In my research, I investigated the chemical degradation of polyurethane structural adhesives that are widely used in the assembly of modern vehicle traction battery packs. The rapid growth of the electric vehicle industry has led to a pressing need for sustainable recycling of end-of-life vehicle traction batteries. These batteries are typically bonded to metallic cooling plates and module frames by high-strength polyurethane structural adhesives, which are designed to withstand vibration, thermal cycling, and mechanical shock. Consequently, the removal of such adhesive becomes a critical bottleneck in the disassembly and recycling process. Traditional physical peeling, mechanical scraping, and thermal decomposition often damage the remaining cells or release hazardous gases. Therefore, I set out to develop a mild, efficient, and chemically controllable method for degrading and removing polyurethane structural adhesives from retired vehicle traction batteries.
The central target of my work was the alcoholysis of polyurethane. I first carried out a systematic study using sodium hydroxide as a conventional catalyst and diethylene glycol as the alcoholysis agent. Then I synthesized several functionalized ionic liquids and compared their catalytic performance with that of sodium hydroxide. Through this comparative approach, I intended to identify a catalyst system that not only accelerates the degradation of the polyurethane adhesive but also operates at sufficiently low temperatures to avoid damaging the valuable components of a vehicle traction battery.
Background and Motivation
The application of structural adhesives in vehicle traction battery modules is essential for improving mechanical integrity and thermal management. Polyurethane adhesives are used because of their excellent adhesion to aluminum and steel, high peel strength, high elongation at break, and fatigue resistance. In addition, polyurethane structural adhesives for electric vehicle batteries are often formulated with flame retardants, thermally conductive fillers, and aging-resistant polyols, which further complicate their removal. When a vehicle traction battery reaches the end of its useful life for automotive service, it still possesses roughly 70–80% of its nominal capacity. Thus, the battery is an attractive candidate for repurposing in stationary energy storage or secondary applications. However, to access the cells and modules, the adhesive layer must be effectively separated from the battery housing and cooling plates.
Several methods for adhesive removal have been proposed. Mechanical methods such as prying, cutting, or abrasive blasting often damage the soft aluminum shells of the battery cells and may cause short-circuiting or leakage of toxic electrolytes. Incineration of entire packs is highly detrimental because polyurethane burns to produce carbon monoxide, hydrogen cyanide, aldehydes, and ammonia, while the metal content becomes oxidized and difficult to recover. Ultrasonic cleaning is a non-destructive option, but it requires the pack to be highly water-resistant, which is rarely the case for standard vehicle traction battery assemblies. Because of these drawbacks, chemical degradation by alcoholysis has emerged as a promising route: the polyurethane network is broken down into soluble oligomers and monomers, allowing the structural adhesive to be rinsed away or mechanically wiped from the surfaces of the battery modules.
My first hypothesis was that a strong base, such as sodium hydroxide, could promote the alcoholysis of the carbamate and urea linkages in polyurethane. Diethylene glycol (DEG) was selected as the degradation solvent because of its low volatility, high boiling point, and strong ability to swell the cross-linked polyurethane network. Diethanolamine (DEA), on the other hand, was used as a comparison because its amino group can also attack the carbonyl carbon of the urethane linkage. During preliminary solubility tests I observed that DEG caused more pronounced swelling of the cured adhesive than DEA and water. This greater swelling rate was attributed to the ability of DEG to form hydrogen bonds with the urethane carbonyl groups, loosening the polymer network and allowing solvent molecules to penetrate more deeply into the bulk material.
Experimental Section
I prepared cured polyurethane blocks from two different two-part commercial systems, designated as black adhesive and white adhesive. These adhesives were supplied as part A (polyol or prepolymer) and part B (isocyanate hardener). I mixed the components in a 1:1 mass ratio and allowed the mixtures to cure for 24 hours under ambient conditions. After curing, I cut the solid polyurethane blocks into small rectangular pieces with similar mass and dimensions. For the three-brand study, I also obtained three commercial polyurethane AB adhesives used for potting, resin bonding, and general structural bonding, cured them by the same procedure, and used them as test specimens to evaluate the universal applicability of the developed degradation formulation.
All degradation experiments were conducted in a three-necked round-bottom flask equipped with a magnetic stirrer and a reflux condenser. The flask was heated by a digitally controlled oil bath. In a typical non-catalytic experiment, 60 mL of the alcoholysis agent was first transferred into the flask, followed by the addition of a known amount of catalyst. The mixture was heated to the desired temperature, and then a weighed piece of cured polyurethane was immersed in the hot solution. After a predetermined reaction time, the remaining solid was removed, washed with ethanol, dried, and weighed again. The mass loss was calculated by the following formula:
$$ \text{Mass loss} = m_1 – m_2 \quad \text{(g)} $$
where \(m_1\) is the initial mass of the cured polyurethane and \(m_2\) is the mass of the residual gel after the degradation test. When I needed to express a degradation rate relative to the initial mass, I used:
$$ \text{Degradation rate} \, (\%) = \frac{m_1 – m_2}{m_1} \times 100 $$.
In the swelling experiments, I immersed small rectangular polyurethane specimens in sealed vials containing 30 mL of water, DEG, or DEA. The vials were placed in a water bath maintained at 30 °C. The samples were removed after five days and then after seven days until the swelling reached equilibrium. The surface liquid was carefully wiped away, and the mass was recorded. The swelling ratio was calculated as:
$$ \text{Swelling ratio} \, (\%) = \frac{m_{\text{swollen}} – m_{\text{dry}}}{m_{\text{dry}}} \times 100 $$.
For the ionic-liquid study, I synthesized a series of Lewis-acidic ionic liquids based on imidazolium cations and metal chloride anions. Specifically, I reacted 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), 1-allyl-3-methylimidazolium chloride ([Amim]Cl), or 1-hexyl-3-methylimidazolium chloride ([Hmim]Cl) with anhydrous ferric chloride, cobalt chloride, or zinc chloride in different molar ratios. The reagents were mixed in a dry three-necked flask under nitrogen and stirred at 50 °C for five hours. The resulting homogeneous liquid was then dried under vacuum at 60 °C for 24 h to remove residual water. In this way I obtained ionic liquids labeled [Bmim][FeCl4], [Amim][FeCl4], [Hmim][FeCl4], [Amim][ZnCl3], and [Amim][CoCl3]. For a given anion fraction, the molar ratio of metal chloride to imidazolium chloride was set to 1:1 to yield the expected anionic species. I determined the composition by electrospray ionization mass spectrometry (ESI-MS). Positive-ion spectra showed the expected imidazolium cations at their corresponding m/z values, while negative-ion spectra confirmed the presence of tetrachloroferrate, trichlorozincate, or trichlorocobaltate anions. The ESI-MS results also revealed small fragment peaks, which may have arisen from reduction or collision-induced dissociation, but the dominant species were consistent with the desired ionic liquid structures.
In the catalytic alcoholysis experiments with ionic liquids, 40 mL of diethylene glycol was placed in the three-necked flask along with a specified mass of ionic liquid. The solution was heated to the target temperature, and then a weighed piece of cured polyurethane was added. After the designated time, the solid residue was extracted, cleaned, and weighed. For the recycling tests, after each reaction the remaining solvent and ionic liquid mixture were retained in the flask; a fresh piece of polyurethane was added, and the reaction was repeated without the addition of new catalyst. When the mass loss decreased significantly after twenty runs, I added fresh diethylene glycol to restore the liquid level and tested again until the activity could no longer be recovered.
I also performed experiments to remove adhesive from an actual bonded metal plate. In these tests, the metal plate with cured polyurethane adhesive on its surface was fully immersed in a solution of DEG containing 2 g of [Amim][ZnCl3] per 60 mL. The system was heated to 50 °C for 40 minutes. After the treatment, the adhesive swelled, softened, and could be scraped away with a rigid plastic spatula. This experiment simulated the practical conditions encountered in the recycling of vehicle traction battery modules, where sensitive battery housings cannot withstand high temperatures.
Sodium Hydroxide Catalytic Degradation of the Adhesive
Using sodium hydroxide as the catalyst, I performed the alcoholysis of polyurethane in two different alcoholysis media: DEA and DEG. In an initial set of experiments, I heated both the black and white adhesives in pure DEA at selected temperatures and recorded the time required for complete disappearance of the solid polyurethane. The results are summarized in Table 1.
| Temperature (°C) | Complete degradation time of black adhesive | Complete degradation time of white adhesive |
|---|---|---|
| 125 | 7.43 h | 8.1 h |
| 145 | 3.2 h | 3.6 h |
| 165 | 1.5 h | 1.7 h |
| 185 | 0.6 h | 0.5 h |
| 205 | 20 min | 18 min |
| 245 | 10 min | 5 min |
Clearly, an increase in the degradation temperature strongly accelerated the alcoholysis reaction. At 125 °C, the white adhesive required more than eight hours to be fully degraded, while at 245 °C complete degradation occurred within five to ten minutes. This behavior is consistent with the Arrhenius law, since higher temperatures increase the fraction of activated molecules and thus the number of effective collisions between the alcohol and the urethane bonds in the cross-linked polymer. The observed difference between the two adhesive formulations suggested that the white adhesive had a slightly higher degree of cross-linking or a different ratio of hard segments to soft segments, which slightly reduced the penetration rate of the solvent. In order to verify this, I carried out swelling tests. The results are presented below.
Table 2. Swelling ratio of black and white polyurethane adhesives after seven days of immersion at 30 °C.
| Solvent | Adhesive color | Initial mass (g) | Equilibrium mass (g) | Swelling ratio (%) |
|---|---|---|---|---|
| DEG | black | 0.298 | 0.305 | 2.35 |
| DEG | white | 0.469 | 0.475 | 1.28 |
| DEA | black | 0.301 | 0.306 | 1.66 |
| DEA | white | 0.536 | 0.542 | 1.12 |
| Water | black | 0.308 | 0.312 | 1.30 |
| Water | white | 0.525 | 0.530 | 0.95 |
The swelling experiments showed that DEG is the most effective swelling agent among the three liquids tested, while water induces very little swelling. The higher swelling rate of the black adhesive in DEG points to a lower cross-link density, which facilitates solvent uptake and explains why it is degraded faster than the white adhesive under the same conditions. From this point onward, I employed the white adhesive as the principal non-standard test specimen because it represents a more challenging material for chemical degradation.
I next examined the effect of varying the DEG molar fraction in a mixture of DEG and DEA. The experiments were carried out at 165 °C for one hour. The mass loss of the polyurethane decreased first and then increased as the DEG percentage increased from zero to 100%. This non-monotonic behavior is due to two different degradation mechanisms. When DEA is abundant, the primary amine groups of DEA undergo a nucleophilic addition to the carbonyl carbon of the urethane group, forming urea linkages and cleaving the network. As the DEA proportion decreases, the concentration of active amine groups is reduced, causing the mass loss to fall. However, when DEG becomes the dominant species, the hydroxyl groups—which are present in double molar quantity per molecule—become the main nucleophiles. The ether oxygen in DEG also promotes solvation of the polymer chains, enhancing the accessibility of the urethane bonds and resulting in a higher mass loss. In direct comparison, pure DEG gave a mass loss of 2.578 g after one hour at 165 °C, whereas pure DEA gave only 1.856 g. Because DEG is also less corrosive and less toxic than DEA, I selected DEG as the preferred degradation solvent for the rest of the research.
I then optimized the sodium-hydroxide-catalyzed alcoholysis in DEG by varying the reaction temperature, the NaOH dosage, and the time. Table 3 lists the effects of temperature at a catalyst loading of 0.2 g and a reaction time of 1 h.
| Temperature (°C) | Mass loss (g) after 1 h |
|---|---|
| 135 | 0.445 |
| 145 | 0.91 |
| 155 | 1.64 |
| 165 | 2.578 |
| 175 | 2.599 |
Because the difference between 165 °C and 175 °C was small, I used 165 °C in subsequent experiments to reduce energy consumption. The effect of NaOH dosage was then tested at 165 °C for 1 h. As shown in Table 4, the maximum mass loss occurred at a NaOH loading of 0.2 g.
| NaOH mass (g) | Mass loss (g) after 1 h |
|---|---|
| 0 | 0.54 |
| 0.1 | 1.63 |
| 0.2 | 2.58 |
| 0.3 | 1.98 |
| 0.4 | 1.56 |
Above 0.2 g, an excess of alkali may shift the equilibrium of the alcoholysis reaction toward the reverse direction or cause secondary saponification reactions that consume the carbonyl groups and form stable carboxylate salts, thereby retarding the desired fragmentation of the polymer. Thus, 0.2 g was the optimum for a 60-mL volume. Finally, I followed the time evolution of the mass loss at 165 °C with 0.2 g NaOH. Mass loss increased steadily from 0.891 g at 20 minutes to about 2.578 g at 60 minutes. The most rapid region was between 30 and 40 minutes, after which the reaction rate slowed because the concentration of unreacted urethane groups in the swollen residual material decreased. Therefore, in later comparative experiments for the ionic liquids I chose a reaction time of 40 minutes, which still lies in the region of high reaction rate but is sufficiently short for reasonable throughput.
I also wanted to prove that the optimum formulation—DEG with 0.2 g NaOH at elevated temperature—is universally applicable to various polyurethane adhesives. I therefore cured three different commercial AB polyurethane products, named PU1, PU2, and PU3, and measured the time needed for their complete degradation at temperatures of 135, 145, 155, 165, and 175 °C. At 165 °C, all three products were fully dissolved in less than 2.5 hours, while at 125 °C, complete degradation required nearly ten hours. This confirmed the broad applicability of the method, although not all adhesives degrade at the same speed. Differences in isocyanate source, polyol backbone, cross-linker content, and inorganic filler content strongly influence the alcoholysis kinetics.
To better understand the nature of the degradation products, I performed Fourier-transform infrared (FTIR) spectroscopy and viscosity measurements. The FTIR spectra of the degradation products obtained at different temperatures all exhibited broad absorption in the 3200–3600 cm-1 region, corresponding to O–H stretching vibrations, and strong bands around 2800–3000 cm-1 for C–H stretching. The peaks near 1000–1100 cm-1 are characteristic of C–O single bonds. The similarity of the FTIR spectra at different temperatures indicates that increasing the reaction temperature did not lead to fundamentally different reaction products; instead, it promoted deeper chain scission and reduced the average molecular weight of the products. In agreement with the FTIR result, viscosity measurements under steady shear flow demonstrated that the degraded products were Newtonian fluids in the tested shear rate range. When the degradation temperature was raised from 135 °C to 175 °C, the viscosity of the product decreased markedly, proving that higher reaction temperatures result in lower molecular weights.
Although the sodium hydroxide system was successful in degrading the polyurethane adhesive, it unavoidably required a temperature of at least 125 °C for reasonable degradation within practical times. This temperature is normally above the safe thermal limit for many vehicle traction battery cells. Therefore, the NaOH-based route, while useful for dismantling battery packs that have already been fully discharged and disassembled into bare metal plates, is not suitable for in-situ treatment of complete vehicle traction battery modules. This limitation motivated me to search for a more efficient catalyst that could promote the alcoholysis of polyurethane at substantially lower temperatures.
Ionic Liquid Catalyzed Degradation of the Polyurethane Adhesive
Ionic liquids are well known in my field for their tunable Lewis acidity, low vapor pressure, excellent thermal stability, and high solubility for polar polymers. In recent years, ionic liquids have been used as recoverable catalysts for the glycolysis and methanolysis of PET and polycarbonate. I decided to transfer this concept to the degradation of polyurethane structural adhesives for vehicle traction battery recycling.
The first series of experiments compared sodium hydroxide with two ionic liquids containing the tetrachloroferrate anion, [Bmim][FeCl4] and [Amim][FeCl4]. In these tests, I used 40 mL of DEG as the degradation solvent, a catalyst loading of 2 g, a reaction temperature of 170 °C, and a reaction time of 40 minutes. The conversion of the cured polyurethane was calculated by the mass-loss formula given above. The representative results are collected in Table 5.
| Catalyst | Degradation rate (%) after 40 min at 170 °C |
|---|---|
| NaOH (0.2 g) | 26.7 |
| [Bmim][FeCl4] (2 g) | 67.4 |
| [Amim][FeCl4] (2 g) | 69.5 |
Clearly, the ionic liquids were much more active than NaOH under comparable conditions. The higher activity of the ionic liquids can be explained by a synergetic activation mechanism. The negatively charged chloride or tetrachloroferrate anion interacts with the hydroxyl hydrogens of DEG, increasing the electron density on the hydroxyl oxygen atom and enhancing its nucleophilicity. In the meantime, the positively charged imidazolium cation coordinates with the carbonyl oxygen atom of the carbamate group. This coordination withdraws electron density from the carbonyl double bond, making the carbonyl carbon more electrophilic. The simultaneous activation allows the formation of a six-membered-ring transition state, which lowers the activation energy of the alcoholysis reaction. The reaction sequence can be drawn as follows:
$$ \mathrm{R^1\!-\!NH\!-\!CO\!-\!O\!-\!R^2} \;+\; \mathrm{HO\!-\!R^3\!-\!OH} \;\xrightarrow[\text{ionic liquid}]{\;170\,^\circ\mathrm{C}}\; \mathrm{R^1\!-\!NH_2} \;+\; \mathrm{HO\!-\!R^3\!-\!O\!-\!CO\!-\!O\!-\!?} \;+\; \mathrm{HO\!-\!R^2} $$.
The exact distribution of products depends on the stoichiometry and the degree of cross-linking, but the overall effect is the scission of the polyurethane main chain into smaller hydroxyl- and amine-terminated fragments that are soluble in DEG.
When comparing the two ionic liquid cations, [Amim]\(^+\) performed slightly better than [Bmim]\(^+\). This can be attributed to the presence of the allyl group in [Amim]\(^+\). The carbon–carbon double bond is electron rich; thus, the positive charge of the imidazolium ring is less evenly distributed and more available for localized electrostatic interaction with the carbonyl oxygen atoms of the polyurethane. The allyl group also exerts a smaller steric hindrance than a butyl group, allowing easier approach to the urethane bond. In contrast, the longer hexyl chain on [Hmim]\(^+\) tends to wrap around and shield the imidazolium ring, reducing its capacity to coordinate with the polymer network. For a fixed anion, the catalytic activity I found was in the order [Amim] > [Bmim] > [Hmim].
I investigated the effect of the molar fraction of the tetrachloroferrate anion in [Bmim]\(_x\)[FeCl4]\(_x\) mixtures by synthesizing ionic liquids with nominal anion mole fractions of 0.4, 0.5, 0.67, and 0.75. The degradation rate at 170 °C for 40 min is shown in Table 6.
| Molar fraction of [FeCl4]\(^-\) | Degradation rate (%) |
|---|---|
| 0.4 | 43 |
| 0.5 | 67.4 |
| 0.67 | 51 |
| 0.75 | 35 |
The optimum was found at an anion mole fraction of 0.5, which corresponds to a stoichiometric 1:1 cation-to-anion ratio. At this ratio, each cation can work together with one anion to activate both the alcohol and the urethane carbonyl group simultaneously. When the anion fraction was below 0.5, there were too few anions to form the transition state with DEG; when the anion fraction exceeded 0.5, the excess chloride or tetrachloroferrate anions may have solvated the cation too tightly and blocked the active coordination sites. This observation is important for designing reusable ionic liquid catalysts for vehicle traction battery adhesive removal.
Next, I varied the amount of [Bmim][FeCl4] added to 40 mL of DEG. The results, in Table 7, show that the degradation rate improved as the ionic liquid mass was increased from 0.5 g to 2 g, and then leveled off above 2 g.
| Ionic liquid mass (g) | Degradation rate (%) |
|---|---|
| 0.5 | 44 |
| 1.0 | 55 |
| 2.0 | 67.4 |
| 3.0 | 69 |
Once the number of catalytic sites saturates the accessible surface area of the swollen polymer, adding more ionic liquid does not substantially increase the reaction rate. Based on this, I used 2 g as the standard catalyst amount for the ionic liquid studies.
The choice of the degradation solvent again influenced the ionic liquid catalyzed reaction. In a side-by-side run at 170 °C for 40 min with [FeCl4]-based ionic liquids, DEG produced a degradation rate roughly 27% higher than that obtained with DEA. This difference is consistent with the stronger nucleophilicity of the hydroxyl oxygen when activated by the ionic liquid and with the lower tendency of DEG to form stable amide or urea side products. Consequently, all the remaining experiments were performed with pure DEG.
To identify the mildest temperature window, I measured the degradation rate at 80, 110, 130, 150, and 170 °C for the three catalysts: NaOH, [Bmim][FeCl4], and [Amim][FeCl4]. The results are presented in Table 8.
| Temperature (°C) | Degradation rate with NaOH (%) | Degradation rate with [Bmim][FeCl4] (%) | Degradation rate with [Amim][FeCl4] (%) |
|---|---|---|---|
| 80 | 1 | 9 | 11 |
| 110 | 4 | 27 | 31 |
| 130 | 9 | 45 | 52 |
| 150 | 16 | 58 | 61 |
| 170 | 26.7 | 67.4 | 69.5 |
These data show clearly that ionic liquids provide a considerably higher degradation rate than NaOH even at low temperature. In particular, at 130 °C, the [Amim][FeCl4] catalyst achieved a 52% degradation rate within 40 minutes, whereas NaOH achieved only 9%. At 80 °C, ionic liquids still caused visible swelling and softening of the adhesive, but not complete dissolution. This suggests that an ionic-liquid-based treatment can be tuned to operate at temperatures compatible with the structural components of a vehicle traction battery.
In order to understand the influence of the cation and anion structures, I extended the ionic liquid family to include [Hmim][FeCl4], [Amim][ZnCl3], and [Amim][CoCl3]. I compared the degradation rates at three temperatures, as shown in Table 9.
| Ionic liquid | Degradation rate at 130 °C (%) | Degradation rate at 150 °C (%) | Degradation rate at 170 °C (%) |
|---|---|---|---|
| [Amim][FeCl4] | 52 | 61 | 69.5 |
| [Bmim][FeCl4] | 45 | 58 | 67.4 |
| [Hmim][FeCl4] | 41 | 54 | 63 |
| [Amim][ZnCl3] | 56 | 65 | 72 |
| [Amim][CoCl3] | 48 | 60 | 68 |
For the iron-based ionic liquids with three different imidazolium cations, the order of activity was [Amim] > [Bmim] > [Hmim]. This trend is explained by charge localization and steric hindrance. The allyl cation [Amim] has a double bond that can participate in conjugation with the imidazolium ring, causing less uniform charge dispersion and hence stronger attraction of the carbonyl oxygen. The hexyl cation [Hmim] is sterically bulkier and therefore retards the formation of the activated complex. For a fixed [Amim] cation, changing the metal chloride anion changed the activity as [ZnCl3]\(^-\) > [CoCl3]\(^-\) > [FeCl4]\(^-\). This trend inversely matches the Lewis acidity of the corresponding metal halides. The lower Lewis acidity of zinc in [ZnCl3]\(^-\) means that the anion does not over-stabilize the negatively charged oxygen atom in the transition state, thus allowing a more balanced electron transfer. By contrast, the strongly Lewis acidic FeCl4 anion binds so tightly to the hydroxyl hydrogen or to negatively charged intermediates that the regeneration of the catalyst is slowed down. Therefore, I selected [Amim][ZnCl3] as the best ionic liquid for further application studies.
Softening and Removal from Bonded Metal Surfaces
A key practical target for the recycling of vehicle traction batteries is to loosen the structural adhesive from the aluminum or steel plate without causing mechanical damage. I simulated this process by bonding pieces of polyurethane adhesive onto a metal plate and then treating the plate with a solution of 60 mL DEG and 2 g [Amim][ZnCl3] at 50 °C for 40 minutes. Under these mild conditions, the adhesive did not fully dissolve but it became heavily swollen, soft, and rubbery. A hard plastic spatula could then easily scrape the softened adhesive from the plate, leaving a clean metallic surface. This experiment demonstrates that the ionic-liquid/DEG system can effectively lower the mechanical removal effort at low temperature, which is highly beneficial for the disassembly of heat-sensitive vehicle traction battery components.
Recycling and Stability of the Ionic Liquid Catalyst
One of my most important tasks was to verify the recyclability of the ionic liquid. I used [Amim][ZnCl3] as the catalyst and repeated the degradation of fresh polyurethane samples in the same DEG/ionic liquid solution. For each cycle, a new piece of cured polyurethane (about 1.2 g) was added, and the system was held at 170 °C for 40 minutes. After each run, the residual gel was removed, and the mass loss was recorded. The results are summarized in Table 10.
| Cycle number | Mass loss (g) | Observed behavior |
|---|---|---|
| 1 | 1.18 | Rapid degradation |
| 5 | 1.16 | Rapid degradation |
| 10 | 1.14 | Rapid degradation |
| 15 | 1.10 | Mostly rapid |
| 20 | 0.92 | noticeably slower |
| 21 | 0.84 | degradation slows |
| 25 | 0.55 | Very weak catalytic activity |
The mass loss remained close to the original value until the twentieth cycle. Thereafter, the activity decreased gradually. I hypothesized that the accumulation of degradation products, especially amine or amide oligomers, shielded the catalytically active sites or changed the pH of the solution. Adding fresh DEG restored some activity at first because it diluted the accumulated oligomer-rich liquid and reduced the viscosity, but after more cycles even fresh DEG did not recover the catalytic performance. Yet ESI-MS and FTIR analysis of the recovered ionic liquid after the twenty-fifth cycle showed that its characteristic absorption bands and ionic fragments were unchanged. This indicates that the ionic liquid itself remained chemically stable and did not decompose during the reaction. The apparent loss of activity was instead the result of the buildup of nonvolatile oligomeric products in the reaction medium. Therefore, an industrial process for the removal of adhesive from vehicle traction battery components would need to include a product-removal step—such as vacuum distillation, solvent extraction, or precipitation of the oligomers—in order to maintain a long service life of the ionic liquid.
Mechanistic Interpretation
From the experimental evidence, I can propose a dual-activation mechanism for the ionic-liquid-catalyzed alcoholysis of polyurethane. The first step involves the coordination of the imidazolium cation with the carbonyl oxygen atom of the carbamate group. This makes the carbonyl carbon more electrophilic. Simultaneously, the metal halide anion forms a hydrogen bond with a hydroxyl group of DEG. This hydrogen bond is actually a delocalized interaction in which the positive charge of the metal center slightly withdraws electron density from the chlorine atoms, increasing the apparent positive character of the hydroxyl hydrogen and thus causing the electron pair on the hydroxyl oxygen to become more available for an attack on the carbonyl carbon. The attack produces a tetrahedral intermediate that then collapses by breaking the C–O single bond. The electron pair of the former C–O bond migrates to the carbonyl oxygen, regenerating a carbonyl group and producing a carbamate-terminated fragment and a hydroxyl-terminated fragment. The same cycle is repeated at other urethane sites in the three-dimensional polymer network. Because the ionic liquid makes both the nucleophile and the electrophile more reactive, the activation barrier is lower than that encountered in the NaOH-catalyzed reaction, where only the nucleophilic strength of the alcohol is enhanced.
This mechanistic explanation accounts for the higher activity of [Amim][ZnCl3] compared to [Amim][FeCl4]. If the metal center is too Lewis acidic, it binds too strongly to the hydroxyl oxygen and hampers the final proton transfer. A mild Lewis acid like ZnCl2 in the form of the trichlorozincate anion provides an optimum hydrogen-bonding capacity without over-stabilizing the tetrahedral intermediate. This is also consistent with the observation that the optimal anion mole fraction is 0.5, where a single imidazolium cation is paired with a single metal-chloride anion. Excess anion would competitively coordinate the cations and reduce the availability of the electrophilic interaction with the urethane carbonyl. This mechanistic insight guides the rational design of future ionic liquid catalysts for the removal of structural adhesives from vehicle traction battery packs.
Conclusions and Outlook
In summary, my research has established two chemical degradation routes for the removal of polyurethane structural adhesives from vehicle traction batteries. Sodium hydroxide in diethylene glycol was shown to be effective at dissolving a broad variety of commercial polyurethane adhesives. The NaOH-catalyzed alcoholysis was optimized with respect to temperature, catalyst dosage, time, and solvent composition. The optimal condition was 165 °C with 0.2 g NaOH in 60 mL of DEG. Although this reaction proceeds efficiently, it still necessitates a high temperature that is not compatible with the thermal stability of most lithium-ion cells inside a vehicle traction battery.
To overcome this temperature limitation, I synthesized imidazolium-based ionic liquids with iron, cobalt, and zinc chloride anions. The ionic liquids acted as dual-functional catalysts that activated both the hydroxyl group of the alcoholysis solvent and the carbonyl group of the urethane linkage. At 170 °C for 40 minutes, the degradation rate increased from 26.7% with NaOH to over 67% with [Bmim][FeCl4] and to nearly 70% with [Amim][FeCl4]. Furthermore, the ionic liquid catalyst remained active for twenty consecutive cycles. More importantly, at only 50 °C, the ionic liquid/DEG solution could soften a cured adhesive enough for it to be removed from metal plates by gentle scraping. This low-temperature operation is a critical advantage for the safe disassembly of vehicle traction battery modules that are destined for reuse or recycling.
I have also learned that the catalytic activity is highly sensitive to the structure of the ionic liquid. Cations with allyl groups and minor steric hindrance perform better, and anions with moderate Lewis acidity, such as [ZnCl3]\(^-\), provide an optimal balance between activation and product release. These findings demonstrate that ionic liquids can be tailored to meet the specific demands of the adhesive-removal process.
Future work should focus on scaling up the ionic-liquid-based process for complete vehicle traction battery packs, developing continuous recycling equipment, and integrating the degradation step with downstream metal recovery. A closed-loop process for vehicle traction battery recycling will require not only efficient adhesive removal but also the ability to recover the multifunctional polyurethane degradation products, which may themselves be used as raw materials for new adhesives or foams. The results of my study provide a practical and environmentally friendly foundation for achieving these goals.