Facile Preparation of Isocyanate-Modified Melamine Sponges as Efficient Absorbents for Oil and Organic Solvents

Abstract

Hydrophobic melamine sponges were prepared via a facile one-step reaction of three isocyanates (i.e., cyclohexyl isocyanate, octadecyl isocyanate, and butyl isocyanate) with hydroxymethyl groups located on the melamine sponge. The modified sponges were characterized by infrared spectroscopy, scanning electron microscope, X-ray photoelectron spectroscopy, and contact angle tester. The results showed that the water contact angle of the modified sponges reached around 140°, and the sponges had a three-dimensional network with high porosity, which can be used as efficient adsorbents to adsorb a variety of oils or organic solvents with different viscosities and densities. The experiments found that the modified sponges had good oil absorption performance, with 62–143 times their weight, for different oil and organic solvents. In addition, after being soaked in strong acid, strong alkali, and 3.5% sodium chloride aqueous solutions for 72 h, the modified sponges’ absorption capacity remained high.

1. Introduction

With the rapid development of the modern economy and industrial production, the problem of water pollution is becoming increasingly serious, mainly from industrial waste liquid pollution and domestic wastewater pollution. On the one hand, a large amount of organic wastewater and oil generated from economic activities is discharged into rivers. On the other hand, people’s daily lives also produce oily wastewater that pollutes the environment, causing incalculable damage to the ecological environment and necessary water resources on which humans rely for survival [1,2,3]. Nowadays, people are increasingly paying attention to water resources due to the problems caused by water pollution, which damages human health, damages the natural environment, and reduces the efficiency of economic activities. In addition to reducing wastewater discharge, how to quickly, efficiently, and cost-effectively fulfill oil–water separation to achieve wastewater recycling is also an important method to solve water pollution problems [4,5,6]. The traditional oil–water separation methods mainly include chemical dispersant degradation, centrifugal method, biological method, and in situ combustion, etc. [7]. Although these methods can be relatively simple for the primary separation of oil and water, some methods require large and expensive process equipment, some are not conducive to oil recovery and utilization, and some can cause secondary pollution to the environment [8]. So, people began to develop new oil–water separation materials with advantages such as high oil–water separation performance, multiple recoveries, low production costs, and simple preparation processes [9].

Hou et al. [10] prepared elastic three-dimensional interpenetrating network carbon foam through direct carbonization of melamine foam. The prepared carbon foam has excellent elasticity, high porosity, low density, high specific surface area, superhydrophobicity, and excellent oil and organic solvent absorption properties. As an adsorbent material, it can absorb 148 to 411 times the weight of an organic solvent. However, preparing this hydrophobic carbon foam requires large instruments and ultra-high temperatures, making it difficult to achieve large-scale production. Xu et al. [11] prepared superhydrophobic foam by a simple three-step operation: first, the original melamine formaldehyde (MF) foam was simply immersed in a dopamine alkaline solution to prepare a polydopamine (PDA) coating; then, silver nanoparticles were formed by reduction and uniformly distributed on the surface of the PDA-coated MF foam; finally, alkanethiol, a hydrophobic substance with low surface energy, reacted with PDA and silver nanoparticles to prepare superhydrophobic MF foam. The foam, with strong absorption capacity and good recoverability, can absorb 60~130 times its weight of oil and organic solvents. Lei et al. [12] prepared a modified sponge with a highly hydrophobic surface and high absorption capacity for organic solvents by immersing the melamine sponge (MS) in a tetrahydrofuran solution containing renewable alkali lignin and carbodiimide-modified diphenylmethane diisocyanate through a simple one-step operation at room temperature, with a volume absorption capacity of up to 98% of its original volume.

In this paper, in order to synthesize hydrophobic materials with a simple preparation process, low cost, excellent oil absorption and oil–water separation performance, and good cycling stability, a cheap and readily available MS with high porosity was used as the substrate to construct a hydrophobic and lipophilic sponge. Lasting hydrophobic performance can be achieved by a one-step reaction of the hydroxymethyl groups on MS with isocyanates to form stable urethane groups. Using a facile preparation procedure, a hydrophobic and lipophilic sponge was prepared by impregnating MS into an isocyanate solution at room temperature. This process is simple and low-cost, providing a promising method for the design of oil absorption materials.

2. Materials and Methods

2.1. Materials

Cyclohexyl isocyanate was purchased from Shanghai Jizhi Biochemical Technology Co., Ltd. (Shanghai, China); octadecyl isocyanate was provided by Beijing Mairuida Technology Co., Ltd. (Beijing, China); butyl isocyanate was supplied by Shanghai Yien Chemical Technology Co., Ltd. (Shanghai, China); petroleum ether was purchased from Shanghai McLean Biochemical Technology Co., Ltd. (Shanghai, China). All reagents were analytical reagents and used without further purification, except for petroleum ether used after dehydration. MS (commercially available product, density: 0.009 g/cm³, porosity: 99.2%) was produced by Puyang Youpin Trading Co., Ltd. (Puyang, Henan, China).

2.2. Modification of MS

The MS was cut into a cylinder with a diameter of 2 cm and a height of 1 cm, washed with anhydrous ethanol and deionized water several times, and then dried in a vacuum. The prepared MS was immersed into the petroleum ether solution of isocyanate at room temperature, and the modified MS was taken out after the reaction for 3 h, washed with petroleum ether several times until the unreacted isocyanate was completely removed, and then dried in a vacuum. The sponges modified with cyclohexyl isocyanate, octadecyl isocyanate, or n-butyl isocyanate were recorded as I-MS1, I-MS2, and I-MS3, respectively.

2.3. Measurement of Oil Absorption Capacity

A sponge with an initial mass of m₀ was submerged in oil or organic solvent. After absorption saturation, the sponge was allowed to stand on a stainless-steel filter for a certain time to remove excess oil or organic solvent, at this time, the mass of the sponge was recorded as m₁. The absorption capacity (G) was calculated by Formula (1):

$$G=\frac{m_1-m_0}{m_0}$$

(1)

2.4. Characterization

Infrared spectra of the sponges were recorded by attenuated total reflection (ATR) Fourier-transform infrared spectroscopy (FTIR, Nicolet iS50, Thermo Fisher, Waltham, MA, USA) using 16 scans over a range of 400–4000 cm⁻¹ with a resolution of 2 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was recorded on an Axis Supra (Shimadzu KRATOS Company, Kyoto, Japan) for the determination of the elemental composition of sponge surfaces using Al Kα (1486.6 eV) radiation. XRD patterns were measured at a 0.02° scan step size, 2°·min⁻¹. The surface morphology of the sponges was observed using a field emission scanning electron microscope (SEM, S-4800, Hitachi, Tokyo, Japan) under 10 kV accelerating voltage. All the samples were sputter-coated with gold before imaging to improve their conductivity and enhance the image quality. The wettability of the sponge was characterized by a contact angle tester (JC2000D5, Shanghai Zhongchen Company, Shanghai, China) according to the Chinese National Standard GB/T 30693-2014 [13]. In order to obtain reliable results as much as possible, the final value of the contact angle for each case was averaged by six measurement results.

3. Results and Discussion

3.1. Preparation Procedure of Hydrophobic I-MS

Due to the hydroxymethyl groups on the surface, the pristine MS has hydrophilic and oleophilic properties and can adsorb both oil and water, which has no ability for oil–water separation. Therefore, it is necessary to be modified to achieve hydrophobic and oleophilic properties. As shown in Scheme 1, a large number of hydroxymethyl groups on the surface of MS can react with isocyanate to form urethane groups at room temperature, and the non-polar groups (-R) are introduced onto the surface. At this time, I-MS obtains good hydrophobic performance.

3.2. FTIR Analysis

To determine the functional groups on the sponge surface, FTIR was employed to detect the chemical components of the sponge. As shown in Figure 1, the FTIR spectra of MS and I-MSs show that the peaks at 3313, 2920, 1552, 1148, and 810 cm⁻¹ are attributed to N-H, C-H, C=N, and C-O stretching vibration peaks, as well as triazine ring deformation vibration peak, respectively [14,15,16]. In addition, the peaks at 974, 1326, and 1460 cm⁻¹ are the deformation vibration of C-H [17]. Compared with MS, the spectra of the I-MSs show a new C-H stretching vibration peak at 2848 cm⁻¹, which is related to the alkyl groups derived from isocyanates [18,19], and a new peak of C=O stretching vibration at 1742 cm⁻¹ that appears in I-MS curves is attributed to carbonyl in the amine ester group. It should be that isocyanate reacts with hydroxyl to form a urethane group, rather than with secondary amine to form a urea group, due to the lower activity of the secondary amine group connected to the triazine ring.

3.3. XPS Analysis

To further elucidate the chemical composition changes on the surface of the sponge, XPS analysis was conducted on the sponge. The full XPS spectra of the sponges are shown in Figure 2a; the peaks located at about 286, 400, and 533 eV are attributed to C1s, N1s, and O1s, respectively. The detailed peak analyses of N1s spectra of the sponges are shown in Figure 2b–e. From the spectra of MS (Figure 2b), it can be seen that the percentage of the peak area at 398.2 eV, corresponding to C=N, is 31.58%, and the percentage of C-N at 399.3 eV is 68.42%. After modification with cyclohexyl isocyanate, as shown in Figure 2c, the two groups of C=N and C-N, with relative percentages of 29.9% and 44.9%, are also shown in the spectrum. In addition, a new group of -CONH-, with a percentage of 25.2%, appears at 399.9 eV, indicating the successful reaction of isocyanate with MS to form urethane groups. The spectra in Figure 2d, e also show the -CONH- group, with 24.1% and 32.8%, respectively.

3.4. SEM

As shown in Figure 3, from the SEM photos, it can be seen that there are many fractures in the outer sponge skeleton, which should be caused by cutting the sponge, while the inner sponge skeleton remains intact. In addition, MS has a three-dimensional network structure with connected pores and high porosity. The apparent density of MS is about 9 mg/cm³, and the density of the melamine resin is approximately 1.15 g/cm³. It can be calculated that the porosity of MS used in this experiment is about 99.2%. Such a high porosity is beneficial for endowing sponges with high absorption capacity for oil or organic solvents. As shown in Figure 3b–d, I-MSs modified with isocyanate still retain the three-dimensional network structure and similar pore structure of MS, which is conducive to maintaining good absorption capacity. Further magnification observation shows that the surface of MS is smooth (Figure 3e), while the skeleton of the modified sponge has many wrinkles, increasing the surface roughness and achieving a hydrophobic effect (Figure 3f–h).

3.5. Wettability Analysis

The wettability tests of pre- and post-modified sponges were conducted, as shown in Figure 4. When the pristine sponge (MS) and the modified sponge (I-MS1) were put into the water, the untreated sponge quickly absorbed water and sank to the bottom due to its good hydrophilicity, while the modified sponge always floated on the water due to its excellent hydrophobicity and water repellency.

Due to the good hydrophilicity, MS can quickly absorb water droplets, and it is very difficult to test the water contact angle (WCA) of MS. The WCAs of the three modified sponges are shown in Figure 5, As the concentration of isocyanate increases, the WCAs of all three sponges first increase and then tend to stabilize and slightly decrease. The maximum WCAs of I-MS1, I-MS2, and I-MS3 are 137°, 143°, and 141°, respectively. At the same concentration, the WCA of the octadecyl isocyanate-modified sponge (I-MS2) was the highest, and that of the cyclohexyl isocyanate-modified sponge (I-MS1) was the lowest. It can be seen that the introduction of carbon chains makes sponges more hydrophobic than alicyclic rings, and long carbon chains have better hydrophobicity than short carbon chains. In addition, compared with cyclohexyl isocyanate, a lower concentration of octadecyl or butyl isocyanate can make the sponge hydrophobic. It is probably because the cyclohexyl with a large steric hindrance has the lowest activity, while the octadecyl or butyl with a small steric hindrance has higher activity.

3.6. Oil–Water Separation

Since the modified sponge has a superhydrophobic feature, its oil/water separation performance was examined using two different oil/water mixtures. Trichloromethane was chosen as heavy oil, and heptane was chosen as light oil. As shown in Figure 6, when the modified sponge (I-MS1) approached the organic solvent dyed red, I-MS1 could quickly and completely absorb the organic solvent from water, whether the organic solvent was above (heptane) or below water (trichloromethane), indicating that the modified sponge has good oil–water separation performance.

3.7. Oil Absorption Performance

To characterize in detail the absorption performance of sponges towards various solvents, the selection of oil and organic solvent is mainly based on the difference in density and viscosity, including heptane, hexadecane, liquid paraffin, pump oil, chloroform, and carbon tetrachloride. The absorption capacity of the three modified sponges is shown in Figure 7. For different oils, the modified sponges have excellent absorption capacity ranging from 62 to 143 times their weight; this can be attributed to the urethane group generated by the reaction between isocyanate and hydroxyl group on the sponge, which enhances the oil absorption capacity of the sponge. In addition, I-MS3 modified with n-butyl isocyanate has the highest absorption capacity, probably because the proportion of the urethane group on I-MS3 is higher than the other two modified sponges, owing to n-butyl with the lowest steric hindrance, while I-MS1 modified by cyclohexyl isocyanate with the highest steric hindrance has the lowest absorption capacity.

Further analysis can observe that the absorption capacity of modified sponges varies widely for different oils and organic solvents. This is because the absorption capacity mainly depends on two factors: firstly, the intrinsic properties of the absorption materials, and secondly, the density and viscosity of oil or organic solvent. The intrinsic properties of sponges include pore volume and hydrophobicity. For the same sponge, the pore volume and hydrophobicity of the modified sponge are fixed, and the absorption capacity mainly depends on the properties of the oil, such as the density and viscosity [20]. The higher the density, the greater the quality of the absorbed oil. For example, n-heptane has the lowest density and possesses the smallest absorption capacity, while tetrachloromethane has the highest density and the largest absorption capacity. In addition, the higher the viscosity of the oil, the better the oil retention of the sponge, which is also beneficial for improving oil absorption capacity. On the other hand, for different modified sponges, the pore volume should not be changed much, so the ability to absorb the same oil is nearly similar, and the little difference in oil absorption may come from the difference in oil retention of the modified sponges.

To further understand the absorption performance of the modified sponges, I-MS3 was selected and compared with other adsorbent materials for oil and organic solvents reported in the literature; the results are shown in

Table 1. Absorption amount of oil by different adsorbent materials.

Absorbent Materials	Solvent/Oil	Absorption Capacity (g/g)	Ref.
MoS2@RTV sponge	CHCl3/gasoline	104/56	[21]
CNT/PDMS-coated PU sponge	Diesel oil/motor oil	25/23	[22]
Graphene foam	Pump oil/toluene	72/68	[23]
Carbon fiber aerogel	CHCl3/olive oil	115/85	[24]
CNF/PVA/GO aerogel	DMF/pump oil	96/60	[25]
NC-GO@NFC	CHCl3/lubricant	75/68	[26]
SMS27K	CHCl3/toluene	100/62	[27]
SiO2@VTMS sponge	Diesel/n-hexane	73/60	[28]
I-MS3	CCl4/CHCl3/pump oil	143/133/86	This work

. It can be seen that I-MS3 not only has a simple, low-cost preparation process but also has excellent absorption performance.

3.8. Recyclability and Stability

Pump oil, heptane, and liquid paraffin were selected to characterize the cyclic oil absorption performance of the modified sponges, as shown in Figure 8. After 20 cycles, the oil absorption capacity of all three modified sponges slightly decreased and remained above 94%, 90%, and 91%, for n-heptane, liquid paraffin, and vacuum pump oil, respectively, which showed that the modified sponges possessed good reusability. The decrease in sponge absorption capacity is related to its elastic deformation. During the cyclic absorption process, repeated external compression caused irreversible deformation and a decrease in the resilience of the sponges, resulting in a decrease in the three-dimensional pore volume and storage space for adsorbed oil inside the sponges. Therefore, the absorption capacity of the sponges for oils or organic solvents shows a decreasing trend with increasing cycles.

The modified sponges were soaked in aqueous solutions with different pH values for three days, and the changes in water contact angle and oil or organic solvent absorption performance were investigated to characterize the stability of the modified sponges, as shown in Figure 9. Compared with untreated sponges, the sponges treated with aqueous solutions of strong acids (pH = 1), strong alkalis (pH = 13), and seawater have a slight decrease in contact angle and oil or solvent absorption performance. The sponges show good stability.

The decrease in water contact angle may be due to the corrosive effect of acid and alkali on the sponge skeleton, which affects the hydrophobicity of the sponges. The impact of seawater on the contact angle is minimal, as modified sponges soaked in seawater produce a silver mirror-like air film on the surface of the sponges, protecting them from the influence of seawater [29].

4. Conclusions

The hydrophobic and lipophilic MS was successfully prepared by a one-step reaction of isocyanate groups with the hydroxymethyl groups located on the surface of the MS. The WCAs of I-MSs were measured to be around 140°, and the modified sponges possess good hydrophobic performance. The prepared hydrophobic I-MSs have excellent oil–water separation and absorption capacity, can absorb 62 to 142 times their mass of oil and organic solvents, and have good reusability and stability.