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Foam-like materials based on whey protein isolate
European Polymer Journal 49 (2013) 3387–3391
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Foam-like materials based on whey protein isolate Hong-Bing Chen a,b, Yu-Zhong Wang a, David A. Schiraldi b,⇑ a Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China b Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USA
a r t i c l e
i n f o
Article history: Received 1 March 2013 Received in revised form 9 July 2013 Accepted 12 July 2013 Available online 26 July 2013 Keywords: Aerogel Whey Protein Clay Thermal Mechanical
a b s t r a c t Low density materials from sustainable whey protein were fabricated through a simple, environmentally-friendly freeze-drying process. Aerogels produced solely from whey protein show poor mechanical properties, consistent with those of films produced from that biopolymer. The compressive moduli of these lamellar materials were increased by more than an order of magnitude by crosslinking, and further increased with increasing aerogel densities. Blending whey protein with alginate allowed for the production of bio-based aerogels with higher mechanical properties than those produced with whey alone, though thermal properties were slightly decreased by blending. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Bio-based polymers are of increasing interest as fossil fuel resources become more costly. Use of materials, such as cellulose, sugar cane, animal proteins, and plant starches and oils, as alternatives to petrochemicals can decrease overall carbon footprints for consumer and industrial products, as well as reducing their environmental impacts [1– 4]. Whey is the liquid remaining after milk has been curdled and strained for cheese production, and is composed primarily of b-lactoglobulin (>50% of the total whey protein), a-lactalbumin, bovine serum albumin (BSA), and immunoglobulins [5,6]. Whey protein is widely used in the food industry, with applications including processed meats, bakery products, pasta, ice cream, and infant foods [7]. As a by-product of the manufacture of cheese or casein, the broad availability in western countries makes whey protein a good feedstock candidate for the production of environmentally friendly-materials. Due to its ⇑ Corresponding author. E-mail address: [email protected] (D.A. Schiraldi). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.07.019
comparatively low molecular weight and specific molecular conformation, whey protein possesses limited mechanical strength and is therefore seldom used other than in the food industry. Modification of the whey protein structure and blending with other materials, including plasticizers, crosslinkers, and other polymers, are possible methods to improve its final properties; in the absence of plasticization, whey protein isolate is known to be a brittle material [8,9]. Heat-induced gels can form when heating whey protein. Denatured whey proteins aggregate irreversibly and eventually form a space-filling, crosslinked gel structure. The gelation mechanisms and associated processes have been extensively studied [10–17]. The higher the ionic strength is, the lower the critical protein concentration necessary for gel formation other than near the isoelectric pH. By means of such facile crosslinking, the mechanical properties of whey protein can be enhanced. We have previously reported an environmentally-friendly process for the preparation of low-density polymer/clay aerogels from aqueous mixtures [18–20], showing great potential as alternatives to polymeric foam and balsa wood. An especially striking family of such aerogels is those produced
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from alginate/clay, exhibiting extremely high mechanical strengths and moduli [20]. In the current work, we report a family of low density materials based on whey protein. Gelation of the whey protein, as well as blending with alginate are explored as methods to enhance the mechanical properties of these whey protein aerogels. 2. Experimental 2.1. Materials Whey protein isolate (WPI), BIPRO with a protein content of >95%, was supplied by Davisco Foods International, Inc. The main proteins are beta-lactoglobulin with a MW of 18,000 Daltons and alpha-lactalbumin at 14,000 Daltons, and used without further modification. Ammonium alginate and sodium chloride (Fisher Scientific), and sodium montmorillonite (Na+-MMT, PGW grade, Nanocor Inc.) were used as received. Deionized water was produced using a Barnstead RoPure reverse osmosis system. 2.2. Aerogel preparation Percentages of WPI, alginate and clay structural components in aqueous suspensions given prior to freeze-drying. For pure WPI aerogels, WPI powder (10, 15, 20 and 25 g, noted as WPI10, WPI15, WPI20 and WPI25) was dissolved into 100 ml DI water. The solutions were poured into 5 g polystyrene vials and frozen in a solid carbon dioxide/ethanol bath. Equivalent batches were prepared for gelling with 100 mM NaCl aqueous solutions, providing sufficient salt to induce gelation of thermally denatured WPI. The WPI solutions were then heated at 80 °C for 30 min to form gels, which were then frozen in a solid carbon dioxide/ethanol bath. For WPI10A2.5C2.5, 2.5 g of Na+-MMT was blended with 100 mL of DI water on the high speed setting of a Waring model MC2 mini laboratory blender for 1 min to obtain a 2.5 wt% clay aqueous suspension. 2.5 g alginate and 10 g WPI were then dissolved into the clay suspensions. These mixtures were then poured into 5 g polystyrene vials and frozen in a solid carbon dioxide/ethanol bath. The preparation of WPI10A5 (10 g WPI, 5 g alginate) followed the same procedure as was used to produce WPI10A2.5C2.5. The frozen samples were dried in a VirTis Advantage freeze dryer, with an initial shelf temperature of 25 °C, condenser temperature of 87 °C and an eventual vacuum of <10 mbar applied to sublime the ice. The freeze drying process was allowed to proceed for 3–4 days to ensure complete solvent removal.
Instron model 5565 universal testing machine, fitted with a 1 kN load cell, at a crosshead of 10 mm min 1. Five samples of each composition were tested for reproducibility, run to 75% compressive strain. The initial compressive moduli were calculated from the slope of the linear portion of the stress–strain curve. The morphological microstructure of the aerogels was characterized with HITACHI S-4500 scanning electron microscope at acceleration voltage of 5 kV. The samples were prepared by fracturing in liquid nitrogen, and then coated with platinum before testing. The thermal stabilities were measured on a TGA Q500 thermogravimetric analyzer (TA Instruments) under a nitrogen flow (40 mL min 1). Approximately 5 mg samples were placed in a platinum pan and heated from ambient temperature to 600 °C at a rate of 10 °C min-1.
3. Results and discussion Whey protein isolate (WPI) is very easy to dissolve in water up to 25% solids concentrations. The addition of other materials, such as alginate or clay, further increases solution viscosity, leading to an unwanted tendency to trap air bubbles in the mixture. Much like blending of egg whites in cooking operations, the applied mixing force tends to denature the WPI protein, while generating foam-like bubbles. Caution needs to be taken in the production of such mixtures, as trapped bubbles will lead to imperfection in the freeze dried aerogels. Our previous research has shown a linear dependency of aerogel bulk densities and mechanical properties on the starting polymer content in the aerogel [19,21]. As shown in Fig. 1, the compressive moduli of pure WPI aerogels show a monotonic increase with increasing polymer content, consistent with previous experience. The densities of WPI10A2.5, WPI10A5 and WPI10A2.5C2.5 were much lower than expected, only 50–70% of their theoretical values, which we attribute to trapped air bubbles which freeze dry into structural voids. Because of these bubbles, thermal gelation of WPI composites failed: during heating, the trapped bubbles would expand and cause flaws in the aerogel structure.
2.3. Characterization The densities of the dried aerogels were calculated from the mass and dimension measurements using mass measurements and digital calipers. Compression testing was conducted on the cylindrical specimens (20 mm in diameter and height) using an
Fig. 1. Compressive modulus of WPI aerogel.
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H.-B. Chen et al. / European Polymer Journal 49 (2013) 3387–3391 Table 1 Mechanical properties of WPI aerogels. Concentration
Property
WPI10 (10%)
WPI15 (15%)
WPI20 (20%)
WPI25 (25%)
Original WPI
Modulus Density M/d
0.18 ± 0.11 0.112 ± 0.003 1.6 ± 1.0
0.35 ± 0.11 0.165 ± 0.003 2.1 ± 0.7
0.68 ± 0.11 0.205 ± 0.003 3.3 ± 0.5
1.6 ± 0.1 0.245 ± 0.003 6.4 ± 0.5
Cross-linked WPI (100 mM NaCl, 80 °C for 30 min)
Modulus Density M/d
2.4 ± 0.1 0.114 ± 0.015 20.9 ± 2.4
4.9 ± 1.9 0.158 ± 0.003 30.7 ± 11.9
11.2 ± 1.7 0.201 ± 0.003 55.9 ± 8.5
18.2 ± 7.0 0.256 ± 0.003 70.9 ± 26.6
Modulus in MPa, density in g/cm3, specific density (M/d) in MPa cm3/g.
Table 1 summarizes the mechanical properties of the WPI aerogels. Pure WPI aerogels are very brittle and show poor mechanical properties, even with increasing polymer content. The moduli of WPI10 and WPI25 are 0.18 ± 0.11 MPa and 1.6 ± 0.13 MPa, respectively, approximately 2% of the values previously observed for alginate aerogels [20]. Crosslinking has been reported previously as a successful approach to enhance mechanical properties of aerogels [22], due to the resultant network structures. In the present case, WPI can be thermally transformed in presence of Na+ ions, consistent with the literature reports of crosslinking, giving rise to ten-fold increases in modulus (Fig. 1). The specific moduli of the crosslinked samples similarly increase ten-fold (Table 1), showing that this is not a densitydriven property enhancement. Another potential method for increasing the mechanical strength of aerogels is to blend polymer with filler; montmorillonite clay has been shown to successfully fill this roll [23,24]. Alginate was chosen as a blending component to increase the mechanical properties of WPI aerogels as well. The aerogel composites containing greater alginate content show higher mechanical properties, as can be seen in Table 2. WPI10C5 monoliths are difficult to handle without significantly damaging the samples, therefore the mechanical properties of this composition could not be accurately obtained. The mechanical properties of both clay and WPI aerogels are inferior to those of alginate aerogels, so it is not surprising that the alginate-free sample was the least robust of the samples tested. The specific moduli of samples with increasing levels of solids increased monotonically, despite trapped air bubbles that were present. The compressive stress–strain curves of the WPI and composite aerogels are illustrated in Fig. 2. Pure WPI aerogels are brittle and collapse when compressed. Thermal crosslinking does not alter the brittle nature of WPI aerogels, despite the increase in modulus that were produced. With the incorporation of alginate into WPI aerogels, a
Fig. 2. Example stress–strain curves of WPI aerogels.
brittle collapse was observed under high strain, despite the increased compressive strengths and moduli. With further addition of clay (as in WPI10A2.5C2.5), the stress– strain curves followed the basic form of classical rigid porous foam behavior: a linear elastic deformation at low strain, followed by a densification region beyond the yield point, as the void space collapses [21,25,26]. Morphological studies of the WPI and WPI composites were conducted using SEM, and the corresponding micrographs are shown in Fig. 3. WPI10 shows a layered architecture that follows the direction of ice crystal growth (Fig. 3A), similar as in previous reports [21]. The thermal crosslinking does not alter the overall isotropic structure (Fig. 3B); apparently this chemical reaction only alters the material at a molecular scale, and does not change the gross morphology of the material. During freezing, ice crystal growth would be expected partly destroy the network structure, and form a micro-scale layered structure. WPI10A5 shows an irregular layered structure with links between the layers (Fig. 3C). The two components of the
Table 2 Mechanical properties of WPI aerogel composites. WPI Alginate
12.5 2.5
10 5
7.5 7.5
10 2.5/Clay 2.5
0 5
10 Clay 5
Modulus Density M/d
0.48 ± 0.06 0.0592 ± 0 8.2 ± 1.0
4.1 ± 1.61 0.098 ± 0.002 41.6 ± 16.9
12.9 ± 4.7 0.129 ± 0.002 100.4 ± 35.9
2.5 ± 0.3 0.082 ± 0.003 30.5 ± 3.6
0.99 ± 0.06 0.047 ± <0.001 21 ± 2
Brittle
Modulus in MPa, density in g/cm3, specific density (M/d) in MPa cm3/g.
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Fig. 3. SEM micrographs of WPI based aerogels (A) WPI 10, (B) denatured WPI10, (C) WPI10C5 and (D) WPI10A2.5C2.5.
aerogel composite, WPI and alginate, are indistinguishable in the SEM micrographs. Fig. 3D shows a micrograph of WPI10A2.5C2.5, a less well defined layered structure possessing a large number of holes. The layer thicknesses of the clay-containing WPI/alginate aerogels were much smaller, perhaps giving rise to the lower compressive modului observed compared with that of WPI10A5.
3.1. Thermal stability The thermal stabilities of WPI, denatured WPI, WPI/ alginate, WPI/alginate/clay aerogels, as measured by TGA are shown in Fig. 4. The onset decomposition temperatures (T5%, T20%), percentage of residue (WR) and highest weight loss temperature (Td max) are given in Table 3. The thermal stability data of alginate aerogels (A5) are also listed as a control. Such bio-based materials, typically have significant quantities of water associated with them, and display similar decomposition patterns with two main steps of weight loss. The first weight loss of WPI based aerogel is observed up to 70 °C, which was likely related to the removal of absorbed or bound water, and monitored by T5%. The second weight loss step of WPI-based aerogel begins at approximately 200 °C, associated with the decomposition of polymer compound; T20% was recorded to quantify this property of the materials. The blending with alginate was found to decrease the thermal stability of the WPI aerogels, perhaps because of the comparatively low thermal stability of alginate (or possibly as a result of a chemical reaction between WPI and alginate). Thermal crosslinking, as expected, increased the thermal stability of WPI aerogel by a modest amount, presumably due to reduced molecular motions available to the polymer. The addition of clay and thermal crosslinking also helps to
Fig. 4. TGA curves of WPI aerogels.
increase the char yield after high temperature degradation of the aerogels. Although A5 has comparatively high residue, the addition of A5 to WPI has lower residue content,
H.-B. Chen et al. / European Polymer Journal 49 (2013) 3387–3391 Table 3 Thermal characteristics of the freeze-dried aerogels.
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References
Samples
T5% (°C)
T20% (°C)
Td
max(°C)
A5 WPI10 DWPI10 WPI10A5 WPI10A2.5C2.5
47.0 53.2 65.5 65.0 118.3
228.2 284.9 289.6 244.5 278.6
245.9 316.9 304.3 310.6 308.9
WR (%) 36.4 20.8 26.3 18.5 29.3
which may attributed to the thermal conductivity change caused by microstructure change.
4. Conclusions Production of foam-like polymer aerogels from a widely available by-product of the cheese making industry was explored in the present study. While aerogels produced solely from whey protein show poor mechanical properties, the compressive moduli of these lamellar materials were significantly enhanced by crosslinking (and their thermal stabilities slightly increased), and further increased with increasing aerogel densities. Blending whey protein with alginate allowed for the production of bio-based aerogels with better mechanical properties than those produced with whey alone, though thermal properties was slightly decreased by blending. Acknowledgements The authors of this paper would like to thank China Scholarship Council for financial support. Author Schiraldi and Case Western Reserve University have a financial interest in a company which could commercialize such products.
[1] Li YD, Zeng JB, Wang XL, Yang KK, Wang YZ. Biomacromolecules 2008;9(11):3157–64. [2] Zhang SD, Wang XL, Zhang YR, Wang YZ. Starch/Staerke 2009;61(11):646–55. [3] Meier MAR, Metzger JO, Schubert US. Chem Soc Rev 2007;36(11):1788–802. [4] Trindade WG, Hoareau W, Razera IAT, Ruggiero R, Frollini E, Castellan A. Macromol Mater Eng 2004;289(8):728–36. [5] Harrington JC, Foegeding EA, Mulvihill DM, Morris ER. Food Hydrocolloids 2009;23(2):468–89. [6] Fox PF, McSweeney PLH. Milk proteins. In: Fox PF, McSweeney PL, editors. Dairy chemistry and biochemistry. London: Blackie Academic and Professional; 1998. p. 146–237. [7] Fitzsimons SM, Mulvihill DM, Morris ER. Food Hydrocolloids 2008;22(3):468–84. [8] Sothornvit R, Krochta JM. J Agric Food Chem 2000;48:3913–6. [9] Zinoviadou KG, Koutsmanis KP, Biliaderis CG. Food Hydrocolloids 2010;24:49–59. [10] Taylor SM, Gladden LF, Fryer PJ. J Dairy Res 1994;61(1):71–81. [11] Tobitani A, Ross-Murphy SB. Macromolecules 1997;30(17):4845–54. [12] Verheul M, Roefs SPFM. J Agric Food Chem 1998;46(12):4909–16. [13] Verheul M, Roefs SPFM. Food Hydrocolloids 1998;12(1):17–24. [14] Mulvihill DM, Kinsella JE. J Food Sci 1988;53(1):231–6. [15] Puyol P, Pérez MD, Horne DS. Food Hydrocolloids 2001;15(3):233–7. [16] Varunsatian S, Watanabe K, Hayakawa S, Nakamura R. J Food Sci 1983;48(1):42–6. [17] Verheul M, Roefs SPFM, de Kruif CG. In: Dickenson E, Lorient D, editors. Food macromolecules and colloids. Cambridge, UK: Royal Society of Chemistry; 1995. p. 437–44. [18] Bandi S, Bell M, Schiraldi DA. Macromolecules 2005;38(22):9216–20. [19] Arndt EM, Gawryla MD, Schiraldi DA. J Mater Chem 2007;17(33):3525–9. [20] Chen H-B, Wang Y-Z, Sánchez-Soto M, Schiraldi DA. Polymer 2012;53:5825–31. [21] Gawryla MD, Nezamzadeh M, Schiraldi DA. Green Chem 2008;10(10):1078–81. [22] Pojanavaraphan T, Magaraphan R, Chiou BS, Schiraldi DA. Biomacromolecules 11(10):2640–6. [23] Somlai LS, Bandi SA, Schiraldi DA, Mathias LJ. AIChE J 2006;52(3):1162–8. [24] Bandi S, Schiraldi DA. Macromolecules 2006;39(19):6537–45. [25] Gawryla MD, van den Berg O, Weder C, Schiraldi DA. J Mater Chem 2009;19(15):2118–24. [26] Johnson JR, Spikowski J, Schiraldi DA. Acs Appl Mater Interfaces 2009;1(6):1305–9.
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Foam-like materials based on whey protein isolate Hong-Bing Chen a,b, Yu-Zhong Wang a, David A. Schiraldi b,⇑ a Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China b Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USA
a r t i c l e
i n f o
Article history: Received 1 March 2013 Received in revised form 9 July 2013 Accepted 12 July 2013 Available online 26 July 2013 Keywords: Aerogel Whey Protein Clay Thermal Mechanical
a b s t r a c t Low density materials from sustainable whey protein were fabricated through a simple, environmentally-friendly freeze-drying process. Aerogels produced solely from whey protein show poor mechanical properties, consistent with those of films produced from that biopolymer. The compressive moduli of these lamellar materials were increased by more than an order of magnitude by crosslinking, and further increased with increasing aerogel densities. Blending whey protein with alginate allowed for the production of bio-based aerogels with higher mechanical properties than those produced with whey alone, though thermal properties were slightly decreased by blending. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Bio-based polymers are of increasing interest as fossil fuel resources become more costly. Use of materials, such as cellulose, sugar cane, animal proteins, and plant starches and oils, as alternatives to petrochemicals can decrease overall carbon footprints for consumer and industrial products, as well as reducing their environmental impacts [1– 4]. Whey is the liquid remaining after milk has been curdled and strained for cheese production, and is composed primarily of b-lactoglobulin (>50% of the total whey protein), a-lactalbumin, bovine serum albumin (BSA), and immunoglobulins [5,6]. Whey protein is widely used in the food industry, with applications including processed meats, bakery products, pasta, ice cream, and infant foods [7]. As a by-product of the manufacture of cheese or casein, the broad availability in western countries makes whey protein a good feedstock candidate for the production of environmentally friendly-materials. Due to its ⇑ Corresponding author. E-mail address: [email protected] (D.A. Schiraldi). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.07.019
comparatively low molecular weight and specific molecular conformation, whey protein possesses limited mechanical strength and is therefore seldom used other than in the food industry. Modification of the whey protein structure and blending with other materials, including plasticizers, crosslinkers, and other polymers, are possible methods to improve its final properties; in the absence of plasticization, whey protein isolate is known to be a brittle material [8,9]. Heat-induced gels can form when heating whey protein. Denatured whey proteins aggregate irreversibly and eventually form a space-filling, crosslinked gel structure. The gelation mechanisms and associated processes have been extensively studied [10–17]. The higher the ionic strength is, the lower the critical protein concentration necessary for gel formation other than near the isoelectric pH. By means of such facile crosslinking, the mechanical properties of whey protein can be enhanced. We have previously reported an environmentally-friendly process for the preparation of low-density polymer/clay aerogels from aqueous mixtures [18–20], showing great potential as alternatives to polymeric foam and balsa wood. An especially striking family of such aerogels is those produced
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from alginate/clay, exhibiting extremely high mechanical strengths and moduli [20]. In the current work, we report a family of low density materials based on whey protein. Gelation of the whey protein, as well as blending with alginate are explored as methods to enhance the mechanical properties of these whey protein aerogels. 2. Experimental 2.1. Materials Whey protein isolate (WPI), BIPRO with a protein content of >95%, was supplied by Davisco Foods International, Inc. The main proteins are beta-lactoglobulin with a MW of 18,000 Daltons and alpha-lactalbumin at 14,000 Daltons, and used without further modification. Ammonium alginate and sodium chloride (Fisher Scientific), and sodium montmorillonite (Na+-MMT, PGW grade, Nanocor Inc.) were used as received. Deionized water was produced using a Barnstead RoPure reverse osmosis system. 2.2. Aerogel preparation Percentages of WPI, alginate and clay structural components in aqueous suspensions given prior to freeze-drying. For pure WPI aerogels, WPI powder (10, 15, 20 and 25 g, noted as WPI10, WPI15, WPI20 and WPI25) was dissolved into 100 ml DI water. The solutions were poured into 5 g polystyrene vials and frozen in a solid carbon dioxide/ethanol bath. Equivalent batches were prepared for gelling with 100 mM NaCl aqueous solutions, providing sufficient salt to induce gelation of thermally denatured WPI. The WPI solutions were then heated at 80 °C for 30 min to form gels, which were then frozen in a solid carbon dioxide/ethanol bath. For WPI10A2.5C2.5, 2.5 g of Na+-MMT was blended with 100 mL of DI water on the high speed setting of a Waring model MC2 mini laboratory blender for 1 min to obtain a 2.5 wt% clay aqueous suspension. 2.5 g alginate and 10 g WPI were then dissolved into the clay suspensions. These mixtures were then poured into 5 g polystyrene vials and frozen in a solid carbon dioxide/ethanol bath. The preparation of WPI10A5 (10 g WPI, 5 g alginate) followed the same procedure as was used to produce WPI10A2.5C2.5. The frozen samples were dried in a VirTis Advantage freeze dryer, with an initial shelf temperature of 25 °C, condenser temperature of 87 °C and an eventual vacuum of <10 mbar applied to sublime the ice. The freeze drying process was allowed to proceed for 3–4 days to ensure complete solvent removal.
Instron model 5565 universal testing machine, fitted with a 1 kN load cell, at a crosshead of 10 mm min 1. Five samples of each composition were tested for reproducibility, run to 75% compressive strain. The initial compressive moduli were calculated from the slope of the linear portion of the stress–strain curve. The morphological microstructure of the aerogels was characterized with HITACHI S-4500 scanning electron microscope at acceleration voltage of 5 kV. The samples were prepared by fracturing in liquid nitrogen, and then coated with platinum before testing. The thermal stabilities were measured on a TGA Q500 thermogravimetric analyzer (TA Instruments) under a nitrogen flow (40 mL min 1). Approximately 5 mg samples were placed in a platinum pan and heated from ambient temperature to 600 °C at a rate of 10 °C min-1.
3. Results and discussion Whey protein isolate (WPI) is very easy to dissolve in water up to 25% solids concentrations. The addition of other materials, such as alginate or clay, further increases solution viscosity, leading to an unwanted tendency to trap air bubbles in the mixture. Much like blending of egg whites in cooking operations, the applied mixing force tends to denature the WPI protein, while generating foam-like bubbles. Caution needs to be taken in the production of such mixtures, as trapped bubbles will lead to imperfection in the freeze dried aerogels. Our previous research has shown a linear dependency of aerogel bulk densities and mechanical properties on the starting polymer content in the aerogel [19,21]. As shown in Fig. 1, the compressive moduli of pure WPI aerogels show a monotonic increase with increasing polymer content, consistent with previous experience. The densities of WPI10A2.5, WPI10A5 and WPI10A2.5C2.5 were much lower than expected, only 50–70% of their theoretical values, which we attribute to trapped air bubbles which freeze dry into structural voids. Because of these bubbles, thermal gelation of WPI composites failed: during heating, the trapped bubbles would expand and cause flaws in the aerogel structure.
2.3. Characterization The densities of the dried aerogels were calculated from the mass and dimension measurements using mass measurements and digital calipers. Compression testing was conducted on the cylindrical specimens (20 mm in diameter and height) using an
Fig. 1. Compressive modulus of WPI aerogel.
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H.-B. Chen et al. / European Polymer Journal 49 (2013) 3387–3391 Table 1 Mechanical properties of WPI aerogels. Concentration
Property
WPI10 (10%)
WPI15 (15%)
WPI20 (20%)
WPI25 (25%)
Original WPI
Modulus Density M/d
0.18 ± 0.11 0.112 ± 0.003 1.6 ± 1.0
0.35 ± 0.11 0.165 ± 0.003 2.1 ± 0.7
0.68 ± 0.11 0.205 ± 0.003 3.3 ± 0.5
1.6 ± 0.1 0.245 ± 0.003 6.4 ± 0.5
Cross-linked WPI (100 mM NaCl, 80 °C for 30 min)
Modulus Density M/d
2.4 ± 0.1 0.114 ± 0.015 20.9 ± 2.4
4.9 ± 1.9 0.158 ± 0.003 30.7 ± 11.9
11.2 ± 1.7 0.201 ± 0.003 55.9 ± 8.5
18.2 ± 7.0 0.256 ± 0.003 70.9 ± 26.6
Modulus in MPa, density in g/cm3, specific density (M/d) in MPa cm3/g.
Table 1 summarizes the mechanical properties of the WPI aerogels. Pure WPI aerogels are very brittle and show poor mechanical properties, even with increasing polymer content. The moduli of WPI10 and WPI25 are 0.18 ± 0.11 MPa and 1.6 ± 0.13 MPa, respectively, approximately 2% of the values previously observed for alginate aerogels [20]. Crosslinking has been reported previously as a successful approach to enhance mechanical properties of aerogels [22], due to the resultant network structures. In the present case, WPI can be thermally transformed in presence of Na+ ions, consistent with the literature reports of crosslinking, giving rise to ten-fold increases in modulus (Fig. 1). The specific moduli of the crosslinked samples similarly increase ten-fold (Table 1), showing that this is not a densitydriven property enhancement. Another potential method for increasing the mechanical strength of aerogels is to blend polymer with filler; montmorillonite clay has been shown to successfully fill this roll [23,24]. Alginate was chosen as a blending component to increase the mechanical properties of WPI aerogels as well. The aerogel composites containing greater alginate content show higher mechanical properties, as can be seen in Table 2. WPI10C5 monoliths are difficult to handle without significantly damaging the samples, therefore the mechanical properties of this composition could not be accurately obtained. The mechanical properties of both clay and WPI aerogels are inferior to those of alginate aerogels, so it is not surprising that the alginate-free sample was the least robust of the samples tested. The specific moduli of samples with increasing levels of solids increased monotonically, despite trapped air bubbles that were present. The compressive stress–strain curves of the WPI and composite aerogels are illustrated in Fig. 2. Pure WPI aerogels are brittle and collapse when compressed. Thermal crosslinking does not alter the brittle nature of WPI aerogels, despite the increase in modulus that were produced. With the incorporation of alginate into WPI aerogels, a
Fig. 2. Example stress–strain curves of WPI aerogels.
brittle collapse was observed under high strain, despite the increased compressive strengths and moduli. With further addition of clay (as in WPI10A2.5C2.5), the stress– strain curves followed the basic form of classical rigid porous foam behavior: a linear elastic deformation at low strain, followed by a densification region beyond the yield point, as the void space collapses [21,25,26]. Morphological studies of the WPI and WPI composites were conducted using SEM, and the corresponding micrographs are shown in Fig. 3. WPI10 shows a layered architecture that follows the direction of ice crystal growth (Fig. 3A), similar as in previous reports [21]. The thermal crosslinking does not alter the overall isotropic structure (Fig. 3B); apparently this chemical reaction only alters the material at a molecular scale, and does not change the gross morphology of the material. During freezing, ice crystal growth would be expected partly destroy the network structure, and form a micro-scale layered structure. WPI10A5 shows an irregular layered structure with links between the layers (Fig. 3C). The two components of the
Table 2 Mechanical properties of WPI aerogel composites. WPI Alginate
12.5 2.5
10 5
7.5 7.5
10 2.5/Clay 2.5
0 5
10 Clay 5
Modulus Density M/d
0.48 ± 0.06 0.0592 ± 0 8.2 ± 1.0
4.1 ± 1.61 0.098 ± 0.002 41.6 ± 16.9
12.9 ± 4.7 0.129 ± 0.002 100.4 ± 35.9
2.5 ± 0.3 0.082 ± 0.003 30.5 ± 3.6
0.99 ± 0.06 0.047 ± <0.001 21 ± 2
Brittle
Modulus in MPa, density in g/cm3, specific density (M/d) in MPa cm3/g.
3390
H.-B. Chen et al. / European Polymer Journal 49 (2013) 3387–3391
Fig. 3. SEM micrographs of WPI based aerogels (A) WPI 10, (B) denatured WPI10, (C) WPI10C5 and (D) WPI10A2.5C2.5.
aerogel composite, WPI and alginate, are indistinguishable in the SEM micrographs. Fig. 3D shows a micrograph of WPI10A2.5C2.5, a less well defined layered structure possessing a large number of holes. The layer thicknesses of the clay-containing WPI/alginate aerogels were much smaller, perhaps giving rise to the lower compressive modului observed compared with that of WPI10A5.
3.1. Thermal stability The thermal stabilities of WPI, denatured WPI, WPI/ alginate, WPI/alginate/clay aerogels, as measured by TGA are shown in Fig. 4. The onset decomposition temperatures (T5%, T20%), percentage of residue (WR) and highest weight loss temperature (Td max) are given in Table 3. The thermal stability data of alginate aerogels (A5) are also listed as a control. Such bio-based materials, typically have significant quantities of water associated with them, and display similar decomposition patterns with two main steps of weight loss. The first weight loss of WPI based aerogel is observed up to 70 °C, which was likely related to the removal of absorbed or bound water, and monitored by T5%. The second weight loss step of WPI-based aerogel begins at approximately 200 °C, associated with the decomposition of polymer compound; T20% was recorded to quantify this property of the materials. The blending with alginate was found to decrease the thermal stability of the WPI aerogels, perhaps because of the comparatively low thermal stability of alginate (or possibly as a result of a chemical reaction between WPI and alginate). Thermal crosslinking, as expected, increased the thermal stability of WPI aerogel by a modest amount, presumably due to reduced molecular motions available to the polymer. The addition of clay and thermal crosslinking also helps to
Fig. 4. TGA curves of WPI aerogels.
increase the char yield after high temperature degradation of the aerogels. Although A5 has comparatively high residue, the addition of A5 to WPI has lower residue content,
H.-B. Chen et al. / European Polymer Journal 49 (2013) 3387–3391 Table 3 Thermal characteristics of the freeze-dried aerogels.
3391
References
Samples
T5% (°C)
T20% (°C)
Td
max(°C)
A5 WPI10 DWPI10 WPI10A5 WPI10A2.5C2.5
47.0 53.2 65.5 65.0 118.3
228.2 284.9 289.6 244.5 278.6
245.9 316.9 304.3 310.6 308.9
WR (%) 36.4 20.8 26.3 18.5 29.3
which may attributed to the thermal conductivity change caused by microstructure change.
4. Conclusions Production of foam-like polymer aerogels from a widely available by-product of the cheese making industry was explored in the present study. While aerogels produced solely from whey protein show poor mechanical properties, the compressive moduli of these lamellar materials were significantly enhanced by crosslinking (and their thermal stabilities slightly increased), and further increased with increasing aerogel densities. Blending whey protein with alginate allowed for the production of bio-based aerogels with better mechanical properties than those produced with whey alone, though thermal properties was slightly decreased by blending. Acknowledgements The authors of this paper would like to thank China Scholarship Council for financial support. Author Schiraldi and Case Western Reserve University have a financial interest in a company which could commercialize such products.
[1] Li YD, Zeng JB, Wang XL, Yang KK, Wang YZ. Biomacromolecules 2008;9(11):3157–64. [2] Zhang SD, Wang XL, Zhang YR, Wang YZ. Starch/Staerke 2009;61(11):646–55. [3] Meier MAR, Metzger JO, Schubert US. Chem Soc Rev 2007;36(11):1788–802. [4] Trindade WG, Hoareau W, Razera IAT, Ruggiero R, Frollini E, Castellan A. Macromol Mater Eng 2004;289(8):728–36. [5] Harrington JC, Foegeding EA, Mulvihill DM, Morris ER. Food Hydrocolloids 2009;23(2):468–89. [6] Fox PF, McSweeney PLH. Milk proteins. In: Fox PF, McSweeney PL, editors. Dairy chemistry and biochemistry. London: Blackie Academic and Professional; 1998. p. 146–237. [7] Fitzsimons SM, Mulvihill DM, Morris ER. Food Hydrocolloids 2008;22(3):468–84. [8] Sothornvit R, Krochta JM. J Agric Food Chem 2000;48:3913–6. [9] Zinoviadou KG, Koutsmanis KP, Biliaderis CG. Food Hydrocolloids 2010;24:49–59. [10] Taylor SM, Gladden LF, Fryer PJ. J Dairy Res 1994;61(1):71–81. [11] Tobitani A, Ross-Murphy SB. Macromolecules 1997;30(17):4845–54. [12] Verheul M, Roefs SPFM. J Agric Food Chem 1998;46(12):4909–16. [13] Verheul M, Roefs SPFM. Food Hydrocolloids 1998;12(1):17–24. [14] Mulvihill DM, Kinsella JE. J Food Sci 1988;53(1):231–6. [15] Puyol P, Pérez MD, Horne DS. Food Hydrocolloids 2001;15(3):233–7. [16] Varunsatian S, Watanabe K, Hayakawa S, Nakamura R. J Food Sci 1983;48(1):42–6. [17] Verheul M, Roefs SPFM, de Kruif CG. In: Dickenson E, Lorient D, editors. Food macromolecules and colloids. Cambridge, UK: Royal Society of Chemistry; 1995. p. 437–44. [18] Bandi S, Bell M, Schiraldi DA. Macromolecules 2005;38(22):9216–20. [19] Arndt EM, Gawryla MD, Schiraldi DA. J Mater Chem 2007;17(33):3525–9. [20] Chen H-B, Wang Y-Z, Sánchez-Soto M, Schiraldi DA. Polymer 2012;53:5825–31. [21] Gawryla MD, Nezamzadeh M, Schiraldi DA. Green Chem 2008;10(10):1078–81. [22] Pojanavaraphan T, Magaraphan R, Chiou BS, Schiraldi DA. Biomacromolecules 11(10):2640–6. [23] Somlai LS, Bandi SA, Schiraldi DA, Mathias LJ. AIChE J 2006;52(3):1162–8. [24] Bandi S, Schiraldi DA. Macromolecules 2006;39(19):6537–45. [25] Gawryla MD, van den Berg O, Weder C, Schiraldi DA. J Mater Chem 2009;19(15):2118–24. [26] Johnson JR, Spikowski J, Schiraldi DA. Acs Appl Mater Interfaces 2009;1(6):1305–9.