Liquid and Supercritical Carbon Dioxide Assisted Blending of Poly

Solubility of Felodipine and Nitrendipine in Liquid and Supercritical Carbon Dioxide by Cloud Point and UV Spectroscopy. Randy D. Weinstein, William H...
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MATERIALS AND INTERFACES Liquid and Supercritical Carbon Dioxide Assisted Blending of Poly(vinyl acetate) and Citric Acid Randy D. Weinstein,* Joseph J. Gribbin, and Daniel Najjar Department of Chemical Engineering, Villanova University,Villanova, Pennsylvania 19085

The use of liquid and supercritical carbon dioxide was explored for the blending of poly(vinyl acetate) and citric acid as a basis for developing a new process for making chewing gum which would allow for flavorings to be released slower during chewing. Mixtures of 75% of polymer and 25% citric acid by mass were blended with carbon dioxide from 5 to 60 min over a temperature range of 25 to 55 °C and a pressure range of 83 to 241 bar in a batch process. Samples were then artificially chewed and citric acid dissolution monitored. Comparisons were made with samples prepared without carbon dioxide. In all cases, carbon dioxide blending produced polymers which retained the critic acid longer and hence produced more desirable products. With carbon dioxide, temperature had the largest effect on citric acid retention while pressure (or density) had only a modest effect. To make the flavoring be retained the longest in the polymer, the highest temperature and pressure (density) should be used with the longest blending time in carbon dioxide. Introduction Carbon dioxide is becoming a viable processing fluid for many polymers, leaving the laboratory and working its way into small- and large-scale commercial polymer production facilities.1-4 Not only is carbon dioxide environmentally friendly, basically inert, nonflammable, inexpensive, and easy to recycle in a closed-loop process, it also has many properties which make it ideal for polymer synthesis and processing. In this work we examine the use of carbon dioxide for the simple blending of poly(vinyl acetate) with citric acid. Many polymers, including poly(vinyl acetate), have been shown to dissolve significant quantities of carbon dioxide.3 Takishma and co-workers5 as well as Sato and co-workers6 were able to load almost 35 wt % of carbon dioxide into poly(vinyl acetate) at relatively modest temperatures and pressures. The high affinity of carbon dioxide for certain polymers has been attributed to the favorable interactions (such as dipole-quadropole, Lewis acid-base, and electron donor-acceptor) between carbon dioxide and specific functional groups in the polymer such as the carbonyl groups found in acetates.7,8 When large quantities of carbon dioxide dissolve in a polymer, the polymer becomes plasticized and its glass transition temperature usually lowered, often significantly.3,9,10 This effect is not due to the hydrostatic pressure, but the actual molecular interactions of carbon dioxide with the polymer. Upon plasticizing with carbon dioxide, the polymer swells and it is possible to load carbon dioxide-soluble dyes,10 surfactants,11 and reactants12 into the polymer, as well as to remove excess * To whom correspondence should be addressed. Tel: (610) 519-4954. Fax: (610) 519-7354. E-mail: randy.weinstein@ villanova.edu.

carbon dioxide-soluble monomers and other compounds from the swollen polymer.13 We have recently begun exploring the use of carbon dioxide for chewing gum processing.14 Chewing gum is typically produced from a synthetic gum base. These gum bases are usually blends of styrene-butadiene copolymers with added poly(vinyl acetate).15 In addition to these polymers, gum bases contain resins, waxes, fats, and emulsifiers to regulate texture and hardness. The final product combines gum base, flavors, and sweeteners in a melt process around 70-115 °C.16 It is also possible to add pharmaceuticals to make a drug delivery device out of chewing gum during the melt process. In our earlier work14 we showed that low-temperature liquid and supercritical carbon dioxide dissolves into a typical gum base up to 10 wt % and hence carbon dioxide has the potential to dissolve carbon dioxidesoluble additives and allow them to diffuse into the gum base without higher temperature melt processing. Unfortunately, citric acid, a typical fruit flavoring used in chewing gum, is not soluble in carbon dioxide. Therefore, we chose to investigate the use of carbon dioxide to plasticize and swell a major component of chewing gum (poly(vinyl acetate)), lower its glass transition temperature, and blend a flavoring (citric acid) into the polymer at relatively low temperatures. When carbon dioxide is released from the system, the poly(vinyl acetate) can return to a nonswollen state (contract) and physically embed the citric acid within itself if the carbon dioxide is released slowly. If quick release occurs, then foaming and permanent expansion of the polymer can occur. This type of blending is advantageous for carbon dioxideinsoluble additives, such as citric acid, which will not dissolve into carbon dioxide and diffuse into the gum base. Blending at low temperature is also desirable because many of the fruit flavors and pharmaceutical

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products cannot withstand normal processing temperatures. The additives either degrade or react with the polymer, producing undesired effects such as a color change or actual breaking down of the polymer.17 To prevent these undesired effects, traditional blending is often performed at lower than ideal temperatures, which does not allow sufficient mixing and embedding of the flavorings with the polymer, and hence these flavorings do not tend to last long in the final gum when is it chewed.17 In this paper we will demonstrate that carbon dioxide can be used to blend poly(vinyl acetate) with citric acid and that the final product has a much longer lasting flavor release than blending performed without carbon dioxide. Experimental Section Materials. Ground poly(vinyl acetate) (M h w ) 25000, M h n ) 15000, and Tg ) 32 °C) and ground anhydrous citric acid were both provided by the Wm. Wrigley Jr. Company as spherical particles with diameters of 1 µm. Carbon dioxide (grade 5) was supplied by BOC Gases. Deionized ultrafiltered water, 5 M aqueous sodium hydroxide, toluene, wash ethanol (95%), and 1% phenolphthalein solution were purchased from Fisher Scientific. All materials were used as received. Carbon Dioxide Blending. Poly(vinyl acetate) (0.75 g) and citric acid (0.25 g) were mixed and placed inside a 10 mL Thar Technologies finger tight vessel which had been preheated in a water bath at the desired temperature of the experiment. A small amount of lowpressure carbon dioxide was quickly flushed through the vessel after it had been sealed and returned to the water bath. An Isco 260D syringe pump was then used to pressure (in less than 20 s) the vessel with carbon dioxide. The pump was set in constant-pressure mode and left running during blending so that as carbon dioxide dissolved into the polymer, the pressure in the vessel remained unchanged. The inlet to the vessel contained about 10 feet of capillary tubing which was also in the water bath to provide preheating of the carbon dioxide if needed. After the fixed exposure time, the pump was turned off and isolated from the vessel which was then quickly depressurized and opened. The blended material was quickly removed from the vessel to prevent it from hardening and sticking to the walls. Some material did remain stuck to the seals and interior of the vessel so complete recovery was not possible. Of the material recovered, 0.5 g was isolated as one piece and used for analysis. This sample contained the same weight ratio of polymer to citric acid that was initially loaded into the vessel. After an experiment, the vessel was cleaned with toluene followed by ethanol and then it was air-dried in an oven at 75 °C. Control experiments were also performed where no carbon dioxide was introduced to the vessel during exposure. All blending was done as batches without stirring. Sample Analysis. Once 0.5 g was recovered as one solid piece, it was placed in our chewing apparatus. This apparatus consisted of a cylinder (5 cm inside diameter and 10 cm long) made of aluminum with a flat bottom. In this cylinder was placed the sample and 20 mL of water. A plunger was snuggly fitted inside the cylinder and was inserted so that it was 2 cm from the bottom at rest. This plunger was automatically controlled to “chew” or pulse downward. A chew event would happen quickly, with the plunger moving downward and back up to rest in less than 0.3 s. A sample was chewed at a

Figure 1. Amount of citric acid released as a function of chew time for different blend exposure pressures (and densities) in supercritical carbon dioxide at 35 °C for 30 min.

rate of 25 times/min. Without the sample in the cylinder the plunger would just touch the bottom of the cylinder on its downward pulse. The plunger also continually rotated at 5 rpm to prevent the sticking of material to it and to provide added sheer.18 The plunger was constructed of polycarbonate. A cover, to contain splashing during chewing, was placed on the cylinder with a hole allowing the plunger motion through it. After the desired chewing time was achieved, 4 mL of liquid was removed from the chewing apparatus. Three drops of phenolphthalein were added and the solution was then titrated with 0.075 M sodium hydroxide solution (made by dilution of a 5 M solution) until neutralization occurred. With the fact that citric acid is a triacid, simple material balances were used to calculate how much citric acid was present in the 4 mL sample, the original 20 mL sample, and hence what percentage of the citric acid was removed from the 0.5 chewed sample. Each sample was only chewed for one chew time and then discarded. The chewing apparatus was thoroughly washed with water and dried after each use. The titration method was verified to be accurate (within 3%) by measuring the mass of citric acid in several prepared solutions of known concentration. Results and Discussion The effect of carbon dioxide pressure, and hence density, was explored on the batch blending of poly(vinyl acetate) and citric acid at 35 °C, a temperature slightly above the glass transition temperature (32 °C) of the polymer. Three pressures were explored, 83, 157, and 241 bar, which correspond to carbon dioxide densities of 0.57, 0.82, and 0.90 g/cm3, respectively. Densities were calculated using the NIST Thermodynamic and Transport Properties of Pure Fluids19 database with the measured temperature and pressure. The percentages of available citric acid released during chewing after 30 min of blend exposure time in carbon dioxide are shown in Figure 1. In all the results reported, each sample was only chewed for one chewing time and then discarded and a fresh sample was used for a different chewing time. Each data point reported is the average of two repeated experiments. In all pressures explored, a significant amount of citric acid is released quickly while chewing. This quick

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Figure 2. Amount of citric acid released as a function of chew time and exposure times in supercritical carbon dioxide at 35 °C and 83 bar.

release is most likely due to the citric acid at or near the surface of the polymer which readily dissolves into water in the chewing apparatus. However, by examining Figure 1, it is clear that as pressure or density is increased during blending, less citric acid is released quickly during chewing. Work by Sato et al.6 has shown that poly(vinyl acetate) swells significantly in carbon dioxide and the amount of swelling tends to increase with increasing applied pressure or density. They found polymer swelling as much as 40% over the range of conditions explored in our experiments. By increasing the carbon dioxide pressure, the poly(vinyl acetate) plasticizes and swells more; therefore, it is able to embed more citric acid within its matrix and leave less on the surface for quick dissolution. As chewing time is increased from the early times, the trend in citric acid retention continues. As pressure is increased, less citric acid is released at a particular chew time. This implies that the polymer matrix is able to retard the diffusion of citric acid out. The citric acid must be bound more tightly to the polymer or the polymer matrix becomes more rigid and ordered as pressure during blending is increased. Most likely, the later is the case as several researches have found polymer structures to change significantly with changes in carbon dioxide processing pressure and density.20-22 The effect of carbon dioxide blend time was also explored on the retention of citric acid. Blend times of 5, 10, 30, and 60 min were explored in supercritical carbon dioxide at 35 °C and at the three different pressures/densities previously explored. Results are shown in Figures 2-4. As blend time is increased, citric acid is retained longer in the polymer. The effects are more pronounced as the pressure during blending is increased. Although many polymers tend to swell and plasticize in carbon dioxide, this is not a fast process and often can take hours for even a small sample to finish swelling.14,23,24 Diffusion of carbon dioxide into a polymer tends to be a slow process. Even samples of poly(vinyl acetate) as small as 0.2 g took over 4 h to finish swelling at some conditions in carbon dioxide.6 Longer blend times are more advantageous for citric acid retention, as the polymer can swell more and entrap the citric acid more inside itself. It appears that retention does not level off with increasing blend time even after 60 min and the

Figure 3. Amount of citric acid released as a function of chew time and exposure times in supercritical carbon dioxide at 35 °C and 157 bar.

Figure 4. Amount of citric acid released as a function of chew time and exposure times in supercritical carbon dioxide at 35 °C and 241 bar.

polymer continues swelling in carbon dioxide. The experiments at 35 °C and 241 bar were also performed at longer times than those shown in Figure 4. From these exposure times (1-24 h), no additional benefits of exposure time were observed after roughly 5 h of blending. In our experiments, the polymer and citric acid were loaded as powders and were expanded into one large pellet inside the high-pressure vessel. The vessel was cylindrical with an inside diameter of 1.9 cm and a height of 3.5 cm and was oriented vertically during blending. The polymer pellet formed would vary in length, but would always be expanded to 1.9 cm wide. The longer the blend time, the longer the pellet would be. The longer pellets would have larger volume to surface area ratios. Hence, as blend time is increased, more of the citric acid is retained within the pellet and less on or near its surface. Before examining the effects of varying the temperature on the blending, we first wanted to verify that carbon dioxide was actually playing a significant role in the process since we were already working at 3 °C above the glass transition temperature of the poly(vinyl acetate). We took the experimental conditions explored

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Figure 5. Amount of citric acid released as a function of chew time and heating times without carbon dioxide at 35 °C.

in Figures 2-4 and repeated them at the same temperature, exposure time, and chewing time, but without the addition of carbon dioxide (or pressure) into the vessel. These results are shown in Figure 5. Clearly, citric acid is not retained for any significant chew time without the use of carbon dioxide-assisted blending. The mere softening of the polymer by raising it above its glass transition temperature does not allow it to strongly embed the citric acid within itself. The actual plasticizing and swelling of the polymer by carbon dioxide is critical in its ability to time release flavorings while being chewed. Furthermore, the time the blend is held at temperature does not appear to have affected citric acid retention unless carbon dioxide is used during blending. This fact reiterates the points that carbon dioxide interactions with the poly(vinyl acetate) are playing a significant role in the final product formed and that the diffusion of carbon dioxide into the polymer is a slow process. The effect of temperature on the blending was explored by first using liquid carbon dioxide at 25 °C and 103 bar, an identical density (0.82 g/cm3) to that of supercritical carbon dioxide at 35 °C and 157 bar. The results using liquid carbon dioxide are shown in Figure 6 and can be compared to those at 35 °C shown in Figure 3. Lower temperature blending appears to do a worse job of retaining citric acid during chewing. To explore this further, various temperatures were used to make samples that were chewed for 30 s (Figure 7) as well as 15 min (Figure 8). All of these samples were blended with carbon dioxide at a fixed density of 0.82 g/cm3 and therefore the pressure varied as the temperature changed in order to obtain this density. At the short chew time of 30 s a significant amount of citric acid is released. At the lowest temperature with a short blend time of 5 min, about 25% of the flavoring is quickly dissolved off. But as temperature is increased and longer blend times used, it is possible to have only abut 10% of the flavoring quickly released. At a fixed blend time, less citric acid is released as the temperature is increased. This trend in citric acid retention becomes more pronounced as the chew time is increased to 15 min (see Figure 8). For the shortest blend time (5 min) at the lowest temperature (25 °C) almost 95% of the citric acid is gone from the sample after 15 min of chewing. When the temperature was increased by 30 °C, only 73% of the flavoring is released. When the blend

Figure 6. Amount of citric acid released as a function of chew time and exposure times in liquid carbon dioxide at 25 °C and 103 bar.

Figure 7. Amount of citric acid released after 30 s of chew time as a function of exposure times and temperature in carbon dioxide at 0.82 g/cm3.

Figure 8. Amount of citric acid released after 15 min of chew time as a function of exposure times and temperature in carbon dioxide at 0.82 g/cm3.

time was increased (to 60 min) as well as the temperature (to 55 °C), only 56% of the citric acid is released after 15 min of chewing. These trends with temperature correlate with the fact that polymer swelling increases with increasing temperature as does the diffusion of

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carbon dioxide into the polymer.6,14 As we have previously explained, the trends with blend time are due to the fact that the polymer swelling is a slow process in carbon dioxide. Summary We explored the batch blending of poly(vinyl acetate) and citric acid in liquid and supercritical carbon dioxide. Higher temperatures and higher densities produced samples which would slowly release the citric acid while being chewed in a more controlled release manner. Longer blend times than 60 min are required to maximize polymer swellings and hence retention of citric acid. Although this simple batch mixing without stirring does not simulate continuous screw blending or mixing with an impeller as might be done in a real process, it does easily and quickly show that compressed carbon dioxide has the potential to be used to make longer lasting chewing gum. Acknowledgment We gratefully acknowledge the Wm. Wrigley Jr. Company for donation of materials. We also would like to thank Los Alamos National Laboratory for support through their Laboratory Education Equipment Gift Program as well as Emily Schmidt and Marek Grinberg, both of Villanova University, for help in construction and testing of the chewing apparatus. Literature Cited (1) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Polymerizations in Supercritical Carbon Dioxide. Chem. Rev. 1999, 99, 543. (2) Cooper, A. I. Polymer Synthesis and Processing Using Supercritical Carbon Dioxide. J. Mater. Chem. 2000, 10, 207. (3) Tomasko D. L.; Li, H.; Liu, D.; Han, X.; Wingert, M. J.; Lee, L. J.; Koelling, K. W. A Review of CO2 Applications in the Processing of Polymers. Ind. Chem. Eng. Res. 2003, 42, 6431. (4) Beckman, E. J. Supercritical and Near-Critical CO2 in Green Chemical Synthesis and Processing. J. Supercrit. Fluids 2004, 28, 121. (5) Takishma, S.; Nakamura, K.; Sasaki, M.; Masuoka, H. Dilation and Solubility in Carbon Dioxide + Poly(vinyl acetate) Systems at High Pressures. Sekiyu Gakkaishi 1990, 33, 332. (6) Sato, Y.; Takikawa, T.; Takishima, S.; Masuoka, H. Solubilities and Diffusion Coefficients of Carbon Dioxide on Poly(vinly acetate) and Polystyrene. J. Supercrit. Fluids 2001, 19, 187. (7) Kazarian, S. G.; Vincent, M. F.; Bright, F. V.; Liotta, C. L.; Eckert, C. A. Specific Intermolecular Interaction of Carbon Dioxide with Polymers. J. Am. Chem. Soc. 1996, 118, 764. (8) Mawson, S.; Johnston, K. P.; Combes, J. R.; DeSimone, J. M. Formation of Poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) Submicron Fibers and Particles from Supercritical Carbon Dioxide Solutions. Macromolecules 1995, 28, 3182.

(9) Chiou, J. S.; Barlow, J. W.; Paul, D. R. Plasticization of Glassy Polymers by CO2. J. Appl. Polym. Sci. 1985, 30, 2633. (10) Kazarian, S. G.; Brantley, N. H.; West, B. L.; Vincent, M. F.; Eckert, C. A. In Situ Spectroscopy of Polymers Subjected to Supercritical CO2: Plasticization and Dye Impregnation. Appl. Spectrosc. 1997, 51, 491. (11) Ma, X.; Tomasko, D. L. Coating and Impregnation of a Nonwoven Fibrous Polyethylene Material with a Nonionic Surfactant Using Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1997, 36, 1586. (12) Kung, E.; Lesser, A. J.; McCarthy, T. J. Composites Prepared by the Anionic Polymerization of Ethyl 2-Cyanoacrylate with Supercritical Carbon Dioxide-Swollen Poly(tetrafluoroethylene-co-hexafluoropropylene). Macromolecules 2000, 33, 8192. (13) Stafford, C. M.; Russell, T. P.; McCarthy, T. J. Expansion of Polystyrene Using Supercritical Carbon Dioxide: Effects of Molecular Weight, Polydispersity, and Low Molecular Weight Components. Macromolecules 1999, 32, 7610. (14) Weinstein, R. D.; Cushnie, E.; Kopec, T. Liquid and Supercritical Carbon Dioxide Loading into Chewing Gum Base. Ind. Chem. Eng. Res. 2003, 42, 5554. (15) Mestres, J. Gum Base Selection and Use. Manuf. Confectioner 2000, Jan 53. (16) Rassing, M. R. Chewing Gum as a Drug Delivery System. Adv. Drug Delivery Rev. 1994, 13, 89. (17) Seielstad, D. Technology Scientist, Wm. Wrigley Jr. Company, Personal Communication, April 18, 2002. (18) Rider, J. N.; Brunson, E. L.; Chambliss, W. G.; Cleary, R. W.; Hikal, A. H.; Rider, P. H.; Walker, L. A.; Wyandt, C. M.; Jones, A. B. Development and Evaluation of a Novel Dissolution Apparatus for Medicated Chewing Gum Products. Pharm. Res. 1992, 9, 255. (19) Lemmon, E. W.; Peskin, A. P.; McLinden, M. O.; Friend, D. G. NIST Thermodynamic and Transport Properties of Pure Fluids - NIST Pure FluidsVersion 5.0. 2000, U.S. Secretary of Commerce Copyright. (20) Arora, K. A.; Lesser, A. J.; McCarthy, T. J. Compressive Behavior of Microcellular Polystyrene Foams Processed in Supercritical Carbon Dioxide. Polym. Eng. Sci. 1998, 38, 2055. (21) Favis, B. D. The Effect of Processing Parameters on the Morpholoy of an Immiscible Binary Blend. J. Appl. Polym. Sci. 1990, 39, 285. (22) Chaudhry, B. L.; Hage, E.; Pessan, L. A. Effects of Processing Conditions on the Phase Morphology of PC/ABS Polymer Blends. J. Appl. Polym. Sci. 1998, 67, 1605. (23) Zhang, Y.; Gangwani, K. K.; Lemert, R. M. Sorption and Swelling of Block Copolymers in the Presence of Supercritical Fluid Carbon Dioxide. J. Supercrit. Fluids 1997, 11, 115. (24) Muth, O.; Hirth, Th.; Vogel, H. Investigation of Sorption and Diffusion of Supercritical Carbon Dioxide into Poly(vinyl chloride). J. Supercrit. Fluids 2001, 19, 299.

Received for review October 18, 2004 Revised manuscript received February 15, 2005 Accepted March 1, 2005 IE0489857