Water absorption and thermomechanical behavior of composites based on thermoplastic starch (TPS) are presented in this work, wherein the concentration of agave bagasse fibers (ABF, 0–15 wt%) and poly(lactic acid) (PLA, 0–30 wt%) is varied. Glycerol (G) is used as starch (S) plasticizer to form TPS. Starch stands as the polymer matrix (70/30 wt/wt, S/G). The results show that TPS hygroscopicity decreases as PLA and fiber content increase. Storage, stress-strain, and flexural moduli increase with PLA and/or agave bagasse fibers (ABF) content while impact resistance decreases. The TPS glass transition temperature increases with ABF content and decreases with PLA content. Micrographs of the studied biocomposites show a stratified brittle surface with a rigid fiber fracture.
Nowadays, the interest in bioplastics is growing for market niches such as packaging, agriculture, or automotive parts among others. They are classified in biodegradable and biobased/nonbiodegradable. In 2014, their total global production capacity reached 1.67 million tons, where 643,000 tons corresponded to biodegradable plastics [
Since starch (S) is an economical biopolymer that is contained in many natural products, it is attractive as a source to make biodegradable plastics [
Starch has to be plasticized to lower its high
Dufresne and Vignon in an early work on starch/fiber composites reported that thermomechanical properties of potato starch films were improved when they were mixed with cellulose nanofibers, showing also a decrease in moisture sensitivity, while maintaining biodegradability. Additionally, they found that increasing the glycerol content the equilibrium moisture increased and that such parameter decreased when the fiber content was augmented [
Huneault and Li studied mixtures of TPS with poly(lactic acid) (PLA) using maleic acid as a compatibilizer. They reported that Young modulus (
Although there are several works that follow the effect of PLA or natural fibers on moisture absorption and mechanical properties of TPS, there are very few reports on the effect of the simultaneous addition of both materials to the TPS. Furthermore, there is only scarce data on properties of TPS composites containing cellulosic fibers that were obtained as a byproduct of an industrial process. Teixeira et al. reported the use of cassava bagasse to obtain fiber reinforced TPS and PLA/TPS blends, but tensile strength did not increase significantly and the fiber essentially acted as a filler [
Using PLA [
In this work, the effects of the amount of PLA and/or agave bagasse fibers on moisture absorption and mechanical and thermal properties of TPS are reported. This is the first report on the application of agave bagasse fibers to reinforce TPS/PLA blends, which are discarded fibers from industrial processes.
The materials used in this work were corn starch (IMSA) with 10% humidity, glycerol QP (Golden Bell Products), agave bagasse (tequilana Weber blue var.) fiber, PLA (Ingeo Biopolymer 3521D Industries Leben), and Magnesium Nitrate (Fermont). To prepare TPS, after the starch was dried for 24 h at 60°C, it was manually mixed with glycerol (30 wt%) until a homogeneous mixture was obtained. TPS was mixed with different amounts of PLA and/or fiber (Table
Composite formulations.
S, g | G, g | TPS |
PLA, g | ABF, g | PLA wt% in polymer blend● | ABF wt% in composite▼ |
---|---|---|---|---|---|---|
70 | 30 | 100 | 0 | 0 | 0 | 0 |
63 | 27 | 90 | 10 | 0 | 10 | 0 |
56 | 24 | 80 | 20 | 0 | 20 | 0 |
49 | 21 | 70 | 30 | 0 | 30 | 0 |
70 | 30 | 100 | 0 | 11.11 | 0 | 10 |
63 | 27 | 90 | 10 | 11.11 | 10 | 10 |
56 | 24 | 80 | 20 | 11.11 | 20 | 10 |
49 | 21 | 70 | 30 | 11.11 | 30 | 10 |
70 | 30 | 100 | 0 | 17.65 | 0 | 15 |
63 | 27 | 90 | 10 | 17.65 | 10 | 15 |
56 | 24 | 80 | 20 | 17.65 | 20 | 15 |
49 | 21 | 70 | 30 | 17.65 | 30 | 15 |
First, composites were dried at 60°C for 24 h; then the material was weighed and placed at 25°C in a closed chamber maintained at a relative humidity of 53% (saturated solution of magnesium nitrate). The composites weight was recorded periodically until a constant weight was obtained.
Mechanodynamic tests were carried out following ASTM D5023-01, using a thermomechanical analyzer (TA Q800 DMA) and the following conditions: temperature range, −85°C to 150°C, heating rate, 2°C/min, three-point bending clamp, and frequency of 1 Hz. Mechanostatic tests were carried out at 25°C following ASTM D638-04 for stress-strain (Instron 4411, crosshead speed: 5 mm/min), ASTM D790-03 for flexure (Instron 4411), and ASTM D6110-04 (Instron, Ceast 9050) for Charpy impact testing.
Thermal behavior of the samples was followed by DSC (Q Series DSC Q100, TA Instruments), using ASTM D3418-03. Heating rate was 10°C/min from 20 to 180°C.
Samples were observed by Field Emission Scanning Electronic Microscopy (FE-SEM (Tescan, Mira3)). The samples were frozen in liquid nitrogen for 5 minutes before fracture. Subsequently, the samples were dried at 60°C before FE-SEM observation.
Figure
Moisture absorption of TPS/PLA/Fiber composites as a function of time for the polymer blend and composites containing 20 wt% PLA.
For moisture absorption, it can also be noticed that an increase in ABF content reduces the equilibrium moisture value (Figure
Equilibrium moisture of polymer blends and composites varying PLA content.
In Figure
Storage modulus as a function of temperature for the polymer blend and composites containing 20 wt% of PLA.
The storage modulus at 25°C of the studied composites is shown in Figure
Storage modulus of polymer blends and composites at 25°C, varying PLA content.
Figure
tan
Using tan
PLA content in polymer blend, wt% | ABF content in composites, wt% | ||
---|---|---|---|
0 | 10 | 15 | |
10 | 120°C | 124°C | 132°C |
20 | 115°C | 120°C | 130°C |
30 | 111°C | 114°C | 127°C |
The DSC thermogram (Figure
PLA content in polymer blend, wt% | ABF content in composites, wt% | ||
---|---|---|---|
0 | 10 | 15 | |
10 | 57.0°C | 57.1°C | 54.5°C |
20 | 57.0°C | 57.3°C | 55.5°C |
30 | 57.0°C | 57.0°C | 56.0°C |
Thermograms of the polymer blend and composites containing 20 wt% of PLA.
The exothermic peak that appears at approximately 100°C is related to PLA crystallization (
PLA content in polymer blend, wt% | ABF content in composites, wt% | ||
---|---|---|---|
0 | 10 | 15 | |
10 | 100.0°C | 100.7°C | 99.1°C |
20 | 99.9°C | 99.8°C | 101.0°C |
30 | 98.9°C | 100.4°C | 100.2°C |
For the polymeric materials of Figure
Figure
The endothermic peaks at the high temperature zone correspond to PLA melting temperature (
PLA content in polymer blend, wt% | ABF content in composites, wt% | ||
---|---|---|---|
0 | 10 | 15 | |
10 | 163.0°C | 164.5°C | 161.0°C |
20 | 165.9°C | 163.6°C | 163.4°C |
30 | 166.1°C | 165.9°C | 163.2°C |
Even though mechanodynamic tests allowed the thermomechanical characterization of extruded materials, mechanostatic tests of some of the composites are included to confirm their moduli values pattern, as well as to determine impact resistance behavior. In Figure
Young modulus of polymer blends and composites at 25°C, varying PLA content.
Figure
Flexural modulus of polymer blends and composites at 25°C, varying PLA content.
In Figure
Impact resistance of polymer blends and composites at 25°C varying PLA content.
In Figure
SEM images of polymeric materials: (a) TPS, (b) polymer blend containing 20 wt% PLA, and (c) 80 TPS/20 PLA w/w, with 15 wt% ABF composite.
Blends of TPS and PLA and composites of TPS/PLA/ABF prepared by extrusion followed by compression molding were characterized. The reinforcing effect of PLA and/or ABF in TPS led to an increase in moduli and a decrease in moisture absorption and impact resistance. The TPS glass transition temperature increased with ABF content and decreased with PLA content.
The reinforcing effect of PLA was enhanced by the incorporation of ABF, although the reduction in impact resistance is not convenient. That kind of behavior is expected generically for rigid materials; nevertheless, the wood appearance and biodegradability, along with the increase in moduli as well as thermal resistance, and the decrease in water absorption justify the production of this type of composites for many applications.
The authors declare that there is no conflict of interests regarding the publication of this paper.
F. J. Aranda-García acknowledges CONACYT for a scholarship.