**3.1. Migration of lignosulfonate**

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

and an exhaust flow of 24 L/s. The sample was placed

with an incident target flux of 35 kW/m2

202 Composites from Renewable and Sustainable Materials

In a previous study, the flammability of epoxy composites containing lignosulfonate was initially measured using a radiative gasification apparatus [25]. This is a nonflaming technique used to study condensed phase flammability reductions in composites. The measured mass loss rate and time to peak mass loss rate are related to the heat release rate and time to peak heat release rate (PHRR) of materials in a standard cone calorimetry experiment. The results indicated that ethoxylated lignosulfonate can reduce the flammability of epoxy composites through the formation of char. Further, the addition of naturally derived gas-forming agents, such as melamine or ammonium tartrate, initially reduced mass loss rates by enhancing the quality of char rather than by inducing intumescence behavior. The addition of these gasforming agents also induces cracks and debonding later in the burning process, leading to a loss of protection partway through the experiment.

The flammability of these materials was re-examined using cone calorimetry. The shape and relative heat release values mirrored the mass loss rate curves obtained using the gasification apparatus for all samples. The strong correlation observed indicates the reduced flammability when adding lignosulfonate is almost entirely through a condensed phase mechanism. The cone calorimetry data are summarized in **Figure 1**. The peak heat release rate (PHRR), which estimates how intense a fire will be and can help predict the time to flashover, is reduced by 24 % by adding 10 % lignin, 34 % by adding 10 % REAX825E, and 36 % by adding 7 % REAX825E + 3 % AT. The addition of ML instead of AT leads to an increase in PHRR compared to pure epoxy. Melamine produces more gases, leading to more cracks to the char layer, reducing its protective ability. The total heat release is reduced by as much as 14 % when using REAX825E. Since MCC suggests that 30 % to 40 % of lignosulfonate will volatize during pyrolysis, this indicates an ability to prevent some of the epoxy from burning. One disadvantage to using lignin is that the time to ignition is significantly reduced. A number of factors contribute to the ignition time, including heat capacity, thermal conductivity, surface emissivity, oligomeric decomposition products, melt viscosity, and permeability of the solid. Pure epoxy is transparent with low surface emissivity, whereas lignin is brown or black in color with high surface emissivity. Accordingly, tign is reduced going from pure epoxy to epoxy + 10 % lignin to epoxy + 10 % REAX825E. The addition of gas-forming compounds, which degrade at temperatures lower than the pure epoxy, leads to a more porous solid as the sample heats up. So, it is not surprising that tign is further reduced going from epoxy + 10 % REAX825E to epoxy + 7 % REAX825E + 3 % AT to epoxy + 7 % REAX825E + 3 % ML.

**Figure 1.** Cone calorimetry data for epoxy + lignosulfonate composites. Error bars represent 2σ.

The observed peeling of the char layer during combustion in the cone calorimeter suggested the lignin migrated and separated from the epoxy matrix during curing. The migration was verified using microcombustion calorimetry. Microcombustion calorimetry is a small scale (3 mg to 10 mg samples) test, where a sample is heated in nitrogen, similar to thermogravimetric analysis, and the pyrolyzed gases are mixed with oxygen and combusted in a separate chamber. The amount of oxygen consumed can be correlated to heat released. The heat release capacity (HRC) and heat of combustion (Hc) measured by the MCC are correlated to several flammability parameters obtained from other measurements, including peak heat release rate (PHRR) and total heat released (THR) from cone calorimetry experiments. As shown in **Table 1**, ethoxylated lignosulfonate has a lower flammability and higher char yield than alkali lignin, which is why it was chosen as the primary flame retardant in this study. Lignin and ethoxylated lignosulfonate have much lower flammabilities than epoxy. The use of these materials in epoxy significantly reduces the HRC by up to 45 % and the Hc by up to 20 %. A 5 mg sample was taken from the top and the bottom of an 8 mm thick epoxy + 10 % REAX825E sample. The results show that the lignosulfonate migrates to the top surface of the epoxy during the curing process. The higher char yield and lower apparent heat capacity are likely due almost entirely to the lignin, rather than uncombusted epoxy. The low Hc and char yield for AT verifies that it has potential as a gas-forming agent in intumescent formulations.


**Table 1.** Microcombustion analysis of epoxy + lignosulfonate samples.

#### **3.2. Variability of lignosulfonate**

The flammability of these materials was re-examined using cone calorimetry. The shape and relative heat release values mirrored the mass loss rate curves obtained using the gasification apparatus for all samples. The strong correlation observed indicates the reduced flammability when adding lignosulfonate is almost entirely through a condensed phase mechanism. The cone calorimetry data are summarized in **Figure 1**. The peak heat release rate (PHRR), which estimates how intense a fire will be and can help predict the time to flashover, is reduced by 24 % by adding 10 % lignin, 34 % by adding 10 % REAX825E, and 36 % by adding 7 % REAX825E + 3 % AT. The addition of ML instead of AT leads to an increase in PHRR compared to pure epoxy. Melamine produces more gases, leading to more cracks to the char layer, reducing its protective ability. The total heat release is reduced by as much as 14 % when using REAX825E. Since MCC suggests that 30 % to 40 % of lignosulfonate will volatize during pyrolysis, this indicates an ability to prevent some of the epoxy from burning. One disadvantage to using lignin is that the time to ignition is significantly reduced. A number of factors contribute to the ignition time, including heat capacity, thermal conductivity, surface emissivity, oligomeric decomposition products, melt viscosity, and permeability of the solid. Pure epoxy is transparent with low surface emissivity, whereas lignin is brown or black in color with high surface emissivity. Accordingly, tign is reduced going from pure epoxy to epoxy + 10 % lignin to epoxy + 10 % REAX825E. The addition of gas-forming compounds, which degrade at temperatures lower than the pure epoxy, leads to a more porous solid as the sample heats up. So, it is not surprising that tign is further reduced going from epoxy + 10 % REAX825E to epoxy + 7 %

REAX825E + 3 % AT to epoxy + 7 % REAX825E + 3 % ML.

204 Composites from Renewable and Sustainable Materials

**Figure 1.** Cone calorimetry data for epoxy + lignosulfonate composites. Error bars represent 2σ.

Additional ethoxylated lignosulfonate was obtained from the same source. Composites were prepared and tested in a cone calorimeter. Even though the exact same product was used, the two lots had significantly different properties. As shown in **Figure 2**, lot #RI27 had a later ignition time, but a 10 % higher PHRR and THR than lot #NK22. The addition of ammonium tartrate lowered the PHRR and reduced the time to PHRR to the same extent, so its interaction with the salt did not change. Most of the difference is due to the natural variability of the lignin composition. The color of lot #RI27 was darker than that of #NK22. In addition, the number of phenolic groups, sulfate groups, or average molecular weight may be different, depending on the source of the lignin and sulfonation process. Although there are variations, the general properties and potential use of lignosulfonate as a flame retardant remain promising.

#### **3.3. Immobilization of lignosulfonate**

To prevent the migration of the lignosulfonate, it was epoxidated using epichlorohydrin. It was then used to prepare epoxy + 10 % eLS composites, which were still opaque, but lighter in color than the REAX825E composites. The addition of glycidyl moieties did add to the fuel content of the lignin. Using MCC, eLS was found to have Hc = 5.1 kJ/g, HRC = 72 J/g · K, Tpeak = 340 °C, and 44 % mass fraction residue. This is only a modest increase in flammability and the immobilization was expected to have a greater effect. As shown in **Figure 3**, by incorporating the lignosulfonate within the epoxy matrix, the char layer was more effective at reducing the heat released. The PHRR was reduced by 63 % and the THR was reduced by 27 % over neat epoxy. The one disadvantage is that the composite ignited the earliest of all composites tested. To improve the efficiency of the epoxidation process, the starting material was dissolved in ethanol and filtered to remove any insoluble material. The solubility of lignin in ethanol decreases as the molecular weight of lignin increases, so this likely removes the highest molecular weight lignin. This may account for the earlier time to ignition, though residual ethanol will also lower this characteristic. The char formed during the decomposition was thicker and did not peel away from the composite, which helps explain how it could effectively reduce the heat released during combustion.

**Figure 2.** Heat release rate as measured in a cone calorimeter of epoxy flame retarded with lignosulfonate, using two different batches of ethoxylated lignosulfonate.

The addition of lignosulfonate significantly changed the glass transition temperatures (Tg). The measured Tg were 81.2 °C for neat epoxy, 88.7 °C for epoxy + 10 % REAX825E, and 75.7 °C for epoxy + 10 % eLS. Increases in Tg are typically associated with enthalpic interactions between the filler and epoxy matrix. This may be π–π interactions between lignosulfonate and DGEBA or hydrogen bond interactions with the poly(propylether)diamine curing agent.

**Figure 3.** Heat release rate as measured in a cone calorimeter of epoxy + epoxidated lignosulfonate.
