Materials. Cassava starch (Dragon fish brand), with an amylose/amylopectin content of 17/83% w/w, 11% moisture content, and a molecular weight of 1.34 × 108 g/mol, was purchased from Tong Chan Registered Ordinary Partnership (Bangkok, Thailand). Glycerol was purchased from Union Science Co., Ltd. (Chiang Mai, Thailand). PEMAH (Fusabond, DuPont, Bangkok, Thailand) with a melt flow index of 1.75 g/10 min and 2% MAH content was obtained from Chemical Innovation Co., Ltd. (Bangkok, Thailand). Cocoon was kindly supplied by Chul Thai Silk Co., Ltd. (Phetchabun, Thailand).
Sample preparation. Sericin extraction was performed using a high temperature-high pressure method.[22,56] Briefly, 45 g of the cocoon pieces (2 × 2 cm) were immersed in distilled water (1.5 L) in a 2.5 L laboratory bottle. Sericin was extracted by autoclaving at 121°C for 30 min. The aqueous solution was filtered through filter paper and stored at 4°C until further use. Cassava starch was mixed with glycerol (70:30% w/w) in a water bath at 80°C for 30 min. During premixing, sericin at 1–10 phr (part/hundred) of TPS was incorporated into the solution. The solutions were then dried at 50°C for 24 h. The dried samples were melt-mixed using an internal mixer (Labo Plastomill; Toyo Seiki Co. Ltd., Tokyo, Japan) to prepare the TPSS. The melt-blending of TPSS with PEMAH was also performed using the same mixer at 160°C for 10 min. The samples were compressed into sheets at 160°C for 3 min. The formulations of the blended samples are listed in Table 1.
Table 1
Composition and codes of thermoplastic starch (TPS) and thermoplastic starch with sericin blending with polyethylene grafted maleic anhydride (PEMAH).
Samples | Composition |
TPS (w%) | PEMAH (w%) | Sericin (phr/TPS) |
TPS | 80 | - | - |
TPS/PEMAH | 80 | 20 | - |
TPSS1/PEMAH | 80 | 20 | 1 |
TPSS2.5/PEMAH | 80 | 20 | 2.5 |
TPSS5/PEMAH | 80 | 20 | 5 |
TPSS10/PEMAH | 80 | 20 | 10 |
Tensile properties. A tensile tester (Tensilion UTM-II-20; Orientec Co. Ltd., Tokyo, Japan) was used to investigate the tensile properties of the samples at a speed of 10 mm/min. The samples were compressed into sheets by compression molding at 160°C for 3 min, followed by conditioning at 50% RH for 48 h at 25°C. The width, length, and thickness were 2 mm × 30 mm ×1 mm. The maximum tensile strength (MPa) and elongation at break were observed for the five specimens for each composition.
Scanning electron microscopy. The morphologies of the samples were observed using a scanning electron microscope (SM-200, Topcon Corp., Tokyo, Japan). The samples were immersed and broken into liquid nitrogen. The fracture surfaces of the samples were coated with a thin layer of gold by sputtering and observed at a voltage of 15 kV.
Differential scanning calorimetry. Differential scanning calorimetry (DSC) (PerkinElmer Pyris Diamond DSC, Connecticut, USA) was used to observe the melting temperatures of the samples. The sample (8–10 mg) was covered with aluminum pans and placed in the DSC chamber with a pan reference. The samples were observed during the first and second scans at a heating rate of 10°C/min. The melting temperature (Tm) was determined from the temperature at the maximum level of the endothermic peak.
Rheology. A rheometer (dynamic analyzer RDA II; Rheometric Scientific Corp., Delaware, USA) was used to determine the complex dynamic viscosity (η*) under a nitrogen atmosphere. Samples with 25 mm diameter and 1 mm thickness were compressed at 160°C for 3 min. The experiments were performed at 160°C with a frequency range of 0.1–1000 rad/s at a 10% strain rate.
Reaction mechanism. Fourier-transform infrared spectroscopy (FTIR-480plus; Jasco Corp., Tokyo, Japan) was used to investigate the reaction between TPS, sericin, and PEMAH. The samples were prepared as thin films by compression molding at 160°C for 3 min. The IR spectrum was measured at 600–4000 cm− 1 with a resolution of 4 cm− 1.
Statistical analysis. All results were analyzed by one-way analysis of variance (ANOVA) using SPSS software. Differences found (P < 0.05) were evaluated using Duncan’s test. Five replicates for each sample were used for the evaluation.