Keywords

Introduction

Ethylene vinyl alcohol (EVOH) copolymers has been widely applied in food packing, formed by random sequences of ethylene and vinyl alcohol [1]. It is one of the lowest oxygen permeability reported among polymers, commonly used in packaging, due to their excellent barrier properties to oxygen and organic compounds [2, 3]. EVOH copolymers with 32 mol% ethylene is considered to have the superior gas barrier properties when compared to most of the polymeric materials, it is outstanding barrier to gas, odor, and flavor, EVOH presents good chemical resistance, high transparency, low thermal stability and harmlessness toward health [4, 5]. However, EVOH copolymers are severe hygroscopicity, and consequently your applications seriously limited. Such properties are caused by strong hydrogen bond interactions, both inter and intra-molecular, which reduce the free volume of the polymer chains [6,7,8]. High density polyethylene (HDPE) that is a thermoplastic resulting from the polymerization of ethylene, and is widely used in various industries due to its mechanical properties, chemical resistance, water impermeability, ease of processing and low cost. HDPE is an inexpensive commodity polymer, but its use in some engineering applications may depend on improving its properties by crosslinking [9].

Blending polymer can be an effective alternative for create new materials with enhanced physical properties and polymer diversification. The small quantity of a barrier material into a low-cost matrix material can lead to a low-cost product with a good improvement in barrier properties, where are prominent in the packaging industry. Environmental concerns have thoughts with EVOH application as a thermoplastic, combining high oil resistance with excellent transparency, superior gas barrier properties and easy processability [10]. Its hydrogen-bonded structure, though beneficial to low permeability, causes moisture absorption, which results in the deterioration of the same property. Thus blending with a hydrophobic polymer, like HDPE, may offset this drawback.

These requirements are met through use of multilayer structures consisting of different polymers, each contributing certain specific functions. Because of the stringent requirement of withstanding the rigors of high temperatures and pressures, relatively few polymers are suitable for retort applications. The most common retortable plastic containers are thermoformed from co-extruded multilayer structures consisting of polyolefins (e.g., polypropylene (PP) and polyethylene (PE)), high density (HD) at the surfaces, with an internal oxygen barrier layer. Relevant work reported by Kamal and coworkers on the PP/EVOH blend compatibilized with PP modified with maleic anhydride (PP-g-MA). The influence of various processing parameters on morphology, permeability and impact strength were examined for extruded films. In another work, Prasad and Jackson reported on the mechanical properties and morphology of melt-blended EVOH in an HDPE matrix using PE-g-MA as a compatibilized [11,12,13]. Nowadays the high-energy radiation is a powerful technique for the modification of polymers, some authors indicates that in certain blends systems the recombination reaction of different macromolecule free radicals induced by irradiation may occur in the interface area, so complex grafting or cross-linking structures may be formed. New structures can improve interphase bonding, and lead to a significant change in mechanical and thermal properties. According with Goulas et al. the effect of radiation on the properties of multilayer films containing EVOH/HDPE blends can be improved by ionizing radiation [14, 15].

Packaging scientists are working on developing solutions like to the moisture sensitivity problem. The incorporation of desiccants in the layers surrounding EVOH to reduce the amount of moisture reaching the EVOH layer, blending of EVOH with other materials to improve properties, use of nanocomposites, etc. [16]. The structural uniqueness of graphite can greatly improve the thermal, mechanical, and gas barrier properties of a polymer if the graphite is well dispersed in the polymer matrix. Graphene oxide is a monolayer of carbon atoms, which may be obtained from exfoliation of graphite and is considered an ultrathin, perfect two dimensional (2D) barrier against gas diffusion [17,18,19,20]. Recently, graphene oxide (GO), which have been prepared by chemically oxidizing graphite to graphite oxide with strong oxidants and ultrasonic cleavage for graphene oxide nanosheets, has gained significant attention for incorporation in different polymers for improve the gas barrier and mechanical properties. Because GO has a large number of polar groups such as hydroxyl, ether, and carboxylate groups, intercalation of water-soluble polymers in GO is possible. EVOH/GO nanocomposites can present enhanced molecular ordering and the long tortuous pathway for water and gas molecules may explain the good water/oxygen barrier properties in flexible films [21].

This study aims to evaluate morphology, thermal, mechanical and barrier properties of HDPE/EVOH/EVA/GO flexible films compatibilizer by ionizing radiation.

Experimental

Material

  • Graphene oxide

  • (GO) nanosheets prepared from conventional flake graphite [15, 16];

  • Ethylene vinyl alcohol copolymer (EVOH) with 32% mol/ethylene (EVAL™ manufactured by Kuraray Co. Ltd.);

  • High density polyethylene (HDPE);

  • Ethylene Vinyl Acetate (EVA).

Preparation of HDPE/EVOH /EVA/GO Flexible Films

The HDPE/EVOH/EVA/GO films were processed by twin-screw extrusion and extrusion blown film processing. The HDPE/EVOH/EVA/GO flexible films with addition of 0.1 wt% of GO nanosheets were prepared by melting extrusion process, using a twin-screw extruder Haake Rheomex P332 with 16 mm and L/D = 25 rate from Thermo Scientific. The temperature profile was of 182/192/197/197/205/205 °C and a screw speed of 30 rpm. The extrudates materials were cooled down for a better dimensional stability, pelletized by a pelletizer, dried again and fed into extrusion blown film, single screw machine (Carnevalli) with 25 mm diameter and flexible film test samples were obtained. The temperature profile used in the blow extrusion process of HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO flexible films were 190/195/210/215/215/220 °C and screw speed was 30 rpm (Table 1).

Table 1 DSC analysis results of EVA/EVOH/HDPE/GO Nanocomposite Flexible Films
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Electron-Beam Irradiation

Part of HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO flexible films were submitted to electron-beam irradiation at 150 kGy, using a 1.5 meV electron beam accelerator, at room temperature in presence of air.

Characterization of GO Nanosheets and HDPE/EVOH/EVA/GO Flexible Films

Mechanical tests: Tensile tests were determined using an INSTRON Testing Machine model 5564, according to ASTM D882-91 in order to evaluate the mechanical behavior of the materials studied. Each value obtained represented the average of five samples.

Differential scanning calorimetry (DSC): analyses were carried out using a Mettler Toledo DSC 822e from 25 to 250 °C at a heating rate of 10 °C/min under nitrogen atmosphere (50 ml/min). DSC analyses of the materials were performed on four samples of the materials. DSC were carried out to obtain melt temperature (Tm) and melting enthalpy (ΔHm).

Thermogravimetric analysis (TG): In this study the TG analyses were made in a Mettler Toledo TGA module “TGA/SDTA851e” from 25 to 500 °C at a heating rate of 10 °C/min under nitrogen atmosphere (50 ml/min).

Field emission scanning electron microscopy (FE-SEM): FE-SEM of irradiated and non-irradiated HDPE/EVOH /EVA and HDPE/EVOH/EVA/GO samples cryofractured under liquid nitrogen were carried out using a JEOL-JSM-6701F, microscope with an accelerating voltage of 1–30 kV, using EDS Thermo-Scientific mod. Noran System Six software, in carbon sputtered samples.

Results and Discussion

DSC analysis results of HDPE/ EVOH and HDPE/EVOH/GO flexible films: Fig. 1 shows the DSC analysis results for HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO flexible films.

Fig. 1
figure 1

DSC analysis results for HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO flexible films

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Thermogravimetric analysis (TG) results: Fig. 2 shows the TG thermograms of HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO flexible films. As can be seen in Fig. 2, the HDPE/EVOH/EVA/GO showed a great reduction in the onset temperature when compared with HDPE/EVOH/EVA without GO addition.

Fig. 2
figure 2

TG analysis results for HDPE/EVOH and HDPE/EVOH/GO flexible films

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The decomposition temperatures and weight loss for HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO flexible films are presents in Table 2.

Table 2 Decomposition temperature and weight loss of HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO flexible films
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Field Emission Scanning Electron Microscopy (FE - SEM) analysis results: FE-SEM micrographs of cryofractured surfaces of irradiated and non-irradiated HDPE/EVOH/EVA blend and HDPE/EVOH/EVA/GO nanocomposite were studied to understand the failure mechanisms and also study possible interaction between different components after electron-beam irradiation. FE-SEM micrographs of HDPE/EVOH/EVA blend and its composite are showed in Fig. 3.

Fig. 3
figure 3

a HDPE/EVOH/EVA blend (2400×), b HDPE/EVOH/EVA blend at 150 kGy (1600×), c HDPE/EVOH/EVA/GO nanocomposite (2500×), d HDPE/EVOH/EVA/GO nanocomposite at 150 kGy (2000×)

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The micrographs of irradiated Blend and nanocomposite showed better compatibilization between the different components when compared with non-irradiated samples.

Tensile tests results: Table 3 presents the results of tensile tests of the HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO flexible films. The results presented shows the average values calculated from the data obtained in tests for five test specimens.

Table 3 Tensile test results of the HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO flexible films
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Conclusions

The results showed that the incorporation of GO nanosheets led to an important increase in Tensile strength at break and Young’s modulus without an important variation in Elongation at break properties. The FE-SEM micrographs of irradiated HDPE/EVOH/EVA and HDPE/EVOH/EVA/GO nanocomposite showed better compatibilization between the different components when compared with non-irradiated samples.