Bio-treatment of poplar via amino acid for interface control in biocomposites

https://doi.org/10.1016/j.compositesb.2020.108276Get rights and content

Highlights

  • A small amount of amino acid improved the fiber/PLA interfacial adhesion.

  • Poplar/PLA composites with 0.1 wt% lysine exhibited best tensile strength and modulus.

  • Poplar/PLA composites with 0.1 wt% lysine exhibited superior rheology for 3D printing.

  • A proposed mechanism in composites: chain extension, cross-linking, and degradation.

Abstract

Advanced biocomposites reinforced by abundant biomass-derived fillers can add a revenue stream to enhance the economic viability of biofuel production chains and the energy efficiency of the composite industry. However, the low stiffness of biopolymers limits their use in structural applications. Poplar fibers (mesh size: <180 μm, Populus spp.), an abundant waste from the wood industry, were used as bio-filler to fabricate high-performance biocomposites based on polylactic acid (PLA), in which the poplar fibers were modified by an amino acid (l-lysine). As a benefit of the amino acid treatment, the tensile Young's moduli of the lysine/poplar/PLA composites increased by up to 68% with the addition of a small amount of lysine, compared with neat PLA. At the same time, the tensile strength, failure strain, and Young's modulus of the poplar/PLA composites all increased after adding only 0.1 wt % of lysine. It has been observed that the lysine content has a significant effect on the decomposition temperature, complex viscosity, storage modulus, crystallization temperature, and crystallinity of composites. The fracture surfaces of the composites with an optimum lysine content had fewer voids and were more compact compared with composites without any lysine. The pores on the surfaces of poplar fibers became more available for the penetration of PLA molecules as a result of lysine addition. Therefore, this study presents a facile method for reinforcing biocomposites with extremely low-cost and environmentally friendly biofillers.

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

References (53)

  • K. Mak et al.

    The effect of wet-dry cycles on tensile properties of unidirectional flax fiber reinforced polymers

    Compos B Eng

    (2020)
  • R. Guo et al.

    Effect of toughening agents on the properties of poplar wood flour/poly (lactic acid) composites fabricated with Fused Deposition Modeling

    Eur Polym J

    (2018)
  • P. Rytlewski et al.

    Flax fibres reinforced polylactide modified by ionizing radiation

    Ind Crop Prod

    (2018)
  • L. Zhou et al.

    Enhancing mechanical properties of poly(lactic acid) through its in-situ crosslinking with maleic anhydride-modified cellulose nanocrystals from cottonseed hulls

    Ind Crop Prod

    (2018)
  • R.B. Rucker et al.

    Cross-linking amino acids in collagen and elastin

    Am J Clin Nutr

    (1978)
  • I. Spiridon et al.

    New opportunities to valorize biomass wastes into green materials

    J Clean Prod

    (2016)
  • I. Armentano et al.

    Multifunctional nanostructured PLA materials for packaging and tissue engineering

    Prog Polym Sci

    (2013)
  • G. Josefsson et al.

    Stiffness contribution of cellulose nanofibrils to composite materials

    Int J Solid Struct

    (2014)
  • R. Scaffaro et al.

    Physical properties of green composites based on poly-lactic acid or Mater-Bi® filled with Posidonia Oceanica leaves

    Compos Part A-Appl S

    (2018)
  • U.K. Komal et al.

    PLA/banana fiber based sustainable biocomposites: a manufacturing perspective

    Compos B Eng

    (2020)
  • Aashima et al.

    Ultrasonication assisted fabrication of l-lysine functionalized gadolinium oxide nanoparticles and its biological acceptability

    Ultrason Sonochem

    (2018)
  • S. Sharma et al.

    Harnessing the ductility of polylactic acid/halloysite nanocomposites by synergistic effects of impact modifier and plasticiser

    Compos B Eng

    (2020)
  • X. Wang et al.

    Mechanical properties, rheological behaviors, and phase morphologies of high-toughness PLA/PBAT blends by in-situ reactive compatibilization

    Compos B Eng

    (2019)
  • W. Xu et al.

    Novel biorenewable composite of wood polysaccharide and polylactic acid for three dimensional printing

    Carbohydr Polym

    (2018)
  • S. Farah et al.

    Physical and mechanical properties of PLA, and their functions in widespread applications - a comprehensive review

    Adv Drug Deliv Rev

    (2016)
  • F.-L. Jin et al.

    Improvement of thermal behaviors of biodegradable poly(lactic acid) polymer: a review

    Compos B Eng

    (2019)
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