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kaolin composites
Materials and Design 37 (2012) 423–428
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Structure and properties of pregelatinized cassava starch/kaolin composites Kaewta Kaewtatip ⇑, Varaporn Tanrattanakul Bioplastic Research Unit, Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
a r t i c l e
i n f o
Article history: Received 14 November 2011 Accepted 24 December 2011 Available online 5 January 2012 Keywords: A. Composites E. Mechanical properties G. Scanning election microscopy
a b s t r a c t Pregelatinized cassava starch/kaolin composites were prepared using compression molding. The morphology of the fractured surfaces, retrogradation behavior, thermal decomposition temperatures and mechanical properties of the composites were investigated using scanning election microscopy (SEM), X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and tensile testing, respectively. The tensile strengths and thermal degradation temperatures of the composites were higher than for thermoplastic starch (TPS). The retrogradation behavior of the composites was hindered by kaolin. The water absorption was measured after aging for 12 and 45 days at a relative humidity (RH) of 15% and 55%. It indicated that all the composites displayed lower water absorption values than TPS. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Synthetic plastics are major materials in daily life. Because of their durable properties, ease of production, convenience and cheapness, synthetic plastics are ideal materials for a large number of applications. Plastics find uses as containers, packaging materials for fast food, picnic tableware and agricultural films or in combination with other materials as parts of transportation vehicles, computers, tools, etc. The tremendous increase in plastic wastes has become a worldwide environmental problem. This results from accumulation of synthetic plastics in nature as a consequence of their excellent mechanical properties as well as their resistance to chemicals, weather and biodegradability. As a result, it is recognized that one solution to alleviate this plastic waste problem is to use more biodegradable polymers. Some biodegradable polymers include polycaprolactone, poly(lactic acid) and bacterial polyesters such as polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV). However, they are not widely used because of their high production costs. In contrast, starch has been considered to be a good candidate for preparing biodegradable products because it is inexpensive, abundant and biodegradable, has versatile uses and is a readily available and renewable agricultural resource [1–3]. Useful starch-based plastic materials can be prepared using common thermoplastic processing such as injection [4,5] and compression molding [6]. However, thermoplastic starch (TPS) has some drawbacks including their poor long-term stability, poor mechanical properties, retrogradation during aging, a strong ability to absorbe water and some difficulties in processing [7–9]. Addition of additives is one of the most effective methods to improve the properties of TPS. TPS composites have been achieved by ⇑ Corresponding author. Tel.: +66 74288360; fax: +66 74446925. E-mail addresses: [email protected], [email protected] (K. Kaewtatip). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.12.039
adding various types of reinforcing agents, including clay, especially montmorillonites [10–14] and sepiolite [15], carbon nanotubes [16,17], cotton fiber [18], cassava bagasse cellulose [19], bacterial cellulose [20], hemicellulose [21] and zein [22]. Some of these TPS composites display marked improvements to their properties such as less moisture sensitivity, better mechanical properties and thermal stability. However, there have been few reports of kaolin/thermoplastic starch composites. Kaolin is an aluminum silicate clay mineral. The structure of kaolin consists of octahedral sheets of AlO2(OH)4 and tetrahedral sheets of SiO4. This mineral is an ideal material for many applications in numerous industries because it is inexpensive, abundant, has versatile uses and is readily available and environmentally friendly [23–25]. Conventional composites of TPS/kaolin blends have been prepared by some of those authors [23,26]. Kaolin can improve the mechanical properties of TPS, while their thermal stability and water resistance remain unchanged [23]. However, improvements to thermal stability were observed in thermoplastic amylose/kaolin composites [27]. The objective of this work was to investigate the effect of kaolin on the properties of thermoplastic starch (glycerol-plasticized TPS; GTPS). The retrogradation behavior, morphology of fractured surfaces, thermal and mechanical properties of the composites were characterized by wide angle X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and tensile testing. The relationship between water absorption and retrogradation behavior was also studied. 2. Experimental details 2.1. Materials Pregelatinized cassava starch (PD 10370) and kaolin (Wizkay TU-90) were kindly supplied by Siam Modified Starch Co., Ltd.
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water absorption can be calculated using the following equation [30,31]:
and WIZARD Chemical Co., Ltd., respectively. Typical properties of Wizkay TU-90 kaolin are as follows: the particle sizes are finer than 1 lm; particles larger than 325 mesh are 0.02% maximum; the brightness 90–93%; free moisture 1% maximum; pH 6–9; bulk density 40–50 lbs/ft3 and specific gravity 2.58. The starch was dried at 105 °C for 48 h in an oven and kept in a desiccator prior to use. The glycerol was from Ajax Finechem.
where w1 was the weight of sample before drying and w2 was the weight of sample after drying. All measurements were performed in triplicate.
2.2. Preparation of composites
3. Results and discussion
Pregelatinized cassava starch, glycerol (45 parts per hundred (phr) of total weight of starch) and kaolin (0 (GTPS), 10, 20, 30, 40, 50 and 60 wt.%, dry starch basis) were mixed in polyethylene bags until a homogeneous material was obtained. The mixtures were thermo-molded (15 cm 15 cm 1 mm) by using a compression molding machine, KT-7014 produced by Kao Tieh Ltd. (Taipei, Taiwan) at 160 °C for 10 min and a pressure of 200 kg/ cm2 was directly applied to the sample.
3.1. Retrogradation behavior
2.3. Tensile tests Tensile tests were conducted on the dog bone-shaped according to ASTM D 412 Die C [28] (as shown in Fig. 1) with a 100 N load cell and a crosshead speed of 50 mm/min. Testing was performed at 25 ± 3 °C and 55 ± 2% RH after equilibrating the samples at 55 ± 2% RH and 25 ± 3 °C for 12 days. Ten specimens were tested for every sample. 2.4. Characterization of composites Wide angle X-ray diffraction (XRD) studies were carried out using a Phillips diffractometer (Model PW 1830) with copper as a target material. The voltage, the current and the wavelength of the X-ray source were 40 kV, 30 mA and 0.154060–0.154438 nm, respectively. The scanning regions of the diffraction angle 2h were 5–50° which covered all the significant diffraction peaks of starch and kaolin crystallites. The samples were conditioned at 55 ± 2% RH and 25 ± 3 °C for 12 days and 45 days before measurements. The B-type crystallinity index (Xc) of the samples was determined by analyzing the intensity of the X-ray scattering, as described by Teixeira et al. [19] and Hulleman et al. [29]. Morphology of the fractured surfaces of the TPS and TPS/kaolin composites was examined by using a scanning electron microscope (SEM) (Quanta 400, FEI). The samples were immersed in liquid nitrogen before fracturing. The specimen was coated with a thin layer of gold. The operating voltage used was 15 kV. The thermal decomposition temperatures of the TPS and TPS/ kaolin composites were obtained using a PerkinElmerÒ TGA 7. The thermogravimetric analyzer (TGA) was operated at a heating rate of 10 °C/min from 50 to 600 °C under a nitrogen atmosphere.
Water absorptionð%Þ ¼ ½ðw1 w2 Þ=w2 100
The wide angle X-ray diffraction pattern of the kaolin is shown in Fig. 2. They indicate that the clay layers are highly ordered because only the sharp peaks corresponding to the kaolin are clear as suggested by Gonzalez et al. [32]. The retrogradation (re-crystallization) phenomena that appears during storage of TPS for a long time affects the final properties of TPS. During long term storage, starch molecules tend to form strong intermolecular and intramolecular hydrogen bonds inducing crystallization that makes the TPS brittle and difficult to control the properties of the final product [33,34]. The extent of the retrogradation phenomena of GTPS and TPS/kaolin composites at different storage times was determined by X-ray diffraction. Fig. 3 presents the wide angle X-ray diffraction pattern of GTPS and TPS/kaolin composites at different storage times. After 12 days (Fig. 3a), the GTPS showed a peak at 2h = 16.9° and 19.8°, which is the B-type and V-type crystallinity, respectively [22,35]. For the composites, all the peaks of kaolin in the composites did not change. This indicated that kaolin formed conventional composites with starch and it is difficult to decide the accurate position for the pattern of V-type crystallinity in GTPS and kaolin in the composites because both of them present a peak at 2h = 19.8°; therefore, this peak was not considered for evaluations. The composite containing 10% kaolin content exhibited a peak at 2h = 22.3°, corresponding to the B-type crystallinity pattern. The composites with 30% and 60% kaolin contents presented no B-type crystallinity pattern to indicate that kaolin could restrain the retrogradation behavior of TPS. All samples presented the B-type crystallinity after aging for 45 days (Fig. 3b). The presence of kaolin has a strong effect on the B-type crystallinity index (Xc). From the data in Table 1 it can see that the B-type crystallinity index of GTPS (0% kaolin) increased from 0.47 to 0.54 during aging from 12 days to 45 days. It is concluded that the storage time induced retrogradation. There is no B-type crystallinity index value related to the peak
2.5. Water absorption measurement Small pieces of samples (dimensions 15 15 1 mm) were stored at 15 and 55% RH for 8, 12, 16, 20 and 45 days before testing, then were dried in the oven at 105 °C for 24 h. These small pieces of sample were weighed immediately after being taken out. The
Fig. 1. Schematic illustration of the tensile test sample.
ð1Þ
Fig. 2. Wide angle X-ray diffraction pattern of kaolin.
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the environmental humidity influenced the water absorption of the GTPS and TPS/kaolin composites [21,23]. Water absorption is an important index when considering the benefits of composites [34]. Figs. 4 and 5 present the effects of the storage condition and storage time on the water absorption of GTPS and TPS/kaolin composites containing different kaolin contents. The water absorption at 55% RH was higher than at 15% RH. Moreover, the water absorption of all composites was less than that of GTPS for both relative humidity conditions due to the barrier properties of the kaolin. The TPS/kaolin composite at 60% kaolin content produced the lowest water absorption. Water absorption of the composites decreased with increasing kaolin contents because the kaolin produces a labyrinth structure that blocks the water molecules from absorption into the TPS products. This result agrees with those from other authors [23,27]. It was concluded that the kaolin acts to obstruct water absorption by the composites and this could have extreme implications for improving the application properties of TPS products. Moreover, water absorption is also dependent on the retrogradation behavior. Table 1 shows the relationship between water absorption and the B-type crystallinity index after aging for 12 and 45 days at 55% RH. It was obvious that the water absorption of the samples that were collected at 12 days had a higher value than those collected at 45 days, and was closely related to the increase in the B-type crystallinity index after storage for 45 days. This effect was attributed to the starch molecules recrystallizing during storage so there are no gaps to contain water molecules because of the hydrophobic behavior of the crystalline zones [36]. This also indicated that the water molecules were expelled from the crystalline zones during storage. 3.3. Morphology
Fig. 3. Wide angle X-ray diffraction pattern of GTPS and TPS/kaolin composites stored at a RH of 55 ± 2% for 12 days (a) and 45 days (b).
at 2h = 17° for all the composites during aging for 12 days but it was detected during aging for 45 days. In the case of aging for 45 days, the addition of the kaolin decreased the B-type crystallinity index from 0.54 (0% kaolin) to 0.28 (60% kaolin). That is because kaolin hindered the formation of hydrogen bonds between starch molecules. Therefore, in this case re-crystallization occurred with difficulty.
The SEM micrographs of the fractured surfaces of the GTPS and TPS/kaolin composites containing different amounts of kaolin (10, 30 and 60 wt.% kaolin) are presented in Fig. 6. The surface of GTPS looks clearly smooth (Fig. 6a), whereas the images of the composites exhibit rough surfaces and gap between the GTPS matrix (dark and smooth areas) and kaolin (white and rough areas) (Fig. 6b–d). The rough surfaces and numbers of gaps increase as the kaolin content increased from 10 to 60 wt.%. These results can be attributed to the kaolin which is poorly dispersed and forms aggregates in the composites. These results are similar to those found in previous work [27]. This indicates the poor interfacial adhesion between TPS
3.2. Water absorption TPS/kaolin composites with different kaolin contents were kept at two relative humidity conditions (15% and 55% RH). Changes in
Table 1 B-type crystallinity index (Xc) related to the peak at 2h = 17° and water absorption (%) after aging for 12 and 45 days at RH = 55 ± 2%. Kaolin content (%)
0 10 30 60 a
N = No peak.
B-type crystallinity index
Water absorption (%)
12 (days)
45 (days)
12 (days)
45 (days)
0.47 Na Na Na
0.54 0.43 0.39 0.28
20.84 20.13 17.66 15.60
20.16 19.17 17.04 14.95 Fig. 4. Water absorption (%) versus storage time for GTPS and TPS/kaolin composites during storage at 15% RH.
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Fig. 5. Water absorption (%) versus storage time for GTPS and TPS/kaolin composites during storage at 55% RH.
and kaolin. This is expected because starch and kaolin are immiscible due to the hydrophobic kaolin phase and the hydrophilic TPS phase. This morphology results in poor mechanical properties of TPS/kaolin composites. 3.4. Tensile properties Figs. 7 and 8 show the tensile strength and elongation at break of GTPS and TPS/kaolin composites with different kaolin contents (10,
20, 30, 40, 50 and 60 wt.%), that were tested after 12 days of stabilization at 55% RH. The TPS/kaolin composites did improve the tensile strength of the TPS, however, the elongation at break decreased. The tensile strength of GTPS was 0.62 MPa (Fig. 7). The maximum tensile strength of TPS/kaolin composites occurred at a kaolin content of 10 wt.% (1.19 MPa) but the maximum tensile strength of corn starch/kaolin composites was obtained from 50 phr kaolin content [23]. In this study, a twofold increase in tensile strength was observed when 10 wt.% of kaolin was added to the GTPS. A further increase in kaolin content such as to 60 wt.% brought about a reduced effect on the tensile strength as evidenced by a slight decrease in the tensile strength. These phenomena could be due to the large aggregates of inorganic filler (kaolin) described by other authors [11,13]. The results are in agreement with the SEM observations. It is well known that the mechanical properties of polymer composites are strongly consistent with their morphology. Fig. 8 presents the elongation at break results of GTPS and TPS/kaolin composites with different kaolin contents. The GTPS shows a 36.77% elongation at break. This was significantly higher than those of the TPS/kaolin composites. The decrease of elongation at break occurred when the kaolin content increased which was the same result as reported by Curvelo et al. [23]. It can be concluded that the elongation at break of TPS/kaolin composites decreased because the addition of inorganic filler to the TPS increased its brittleness, as shown by other research groups [11,37]. 3.5. Thermal stability Fig. 9 displays the TGA thermograms of the TPS and TPS/kaolin composites with different kaolin contents (10, 30 and 60 wt.%). There were three steps of weight loss in the samples. First, the weight loss from 50 °C to 110 °C, that was assigned to water
Fig. 6. SEM micrographs of TPS (a) and TPS/kaolin composites with: 10 wt.% (b), 30 wt.% (c) and 60 wt.% (d) of kaolin.
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Fig. 7. The tensile strength of GTPS and TPS/kaolin composites.
Fig. 9. TGA thermograms of GTPS (a) and TPS/kaolin composites with different kaolin contents: 10 wt.% (b), 30 wt.% (c) and 60 wt.% (d).
Table 2 The temperature values of GTPS and TPS/kaolin composites at maximum weight loss (Tmax). Kaolin content (wt.%)
Tmax (°C)
0 10 30 60
333 336 339 342
4. Conclusions
Fig. 8. The elongation at break of GTPS and TPS/kaolin composites.
evaporation [3,38,39]. The second step involved the volatilization of glycerol at the temperature of 150–250 °C [20,40]. A significant weight loss that occurred in the third step in the temperature range of 280–380 °C could be attributed to the decomposition of the starch in the TPS and composites [15,20,39]. However, the weight loss percentage (Wight (%)) in the thermograms of the samples was different. This result indicated that the TPS and TPS/kaolin composites with a kaolin content equal to a 10 wt.% and 30 wt.% were degraded more rapidly than the TPS/kaolin composites of 60 wt.% kaolin. The temperature values of TPS and TPS/kaolin composites at their maximum weight loss are presented in Table 2. As the kaolin content increases, the temperature values of maximum weight loss increases. This indicates that the addition of kaolin improved the thermal stability of the TPS. Compared to other work [27], the temperature values at maximum weight loss of the amylose/kaolin composite containing 20% kaolin was 325 °C whereas the temperature values at maximum weight loss of the composite containing 10%, 30% and 60% kaolin in this study was 336 °C, 339 °C and 342 °C, respectively. The present result contrasts with the previous result which was unable to show an improvement in the thermal stability of the TPS with kaolin [23].
Compression-molded thermoplastic starch was prepared from pregelatinized cassava starch using kaolin as an additive. The results from XRD confirmed that the retrogradation behavior effect was dependent on the water absorption properties of the samples. Kaolin prevented the retrogradation and water absorption of TPS as it hindered the intermolecular and intramolecular hydrogen bonds between starch molecules and at the same time expelled water molecules from crystalline zones during storage. The maximum tensile strength of TPS/kaolin composites (1.19 MPa) was derived when 10 wt.% of kaolin was used. Moreover, when the amount of kaolin increased further the composites became very brittle. From the SEM results it seems that kaolin is prone to form aggregate structures in the composites when the amount of kaolin was increased. The thermal stability of TPS was improved by the addition of the kaolin because it acts as a heat barrier. Acknowledgements We would like to thank the Prince of Songkla University, the Faculty of Science, the Office of the Higher Education Commission and the Thai Research Fund (TRF) Contract No. MRG5480088 for their financial support and Siam Modified Starch Co., Ltd. and WIZARD Chemical Co., Ltd., for their pregelatinized cassava starch and Wizkay TU-90 kaolin for support. Thanks to Dr. Brian Hodgson for assistance with the English. References [1] Chen L, Qiu X, Deng M, Hong Z, Luo R, Chen X, et al. The starch grafted poly (L-lactide) and the physical properties of its blending composites. Polymer 2005;46:5723–9.
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Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Structure and properties of pregelatinized cassava starch/kaolin composites Kaewta Kaewtatip ⇑, Varaporn Tanrattanakul Bioplastic Research Unit, Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
a r t i c l e
i n f o
Article history: Received 14 November 2011 Accepted 24 December 2011 Available online 5 January 2012 Keywords: A. Composites E. Mechanical properties G. Scanning election microscopy
a b s t r a c t Pregelatinized cassava starch/kaolin composites were prepared using compression molding. The morphology of the fractured surfaces, retrogradation behavior, thermal decomposition temperatures and mechanical properties of the composites were investigated using scanning election microscopy (SEM), X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and tensile testing, respectively. The tensile strengths and thermal degradation temperatures of the composites were higher than for thermoplastic starch (TPS). The retrogradation behavior of the composites was hindered by kaolin. The water absorption was measured after aging for 12 and 45 days at a relative humidity (RH) of 15% and 55%. It indicated that all the composites displayed lower water absorption values than TPS. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Synthetic plastics are major materials in daily life. Because of their durable properties, ease of production, convenience and cheapness, synthetic plastics are ideal materials for a large number of applications. Plastics find uses as containers, packaging materials for fast food, picnic tableware and agricultural films or in combination with other materials as parts of transportation vehicles, computers, tools, etc. The tremendous increase in plastic wastes has become a worldwide environmental problem. This results from accumulation of synthetic plastics in nature as a consequence of their excellent mechanical properties as well as their resistance to chemicals, weather and biodegradability. As a result, it is recognized that one solution to alleviate this plastic waste problem is to use more biodegradable polymers. Some biodegradable polymers include polycaprolactone, poly(lactic acid) and bacterial polyesters such as polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV). However, they are not widely used because of their high production costs. In contrast, starch has been considered to be a good candidate for preparing biodegradable products because it is inexpensive, abundant and biodegradable, has versatile uses and is a readily available and renewable agricultural resource [1–3]. Useful starch-based plastic materials can be prepared using common thermoplastic processing such as injection [4,5] and compression molding [6]. However, thermoplastic starch (TPS) has some drawbacks including their poor long-term stability, poor mechanical properties, retrogradation during aging, a strong ability to absorbe water and some difficulties in processing [7–9]. Addition of additives is one of the most effective methods to improve the properties of TPS. TPS composites have been achieved by ⇑ Corresponding author. Tel.: +66 74288360; fax: +66 74446925. E-mail addresses: [email protected], [email protected] (K. Kaewtatip). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.12.039
adding various types of reinforcing agents, including clay, especially montmorillonites [10–14] and sepiolite [15], carbon nanotubes [16,17], cotton fiber [18], cassava bagasse cellulose [19], bacterial cellulose [20], hemicellulose [21] and zein [22]. Some of these TPS composites display marked improvements to their properties such as less moisture sensitivity, better mechanical properties and thermal stability. However, there have been few reports of kaolin/thermoplastic starch composites. Kaolin is an aluminum silicate clay mineral. The structure of kaolin consists of octahedral sheets of AlO2(OH)4 and tetrahedral sheets of SiO4. This mineral is an ideal material for many applications in numerous industries because it is inexpensive, abundant, has versatile uses and is readily available and environmentally friendly [23–25]. Conventional composites of TPS/kaolin blends have been prepared by some of those authors [23,26]. Kaolin can improve the mechanical properties of TPS, while their thermal stability and water resistance remain unchanged [23]. However, improvements to thermal stability were observed in thermoplastic amylose/kaolin composites [27]. The objective of this work was to investigate the effect of kaolin on the properties of thermoplastic starch (glycerol-plasticized TPS; GTPS). The retrogradation behavior, morphology of fractured surfaces, thermal and mechanical properties of the composites were characterized by wide angle X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and tensile testing. The relationship between water absorption and retrogradation behavior was also studied. 2. Experimental details 2.1. Materials Pregelatinized cassava starch (PD 10370) and kaolin (Wizkay TU-90) were kindly supplied by Siam Modified Starch Co., Ltd.
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water absorption can be calculated using the following equation [30,31]:
and WIZARD Chemical Co., Ltd., respectively. Typical properties of Wizkay TU-90 kaolin are as follows: the particle sizes are finer than 1 lm; particles larger than 325 mesh are 0.02% maximum; the brightness 90–93%; free moisture 1% maximum; pH 6–9; bulk density 40–50 lbs/ft3 and specific gravity 2.58. The starch was dried at 105 °C for 48 h in an oven and kept in a desiccator prior to use. The glycerol was from Ajax Finechem.
where w1 was the weight of sample before drying and w2 was the weight of sample after drying. All measurements were performed in triplicate.
2.2. Preparation of composites
3. Results and discussion
Pregelatinized cassava starch, glycerol (45 parts per hundred (phr) of total weight of starch) and kaolin (0 (GTPS), 10, 20, 30, 40, 50 and 60 wt.%, dry starch basis) were mixed in polyethylene bags until a homogeneous material was obtained. The mixtures were thermo-molded (15 cm 15 cm 1 mm) by using a compression molding machine, KT-7014 produced by Kao Tieh Ltd. (Taipei, Taiwan) at 160 °C for 10 min and a pressure of 200 kg/ cm2 was directly applied to the sample.
3.1. Retrogradation behavior
2.3. Tensile tests Tensile tests were conducted on the dog bone-shaped according to ASTM D 412 Die C [28] (as shown in Fig. 1) with a 100 N load cell and a crosshead speed of 50 mm/min. Testing was performed at 25 ± 3 °C and 55 ± 2% RH after equilibrating the samples at 55 ± 2% RH and 25 ± 3 °C for 12 days. Ten specimens were tested for every sample. 2.4. Characterization of composites Wide angle X-ray diffraction (XRD) studies were carried out using a Phillips diffractometer (Model PW 1830) with copper as a target material. The voltage, the current and the wavelength of the X-ray source were 40 kV, 30 mA and 0.154060–0.154438 nm, respectively. The scanning regions of the diffraction angle 2h were 5–50° which covered all the significant diffraction peaks of starch and kaolin crystallites. The samples were conditioned at 55 ± 2% RH and 25 ± 3 °C for 12 days and 45 days before measurements. The B-type crystallinity index (Xc) of the samples was determined by analyzing the intensity of the X-ray scattering, as described by Teixeira et al. [19] and Hulleman et al. [29]. Morphology of the fractured surfaces of the TPS and TPS/kaolin composites was examined by using a scanning electron microscope (SEM) (Quanta 400, FEI). The samples were immersed in liquid nitrogen before fracturing. The specimen was coated with a thin layer of gold. The operating voltage used was 15 kV. The thermal decomposition temperatures of the TPS and TPS/ kaolin composites were obtained using a PerkinElmerÒ TGA 7. The thermogravimetric analyzer (TGA) was operated at a heating rate of 10 °C/min from 50 to 600 °C under a nitrogen atmosphere.
Water absorptionð%Þ ¼ ½ðw1 w2 Þ=w2 100
The wide angle X-ray diffraction pattern of the kaolin is shown in Fig. 2. They indicate that the clay layers are highly ordered because only the sharp peaks corresponding to the kaolin are clear as suggested by Gonzalez et al. [32]. The retrogradation (re-crystallization) phenomena that appears during storage of TPS for a long time affects the final properties of TPS. During long term storage, starch molecules tend to form strong intermolecular and intramolecular hydrogen bonds inducing crystallization that makes the TPS brittle and difficult to control the properties of the final product [33,34]. The extent of the retrogradation phenomena of GTPS and TPS/kaolin composites at different storage times was determined by X-ray diffraction. Fig. 3 presents the wide angle X-ray diffraction pattern of GTPS and TPS/kaolin composites at different storage times. After 12 days (Fig. 3a), the GTPS showed a peak at 2h = 16.9° and 19.8°, which is the B-type and V-type crystallinity, respectively [22,35]. For the composites, all the peaks of kaolin in the composites did not change. This indicated that kaolin formed conventional composites with starch and it is difficult to decide the accurate position for the pattern of V-type crystallinity in GTPS and kaolin in the composites because both of them present a peak at 2h = 19.8°; therefore, this peak was not considered for evaluations. The composite containing 10% kaolin content exhibited a peak at 2h = 22.3°, corresponding to the B-type crystallinity pattern. The composites with 30% and 60% kaolin contents presented no B-type crystallinity pattern to indicate that kaolin could restrain the retrogradation behavior of TPS. All samples presented the B-type crystallinity after aging for 45 days (Fig. 3b). The presence of kaolin has a strong effect on the B-type crystallinity index (Xc). From the data in Table 1 it can see that the B-type crystallinity index of GTPS (0% kaolin) increased from 0.47 to 0.54 during aging from 12 days to 45 days. It is concluded that the storage time induced retrogradation. There is no B-type crystallinity index value related to the peak
2.5. Water absorption measurement Small pieces of samples (dimensions 15 15 1 mm) were stored at 15 and 55% RH for 8, 12, 16, 20 and 45 days before testing, then were dried in the oven at 105 °C for 24 h. These small pieces of sample were weighed immediately after being taken out. The
Fig. 1. Schematic illustration of the tensile test sample.
ð1Þ
Fig. 2. Wide angle X-ray diffraction pattern of kaolin.
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the environmental humidity influenced the water absorption of the GTPS and TPS/kaolin composites [21,23]. Water absorption is an important index when considering the benefits of composites [34]. Figs. 4 and 5 present the effects of the storage condition and storage time on the water absorption of GTPS and TPS/kaolin composites containing different kaolin contents. The water absorption at 55% RH was higher than at 15% RH. Moreover, the water absorption of all composites was less than that of GTPS for both relative humidity conditions due to the barrier properties of the kaolin. The TPS/kaolin composite at 60% kaolin content produced the lowest water absorption. Water absorption of the composites decreased with increasing kaolin contents because the kaolin produces a labyrinth structure that blocks the water molecules from absorption into the TPS products. This result agrees with those from other authors [23,27]. It was concluded that the kaolin acts to obstruct water absorption by the composites and this could have extreme implications for improving the application properties of TPS products. Moreover, water absorption is also dependent on the retrogradation behavior. Table 1 shows the relationship between water absorption and the B-type crystallinity index after aging for 12 and 45 days at 55% RH. It was obvious that the water absorption of the samples that were collected at 12 days had a higher value than those collected at 45 days, and was closely related to the increase in the B-type crystallinity index after storage for 45 days. This effect was attributed to the starch molecules recrystallizing during storage so there are no gaps to contain water molecules because of the hydrophobic behavior of the crystalline zones [36]. This also indicated that the water molecules were expelled from the crystalline zones during storage. 3.3. Morphology
Fig. 3. Wide angle X-ray diffraction pattern of GTPS and TPS/kaolin composites stored at a RH of 55 ± 2% for 12 days (a) and 45 days (b).
at 2h = 17° for all the composites during aging for 12 days but it was detected during aging for 45 days. In the case of aging for 45 days, the addition of the kaolin decreased the B-type crystallinity index from 0.54 (0% kaolin) to 0.28 (60% kaolin). That is because kaolin hindered the formation of hydrogen bonds between starch molecules. Therefore, in this case re-crystallization occurred with difficulty.
The SEM micrographs of the fractured surfaces of the GTPS and TPS/kaolin composites containing different amounts of kaolin (10, 30 and 60 wt.% kaolin) are presented in Fig. 6. The surface of GTPS looks clearly smooth (Fig. 6a), whereas the images of the composites exhibit rough surfaces and gap between the GTPS matrix (dark and smooth areas) and kaolin (white and rough areas) (Fig. 6b–d). The rough surfaces and numbers of gaps increase as the kaolin content increased from 10 to 60 wt.%. These results can be attributed to the kaolin which is poorly dispersed and forms aggregates in the composites. These results are similar to those found in previous work [27]. This indicates the poor interfacial adhesion between TPS
3.2. Water absorption TPS/kaolin composites with different kaolin contents were kept at two relative humidity conditions (15% and 55% RH). Changes in
Table 1 B-type crystallinity index (Xc) related to the peak at 2h = 17° and water absorption (%) after aging for 12 and 45 days at RH = 55 ± 2%. Kaolin content (%)
0 10 30 60 a
N = No peak.
B-type crystallinity index
Water absorption (%)
12 (days)
45 (days)
12 (days)
45 (days)
0.47 Na Na Na
0.54 0.43 0.39 0.28
20.84 20.13 17.66 15.60
20.16 19.17 17.04 14.95 Fig. 4. Water absorption (%) versus storage time for GTPS and TPS/kaolin composites during storage at 15% RH.
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Fig. 5. Water absorption (%) versus storage time for GTPS and TPS/kaolin composites during storage at 55% RH.
and kaolin. This is expected because starch and kaolin are immiscible due to the hydrophobic kaolin phase and the hydrophilic TPS phase. This morphology results in poor mechanical properties of TPS/kaolin composites. 3.4. Tensile properties Figs. 7 and 8 show the tensile strength and elongation at break of GTPS and TPS/kaolin composites with different kaolin contents (10,
20, 30, 40, 50 and 60 wt.%), that were tested after 12 days of stabilization at 55% RH. The TPS/kaolin composites did improve the tensile strength of the TPS, however, the elongation at break decreased. The tensile strength of GTPS was 0.62 MPa (Fig. 7). The maximum tensile strength of TPS/kaolin composites occurred at a kaolin content of 10 wt.% (1.19 MPa) but the maximum tensile strength of corn starch/kaolin composites was obtained from 50 phr kaolin content [23]. In this study, a twofold increase in tensile strength was observed when 10 wt.% of kaolin was added to the GTPS. A further increase in kaolin content such as to 60 wt.% brought about a reduced effect on the tensile strength as evidenced by a slight decrease in the tensile strength. These phenomena could be due to the large aggregates of inorganic filler (kaolin) described by other authors [11,13]. The results are in agreement with the SEM observations. It is well known that the mechanical properties of polymer composites are strongly consistent with their morphology. Fig. 8 presents the elongation at break results of GTPS and TPS/kaolin composites with different kaolin contents. The GTPS shows a 36.77% elongation at break. This was significantly higher than those of the TPS/kaolin composites. The decrease of elongation at break occurred when the kaolin content increased which was the same result as reported by Curvelo et al. [23]. It can be concluded that the elongation at break of TPS/kaolin composites decreased because the addition of inorganic filler to the TPS increased its brittleness, as shown by other research groups [11,37]. 3.5. Thermal stability Fig. 9 displays the TGA thermograms of the TPS and TPS/kaolin composites with different kaolin contents (10, 30 and 60 wt.%). There were three steps of weight loss in the samples. First, the weight loss from 50 °C to 110 °C, that was assigned to water
Fig. 6. SEM micrographs of TPS (a) and TPS/kaolin composites with: 10 wt.% (b), 30 wt.% (c) and 60 wt.% (d) of kaolin.
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Fig. 7. The tensile strength of GTPS and TPS/kaolin composites.
Fig. 9. TGA thermograms of GTPS (a) and TPS/kaolin composites with different kaolin contents: 10 wt.% (b), 30 wt.% (c) and 60 wt.% (d).
Table 2 The temperature values of GTPS and TPS/kaolin composites at maximum weight loss (Tmax). Kaolin content (wt.%)
Tmax (°C)
0 10 30 60
333 336 339 342
4. Conclusions
Fig. 8. The elongation at break of GTPS and TPS/kaolin composites.
evaporation [3,38,39]. The second step involved the volatilization of glycerol at the temperature of 150–250 °C [20,40]. A significant weight loss that occurred in the third step in the temperature range of 280–380 °C could be attributed to the decomposition of the starch in the TPS and composites [15,20,39]. However, the weight loss percentage (Wight (%)) in the thermograms of the samples was different. This result indicated that the TPS and TPS/kaolin composites with a kaolin content equal to a 10 wt.% and 30 wt.% were degraded more rapidly than the TPS/kaolin composites of 60 wt.% kaolin. The temperature values of TPS and TPS/kaolin composites at their maximum weight loss are presented in Table 2. As the kaolin content increases, the temperature values of maximum weight loss increases. This indicates that the addition of kaolin improved the thermal stability of the TPS. Compared to other work [27], the temperature values at maximum weight loss of the amylose/kaolin composite containing 20% kaolin was 325 °C whereas the temperature values at maximum weight loss of the composite containing 10%, 30% and 60% kaolin in this study was 336 °C, 339 °C and 342 °C, respectively. The present result contrasts with the previous result which was unable to show an improvement in the thermal stability of the TPS with kaolin [23].
Compression-molded thermoplastic starch was prepared from pregelatinized cassava starch using kaolin as an additive. The results from XRD confirmed that the retrogradation behavior effect was dependent on the water absorption properties of the samples. Kaolin prevented the retrogradation and water absorption of TPS as it hindered the intermolecular and intramolecular hydrogen bonds between starch molecules and at the same time expelled water molecules from crystalline zones during storage. The maximum tensile strength of TPS/kaolin composites (1.19 MPa) was derived when 10 wt.% of kaolin was used. Moreover, when the amount of kaolin increased further the composites became very brittle. From the SEM results it seems that kaolin is prone to form aggregate structures in the composites when the amount of kaolin was increased. The thermal stability of TPS was improved by the addition of the kaolin because it acts as a heat barrier. Acknowledgements We would like to thank the Prince of Songkla University, the Faculty of Science, the Office of the Higher Education Commission and the Thai Research Fund (TRF) Contract No. MRG5480088 for their financial support and Siam Modified Starch Co., Ltd. and WIZARD Chemical Co., Ltd., for their pregelatinized cassava starch and Wizkay TU-90 kaolin for support. Thanks to Dr. Brian Hodgson for assistance with the English. References [1] Chen L, Qiu X, Deng M, Hong Z, Luo R, Chen X, et al. The starch grafted poly (L-lactide) and the physical properties of its blending composites. Polymer 2005;46:5723–9.
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