Abstract

Raw material and rheology sample preparation

Am-LS lye material (∼4 liters) was sampled at LTF and immediately cooled on ice for shipment to LignoTech USA (Rothschild, Wisconsin). Materials were kept refrigerated (∼10°C) prior to sample preparation. The thick Am-LS lye had a pH of ∼1.8 to 2.2 and a dissolved solids content of ∼61 to 62 wt%.

To prepare samples for rheological analysis, thick liquor was heated to ∼50°C and neutralized with aqua ammonia (28% NH₃ in H₂O) to a desired pH. Following neutralization, the dissolved solids content was gravimetrically measured by drying a sample in an oven at 110°C for 1 hour. Dissolved solids results were considered prior to further addition of water to dilute the material to the desired levels. For the samples that included a cross-linking agent (glyoxal [40 wt% aqueous solution], Sigma-Aldrich), glyoxal was added after neutralization but prior to additional dilution due to its water content.

Polymerization rheology

Additional analytical techniques

Quantities of monomeric sugars and molecular weights of progressively cooked Am-LS samples were measured using an ion chromatography system calibrated with commercial standard monosaccharides. Molecular weight analysis was performed as described in the literature (Fredheim et al. 2002, 2003). Dissolved solids content, as indicated in the “Raw material and rheology sample preparation” section, were measured using the 1-hour ovendrying method. Finally, sample pH values were monitored using a standard electrode pH meter calibrated immediately prior to reading with valid pH 4, 7, and 10 calibration standards.

Rheological observation of polymerization

Rheology was employed to quantify the polymerization behavior of Am-LS, which enabled real-time observation and quantification of the polymerization reactions in the form of a viscosity response. The rheometer setup forces an Am-LS sample to polymerize in a closed and controlled environment that can be cleaned afterward and reset for analysis of additional samples. This rheometer configuration essentially serves as a pressurized reaction vessel with the added capability of being able to “look inside” through the lens of accurate viscosity readings, beyond only recording temperature and pressure.

As Figure 3 showed, the onset of polymerization is observed on the rheometer as an increase in measured viscosity at an exponential rate. Description of the polymerization behavior observed using the rheometer is hereafter interchangeably referred to as gelling. Use of exchangeable vocabulary is founded upon the physical nature of the lignosulfonate in the measuring system after cooling, since it has physically gelled as a function of lignin polymerization. A collage of images of gelled Am-LS withdrawn from the measurement system after having cooled is compiled in Figure 4. The black lignin gel is a nonflowing and moldable semisolid. Of the gelled material after cooling, only a small (yet unknown) portion remains soluble. The remainder of the mass can be described as flakes and globular agglomerates. Future work will seek to better describe the gelled material; however, no further analyses of its properties are included in this work.

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It became crucial to identify a time point on the Am-LS exponential viscosity curve that is able to quantify the onset of polymerization, but early enough to avoid formation of a cooked material that might problematically befoul the measuring apparatus. For this reason, a standard test was programmed that halted measurement of viscosity at the analysis temperature once recording a viscosity reading ≥150 mPa s. By defining a cutoff point, it became possible to define two time-based responses. The simplest response involves the time taken for a sample to reach said cutoff point (150 mPa s), defined as the “gel time.” All materials withdrawn from the cylinder at this point appear similar to what is shown in Figure 4. A second response intended to quantify the rate of gelation was also defined. Specifically, the “gel rate” was defined as the time taken for a sample to reach 150 mPa s subtracted from the time taken to reach 100 mPa s. It is thought that this gap in time could provide insight into the rate at which polymerization proceeds.

It is important to note that the defined points on the viscosity curve were not adjusted according to varying analysis temperature. In reality, the gel time endpoint of a sample at 150°C would translate to a higher viscosity than a sample that reaches the same endpoint at a lower temperature. However, this was deemed negligible on account of the minor difference in measured viscosity once temperatures rose above ∼120°C. For instance, a sample at 52 percent dissolved solids saw its viscosity decrease from 10.5 to 9.8 mPa s, while the cylinder temperature ramped from 135°C to 150°C. This minor difference (0.7 mPa s) suggests that the difference between 150 mPa s as measured at 135°C and 150°C is likely within the instrument's range of error; thus, it is not required to be corrected for.

Designed experiment and the predictive models rendered

After gaining an understanding of how polymerization translates to responses from the rheometer, the next task was to develop and perform a designed experiment capable of quantifying the influence of key parameters upon Am-LS gelation. The factors included were pH, percent dry matter, temperature, and addition of glyoxal (cross-linking agent). The assay responses were the previously discussed gel time (defined as time to 150 mPa s in h) and gel rate (defined as [time to 150 mPa s − time to 100 mPa s] in h). Table 1 details the experimental layout as well as the parameters of the factors studied in the designed experiment. Four identical center point samples were also analyzed to estimate for response surface curvature.

Table 1 Randomized two-level factorial experiment for evaluating ammonium lignosulfonate polymerization.^a

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The effect of glyoxal addition was integrated into the design in an effort to identify the contribution of potential cross-linking agents. Glyoxal itself is a di-aldehyde that can facilitate acid-catalyzed cross-linking through a mechanism similar to formaldehyde (Bialski et al. 1986). Glyoxal was used as a cross-linker in place of formaldehyde, taking into consideration laboratory handling safety with respect to our rheometer configuration. In the event that Am-LS is to be implemented as a gelling component in formaldehyde-free systems, then the results from glyoxal addition will be demonstrative of implementation of cross-linking agents not subject to stringent regulation. Some examples of these types of cross-linkers include glutaraldehyde, succinaldehyde, and 2,5-diformylfuran.

After completing the schedule of rheology analyses across the 20 samples designated by the experiment, the analytical software was able to generate statistically significant models for predicting the two analyzed responses. Prior to presenting any results, it is first important to note that no significant response curvature was found for either response (data not shown). In addition, the software advised that certain mathematical transformations to the data would improve the model accuracy for both responses. Based on these suggestions, gel time responses underwent an inverse data transformation, while gel rate responses were treated with a log₁₀ data transformation. Highlights from the software-provided ANOVAs for both responses are shown in Table 2. Both models computed by the software were significant; however, the amount of significant two-factor interactions differs between both responses.

Table 2 Analysis of variance for gel time and gel rate.

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Understanding the gel time response

Table 2 shows that all four primary factors significantly contribute to gel time. Temperature was revealed to be involved in significant two-factor interactions with every other factor: higher temperature and lower pH decreases gel time, higher temperature and higher dry matter decreases gel time, and higher temperatures plus the addition of cross-linker decreases gel time. These relationships indicate that the most influential factor promoting polymerization of Am-LS is the temperature at which it is held, since it significantly interacts with every other controllable parameter of a given Am-LS material. For instance, gel time decreases by ∼5.5 hours when moving from 135°C to 150°C at 58 percent dry matter, pH 6, and no added cross-linking agent. In comparison, gel time only decreases by ∼4 hours when changing glyoxal addition from 0 up to 3.9 percent at 135°C, 58 percent dry matter, and pH 6.

A significant two-factor interaction was also identified between pH and glyoxal dosage. This was expected, since cross-linking reactions involving glyoxal require protonation of the carbonyl oxygen. After protonation, the partially positive carbonyl carbon can then undergo electrophilic aromatic substitution with a nearby free aromatic carbon. Given that the pKa of protonated aldehydes is ∼−8, it is logical that lower pH correlates with greater response influence by aldehyde-bearing cross-linkers. It is also worth noting that greater amounts of hydronium ions are auto-ionized in the aqueous system at increasing temperatures, which provides an explanation for the significance of the temperature-glyoxal interaction (A to D, P = 0.0221).

The results obtained were not surprising, but rather demonstrative of the significance of each of the tested variables around controlling the time required for Am-LS to undergo gelation-inducing polymerization. This information should prove valuable for: (1) future product development, (2) future technologies for creating specialty products, and (3) the necessary process parameters for avoiding Am-LS gelation.

Understanding the gel rate response

The gel rate response was defined with intent to describe the rapidity with which polymerization takes place as it begins. Gel rate's crude definition (time to 150 mPa s minus time to 100 mPa s in h) likely contributed to the model containing only a single significant two-factor interaction. In addition, a strong level of deviation was recorded for the four identical center point samples (average center point gel rate measured in quadruplicate: 0.12 ± 0.05 h). Nevertheless, the model predicting the defined gel rate is considered to be statistically significant (P = 0.0003) but it is driven by fewer factors than the model for gel time.

The primary takeaways regarding gel rate found in Table 2 include the revelation that the only critical factors are temperature, pH, and addition of glyoxal. Percentage dry matter was shown to have virtually no contribution to gel rate (P = 0.9401). Only a single two-factor interaction was found to be significant: the interaction between temperature and glyoxal addition. This information shows that the rate at which gelling proceeds is controlled by higher temperatures, lower pH levels, and levels of cross-linking agent addition (should they be added).

Gel rates ranged from 0.02 hour (1.2 min) to 0.73 hour (43.8 min). From a practical perspective, the extended gelation rates and the samples they were exhibited by could prove useful in instances where gelation is welcome, but a level of control on its speed is important. For example, enhanced oil recovery involving gel-prone Am-LS requires extended gel rates to allow for an Am-LS liquid product to be delivered to the destination where it is intended to fully gel (Felber et al. 1977; Lawrence and Felber 1981a, 1981b). This sort of requirement means use of an Am-LS that has been sequentially treated to lower gel time (a thermal priming step) and then modified (cooled and/or neutralized) to ensure the gel rate is slow enough to prevent the product from setting in unintended locations.

Thermal priming demonstration and investigation

After successfully constructing a predictive model for describing the gelation behavior of Am-LS, it next became necessary to further investigate the concept of priming Am-LS to create a stable material with a shortened gel time. This approach would be necessary for delivering Am-LS materials into applications that are not capable of inducing gelation from a standard Am-LS product by their own process dynamics. Furthermore, a priming step could potentially be implemented into an industrial process.

The rheometer was again used to demonstrate the concept of thermal priming. This demonstration involved thermally treating Am-LS samples from a master batch for fractions of the studied sample's gel time. After the predetermined time had passed, samples were cooled to monitor for viscosity stability. In addition, cooked liquid samples were analyzed for molecular weight, sugar content, and pH. The Am-LS makeup chosen for this analysis was customized to be as follows: pH 3, 55 percent dissolved solids, no added cross-linker, cooked at 135°C. This Am-LS formulation was chosen to be approximately “medium” in terms of reactivity toward polymerization, based off the predicted gel times and rates defined obtained from the response surface models. Importantly, the rheometer was programmed for these experiments to have a temperature ramp-up rate that was identical in time to what was manually recorded for a 2-liter Parr reactor vessel loaded with the same Am-LS: 20°C to 135°C in 1.43 hours, 80.4°C/h. The intent behind programming the rheometer to more accurately simulate a larger scale reactor was to move toward scaled-up production of thermally primed prototypes.

It was found that the gel time for the formulated Am-LS was 4.95 hours, a timespan that includes 1.43 hours of abated ramp-up. From this information, samples were cooked for fractions of this time: 3/8, 1/2, 5/8, and 3/4 of the gel time. These fractional times translate to 1.86, 2.47, 3.09, and 3.71 hours, respectively. After cooking, the samples were first analyzed for viscosity stability. This evaluation was intended to gauge whether simply cooling Am-LS after thermal priming was capable of stifling further polymerization of the material. It would also be possible to limit further reactions by means of dilution or neutralization. However, the rheometer is not configured in a manner that makes such demonstrations possible. Spray drying thermally primed Am-LS into powder could also serve as a means for halting further reactivity.

Figure 5 displays the rheology of the materials cooked for various fractions of their gel time. Inclusion of the temperature data on the secondary vertical axis is intended to explain that the sudden elevation in viscosity observed at specific times was solely induced by cooling the instrument, as opposed to a sudden onset of polymerization. The key takeaway from this demonstration is twofold: first, the viscosity of thermally primed samples at 70°C does increase in line with extended cooking times, as expected. The second and more critical conclusion is that despite the elevations in viscosity, these viscosities remain relatively constant over an hour after thermal priming. Average viscosity over the holding period is also listed in Table 3. Specifically, the viscosity of the solutions increased from 70 mPa s at 70°C for the uncooked material up to 2270 mPa s at 70°C for the material cooked for 3.7 hours. Results suggest that heating a material short of its gelation period can indeed produce a material with a newly shortened gel time as indicated by the drastic build-ups in viscosity that are beginning to be observed as cooking time extends. Furthermore, polymerization reactions can be stifled during storage and transit assuming the material was not too severely cooked and appropriate actions are taken to halt further reaction (e.g., dilution, cooling, neutralization, spray drying). Therefore, delivering products with a shortened gel time is indeed a feasible concept that warrants study at larger scales of production. It will also be important to take samples that have been primed and prove that their gelation can be reactivated in short time through additional heating.

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Table 3 Changes to ammonium lignosulfonate properties over the time course of thermal priming.

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After demonstrating viscosity stability, the samples were subjected to further characterization in an effort to better understand the effects of thermal priming. Some conventional methods of lignosulfonate characterization were applied, including measuring pH, lignosulfonate molecular weight, and monomeric sugar content. First, it was found that the pH of the sample progressively dropped as cooking time was extended, with the most significant drop occurring between the uncooked sample and the sample cooked for 1.9 hours (0.6 pH units). pH decline appeared to level out at pH 1.9 once the samples reached ∼3 hours of cooking. These results suggest that acidic compounds are being generated during the cook, and we conjecture that this effect is derived from degradation of the monosaccharides present. To confirm, the monosaccharide contents in each sample were also considered. Because this is a softwood lignosulfonate, the prevailing carbohydrates in the material are the hexoses glucose and mannose derived from glucomannan hemicelluloses. Xylose was next in abundance, followed by minor (<1.0 wt% on dry matter) quantities of rhamnose, galactose, and arabinose. Quantities of all of these sugars were also found to decline as cooking time progresses, with the most significant decrease taking place between the uncooked sample and the sample cooked for 1.9 hours. This trend matches up with what was observed for pH decline, possibly indicating that the sugars rapidly react to form a variety of pH-lowering byproducts. The most reactive sugar was xylose, which experienced a relative decrease of ∼72 percent over the first 1.9 hours of cooking. Furthermore, xylose content was decreased by ∼94 percent over the course of the cooks. All other sugars only saw their abundances reduced by ∼65 to 75 percent in the sample cooked the longest time. These changes suggest that the generation of furfural from dehydration of xylose is likely a major reaction taking place, presenting a chemical species into the system that may be reacting with lignin as a hydrophobic cross-linker. Future work should seek to characterize some of these sugar-derived byproducts, including the presence of sugar-derived furans.

It is important to note that while the sugar contents are greatly diminished at the conclusion of the longest cook conducted, they are not completely removed. Instead, it could be that at a certain point the majority of the reactivity taking place occurs around the lignosulfonate molecules. Understanding changes to lignosulfonate molecular weight can help identify at which point the lignin begins undergoing significant polymerization, whether it be interlignin polymerization, lignin reacting with the previously discussed sugar byproducts, or a combination of the two. Table 3 shows that the molecular weight certainly increases over prolonged cooking times; however, the greatest increases are observed in the final intervals of cooking time (3.1 vs. 3.7 h). This finding is in agreement with previous conjecture concerning the order in which the components of Am-LS react: first the sugars followed by the lignosulfonates. A plot comparing Mw_w and total sugar content over cooking time is included as Figure 6, which clearly demonstrates the rapid increase in lignosulfonate's molecular weight beginning once sugar content was decreased by ∼50 percent.

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In all, a significant amount of elucidative information was obtained through the thermal priming exercise. First, a proof of concept was demonstrated for thermal priming along the basis of sample rheology, where it was found that reactivity can be induced in Am-LS materials and then stifled by cooling. Next, insight was gained into the interplay between sample pH, sugar content, and lignosulfonate molecular weight over the time course of a cook. It was found that these three properties are likely interrelated and therefore warrant further investigation.

Commercial applications of interest

It is most important to translate the findings of this work into applications where this chemistry can be manipulated into performing functions that the existing lignosulfonate products are unable to perform. The first conceivable application where this phenomenon could prove beneficial is as a hydrophobic pellet or agglomerate binder. Specifically, Am-LS could be subjected to a predetermined thermal priming regime at a low pH (and possibly with an added cross-linker) to reach a point short of initiating polymerization. Through cooling, pH elevation, dilution, and/or spray drying, this product could then be implemented into a pelleting or agglomeration process, upon which the heat of the process would immediately restart the polymerization process. Some of the industries where hydrophobic pellet or agglomerate binders would prove advantageous include aquaculture, agrochemicals, wood, and metallurgy wastes (Blackmore 1986, Buzulukov et al. 2000, Komarov and Fomin 2002).

Another application for this technology is enhanced oil recovery. This is an application where Am-LS has been used in the past (Felber et al. 1977; Lawrence and Felber 1981a, 1981b; Mickus et al. 1988; Hornof 1990; Popova et al. 1991; Gasumov et al. 2009). Specifically, a certain formulation of Am-LS product is piped down into the porous earth and allowed to gel within and effectively seal the subterranean fractures that contribute to lower levels of crude oil recovery. An Am-LS product for this application would have to be formulated to not be prone to gelation during downward piping but have enough polymerization-inducing properties to allow for gelation to occur at the temperatures and pressures experienced underground.

It is also worth investigating whether a gel-prone Am-LS could serve as a sustainable alternative to low-grade adhesives. It is possible that adhesives not intended to be long-lived and/or remarkably strong could be substituted with a well-formulated Am-LS product. This sort of compromise could allow for Am-LS products to enter into new markets, mostly as environmentally friendly alternatives to higher-performing synthetic adhesives. Single-use products in particular come to mind, especially those produced by the papermaking industry. Furthermore, it is also possible that novel Am-LS products could be implemented as resin extenders in the adhesive systems used for wood board products like fiberboard, waferboard, oriented strand board, and particleboard (Philippou et al. 1982, Peng and Riedl 1994, Snopkov et al. 1995, Janowiak and Carlson 1998, Alonso et al. 2004, Yuan et al. 2014). These adhesive systems, typically involving urea-formaldehyde, phenol-formaldehyde, or pMDI systems, could warrant a lower dosage if a gel-prone Am-LS product is introduced to the fiber either prior to adhesive application or during adhesive application.