Finite Element Analysis for Thermoforming Process of Starch/ Biodegradable Polyester Blend
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AIJSTPME (2012) 5(2): 33-37 Finite Element Analysis for Thermoforming Process of Starch/ Biodegradable Polyester Blend Thongwichean T. Agricultural Machinery Technology Department, Rajamangala University of Technology Srivijaya Rattaphum College, Thachamuang Rattaphum Songkla 90180. Thailand Phalakornkule C. Department of Chemical Engineering, King Mongkut’s University of Technology North Bangkok, 10800. Thailand Chaikittiratana A.* Department of Mechanical and Aerospace Engineering, King Mongkut’s University of Technology North Bangkok, 10800. Thailand. *Corresponding author, email: [email protected] Abstract The objective of this work is to study the behaviour during sheet thermoforming process of a Tapioca starch- biodegradable polyester (EnpolTM) blend with the mixing ratio of 50:50 by weight. The mechanical behavior of the material extruded in the form of thin sheet was studied by means of compression tests with varying strain rates at temperatures ranging from between 363 K to 393 K. The Elastic–Perfectly Plastic material model was used to capture the compressive deformation behavior of the material. It was found that the model described reasonably well the behavior of the material and the 2D Finite element simulation with Elastic–Plastic material model gave good representation of the real thermoforming process. Keywords: Bioplastics, Thermoforming, Finite Element Analysis 1 Introduction Bioplastics is derived from renewable resources and applications such as bone replacement, tissue could be degraded more easily than petroleum-based scaffolds and drug-delivery systems [2]. plastics. With the growing environmental concerns Starch-based bioplastics can be processed by several and the realization that petroleum resources are techniques traditionally undertaken with petroleum- limited, a wide range of biodegradable plastics have based plastics. However, processing of starch-based been developed in the last two decades in order to bioplastics could be very different from petroleum- reduce the amount of long-lived petroleum-based based plastics because of the highly-branched plastics wastes. Starch-based bioplastics is one class molecular structure and the hydrophilic nature of of bioplastics, which can be further divided into three starch. A number of computational modeling of categories [1]: (1) thermoplastic starch materials; thermoforming processes of petroleum-based modified starch with plasticizing additives; (2) plastics have been developed and proposed blends of starch and biodegradable plastics; (3) ([3],[4],[5],[6],[7],[8]), but only few for starch-based plastics whose monomers are biochemically derived bioplastics. Szegda et al. [1] employed finite element from starch. Among these materials, the blends of techniques to model thermoforming processes of starch and biodegradable plastics have been PlanticTM, commercial starch-based thermoplastics, developed into packaging materials and biomedical in order to predict its thermoformed structure. Even 33 Thongwichean T. et al. / AIJSTPME (2012) 5(2): 33-37 though the prediction did not perfectly match the 2.3 Differential Scanning Calorimeter test experimental data, the model could capture some The thermal analysis was performed using a Mettler features of the thickness variation. Further model Toledo Differential Scanning Calorimeter (DSC) modifications such as the considerations of non- model DSC 822 at the heating rate of 20oC/min. Data uniform initial thickness and the moisture and from DSC showed that the blend had one temperature effects were necessary to improve the endothermic melting peak between 383-403 K with model predictions. the highest point located at 391 K. The paper is among the first attempts to develop a simple computational modeling based on finite 2.4 Mechanical Testing element techniques to predict the behavior of a blend of starch/biodegradable plastic, EnpolTM. The In order to determine the mechanical deformation mechanical behavior of the material extruded to the behaviour of the biodegradable material, the uniaxial form of thin sheet was studied by means of compression tests were performed at various compression tests at temperatures from 363 toto 393 temperatures ranging from 363 K to 393 K with K and at strain rates of 0.1 and 0.5 s-1. Elastic-plastic strain rates of 0.1 and 0.5 s-1. The compression tests material models was used to capture the compressive were carried out according to ASTM D695 using an behavior of the material and 2-dimensional finite universal testing machine (Instron model 5567) element simulation of the sheet thermoforming equipped with a temperature control chamber. Two process was presented. thin square samples described earlier were stacked up on a compression platen as shown in Figure 2. The 2 Experimental Work dependence on temperature and strain rate of the deformation is shown in a compressive engineering 2.1 Materials stress vs. engineering strain plot given in Figure 3. The polymer in this study was tapioca starch- As temperature increases, the material behaves as a EnpolTM blend, provided by DES Co.Ltd.(Thailand). softer material and yield stress decreases. On the EnpolTM is a fully biodegradable aliphatic polyester other hand, the yield strain values at different resin developed by IRe Chemical Ltd. (Seoul, temperatures are not affected by temperature and Korea). The ratio of tapioca starch to EnpolTM was yielding occurred at approximately the engineering 50:50 by weight. strain of 0.07. As the strain rate increases the material becomes stiffer but only very slightly, thus it can be said that the deformation behaviour is almost 2.2 Samples preparation unaffected by strain rates. When true stress are The tapioca starch-EnpolTM blend was extruded into plotted against true strain at different temperatures thin strips with cross-sectional area of 2mm x 20mm, and a strain rate of 0.5 S-1 in Figure 4, it can be using a twin screw extruder (HAAKE Polylab, observed that the deformation undergoes without Rheomex CTW 100P). To eliminate moisture increase in stress after yielding and that it deforms in content, the material was dried in a hopper drier at elastic-perfectly plastic manner. From the tests 80oC for 4 hours, prior to the extrusion process. The conducted, the optimum processing condition was extruded material, shown in Figure 1, was then cut in achieved at 393K with strain rate of 0.5 S-1. At this to small thin square test pieces with dimension of condition, the compressed specimen was easily 12x12x2 mm for compression tests. deformed and had good surface appearance. For other conditions, cracks appeared on the specimen's surface. These test results also provided material's properties used in the subsequent finite element modelling. 2.5 Sheet thermoforming Sheet thermoforming was performed by a match Figure 1: Thin strips of extruded tapioca starch- mould compression method with die and punch EnpolTM blend. configuration shown in Figure. 5. Before subjected to compression, a strip of specimen was heated at 393 K 34 Thongwichean T. et al. / AIJSTPME (2012) 5(2): 33-37 for 20 minutes. The compression sheet thermoforming process was carried out at 393 K and the punch was set to travel 10mm in 4 seconds. An example of stamped specimen strip is shown in Figure 6. Compressing load Stack of small thin samples Compressing plattens Figure 5: Sheet thermoforming test arrangement 4 mm. Figure 6: Specimen after the sheet thermoforming process Figure 2: Compression test arrangement 3 Finite Element Simulation 3.1 Material Model 60 Since the deformation behavior was found to be 50 temperature dependence, strain rate insensitive with no strain hardening after yield, thus it was modeled 40 with a standard isotropic elastic-perfectly plastic material model inbuilt in a commercial finite element 30 -1 -1 90363 อK ง ศาเซ 0.5ลเซ sยี ส έ = 0.5 s package ABAQUS 6.5 [9]. The input material's -1 -1 363 K 0.1 s έ = 0.1 s 90 องศาเซลเซยี -1ส properties needed are Young's modulus, Poisson's 373 K 0.5 s -1 20 100 องศาเซลเซยี ส έ = 0.5 s -1 -1 10037 3 อKง ศาเซ ล0.เ1ซ sยี ส έ = 0.1 s Ratio and true yield stress. Young's modulus and true -1 -1 11038 3 อKง ศาเซ ล0.เ5ซ sยี ส έ = 0.5 s -1 -1 yield stress are temperature dependent and were 10 11038 3 อKง ศาเซ ล0.เ1ซ sยี ส έ = 0.1 s -1 393 K 0.5 s-1 έ = 0.5 s 120 องศาเซลเซยี ส determined from true stress-strain curves shown in -1 -1 Compression Engineering Stress (MPa) 12039 3 อKง ศาเซ ล0.เ1ซ sยี ส έ = 0.1 s 0 Figure 4. These values are tabulated in Table 1. 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 Poisson's ratio of the materials was assumed to be Compression Engineering Strain 0.4. A comparison between the experimental data Figure 3: Plot of compressive engineering stress vs. and finite element simulation using a single 3D engineering strain at different temperatures and strain element (C3D8H) with elastic-perfectly plastic rates material model is given in Figure 7 and a very good agreement of to the data can be observed. 55 50 35 45 30 40 25 35 30 20 363 K Data 25 ทดสอบที่ 90 องศาเซลเซียส 15 373 K Data -1 20 ทดสอบที่ 100 องศาเซลเซียส 9036 3อ งKศ าเซล 0.5เซีย sส ทด3ส8อ3บ Kท ี่ Data110 องศาเซลเซียส 10 100373 อKง ศาเซ 0.5ลเซ sีย-1ส 15 ทด3ส9อ3บ Kท ี่ Data120 องศาเซลเซียส 363 K Simulation -1 แบบจ าลองที่ 90 องศาเซลเซียส 383 K 0.5 s 10 5 110 องศาเซลเซียส แบ3บ7จ3 า Kล อ Simulationงที่ 100 อ งศาเซลเซียส 393 K 0.5 s-1 383 K Simulation Compression True Stress (MPa) Stress True Compression 5 120 องศาเซลเซียส แบบจ าลองที่ 110 องศาเซลเซียส 393 K Simulation 0 Compression Engineering Stress (MPa) แบบจ าลองที่ 120 องศาเซลเซียส 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Compression True Strain Compression Engineering Strain Figure 4: Plot of compressive true stress vs.