Mingzhi Xuab,
Junjia Bianab,
Changyu Han*a and
Lisong Dong*a
aKey Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People's Republic of China. E-mail: cyhan@ciac.ac.cn; dongls@ciac.ac.cn; Tel: +86-431-85262244 Tel: +86-431-852622890
bUniversity of Chinese Academy of Sciences, No. 19A Yuquanlu, Beijing 100049, People's Republic of China
First published on 22nd August 2016
Frequent oil spillages and industrial discard of many organic solvents have created severe environmental and ecological problems. Therefore, it is imperative to find effective absorbent materials with high performance. Simultaneously, a green process of preparing such absorbent materials should be developed. Herein we present a facile approach to prepare open-cell polypropylene (PP)/starch blend foams with low density by twin-screw extrusion using water as a physical blowing agent and starch as an effective water carrier. The cell diameter of the prepared PP/starch blend foams was controlled by using different nucleating agents and changing the die geometry. Foams with mean cell diameter in the range of 0.4–4.5 mm and open-cell content larger than 90% were successfully obtained. Moreover, a remarkable improvement of hydrophobicity of the foams was obtained when decreasing the cell diameters. Consequently, the water contact angle and oil recovery efficiency were increased up to 142.2° and 98.4%, respectively, when the mean cell diameter was reduced to 0.4 mm. These characteristics make this foam a promising candidate absorbent material for use in oil-spill cleanup.
The commonly used oil absorbents include porous inorganic particles,8,9 fibers derived from both natural and synthetic polymers10,11 and sponges.12–15 However, these materials often suffer from problems of poor buoyancy, low oil sorption capacity and unsatisfactory selectivity. Recently, various advanced absorbent materials with superhydrophobicity and excellent oil sorption capacity have been developed to solve the above mentioned problems.16–19 For example, superhydrophobic graphene-based sponges were successfully fabricated and the obtained absorbent demonstrated high selectivity, good recyclability and excellent oil absorption capacity.16
Polymer foams with low density and high open-cell content are attractive candidate oil absorbent materials since they are usually cheap and the production process is easy to scale up. The blowing agents are usually used for large-scale production of polymer foams in industry including both chemical and physical blowing agents. Traditional physical blowing agents, mainly including volatile organic hydrocarbons and their derivatives, are usually harmful to the environment and/or flammable.20–22 Recently, we reported a facile and green extrusion foaming method for the production of polypropylene (PP)/starch blend foams using water as a blowing agent.23 The results showed that PP/starch blend foams with low density and high open-cell content could be severed as a potential oil absorbent material. However, the hydrophobicity of the above mentioned foams needs to be further improved to better satisfy the oil-spill cleanup application.
Many previous studies showed that surfaces with good hydrophobicity could be obtained by the combination of low surface energy and proper surface roughness.24–27 In the case of polymer foams, their cellular structure could provide a natural rough surface. Therefore, many researchers have tried to improve polymer foams' hydrophobicity by surface chemical modification.28–30 For example, Zhou et al. prepared superhydrophobic sponge by a vapor-phase deposition process.28 Surprisingly, very few literatures investigated the relationship between the cellular structure and hydrophobicity of polymer foams, although this may offer us a facile and green method of preparing polymer foams with good hydrophobicity without any chemical modifications.
Therefore, in this study, open-cell PP/starch blend foams with low density were first prepared by facile twin-screw extrusion using water as a physical blowing agent. Then we presented a simple and inexpensive method for the fabrication of hydrophobic open-cell foams without involving any chemical reaction by only tailoring the cell morphology. The cell diameter of the prepared PP/starch blend foams was tailored by adding different nucleating agents and altering the die geometry. Then, the relationship between cell diameter and hydrophobicity of the obtained foams was studied. Finally, the oil absorbing behavior of the prepared foams was further estimated in detail.
A co-rotating twin-screw extruder (ZSK-35) was used in this study for extrusion foaming process. And a filamentary die with varying diameter was equipped at the end of the extrusion foaming system. The first three extruder segments were used to gelatinize the starch in which the temperatures were set at 60/80/100 °C. Then the temperature was increased to 190 °C for melting and mixing zone of the extruder. Finally, the temperature was progressively decreased to 170 °C during the cooling zone. The die temperature was kept at 150 °C. The screw speed was fixed at 300 rpm for all experiments and the feeding rate was estimated to be in the range of 16.2–16.8 kg h−1. The foaming procedure was described in detail in our previous work.23
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The foam samples were cut by a sharp blade or cryogenically fractured and then coated with gold before the observation by a scanning electron microscope (SEM). Image J software was used to analyze the obtained micrographs in order to gain cell parameters. The mean cell diameter (D) was calculated as the average diameter of over 100 cells while the cell density (Nc) was determined with:
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To test the open-cell content of the obtained foams, Ultrapyc 1200e was used and over 3 samples were tested for each formulation. In the equilibrium stage, a low gas (N2) pressure (0.1 MPa) was set to minimize the rupture of the cells. Then measurements were carried out after the pressure reached an equilibrium value for 5 min.
Contact angle measurements were carried out using a contact angle goniometer (DSA 100, Kruss GmbH, Hamburg, Germany) at 25 °C. Water droplets were dropped carefully to the surface of the foams and the water contact angle values were determined by at least five separate measurements at different positions of a single sample.
During the oil sorption capacity tests, foam samples were immersed into 150 mL of oil in a glass beaker of 500 mL for 30 min. The oil sorption capacity was calculated according to following equation:
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To test the foams' selectivity of water and oil, a certain amount of pump oil (4.0 g, 8.0 g and 16 g) was poured into a 100 mL beaker which contained 50 mL deionized water and then placed on a shaker apparatus. The foam samples was first weighed and then added to the oil–water mixture, which was shaken for 30 min at three different frequencies: 0, 90 or 150 cycles per min. After that, the wetted sorbent was removed from the mixture and weighed. The water sorption content of the sorbent was determined according to ASTM D95. Then the net oil sorption was determined by subtracting the water content from the total amount of the sorption. Finally, the foams' selectivity could be estimated by oil recovery efficiency (R) which is calculated by following equation according to previous work:31
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Fig. 1 Cellular morphology of PP/starch blend foams without or with different kinds of nucleating agents: (a) without nucleating agent; (b) 2 wt% CaCO3; (c) 2 wt% MMT; (d) 2 wt% talc. |
It has been proved that the addition of nucleating agents was an effective way to increase nucleating rate and improve the cellular structure.32,33 In this study, three typical inorganic particles (talc, CaCO3 and MMT) were selected to test their effectiveness when severing as nucleating agents. As seen in Fig. 1b–d, with the addition of nucleating agents, the cell size was significantly reduced and the cell size distribution became much more uniform. With the same loading, however, different nucleating agent showed quite different efficiency in increasing the nucleation rate of PP/starch blend system. The results showed that MMT was the least effective nucleating agent (D = 2.0 mm). This could be attributed to the high hydrophilicity nature of MMT particles that made it difficult for water molecules to form gas nuclei on their surface.34 Considering the particle size of CaCO3 and talc was comparable, their surface geometries might be the key factor that determined their efficiency in enhancing nucleation. It turned out that scaly talc demonstrated higher efficiency (D = 0.8 mm) when compared with spherical CaCO3 (D = 1.8 mm). This result was consistent with one previous study reported by Leung et al.35 According to the above discussion, talc was the best nucleating agent among the three studied ones and was selected as the nucleating agent in following sections.
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Fig. 2 (a) Dependence of mean cell diameter and cell density on talc loading; (b) dependence of volume expansion ratio on talc loading. |
It's noteworthy that if the talc loading was too high (5 wt%), the volume expansion ratio of PP/starch blend foams decreased significantly, as seen from Fig. 2b. It was found that the foams shrank a lot during extrusion foaming process when the talc content reached 5 wt% that led to the decease of volume expansion ratio. Apparently, the existence of inorganic particles within cell walls, especially at high loadings, would accelerate the cell wall rupture process since they could serve as potential defects.36 Therefore, more cell walls would rupture at early stage of cell growth process leading to the shrinkage of extruded foams. According to the above discussion, other strategy should also be applied so as to further reduce the cell size and increase cell density without apparent reduction of volume expansion ratio.
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Then the pressure drop rate (dP/dt) can be estimated by:39
![]() | (6) |
Based on theoretical calculation, high pressure drop rate can be achieved by the reduction of die diameter for a given polymer/gas solution passing a filamentary die. Therefore, a series of filamentary dies with varying die diameters and fixed length–diameter ratio (l/2r = 2.5) were used to study the effect of die geometry on the cellular structure of PP/starch blend foams using water as a blowing agent in this study. In this series of experiments the talc content was fixed at 2 wt%. As seen from Fig. 3, SEM observation of the cellular structure clearly showed that with decreasing die diameters, the mean cell size decreased and the cell density increased progressively due to the increased nucleation rate induced by rapid pressure drop. When the die diameter was reduced to 1.0 mm, the mean cell diameter of PP/starch blend foams could be further reduced to 0.4 mm and the cell density was increased up to 6.0 × 105 #/cm3 (Fig. 4). In addition, the volume expansion ratio of the foams remained almost unchanged with varying die diameters.
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Fig. 3 Cellular morphology of PP/starch blend foams extruded with different dies: (a) 2r = 4.0 mm; (b) 2r = 3.0 mm; (c) 2r = 2.0 mm; (d) 2r = 1.0 mm. |
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Fig. 5 Open-cell content of PP/starch blend foams prepared with varying talc contents and die geometries. |
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Fig. 6 Water contact angle of PP/starch blend foams with different mean cell diameters and comparable apparent densities (20–23 kg m−3). |
The Cassie and Baxter model was often applied to explain the improved hydrophobicity of the rough surfaces with micro- or nanostructure when water droplets were much larger than the structures presented on substrate surface.41 Considering the cell size (0.4–4.5 mm) was comparable with water droplets, the Cassie and Baxter model was no longer suitable to correlate the improved hydrophobicity of PP/starch blend foams with reduced mean cell diameters. Herein, the cells were simplified into capillary tubes. When a water droplet was put on the top surface of the capillary tube, it would enter the tube due to its own gravity. Then, a meniscus with the shape depicted in Fig. 7 formed at the interface between the water and air since the inner surface of the capillary tube (i.e. cell) was mainly composed of hydrophobic PP resin. Meanwhile, a capillary pressure (ΔP) would generate, which could hinder the entering of water droplet into tube. When water droplet's gravity was larger than a single capillary pressure, it would go down and contact with more cells generating larger capillary pressure until the equilibrium was reached. According to Laplace equation:
![]() | (7) |
![]() | (8) |
Rm can be calculated by following equation:
Rm = a![]() | (9) |
The water-repellent behavior of the obtained PP/starch blend foams was also tested to further confirm the hydrophobicity of them. As seen in Fig. 8a, foam sample with D = 1.8 mm only could support an approximately hemispherical dyed water droplet on its surface. And the water droplet only could partially be absorbed by a piece of filter paper. On the contrary, foam sample with D = 0.4 mm could support a nearly spherical water droplet on its surface and the droplet could easily be absorbed by the filter paper almost totally (Fig. 8b).
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Fig. 8 Water-repellent behavior of PP/starch blend foams with different mean cell diameters: (a) D = 1.8 mm; (b) D = 0.4 mm. |
To test the PP/starch blend foams' selectivity of water and oil quantitatively, the oil recovery efficiency of them was determined and the obtained results were demonstrated in Fig. 10. One could see that the selectivity of foams with large cell size (D = 4.5 mm) were very poor and they would absorb a certain high amount of water (24.4%) when they were immersed into oil–water mixtures. On the contrary, foams with small cell size (D = 0.4 mm) only absorb a little of water and the oil recovery efficiency was up to 98.4%. The foams' selectivity was positively correlated with their hydrophobicity which improved progressively as their mean cell diameter decreased, thus leading to the increasing oil recovery efficiency.
It is noteworthy that the selectivity of the PP/starch blend foams was also affected by some other factors such as the viscosity of the oils, the thickness of the oil films and the shaking frequency. It was found that the oil film was more easily broken into pieces by turbulence of water when the viscosity of the tested oil was lower and/or the shaking frequency was higher. Therefore, the foam samples would have more opportunity to contact with water and absorb more water, which led to the decreased oil recovery efficiency (Fig. S2 and S3†). On the other hand, with increasing thickness of oil films, the foam sample were more likely surrounded by oils and absorb more oil leading to the increased oil recovery efficiency (Fig. S4†). Besides, it was also found that the selectivity of the foams with larger cell size was more sensitive to testing conditions due to their poor hydrophobicity. On the contrary, foams with small cell size (D = 0.4 mm) and good hydrophobicity remained a relatively high oil recovery efficiency (>95%) under varying testing conditions.
Fig. 11 described the sorption capacity of the foam samples (D = 0.4 mm, ρf = 22 kg m−3) quantitatively. It was found that PP/starch blend foam demonstrated good sorption capacity toward a wide range of oils and organic solvents. It could absorb oil and solvents up to 32–60 times of its own weight depending on the density of the liquids. It is noteworthy that since the volume expansion ratio and the open-cell content of PP/starch foams with different cell size were comparable, their sorption capacities were very similar.
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Fig. 11 Sorption capacities of PP/starch blend foams toward several common oils and organic solvents. |
The reusability of the PP/starch blend foams (D = 0.4 mm, ρf = 22 kg m−3) was also estimated using pump oil as an example and the obtained results were shown in Fig. 12. It was found that the foam samples could sustain cyclic compression and maintained an acceptable oil sorption capacity (31.8 g g−1) after 10 cycles. The reduction of the foams' sorption capacity during cycles was attributed to the irreversible deformation of the foams caused by cyclic compression. In addition, the water contact angle of the foam sample decreased slightly to 139.6° after 10 cycles since a small part of the cells were destroyed during cyclic compression.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19642j |
This journal is © The Royal Society of Chemistry 2016 |