Reinforcing abilities of microfibers and nanofibrillated cellulose in poly(lactic acid) composites

2.1 Material

RF with the length of approximately 1 cm and diameters of 11.3±1.7 μm were purchased from a local market in Bangkok, Thailand, as shown in Figure 1. PLA, 4043, was supplied by NatureWorks company, USA.

Figure 1:

Scanning electron micrograph of regenerated fibers.

2.2 Preparation of cellulose mats

One-centimeter-long RF were firstly washed with distilled water to remove impurities. Then, 1 g of the fibers was soaked in 100 ml distilled water for 24 h, and was subsequently mechanically treated using a high-speed blender with 20,000 rpm (PANASONIC MXAC400) for 60 min. The treated fiber suspension was poured into a Petri dish, and then dried in an oven at a temperature of 60°C for 7 days to form a mat of NFC with a thickness of ~270 μm. Meanwhile, mats of the untreated fibers were also prepared as follows. After 24 h of soaking, 1 g of RF in 100 ml distilled water was stirred for 1 min, and then poured into a Petri dish and kept in an oven at 60°C for 7 days. A mat of RF was finally prepared.

2.3 Preparation of composites

Prior to PLA film preparation, PLA pellets were oven-dried at 60°C for 24 h. PLA films were produced using compression molding. The PLA pallets were first pre-melted at a temperature of 180°C for 3 min without any pressure applied, and then compressed at a pressure of 10 MPa for 3 min. A transparent PLA film was formed. After that, a mat of treated or untreated fibers was placed between two PLA films, and a composite was formed using compression molding at a temperature of 180°C and a pressure of 10 MPa for 3 min. The preparation step of the composite with a cellulose mat is shown in Figure 2. A composite with a thickness of ~300 μm was produced. Composites reinforced with RF and NFC mats were abbreviated to C-RF and C-NFC, respectively. Each composite contained approximately 50 wt% of reinforcement.

Figure 2:

Preparation of PLA composites reinforced with a cellulose mat.

2.4 Degree of crystallinity

An X-ray diffractometer (Bruker AXS, D8DISCOVER) with a Goebel mirror and CuK_α with a wavelength of 0.1540 nm generated at 40 kV and 40 mA was used to study the crystallinity of untreated fibers and NFC. Samples were scanned from 10° to 50° at an increment of 0.02°. The degree of crystallinity of the cellulose samples (χ_c) was calculated using Segal’s method [3], [17]:

where I₂₀₀ represents the highest intensity of the 200 lattice peak, crystalline and amorphous regions, and I_AM represents the lowest intensity between the 200 and 110 peaks, the amorphous region [3], [17].

2.5 Thermal stability

Thermogravimetric analysis was performed using a simultaneous thermal analyzer (NETZSCH TG 209) from 30°C to 700°C at a heating rate of 10°C min⁻¹ under a nitrogen atmosphere. Approximately 8 mg of the sample was placed in an alumina crucible.

2.6 Morphology

The morphology of original RF and NFC was studied using a scanning electron microscope (SEM JSM6610LV) equipped with a secondary electron detector at an accelerating voltage of 10 kV. Before the analysis, the samples were sputter-coated with a thin layer of gold to avoid charging. The fractured surfaces of cellulose mats and composites after tensile deformation were also investigated.

2.7 Mechanical properties

The mechanical properties of NFC mats, neat PLA films and composites were determined using a LLOYD LR 50 K universal testing machine equipped with a load cell of 1 kN. Samples with the width of 5 mm were tested with a gauge length of 40 mm and a crosshead speed of 1 mm min⁻¹. The width and thickness of the specimens were measured using a vernier caliper and micrometer, respectively. Young’s modulus was calculated from the initial part of the slope of the stress-strain curve. At least five samples were tested for each material to obtain means and standard deviations.

3.1 Fiber characterizations

X-ray diffraction patterns for RF and NFC mats are shown in Figure 3. Peaks located at 2θ of around 12°, 20° and 22°, corresponding to crystal planes of 110, 11̅0 and 200, respectively, are typical for a cellulose II structure [1], [3]. The degree of crystallinity for RF was 70.6±0.3%, while a value of 69.4±0.2% for the degree of crystallinity of NFC was measured. The slight decrease in the degree of crystallinity obtained from NFC was an indication of the degradation of cellulose during mechanical treatment [3]. A similar result was previously reported for NFC prepared from lyocell fibers using the combined homogenization and sonication [1]. The degree of crystallinity obtained from the treated fibrils was 75.5%, compared with a value of 80.6% for that of intact lyocell fibers. Uetani and Yano [12] found that the use of high-speed blending caused less damage to cellulose fibers, compared with grinding [12]. Scanning electron micrographs of NFC after 60 min of mechanical treatment are shown in Figure 4. NFC with diameters in the range of 10–500 nm can be observed, while the diameters of untreated RF were 11.3±1.7 μm. The structure of NFC prepared using high-speed blending was considered to be a network rather than individual nanofibers. Some micron-sized fibers were still observed. The combination of chemical pre-treatment and high-speed blending is our future topic to reduce time consumption and obtain larger amounts of NFC fibrils.

Figure 3:

X-ray diffraction patterns of original regenerated fibers and NFC.

Figure 4:

Scanning electron micrographs of NFC treated for 60 min.

3.2 Thermal stability

Thermal stability of composites reinforced with two different types of cellulose mats, neat PLA and cellulose mats in the range of 30°C–700°C is shown in Figure 5. A small weight loss from room temperature to around 100°C was found for both RF and NFC mats and composites reinforced with these mats, but this behavior could not be found for neat PLA. This was due to the evaporation of moisture absorbed by cellulose. The onset degradation temperature of RF was 322.0°C; on the other hand, after mechanical treatment for 60 min the onset degradation temperature of NFC was slightly decreased to 320.3°C. The decrease was caused by the cellulose degradation, as confirmed by the X-ray diffraction results. In RF and NFC mats, there are two decomposition stages. The first degradation stage was found between 270°C and 330°C due to the cellulose main chain decomposition, and the second degradation stage presented at 360°C was attributed to the scission of char residuals to form gaseous products with a low molecular weight [18].

Figure 5:

Thermogravimetric analysis curves of mats of RF and NFC, PLA composites reinforced with these mats and neat PLA.

The onset degradation temperature of PLA was found to be ~344°C. The addition of both NFC and RF in the PLA resin led to a decrease in the degradation temperature due to the fact that NFC and RF had a lower degradation temperature than the PLA. This may be because cellulose fibers started to degrade before PLA. Similarly, with the presence of cellulose in poly(vinyl alcohol) [19] and low-density polyethylene [10], the degradation temperature of the composites was shifted to a lower temperature.

3.3 Morphology

After tensile deformation, the fractured surfaces of the cellulose mats and composites reinforced with RF and NFC mats were investigated. Figure 6 shows the SEM images of the fractured surfaces of RF and NFC mats, while the SEM images of the fractured surfaces of C-RF and C-NFC after tensile deformation are shown in Figure 7. The failure mechanism of both cellulose mats was fiber pull-out, indicating less bonding between fibers, as large pores between fibers could be found. On the other hand, the breakage of fibers was observed for dense cellulose nanofiber networks with high interaction between fibrils [9], [20].

Figure 6:

SEM images of fractured surfaces of the (A) RF mat and (B) NFC mat after tensile deformation at the same magnification.

Figure 7:

SEM images of fractured surfaces of composites reinforced with the (A) RF mat and (B) NFC mat after tensile deformation at the same magnification and those of composites reinforced with the (C) RF mat and (D) NFC mat at a higher magnification.

The fractured surfaces of the composites reinforced with RF and NFC mats were found to be different. The fiber pull-out mechanism was observed for C-RF. This was an indication of poor adhesion between the reinforcement and resin [21], [22]. Voids between the fibers and resin could be observed, as shown in Figure 7C. Samat et al. [23] found that the fiber pull-out of microcrystalline cellulose from polypropylene was caused by voids and cavities around the fibers. This induced poor interaction between polypropylene and microcrystalline cellulose. Also, the length of RF was as short as 1 cm. These reasons supported the lower values of the mechanical properties of C-RF, compared with C-NFC. The stress transfer from the matrix to fibers can be hindered by the poor interaction between the matrix and fibers [24]. The better interaction between the NFC and PLA resin, on the other hand, was observed from the fractured surface of C-NFC. This allowed stress to efficiently transfer from the resin to the reinforcement. The fractured surface of the composites reinforced with cellulose mats was found to depend on the interaction between fibers and resin and the amount of the resin penetrated into a mat. Cellulose fibers within a mat can be totally covered by the resin if the pore size between the fibers in the mat is large enough. In this work, the interaction between fibrils within the NFC mat is not strong, and an NFC mat has high porosity. These allowed the resin to easily penetrate into the mat and to interact with the fibrils. However, only interfacial interaction between the top layer of the cellulose network and the resin occurred when the dense network with less porosity was embedded in the polymer resin [25]. The surface modification of NFC would be our future topic to prepare composites with the better adhesion between the fibers and matrix in order to enhance the mechanical properties of the composites.

3.4 Mechanical properties

Values of RF mats for tensile properties were not able to be measured although 1 N load cell was used. This may be due to the fact that in the preparation step, the fibers were not disintegrated, and the overall weight of the fibers used to prepare a mat was only 1 g. A mat with high porosity and less interaction between fibers was formed. On the other hand, after disintegration nano-sized fibrils were fibrillated from the microfibers. The NFC mat had smaller pore sizes and stronger bonding between fibrils. This resulted in the improvement of mechanical properties. The tensile properties of cellulose mats, composites and neat PLA are summarized in Table 1. The tensile strength and Young’s modulus of the NFC mats were 2.6 MPa and 0.2 GPa, respectively. These results are close to those of NFC mats prepared from lyocell fibers using the combined homogenization and ultrasonication. The tensile strength and Young’s modulus of the NFC prepared from lyocell fibers were 6.8 MPa and 0.6 GPa, respectively [1]. The mechanical properties of cellulose mats are found to depend on the fiber morphology (width and length) and the amount of interaction between fibers [1], [26]. González et al. [26] evaluated the mechanical properties of papers with different contents of cellulose nanofibers. The mechanical properties of the paper increased considerably when the content of nanofibers was higher because the interaction between fibrils was improved by the high specific surface area of the nanofibers.

Stress-strain curves of the neat PLA, C-RF, C-NFC and NFC mat are presented in Figure 8. C-NFC showed the improvement in tensile strength and Young’s modulus. After embedding the NFC mat in the PLA resin, the tensile strength and Young’s modulus of the resin were increased by 18% and 42% from 32.8 MPa and 2.4 GPa to 38.7 MPa and 3.4 GPa, respectively. Strain at break, however, decreased slightly due to embrittlement of the material. It is worth mentioning that although the NFC mat had lower mechanical properties than PLA, an individual fiber of NFC has superior mechanical properties. A value of Young’s modulus of an individual fiber of NFC prepared from lyocell fibers was reported to be ~39 GPa [14]. Also, the reduction of fiber size has been reported to increase Young’s modulus of the fibers due to the lateral crystal dimension packing [14]. During the composite preparation, the matrix was penetrated through pores between fibers and separated NFC fibrils in the mat form to random-orientation NFC fibrils dispersed in the PLA resin. The improvement in the mechanical properties of the composites in this work is an indication that NFC prepared using high-speed blending is promising reinforcement used to prepare biodegradable composites. However, compared with those of C-NFC, the tensile strength and Young’s modulus of C-RF (31.5 MPa and 2.8 GPa) were considerably lower. The lower results may be due to the fact that untreated fibers had lower surface area and aspect ratio and less adhesion between fibers and resin. When cellulose fibers are used as reinforcement, its size and aspect ratio should be taken into account. Likewise, the fibrillation of original fibers to be nano- and micron-sized fibers can reduce defect points along the fibers as these points act as stress concentrations and crack initiators of debonding [4], [27].

Figure 8:

Stress-strain curves of the neat PLA, C-RF, C-NFC and NFC mat.