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Castor oil-based cationic waterborne polyurethane dispersions: Storage stability, thermo-physical properties and antibacterial properties
Industrial Crops & Products 117 (2018) 169–178
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Castor oil-based cationic waterborne polyurethane dispersions: Storage stability, thermo-physical properties and antibacterial properties Haiyan Lianga, Lingxiao Liua, Jingyi Lua, Moutong Chenb, Chaoqun Zhanga,
T
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a
College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China Guangdong Institute of Microbiology, State Key Laboratory of Applied Microbiology Southern China, Guangdong Provincial Key Laboratory of Microbial Culture Collection and Application, Guangdong Open Laboratory of Applied Microbiology, Guangzhou, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Castor oil Waterborne polyurethane Antibacterial properties Thermo-mechanical properties
In this study, castor oil and its derivative were used to prepare cationic waterborne polyurethane dispersion using N-methyl diethanolamine (MDEA) as ion center. The effect of polyol functionalities and the ionic chain extender contents on the particle size and antibacterial properties of the polyurethane dispersions, and thermomechanical, thermal stability, mechanical properties, water/solvent uptakes of the resulting polyurethane films were thoroughly investigated. Moreover, all the polyurethane dispersions exhibit enhanced antibacterial activity against Vibrio parahaemolyticus with the rise of MDEA content and the reduction of polyol functionalities, but rarely antibacterial activity to Listeria monocytogenes. To the best of my knowledge, this work for the first time extensively investigate the structure-property relationships of castor oil based cationic waterborne polyurethane dispersions and their polyurethane films.
1. Introduction Polyurethanes (PUs) are one of the most versatile polymers and have been widely used in various fields such as coatings, adhesives, clothing, paints and foams because of their excellent chemical resistance, tunable thermo-mechanical properties and good process ability. (Gurunathan et al., 2013; Schmidt et al., 2017) However, traditional polyurethane products usually contain large amount of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs), which are mostly released into the air during practical application, leading to a big health threat for producers and users. With the introduction of increasingly strict legislation and consumer demands towards reducing the VOC and HAPs, a current renaissance of environmentally friendly waterborne polyurethanes has occurred in a global scale, aiming to partially or completely replace solvent-based polyurethanes for application in ink, adhesives and coatings. (Gogoi and Karak, 2014) In order to disperse polyurethanes in aqueous as a stable phase, hydrophilic groups (carboxylic and sulfonic acid, amine) are usually incorporated into the side chain or backbone of the polymers, which leads to the formation of waterborne polyurethanes. Much progress have been made recently to formulate waterborne polyurethanes with high solid content and high performance. (Burja et al., 2015; Peng et al., 2015; Peng et al., 2014; Zhang et al., 2015) For example, Li and his co-workers synthesized a cationic waterborne polyurethane with a
⁎
high solid content (42.06–52.18%) via the combination of cationic ionic (N-methyl diethanolamine) and nonionic segments (Poly(oxyethylene alkyl amine)). (Li et al., 2017) Yong et al. have synthesized a solvent-free and chemical matt waterborne polyurethanes, simultaneously using sulfonic (sodium 2-[(2-amino ethyl) amino] ethane sulfonate) and carboxylic acid (2,2-Bis(hydroxymethyl) pro-pionic acid) as hydrophilic chain extender (Yong et al., 2015). And Yu et al. successfully prepared a series of high waterproof carboxylic acid type waterborne polyurethane films with introduction of fluorine and siloxane to increase the cross-linking density of the materials. (Yu et al., 2016) Up to date, most of the raw materials (Polyols, isocyanates, chain extenders) used for polyurethane dispersions (PUDs) are derived from petrochemical feedstock, which are widely regarded as nonrenewable and major source of criteria air pollutants. (Alam et al., 2014; Zhu et al., 2016) With the depletion of the world crude oil stock and increasing environmental concerns, efforts on a global scale are dedicated to find a renewable resource (such as cellulose, natural oils, lignin, and so on) for bio-based polyurethanes to replace petroleum based counterparts. (Gaikwad et al., 2015; Zhang et al., 2014) Vegetable oils as a kind of typical renewable biomass resources are among the most promising for polyol synthesis due to its low cost, and readily available. (Pawar et al., 2015) Vegetable oils are triglyceride of fatty acid that usually bears 12–22 carbon atoms and 0–3 carbon–carbon double bonds. Except for castor oil, most vegetable oils do not contain hydroxyl groups.
Corresponding author. E-mail addresses: [email protected], [email protected] (C. Zhang).
https://doi.org/10.1016/j.indcrop.2018.02.084 Received 7 January 2018; Received in revised form 26 February 2018; Accepted 28 February 2018 0926-6690/ © 2018 Elsevier B.V. All rights reserved.
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(Gurunathan et al., 2015) The reactive ester and carbon–carbon double bonds in triglyceride oils offer several routes to introduce hydroxyl groups necessary in polyols for PU synthesis, including epoxidation/ ring opening, ozonolysis/reduction, hydroformylation/reduction, transesterification, thiol-ene click reactions. (Bullermann et al., 2013; Feng et al., 2017; Zhang et al., 2013) Anionic and cationic PU dispersions have been synthesized from different vegetable oil based polyols. It is found that the hydrophobic nature of triglycerides and long fatty acid chains endow the resulting PU films excellent chemical and physical properties, including enhanced hydrolytic, flexibility and toughness. (Zhang et al., 2017) Lu et al. successfully prepared soybean oil-based cationic waterborne PU films with tensile strengths from 5.7 to 23.2 MPa and elongation at break from 235 to 291%. Moreover, the effect of polyols functionalities on the size of the polyurethane particle and the thermo-mechanical properties of the PU films were studied and discussed. (Lu and Larock, 2010) Fu and his co-worker synthesized a castor oil-based anionic waterborne PU film with a high flexibility (1 mm) and excellent chemical resistance (1.75% water absorption and 90% toluene absorption for 168 h). (Fu et al., 2014) Saalah et al. investigated the effect of the OH number, DMPA content and hard segment content on the stability of the anionic waterborne PU dispersions from jatropha oil, as well as the physical, mechanical and thermal properties of the resulting films. The resulting PU film exhibited excellent hydrophobicity, with a contact angle of 90° or more, indicating a nonwetting surface. (Saalah et al., 2015) To the best of my knowledge, the effect of polyols functionalities and chain extender content on the size of the anionic polyurethane particle and the thermo-mechanical properties of the resulting films have been widely investigated. But their effects on the performance of castor oil-based cationic polyurethane have not been reported previously. In this study, cationic waterborne PUDs were successfully prepared from castor oil and its derivative using N-methyl diethanolamine (MDEA) as ion center. The effect of polyol functionalities and MDEA contents on the physical properties and antibacterial properties of the PUDs and the thermo-mechanical properties of the resulting films were investigated. The particle size and zeta potential of the PUDs were characterized by zeta-sizer while the thermo-mechanical properties of the resulting PU films were characterized by DMA, TGA, tensile testing. In addition, the antibacterial properties of the resulting PUDs were tested against Vibrio parahaemolyticus and Listeria monocytogenes. The work extensively investigated the structure-property relationships of castor oil based cationic waterborne polyurethane dispersions and their films.
Table 1 Compositions of the waterborne PUs. Samples
OH number of castor oil (mg KOH per g)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19 a b
164 164 164 164 208 208 208 208
Molar ratio NCO
OHa
OHb
1.7 1.85 2.0 2.2 1.7 1.85 2.0 2.2
1 1 1 1 1 1 1 1
0.69 0.84 0.99 1.19 0.69 0.84 0.99 1.19
Hydroxyl molar ratio of castor oil. Hydroxyl molar ratio of the MDEA.
The molar ratio of the raw materials is summarized in Table 1. After the reaction continued for 30 min, 30 ml MEK was added to reduce the viscosity. The mixture continued to react under stirring for 2 h at 78 °C. After the reactants were cooled down to room temperature, acetic acid was added as neutralizer under stirring for 30 min. Finally, 90 ml distilled water was added for emulsification for 2 h. The waterborne PUDs with a solid content of 10–12 wt% was obtained after MEK was removed by rotary evaporator. The PU films were obtained after their corresponding waterborne PUDs were casted and dried in a silicon mold at room temperature for at least 48 h (See Scheme 1). 2.3. Characterization The stability of waterborne PU dispersion was evaluated by centrifuging the emulsion at 3000 r/min for 30 min on Tomos 3–18. A Zeta-sizer Nano ZSE (Malvern Instruments) was used to measure the size distribution and zeta potential of the PU dispersions which was diluted with distilled water to about 0.01 wt% before test. The mechanical properties of the WPU films were determined using an electronic universal testing machine (UTM-4204) with a crosshead speed of 100 mm/min. The waterborne polyurethane dispersions were casted into a silicon mold and allowed them dry in room temperature for PU films. All the samples with a length of 30 mm and a width of 10 mm were dried at 60 °C for 12 h before test. An average value of at least three replicates of each PU sample was taken. The dynamic mechanical behavior of the resulting films was determined using a Netzsch DMA 242C dynamic mechanical analyzer in tensile mode at 1 Hz. The samples with a length of 8 mm and a width of 6 mm were heated from −60 to 120 °C at a rate of 5 °C min−1. For this study, the glass transition temperatures (Tg) of the PU films were obtained from the peaks of the loss factor curves. Thermo-gravimetric analysis (TGA) of the PU films was carried out on a Netzsch- STA 449C thermal analyzer. The samples were heated from 30 to 700 °C at a heating rate of 20 °C min −1 in a nitrogen atmosphere. Generally, 8–14 mg samples were used for the test. The chemical resistance of the PU films against distilled water and ethanol was conducted according to the methods previous reported (Wang et al., 2015a; Xia and Larock, 2011). All the square films with a length of 10 mm were dried at 60 °C for 12 h before test. An average value of more than four replicates of each sample was taken. The PU films with known weight (m0) were immersed in a distilled water bath at room temperature for 20 days for water absorption. It was also immersed in absolute ethanol for 96 h for ethanol uptake. Then the films were weighed (m1) and dried at 60 °C for at least 48 h. The dried films were then weighed (m2) and the percentage of water absorption (WA), ethanol absorption (WE), and weight losses (Wx, Wy) of the resulting films in water and ethanol were calculated as follows:
2. Experimental 2.1. Materials Castor oil (OH number: 164 mg KOH/g) and its derivative (OH number: 208 mg KOH/g) were provided by Fuyu Chemical Company, and Guangzhou Xinye Trading Company, respectively. In this study, they are coded as castor oil-164, and castor oil-208. Isophorone diisocyanate (IPDI) and N-methyl diethanolamine (MDEA) were purchased from Wengjiang Chemical Reagent Company. Dibutyltin dilaurate (DBTDL) was purchased from Fuchen Chemical Reagent Factory. Acetic acid, and methyl ethyl ketone (MEK) were purchased from Aladdin reagent and Tianjin Hongda Chemical reagent, respectively. All materials were used as received without further purification. 2.2. Preparation of castor oil-based waterborne PUDs Firstly, castor oil (6 g) and IPDI were mixed in a double neck round bottom flask equipped with mechanical stirrer for 10 min at 78 °C. One drop of DBTDL was added in the mixture to allow the reaction to proceed for 10 min. Then, MDEA was added under high-speed stirring.
WA (%) = 170
(m1 − m 0) × 100% m0
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Scheme 1. The representative synthesis route of the castor oil-based cationic waterborne PUDs.
Wx (%) =
(m 0 − m2) × 100% m0
diffusion method (Chen et al., 2014a; Xia et al., 2012). Briefly, the concentrations of cell suspension were adjust to 0.5 McFarland turbidity (about 108 CFU/mL), and then spread evenly to the MuellerHinton agar. PUDs dispersions (20 μg) were added to paper discs (6 mm, Oxoid Ltd). The pH of the PUDs ranges from 6 to 6.5, similar to the pH of water, which excludes the influence of the pH on the antibacterial action. The M-H agar plates were incubated at 37 °C for 24 h. Zones of inhibition were measured with a precision caliper to the nearest 0.01 mm. Each sample was repeated for three times.
Where m1 is the weight of the film had immersed in water and m2 is the weight of the dried film after test.
WE (%) =
(m1 − m 0) × 100% m0
Wy (%) =
(m 0 − m2) × 100% m0
3. Results and discussion
Where m1 is the weight of the film had immersed in absolute ethanol and m2 is the weight of the dried film after test. The contact angle of water and ethanol droplets on the resulting films were measured with a contact angle goniometer (Powereach JC2000C1) using the sessile-drop method at room temperature. All the samples were dried at 60 °C for at least 48 h before test. The antibacterial ability of the resulting PUDs against Vibrio parahaemolyticus and Listeria monocytogenes were performed using disk
3.1. Appearance and size of the PUDs The images of PUDs from castor oil-164 (a) and castor oil-208 (b) are shown in Fig. 1. With the increase of MDEA contents, the transparency of the resulting PUDs gradually increases from milky white opacity to transparent. In addition, the PUDs from castor oil-208 are much more transparent than the PUDs from castor oil-164 when the 171
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Fig. 1. The appearance of waterborne PUDs (a) The hydroxyl ratios of MDEA and castor oil-164: (1) 0.69 (2) 0.84 (3) 0.99 (4) 1.19; (b) The hydroxyl ratios of MDEA and castor oil-208: (1) 0.69 (2) 0.84 (3) 0.99 (4) 1.19.
hydroxyl ratio of MDEA and polyols are the same. As a typical internal emulsifier, MDEA provides large amounts of hydrophilic groups and enables the hydrophobic hard and soft segments of polyurethane disperse in the water. Therefore, the more MDEA content in the polymer chains, the more hydrophility of the resulting PUDs and the more transparency of the resulting PUDs will be. With the OH number of castor oil increase, the content of MDEA used for the PUDs rises accordingly, leading to the enhancement of transparence when equal hydroxyl molar ratio of the MDEA are applied. In addition, the castor oil based PUDs demonstrate excellent storage stability. There is no obvious perception or stratification when the PUDs were centrifuged at 3000 rpm for 30 min and storage for 12 months at room temperature. In order to investigate the mechanism underlying their unique appearance and the excellent storage stability, the average size and its distribution of the PUDs were characterized and shown in Table 2 and Fig. 2. With the rise of MDEA content, the particle size of waterborne PUD lessens. (Wang et al., 2015b) As the hydroxyl molar ratios of the MDEA shift from 0.69 to 1.19, the particle size of the PUD from castor oil-164 reduces from 73.3 nm to 21.3 nm due to the enhancement of hydrophility. Besides, in the same molar ratios of MDEA, the particle size of the PUDs from castor oil-208 is smaller than that from castor oil164 since higher content of MDEA were used in the former formulation when equal hydroxyl molar ratio of the MDEA are applied. Moreover, as the OH number of castor oil increases, the cross-linking density (see Table 4) of the waterborne polyurethane increases, resulting in reduction of the particle size. (Lu and Larock, 2008) The particle sizes of the PUDs from castor oil-208 and castor oil-164 at MDEA ratios of 9.08 wt% are 22.8 nm, and 25.8 nm, respectively. Similarly, the zeta potential of the PUDs reduces with the increase of MDEA under otherwise equal conditions. As the MDEA content increase to 10.21 wt% from 6.95 wt%, the zeta potential of the PUDs from castor
Fig. 2. The particle size distribution of castor oil based waterborne PUDs with different hydroxyl molar ratios of MDEA: (a) PUDs from castor oil-164; (b) PUDs from castor oil208.
oil-164 decrease to 45.5 mV from 73.2 mV. In the same molar ratio of MDEA, the zeta potential of the PUDs from castor oil-208 is lower than that from castor oil-164. The results show that the particle size of waterborne PUD has remarkable impact on the zeta potential and appearance of the emulsion. The smaller of the particle size is, the lower zeta potential and more transparent of the PUDs will be, and vice versa. 3.2. Properties of the PU films The mechanical, thermo-mechanical properties and thermal stability of the casting PU films were characterized by tensile testing, DMA, and TGA. Stress-strain curves for PU films from castor oil-164 and castor oil208 are shown in Fig. 3. The Young’s modulus, toughness, tensile strength and elongation at break of the PU films are summarized in Table 2. With the molar ratios of MDEA increase from 0.69 to 1.19, the
Table 2 The stability, particle size and zeta potential of the waterborne PUDs. Samples
MDEA content (wt%)
Appearance
Storage life
Z-average size (nm)
Zeta Potential (mV)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
6.95 8.07 9.08 10.21 7.87 9.08 10.16 11.46
Milky white Milky white translucent with yellow light Pale white translucent Milky white with blue light Pale blue transparent Pale blue transparent Micro-blue transparent
> 12 months > 12 months > 12 months > 12 months > 12 months > 12 months > 12 months > 12 months
73.3 51.2 25.8 21.3 58.7 22.8 20.0 17.2
73.2 63.5 46.3 45.5 39.7 31.7 27.6 26.8
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Table 3 Mechanical Properties of the waterborne PU films. Samples
Young’s modulus (MPa)
Tensile strength (MPa)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
1.10 ± 0.06 1.97 ± 0.07 2.37 ± 0.12 2.76 ± 0.77 7.99 ± 2.02 11.85 ± 2.40 19.93 ± 0.47 34.50 ± 2.21
0.71 0.90 0.93 0.99 1.84 2.41 3.46 3.95
± ± ± ± ± ± ± ±
0.08 0.11 0.09 0.41 0.24 0.22 0.05 1.45
Elongation at break (%)
Toughness (MPa)
585 506 565 520 421 487 505 646
2.10 ± 0.47 2.41 ± 0.55 3.05 ± 0.24 2.71 ± 0.74 5.77 ± 1.33 8.16 ± 1.58 9.43 ± 1.51 16.51 ± 6.21
± ± ± ± ± ± ± ±
145 6.95 115 113 2.12 35.7 90.7 105
Fig. 3. Stress-strain curves for PU films from castor oil-164 (a) and castor oil-208 (b) with different content of MDEA.
tensile strength of the castor oil-164 based film improves from 0.71 to 0.99 MPa, and the Young’s modulus rises from 1.10 to 2.76 MPa, while there is no big change in elongation at break. Moreover, when the molar ratios of MDEA enhance from 0.69 to 1.19, a sharply shift in the tensile strength from 1.84 to 3.95 MPa, Young’s modulus from 7.99 to 34.5 MPa, elongation at break from 421 to 646% are observed for the PU films from castor oil-208. The increases of tensile strength and Young’s modulus can be explained by the fact that the rise of the hard segment content contributes to the stiffness of the network structure. And the difference in elongation at break of the resulting PU films results from different functionalities of the castor oil and the hard segments of the materials. As expected, an increase of OH number of the polyols (castor oil) improves the tensile strength and Young’s modulus of the resulting PU films. In the same content of MDEA, the PU films from castor oil-208 have higher tensile strength than the PU films from castor oil-164 due to their higher cross-linking density. In detail, the PU208-0.84 film exhibits a tensile strength of 2.41 MPa, while the PU164-0.99 film exhibits a tensile strength of 0.93 MPa with the same 9.08 wt% MDEA content. This finding is similar to the result that reported by Gaddam et al. (Gaddam et al., 2017) Stress-strain test also afford the toughness of the materials, which regard as the characteristics of a material’s resistance to fracture when stressed. As shown in Table 3, the toughness of the films from castor oil208 significantly rises from 5.77 to 16.51 MPa, with the molar ratio of MDEA increase from 0.69 to 1.19. Because increasing MDEA content leads to a rise of hard segment content, resulting in an improvement of hydrogen bonding interactions and toughness of PU films. (Lu and Larock, 2008) In addition, an increase of OH number of the castor oil also improves the toughness of the films. The PU208-0.69 with 48.1 wt
Fig. 4. Storage modulus and loss factor (tan δ) as a function of the temperature for PU films from castor oil-164 (a) and castor oil-208 (b) with different content of MDEA.
% hard segment content exhibits higher toughness (5.77 MPa) than PU164-1.19 (2.71 MPa) with 50.6 wt% hard segment content, resulting from the enhancement of the cross-linking densities. The storage modulus (E’) and tan δ of the PU films from castor oil164 (a) and castor oil-208 (b) with different molar ratio of MDEA are shown in Fig. 4. At the temperature from −60 to 0 °C, the E’ and tan δ of the resulting films both decreased slightly and the films are in the glassy state. Then, a rapid drop in the E’ is observed in the temperature from 10 to 50 °C for all of the resulting films, corresponding to the energy dissipation of the materials that is shown as the peak maximum in the tan δ curve. (Nguyen Dang et al., 2016) And the temperature associated with the maximum in the tan δ curve is the glass-transition temperature (Tg). All of the films show only one peak, indicating the homogeneous properties of the waterborne PU films. (Feng et al., 2017) The Tg and E’ at 25 °C of the PU films are shown in Table 4. As the MDEA content increases, both the Tg and E’ of the PU films increase, owing to the rise of hard segment content. (Chen et al., 2014b; Gurunathan et al., 2013) For example, with the MDEA content shift from 7.87 to 11.46 wt%, a rise of the Tg from 29.5 to 39.7 °C and an
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Table 4 DMA and TGA data of the waterborne PU films. Samples
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
Hard segment content (wt%)
ve (mol/ m3)
DMA Tg (°C)
E’ at 25 °C (MPa)
T5 (°C)
T50 (°C)
Tmax (°C)
42.2 45.0 47.6 50.6 48.1 50.9 53.5 56.5
64.9 62.3 58.2 59.0 60.3 60.6 60.7 129.8
17.2 18.7 27.1 25.0 29.5 33.2 33.8 39.7
3.4 3.9 21.3 13.4 49.2 174.4 222.1 431.5
288 290 290 278 287 269 282 266
376 383 378 371 371 359 364 351
470 472 472 474 477 477 477 477
TGA
increase of the E’ at 25 °C from 49.2 to 431.5 MPa are observed for the PU films from castor oil-208. Additionally, the PU films from castor oil208 obviously exhibit higher Tg and storage modulus than the PU films from castor oil-164 under otherwise equal conditions. In detail, the Tg and E’ at 25 °C of the PU films from castor oil-208 with 48.1 wt% hard segment content are 29.5 °C and 49.2 MPa, respectively, while the Tg and E’ at 25 °C of the PU films from castor oil-164 with 50.6 wt% hard segment content are 25 °C and 13.4 MPa. This can be explained by the higher cross-linking densities of the resulting PU films from castor oil208 than those from castor oil −164. Cross-linking densities (ve) of all the resulting films were calculated from the rubbery plateau modulus according to the kinetic theory of rubber elasticity:(Andjelkovic et al., 2005) E’ = 3 veRTWhere T is the absolute temperature at Tg + 30 °C, and E’ is the storage modulus at T. R is the gas constant. As shown in Table 4, the cross-linking densities of the polyurethanes increase with the increase of the polyol functionalities. And the PU208-0.69 shows a higher cross-linking density (60.3 mol/m3) than PU164-1.19 (59.0 mol/ m3). High cross-linking densities usually restrict the movement of the polymer chain, resulting in a high Tg and storage moduli as discussed above. (Lu and Larock, 2008) TGA curves of PU films from castor oil-164 (a) and castor oil-208 are shown in Fig. 5. The TGA data of the resulting films are summarized in Table 3. All the PU films exhibit three stages of thermal degradation process. The first stage at 200–300 °C is attributed to the dissociation of the labile urethane bonds, forming isocyanates, alcohols, primary amines, secondary amines, olefins, and carbon dioxide. (Gurunathan and Chung, 2016; Xia and Larock, 2011) With the increase of MDEA content, the thermal stability of PU in this stage decrease due to the rise of labile urethane bonds. The same trend is observed for the thermal stability as a function of OH number. T5 of the PU films from castor oil164, generally considered as the onset decomposition temperature, decrease from 288 to 278 °C when the molar ratios of MDEA increase from 0.69 to 1.19 (the hard segment increases from 42.2 to 50.6%). Degradation in the range 300–450 °C, the fastest degradation process, relates to castor oil chain scission. There is no big difference for the PUs with different OH numbers and MDEA content. An increase of the OH number of the castor oil and MDEA content rise the crosslinking density of the resulting PU films, leading to an improvement of the thermal stability of the PU films. (Chen et al., 2014b) These enhancement of thermal stability compensate the low thermal stability induced from the high labile urethane bonds as the OH numbers of the castor oils and MDEA content increase, resulting in no significant difference of maximum degradation temperatures (Tmax) of PUs. The last step above 450 °C was assigned to further thermo-oxidation of the PU films in air.
Fig. 5. TGA curves of PU films from castor oil-164 (a) and castor oil-208 (b) with different content of MDEA.
absorption are observed for the waterborne PU films: a fast absorption stage, a slow absorption stage, and an equilibrium absorption stage, which was consistent with other researches. (Wang et al., 2015b) Firstly, the water absorptions of the films increase rapidly with immersion time, then the absorption rate slow down. Finally, the absorption curves reach a plateau, corresponding to the absorption equilibrium. In this study, two factors play a key role in influencing the hydrophility of the PU films. Firstly, with the MDEA content in the PU films increases, the hydrophilic ionic group increases, resulting in a rise in water absorption of the films. (Saetung et al., 2012) In detail, after immersion for 20 days, the water absorption of PU164-0.69 reached 6.37% (6.95 wt% MDEA content), while the water absorption of PU1641.19 reached 14.1% (10.21 wt% MDEA content). Meanwhile, as shown in Table 5, the weight loss of the PU films after immersion in water for 20 days increase from 0.44 to 0.52 wt% with MDEA content increase from 6.95 to 10.21 wt%, and the water contact angle decrease from 51.25° to 36.85°. These results both indicate the improvement in hydrophility of the films. On the other hand, higher cross-linking of the PU films can be obtained by increasing the OH functionality of the castor oil, which leads to an increase in hydrophobicity. (Gurunathan and Chung, 2016; Xia and Larock, 2011) Therefore, PU208-0.69 exhibited higher water contact angle than PU164-0.69 (58.80° and 51.25°, respectively), although PU208-0.69 has higher MDEA content than PU164-1.19 (7.87 and 6.95 wt%, respectively). And the water contact angle images of PU164-0.99 and PU208-0.84 are shown in Fig. 7(a) and (b). With MDEA content of 9.08 wt%, PU208-0.84 obviously exhibited better hydrophobicity than PU164-0.99, resulting from the increase of cross-linking density. But higher cross-linking densities lead to more pore in the PU films, resulting in a higher water absorption of the films
3.3. Chemical resistance and contact angle of the PU films As shown in Fig. 6(a) and (b) and Table 5, three stages of water 174
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Fig. 6. Water absorption and ethanol absorption for PU films from castor oil-164 and castor oil-208 with different content of MDEA.
the ethanol uptake and ethanol contact angle of the films are mainly influenced by the MDEA content and polyol functionalities. As MDEA content and polyol functionalities increases, both the ethanol uptake and ethanol contact angle of the film increases, resulting from the increase of hydrophilic ionic group and cross-linking densities (Xia and Larock, 2011). With MDEA content rises from 6.95 to 10.21 wt%, the ethanol uptake of the films from castor oil-164 increases from 107.79 to 160.51 wt%, and the ethanol contact angle of them decrease from 28.40° to 20.75°. Additionally, with 9.08 wt% MDEA content, PU2080.84 exhibited higher ethanol uptake and ethanol contact angle than PU164-0.99 (ethanol uptake are 217.03 wt% and 133.64 wt%, ethanol contact angle are 24.75° and 23.45°, respectively). This increase of ethanol uptake can be explained by the increasing pore in the films with polyol functionalities rise(Wang et al., 2015a), resulting from the shift in cross-linking density as mention above. Compared with other Petroleum based waterborne PU films, the ethanol absorptions of the resulting films were relatively low, contributed to the hydrophobic nature of the soft segment in the PU films. The waterborne anionic PU films
(Wang et al., 2015a). In detail, with same MDEA content, the water absorption of PU164-0.99 is 7.97 wt%, while that of PU208-0.84 is 14.74 wt%. Compared with other petroleum based waterborne PU films, the water absorptions of the resulting films were relatively low, contributing by the hydrophobic nature of the castor oil based soft segment. (Bullermann et al., 2013) The waterborne anionic PU films from polytetramethylene ether glycol (PTMG) have 48.24 wt% water uptake for 24 h, while the maximum water absorption for 20 days in this study is 23.68 wt%. (Zhong et al., 2017) Moreover, Luo et al. also prepared a waterborne PU film from PTMG with 48 wt% water uptake for 48 h, (Luo et al., 2011) and Saetung et al. prepared a waterborne PU film from hydroxytelechelic natural rubber with 30 wt% water uptake for 7 days. (Saetung et al., 2012) As shown in Fig. 6(c) and (d) and Table 4, the ethanol absorption of the resulting films clearly increases with immersion time and reaches maximum before 24 h. Then, the ethanol absorption decreases with immersion time due to the weight loss of the PUs resulting from partly dissolution of the small molecular material. Similar to the hydrophility, Table 5 Chemical resistance and contact angle of the waterborne PU films. Samples
Water absorption for 20 days (%)
Weight loss in water (%)
Water contact angle (θw, deg)
Ethanol absorption for 96 h (%)
Weight loss in ethanol (%)
Ethanol contact angle (θe, deg)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
6.37 ± 0.55 7.28 ± 0.35 7.97 ± 0.17 14.1 ± 2.36 12.28 ± 1.12 14.74 ± 0.63 18.24 ± 0.77 23.68 ± 1.08
0.44 0.51 0.61 0.64 0.64 0.80 0.53 0.70
51.25 50.75 47.80 36.85 58.80 56.25 51.00 33.75
107.79 120.06 133.64 160.51 203.38 217.03 218.07 330.48
32.9 ± 0.37 31.83 ± 0.37 33.69 ± 0.27 40.27 ± 1.09 29.74 ± 0.78 30.23 ± 0.36 25.87 ± 0.56 39.01 ± 0.75
28.40 27.10 23.45 20.75 21.60 24.75 24.50 16.7
± ± ± ± ± ± ± ±
0.31 0.09 0.09 0.15 0.11 0.23 0.08 0.18
175
± ± ± ± ± ± ± ±
6.88 5.79 2.60 6.60 8.08 16.72 11.88 10.48
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Fig. 7. Water contact angle images of PU films: (a) PU164-0.99 (b) PU208-0.84, and ethanol contact angle images of PU films: (c) PU164-0.99 (d) PU208-0.84.
from Polypropylene glycol (PPG) have 2705 wt% ethanol uptake for 48 h, while the maximum water absorption for 96 h in this study is 330.48 wt%. (Lai et al., 2003)
Table 6 Zone of inhibition (diameter) against Vibrio parahaemolyticus for the PUDs. Samples
Zone of Inhibition (mm)
Castor oil content (wt%)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
11.40 ± 0.34 10.79 ± 0.45 9.36 ± 0.44 12.29 ± 0.31 11.93 ± 0.26 10.66 ± 0.34 11.34 ± 0.57 11.94 ± 0.23
0.58 0.55 0.52 0.49 0.52 0.49 0.47 0.44
3.4. Antibacterial properties Polymeric materials containing quaternary ammonium have been extensively studied and used in antibacterial-relevant application. (Chen et al., 2016; Ren et al., 2017; Xue et al., 2015) It’s found that the positively charged quaternary ammonium groups destructive interact with negatively charged bacterial cells and/or cytoplasmic membranes. (Muñoz-Bonilla and Fernández-García, 2012) And castor oil is a typical natural antimicrobial agents (antibacterial, antiviral, and antifungal), which is useful for a variety of skin conditions, including dermatosis, skin infections, wound healing, itching, sebaceous cysts, and even hair Fig. 8. Comparative antibacterial activity of the PUDs against Vibrio parahaemolyticus and Listeria monocytogenes. (a) and (b) show the results against Vibrio parahaemolyticus (Gram-negative, ATCC17802). (c) and (d) show the results against Listeria monocytogenes (Gram-positive, ATCC19115). The boundaries of the inhibition zone are indicated by arrows. (1) PU164-0.69 (2) PU164-0.84 (3) PU164-0.99 (4) PU1641.19 (5) PU208-0.69 (6) PU208-0.84 (7) PU208-0.99 (8) PU208-1.19.
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loss. (Yari et al., 2014) Hence, in this study, the MDEA content, which provide quaternary ammonium groups, and castor oil content may pay a key role in influencing the antibacterial properties of the resulting polymer. Table 6 and Fig. 8(a) and (b) show the antibacterial activity of the PUDs against Vibrio parahaemolyticus. PU164-1.19 and PU208-1.19 exhibit the best antibacterial activity among the PUDs from castor oil164 and castor oil-208 due to their highest MDEA content, which indicate that the rise of quaternary ammonium groups improves the antibacterial activity of the PUDs. In addition, PU164-0.69 with the lowest MDEA content, shows a higher antibacterial activity than PU164-0.84 and PU164-0.99. This result may cause by the highest castor oil content of PU164-0.69. But PU208-1.19 with 11.46 wt% MDEA content exhibits poor antibacterial properties than PU164-1.19 with 10.21 wt% MDEA content (zone of inhibition are 11.94 mm and 12.29 mm, respectively). As mentioned above, the increase of the polyol functionalities increase the cross-linking densities of the resulting polymers, resulting in poor mobility of the PUDs from castor oil-208 than the PUDs from castor oil164. Hence, PU164-1.19 exhibits more effective physical interaction with target bacteria than PU208-1.19, resulting in better antibacterial activity. (Garrison et al., 2014) Fig. 8(c) and (d) show the antibacterial activity of the resulting PUDs against Listeria monocytogenes. The resulting PUDs demonstrate rarely antimicrobial activity toward Listeria monocytogenes, which may be corresponded to the poor diffusivity of the PUDs. Listeria monocytogenes have a thicker cell wall than gram-negative bacteria (Vibrio parahaemolyticus), (Garrison et al., 2014; Silhavy et al., 2010) which demand a higher diffusivity of the PUDs to interact with the target substance. This can explain the different antimicrobial activities of PUDs toward Listeria monocytogenes and Vibrio parahaemolyticus.
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4. Conclusions Castor oil and its derivative were used to prepare cationic water polyurethane dispersion. The effect of polyol functionalities and the ionic chain extender contents on the particle size and antibacterial properties of the polyurethane dispersions, and thermo-mechanical, thermal stability, mechanical properties, water/solvent uptakes of the resulting polyurethane films were investigated. It is found the castor oil based PUDs demonstrate excellent storage stability. There is no obvious perception or stratification when the PUDs were centrifuged at 3000 rpm for 30 min and storage for 12 months at room temperature. High hydroxyl number of polyols leads to high cross-linking densities of the resulting polyurethane films, resulting in high tensile strength, Tg, and low hydrophility. Increasing ionic chain extender contents result in increasing hydrophility of the polyurethane films therefrom, leading to high tensile strength, Tg, water uptakes and solvent uptakes. Furthermore, the polyurethane dispersions were found to exhibit an increasing antibacterial activity against Vibrio parahaemolyticus with the rise of MDEA content and the reduction of polyol functionalities, but rarely antibacterial activity to Listeria monocytogenes. These new castor oil-based cationic waterborne polyurethanes with good mechanical properties and antibacterial properties can potentially be used as coating in medical instruments, food processing, or added into paints for pathogen control. Acknowledgements This work was supported by the National Natural Science Foundation (No. 51703068), the Guangdong Province Science & Technology Program (2017A010103015). References Alam, M., Akram, D., Sharmin, E., Zafar, F., Ahmad, S., 2014. Vegetable oil based ecofriendly coating materials: a review article. Arab. J. Chem. 7, 469–479.
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Castor oil-based cationic waterborne polyurethane dispersions: Storage stability, thermo-physical properties and antibacterial properties Haiyan Lianga, Lingxiao Liua, Jingyi Lua, Moutong Chenb, Chaoqun Zhanga,
T
⁎
a
College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China Guangdong Institute of Microbiology, State Key Laboratory of Applied Microbiology Southern China, Guangdong Provincial Key Laboratory of Microbial Culture Collection and Application, Guangdong Open Laboratory of Applied Microbiology, Guangzhou, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Castor oil Waterborne polyurethane Antibacterial properties Thermo-mechanical properties
In this study, castor oil and its derivative were used to prepare cationic waterborne polyurethane dispersion using N-methyl diethanolamine (MDEA) as ion center. The effect of polyol functionalities and the ionic chain extender contents on the particle size and antibacterial properties of the polyurethane dispersions, and thermomechanical, thermal stability, mechanical properties, water/solvent uptakes of the resulting polyurethane films were thoroughly investigated. Moreover, all the polyurethane dispersions exhibit enhanced antibacterial activity against Vibrio parahaemolyticus with the rise of MDEA content and the reduction of polyol functionalities, but rarely antibacterial activity to Listeria monocytogenes. To the best of my knowledge, this work for the first time extensively investigate the structure-property relationships of castor oil based cationic waterborne polyurethane dispersions and their polyurethane films.
1. Introduction Polyurethanes (PUs) are one of the most versatile polymers and have been widely used in various fields such as coatings, adhesives, clothing, paints and foams because of their excellent chemical resistance, tunable thermo-mechanical properties and good process ability. (Gurunathan et al., 2013; Schmidt et al., 2017) However, traditional polyurethane products usually contain large amount of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs), which are mostly released into the air during practical application, leading to a big health threat for producers and users. With the introduction of increasingly strict legislation and consumer demands towards reducing the VOC and HAPs, a current renaissance of environmentally friendly waterborne polyurethanes has occurred in a global scale, aiming to partially or completely replace solvent-based polyurethanes for application in ink, adhesives and coatings. (Gogoi and Karak, 2014) In order to disperse polyurethanes in aqueous as a stable phase, hydrophilic groups (carboxylic and sulfonic acid, amine) are usually incorporated into the side chain or backbone of the polymers, which leads to the formation of waterborne polyurethanes. Much progress have been made recently to formulate waterborne polyurethanes with high solid content and high performance. (Burja et al., 2015; Peng et al., 2015; Peng et al., 2014; Zhang et al., 2015) For example, Li and his co-workers synthesized a cationic waterborne polyurethane with a
⁎
high solid content (42.06–52.18%) via the combination of cationic ionic (N-methyl diethanolamine) and nonionic segments (Poly(oxyethylene alkyl amine)). (Li et al., 2017) Yong et al. have synthesized a solvent-free and chemical matt waterborne polyurethanes, simultaneously using sulfonic (sodium 2-[(2-amino ethyl) amino] ethane sulfonate) and carboxylic acid (2,2-Bis(hydroxymethyl) pro-pionic acid) as hydrophilic chain extender (Yong et al., 2015). And Yu et al. successfully prepared a series of high waterproof carboxylic acid type waterborne polyurethane films with introduction of fluorine and siloxane to increase the cross-linking density of the materials. (Yu et al., 2016) Up to date, most of the raw materials (Polyols, isocyanates, chain extenders) used for polyurethane dispersions (PUDs) are derived from petrochemical feedstock, which are widely regarded as nonrenewable and major source of criteria air pollutants. (Alam et al., 2014; Zhu et al., 2016) With the depletion of the world crude oil stock and increasing environmental concerns, efforts on a global scale are dedicated to find a renewable resource (such as cellulose, natural oils, lignin, and so on) for bio-based polyurethanes to replace petroleum based counterparts. (Gaikwad et al., 2015; Zhang et al., 2014) Vegetable oils as a kind of typical renewable biomass resources are among the most promising for polyol synthesis due to its low cost, and readily available. (Pawar et al., 2015) Vegetable oils are triglyceride of fatty acid that usually bears 12–22 carbon atoms and 0–3 carbon–carbon double bonds. Except for castor oil, most vegetable oils do not contain hydroxyl groups.
Corresponding author. E-mail addresses: [email protected], [email protected] (C. Zhang).
https://doi.org/10.1016/j.indcrop.2018.02.084 Received 7 January 2018; Received in revised form 26 February 2018; Accepted 28 February 2018 0926-6690/ © 2018 Elsevier B.V. All rights reserved.
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(Gurunathan et al., 2015) The reactive ester and carbon–carbon double bonds in triglyceride oils offer several routes to introduce hydroxyl groups necessary in polyols for PU synthesis, including epoxidation/ ring opening, ozonolysis/reduction, hydroformylation/reduction, transesterification, thiol-ene click reactions. (Bullermann et al., 2013; Feng et al., 2017; Zhang et al., 2013) Anionic and cationic PU dispersions have been synthesized from different vegetable oil based polyols. It is found that the hydrophobic nature of triglycerides and long fatty acid chains endow the resulting PU films excellent chemical and physical properties, including enhanced hydrolytic, flexibility and toughness. (Zhang et al., 2017) Lu et al. successfully prepared soybean oil-based cationic waterborne PU films with tensile strengths from 5.7 to 23.2 MPa and elongation at break from 235 to 291%. Moreover, the effect of polyols functionalities on the size of the polyurethane particle and the thermo-mechanical properties of the PU films were studied and discussed. (Lu and Larock, 2010) Fu and his co-worker synthesized a castor oil-based anionic waterborne PU film with a high flexibility (1 mm) and excellent chemical resistance (1.75% water absorption and 90% toluene absorption for 168 h). (Fu et al., 2014) Saalah et al. investigated the effect of the OH number, DMPA content and hard segment content on the stability of the anionic waterborne PU dispersions from jatropha oil, as well as the physical, mechanical and thermal properties of the resulting films. The resulting PU film exhibited excellent hydrophobicity, with a contact angle of 90° or more, indicating a nonwetting surface. (Saalah et al., 2015) To the best of my knowledge, the effect of polyols functionalities and chain extender content on the size of the anionic polyurethane particle and the thermo-mechanical properties of the resulting films have been widely investigated. But their effects on the performance of castor oil-based cationic polyurethane have not been reported previously. In this study, cationic waterborne PUDs were successfully prepared from castor oil and its derivative using N-methyl diethanolamine (MDEA) as ion center. The effect of polyol functionalities and MDEA contents on the physical properties and antibacterial properties of the PUDs and the thermo-mechanical properties of the resulting films were investigated. The particle size and zeta potential of the PUDs were characterized by zeta-sizer while the thermo-mechanical properties of the resulting PU films were characterized by DMA, TGA, tensile testing. In addition, the antibacterial properties of the resulting PUDs were tested against Vibrio parahaemolyticus and Listeria monocytogenes. The work extensively investigated the structure-property relationships of castor oil based cationic waterborne polyurethane dispersions and their films.
Table 1 Compositions of the waterborne PUs. Samples
OH number of castor oil (mg KOH per g)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19 a b
164 164 164 164 208 208 208 208
Molar ratio NCO
OHa
OHb
1.7 1.85 2.0 2.2 1.7 1.85 2.0 2.2
1 1 1 1 1 1 1 1
0.69 0.84 0.99 1.19 0.69 0.84 0.99 1.19
Hydroxyl molar ratio of castor oil. Hydroxyl molar ratio of the MDEA.
The molar ratio of the raw materials is summarized in Table 1. After the reaction continued for 30 min, 30 ml MEK was added to reduce the viscosity. The mixture continued to react under stirring for 2 h at 78 °C. After the reactants were cooled down to room temperature, acetic acid was added as neutralizer under stirring for 30 min. Finally, 90 ml distilled water was added for emulsification for 2 h. The waterborne PUDs with a solid content of 10–12 wt% was obtained after MEK was removed by rotary evaporator. The PU films were obtained after their corresponding waterborne PUDs were casted and dried in a silicon mold at room temperature for at least 48 h (See Scheme 1). 2.3. Characterization The stability of waterborne PU dispersion was evaluated by centrifuging the emulsion at 3000 r/min for 30 min on Tomos 3–18. A Zeta-sizer Nano ZSE (Malvern Instruments) was used to measure the size distribution and zeta potential of the PU dispersions which was diluted with distilled water to about 0.01 wt% before test. The mechanical properties of the WPU films were determined using an electronic universal testing machine (UTM-4204) with a crosshead speed of 100 mm/min. The waterborne polyurethane dispersions were casted into a silicon mold and allowed them dry in room temperature for PU films. All the samples with a length of 30 mm and a width of 10 mm were dried at 60 °C for 12 h before test. An average value of at least three replicates of each PU sample was taken. The dynamic mechanical behavior of the resulting films was determined using a Netzsch DMA 242C dynamic mechanical analyzer in tensile mode at 1 Hz. The samples with a length of 8 mm and a width of 6 mm were heated from −60 to 120 °C at a rate of 5 °C min−1. For this study, the glass transition temperatures (Tg) of the PU films were obtained from the peaks of the loss factor curves. Thermo-gravimetric analysis (TGA) of the PU films was carried out on a Netzsch- STA 449C thermal analyzer. The samples were heated from 30 to 700 °C at a heating rate of 20 °C min −1 in a nitrogen atmosphere. Generally, 8–14 mg samples were used for the test. The chemical resistance of the PU films against distilled water and ethanol was conducted according to the methods previous reported (Wang et al., 2015a; Xia and Larock, 2011). All the square films with a length of 10 mm were dried at 60 °C for 12 h before test. An average value of more than four replicates of each sample was taken. The PU films with known weight (m0) were immersed in a distilled water bath at room temperature for 20 days for water absorption. It was also immersed in absolute ethanol for 96 h for ethanol uptake. Then the films were weighed (m1) and dried at 60 °C for at least 48 h. The dried films were then weighed (m2) and the percentage of water absorption (WA), ethanol absorption (WE), and weight losses (Wx, Wy) of the resulting films in water and ethanol were calculated as follows:
2. Experimental 2.1. Materials Castor oil (OH number: 164 mg KOH/g) and its derivative (OH number: 208 mg KOH/g) were provided by Fuyu Chemical Company, and Guangzhou Xinye Trading Company, respectively. In this study, they are coded as castor oil-164, and castor oil-208. Isophorone diisocyanate (IPDI) and N-methyl diethanolamine (MDEA) were purchased from Wengjiang Chemical Reagent Company. Dibutyltin dilaurate (DBTDL) was purchased from Fuchen Chemical Reagent Factory. Acetic acid, and methyl ethyl ketone (MEK) were purchased from Aladdin reagent and Tianjin Hongda Chemical reagent, respectively. All materials were used as received without further purification. 2.2. Preparation of castor oil-based waterborne PUDs Firstly, castor oil (6 g) and IPDI were mixed in a double neck round bottom flask equipped with mechanical stirrer for 10 min at 78 °C. One drop of DBTDL was added in the mixture to allow the reaction to proceed for 10 min. Then, MDEA was added under high-speed stirring.
WA (%) = 170
(m1 − m 0) × 100% m0
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Scheme 1. The representative synthesis route of the castor oil-based cationic waterborne PUDs.
Wx (%) =
(m 0 − m2) × 100% m0
diffusion method (Chen et al., 2014a; Xia et al., 2012). Briefly, the concentrations of cell suspension were adjust to 0.5 McFarland turbidity (about 108 CFU/mL), and then spread evenly to the MuellerHinton agar. PUDs dispersions (20 μg) were added to paper discs (6 mm, Oxoid Ltd). The pH of the PUDs ranges from 6 to 6.5, similar to the pH of water, which excludes the influence of the pH on the antibacterial action. The M-H agar plates were incubated at 37 °C for 24 h. Zones of inhibition were measured with a precision caliper to the nearest 0.01 mm. Each sample was repeated for three times.
Where m1 is the weight of the film had immersed in water and m2 is the weight of the dried film after test.
WE (%) =
(m1 − m 0) × 100% m0
Wy (%) =
(m 0 − m2) × 100% m0
3. Results and discussion
Where m1 is the weight of the film had immersed in absolute ethanol and m2 is the weight of the dried film after test. The contact angle of water and ethanol droplets on the resulting films were measured with a contact angle goniometer (Powereach JC2000C1) using the sessile-drop method at room temperature. All the samples were dried at 60 °C for at least 48 h before test. The antibacterial ability of the resulting PUDs against Vibrio parahaemolyticus and Listeria monocytogenes were performed using disk
3.1. Appearance and size of the PUDs The images of PUDs from castor oil-164 (a) and castor oil-208 (b) are shown in Fig. 1. With the increase of MDEA contents, the transparency of the resulting PUDs gradually increases from milky white opacity to transparent. In addition, the PUDs from castor oil-208 are much more transparent than the PUDs from castor oil-164 when the 171
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Fig. 1. The appearance of waterborne PUDs (a) The hydroxyl ratios of MDEA and castor oil-164: (1) 0.69 (2) 0.84 (3) 0.99 (4) 1.19; (b) The hydroxyl ratios of MDEA and castor oil-208: (1) 0.69 (2) 0.84 (3) 0.99 (4) 1.19.
hydroxyl ratio of MDEA and polyols are the same. As a typical internal emulsifier, MDEA provides large amounts of hydrophilic groups and enables the hydrophobic hard and soft segments of polyurethane disperse in the water. Therefore, the more MDEA content in the polymer chains, the more hydrophility of the resulting PUDs and the more transparency of the resulting PUDs will be. With the OH number of castor oil increase, the content of MDEA used for the PUDs rises accordingly, leading to the enhancement of transparence when equal hydroxyl molar ratio of the MDEA are applied. In addition, the castor oil based PUDs demonstrate excellent storage stability. There is no obvious perception or stratification when the PUDs were centrifuged at 3000 rpm for 30 min and storage for 12 months at room temperature. In order to investigate the mechanism underlying their unique appearance and the excellent storage stability, the average size and its distribution of the PUDs were characterized and shown in Table 2 and Fig. 2. With the rise of MDEA content, the particle size of waterborne PUD lessens. (Wang et al., 2015b) As the hydroxyl molar ratios of the MDEA shift from 0.69 to 1.19, the particle size of the PUD from castor oil-164 reduces from 73.3 nm to 21.3 nm due to the enhancement of hydrophility. Besides, in the same molar ratios of MDEA, the particle size of the PUDs from castor oil-208 is smaller than that from castor oil164 since higher content of MDEA were used in the former formulation when equal hydroxyl molar ratio of the MDEA are applied. Moreover, as the OH number of castor oil increases, the cross-linking density (see Table 4) of the waterborne polyurethane increases, resulting in reduction of the particle size. (Lu and Larock, 2008) The particle sizes of the PUDs from castor oil-208 and castor oil-164 at MDEA ratios of 9.08 wt% are 22.8 nm, and 25.8 nm, respectively. Similarly, the zeta potential of the PUDs reduces with the increase of MDEA under otherwise equal conditions. As the MDEA content increase to 10.21 wt% from 6.95 wt%, the zeta potential of the PUDs from castor
Fig. 2. The particle size distribution of castor oil based waterborne PUDs with different hydroxyl molar ratios of MDEA: (a) PUDs from castor oil-164; (b) PUDs from castor oil208.
oil-164 decrease to 45.5 mV from 73.2 mV. In the same molar ratio of MDEA, the zeta potential of the PUDs from castor oil-208 is lower than that from castor oil-164. The results show that the particle size of waterborne PUD has remarkable impact on the zeta potential and appearance of the emulsion. The smaller of the particle size is, the lower zeta potential and more transparent of the PUDs will be, and vice versa. 3.2. Properties of the PU films The mechanical, thermo-mechanical properties and thermal stability of the casting PU films were characterized by tensile testing, DMA, and TGA. Stress-strain curves for PU films from castor oil-164 and castor oil208 are shown in Fig. 3. The Young’s modulus, toughness, tensile strength and elongation at break of the PU films are summarized in Table 2. With the molar ratios of MDEA increase from 0.69 to 1.19, the
Table 2 The stability, particle size and zeta potential of the waterborne PUDs. Samples
MDEA content (wt%)
Appearance
Storage life
Z-average size (nm)
Zeta Potential (mV)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
6.95 8.07 9.08 10.21 7.87 9.08 10.16 11.46
Milky white Milky white translucent with yellow light Pale white translucent Milky white with blue light Pale blue transparent Pale blue transparent Micro-blue transparent
> 12 months > 12 months > 12 months > 12 months > 12 months > 12 months > 12 months > 12 months
73.3 51.2 25.8 21.3 58.7 22.8 20.0 17.2
73.2 63.5 46.3 45.5 39.7 31.7 27.6 26.8
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Table 3 Mechanical Properties of the waterborne PU films. Samples
Young’s modulus (MPa)
Tensile strength (MPa)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
1.10 ± 0.06 1.97 ± 0.07 2.37 ± 0.12 2.76 ± 0.77 7.99 ± 2.02 11.85 ± 2.40 19.93 ± 0.47 34.50 ± 2.21
0.71 0.90 0.93 0.99 1.84 2.41 3.46 3.95
± ± ± ± ± ± ± ±
0.08 0.11 0.09 0.41 0.24 0.22 0.05 1.45
Elongation at break (%)
Toughness (MPa)
585 506 565 520 421 487 505 646
2.10 ± 0.47 2.41 ± 0.55 3.05 ± 0.24 2.71 ± 0.74 5.77 ± 1.33 8.16 ± 1.58 9.43 ± 1.51 16.51 ± 6.21
± ± ± ± ± ± ± ±
145 6.95 115 113 2.12 35.7 90.7 105
Fig. 3. Stress-strain curves for PU films from castor oil-164 (a) and castor oil-208 (b) with different content of MDEA.
tensile strength of the castor oil-164 based film improves from 0.71 to 0.99 MPa, and the Young’s modulus rises from 1.10 to 2.76 MPa, while there is no big change in elongation at break. Moreover, when the molar ratios of MDEA enhance from 0.69 to 1.19, a sharply shift in the tensile strength from 1.84 to 3.95 MPa, Young’s modulus from 7.99 to 34.5 MPa, elongation at break from 421 to 646% are observed for the PU films from castor oil-208. The increases of tensile strength and Young’s modulus can be explained by the fact that the rise of the hard segment content contributes to the stiffness of the network structure. And the difference in elongation at break of the resulting PU films results from different functionalities of the castor oil and the hard segments of the materials. As expected, an increase of OH number of the polyols (castor oil) improves the tensile strength and Young’s modulus of the resulting PU films. In the same content of MDEA, the PU films from castor oil-208 have higher tensile strength than the PU films from castor oil-164 due to their higher cross-linking density. In detail, the PU208-0.84 film exhibits a tensile strength of 2.41 MPa, while the PU164-0.99 film exhibits a tensile strength of 0.93 MPa with the same 9.08 wt% MDEA content. This finding is similar to the result that reported by Gaddam et al. (Gaddam et al., 2017) Stress-strain test also afford the toughness of the materials, which regard as the characteristics of a material’s resistance to fracture when stressed. As shown in Table 3, the toughness of the films from castor oil208 significantly rises from 5.77 to 16.51 MPa, with the molar ratio of MDEA increase from 0.69 to 1.19. Because increasing MDEA content leads to a rise of hard segment content, resulting in an improvement of hydrogen bonding interactions and toughness of PU films. (Lu and Larock, 2008) In addition, an increase of OH number of the castor oil also improves the toughness of the films. The PU208-0.69 with 48.1 wt
Fig. 4. Storage modulus and loss factor (tan δ) as a function of the temperature for PU films from castor oil-164 (a) and castor oil-208 (b) with different content of MDEA.
% hard segment content exhibits higher toughness (5.77 MPa) than PU164-1.19 (2.71 MPa) with 50.6 wt% hard segment content, resulting from the enhancement of the cross-linking densities. The storage modulus (E’) and tan δ of the PU films from castor oil164 (a) and castor oil-208 (b) with different molar ratio of MDEA are shown in Fig. 4. At the temperature from −60 to 0 °C, the E’ and tan δ of the resulting films both decreased slightly and the films are in the glassy state. Then, a rapid drop in the E’ is observed in the temperature from 10 to 50 °C for all of the resulting films, corresponding to the energy dissipation of the materials that is shown as the peak maximum in the tan δ curve. (Nguyen Dang et al., 2016) And the temperature associated with the maximum in the tan δ curve is the glass-transition temperature (Tg). All of the films show only one peak, indicating the homogeneous properties of the waterborne PU films. (Feng et al., 2017) The Tg and E’ at 25 °C of the PU films are shown in Table 4. As the MDEA content increases, both the Tg and E’ of the PU films increase, owing to the rise of hard segment content. (Chen et al., 2014b; Gurunathan et al., 2013) For example, with the MDEA content shift from 7.87 to 11.46 wt%, a rise of the Tg from 29.5 to 39.7 °C and an
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Table 4 DMA and TGA data of the waterborne PU films. Samples
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
Hard segment content (wt%)
ve (mol/ m3)
DMA Tg (°C)
E’ at 25 °C (MPa)
T5 (°C)
T50 (°C)
Tmax (°C)
42.2 45.0 47.6 50.6 48.1 50.9 53.5 56.5
64.9 62.3 58.2 59.0 60.3 60.6 60.7 129.8
17.2 18.7 27.1 25.0 29.5 33.2 33.8 39.7
3.4 3.9 21.3 13.4 49.2 174.4 222.1 431.5
288 290 290 278 287 269 282 266
376 383 378 371 371 359 364 351
470 472 472 474 477 477 477 477
TGA
increase of the E’ at 25 °C from 49.2 to 431.5 MPa are observed for the PU films from castor oil-208. Additionally, the PU films from castor oil208 obviously exhibit higher Tg and storage modulus than the PU films from castor oil-164 under otherwise equal conditions. In detail, the Tg and E’ at 25 °C of the PU films from castor oil-208 with 48.1 wt% hard segment content are 29.5 °C and 49.2 MPa, respectively, while the Tg and E’ at 25 °C of the PU films from castor oil-164 with 50.6 wt% hard segment content are 25 °C and 13.4 MPa. This can be explained by the higher cross-linking densities of the resulting PU films from castor oil208 than those from castor oil −164. Cross-linking densities (ve) of all the resulting films were calculated from the rubbery plateau modulus according to the kinetic theory of rubber elasticity:(Andjelkovic et al., 2005) E’ = 3 veRTWhere T is the absolute temperature at Tg + 30 °C, and E’ is the storage modulus at T. R is the gas constant. As shown in Table 4, the cross-linking densities of the polyurethanes increase with the increase of the polyol functionalities. And the PU208-0.69 shows a higher cross-linking density (60.3 mol/m3) than PU164-1.19 (59.0 mol/ m3). High cross-linking densities usually restrict the movement of the polymer chain, resulting in a high Tg and storage moduli as discussed above. (Lu and Larock, 2008) TGA curves of PU films from castor oil-164 (a) and castor oil-208 are shown in Fig. 5. The TGA data of the resulting films are summarized in Table 3. All the PU films exhibit three stages of thermal degradation process. The first stage at 200–300 °C is attributed to the dissociation of the labile urethane bonds, forming isocyanates, alcohols, primary amines, secondary amines, olefins, and carbon dioxide. (Gurunathan and Chung, 2016; Xia and Larock, 2011) With the increase of MDEA content, the thermal stability of PU in this stage decrease due to the rise of labile urethane bonds. The same trend is observed for the thermal stability as a function of OH number. T5 of the PU films from castor oil164, generally considered as the onset decomposition temperature, decrease from 288 to 278 °C when the molar ratios of MDEA increase from 0.69 to 1.19 (the hard segment increases from 42.2 to 50.6%). Degradation in the range 300–450 °C, the fastest degradation process, relates to castor oil chain scission. There is no big difference for the PUs with different OH numbers and MDEA content. An increase of the OH number of the castor oil and MDEA content rise the crosslinking density of the resulting PU films, leading to an improvement of the thermal stability of the PU films. (Chen et al., 2014b) These enhancement of thermal stability compensate the low thermal stability induced from the high labile urethane bonds as the OH numbers of the castor oils and MDEA content increase, resulting in no significant difference of maximum degradation temperatures (Tmax) of PUs. The last step above 450 °C was assigned to further thermo-oxidation of the PU films in air.
Fig. 5. TGA curves of PU films from castor oil-164 (a) and castor oil-208 (b) with different content of MDEA.
absorption are observed for the waterborne PU films: a fast absorption stage, a slow absorption stage, and an equilibrium absorption stage, which was consistent with other researches. (Wang et al., 2015b) Firstly, the water absorptions of the films increase rapidly with immersion time, then the absorption rate slow down. Finally, the absorption curves reach a plateau, corresponding to the absorption equilibrium. In this study, two factors play a key role in influencing the hydrophility of the PU films. Firstly, with the MDEA content in the PU films increases, the hydrophilic ionic group increases, resulting in a rise in water absorption of the films. (Saetung et al., 2012) In detail, after immersion for 20 days, the water absorption of PU164-0.69 reached 6.37% (6.95 wt% MDEA content), while the water absorption of PU1641.19 reached 14.1% (10.21 wt% MDEA content). Meanwhile, as shown in Table 5, the weight loss of the PU films after immersion in water for 20 days increase from 0.44 to 0.52 wt% with MDEA content increase from 6.95 to 10.21 wt%, and the water contact angle decrease from 51.25° to 36.85°. These results both indicate the improvement in hydrophility of the films. On the other hand, higher cross-linking of the PU films can be obtained by increasing the OH functionality of the castor oil, which leads to an increase in hydrophobicity. (Gurunathan and Chung, 2016; Xia and Larock, 2011) Therefore, PU208-0.69 exhibited higher water contact angle than PU164-0.69 (58.80° and 51.25°, respectively), although PU208-0.69 has higher MDEA content than PU164-1.19 (7.87 and 6.95 wt%, respectively). And the water contact angle images of PU164-0.99 and PU208-0.84 are shown in Fig. 7(a) and (b). With MDEA content of 9.08 wt%, PU208-0.84 obviously exhibited better hydrophobicity than PU164-0.99, resulting from the increase of cross-linking density. But higher cross-linking densities lead to more pore in the PU films, resulting in a higher water absorption of the films
3.3. Chemical resistance and contact angle of the PU films As shown in Fig. 6(a) and (b) and Table 5, three stages of water 174
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Fig. 6. Water absorption and ethanol absorption for PU films from castor oil-164 and castor oil-208 with different content of MDEA.
the ethanol uptake and ethanol contact angle of the films are mainly influenced by the MDEA content and polyol functionalities. As MDEA content and polyol functionalities increases, both the ethanol uptake and ethanol contact angle of the film increases, resulting from the increase of hydrophilic ionic group and cross-linking densities (Xia and Larock, 2011). With MDEA content rises from 6.95 to 10.21 wt%, the ethanol uptake of the films from castor oil-164 increases from 107.79 to 160.51 wt%, and the ethanol contact angle of them decrease from 28.40° to 20.75°. Additionally, with 9.08 wt% MDEA content, PU2080.84 exhibited higher ethanol uptake and ethanol contact angle than PU164-0.99 (ethanol uptake are 217.03 wt% and 133.64 wt%, ethanol contact angle are 24.75° and 23.45°, respectively). This increase of ethanol uptake can be explained by the increasing pore in the films with polyol functionalities rise(Wang et al., 2015a), resulting from the shift in cross-linking density as mention above. Compared with other Petroleum based waterborne PU films, the ethanol absorptions of the resulting films were relatively low, contributed to the hydrophobic nature of the soft segment in the PU films. The waterborne anionic PU films
(Wang et al., 2015a). In detail, with same MDEA content, the water absorption of PU164-0.99 is 7.97 wt%, while that of PU208-0.84 is 14.74 wt%. Compared with other petroleum based waterborne PU films, the water absorptions of the resulting films were relatively low, contributing by the hydrophobic nature of the castor oil based soft segment. (Bullermann et al., 2013) The waterborne anionic PU films from polytetramethylene ether glycol (PTMG) have 48.24 wt% water uptake for 24 h, while the maximum water absorption for 20 days in this study is 23.68 wt%. (Zhong et al., 2017) Moreover, Luo et al. also prepared a waterborne PU film from PTMG with 48 wt% water uptake for 48 h, (Luo et al., 2011) and Saetung et al. prepared a waterborne PU film from hydroxytelechelic natural rubber with 30 wt% water uptake for 7 days. (Saetung et al., 2012) As shown in Fig. 6(c) and (d) and Table 4, the ethanol absorption of the resulting films clearly increases with immersion time and reaches maximum before 24 h. Then, the ethanol absorption decreases with immersion time due to the weight loss of the PUs resulting from partly dissolution of the small molecular material. Similar to the hydrophility, Table 5 Chemical resistance and contact angle of the waterborne PU films. Samples
Water absorption for 20 days (%)
Weight loss in water (%)
Water contact angle (θw, deg)
Ethanol absorption for 96 h (%)
Weight loss in ethanol (%)
Ethanol contact angle (θe, deg)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
6.37 ± 0.55 7.28 ± 0.35 7.97 ± 0.17 14.1 ± 2.36 12.28 ± 1.12 14.74 ± 0.63 18.24 ± 0.77 23.68 ± 1.08
0.44 0.51 0.61 0.64 0.64 0.80 0.53 0.70
51.25 50.75 47.80 36.85 58.80 56.25 51.00 33.75
107.79 120.06 133.64 160.51 203.38 217.03 218.07 330.48
32.9 ± 0.37 31.83 ± 0.37 33.69 ± 0.27 40.27 ± 1.09 29.74 ± 0.78 30.23 ± 0.36 25.87 ± 0.56 39.01 ± 0.75
28.40 27.10 23.45 20.75 21.60 24.75 24.50 16.7
± ± ± ± ± ± ± ±
0.31 0.09 0.09 0.15 0.11 0.23 0.08 0.18
175
± ± ± ± ± ± ± ±
6.88 5.79 2.60 6.60 8.08 16.72 11.88 10.48
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Fig. 7. Water contact angle images of PU films: (a) PU164-0.99 (b) PU208-0.84, and ethanol contact angle images of PU films: (c) PU164-0.99 (d) PU208-0.84.
from Polypropylene glycol (PPG) have 2705 wt% ethanol uptake for 48 h, while the maximum water absorption for 96 h in this study is 330.48 wt%. (Lai et al., 2003)
Table 6 Zone of inhibition (diameter) against Vibrio parahaemolyticus for the PUDs. Samples
Zone of Inhibition (mm)
Castor oil content (wt%)
PU164-0.69 PU164-0.84 PU164-0.99 PU164-1.19 PU208-0.69 PU208-0.84 PU208-0.99 PU208-1.19
11.40 ± 0.34 10.79 ± 0.45 9.36 ± 0.44 12.29 ± 0.31 11.93 ± 0.26 10.66 ± 0.34 11.34 ± 0.57 11.94 ± 0.23
0.58 0.55 0.52 0.49 0.52 0.49 0.47 0.44
3.4. Antibacterial properties Polymeric materials containing quaternary ammonium have been extensively studied and used in antibacterial-relevant application. (Chen et al., 2016; Ren et al., 2017; Xue et al., 2015) It’s found that the positively charged quaternary ammonium groups destructive interact with negatively charged bacterial cells and/or cytoplasmic membranes. (Muñoz-Bonilla and Fernández-García, 2012) And castor oil is a typical natural antimicrobial agents (antibacterial, antiviral, and antifungal), which is useful for a variety of skin conditions, including dermatosis, skin infections, wound healing, itching, sebaceous cysts, and even hair Fig. 8. Comparative antibacterial activity of the PUDs against Vibrio parahaemolyticus and Listeria monocytogenes. (a) and (b) show the results against Vibrio parahaemolyticus (Gram-negative, ATCC17802). (c) and (d) show the results against Listeria monocytogenes (Gram-positive, ATCC19115). The boundaries of the inhibition zone are indicated by arrows. (1) PU164-0.69 (2) PU164-0.84 (3) PU164-0.99 (4) PU1641.19 (5) PU208-0.69 (6) PU208-0.84 (7) PU208-0.99 (8) PU208-1.19.
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loss. (Yari et al., 2014) Hence, in this study, the MDEA content, which provide quaternary ammonium groups, and castor oil content may pay a key role in influencing the antibacterial properties of the resulting polymer. Table 6 and Fig. 8(a) and (b) show the antibacterial activity of the PUDs against Vibrio parahaemolyticus. PU164-1.19 and PU208-1.19 exhibit the best antibacterial activity among the PUDs from castor oil164 and castor oil-208 due to their highest MDEA content, which indicate that the rise of quaternary ammonium groups improves the antibacterial activity of the PUDs. In addition, PU164-0.69 with the lowest MDEA content, shows a higher antibacterial activity than PU164-0.84 and PU164-0.99. This result may cause by the highest castor oil content of PU164-0.69. But PU208-1.19 with 11.46 wt% MDEA content exhibits poor antibacterial properties than PU164-1.19 with 10.21 wt% MDEA content (zone of inhibition are 11.94 mm and 12.29 mm, respectively). As mentioned above, the increase of the polyol functionalities increase the cross-linking densities of the resulting polymers, resulting in poor mobility of the PUDs from castor oil-208 than the PUDs from castor oil164. Hence, PU164-1.19 exhibits more effective physical interaction with target bacteria than PU208-1.19, resulting in better antibacterial activity. (Garrison et al., 2014) Fig. 8(c) and (d) show the antibacterial activity of the resulting PUDs against Listeria monocytogenes. The resulting PUDs demonstrate rarely antimicrobial activity toward Listeria monocytogenes, which may be corresponded to the poor diffusivity of the PUDs. Listeria monocytogenes have a thicker cell wall than gram-negative bacteria (Vibrio parahaemolyticus), (Garrison et al., 2014; Silhavy et al., 2010) which demand a higher diffusivity of the PUDs to interact with the target substance. This can explain the different antimicrobial activities of PUDs toward Listeria monocytogenes and Vibrio parahaemolyticus.
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4. Conclusions Castor oil and its derivative were used to prepare cationic water polyurethane dispersion. The effect of polyol functionalities and the ionic chain extender contents on the particle size and antibacterial properties of the polyurethane dispersions, and thermo-mechanical, thermal stability, mechanical properties, water/solvent uptakes of the resulting polyurethane films were investigated. It is found the castor oil based PUDs demonstrate excellent storage stability. There is no obvious perception or stratification when the PUDs were centrifuged at 3000 rpm for 30 min and storage for 12 months at room temperature. High hydroxyl number of polyols leads to high cross-linking densities of the resulting polyurethane films, resulting in high tensile strength, Tg, and low hydrophility. Increasing ionic chain extender contents result in increasing hydrophility of the polyurethane films therefrom, leading to high tensile strength, Tg, water uptakes and solvent uptakes. Furthermore, the polyurethane dispersions were found to exhibit an increasing antibacterial activity against Vibrio parahaemolyticus with the rise of MDEA content and the reduction of polyol functionalities, but rarely antibacterial activity to Listeria monocytogenes. These new castor oil-based cationic waterborne polyurethanes with good mechanical properties and antibacterial properties can potentially be used as coating in medical instruments, food processing, or added into paints for pathogen control. Acknowledgements This work was supported by the National Natural Science Foundation (No. 51703068), the Guangdong Province Science & Technology Program (2017A010103015). References Alam, M., Akram, D., Sharmin, E., Zafar, F., Ahmad, S., 2014. Vegetable oil based ecofriendly coating materials: a review article. Arab. J. Chem. 7, 469–479.
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