Property comparison of thermoplastic starch reinforced by cellulose nanofibrils with different chemical compositions

Abstract

Masterbatch composites made from starch/glycerol mixtures having 50 parts per hundred resin (phr) of three cellulose nanofibrils (CNFs) with different chemical compositions were prepared by pre-gelatinizing starch to create better dispersion of CNFs. The CNF contents were adjusted to 1, 5, 10, and 30 phr by adding ungelatinized starch and glycerol to the obtained masterbatch composite. The composite was then extruded at 150 ºC using a twin-screw extruder. The average diameters of the lignocellulose nanofibrils (LCNF), holocellulose nanofibrils (HCNF), and pure cellulose nanofibrils (PCNF) were 53.1, 24.4, and 22.4 nm, respectively. By increasing the CNF content in all nanocomposites, the tensile strength and elastic modulus were improved, whereas the elongation at break was diminished. Tensile properties were higher in the order of thermoplastic starch (TPS)/HCNF > TPS/PCNF > TPS/LCNF nanocomposites when the same CNF content was used. The addition of LCNF and PCNF also improved the thermal and moisture stability, whereas a negative effect was found in the TPS/HCNF nanocomposite. The effect of the LCNF on the thermal and water stability was greater than that of the HCNF and PCNF composites. The water uptake of the TPS/HCNF nanocomposite was higher than that of the TPS without CNFs.

Full Article

Property Comparison of Thermoplastic Starch Reinforced by Cellulose Nanofibrils with Different Chemical Compositions

Chan-Woo Park,^a Song-Yi Han,^a Pureun-Narae Seo,^a Won-Jae Youe,^a,b Yong Sik Kim,^a Seon-Kang Choi,^c Nam-Hun Kim,^a and Seung-Hwan Lee ^a,*

Masterbatch composites made from starch/glycerol mixtures having 50 parts per hundred resin (phr) of three cellulose nanofibrils (CNFs) with different chemical compositions were prepared by pre-gelatinizing starch to create better dispersion of CNFs. The CNF contents were adjusted to 1, 5, 10, and 30 phr by adding ungelatinized starch and glycerol to the obtained masterbatch composite. The composite was then extruded at 150 ºC using a twin-screw extruder. The average diameters of the lignocellulose nanofibrils (LCNF), holocellulose nanofibrils (HCNF), and pure cellulose nanofibrils (PCNF) were 53.1, 24.4, and 22.4 nm, respectively. By increasing the CNF content in all nanocomposites, the tensile strength and elastic modulus were improved, whereas the elongation at break was diminished. Tensile properties were higher in the order of thermoplastic starch (TPS)/HCNF > TPS/PCNF > TPS/LCNF nanocomposites when the same CNF content was used. The addition of LCNF and PCNF also improved the thermal and moisture stability, whereas a negative effect was found in the TPS/HCNF nanocomposite. The effect of the LCNF on the thermal and water stability was greater than that of the HCNF and PCNF composites. The water uptake of the TPS/HCNF nanocomposite was higher than that of the TPS without CNFs.

Keywords: Thermoplastic starch; Lignocellulose nanofibril; Holocellulose nanofibril; Twin-screw extrusion; Nanocomposite

Contact information: a: College of Forest and Environmental Science, Kangwon National University, Chun-Cheon 200-701, Republic of Korea; b: Division of Wood Chemistry, National Institute of Forest Science, Seoul, 02455, Republic of Korea; c: Department of Agricultural Life Industry, Kangwon National University, Chun-Cheon 200-701, Republic of Korea; *Corresponding author: lshyhk@kangwon.ac.kr

INTRODUCTION

In recent years, environmental pollution caused by petroleum-derived plastic wastes has worsened. As an alternative to petroleum-derived plastics, bio-based plastics based on natural polymers such as starch, cellulose, lignin, and protein have attracted attention (Chang et al. 2010; Liu et al. 2010; Girones et al. 2012; Cobut et al. 2014; Balakrishnan et al. 2017; Drakopoulos et al. 2017). Specifically, starch is a highly advantageous material for bio-based plastics because of its low cost, biodegradability, biocompatibility, and high applicability (Karimi et al. 2014; Müller et al. 2014; Khan et al. 2017). Also, it is one of the most abundant biopolymers in nature, mostly found in corn, potato, rice, fruit, and other plants. It is a homopolymer of α-D-glucose, which consists of amylose and amylopectin with semi-crystallinity (Li and Huneault 2011; Mendes et al. 2016). Starch can be gelatinized in the presence of plasticizers, such as water, glycerol, and sorbitol (Martins et al. 2009; Ghanbari et al. 2018). Upon heating (90 to 180 ºC) and mechanical shearing, the structure and crystallinity of starch granules can be disrupted due to the starch-plasticizer interaction (Carmona et al. 2015; González et al. 2015). Thermoplastic starch (TPS) is the most widely used bioplastic and has a high annual growth rate.

However, TPS has some drawbacks, such as a low water resistance and low mechanical properties, which limits its range of application (Belhassen et al. 2014; Karimi et al. 2014; Müller et al. 2014; Balakrishnan et al. 2017). Various reinforcing fillers, such as montmorillonite, clay, carbon nanotube, and cellulosic fiber, can be used to improve the properties of TPS (Huang et al. 2004; Dean et al. 2007; Martins et al. 2009). Recently, natural cellulosic fiber, especially nanocellulose, has gained attention as a reinforcing filler for TPS (Hietala et al. 2013; Nasri-Nasrabadi et al. 2014; González et al. 2015; Drakopoulos et al. 2017; Kargarzadeh et al. 2017; Ghanbari et al. 2018; Fazeli et al. 2018; Fazeli et al. 2019).

Nanocellulose can be prepared from lignocellulosic biomass by mechanical fibrillation or hydrolysis. Because the nanocellulose has beneficial properties, such as high strength (10 GPa), elastic modulus (130 to 140 GPa), large specific surface area, and high thermal stability (Samir et al. 2005; Eichhorn et al. 2010; Lee et al. 2010; Balakrishnan et al. 2017), it can greatly enhance various properties of TPS (Hietala et al. 2013; Nasri-Nasrabadi et al. 2014). Hietala et al. (2013) prepared TPS/cellulose nanofibril (CNF) composites from a mixture of potato starch/sorbitol/stearic acid/CNF by twin-screw extrusion between 80 and 110 ºC; the tensile properties of the TPS/CNF composite were improved as the CNF content increased. Furthermore, the addition of CNF reduced the moisture sensitivity, which is one of the largest disadvantages of TPS composites. The moisture diffusion coefficient and moisture equilibrium content also decreased with increasing CNF content.

Previous research (Park et al. 2017a,b) has focused on the characterization and application of lignocellulose nanofibrils (LCNF) containing both lignin and hemicellulose, and holocellulose nanofibrils (HCNF) with a high hemicellulose content without lignin. The properties of CNF and its reinforced composites can be controlled by adjusting the chemical composition. The hydrophobic lignin on the surface of LCNF improves the thermoflowability, water stability, and mechanical properties, but lowers the dispersibility in hydrophilic polymers. HCNF has a core-shell structure in which the hemicellulose covers the cellulose core and has a high potential as a reinforcing filler for hydrophilic polymers (Galland et al. 2015; Park et al. 2017a, b). Hemicellulose can act as an adhesive between the nanofibril and the hydrophilic matrix polymer because of its strong hydrophilic properties, thereby improving the strength and hardness of the composites.

In this study, three types of CNFs with different chemical compositions, i.e., LCNF, HCNF, and pure cellulose nanofibril (PCNF), were used as the reinforcing fillers for TPS. The effects of the chemical composition and content of these CNFs on the mechanical properties, thermal properties, and water stability of the CNF-reinforced TPS composites were compared.

EXPERIMENTAL

Materials

For the CNF preparation, yellow poplar (Liriodendron tulipifera L.), which consists of 45.4% cellulose, 26.3% hemicellulose, and 28.3% lignin (Park et al. 2017b), was obtained from the Experimental Forest of Kangwon National University (Chuncheon, Republic of Korea). Corn starch, glycerol, sodium chlorite, acetic acid, sodium hydroxide, and tert-butyl alcohol were purchased from Daejung Chemicals & Metals Co., Ltd. (Siheung, Republic of Korea) and used without further purification.

Delignification and alkaline treatment

Delignification to obtain holocellulose was conducted according to the following process. Wood powder (20 g) was added into distilled water (1,200 mL) and kept in a water bath at 80 ºC while being stirred at 150 rpm. The delignification reaction was initiated by adding sodium chlorite (8 g) and acetic acid (1,600 µL) to the suspension. The reactant was continuously stirred for 1 h. The same amount of sodium chlorite and acetic acid was added every hour, and the process was repeated 7 times. The obtained residue, namely holocellulose, was purified by vacuum-filtration with distilled water. The pure cellulose was prepared from the obtained holocellulose through successive alkaline treatment. The holocellulose (30 g) was poured into a 17.5% sodium hydroxide solution (750 mL). The reaction was performed for 50 min while being stirred at 150 rpm at a temperature between 20 and 23 ºC. At the end of the reaction time, 10% acetic acid (750 mL) was added to the solution for neutralization. The reactant was vacuum-filtrated and washed with distilled water.

Methods

Preparation of CNFs

The wood powder for LCNF preparation was suspended in water to obtain a concentration of 5.0 wt.% (3,000 mL), and holocellulose and pure cellulose for HCNF and PCNF preparation were suspended to obtain a concentration of 1.0 wt.% (1,500 mL). The suspensions were subjected to wet disk milling (WDM) (Supermasscolloider, MKCA6-2, Masuko Sangyo Co. Ltd., Kawaguchi, Japan). The rotational speed was set to 1800 rpm, and the clearance between the upper and lower disks was reduced to between 80 and 150 µm from the zero point, at which the disks would begin to rub.

The operation for LCNF was repeated until the 15th pass was completed, and the operations for HCNF and PCNF were repeated until the 5th pass. The duration was recorded for each number of WDM passes, and each WDM time (h/kg) was calculated based on the solid weight.

Preparation of TPS/CNF composites

First, masterbatch composites with a high content of CNFs (50 phr based on starch and glycerol weight) were prepared according to the following method. Starch and glycerol were mixed to obtain a 75/25 ratio, respectively, and added into the CNF suspensions in water (1.5 wt. %).

Gelatinization was performed at 85 ºC for 90 min with a stirring speed of 150 rpm, and then the mixtures were dried on polytetrafluoroethylene (PTFE) sheets in an oven dryer at 60 ºC for 24 h. Next, the mixture of the ungelatinized starch/glycerol (75/25) was added into the masterbatch composite to dilute the CNF content from 50 phr to 1, 5, 10, and 30 phr, consecutively (Table 1). Then, the mixtures were extruded using a twin-screw extruder (BA-11, Bautek Co., Ltd., Pochen, Republic of Korea) with a 40 length/diameter (L/D) ratio at 150 ºC with a rotational speed of 100 rpm.

Table 1. Material Formulation for TPS/CNF Composites

Morphological observation

The CNF samples for the scanning electron microscope (SEM) observation were prepared according to the following method. The LCNF, HCNF, and PCNF suspensions were diluted to 0.001 wt.% and sonicated using an ultrasonicator (VCX130PB, Sonics & Materials Inc., Newtown, CT, USA) for 1 min. The suspensions were vacuum-filtrated on a PTFE membrane filter. The filtrated products, which were stacked on the PTFE filter, were immersed in tert-butyl alcohol for 30 min. This immersion procedure was repeated three times to completely exchange the water with the tert-butyl alcohol. The CNFs were freeze-dried using a freeze dryer (FDB-5502, Operon Co. Ltd., Gimpo, Republic of Korea) at -55 ºC for 3 h to prevent the aggregation of CNFs.

The freeze-dried CNF samples and fractured TPS/CNF nanocomposites were then coated with iridium using a high-vacuum sputter coater (EM ACE600, Leica Microsystems, Ltd., Wetzlar, Germany). The coating thickness was approximately 2 nm. The morphologies of the CNFs and TPS were observed using a SEM (S-4800, Hitachi Co. Ltd, Tokyo, Japan) in the Central Laboratory at Kangwon National University. The diameter of individual fibers was measured at least 400 times on each sample by ImageJ software (National Institute of Health, U.S.A.)

Thermogravimetric (TG) analysis

TG analysis of the TPS/CNF nanocomposites was conducted using a TG analyzer (Q2000, TA Instruments Inc., New Castle, DE, USA) in the Central Laboratory of Kangwon National University. The samples (5 to 10 mg) were heated on a platinum pan under a nitrogen atmosphere. The range of the scanning temperature was from 25 to 500 ºC, with a heating rate of 10 ºC/min. The derivative TG (DTG) analysis was conducted by measuring mass loss with respect to temperature.

Tensile properties

TPS/CNF nanocomposites were hot-pressed at 150 ºC for 1 min for sheet formation. For tensile testing, the specimens were prepared from the sheet according to Type V dimensions described by the American Society for Testing and Materials D638 standard and were kept in a thermos-hygrostat (SJ-109, SoJung Measuring Instrument Company Co. Ltd., Anyang, Republic of Korea) at 25 ºC and 65% relative humidity (RH) to standardize the effect of RH on the tensile properties. The tensile test was conducted using a tensile testing machine (H50K, Hounsfield Test Equipment, Redhill, UK) with a cross-head speed of 10 mm/min. At least 9 specimens of each sample were tested, and the average values were taken.

Water uptake

TPS/CNF nanocomposite sheets prepared by hot-pressing at 150 ºC were cut to dimensions of 10 x 10 mm with a 2 mm thickness. The specimens were air-dried at 80 ºC for 12 h and vacuum-dried at 40 ºC until the weight of the composites was constant. Then, the samples were conditioned at 25 ± 2 ºC with 43, 59, 75, and 98% RH, consecutively. The RH was adjusted using saturated solutions of potassium carbonate, sodium bromide, sodium chloride, and potassium sulfate, respectively. The weight of the samples was measured until it was constant, and the water uptake ( was calculated by the following equation,

(1)

where is the initial weight and is the constant weight of humid specimens.

RESULTS AND DISCUSSION

Morphological Characteristics of LCNF, HCNF, and PCNF

The morphological characteristics of the LCNF, HCNF, and PCNF prepared at similar WDM times of 6 to 7 h/kg are shown in Fig. 1. In the LCNF, incompletely defibrillated fibrils of 100 nm thickness, which were covered with lignin-like particles, were present alongside 20 nm thick fibers, while the HCNF and PCNF had uniform fiber morphologies with a diameter of 20 to 35 nm. The average diameters of the LCNF, HCNF, and PCNF were 53.1 ± 24.1, 24.4 ± 9.2, and 22.4 ± 8.4 nm, respectively. Although three types of CNFs were defibrillated at similar WDM times, the diameter was larger in the order of LCNF > HCNF > PCNF. This phenomenon was due to the improvement in defibrillation efficiency resulting from the removal of lignin and hemicellulose. Because the existence of lignin and hemicellulose disturbs the defibrillation of cellulose microfibrils, narrower fibers with a uniform morphology can be obtained after chemical pretreatment for the removal of lignin and hemicellulose, especially lignin.

Fig. 1. Morphological characterization of (a) lignocellulose nanofibril (LCNF), (b) holocellulose nanofibril (HCNF), and (c) pure cellulose nanofibril (PCNF)

Properties of TPS/CNF Masterbatch Composite

To enhance the dispersity of CNFs in the TPS matrix, starch was pre-gelatinized in the presence of glycerol and CNF suspensions of water, which resulted in a masterbatch composite with a 50 phr CNF content. Figure 2 shows the morphological characteristics of the fractured surface of the neat TPS and TPS/CNF masterbatch composite. The neat TPS without CNFs had a smooth fractured surface with a hyphae-like pattern, which may have been due to the recrystallization of starch (Wang et al. 2012). On the other hand, the masterbatch composite had a rough fractured surface that showed both aggregated and individual CNFs. The difference in dispersity among the CNFs with different chemical compositions was not significant.

Fig. 2. Morphological characterization of thermoplastic starch (TPS)/cellulose nanofibril (CNF) masterbatch composites with 50 phr in (a) Neat TPS without CNFs, (b) TPS/lignocellulose nanofibril (LCNF), (c) TPS/holocellulose nanofibril (HCNF), and (d) TPS/pure cellulose nanofibril (PCNF) masterbatch composites

Figure 3 indicates the effect of the addition of CNFs with different chemical compositions on tensile strength, elastic modulus, and elongation at break of the TPS/CNF masterbatch composites. The addition of all CNFs improved the tensile strength and elastic modulus, which were higher in the order of TPS/HCNF > TPS/PCNF > TPS/LCNF masterbatch composites. Because the tensile properties of the TPS may have been influenced by the hydrogen bonding between starch molecules, the hydrophilicity of the LCNF, HCNC, and PCNF, dependent on their chemical composition, was an important factor for the tensile properties of the masterbatch composite. In previous studies (Kim et al. 2017; Park et al. 2017a,b), the tensile strength and elastic modulus of nanopapers obtained from LCNF, HCNF, and PCNF were higher in the order of HCNF > PCNF > LCNF, thereby showing a similar tendency with the tensile properties of the TPS/CNF masterbatch composite. Because of the hydrophobicity of the lignin on the surface of LCNF, the nanopaper from the LCNF had a lower tensile strength and elastic modulus than those of the HCNF and PCNF, which was attributed to a lack of hydrogen bonding between the cellulose molecules. In the case of HCNF with a core-shell structure, hemicellulose is present on the surface of cellulose and may act as an adhesive between the cellulose fibrils to improve tensile properties (Galland et al. 2015; Prakobna et al. 2016). This characteristic of HCNF could have contributed to the improvement in tensile properties of TPS. The elongation at break of TPS was 11.5%, which decreased to less than 3% with CNF addition.

Fig. 3. (a) Tensile strength, elastic modulus, and (b) elongation at break of thermoplastic starch (TPS)/cellulose nanofibril (CNF) masterbatch composites with lignocellulose nanofibril (LCNF), holocellulose nanofibril (HCNF), and pure cellulose nanofibril (PCNF) contents of 50 phr