Bio-treatment of poplar via amino acid for interface control in biocomposites
Graphical abstract
Introduction
Biomass (e.g., wood and grass) is an abundant sustainable feedstock to produce both biomaterials and bioenergy [1]. Using as a feedstock the parts of the biomass that are not optimal for pyrolysis/biofuel production can probably enhance the economic viability of the bioenergy production. Biocomposites offer several advantages, including better formability, abundance, renewable nature, and cost-effectiveness [[2], [3], [4]]. Biocomposites are emerging ecofriendly materials for many applications in the automotive, aerospace, marine, sporting goods, and electronic industries [5]. Biocomposites also have found a place in the emerging manufacturing technology of large-scale polymer additive manufacturing (LSAM) [6,7]. LSAM is a creative and novel additive manufacturing technology that builds large objects without traditional casting or cutting processes. It can produce components with a gradient function and complex shape. However, the use of biocomposites in LSAM applications is currently limited by the lack of sufficient knowledge and development in the area [8,9].
Polylactic acid (PLA) is a biobased polyester derived from the polymerization of lactide. Its tensile strength and stiffness are about 50–60 MPa and 3.5 GPa, respectively. These mechanical properties and its compostability make PLA an attractive polymer for many applications, especially in packaging [[10], [11], [12]]. Glass fiber (GF) is a common fibrous reinforcement used to improve the mechanical properties of polymers such as acrylonitrile butadiene styrene (ABS) and polypropylene (PP) [10]. However, the manipulation of GF can cause silicosis and dermatitis and increase maintenance costs for the processing apparatus. Replacing GF with natural fibers can minimize the environmental drawbacks of GF. The use of lignocellulosic fibers as reinforcements for composites has expanded rapidly because of their low cost, renewable nature, low weight, and high stiffness. Various lignocellulosic fibers such as flax, bamboo, kenaf, wood, and coir fibers have been used to produce biocomposites with PLA [10]. However, there is a need to improve the mechanical properties (including strength and stiffness) of biocomposites to enable their use in load-bearing applications. The mechanical properties of composites are crucial for their use in structural/semi-structural applications such as automobiles, aircraft, and medical devices [13].
Poplar, pine, and switchgrass are widely used in the United States for producing biofuels and biomaterials. Poplar can economically grow on marginal crop lands or forest lands [14]. It is a fast-growing wood with high carbohydrate content (cellulose and hemicellulose) [15]. Cellulose and hemicellulose both contain hydroxyl groups [16]. The poplar studied in our previous research [17] contained 39–48% of glucan, 27–29% of lignin, and 13–16% of xylan, respectively. In addition, the poplar microstructure consists of pores and hollow channels, which are available for penetration by polymer molecules [17].
Marathe et al. [18] utilized borassus fibers to reinforce PLA with 5, 10, and 15 wt % of borassus. The borassus/PLA composites’ tensile strength was enhanced by 34% as the borassus content increased. In addition, reinforcement with borassus fibers increased the crystallinity of PLA. Guo et al. [19] added 10 wt % of thermoplastic polyurethane (TPU) into poplar wood flour/PLA composites. TPU (melting point = 150–230 °C, density = 1.19 g/mL) is a common thermoplastic elastomer used to toughen PLA. The addition of TPU improved the interfacial bonding between the poplar wood flour and PLA, as observed by scanning electron microscopy (SEM) morphologies. With the addition of TPU, the tensile strength, impact strength, complex viscosity, and storage modulus of the composites increased.
Gowman et al. [20] compounded bio-based poly(butylene succinate) (BioPBS) with grape pomace (particle size: < 1 mm), a major byproduct of the grape juice and wine industries, via a melt extrusion-injection molding method. With the addition of grape pomace, the flexural and impact strengths of the composites were improved. Grape pomace can act as a reinforcing phase for BioPBS to produce composites with enhanced thermo-mechanical properties. Rytlewski et al. [21] used flax fibers (20 wt %) to reinforce PLA with the addition of triallyl isocyanurate (TAIC, 1 wt %), an appropriate cross-linking agent for PLA. The addition of TAIC improved the tensile strength, Young's modulus, and failure strain of the flax/PLA composites. However, applications of PLA are still limited by its brittleness and relatively low stiffness. Chemical modification (e.g., grafting with monomers and copolymerization) and polymer blending are currently major methods of improving the properties of PLA [22]. Amino acids (e.g., lysine), which consist of both amino (-NH2) and carboxyl (-COOH) groups, can be used as cross-linking agents [23,24]. Nevertheless, studies and understanding of the amino acid effects on the thermo-mechanical properties of poplar/PLA composites are lacking.
In the present study, a small amount of lysine was added to modify poplar/PLA composites to improve their thermo-mechanical properties and 3D printability. To optimize the lysine content of the composites, different amounts of lysine (0.05–1.0 wt %) were investigated. The novelty of this study was to utilize lysine as a cross-linking agent to improve the interfacial adhesion between poplar fibers and the PLA matrix. The mechanism of the reactions (e.g., chain extension, cross-linking, and degradation) in the lysine/poplar/PLA composites is investigated and discussed. Different characteristics of the composites are investigated, including surface morphology, chemical structure, and thermo-mechanical properties (including thermogravimetric analysis [TGA], differential scanning calorimetry [DSC] analysis, dynamic mechanical analysis [DMA], and tensile stress-strain).
Section snippets
Materials
PLA (Lot No. EE0928B131, biopolymer 4043D, NatureWorks LLC) was used as a matrix. Lysine (l-lysine, white powder, assay: ≥ 98%, lot #BCBW8728, CAS: 56-87-1, melting point: 215 °C, solubility: H2O soluble) was purchased from Sigma-Aldrich Co. (St. Louis, MO). Poplar fibers (Populus spp., mesh size: <180 μm) derived from the Center for Renewable Carbon (CRC) at the University of Tennessee, Knoxville. All raw materials were dried at 80 °C without further purifying before use.
PLA-based composite preparation
The PLA-based
Tensile property analysis
Fig. 1B and C shows the tensile strength and Young's modulus results for neat PLA and the PLA-based composites. The tensile strength of poplar/PLA composites was enhanced slightly from 54 to 60 MPa with the addition of lysine at up to 0.1 wt %. However, a further increase of lysine content to 1.0 wt % significantly decreased the tensile strength to 44 MPa. Similarly, the Young's modulus of the poplar/PLA-based composite was enhanced by 16% with adding lysine at up to 0.1 wt %, but a further
Conclusions
It was shown that lysine can be used as an agent (via, e.g., chain extension, cross-linking, and degradation) to modify poplar/PLA biocomposites to improve their thermo-mechanical properties. The addition of lysine led to reactions between -COOH groups and -OH or -NH2 groups. The lysine content (0.05–1.0 wt %) was optimized to achieve a 68% increase in the Young's moduli of lysine/poplar PLA composites compared with neat PLA. With the addition of 0.1 wt % of lysine, not only the Young's modulus
CRediT authorship contribution statement
Xianhui Zhao: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing. Kai Li: Conceptualization, Writing - review & editing. Yu Wang: Conceptualization, Writing - review & editing. Halil Tekinalp: Methodology, Writing - review & editing. Alan Richard: Investigation. Erin Webb: Resources, Funding acquisition. Soydan Ozcan: Conceptualization, Writing - review & editing, Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research is sponsored by the US Department of Energy (DOE), FY 2019 BETO Project, under Contract 2.5.6.105 with UT-Battelle, LLC. This manuscript was authored by UT-Battelle LLC under contract DE-AC05-00OR22725 with DOE. The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do
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