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polyvinyl alcohol foams
Nuclear Instruments and Methods in Physics Research B 211 (2003) 244–250 www.elsevier.com/locate/nimb
The effect of electron beam irradiation on preparation of sago starch/polyvinyl alcohol foams Benchamaporn Wongsuban a,*, Kharidah Muhammad a, Zulkafli Ghazali b, Kamaruddin Hashim b, Muhammad Ali Hassan c a
c
Department of Food Science, Faculty of Food Science and Biotechnology, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia b Malaysian Institute for Nuclear Technology Research, MINT, Bangi, 43000 Kajang, Malaysia Department of Biotechnology, Univerisiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Received 21 February 2003
Abstract Blends of sago starch (SS)/polyvinyl alcohol (PVA) were irradiated with doses ranging from 10 to 30 kGy. Foams were then produced from these irradiated blends using a microwave. Changes in the degree of crosslinking, gel strength, thermal stability morphology of blends and linear expansion of foam with increasing irradiation doses were subsequently investigated. It was observed that the degree of crosslinking was important in maximizing the positive effect on foams produced. The gel strength of SS/PVA blends was affected by the irradiation. The crosslinking by the irradiation enhanced the thermal stability of SS/PVA blends. The results also revealed that the highest linear expansion of foams could be produced by irradiation blends at 15 kGy. Changes in blend morphology were observed upon irradiation. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Gel content; Crosslinking; Thermal stability; Biodegradable; Morphology
1. Introduction Owing to the rapid population and economic growth, many countries are faced with environmental problems such as that related to garbage [1,2]. One of the items that is contributing further to this problem would be the foams used in packaging. Several serious problems are being faced by the foam industries. These include flam-
*
Corresponding author. Tel.: +76-03-8948-6389; fax: +7603-8942-3552. E-mail address: [email protected] (B. Wongsuban).
mability (combustibility), waste disposal (biodegradability) or recycling wherever possible or economically feasible and the problem of ozone depletion by fluorocarbon (CFC) blowing agents [3]. The development of biodegradable foam would thus be a step forward in the right direction for the aforementioned industry. Utilization of annually renewable agriculturally derived products such as starch as extenders and replacements for synthetic, petroleum-based polymer is currently an active area of research [4,5]. Starch and its derivatives are currently used as plastic materials for disposable items such as packaging loose-fills and picnic tableware [6]. Use of polysaccharides in
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01250-3
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
plastics not only reduces our dependence on petrochemical-derived monomers, but the polysaccharide portion will also biodegrade, causing the finished plastic article to lose its integrity and be reduced to the small particles [7]. Electron beam irradiation of polymer has been employed in the production of foam. High-energy irradiation typically produces free radicals can readily interact with each other forming crosslinking and strengthening the polymer structure. Crosslinking will also enhance the resistance of the cellular product to thermal collapse [8]. Therefore, the objective of this study was to determine the effect of irradiation doses and sago starch (SS)/polyvinyl alcohol (PVA) blends on the foam produced.
The gel content was calculated using the equation below: %Gel ¼ ðweight of dry gel after extraction =weight of initial polymerÞ 100: 2.2. Gel strength determination of SS/PVA blends The gel strength of the blend was determined based on the force required to penetrate it and measured using a Texture Analyser (TAXT2). The testing conditions were as follows: Pre-test speed of 10 mm/s, test speed of 1 mm/s, post-test speed of 10 mm/s, penetration distance 2 mm using a cylindrical probe (15 mm diameter). 2.3. Thermogravimatric analysis of SS/PVA blends
2. Materials and methods SS was donated by Nitsei Sago Industries Sdn. Bhd, Mukah, Sarawak and PVA was purchased from Kuraray Poval Co. Ltd., Japan. 10/30/100, 15/25/100, 20/20/100, 25/15/00 and 30/10/100 of SS/PVA/water mixtures were prepared by solubilised PVA in distilled water at 121 °C for 10 min. They were then left to cool. SS was then mixed with aqueous PVA before the irradiation. Thirty milliliter of blend was poured into a square petri dish (10 cm 10 cm). They were then irradiated at 10, 15, 20, 25 and 30 kGy. The conditions of the electron beam machine were as follows: Accelerate voltage Beam current Speed
245
2.0 MeV 10.0 mA 0.94 m/min
The irradiated blends were then foamed by microwave heating for 5–8 min. In this study, the distilled water acted as the blowing agent. 2.1. Gel content determination The sample obtained after moisture content of the irradiated SS/PVA blends (in the form of gels) was determined. They were then weighed, placed in between steel nets and then autoclaved for 1 h. the autoclaved gel were later washed with distilled water and dried in an oven at 60 °C overnight.
Thermogravimetric analysis (TGA) is a test procedure in which change in the weight of specimen is monitored as the specimen is progressively heated [9]. TGA was performed on a thermogravimetric analyser TGA7 (Perkin Elmer) at a heating rate of 5 °C/min over the temperature range from 30 to 500 °C. Data was computed by the Pyris software. 2.4. Linear expansion determination of foams The percentage linear expansion was obtained on foaming the SS/PVA blends in a microwave. The unfoamed blends were ruled with a line across using a fine oil pen. Each line was measured before and after forming. The percentage linear expansion was calculated as follows: Percentage linear expansion ¼ ððLength after foaming Length before foamingÞ 100Þ =ðLength before foamingÞ 2.5. Scanning electron microscopy Blends were cryogenically frozen and manually fractured in liquid nitrogen for 2–3 min. They were then cut, mounted on aluminium stubs and viewed
246
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
using an environment scanning electron microscope model Philips XL30/TMP.
3. Results and discussion 3.1. Gel content A major practical use of high-energy radiation to modify materials has been in the crosslinking of polymers. Generally, the extent of radiationinduced crosslinking of polymers can be estimated from gel content determination [10,11]. The gel content analysis of irradiated polymers allows to estimate important radiation parameters as yield of crosslinking and degradation, gelation dose, etc., and to correlate these with some physico-chemical properties [12]. Thus, in order to elucidate radiation-induced crosslinking, the gel content was determined and the results were plotted in Fig. 1. Apparently, it was observed that the gel content was increased when the irradiation dose was increased. This phenomenon showed that PVA was crosslinkable polymer as reported by other researchers [12,13]. The dramatically increased was observed at 10–20 kGy. Thereafter, it reached the plateau. This indicated that the crosslinking was saturated. It can be seen that when the PVA content in the blend was in-
creased from 10% to 20% (blends 10% SS:30% PVA, 15% SS:25% PVA and 20% SS:20% PVA), the saturated point was at 15 kGy while it was at 20 kGy when the PVA content in the blend was 25–30% (blends 25% SS:15% PVA and 30% SS:10% PVA). The difference might be due to higher PVA content in the blend. Therefore, higher dose was required. The gel content of blends containing 25–30% PVA were found to be lower than that of blends containing 10–20% PVA. This was probably due to efficiency of PVA to form the crosslinking. Suggesting that SS enhanced the crosslinking ability of blends. Surface grafting of SS onto PVA may be have taken place. The free radical in the starch was suspected to be occurred more in the amylose part since in the crystalline regions (amylopectin), molecular chains are more tightly packed and less mobile than the amorphous regions, which can be considered as semi-solid. Crosslinks (which require a considerable local rearrangement of molecular chains) may therefore be expected to occur preferentially in amorphous region [12]. Optimum crosslinking is important for the foam production. Crosslinking not only stabilizes bubbles during expansion but also enhances the resistance of the cellular products to thermal collapse. This enhanced resistance to collapse is necessary for the applications [8].
120 10% PVA
Gel content (%)
100
20% PVA 30% PVA 30% SS : 10% PVA
80 60
25% SS : 15% PVA 20% SS : 20% PVA
40
15% SS : 25% PVA 10% SS : 30% PVA
20 0 0
10
15
20
25
30
Dose (kGy)
Fig. 1. Effect of irradiation doses on the gel content of SS–PVA blends.
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
247
2
Gel strength (g/cm )
3500 3000
30% SS : 10% PVA
2500
25% SS : 15% PVA 20% SS : 20% PVA
2000
15% SS : 25% PVA 10% SS : 30% PVA
1500
10% PVA 1000
20% PVA 30% PVA
500 0
10
15
20 Dose (kGy)
25
30
Fig. 2. Effect of irradiation doses on the gel strength of SS–PVA blends.
3.2. Texture of blends
3.3. Thermal stability of blends
Fig. 2 shows the variation in gel strength of SS/ PVA blends with respect to the irradiation dose. It is apparent that blends showed a gradual increment in gel strength until dose of 25 kGy. A gradual increment in gel strength with dose was also observed for irradiated aqueous PVA, indicating that SS/PVA blends were rather harder than irradiated PVA. The surface of irradiated blends was also observed to be less sticky. This implied that the sago enhanced the gel strength of the blends and might be due to the effect of crosslinking. The overall effect of crosslinking is that the molecular weight of the polymer steadily increases with dose, leading to branched chains, until ultimately a tri-dimensional network is formed when on the average each polymer chain is linked to another chain [13]. Polymer crosslinking may be defined as any process whereby a weak, plastic material is transformed into a stronger, extensible, more rubber like material [14]. PVA is categorised as the crosslinking polymer, therefore, the gel content was found to increase when the irradiation dose was increased [12]. The relationship of gel strength and gel content of blends seemed to be directly proportional up to 25 kGy for all blends. Thereafter, the gel strength in blends dropped. This indicated that at higher irradiation dose rate, chain scission is predominant over crosslinking and hence the polymer degrades and lost its strength.
The thermogravimetric thermograms of SS, SS/ PVA blends and PVA are shown in Fig. 3(a). The PVA clearly showed its superior thermal stability over the SS/PVA gel sample and SS. It can be seen that the decomposition of the gel sample was shifted to higher temperature as compared to SS alone. The gel sample showed four stages decomposition as can be seen clearly in Fig. 3(b). Its decomposition was initiated at different temperatures. At 50% weight loss, gel sample degraded at higher temperature. Thus, it degraded over a longer time than SS, which means that the thermal stability is improved by irradiation. This was probably due to the higher crosslinking that occurred in the blends irradiated at higher dose, thus resulting in the higher thermal stability of blends. This lead to the conclusion that the thermal stability was improved by the irradiation. 3.4. Expansion ratio of foams Fig. 4 depicts the expansion ratio of the SS/ PVA foams. The maximum expansion ratio for all blends was obtained when they were irradiated at 15 kGy. This value coincided with when the crosslinking reached the saturated point. Crosslinking will also enhance the resistance of the cellular product to thermal collapse [8]. When the irradiation dose was further increased to 30 kGy, the linear expansion of the foams tended to be
248
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
Fig. 3. (a) Thermogravimetric thermograms of PVA, SS and gel fraction of 20% SS:20% PVA irradiated at 30 kGy. (b) Thermogravimetric thermograms the gel fraction of 20% SS:20% PVA irradiated at 30 kGy.
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
249
120
Linear expansion (%)
100 10% SS : 30% PVA 80
15% SS : 25% PVA 20% SS : 20% PVA
60
25% SS : 15% PVA 30% SS : 10% PVA
40 20 0 0
10
15
20
25
30
Dose (kGy) Fig. 4. Effect of irradiation doses on the linear expansion of SS–PVA blends.
decreased. This was due to the fact that more crosslinking occurred with increment in the irradiation dose resulting in less expansion of the foam. The crosslinked polymer inhibited the mobility of the chain. Furthermore, it might be due to the degradation of the sago granule and the scission of the PVA chains in the system. The expansion of foam seemed to be very much dependent on the level of crosslinking density in the blends. The ability of the foams to expand was restricted by an increase in crosslinking density. The gel strength of the blends increases as the crosslinking density increased. As mentioned earlier, the more developed the network linking the molecular chains the more limited the mobility of the molecular chains thus producing a restraining effect of foam development. With more dense networks the less is the ability of the gases to expand and form the foam cells. Hence, the expansion value reduced. At 15 kGy, the comparison between the blends was made. It was found that the maximum expansion was blends containing 30% SS followed by 25%, 20%, 15% and 10%, respectively. This showed that the increment of the SS in the blends enhanced the foam expansion.
5(A) and (B). The starch granules scattered among the PVA. It is interesting to note that the irradiation change the phase morphology of the blends. The scanning electron micrographs revealed that at 10 kGy irradiation, the blend showed that granules were found embedded in a matrix whereas at 20 and 30 kGy, the granules were on surface. The fracture surface of SS/PVA irradiated at 30 kGy appeared rougher and more brittle than that irradiated at 10 and 20 kGy. Fig. 5(C) showed more crack appear on the surface. The development of these fracture patterns was attributed to the presence of excessive crosslinking. The surface of the irradiated sample at 10 and 20 kGy showed more regular surface. There was no crack observed, implying a lower degree of crosslinking to resist the crack propagation. This was reflected by the lower strength of the blend. This was due to the fact that there was more crosslinking formed when the blends were exposed to higher dose as compared to that exposed to lower dose [15,16]. Therefore the SEM studies further confirmed the occurrence of irradiation-induced crosslinking in the blend [17].
3.5. Scanning electron microscope studies
4. Conclusions
Microscopy can be used as a tool to obtain information of domain size and size distribution in the blends. Blends morphology is shown in Fig.
The gel content determination confirmed that the crosslinking occurred in the SS/PVA blends. An irradiation dose of 15 kGy was found to be
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B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
was affected by the irradiation dose as the results of higher gel content were obtained. This phenomenon also lead to the higher thermal stability of the blends. The change of blend morphology was observed when they were exposed to higher irradiation dose.
Acknowledgements The authors are grateful to the IRPA for the financial support. A special appreciation goes to Ms. Chantara Thevy Ratnam and Ms. Sarada Binti Idris for their assistance during the experiment.
References
Fig. 5. Scanning electron micrograph (100 magnification) of blends prepared using 20% SS:20% PVA irradiated at (A) 10 kGy, (B) 20 kGy and (C) 30 kGy.
suitable in the production of biodegradable foam from SS/PVA blends since results revealed that the maximum linear expansion was obtained. The increment of the SS in the blends enhanced the linear expansion of foams. The gel strength of the blends
[1] C. Tilman, R. Sandhu, Resour. Conserv. Recycl. 24 (1998) 183. [2] C. Defeuilley, S. Lupton, Resour. Conserv. Recycl. 24 (1998) 217. [3] K.C. Frisch, D. Klempner, in: D. Klempner, K.C. Frisch (Eds.), Handbook of Polymeric Foams and Foam Technology, Hanser publishers, New York, USA, 1991. [4] A. Tsiapouris, L. Like, Starch/ St€arke 52 (2000) 53. [5] J.-P. Onteniente, B. Abbes, L.H. Safa, Starch/St€arke 52 (2000) 112. [6] S.-T. Lim, J.-L. Jane, Starch/St€arke 48 (1996) 444. [7] D.R. Patil, M.N. Crookston, G.F. Fanta, Starch/St€arke 46 (1994) 142. [8] Z. Ghazali, A.F. Johnson, K.Z. Dahlan, Radiat. Phys. Chem. 55 (1999) 73. [9] V. Shah, Handbook of Plastics Testing Technology, 2nd ed., Wiley–Interscience Publication, USA, 1998, p. 171. [10] C.T. Ratnam, K. Zaman, Angew. Makromol. Chem. 269 (1999) 42. [11] J.M. Rosiak, Radiat. Phys. Chem. 51 (1) (1998) 13. [12] A. Chalesby, in: Atomic Radiation and Polymers, Pergamon Press Ltd., Great Britain, 1960, p. 426. [13] A. Chapiro, in: Radiation Chemistry of Polymeric Systems, John Wiley & Sons Inc., Great Britain, 1962, p. 339. [14] C.P. Park, in: D. Klempner, K.C. Frisch (Eds.), Handbook of Polymeric Foams and Foam Technology, Hanser Publishers, Germany, 1991, p. 187. [15] C.T. Ratnam, M. Nasir, A. Baharin, K. Zaman, Polym. Int. 50 (2001) 503. [16] C.T. Ratnam, F. Yoshii, K. Makuuchi, K. Zaman, J. Appl. Polym. Sci. 72 (1999) 1421. [17] C.T. Ratnam, M. Nasir, A. Baharin, K. Zaman, J. Appl. Polym. Sci. 81 (2001) 1926.
The effect of electron beam irradiation on preparation of sago starch/polyvinyl alcohol foams Benchamaporn Wongsuban a,*, Kharidah Muhammad a, Zulkafli Ghazali b, Kamaruddin Hashim b, Muhammad Ali Hassan c a
c
Department of Food Science, Faculty of Food Science and Biotechnology, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia b Malaysian Institute for Nuclear Technology Research, MINT, Bangi, 43000 Kajang, Malaysia Department of Biotechnology, Univerisiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Received 21 February 2003
Abstract Blends of sago starch (SS)/polyvinyl alcohol (PVA) were irradiated with doses ranging from 10 to 30 kGy. Foams were then produced from these irradiated blends using a microwave. Changes in the degree of crosslinking, gel strength, thermal stability morphology of blends and linear expansion of foam with increasing irradiation doses were subsequently investigated. It was observed that the degree of crosslinking was important in maximizing the positive effect on foams produced. The gel strength of SS/PVA blends was affected by the irradiation. The crosslinking by the irradiation enhanced the thermal stability of SS/PVA blends. The results also revealed that the highest linear expansion of foams could be produced by irradiation blends at 15 kGy. Changes in blend morphology were observed upon irradiation. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Gel content; Crosslinking; Thermal stability; Biodegradable; Morphology
1. Introduction Owing to the rapid population and economic growth, many countries are faced with environmental problems such as that related to garbage [1,2]. One of the items that is contributing further to this problem would be the foams used in packaging. Several serious problems are being faced by the foam industries. These include flam-
*
Corresponding author. Tel.: +76-03-8948-6389; fax: +7603-8942-3552. E-mail address: [email protected] (B. Wongsuban).
mability (combustibility), waste disposal (biodegradability) or recycling wherever possible or economically feasible and the problem of ozone depletion by fluorocarbon (CFC) blowing agents [3]. The development of biodegradable foam would thus be a step forward in the right direction for the aforementioned industry. Utilization of annually renewable agriculturally derived products such as starch as extenders and replacements for synthetic, petroleum-based polymer is currently an active area of research [4,5]. Starch and its derivatives are currently used as plastic materials for disposable items such as packaging loose-fills and picnic tableware [6]. Use of polysaccharides in
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01250-3
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
plastics not only reduces our dependence on petrochemical-derived monomers, but the polysaccharide portion will also biodegrade, causing the finished plastic article to lose its integrity and be reduced to the small particles [7]. Electron beam irradiation of polymer has been employed in the production of foam. High-energy irradiation typically produces free radicals can readily interact with each other forming crosslinking and strengthening the polymer structure. Crosslinking will also enhance the resistance of the cellular product to thermal collapse [8]. Therefore, the objective of this study was to determine the effect of irradiation doses and sago starch (SS)/polyvinyl alcohol (PVA) blends on the foam produced.
The gel content was calculated using the equation below: %Gel ¼ ðweight of dry gel after extraction =weight of initial polymerÞ 100: 2.2. Gel strength determination of SS/PVA blends The gel strength of the blend was determined based on the force required to penetrate it and measured using a Texture Analyser (TAXT2). The testing conditions were as follows: Pre-test speed of 10 mm/s, test speed of 1 mm/s, post-test speed of 10 mm/s, penetration distance 2 mm using a cylindrical probe (15 mm diameter). 2.3. Thermogravimatric analysis of SS/PVA blends
2. Materials and methods SS was donated by Nitsei Sago Industries Sdn. Bhd, Mukah, Sarawak and PVA was purchased from Kuraray Poval Co. Ltd., Japan. 10/30/100, 15/25/100, 20/20/100, 25/15/00 and 30/10/100 of SS/PVA/water mixtures were prepared by solubilised PVA in distilled water at 121 °C for 10 min. They were then left to cool. SS was then mixed with aqueous PVA before the irradiation. Thirty milliliter of blend was poured into a square petri dish (10 cm 10 cm). They were then irradiated at 10, 15, 20, 25 and 30 kGy. The conditions of the electron beam machine were as follows: Accelerate voltage Beam current Speed
245
2.0 MeV 10.0 mA 0.94 m/min
The irradiated blends were then foamed by microwave heating for 5–8 min. In this study, the distilled water acted as the blowing agent. 2.1. Gel content determination The sample obtained after moisture content of the irradiated SS/PVA blends (in the form of gels) was determined. They were then weighed, placed in between steel nets and then autoclaved for 1 h. the autoclaved gel were later washed with distilled water and dried in an oven at 60 °C overnight.
Thermogravimetric analysis (TGA) is a test procedure in which change in the weight of specimen is monitored as the specimen is progressively heated [9]. TGA was performed on a thermogravimetric analyser TGA7 (Perkin Elmer) at a heating rate of 5 °C/min over the temperature range from 30 to 500 °C. Data was computed by the Pyris software. 2.4. Linear expansion determination of foams The percentage linear expansion was obtained on foaming the SS/PVA blends in a microwave. The unfoamed blends were ruled with a line across using a fine oil pen. Each line was measured before and after forming. The percentage linear expansion was calculated as follows: Percentage linear expansion ¼ ððLength after foaming Length before foamingÞ 100Þ =ðLength before foamingÞ 2.5. Scanning electron microscopy Blends were cryogenically frozen and manually fractured in liquid nitrogen for 2–3 min. They were then cut, mounted on aluminium stubs and viewed
246
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
using an environment scanning electron microscope model Philips XL30/TMP.
3. Results and discussion 3.1. Gel content A major practical use of high-energy radiation to modify materials has been in the crosslinking of polymers. Generally, the extent of radiationinduced crosslinking of polymers can be estimated from gel content determination [10,11]. The gel content analysis of irradiated polymers allows to estimate important radiation parameters as yield of crosslinking and degradation, gelation dose, etc., and to correlate these with some physico-chemical properties [12]. Thus, in order to elucidate radiation-induced crosslinking, the gel content was determined and the results were plotted in Fig. 1. Apparently, it was observed that the gel content was increased when the irradiation dose was increased. This phenomenon showed that PVA was crosslinkable polymer as reported by other researchers [12,13]. The dramatically increased was observed at 10–20 kGy. Thereafter, it reached the plateau. This indicated that the crosslinking was saturated. It can be seen that when the PVA content in the blend was in-
creased from 10% to 20% (blends 10% SS:30% PVA, 15% SS:25% PVA and 20% SS:20% PVA), the saturated point was at 15 kGy while it was at 20 kGy when the PVA content in the blend was 25–30% (blends 25% SS:15% PVA and 30% SS:10% PVA). The difference might be due to higher PVA content in the blend. Therefore, higher dose was required. The gel content of blends containing 25–30% PVA were found to be lower than that of blends containing 10–20% PVA. This was probably due to efficiency of PVA to form the crosslinking. Suggesting that SS enhanced the crosslinking ability of blends. Surface grafting of SS onto PVA may be have taken place. The free radical in the starch was suspected to be occurred more in the amylose part since in the crystalline regions (amylopectin), molecular chains are more tightly packed and less mobile than the amorphous regions, which can be considered as semi-solid. Crosslinks (which require a considerable local rearrangement of molecular chains) may therefore be expected to occur preferentially in amorphous region [12]. Optimum crosslinking is important for the foam production. Crosslinking not only stabilizes bubbles during expansion but also enhances the resistance of the cellular products to thermal collapse. This enhanced resistance to collapse is necessary for the applications [8].
120 10% PVA
Gel content (%)
100
20% PVA 30% PVA 30% SS : 10% PVA
80 60
25% SS : 15% PVA 20% SS : 20% PVA
40
15% SS : 25% PVA 10% SS : 30% PVA
20 0 0
10
15
20
25
30
Dose (kGy)
Fig. 1. Effect of irradiation doses on the gel content of SS–PVA blends.
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
247
2
Gel strength (g/cm )
3500 3000
30% SS : 10% PVA
2500
25% SS : 15% PVA 20% SS : 20% PVA
2000
15% SS : 25% PVA 10% SS : 30% PVA
1500
10% PVA 1000
20% PVA 30% PVA
500 0
10
15
20 Dose (kGy)
25
30
Fig. 2. Effect of irradiation doses on the gel strength of SS–PVA blends.
3.2. Texture of blends
3.3. Thermal stability of blends
Fig. 2 shows the variation in gel strength of SS/ PVA blends with respect to the irradiation dose. It is apparent that blends showed a gradual increment in gel strength until dose of 25 kGy. A gradual increment in gel strength with dose was also observed for irradiated aqueous PVA, indicating that SS/PVA blends were rather harder than irradiated PVA. The surface of irradiated blends was also observed to be less sticky. This implied that the sago enhanced the gel strength of the blends and might be due to the effect of crosslinking. The overall effect of crosslinking is that the molecular weight of the polymer steadily increases with dose, leading to branched chains, until ultimately a tri-dimensional network is formed when on the average each polymer chain is linked to another chain [13]. Polymer crosslinking may be defined as any process whereby a weak, plastic material is transformed into a stronger, extensible, more rubber like material [14]. PVA is categorised as the crosslinking polymer, therefore, the gel content was found to increase when the irradiation dose was increased [12]. The relationship of gel strength and gel content of blends seemed to be directly proportional up to 25 kGy for all blends. Thereafter, the gel strength in blends dropped. This indicated that at higher irradiation dose rate, chain scission is predominant over crosslinking and hence the polymer degrades and lost its strength.
The thermogravimetric thermograms of SS, SS/ PVA blends and PVA are shown in Fig. 3(a). The PVA clearly showed its superior thermal stability over the SS/PVA gel sample and SS. It can be seen that the decomposition of the gel sample was shifted to higher temperature as compared to SS alone. The gel sample showed four stages decomposition as can be seen clearly in Fig. 3(b). Its decomposition was initiated at different temperatures. At 50% weight loss, gel sample degraded at higher temperature. Thus, it degraded over a longer time than SS, which means that the thermal stability is improved by irradiation. This was probably due to the higher crosslinking that occurred in the blends irradiated at higher dose, thus resulting in the higher thermal stability of blends. This lead to the conclusion that the thermal stability was improved by the irradiation. 3.4. Expansion ratio of foams Fig. 4 depicts the expansion ratio of the SS/ PVA foams. The maximum expansion ratio for all blends was obtained when they were irradiated at 15 kGy. This value coincided with when the crosslinking reached the saturated point. Crosslinking will also enhance the resistance of the cellular product to thermal collapse [8]. When the irradiation dose was further increased to 30 kGy, the linear expansion of the foams tended to be
248
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
Fig. 3. (a) Thermogravimetric thermograms of PVA, SS and gel fraction of 20% SS:20% PVA irradiated at 30 kGy. (b) Thermogravimetric thermograms the gel fraction of 20% SS:20% PVA irradiated at 30 kGy.
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
249
120
Linear expansion (%)
100 10% SS : 30% PVA 80
15% SS : 25% PVA 20% SS : 20% PVA
60
25% SS : 15% PVA 30% SS : 10% PVA
40 20 0 0
10
15
20
25
30
Dose (kGy) Fig. 4. Effect of irradiation doses on the linear expansion of SS–PVA blends.
decreased. This was due to the fact that more crosslinking occurred with increment in the irradiation dose resulting in less expansion of the foam. The crosslinked polymer inhibited the mobility of the chain. Furthermore, it might be due to the degradation of the sago granule and the scission of the PVA chains in the system. The expansion of foam seemed to be very much dependent on the level of crosslinking density in the blends. The ability of the foams to expand was restricted by an increase in crosslinking density. The gel strength of the blends increases as the crosslinking density increased. As mentioned earlier, the more developed the network linking the molecular chains the more limited the mobility of the molecular chains thus producing a restraining effect of foam development. With more dense networks the less is the ability of the gases to expand and form the foam cells. Hence, the expansion value reduced. At 15 kGy, the comparison between the blends was made. It was found that the maximum expansion was blends containing 30% SS followed by 25%, 20%, 15% and 10%, respectively. This showed that the increment of the SS in the blends enhanced the foam expansion.
5(A) and (B). The starch granules scattered among the PVA. It is interesting to note that the irradiation change the phase morphology of the blends. The scanning electron micrographs revealed that at 10 kGy irradiation, the blend showed that granules were found embedded in a matrix whereas at 20 and 30 kGy, the granules were on surface. The fracture surface of SS/PVA irradiated at 30 kGy appeared rougher and more brittle than that irradiated at 10 and 20 kGy. Fig. 5(C) showed more crack appear on the surface. The development of these fracture patterns was attributed to the presence of excessive crosslinking. The surface of the irradiated sample at 10 and 20 kGy showed more regular surface. There was no crack observed, implying a lower degree of crosslinking to resist the crack propagation. This was reflected by the lower strength of the blend. This was due to the fact that there was more crosslinking formed when the blends were exposed to higher dose as compared to that exposed to lower dose [15,16]. Therefore the SEM studies further confirmed the occurrence of irradiation-induced crosslinking in the blend [17].
3.5. Scanning electron microscope studies
4. Conclusions
Microscopy can be used as a tool to obtain information of domain size and size distribution in the blends. Blends morphology is shown in Fig.
The gel content determination confirmed that the crosslinking occurred in the SS/PVA blends. An irradiation dose of 15 kGy was found to be
250
B. Wongsuban et al. / Nucl. Instr. and Meth. in Phys. Res. B 211 (2003) 244–250
was affected by the irradiation dose as the results of higher gel content were obtained. This phenomenon also lead to the higher thermal stability of the blends. The change of blend morphology was observed when they were exposed to higher irradiation dose.
Acknowledgements The authors are grateful to the IRPA for the financial support. A special appreciation goes to Ms. Chantara Thevy Ratnam and Ms. Sarada Binti Idris for their assistance during the experiment.
References
Fig. 5. Scanning electron micrograph (100 magnification) of blends prepared using 20% SS:20% PVA irradiated at (A) 10 kGy, (B) 20 kGy and (C) 30 kGy.
suitable in the production of biodegradable foam from SS/PVA blends since results revealed that the maximum linear expansion was obtained. The increment of the SS in the blends enhanced the linear expansion of foams. The gel strength of the blends
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