Research Article pubs.acs.org/journal/ascecg
Starch-Based Biofoams Reinforced with Lignocellulose Nanofibrils from Residual Palm Empty Fruit Bunches: Water Sorption and Mechanical Strength Mariko Ago,*,†,‡ Ana Ferrer,‡ and Orlando J. Rojas*,†,‡ †
Bio-Based Colloids and Materials and Centre of Excellence on “Molecular Engineering of Biosynthetic Hybrid Materials Research” (HYBER), Department of Forest Products Technology, Aalto University, Vuorimiehentie 1, P.O. Box 16300, FIN-00076 Aalto, Espoo, Finland ‡ Departments of Forest Biomaterials and Chemical and Biomolecular Engineering, North Carolina State University, 2820 Faucette Drive, Campus Box 8005, Raleigh, North Carolina 27695-8005, United States ABSTRACT: Lignin-containing cellulosic nanofibrils (LCNF) were extracted from residual oil palm empty fruit bunches (EFB), an abundant but underutilized bioresource, by using a set of sulfur-free fractionation methods. The various types of isolated LCNF were used to reinforce starch-based biofoams. The incorporation of LCNF achieved remarkable increases, by a factor of 44 and 66, of the Young’s modulus and yield stress in compression mode, respectively. In addition, owing to the relatively lower hydrophilicity of residual lignin, water sorption by the composite biofoams was reduced with LCNF loading. The starch/LCNF nanocomposite biofoams displayed mechanical properties similar to those of polystyrene foams and therefore can potentially represent a sustainable and green alternative for packaging and insulation materials. KEYWORDS: Empty palm fruit bunch fibers, EFB, Cellulose nanofibrils, CNF, Lignocellulose, Aerogel, Water sorption, Foams, Hydrophobicity
■
INTRODUCTION The extensive utilization of oil palm trees worldwide has led to a substantial generation of residual biomass. Malaysia alone produces about 20 million tons annually of oil palm solid streams (ca. 90% of the total feedstock mass);1 they include trunks, fronds, and empty fruit bunches (EFB).2 This vast resource poses challenges in replanting operations and represents an environmental burden. Therefore, integrated utilization and valorization of palm tree residues is a necessity. Recently, EFB have been proposed for incorporation in composites,3−6 pulp and paper,7−9 as well as in the production of chemicals10 and other valuable materials and precursors.11,12 Ferrer et al. reported on the deconstruction of EFB into lignocellulosic nanofibrils (LCNF) and their use in strong nanopapers or films.13 Their water absorption was found to be lower compared to that of films produced from lignin-free cellulose nanofibrils (CNF), for example, those obtained from bleached wood fibers. Together with other properties, LCNF from EFB compares favorably for packaging applications, and therefore, it represents an attractive alternative in the synthesis of new materials.8 Given their unique mechanical and morphological characteristics, cellulose nanofibrils have been utilized in the manufacture of low density porous materials,14−17 which typically exhibit high specific surface area, low thermal conductivity, and low dielectric permittivity.18−21 Relevant to this work, LCNF have © XXXX American Chemical Society
been processed with ionic liquids for the synthesis of aerogels.22−24 These materials can be utilized for thermal and sound insulation, fluid sorption, and food packaging. In the latter case, starch is of great interest given its low cost, availability, and compostability.25−28 Unfortunately, critical limitations arise from starch’s high hydrophilicity and poor mechanical integrity.29,30 In fact, starch-based porous materials lack mechanical strength in wet condition and display limited thermal resistance. Such drawbacks can be addressed by incorporation of cellulosic fibrils, especially if they contain residual lignin, which may endow the system with improved water resistance.31,32 Moreover, it is expected that lignincontaining nanocellulose combined with starch is compostable and completely biodegradable, adding to its potential for packaging. Thus, this work addresses the synthesis of a new class of biofoams consisting of starch (amylopectin) reinforced with LCNF from EFB, which were tested for their mechanical strength and water resistance.
■
MATERIALS AND METHODS
Fiber Fractionation from EFB. Empty fruit bunches (EFB) from a Malaysian oil palm mill was supplied by Straw Pulping Engineering S.L. (Zaragoza, Spain). Three different fiber types were produced from
Received: June 9, 2016 Revised: August 16, 2016
A
DOI: 10.1021/acssuschemeng.6b01279 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Table 1. Composition (wt %) of Fibers Produced from EFB via NaOH-AQ (N), Milox (M), and Fomosolv (F) processes fiber
α-cellulose
hemicellulose
holocellulose
lignin
extractives (ethanol)
ash
NaOH-AQ, N Milox, M Fomosolv, F
81.8 63.4 75.8
15.9 22.1 6.2
97.7 85.5 82
2.3 6.2 9
2.5 5.1 8.6
1.0 1.6 1.7
Figure 1. AFM height images of lignocellulose nanofibrils, LCNF. BP nanofibrils from reference lignin-free fibers (a) as well as EFB nanofibrils obtained by the given processing method: N (b), M (c), and F (d) (Table 1). The scan size in all images is 5 μm × 5 μm. EFB after the following sulfur-free treatments:7,8 (a) 15% aqueous NaOH and 1% anthraquinone (AQ) for 30 min at 170 °C with a liquid-to-solid (H) ratio of 10 (NaOH-AQ process, thereafter referred to as N); (b) formic acid (53%) and hydrogen peroxide (3%) for 165 min at 80 °C, H = 10 (Milox process, thereafter referred to as M), and (c) formic acid (92.5%) and hydrochloric acid (0.075%) for 60 min at 100 °C, H = 10 (Fomosolv process, thereafter referred to as F). Table 1 includes the chemical composition of these EFB-derived fibers (N, M, and F). Lignocellulose Nanofibrils. Lignocellulose nanofibrils (LCNF) were obtained from never dried fibers N, M, and F and dispersed in deionized water. They were first stirred for 20 min by using 1 M HCl solution followed by washing. The fibers were suspended in NaHCO3 solution (1 M) in order to convert carboxyl groups to their sodium form. Finally, the fibers were washed until the conductivity of the filtrate was
