Research Article | Open Access
Cu Nanoparticle-Modified High-Density Polyethylene Monofilament and Its Antifouling Performance on Fishing Netting
Cu nanoparticles (CuNPs) were modified by organic surfactant, then CuNP-modified high-density polyethylene (CuNP/HDPE) monofilaments were prepared by melt spinning. The effect of CuNP content on the morphology and properties of nanocomposite monofilaments was investigated. FT-IR and dynamic light scattering proved the successful surface modification for CuNP. Scanning electron microscope was used to observe the dispersed behavior of the CuNP. When the CuNP content was less than 1.0 wt%, the CuNPs were well dispersed in these nanocomposite monofilaments, and the increase of crystallization rate, the breaking strength, and knot strength were observed by differential scanning calorimeter and tensile test. Therefore, nanocomposite monofilaments showed comparable properties at low CuNP contents. With increasing CuNP content, the width of peak and for α-relaxation from dynamic mechanical analysis were increased, indicating more amorphous components near the crystalline regions. In addition, burst release of Cu ions in seawater was observed. The coupon tests demonstrated that CuNPs could significantly improve antibiofouling performance of CuNP/HDPE fishing netting. CuNP/HDPE fishing netting have a strong potential for using in marine antifouling mitigation.
High-density polyethylene (HDPE) resin is the most commonly used materials in fisheries because of its excellent fishing performance, such as good toughness, high strength, low density, smooth surface, minimal hygroscopicity, good water filtration, wear resistance, and aging resistance [1–5]. HDPE monofilament is the preferred material for making trawls, purse seines, fixed nets, aquaculture cages, and various fishing nets [6–8]. The HDPE fishing gear possesses satisfactory economic efficiency, fishery harvesting performance, and operating efficiency. With the development of aquatic science, the industry establishes higher requirements for the performance of fishing gear. Ordinary polyethylene fibers can no longer meet the special needs of existing fishery specialization. The main problems of conventional polyethylene fiber netting materials for traditional aquaculture purse seines are as follows: on the one hand, conventional polyethylene netting materials are prone to be damaged under severe sea conditions, such as typhoon [9, 10] and on the other hand, after placing traditional aquaculture purse seines equipped with ordinary fiber netting in the sea area for a period of time, they will be covered with marine fouling organisms. These organisms affect not only the exchange of water inside and outside but also the growth of cultured fish, causing these fish to become sick . Moreover, drag acting on the net increases with increasing level of biofouling . Even if antifouling coatings are used, the antipollution effect of the netting can only be maintained for 6–12 months . The netting needs to be treated again after coating for a period of time, leading to complicated and costly operation. Therefore, higher requirements are placed on the mechanical strength and antifouling performance of fishing netting materials. Modification is the most economical and feasible way to improve the comprehensive and cost performance of conventional materials [14, 15].
Metallic Cu exhibits a relatively good inhibitory effect on bacteria and can simultaneously prevent the growth of marine organisms. The application of Cu has achieved satisfactory results in the field of fishery engineering, especially in aquaculture cages or aquaculture seines [16, 17]. Long-term antifouling performance tests indicate that Cu and its alloys are resistant to fouling mainly due to corrosion to release Cu ions in seawater. Noteworthy, since the 19th century, dispersions of copper or copper oxide particles into organic matrixes have been employed as antifouling coatings by the paint industry. Such paints after immersion in seawater provoked massive release of copper but it quickly failed in a short time . And the coating is easy to fall off and causes pollution. A worldwide attempt to eliminate or reduce the use of such large-scale coatings is being promoted . However, Cu/polymer nanocomposites can be a valid option for antifouling purposes. The use of Cu nanoparticle- (CuNP-) modified polymer materials combines the excellent properties of both and obtains a composite material with low cost, low density, and excellent performance. The addition of CuNP to the polymer improves its mechanical properties and thermal stability and imparts certain antiseptic and antibacterial effects on the resin itself. Recently, many reports appeared in literature on the study for nanocomposites containing CuNP [20–24]. Molefi et al.  investigated polyethylene/CuNP melt mixing with 0~5 vol.% CuNP, and they found that the tensile modulus generally increased with increasing CuNP content. When exposed to seawater, CuNP are oxidized into free Cu ions, which play a role in antimicrobial and antifouling [25, 26]. As we know, the distribution of CuNP can be improved after surface treatment. Anyaogu et al.  reported a functionalized CuNP/polymer composite based on surface-modified CuNP with acrylic monomers and observed the antimicrobial activity of CuNP/polymer composites.
In the present study, the nanocomposite monofilaments were prepared in such a way that surface-modified CuNPs were added to blending in melt spinning. The effect of CuNP content on morphology and properties of CuNP/HDPE nanocomposite monofilaments was investigated, and the Cu ion release characteristics of CuNP/HDPE nanocomposite monofilaments were explored by ultraviolet-visible spectrophotometer.
2.1. Materials and CuNP/HDPE Nanocomposite Monofilament Preparation
HDPE (5000S) with the melt flow index of 0.9 g/10 min was supplied by Sinopec Yangzi Petrochemical Co., Ltd, China. CuNP with 20 nm was provided by Suzhou Canfuo Nanotechnology Co., Ltd, China. Tween-20 agent and ethanol were purchased from Sinopharm Chemical Reagent Co., China. The surface-modified CuNP was prepared by dissolving Tween agent in ethanol, stirred evenly for 5 min, and then added CuNP powders, shaken for 30 min, and stirred for another 1 h. The stirrer was washed and dried at 80°C under vacuum.
With spinning-grade HDPE as matrix, CuNP/HDPE nanocomposite system was obtained by adding surface-modified CuNP and other processing aids. These mixes were blended and extruded through a twin-screw extruder. Then, the systems were treated by melt spinning, and CuNP/HDPE primary fibers were obtained by single-screw extrusion. CuNP/HDPE nanocomposite monofilaments were prepared by multistage draw production. The suitable spinning temperature was 250°C–270°C. The water bath temperature was 98°C, and the draw ratio was 8.0, under which the nanocomposite monofilaments are continuously melt-spun. The diameter of the nanocomposite monofilaments was approximately 0.20 mm. The weight ratios of CuNP to HDPE were 0.5 wt%, 1.0 wt%, and 2.0 wt%. Correspondingly, the nanocomposite monofilaments were named as CuNP/HDPE-0.5, CuNP/HDPE-1.0, and CuNP/HDPE-2.0, respectively.
FT-IR spectra were measured using a Nicolet spectrometer (model 560, Nicolet Instruments, USA). The samples were scanned 32 times at a resolution of .
The particle size distribution of the suspended nanoparticles (dispersion in H2O) was obtained by dynamic light scattering (DLS) (Brookhaven BI-200SM Goniometer, USA).
The microstructures of the monofilaments were examined using a scanning electron microscope (SEM) (6360LA, JEOL Ltd., Japan) operated at an acceleration voltage of 15 kV.
Dynamic mechanical analysis (DMA) was used a mode of tensile film clamp (242C, NETZSCH Instruments, Germany) under the frequency of 1 Hz and the amplitude of 30 μm for all samples. The samples were initially cooled from ambient temperature to −170°C and then held at this temperature for 10 min. Then, the samples were heated to 140°C at a heating rate of 3°C/min.
Thus, loss factor, , is expressed as follows: where is the storage modulus and is the loss tangent modulus. is the most fundamental dynamic property of a viscoelastic material.
Differential scanning calorimetry (DSC) was applied to investigate the melting and crystallization behavior of the monofilaments using the DSC thermal analyzer (204F1, NETZSCH Instruments, Germany). The samples were scanned at a heating and cooling rate of 10°C/min in nitrogen atmosphere. The degree of crystallinity was calculated via the total enthalpy method, according to the following expression: where is the observed heat of fusion values and is the specific enthalpy of melting for 100% crystalline polymer. The value of 287 J/g was used for PE .
The tensile properties were investigated using the electron tensile tester (4466, Instron Instruments, USA) at a crosshead speed of 300 mm/min on a 750 mm long specimen according to SC/T 5005-2014 under ambient conditions. Results are the average of at least 10 specimens.
A linear wear tester machine (5750, Taber Instruments, USA) was employed to measure the wear resistance of monofilament samples. The test was conducted at 25°C under a pressure of 1.0 cN/dtex. The degree of wear resistance () is related to the wear broken times, expressed as follows:
The release of Cu ion of varying monofilament samples was investigated when exposed to artificial seawater. 2 g monofilament samples were immersed in the 200 ml artificial seawater. Artificial seawater was prepared by using sea salt and distilled water according to ASTM standard D1141-98. Salinity was constantly monitored and held between 33% and 34%, and the pH was kept between 8.10 and 8.15. During immersion, the samples were gently stirred by a magnetic stirrer. The solubility of Cu based on inorganic speciation considerations is about 0.5 mg L−1 . The Cu ion concentrations in seawater are far less than the saturation in the present study. Ultraviolet-visible spectrophotometer can be used to quantify the Cu ion concentration . Absorbance measurements were made at 1 nm intervals from 350 nm to 600 nm. The absorbance was measured using a spectrophotometer (i8, Hanon Instruments, China) to established the standard curve of Cu ion.
Coupon test was carried out by replacing some cage netting with CuNP/HDPE netting in the shallow sea area of the South China Sea. The samples were drawn in 1-, 3-, and 6-month intervals. Four 0.5-square mesh samples were randomly cut from the CuNP/HDPE netting and HDPE netting at 1 m depth. The retrieved mesh was cleaned by immersing in the water to remove sediments and other dirt. Then, weights were taken to assess the accumulation of fouling organisms. Results are the average of four mesh samples.
3. Results and Discussion
3.1. Surface Modification of CuNP
CuNPs before and after surface modification are characterized by FT-IR (Figure 1). The modified CuNP shows a distinct absorption band between 1500 and 1700 cm−1, corresponding to the stretching vibration of the -COO group in Tween agent. In addition, the broad absorption band at 3700–3000 cm−1 is assigned to the stretching vibration of the hydroxyl group (-OH). This band clearly migrates to high wave number after modification. Tween agent is speculated to be adsorbed on the surface of CuNP. Figure 2 shows the DLS analysis for before and after modification of CuNP. The particle size of CuNP is 20 nm. A large amount of agglomeration is found due to the high surface energy, and the average particle size is 170 nm. The average particle size of CuNP modified by Tween agent is 105 nm. It can be clearly seen that the tendency of agglomeration is weakened, resulting from the adsorption of surfactant. Surface-modified CuNP are used in our experiment given the importance of the dispersibility of the microspheres in the particle filling system.
SEM images are used to visually assess nanofiller dispersion within the matrix. The morphology of CuNP/HDPE nanocomposite monofilaments is investigated by SEM, and typical images are given in Figure 3. CuNPs are dispersed in the HDPE monofilament matrix as microaggregates. When 0.5 wt% CuNPs are added and the dispersion size is about 200 nm, the dispersed particles are substantially spherical. When 2.0 wt% CuNPs are added and the dispersion size is about 300 nm–1.2 μm (Figure 3(c)), the aggregates show irregular and nonspherical structure. This finding indicates that CuNPs are not completely dissociated and dispersed because of the high melt viscosity during melt spinning. The aggregation of CuNP is also attributed to the strong van der Waals interactions between the nanoparticles.
The DSC heating curves of CuNP/HDPE nanocomposite monofilaments are shown in Figure 4(a), and the dependence of CuNP content on calculated are plotted in Figure 4(b). The CuNP content has no significant effect on the melting temperature of HDPE matrix. As the CuNP content increases, the of the nanocomposite monofilaments increases first and then decreases. When the CuNP content is 0.5 wt%, CuNP/HDPE nanocomposite monofilament has the largest of 61%. It should be noted that for other systems, an increase of crystallization rate also was observed for low concentrations of nanofiller [31, 32]. It was argued that the nanoparticles could act as additional nucleation sites. On the other hand, the nanomaterials added hinder the crystallization of polyethylene segments. As the CuNP content further increases, the introduction of a large amount of CuNP causes agglomeration and reduces the interaction among HDPE macromolecular chains, resulting in a decrease in the of the CuNP/HDPE monofilaments. A similar behavior was observed for nanocomposites based on polyethylene containing other nanofillers [33, 34].
3.4. Mechanical Properties
The mechanical properties of CuNP/HDPE nanocomposite monofilaments were compared to analyze the effect of the CuNP content. Figure 5 shows the effect of the CuNP content on the mechanical properties of CuNP/HDPE nanocomposite monofilaments. All nanocomposite monofilaments show higher elastic modulus than pure HDPE monofilaments, and the elastic modulus of the nanocomposite monofilaments increases first and then decrease with increasing CuNP content (Figure 5(a)). A similar behavior has also been observed for nanoparticle-reinforced nanocomposites . In general, the improvement of elastic modulus is attributed to the good dispersion of nanoparticles and good interfacial adhesion between the particles and the matrix so that the mobility of polymer chains is restricted under loading . Knot strength is an important property for fishing netting and is associated with the toughness of polymer . With increasing CuNP content, the breaking strength and knot strength of CuNP/HDPE nanocomposite monofilaments show the same trend as the elastic modulus. When the CuNP content is 0.5–1.0 wt%, the CuNP/HDPE nanocomposite monofilament exhibits better breaking strength and knot strength than pure HDPE samples. This finding is related to the dispersion state of nanoparticles and the microstructure of the aggregation state. When CuNPs are uniformly dispersed in the HDPE matrix with good interfacial bonding, significant nanoenhancement and toughening effects are produced, resulting in an increase of breaking strength and knot strength. When the CuNP content is 2.0 wt%, agglomeration is intensified, forming larger agglomerates that become defects. Moreover, a large stress concentration occurs when subjected to force, resulting in the decrease in the elastic modulus, breaking strength, and knot strength. Therefore, the mechanical properties of monofilaments can be further optimized by adjusting the dispersion and aggregation microstructure of the particles.
3.5. Viscoelastic Properties
The viscoelastic properties of CuNP/HDPE nanocomposite monofilaments are studied by dynamic mechanical analysis. Figure 6 shows the temperature and frequency dependence of the dynamic mechanical properties of CuNP/HDPE nanocomposite monofilaments. The of the CuNP/HDPE nanocomposite monofilaments increases with increasing CuNP content (Figure 6(a)), which reflects higher elastic modulus of the nanocomposite monofilaments . Two transition peaks are detected in HDPE and CuNP/HDPE nanocomposite monofilaments in a wide range of test temperatures (Figure 6(b)). The weaker peak appearing at low temperature corresponds to the glass transition of polyethylene . With increasing CuNP content, the glass transition temperature () of CuNP/HDPE nanocomposite monofilament increases and the maximum loss factor () decreases. In general, the shape and position of peak are closely related to crosslinking density and molecular structure [40, 41]. This finding shows that CuNP exerts a strong interaction with HDPE after surface modification. The increase in the CuNP content increases the binding of macromolecular chains, resulting in a decrease in and a tendency of toward high temperature. The transition in high-temperature zone (α transition) corresponds to the movement of restricted amorphous structure near the crystallization zone of polyethylene [23, 38]. With increasing CuNP content, the α peak transition temperature () of CuNP/HDPE nanocomposite monofilament does not change much. When the CuNP content is 2.0 wt%, the peak widens significantly, indicating more amorphous structure in the nanocomposite monofilaments. CuNPs exert a destructive effect on the crystalline structure of polyethylene and increase the mobility of the polyethylene molecular chain. This finding is consistent with the crystallinity results.
The of CuNP/HDPE nanocomposite monofilaments move toward high temperature as frequency increases (Figure 6(c)). The relationship between and temperature is shown in Figure 6(d). The temperature dependence of of CuNP/HDPE nanocomposite monofilaments fits the Arrhenius equation. where is the pre-exponential factor, is the activation energy, is the Boltzman constant, and is the temperature.
By fitting of the Arrhenius equation, the relationship between the corresponding activation energy of α-relaxation () and the content of CuNPs can be obtained (Figure 6(e)). As the CuNP content increases, the value gradually increases, which is related to more limiting molecular motions near the crystalline regions.
3.6. Wear Resistance
Figure 7 shows the wear resistance of CuNP/HDPE nanocomposite monofilaments. The (wear broken times per unit linear density of CuNP/HDPE monofilament) of CuNP/HDPE-1.0 is 115% higher than that of pure HDPE monofilaments under the same wear test conditions. The of CuNP/HDPE-2.0 with the CuNP agglomeration only increases by 20.7% compared with that of HDPE monofilaments. Therefore, the content and dispersion form of CuNPs significantly affect the wear resistance of monofilaments.
3.7. The Release of Cu Ions in Sea Water
The Cu ions’ concentration is determined according to the standard correspondence between the Cu ion concentration and the solution absorbance which measured by ultraviolet-visible spectrophotometer. As a matrix material, HDPE provides a permeability channel for Cu ions. At the same time, the barrier of the matrix to CuNPs can effectively control the release rate of particles thereby controlling the release of the Cu ions . Nanocomposite monofilaments (0.5 and 1.0 wt%) of the same weight are immersed in quantitative seawater at room temperature to measure the concentration of Cu ions at different time periods and the Cu ion concentrations with different soak times are shown in Figure 8. Cu ion concentration increases with the increase of CuNP content. This is attributed to the increase in the amount of amorphous parts previously observed by DSC and DMA, and these amorphous parts could facilitate the diffusion of Cu ions. Such behaviors are in agreement with that of many other composites reported previously [43, 44], in which the ion release from polymer composites could be improved by lowering its crystallinity. The crystallinity affects the water uptake, which in turn controls the ion release. Moreover, the higher release of nanocomposite at higher CuNP content is also due to existing the larger amount of CuNP on the surface. In addition, it can be observed that CuNP/HDPE nanocomposite monofilament samples release a large amount of Cu ions in the first few weeks. One month later, the Cu ion release rate becomes weaker. This is because of the “burst effect” of Cu ions for composites .
3.8. Evaluation of the Antifouling Effect of Fishing Netting
CuNP/HDPE-1.0 nanocomposite monofilaments with optimal overall performance were twisted into a netting twine to prepare knotted netting with a mesh length of 25 mm. Coupon test was used to evaluate antifouling performance from May to October 2018 in the shallow sea area of the South China Sea.
As shown in the photos of several sets of netting in Table 1, the attached fouling organisms on CuNP/HDPE netting are less than the HDPE netting at the same period of time. After six months, HDPE netting is completely covered by the attached fouling organisms, and the surface of the CuNP/HDPE netting also has some shellfish and algae attached. The netting of the same area is cropped and weighed. The results show that CuNP/HDPE nanocomposite mesh shows lower weight gain than the HDPE mesh (reduced by 32%). Hence, the CuNP/HDPE netting has a certain repellent effect on attaching organisms.
We can conclude that the content of CuNP has significant effects on the morphology and properties of CuNP/HDPE nanocomposite monofilaments. When the CuNP content was ranging from 0.5 wt% to 1.0 wt%, CuNP presented nanoscale and uniform distribution in the HDPE monofilament matrix. And the , , , and of nanocomposite monofilaments were higher than that of HDPE monofilament. Moreover, the mechanical properties and wear resistance of CuNP/HDPE nanocomposite monofilaments with low CuNP contents were superior to those of HDPE monofilaments. When CuNP content increased to 2.0 wt%, the mechanical properties and wear resistance decreased resulting from the aggregation of CuNP. In addition, Cu ion concentration increased with the increase of CuNP content, which attributed to the increase in the amount of amorphous parts. Burst release of Cu ions was observed in the first few weeks. Coupon tests observed that CuNP/HDPE nanocomposite mesh showed less weight gain than HDPE mesh during the same period of time; therefore, the CuNP/HDPE nanocomposite netting had better antifouling performance than pure HDPE netting. This study highlights the CuNP/HDPE netting to use as a potential fishing netting to prevent fouling in the aquaculture cage nets.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 31502213, 31872611, and 31972844), Central Public-interest Scientific Institution Basal Research Fund, CAFS (Grant No. 2019CY0304), Special Scientific Research Funds for Central Non-profit Institutes (East China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences) (Grant No. 2019T04), and Earmarked Fund for China Agriculture Research System (Grant No. CARS-50).
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