Scale Deposition Inhibiting Composites by HDPE/Silicified

materials

Article Scale Deposition Inhibiting Composites by HDPE/Siliciﬁed Acrylate Polymer/Nano-Silica for Landﬁll Leachate Piping

Min Li 1, Rui Zhao 1,*, Sude Ma 2 and Tianxue Yang 3 1 Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 611756, China; [email protected] 2 School of Materials Science and Engineering, Xihua University, Chengdu 610039, China; [email protected] 3 State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China; [email protected] * Correspondence: [email protected]; Tel.: +86-028-66367628

Received: 12 June 2020; Accepted: 5 August 2020; Published: 7 August 2020

Abstract: Scaling commonly occurs at pipe wall during landfill leachate collection and transportation, which may give rise to pipe rupture, thus posing harm to public health and environment. To prevent scaling, this study prepared a low surface energy nanocomposite by incorporating silicone-acrylate polymer and hydrophobically modified nano-SiO2 into the high-density polyethylene (HDPE) substrate. Through the characterization of contact angle, scanning electron microscopy and thermogravimetry, the results showed that the prepared composite has low wettability and surface free energy, excellent thermal stability and acid-base resistance. In addition, the prepared composite was compared with the commercial HDPE pipe material regarding their performance on anti-scaling by using an immersion test that places their samples into a simulated landfill leachate. It was apparent that the prepared composite shows better scaling resistance. The study further expects to provide insight into pipe materials design and manufacture, thus to improve landfill leachate collection and transportation.

Keywords: Anti-scaling; surface free energy; Leachate; Pipe; siliciﬁed acrylate copolymer; HDPE

1. Introduction Landﬁll leachate has high chemical oxygen demand and complex constitutions, due to the large number of organic and inorganic pollutants contained [1]. Generally, it is transferred for advanced treatment by using a pipe system [2]. High density polyethylene (HDPE) is a common drainage material, which has been widely used for leachate piping, due to its excellently mechanical properties [3]. However, scaling often occurs at its inner wall during the piping, which may result in pipe rupture, thus posing a risk to human health and the environment [4]. The pipe scale formation is considered as a joint reaction between the leachate and piping materials [5]. To mitigate clogging consequences, conventional engineering measures have mainly focused on pipe cleaning, including aeration in the pipeline [6], adjustment of hydraulic loading [7], high-pressure water jetting [8], addition of scale inhibitors [9], etc. However, such engineering measures are constrained by their implementation of frequency and associated operation costs, which may not substantially prevent pipe scaling [9,10]. Development of anti-fouling materials has aroused great attentions in recent years regarding the reinforcement of long-term performance on sewage piping stability. A number of studies have indicated that minimization of material surface energy is eﬀective at reducing pipe scale by using functional

Materials 2020, 13, 3497; doi:10.3390/ma13163497 www.mdpi.com/journal/materials Materials 2020, 13, 3497 2 of 14 coatings [11–13]. Siloxane polymer and acrylate polymer are typical materials with low surface energy, which have been widely selected as substrates for coatings preparation due to their high bond energy, water repellency, and oxidation stability [14–16]. For instance, Yee et al. prepared polydimethylsiloxane coatings that demonstrated weak adhesion to the calcium carbonate deposition [17]. Ba et al. reported a polydimethylsiloxane fouling release coating, which is efficient at decreasing the adhesion of biofilm in sewage pipes [18]. Xie et al. grafted a telomer to prepare a bis-silanol terminated poly (dimethylsiloxane) as a silicone coating in the application of pipe antifouling [19]. Friis et al. applied methoxysilane compound as the precursor to improving the anti-scaling property of poly (oligo ethyleneglycol) methacrylate based coatings via surface-initiated polymerization [20]. Superhydrophobic nano silica, as a functional filler, is commonly used in preparation of the self-cleaning composite coatings because of its high stability and excellent compatibility with other basis materials [21,22]. Song et al. prepared heptadecafluorodecyltriisopropoxysilane (FPS)-SiO2 coatings on stainless steel substrates to improve anti-corrosion performance on geothermal water pipeline [23]. Wang et al. introduced super-amphiphobic coatings based on highly fluorinated and hierarchical-structure halloysite nanotubes (HNTs)/SiO2 powder that could be applied as self-cleaning substrates [24]. Qian et al. further added carbon nanofillers into fluorinated ethylene propylene and SiO2 composites to improve their scale-inhibition properties by using a test of saturated CaCO3 deposition [25]. However, the coatings may be easily peeled off due to different stress forces between substrates and coatings [12,26]. In particular, they may be not suitable for leachate pipes, due to the complex fluid-structure interaction [3]. This study aims to develop a low surface free energy acrylate copolymer to improve the anti-scaling performance on pipe material. Although HDPE pipes are prone to scaling during landfill leachate collection and transport, they are a commonly selected polymer matrix which can be integrated with functional fillers (e.g., inorganic fillers, nanoparticles, coupling agents, etc.) to improve their performances on specific engineering practice [27–29]. The copolymer and hydrophobically modified nano-SiO2 in this study are used as HDPE fillers to prepare nanocomposites with different components. The performances on morphology and thermal behavior of the composites are characterized by scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The surface free energy test and the leaching experiment by placing the prepared composite into a simulated landfill leachate are implemented to investigate the anti-scaling performance. It is expected that this study may provide insight into the scaling prevention during the transport in the pipe, to ensure safe operation of landfill disposal.

2. Materials and Methods

2.1. Materials Butyl methacrylate (BMA), glycidyl methacrylate (GMA), azodiisobutyronitrile (AIBN) and other inorganic reagents were purchased from Kelong Corporation Ltd., Chengdu, China. Vinyltrimethoxysilane (VTMO) and nanosilica were purchased from Keyan Corporation Ltd. Shanghai. High density polyethylene (HDPE) resin was obtained from SINOPEC Corporation Ltd. HDPE pipe materials were purchased from a local Pipe Supplier in Chengdu City, China with an internal diameter of 48 mm and a wall thickness of 2.0 mm. The initiator, Azodiisobutyronitrile (AIBN), was puriﬁed by recrystallization using absolute ethanol.

2.2. Experiment Design The experiment design is shown in Figure1. First, the low surface energy copolymers were synthesized by selection of appropriate polymerization monomers. Then the Fourier transform infrared (FTIR) test, contact angle (CA) measurement, thermal analysis and SEM were used to characterize the prepared copolymers in order to identify the optimal composition for the synthesis. Through an Materials 2020, 13, 3497 3 of 14 immersion experiment where the prepared composite was placed into the simulated landﬁll leachate, the compositeMaterials performance 2020, 13, x on scaling resistance was compared with that of the commercial HDPE pipe. 3 of 14

Figure 1. ExperimentFigure 1. Experiment design for design the composite for the composite preparation preparation and characterization. and characterization. 2.3. Preparation of Silicified Acrylate Copolymers 2.3. Preparation of Silicified Acrylate Copolymers The polymers of silicified acrylate copolymer (abbreviated to PSAs) were synthesized by bulk free The polymers of silicified acrylate copolymer (abbreviated to PSAs) were synthesized by bulk radical polymerization, and the preparation details were given as follows. A quarter of the mixture of free radical polymerization, and the preparation details were given as follows. A quarter of the monomer VTMO and BMA was added to a neck flask with agitator, reflux condenser, thermometer mixture of monomer VTMO and BMA was added to a neck flask with agitator, reflux condenser, and drip funnel, and the initiator AIBN with 0.3% of the mass of the monomer was then added. thermometer and drip funnel, and the initiator AIBN with 0.3% of the mass of the monomer was then The solution was degassed with nitrogen, and the reaction mixture was stirred at 75 C for 30 min. added. The solution was degassed with nitrogen, and the reaction mixture was◦ stirred at 75 °C for 30 The rest of the mixture of VTMO and BMA, GMA and the initiator AIBN with 0.2% of the mass of min. The rest of the mixture of VTMO and BMA, GMA and the initiator AIBN with 0.2% of the mass the monomer were added into the flask one by one. After dropping, the reaction mixture was kept at of the monomer were added into the flask one by one. After dropping, the reaction mixture was kept 75 C for 40 min. Once the temperature of the reaction system began to rise, the heating was stopped. ◦ at 75 °C for 40 min. Once the temperature of the reaction system began to rise, the heating was The mixture was transferred to a plate and polymerized at 70 C for 20 h, then heated to 120 C for 2 h. stopped. The mixture was transferred to a plate and◦ polymerized at 70 °C for 20 h◦, then heated to 120 Table1 shows the chemical compositions of di fferent mixtures predefined in this study. °C for 2 h. Table 1 shows the chemical compositions of different mixtures predefined in this study.

Table 1. Compositions of polymers of siliciﬁed acrylate copolymer with diﬀerent monomers content. Table 1. Compositions of polymers of silicified acrylate copolymer with different monomers Proportion ofcontent Constituents. (%) PSAs VTMOProportion BMA of Constituents GMA (%) PSAs PSA0 0VTMO 70BMA30 GMA PSA1PSA0 10 7070 29 30 PSA2 2 70 28 PSA5PSA1 51 7070 25 29 PSA10PSA2 102 7070 20 28 PSA15PSA5 155 7070 15 25 PSA20PSA10 2010 7070 10 20 PSA15 15 70 15 2.4. Preparation of Superhydrophobic VTMO-SiOPSA202 20 70 10

A quantity of 2.5 g of SiO2 was dispersed into 100 mL of pure ethanol, and stirred for 30 min, 2.4. Preparation of Superhydrophobic VTMO-SiO2 to obtain a SiO2 suspension with a mass concentration of 2.5%. A quantity of 10 g VTMO was dissolved into 80 mL of ethanolA quantity solution of 2.5 (EtOH g of SiO/H22O was= 3:1), dispersed and acetic into acid100 mL was of used pure to ethanol, adjust its and pH stirred to 4. They for 30 min, to were stirredobtain for 30 a SiO min,2 suspension to preparethe with hydrating a mass concentration solution for VTMO. of 2.5%. Then, A quantity the prepared of 10 g suspension VTMO was of dissolved VTMO-SiOinto2 was 80 mL further of ethanol mixed solution with the (EtOH/H VTMO solution,2O = 3:1), byand which acetic ammoniaacid was used solution to adjust was usedits pH to to 4. They were stirred for 30 min, to prepare the hydrating solution for VTMO. Then, the prepared suspension of VTMO-SiO2 was further mixed with the VTMO solution, by which ammonia solution was used to adjust its pH to 10. This reaction was implemented by the magnetic stirring at 60 °C for 3 h. Once the reaction had been completed, the mixture was centrifuged by the speed of 10,000 r/min and lasted for 10 min, then repeated washing with pure ethanol and centrifuged for 3 times, to obtain a modified

Materials 2020, 13, 3497 4 of 14

adjust its pH to 10. This reaction was implemented by the magnetic stirring at 60 ◦C for 3 h. Once the reaction had been completed, the mixture was centrifuged by the speed of 10,000 r/min and lasted for 10 min, then repeated washing with pure ethanol and centrifuged for 3 times, to obtain a modiﬁed wet sample of VTMO-SiO2. The wet sample was dried at 80 ◦C for 2 h to prepare the VTMO-SiO2 powder.

2.5. Preparation of Composite Material

MaterialsTable 20202, shows13, x the di ﬀerent mass ratios of PSA, VTMO-SiO2 and HDPE pellets for the composite4 of 14 preparation. The composite containing 1% VTMO-SiO2 was entitled HDPE/PSA/SiO2(1%) (HPS1), and similarwet sample nomenclature of VTMO was-SiO used2. The for wet other sample proportions was dr ofied the at VTMO-SiO80 °C for 22 .h The to prepare premixed the materials VTMO- wereSiO2 compoundedpowder. by the rotating and meshing twin-screw extruder, by which the temperature was set at 190 ◦C. The preparation process was shown in Figure2. 2.5. Preparation of Composite Material Table 2. Diﬀerent mixture ratios for the composite preparation. Table 2 shows the different mass ratios of PSA, VTMO-SiO2 and HDPE pellets for the composite preparation. The composite containing 1% VTMO-SiO2 Proportionwas entitled of Constituents HDPE/PSA/SiO (%) 2(1%) (HPS1), Samples Composition Labeling 2 and similar nomenclature was used for other proportionsPSA of the VTMO HDPE-SiO . The VTMO-SiO premixed2 materials were compounded by the rotating and meshing twin-screw extruder, by which the temperature was HDPE/PSA/SiO2(1%) HPS1 9 90 1 set at 190 °C. The preparation process was shown in Figure 2. HDPE/PSA/SiO2(5%) HPS5 15 80 5 HDPE/PSA/SiO2(9%) HPS9 11 80 9

Step 1

Step 2

Step 3

HPS

Figure 2. The preparation procedure for the HDPE/PSA/SiO2 composite. Figure 2. The preparation procedure for the HDPE/PSA/SiO2 composite.

Table 2. Different mixture ratios for the composite preparation.

Proportion of Constituents (%) Samples Composition Labeling PSA HDPE VTMO-SiO2 HDPE/PSA/SiO2(1%) HPS1 9 90 1 HDPE/PSA/SiO2(5%) HPS5 15 80 5 HDPE/PSA/SiO2(9%) HPS9 11 80 9

Materials 2020, 13, 3497 5 of 14

2.6. Characterization

1 FTIR spectra is investigated by a Bruker Vector22 FTIR spectrometer within 400 cm− and 1 4000 cm− . KBr powder was mixed with the samples, which were pressed into the flakes. The static contact angle (CA) is tested by the DSA25 (KRÜSS GmbH, Hamburg, Germany) typed contact angle tester. The static contact angles in five different positions were investigated, and the average result was reported. The droplet volume was 4 µL used in the contact angle tester. Surface free energy is derived from the measurement of contact angles, given as follows [30]:

= d + p σsolid σsolid σsolid (1) q q ( + ) = d d + p p σliquid 1 cosθ 2 σsolidσliquid 2 σsolidσliquid (2) where σ is the surface free energy, σd and σp denote the surface energy contributions from the dispersive d p and the polar component; σsolid and σsolid are measured by the contact angles (θ) between the solid surface d p and two probing liquids (H2O and CH2I2) of the known surface energy components (σliquid, σliquid). The surface microstructure is investigated by field emission scanning electron microscope (SEM) (QUANTA FEG250, FEI). A digital camera by Huawei Corporation Ltd. (Shenzhen, China) was used to capture optical images of the sample. Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) are performed by using the thermal gravimetric analyzer (TG209F1, NETZSCH Group, Bavaria, Germany) in a temperature range between 30 ◦C and 600 ◦C (10 ◦C/min) under a dynamic N2 atmosphere. To study the anti-scaling performance on the prepared composite, an immersion experiment is designed. Although landfill leachate may have different characteristics, they are generally highlighted by high chemical oxygen demand, high salinity and high colority [1]. Moreover, existing studies indicated that the main scaling composition in the leachate pipe was calcium carbonate [4,31]. In such a context, the design of main parameters related to the water quality of the simulated leachate is given in Table3. The leachate was made by reagents of analytical grade and deionized water, in which the addition of the reagents was determined by the simulated leachate concentration. Among them, chemical oxygen demand (COD) was prepared by glucose (1 g glucose equivalent to 1.067 g COD), 2+ calcium ion (Ca ) by CaCl2, and suspended solids by humus. The pH of the leachate in 4.0 and 9.18 was adjusted by adding phosphate and borax buffer, respectively. The composite and commercial HDPE tubes were cut into samples respectively, with the same weight and the same inner wall area size, placed into the same prepared simulated leachate, to investigate their scaling performances with different immersing times.

Table 3. Water quality parameters for the simulated leachate.

Main Parameters Suspended COD (mg/L) pH Ca2+ (mg/L) References Solids (mg/L) Common leachate 5800–117,000 3.8–9.0 250–7200 20–280 [1,9,32] Simulated leachate 35,000 4.0–10.0 2500 100

3. Results and Discussion

3.1. Contact Angle and Surface Energy The contact angles of different silicified acrylate polymers (PSAs) are presented in Table4. The CA of the polymer is 92.1◦ without adding VTMO monomer. As the silicon monomer content increases, the CA of the polymer raises gradually. When the content of VTMO monomer increases to 10%, the CA reaches the peak of 104.9◦. When the addition of VTMO continues increasing, the CA decreases slightly, Materials 2020, 13, 3497 6 of 14 and its change is not significant. The prepared polymer shows the optimal surface hydrophobic property with 10% VTMO content. Therefore, PSA10 is used for the preparation of composite.

Table 4. Contact angle of diﬀerent PSAs copolymers.

PSAs CA (◦) PSA0 92.1 1.2 ± PSA1 97.5 1.0 ± PSA2 98.8 0.8 ± PSA5 100.5 1.0 ± PSA10 104.9 1.5 ± PSA15 101.9 1.0 ± PSA20 102.7 0.9 ±

The CA and surface free energy regarding diﬀerent materials are presented in Table5. It is clear that the surface free energy of pure HDPE is 37.0 mN/m. When the addition of hydrophobic SiO2 is increased from 1% to 5%, the wettability and surface free energy of the prepared composite are gradually deceased from 37.0 mN/m to 23.5 mN/m. With continuing to increase addition of SiO2, the wettability and surface free energy of the composite have not varied. This indicates that the addition of SiO2 at 5% is the optimal addition for the composite.

Table 5. Contact angle and surface free energy of diﬀerent substrates.

CA (◦) Substrates σsolid(mN/m) H2O CH2I2 HDPE 86.6 45.4 37.0 HPS1 106.1 56.5 32.2 HPS5 107.1 69.7 23.5 HPS9 103.5 53.3 33.8 PSA10 104.9 54.7 33.0

As the suspended solids in wastewater may cause wearing on pipe wall, it is necessary to investigate the wear resistance of the prepared material. The CA of the HPS composites with adding diﬀerent mass ratio of VTMO-SiO2 is shown in Figure3. It is identiﬁed that the CA of the composite is increased by 600 mesh sandpaper, and its hydrophobic property thus improves. When the hydrophobic silica content is 5%, the CA is increased to 127.8◦. This may be due to the fact that more nano columnar structures are formed on the surface of the sanded composites. The valleys and grooves in the micro-nano surface may give rise to air cushion to expand the area for air-liquid contact, thus increasingMaterials 2020 the, 13, contactx angle of the solid surface [33]. 7 of 14

140 Worn samples original samples 130 127.8 128.4

120 117

110 CA(degree) 106.1 107.1 103.5

100

90 HPS1 HPS5 HPS9 Comoposites

Figure 3. Change in contact angle ( (CA)CA) as various nano nano-silica-silica loadings for the HPS composites.composites.

VTMOS-SiO2

n(OH)

SiO2

Transmittance(%) Transmittance(%) HDPE PSA (a) HPS (b)

500 1000 1500 2000 2500 3000 3500 4000 500 1000 1500 2000 2500 3000 3500 4000 -1 Wavenumber(cm ) Wavenumber (cm-1)

(a) (b)

Figure 4. Fourier transform infrared (FTIR) spectra of different materials (a) pure high-density polyethylene (HDPE), copolymer PSA and composite HPS; (b) nano silicon.

Materials 2020, 13, x 7 of 14

140 Worn samples original samples 130 127.8 128.4

120 117

110 CA(degree) 106.1 107.1 103.5

100

90 HPS1 HPS5 HPS9 Comoposites

Figure 3. Change in contact angle (CA) as various nano-silica loadings for the HPS composites.

3.2. Fourier Transform Infrared (FTIR) The FTIR spectrum of the silicone-acrylate polymer and composites are shown in Figure 4. According to the PSA curve (Figure 4a), the bands at 2856 cm−1 and 2995 cm−1 are identified as C-H stretching vibrations of -CH2 and -CH3, respectively [34]. The carbonyl C=O stretching vibration occurs at an absorption peak of 1738 cm−1, whilst the epoxy group symmetrical stretch vibration occurs at an absorption peak of 854 cm−1. The bands at 1068 cm−1 and 1139 cm−1 are assigned to the Si- O symmetric stretching vibration and Si-O asymmetric absorption, respectively [35]. No obvious Materialsabsorption2020, 13 peak, 3497 is observed at 1640 cm−1 in the curve, indicating that the C=C double bond7 of has 14 disappeared and the monomer has polymerized. Compared with the HDPE sample, the prepared composite displays an apparent Si-O-C with absorption peaks at 790 cm−1, and the strong and wide 3.2. Fourier Transform Infrared (FTIR) absorption band is Si-O-Si antisymmetric stretching vibration at 1109 cm−1 [36], which indicates a formationThe FTIR of the spectrum hybrid material of the silicone-acrylateHPS. polymer and composites are shown in Figure4. 1 1 AccordingIn Figure to the 4b, PSA as curve compared (Figure with4a), that the bandsof the atunmodified 2856 cm − SiOand2, 2995 it is cm obvious− are that,identiﬁed after as VTMO C-H −1 stretchingmodification, vibrations the width of peak -CH2 ofand the -CHSiO-3H, respectively stretching band [34]. at The3489 carbonyl cm , the Canti=O-symmetric stretching stretching vibration 1 −1 occursvibration at anof SiO absorption-H, and the peak water’s of 1738 H-O cm-H− bending, whilst vibration the epoxy near group at 1635 symmetrical cm on the stretch surface vibration of silica 1 1 1 occursnanoparticles at an absorption decrease peak significantly. of 854 cm These− . The two bands peaks at 1068basically cm− disappearedand 1139 cm −in are the assigned modified to SiO the2, Si-Oindicating symmetric that Sistretching-OH has vibration fully condensed and Si-O into asymmetric Si-O-Si bonds absorption, during respectively the process [ 35 of]. hydrophobic No obvious 1 −1 absorptionmodification peak [35,37]. is observed Compared at 1640with cmthat− ofin the the pure curve, SiO indicating2, small peaks that at the 2964 C= cmC double emerged bond in hasthe disappearedFTIR spectrum and of thethe monomerVTMO-SiO has2, which polymerized. corresponds Compared to characteristic with the Si HDPE-C stretching sample, vibrations the prepared [38]. 1 compositeSuch findings displays further an apparent highlight Si-O-C that the with hydroxyl absorption groups peaks have at 790 been cm replaced− , and the successfully strong and by wide the 1 absorptionhydrocarbyl band groups is Si-O-Si, due to antisymmetric the introduction stretching of VTMO. vibration at 1109 cm− [36], which indicates a formation of the hybrid material HPS.

VTMOS-SiO2

n(OH)

SiO2

Transmittance(%) Transmittance(%) HDPE PSA (a) HPS (b)

500 1000 1500 2000 2500 3000 3500 4000 500 1000 1500 2000 2500 3000 3500 4000 -1 Wavenumber(cm ) Wavenumber (cm-1)

(a) (b)

FigureFigure 4. 4. Fourier transform infrared ( (FTIR)FTIR) spectra of didifferentﬀerent materials materials ( (aa)) pure pure high-densityhigh-density polyethylenepolyethylene (HDPE),(HDPE), copolymercopolymer PSAPSA andand compositecompositeHPS; HPS;( (bb)) nano nano silicon. silicon.

In Figure4b, as compared with that of the unmodified SiO 2, it is obvious that, after VTMO 1 modification, the width peak of the SiO-H stretching band at 3489 cm− , the anti-symmetric stretching 1 vibration of SiO-H, and the water’s H-O-H bending vibration near at 1635 cm− on the surface of silica nanoparticles decrease significantly. These two peaks basically disappeared in the modified SiO2, indicating that Si-OH has fully condensed into Si-O-Si bonds during the process of hydrophobic 1 modification [35,37]. Compared with that of the pure SiO2, small peaks at 2964 cm− emerged in the FTIR spectrum of the VTMO-SiO2, which corresponds to characteristic Si-C stretching vibrations [38]. Such findings further highlight that the hydroxyl groups have been replaced successfully by the hydrocarbyl groups, due to the introduction of VTMO.

3.3. Thermogravimetric Analysis The TGA and DTG results of the copolymer PSA, pure HDPE and the HDPE composite with addition of PSA and SiO2 are shown in Figure 5. For the copolymer PSA, as measured in untreated copolymer, there is 6.5% weight loss at the temperature range of 130–210 °C, due to the removal of unreacted monomers residuals. The main degradation stage of copolymer is in the range of 240–410 °C, and its maximum pyrolysis rate occurs at 341°C, with weight loss reaching 87.4%. The PSA is completely degraded at 420 °C, shown in Figure 5a. According to the DTG of PSA (Figure 5b), the first decomposition shoulder peak at 215 °C is the pyrolytic polymerization of the polymer, and the second major decomposition peak at 341 °C is the polymer decomposition. The DTG curve shows strong and sharp endothermic peak at 468 °C for the pure HDPE and 466 °C for the composite (Figure 5b). This fact implies that the thermal decomposition temperature of the prepared composite is not affected by the addition of acrylate with lower decomposition temperature, which demonstrates excellent thermal stability. In addition, there is only one obvious decomposition peak in the DTG curve of the HPS composite (Figure 5b), indicating that the polymer may cross‐link under the action of residual siloxane and initiator in the process of melt blending [39]. Cross‐linking of polymers is expected to minimize solvent uptake, avoid crystallinity and fix their surface in a solution environment [40]. Materials 2020, 13, 3497 8 of 14

100 0

80 -5

60 -10 40 PSA Weight loss(%) Weight PSA -15 HPS 20 HPS (%/min) Weight Deriv. HDPE HDPE -20 0 (a) (b)

0 100 200 300 400 500 600 0 100 200 300 400 500 600 Temperature(℃) Temperature (℃)

(a) (b)

Figure 5. Thermal analysis of diﬀerent materials: (a) thermogravimetry (TG) and (b) derivative thermogravimetry (DTG).

TheFigure DTG 5. curve Thermal shows analysis strong of and different sharp materials: endothermic (a) thermogravimetry peak at 468 ◦C for (TG) the and pure (b) HDPE derivative and 466 ◦C for the composite (Figure5b). This factthermogravimetry implies that the (DTG). thermal decomposition temperature of the prepared composite is not affected by the addition of acrylate with lower decomposition temperature, which3.4. SEM demonstrates Analysis excellent thermal stability. In addition, there is only one obvious decomposition peakThe in the typical DTG SEM curve images of the of HPS different composite samples (Figure are 5shownb), indicating in Figure that 6. It the can polymer be seen maythat the cross-link surface underof PSA10 the is action smooth, of residual without siloxane obvious and surface initiator features in the (Figure process 6a). of The melt particle blending size [39 of]. hydrophobically Cross-linking of polymersmodified nano is expected silica powder to minimize is about solvent 100 nm uptake, (Figure avoid 6b) crystallinity and the static and water fix their CA is surface tested inup a to solution 154.5°, environmentwhich can be [ 40considered]. as superhydrophobic surface. There is a slight aggregation of particles on the surface of HDPE composite after adding polymer PSA10 and superhydrophobic nano‐SiO2 3.4. SEM Analysis (Figure 6c), which may be attributed to the Van der Waals bond between nanoparticles [27]. The agglomeratedThe typical nanoparticles SEM images ofare di ffdistributederent samples in arebright shown spots in Figurein the6 .dark It can HDPE be seen matrix, that the and surface the ofdistribution PSA10 is smooth, of particles without in the obvious whole surface matrix featuresis relatively (Figure uniform,6a). The which particle may size lead of hydrophobicallyto an increase in modifiedcavitation nano in the silica material powder surface, is about resulting 100 nm in (Figure an increase6b) and of the CA static to further water CAdemonstrate is tested up an to excellent 154.5 ◦, whichhydrophobicity can be considered [33]. as superhydrophobic surface. There is a slight aggregation of particles on the surface of HDPE composite after adding polymer PSA10 and superhydrophobic nano-SiO2 (Figure6c), which may be attributed to the Van der Waals bond between nanoparticles [27]. The agglomerated nanoparticles are distributed in bright spots in the dark HDPE matrix, and the distribution of particles Materialsin the whole 2020, 13 matrix, x is relatively uniform, which may lead to an increase in cavitation in the material9 of 14 surface, resulting in an increase of CA to further demonstrate an excellent hydrophobicity [33].

(a) (b) (c)

10 μm 10 μm 100 nm

Figure 6. SEM images of diﬀerent samples: (a) PSA10; (b) VTMO-SiO2; and (c) HPS5. Figure 6. SEM images of different samples: (a) PSA10; (b) VTMO-SiO2; and (c) HPS5. 3.5. Acid and Alkali Resistance 3.5. Acid and Alkali Resistance It is signiﬁcant to test the acid and alkaline resistance of the prepared material in order for possible engineeringIt is significant practice on to wastetest the water acid piping. and alkaline High concentrated resistance of acid the solutionprepared (HCl, material pH = in2) orderand basic for possiblesolution (NaOH,engineering pH = practice12) have on been waste introduced water piping. to assess High the concentrated stability of the acid hydrophobic solution (HCl, ability pH of = the 2) andHPS5 basic composite solution under (NaOH, extreme pH = environment.12) have been Afterintroduced being immersedto assess the in thestability acid andof the alkali hydrophobic solutions ability of the HPS5 composite under extreme environment. After being immersed in the acid and alkali solutions for 15 days, there is no obvious changes of CA, shown in Figure 7. This indicates that the prepared composite material shows certain potentials in acid and alkaline resistance.

120 120

115 115

)

o o

( 110

105 105

100 100

Contact angle Contact angle

95 95 (a) (b) 90 90 0 3 6 9 12 15 0 3 6 9 12 15 Time (days) Time(days)

(a) (b)

Figure 7. Contact angle changes after immersed in simulated leachate (a) pH = 2.0, (b) pH = 12.0.

3.6. Anti-Scaling Properties A comparative test is designed to investigate the adhesion of the material surface to the fouling by application of scale dust washing. The dried scale samples were taken from the landfill leachate pipeline in Chengdu city, ground and passed through 100 mesh sieve, then sprayed on the surface of the unmodified HDPE resin and the prepared composite, respectively. After the water droplets flowed through the layer of the scale samples, the residuals on the material surface were investigated. Figure 8 shows that nearly all the scaling dusts on the surface of the prepared composite are washed away by water droplets, whilst there are still certain scaling dusts remained on the surface of HDPE. It is indicated that the HDPE substrates are prone to scaling. Existing studies have addressed that low surface energy substrate is effective to decrease fouling adhesion [25,41,42]. This study identifies that the surface free energy of the prepared composite is lower than that of the commercial HDPE pipe (shown in Table 4), indicating better scaling resistance.

Materials 2020, 13, x 9 of 14

(a) (b) (c)

10 μm 10 μm 100 nm

Figure 6. SEM images of different samples: (a) PSA10; (b) VTMO-SiO2; and (c) HPS5.

3.5. Acid and Alkali Resistance It is significant to test the acid and alkaline resistance of the prepared material in order for possible engineering practice on waste water piping. High concentrated acid solution (HCl, pH = 2) and basic solution (NaOH, pH = 12) have been introduced to assess the stability of the hydrophobic Materials 2020, 13, 3497 9 of 14 ability of the HPS5 composite under extreme environment. After being immersed in the acid and alkali solutions for 15 days, there is no obvious changes of CA, shown in Figure 7. This indicates that forthe 15prepared days, there composite is no obvious material changes shows certain of CA, potentials shown in in Figure acid 7and. This alkaline indicates resista thatnce. the prepared composite material shows certain potentials in acid and alkaline resistance.

120 120

115 115

)

o o

( 110

105 105

100 100

Contact angle Contact angle

95 95 (a) (b) 90 90 0 3 6 9 12 15 0 3 6 9 12 15 Time (days) Time(days)

(a) (b)

FigureFigure 7.7. ContactContact angleangle changeschanges afterafter immersedimmersed inin simulatedsimulated leachateleachate ((aa)) pHpH= = 2.0,2.0, ((bb)) pHpH = 112.0.2.0.

3.6. Anti-Scaling Properties 3.6. Anti-Scaling Properties A comparative test is designed to investigate the adhesion of the material surface to the fouling by application of scale dust washing.washing. The dried scale samples were taken from the landfilllandfill leachate pipeline in Chengdu city, groundground andand passedpassed throughthrough 100100 meshmesh sieve,sieve, thenthen sprayedsprayed onon the surface of the unmodified unmodified HDPE HDPE resin resin and and the the prepared prepared composite, composite, respectively. respectively. After Afterthe water the droplets water droplets flowed throughflowed through the layer the of layer the scale of the samples, scale samples, the residuals the residuals on the materialon the material surface surface were investigated. were investigated. Figure8 8 shows shows thatthat nearlynearly allall thethe scalingscaling dustsdusts onon thethe surfacesurface ofof thethe preparedprepared compositecomposite areare washed awayaway byby waterwater droplets,droplets, whilst whilst there there are are still still certain certain scaling scaling dusts dusts remained remained on on the the surface surface of HDPE.of HDPE. It is It indicated is indicated that the that HDPE the HDPE substrates substrates are prone are to prone scaling. to Existing scaling. studies Existing have studies addressed have thataddressed low surface that low energy surface substrate energy substrate is effective is effective to decrease to decrease fouling adhesionfouling adhesion [25,41,42 [25,41,42].]. This study This identifies that the surface free energy of the prepared composite is lower than that of the commercial Materialsstudy identifies 2020, 13, x that the surface free energy of the prepared composite is lower than that10 of of the 14 HDPEcommercial pipe (shownHDPE pipe in Table (shown4), indicating in Table 4), better indicating scaling better resistance. scaling resistance.

(a) (b)

Figure 8. AntiAnti-scaling-scaling properties properties of of ( (a)) HDPE HDPE and and ( (b)) HPS5 HPS5 composite composite.. The scales are measured by comparing the weight of HDPE samples and the composite samples, The scales are measured by comparing the weight of HDPE samples and the composite samples, as shown in Figure9. Both of the two materials occurs weight increase after soaking in the same as shown in Figure 9. Both of the two materials occurs weight increase after soaking in the same simulated leachate, and the variation of HDPE is greater than that of the composite material. Among simulated leachate, and the variation of HDPE is greater than that of the composite material. Among them, the HDPE shows significant weight increase in the first period when placing into the simulated them, the HDPE shows significant weight increase in the first period when placing into the simulated leachate in pH of 4 (see Figure9a). The reason may be the fact that the inorganic filler in HDPE is leachate in pH of 4 (see Figure 9a). The reason may be the fact that the inorganic filler in HDPE is corroded under strong acid environment [43]. corroded under strong acid environment [43].

1.2 1.2 (a) (b)

0.9 0.9

)

2 2

0.6 0.6

mg/cm

(

Δwt Δwt Δwt 0.3 0.3 HDPE HDPE HPS HPS 0.0 0.0

0 7 14 21 28 35 42 49 0 7 14 21 28 35 42 49 Time (days) Time(days)

1.2 (c)

0.9

) 2

0.6

mg/cm

( Δwt Δwt 0.3 HDPE HPS 0.0

0 7 14 21 28 35 42 49 Time(days)

Figure 9. Weight of material samples in simulated leachate under different conditions: (a) pH = 4.0; (b) pH = 6.8; (c) pH = 10.0.

Figure 10 shows the SEM images of different materials immersed in the simulated leachate with pH of 4.0 after 14 days. There are columnar or square crystals on the surface of HDPE sample (Figure 10b). Such crystals may be encapsulated by the bio-films, since it has been further identified by the

Materials 2020, 13, x 10 of 14

(a) (b)

Figure 8. Anti-scaling properties of (a) HDPE and (b) HPS5 composite.

The scales are measured by comparing the weight of HDPE samples and the composite samples, as shown in Figure 9. Both of the two materials occurs weight increase after soaking in the same simulated leachate, and the variation of HDPE is greater than that of the composite material. Among them, the HDPE shows significant weight increase in the first period when placing into the simulated leachate in pH of 4 (see Figure 9a). The reason may be the fact that the inorganic filler in HDPE is corroded under strong acid environment [43]. Materials 2020, 13, 3497 10 of 14

1.2 1.2 (a) (b)

0.9 0.9

)

2 2

0.6 0.6

mg/cm

(

Δwt Δwt Δwt 0.3 0.3 HDPE HDPE HPS HPS 0.0 0.0

0 7 14 21 28 35 42 49 0 7 14 21 28 35 42 49 Time (days) Time(days)

1.2 (c)

0.9

) 2

0.6

mg/cm

( Δwt Δwt 0.3 HDPE HPS 0.0

0 7 14 21 28 35 42 49 Time(days) Figure 9. Weight of material samples in simulated leachate under different conditions: (a) pH = 4.0; Figure 9. Weight of material samples in simulated leachate under different conditions: (a) pH = 4.0; (b) pH = 6.8; (c) pH = 10.0. (b) pH = 6.8; (c) pH = 10.0. Figure 10 shows the SEM images of different materials immersed in the simulated leachate MaterialsFigure 2020 ,10 13 ,shows x the SEM images of different materials immersed in the simulated leachate11 with of 14 with pH of 4.0 after 14 days. There are columnar or square crystals on the surface of HDPE sample pH of 4.0 after 14 days. There are columnar or square crystals on the surface of HDPE sample (Figure (Figure 10b). Such crystals may be encapsulated by the bio-films, since it has been further identified by 10bsurface). Such of commercialcrystals may HDPE be encapsulated pipe which byis adheredthe bio-films, to gelatinous since it hassubstances, been further shown identified in Figure by 11. the A the surface of commercial HDPE pipe which is adhered to gelatinous substances, shown in Figure 11. possible reason for this may be that the immersion experiment is not implemented under an aseptic A possible reason for this may be that the immersion experiment is not implemented under an aseptic environment, by which the organic constituents in the simulated leachate, e.g., glucose, may provide environment, by which the organic constituents in the simulated leachate, e.g., glucose, may provide carbon source and phosphorus as the nutrients to nourish microorganisms. They are further adhered carbon source and phosphorus as the nutrients to nourish microorganisms. They are further adhered to to the immersed material samples to promote their metabolisms, ultimately forming bio-films. the immersed material samples to promote their metabolisms, ultimately forming bio-films. However, However, there is no obvious crystal formation on the surface of the prepared composite, only a few there is no obvious crystal formation on the surface of the prepared composite, only a few rod-shaped rod-shaped and lamellar scales (Figure 10c). This may be related to the good hydrophobicity and low and lamellar scales (Figure 10c). This may be related to the good hydrophobicity and low surface free surface free energy of the prepared composite. The hydrophobic material surface may inhibit crystal energy of the prepared composite. The hydrophobic material surface may inhibit crystal nucleation, nucleation, and thus has excellent anti-scaling properties [40,44,45]. and thus has excellent anti-scaling properties [40,44,45].

(a) (b) (c)

10 μm 10 μm 10 μm Figure 10. SEM images of the material samples immersed in the simulated leachate solution (pH = 4.0) Figure 10. SEM images of the material samples immersed in the simulated leachate solution (pH = for 14 days: (a) Original HDPE; (b) HDPE after immersion; (c) HPS5 composite. 4.0) for 14 days :(a) Original HDPE; (b) HDPE after immersion; (c) HPS5 composite.

HPS HDPE

Figure 11. Material samples immersed in the simulated leachate (pH = 4.0).

3.7. Discussion In this work, copolymers have been polymerized by bulk polymerization without addition of organic solvents, which may decrease environmental impact during the preparation process. The addition of modified nano-silica is identified to improve the wear resistance, hydrophobicity and thermal stability of the prepared composite. The bifunctional groups of siloxanes react with nano- silica and acrylate, to produce a chemical bond between polymer and filler. Thus, the distribution of filler in the polymer is more uniform and stable [46]. The surface energy of the composite is as low as 23.5 mN/m when the content of PSA and nano- SiO2 is added as 15%, 5%, respectively. Azimi et al. identified that if surface energy is below 25 mN/m, the nucleation of scale will be significantly decreased [11]. Such conclusion may imply that our prepared composite has potentials in resistance of scaling deposition. Furthermore, the study fabricates the organic/inorganic cross-linked composite materials to replace conventional anti-scaling coatings, which may improve the lifespan of pipe materials. The immersion experiment results shows that the scaling on the surface of the prepared composite is less than that of commercial HDPE pipe. It further verifies that the anti-scaling performance may have strong relations with the hydrophobicity and surface energy regarding the prepared composite, i.e., the better hydrophobicity and the lower surface energy may result in less scaling happened. The composite demonstrates a strong adaptability to the extreme acid and alkaline environment, indicating it may have wide applications to the collection and transport of wastewater from various

Materials 2020, 13, x 11 of 14 surface of commercial HDPE pipe which is adhered to gelatinous substances, shown in Figure 11. A possible reason for this may be that the immersion experiment is not implemented under an aseptic environment, by which the organic constituents in the simulated leachate, e.g., glucose, may provide carbon source and phosphorus as the nutrients to nourish microorganisms. They are further adhered to the immersed material samples to promote their metabolisms, ultimately forming bio-films. However, there is no obvious crystal formation on the surface of the prepared composite, only a few rod-shaped and lamellar scales (Figure 10c). This may be related to the good hydrophobicity and low surface free energy of the prepared composite. The hydrophobic material surface may inhibit crystal nucleation, and thus has excellent anti-scaling properties [40,44,45].

(a) (b) (c)

10 μm 10 μm 10 μm

MaterialsFigure2020 ,10.13, 3497SEM images of the material samples immersed in the simulated leachate solution (pH 11= of 14 4.0) for 14 days :(a) Original HDPE; (b) HDPE after immersion; (c) HPS5 composite.

HPS HDPE

Figure 11.11. Material samples immersed in the simulated leachateleachate (pH(pH == 44.0)..0).

3.7. Discussion In this work, copolymers copolymers have have been been polymerized polymerized by by bulk bulk polymerization polymerization without without addition addition of oforganic organic solvents, solvents, which which may may decrease decrease environmental environmental impact impact during during the preparation the preparation process. process. The Theaddition addition of modified of modified nano nano-silica-silica is identified is identified to to improve improve the the wear wear resistance, resistance, hydroph hydrophobicityobicity and thermal stabilitystability of of the the prepared prepared composite. composite. The The bifunctional bifunctional groups groups of siloxanes of siloxanes react react with with nano-silica nano- andsilica acrylate, and acrylate, to produce to produce a chemical a chemical bond between bond between polymer polymer and filler. and Thus, filler. the Thus, distribution the distribution of filler of in thefiller polymer in the polymer is more uniformis more uniform and stable and [46 st].able [46]. The surface surface energy energy of of the the composite composite is as is aslow low as 23.5 as 23.5 mN/m mN when/m when the content the content of PSA of and PSA nano and- nano-SiOSiO2 is added2 is addedas 15%, as 5%, 15%, respectively. 5%, respectively. Azimi et Azimi al. identified et al. identified that if surface that ifenergy surface is below energy 25 is mN/m, below 25the mN nucleation/m, the nucleation of scale will of scale be significantly will be significantly decreased decreased [11]. Such [11]. conclusion Such conclusion may mayimply imply that that our ourprepared prepared composite composite has has potentials potentials in in resistance resistance of of scaling scaling deposition. deposition. Furthermore, Furthermore, the the study fabricates the organic/inorganic organic/inorganic cross-linkedcross-linked compositecomposite materials to replace conventional anti anti-scaling-scaling coatings,coatings, which may improve thethe lifespanlifespan ofof pipepipe materials.materials. The immersion immersion experiment experiment results results shows shows that thethat scaling the scaling on the surfaceon the of surface the prepared of the composite prepared iscomposite less than is that less of commercial than that of HDPE commercial pipe. It HDPEfurther pipe. verifies It that further the anti-scaling verifies that performance the anti-scaling may haveperformance strong relations may have with strong the hydrophobicity relations with the and hydrophobicity surface energy regardingand surface the energy prepared regarding composite, the i.e.,prepared the better composite, hydrophobicity i.e., the better and the hydrophobicity lower surface and energy the may lower result surface in less energy scaling may happened. result in less scalingThe happened. composite demonstrates a strong adaptability to the extreme acid and alkaline environment, indicatingThe composite it may have demonstrates wide applications a strong to adaptability the collection to the and extreme transport acid of and wastewater alkaline environment, from various emissionsindicating sources.it may have However, wide applications the mechanism to the of col scalelection formation and transport is still uncertain,of wastewater which from lays various out a room for investigation on interaction between scaling and material surface, especially on how leachate a ffects such interaction.

4. Conclusions This study prepares a low surface energy silicified acrylate copolymer by bulk polymerization. Such copolymer is added by melt blending with HDPE resin and different content of hydrophobic nano-SiO2 to prepare a nano-composite, entitled HDPE/PSA/SiO2 composite. Through the characterization of FTIR, TG, CA, SEM, etc., it is confirmed that PSA and hydrophobic nano-SiO2 are successfully synthesized, such that there is no other organic solvent in addition to the reaction monomer. The thermal analysis results show that the addition of silicified acrylate polymer and nano-SiO2 have little influence on the thermal decomposition temperature, to confirm that the prepared composite has good thermal stability. The surface free energy is decreased to 23.5 mN/m for the composite, and its water contact increases to 107.1◦. It thus implies that 5% hydrophobic nanoparticles is the best percentage of addition. HDPE/PSA/SiO2(5%) is selected to compare its acid and alkali resistance with the common commercial HDPE pipe material. The composite has good wear resistance and strong adaptability to extreme acid and alkali environment. Through an immersion experiment by placing the prepared composite and the commercial HDPE samples into the simulation leachate, the results show that the former has better anti-scaling property due to its low surface energy and wettability. It is thus indicated that the composite material may have perspectives on engineering application to various wastewater piping transport. Materials 2020, 13, 3497 12 of 14

There is room for further study. First, the interaction between scaling particles and material surface, as well as how various water quality may affect such interaction still need to be investigated. Further study should center on the analysis of possible mechanism of scaling. In addition, other properties of composite materials, such as mechanical properties, aging resistance, etc., are expected to be tested to improve its feasibility in engineering application. Furthermore, it is also important to consider the verification of the practical application value of the material in the simulated wastewater flow pipeline experiment.

Author Contributions: R.Z., S.M. and T.Y. were conceptualized in the experiment; M.L. implemented the experiment and data analysis; M.L. prepared the original draft; R.Z. reviewed and edited the paper. All authors have read and agreed to the published version of the manuscript. Funding: This study is funded by National Natural Science Foundation of China (No. 41571520; 51608499), Sichuan Young Talent Scientific Funding (No. 2019JDJQ0020), Sichuan Provincial Key Technology Support (No.2019JDTD0024; 2019ZHCG0048), Sichuan Province Circular Economy Research Centre Fund (No. XHJJ-1802). Conflicts of Interest: The authors declare no conflict of interest.

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