Superheated Water and Ethanol As Green Additives to Process Poly(2

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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Superheated Water and Ethanol As Green Additives to Process Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) Md Arifur Rahman,† Matthew Lok,‡ and Alan J. Lesser*,† †

Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States

‡

S Supporting Information *

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 poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) requires high processing temperature and toxic plasticizers due to its high glass transition temperature (Tg) and phenylene oxide groups in the backbone. Very few research works have reported the use of superheated liquids to process high Tg, intractable polymers. This research work presents a systematic study to explore the advantages of processing PPO with superheated liquids composed of two nontoxic and ubiquitous polar liquids, ethanol and water. Microcellular foams of PPO having a density range from 0.13 to 0.56 g/cm3 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 150−180 °C below the conventional extrusion temperature for PPO.

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efficient methods”.6,7 Apart from being the greenest solvent, water in superheated or supercritical states can be very useful for their unique properties in comparison with ambient water, in many physical and chemical processing of organic compounds and polymers as well.8−10 However, little attention has been given to the potential application of superheated water or superheated liquids for energy efficient and environment friendly processing of polymers. This manuscript presents one of the few systematic attempts to utilize the unique properties of superheated hydrogen bonded liquids, such as water and ethanol, to process PPO in subcritical and supercritical CO2 comedia. Many studies inferred that hydrogen bonded liquids like water or ethanol in their superheated state (below the critical point) can act as green solvents for many polymers. This is because superheated water exhibits a density and polarity similar to the acetonitrile at room temperature.10 With the increase in temperature, superheated water and mostly superheated hydrogen bonded liquids exhibits significant drop in dielectric constant, increasing the solubility of organic compounds in superheated water.11,12 In addition, low pKw due to higher ionic product, especially in the near critical range, superheated water or ethanol can act both as strong acid and

INTRODUCTION Many high performance polymers such as poly(ether sulfone) (PES), polyphenylene oxide (PPO), and Poly(ether imide) (PEI) undergoes thermos-oxidative degradation during processing such as melt extrusion or injection molding. Such thermos-oxidative degradation causes enormous loss in thermal and physical properties of such highly intractable polymers. Once such polymer is PPO 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 resistant, high elastic modulus, transparent and high heat distortion temperature, PPO could not be processed without additive as it undergoes thermo-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. Water, being ubiquitous, nontoxic, low cost, nonflammable, and most importantly, environmentally benign, provides opportunity for clean processing and pollution remediation. Among the 12 principles of green chemistry, use of water satisfies the desire for utilizing “safer solvents” and “energy © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

January 7, 2018 February 11, 2018 March 1, 2018 March 1, 2018 DOI: 10.1021/acs.iecr.8b00007 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Generalized sketch of batch (a) foaming and (b) pressure driven extrusion setup.

base.12 Since superheated water or ethanol can act like organic solvents they can aid in processing of polymers, especially if the polymer has polar groups.13 Rastogi et al. first time reported the potential use of superheated water to process polymers such as polyamides.14−18 Rastogi et al. reported dissolution of PA 4,6 in superheated water and inferred that this was due to the weakening of hydrogen bonding at the superheated state.14 Lesser et al. recently further investigated the use of superheated liquids to process a series of different polyamides.19 They reported that 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. 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.20,21 Another recent work by Lesser et al. observed the syngersitic effect of superheated water/supercritical carbon dioxide comedia on the production of microcellular foams of PES. Addition of superheated water increased the total porosity by 23%.22 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. A reaction is not needed in superheated water processing, so the process does not need to worry about the 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 with subcritical and supercritical CO2 comedia as additive to foam and extrude PPO. Introducing the 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, nonwoven products (fabrics), and ultrafiltration 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 processing of PPO in the presence of subcritical and supercritical CO2 comedia.

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EXPERIMENTAL SECTION Materials. Poly(2,6 dimethyl-1,4-phenylene oxide) (PPO) was supplied by GE Co. Carbon-dioxide was supplied by Airgas. Water (distilled water) and Ethanol (from SigmaAldrich) 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 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 to 250 °C at 10 °C/min. Thermogravimetric analyses (TGA) were performed on neat and processed PPO at a heating rate of 10 °C/min in the heating range of 30−1000 °C. The tests were conducted under nitrogen gas. Gel Permeation Chromatography. Three mg/mL solutions were made with spec grade THF and filtered through a 0.45 um PTFE filter. An Agilent Technologies 1260 Infinity series with THF as the eluent solvent at a flow of 1 mL/min. Polystyrene standards were used. Batch Extrusion and Foaming of PPO. Cylindrical reactors made of stainless steel were used as the reactors to process 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 in order to maintain isobaric condition. Carbon dioxide (CO2) 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 B

DOI: 10.1021/acs.iecr.8b00007 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

PPO decreases to 175 and 110 °C with water and ethanol, respectively, in superheating condition. Such depression of Tg of PPO with superheated ethanol or water can be attributed to their solvation or plasticization effect on PPO. However, this requires further investigation to understand 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 Tg 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 °C. However, PPO could not be processed with superheated water. Ethanol/water (50/50, 70/30) mixtures were used successfully to process PPO. In order to extrude PPO with superheated liquids, a specific pressure and temperature needs to be maintained in the reactor for a specific period. Upon depressurization, melt flow of the PPO can be obtained (Please see Supporting Information Video Clip S1 shows the pressure-driven extrusion of PPO in superheated ethanol/subcritical CO2 and Supporting Information Figure S1 shows the image of extrudates). However, the PPO extrudate obtained are porous which is formed as the consequence of the evaporation of liquids and release of CO2 gas during depressurization as the vapor pressure of liquids decreases at room temperature. Figure 3 shows the SEM

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 reactor. 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 3575−93, W−B) was used to measure the overall density of the foam/film samples. Scanning Electron Microscope. In order 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. The average cell size distribution in PPO foams was determined from the SEM micrographs. Compression Test. Uniaxial compression tests (Instron) were carried out on cylindrical foam samples having a dimension of 15 (dia) × 15 mm at a speed of 1 mm/min.

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RESULTS AND DISCUSSION Influence of Superheated Ethanol and Water on the Tg of PPO. The processability of PPO is mainly limited due to its high Tg and presence of ether group which can undergo thermo-oxidative degradation during processing. In order to observe the effect of superheated ethanol or water to the Tg 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 Tg at 215 °C whereas Tg of

Figure 2. DSC thermograms showing the interaction of superheated water and ethanol with PPO. Thermograms show the change in glass transition of PPO temperature with superheated ethanol and water.

Figure 3. SEM images PPO extrudates obtained at different temperatures with superheated ethanol. C

DOI: 10.1021/acs.iecr.8b00007 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

subcritical pressure (800 psi), at 120, 130, and 140 °C. Images produced from an SEM analysis on PPO foams processed at the designated temperatures are shown in Figure 4. SEM

images of porous extrudates. Pores in the surfaces as well as at the cross-section of extrudates are evident. The average size of pores at the cross-section of extrudates range from 10 to 50 μm whereas larger pores (