Towards green polyurethane foams via renewable castor oil-derived polyol and carbon dioxide releasing blowing agents from alkylated polyethylenimines

Polymer 116 (2017) 240e250

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Towards green polyurethane foams via renewable castor oil-derived polyol and carbon dioxide releasing blowing agents from alkylated polyethylenimines Chao Liu, Yuanzhu Long, Jiang Xie, Xingyi Xie* College of Polymer Science and Engineering, Sichuan University, Chengdu, Sichuan 610065, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 September 2016 Received in revised form 21 March 2017 Accepted 29 March 2017 Available online 30 March 2017

Alternatives to climate changing blowing agents (e.g. chlorofluorocarbons) and petroleum sourced raw materials (e.g. polyether polyols) are very attractive in the polyurethane (PU) foam industry. We explore a series of CO2 adducts from a branched polyethylenimine (bPEI) alkylated with C4 to C16 alkyl glycidyl ethers. These adducts can serve as CO2 releasing blowing agents during the exothermic polymerisation of PUs. A 13% alkylation of the bPEI amine groups enhances the dispersibility of the resultant CO2 adducts into the PU foaming mixtures containing a castor oil-derived polyol. In particular, the CO2 adduct with a C8 alkyl (2-ethylhexyl) side chain possesses the best dispersibility and is among the most effective in decreasing the foam density. It generates PU foams whose density and compressive strength are almost suitable for thermal insulation of underground steel pipes. This is the first report showing that PU foams from biomass polyols can be blown by CO2 adducts that are environmentally neutral. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Alkylated polyethylenimine CO2 adducts Biomass polyurethanes

1. Introduction Polyurethane (PU) foams, as the most used cellular materials, have raised serious ecological concerns. One major concern is that their traditional blowing agents, like chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), are climate changing substances that destroy the stratospheric ozone layer and contribute to global warming [1]. Alternatively, hydrofluorocarbons (HFCs) possess nearly zero ozone depletion potential, but are still greenhouse gases, similar to CFCs and HCFCs [2,3]. The recently introduced, unsaturated hydrofluoroolefins (HFOs, e.g. 1-chloro-3,3,3trifluoropropene) are promising due to their short atmospheric lifetime and thus minor impact on ozone loss and global warming [4e7]. However, little is known about the environmental impact of their final degradation products. Some HFOs have been reported to result in stable trifluoroacetic acid that pollutes water and soil [8,9]. Another ecological concern is the petroleum sourced polyols used as the soft segments in PUs, which are not renewable or sustainable. This field has stimulated significant interest in exploring biomass-based polyols to replace the current polyether

* Corresponding author. E-mail addresses: [email protected] (C. Liu), [email protected] (Y. Long), [email protected] (J. Xie), [email protected], [email protected] (X. Xie). http://dx.doi.org/10.1016/j.polymer.2017.03.079 0032-3861/© 2017 Elsevier Ltd. All rights reserved.

polyols [10e16]. Among them, castor oil is a natural triglyceride with a hydroxyl functionality of about 2.7, since over 90% of its fatty acids is ricinoleic acid which contains a secondary hydroxyl group appended to the C18 fatty acid backbone [10]. This naturally hydroxylated oil has been directly used as a PU polyol for decades [17,18]. Recently, other unsaturated triglycerides from soybean [13,14], cottonseed [15], and even algae [16], have been epoxidised and then transformed into hydroxylated oils to serve as biomass polyols for PU foams. In response to the climate change concerns associated with traditional PU blowing agents, we recently explored a series of CO2 releasing blowing agents [19e21]. In summary, a branched polyethylenimine (bPEI, Mw ¼ 25000 Da) was grafted with a polypropylene glycol chain (PPG, Mn ¼ 396 Da, with a grafting rate from 9% to 20%) [19,20] or a C15 alkyl chain from palmitic acid (with a grafting rate of 12%) [21], followed by saturation with CO2 to form thermally unstable CO2 adducts. These adducts then released CO2 to blow PUs during the exothermic polymerisation. These hydrophobically modified blowing agents are well dispersed in the PU raw material mixture containing polyether polyols and are therefore very suitable for PU foams from polyether polyols. To the best of our knowledge, CO2 releasing blowing agents designed for biomass polyol-based PU foams have so far never been reported. In this study, we attempted to graft bPEI with various alkyl side

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chains (from C4 to C16) to test the feasibility of the resulting CO2 adducts (Cn-bPEI-CO2s, where n represents the carbon number in each side chain) as novel blowing agents for PU foams from biomass polyols like castor oil. The side chain length was optimised based on the raw material compatibility and the foaming efficiency of Cn-bPEI-CO2 blowing agents. This original study might pave the way towards the next generation of PU foams that are more sustainable and more environment-friendly.

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neutral. The washed organic phase was dried with 10 g of Na2SO4 overnight, followed by vacuum rotary evaporation at 90  C for 1 h to remove the solvent and the unreacted epichlorihydrin. For C12EPO, the yield was 78%, with 10% C12-OH present by mole. For C16EPO, the yield was 77%, with 12% C16-OH present by mole (the content of remaining alkyl alcohol was tested by 1H NMR spectroscopy, see Fig. 2b). 2.3. Grafting bPEI with Cn-EPOs

2. Materials and methods 2.1. Materials 1-Dodecanol and 1-hexadecanol (C12-OH and C16-OH, respectively, from Guangfu Fine Chemicals, Tianjin, China) were reacted with epichlorohydrin (Bodi Chemical Holding Co., Tianjin, China) to synthesise the C12 and C16 alkyl glycidyl ethers, respectively. Tetrabutyl ammonium bromide (TBAB, Bodi Chemical Holding Co., Tianjin, China) was used as a catalyst. n-Butyl glycidyl ether (C4EPO) and 2-ethylhexyl glycidyl ether (C8-EPO) were purchased from TCI, Shanghai, China and Solarbio Life Sciences, Beijing, China, respectively. Branched polyethylenimine (bPEI, Mw ¼ 25,000 Da, from Sigma-Aldrich, St. Louis, MO, USA) and compressed CO2 (99%, from Qiaoyuan Gas Co. Ltd., Dujiangyan, Sichuan, China) were commercially available. A biomass polyol Polycin® T-400 was bought from Vertellus Performance Materials Inc., Greensboro, NC, USA. It was obtained by alcoholysis of castor oil with glycerol to improve the hydroxyl value (Table 1). Dimethyl methylphosphonate from Letai Chemicals, Beijing, China, served as both flame retardant and diluent. Other raw materials used to prepare the PU foams (Table 1) were all from Chengdu Advanced Polymer Science and Technology Co., Ltd., Chengdu, Sichuan, China. 2.2. Synthesis of n-dodecyl and n-hexadecyl glycidyl ether (C12and C16-EPO) All the synthesis routes are illustrated in Fig. 1. In a three-necked round-bottomed flask equipped with a thermometer and a mechanical stirrer, 0.1 mol of C12-OH (18.63 g) or C16-OH (24.25 g), 0.15 mol of NaOH powder (6 g), 4.8 mmol of TBAB (1.55 g) and 100 mL of petroleum ether were added and the temperature was raised to 50  C to dissolve the alkyl alcohol. Then, 0.18 mol of epichlorohydrin (16.65 g) was dripped into the flask in 30 min and the mixture was stirred for an additional 4 h. The residual NaOH and by-product NaCl were removed by filtration. The filtrate was repeatedly washed with distilled water until the water phase was

Ten grams of bPEI was dissolved in 100 mL of ethanol in a threenecked round-bottomed flask and then one of Cn-EPOs (n ¼ 4, 8, 12 and 16) was added under stirring. The molar percentage of the epoxy group in the Cn-EPO relative to all amine groups in bPEI was set to 13%, meaning that the theoretical grafting rate of all samples was 13%. The reaction mixture was stirred at 50  C for 12 h, followed by rotary evaporation at 60  C for 2 h to remove the ethanol. The crude product was dissolved in ethanol/petroleum ether (1:3, v/v) and thereafter distilled water was added until phase separation occurred. The top layer was discarded. The bottom viscous stuff was washed with petroleum ether to remove Cn-OH and/or Cn-EPO impurity, followed by rotary evaporation at 70  C for 3 h. The alkylated bPEI (i.e. Cn-bPEI, where n represents the carbon number in the alkyl chain) was obtained by vacuum drying at 80  C for 10 h. All Cn-bPEIs are viscous liquids at room temperature, except for C16-bPEI which is a waxy solid. 2.4. Synthesis of CO2 adducts of bPEI and Cn-bPEI Pure bPEI and C4-bPEI absorb CO2 very slowly (the strong intermolecular H-bonding hindered the permeation of CO2 and such H-bonding could be weakened by bulkier alkyl side chains). In contrast, their 30% (w/v) solutions in ethanol were transformed into white precipitates in 5 min by purging with a CO2 flow. Each precipitate was transferred onto a watch glass to evaporate ethanol at 30  C for 96 h in a vacuum oven. The C8- and C12-bPEI easily spread on watch glasses and quickly solidified in 2 min in the same CO2 flow. C16-bPEI was melted at above 30  C and transformed into a white solid in 1 min by absorbing CO2. All the primary CO2 adducts prepared above were placed in a steel container and further reacted with CO2 at 0.5 MPa for 1 d. Once the CO2 pressure dropped to about 0.2 MPa, more CO2 was introduced to bring the pressure up to 0.5 MPa. The resulting CO2 adducts were ground into powders and saturated with 0.5 MPa CO2 until a constant weight was reached. This generally took an additional day. The resulting product was designated by adding “-CO2” after the corresponding sample code of pristine polyethylenimines,

Table 1 Formulation of polyurethane foams. Raw material

White component Polycin® T-400 Silicone L-3102 Stannous octoate (T-9) Dimethyl methylphosphonate (DMMP) Propylene glycol Diethanol amine Blowing agent (total weight) Black component PMDIa Isocyanate (NCO) indexb a b

Description

Formulation (g) I

II

Castor oil based derivative, OH value 400 mg KOH/g and functionality 3 Foam stabiliser Catalyst Diluent and flame retardant Chain extender Crosslinker bPEI-CO2 or Cn-bPEI-CO2

8.50 0.20 0.04 2.0 0 0 0.50 (11.24)

8.50 0.32 0.03 2.5 0.1 0.5 1.6 (13.55)

NCO content 31 wt %

10.0 1.08

17.0 1.55

Polymeric 4,40 -diphenylmethane diisocyanate. NCO indices were calculated including the water impurity (0.08 g) in Polycin® T-400, assuming that the blowing agents did not take part in the polymerisation of PUs.

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Fig. 1. Synthesis of alkylated bPEIs and their CO2 adducts. Some protons and carbons are labelled for NMR assignments in Figs. 2, 3 and 5. RT: room temperature.

Fig. 2. FTIR and 1H NMR spectra of C16-bPEI, compared with those of the intermediate (C16-EPO) and raw materials (C16-OH and bPEI). (a) Adsorptions from the epoxide ring and the C16 alkyl side chain are indicated by * and # signs, respectively. (b) The peak area ratio of signal g of the epoxide ring to the signal a of the C16 alkyl chain is 0.88:3, while the theoretical ratio is 1:3. This shows the presence of 12% C16-OH in C16-EPO by mole.

for example, bPEI-CO2, C4-bPEI-CO2 and so on. 2.5. Dispersibility test of Cn-bPEI-CO2 samples in the PU foaming system Each blowing agent (0.1 g) was mixed with 5 g of the other, liquid components of the white component (Formulation I, Table 1)

by magnetically stirring at 300 rpm for 1 min. The macroscopic and microscopic images (200  ) were recorded by a digital camera and an Olympus BX 43 light microscope (Olympus, Japan), respectively. To further enhance the dispersibility of the blowing agents, each mixture was further magnetically stirred at 300 rpm for 12 h and rested for another 60 h (3 d in total). Finally, both macroscopic and microscopic photographs were recorded again.

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2.6. Preparation of PU foams The formulations for PU foams are listed in Table 1. Foaming efficiencies could be compared by changing the blowing agent type within the same set of formulations (Formulation I or II). The blowing agent dosage was increased from 0.5 g in Formulation I to 1.6 g in Formulation II to lower the foam density. As more CO2 would be released in the case of Formulation II, more foam stabiliser was used accordingly. For the same reason, the polymerisation speed was decreased by using less organotin catalyst (T-9), allowing a longer time to release CO2 prior to foam solidification. An additional chain extender and crosslinker were used to strengthen the rising foams to largely avoid bubble breakage during the foaming process. A higher isocyanate index was used in Formulation II assuming that the blowing agent would consume some isocyanate groups after CO2 release. To prepare the PU foams, the raw materials constituting the white component (Table 1) were homogenised in a plastic cup by mechanically stirring at 800 rpm for 2 min (Formulation I) or 1000 rpm for 1.5 min (Formulation II). Then, the black component PMDI (polymeric 4,4'-diphenylmethane diisocyanate) was weighed and added into the homogenised white component, followed by another homogenisation at 1500 rpm for 30 s. The foaming mixture then rose freely and gradually became tack free. The as-prepared foams were annealed at 70  C for 3 h and then stored in a silicone gel desiccator at room temperature. 2.7. Characterisation All the characterisation methods are detailed in the Supporting Information. In brief, 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy were used to characterise the chemical structures of target products. All the CO2 adducts were characterised by thermogravimetry (TG) and differential scanning calorimetry (DSC), both being performed from 30 to 300  C at 10  C/min under a nitrogen flow of 100 mL/min unless otherwise specified. The PU foam morphology was examined under a scanning electron microscope. Five replicates (n ¼ 5) were used to test the PU foam density and mechanical properties. The former was calculated based on the precise weight (±0.1 mg) and dimensions (±0.1 mm) of samples with a size of about 30 mm  30 mm  20 mm. The same sized samples were compressed along the 20-mm edges (i.e., the foam rise direction) on an Instron testing machine at a strain rate of 3 mm/min. 2.8. Statistical analysis The densities and mechanical strengths of different samples were compared using a two-tailed Student t-test and a significance level of p < 0.05 was accepted. 3. Results 3.1. Chemical structure of Cn-bPEIs Fig. 2 compares the FTIR and 1H NMR spectra of C16-EPO and C16-bPEI with their respective raw materials. Compared with C16OH, C16-EPO lost the hydroxyl stretching at about 3330 cm1 and had new IR bands at 912, 847 and 766 cm1 (indicated by *, Fig. 2a). The latter bands correspond to the asymmetric and symmetric stretches of the epoxide ring [22]. Accordingly, five proton signals (HeeHi, Fig. 1) characteristic of the glycidyl ether group appeared in the NMR spectrum (Fig. 2b) and were scattered within 2.5e3.8 ppm [23]. The signals of the C16 alkyl chain (HaeHc) remained unchanged except that the a-CH2 signal moved upfield from 3.64 ppm

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(signal d1 in C16-OH) to 3.50 ppm (signal d2 in C16-EPO). However, a weak d1 peak was still found in the C16-EPO spectrum, indicating the presence of the C16-OH impurity, the molar content of which was 12%. This was measured by the relative peak area ratio of two chosen proton signals (g and a, Fig. 2b). The spectroscopic changes between C12-OH and C12-EPO were similar and the residual C12OH in C12-EPO was estimated as 10% by mole. Apart from the NeH stretching at 3358 cm1 and bending at 1652, 1582 and 1467 cm1 from the bPEI backbones [24,25], C16bPEI displayed additional eCH3 bending at 1357 cm1 and CeOeC stretching at 1117 cm1 (indicated by #, Fig. 2a), matching the alkyl ether side chain structure shown in Fig. 1. Likewise, the 1H NMR spectrum of C16-bPEI demonstrated characteristic proton signals from both bPEI backbones (NeCH2, chemical shift at 2.3e2.9 ppm) and the C16 alkyl side chains (Ha eHd, Figs. 1 and 2b). The presence of new signals associated with the linking groups (Ha, Hb and Hg, Figs. 1 and 2b) between the bPEI backbone and the C16 alkyl side chain confirms the formation of the alkylated bPEI (i.e. C16-bPEI). Moreover, the disappearance of the d1 peak in the C16bPEI spectrum indicated the high purity of C16-bPEI without any C16-OH present. The same phenomenon occurred in C12-bPEI. As shown in Fig. 3a, all the FTIR spectra of Cn-bPEIs (n ¼ 4, 8, 12, 16) are similar, showing absorptions related to the alkyl ether side chains (again indicated by #). 1H NMR spectroscopy is more powerful in distinguishing the alkyl structure among Cn-bPEIs (Fig. 3b). The shortest C4 alkyl chain demonstrated independent proton signals, in comparison with the C12 (or C16) chain whose middle methylene signals overlapped (compare peak b in different samples, Fig. 3b). The branching structure in the C8 alkyl chain (Fig. 1) is responsible for the unique signal pattern in the C8-bPEI spectrum (Fig. 3b). The NeCH2 signals, resulting from the bPEI backbone and the side chains (Hg, Fig. 1) of Cn-bPEIs, were similar (Fig. 3b) and overlapped with the active proton signals (NeH and OeH, Fig. 1) that were shifted downfield by addition of CD3OD in the NMR solvent CD3Cl (Fig. S2b). The relative area of pure NeCH2 signals was used to calculate the alkyl grafting rate (See Supporting Information), which is clearly consistent with the theoretical grafting rate of 13% (Table 2). With the increase of alkyl chain length from C4 to C16, the side chain content increased from 27.9% to 48.0% (Table 2) accordingly. 3.2. Chemical structure and CO2 release of Cn-bPEI-CO2 samples The bands from the alkyl ether side chains remained in the FTIR spectra of the Cn-bPEI-CO2 samples (indicated by *, Fig. 4). In addition, a group of strong IR bands at 1630, 1570, 1477 and 1413 cm1, not seen in the Cn-bPEI spectra (Fig. 3a), were observed and assigned to alkylammonium NeH bending, carbamate C]O stretching and the asymmetrical and symmetrical skeletal stretching of the carbamate anions, respectively [19e21]. This observation confirms the formation of alkylammonium cations and carbamate anions (>NCOO and eNHCOO) in the bPEI backbones (Fig. 1) due to CO2 adduction. Among the synthesised CO2 adducts, bPEI-CO2 and C4-bPEI-CO2 are water soluble due to their zwitterionic structures as shown in Fig. 1 (we have not yet found solvents for the other samples). Thus, they were analysed by 1H and 13C NMR spectroscopy to further reveal the structural change before and after CO2 adduction. As shown in Fig. 5a, the multiple peaks from the backbone N-CH2 protons expanded obviously downfield after CO2 adduction and spanned from 2.3 to 3.5 ppm, while the initial range was from 2.3 to 2.7 ppm before CO2 adduction. Most of the proton signals (peaks aec and b) associated with the butyl ether side chain in C4-bPEICO2 were not affected by CO2 adduction. The observed downfield expansion is not surprising since a similar spectroscopic change has

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Fig. 3. FTIR and 1H NMR (in CDCl3) spectra of bPEI and Cn-bPEIs. The IR bands from the alkyl ether side chains are indicated by # signs.

Table 2 Important parameters of both Cn-bPEIs and their CO2 adducts. Material

Cn-bPEI

bPEI-CO2 or Cn-bPEI-CO2

Parameter

Grafting rate y (%) Theoretical Measured Side chain content (wt. %) Theoretical CO2 content (%) Measured weight loss (%) Side chain content (wt. %) Measured DH (J/g) Normalised DH (J/g)

bPEI-CO2

33.8 32.2 361

Grafting alkyl side chain C4

C8

C12

C16

13 12.8 27.9 26.9 22.9 21.5 155 197

13 12.7 35.5 24.8 20.2 28.1 223 310

13 13.0 42.3 22.8 17.7 34.8 143 219

13 13.3 48.0 21.0 14.7 40.9 83 140

All the calculations are shown in Supporting Information. The measured grafting rate was calculated based on 1H NMR spectra. The theoretical CO2 contents are calculated based on the fact that two amino groups or two repeating units capture one CO2 molecule. The normalised DH is calculated by excluding the weight of the alkyl side chain in each Cn-bPEI-CO2 and is just based on the weight of bPEI-CO2 backbone.

been observed in CO2 adducts of low molecular weight polyamines [26,27]. For example, the initial NeCH2 signals in aminoethyl piperazine distribute from 2.2 to 2.6 ppm and then move downfield and range from 2.4 to 3.4 ppm due to adduction with CO2 [27].

Fig. 4. FTIR spectra of Cn-bPEI-CO2. The asterisks indicate absorptions related to the alkyl ether side chains.

Eight carbon signals were detected in bPEI (the assignment is shown in Fig. 5b), clearly indicating the branching backbone structure [28]. After absorption of CO2, the backbone carbon signals combined to roughly form three groups, as a whole shifting upfield slightly. This upfield shift has also been found during the CO2 adduction with low molecular weight polyamines [27,29]. New signals from carbamate and bicarbonate groups appeared at 164.5 and 161.0 ppm [27,29], respectively, due to CO2 adduction. These signals were seen in the C4-bPEI-CO2 spectrum as well. As expected, the side chain carbon signals (aed and b) were barely affected by CO2 adduction. The appearance of a bicarbonate signal was due to the partial hydrolysis of carbamate anions in the NMR solvent D2O [27]. Fig. 6 shows the TG and DSC curves of the CO2 adducts. The control sample C16-bPEI displayed a negligible weight loss before 200  C. All the CO2 adducts showed a significant weight loss that was lower than the corresponding theoretical CO2 content (Table 2). Accordingly, each CO2 adduct demonstrated a broad endothermic process (Fig. 6b), which spanned the same temperature range as the weight loss process (Fig. 6a). Therefore, both the weight loss and the endothermic process could be associated with the CO2 release upon heating. The increase of side chain length decreased the measured weight loss, since the side chain cannot react with CO2. The measured enthalpy change (DH) was normalised by excluding the side chain weight, but was still lower than that of bPEI-CO2 (Table 2). Apart from the main and broad endothermic peak, both C12-

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Fig. 5. 1H and 13C NMR spectra of bPEI, C4-bPEI and their CO2 adducts in D2O. Note that eight carbon signals were detected in bPEI and are labelled according to the type of the adjacent amino groups. The numbers 1, 2, and 3 illustrate primary, secondary, and tertiary amino groups, respectively. For example, C2e1 represents the underlined carbon in eNHCH2CH2NH2. Likewise, the other carbon is denoted as C1e2.

bPEI-CO2 and C12-bPEI-CO2 showed a typical amorphous XRD pattern (Fig. S3). By the way, C16-bPEI showed a melting process in the DSC curve too, with a Tm of 28.7  C (Fig. S4). The other samples from C4- to C12-bPEI were liquids at room temperature. 3.3. Dispersibility of the CO2 adducts in the PU raw materials Fig. 7 shows macroscopic and microscopic photographs of the white components (Formulation I, Table 1) containing different blowing agents. The control sample without any blowing agent (Blank) was clear for the entire observation time. For the same period, both bPEI-CO2 and C16-bPEI-CO2 samples, conversely, were turbid and their microscopic images consisted of relatively large particles. Samples containing C4-, C8- and C12-bPEI-CO2 were much clearer. In particular, C8-bPEI-CO2 seemed to dissolve in the white component, without suspending particles both macroscopically and microscopically. Taking into consideration both macroscopic and microscopic observations, the dispersibility of all the blowing agents can be ranked as C8-bPEI-CO2 > C12-bPEICO2 > C4- bPEI-CO2> C16-bPEI-CO2 z bPEI-CO2. 3.4. PU foaming with the CO2 adducts

Fig. 6. TG and DSC curves of various Cn-bPEI-CO2 samples and the C16-bPEI control. Tg: glass transition temperature; Tm: melting temperature.

and C16-bPEI-CO2 samples showed a baseline drop in the corresponding DSC curve (see arrowed line in Fig. 6b), typical for a glass transition. C16-bPEI-CO2 was more unique, showing a melting temperature (Tm) of about 54.7  C. The crystalline structure was further confirmed by a sharp peak at 2q of 21.42 in the X-ray diffraction (XRD) pattern of C16-bPEI-CO2 (Fig. S3). In contrast, both

Fig. 8 presents the FTIR spectra of PU foams blown by different blowing agents (Formulations I and II, Table 1). All the spectra are similar, in particular, there is no obvious difference between Blank and the CO2 adduct blown samples. This is not surprising taking into consideration that each blowing agent accounted for only 2.4% (Formulation I) or 5.2% (Formulation II) by weight in the corresponding formulation (Table 1). Characteristic bands of PUs were observed, such as NeH stretching at 3400 cm1, C]O stretching at 1729 cm1, benzene ring vibration at 1600 cm1 and CeN stretching in the urethane group at 1221 cm1. Residual eNCO groups were observed at 2276 cm1 because of the overdosed PMDI in the formulation (isocyanate index was 1.08 or 1.55, Table 1). The peak at 2137 cm1 was related to carbondiimide (eN]C]Ne) groups that usually exist in the PMDI raw material. The peak height ratio of both groups (i.e. H2276/H2137) shows the relative content of residual eNCO groups in the corresponding foam. Although foams based on Formulation I possessed far fewer eNCO groups than those based on Formulation II, both sets of foams demonstrated a similar change trend in H2276/H2137 value. The highest and lowest values were always found in the corresponding blank and bPEI-CO2

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Fig. 7. Evolution of macroscopic and microscopic photographs of the white components (Formulation I, Table 1) with and without blowing agent.

blown samples, respectively. PU foams from C8- and C12-bPEI-CO2 possessed relatively lower H2276/H2137 values than those from C4and C16-bPEI-CO2. Fig. 9 further reveals the morphological change across the PU foams. Among the 0.5 g blowing agent group (Formulation I, Table 1), the foams blown by C8-bPEI-CO2 displayed the finest and most homogeneous pores. In the 1.6 g dosage group (Formulation II, Table 1), the foams blown by C8- and C12-bPEI-CO2 were similar in morphological homogeneity, while those blown by bPEI-CO2, C4and C16-bPEI-CO2 were relatively inhomogeneous. Fig. 10 shows the density and mechanical strength of PU foams based on Formulations I and II (Table 1). All foams blown by the CO2

adducts displayed a lower density than the blank foam blown by water impurity in the raw materials. In the case of Formulation I containing 0.5 g of blowing agent, foams from bPEI-CO2 and C8bPEI-CO2 showed a significantly lower density than those from other CO2 adducts (Fig. 10a). For PU foams blown by 1.6 g of blowing agent (Formulation II), their density increased with increasing grafting side chain length in the blowing agents (Fig. 10c). The compressive strength positively correlated with the foam density (Fig. 10b and d). It scattered relatively narrowly and uniformly at a blowing agent dosage of 0.5 g. When the dosage was increased to 1.6 g, the samples obviously divided into high and low strength groups as the foams blown by C8-, C12- and C16-bPEI-CO2 were

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Fig. 8. FTIR spectra of PU foams blown by bPEI-CO2 and Cn-bPEI-CO2s at dosages of 0.5 and 1.6 g (Table 1). The peak height ratios of yeNCO at 2276 cm1 and yeN]C]Ne at 2137 cm1 are shown, based on local baselines.

Fig. 9. Morphology of PU foams blown by various blowing agents at 0.5 g (Formulation I, Table 1) or 1.6 g (Formulation II, Table 1) loading.

much stronger than those by bPEI-CO2 and C4-bPEI-CO2. It is reasonable that the increased volume between the blank sample and the CO2 adduct blown sample represents the entrapped CO2 volume in the corresponding foam. Similarly, the percentage of the entrapped CO2 volume relative to the theoretically released CO2 volume of each blowing agent indicates the foaming efficiency. As shown in Table 3, the foaming efficiency depended on the blowing agent type and dosage. However, C8-bPEI-CO2 demonstrated the highest foaming efficiency, regardless of the blowing agent dosage. 4. Discussion 4.1. Side chain length effect on CO2 absorption and release We changed the side chain length in Cn-bPEIs and then prepared their CO2 adducts as new blowing agents for PUs. Their chemical structures were confirmed by FTIR and NMR spectra (Figs. 2 and 5). The consistency between the measured and theoretical grafting rate of Cn-bPEIs (Table 2) clearly indicates the

successful material synthesis. The grafting rate was maintained at about 13% since our unpublished data shows that a grafting rate between 10% and 13% is optimal in terms of raw material compatibility and foaming efficiency. The measured weight loss in each Cn-bPEI-CO2 (Fig. 6 and Table 2) indicated its real CO2 content. The gap between the real CO2 content and theoretical CO2 content increased at increasing the alkyl side chain length (Table 2). As a matter of fact, the degree of CO2 saturation (equalling real CO2 content divided by theoretical CO2 content) reached 95% in bPEI-CO2, and decreased to 85%, 81%, 78% and 70% for C4-, C8-, C12- and C16-bPEI-CO2, respectively (Table 2). This phenomenon might be related to the strong hydrophobic association among the alkyl side chains which decreased the mobility of the Cn-bPEI macromolecules, in particular for those with long alkyl side chains. With the advance of the CO2 adduction, the consequent zwitterionic backbones increased in rigidity, further reducing the macromolecular mobility. The highly immobilised molecular chains at the final stage of CO2 adduction made it difficult for the remote and isolated amino groups to react with CO2

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Fig. 10. Effect of blowing agent type and loading on the density and mechanical property of PU foams. ***: P < 0.001; **: P < 0.01; *: P < 0.05. NS: not statistically significant.

Table 3 Entrapped CO2 compared with the theoretically released CO2 during the foaming process. Blowing agenta 0.5 g, Formulation I Blank bPEI-CO2 C4-bPEI-CO2 C8-bPEI-CO2 C12-bPEI-CO2 C16-bPEI-CO2 1.6 g, Formulation II Blank bPEI-CO2 C4-bPEI-CO2 C8-bPEI-CO2 C12-bPEI-CO2 C16-bPEI-CO2

Density (kg/m3)

Foam volume (cm3)

Entrapped CO2b (ml)

Theoretically released CO2c (ml)

Foaming efficiencyd (%)

127.8 ± 1.1 89.6 ± 1.0 93.6 ± 1.2 89.7 ± 1.8 96.1 ± 1.7 99.7 ± 2.9

166.20 237.05 226.92 236.79 221.02 213.04

70.85 60.72 70.59 54.82 46.84

112.01 79.66 70.27 61.57 51.13

63.3 76.2 100.0 89.0 91.6

108.9 ± 4.1 59.0 ± 2.2 61.8 ± 1.6 64.8 ± 1.5 72.7 ± 3.6 75.9 ± 1.4

280.53 517.80 494.34 471.45 420.22 402.50

237.27 213.81 190.92 139.69 121.97

358.59 248.09 218.84 191.75 159.25

66.2 86.2 87.2 72.8 76.6

a

No blowing agent was used in blank samples. Calculated by subtracting blank foam volume from each CO2 adduct-blown foam volume. c Calculated based on the moles of CO2 (CO2 content from TG curve) in each blowing agent, the highest temperature of foaming process (about 100  C and 90  C for Formulation I and II, respectively) and the standard air pressure (P ¼ 101325 Pa), using the ideal gas law as PV ¼ nRT. d Obtained from entrapped CO2 volume divided by theoretically released CO2 volume. b

since CO2 adduction usually involves two amino groups. For C16bPEI-CO2, the crystallisation of the C16 alkyl chains (Fig. 6b) further hindered the CO2 adduction, with about 30% of the amino

groups in the original C16-bPEI backbone not reacting with CO2 (Table 2). The observed glass transition in C12- and C16-bPEI-CO2 (Fig. 6b)

C. Liu et al. / Polymer 116 (2017) 240e250

could be associated with the thermal relaxation of the alkyl microdomains in these materials. For CO2 adducts with shorter alkyl side chains, the weaker side chain association and lower side chain content might hinder the microphase separation. In addition, the C16 side chains crystallised in both C16-bPEI and C16-bPEI-CO2 (Figs. S4 and S3). The phenomena of microphase separation and side chain crystallisation have been observed in various comb-like polymers [30], including alkylated polyethylenimines [31]. The alkylation can preferentially consume primary amines in the original bPEI due to steric hindrance in the secondary amines. The steric hindrance of the alkylation agent itself can also inhibit grafting onto the secondary amines, meaning that a long alkyl chain tends to graft onto primary amines, rather than secondary amines. Consequently, the amount of primary amine-derived carbamate anions (eNHCOO) decreased in sequence of bPEI-CO2, C4-, C8-, C12- to C16-bPEI-CO2. So did the upper limit of the CO2 releasing temperatures across these samples (Fig. 6). This was related to the fact that the eNHCOO groups were more difficult to decompose upon heating than the >NCOO groups (derived from secondary amines) due to the extra H-bonding among the eNHCOO groups. 4.2. Effects of the side chain length on the foaming process The alkyl side chains in Cn-bPEI-CO2s tend to swell into the liquid portion of the white component containing a castor oilderived polyol (Polycin® T-400, Table 1). This polyol, except for the hydroxyl groups, is structurally similar to alkyl chains. Thus, the Cn-bPEI-CO2 samples are generally more dispersible in the white component than bPEI-CO2 (Fig. 7). C16-bPEI-CO2 is an exception whose crystallised C16 side chains (Fig. 6b and Fig. S3) can largely hinder the macromolecular reorganisation to disperse into the white component. Among all the CO2 adducts investigated, C8bPEI-CO2 demonstrated the best dispersibility into the white component (Fig. 7). One possible reason was that this blowing agent possessed a suitable alkyl chain length. Another reason might come from the branched C8 alkyl structure (Fig. 1) which loosened the side chain association, enhanced the macromolecular mobility and thereby favoured a fast dispersion into the white component. The dispersibility of the blowing agent could affect the followed foaming process. The FTIR spectra (Fig. 8) and morphological observations (Fig. 9) clearly indicated the successful preparation of PU foams. During the exothermic polymerisation of the PUs, the blowing agent released CO2 and gradually restored its polyamine structure. The restored amino groups could react with the isocyanate groups at the ends of the growing PU chains, as assumed previously [19e21]. This assumption was proved in the current study by FTIR spectroscopy. The obvious decrease in peak height ratio of H2276/H2137 from the blank foams to bPEI-CO2 blown foams (Fig. 8) indicated far fewer isocyanate groups left in the latter. All the alkylated CO2 adducts (Cn-bPEI-CO2s) possessed higher H2276/ H2137 values than bPEI-CO2 because the steric hindrance from the alkyl side chains inhibited the restored amino groups from reacting with the isocyanate groups, at least to some extent. Apart from this steric hindrance, the dispersion of blowing agent into the foaming mixture could also play a role. The relatively good dispersibility of C8- and C12-bPEI-CO2 (Fig. 7) could enlarge the interface area between the blowing agent and PU foaming mixture, thus enhancing the reaction possibility between the restored amines and isocyanate groups. Therefore, relatively lower H2276/H2137 values were found in both the C8- and C12-bPEI-CO2 blown foams (Fig. 8). Despite the delicate variation in H2276/H2137 values, the FTIR spectra (Fig. 8) overall indicated that the degree to which the CO2 adducts interfered with the PU chain growth was very small because an obvious isocyanate adsorption at 2276 cm1 could still be observed at an isocyanate index as low as 1.08 (Formulation I,

249

Table 1). There is no need to adopt high isocyanate indices for future formulation designs using Cn-bPEI-CO2 as blowing agents. It can be deduced that C8-bPEI-CO2 must have generated the finest particles in the foaming mixture because it exhibited the best dispersibility (Fig. 7). During the foaming process, these dispersed particles on the one hand released CO2 to blow the PU, and on the other hand, they served as nuclei of the gas phase. As a result, C8bPEI-CO2 blown foams displayed the finest and most homogeneous pores among the foams investigated (Fig. 9), particularly in the 0.5 g dosage group (Formulation I, Table 1). The observed large pores in others were due to the large particles of blowing agent in the corresponding foaming system (Fig. 7). The foaming efficiency of the CO2 adducts (Table 3) can also be related to their dispersibility in the foaming mixtures. The large dispersed particles could generate large CO2 bubbles that might have broken up before the foam set. The good dispersibility of C8bPEI-CO2 would inhibit the formation of large bubbles with a high potential to break. Therefore, this blowing agent displayed a relatively high foaming efficiency (Table 3). Another factor affecting the foaming efficiency was the CO2 releasing temperatures of the blowing agents. For instance, bPEI-CO2 required a relatively higher temperature to completely release CO2 than others (Fig. 6). This might cause insufficient CO2 release before the foam solidification, lowering its foaming efficiency, in addition to its poor dispersibility. The third factor was the foaming art. This can be seen from the change of foaming efficiency from Formulation I to Formulation II (Table 3). The polymerisation speed was lowered in the latter because less T-9 catalyst was used (Table 1). This slowdown increased the foaming efficiency of both bPEI-CO2 and C4-bPEI-CO2, possibly because the more sufficient release of CO2 played a major role. On the contrary, C8-, C12- and C16-bPEI-CO2 showed a decrease in foaming efficiency. The breakage of large bubbles (due to long term bubble growth) might cause this phenomenon. Increasing the amount of foam stabiliser in the formulation may relieve this bubble breakage. The foam density depends on the CO2 content (Table 2) and the foaming efficiency (Table 3) of each blowing agent. At a blowing agent dosage of 0.5 g (Formulation I, Table 1), bPEI-CO2 and C8bPEI-CO2 generated foams with comparable densities, which were lower than other samples (Fig. 10a), despite much more CO2 being absorbed in the former (Table 2). For the 1.6 g dosage group (Formulation II, Table 1), C8-bPEI-CO2 was not the most effective in decreasing the foam density (Table 3 and Fig. 10c). However, C8bPEI-CO2 blown foams were much stronger than those from bPEICO2 and C4-bPEI-CO2 (Fig. 10d). The density increased only by 5% from C4-bPEI-CO2 blown foams to C8-bPEI-CO2 blown foams (Table 3), while the compressive strength increased by about 50% between the two samples (Fig. 10d). The more homogeneous pores observed in foams from C8-bPEI-CO2 (Fig. 9) accounted for this unusual increase in mechanical strength. Overall, alkylation of bPEI is important to make the resultant CO2 adducts dispersible in the PU foaming mixture that contains biomass-based polyols like castor oil. Good dispersibility of the CO2 adducts is essential for generating strong PU foams with homogeneous pores. Among the investigated CO2 adducts, C8-bPEI-CO2 is the most suitable blowing agent for PU foams from castor oilderived polyols, in terms of raw material compatibility, high foaming efficiency and good mechanical strength. 4.3. Remarks and future works Previous works have involved the alkylation of polyethylenimines to form amphiphilic core-shell nanocarriers for guest molecule encapsulation where long alkyl chains from C12 to C18 are frequently used and the alkylation degree ranges from 15%

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to 86% [32e34]. These alkylated polyethylenimines are not suitable for encapsulating CO2 to blow PUs because the long alkyl chain and high grafting rate compromise the CO2 capacity. Therefore, we synthesised a series of alkylated bPEIs at a relatively low grafting rate of 13% by changing the alkyl length from C4 to C16. These bPEIs and their CO2 adducts were systematically characterised. As expected, these CO2 adducts are chemically compatible with PU foaming mixtures containing a castor oil-based polyol and can be used as CO2 releasing blowing agents for PUs. In this work, the best blowing agent with a C8 alkyl side chain (C8-bPEI-CO2) possesses a CO2 content of 20.2% (Table 2). In comparison, the reported CO2 content in another PPG-grafted counterpart is only 17.6% [20]. The latter CO2 adduct is optimised for traditional PU foams from polyether polyols, rather than PU foams from biomass polyols. To the best of our knowledge, this is the first report of CO2 releasing blowing agents for biomass-based PU foams. The blowing agents we developed emit nothing but CO2 into the PU foams and later into the atmosphere. CO2 can come from the air and/or industrial waste gases, therefore the new blowing agents are environmentally neutral, avoiding the risks of ozone depletion and global warming that are usually associated with traditional HCFC and HFC blowing agents. Moreover, the optimised blowing agent C8-bPEI-CO2 has been approved to be suitable for castor oil-based PU foams. Therefore, the PU foams from the environmentfriendly blowing agent and the renewable polyol in this study provide a new avenue towards much greener PU foams. The density and compressive strength of the current foams from C8-bPEI-CO2 (about 65 kg/m3 and 300 kPa, Fig. 10) almost meet the required values (40e60 kg/cm3 and >100 kPa) of PU foams for thermal insulation of underground steel pipes (see Chinese standard SY/T 0415). Future work can optimise the foam preparation art to achieve better properties. With the emergence of more and more plant oil-derived polyols for PUs [11,12], the following years will see attempts to apply these new CO2 adducts to blow PUs from those biomass polyols, with detailed studies about the structure-property relationships. 5. Conclusions A series of CO2 adducts from bPEIs with C4 to C16 alkyl side chains were synthesised as environmentally friendly blowing agents for PUs based on a castor oil-derived polyol. The alkylation favoured the dispersibility of the blowing agents into the PU foaming mixtures. Overall, the CO2 adduct containing a C8 (2ethylhexyl) side chain (C8-bPEI-CO2) was the best in terms of good raw material compatibility and high foaming efficiency. In addition, the foams from this blowing agent possessed the most homogeneous pores, showing a higher compressive strength than other inhomogeneous foams with a similar foam density. As a first report regarding PU foams based on CO2 releasing blowing agents and renewable polyols, this study provides a new avenue towards the next generation of green PU foams. Acknowledgments We thank Professor Yajiang Huang at Sichuan University, China

for FTIR spectroscopy and are grateful for financial support from the National Natural Science Foundation of China (Grant No. 51173111).

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2017.03.079.

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