1. Introduction
Replacing petro-fossil carbon with bio-based carbon in polymers offers a reduced material carbon footprint and managed end of life [
1,
2,
3]. The most widely studied and commercial bioplastic is polylactide (PLA) polymer. It is manufactured commercially by NatureWorks LLC, MN, USA (
https://www.natureworksllc.com/) (accessed on 2 June 2021) (150 kton plant in Blair, Nebraska), and Total Corbion (
https://www.total-corbion.com/) (accessed on 22 September 2021) (a total capacity of 175 kton with plants in Thailand, and France). PLA is 100% bio-based and at its end-of-life recyclable [
4] or industrially compostable. However, several property deficiencies in PLA have restricted its use in many packaging applications—primarily a low percentage of crystallinity and slow rate of crystallization. Nucleating agents like talc, nanocrystalline cellulose, hydrazine, PDLA, and other molecules have been used for increasing the crystallization rate and percent crystallinity of neat PLA [
5,
6,
7,
8,
9,
10,
11,
12].
PLA has mechanical and barrier properties comparable to polystyrene (PS) and thermal properties similar to polyethylene terephthalate (PET). The water vapor permeability of PLA films is low (1–4 × 10
−14 kg·m
2/s·m·Pa) [
13,
14,
15,
16] because of its hydrophobic nature. However, the oxygen permeability of PLA is very high as compared to PET, which limits its use in many packaging applications [
17,
18,
19].
Table 1 shows a comparison of the literature values of mechanical, thermal, barrier, and tensile properties of some commonly used polymers in packaging including PLA, Polyethylene terephthalate (PET), Low density polyethylene (LDPE), polystyrene (PS), polypropylene (PP) and starch.
Starch is an abundant, inexpensive, 100% bio-based, and completely biodegradable polymer. Starch consists of two main units—(1) amylose which is a linear polymer containing chains of α-1,4-anhydroglucose units which are mainly responsible for film-forming abilities and (2) amylopectin which is a highly branched polymer containing α-1,4-anhydroglucose units and α-1,6-glycosidic branched chains [
18,
19,
20]. The ratio of amylose and amylopectin is different in different starches [
13]. The melting temperature of pure starch is above its decomposition temperature. Therefore, it does not flow on thermal processing [
21]. To make starch processable, plasticizers such as water, glycerol, sorbitol are used [
21,
22]. However, such thermoplastic starches (TPS) have poor dimensional stability and reduced mechanical properties with time [
21,
22]. More problematic is the leaching of the plasticizer (glycerol for example) over time contributing to brittleness and making the film surface tacky and unusable. In our group, we have synthesized a maleated thermoplastic starch (MTPS) using reactive extrusion (REX) in which the glycerol plasticizer is covalently bonded to the starch, thereby eliminating glycerol migration and maintaining good processability [
23,
24]. The structure of the glycerylated starch polymer is shown below in
Figure 1 and described in our earlier papers [
23,
24,
25].
REX offers several advantages over traditional batch and flow reactors (CSTR, PFR) like fast reaction time, enhanced heat and mass transfer, better mixing and does not require any solvents [
26]. Starch based films have shown some desirable properties like high barrier to oxygen and CO
2 which is useful in packaging [
27,
28]. The oxygen permeability of starch films ranges between 0.4–2.5 × 10
−13 cm
3/m·s·Pa. Because of these advantages, different types of starch are often blended with PLA to reduce its cost and improve properties. However, pure starch and PLA blends are thermodynamically immiscible due to the hydrophobic nature of PLA and the hydrophilic nature of starch. Hence, the resulting system shows reduced strength and ductility compared to neat PLA. Several strategies have been tried to improve the compatibility by modifying either PLA or starch [
29,
30,
31]. Studies have also shown the effect of starch and thermoplastic starch as a completely bio-based and biodegradable nucleating agent for PLA as opposed to inorganic talc [
32]. Sun et al. studied the crystallization kinetics of PLA and starch composites and found that the addition of 1% of starch increased the crystallization rate considerably [
33]. Jang et al. studied the thermal properties and morphology of PLA/starch blends using MA as compatibilizer and it was found that MA modified starch was much more compatible with PLA than pure starch [
34]. Starch is hydrophilic and highly water sensitive. However, encapsulating the starch within the hydrophobic PLA matrix can mitigate this issue. This is in fact observed in several starch-based blends with various polyesters [
35,
36]. Multilayer films of starch and PLA have higher oxygen and moisture barrier compared to neat PLA [
37,
38,
39]. There are no reports on the compatibilized blends of maleated thermoplastic starch and PLA and their effect on the properties like crystallinity, crystallization rate, barrier, thermal, mechanical, and biodegradability.
In this paper, we report on using inexpensive, REX modified thermoplastic starch particles in the PLA matrix to increase the rate of crystallization and percent crystallinity of PLA. The MTPS-filled PLA polymer films were found to improve oxygen and water vapor permeability without any effect on biodegradability. Crystallinity, crystallization kinetics, and barrier properties were studied and compared with neat PLA. Mechanical and thermal properties as well as morphology of the MTPS-filled thermoplastic PLA were also analyzed. This MTPS could be used as bio-based and biodegradable nucleating agent with a responsible end of life option and a replacement for talc of inorganic origin.
2. Materials and Methods
2.1. Materials
High amylose corn starch with an initial moisture content of 12.8% (w/w) was obtained from National Starch (Bridgewater, NJ, USA). Glycerol was obtained from J.T. Baker (Phillipsburg, NJ, USA) and was used as received. 2,5-bis(tert-butyl-2,5-dimethylhexane), 90% (Luperox 101), and Maleic anhydride (MA) were obtained from Sigma–Aldrich (Milwaukee, WI, USA). IngeoTM biopolymer 3001D, a commercially available semi-crystalline grade of polylactide (PLA) was supplied from NatureWorks LLC (Minnetonka, MN, USA). It had a molecular weight Mw of 128,000 Da and polydispersity of 1.52. It was prepared from the polymerization of L-lactide and had a meso content of 9%.
2.2. Preparation of Maleated Thermoplastic Starch (MTPS) and Polylactide (PLA)/MTPS Blends
MTPS was prepared in a co-rotating twin-screw CENTURY ZSK-30 extruder (MI, USA). The screw diameter and transport length were 30 mm and 1260 mm respectively with L/D ratio of 42. Inherent moisture of starch is reduced before the reactive extrusion because it can interfere with the reactivity of glycerol and can cause foaming of the extrudate. Therefore, the corn starch was dried for 48 h in the oven at a temperature of 65 °C to reduce its moisture content below 1%. MA was used as a promoter for enhanced grafting of glycerol on starch. The details for the reaction chemistry can be found in previous work by Raquez et al. [
23,
24]. MA (2% by wt.) was ground to fine the powder using a mortar and pestle and was premixed with dry starch (800 g). Luperox (1.1 g) was mixed with glycerol (200 g) and the mixture was then fed into the extruder directly via a peristaltic pump. The feeder was calibrated to get the ratio of 80:20 (starch: glycerol) [
16]. The temperature profile was set as 70/90/110/120/130/140/150/150/150/140 °C from the feed port to the die. The screw speed was set at 100 rpm and the melt temperature was 150 °C. The vent port was kept open to remove any moisture and water formed during the reaction. The extrudate coming out of the extruder was air-cooled and pelletized simultaneously using Scheer Bay pelletizer as shown in
Figure 2.
Both polylactide and MTPS quickly absorb moisture from the atmosphere. Therefore, they were dried at 55 °C for 12 h before reactive extrusion. Then, MTPS and PLA pellets were mixed in various proportions of 1–10 wt. % in an aluminum tray before feeding. The detailed compositions are listed in the table below (
Table 2).
The temperature profile used on the extruder going from the feed section to the die is as follows: 150/160/165/170/180/180/175/175/160/155 °C. These temperatures were selected based on the processing temperatures required for semicrystalline PLA. The screw speed and throughput were 100 rpm and 130 g/min. The extrudate was quenched in a water bath and was then pelletized. The resulting pellets were dried overnight in an oven at 50 °C and then stored in vacuum-sealed bags before using for any further characterization.
2.3. Soxhlet Extraction
Selective solubility of glycerol in acetone was used to establish and determine percent covalent grafting of glycerol [
23,
24]. The MTPS pellets prepared were ground to a fine powder and about 5 g of sample was put in a pre-dried and pre-weighed cellulose extraction thimble. The thimbles were then inserted in the soxhlet extractor connected to a 500 mL round bottom flask containing around 200–250 mL acetone. The flasks were heated, and the solvent was allowed to reflux. The extraction was continued for 72 h. After the extraction, the thimbles were removed; residue and extract were separated and dried overnight at 70 °C. The dried thimble with residue was weighed again and the weight change in the residue was calculated. The reproducibility of the results was confirmed by testing three replicates for each sample. It was expected that the covalently grafted glycerol will not get extracted in acetone and there will be a weight gain in the residue. Percent grafting was calculated from the mass balance as shown in Equation (1).
where,
W1 is the weight of glycerol present in the sample originally and
W2 is the glycerol in the extract after 72 h. i.e., free glycerol.
2.4. Thermal Analysis
The degradation temperature of samples was obtained by thermogravimetric analysis (TGA). TGA measurements of all the samples were conducted under an inert atmosphere of nitrogen using a TGA Q50 (TA Instruments, New Castle, DE, USA). The general sample weight used was 5–7 mg. The sample was placed in an aluminum pan and was heated to 600 °C at the rate of 10 °C/min. The weight loss (%) of a sample as a function of temperature (°C) was obtained from this analysis. Also, the thermal transitions of the samples were obtained by using a differential scanning calorimeter (DSC). The sample was heated to 200 °C in DSC Q20 (TA Instruments, New Castle, DE, USA) with a heating rate of 10 °C/min and held for 5 min to erase thermal history. The sample was then cooled back to 20 °C and heated again to 200 °C with a heating rate of 10 °C/min. The glass transition temperature (T
g), melting temperature (T
m), the crystallinity of samples (%X
c), enthalpy of melting (Δ
Hm), and enthalpy of cold crystallization (Δ
Hc) were calculated using TA universal analysis 2000 software. The % crystallinity of PLA samples was calculated from the formula given by Bher et al., 2017 [
40,
41].
where, Δ
Hm and Δ
Hc are enthalpies of melting and crystallization respectively.
is the weight fraction of MTPS in the blends and Δ
Ho is the enthalpy of melting for 100% crystalline PLA which was obtained from the literature as 93.1 J/g [
29,
33,
41].
2.5. Isothermal Crystallization Analysis
To study the isothermal crystallization kinetics, the samples were heated to 200 °C and maintained for 5 min at that temperature to remove any thermal history. Then they were cooled to the desired crystallization temperatures (90, 95, 100, 105 and 110 °C) at a rate of 20 °C/min and held at that temperature till crystallization was complete, then heated again to 200 °C to obtain the melt temperature and final crystallinity after annealing.
2.6. Polarized Optical Microscopy (POM)
POM observation was performed on an Olympus BH-2 microscope (Olympus corp., Tokyo, Japan) with crossed-polarizers, equipped with a digital camera system and a Mettler Toledo FP82 (Columbus, OH, USA) hot stage. All the samples were first inserted between two microscope coverslips and squeezed at 200 °C to obtain a thin slice. The films were held at 200 °C for 2 min to achieve thermal equilibrium. This was followed by rapid cooling to the selected crystallization temperature of 105 °C. The polarized optical micrographs during isothermal crystallization were recorded after every 90 s to monitor the formation and growth of crystallites.
2.7. Mechanical Properties
The injection molded test bars were prepared using a tabletop DSM 15 cc mini extruder (DSM Research B. V., Sittard-Geleen, The Netherlands) and 3.5 cc mini-injection molder (DACA Instruments, Santa Barbara, CA, USA). The injection pressure was set as 140 psi and the cylinder and mold temperatures were 200 and 65 °C, respectively. The samples were stored for 2 days at 25 °C in a humidity chamber with RH of 50% before any analysis. Tensile testing was performed using an Instron model 5565-P6021 (Instron, Norwood, MA, USA) with a 5 kN load cell and grip separation speed of 12.5 mm/min as per ASTM D882. Data from five samples of each formulation were averaged and compared with the properties of neat PLA.
2.8. Scanning Electron Microscopy
A JOEL 6610 LV scanning electron microscope (JEOL Ltd., Tokyo, Japan) was used to study the dispersion of MTPS in PLA using the tensile fracture surfaces of all samples. The tensile bars were immersed in liquid nitrogen for ~2 min and then fractured. Fracture surfaces were mounted on aluminum stubs using high vacuum carbon tabs and coated with gold using a sputter coater. A different set of bar specimens were also treated with 6 N HCl for 12 h to remove the MTPS phase from the samples and air-dried for 12 h in a fume hood. Then they were mounted on aluminum stubs as explained before and examined using JOEL at 500× magnification at 10 kV.
2.9. Barrier Properties
The barrier properties were measured using Mocon instruments (OX-TRAN Model 2/21 and PERMATRAN-W Model 3/33, Lyons, CO, USA). All the measurements were undertaken at 50% RH for oxygen and 100% for water vapor. Circular films of 3.14 cm
2 area were used. The thickness of the samples was measured using a micrometer (TMI) and was used to calculate the permeability to oxygen and moisture. Water vapor permeability (
WVP) is given as:
Oxygen permeability (
OP) was calculated from oxygen transmission rate (
OTR) data using Equation (4):
where,
WVTR is water vapor transmission rate,
OTR is oxygen transmission rate,
Th (m) was the thickness of the sample and Δ
P was the pressure difference between both sides of the sample (Pa) [
42].
2.10. Aqueous Biodegradability Testing
The biodegradability of neat PLA and PLA + 5% MTPS samples was tested in an aqueous environment. All the tests were performed in an aerobic environment at 30 °C. A respirometric mineralization test system for calculating CO
2 evolution was set up based on International Standard ISO 14852. The system comprised blank, positive reference (cellulose) and the test material (PLA and PLA + 5%MTPS) for all the runs. All the samples, blanks, and references were run in duplicates. An optimized test medium containing all the nutrients and buffers was prepared according to the ISO standard.
Table 3 gives the detailed composition of the mineral solution prepared for all the tests.
Wastewater inoculum was added to all the flasks to obtain the concentration of 5%
v/v in the test medium as described in ISO 14852. Then the polymer samples were added to these flasks, and they were subjected to the test conditions. A solution of 1 N NaOH was used for trapping the CO
2 generated from test flasks. CO
2 trapping is a two-step process as shown below:
1 g of sample was taken from each of the 50 mL NaOH trapping solution and titrated with 0.1 N standardized hydrochloric acid (HCl) solution to find the amount of CO
2 trapped. The reactions are as follows:
The titrations were done with the help of an auto titrator to get the volumes of HCl,
V1 and
V2 required for reactions 1 and 2 respectively. The amount of HCl consumed can be used to calculate the
mmoles of CO2 evolved using the following Equation:
The percentage biodegradation (
%B) was further calculated by the following Equation:
is the amount of carbon dioxide that evolved in a test flask between the start of the test and time
t;
is the amount of carbon dioxide that evolved in a blank flask between the start of the test and time
t;
ThCO2 is the theoretical amount of carbon dioxide that evolved from the test material. All the values were expressed as
mmoles of CO2. The samples were replaced every 2–3 days in the starting phase when the rate of biodegradation was expected to be maximum and weekly or biweekly in the end [
43,
44]. Plots of cumulative CO
2 evolution for all the samples and blanks and % biodegradation vs. time were made for all the samples and compared for any differences between PLA and PLA + 5% MTPS.