported in Table 1. The density of extrudates range from 0.4 to 0.6 g/cm 3. The viscosity of PPO melt flow can be approximated from the mass flow rate of PPO through the exit point of the reactor. The capillary flow of the viscous PPO fluid can be measured using the Poiseullie s equation assuming the existence of laminar flow of a Newtonian fluid through the cylindrical reactor. Q =.. (1) Where ΔP is the change in pressure along the capillary, ɳ is the viscosity, L is the length of the capillary, Q is the volumetric flow rate, and r is the capillary radius. The volumetric flow rate is known by timing the continuous production of the extrudate. Table 1 reports the dynamic viscosity calculated at each temperature for PPO processed either with ethanol or ethanol/water mixtures. Two important things can be noticed; with superheated ethanol PPO can be processed at 160 ⁰C which is about 60 ⁰C lower than the Tg of neat PPO and 150 to 160 ⁰C lower than the conventional processing temperature for PPO (i.e ⁰C). In addition, PPO can also be processed with superheated ethanol/water mixtures (50, 70% ethanol) at 220 ⁰C. Table 1. Density of extrudates and dynamic viscosity of PPO processed with superheated ethanol and ethanol/water mixtures. Superheated Liquids T (⁰C) Ethanol 160 Processing Condition P (psi) ρ (g/cm 3 ) ɳ (Pa s) Ethanol Ethanol Ethanol Ethanol/water (30/70) 220 na na Ethanol/water (50/50) Ethanol/water (70/30) Structure-property relationship of PPO foams processed with superheated ethanol and ethanol/water mixtures. PPO foams were produced with superheated ethanol at the subcritical pressure (800 psi), at 120, 130 and 140 ⁰C. Images produced from an SEM analysis on PPO foams processed at the designated temperatures are shown in Figure 4a. SEM images show that closed cell PPO foams can be produced in superheated ethanol or water at the subcritical pressure. Processing temperature of 130⁰C and 140⁰C are 20⁰C and 30⁰C above the observed glass transition temperature, which justifies that PPO can be foamed at a very low temperature in superheating condition at subcritical pressure. SEM images also shows that cell sizes range from µm. To produce microcellular PPO foams, supercritical CO 2 was used with superheated ethanol, water and ethanol/water mixtures. PPO foams with closed cells, having cell size ranging from µm can be produced with superheated liquids at supercritical pressure (4000 psi). Density of PPO foams processed with subcritical or supercritical CO 2, superheated water, and superheated ethanol are shown in Table 2. It can be noticed that foams produced at subcritical pressure with superheated ethanol exhibit a density of 0.13 g/cm 3. This indicates that PPO foams produced at subcritical pressure exhibit porosity around 90%. On the other hand, PPO foams produced with superheated ethanol, water and ethanol/water mixture produce microcellular foams with slightly increased density. PPO foams produced with water exhibits higher density than other foams which may be due to their low cell density. Table 2. Bulk density and average cell size of PPO foams produced with superheated liquids. PPO Foams Neat PPO Macrocellular Microcellular Foaming Process ρ (g/cm 3 ) Cell size (µm) Liquids T P (⁰C) (psi) Ethanol Ethanol Ethanol Ethanol/ Water (70/30) Water It is known that the cell size, shape and density affect the mechanical properties of foams. Uniaxial compression SPE ANTEC Anaheim 2017 / 2456

tests were carried out on PPO foams. Figure 5 (a, b) shows the stress-strain behavior, Specific modulus and stress of PPO foams as the function of their relative density.

4 tests were carried out on PPO foams. Figure 5 (a, b) shows the stress-strain behavior, Specific modulus and stress of PPO foams as the function of their relative density. The effect of cell size and density is clear from the stress-strain plot. The microcellular PPO foams (processed with superheated water, cell size µm) shows higher yield stress than the macroscopic cells ( µm) containing PPO. As expected, macrocellular PPO foams exhibit large progressive buckling after yield which can be attributed to their uniform cell size distribution. PPO foams also exhibit high specific modulus and specific stress as a function to their relative density. The specific modulus and specific strength of PPO foams are comparable to PS based microcellular foams produced by supercritical CO 2. Conclusions PPO was successfully processed with superheated ethanol, water and ethanol/water mixtures. With superheated ethanol PPO was extruded at significantly low temperature which is also 150 ⁰C lower than the conventional extrusion temperature for PPO. In addition, continuous production of porous PPO was possible through the pressure extrusion method. Most importantly, microcellular foams of PPO with the density ranging from 0.13 to 0.5 g/cm 3 can be produced with the aid of superheated water, ethanol or water/ethanol mixtures. The PPO microcellular foams can be used in many different applications because of the foam s ability to exhibit high specific strength. 8. Brunner, G. The Journal of Supercritical Fluids 47.3 (2009): Pijpers, M. F. J., V. B. F. Mathot, and R. L. Scherrenberg. Technical Report RC DSM Research, Lesser, A. J., & Evans, G. C. U.S. Patent No. US 2016/ A1. (2016) 11. G.C Evans. Engineering polymers through impact modification and superheated liquid processing. Diss. UMA Wevers, Marjoleine GM, et al. Progress in Understanding of Polymer Crystallization. Springer Berlin Heidelberg, Wevers, Marjoleine GM, Thijs FJ Pijpers, and Vincent BF Mathot. Thermochimica acta (2007): Arora, KA., Lesser, AJ., McCarthy T. Polymer Eng. Sci. 38 (1998), Ruoij, JAR., Viot, P., Dumon, M. Cellular Polymers 28 (2009), 363. Acknowledgements The authors gratefully acknowledge the Center for University of Massachusetts Industrial Research on Polymers (CUMIRP) research Clusters M for their financial support. References 1. Hay, Allan S. U.S. Patent 3,306,875. (1967). 2. Hay, A. S., and R. F. Clark. Macromolecules 3.5 (1970): Cizek, Eric P. U.S. Patent No. 3,383,435. (1968). 4. R. Nagelsdiek, H. Keul and H. Hocker, Polym Int 55: (2006) 5. S. Y. HOBBS, M. E. J. DEKKERS, J. MAT SCI, 24 (1989) Galkin, Aleksandr Aleksandrovich, and Valery Vasil'evich Lunin. Russian Chemical Reviews 74.1 (2005): Dannhauser, Walter, and Lowell W. Bahe. III. The Journal of Chemical Physics40.10 (1964): Figure 1. Generalized sketch of batch (a) foaming and (b) pressure drive extrusion setup. SPE ANTEC Anaheim 2017 / 2457

Images at the top shows the morphology of PPO foam processed at 130 ⁰C (a)

5 Figure 2. DSC thermograms of PPO with superheated ethanol (up), superheated water (middle), and neat (bottom). Figure 3. SEM images PPO extrudates obtained at different temperatures with superheated ethanol. Figure 4. SEM images showing the cellular morphology of PPO foams. Images at the top shows the morphology of PPO foam processed at 130 ⁰C (a) and at 140 ⁰C, (850 psi) with superheated ethanol and subcritical CO 2 (850 psi). Bottom two images exhibit the porous morphology of microcellular PPO processed with ethanol (c) and water (d) with supercritical CO 2. SPE ANTEC Anaheim 2017 / 2458

6 Figure 5. Compression properties of PPO foams produced with superheated ethanol and water. (a) Stressstrain behavior of PPO foams. (b) Specific modulus and specific stress of PPO foams and comparison with PS based microcellular foams [14,15]. SPE ANTEC Anaheim 2017 / 2459

POLY(LACTIC ACID) BASED SINGLE COMPOSITES

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