4736
Ind. Eng. Chem. Res. 2008, 47, 4736–4742
Expansion, Morphological, and Mechanical Properties of Starch-Polystyrene Foams Containing Various Additives† Heartwin A. Pushpadass, Robert W. Weber, and Milford A. Hanna* Industrial Agricultural Products Center and Department of Biological Systems Engineering, UniVersity of NebraskasLincoln, Lincoln, Nebraska 68583-0730
Starch and polystyrene (PS) were mixed at 70:30, 80:20, and 85:15 ratios with 0.5 and 1% talc and extruded into loose-fill packaging foams. Azodicarbonamide (ADC), at 0, 0.2, and 0.4 wt %, and citric acid, at 0, 0.25, and 0.5%, were added as blowing agents to enhance the radial expansion and functional properties of foams. As the concentration of ADC in the starch-PS mixtures was increased from 0 to 0.2%, the expansion ratios increased, and consequently the bulk densities decreased. However, with a further increase in the concentration of ADC to 0.4%, the expansion ratios dropped considerably. The effects of citric acid on radial expansion and other properties were similar to those of ADC. Compared to ADC, citric acid produced foams with higher radial expansion and correspondingly lower densities. Extrudates containing citric acid had largesized cells, but the cell walls were structurally damaged due to degradation of starch. The spring indices of foams treated with various additives were not significantly different, indicating that spring index may not be a reliable measure of the elasticity. On the other hand, compressibility and modulus of elasticity varied depending on the starch, talc, and blowing agents. Foams extruded with ADC were superior in terms of their cushioning ability and other functional properties. 1. Introduction Expanded polystyrene (PS), derived from petroleum products, is the most widely used polymer to produce loose-fill packaging foams. However, the petro-based foams are not biodegradable, and a significant portion of them is used once and then discarded. In the future, it may be untenable to dispose of them, and hence, sustainability, recyclability, and ecological implications may become major considerations. Starch serves as an alternative to the conventional PS polymer to produce loosefill packaging foams. Starch, by itself, is not suitable for packaging foams due to its poor mechanical properties and hydrophilic nature. However, it can be used after suitable modifications or can be used in combination with synthetic polymer resins as reported by Bhatnagar and Hanna.1 The minimum threshold level of the synthetic copolymer added to starch is 25-30 wt %. Any further increase in starch content in foams impairs the physical and mechanical properties. Therefore, the challenge is to make loose-fill packaging foams with higher levels of starch, without compromising essential functional properties. Many chemical blowing agents for extrusion of synthetic foams are available in the market, but their limited understanding in extrusion of starch-based foams has restrained their applications. Citric acid, sodium bicarbonate, and azodicarbonamide (ADC) could be added as blowing agents for starch-based foams to improve the cell growth and expansion characteristics of the starch-PS melt. The blowing agents, under the heat and pressure during extrusion, release CO2 as a foaming agent as well as oxidize the methyl groups of the styrene to form aldehyde groups which react with similar groups on starch.2 Talc also is added to favor cell nucleation and produce foams with a uniform cell size distribution. Unlike blowing agents, nucleating agent like talc facilitates the formation of small-sized cells in the * Corresponding author. E-mail:
[email protected]. Phone: +1-402472-1634. Fax: +1-402-472-6338. † A contribution of the University of Nebraska Agricultural Research Division, Lincoln, NE 68583.
extrudates, which in turn are expected to impart better mechanical properties. Bhatnagar and Hanna1 extruded 25% amylose corn starch with PS and poly(methyl methacrylate) in ratios of 70:30 where bicarbonate, urea, and siloxane were used as blowing and cross-linking agents. Chinnaswamy and Hanna3 mixed sodium chloride, sodium carbonate, and urea as additives with corn starch and extruded them at 140 °C to obtain expansion ratios of 13 to 16.9. The physical, mechanical, and thermal properties of starch-based foams depend upon the type of additives used. Xu and Hanna4 reported that the texture of starch-acetate foams was affected strongly by the type of blowing agent. Higher compatibility between blowing agent and starch acetate increased the viscosity of the melt, which allowed it to hold more vaporized gas and, therefore, expand more. Wang and Shogren5 reported that 0.5-1% addition of talc was optimum for maximum radial expansion of the foams. The mechanical properties of starch-based foams containing various additives were studied by Bhattacharya and Hanna,6 Hayter et al.,7 Hutchinson et al.,8 Chinnaswamy and Hanna,3 Moore et al.,9 Warburton et al.,10 Cha et al.,11 and Shogren and Willet.12 Although studies have reported the effect of talc and other chemical blowing agents on expansion and properties of starchbased foams, literature pertaining to extrusion of starch-PS foams with chemical blowing agents is limited. Furthermore, experiments were conducted only at a fixed content of starch or PS, and consequently, the behavior of talc and blowing agents on cell growth, expansion, and functional properties of foams at various starch and PS contents was not studied. This paper focuses on the radial expansion and properties of starch-based foams containing varying levels of starch and PS. In addition, the role of talc and chemical blowing agents such as ADC and citric acid on the radial expansion, densities, and mechanical properties of foams are discussed.
10.1021/ie071049h CCC: $40.75 2008 American Chemical Society Published on Web 06/18/2008
Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4737 Table 1. Independent Factors and Their Levels ingredient
levels
starch:PS ratio talc blowing agent concentration of blowing agent
70:30, 80:20, and 85:15 0.5 and 1% two types ADC: 0, 0.2, and 0.4% citric acid: 0, 0.25, and 0.5%
2. Experimentation Native normal corn starch (Tate & Lyle Ingredients Americas Inc., Decatur, IL) was agglomerated and dried to about 6% moisture content (d.b.) prior to extrusion. Starch, adjusted to 18% moisture content (d.b.), was mixed with PS (Dow Styron 685D, Dow Chemical, Midland, MI) having an average molecular weight of 120 000, at ratios of 70:30, 80:20, and 85:15 by weight. For discussion purposes, these mixtures will be henceforth referred to as 70%, 80%, and 85% starch samples, respectively. Talc (Barrett Minerals, Inc., Dillon, MT) was added as a nucleating agent13 while ADC (Crompton Corp., Middleburry, CT) and citric acid (Fisher Scientific, NJ) were used as chemical blowing agents. The different ingredients and their concentration levels used are listed in Table 1. Before extrusion, the ingredients were mixed in a Hobart mixer (model C-100, Hobart Corp., Troy, OH) for 2 min and stored overnight for equilibration of moisture. Extrusion was done in a twin-screw extruder (model CTSE-V, C.W. Brabender Instruments, Inc., S. Hackensack, NJ). Temperatures of 50, 140, 140, and 140 °C were maintained in the feed, metering, compression, and die sections of the extruder, respectively. The feed rate in the extruder was maintained constant by a volumetric feeder (PW40PLUS-0, Brabender Technologie Inc., Ontario, Canada). A die nozzle of 3 mm diameter and 13.8 mm land length was used to produce cylindrical extrudates, which were then cut at the die end using an adjustable speed rotary knife into loosefill foams of about 30-35 mm length. During extrusion, the temperature of the viscous melt, the pressure, and the torque inside the barrel were monitored and recorded. 2.1. Physical Properties of Foams. The radial expansions (expansion ratios) of the extruded foams were computed by dividing the cross-sectional area of the extrudates by the crosssectional area of the die nozzle. Each reported value was an average of 20 observations. The bulk densities of the starch foams were measured using a wooden box of 2.84 × 10-2 m3 volume (0.305 × 0.305 × 0.305 m). The unit densities of the foams were determined by the glass bead displacement method7 with modifications.6 The glass beads used for volume displacement had diameters in the range of 1.0-1.05 mm. The measurements on densities were replicated at least five times. 2.2. Morphology of Foams. The microstructures of the foams were examined using a variable pressure scanning electron microscope (SEM) (model S-3000N, Hitachi High Technologies America, Inc., CA). Thin cross sections of the extrudates were mounted on stubs with double-sided adhesive tapes and then sputter-coated under vacuum to make the sample conductive prior to examination. Images were then acquired at 50× magnification and at 1280 × 960 pixel resolution. 2.3. Mechanical Properties of Foams. Bulk spring indices of the foams were measured using an Instron universal testing machine (model 5566, Instron Engineering Corp., Canton, MA) equipped with a 10 kN load cell. A cylindrical probe was used to compress the samples to 80% of their original dimension at a loading rate of 30 mm/min. A cylindrical container with a volume of 434.4 cm3 (74 mm diameter and 101 mm depth) was used to hold the samples. The initial force required to compress a sample and the force required to recompress the
same sample 1 min after releasing the initial load were recorded. Spring indices were computed by dividing the values of the recompression force by the values of the initial compression force. Young’s modulus of elasticity also was computed from loading curves of the bulk spring index test. The measurements were repeated five times, and the values were averaged. 2.4. Statistical Design. There were 18 treatment combinations for each blowing agent. Using talc content as a block, all the samples were extruded in 2 days. Data were analyzed using the Statistical Analysis Software (SAS) package14 (version 9.1, SAS Institute Inc., Cary, NC). The “Proc Mixed” procedure of SAS was used to analyze the data and to compare the treatments with the significance level of R e 0.05. 3. Results and Discussion 3.1. Physical Properties of Foams. 3.1.1. Expansion Ratio. The expansion ratios of foams extruded at different starch contents are presented in Table 2. The main effects of starch, talc, and blowing agents on the radial expansion of foams were found to be highly significant (P < 0.0001). Regardless of the talc content, the expansion ratio remained nearly constant as the starch content increased from 70 to 80% but decreased significantly with further increase in starch content to 85% (Figure 1). In other words, the expansion ratios were not compromised when the PS content was reduced from 30 to 20%, but they fell considerably with further reduction in PS content from 20 to 15%. It can be reasoned that the melt viscosity may have increased considerably at 85% starch content due to swelling of starch, thereby limiting the radial expansion of the foams. The expansion ratios of foams extruded with 1% talc were much lower than their counterparts containing 0.5%, indicating that the talc had a negative effect on expansion properties of starch foams. In an earlier study done on starch-PS foams with 1-3% talc (Pushpadass et al.15), the optimum level of talc was found to be 1%. However, from the results of this work, it could be concluded that addition of talc should be restrained to 0.5%. Surging and increased torque requirements were observed during extrusion of the starch-PS mixtures containing talc, which were indicators of high flow rate and viscosities. However, with increasing talc content, there were no distortions in the shape of the extrudates as observed by Bhatnagar and Hanna.16 Incorporation of ADC in the starch-PS mixtures was found to reduce the torque and pressure in the extruder, which could be taken as indirect evidence of reduction in melt viscosity. According to Marrazzo et al.,17 plasticization induced by the blowing agent results in reduction of the viscosity and temperature of the polymer melt. ADC as a blowing agent had a mixed response on the expansion ratios of foams. At both levels of talc, increasing the concentration of ADC from 0 to 0.2% significantly increased the radial expansion in 70 and 80% starch mixtures (Table 2). However, when the ADC content was further increased to 0.4%, the radial expansion dropped considerably in all cases. At 0.4% concentration, ADC favored the formation of greater numbers of cells, which did not grow and expand well. However, the uniformity of the cells in the extrudates increased with the addition of ADC (Figure 2A-C). Thus, it is speculated that the excess ADC functioned more as a nucleating agent rather than a blowing agent. It can be agreed that PS responded more to the effect of ADC than did starch, or conversely, PS was intrinsically a better expanding polymer than starch. For this reason, ADC is used widely as a blowing agent in the manufacture of synthetic foams.
4738 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 Table 2. Expansion Ratios and Bulk Densities of Foams starch:PS ratio
talc (%)
concn of blowing agent (%)
70:30 70:30 70:30 70:30 70:30 70:30 80:20 80:20 80:20 80:20 80:20 80:20 85:15 85:15 85:15 85:15 85:15 85:15
0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 1.0 1.0 1.0
0.00 0.20 0.40 0.00 0.20 0.40 0.00 0.20 0.40 0.00 0.20 0.40 0.00 0.20 0.40 0.00 0.20 0.40
70:30 70:30 70:30 70:30 70:30 70:30 80:20 80:20 80:20 80:20 80:20 80:20 85:15 85:15 85:15 85:15 85:15 85:15
0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 1.0 1.0 1.0
0.00 0.25 0.50 0.00 0.25 0.50 0.00 0.25 0.50 0.00 0.25 0.50 0.00 0.25 0.50 0.00 0.25 0.50
expansion ratio
bulk density (kg/m3)
unit density (kg/m3)
14.8 ( 0.2 14.0 ( 0.4 16.7 ( 0.1 16.6 ( 0.2 15.3 ( 0.4 18.2 ( 0.4 15.0 ( 0.5 14.4 ( 0.3 18.1 ( 0.2 16.8 ( 0.2 16.5 ( 0.3 19.8 ( 0.7 16.4 ( 0.2 16.1 ( 0.3 19.9 ( 0.2 17.7 ( 0.6 18.3 ( 0.5 21.5 ( 1.3
22.7 ( 1.6 24.3 ( 1.9 29.8 ( 3.1 25.2 ( 1.4 24.5 ( 1.6 31.9 ( 3.6 24.0 ( 2.7 26.4 ( 1.1 32.5 ( 3.8 27.3 ( 2.2 27.5 ( 1.8 34.4 ( 3.3 27.3 ( 2.4 28.9 ( 3.2 34.7 ( 4.5 30.7 ( 2.7 31.1 ( 1.6 36.2 ( 3.7
14.8 ( 0.2 12.9 ( 0.3 13.3 ( 0.3 16.6 ( 0.2 14.2 ( 0.5 15.6 ( 1.1 15.0 ( 0.5 13.7 ( 0.6 15.6 ( 0.6 16.8 ( 0.2 14.7 ( 0.1 15.6 ( 0.2 16.4 ( 0.2 14.0 ( 0.3 15.2 ( 0.1 17.7 ( 0.6 15.0 ( 0.2 15.6 ( 0.3
22.7 ( 1.6 20.6 ( 1.3 20.8 ( 0.4 25.2 ( 1.4 22.0 ( 1.4 26.2 ( 1.1 24.0 ( 2.7 24.3 ( 1.9 27.0 ( 3.3 27.3 ( 2.2 25.0 ( 1.1 27.4 ( 2.4 27.3 ( 2.4 25.1 ( 3.1 26.6 ( 1.5 30.7 ( 2.7 25.8 ( 0.6 27.1 ( 1.4
blowing agent: ADC 46.2 ( 3.1 49.4 ( 3.5 40.5 ( 4.7 40.4 ( 2.9 43.1 ( 4.1 30.4 ( 2.0 45.5 ( 3.3 47.5 ( 2.8 33.5 ( 3.5 40.5 ( 2.6 40.8 ( 2.8 26.5 ( 1.9 40.7 ( 2.6 40.9 ( 3.6 26.3 ( 1.3 36.5 ( 3.9 33.8 ( 3.1 22.1 ( 2.2 blowing agent: citric acid
Zhou and Hanna18 postulated that the properties of the blowing agent and operating conditions govern the solubility of the polymers, the nucleation process, and cell growth, which affected the expansion volume. As per the classic nucleation theory, a higher blowing agent concentration will result in higher nucleation rate, assuming other operating conditions constant.19 According to Lee et al.,20 the effects of blowing agent concentration on nucleation, cell growth, and cell merging resulted in optimum expansion ratio and cell density. Like ADC, citric acid as a blowing agent also had a mixed effect on the expansion properties of foams (P < 0.0001). With increase in citric acid content from 0 to 0.25%, the radial expansion increased at all levels of starch and talc, but it
Figure 1. Effect of starch content on radial expansion of foams.
46.2 ( 3.1 57.2 ( 4.8 53.8 ( 5.1 40.4 ( 2.9 51.0 ( 4.3 55.0 ( 2.9 45.5 ( 3.3 56.6 ( 3.3 47.6 ( 4.7 40.5 ( 2.6 49.1 ( 4.1 45.7 ( 3.6 40.7 ( 2.6 53.6 ( 3.3 44.7 ( 4.4 36.5 ( 3.9 48.3 ( 3.0 46.0 ( 3.2
decreased with further increase in citric acid to 0.5%. The radial expansion dropped at 0.5% citric acid concentration plausibly due to the melt’s inability to hold the heavily expanding cells. Under such conditions, cell coalescence occurred, leading to postextrusion shrinkage and reduced expansion. Another valid reason for the drop in viscosity was the variation in the degree of gelatinization of starch at different concentrations of citric acid.3 Also, it is speculated that citric acid degraded the starch molecules under the intense pressure and heat in the extruder, which was presumed to have a detrimental effect on the expansion properties of starch.21 These results are similar to those of Chinnaswamy and Hanna,3 who observed significant reduction in the expansion ratio of corn starch extruded with sodium bicarbonate and urea. But, Lai et al.22 reported that addition of sodium bicarbonate to wheat starch improved the expansion characteristics of extrudates but weakened their structure. The increase in radial expansion at 70% starch and 1% talc was attributed to the better elasticity and viscosity characteristics of the starch-PS melt, which was able to expand to an optimum level as compared to other mixtures. Unlike the case of ADC-based foams, the radial expansion of citric acid foams was statistically the same across the range of starch content. The citric acid containing foams manifested higher expansion ratios than corresponding ADC blown counterparts. This was presumably due to the lower decomposition temperature of citric acid (compared to that of ADC) as well as the chemical changes induced in the starch-PS melt. From
Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4739
Figure 2. Morphology of foams extruded from 70% starch, 0.5% talc, and containing 0% (A), 0.2% (B), and 0.4% (C) ADC.
the results, it can be concluded that citric acid functioned as an effective blowing agent, and the cells expanded in volume depending on the strength and viscosity of the melt. According to Dixon et al.,23 the chemical blowing agents with higher decomposition temperatures (like ADC) and higher rates of gas evolution generally produced foams with smaller cell size and higher cell density. Unpublished differential scanning calorimetry data also showed that citric acid decomposed into gaseous products at the extrusion temperatures tested, which corroborates the assumption. On the other hand, ADC decomposed at a temperature range of 160-170 °C. Zhou and Cong24 reported that the expansion ratio and cell structure of extrudates were influenced by the decomposition feature of the blowing agent as well as its dependence on foaming temperature. From the standpoint of radial expansion, it could be concluded that citric acid was a better blowing agent than ADC. 3.1.2. Bulk Density. It is an important factor in determining the commercial acceptance of loose-fill foams. Expansion ratio and bulk density are related inversely. The bulk densities of foams were markedly affected (P < 0.0001) by percentages of starch, talc, type of blowing agent, and its concentration. Inverse
relationships were manifested between starch content and bulk density. Keeping other conditions constant, the differences in bulk densities between 70 and 80% starch-based foams were much smaller when compared to the values observed between 80 and 85% starch-containing foams (Table 2). This was reasonable because the radial expansion of the foams were matched closely at those starch contents. This also demonstrated that the bulk densities of the extrudates were severely impaired at starch contents above 80%. Foams with 1% talc as nucleating agent possessed much higher bulk densities compared to those containing 0.5% talc. As the talc level increased, the radial expansion reduced because the talc favored the formation of large number of cells in the extrudates that did not grow and expand more. These results are in agreement with the range of values reported by Bhatnagar and Hanna16 for extrusion of corn starch with 0-5% talc. However, in the presence of talc, the smaller and more uniform cell sizes did not result in lower densities as reported by Bhatnagar and Hanna.16 Consequent to the increase in expansion ratio with increase in ADC from 0 to 0.2%, bulk densities of the extrudates decreased. However, with further increase in the concentration of ADC, the bulk densities of foams increased sharply in all cases. In general, the effect of citric acid on the bulk density was similar to that of ADC. For both blowing agents, the bulk densities were lowest for 70% starch mixture extruded with 0.5% talc and 0.25% citric acid. Regardless of the starch or talc contents, the bulk densities of foams made with citric acid were significantly lower than those containing ADC as the blowing agent. The bulk densities of ADC and citric acid containing foams ranged from 14.0 to 21.5 and 12.9 to 15.6 kg/m3, respectively. The effects of starch, talc, blowing agent, and their concentrations on unit densities of foams were similar to the trends observed for bulk density. In general, the unit densities were 1.5-1.8 times higher than the corresponding bulk densities obtained. However, the densities of starch foams were still 4-6 times greater than that of commercial E-shaped expanded bead PS foams. 3.2. Morphology of Foams. The number and diameter of the cells in a foam govern the distribution and uniformity of cells. In general, the starch foams were characterized by large and irregular cells, which were randomly distributed. Starch had a negative impact on the cell growth and diameters of the foams. Similarly, as the talc concentration was increased from 0.5% to 1%, the cell diameters decreased, and correspondingly, the number of cells increased.18 The effect of ADC on the morphology of extrudates from 70% starch and 0.5% talc is illustrated in Figure 2A-C. It can be inferred that, with increasing ADC, the mean cell diameter decreased but uniformity of the cells increased. Foams made with 0.4% ADC had much smaller cell size compared to their respective counterparts extruded with 0 and 0.2% ADC. As discussed before, it is speculated that the nucleating effect of excess nondegraded ADC may have facilitated the formation of a large number of smallsized cells. The cell diameters of foams extruded from 80% starch and 0.5% talc nearly matched the diameters of foams obtained from 70% starch mixtures (Figures 2A-C vs 3A-C). SEM micrographs also showed that the cell sizes of citric acid foams were much bigger than those containing ADC as a blowing agent (Figure 4A,B). The effect of citric acid was so phenomenal that there were no marked differences in cell diameters among the foams containing citric acid as blowing agent. However, the wall thickness to diameter ratios of cells in citric acid-based foams were much smaller compared to those extruded with
4740 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008
Figure 4. Morphology of foams extruded from 70% starch, 0.5% talc, and containing 0.25% (A) and 0.5% (B) citric acid.
Figure 3. Morphology of foams extruded from 80% starch, 0.5% talc, and containing 0% (A), 0.2% (B), and 0.4% (C) ADC.
ADC. In addition, when citric acid was used as blowing agent, the cell walls were characterized by fissures or cracks, which confirmed the structural damage caused by citric acid. It could be reasoned that the structural damage in the cell walls was due to extensive degradation in starch. 3.3. Compressive Strength and Bulk Spring Index. Spring index is a measure of the ability of a material to recover its original shape after it is deformed. Thus, it refers to the elastic and resilience characteristics of a material. More elastic and resilient foams can absorb more energy during impact and vibration. The compressive force, bulk spring indices, and Young’s modulus of elasticity of the starch foams are presented in Table 3. In general, the spring indices of foams containing ADC were higher than their respective counterparts blown with citric acid. The higher spring indices of foams containing ADC as a blowing agent may be attributed, in part, to their comparatively lower expansion ratios. The ADC-based foams were resilient, structurally rigid, and not crunchy. On the other hand, the citric acid containing foams were crunchy and lacked resilience. The lower spring indices of foams extruded with citric acid were due to the structural damage to the cell walls caused
by degradation of starch. For each blowing agent, despite the considerable differences in the compressive forces, the bulk spring indices were statistically the same or matched closely, regardless of the starch and talc contents. It was not possible to make meaningful comparisons between various treatments despite the significant effects of various ingredients on the radial expansion and other physical and morphological properties. Hence, the bulk spring index may not be a valid measure of elasticity or cushioning properties of starch-based foams. Therefore, the compressive force was used as a criterion for comparing the effects of various ingredients. Compressibility of extrudates is a direct measurement of the ability of a material to deform under force and is related physically to the relative hardness or softness of the product. Compressive force may be an indicator of the cushioning ability of foams. Regardless of the blowing agent, the compressibilities of foams were affected significantly (P < 0.0001) by the concentrations of starch, talc, and blowing agent. The compressive force requirement increased with increase in the percentage of starch and talc. It can be inferred that both starch and talc increased the cushioning ability, which is a desired trait for loose-fill packaging foams. When ADC was used as a blowing agent, the compressive forces required to deform the foams increased steadily with increasing concentration of ADC, implying that the ADC increased the cushioning ability of the foams considerably. Bhatnagar and Hanna1 obtained spring indices of 0.97 and 0.96 and compressibility values of 97 and 136 kPa for commercial PS and starch foams, respectively. With increase in citric acid concentration from 0 to 0.25%, the compressive forces required to deform the samples reduced significantly. However, when the citric acid content was increased from 0.25 to 0.5%, the compressive force increased or decreased depending on the starch and talc contents. Using sodium bicarbonate and calcium bicarbonate as additives for corn grits, Lui and Peng25 observed
Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008 4741 Table 3. Compressibility and Bulk Spring Indices of Foams starch:PS ratio
talc (%)
concn of blowing agent (%)
70:30 70:30 70:30 70:30 70:30 70:30 80:20 80:20 80:20 80:20 80:20 80:20 85:15 85:15 85:15 85:15 85:15 85:15
0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 1.0 1.0 1.0
0.00 0.20 0.40 0.00 0.20 0.40 0.00 0.20 0.40 0.00 0.20 0.40 0.00 0.20 0.40 0.00 0.20 0.40
70:30 70:30 70:30 70:30 70:30 70:30 80:20 80:20 80:20 80:20 80:20 80:20 85:15 85:15 85:15 85:15 85:15 85:15
0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 1.0 1.0 1.0 0.5 0.5 0.5 1.0 1.0 1.0
0.00 0.25 0.50 0.00 0.25 0.50 0.00 0.25 0.50 0.00 0.25 0.50 0.00 0.25 0.50 0.00 0.25 0.50
compressive force (kPa)
bulk spring index
Young’s modulus (kPa)
0.96 ( 0.03 0.96 ( 0.01 0.96 ( 0.00 0.96 ( 0.00 0.96 ( 0.02 0.94 ( 0.02 0.96 ( 0.01 0.96 ( 0.01 0.96 ( 0.01 0.95 ( 0.01 0.98 ( 0.02 0.97 ( 0.02 0.95 ( 0.00 0.96 ( 0.00 0.95 ( 0.00 0.97 ( 0.03 0.96 ( 0.00 0.95 ( 0.00
390.4 ( 29.1 462.4 ( 27.2 562.7 ( 24.0 468.3 ( 12.7 520.5 ( 26.5 684.3 ( 31.7 460.7 ( 9.8 493.8 ( 22.3 683.1 ( 3.5 603.0 ( 25.7 617.1 ( 22.8 793.1 ( 5.4 442.1 ( 10.2 507.9 ( 17.5 805.1 ( 7.3 679.2 ( 29.9 684.5 ( 4.9 793.7 ( 29.7
0.96 ( 0.03 0.93 ( 0.01 0.95 ( 0.03 0.96 ( 0.00 0.94 ( 0.01 0.91 ( 0.00 0.96 ( 0.01 0.92 ( 0.02 0.92 ( 0.01 0.95 ( 0.01 0.94 ( 0.03 0.94 ( 0.03 0.95 ( 0.03 0.93 ( 0.01 0.93 ( 0.04 0.97 ( 0.03 0.96 ( 0.04 0.93 ( 0.00
390.4 ( 29.1 335.3 ( 16.5 391.0 ( 27.1 468.3 ( 12.7 471.6 ( 3.7 491.6 ( 31.5 460.7 ( 9.8 422.2 ( 26.5 463.2 ( 21.5 603.0 ( 25.7 435.6 ( 6.9 470.9 ( 17.6 442.1 ( 10.2 459.0 ( 4.5 411.2 ( 18.5 679.2 ( 29.9 548.3 ( 30.9 483.4 ( 21.6
blowing agent: ADC 90.2 ( 3.1 101.8 ( 2.4 127.3 ( 2.8 107.2 ( 2.3 115.1 ( 3.2 149.7 ( 3.6 104.6 ( 0.6 116.3 ( 2.3 150.1 ( 5.9 126.2 ( 2.3 132.8 ( 2.3 173.3 ( 5.8 104.7 ( 2.3 121.2 ( 3.9 176.1 ( 7.7 145.8 ( 5.9 158.3 ( 4.4 170.5 ( 2.7 blowing agent: citric acid
that the change of additives had a significant effect on the compressibility but no effect on the spring index of the extrudates. The structure of the foams containing ADC remained intact after application of the compressive force. In contrast, with the addition of citric acid, the structural integrity of the foams was impaired, and therefore, the foams crumbled under the compressive load, losing the original size and shape. The disintegration of structure and mechanical damage were reflected on the relatively lower compressive forces requirement to deform the foams to 80% level. In all cases, the compressive force observed for ADC blown foams were significantly higher than those of the citric acid blown foams. The compressive force of ADCbased foams ranged from 90.2 to 173.3 kPa while those of citric acid-based foams ranged from 64.7 to 106.4 kPa. In other words, under the same compressive force, ADC-based foams would compress less than the foams extruded with citric acid. According to Bhatnagar and Hanna,16 foams possessing higher spring indices and lower compressibilities would be more suitable for packaging purposes. Young’s modulus is the ratio of the foam’s ability to resist deformation at different compression levels. In general, the modulus increased with increasing concentrations of starch, talc, and blowing agent. As expected, foams extruded with ADC exhibited higher modulus than their corresponding counterparts containing citric acid. The Young’s modulus values of ADC and citric acid containing foams ranged from 462.4 to 890.1 and 335.3 to 548.3 kPa, respectively. The combination of very
90.2 ( 3.1 64.7 ( 2.5 74.1 ( 1.7 107.2 ( 2.0 93.3 ( 2.0 104.5 ( 2.9 104.6 ( 0.6 79.7 ( 2.6 81.7 ( 2.3 126.2 ( 2.3 84.8 ( 3.1 86.5 ( 2.9 104.7 ( 2.3 85.8 ( 3.3 67.9 ( 4.9 145.8 ( 5.9 106.4 ( 4.1 96.7 ( 4.9
large cells, improper ratio of cell wall thickness to diameter, and weaker cell walls of citric acid containing foams were the probable reasons for the poor resilience and mechanical properties.26 ADC as a blowing agent imparted better mechanical properties to starch foams than did citric acid, and the ADCbased foams were functionally superior. This also confirmed that the ADC-based foams were comparatively more suited for packaging purposes as they are expected to offer better cushioning and resistance to load. 4. Conclusions This study focused on the radial expansion, morphology, and selected properties of starch foams at different concentrations of starch, talc, and blowing agents. The percentage of starch in the raw material played an important role in the expansion and functional properties of foams. Although 70% starch (or 30% PS) gave better expansion and properties, the study demonstrated that it was feasible to produce loose-fill packaging foams from 80% starch mixtures with desirable qualities. Moreover, foams containing 80% starch would definitely be more biodegradable and eco-friendly. ADC and citric acid as blowing agents produced a mixed response on expansion properties, depending on their concentrations as well as the concentrations of starch and talc. On the other hand, citric acid was much more effective than ADC as a blowing agent, resulting in foams with higher expansion and lower bulk densities. However, when citric acid
4742 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008
was used as a blowing agent, there was strong evidence for degradation in starch. Foams containing citric acid crumbled badly in the compressive test and hence may not be suitable for cushioning purposes. Even though talc had a negative effect on the radial expansion and densities of starch foams, it enhanced the cushioning ability and the modulus of elasticity considerably. ADC at 0.25 wt % was a reasonably good blowing agent, producing foams with better physical and mechanical properties. Acknowledgment This research paper is a contribution of the University of Nebraska Agricultural Research Division, supported in part by funds provided through the Hatch Act. Mention of a trade name, proprietary products, or company name is for presentation clarity and does not imply endorsement by the authors or the University of Nebraska. Literature Cited (1) Bhatnagar, S.; Hanna, M. A. Physical, mechanical, and thermal properties of starch-based plastic foams. Trans. ASAE 1995, 38, 567–571. (2) Chinnaswamy, R.; Hanna, M. A. Biodegradable polymers. US Patent No. 5496895, 1992. (3) Chinnaswamy, R.; Hanna, M. A. Expansion, color and shear strength properties of corn starches extrusion cooked with urea and salts. Starch/ Sta¨rke 1988, 40, 186–190. (4) Xu, Y.; Hanna, M. A. Physical, mechanical, and morphological characteristics of extruded starch acetate foams. J. Polym. EnViron. 2005, 13, 221–230. (5) Wang, L.; Shogren, R. L. Preparation and properties of corn starchbased loose-fill foams. In Proceedings of the 6th Annual Meeting of the Bio/Environmentally Degradable Polymer Society, St. Paul, MN, 1997. (6) Bhattacharya, M.; Hanna, M. A. Textural properties of extrusion cooked corn starch. Lebensm.-Wiss. u.-Technol. 1987, 20, 195–201. (7) Hayter, A. C.; Smith, A. C.; Richmond, P. The physical properties of extruded food foams. J. Mater. Sci. 1986, 21, 3729–3736. (8) Hutchinson, R. J.; Siodlak, G. D. E.; Smith, A. C. Influence of processing variables on the mechanical properties of extruded maize. J. Mater. Sci. 1987, 22, 3956–3962. (9) Moore, D.; Sanei, A.; Van Hecke, E.; Bouvier, J. M. Effect of ingredients on physical/structural properties of extrudates. J. Food Sci. 1990, 55, 1383–1402.
(10) Warburton, S. C.; Donald, A. M.; Smith, A. C. Structure and mechanical properties of brittle starch foams. J. Mater. Sci. 1992, 27, 1469– 1474. (11) Cha, J.; Chung, D. S.; Seib, P. A. Effects of extrusion temperature and moisture content on mechanical properties of starch-based foams. Trans. ASAE 1999, 42, 1765–1770. (12) Shogren, R. L.; Willett, J. L. Processing and properties of extruded starch/polymer foams. Polymer 2002, 43, 5935–5947. (13) Cha, J.; Chung, D. S.; Seib, P. A. Rheological properties of blend melts for starch-based foams in extrusion processing. Trans. ASAE 1999, 42, 1801–1807. (14) Statistical Analysis Software (SAS) v. 9.1. SAS Institute, Cary, NC, 2005. (15) Pushpadass, H. A.; Suresh Babu, G.; Weber, R. W.; Hanna, M. A. Extrusion of starch-based loose-fill packaging foams. Accepted for publication in Packag. Technol. Sci. (16) Bhatnagar, S.; Hanna, M. A. Effect of talc on properties of corn starch extrudates. Starch/Sta¨rke 1996, 48, 94–101. (17) Marrazzo, C.; Maio, E. D.; Iannace, S. Foaming of synthetic and natural biodegradable polymers. J. Cell. Plast. 2007, 43, 123–133. (18) Zhou, J.; Hanna, M. A. Effects of the properties of blowing agents on the processing and performance of extruded starch acetate. J. Appl. Polym. Sci. 2005, 97, 1880–1890. (19) Blander, M.; Katz, J. L. Bubble nucleation in liquids. AIChE J. 1975, 21, 833–848. (20) Lee, M.; Tzoganakis, C.; Park, C. B. Effects of supercritical CO2 on the viscosity and morphology of polymer blends. AdV. Poly. Technol. 2000, 19, 300–311. (21) Davidson, V. J.; Paton, D.; Diosady, L. L.; Larocque, G. Degradation of wheat starch in a single screw extruder: characteristics of extruded starch polymers. J. Food Sci. 1984, 49, 453–458. (22) Lai, C. S.; Guetzlaff, J.; Hoseney, R. C. Role of sodium bicarbonate and trapped air in extrusion. Cereal Chem. 1989, 66, 69–73. (23) Dixon, D.; Martin, P. J.; Harkin-Jones, E. Predicting the performance of chemical blowing agents using thermal analysis techniques. J. Cell. Plast. 2000, 36, 310–326. (24) Zhou, Q.; Cong, C. Exo-endothermic blowing agent and its foaming behavior. J. Cell. Plast. 2005, 41, 225–234. (25) Lui, W.; Peng, P. The effects of die shapes and additives on the physical and biodegradable properties of biodegradable cushioning extruded foams. Packag. Technol. Sci. 2003, 16, 1–8. (26) Tsai, J.; Kulp, C. L.; Maliczyszyn, W.; Altieri, P. A.; Rawlins, D. C. Starch foam products with improved flexibility/compressibility and the method of preparation thereof. US Patent No. 5756556, 1996.
ReceiVed for reView August 1, 2007 ReVised manuscript receiVed March 12, 2008 Accepted May 3, 2008 IE071049H
