Abstract
Cellulosic henequen fibers were subjected to steam explosion and impregnated with polyethylene glycol (PEG) to improve fiber-matrix compatibility in polylactic acid (PLA) composites. Through Fourier-transform infrared spectroscopy (FTIR) it was shown that the hydroxyl, methyl, and ether functional groups were increased after the steam explosion treatment. Changes in the cellulose morphology caused by the steam explosion and impregnation with PEG were observed via scanning electron microscopy (SEM). Good adhesion of the treated cellulose and the PLA matrix was observed through improvement of the tensile strength and Young’s modulus of the PLA composite. The PEG impregnated into the fiber plasticized the PLA matrix and reduced the Tg from 59 °C to 52 °C. The increase in crystallinity confirmed the cellulose fibers induced nucleation of the PLA, which resulted in greater rigidity of the PLA composites.
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The Effects of Henequen Cellulose Treated with Polyethylene Glycol on Properties of Polylactic Acid Composites
Francisco J. Moscoso-Sánchez,a,* Abraham Alvarado,b Liliana Martínez-Chávez,c Rosaura Hernández-Montelongo,b Victor V. Fernández Escamilla,d and Gonzalo Canche Escamilla e
Cellulosic henequen fibers were subjected to steam explosion and impregnated with polyethylene glycol (PEG) to improve fiber-matrix compatibility in polylactic acid (PLA) composites. Through Fourier-transform infrared spectroscopy (FTIR) it was shown that the hydroxyl, methyl, and ether functional groups were increased after the steam explosion treatment. Changes in the cellulose morphology caused by the steam explosion and impregnation with PEG were observed via scanning electron microscopy (SEM). Good adhesion of the treated cellulose and the PLA matrix was observed through improvement of the tensile strength and Young’s modulus of the PLA composite. The PEG impregnated into the fiber plasticized the PLA matrix and reduced the Tg from 59 °C to 52 °C. The increase in crystallinity confirmed the cellulose fibers induced nucleation of the PLA, which resulted in greater rigidity of the PLA composites.
Keywords: Biodegradable; Biopolymers; Composites; Polylactic acid; Polyethylene glycol; Henequen fibers
Contact information: a: Departamento de Química, Centro Universitario de Ciencias Exactas e Ingenierías, Universidad de Guadalajara, Blv. Marcelino García Barragán 1421, Col. Olímpica, 44430, Guadalajara, Jalisco, México; b: Departamento de Mecánica Eléctrica y Electrónica, Centro Universitario de Ciencias Exactas e Ingenierías, Blv. Marcelino García Barragán 1421, Col. Olímpica, 44430, Guadalajara, Jalisco, México; c: Departamento de Farmacobiología, Centro Universitario de Ciencias Exactas e Ingenierías, Blv. Marcelino García Barragán 1421, Col. Olímpica, 44430, Guadalajara, Jalisco, México; d: Departamento de Ciencias Tecnológicas, Centro Universitario de la Ciénega, Av. Universidad 1115, Col. Lindavista, 47820, Ocotlán, Jalisco, México; e: Unidad Académica de Materiales, Centro de Investigación Científica de Yucatán, Calle 43 No. 130 between 32th and 34th street, Chuburná de Hidalgo, 97205, Mérida, Yucatán, México;
* Corresponding author: francisco.moscoso@academicos.udg.mx
INTRODUCTION
In recent years, great emphasis has been given to the development of composites with biodegradable thermoplastic matrices and fillers from renewable resources; a goal of these efforts is to decrease the contributions of plastics to environmental pollution (Ibrahim et al. 2014; Jia et al. 2014; Bourmaud et al. 2015; Rubio-López et al. 2017). The use of cellulosic or lignocellulosic materials (cellulose fibers, wood fibers, nut flour, sisal or jute fiber, etc.) as fillers or reinforcement for polymeric matrices has been increasing (An and Ma 2017; Chaitanya et al. 2017; Moustafa et al. 2017). Advantages of these materials are their low density, flexibility in their processing, reduced equipment wear, and their biodegradability. Lignocellulosic materials are relatively inexpensive and obtained from renewable natural resources (Canché-Escamilla et al. 1999). Cellulose is the most abundant natural biopolymer on Earth and is renewable, biodegradable, and nontoxic. It is a natural polymer composed of units of β-D-glucopyranose, which contains three hydroxyl groups per anhydroglucose unit (AGU) (Peng et al. 2011). The attractive mechanical properties of cellulose fibers, i.e., high strength in combination with low weight, are desirable to utilize in more sophisticated applications beyond conventional paper and board.
Early reports on the use of natural fibres in composites date back to 1970 and 1980 (Huber et al. 2012). Since then, modern advances in the development of cellulose fiber reinforced polymer composites have been the subject of several hundred studies. Due to the independence of cellulosic fibers of crude oil and their vast availability, an improved CO2 balance compared with composites made from industrially made fibers and fillers and good mechanical properties, cellulose-containing composites have generated much interest amongst various industries, especially the automotive industry (Huber et al. 2012). Lignocellulosic fiber, such as that obtained from henequen (Agave fourcroydes), is an unconventional source of cellulose. Agave fourcroydes belongs to the family Asparagaceae and is native and cultivated mainly in the state of Yucatán, Mexico for the use of the fiber contained in the leaves of the plant. It is used to make ropes, carpets, and sacks to pack and transport seeds, as well as for the manufacture of fabrics. It has been reported that henequen fiber has a high cellulose content (60%) compared to wood (40%) used in the production of cellulose pulps (Aguilar-Vega and Cruz-Ramos 1995). Although there are many studies on obtaining cellulose from henequen fibers, it is not yet a commercial source.
Different biodegradable polymers, such as polylactic acid (PLA) (Pracella et al. 2014), poly(ε-caprolactone) (PCL), polyhydroxybutyrate (PHB), and poly(hydroxyl-butyrate-co-hydroxyvalerate) (PHBV), have been used as matrices in composites with natural fibers (Csikós et al. 2015), such as cellulose microfibers or nanofibers as reinforcement. The total production capacity of bioplastics in 2017 was 2.05 million tons. PLA occupied the 10% of the raw material used for bioplastics production (Gaille 2018). Polylactic acid, a biodegradable polymer, has entered the market quite rapidly due to its advantageous properties. It is produced from natural feedstock, is compostable, and has good stiffness and strength. The world PLA market was $1.49 billion dollars in 2013 and in 2016 was 3.74 billion dollars. The United States is the major exporter of PLA; in 2016 it exported PLA worth US$ 116.47 million, while other countries are exporting very small quantities. The price of PLA in 2017 in the USA was $2.04 per ton in January, and by October it was $1.91 per ton (Baron 2018). The demand for PLA is increasing along with the production capacity.
Despite its advantages, PLA also has some drawbacks for its application in composite materials. Due to its hydrophobicity, PLA has poor compatibility with hydrophilic reinforcements such as cellulose; therefore, remedies are necessary to improve this compatibility. Different types of reactions can be applied for the covalent modification of cellulose, including etherification, acetylation, silylation, amidation, and urethane formation as a method to introduce small molecules compatible with PLA (Chaitanya et al. 2017). Apart from small molecules, it is possible to graft polymers on a cellulose surface. The grafting of polymers allows increased compatibility between cellulose and a polymer matrix due to decreased surface energy and the possible formation of entanglements between the grafted polymers and the matrix (Carlsson 2014). However, there are simple non-covalent methods, such as filling the porous fiber structure with polymers (pre-impregnation) (De Cuadro et al. 2015). Steam explosion is a promising technology that may have wide applications for lignocellulosic materials (Boonterm et al. 2016; Feng et al. 2017). Steam explosion is an efficient physical–chemical method that can destroy the structure and separate the components of fibers (Deepa et al. 2011) by increasing superficial area and accessibility to functional modification. Polyethylene glycol (PEG) is a water-soluble polyether polymer that can be used in biodegradable PLA-cellulose composites due to its good compatibility with PLA and its capacity to impregnate the cellulose fiber. Researchers have used steam explosion as a means to study biomass (Deepa et al. 2011), and other researchers have been blending polymers using biocompatible plasticizer and incorporation of filler materials. Chemical incompatibilities between reinforcement and matrix phases can also be removed by utilizing cellulose for both components (Mohapatra et al. 2014).
The aim of this study was to use PEG, as a modifier of the surface of henequen cellulose fibers subjected to steam explosion treatment, to improve cellulose-matrix compatibility in the PLA composite, and to study the effects on mechanical and thermal properties. The cellulose composites were analyzed via scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), thermal analysis, differential scanning calorimetry (DSC), and their mechanical properties.
EXPERIMENTAL
Materials
Cellulose extracted from the henequen fiber (Agave fourcroydes) was obtained using a method developed at the Scientific Research Center of Yucatán (CICY), Mexico (Canché-Escamilla et al. 2005). The process consists of four steps: (1) mild acid hydrolysis with 0.4% H2SO4 at boiling temperature, (2 ) treatment with 3.5% NaClO at 30 ° C, (3) an alkaline extraction with 20% NaOH at room temperature, and (4) bleaching with 0.5% NaClO at room temperature. After each step, the material was washed until neutral pH was obtained. The cellulose was dried at room temperature. The cellulose content was not determined in this work. However, when the same method was used by Cazurang-Martínez et al. (1990), they reported that the henequen cellulose obtained had a content of 96% cellulose and 4% hemicellulose. Polyethylene glycol 400 from Sigma-Aldrich (St. Louis, MO, USA) and deionized water were used for all of the impregnation experiments. The reagents were used as received without any additional purification. Polylactic acid (Ingeo™ Biopolymer 2003D, specific gravity: 1.24, melt flow rate: 6 g/10 min (210 °C, 2.16 kg), extrusion grade was supplied by NatureWorks (Minnetonka, MN, USA) and used as received without further purification.
Henequen cellulose treated by steam explosion and PEG impregnation
The boiler was filled with double-distilled water until reaching 10 lbf/in2 (psi) of generated steam pressure. The henequen cellulose was loaded in a tank, hermetically closed, and the valves that conduct the steam to the deposit were opened. The cellulose was allowed to stand for 5 min at a constant 10 psi steam pressure. Once this time elapsed, the valve located in the lower part of the tank was immediately opened, creating a sudden change in pressure and violently expelling the cellulose towards a container located in the lower part of the equipment. This procedure was repeated once. The wet cellulose was placed in a 70 °C oven until a constant weight was obtained.
Impregnation of PEG into steam-exploded cellulose
A solution of PEG/water with a 1:1 ratio was prepared. Afterwards, the PEG/H2O solution and cellulose were deposited in the reactor to prepare the cellulose/PEG required. The mixture was agitated for 2 h at room temperature. The resulting mixture was filtered, poured into a vessel open, and placed in a 150 °C oven for 2 h to evaporate the water. Then, the materials were removed and dried at room temperature. Subsequently, the cellulose with PEG was placed in a beaker with 1 L of double-distilled water and stirred for 2 h to remove the remaining PEG. This procedure was performed three times, replacing the old water and adding new. After removing the largest amount of residual PEG, the cellulose was placed in an oven to dry at 105 °C for 5 d, to finally obtain material at a constant weight with its chemical treatment. The calculations determined that approximately 92% by weight of the PEG was added to the cellulose, when it was realized the chemical treatment. Figure 1 shows a photograph of the cellulose subjected to steam explosion, without PEG (CEST) and with PEG (CETQ), accordingly.
Fig. 1. Cellulose subjected to steam explosion without PEG (CEST) and with PEG (CETQ)
Elaboration of composite
All the composites were prepared by mixing in a Haake Rheocord Fision 9000 (Haake, Karlsruhe, Germany) batch mixer with a 60 cm3 chamber capacity with roller rotors. The PLA was preheated and set at 160 °C and 20 rpm for 3 min. Then the cellulose was gradually introduced into the mixer at a constant rate to avoid overflow problems. After achieving the mixing time requirements, approximately 5 min, the mixer was turned off, and the composites were collected immediately in a vessel and allowed to dry at room temperature.
A Carver compression molder (Carver Inc., Wabash, IN, USA) was used to prepare 3-mm-thick, 15 cm × 15 cm plates. The plates were annealed in the molder at 160 °C for 3 min with 100 bar pressure. Subsequently, the pressure was removed for 1 min and resumed at 100 bar for 3 min; this procedure was repeated three times. In the next procedure, the heat continued at 160 °C for 3 min at 200 bar pressure. Subsequently, the pressure was removed for 1 min and resumed at 100 bar for 3 min; this procedure was repeated three times. To conclude, the heat control was disconnected, and the plate was allowed to cool at 200 bar pressure at room temperature. The code used to identify the samples was as follows: CEL is cellulose, CEST is cellulose by steam explosion, CETQ is cellulose with PEG treatment, CESTQ is cellulose by steam explosion with PEG treatment, and % is percent by weight. The samples are presented according to the relation of ‘‘X” % polylactic acid/ “Y” % treated Cellulose and “Z” % is the amount added of polyethylene glycol into cellulose (wt% ). One example is 95% PLA/5% CESTQ_40% PEG. That is, 95% by weight PLA and 5% by weight of cellulose. The last part is the 40% weight ratio of PEG added to cellulose.
Methods
Scanning electron microscopy (SEM)
The morphologies of the fibers and of the composites were examined using a Hitachi TM-1000 field emission scanning electron microscope (Hitachi, Tokyo, Japan). Specimens, after tensile testing, were fractured in liquid nitrogen and coated with gold prior to imaging (SPi, West Chester, PA, USA).
FTIR-attenuated total reflectance (ATR)
The FTIR analysis of the surface modification of the composites was performed using a Thermo Scientific iS5 Nicolet (Thermo Fisher Scientific, Madison, WI, USA) with ATR. The spectra were obtained at a 4 cm-1 resolution, with 64 scans in the standard wavenumber range from 400 cm-1 to 4000 cm-1. All the samples were oven-dried at 60 °C for 24 h before testing.
Mechanical properties
The test pieces for measuring the mechanical properties were cut by a laser machine at a 5 mm/min speed with a 100% cutting intensity. All of the test specimens were arranged for 7 d at room temperature prior to testing.
The tensile tests were conducted in an Instron 3345 universal testing machine (Instron, Norwood, MA, USA), equipped with a 1-kN electronic load cell and mechanical clamp grips.
The testing was performed following the ASTM D638 (2001) standard at a crosshead speed of 1 mm/min and a distance between mechanical clamp grips of 25.4 mm. The flexural test samples were performed following the procedure of the ASTM D790 (2001) standard for plastic materials with and without reinforcement, using a three-point contact system, in the universal testing machine at 1 kN at a speed of 1 mm/min.
Differential scanning calorimetry (DSC) analysis
The DSC measurements were taken with a Discovery TA (model: Q100, TA Instruments, New Castle, USA). The samples were dried for 24 h in an oven at 60 °C prior to DSC analysis. For the first scan, the samples were heated from 0 °C to 200 °C and kept at 200 °C for 1 min to remove the internal moisture and volatile small molecules. For the second scan, the samples were cooled to 0 °C, kept for 1 min, and subsequently heated to 200 °C, kept for 1 min, before finally cooling to 0 °C. Both the heating and cooling rates were 10 °C/min during the scans.
The degree of crystallinity (Xc) of the PLA and its composites was calculated from the second thermal scan as follows,
(1)
where Hm and Hmc are the melting enthalpies for PLA composites and the 100% crystalline PLA, respectively, and w is the mass fraction of PLA in the composite. The melting enthalpy of a totally crystalline PLA material (Hmc) was considered to be 93 J/g.
RESULTS AND DISCUSSION
Microphotographs of Fibers
The SEM micrographs in Fig. 2 show the effects of the pretreatment on the structure of the fibers. Figure 2A shows the raw, unbroken henequen fibers that are well-defined. Figure 2D shows the raw henequen fibers at 2500× magnification; the untreated superficial layer was flat and smooth. In Fig. 2B, the henequen cellulose fibers show a changed morphology. This change occurred during the course of pretreatment with sulfuric acid solution and delignification with sodium hydroxide and steam explosion.
Fig. 2. Scanning electron micrographs of the different fibers used 250X: A) raw henequen cellulose fibers, B) henequen cellulose fiber with pretreatment acid/base and steam explosion, C) cellulose fiber with pretreatment acid/base and steam explosion, and chemical treatment with PEG. High magnification scanning electron micrographs to 2500X, D) raw henequen cellulose fibers, E) henequen cellulose fiber with pretreatment acid/base and steam explosion, and F) cellulose fiber with pretreatment acid/base, steam explosion and chemical treatment with PEG.
The removal of the extractives from the superficial layer was able to increase the contact area because the fibrils became more exposed (Sreekumar et al. 2009; Pereira et al. 2011). As shown in Fig. 2E (high magnification), the fibers pretreated with acid/base and steam explosion exhibited shortening and thinning. Afterwards, typical organic cells, parenchyma, and defibrillated fibers could be observed in the morphology. Figure 2C shows the surface of henequen cellulose pretreated with acid/base, steam explosion, and chemical treatment with PEG. This yielded a superficial texture different than that of the raw fiber and the pretreated fiber, which indicated that the PEG present on the surface somehow interacted between PLA and cellulose (Taib et al. 2010). The fibers in the micrographs of Fig. 2C appear small in comparison to Figs. 2A and 2B; this size reduction could have been caused by the mechanical agitation of the water/PEG solution and henequen cellulose fibers during the thermal chemical treatment. Being broken into small pieces enhances contact between the solution and the cellulose fibers (Espitia Sibaja 2010). The superficial layer of henequen cellulose pretreated with acid/base steam explosion and chemically treated with PEG, at a magnification of 2500× with the fiber covered with PEG, is shown in Fig. 2F.
FTIR Analysis
The changes in the fiber molecular structures treated by steam explosion were revealed by FTIR analysis. In Fig. 3, the samples prepared under steam pressure gave a very similar spectral pattern to that of henequen cellulose. The only difference observed was the peak intensity, indicating that the treatment of steam pressure did not result in any noticeable changes in the macromolecular structure of the henequen cellulose. The magnitudes of all the spectra were normalized relative to the base line. The reason for of choosing the region at 1024 cm-1 was that all of the samples showed too high absorbance in this area. The normalized in-phase spectra were then used for the peak height calculations, which were required for the cellulose estimation, in accordance with Akerholm et al. (2004).