5554
Ind. Eng. Chem. Res. 2003, 42, 5554-5558
Liquid and Supercritical Carbon Dioxide Loading into Chewing Gum Base Randy D. Weinstein,* Emily Cushnie, and Thomas C. Kopec Department of Chemical Engineering, Villanova University, Villanova, Pennsylvania 19085
The sorption and loading of liquid and supercritical CO2 into chewing gum base spheres with radii of 0.5 cm was experimentally explored over the temperature and pressure ranges of 2545 °C and 70-276 bar, respectively. Maximum loading amounts were found to be independent of temperature and to increase with increasing CO2 density. The time required to achieve maximum loading decreased with increasing temperature. A loading as high as 0.094 g of CO2 per gram of gum base was achieved. The sorption process was modeled by unsteady-state radial Fickian diffusion assuming a constant diffusion coefficient at a particular temperature and pressure (CO2 density). Volume changes of the polymer blend spheres under the conditions explored were small and ignored in the modeling. Sorption diffusion coefficients were on the order of 10-10 m2/s and increased with increasing temperature and CO2 density. Introduction Chewing gum is typically produced from a gum base with added sweeteners and flavors.1 Most synthetic gum bases are blends of styrene-butadiene copolymers with added polyvinyl acetate to reduce tackiness. In addition to these polymers, gum bases usually contain resins, waxes, fats, and emulsifiers to regulate texture and hardness.2 During manufacturing, the gum base is typically blended by heating the ingredients to around 70-115 °C with mixing.2,3 Usually late in the mixing process, flavorings and sweeteners are added to the hot gum base to produce the final specialized product,2 which is then cooled and packaged. With the addition of pharmaceuticals such as cold relief ingredients,4 caffeine,5 nicotine,6 and many others2 to the hot polymer blend, chewing gum can also be used as a drug delivery device. Often, the gum base itself is cooled and stored as generic stock to later be heated and blended with the desired flavors, sweeteners, or active ingredients. When typical gum is chewed, there is generally a fast release of sweetness and flavorings over the first few minutes, providing adequate taste that then drops off to low levels after about 5 min.2 Methods of sustained release of sweeteners and flavors have been improved over the years by using altered manufacturing techniques, including careful selection of the particle size of the flavorings added to the melt,7 microencapsulation of the flavors,8 trapping of the flavoring in hydrophobic matrixes,9 use of water-insoluble porous beads to hold flavors,10 and most likely trade secrets not known in the public domain. Interestingly, mint flavors tend to last for a significant period of time when gum is chewed, whereas fruit flavors are quickly exhausted. It is believed that the poor performance of fruit flavors in chewing gum is partly due to the lower-temperature conditions under which the final blend is made for gum with these flavorings. Fruit flavors need to be blended at lower temperatures than mint flavors because, at higher temperatures, the fruit flavors, which are organic * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: (610) 519-4954. Fax: (610) 519-7354.
acids, tend to degrade and discolor the gum, producing an undesirable product.11 A possible method of improving fruit flavor release in chewing gum would be to lower the glass transition temperature of the gum base and/or to plasticize and soften the gum base to allow for the loading of fruit flavors under lower-temperature conditions that would mimic the higher-temperature blend conditions used when mint flavors are formulated. Also, pharmaceuticals that cannot be processed at elevated temperatures could be blended into chewing gum under these conditions. We propose the use of compressed CO2 to plasticize gum base and then to transport fruit flavors into the gum. The high diffusivity, high density, and low viscosity of liquid and supercritical CO2 make it an ideal plasticizing agent.12 In addition, CO2 is benign and therefore can be easily used in food and pharmaceutical processing without the need of additional purification steps. A simple drop in pressure allows the sorbed CO2 to be released from the gum, leaving behind the sorbed flavorings and pharmaceuticals. The study presented here reports the results of CO2 sorption into a standard gum base, which will lay the groundwork for future work aimed at blending flavors into the base using CO2. Many polymers have been shown to be highly plasticized by CO2,13 including the polymers that make up the bulk of gum base, namely, polystyrene,14,15 polybutadiene,16 and polyvinyl acetate.15,17 Carbon dioxide has even been able to dissolve some polymers completely, particularly polymers with sulfones,13 carbonyls,18 and fluorinated groups.19 Carbon dioxide’s ability to dissolve certain polymers has been attributed to the favorable interactions (such as dipole-quadropole) between CO2 and these specific functional groups.13,21,22 Carbon dioxide has also been found to lower the glass transition temperature of many polymers significantly,13,21,22 sometimes as much as 70 °C.14 Because of the favorable interactions of CO2 with polymers, CO2 has been used for a variety of polymer modification processes such as loading plasticizers,23 dyes,23 surfactants,24 and reactants25 into polymers; removing excess monomers and other compounds from polymers;26 and blending polymers.27,28
10.1021/ie030449b CCC: $25.00 © 2003 American Chemical Society Published on Web 10/04/2003
Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5555
would also be the mass of CO2 in the sphere after the fixed exposure time to CO2. However, we were only able to obtain the mass in the sphere after about 30 s (and later) of the desorption process. Figure 1 shows a set of typical desorption data. If we can accurately model the desorption process, then we can extrapolate the data shown in Figure 1 to time zero and obtain the amount of CO2 loaded into the sphere at the onset of the depressurization process. Following the example of Muth et al.,30 who studied sorption of CO2 into poly(vinyl chloride) flat plates, we can model our process as Fickian diffusion into or out of a sphere, as follows31
Figure 1. Typical desorption data. Sphere exposed to CO2 at 207 bar and 25 °C for 4 h.
In this paper, we present the results of the sorption of CO2 into a standard gum base at temperatures between 25 and 45 °C and pressures between 70 and 275 bar. Maximum loading amounts are obtained as a function of temperature and pressure (and CO2 density). The unsteady-state diffusion of CO2 into the spherical gum base is modeled, and diffusion coefficients are calculated. Experimental Section Materials. Standard chewing gum base was provided in spheres with radii of 0.50 ( 0.02 cm by the Wm. Wrigley Jr. Company (actual composition is proprietary). Because these spheres were not perfect, several diameter measurements where made of each with a venire caliber, and the results were then averaged. A fresh gum base sphere was washed in CO2 (grade 5, BOC Gases) at 100 bar for approximately 1 h at the temperature at which the sphere was to be used to remove unbound solids and relieve any residual stresses.29 There was less than 1% weight loss after initial washing and insignificant weight loss after subsequent exposures to CO2. Each sphere of the homogeneous blend was used for no more than 10 exposures of CO2. Carbon Dioxide Loading. After the gum base spheres had been prewashed with CO2 and their mass and diameter measured, the spheres were then sealed in a 10-mL Thar Technologies fingertight vessel. The vessel was placed in a water bath to achieve the desired temperature. A small amount of low-pressure CO2 was flushed through the vessel before pressurization. An Isco 260D syringe pump was used to quickly (less than 15 s) pressurize the vessel after it had been preheated. The inlet to the vessel contained about 10 feet of tubing that was also in the water bath to provide preheating of the CO2 if needed. After the fixed exposure time, the vessel was quickly depressurized and opened. The gum base sphere was transferred to an analytical balance (accurate to 0.05 mg), and its mass was monitored as the CO2 was released from the sphere. Using the known mass of the gum base sphere, the amount of CO2 in the sphere at a particular desorption time could be calculated. The time from initial depressurization to the first mass measurement of the sphere was usually 30 s. Mass and volume changes of the sphere were negligible before and after exposure. Data Analysis. We desired the mass of CO2 in the sphere at the initial point of depressurization, which
M Mo
)1-
6
∞
∑
[
exp
π2n)1
(
a2
]
)
-Dn2π2t
n2
(1)
which, for small times, can be simplified to
M Mo
()[
)6
Dt a2
0.5
1
π0.5
∞
+2
( )]
∑ ierfc n)1
na
(Dt)0.5
-
3Dt a2
(2)
where t is time (either desorption time or adsorption time), a is the sphere radius, D is the diffusion coefficient, M is the amount of CO2 in the sphere at a particular time per unit mass of sphere, and Mo is the amount of CO2 in the sphere at maximum loading per unit mass of sphere. The radius of the sphere, the diffusion coefficient, and the external CO2 concentration or density must remain constant for eqs 1 and 2 to accurately model the masstransfer process into or out of the sphere. We observed negligible volume change of the sphere before and after CO2 exposure. Also, for several experiments, instead of measuring the mass of the sphere as the CO2 was released, we measured the radius of the sphere during the desorption process and found negligible change. As discussed previously, many polymers swell significantly during CO2 sorption. The gum base spheres did not swell much, most likely because of the fillers, waxes, and resins in the gum base causing it to maintain its shape. Therefore, the assumption of a constant sphere radius is justified for our experiments. During sorption, the pressure was kept constant in the vessel by the Isco syringe pump, and the temperature was kept constant by the water bath. Hence, the bulk CO2 concentration was also kept constant. Desorption of CO2 out of the sphere was into ambient air above the analytical balance. Because the quantity of CO2 being released from a typical sphere was so low, assuming that the concentration of CO2 in air is constant should be valid. Therefore, the assumption of a constant external CO2 concentration is appropriate. The one drawback of the model presented in eqs 1 and 2 is the requirement of a constant diffusion coefficient. In all likelihood, the spheres do swell, contract, and/or change morphology slightly during CO2 sorption or desorption, and hence, the diffusion coefficient could change over the process. However, as others30,32 have shown and as we will show in this paper, the assumption of a constant diffusion coefficient provides a simple avenue for accurately modeling CO2 loading into polymers, even though it might not be the perfect description of the actual process.
5556 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003
Figure 2. Data fit to eq 4 (with R2 ) 0.998) to obtain initial loading of 0.064 g of CO2/g of gum base when the sphere was exposed to CO2 at 207 bar and 25 °C for 4 h.
Figure 3. Loading of liquid CO2 into a single gum base sphere at 25 °C as a function of pressure.
For typical polymers, the diffusion coefficient of CO2 into or out of a polymer at low temperatures has been found to be around 10-9-10-11 m2/s or even smaller.15,30 Our spheres were small (radius of 0.5 cm), and therefore, for short times, it is possible to approximate eq 2 as
6 Dt 0.5 3Dt M ) - 2 Mo a π a
( )
(3)
as the integrated complimentary error function of anything larger than 5 is essentially zero. Therefore, if desorption data such as the set shown in Figure 1 are replotted versus the square root of time, the early-time data (less than 100 s was used here) should be modeled by a second-order polynomial given that eq 3 can be written as
M ) C1t0.5 + C2t Mo
(4)
with C1 ) (6/a)(D/π)0.5 and C2 ) -3D/a2. The data can then be accurately extrapolated back to time zero in the desorption process to obtain the mass of CO2 sorbed into the sphere at the end of the exposure to CO2 (see Figure 2). This approach provides a relatively simple method for obtaining the CO2 loading into polymers without the need for highly specialized equipment. Such a method would not be suitable for polymers that undergo large volume changes in either the sorption or desorption process or for processes that take a long time to depressurize the sample. Other investigators have employed gravimetric14,15,22,29 and dielectric12 measurements to obtain loading information in polymers without the need to have constant geometric shapes and diffusion coefficients or the need to depressurize their samples. The values of the two constants found by fitting the desorption data with eq 4 could be used to find the diffusion coefficient of CO2 out of the sphere into the atmosphere if that information were desired. We were instead interested in the diffusion coefficients of CO2 into the gum base spheres as a function of temperature and pressure. Therefore, we exposed spheres for varying amounts of time at a particular temperature and pressure, obtaining the loading of CO2 as a function of exposure time. We then used a linear least-squares fit of the data with eq 1 to obtain the diffusion coefficient,
Figure 4. Loading of supercritical CO2 into a single gum base sphere at 35 °C as a function of pressure.
D, and maximum loading of CO2, Mo, at a particular temperature and pressure. Results and Discussion At 25 °C, gum base spheres were exposed to liquid CO2 at 70, 138, and 207 bar (0.72, 0.86, and 0.92 g/cm3, respectively) for periods of time between 5 min and 24 h to obtain the mass of CO2 loaded into the sphere as a function of time (see Figure 3). Initial loading was rapid, with maximum loading achieved within about 8 h. As the pressure, and hence the density, was increased, the loading of CO2 into the sphere increased; however, the time required to achieve the maximum loading at a particular density was not affected by the actual CO2 density. The highest loading of 0.092 g of CO2 per gram of gum base was achieved at the highest density (0.92 g/cm3) at 25 °C. Experiments at several exposure times were repeated to examine the reproducibility and accuracy of the results. Typical loading amounts were found to be within 5% (95% confidence limits), and average values are reported in Figure 3. The effect of temperature on the loading of CO2 into gum base was explored by exposing spheres to supercritical CO2 at 35 and 45 °C and pressures of 103, 138, and 207 bar. At 35 °C, these pressures correspond to CO2 densities of 0.71, 0.79, and 0.87 g/cm3, respectively, and at 45 °C, they correspond to densities of 0.49, 0.66, and 0.82 g/cm3, respectively. Loading data from 5 min to 24 h are shown in Figures 4 and 5. Again, as pressure (and hence density) is increased at a particular tem-
Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5557
Figure 5. Loading of supercritical CO2 into a single gum base sphere at 45 °C as a function of pressure.
Figure 6. Sorption isotherms of CO2 into a single gum base sphere as a function of temperature.
perature, the maximum loading of CO2 increases. At 35 °C, the maximum loading is achieved after about 5 h of exposure, whereas at 45 °C, the maximum loading is obtained after about 4 h. The times required to achieve the maximum loading at these elevated temperatures with supercritical CO2 are significantly shorter than the time found at 25 °C with liquid CO2 (about 8 h). This might be due to one or more of the facts that the selfdiffusivity of CO2 increases at higher temperatures; the polymer relaxes more at higher temperatures, thereby providing more open spaces within it; and/or the glass transition temperature of the polymer is depressed in CO2. As the temperature is increased, the maximum loading within the gum base decreases at a particular pressure. This can be observed in the sorption isotherms shown in Figure 6. To obtain the data in Figure 6, a sphere was exposed to CO2 at a fixed temperature and pressure for at least 18 h and no more than 24 h. This time was deemed sufficient given that the maximum loading was achieved in the gum base within as little as 4 h and at most 8 h. The measurement at each pressure was repeated twice, and average values are reported. Loading increased up to about 200 bar. Loading did not seem to increase with any additional increase in pressure above 200 bar. Depending on the specific polymer and temperature explored, other investigators have observed CO2 uptake to continually increase with increasing pressure,14,15,29,30 as well as to level off at a particular pressure.14,29 For the temperatures we explored, the density of CO2 does not increase significantly with increasing pressure
Figure 7. Effect of density on the CO2 loading of a single gum base sphere. Table 1. Calculated Sorption Diffusion Coefficients (D) temp (°C)
pressure (bar)
CO2 density (g/cm3)
max CO2 loading (g of CO2/ g of gum base)
D × 1010 (m2/s)
25 25 25 35 35 35 45 45 45
70 138 207 103 138 207 103 138 207
0.72 0.86 0.92 0.71 0.79 0.87 0.49 0.66 0.82
0.073 0.089 0.094 0.070 0.082 0.090 0.069 0.075 0.079
3.5 4.2 4.8 5.0 5.5 5.8 6.0 6.5 6.5
above 200 bar. This could be why we did not see loading increase as pressure was increased above this level. It therefore might be more appropriate to examine the maximum loading amounts as a function of density as opposed to pressure. Figure 7 shows that the maximum loading amounts are a strong function of density and do not appear to be affected much by temperature. As discussed previously, the time required to achieve maximum loading was significantly affected by temperature. It is possible to use eq 1, the data presented in Figures 3-5, and a linear least-squares regression to find the diffusion coefficient for CO2 sorption into the gum base sphere at a particular temperature and pressure. The lines on Figures 3-5 represent the model in eq 1 being used with the calculated diffusion coefficient (Table 1) and the maximum loading of CO2 found from the data. The model appears to provide an excellent fit to the experimental loading data, although several assumptions were used in the model (constant radius of the gum base sphere and constant diffusion coefficient) that might not be exactly representative of the actual sorption process. At a fixed temperature, the diffusion coefficient tends to increase with increasing pressure (or density), a phenomena that has also been observed by other investigators of CO2 sorption into polymers.30,32 As expected, the diffusion coefficient tends to increase with increasing temperature at a fixed density. Because we found sorption diffusion coefficients on the order of 10-10 m2/s, and because desorption diffusion coefficients out of polymers into the atmosphere tend to be of the same order of magnitude as or smaller than the sorption diffusion coefficients,30,32 the assumptions used to find the CO2 loading amounts from the desorption data by simplifying eq 2 into eq 3 should be valid.
5558 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003
Summary We explored the sorption of liquid and supercritical CO2 into spherical standard gum base over the ranges of 25-45 °C and 70-276 bar. Maximum loading amounts were found to increase with increasing density and did not appear to be affected by temperature. The highest loading of 0.094 g of CO2 per gram of gum base was achieved at 25 °C and 276 bar. The time required to reach maximum loading decreased with increasing temperature and was independent of density. The sorption diffusion coefficient was found to be on the order of 10-10 m2/s and to increase with increasing density and temperature. A simple Fickian diffusion model captured the transient loading into the spherical gum base. The experimental method for obtaining and modeling CO2 loading is a simple, relatively inexpensive, and accurate process as long as minimal swelling of the sample is observed. Acknowledgment We gratefully acknowledge the Wm. Wrigley Jr. Company for donation of the gum base, as well as for input into the early stages of this research. We also thank Los Alamos National Laboratory for support through their Laboratory Education Equipment Gift Program. Literature Cited (1) Mestres, J. Gum Base Selection and Use. Manuf. Confect. 2000, 80, 53. (2) Rassing, M. R. Chewing Gum as a Drug Delivery System. Adv. Drug Delivery Rev. 1994, 13, 89. (3) Schlager, N., Ed. How Products are Made; Gale Research: Detroit, MI, 1994; Vol. 1. (4) Gasco-Lopez, A. I.; Izquierdo-Hornillos, R.; Jiminez, A. Development and Validation of a High-Performance Liquid Chromatography Method for the Determination of Cold Relief Ingredients in Chewing Gum. J. Chromatogr. A 1997, 775, 179. (5) Kamimori, G. H.; Karyekar, C. S.; Otterstetter, R.; Cox, D. S.; Balkin, T. J.; Belenky, G. L.; Eddington, N. D. The Rate of Absorption and Relative Bioavailability of Caffeine Adminstered in Chewing Gum Versus Capsules to Normal Healthy Volunteers. Int. J. Pharm. 2002, 234, 159. (6) Fagerstrom, K. O.; Tonnesen, Ph. Nicotine Chewing Gum and Nictoine Patch. Wein. Med. Wschr. 1995, 145, 77. (7) Mackay, D. A. M.; Clark, K. W.; Witzel, F.; Schoenholz, D. Long-lasting Flavored Chewing Gum Including Chalk-free Gum Base. Eur. Patent 4,064,274, 1977. (8) Cappellari, R. Chewing Gum. Eur. Patent 5,217,109, 1986. (9) Sharma, S. C.; Yang, K. Y. Chewing Gum Compositions Containing Novel Sweetener Delivery Systems and Method of Preparation. U.S. Patent 4,597,970, 1986. (10) Broderich, K. B.; Record, D. W. Gum Composition Containing Dispersed Porous Beads Containing Active Chewing Gum Ingredients and Methodology. U.S. Patent 5,139,787, 1992. (11) Seielstad, D. Technology Scientist, Wm. Wrigley Jr. Company, Chicago, IL. Personal Communication, Apr 18, 2002. (12) Goel, S. K.; Beckman, E. J. Plasticization of Poly(methyl methacrylate) (PMMA) Networks by Supercritical Carbon Dioxide. Polymer 1993, 34, 1410. (13) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Polymerizations in Supercritical Carbon Dioxide. Chem Rev. 1999, 99, 543.
(14) Wissinger, R. G.; Paulaitis, M. E. Swelling and Sorption in Polymer-CO2 Mixtures at Elevated Pressures. J. Polym. Sci. B 1987, 25, 2497. (15) 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. (16) Shenoy, S.; Woerdeman, D.; Garach-Domech, A.; Wynne, K. J. Quantifying Polymer Swelling Employing a Linear Variable Differential Transformer: CO2 Effects on SBS Triblock Copolymer. Macromol. Rapid Commun. 2002, 23, 1130. (17) 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. (18) Fried, J. R.; Li, W. High-Pressure FTIR Studies of GasPolymer Interactions. J. Appl. Polym. Sci. 1990, 41, 1123. (19) Rindfleisch, F.; DiNoia, T. P.; McHugh, M. A. Solubility of Polymers and Copolymers in Supercritical CO2. J. Phys. Chem. 1996, 100, 15581. (20) Shah, V. M.; Hardy, B. J.; Stern, S. A. Solubility of Carbon Dioxide, Methane, and Propane in Silicone Polymers: Effect of Polymer Backbone Chains. J. Polym. Sci. B 1993, 31, 313. (21) Chiou, J. S.; Barlow, J. W.; Paul, D. R. Plasticization of Glassy Polymers by CO2. J. Appl. Polym. Sci. 1985, 30, 2633. (22) Condo, P. D.; Johnston, K. P. In Situ Measurement of the Glass Transition Temperature of Polymers with Compressed Fluid Diluents. J. Polym. Sci. B 1994, 32, 523. (23) 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. (24) 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. (25) 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. (26) 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. (27) Elkovitch, M. D.; Tomasko, D. L.; Lee, L. J. Supercritical Carbon Dioxide Assisted Blending of Polystyrene and Poly(methyl methacrylate). Polym. Eng. Sci. 1999, 39, 2075. (28) Elkovitch, M. D.; Tomasko, D. L.; Lee, L. J. Effect of Supercritical Carbon Dioxide on Morphology Development During Polymer Blending. Polym. Eng. Sci. 2000, 40, 1850. (29) 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. (30) 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. (31) Crank, J. The Mathematics of Diffusion, 2nd ed.; Claredon Press: Oxford, U.K., 1975. (32) von Schnitzler, J.; Eggers, R. Mass Transfer in Polymers in a Supercritical CO2 Atmosphere. J. Supercrit. Fluids 1999, 16, 81.
Received for review May 28, 2003 Revised manuscript received August 27, 2003 Accepted September 3, 2003 IE030449B
