Depending on the source (softwood, hardwood or annual crop) or isolation method (kraft, organosolv, sulfite, soda or enzymatic-hydrolysis), there are significant variations in lignin structures and properties. In this work, phenol was entirely replaced with nine different lignin samples in phenolic adhesive formulations to produce lignin-based resins and adhesives. Properties of these novel lignin-based resins, as well as their starting lignin, were evaluated by advanced analytical methods to assess the suitability of each lignin for phenol formaldehyde (PF) adhesive application. The hydroxyl content of lignins was determined with phosphorous nuclear magnetic resonance spectroscopy (31P NMR) and p-coumaric and ferulic acid content with high performance liquid chromatography (HPLC). Properties of lignin-based resins and adhesives were evaluated for solid content, viscosity, gelation time, pH, and free formaldehyde content using the appropriate ASTM standard test methods. Results showed that lignin samples with higher p-hydroxyphenyl or higher p-coumaric acid contents were better candidates for replacing 100% of phenol in phenolic adhesive formulations. Two-way ANOVA statistical analyses showed that a biorefinery corn stover lignin was the best lignin for this application. Additionally, our results showed that replacing 100% of phenol with lignin in phenolic adhesive formulation would reduce the consumption of formaldehyde by 50% on weight basis.
Keywords: Lignin, Bioabsed adhesive, Phenolic resin, and Biorefninery.
Wood product industries widely use phenol formaldehyde (PF) adhesives as a binder in manufacturing of OSB and plywood (F.P.L, 1974). PF resins are classified as exterior adhesives due to their resistance to temperature, water, and weathering, but their low formaldehyde emissions make them also suitable for indoor applications (Yang et al., 2015). Two main types of PF resins are resol and novolac, whose differences depend on the reaction condition and phenol to formaldehyde molar ratios. When the ratio of phenol to formaldehyde (P/F ratio) is less than one and a base is used as a catalyst, resol is created, while novolac is formed when the ratio of phenol to formaldehyde is greater than one and an acid is used as a catalyst (Higuchi et al., 2001).
The fluctuations in the price phenol with oil and gas prices and long-term exposure to phenol throughout the production process has motivated many researchers and industries to work on phenol replacement, using renewable alternatives (Pilato, 2010). Lignin is an amorphous biopolymer with a backbone of phenyl propane units, making it an exceptional substitute for phenol in the production of phenolic resin (Banoub et al., 2015; Calvo‐Flores and Dobado, 2010; Delmas, 2008; Pilato, 2010). However, lignin’s high molecular weight, low reactivity, and high polydispersity restrict application of lignin in phenol formaldehyde adhesive production (Hon, 1995). Along with its high molecular weight, the complex 3D structure of lignin limits the accessibility of its reactive sites by formaldehyde in phenolic resin formulation. The availability of aromatic C3 and C5 (ortho) positions in the aromatic structure of lignin plays an important role in defining the lignin reactivity towards formaldehyde. In syringyl units, both ortho positions (C3 and C5 positions) are occupied by methoxyl groups, while there are one and two free ortho positions in guaiacyl and p-hydroxyphenyl, respectively. The ratio of monolignols in lignin differs depending on their source (softwood, hardwood, or annual crop) and isolation method (kraft, organosolv, sulfite, soda or enzymatic hydrolysis). The syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units exist in both woody plants and annual crops at different ratios. Hardwoods contains both syringyl and guaiacyl units, while softwoods mainly contain guaiacyl units with traces of p-hydroxyphenyl units (Laurichesse and Averous, 2014). On the other hand, annual crops contain all the three lignin groups along with p-coumaric and ferulic acids.
Given that lignins from annual crops have higher amounts of p-hydroxyphenyl with two free ortho positions available for reaction with formaldehyde, they are better candidates for this application (Doherty et al., 2011; Hu et al., 2011; Wang et al., 2009). In general, lignin reactivity is lower than phenol due to its macromolecular structure, steric hindrance, and less reactive sites in the structure. Demethylation, phenolation in acidic or basic media, and methylolation techniques have been used by previous researchers to increase the reactivity of phenol with formaldehyde (Alonso et al., 2004; Alonso et al., 2005; Doherty et al., 2011; Hu et al., 2011; Lee et al., 2012; Lin et al., 2006; Mansouri et al., 2007; Shahid et al., 2014; Vázquez et al., 1997). However, any modification process requires additional energy, cost, and time, which is not desired by industry.
In previous studies, up to 50% of phenol has been replaced by unmodified lignin in the adhesive formulation of wood composite products without negatively affecting the mechanical properties of the panels (Khan and Ashraf, 2007; Wang et al., 2009). Zhao et al. (2016) have recently shown that panels made with adhesives synthesized from the substitution of 70% of phenol with a modified (phenolated) lignin can maintain their mechanical properties as required by Chinese standards. Tachon et al. (2016) also synthesized a phenolic resin replacing 70% of the phenol with unmodified organosolv wheat straw lignin, showing that the physical, chemical, thermal, and mechanical properties of their formulated resin was similar to the standard phenolic resin. In our previous study, we were able to replace 100% of phenol with an unmodified lignin. The resulting lignin-based adhesive had similar wet and dry shear strength to a commercial phenol resorcinol formaldehyde adhesive (Kalami et al., 2017).
The main objective of this present study was to test the reactivity of a wide range of lignins from different resources (hardwood, softwood, wheat straw, and corn stover) and isolation processes (kraft, organosolv, soda, sulfite, and enzymatic hydrolysis) toward formaldehyde to determine which lignin is most suitable to replace phenol in phenolic adhesive formulations.
2. Materials and Methods
In this work, nine different lignin samples were used as phenol replacements in phenolic resin formulation. The abbreviated name, biomass source, and extraction method of each lignin sample are presented in Table 1. All other chemicals and reagents were supplied by Sigma Aldrich or Fisher Scientific and used without further purification.
Table 1: Description of lignin samples.
2.2. Lignin Characterization
2.2.1. Moisture Content (%MC)
The moisture content of each lignin sample was measured gravimetrically by drying samples to a constant weight in an air oven at 80 °C and 100 °C for 3 h and 1 h, respectively. Since it has been reported that lignin might partially degrade at 100 °C (Comyn, 1990), the moisture contents of samples were also calculated at 80 °C for three hours. Briefly, about 0.5 g of each lignin sample (5 replicates) was placed in an aluminum pan, weighed, and heated as stated above. Pans were then removed and placed in a desiccator to cool before measuring their final weight. The corresponding weight loss was attributed to the moisture content, which was determined using the following equation:
% MC= (w1–w2)/ w2 × 100%
where w1 and w2 are the initial weight of sample (g) and the oven dry weight of sample (g), respectively.
2.2.2. Ash Content
The ash content of the lignin samples was gravimetrically determined according to TAPPI- T 211 om-93 method. First, crucibles were pre-dried to a constant weight using a Sybron Thermolyne Furnatrol muffle furnace at 250 °C, cooled to the room temperature in a desiccator, then weighed to the nearest 0.1 mg. About 1 g of each oven dried lignin sample (i.e. dried at 100 °C for 1 h) was transferred to a pre-weighed crucible and placed in the furnace. The temperature was initially kept at 250 °C for 15 min to prevent flaming followed by a calcination step at 525 °C for 4 hours, and cooling naturally to 100 °C. Then, the samples were transferred into a desiccator before measuring their weights. The ash content was calculated based on the following equation:
Ash content (%) = (weight of Ash (g) / weight of the oven-dried sample (g)) × 100%
2.2.3. Analysis of Mn, Mw, and PDI of Lignin
Molecular size distribution of lignin samples was determined using Gel Permeation Chromatography (GPC). Lignin samples were dissolved in THF (HPLC grade). Samples were then filtered using a syringe filter (PTFE, 0.45 μm). The filtrate was injected into the GPC system (Waters, Milford, MA, USA) which included a separation module (Waters e2695), column oven, PDA detector (Waters 2998), and refractive index detector (Waters 2414). The mobile phase was THF (HPLC grade) with a flow rate of 1 ml/min. The column used was 300 mm × 7.8 mm Ultyragel THF 500 Å from Waters. Polystyrenes with specific molecular weights (162, 370, 580, 1000, 1300, 2000, 3000, 5000, 7000, 10000 Da) were used as calibration standards. All molecular weights of samples were calculated using Empower GPC Software (Wu, 2003).
2.2.4. Elemental Analysis
The weight percent of hydrogen (H), sulfur (S), carbon (C) and nitrogen (N) contents of the lignin samples were measured by a CHNS Automatic Analyzer (CHNS 932, LECO, Atlantic Microlab, USA). The amount of oxygen was determined based on the sum of all elements plus ash and then subtracted from 100.
2.2.5. Quantitative 31P NMR Analysis
31P NMR analysis with Granata and Argyropoulos (1995) method was used to quantitatively measure the Phenolic hydroxyl content of lignin samples. Briefly, 500 μl of deuterated chloroform and anhydrous pyridine in a 1.0:1.6 (v/v) ratio were used to dissolve 30.0- 40.0 mg of lignin samples. Then, 22.0 mg/ml of cyclohexanol as an internal standard and 5.6 mg/ml of chromium acetylacetonate as a relaxation reagent were added, followed by adding 2-chloro-4, 4, 5, 5 tetramethyl 1, 3, 2 dioxaphospholane (TMDP) as a phosphitylating reagent. The 31P NMR spectra were obtained on a 500 MHz Bruker Avance spectrometer using 90° pulses angle and 10 s pulse delay for 256 scans at room temperature.
2.2.6. HPLC Analysis
Alkaline saponification was used to determine p-coumaric acid (p-CA) and ferulic acid (FA) content in lignin samples based on the procedure published by Crowe et al.( 2017). Briefly, 0.5 g of lignin sample was loaded in a pressure tube, with the addition of 25 ml of 3 M NaOH. The sealed tube was then transferred to a hot water digester (Model MK800D-2, M/K Systems Inc), and treated at 121 °C for 1h. After treatment, liquid samples were centrifuged at 10,000 rpm for 3 min (Centrifuge 5804 R, Eppendorf). In the next step, 1 ml of the liquor was then adjusted to about pH 1.5 with 72% (w/w) sulfuric acid, followed by centrifugation at 10,000 rpm for 3 min. The supernatant was analyzed by HPLC (Waters e2695) equipped with an Aminex HPX-87H column (Bio-Rad, Hercules) using an aqueous mobile phase containing 5 mM sulfuric acid and a 10% (v/v) acetonitrile. p-CA and FA quantification was performed using a photodiode array (PDA) detector (Waters 2414) with a UV wavelength of 315 nm.
2.3. Preparation of Phenolic Resins
Lignin-based phenolic resins were formulated by substituting 100% of phenol with different lignin samples. Lignin samples were mixed with 1 M NaOH and formaldehyde (37%) solution in a round-bottom three-neck flask equipped with a thermometer and a reflux condenser. The molar ratio of formaldehyde to phenolic hydroxyl in lignin was kept at 2:1 for all the formulated lignin-based resins, which resulted in different masses of the various lignins used listed in Table 1 based upon their phenolic hydroxyl content (mmol/g). The solution was stirred with the rate of 400 rmp at 65 ºC for 30 min, followed by adding 1 M NaOH solution. The system was then kept at 90 °C for two hours. After the reaction was complete, the system was cooled down to the ambient temperature and the resin was kept in the freezer to prevent further condensation reactions. Phenol formaldehyde resin was also prepared in the lab with the molar ratio of phenol to formaldehyde equals to 1:2 to compare the formaldehyde consumption in 100% lignin-based with pure phenol-formaldehyde resins.
2.4. Phenolic Resin and adhesive Properties
2.4.1. Solid Content
The solid content (SC) of liquid phenolic resins and adhesives was determined according to ASTM D4426-01 method. First, disposable aluminum pans were placed on a heater at 270 °C for about 15 seconds to flash off any excess oil from the surface, which could have remained during manufacturing. The preheated dishes were transferred into a desiccator for 5 minutes before measuring their weights. Then, about 1 gram of resin was placed at the center of each dish and kept in an oven at 125 °C for 105 min (5 replicates for each sample). After cooling the samples in the desiccator, their weights were measured, and solid contents were calculated using the following equation:
% SC= (weight of oven dried resin (g) / weight of initial resin (g)) × 100%
2.4.2. pH and Viscosity
Mettler ToledoTM S220Seven Compact digital pH meter was used to measure the pH of all resins and adhesives at room temperature by inserting the pH electrode into the samples. The pH was recorded to one decimal place after vigorously agitating the samples for 10 to 15 seconds. The viscosity of formulated resins and adhesives was measured at ambient temperature (23 °C) using a Discovery HR-2 Hybrid Rheometer at the shear rate of 1000 s-1.
Since there is not any available standard test method to determine the alkalinity of resins and adhesives, the alkalinity of phenolic resins was measured based on the standard test method for water alkalinity (ASTM D1067) which was recommended by Lorenz and Christiansen (1995). Depending on the expected amount of NaOH in phenolic resins, different amounts of PF resins ranging from 2 to 10 g were dissolved in 100 ml distilled water and then titrated to a pH 3.5 with 0.1 N HCl solution. The alkalinity was determined using the following equation:
% NaOH = (Vml HCl used × 0.4) / weight of PF resin (g)
2.4.4. Gelation Time
Gelation time is considered as the time when the resin changes from a liquid state to a flexible gel state. The gel time of resin samples with Pizzi and Mittal (2011) procedure was measured by visual assessment. Briefly, a glass test tube equipped with a glass rod was filled with about 1 g of resin and then immersed in a boiling water bath, the gelation time was then recorded from the time point that the test tube was submerged in the boiling water to the time point that the resin tends to hold the rod. During the test, the resin was stirred in the tube by raising and lowering the glass rod, and the stopwatch started immediately until the gel was formed.
2.4.5. Free Formaldehyde Content
The free formaldehyde percent of formulated phenolic resins was determined according to European Standard DIN EN ISO 9397. Potentiometric titration was applied to quantitatively measure the amount of HCl released through the reaction of hydroxylamine hydrochloride (NH2OH HCl) with formaldehyde toward the formation of formaldoxime (CH2 = NOH). 100 ml of distilled water was added to an adequate amount of phenolic resins, and pH was adjusted to 4 by 0.1 N HCl. Afterwards, 20 ml of 10% hydroxylamine hydrochloride was added to the resins and the solution was titrated to pH = 4 using 0.1 N NaOH solution. Finally, the percentage of free formaldehyde in the resin was calculated using following equation:
% CH2O = (Vml (NaOH) × N (NaOH) × 3.003)/ weight of resin sample (g)
2.4.6. Thermal Property
The curing temperature of the freeze-dried uncured lignin-based adhesives was measured by DSC 3, Mettler Toledo, using a single heating rate method. About 5 mg of adhesive samples was placed in a Tzero aluminum pan and heated from ambient temperature to 250 °C under the flow of nitrogen gas (15 ml/min) at a heating rate of 10 °C/min.
2.5. Resin and Adhesive Performance
2.5.1. Water Resistance
In order to evaluate the water resistance performance of PF resins, about 5 ml of formulated resin was mixed with 0.1 g sawdust in an aluminum dish. The sample was cured and dried in an oven at 130 °C for 1 hour. Afterwards, the dried sample was submerged into about 100 ml of distilled water at room temperature, and left there for 24 hours.
2.5.2. Plywood Preparation
To formulate adhesive (or glue mix as it is called by industry), 0.65 g of wheat flour was dissolved in 1.80 g distilled water and then alder bark (Modal) was gradually added to the solution and stirred for about 2 minutes. Afterward, 6.6 g of a prepared resin, and 0.03 g NaOH solution were added to the mixture and stirred for about five minutes. Finally, to test the performance of synthesized adhesives, shear lap samples were prepared by applying 0.10-0.12 g of formulated adhesives on one fourth of the surface of Douglas fir veneer samples (measuring 25.4mm × 102mm × 3.17mm) as indicated in ASTM D5868-01. In order to make plywood samples, A SATEC Universal mini hot press was used by pressing the two veneers at 175°C under 1400 kPa for 4 min.
2.5.3. Shear Lap Test
The Instron universal tensile strength machine was used to measure the lap shear strength of bonded veneers according to the ASTM D5868 – 01 methods. Ten replicates of lap shear strength measurements were conducted for each adhesive formulation. In the next step, the images of detached veneers were taken using Canon CS100 camera. For quantitative data, images were analyzed using ImageJ software following the procedure described by Nejad and Cooper (2011).
2.6. Statistical Analysis
To test the effects of different type lignin-based resins on shear strength of plywood samples, two-way analyses of variance (ANOVA) were performed at 95% confidence level using SAS software. Pearson’s correlation matrix (SPSS software) was also conducted to assess the correlation between properties of lignin samples, lignin-based resins and adhesives with adhesives performances.
3. Results and Discussion
3.1. Characterization Results of Lignin Samples
3.1.1. The properties of Lignin Samples
Physico-chemical characteristics of lignin samples are summarized in Table 2. The moisture content (MC) of lignin samples was higher at 100 °C (1h) compared to 80 °C (3h). This could be due to either the loss of some volatile organic compounds within lignin samples at 100 °C (Comyn, 1990), or because of heating lignin at 80 °C for 3 hours was sufficient to evaporate all the moisture from samples.
Measured lignin properties, values in parenthesis are standard deviation based on five replicate analyses.
Note: SW=Softwood, HW=Hardwood, CS=Corn Stover, WS=Wheat Straw, Kr=kraft, EH=Enzymatic Hydrolysis, OS=organosolv, So=Soda, Su=Sulfite.
As shown in Table 2, lignosulfonate (L6- Su-SW) and organosolv (L5- Os-HW) lignins had the highest and lowest amount of ash content, respectively. In general, the ash content variation in different lignin samples can be attributed to the original source and isolation method of biomass samples (Constant et al., 2016). Lignosulfonate and kraft lignins use sulfur dioxide (SO2) and sodium sulfide (Na2S) in their extraction methods, respectively. Therefore, the higher ash content in these lignins could be a result of the high amount of sulfur (Table 2), as well as sodium or potassium included after the neutralization of alkaline liquor during precipitation (Constant et al., 2016). Although lignins numbered 3 and 8 are both softwood kraft lignins, their inorganic mineral contents (or ash content) were different; this could be due to the different conditions (temperature, pH, or time) used to separate lignin from black pulping liquors. Applying the sulfur-free isolation technique like organosolv process can lead to lignin with low ash content, as indicated in as content analysis of lignins numbered 4, 5, and 7 that are all isolated through organosolv process. The organosolv corn stover lignin (L7- Os-CS) had higher ash content than softwood and hardwood organosolv samples (L4- Os-SW and L5- Os-HW). This might be explained by the fact that usually there are more silica in annual crops than in woody biomass (Sorek et al., 2014). Additionally, the relatively high amount of ash (1.78%) in wheat straw soda lignin (L9- So-WS) could be attributed to either high amount of silica in the wheat straw lignin (Sorek et al., 2014), or sulfur in sulfuric acid that is used to precipitate lignin in the neutralization step (Constant et al., 2016).
The structure of lignins and their molecular weights highly depend on two factors: lignin source and the extraction method. Due to the high solubility of lignosulfonate (L6- Su-SW) in water, it was not possible to perform the acetylation in order to measure the molecular weight of lignosulfonate sample. As seen in Table 2, two hardwood lignin samples used in this study (L1 and L5) had higher syringyl (S) units in their structures and lower molecular weights compared to other lignin samples as expected. The lower molecular weight of hardwood lignins is due to the occupation of two ortho positions in their S units by methoxyl groups, resulting in the formation of weak ether linkages (β-O-4, α-O-4, and 4-O-5) (El Mansouri et al., 2011). In contrast, guaiacyl and p-hydroxyphenyl with one and two empty ortho positions, respectively, can form a stable C-C bond during biosynthesis in the plant, which is not easy to cleave during the pulping process. Consequently, the molecular weight of lignins with high amounts of G or H units are expected to be higher than that of lignin with more S units (Brunow et al., 1999; El Mansouri et al., 2011). It should also be noted that even though two kraft softwood lignin samples ((L3 and L8) used in this study have higher guaiacyl and hydroxyphenyl (G and H) units than organosolv softwood lignin (L4), kraft lignins had lower molecular weights (L8 = 4600 and L3 = 4590 g/mol) and polydispersity index (PDI) than organosolv softwood lignin (5770 g/mol). This could be because the majority of β-O-4 linkages are broken during the kraft process when compared to a milder process such as organosolv. Molecular weight decreased in annual crop lignins in the following order: organsolv corn stover (L7- Os-CS) > enzymatic corn stover (L2- EH-CS) > soda wheat straw (L9- So-WS). Similarly, the lower molecular weight of lignin number 9 (soda wheat straw) compared to the other annual crop lignins could be explained by the higher syringyl functional units in this lignin.
Additionally, the enzymatic corn stover lignin had the lowest PDI, which shows that this lignin has more homogenous molecular size distributions than other lignins. This is a valuable indictor, which significantly affects the production of a resin with more reproducible and predictable properties.
3.1.2. 31P NMR and HPLC Analyses
The 31P NMR results of lignin samples were used to calculate the phenolic hydroxyl content of lignin (Table 3) to determine the amount of lignin needed to react with formaldehyde at a molar ratio of 1:2 (lignin: formaldehyde).
Generally, annual crop lignins have higher amounts of H units than woody lignins (Doherty et al., 2011). The higher amount of p-hydroxyphenyl in enzymatic corn stover lignin (L2- EH-CS) than all other lignin samples might be associated with the highly esterified p-coumaric acid (Table 2). In fact, the sharp peak at 137.7 ppm in Fig. 1 corresponds to p-coumaric acid and the broad peaks in the region between 138.5-137.3 ppm are assigned to p-hydroxyphenyl (El Hage et al., 2009). Although sample numbers L2 (EH-CS) and L7 (Os-CS) are both corn stover lignins, it can be seen how the extraction processes could have a major impact on the structure of lignin. As a result, the corn stover lignin isolated through enzymatic hydrolysis (H: 0.81 and G: 0.68 mmol/g) had much more reactive sites (H and G) compared to corn stover isolated through organosolv (H: 0.14 and G: 0.60 mmol/g). The obtained results indicate that probably more ether linkages are broken during dilute acid-pretreatment and enzymatic hydrolysis process (Crestini and Argyropoulos, 1997).
Phenolic acids and phenolic hydroxyl contents of lignin samples obtained by HPLCand 31P NMR, respectively.
a Values in parenthesis are standard deviation based on five replicate analyses.
Note: SW=Softwood, HW=Hardwood, CS=Corn Stover, WS=Wheat Straw, Kr=kraft, EH=Enzymatic Hydrolysis, OS=organosolv, So=Soda and Su=Sulfite.
As expected, softwood lignins contain more guaiacyl units, but the amount varies based on isolation methods. The kraft lignins had higher G units than organosolv and lignosulfonate softwood lignins. The two softwood kraft lignins (L3- Kr-SW and L8- Kr-SW) contained almost an equal amount of both H and G units, but the amount of S unit in lignin L8 (0.56 mmol/g) was twice that of L3 (0.28 mmol/g). This difference may be due to the different type of softwood used in these two lignins or different extraction conditions (time, temperature or pH). The kraft pulping process has been reported to induce the most condensed phenolic groups than other isolation methods (Crestini and Argyropoulos, 1997). Our data also show that the amount of condensed phenolic (Table 3) was much higher in kraft softwood and hardwood lignins (L1, L3, and L5) than others. HPLC results showed that phenolic acids only existed in two annual crop lignins (L2- EH-CS and L9- So-WS). Ferulic and p-coumaric acids are covalently linked to the lignin side chains via ether and ester bonds, respectively. The higher amount of p-coumaric acid (p-CA) in enzymatic hydrolysis corn stover lignin (L2) versus soda wheat straw lignin could be attributed to the saponification reaction that occurs during alkaline (soda) isolation method resulting in removal of p-CA acid from biomass (JÚNIOR, 1985).
Fig 1. Quantitative 31P NMR of L2- EH-CS.
3.2. Resins Characteristics and Performances
Solid content, pH, alkalinity, viscosity, gelation time, and free formaldehyde content of prepared lignin-based resins data are summarized in Table 4.
Measured properties of formulated lignin-based resins, average (standard deviations) based on five replicates.
a PRF: Commercial phenol resorcinol formaldehyde.
Note: SW=Softwood, HW=Hardwood, CS=Corn Stover, WS=Wheat Straw, Kr=kraft, EH=Enzymatic Hydrolysis, OS=organosolv, So=Soda, Su-Sulfite and PRF=Phenol Resorcinol Formaldehyde.
To have high water resistance and to avoid precipitation of lignin samples, the pH of all lignin-based resol resins was maintained in the range between 9-11 (Pizzi and Mittal, 2003). Overall, lignin-based resins with higher pH had higher viscosities and shorter gelation time (Table 4). For instance, organosolv hardwood lignin (L5- Os-SW) with pH 10.9 had the highest viscosity (440 mPa. s) and shortest gelation time (6.2 min). This observation is in agreement with previous reported studies in which an increase in the pH of the resin has shown to increase the resin viscosity and reduce the gelation time, causing a rapid thickening (Dunky, 2003). The alkalinity of our lignin-based phenolic resins ranged from 1.9 to 2.7%. Lignin-based adhesives formulated with kraft and soda lignins had much higher alkalinity than the others, resulting from sodium hydroxide, sodium sulfide, and sodium carbonate used in the kraft and soda pulping processes.
Additionally, our results showed that lignins with higher reactive sites (H and G units, Table 3) were more reactive toward formaldehyde and resins made with these lignin samples had less remaining free formaldehyde following the formulation reaction (Table 4). In contrast, lignins containing higher syringyl content had higher free formaldehyde content. For example, the hardwood lignins with high syringyl units (L1- Kr-HW = 2.40 and L5- SW-HW = 1.55 mmol/g) had the lowest reactive sites (low p-hydroxyphenyl and guaiacyl content), and thus had the highest free formaldehyde contents (L1 = 2.4% and L5 = 1.2%). Lignin L2, an enzymatic corn stover lignin, had the highest hydroxyphenyl content (H = 0.81 mmol/g) and lowest free formaldehyde content (0.1%). This indicates that more formaldehyde took part in the hydroxy-methylated reaction during resin formulation; therefore, the unreacted formaldehyde was lower in the developed lignin-based resin. The second most reactive lignins were the two softwood kraft lignins (L3 and L8), which had the highest guaiacyl content (L3 = 1.92 and L8 = 2.18 mmol/g) and the lowest free formaldehyde content after lignin-2 (L3 = 0.3%, L8 = 0.5%). The existence of one free ortho position in guaiacyl structure makes softwood lignins more reactive toward formaldehyde; thereby, resulting in a resin formulation with lower free formaldehyde content.
Another important parameter that effects adhesive performance and its penetration into the wood is the viscosity (Haupt and Sellers, 1994). The correlation analysis results showed that there is a strong positive relationship (r = 0.93) between lignin molecular weights (Table 2) and the viscosity of developed lignin-based resins; it should be noted that L5 was removed from the regression analysis as it was an outlier. The lower viscosity of kraft hardwood lignin (L1) and higher viscosity of organosolv lignin (L7) is consistent with previous studies (Christiansen and Gollob, 1985; Pang et al., 2017).
Solid content is also another important factor in the formulation of phenolic adhesives, since it can significantly affect the spread rate of the glue and the final cost calculation (Haupt and Sellers, 1994). The solid content of lignin-based resins ranged from 15 to 29%. Unlike phenol formaldehyde resin (Hon, 1995; Shahid et al., 2014), there was not a linear correlation (r = 0.15) between the molecular weight of lignin samples (Table 2) and the solid contents of lignin-based resins (Table 4).
The water immersion test consisting of a mixture of cured resins and sawdust was a simple method to quickly identify the water-resistance of lignin-based resins in this prototype study. Resin prepared from biorefinery corn stover lignin (L2- EH-CS) showed the highest water resistance and remained intact even after 2 weeks of water immersion. High water resistance can be explained by the fact that the resin made with this enzymatic corn stover lignin (L2- EH-CS) had higher reactivity toward formaldehyde, which resulted in a higher degree of crosslinking when cured.
Physical, chemical, and thermal properties of lignin based phenolic adhesives are measured and presented in Table 5. The presence of at least one distinctive exothermic peak in DSC diagrams is a characteristic of curing or crosslinking of resol phenolic adhesives, as a result of condensation polymerization reactions in the adhesives (Chai et al., 2016; Christiansen and Gollob, 1985).
Characteristics of formulated lignin-based adhesives, average (standard deviations) based on five replicates.
a PRF: Commercial phenol resorcinol formaldehyde.
Note: SW=Softwood, HW=Hardwood, CS=Corn Stover, WS=Wheat Straw, Kr=kraft, EH=Enzymatic Hydrolysis, OS=organosolv, So=Soda, Su-Sulfite and PRF=Phenol Resorcinol Formaldehyde.
The main exothermic peak of all formulated lignin-based adhesives ranged between 121 °C to 168 °C (Table 5) which is less than the curing temperature of the commercial phenol resorcinol formaldehyde adhesive at 195 °C used in this study. The curing temperature of 100% lignin-based resins developed in this study was lower than previous studies (Fechtal and Riedl, 1993; Kalami et al., 2017; Mattos et al., 2016; Nair et al., 2001; Sarkar and Adhikari, 2000) in which phenol was partially replaced by lignin. Since the concentration of free reactive sites in all the lignin samples used in this study were the same, the curing temperature varies in prepared lignin-based adhesives. The hardwood lignins (L1- Kr-HW and L5- Os-HW) had the highest syringyl content (L1 = 2.40 and L5 = 1.55 mmol/g) in which both ortho positions are occupied by methoxyl groups. As a result, there were more free formaldehyde in the adhesive formulation that lead to lower curing temperatures (Christiansen and Gollob, 1985). On the contrary, L2 (EH-CS) contained the highest p-hydroxyphenyl (0.81 mmol/g) with two free ortho positions to react with formaldehyde. Consequently, adhesives made with L2 had the least free formaldehyde content and the maximum curing temperature (168°C). This indicates that by slightly increasing the amount of formaldehyde we can significantly reduce the curing temperature to meet the temperature required for curing of plywood or OSB by industry.
DSC curing plots of biorefinery corn stover and softwood kraft lignin (L3- Kr-SW) are shown in Fig. 2. The broad exothermic peak of both lignin-based adhesives (L2- EH-CS and L3- Kr-SW) in comparison to the commercial adhesive (PRF) could be attributed to either a higher molecular weight of lignin compared to phenol, or the overlapping of curing reaction with the sharp endothermic peak from the vaporization of produced water (Srinivasan and Bandyopadhyay, 2016).
Fig 2. DSC thermograms of L2- EH- CS, L3- Kr- SW, and commercial adhesives.
3.3.2. Adhesives Performance
The results of the dry shear strength of plywood specimens prepared from different lignin-based adhesives and the commercial PRF adhesive are shown in Fig. 3. The shear strength data, percentage failure (based on image analysis results), and Tukey grouping statistical analysis results are also presented in Table 6. In addition, the p-values results of two-way ANOVA test for the shear strength data of phenolic adhesives are shown in Table 7.
Fig. 3. Shear strength of 100 % lignin-based PF adhesives compared to PRF adhesive.
Note: SW=Softwood, HW=Hardwood, CS=Corn Stover, WS=Wheat Straw, Kr=kraft, EH=Enzymatic Hydrolysis, OS=organosolv, So=Soda, Su-Sulfite and PRF=Phenol Resorcinol Formaldehyde.
As can be observed in Tables 6 and 7, based on two-way ANOVA, statistically there is no significant difference between the mean shear strength of commercial PRF (3.6 MPa) and that of the next highest shear strength of the developed 100% lignin-based adhesive of enzymatic corn stover lignin L2 (3.2 MPa). It should be noted that there is no significant difference between adhesives marked with the same letters.
The two-way ANOVA results (Table 7) also indicate that resin types have a significant effect on the shear strength of the adhesives (p < 0.05), while the amount of resin does not have significant effect on adhesion strength. Also, there is no significant interaction effect between the amount and type of resins (p > 0.05).
Overall, the statistical analysis result of shear strength data confirms that our initial hypothesis that the lignin samples with higher p-hydroxyphenyl and guaiacyl content, which have two and one free ortho positions in their structures to react with formaldehyde are more suitable for formulating lignin-based phenolic adhesives was correct.
|Resin x Amount||27||0.1230|
Table 8 shows the Pearson’s correlation matrix between the properties of lignin-based resin, lignin-based adhesive, and the phenolic and ash content of various lignins. Each random variable (Xi) in the table was correlated with other values (Xj). The positive correlation coefficient (r) corresponds to a direct relationship between two parameters, and the negative r indicates that when one property increases the other decreases. In Table 8, those values with p-value of less than 0.05 and 0.01 are shown with bold font indicating confidence interval of 95% and 99%, respectively.
There is a significant positive correlation between adhesive shear strength with adhesive curing temperature (p < 0.05, r = 0.75), the p-hydroxyphenyl content of lignin samples (p < 0.05, r = 0.78), and phenolic acids content of lignin samples (p-CA: p < 0.05, r = 0.75 and FC: p < 0.05, r = 0.74). The statistical results confirm that lignins with higher p-hydroxyphenyl and p-coumaric acid content are better candidates for replacing phenol in PF adhesive formulations. Also, there was a strong negative correlation (r = −0.86) between adhesives shear strength and the free formaldehyde content of resin, indicating that resins with lower residual free formaldehyde contents formed stronger adhesives. Additionally, there was a strong negative correlation (r = −0.94) between adhesive curing temperature and syringyl content of the lignin samples (p < 0.01, r = −0.94). As predicted hardwood, lignins with higher syringyl content had more residual free formaldehyde content (low reactivity with formaldehyde); thus, they were not suitable for this application.
Moreover, our statistical analysis results showed that there was no significant correlation between adhesive shear strength and lignin ash content (p = 0.51, r = 0.28), which means the developed lignin-based adhesives were not sensitive to the ash (sulfur) content of the lignin. This is particularly important for lignin producers since high purity lignin is not a requirement for favorable lignin-based adhesives for this application.
Notably, the amount of formaldehyde consumption was nearly decreased by half (50% on a weight basis) when substituting 100% of phenol with lignin in PF adhesive formulation. For instance, in 1 g of developed lignin-based resin using the corn stover enzymatic lignin (L2- EH-CS), only 0.35 g of formaldehyde was required for the optimal 1:2 molar ratio of phenolic hydroxyl content to formaldehyde. Meanwhile, for 1 g of phenol formaldehyde resin developed in the lab, 0.63 g of formaldehyde was required to achieve the 1:2 molar ratio.
Correlation matrix showing Pearson’s r for the properties of lignin-based resin, lignin-based adhesive, and the phenolic and ash content of lignins.
SS: Shear Strength, R pH: Resin pH, G T: Gelation Time, F: Free Formaldehyde Content, CT: Curing Temperature, S: Syringyl, G: Guaiacyl, H: Hydroxy Phenyl, T ph: Total Phenolic, FA: Ferulic acid, and p-CA: p-Coumaric acid.
Depending on the source of biomass and isolation process, the structures and properties of lignin highly differs. In general, lignin with a closer backbone structure to phenol are more suitable for replacing 100% of the phenol portion in the phenolic adhesive formulations. The statistical analysis of shear strength data of lignin-based adhesives shows that 1) lignins with high concentration of p-hydroxyphenyl or p-coumaric acid (mainly lignins from annual crops) are best candidates for formulating lignin-based phenolic adhesive; 2) hardwood lignins which are rich in syringyl groups whose two ortho positions are occupied with methoxyl groups are not reactive toward formaldehyde, thus not suitable for this application; and 3) softwood lignins with high guaiacyl content have the potential to replace 100% of phenol in PF formulation, but more in depth research is needed to optimize both the reaction process and adhesive formulation to improve adhesive performance.
In addition to replacing 100% of phenol with a sustainable raw material (lignin), our study showed that on a weight basis the formaldehyde consumption is also reduced by about 50%. This means that the developed 100% lignin-based adhesive not only has similar performance as that of commercially formulated phenol resorcinol formaldehyde, it is significantly less toxic, less expensive and more environmentally friendly than the current phenol formaldehyde adhesive on the market.
The authors would like to thank Maryam Arefmanesh for PNMR analysis, and Ravikiran Relangi for mechanical testing and image analysis. We are also grateful to technical advisors from Oxiquim, Willamette Valley, Arclin, GP Chemical, Henkel and Hexion for their invaluable technical advice throughout the project. We appreciate the funding support from the Wood-Based Composites Center, a National Science Foundation Industry/University Cooperative Research Center (Award 1624536-IIP), POET LLC and U.S. Department of Agriculture (USDA); Agriculture Research Service (ARS) under Agreement No. 58-0204-6-001.
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