Thermosetting polyurethanes prepared with the aid of a fully bio-based emulsifier with high bio-content, high solid content, and superior mechanical properties

In this paper, a novel, fully bio-based emulsifier has been successfully prepared from epoxidized soybean oil and glutaric acid through a solvent-free and self-catalysis method. The effects of reaction time and carboxyl : epoxy molar ratios on the structures of the emulsifier were systematically investigated. This novel emulsifier exhibits properties similar to those of dimethylolpropionic acid (DMPA) and dimethylolbutanoic acid (DMBA), with hydroxyl groups serving as crosslinking agents and the carboxylic acid group acting as an ionic segment. In order to validate the robustness of this emulsifier, a series of anionic, waterborne polyurethane dispersions were prepared from typical polyols (vegetable oil- and petroleum-based). The structure of this emulsifier and its good compatibility with bio-based polyols confer the resulting dispersions excellent storage stability and high solid content (up to 45%). The prepared waterborne polyurethane films exhibited higher toughness and thermal stability than traditional solvent-based polyurethane films and waterborne polyurethane films prepared from DMPA and DMBA. Moreover, a high bio-based content of up to 74% was achieved for the prepared polyurethanes. This new environmentally friendly, liquid emulsifier, prepared using a solvent-free and self-catalysis synthetic route, can potentially replace typical petroleum-based emulsifiers for the production of waterborne polyurethanes.

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Introduction

Polyurethanes (PUs) are among the most versatile polymers and have been widely used in various applications, such as leather, coatings, foams, and inks.^1,2 However, conventional PU products usually contain large amounts of organic solvents, which is hazardous to human health and the environment. Increasing concern with the emission of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) has led to strong interest in the development of waterborne PU products.

Waterborne PUs are environmentally friendly and exhibit exceptional performance, such as tunable mechanical properties and adhesion to various substrates.^3,4 Generally, polyurethanes, formed by step-growth polymerization of diisocyanates with polyols, are hydrophobic. Therefore, the use of emulsifiers is necessary in order to disperse the polymers in water. The introduction of internal emulsifiers in the polymer was found to be more advantageous compared to external emulsifiers.⁵ Dimethylolpropionic acid (DMPA) and dimethylolbutanoic acid (DMBA) are two internal emulsifiers that have been widely used as hydrophilic chain extenders in the synthesis of anionic PUDs. The main advantage of these internal emulsifiers is that their primary hydroxyl groups are much more reactive than the carboxylic acid group toward isocyanate. Therefore, hydroxyl groups preferentially react with isocyanates to form a large molecular polyurethane backbone and carboxylic acid groups provide surface charge for the stabilization of the resulting polyurethane in the aqueous phase. Several PUD systems have been synthesized with these two internal emulsifiers and various polyols, including petroleum- and bio-based polyols.

However, the use of these two internal emulsifiers leads to some drawbacks. For instance, the high melting point (185–190 °C and 108–115 °C, respectively) and their low solubility leads to the heavy use of solvents such as N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP) during the synthesis of PUDs.^6–8 These solvents are not only hazardous but also difficult to be completely removed from the PUDs. In addition, the high melting points of DMPA and DMBA lead to the need for a high homogenization temperature, which results in an undesired extended cooling time.⁵ Furthermore, the different reactivities of hydroxyl groups from these two reactants, in comparison with other polyols, toward isocyanates lead to an uneven distribution of carboxylic acid groups throughout the PU backbone, resulting in poor stability and performance.⁹

Therefore, the development of liquefiable emulsifiers with compatible reactivity with polyols has attracted much attention. Tatai et al. synthesized a biodegradable emulsifier from ethylene glycol and lactic acid.¹⁰ Wu and Chen prepared liquefiable dimethylol propionic acid (LDMPA) by a condensation reaction between DMPA and ε-caprolactone and successfully synthesized a series of PUDs.¹¹ Brannigan et al. prepared an emulsifier from amino acid-derived diols, and thermoplastic polyester-urethanes therefrom.¹² Fu et al. prepared new hydrophilic emulsifier from castor oil (CCE) and 3-mercaptopropionic acid.¹³ Karak reported the synthesis of environmentally friendly polyurethane dispersions derived from a biobased chain extender prepared from citric acid and glycerol.¹⁴ However, the syntheses of the above emulsifiers involve the use of large amounts of organic solvents, which increases both process cost and environmental burden. In addition, most starting materials utilized are derived from petroleum feedstock, so the bio-content of the waterborne PUD system is limited.

With increasing environmental concerns and rapid depletion of petroleum feedstock, the utilization of renewable resources for the synthesis of chemicals to replace their petroleum-based counterparts has attracted much attention from industry and academia. Triglycerides are among the most promising options with several advantages over other starting materials, such as ready availability, low cost, and renewable characteristics. In this study, a new internal emulsifier was synthesized from soybean oil and glutaric acid through a solvent-free and catalyst-free method. The reaction conditions of the synthesis were optimized, and the structure of the bio-based chain extender was elucidated. The resulting emulsifier exhibits similar properties to those of DMPA and DMBA, with the hydroxyl group serving as a crosslinking agent and the carboxylic acid group acting as an ionic segment. A series of bio-based anionic waterborne polyurethane dispersions were prepared from this new emulsifier and typical bio-based and petroleum-based polyols. The particle size and zeta potential of PUDs were determined using a zeta-sizer. The thermal and mechanical properties of the PU films were studied by dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA) and tensile testing. The effect of the functionalities and chemical structure of the emulsifier and polyols on the properties of the polyurethane dispersions and the resulting films was investigated and discussed.

The work described in this manuscript has several advantages in comparison with the previously published work.¹⁵ Most notably, this work brings improvements in the use of green and sustainable raw materials, proposes a simplified preparation method, and leads to higher performance of the final polyurethane films obtained. Glutaric acid is a natural, renewable, weak diacid that is widely used in the production of polyester polyols and polyamides. The utilization of this natural raw material leads to a fully bio-based emulsifier, as opposed to the use of ortho-phosphoric acid,¹⁵ which is mainly produced from petroleum feedstock, resulting in emulsifiers that are only partially bio-based. Additionally, the utilization of a strong acid represents a scale-up issue associated with the potential corrosion of equipment. Another advantage of the currently proposed system in comparison with previous approaches¹⁵ is the use of a solvent-free procedure, therefore meeting the Green Chemistry criteria. Furthermore, in order to validate the robustness of the proposed emulsifier, a series of anionic, waterborne polyurethane dispersions are prepared using typical vegetable oil- and petroleum-based polyols, whereas previously published data rely solely on the use of 1,6-hexanediol.¹⁵ Finally, in order to improve the performance of the resulting polyurethanes, APTES was previously incorporated in the polymer chains,¹⁵ which increased the complexity of the polymer preparation. In the work presented here, the use of APTES is not necessary. Higher solid and bio-based contents are obtained, along with a higher tensile strength.

Experimental

Materials

Epoxidized soybean oil (ESO) with approximately 3.9 oxirane rings per triglyceride (as determined from the NMR spectrum of the epoxidized soybean oil) was bought from Hairma Chemical (Guangzhou, China). This molar ratio of oxirane rings per triglyceride translates into an oxirane value of 6.6, which is typical of commercial epoxidized soybean oil. Linseed oil-based polyol (LOP),¹⁶ soy-castor oil-based polyol (SCP), and linseed-castor oil-based polyol (LCP) were prepared previously in our research lab.^17,18 Castor oil (CO) and sodium chloride (NaCl) were provided by Fuyu Chemical Company (Tianjin, China). Polypropylene glycol (PPG, M_w = 200 g mol⁻¹ and 800 g mol⁻¹) was obtained from Jining Huakai Resin Co., Ltd (Jining, China). Polycarbonate diol (PCDL, M_w = 500 g mol⁻¹ and 1000 g mol⁻¹) was purchased from Asahi Kasei Corporation (Shanghai, China). The structure and hydroxyl numbers of all polyols used in this study are shown in Fig. 1. Isophorone diisocyanate (IPDI) was obtained from Wengjiang Chemical Reagent Company (Guangzhou, China). Dibutyltin dilaurate (DBTDL) and dibutylamine were purchased from Fuchen Chemical Reagent Factory (Tianjin, China). Methyl ethyl ketone (MEK) was purchased from Tianjin Hongda Chemical Reagent Co. (Tianjin, China). Triethylamine (TEA), magnesium sulfate (MgSO₄), glutaric acid (99%), and ethyl acetate were purchased from Aladdin Reagent Co. (Shanghai, China). Absolute ethanol was purchased from Tianjin Damao Chemical Reagent Co. (Tianjin, China). All materials were used as received without further purification.

Fig. 1 Representative structure of all polyols used in this study.

Synthesis of the emulsifier from ESO and glutaric acid

The bio-based emulsifier was prepared by a ring-opening reaction between ESO and glutaric acid. Firstly, ESO was added dropwise into a double neck round-bottom flask at 109 °C under vigorous stirring with excess glutaric acid. After the reaction continued for a certain period of time, the reactant was diluted with ethyl acetate, washed with saturated NaCl solution at least four times, dried with anhydrous MgSO₄ and filtered. Finally, the emulsifier was obtained after the removal of the organic solvent by rotary evaporation and drying under vacuum at 45 °C overnight. Scheme 1 shows the schematic for the preparation of the emulsifier.

Scheme 1 Schematic route for the preparation of the bio-based emulsifier.

Synthesis of waterborne polyurethane dispersions

Firstly, the polyol, IPDI and DBTDL (2 wt% of the polyol) were fed into a double neck round-bottom flask under vigorous stirring and maintained at 78 °C for 10 min–3 h. Then, the emulsifier was added in the mixture. The molar ratios of the NCO groups of IPDI, the OH groups of the polyol and the OH groups of the emulsifier are listed in Table 1. After 3 h, 9–15 ml of MEK was added to the reaction mixture in order to reduce the viscosity of the system. After another 1 h, the mixture was cooled to room temperature and then neutralized with triethylamine (1 equivalent to the acid number of the emulsifier), followed by dispersion at high speed (400 rpm to 600 rpm) with distilled water for 30 min. Finally, the PUDs were obtained after the removal of MEK using a rotary evaporator. The solid contents of the PUDs were 32%–45%.

Table 1 Composition of waterborne polyurethane dispersions

Sample	OH number of the polyols (mg KOH per g)	Emulsifier content (%)	Molar ratio
			NCO : OH^a : OH^b
a Hydroxyl molar equivalents of the polyol. b Hydroxyl molar equivalents of the emulsifier.
PU-SCP	149	43.99	2 : 1 : 0.99
PU-SCPII	149	37.73	1.7 : 1 : 0.69
PU-LCP	186	46.62	2 : 1 : 0.99
PU-LCPII	186	40.55	1.7 : 1 : 0.69
PU-CO	164	45.0	2 : 1 : 0.99
PU-LOP	305	51.6	2 : 1 : 0.99
PU-PPG200	530	55.7	2 : 1 : 0.99
PU-PPG800	152	44.1	2 : 1 : 0.99
PU-PCDL500	224	48.6	2 : 1 : 0.99
PU-PCDL1000	112	39.9	2 : 1 : 0.99

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Preparation of PUD films

The resulting PUDs were poured into silicon molds and dried at room temperature to obtain films for further analysis. All the samples were dried at 60 °C for more than 12 h before the test.

Characterization

All FT-IR spectra were recorded on a Nicolet iS10 FT-IR system (Thermo Fisher, America). The ¹H NMR spectroscopic analyses of the monomers and the emulsifier were conducted by using a Bruker AV 600 spectrometer at 600 MHz. The molecular weight of the monomers and the emulsifier were determined using a THF-eluted Shimadzu Prominence GPC, equipped with an RID-10A refractive index detector. The eluent used was tetrahydrofuran with Shodex KF804L and 802.5 columns. The column flow rate and temperature were 1.0 ml min⁻¹ and 40 °C, respectively. Rheological investigation of the monomers and the emulsifier was conducted with a Kinexus Pro rotational rheometer (Malvern, UK). The hydroxyl and acid numbers of the emulsifier were determined using the Unilever method and the AOCS Official Method Te 1a-64, respectively.

In order to study the reactivity of the carboxylic acid groups during the synthesis of the PUDs, samples were collected every one hour before neutralization. The residual COOH in the polymer was measured by KOH titration. In detail, the reaction device including a double neck round-bottom flask and a stirring paddle was weighed as m_o before the preparation of the polymer, and the total weight (m₁) of the reaction device and the polymer was obtained before titration. The polymer was accurately weighed in a dried flask and coded as m₃. Then, 5 mL of MEK and 5 mL of absolute alcohol were added to dilute the polymer. In addition, excess dibutylamine solution was weighed to expend the residual NCO groups. After adding 3 drops of 0.01 g mL⁻¹ phenolphthalein pyridine solution as an indicator, the sample was titrated with 0.5 mol L⁻¹ KOH solution. Moreover, a blank titration was carried out without the polymer.

The emulsifier content in the sample is calculated using eqn (1). The relative content of unreacted –COOH remaining in the reaction mixture is calculated using eqn (2).

where M is the emulsifier content in the sample, m₂ is the weight of the emulsifier used in the reaction, V is the volume of KOH consumed during the titration (mL), and C1 is the acid number of the emulsifier used in the study.

A Tomos 3–18 centrifuge was used to characterize the stability of the PUDs by centrifuging the samples at 3000 rpm for 30 min. The particle size distributions and zeta potential of the PUDs were measured with a zeta-sizer Nano ZSE (Malvern Instruments), and the samples were diluted to approximately 0.01 wt% before the test. The water contact angle of the samples was measured with a contact angle goniometer (Powereach JC2000C1) using the sessile drop method at room temperature. An average contact angle value of more than three replicates of each sample was taken. TGA of the monomers and the resulting polyurethane films was carried out on a NETZSCH Scientific Instrument STA499C system from 30 °C to 700 °C at a heating rate of 10 °C min⁻¹ under nitrogen. Generally, 8–10 mg samples, dried at 60 °C for more than 12 h, were used in the test.

Tensile stress–strain test of the PU films were performed on an electronic universal testing machine (Shimadzu AGS-X) at an extension rate of 100 mm min⁻¹. The length and width of the samples were 25 mm and 10 mm, respectively. DMA of the resulting films was carried out using a Netzsch DMA 242C dynamic mechanical analyzer with temperatures ranging from −60 °C to 120 °C, at a heating rate of 5 °C min⁻¹ (tensile mode at 1 Hz). The size of the samples was 20 mm × 6 mm (length × width), and the glass transition temperatures (T_gs) of the resulting films were obtained from the peak maximum of the Tan delta curves.

Results and discussion

Preparation and properties of the emulsifiers

According to the TGA results of glutaric acid (Fig. S1†), onset decomposition occurs at 155 °C. Thus, the reaction temperature was chosen to be 109 °C, higher than its melting temperature (95–98 °C) and significantly lower than its decomposition temperature. In order to investigate the effects of reaction time and stoichiometry on the structures of the final emulsifier, seven reaction times were selected (0 min, 30 min, 60 min, 90 min, 120 min, 150 min, and 180 min) and four carboxyl : epoxy ratios (2 : 1, 3 : 1, 4 : 1 and 5 : 1) were studied. During the ring-opening reaction, epoxidized soybean oil was added dropwise into the glutaric acid solution. The overall mixture contained excess glutaric acid. It is expected that only one of the carboxyl groups of glutaric acid participates in the ring-opening reaction, while the other carboxyl group remains intact to provide surface charge when used as an emulsifier. For the emulsifier prepared at different reaction times, a carboxyl : epoxy mole ratio of 4 : 1 was used. The obtained emulsifier was named EG-T, where T is the reaction time (T = 30 min, 60 min, 90 min, 120 min, 150 min, or 180 min).

As shown in Fig. 2a, once the reaction time increased to 90 min, the epoxy group in the molecule of epoxidized soybean oil underwent complete ring-opening, as evidenced by the complete disappearance of the signal at 2.8–3.2 ppm.¹⁸ At the same time, new peaks are observed between 4.6 ppm and 5.0 ppm, corresponding to the tertiary hydrogen atoms adjacent to the newly formed ester groups¹⁹ resulting from the ring-opening initiated by glutaric acids. Other new peaks are seen between 3.7 ppm and 4.0 ppm, possibly representing the methyne hydrogen attached to the carbon adjacent to the hydroxyl group.¹⁷ Therefore, a reaction time of 90 min was used to optimize the stoichiometry of the two reactants. The obtained emulsifier was named EG-R, where R is the molar ratio between carboxyl and epoxy groups (2 : 1, 3 : 1, 4 : 1, 5 : 1, R = 2, 3, 4, and 5).

Fig. 2 ¹H NMR spectra of the emulsifier prepared for (a) different reaction times and (b) different carboxyl : epoxy molar ratios.

As the carboxyl : epoxy molar ratio increases, as shown in Fig. 2b, the peaks at 2.8–3.2 ppm corresponding to epoxy groups decrease when the reaction time was fixed at 90 min. The epoxy groups were totally consumed when the carboxyl : epoxy molar ratio reached 3. FTIR results also confirm the successful ring-opening reaction initiated by glutaric acid and the new formation of hydroxyl groups. More details are provided in the ESI.†

In order to further confirm the successful grafting of glutaric acid on ESO, GPC of EG-T for different reaction times was carried out. The results are shown in Fig. 3. Compared to the peaks for glutaric acid and ESO, the peaks for EG-T become broadened and shift to lower retention times, representing an increase in molecular weight. Apart from the main peaks at approximately 16 min, there are another two peaks at 14.5 min and 15 min, indicating the formation of oligomers. The possible reason is that two carboxylic acid groups in one glutaric acid molecule both participate in the ring-opening reaction, resulting in the partial connection of two ESO molecules.¹⁷ Also, the newly formed hydroxyl group can lead to pre-crosslinking of the emulsifier. As the reaction time increases, the main peaks of the emulsifier shift to lower retention times, indicating an increase in molecular weight. However, as the reaction time increases from 90 min to 180 min, the GPC peaks almost overlap with each other, indicating no significant change in the molecular weight. This occurs due to the complete reaction of the epoxy groups (as shown in the NMR results) when the reaction time exceeds 90 min and the carboxyl : epoxy molar ratio is 4 : 1, leading to no more glutaric acid grafting on ESO.

Fig. 3 GPC curves of the emulsifier prepared at different reaction times.

The acid content is very crucial for an internal emulsifier which provides surface charge after neutralization to disperse the polyurethane backbone in the aqueous phase. Thus, the effect of the carboxyl : epoxy molar ratio on the acid number of the emulsifier was studied. As shown in Fig. 4, the acid number of the emulsifier increases with the increase in reaction time, and it exhibits three time-dependent stages: rapid increase (0–30 min), slow increase (30–60 min) and equilibrium stage (60–180 min). This result is consistent with the results of ¹H NMR and FTIR. As glutaric acid reacts with the epoxy groups and grafts on ESO,²⁰ the acid number of the emulsifier increases until all epoxy groups are consumed.

Fig. 4 The acid number of the emulsifier as a function of the reaction time at different carboxyl : epoxy molar ratios.

With the increase in the carboxyl : epoxy molar ratio, the acid number gradually increases at a consistent reaction time. For example, the acid number was 73.2 mg KOH per g with a carboxyl : epoxy molar ratio of 2, and 104.6 mg KOH per g when the carboxyl : epoxy molar ratio was 5 at a consistent reaction time of 180 min. When the carboxyl : epoxy molar ratio was >4, the acid number of the samples did not change significantly at a reaction time of 180 min. The rheological behavior of the emulsifier prepared at different reaction times and with different carboxyl : epoxy molar ratios was also studied and is shown in Fig. S4 and Table S1.†

According to the above discussion, the reaction time of 90 min and the carboxyl : epoxy molar ratio of 4 : 1 were set as optimized reaction conditions in order to obtain emulsifiers with the highest hydroxyl and acid numbers. These conditions also maintain the highest atom efficiency and the lowest energy consumption for the system. The characteristics of the emulsifier prepared under the optimized conditions are listed in Table 2. The resulting green, 100% bio-based emulsifier, together with typical polyols (vegetable oil- and petroleum-based), was used to prepare anionic waterborne polyurethane dispersions.

Table 2 Characteristics of the optimal emulsifier

Properties	Experimental value
Physical state at 30 °C	Liquid
Viscosity at 30 °C at 25 s^−1 shear rate (Pa s^−1)	55.64
Acid value (mg KOH per g)	119.15 ± 1.76
OH value (mg KOH per g)	99.58 ± 2.11
M w/Mn	5331/2980
PDI (Mw/Mn)	1.7889

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Preparation and properties of anionic waterborne polyurethane dispersions

In order to confirm its viability, typical vegetable oil- and petroleum-based polyols (polyester and polyether polyols) were used to prepare anionic waterborne polyurethane dispersions with the new emulsifier. The compatibility between the emulsifier and the polyols was confirmed by the visual observation of the mixtures. No phase separation or precipitates were observed for several months. Four vegetable oil-based polyols (CO, LOP, SCP, and LCP) and four petroleum-based polyols (PCDL 500, PCDL 1000, PPG 200, and PPG 800) were used in the study. Castor oil is the only vegetable oil naturally containing hydroxyl groups. LOP is a vegetable oil-based polyol prepared by the thiol–ene photo-click reaction.¹⁶ SCP and LCP are fully bio-based polyols with pre-crosslinking structures previously developed by our research group.^17,18 PCDL 500 and PCDL 1000 are petroleum-based polyester polyols with molecular weights of 500 g mol⁻¹ and 1000 g mol⁻¹, respectively. PPG 200 and PPG 800 are petroleum-based polyether polyols with molecular weights of 200 g mol⁻¹ and 800 g mol⁻¹, respectively. It is worth noting that the synthesis of anionic waterborne polyurethane dispersions from SCP and LCP with DMBA and DMPA was unsuccessful due to local over-crosslinking induced by the non-uniform distribution of soft and hard segments. The effect of the new emulsifier and the polyols’ structures on the final properties of polyurethane dispersions and films was systematically studied.

A representative schematic for the synthesis of anionic waterborne polyurethane dispersions with the new emulsifier and castor oil is outlined in Fig. 5. The NCO groups of IPDI react with the OH groups from polyols to form the prepolymer followed by the reaction with OH groups from the emulsifier. The anionic waterborne polyurethane dispersions are obtained after deprotonation and dispersion. FTIR spectroscopy measurements were carried out to investigate the mechanism of the reaction of the prepolymer with the emulsifier. As shown in Fig. 6, the N–H stretching peak gradually increased and shifted from 3411 cm⁻¹ to 3360 cm⁻¹ with increasing reaction time from 0 h to 5 h, indicating the formation of new urethane groups (the FTIR spectrum of the polymer obtained from the reaction of castor oil–IPDI prepolymers and the emulsifier at different times is normalized with the peak of –CH as the standard). In addition, the NCO stretching peaks at 2260 cm⁻¹ decrease drastically in the first hour of the reaction. No significant change was observed after 3 h, possibly due to reaction completion.

Fig. 5 Representative synthesis of an anionic waterborne polyurethane dispersion with the new emulsifier and castor oil.

Fig. 6 FTIR spectra of the polymer obtained from the reaction of castor oil–IPDI prepolymers and the emulsifier at different times.

One advantage of using DMPA and DMBA as the internal emulsifiers is that the carboxylic acid groups do not react with the isocyanate groups, remaining free to provide the surface charge.²¹ In order to confirm that the –COOH groups remain intact during the synthesis, KOH titration was performed. It was found that approximately 12–17% of –COOH groups have been consumed during the reaction with diisocyanates. The possible reaction is the formation of carbamic-carboxylic anhydride through condensation between COOH and NCO,²² as evidenced by the appearance of a signal at approximately 1790 cm⁻¹ (corresponding to carbonyl from anhydrides). Then, these peaks gradually decrease and disappear after 5 h. In addition, the amide carbonyl stretch at 1594–1673 cm⁻¹ appeared after 1 h and gradually increased. The carbamic-carboxylic anhydride intermediates quickly led to the formation of amide and CO₂, as shown in Fig. 7.^22,23

Fig. 7 The possible reaction between NCO groups from IPDI and carboxylic acid groups from the emulsifier.

Anionic waterborne polyurethane dispersions have been successfully prepared from the emulsifier and typical bio- and petroleum-based polyols as shown in Fig. 8. The effect of different molar ratios between the hydroxyl groups of the polyols and the hydroxyl groups of the emulsifier on the properties of the polyurethane dispersions and the resulting PU films were also systematically studied in this work.

Fig. 8 (a) PUDs from the emulsifier and the vegetable oil-based polyols: (1) SCP, (2) SCPII, (3) LCP, (4) LCPII, (5) CO, and (6) LOP; (b) PUDs from the emulsifier and the petroleum-based polyols: (1) PPG200, (2) PPG800, (3) PCDL500, and (4) PCDL1000. Particle size distribution of (c) PUDs from the vegetable oil-based polyols, and (d) from the petroleum-based polyols.

The appearance of the resulting PUDs gradually changes from milky white to transparent with increasing hydroxyl number of the polyols (Fig. 8). For example, the PU from LOP (305 mg KOH per g) exhibits a transparent appearance while the PU from SCP (149 mg KOH per g) exhibits a milky white appearance. With an increase in the emulsifier content, the appearance of the resulting PUDs becomes more transparent. For instance, the appearance of the PUD from LCP (46.42% emulsifier content) is much more transparent than that of the PUD from LCPII (40.55% emulsifier content). All the dispersions from petroleum-based polyols are translucent yellow solutions. Generally, the carboxylic acid group in the emulsifier provides surface charge for the polyurethane backbone followed by self-assembly into polyurethane micelles in the water phase. The appearance of the dispersion is linked to the particle size. The smaller the particle sizes, the more transparent the PUDs.

The particle size distribution and the zeta potential of the PUDs were investigated using a zeta-sizer. As expected, increases in the hydroxyl number of the polyols and in the emulsifier content resulted in a decrease of the particle size of the final PUDs. For example, as the emulsifier content decreases from 46.62% to 40.55%, the particle size of the emulsion from LCP increases from 31.4 nm to 227.6 nm. Two factors affect the particle size of the PUDs. On the one hand, the high OH number leads to high crosslinking density, leading to large particle size.²⁴ On the other hand, the more the emulsifier employed, the higher the surface charge density of the particles. Thus, the particles are not easily aggregated due to electrostatic repulsion, leading to small particle sizes.²⁵

According to the results observed, the emulsifier content is the main factor affecting the size of PUDs.²⁶ As shown in Table 3, an increase in the OH number from 149 mg KOH per g to 305 mg KOH per g results in a decrease in the particle size of the emulsion from 187.9 nm to 58.8 nm. This arises from the fact that with the increase in the OH number, more emulsifier is used in the PUDs (considering the same ratios of OH groups from the polyols and OH groups from the emulsifier), leading to the small size of the PUDs, compensating for the increase of the particle size due to the high crosslinking densities. However, compared with the particle size of the resulting emulsion from vegetable oil, the PU from LCP exhibits a smaller particle size (31.4 nm), possibly because of the pre-crosslinking structures of LCP.¹⁸ All the particle sizes of the resulting emulsions from petroleum-based polyols follow a similar trend.

Table 3 Properties of PUDs with the new emulsifier

Sample	Appearance	Peak 1/percent (nm/%)	Peak 2/percent (nm/%)	Z-Average size (nm)	Zeta potential (mV)
PU-SCP	Milky white with blue light	215.6/100	—	187.9	−42.4
PU-SCPII	Milky white	295.5/100	—	295.5	−41.7
PU-LCP	Transparent with blue light	38.5/100	—	31.4	−31.8
PU-LCPII	Milky white	252.1/100	—	227.6	−43.5
PU-CO	Translucent with yellow light	148.9/91.5	18.9/8.5	97.2	−60.9
PU-LOP	Transparent	97.2/92.1	13.5/7.9	58.8	−42.4
PU-PPG200	Transparent	10.2/86.1	237.1/13.9	121.6	−35.6
PU-PPG800	Transparent with yellow light	337.2/52.1	62.9/47.9	125.4	−37.3
PU-PCDL500	Transparent	32.6/79.3	242.9/20.7	31.2	−71.6
PU-PCDL1000	Translucent with yellow light	177.1/94.5	18.08/5.5	105.9	−45.7

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The zeta potentials of the PUDs are also shown in Table 3. All the samples exhibit zeta potentials below −31.8 mV, indicating that these dispersions are very stable.²⁷ When the polyol functionality increases, the zeta potential of the PUDs decreases. The chemical structure of this emulsifier and its good compatibility with the polyols contribute to the resulting dispersions’ excellent stability. No precipitation or stratification of the samples is observed after centrifugation at 3000 rpm for 30 min, further indicating the excellent stability of all PUDs prepared.

Polyurethane properties

Fig. 9 shows the storage modulus and tan δ as functions of temperature for PU films prepared from the emulsifier and different polyols in the temperature range from −50 °C to 100 °C. At temperatures lower than −25 °C, all films are in the glassy state and their storage modulus decreases slightly as the temperature increases. From −25 °C to 50 °C, a rapid decrease (three orders of magnitude) is observed for the storage modulus curves, corresponding to the relaxation process of the polymer chain. At this stage, a peak maximum was observed in the tan δ curves, corresponding to the glass transition temperature (T_g). All films show one tan δ peak, indicating the homogeneous nature of the polyurethane films.^16,28 When the temperature increases above 50 °C, all curves exhibit a rubbery plateau for the storage modulus.

Fig. 9 Storage modulus and tan δ as functions of temperature for PU films from the emulsifier and (a) vegetable oil-based polyols, or (b) petroleum-based polyols.

Polyurethane films from LOP exhibited the highest storage modulus while polyurethane films from CO exhibited the lowest storage modulus due to the different crosslinking densities induced from the different OH numbers of these two polyols. For the polyurethane films from LCP and SCP, the storage modulus decreased with increasing emulsifier content due to the low functionality and soft fatty acid chains of the emulsifier. Polyurethane films from LCP and SCP have the highest T_gs due to the high OH numbers and pre-crosslinking densities of these two polyols.¹⁹ As the crosslinking density increases, molecular motions of the polymer chains become more restricted, leading to higher T_gs. Waterborne polyurethane films from LCP exhibit lower T_gs than solvent-based polyurethane films from the same polyols (33.67 vs. 53.46), while waterborne polyurethane films from SCP and CO exhibit higher T_gs than their solvent-based polyurethane films (36.14 vs. 31.3 and 7.09 vs. 4.6, respectively).

The hydroxyl number of the emulsifier is much lower than that of LCP. The introduction of the emulsifier decreases the crosslinking density and the content of rigid rings, resulting in the low T_gs of the resulting PU films. However, the long, flexible fatty acid chain of the emulsifier facilitates the phase compatibility between hard and soft segments, leading to higher T_gs for the waterborne samples prepared with the emulsifier in comparison with the solvent-based polyurethane films from LCP and CO, although the hydroxyl number of the emulsifier is slightly lower than that of the two polyols.^17,18 When traditional emulsifiers, such as DMPA and DMBA, are used, the polyols function as the soft segment while diisocyanate and emulsifiers act as the hard segment.²⁴ In this circumstance, the phase compatibility between hard and soft segments is compromised due to the polarity difference between hard and soft segments. In this work, both the emulsifier and polyols function as the soft segment, while the diisocyanates act as the hard segment in the polymer chain. The introduction of carboxyl groups in the chains of the emulsifier leads to a decrease in the polarity difference between soft and hard segments and results in an even distribution of hydrogen bonds throughout the polymer chain, therefore increasing the phase compatibility. In addition, the branched chain structure of the emulsifier can inhibit the packing of the segments, resulting in higher phase compatibility.⁷

Similarly, the storage modulus and T_g of polyurethanes from PPG and PCDL increase with increasing OH number of the polyol. Although PPG800 has a higher OH number than PCDL1000, polyurethane films from PPG800 exhibit a lower storage modulus and T_g than those from PCDL1000. Indeed, the lower cohesive energy (4.2 kJ mol⁻¹) of the ether group when compared to the ester group (12.2 kJ mol⁻¹) leads to higher intermolecular interaction and rigidity of PU films from PCDL1000.²⁹

Fig. 10 shows the TGA results, under nitrogen, of the polyurethane films from different polyols. The degradation of polyurethane films at temperatures between 150 °C and 300 °C resulted from the decomposition of the labile urethane groups.³⁰ Thus, polyurethane films from LOP and PPG200 lost the highest weight, while polyurethane films from SCP and PCDL 1000 lost the lowest weight in this temperature range. The thermal stability of the polyurethane films decreased with lower emulsifier and higher polyol contents because the number of hydroxyl groups of the emulsifier is much lower than that of LCP and SCP. Decomposition at the 300–500 °C temperature range can be attributed to the chain scission of the polyols.³¹ Polyurethane films from LCP and SCP exhibit the highest thermal stability at that range due to their pre-crosslinked structures. Waterborne polyurethane films showed higher thermal stability than solvent-based polyurethane films with the same polyols (LCP, SCP, and CO).^17,18 This can be attributed to the fact that the use of the emulsifier with a low hydroxyl number leads to a high content of the thermally stable fatty acid chain and a low content of the labile urethane.

Fig. 10 TGA curves for the PU films from the emulsifier and (a) vegetable oil-based polyols and (b) petroleum-based polyols.

Polyurethane films from polyester polyols showed higher thermal stability than polyether polyols due to more extensive hydrogen bonding formed between the urethane NH and the ester groups of polyester chains.³² No further decomposition of the polyurethane films was observed above 550 °C. The temperatures corresponding to 5% and 50% decomposition of the polyurethane films (T₅ and T₅₀, respectively) are summarized in Table 4.

Table 4 DMA, cross-linking density (υ_e), and TGA data of the PU films

Sample	DMA		υ e (mol m^−3)	TGA
	T g (°C)	E′ at 25 °C (MPa)		T 5	T 50	T max
PU-SCP	31.67	9.80	79.66	289.2	390	389
PU-SCPII	36.14	14.34	127.07	279	386	386
PU-LCP	27.75	10.28	94.09	284	388.2	384
PU-LCPII	33.67	19.47	141.65	287.6	393.7	366
PU-CO	7.09	1.14	103.91	250	382	387
PU-LOP	29.55	16.72	143.9	176	377	378
PU-PPG200	33.46	58.78	181.26	173	384	394
PU-PPG800	−6	0.84	87.46	257	383	387
PU-PCDL500	15.19	3.69	109.9	192	371	398
PU-PCDL1000	5.57	1.46	103.15	246	334	321

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The tensile stress–strain curves of PU films from different polyols are shown in Fig. 11. Table 5 summarizes the Young's moduli, tensile strength, elongation at break and toughness of the PU films. The PU films from LCP and SCP show the highest tensile strength, possibly due to the pre-crosslinking structures and high functionalities of the polyols. For this type of polyurethane film, the tensile strength increases and the elongation decreases with a decrease in the emulsifier content. That is because the emulsifier has lower hydroxyl numbers than the polyols. High emulsifier content leads to low polyol content, resulting in low crosslinking densities and rigid carbon ring content. Generally, high cross-linking density imparts high tensile strength and low elongation at break.

Fig. 11 Mechanical properties of PU films from the emulsifier and the vegetable oil-based polyols (a) or petroleum-based polyols (b); mechanical comparison of waterborne polyurethane films from EG (labeled in red), DMPA (labeled in green), DMBA (labeled in blue) and solvent-based polyurethane films (labeled in black) (c) and (d).^17,18,33

Table 5 Mechanical properties and contact angles of the PU films

Samples	Hard segment content (wt%)	Tensile strength (MPa)	Elongation at break (%)	Young's modulus (MPa)	Toughness (MPa)	Water contact angle (θ, °)	Ethanol contact angle (θ, °)
PU-SCP	17.53	6.97 ± 0.42	215 ± 5	49.42 ± 1.79	12.36 ± 0.44	73.88 ± 0.12	29 ± 2.25
PU-SCPII	18.34	11.83 ± 0.11	187 ± 31	96.01 ± 6.22	18.80 ± 2.79	80.88 ± 0.62	29.75 ± 1.5
PU-LCP	18.58	15.60 ± 1.20	165 ± 5	217.00 ± 2.20	21.88 ± 1.06	71.13 ± 1.37	27.88 ± 1.12
PU-LCPII	19.70	17.50 ± 0.50	249 ± 9	158.70 ± 7.00	41.61 ± 4.20	72.5 ± 0	28 ± 3.5
PU-CO	17.95	2.41 ± 0.034	346 ± 11	3.44 ± 0.005	4.31 ± 0.055	77 ± 4.5	28 ± 3
PU-LOP	19.36	8.17 ± 0.55	98 ± 6	49.9 ± 0.15	6.87 ± 0.85	82.25 ± 2.75	28.13 ± 1.63
PU-PPG200	22.18	6.28 ± 0.35	84.57 ± 5.79	66.34 ± 1.43	5.02 ± 0.48	86.63 ± 6.63	36.83 ± 1.22
PU-PPG800	17.57	1.02 ± 0.056	453.98 ± 4.51	2.03 ± 0.035	2.48 ± 0.045	76.63 ± 1.13	27.58 ± 1.06
PU-PCDL500	19.39	8.07 ± 0.71	34.77 ± 4.11	93.02 ± 1.34	2.07 ± 0.16	82.08 ± 4.06	27 ± 2
PU-PCDL1000	15.91	1.32 ± 0.031	197.71 ± 19.67	7.61 ± 1.34	2.62 ± 0.11	71.5 ± 3.25	26.75 ± 2.75

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LOP has a similar structure to that of CO, but it has a higher hydroxyl number, resulting in the higher tensile strength of PU when the emulsifier contents are the same. Waterborne polyurethane films from LCP, SCP, and CO exhibit tensile strengths of 17.5 MPa, 11.83 MPa, and 2.41 MPa, respectively, which are comparable to those of solvent-based polyurethane films from the same polyols (17.3 MPa, 11.6 MPa, and 1.6 MPa, respectively) without the bio-based emulsifier. The preparation of waterborne polyurethane dispersions from LOP (or LCP or SCP) and traditional petroleum-based emulsifiers (DMPA or DMBA) failed due to uneven distribution of carboxylic acid groups throughout the PU backbone as discussed in the introduction part. While the tensile strengths of waterborne polyurethane films from CO and petroleum-based emulsifiers (DMPA, DMBA) are 12.91 MPa and 12.47 MPa,³³ resulting from the rigid structure of petroleum-based emulsifiers. However, polyurethane films from LCP exhibit the highest elongation at break (249%), which is significantly higher than the observed value for solvent-based polyurethane films from the same polyols (98%). Most interestingly, waterborne polyurethane films from these three polyols exhibit significantly higher toughness than the solvent-based polyurethane films from the same polyols (41.61 MPa vs. 10.15, 18.8 MPa vs. 13.85 MPa, 4.31 MPa vs. 2.51 MPa, respectively) (as shown in Fig. 11b). These improvements of the mechanical properties can be attributed to the fact that the long, flexible fatty acid chain of the emulsifier prevents aggregation of the hard segments in the polyurethane and facilitates the phase compatibility between hard and soft segments, leading to an increase in the elongation at break and toughness.

For the PUs from the petroleum-based polyols, the same trend is observed. With the increase in the hydroxyl numbers of the polyols, the tensile strength of the PU films increases and the elongation decreases. It is reported that PUs from polyester polyols exhibit higher mechanical properties than PUs from polyether polyols. Polyester polyols are stronger hydrogen bond acceptors than polyether polyols, and more hydrogen bonding is formed between the urethane bonding and the ester groups of polyester chains, leading to a lower degree of phase separation and better crystalline structure of PU from polyester polyols compared with PU from polyether polyol.³⁴ This can explain the fact that PU films from PCDL showed the highest tensile strength although its hydroxyl number is not the highest.

Fig. 12 shows the water and ethanol contact angle of PU films from the emulsifier and different polyols. Table 5 summarizes the water and ethanol contact angle data of the PU films. For the PUs from the vegetable oil-based polyols, with an increase in the hydroxyl number of the polyols, the resulting PU films from different polyols exhibit increasing water contact angles (PUs from LCP are the exceptions). PU films from LOP exhibit the highest water contact angles (82.25°), while PU films from SCP exhibit the lowest water contact angles (73.88°). A higher OH number of the polyol results in higher cross-linking density, which prevents the penetration of water.³⁵ When the emulsifier content increases, the water contact angles of the resulting PU films increase with the same polyols. Although the ionic segments increase with increasing emulsifier content, the content of the hydrophobic triglyceride structure from the vegetable oil-based emulsifier also increases, leading to an increase in the water contact angles. The ethanol contact angles of the resulting PU films exhibit a very similar behavior.

Fig. 12 Water (a) and ethanol (b) contact angles of PU films from the emulsifier and vegetable oil-based polyols. Water (c) and ethanol (d) contact angles of PU films from the emulsifier and petroleum-based polyols.

For PUs from the petroleum-based polyols, a similar trend is observed. Increases in the hydroxyl numbers of the polyols and the emulsifier content result in increasing water and ethanol contact angles for PU films from different polyols. For example, when the polyol functionality increases from 152 mg KOH per g to 530 mg KOH per g, the water contact angle of the PU film from PPG increases from 76.63° to 86.63°, and the ethanol contact angle increases from 27.58° to 36.83°. The water and ethanol contact angles of the PU film from PPG200 is the largest, which is attributed to the fact that the hydroxyl number of PPG200 is significantly higher than that of the other polyols, resulting in the highest cross-linking density (Table 4).

Several factors influence the contact angle of polyurethane films, such as the polarity of the polyols, the cross-linking density of polyurethane,³⁵ the emulsifier content, etc. Considering solely the difference in polarity, the water contact angle of polyester polyol-based systems would be expected to be lower than that of polyether-based systems. However, the cross-linking density of PU-PCDL500 (109.9 mol m⁻³) is significantly higher than that of PU-PPG800 (87.46 mol m⁻³), as shown in Table 4. In addition, the emulsifier content of PU-PCDL500 (48.64%) is higher than that of PU-PPG800 (44.07%). The combination of these two factors is believed to account for the contact angle of PU-PCDL500 being larger than the contact angle of PU-PPG800.

Conclusions

A solvent-free and self-catalysis method was developed to prepare a fully bio-based internal emulsifier from soybean oil and glutaric acid. It is found that the reaction time of 90 min and the molar ratio of epoxy groups to glutaric acid of 4 : 1 are optimized reaction conditions, which result in the highest OH number and acid number of the emulsifier, as well as the highest atom efficiency, and the lowest energy consumption. This novel emulsifier exhibits similar properties to those of DMPA and DMBA, in which hydroxyl groups serve as cross-linking agents and the carboxylic acid group acts as an ionic segment. A series of bio-based anionic waterborne polyurethane dispersions from this emulsifier and typical polyols (vegetable oil- and petroleum-based polyols) were successfully prepared. Due to the chemical structure of this emulsifier and its good compatibility with bio-based polyols, the resulting dispersions demonstrate excellent stability and solid content up to 45%. The resulting PU films exhibited higher tensile strength, toughness and thermal stability than the solvent-based polyurethane films prepared without this novel emulsifier. The bio-content of the resulting polyurethane films is 74%. The synthetic route developed in this study has various advantages over traditional methods, including the absence of a solvent and a catalyst, the high bio-based content of the products, and the ease of preparation, offering a new platform for environmentally friendly emulsifiers for waterborne polyurethanes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was sponsored by the National Natural Science Foundation (51703068), the Guangdong Province Science & Technology Program (2017A010103015 and 2018B030306016), and the Program of the Science, Technology Department of Guangzhou, China (201803020039) and the Open Project of Jiangsu Key Laboratory for Biomass Energy and Materials (JSBEM201803).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8gc03560a

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