**1. Introduction**

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

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 additional liquid–liquid extraction step to remove it.

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].

There have been a variety of approaches to overcome this obstacle. In most studies, a covalently bonded coupling agent is used to improve the adhesion between the crystal and the polymer. Another approach for is to use solvent exchange and solvent casting or thermosetting with the aid of a solvent. In a two-component thermoset, each component can have significantly different chemistries and surface energies, creating opportunities to use processing techniques to improve dispersion of less compatible fillers. One of the most commonly studied epoxy composites are based on diglycidyl ether of bisphenol A epoxy resin and aliphatic diamines. Polyether diamines are typically used to improve flexibility, toughness, and color fastness of the composite. These diamines are significantly more hydrophilic than the epoxy resin or the final epoxy composite, and will have better adhesion with cellulose. Tang and Weder [38] prepared cellulose nanocrystal–epoxy composites by solvent exchange into dimethylformamide and Peng et al. [39] used acetone to create a nanocrystal gel network prior to adding a hydrophilic curing agent, followed by an epoxy resin. These approaches all suffer from laborious and high-energy processing steps. Further, the solvent is difficult to remove, can lead to the loss of desirable composite properties, and add an environmental burden to the process. And, Emami et al. [40] used a polypropylene oxide–polyethylene oxide block copolymer as a surfactant. Although dispersion was improved, there was a loss in tensile strength and elongation by adding the surfactant. We have recently developed a simple, scalable, ionexchange method to alter the surface energy of cellulose nanocrystals while retaining much of the cellulose surface intact [41].

In this study, we modified wood components using simple procedures to improve their dispersioninepoxymatrices.Lignosulfonatewasusedasaflameretardantinepoxycomposites. The lignosulfonate was epoxidized using epichlorohydrin and sodium hydroxide, then fractionated by extraction in ethanol to minimize migration in the epoxy during curing. The flammability was examined using microcombustion and cone calorimeters and the glass transition temperature was measured using differential scanning calorimetry (DSC). We also prepared cellulose nanocrystal–epoxy composites. A novel ion exchange method was used to eliminate the need for a reaction solvent, reduce the aggregation of the nanocrystals, and improve dispersion throughout the epoxy matrix. The glass transition temperatures were measured using DSC and the dispersion was visualized using confocal fluorescent microscopy.
