BIO-BASED POLYAMIDES REINFORCED WITH CELLULOSE NANOFIBERS—PROCESSING AND CHARACTERIZATION Jennifer H. Zhu, Alper Kiziltas, Ellen C. Lee, Deborah Mielewski Materials Research and Advanced Engineering, Ford Motor Company, Dearborn, MI, 48124 Abstract Bio-based polyamides are among the most promising families of bioplastics based on fully or partially derived renewable sources because of their low density, good mechanical and thermal properties, and durability. Bio-based polyamides (e.g., PA 11, PA 1010, and, to a lesser extent, PA 610) are also key resins for automotive applications because their continuous operating temperatures are comparable to the widely used PA6 and PA66. In this study, cellulose nanofibers (CNF) have been successfully dispersed in bio-based polyamide matrices (PA610 and PA1010) by conventional melt processing. The effects of CNF contents on the mechanical (tensile, flexural, and impact) and thermal (crystallization behavior and thermal stability) properties were investigated. The results indicate that these CNF fillers can be efficiently incorporated into the bio-based polymer matrices without the need for coupling agents, surface modifications or surfactants. The distribution and dispersion of the particles within the polymer matrix were studied using scanning electron microscopy. The composites produced here using bio-based polyamides have good mechanical and thermal properties and could be especially useful in applications within or near the engine. Introduction Traditional high performance polymer composites have been made in the past using inorganic fillers such as carbon fibers, glass fibers, and talc fillers. Although these composites have good mechanical and thermal properties, they are not renewable and not biodegradable. Current research focuses upon finding natural, environmentally-friendly, and renewable fibers to replace traditional fillers. The cellulose from natural fibers such as kenaf, flax, wood, and hemp has been successfully incorporated into polymer composites that demonstrate good mechanical behavior and are also lighter than traditional composites. Cellulose fibers are nontoxic materials that have a high specific strength and stiffness and a lower density than traditional glass or talc filled composites. However, cellulose fibers have high moisture absorption and have limited thermal stability at higher processing temperatures [1]. Beyond a temperature of 200 °C, degradation will start to occur under sustained exposure. Polyamides are of particular interest as a polymer matrix due to their mechanical and thermal properties and relative ease of processing. It is used in a variety of packaging and auto applications. In addition, they pair well with cellulose because they both have hydrogen bonding, which can lead to stronger interactions between filler and matrix. The use of micro and nanocellulose as a reinforcing agent in polyamides has been a well researched subject. In 1984, Klason found that upon putting cellulose in PA 6 matrix, although the elastic modulus of the composites increased slightly, the strength and elongation decreased as a result of cellulose’s thermal instability [2]. Thermal degradation is the primary cause of difficulty when working with cellulose fibers, since its deterioration leads to reduced mechanical properties. In 1985, Zadorecki used reaction injection molding to make cellulose PA 6 composites, and although the lower temperature and pressure successfully reduced degradation, the molding method is fundamentally flawed and prevents highly loaded composites from being made [3]. In 2003, Page 1 Winata et al used Mucell technology to create a cellulose and PA 6 composite [4]. Although this successfully lowered the processing temperature, it allowed for cell nucleation to occur which worsened the mechanical properties. However, as Xu noted in 2008, if a workaround can be found to circumvent thermal degradation, cellulose fibers can be a much better reinforcing agent than traditional glass fibers due to its lower density, improved flexibility, and inherent renewability [5]. In 2014, Lee et al successfully incorporated microcrystalline cellulose (MCC) into a PA 6 matrix to increase the tensile strength of the composites by more than two times the neat polyamide [6]. Kiziltas et al also incorporated MCC in PA6 to create engineering thermoplastic composites with inmproved tensile and flexural properties [7]. The use of nanocellulose in polymer composites is a more recent topic of study. Although nanocellulose has been successfully used in different polymer matrices including polyethylene (PE), polymethylmethacrylate (PMMA), polypropylene (PP), and polystyrene (PS), most of these composites were created through methods other than melt blending, due to the challenges with thermal stability. Yousefian and Rodrigue successfully used melt compounding to create nanocrystalline cellulose (NCC) and PA 6 composites with improved mechanical properties [8]. The NCC was created via acid hydrolysis of a wood pulp. According to Yousefian, tensile strength and modulus increased by about 10% with only a 3% loading level of nanocrystalline cellulose (NCC) . However, after 3%, property improvement hit a plateau, likely due to poor filler dispersion at higher loading levels. Flexural modulus improved by 41% at 3% loading level. These NCC composites improved more than composites made from other natural fibers like hemp, flax, and kenaf. However, increasing loading level resulted in a reduction of composite impact strength [8]. In addition to using a renewably sourced filler, if the polymer (such as polyamide) used for the matrix was renewable, then the entire composite can be made in an environmentally friendly manner. Neat nylon 11 is an example of a biobased engineering thermoplastic that is currently used in 95% of Ford vehicles for fuel line applications due to the good mechanical, thermal, and processing properties of polyamides [9]. In addition, there are biobased polyamides such as PA 1010 and PA 610; the former of which is 100% based on castor oil, and the latter is 62% based on castor oil [10]. Both of these polyamides (and to a lesser extent, PA 610) have lower melting temperature, density, and moisture absorption than PA 6 and PA 66. Unfortunately, these materials are currently very expensive compared to tradition nylon materials. Costs can be reduced if less polymer material is used and reinforcing fillers such as cellulose nanofibers are added to strengthen the material. The resulting biobased composite has the potential of lowering costs while further strengthening the composite material. If these cellulose based composites can be successfully made with improved mechanical properties and without thermal degradation, then fully or partially biobased polyamide composites can be used to replace the glass-reinforced nylon composites that are currently used in engine-related vehicle applications, including engine intake manifolds and fuel-line applications. Preliminary results of using cellulose fibers to reinforce PA 1010 and PA 610 show that the addition of cellulose increased stiffness and strength of the composite material [11, 12, 13]. However, little to no published work could be found that explores the production and properties of ultrafine cellulose nanofiber reinforced PA 610 and PA 1010 composites. Page 2 Materials and Methods Materials Semicrystalline cellulose nanofibers, grade UFC 100, were kindly obtained from JRS Arbocel. According to JRS, it is the finest cellulose fiber available on the market, with an average particle size of 8 um and a diameter of approximately 2 um. It has a bulk density ranging from 150 to 220 g/L and a specific density of 1.4 g/cm3. Two bio-based polyamides were studied as matrices for the cellulose nanofibers, PA1010 and PA610. Both are kindly supplied by Evonik Vestamid. PA1010 is 100% based on natural resources, as both of its monomers are derived from castor oil, giving it a carbon footprint of 4.0 kg CO2eq. It is a semicrystalline material with high mechanical strength and good for use at high temperatures. Similarly, PA610 is also a biobased polyamide, but only 62% based on castor oil. It has a carbon footprint of 4.6 kg CO2eq, slightly larger than that of PA1010. Like PA1010, it is semicrystalline and has high mechanical strength (slightly higher than PA1010) and good thermal properties. Sample Preparation and Processing Conventional melt processing was used to disperse cellulose nanofibers into the bio-based polyamide matrices. Prior to extrusion, cellulose and polymer was dried in an oven at 70 °C overnight. Cellulose and polyamide were fed into the extruder hopper using K-Tron feeders to create a masterbatch. Single screw extrusion was carried out on a Davis-Standard machine. The melt temperature was set to 446 °F and decreased by 5 in each zone. The extrudant was solidified in a water bath and pelletized. Masterbatch pellets were mixed with pure polyamide pellets and extruded to create extrudant composites with 2, 4, 6, and 8 weight percent of cellulose nanofibers. In addition, 10 and 20 weight percent samples were made for the PA 610 matrix. A constant screw speed of 40 was used for all extrusion. The pelletized extrudant was dried again in an oven at 80 °C overnight in preparation for injection molding. Samples were made according to ASTM standards for tensile, flexural, and impact testing in a Boy Machines Model 80M injection molder. The injection molding parameters used to create good PA1010 and PA610 samples are shown below in Table I. Page 3 Table I. Injection molding processing conditions for PA1010 and PA610. PA1010 PA610 Shot size 24 mm 24 mm Injection Pressure 800|1300 psi 2000|2000 psi Hold Pressure 450|550|750|1000 psi 750|850|950|1000 psi MPP Back Pressure 50|50 psi 50|50 psi Mold Temperature 160 °F 180 °F Temperatures 470|470|465|460|455 °F 510|510|505|500 °F Injection Speed 2|2|2 mm/s 100|100|100 mm/s Screw Speed 50|50 1/min 100|100 1/min Cooling Time 30 s 30 s Decompression 1 mm 1 mm Testing and Characterization After the injection molded samples have rested for a week to allow samples to reach equilibrium, their mechanical, thermal, rheological, and morphological properties were evaluated.