This research focuses on synthesis and characterization of sago starch-mixed LDPE biodegradable polymer. Firstly, the effect of variation of starch content on mechanical property (elongation at break and Young’s modulus) and biodegradability of the polymer was studied. The LDPE was combined with 10%, 30%, 50%, and 70% of sago for this study. Then how the cross-linking with trimethylolpropane triacrylate (TMPTA) and electron beam (EB) irradiation influence the mechanical and thermal properties of the polymer was investigated. In the 2nd study, to avoid overwhelming of data LDPE polymer was incorporated with only 50% of starch. The starch content had direct influence on mechanical property and biodegradability of the polymer. The elongation at break decreased with increase of starch content, while Young’s modulus and mass loss (i.e., degradation) were found to increase with increase of starch content. Increase of cross-linker (TMPTA) and EB doses also resulted in increased Young’s modulus of the polymer. However, both cross-linking and EB irradiation processes rendered lowering of polymer’s melting temperature. In conclusion, starch content and modification processes play significant roles in controlling mechanical, thermal, and degradation properties of the starch-mixed LDPE synthetic polymer, thus providing the opportunity to modulate the polymer properties for tailored applications.
The development of innovative biopolymers has been underway for a number of years and continues to be an area of interest for the modern scientists. In 1996, shipments from the Canadian Plastic Industry increased by 10.6% from 1995 levels to $9.1 billion [
Starch is a hydrocolloid biopolymer that can be found in a variety of agricultural feedstocks such as wheat, corn, rice, beans, and potatoes [
In the 1st phase of the experiment, five types of LDPE polymer samples were produced in combination with various percentages of starch. Low-density polyethylene (LDPE) and sago starch (Igan Sago Industries Sdn. Bhd.) were weighed by a toploader balance (AND, model GF-3000) with various percentages of LDPE and starch as reported in Table
Relative percentage of LDPE and starch of polymer samples.
Sample ID | % LDPE | % Starch |
---|---|---|
LDPE00-Starch0 | 100 | 0 |
LDPE90-Starch10 | 90 | 10 |
LDPE70-Starch30 | 70 | 30 |
LDPE50-Starch50 | 50 | 50 |
LDPE30-Starch70 | 30 | 70 |
In the 2nd phase, the effects of crosslinking with trimethylolpropane triacrylate (TMPTA) and electron beam (EB) irradiation on the mechanical and thermal properties of the polymer samples were investigated. To avoid overwhelming of data, the polymer samples used in this second phase experiments consisted of 50% LDPE and 50% starch. Various percentages of TMPTA were added to the LDPE-starch blends during the blending process and then irradiated accordingly with various doses of EB as presented in Table
Polymer composition with TMPTA cross-linker and EB irradiation doses.
Sample ID | % LDPE | % Starch | % TMPTA | EB irradiation (kGy) |
---|---|---|---|---|
LDPE50-STARCH50 | 50 | 50 | 0 | 0 |
TMPTA1-DOSE0 | 50 | 50 | 1 | 0 |
TMPTA3-DOSE0 | 50 | 50 | 3 | 0 |
TMPTA5-DOSE0 | 50 | 50 | 5 | 0 |
TMPTA1-DOSE10 | 50 | 50 | 1 | 10 |
TMPTA3-DOSE10 | 50 | 50 | 3 | 10 |
TMPTA5-DOSE10 | 50 | 50 | 5 | 10 |
TMPTA1-DOSE30 | 50 | 50 | 1 | 30 |
TMPTA3-DOSE30 | 50 | 50 | 3 | 30 |
TMPTA5-DOSE30 | 50 | 50 | 5 | 30 |
TMPTA1-DOSE50 | 50 | 50 | 1 | 50 |
TMPTA3-DOSE50 | 50 | 50 | 3 | 50 |
TMPTA5-DOSE50 | 50 | 50 | 5 | 50 |
TMPTA1-DOSE70 | 50 | 50 | 1 | 70 |
TMPTA3-DOSE70 | 50 | 50 | 3 | 70 |
TMPTA5-DOSE70 | 50 | 50 | 5 | 70 |
TMPTA1-DOSE100 | 50 | 50 | 1 | 100 |
TMPTA3-DOSE100 | 50 | 50 | 3 | 100 |
TMPTA5-DOSE100 | 50 | 50 | 5 | 100 |
Upon completion of synthesis process, the polymer samples were cut into dumb-bell-shaped by a dumb-bell cutter (model SDL-100; Dumb-Bell Co., Ltd.) according to ASTMD 1882L standard. The dumb-bell-shaped samples had smooth surface especially, in the neck section that avoided stress concentration during mechanical test.
Tensile test was carried out to investigate the influence of starch content and further the effect of modification processes, namely, crosslinking with TMPTA and EB irradiation on the polymer’s mechanical property. The dumb-bell-shaped sample thicknesses were measured by a thickness gauge (Mitutoyo, model EMD-57B-11M) and were keyed in the test system. Tensile test was performed by using a uniaxial testing system (Instron 3365) and a 5 kN load cell (Canton, MA, USA) according to ASTMD1882L standard. The cross-head speed was set at 10 mm/min. For each type of polymer, five samples were tested. Young’s modulus was calculated as stress divided by strain to evaluate the mechanical property of the polymer samples. A Student’s
The in vitro degradation study was performed on the dumb-bell-shaped LDPE polymer samples containing various percentages of starch (10%, 30%, 50%, and 70%) by burying the polymer samples at the exterior under the soil at a depth of 2 feet for a period of one month. Each type of polymer had five samples. This study was to primarily investigate the variation of degradation kinetics due to the variation of starch content. The degradation phenomenon was evaluated through mass loss and change in surface morphology. Prior to burial, the polymer samples were characterized by weighing and recording their initial mass using an electronic balance with a resolution of 0.1 mg. After one month of burial, the samples were dug out and cleaned to ensure complete removal of soil/mud. Samples were then placed in an area with sufficient ventilation for natural drying. The dried degraded samples were weighed using the same electronic balance as carried out before starting degradation. Subsequently, the percentage of mass loss of respective sample was measured as follows:
The thermal characteristic of cross-linked and EB irradiated polymer sample was determined by differential scanning calorimeter (TA Instruments DSC 2910, New Castle, DE, USA). An indium standard was used to calibrate the instrument. The sample weight of 5 mg was taken for scanning. All samples were placed in aluminum pans and scanned from −70°C to 200°C at a rate of 10°C/min, using argon as purge gas. Five samples for each type of polymer were analyzed. The DSC analysis provided the melting point of the polymer.
The mechanical experiment (tensile test) initially investigated the influence of starch content and further the effect of modification processes, namely, cross-linking with TMPTA and EB irradiation on the polymer’s mechanical property. The starch content and modification process (crosslinking and EB irradiation) had direct influences on the mechanical characteristics of the starch-incorporated LPDE polymer. The elongation at break of the polymer samples decreased with the increase of starch content, while Young’s modulus increased with the increase of starch content as demonstrated in Figures
Variation of elongation with starch content of LDPE/starch blends without modification.
Variation of Young’s modulus with starch content of LDPE/starch blends without modification.
Unlike the elongation, Young’s modulus of the synthesized polymer increased with the increase of starch content (Figure
It was observed that the increase of TMPTA cross-linker resulted in decrease of elongation and consequently, increased Young’s modulus as shown in Figures
Variation of elongation with TMPTA cross-linker and electron beam irradiation.
Variation of Young’s modulus with TMPTA cross-linker and electron beam irradiation.
The effects of TMPTA cross-linker and EB irradiation on the thermal property (melting temperature) of the polymer were determined via DSC analysis. The overall influence of TMPTA cross-linker and EB irradiation on the polymer’s melting temperature is presented in Figure
Influence of TMPTA cross-linker and electron beam irradiation on the polymer’s melting temperature.
Technically, all polymers regardless of their chemical structure or origin degrade under appropriate conditions. However, the term “nondegradable polymer” is meant to indicate the polymer that does not degrade during use or even after very long-term use (e.g., decades to centuries) rather than not to degrade at all [
The mass of LPDE polymer samples (before and after degradation) containing various percentages of starch.
Starch Content (%) | Mass (before degradation) (g) | Mass (after degradation) (g) | Mass loss (%) |
---|---|---|---|
0 | 0.464 | 0.464 | 0.00 |
10 | 0.452 | 0.452 | 0.05 |
30 | 0.492 | 0.496 | 3.81 |
50 | 0.53 | 0.528 | 5.37 |
70 | 0.632 | 0.562 | 11.07 |
Pure LDPE and low starch containing (e.g. 10%) LDPE showed no apparent mass loss. This result is in agreement with the fact that the pure LDPE is considered to be technically non-degradable, and there should not be any mass loss. Virtually the LDPE polymer sample containing 10% starch should demonstrate some mass loss (i.e., degradation). However, due to time constraints (e.g., too short burial duration) the degradation might not be observed appreciably. Besides, the presence of starch (only 10%) might be too low to render the polymer degrade. The LDPE polymer samples containing 30%, 50%, and 70% starch demonstrated mass loss of 3.81%, 5.37%, and 11.07%, respectively. These results reveal that higher starch content enhances the degradation kinetics and thus increases mass loss. This could be due to the hydrophilic nature of starch that has been argued in another study [
Physical appearances of LDPE polymer samples containing various percentages of starch after a one-month long burial that demonstrate different levels of degradation: (a) Pure LDPE, (b) LDPE 90 and Starch 10, (c) LDPE 70 & Starch 30, (d) LDPE 50 & Starch 50, and (e) LDPE 30 & Starch 70.
The LDPE polymer was successfully synthesized incorporating sago starch and utilizing some modification processes like, TMPTA cross-linker and EB irradiation. The overall process provides the freedom to develop a large variety of polymers with various mechanical, thermal, and degradation properties. The characterization results indicate that the variations of starch content, percentage of TMPTA crosslinker, and EB irradiation dose are very effective means to modulate the polymer’s mechanical, thermal, and degradation properties. The polymer’s strength (i.e., Young’s modulus) increased with the increase of starch content, percentage of TMPTA, and EB irradiation dose, while the ductility of the polymer decreased with the increase of the said parameters. The melting temperature decreased with the increase of TMPTA percentage and EB irradiation dose. The degradation of the polymer was enhanced with the increase of starch content. In conclusion, the starch-incorporated LDPE polymer properties could be modulated by manipulating the starch content and modification processes (e.g., TMPTA crosslinker and EB irradiation dose) for tailored applications.
The authors would like to clarify that there is no direct financial relation with the software and corporation mentioned in this paper that might lead to a conflict of interests for any of the authors.