**2. Materials and methods1**

**1. Introduction**

200 Composites from Renewable and Sustainable Materials

Wood is comprised of three of the most abundant natural polymers on earth: cellulose, lignin, and hemicellulose. Wood fibers and extracted cellulose fibers have good structural characteristics, and their use in polymers as an inexpensive fillerfor strength and stiffness improvements is well documented [1–5]. Lignin can also be used as a reinforcing filler, though its irregular structure andnatural compositionvariability canleadto reducedtoughness, while thephenolic moieties can promote heat instability [6–9]. Nevertheless, there are benefits, such as sustainability, costreduction,andimprovedstiffness,toincorporatingwood-basedfibers intopolymers. Fiber composites can be prepared using either thermoplastics or thermosets. Thermoplastics have higher impact strength, are recyclable, and can be molded into a variety of shapes, while thermosets are typically stronger, easier to process, and less expensive. Most fiber-reinforced plastics in use today are thermosets. One of the most versatile and widely used thermosets is bisphenol-A-based epoxy systems [10, 11]. These polymers suffer from poor impact strength

Lignin has been used to increase stiffness and toughness in thermoset composites [7, 9, 12]. The addition of lignin was found to toughen neat epoxy [12] and hemp-reinforced epoxy composites [13]. And, lignin was found to increase the flexural strength in flax-epoxy composites [14]. Lignin has also been used as a condensed phase flame retardant. Initially, it was used as an additive to other flame retardants to reduce the flammability of polypropylene [15–17] and polylactic acid [18, 19]. More recently, lignin has been modified with phosphorous to enhance its flame retardancy, then blended with epoxy [20, 21], polybutylene succinate [22], poly(acrylonitrile-butadiene-styrene) [23], and wood plastic composites [24]. And, lignosulfonate has been used unmodified to reduce the flammability of epoxy composites [25]. More often, however, lignin is used as a raw material for the preparation of polymers. Due to the phenolic structure of lignin, it is most often used as a prepolymer for epoxy composites. Typically, the lignin is depolymerized, then epoxidated with epichlorohydrin to produce polyglycidyl phenolic monomers and oligomers [11, 26, 27]. In some cases, it has been aminated to act as a hardener in epoxy composites [28–30]. Recently, lignin has been fractionated using ethanol, then epoxidated using epichlorohydrin in the presence of a phase transfer salt, such as tetramethylammonium hydroxide [31, 32]. This isolates the lower molecular weight lignin to produce a liquid epoxy and minimize segregation between the lignin and the epoxy composite. The use of the phase transfer salt does add toxicity to the process and requires an

The main component of plant fiber that adds strength and stiffness is cellulose, making it a good reinforcement agent for polymer nanocomposites. Recently, cellulose nanocrystals, which are the short, highly crystalline regions in a cellulose fiber, have been identified as good candidates to add reinforcement while simultaneously reducing the weight and increasing the sustainability of the reinforced composite [5, 33–35]. Unfortunately, cellulose is hydrophilic while most polymers are hydrophobic, leading to poor adhesion and water absorption problems when blending the two. In addition, when the cellulose nanocrystals are dried they

aggregate, and it is extremely difficult to reseparate these aggregates [36, 37].

and are typically reinforced with glass, polymer, or carbon fibers.

additional liquid–liquid extraction step to remove it.

All the chemicals were used as-received unless otherwise indicated. A sulfonated ethoxylated kraft lignin (REAX825E) was provided by MeadWestvaco Corporation (Richmond, VA). Two forms of cellulose nanocrystals were obtained from the University of Maine: (a) an aqueous solution prepared using sulfuric acid, neutralized to the sodium form, and containing 0.95 % mass fraction sulfur on a dry basis and (b) freeze-dried powder, freeze-dried from an aqueous

<sup>1</sup> The policy of NIST is to use metric units of measurement in all its publications, and to provide statements of uncertainty for all original measurements. In this document however, data from organizations outside NIST are shown, which may include measurements in non-metric units or measurements without uncertainty statements.

solution containing a 9 % mass fraction t-butanol. Ammonium polyphosphate (APP, Clariant EXOLIT AP422,(NH4PO3)1000+) was used as control filler. Ammonium tartrate (AT, Aldrich) and melamine (ML, Melamine 003 fine, DSM) were used as blowing agents. An epoxy monomer (DER331, Dow Plastics) was cross-linked with a diamine-terminated polypropylene glycol (JA230,JeffamineD230,HuntsmanCorp.).Alkalilignin,Dowex50W-X2(50–100mesh,H+ form) cation exchange resin and Rhodamine 6G (Rh, 95 %) were obtained from Sigma Aldrich. Methyltriphenylphosphonium bromide (MePh3PBr, 98 + %) was supplied by Alfa Aesar.

Lignin sulfonate (Reax 825E, 20.6 g) was dispersed in 400 mL of 15 % mass fraction sodium hydroxide solution and stirred for 3 h. After stirring, the solution was filtered using vacuum filtration and a glass fiber filter to remove any undissolved lignin, yielding a dark red solution. The lignin solution was then heated to 50 °C and epichlorohydrin was added (60 g, 0.63 mol). The reaction was stirred for 4 h and then brought to room temperature. The reaction solution was again filtered to remove any precipitates. The majority of water and unreacted epichlorohydrin was removed under high vacuum. The glycidyl lignin sulfonate was extracted from the resulting opaque solid using ethanol (200 proof). The excess sodium hydroxide was removed through repetitive washings of the lignin sulfonate with 50/50 ethanol/isopropanol (volume fraction). Excess ethanol was removed under vacuum at 50 °C. The product was isolated as a viscous red oil (30.8 g). Epoxide content (275 g/equiv) was determined by titration with 0.1 N HBr in acetic acid using a crystal violet endpoint. Cellulose nanocrystals were exchanged using the process described previously [41]. The epoxy composites were prepared by in the following manner. For most composites, fillers were added to either the epoxy resin or amine curing agent using a high shear mixer (FlackTek Inc. SpeedMixer) for 10 min at 260 rad/s (2500 rpm). The other epoxy composite component was added in stoichiometric amounts and mixed again in a high shear mixer for 10 min at 2500 rpm. For some of the composites, fillers were added using a mechanical stirrer and final mixtures were placed in a sonication bath for 30 min. For all composites, the mixture was degassed for 5 min under vacuum and immediately transferred to silicone molds for cone calorimetry (25 g, 75 cm diameter disk), optical properties (22 mm diameter, 1 mm thick), tensile properties (type V, ASTM D 638-02a), and water absorption analysis (51 mm diameter × 3.2 mm thick). Samples were also extracted using a 1 mL Teflon syringe before and after transferring to the silicone molds for thermal property measurements. These syringes were kept upright during the curing process. All epoxy samples were cured at room temperature for 24 h and 80°C for 2 h. For lignosulfonate containing composites, the total filler content was kept constant at 10 % mass fraction, and for cellulose containing composites, the filler content varied between 0.5 % and 5 % mass fraction.

Combustion properties were examined using microcombustion calorimetry (MCC) and cone calorimetry. The microcombustion calorimetry (MCC) samples (5 ± 0.1 mg) were tested with a Govmark MCC-2 microcombustion calorimeter at 1°C/sec heating rate under nitrogen from 200 to 600°C using method A of ASTM D7309 (pyrolysis under nitrogen). Each sample was run in triplicate to evaluate reproducibility of the flammability measurements. Cone calorimetry was conducted according to a standard testing procedure (ASTM E-1354-07) on a National Institute of Standards and Technology (NIST) prototype calorimeter. The cone was operated with an incident target flux of 35 kW/m2 and an exhaust flow of 24 L/s. The sample was placed in a pan constructed from heavy-gauge aluminum foil (Reynolds Heavy Gauge Aluminum Foil). The sides and bottom of the sample were covered by aluminum foil so that only the top surface of the sample was exposed to the Cone heater. The aluminum foil height was 5 mm higher than the sample to allow for expansion of intumescing samples. Exposure to the 35 kW/m2 external heater caused pyrolysis of the sample. Once sufficient fuel (pyrolysis products) was released, ignition occurred, which was activated by a spark igniter. The test was over when there were no visible flames. The standard measurement uncertainty was ±10 % of the reported reduction values and ±2 s in time.

Optical images were obtained using a Zeiss ID03 inverted microscope, equipped with LD10, LD20, and LD32 phase contrast objectives and an AmScope 5.0MP Microscope USB Camera. A confocal laser scanning microscope (LSM 510 META Carl Zeiss, Germany) was used to examine the aggregation and dispersion of cellulose in epoxy. The excitation source was a 405 nm diode laser (30 mW) and an emission band pass filter (420–480 nm) was used. Images were collected at 5×, 50×, and 100× magnification. A TA Instruments Q-2000 differential scanning calorimetry (DSC) was used to determine extent of curing and glass transition temperatures. For each epoxy, 5.0 mg ± 0.4 mg samples were placed in aluminum pans with unsealed lids and the cell was purged with nitrogen at a flow rate of 50 mL/min. The samples were equilibrated at −30 °C, heated to 200 °C at a scan rate of 10 °C/min, and cooled to −30 °C. The cycle was repeated five times, with the final four showing no changes between cycles. The uncertainties are σ = ±0.4° C for the reported temperatures and σ = ±0.3 J/g for the heat flow. Tensile tests were performed on an MST Criterion Model 45 hydraulically driven test frame with a 5 kN load cell in accordance to the ASTM D 638-02a norm at a speed rate of 0.05 mm/s with a gauge length of 14 mm. All tests were averaged over a minimum of five measurements for each sample with a standard deviation of σ = ±10 %. Water absorption was measured after submersion in water for 1 day at 23°C, according to the ASTM D570 standard.
