Utilization of lignin extracted from Thai ago-waste as UV-blocking agent for BG-lignin/PLA composite films

The improving properties of poly(lactic acid), PLA, by utilizing natural resources attracted great intrigue to make such a green composite material that can be used as a commercial product in human life [1]. Lignin is one of biopolymer that can be used as bio-based filler and multifunctional bio-additive in a polymer composite. The most exciting properties of lignin that can be provided in a polymer composite is UV absorption and anti-oxidation [2]. In this current study, organosolv lignin extracted from sugarcane bagasse (BG-lignin) was utilized as multifunctional bio-additive in PLA for improving the UV absorption. The physicochemical and thermal properties of BG-lignin were determined using several techniques including SEM, GPC, quantitative 31P NMR, and DSC. BG-lignin at different loading contents (0.1, 0.2, 0.5, and 1 wt.%) was mixed with PLA via melt-extrusion. The attained compounds were converted to composite films via blown film extrusion. With the 0.5 wt.% loading content, the PLA composite films (0.5BG-lignin/PLA) absorbed almost all UV radiation which exhibits almost 70% blocking of UVB. The onset oxidation temperature of the PLA/0.5BG composite film increased by 34% as compared to that of the neat PLA film. Adding of BG-lignin enhanced tensile strength and Young’s modulus but did not favour to elongation at break.


Introduction
Ultraviolet (UV) radiation is high-energy-visible light with a wavelength from 200 to 400 nm and present in sunlight about 10% of total electromagnetic radiation; the long term of UV radiation exposure will cause photodegradation of polymeric materials [3]. Therefore, being an improvement of UVshielding polymer composite is a desirable interest. Lignin is a polyphenolic compound and the second most abundant renewable biomaterial after cellulose. It is the main component of the plant cell wall to the cellulose and hemicellulose for mechanical support in their structure [4]. Lignin has an aromatic structure and many functional groups such as hydroxyls, methoxy, carbonyl, and carboxyl, etc. In the academic area, lignin has been recognized as the bio-multifunctional additive that can develop plastic composite properties, especially, antioxidant [2] and UV-shielding ability [5]. Sugarcane bagasse is one of the most agricultural waste in Thailand. It could also be one of the most promising resource for lignin IOP Publishing doi:10.1088/1757-899X/1234/1/012018 2 extraction. Utilization of lignin for high value-added applications is one of the great challenges, especially in the application for a biodegradable single-use plastics. Poly(lactic acid), PLA, has become an alternative choice to replacing petroleum-based plastics. However, poor UV absorption of PLA films might not be suitable for UV-sensitive products. Using of lignin as UV-absorber in PLA films sounds to be a promising choice to attain fully biodegradable PLA composite films.
The objective of this current work is to utilization of organosolv lignin extracted from sugarcane bagasse as multifunctional bio-additives for PLA films. BG-lignin/PLA composite films were prepared via ubiquitous melt-extrusion. Different contents of BG-lignin (0.1, 0.2, 0.5, and 1 wt.%) were incorporated into PLA. Properties of the obtained BG-lignin/PLA composite films including film color, mechanical properties, UV-blocking ability, and antioxidant activity were investigated in comparison with the neat PLA film.

Materials
Organosolv sugarcane bagasse-lignin (BG-lignin) was kindly supported from the Biorefinery and Biological Technology Research Group (National Center for Genetic Engineering and Biotechnology (BIOTEC)). PLA (4043D) used in this study is a product of NatureWork LLC. Its number-average molecular weight (Mn) is 160,000 g/mol containing 94% L-lactic acid content and the melt flow rate is 6.0 g/10 minutes at 210ºC.

Characterizations of BG-lignin
The morphology of BG-lignin was observed by field-emission scanning electron microscope (FE-SEM, model SU5000, Hitachi, Japan). The molecular weight distribution was thoroughly analyzed by gel permeation chromatography (GPC, Waters e2695 separation module, USA). Quantitative phosphorus-31 nuclear magnetic resonance spectroscopy, 31 P NMR, (AV-500 Bruker Biospin, USA) were used for determining chemical characteristic. Protocol of 31 P NMR solution preparation was followed steps reported in Nature protocol [7]. Glass transition temperature (Tg) of the BG-lignin was measured in a temperature scan at heating rate of 10ºC/minute using differential scanning calorimetry (DSC-1, Mettler Toledo, Switzerland).

Preparation of BG-lignin/PLA composite films
Pre-dried PLA pellets were premixed with the 2 wt.% of BG-lignin powders prior to load to a feeder of the single-screw extruder (Thermo-Haake Rheomix OS, Germany) equipped with 3-mm rod die. Temperature profile was set as 170, 180, 185ºC and die temperature was 180ºC. Rotating speed was 45 rpm. The extrudates were extruded through a water bath and was then pelletized into 3-mm pellets. Attained 2 wt.% BG-lignin/PLA pellets were diluted to the desired loading contents (0.1, 0.2, 0.5, and 1 wt.%) by adding of pristine PLA. BG-lignin/PLA composite films were manufactured using the same single-screw extruder equipped with blown film (34-mm inner diameter and 1-mm die gap). Screw speed was kept at 60 rpm to fabricate film with the width and thickness of 13 cm and 35 µm, respectively.

Characterizations of BG-lignin/PLA composite films
Properties of the obtained neat PLA film and its composite films with BG-lignin/PLA were conducted. Color of the films was determined by datacolor spectrophotometer (datacolor 650, Pakisatan). Mechanical properties including tensile strength, Young's modulus and elongation at break were performed using a universal testing machine (Instron 4502 series, USA) by following ASTM D882 at the cross-head speed of 400 mm/10 minute. Light transmittance of all the films was measured using a UV spectrometer (UV-3600Plus, Japan) ranges from 200-800 nm. The oxidative induction temperature (OITtemp) was measured by using DSC under oxygen atmosphere, a DSC protocol as described in literature [8].

Results and discussion
SEM images of BG-lignin are presented in Figure 1. BG-lignin particles seem to be constructed by the accumulation of small lignin particles (see Figure 1a). The surface and shape of BG-lignin particles were evaluated at high magnification of 20,000x (see Figure 1b). BG-lignin were irregular in their shape, which varied diameter from sub-micrometer to a few microns. Molecular weight (Mw) and polydispersity index (PDI) of BG-lignin were evaluated by the GPC technique using the polystyrene as a standard polymer. Mw and the PDI of BG-lignin are 1,638 g/mol and 1.62, respectively. The phenolic hydroxy group was considered as an essential functional group due to the UV-absorption of its active group, which was investigated by 31 P NMR, BG-lignin exhibits high content of phenolic to aliphatic hydroxyl groups, which is 1.82 mmol/g. The content of the phenolic hydroxyl group could be correlated to the active functions of lignin including UV-shielding and antioxidant [2]. Tg of BG-lignin was detected at 135ºC.  UV-visible spectra of the neat PLA and BG-lignin/PLA composite films were plotted as a function of wavelength as shown in Figure 2. The neat PLA film exhibits a nearly transparent character at 90% transmittance in visible region (400-800 nm). High 90% transmittance was still detected in UV region, this indicates poor UV absorption of PLA film. Adding small amount of BG-lignin, the composite films show an improvement of UV absorption. It is due to the phenolic hydroxyl groups of the BG-lignin.
Increasing of BG-lignin effected directly to increase the UV absorption, while the transparency was satisfied [2]. However, loading content of BG-lignin resulted in film color as more brownish was noticed. Composite film with 0.5 wt.% BG-lignin (0.5BG-lignin/PLA) is expected as a suitable film which exhibits almost 70% blocking of UVB (315-280 nm).
As the results shown in Table 1, antioxidant activity for the neat PLA and its composite films was considered using OITtemp. The OITtemp of each film was evaluated on the onset temperature of exothermal characteristic. An antioxidant activity (%) was calculated based on the neat PLA film. Additional BGlignin can enhance significantly on the OITtemp and the anti-oxidation activity. This can be explained by the phenolic hydroxy groups in lignin acting as free radical scavenger that can promote anti-oxidation activity of the composite films.

Conclusions
In this current work, we have demonstrated the utilization of lignin extracted from Thai ago-waste, sugarcane bagasse, as multifunctional bio-additive in PLA film. BG-lignin/PLA composite films achieve excellent in UV-shielding ability and anti-oxidation. The composite film with 0.5 wt.% BGlignin (0.5BG-lignin/PLA) exhibits almost 70% blocking of UVB while film transparency and color are satisfied. The OITtemp of the 0.5BG-lignin/PLA film become to 289ºC which is 74ºC higher than that of the neat PLA. Mechanical properties of the composite films were kept in the acceptable criteria. Therefore, BG-lignin can be an alternative bio-additive providing multifunctional for PLA films.