Infusing High-density Polyethylene with Graphene-Zinc Oxide to Produce Antibacterial Nanocomposites with Improved Properties

Nanocomposites of high-density polyethylene (HDPE) modified with 0.2 phr graphene-zinc oxide (GN-ZnO) exhibited optimal mechanical properties and thermal stability. Two other nano-materials—GN and nano-ZnO—were also used to compare them with GN-ZnO. Increasing the content of GN-ZnO gradually enhanced the antibacterial and barrier properties, but the addition of 0.3 phr GN-ZnO led to agglomeration that caused defects in the nanocomposites. Herein, we investigated the antibacterial and barrier properties of HDPE nanocomposites infused with different nanoparticles (GN, ZnO, GN-ZnO) of varying concentrations. HDPE and the nanoparticles were melt-blended together in a Haake-Buchler Rheomixer to produce a new environment-friendly nano-material with improved physical and chemical properties. The following characterizations were conducted: tensile test, thermogravimetric analysis, morphology, differential scanning calorimetry, X-ray diffraction, antibacterial test, and oxygen and water vapor permeation test. The results showed that the crystallinity of HDPE was affected with the addition of GN-ZnO, and the nanocomposites had effective antibacterial capacity, strong mechanical properties, high thermal stability, and excellent barrier performance. This type of HDPE nanocomposites reinforced with GN-ZnO would be attractive for packaging industries.


INTRODUCTION
Polymers are widely used in the beverage and food packaging industries because of their low density, low cost, substantial chemical inertness, and easy processability. [1][2][3] One of the most important requirements for packaging is high barrier performance. Plastics with excellent barrier properties are used in industrial applications that process materials which include medical and pharmaceutical products, electronic gadgets, and chemicals. [4][5][6] Polyolefins (e.g., polyethylene (PE) and polypropylene) are one type of the most widely used polymers in many fields including food packaging industries, agricultural firms, water piping, and tank-container storage. [7][8][9][10] As a polymer matrix, high-density PE (HDPE) exhibits strong mechanical properties, and it has a combination of attractive features such as low production cost and high resistance to thermal deformation. [11] Nanocomposites consist of a polymer matrix and an organic or inorganic filler in the nanometer scale (1-100 nm). [12,13] At present, improvements in the properties of polymers (PE or HDPE, among others) are beneficial to numerous applications. The integration of nano-fillers into polymers has attrac-ted substantial interests.
Graphene (GN) was isolated in the mid-20 th century. [14] Since then, polymeric nanocomposites containing GN and GN derivatives have been the subject of intense investigations. The reason is that they exhibit excellent electrical properties and thermal conductivity, as well as high aspect ratio. [15][16][17] Nanocomposites of polar polymers with GN and its derivatives have been widely studied. Examples of these polymers are as follows: poly(methyl methacrylate), [18,19] poly(vinyl alcohol), [20,21] poly(vinyl chloride), [22,23] and poly(ethylene terephthalate). [24,25] Few studies have been conducted on the addition of GN or a derivative of GN to nonpolar polyolefins such as polystyrene [26] and polypropylene, [27][28][29] because dispersing it in melt-blended nonpolar polymers is a major challengenano-sheets have thermodynamic tendencies to form aggregates. [30] Because of the agglomeration, the interfacial adhesion in nonpolar polymers is poor (unlike in the case of polar polymers). Therefore, the modification of GN is an essential strategy to effectively mix GN with polyolefins.
Nanoparticles of metal oxides have high surface energies, small sizes, large surface atomic ratios, and large specific surface areas. [31,32] For these reasons, they have been widely used in several fields such as photocatalysis, [33] sunscreen cosmetics, [34,35] antibacterial materials, [36] and solar batteries. [37] Zinc oxide (ZnO) is an n-type semiconductor with a band gap of 3.37 eV. The free exciton binding energy of ZnO at room temperature is greater than 60 MeV. [38,39] Its special photoelectric properties make it one of the most important semiconductors in the field of visible light bacteriostasis and photocatalytic deoxidation. [40,41] When the size of ZnO is reduced to a nanometer level, it can adsorb bacteria on its surface and inhibit the growth of most bacteria or even kill them. [42] This activity is largely attributed to the excellent hydrophobicity and oxidation capacity and high specific surface area of ZnO. [43] Introducing additives (cross-linking agents) and inorganic fillers (GN, carbon nanotubes, ZnO) to PE or HDPE can improve the properties and performance of the polymer. [44][45][46] GN can effectively reinforce the polymer, but it is a weak antibacterial agent; thus, using it as a filler, especially in small amounts, would impart little or no antibacterial properties. On the other hand, nano-ZnO has an excellent antibacterial capacity, albeit its poor reinforcement ability. Combining the advantages of these two nano-materials would therefore make an effective nano-filler in the form of complexes of GN-ZnO.
The addition of GN-ZnO to HDPE produced a new nanocomposite (GN-ZnO/HDPE), which should supersede the original materials. Physical methods were used to infuse the conductive filler into the polymer. GN-ZnO/HDPE had advantages in terms of physical and chemical properties and synergistic effects, and it had antibacterial and conductive properties.
To evaluate the efficiency of the new nano-filler (GN-ZnO), it was compared with GN as a single filler, as well as with nano-ZnO alone. The mechanical properties, thermal stability, and antibacterial capacity of GN/HDPE, ZnO/HDPE, and GN-ZnO/HDPE were examined. The HDPE nanocomposites would have potential applications as packaging materials because of improved tensile and barrier properties, crystallinity, and antimicrobial activity.

EXPERIMENTAL Materials
HDPE, known for its suitability in food packaging, was supplied by China Petroleum and Chemical Corporation. It was in a white granular form, with a density of 0.955 g·cm −3 . GN-ZnO (a specially-tailored new nano-filler) was custom-made by Apex Nano Co., Ltd. (New Taipei City, Taiwan Province, China). The ratio of GN to ZnO was roughly 1:2, which was evaluated using energy-dispersive X-ray spectroscopy (EDS) (Fig. S1 and Table S1 in the electronic supplementary information, ESI).

Preparation of Nanocomposites
A Haake-Buchler Rheomixer (Type 600), with a volume of 69 cm 3 , was used to blend HDPE with different nanoparticles of varying concentrations (0.1, 0.2, and 0.3 phr) at 170 °C and 120 r·min -1 for 5 min to produce nanocomposites. The following nano-fillers were considered: GN, ZnO, and GN-ZnO. The melting temperature and torque were recorded by a computer. Nanocomposites of HDPE with 0.1, 0.2, and 0.3 phr GN-ZnO were designated as 0.1phrGN-ZnO/HDPE, 0.2phrGN-ZnO/HDPE, and 0.3phrGN-ZnO/HDPE, respectively.

Mechanical testing
The mechanical properties of all samples were determined with the use of an FBS10KNW electronic universal testing machine (Xiamen Forbes Tensile Equipment Co., Ltd., China) operated at a speed of 50 mm·min −1 , according to the GB/T1040-2006 standard for testing the tensile strength of plastics. At least 5 samples were measured, and the obtained data were averaged. The total length of the dumbbell-shaped tensile samples was 115 mm, and the tensile section had the following dimensions: length, 30 mm; width, 4.3 mm; thickness, 1.4 mm. The load was applied along the axial direction until the sample broke. Tensile strength was measured at a certain test temperature and humidity.

Thermal gravimetry
A thermal gravimetric analyzer (Model HTG-1, Beijing Hengjiu Experimental Equipment Co., Ltd., China) was employed to evaluate the decomposition behavior of the samples. The heating rate was set equal to 10 °C min −1 . Heating was started from room temperature, and was stopped when the temperature reached 750 °C. Origin software was run to obtain a diagram of differential thermal gravimetry (DTG) for the tested samples (a plot of temperature versus rate of weight loss).

Field emission scanning electron microscopy-energydispersive X-ray spectroscopy
The morphology and elemental surface analysis of the HDPE nanocomposites were characterized using field emission scanning electron microscopy-EDS (FESEM-EDS, MIRA3 FEG-SEM, Tescan, Czech). Before the samples were examined, they were sputtered with gold dust to make them conductive.
In EDS, the characteristic X-ray energy emitted by different elements is unique, and the unique energy for Zn was the basis for determining its dispersion in the nanocomposites.

Differential scanning calorimetry
Data on the crystallization and melting temperatures, as well as the crystallization and melting enthalpies of the nanocomposites were recorded through differential scanning calorimetry (DSC, 200F3, Netzsch, Germany). At a rate of 10 °C·min −1 , heating was begun from room temperature to 180 °C. Then, the temperature was held at 180 °C for 3 min. Afterward, cooling was applied from 180 °C down to room temperature at a rate of 10 °C·min −1 . Samples and reference materials were separately placed in two crucibles, and they were heated at the same rate of 10 °C·min −1 . To eliminate the influence of previous thermal history of the samples, heating and cooling were conducted twice.

X-ray diffraction
The degree of crystallinity of the nanocomposites was investigated using X-ray diffraction (XRD, D8 Advance, Bruker, Germany) equipped with monochromatic Cu-Kα radiation (λ = 0.154 nm) under a voltage of 40 kV and a current of 40 mA. All samples were analyzed in continuous scan mode, with 2θ ranging from 5° to 90°.

Antibacterial test
The antimicrobial properties of GN/HDPE, ZnO/HDPE, and GN-ZnO/HDPE were tested on Escherichia coli. A qualitative analysis was performed. The main steps were as follows.
(1) Solid medium configuration. Beef paste peptone was sterilized at a high temperature and pressure in a kettle at 121 °C for 1 h. Then, the liquid was poured into a disposable Petri dish to a height one-third of the dish.
(2) Inoculation with E. coli. E. coli was used to activate the liquid medium (activation took 24 h). The culture medium was cooled and placed in an ultraviolet incubator. E. coli was inoculated with beef paste peptone through a dilution-coating method, and it was placed in a constant temperature and humidity incubator set at 37 °C and 30% humidity.
(3) Gathering of experimental results. After the bacterial culture that lasted for 24 h, bacteriostasis in the Petri dish was observed, and experimental data were recorded.

Water vapor transmission tester
To measure the water vapor permeability, a transmission tester (Labthink W3/060, Jinan, China) was used. The test environment temperature and humidity were controlled. As such, a fixed humidity difference was attained across the sample. The water vapor passed through the sample toward the dry side. Hence, the moisture was reduced, and the weight of the sample cup decreased. Analysis of the sample was conducted to determine the water vapor permeability and the permeability coefficient. The test interval was 30 min, with the temperature set at 25 °C and the humidity maintained at 90%.

Oxygen permeameter
Pretreated samples were sandwiched between the upper and the lower test chambers of a differential pressure gas permeameter (Labthink VAC-V2, Jinan, China). First, the low-vacuum chamber was emptied. Then, the high-pressure chamber was filled with oxygen at a certain pressure. Thus, the gas permeated from the high-pressure chamber to the low-pressure chamber under the action of a pressure difference. The permeability coefficient of the tested sample was obtained by measuring the lateral pressure at a low pressure. Degassing time in the chamber was 5 h. The operating temperature was controlled at 23 °C, with the humidity set at 50%. Table 1 presents a comparison of the mechanical properties of HDPE modified with different nanoparticles (GN, ZnO, and GN-ZnO). Nanocomposites even with a low content of 0.1 phr GN indicated improvement in the properties, relative to neat HDPE, signifying that GN had a reinforcing effect. By contrast, HDPE filled with only ZnO (at all concentrations of 0.1, 0.2, and 0.3 phr) exhibited inferior tensile strength and elongation at break, demonstrating that ZnO effected no reinforcement because it failed to bind with the nonpolar HDPE. [47] A combination of GN and ZnO as GN-ZnO nano-fillers also made HDPE tougher. For example, the HDPE nanocomposite with 0.2 phr GN-ZnO yielded a tensile strength that was 10% higher than that of neat HDPE, and an elongation at break that was 62% higher. However, adding 0.3 phr GN-ZnO reduced the mechanical properties; hence, 0.2 phr GN-ZnO could be considered the optimum content. A high concentration of the nano-filler would cause agglomeration, leading to defects in the nanocomposite.

Mechanical Properties
The superiority of GN-ZnO/HDPE over the other two nanocomposites can be better visualized in Fig. 1. For each nanocomposite, the trend of data on tensile strength (Fig. 1a) was consistent with that on elongation at break (Fig. 1b). A peak occurred at 0.2 phr GN-ZnO (i.e., the mechanical properties were optimal). The tensile strength peaked at 30 MPa, whereas the elongation at break peaked at 52.43%. For the case of 0.3 phr GN-ZnO, the dip in tensile strength or elongation at break might be due to defects because of the agglomeration. Fig. 2 illustrates the DTG curves for HDPE modified with different nano-fillers of varying concentrations (inset shows magnified peaks). DTG is the first-order differential weight loss with respect to temperature or time. The lowest peak is associated with the fastest rate of weight loss, and a peak corresponds to the decomposition temperature. HDPE had the lowest thermal decomposition temperature (480.0 °C). The thermal degradation temperatures of GN/HDPE nanocomposites were 483.0, 480.5, and 475.5 °C when the content of GN was 0.1, 0.2, and 0.3 phr, respectively. If ZnO replaced GN as the nano-material present in the nanocomposites, the corresponding thermal degradation temperatures were 478.7, 482.1, and 484.9 °C.

Thermal Decomposition Temperatures
In the case of GN-ZnO as the nano-filler, varying its content from 0.1 phr to 0.3 phr contributed to a consistent increase in the thermal decomposition temperature relative to that of neat HDPE. The inset demonstrates that the thermal decomposition temperatures were 482.5, 485.0, and 485.0 °C at 0.1, 0.2, and 0.3 phr GN-ZnO, respectively.
In summary, our specially-tailored new nano-filler (GN-ZnO) effected the greatest improvement in the thermal stability of HDPE, indicating its advantage over the single nano-materials (GN and ZnO). Fig. 3 illustrates the FESEM images of fractured surfaces of neat HDPE and GN-ZnO/HDPE nanocomposites. The surface of HDPE resembled untangled vertical threads of fiber (Fig. 3a). Modifying HDPE by blending it with even a small amount of GN-ZnO drastically changed its morphology to that of a dense structure. In Fig. 3(b), 0.1 phr GN-ZnO apparently cemented the loose fibers of HDPE. The nanocomposite morphology became denser with the addition of 0.2 phr GN-ZnO (Fig. 3c). However, the presence of a greater amount of GN-ZnO (0.3 phr) led to agglomeration (Fig. 3d).

Morphology
EDS was used to analyze the distribution of Zn in GN-ZnO/ HDPE nanocomposites, as a way of demonstrating the dispersion of the nano-fillers.    which was unfilled with GN-ZnO; therefore, no Zn was detected in the EDS image (Fig. 4a'). Zn was uniformly dispersed in the nanocomposites when the concentrations of GN-ZnO were 0.1 phr (Fig. 4b') and 0.2 phr (Fig. 4c'). However, when the content of GN-ZnO increased to 0.3 phr, agglomeration occurred (Fig. 4d').

Differential Scanning Calorimetry Data
The data in Table 2 (obtained from Fig. 5) show that the melting point of each GN-ZnO/HDPE nanocomposite was lower than that of neat HDPE, because GN-ZnO promoted nucleation (or the generation of a higher number of crystal core). Enhanced nucleation hindered crystallization; thus, crystallization temperatures were higher. A slower crystallization rate caused HDPE to melt at a lower temperature in the heating process. From Table 2
According to Fig. 6 and Table 3, GN-ZnO registered only one characteristic peak at 11.06°, and the characteristic peaks of HDPE were 21.23°, and 23.51°. With the addition of GN-ZnO, shifts in the characteristic peaks of the resultant nanocomposites were only small, and the characteristic peak of ZnO was not observed. A probable reason is that only small amounts of GN-ZnO were added, so the presence of ZnO could not be detected.  Antimicrobial Activity Fig. 7 shows images of E. coli colonies in each dish, arranged in rows and columns. HDPE nanocomposites containing different nano-fillers, namely GN, ZnO, and GN-ZnO, were aligned in the first, second, and third rows, respectively. The samples were grouped in four columns according to the nano-filler content (0, 0.1, 0.2, 0.3 phr): the first column, HDPE; the second column, 0.1phr(nano-filler)/HDPE; the third column, 0.2phr-(nano-filler)/HDPE; the fourth column, 0.3phr (nano-filler)/HDPE.
In the middle of each dish was the sample with a round shape. HDPE by itself and GN/HDPE nanocomposites had no antibacterial effect. However, the nanocomposites containing ZnO and GN-ZnO had effective inhibitory action on the growth of E. coli. Higher amounts of ZnO and GN-ZnO had more effective antibacterial effects, as indicated by the growing dimension of inhibition area around the sample. Because of the strong photocatalytic ability of ZnO, it exhibited greater antibacterial activity than GN-ZnO. Fig. 8 plots data on instantaneous (Fig. 8a) and average (Fig. 8b) water vapor permeability coefficients for HDPE and its nanocomposites. Fig. 8(b) indicates a peak at 0.1 phr content of GN-ZnO. A higher content led to a decrease in the permeability coefficient, the lowest being at 0.3 phr GN-ZnO. The average    (Fig. 8b) might be attributed to the hydrophobicity of the polyolefin HDPE polymer and its lack of functional groups. As such, ZnO did not readily bind with HDPE, and interfacial defects were formed between HDPE and GN-ZnO. However, the nanocomposites were much less permeable than neat HDPE.

Water Vapor Permeability
The permeability of 0.2phrGN-ZnO/HDPE was lower than that of neat HDPE, which may be due to the dispersion of GN in HDPE. Fig. 9 depicts that the presence of GN-ZnO lengthened the path of water vapor permeation and enhanced the barrier performance of the nanocomposite.

Oxygen Permeability
The effect of filling HDPE with GN-ZnO was to lower the oxygen transmission coefficient (Fig. 10). The lowest transmission coefficient was when 0.2 phr GN-ZnO was added to HDPE. However, the addition of 0.3 phr GN-ZnO led to an increased transmission coefficient of oxygen. The reason is the agglome- ration of GN at this high content of GN-ZnO. Hence, the pathway for the passage of oxygen was shorter. Nonetheless, the coefficient was still lower than that for neat HDPE.

CONCLUSIONS
In this present study, HDPE was selected as the polymer matrix and GN-ZnO as an additive or a filler for modifying HDPE. The obtained nanocomposites were examined in terms of the following parameters: mechanical properties, morphology, thermogravimetric data, crystallinity, antimicrobial activities, and barrier performance. The results are summarized as follows: (1) The tensile strength and elongation at break of the nanocomposites were considerably improved when 0.2 phr GN-ZnO was added, but increasing the concentration further to 0.3 phr led to the reduction of the mechanical properties. At this higher concentration, FESEM images showed the incompatibility between HDPE and GN-ZnO.
(2) The thermal decomposition temperature of HDPE gradually increased with the addition of GN-ZnO. Hence, filling HDPE with certain amounts of GN-ZnO improved the thermal stability of the nanocomposites.
(3) The crystallinity of HDPE decreased when GN-ZnO was incorporated, which may be caused by the incomplete crystallization of the HDPE nanocomposites.
(4) Nanocomposites of HDPE containing different nanoparticles (GN, ZnO, GN-ZnO) exhibited different degrees of inhibitory effects on the growth of E. coli. The addition of GN did not show any antibacterial property, but modifying HDPE with ZnO and GN-ZnO produced an effect. The antibacterial activity of ZnO/HDPE was stronger than that of GN-ZnO/HDPE. Adding a higher content of GN-ZnO demonstrated superior bacterial inhibition.
(5) The addition of GN-ZnO could reduce the water vapor and oxygen permeability and enhance the barrier performance of the nanocomposites.

Electronic Supplementary Information
Electronic supplementary information (ESI) is available free of charge in the online version of this article at https://doi.org/10.1007/s10118-020-2392-z.

ACKNOWLEDGMENTS
The authors would like to acknowledge the financial support from the following organizations: Wuliangye Group Co., Ltd.