Abstract
Hydrophobic materials have promising applications. However, fluorine present in hydrophobic materials can lead to health risks and environmental pollution. This study investigated an environmentally friendly route to produce fluorine-free hydrophobic coatings with good wear resistance. Wear-resistant hydrophobic coatings were prepared by introducing different ratios of spirocyclic alkoxysilane into the epoxy resin. Characterization by Fourier transform infrared spectroscopy, water contact angle measurement, scanning electron microscope, tape adhesion test, and abrasion testing showed that the multi-methyl group and double-ring rigid skeleton structure of spirocyclic alkoxysilane can remarkably improve the hydrophobic performance and wear resistance of the epoxy resin. The modified hydrophobic coatings can withstand more than 1600 instances of controlled rubbing. Furthermore, the maximum water contact angle can reach 110° after substantial abrasion. The novelty of the work is that the authors successfully synthesized spirocyclic alkoxysilane for the first-time using corn straw ash. The prepared spirocyclic alkoxysilane was then used as a modifier to prepare hydrophobic epoxy resin coating. This approach can broaden the utilization of corn straw ash. Therefore, this method can achieve a green and low-cost preparation of wear-resistant hydrophobic coatings without the introduction of fluorine.
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Fluorine-free and Durable Hydrophobic Coating Made with Corn Straw-derived Silane Modified Epoxy Resin
Wenlong Liu,a,b Xingwen Zhang,b,* Hongyu Ren,c,* Xingcheng Hu,b Xinyu Yang,b and Hui Liu a
Hydrophobic materials have promising applications. However, fluorine present in hydrophobic materials can lead to health risks and environmental pollution. This study investigated an environmentally friendly route to produce fluorine-free hydrophobic coatings with good wear resistance. Wear-resistant hydrophobic coatings were prepared by introducing different ratios of spirocyclic alkoxysilane into the epoxy resin. Characterization by Fourier transform infrared spectroscopy, water contact angle measurement, scanning electron microscope, tape adhesion test, and abrasion testing showed that the multi-methyl group and double-ring rigid skeleton structure of spirocyclic alkoxysilane can remarkably improve the hydrophobic performance and wear resistance of the epoxy resin. The modified hydrophobic coatings can withstand more than 1600 instances of controlled rubbing. Furthermore, the maximum water contact angle can reach 110° after substantial abrasion. The novelty of the work is that the authors successfully synthesized spirocyclic alkoxysilane for the first-time using corn straw ash. The prepared spirocyclic alkoxysilane was then used as a modifier to prepare hydrophobic epoxy resin coating. This approach can broaden the utilization of corn straw ash. Therefore, this method can achieve a green and low-cost preparation of wear-resistant hydrophobic coatings without the introduction of fluorine.
DOI: 10.15376/biores.18.1.2011-2023
Keywords: Plant biomass; Corn straw ash; Hydrophobic Coating; Biosilica; Fluorine-free
Contact information: a: School of Energy Science and Engineering, Harbin Institute of Technology, No. 92, West Dazhi Street, Harbin 150001, China; b: School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92, West Dazhi Street, Harbin 150001, China; c: College of Resources and Environment, Northeast Agriculture University, No. 600, Changjiang Street, Harbin 150001, China;
* Corresponding authors: zhangxingwen@hit.edu.cn; renhongyu@163.com
INTRODUCTION
Hydrophobic materials have become a popular research topic in recent years because of their promising applications in self-cleaning (Ganesh et al. 2011), anti-fogging (Lai et al. 2012), hydrophobic (Liu et al. 2022), and anti-fingerprint (An et al. 2018) applications. The static water contact angle (WCA) of hydrophobic surfaces is greater than 90°. Because of the small atomic radius and high electronegativity of fluorine, materials containing it are often used to prepare hydrophobic materials (Nishino et al. 1999). However, fluorine-containing materials are expensive; they also can accumulate in living organisms and pollute the environment (Li et al. 2015). Dental fluorosis has been linked to fluoride levels between 1.5 and 4 mg L-1 (Salah and Arab 2007). Skeletal fluorosis develops with sustained exposure to greater concentrations of fluorine sewage (4 to 10 mg L-1) (Raja Reddy 2009; Dhorge et al. 2017). Thus, eliminating F-containing materials to reduce environmental concerns and using silicon-containing materials incorporated into alkyl-based polymer materials to promote hydrophobicity have become a research hotspot in recent years (Tang et al. 2015, 2016; Lin et al. 2019). The synthesis of silanes containing polymethyl structure from agricultural waste corn straw ash can significantly reduce the synthesis cost and environmental pollution. Moreover, there are few studies on the use of these silanes as modifiers to modify the hydrophobicity of epoxy resins. In addition, the surface of the hydrophobic coating is often damaged by wear and tear, resulting in the reduction or even loss of the hydrophobic effect of the coating. Therefore, it is of great importance to develop low-cost, fluorine-free, environmentally friendly, and wear-resistant hydrophobic coating preparation methods.
Here, the authors used corn straw ash as a reactant to synthesize the spirocyclic alkoxysilane by utilizing an environmentally friendly method (Laine et al. 2016; Putro et al. 2021) for the first time. Then, the prepared spirocyclic alkoxysilane was used, for the first time, as a modifier to fabricate an epoxy hydrophobic coating. The structure and properties of the material were characterized by Fourier transform infrared spectroscopy, WCA measurement, scanning electron microscopy (SEM), tape adhesion test, and abrasion testing. Furthermore, the effects of modifier addition and its mechanisms were investigated. Low-cost, fluorine-free, environmentally friendly, and wear-resistant hydrophobic coatings were successfully prepared in this study, which has broadened the application of corn straw ash.
EXPERIMENTAL
Methanol (CH₃OH, 99.8%), ethanol (C₂H₅OH, 99.5%), hexane (C6H14, 99.5%), and 2-methyl-2,4-pentanediol (C6H14O2, 99%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Corn straw ash was received from the Xing Fu Farm (Qiqihar, China). Low molecular polyamide (99%) was purchased from Beijing Xiangshan Lianhe Auxillary Factory (Beijing, China). Epoxy resin (98%) was purchased from Nantong Xingchen Synthetic Material Co., Ltd. (Nantong, China). Glass substrates (76.2 mm × 25.4 mm × 1.2 mm) were purchased from Yancheng Fei Zhou Glass & Plastic Co., Ltd. (Yancheng, China). All reagents were used as received.
Methods
Synthesis of spirocyclic alkoxysilane
First, 500 g of corn straw ash was filled into a 2-L three-necked flask, and later 1.2 L of 2-methyl-2,4-pentanediol was added to the three-necked flask. Then, the reactants were stirred for 0.5 h, to make corn straw ash homogeneously disperse in the 2-methyl-2,4-pentanediol. Afterward, the reaction was completed at 180 °C under nitrogen and stirred for 48 h. The mixed solution of spirocyclic alkoxysilane and 2-methyl-2,4-pentanediol was collected by distillation. Later, the mixed solution was washed with hexane (1.2 L) and water (3 × 1.2 L), and then the hexane layer was collected and dried with sodium sulfate. Next, the sodium sulfate was removed by filtration. After that, the filtrate was placed in a rotary evaporator to remove hexane. Finally, the product spirocyclic alkoxysilane was collected and obtained. 0.67 g of spirocyclic alkoxysilane was synthesized from 1 g of corn straw ash as a reactant.
Preparation procedure for coatings
Firstly, the epoxy resin (30 g) and curing agent (Low molecular weight polyamid 10 g) were weighed in beakers according to the mass ratio of 3:1. Then, 30 g and 10 g of epoxy resin and curing agent, respectively, were added to the beakers with 60 g and 20 g ethanol. The beakers were sealed and placed in an oven at 60 °C for 30 min. After that, the epoxy resin and curing agent were mixed well. Spirocyclic silane with mass percentages of 10%, 20%, 30%, and 40% was added. The epoxy resin coating solutions with 10 wt%, 20 wt%, 30 wt%, and 40 wt% of spirocyclic alkoxysilane and a control group (epoxy resin without added spirocyclic silane) were prepared. Glass substrates were ultrasonically cleaned with deionized water for 5 min. Then, glass substrates were ultrasonically cleaned with ethanol for 5 min. Finally, glass substrates were ultrasonically cleaned with acetone for 5 min. Glass substrates were thoroughly dried before spin coating. A KW-4A (Spin Coater, Beijing, China) spin coater was used to spin film at 2000 revolutions per minute for 15 s. Then, the film was cured at 80 °C for 16 h and 120 °C for 4 h.
Characterization
Fourier transform infrared spectroscopy (NICOLET IS50, Waltham, MA, USA) was used to analyze the chemical groups of the samples by the KBr squash method. A SEM (Quanta 200FEG, Thermo Fisher Scientific, Waltham, USA) was adopted to observe the morphologies of the coatings with an acceleration voltage of 20 kV. Morphologic photographs were taken at a magnification of 200×. Nuclear magnetic resonance (NMR, Bruker Avance NEO 600, Bruker Corporation, Karlsruhe, Germany) and mass spectrometry (MS, Waters Quattro Premier XE, Milford, MA, USA) were used to analyze the chemical structure of the as-obtained spirocyclic alkoxysilane.
The WCA of the coating was measured by a contact angle meter (FCA2000A, Shanghai Aifis Precision Instruments Co., Shanghai, China). Three different areas in each coating were selected to repeat the WCA measurements. The average of the three tests was taken as the WCA for each coating. The error bars represent the standard deviation of the three tests for each coating.
An H-18 device purchased from Taber (North Tonawanda, NY, USA) was used to test the wear resistance of coatings. The coating was subjected to a pressure of 2 kPa. The rubbing speed was 50 mm/s. The numbers of rubbing actions were 400, 800, 1200, 1600, and 2000. Three different areas in each coating were selected to repeat the wear resistance test. The average of the three tests is taken as the WCA for each coating. The error bars represent the standard deviation of the three tests for each coating. In a review of the durability of hydrophobic coatings, Milionis suggests that this method may become a universal standard for evaluating the durability of hydrophobic coatings in the future (Milionis et al. 2016). Coating adhesion was evaluated according to ASTM D3359 (2009) (Krug et al. 2019).
RESULTS AND DISCUSSION
Synthesis of Spirocyclic Alkoxysilane
Figure 1 shows the infrared spectrum of the corn straw ash. The peak located at ~1100 cm-1 corresponds to the absorption peak of Si-O stretching vibration. In addition, the peak at ~3100 cm-1 corresponds to the stretching vibration absorption peak of OH.
Fig. 1. Infrared spectroscopy of corn straw ash
The infrared spectrum of the product is shown in Fig. 2(a), in which the peak at 961 cm-1 was ascribed to the stretching vibration of Si-O-C; the peaks at 996 cm-1 and 1054 cm-1 were attributed to the stretching vibration of Si-O (Liu et al. 2022). The mass spectrum of the product is shown in Fig. 2(b), from which it can be seen that the signals at m/z = 283.3 and 278.3 were respectively assigned to the molecular weight of spirocyclic alkoxysilane (Na+ adduct) and spirocyclic alkoxysilane (NH4+ adduct).
Fig. 2. Spirocyclic alkoxysilane (a) Infrared spectrum, (b) mass spectrum
The 1H NMR spectrum of spirocyclic alkoxysilane is shown in Fig. 3(a), from which it can be seen that the signals at 1.33 ppm and 1.22 ppm can be attributed to -CH3; the signals at 1.62 ppm and 1.47 ppm were assigned to -CH2-. The 29Si NMR spectrum of spirocyclic alkoxysilane is shown in Fig. 3(b), and a sharp signal at -81.65 ppm can be seen in the spectrum, whose chemical shift was the same as that of Q-type siloxane. The 13C NMR spectrum of spirocyclic alkoxysilane is shown in Fig. 3(c). In the 13C NMR spectrum, the peak at 23.93 ppm was attributed to -CH3; the peaks at 32.04 ppm and 27.79 ppm were attributed to (CH3)2; the peaks at 49.29 ppm and 48.10 ppm corresponded to -CH2-; the peak at 67.34 ppm corresponded to -CHMeO; the peak at 74.12 ppm was assigned to -CMe2O. The above results combined with the IR and MS results indicate that the spirocyclic alkoxysilane structure was successfully synthesized (Laine et al. 2016).
Fig. 3. NMRs of spirocyclic alkoxysilane (a) 1H NMR, (b) 29Si NMR, and (c) 13C NMR
Synthesization of Coatings
Figure 4 shows the infrared spectra of epoxy resin (EP), spirocyclic alkoxysilanes (SP), and spirocyclic alkoxysilane modified epoxy resin (EP-SP). In the spectrum of epoxy resin, which is shown in black, the peak at 826 cm-1 is the absorbance peak of p-disubstituted benzene. The peak at 1235 cm-1 is the stretching vibration absorbance peak of Ar-O-R. The peak at 1180 cm-1 is the absorbance peak of C-N formed by ring-opening polymerization after the reaction between the epoxy resin and curing agent, and additionally, there is no obvious infrared absorption peak of the epoxy group in the infrared spectra, indicating that the curing of epoxy resin is relatively complete. The peaks at 1507 cm-1 and 1606 cm-1 are absorbance peaks of the benzene ring. The peak at 2924 cm-1 is the absorbance peak of -CH2. The broad absorption peak at 3362 cm-1 is the absorbance peak of the stretching vibration of the hydroxyl group. In the red spectrum of spirocyclic alkoxysilanes, the peak at 961 cm-1 is the absorbance peak of Si-O-C. The peaks at 996 and 1054 cm-1 are the absorbance peaks of Si-O. There is no absorbance peak near 3300 cm-1. In the spectrum of EP-SP, the peak at 825 cm-1 is the absorbance peak of the p-disubstituted benzene structure. The broad peak at 1081 cm-1 is the absorbance peak of Si-O. The peak at 1245 cm-1 is the stretching vibration absorbance peak of Ar-O-R. The peaks at 1508 and 1606 cm-1 are the absorbance peaks of the benzene ring. The peak at 1181 cm-1 is the absorbance peak of the C-N bond formed by ring-opening polymerization after the reaction between the epoxy resin and curing agent, and in addition, no obvious infrared absorbance peak of the epoxy group can be seen in the infrared spectrum, which indicates that the addition of spirocyclic alkoxysilane did not influence the curing effect of the curing agent in the original system of the epoxy resin. The curing of epoxy resin remained relatively complete after modification. The peak at 2920 cm-1 is the absorbance peak of -CH2. Compared with the IR spectrum of the pure epoxy resin, the IR spectrum of the spirocyclic alkoxysilane modified epoxy resin showed a broader peak near 1081 cm-1, indicating that spirocyclic alkoxysilane had been successfully introduced into the epoxy resin. Further supporting this finding, the other peaks in the IR spectrum and the main absorbance peaks of the pure epoxy resin have similar positions and shapes, indicating that the addition of spirocyclic alkoxysilane does not destroy the main structure of the epoxy resin. Compared with the pure epoxy resin, the hydroxyl absorbance peak near 3300 cm-1 of the spirocyclic alkoxysilane modified epoxy resin is remarkably reduced, indicating that the hydroxyl content in the cured epoxy resin film layer is relatively low, which is beneficial to enhancing the WCA and reducing the contact angle hysteresis for the epoxy resin coating.
Fig. 4. IR spectra of the EP, SP, and EP-SP
Figure 5 shows the SEM image of the spirocyclic alkoxysilane modified epoxy resin. It can be seen that the surfaces of the control group, 10%, 20%, and 30% spirocyclic alkoxysilane modified epoxy resin film layers were flat, dense, and homogeneous, supposing that the curing effect of epoxy resin is pretty good at this time and showing great compatibility between spirocyclic alkoxysilane and epoxy resin. The surface of the epoxy resin layer modified by 40% spirocyclic alkoxysilane showed more small black defects. The main reason for this is that the spirocyclic alkoxysilane does not contain active groups, and the epoxy resin was mainly modified by physical doping. When the addition ratio of spirocyclic alkoxysilane reached 40%, partial spirocyclic alkoxysilane agglomerated in the epoxy resin and formed defects.
Fig. 5. SEM images of coatings: (a) control, (b) 10%, (c) 20%, (d) 30%, (e) 40%, and (f) detail view; Magnification 200×, scale bar 500 μm
Hydrophobic Performance
Figure 6 shows the hydrophobic performance of the modified epoxy resin.
Fig. 6. Hydrophobic performance of modified epoxy resin
It can be observed that with an increased spirocyclic alkoxysilane doping ratio, the WCA initially showed an upward trend. When the doping ratio of spirocyclic alkoxysilane was 30%, the WCA of the modified epoxy resin reaches its maximum value. This value was higher than the WCA of resin film previously reported in the literature (Liu et al. 2022). The possible reasons why the WCA of the modified epoxy resin was noticeably higher are as follows: Firstly, spirocyclic alkoxysilane contains 6 methyl groups per molecule. As it is introduced into the epoxy resin, the proportion of methyl groups in the epoxy resin remarkably increases. Due to the multi-methyl group structure of spirocyclic alkoxysilane, with an increased doping ratio of spirocyclic alkoxysilane, the WCA of the modified resin gradually increases within a certain range. Secondly, the unique silica-oxygen bond core of spirocyclic alkoxysilane and the rigid skeleton structure of the double ring contribute to an enhancement of the hydrophobicity of the epoxy resin in both composition and structure. As a result, spirocyclic alkoxysilane does not contain reactive groups, and it predominantly modifies the epoxy resin by physical doping. When the spirocyclic alkoxysilane doping ratio exceeds 30%, the high amount may lead to agglomeration in the resin, resulting in a change of the surface micro-nano structure of the modified epoxy resin. Thus, the hydrophobicity of the coating decreases.
Figure 7 shows the contact angle hysteresis data of the epoxy resin film layer.
Fig. 7. Contact angle hysteresis of coatings
Figure 7 illustrates that as the addition ratio of spirocyclic alkoxysilane was gradually increased, the contact angle hysteresis of the film layer gradually decreased at first, it reached the minimum point when the addition ratio was 30%, and then it increased after that. The main reason is that spirocyclic alkoxysilane was not introduced in the control group. Further, the epoxy resin still contained a large quantity of hydroxyl and amino groups after curing. These hydroxyl and amino groups could combine with water and thus induce molecular rearrangement on the surface. As the addition ratio of spirocyclic alkoxysilane gradually increased, the proportion of hydroxyl groups in the cured film layer was relatively reduced. Thus, the polymer composition became more fixed, and the water-induced molecular rearrangement was greatly reduced (Lin et al. 1996). Therefore, the contact angle hysteresis of the modified epoxy resin film layer gradually decreased at first. When the addition ratio of spirocyclic alkoxysilane exceeded 30%, the surface morphology and roughness of the film layer changed due to the agglomeration of spirocyclic alkoxysilane. As a result, the contact angle hysteresis of the film layer then increased.
Wear Resistance
Figure 8 shows the wear resistance data of spirocyclic alkoxysilane modified epoxy resin.
Fig. 8. Wear resistance of the modified epoxy resin
From Fig. 8, it can be seen that with increased exposure to bouts of abrasion, the WCA of each group of epoxy resin showed a gradually increasing trend. When the rounds of friction number were 1200, the WCA of each group of epoxy resin reached the corresponding maximum value. After that, with the times of friction increasing, the WCA of each group of epoxy resin gradually decreased. When the number of instances of friction was 2000, the WCA of each group of epoxy resin was smaller than the corresponding group without friction being applied, which indicates that the hydrophobic of the coating had been destroyed. The WCA of the epoxy resin coating without the addition of spirocyclic alkoxysilane gradually decreased with the increased exposure to bouts of friction, and a more obvious breakage occurred after 1200 applications of friction. From Fig. 8, it can be seen that the WCA of epoxy resin modified by 20% and 30% doping of spirocyclic alkoxysilane decreased remarkably less than other groups; those specimens were able to withstand more than 1600 applications of friction. This value is significantly better than the applications of friction of resin coating previously reported in the literature (Liu et al. 2022). Furthermore, the epoxy resin modified by 30% doping of spirocyclic alkoxysilane was able to reach the maximum WCA at more than 110° after 1200 applications of friction.
The main reasons for the improvement of the wear resistance of the modified epoxy resin are as follows: Firstly, in the modified epoxy resin system, silicon-oxygen bonds are introduced due to the addition of spirocyclic alkoxysilane. The bond energy of silicon-oxygen bonds is 422.5 kJ/mol, while the bond energy of carbon-carbon bonds is 334.7 kJ/mol. Therefore, the bond energy of silicon-oxygen bonds is much stronger than that of carbon-carbon bonds, resulting in the increase of wear resistance of the modified epoxy resin. Secondly, the unique double-ring rigid skeletal structure of spirocyclic alkoxysilane can play the role of “anchor point” (Grassie et al. 1980) in the epoxy resin system. Meanwhile, the infrared spectroscopy (Fig. 3) suggests that the solidification of epoxy resin has not been destroyed by the addition of spirocyclic alkoxysilane and the modified epoxy resin still maintains a high crosslink density.
Figure 9 shows the SEM of the spirocyclic alkoxysilane modified epoxy resin film layer after 2000 wear cycles.
Fig. 9. SEM images of coatings after 2000 wear cycles: (a) control, (b) 10%, (c) 20%, (d) 30%, and (e) 40%; magnification 200×, scale bar 500 μm
It can be seen that the film layer of the control group was the most severely broken, with a large area of rupture and peeling of the film layer. With the increase of the addition ratio of spirocyclic alkoxysilane, the breakage of the epoxy resin layer was gradually discouraged. The lowest degree of breakage was reached when the addition ratio was 30%, indicating that this kind of film layer had the best wear resistance. When the addition ratio of spirocyclic alkoxysilane exceeded 30%, the breakage of the film layer was further aggravated. The possible reasons are given as follows: Firstly, the bond energy of the silicon-oxygen bond is higher than that of the carbon-oxygen bond. Secondly, the spirocyclic alkoxysilane has the rigid structure of a double-ring skeleton. When the addition ratio of spirocyclic alkoxysilane did not exceed 30%, the spirocyclic alkoxysilane was homogenously dispersed in the epoxy resin film layer. In addition, the spirocyclic alkoxysilane could be used as an anchor point to remarkably improve the wear resistance of the film layer. When the addition ratio of spirocyclic alkoxysilane exceeded 30%, the spirocyclic alkoxysilane agglomerated in the film layer and formed defects, which led to a decrease of the wear resistance of the epoxy resin film layer.
Coating adhesion was evaluated according to ASTM D3359 (2009) (Krug et al. 2019). Table 1 shows the tape adhesion test of the epoxy resin film layer. It can be seen from the table that, when the doping ratio of spirocyclic alkoxysilane was lower than 30%, the epoxy resin film layer showed good adhesion, and each test of the adhesion strength reached level 4B (excellent adhesion strength), which shows promising compatibility between spirocyclic alkoxysilane and epoxy resin. It was found that proper addition of spirocyclic alkoxysilane did not affect the adhesion of the layer. When the doping ratio exceeded 30%, the adhesion strength of the epoxy resin film was slightly reduced to 3B (good adhesion strength). The main reason for the above fact is that too much spirocyclic alkoxysilane was agglomerated in the epoxy resin film layer, which led to more defects and degraded the adhesion of the film. But the adhesion of the spirocyclic alkoxysilane modified epoxy resin film was significantly better than that of the resin film previously reported in the literature (Liu et al. 2022).
Table 1. Tape Adhesion Test of Coatings
CONCLUSIONS
- The novelty of this work is that spirocyclic alkoxysilane synthesized from corn straw ash was used for the first time to modify epoxy resin and prepare wear-resistant hydrophobic material.
- The 30% spirocyclic alkoxysilane modified epoxy resin coating exhibited the best hydrophobic performance.
- The coating was able to withstand more than 1600 applications of friction, and the maximum water contact angle (WCA) reached 110°.
- The multi-methyl group and double-ring rigid skeleton structure of spirocyclic alkoxysilane can remarkably improve the hydrophobic performance and wear resistance of epoxy resin.
ACKNOWLEDGMENTS
The authors are grateful for the support of the Major Project of the Ministry of Science and Technology of China, Grant No. 2017YFB0307700.
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Article submitted: October 24, 2022; Peer review completed: December 31, 2022; Revised version received and accepted: January 19, 2023; Published: January 25, 2023.
DOI: 10.15376/biores.18.1.2011-2023