CO₂-releasing blowing agents from modified polyethylenimines slightly consume isocyanate groups while foaming polyurethanes

2.1 Synthesis of hydrophobically modified PEI-based CO₂ adducts

The chemical structure of the studied CO₂ adducts (PPGx-1:y-zPEI-CO₂), together with the corresponding PEI precursors (PPGx-1:y-zPEI), are displayed in Fig. 1A. The side chains attached to the branched PEI backbones were poly(propylene glycol) (PPG) oligomers with n-butoxyl terminal groups, whose polymerization degree (x) varied from 1 to 5. Other changing parameters included the molar ratio between the side chains (R’s) and the N atoms in PEI backbones (R: N = 1: y), as well as the backbone molecular weight (z). Thus the grafting rate of the PPG side chains can be calculated as (1/y) × 100%. The samples were named according to these parameters, as shown in Fig. 1A. Note that for PPG5 side chains, the precise average polymerization degree was 4.58.

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Fig. 1 (A) The CO₂ adducts (PPGx-1:y-zPEI-CO₂) release CO₂ to blow PU and restore their original polyamine structures (PPGx-1:y-zPEI). The values of the repeating number x are precise, except for that of PPG5 side chains, whose x = 4.58. The protons from the side chains are labeled for the assignments of corresponding ¹H NMR spectra (see Fig. 5). (B) A schematic diagram shows how a molecule of the polyamines (PPGx-1:y-zPEI) can be incorporated into a growing PU network.

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As the first step of the material synthesis, the monoalcohol of each PPG oligomer (from Sigma-Aldrich, USA) was reacted with epichlorohydrin to transform the hydroxyl group of the PPG oligomer into a glycidyl ether group. This group was then ring-opened by the primary and secondary amines in branched PEIs (number average molecular weight M_n = 0.5 k–25 k Da, from Gobekie New Materials Science and Technology Co. Ltd., Shanghai, China), forming corresponding PPG-grafted PEIs (PPGx-1:y-zPEI). Finally, the as-synthesized grafted PEIs chemically absorbed CO₂ to form the corresponding CO₂ adducts (PPGx-1:y-zPEI-CO₂). All the material syntheses have been detailed previously (Long et al., 2014, 2018). Their chemical structures were characterized by ¹H NMR and FTIR spectroscopy. The CO₂ content of each CO₂ adduct was measured by thermogravimetry (TG) analysis (see Section 2.4).

2.2 PU foam preparation

The raw materials for PU foams are listed in Table 1. The crosslinkers, triethanolamine and glycerol, were bought from Kelong Chemical Reagent Plant (Chengdu, Sichuan, China). The other raw materials except for the blowing agents were purchased from Chengdu Advanced Polymer Technology Co., Ltd. (Chengdu, Sichuan, China). Among them, the polyether diol PPG 2000 served as a diluent in the early stage of the foaming process because of its low viscosity and then as one of the soft segments of the final PU foams. Another soft segment polyether polyol was PPG 4110. No flame retardants or plasticizers were used because they might be extracted from the resultant foams and thus disturb the analyses of the foam extracts (see Section 2.3). The catalyst triethanolamine served as a crosslinker as well, which could be incorporated into the foam structure and finally lost its extractability.

To prepare PU foams, each white component with PPGx-1:y-zPEI-CO₂ or 25kPEI-CO₂ as the blowing agent was magnetically stirred at 400 rpm for 10 min in a plastic cup and then the black component PMDI (Table 1) was added and the whole mixture was homogenized mechanically at 400 rpm for 30 s to allow free foaming at room temperature. The temperature of the foaming system peaked at about 100 °C, as measured by a thermometer. The white components before mixing with PMDI were diluted 20 times with the blank white component (without blowing agent added, Table 1) for morphology observation (see Section 2.4). Finally, the foams obtained were aged at least for 3 days and characterized by FTIR spectroscopy. The blank foam without added blowing agent served as a control.

Note that the hydroxyl number in the blank white component was determined according to ASTM D1957-90. This number (450.3 mg KOH/g) including the contribution of the water impurity (0.92 wt%) was used to calculate the real NCO index (1.103) of the blank foam, which was much lower than the nominal NCO index (1.264) calculated from the raw material nominal hydroxyl numbers provided by the suppliers (see Table S1).

2.3 Preparation of the foam extracts and analysis of the same

The obtained foams were brittle enough to be ground into powder; then they were dried at 80 °C for 8 h to release completely any CO₂ that might be left in the blowing agents. The dried samples (about 3.5 g) were weighed using an analysis balance and then incubated in ethylenediamine (EDA) vapor for 12 h. To obtain the vapor, 5 g EDA was added to a 30-L glass container, then a vacuum was applied and finally the glass container was sealed. The EDA vapor incubation could eliminate all the NCO groups left in the foams because the amine groups in EDA reacted with the NCO groups very rapidly. The NCO-free samples were vacuum-dried at 80 °C for another 4 h to remove any residual EDA.

Each PU sample treated as described above was carefully transferred into a 250-mL Soxhlet extractor where about 120 mL of ethanol was refluxed for 30 h to extract the soluble substances from the PU powder. The extracted PU powder was dried again until a constant weight was achieved. The weight decrease due to the ethanol extraction was recorded as the total weight of the extract. This extraction was repeated four times for each type of foams (n = 4). Into three of the corresponding extract solutions was added precisely weighed (about 1.0 g) 1,4-bis(methoxymethyl)benzene (>99%, Adamas-beta, Shanghai, China) as an external standard. Then these solutions were rotary evaporated prior to analysis using ¹H NMR spectroscopy. The fourth extract solution, without the external standard added, was analyzed by ¹H NMR spectroscopy as well.

2.4 Instrumental characterizations

¹H NMR spectroscopy was performed on a Bruker AV II 400 MHz ¹H NMR spectrometer (Bruker Corp., Switzerland), using CDCl₃ as the solvent. FTIR spectra of PU foams were collected on a Nicolet 560 FTIR spectrophotometer (Nicolet Instrument Corp., USA) in a transmission mode, using KBr discs containing the PU powders. The obtained spectra can be transformed to absorbance mode using Nicolet Omnic 8.0 software for quantitative analysis of the NCO groups. TG analysis was performed on a TG 209F1 apparatus (Netzsch instruments, Germany), operated from 30 to 200 °C at 10 °C/min under a nitrogen flow of 100 mL/min. Morphology of the CO₂ adducts-containing white components was observed under an Olympus BX 43 light microscope (Olympus, Japan). Morphological changes in PU foams due to the ethanol extraction were observed by a JSM-9600 scanning electron microscope (SEM, Jeol, Japan). Note that the foams for this purpose were not incubated in the EDA vapor (see Section 2.3), as a drop of water (about 0.1 g) was added to the foaming mixture (Table 1) and no NCO group was left in the PU foams obtained.

2.5 Statistical analysis

Data were expressed as mean ± SD, with at least three parallel samples being used. Two-tailed Student's t-tests were used to compare selected data; and p < 0.05 (*) was accepted for statistical significance.

3.1 Chemical compositions of PPGx-1:y-zPEI-CO₂s

The synthesis and characterization of PPG-grafted PEIs and their CO₂ adducts have been detailed in our previous publications (Long et al., 2014, 2018). In this study, we changed the PPG side chain length (x), the side chain grafting rate (equaling 1/y), and the molecular weight (z) of the PEI backbone to obtain three series of CO₂ adducts, as shown in Table 2 (25kPEI and its CO₂ adduct served as controls). The purpose was to test the effects of such structural parameters on the PU foaming process. The chemical structures of the PPG-grafted PEIs were confirmed by both FTIR and ¹H NMR spectroscopy (see Figs. S1 and S2). Based on peak areas of selected NMR signals from the side and main chains, the PPG grafting rates could easily be calculated (see Supporting Information) and were found to be consistent with the theoretical values (Table 2). The chemical structure of the CO₂ adducts was characterized just by FTIR spectroscopy (Fig. S1). ¹H NMR spectroscopy could not be performed, as no solvent has been found for these CO₂ adducts. Their CO₂ contents were measured by TG analysis (Fig. S3) and were consistent with the theoretical values as well.

3.2 Morphology of the white components and the PU foams

As shown in Fig. 2A and B, the decrease in the PPG side chain length (x) or in the grafting rate (equaling 1/y) caused an increase in the particle size of the dispersed blowing agents in the corresponding white components. In contrast, the decrease in the PEI backbone molecular weight (z) reduced the particle size and narrowed the particle size distribution markedly. In addition, the dispersed particles were irregular in shape at relatively high molecular weights of the PEI backbones while they became spherical at lower PEI molecular weights (e.g., 1 k and 0.5 k Da).

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Fig. 2 (A) Morphology of the dispersed CO₂ adducts in the corresponding white components, revealed by light microscopy at an original magnification of 200×. (B) The particle size of the dispersed CO₂ adducts. The data were obtained from 10 randomly selected views under the light microscope. The vertical lines show the particle size ranges (not error bars); the black squares indicate the number mean particle sizes. (C) Morphology of selected PU foams before and after ethanol extraction. The CO₂-released blowing agents were well dispersed into the PU foams. Otherwise more holes should be observed in the pore walls after ethanol extraction.

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Interestingly, the blowing agent particles were not observed in the PU foams after they released CO₂ to blow the PUs (Fig. 2C), showing that the CO₂-free blowing agents (which are viscous liquids) are miscible with the PUs. If they were not miscible and just dispersed in droplets into the foams, there should be more holes observed in the pore walls after ethanol extraction, when compared to the original foams. This was not the case. The distortion of the pores after ethanol extraction (Fig. 2C) was due to the contraction caused by the ethanol evaporation at 60 °C prior to SEM observation.

3.3 Chemical structure of the PU foams

FTIR spectra of the as-prepared PUs were similar and selected spectra are compared in Fig. 3A. Characteristic absorptions associated with PUs were found at 3416, 1724, 1612, and 1229 cm⁻¹, which can be assigned to N—H stretch, urethane C⚌O stretch, benzene ring vibration (from PMDI), and urethane C—N stretch, respectively (Xie et al., 2009). Compared to the blank foam, the foams blown by CO₂ adducts displayed a more prominent peak at 1310 cm⁻¹ (indicated by *, Fig. 3A), showing the contribution of the aliphatic C—N vibration from the PEI segments (Liu, et al., 2017). All the specimens showed a C—O—C stretch at 1080 cm⁻¹, due to the presence of polyether segments (from PPG 4110 and PPG 2000) in the PU chains (Liu, et al., 2017). However, the foam blown by PPG1-1:9-25kPEI-CO₂ displayed the most prominent C—O—C stretch among the three specimens (see the peak indicated by # in Fig. 3A), due to the extra contribution from the PPG side chains of the blowing agent.

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Fig. 3 (A) Representative IR spectra of PU foams blown by different CO₂ adducts. (B) The absorbance ratios between υ_–NCO at 2279 cm⁻¹ and υ_–N=C=N– at 2152 cm⁻¹ and the PEI amine reaction extents (see the right Y-axis). The A₂₂₇₉/A₂₁₅₂ ratios were averaged from three measurements (n = 3). *: p < 0.05, compared with the A₂₂₇₉/A₂₁₅₂ value of the control b, i.e. 25kPEI-CO₂ blown foam. These ratios represent the numbers of NCO groups left in the corresponding foams and can be used to calculate the reaction extent of the PEI amine groups during PU foaming (Supporting information).

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Absorption from the residual NCO groups was evident at 2279 cm⁻¹ (Fig. 3A) (Liu, et al., 2017). The peak at 2152 cm⁻¹ was due to the stretch of carbodiimide (—N⚌C⚌N—) groups from the raw material PMDI (Liu et al., 2017). The number of —N⚌C⚌N— groups should remain unchanged before and after PU foaming. According to Lambert–Beer’s law, the absorbance ratio of the both groups (i.e. A₂₂₇₉/A₂₁₅₂) should be linear with the number of the residual NCO groups in the corresponding foam. For the blank foam, the absorbance ratio A₂₂₇₉/A₂₁₅₂ was 3.25, corresponding to the overdosed NCO groups of 7.56 mmol (see footnote c, Table 1) that was left in the foam. As shown in Fig. 3B (see the bars), the 25kPEI-CO₂ blown foam demonstrated an obvious decrease in A₂₂₇₉/A₂₁₅₂ value (or in residual NCO amount), compared to that of the blank foam. The other CO₂-adduct blown foams showed a similar decrease as well, but to a lesser extent. The decreased number of NCO groups was due to the reaction with the PEI amine groups from the corresponding CO₂-released blowing agent during the PU foaming (Fig. 1B). One may argue that the secondary hydroxyl group from the side chain (see the R chain in Fig. 1A) could react with the NCO groups as well. However, this reaction was minor enough to be ignored, as amine groups usually have several orders of magnitude higher reactivity toward NCO groups than do hydroxyl groups (Hepburn, 1992). The percentage of the reacted amine groups with respect to the total amine groups in the CO₂-released blowing agent can be defined as the amine reaction extent, which varied narrowly in the range of 0.4–1.5% (Fig. 3B, indicated by triangles). The calculation of these values can be found in Supporting Information.

It is worth noting that the total amine content of the blowing agents could be reduced after PPG grafting. Not surprisingly, some PPG-grafted blowing agents (see those with z = 1 k and 0.5 k, Fig. 3B) possessed close to or even higher amine reaction extents, compared with that of 25kPEI-CO₂ (control b, Fig. 3B), although their real reacted amine groups were fewer than those in the latter blowing agent.

3.4 Quantification analysis of the PU foam extracts

Before extraction with ethanol, the PU foams in powder form were heated to remove any residual CO₂ in the blowing agents and thus the blowing agents could completely restore the polyamine structure of corresponding PPG-grafted PEIs (see Section 2.3). The restored PEIs that had not reacted with NCO groups during the PU foaming could be extracted into the ethanol if no further reaction occurred during the extraction. This condition could be guaranteed because the EDA treatment had depleted any residual NCO groups prior to the extraction, as proved by the disappearance of the NCO absorption at 2279 cm⁻¹ (compare curves a and b, Fig. 4). There was no obvious change in the FTIR spectra before and after the ethanol extraction (see curves b and c, Fig. 4).

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Fig. 4 FTIR spectra of both the blank foams and the PPG1-1:9-25kPEI-CO₂ blown foams. The υ_–NCO absorption in the original foams disappeared after being treated with EDA. (a) Original foams; (b) EDA treated foams; (c) the foams after being extracted with ethanol.

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Fig. 5A shows the assignments of the ¹H NMR spectra of selected PU foam extracts. Signals from repeating units of PPG segments were identified in the blank foam, showing that polyether polyols (mainly from the PPG 2000 raw material) had been partially extracted into the ethanol. For the foams blown by the CO₂ adducts, corresponding CO₂-free blowing agents were found as well (e.g. 25kPEI and PPG1-1:9-25kPEI, Fig. 5A). Fig. 5B shows the ¹H NMR spectra after addition of the external standard to the extracts. The identified substances in the extracts could be quantitatively analyzed from the areas of their selected peaks (peak s, peak s + g, or peak a) relative to the area of peak t from the external standard (Supporting Information). This peak was chosen because it does not overlap with the other proton signals.

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Fig. 5 ¹H NMR spectra of selected PU foam extracts, labeled by the used blowing agent in CO₂-free form. (A) The spectra of the original extracts. For the assignments please also refer to Fig. 1A. (B) The spectra obtained after adding the external standard. The corresponding spectra from the other foams can be found in Supporting information. The masses of the PU foams and the external standard (m_PU and m_ex) are shown on each spectrum.

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The compositions of the extracts from different PU foams are compared in Fig. 6A. For all the extracts, the total weights (by weighing) were slightly higher than the sum of the weights of the identified substances (by ¹H NMR spectroscopy). This was natural because the ¹H NMR spectra also demonstrated an unknown substance (indicated by &) and/or traces of ethanol (indicated by #) present in the extracts (Fig. 5A). The weight gap between each extracted blowing agent and feed blowing agent (both should be CO₂ free) came from the immobilized blowing agent in the corresponding PU foam (Tables S3 and S4). The percentage of the mass of each immobilized blowing agent with respect to the feed mass, or the immobilized rate, changed considerably from sample to sample (Fig. 6B). The blowing agent without PPG grafting (i.e. 25kPEI-CO₂) demonstrated the highest immobilized rate. The decrease in the polymerization degree (x) or grafting rate (equaling 1/y) of the PPG side chains caused an increase in the immobilized rate. A relatively sharp decrease in the immobilized rate could be found when the molecular weight of the PEI backbone decreased from 25 k to 5 k Da while a further decrease in the molecular weight hardly affected the immobilized rate.

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Fig. 6 (A) Mass percentages of the extracted substances with respect to the corresponding PU foam. The darkest bars represent the total percentages of the extracts (n = 4). The lengths of gray bars and light gray bars (n = 3) demonstrate the contributions of the CO₂-free blowing agents and PPG polyols, respectively. (B) Mass percentage of the immobilized blowing agents changed with the structure of the used blowing agent (n = 3).

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As one repeating unit of the PEI backbone contains one amine group, the immobilized rate equals the mole percentage of the amine groups being captured into the foam structure. The ratio between the captured and the reacted amine groups (i.e. the ratio between the immobilized rate and the amine reaction extent) reveals the average polymerization degree of PEI segments between neighboring reacted amines. This degree of polymerization or the PEI segment molecular weight can show the reacted amine density along PEI backbones in the immobilized blowing agents, which are compared in Table 3. Also, the average number of reacted points per PEI backbone can be achieved by dividing the polymerization degree of the whole PEI backbones with that of the PEI segments between two neighboring reacted amines. As shown in Table 3, the PEI backbone molecular weight (z) was the major factor that affected the number of reacted amines per PEI backbone.