Mechanical and physical properties of polyethylene/sour cherry shell powder bio‐composite as potential food packaging

Abstract Eco‐friendly composite materials have received more attention in recent years, and research has shown that biostructures have great potential as a solution to meet the needs of sustainability in product design and potential food packaging. In this study, the mechanical, thermal, morphological, water absorption properties and water vapor permeability of bio‐composites based on polyethylene and sour cherry shell powder (SCS) (0% –7.5%) have been investigated. It was observed that 2.5% of sour cherry shell increased elastic modulus and tensile strength and improved mechanical properties. Composites without adding sour cherry shell show 1.28% water absorption. Decreased water absorption was observed for treated composites containing 2.5% of sour cherry shell, and its amount was 1.26%. The presence of sour cherry shell increased the bio‐composite resistance to moisture absorption, but the addition had little effect on the thermal process properties of polyethylene. Vapor permeability in sour cherry shell / polyethylene bio‐composites, which was a significant difference between samples with sour cherry shells (2.5%–7.5%) and samples without sour cherry shells. Results indicated that polyethylene/sour cherry shell composites could be used to replace polyethylene in application such as stretch film, shrink film, and bags of fruit.


| Bio-composite preparation
Sour cherry shells were dried in the shade at ambient temperature.
After breaking the seed, the separated shells were crushed and powdered with a blender (Moulinex, Spain) and sifted with a 325 U.S. MESH sieve. According to the (Table 1), PE mix (as a matrix contains 83% of low-density polyethylene, 13% of linear low-density polyethylene, and 4% of maleic anhydride polyethylene ) and SCS (0%-7.5%) were extruded ( at 165°C, pressure of 85 bar and screw speed of 100 rpm into a double-helical extrusion machine (ZSK25, Germany).
Then, the bio-composite was prepared using a single-screw extruder with blowing die film (HAK, Germany) at a temperature of 140 to 170°C with a screw speed of 37 rpm and tensile speed of 100 m / min. The calculated inflation rate was 500%.

| Mechanical testing
Tensile test was performed using Universal Testing Machine (model 1,432, China) at a speed of 5 mm/min according to ASTM D638. The composite sheet was cut into dumbbell-shaped specimens using a dumbbell cutter (Leader Technology Scientific (M) Sdn. Bhd., Balakong, Malaysia). For each composition, 3 dumbbellshaped samples (were prepared 20*20 cm 2 ) were cut and labeled.
The thickness of each specimen had an average thickness of 1.00 mm and was measured using digital thickness gages (JD200, Checkline). The film was subjected to a 500 N load cell, 1,000 mm of extension range, with 26 mm of gauge length, and a crosshead speed of 5 mm/min until the specimen fractured. All tests were conducted at room temperature, and the average values for the tensile strength and elongation at the break of the 3 repeated specimens were summarized for each composition from the stress-strain curve.

| Moisture absorption
Moisture absorption of bio-composite was analyzed based on Angles and Dufresne (2000) method. Samples of bio-composite with dimensions of (20 × 20) mm 2 were prepared and placed in a desiccator containing calcium sulfate with relative humidity (RH) 0% for 24 hr.
After initial weighing, the samples were transferred to a desiccator containing a saturated solution of calcium nitrite at RH = 55% and placed at a temperature of 20-25°C. Then, the weight of the samples was measured at different times until reaching a constant weight and the amount of moisture absorption was calculated from the follow-

| Water Vapor Transmission Rate
Water vapor transmission rate (WVTR) was measured by Javanmard (2008) method. In summary, the bio-composite was tested on a cup containing 12 ml of distilled water. Within 2 hr, steady-state conditions were assumed to have occurred. The cells were stored in a temperature and humidity controlled room (a/c unit supplied by Denco Ltd., Herts.UK; conditions: 50 ± 5 percentage relative humidity, 23 ± 2°C), and a fan set at an air velocity of 154 mm/min was placed over the cells to ensure uniform movement of air. Eight weight measurements were then recorded over a 24 hr period with intervals of greater than 1.5 hr between readings. At least three replicates of the film samples were tested.
where WVTR is water vapor transmission rate (g H2O mm cm −2 ), x is the average thickness of the film (mm), and A is the permeation area (cm 2 ).

| Thermal Properties
The samples were subjected to thermal analysis (differential scanning calorimetry) by a Toledo machine (SDTA 851, Switzerland). The rate of temperature increase was selected linearly at 20°C /min. The atmosphere used was nitrogen gas, and the applied temperature was from ambient temperature up to 500°C. The results of thermal analysis were expressed in a table (4).

| Morphological analysis
The tensile fracture surfaces of PE/SCS bio-composites with filler contents of 0, 2.5, 5, and 7.5% were examined with a scanning electron microscope (SEM) (model WEGA-II TESCAN, Czech Republic).
Samples surface to avoid electric charge during experiments were coated with gold for 10 nm and analyzed at 15 keV.

| Statistical analysis
Results were expressed as the mean and standard deviation of three independent replicates. All the data were statistically analyzed using one-way analysis of variance (ANOVA) through Duncan post hoc at p <.05. All of the statistical analyses were performed using Minitab software version 16. Figure 1 shows the stress-to-strain diagram from the tensile test for four samples, in which the vertical axis represents the stress and the horizontal axis shows the strain. Tensile strength in the reinforced polymer containing 2.5% of sour cherry shell powder was the highest value. This resistance decreased with increasing amount of sour cherry shell powder, and the lowest tensile strength was seen in the composite containing 7.5% of filler. Due to the curved slope the stress-strain curve, the reinforced polymer containing 2.5% of sour cherry shell had the highest elastic modulus (Table. 2). The effect of increasing sour cherry shell powder was loading on the tensile strength of PE/SCS composites.  (Fu et al., 2008). Tensile strength for polypropylene composite reinforced with Argan nuts shell was improved with increasing particle loading compared to neat polypropylene (Essabir et al., 2013). Muniyadiet al. (2018)      composites as compared to control sample. This indicates that the presence of sour cherry shell increases the bio-composite resistance to moisture. As the percentage of sour cherry shell increases to 7.5%, moisture absorption increases. This result shows that adding sour cherry shell to a certain extent can improve the moisture barrier property and will have the opposite effect in higher percentages. Vilaseca et al., (2007) reported similar results of the effect of hemp fibers on reducing the moisture absorption of starch film. The properties of luffa fiber-treated composites showed that the reduction in water absorption was due to better adhesion between the fiber and the matrix (Demir et al., 2006). Hussein et al., (2011 concluded that composites (high-density polyethylene/egg shell powder) with higher filler content show more water absorption. Table 3 shows the mean of water vapor transmission rate in sour cherry shell / polyethylene bio-composites. There was a significant difference between bio-composites loaded with sour cherry shells powder (2.5%-7.5%) and samples without sour cherry shells. The results showed that the addition of sour cherry shells to PE matrix reduced water vapor transmission rate. Alias et al., (2017) showed that by adding palm kernel shell powder, water vapor permeability in polyvinyl alcohol composites was reduced. This is due to the structure and size of the palm kernel shell powder and the smaller distance between the chains. Table 4 illustrates the DSC results for the bio-composite containing 0 to 7.5% SCS powder. As the percentage of sour cherry shells in the composite increases, the melting temperature and crystallization temperature increase. Enthalpy of melting and degree of crystallinity also showed a decreasing but slow trend with increasing containing of sour cherry shell powder. As the SCS powder increased above 2.5 wt.%, the segmental mobility of the PE chains reduced due to stiffening in the PE network, which gradually hinders the rate of crystallization; therefore, the T c and T m decreased slightly and resulted in the enthalpy of melting and degree of crystallinity gradually reduced. The SEM morphology also confirmed the presence of SCS powder agglomerates at PE/SCS-7.5%, which could inhibit the crystallization rate and result in a reduction in the degree of crystallinity. Muniyadi et al., (2018) showed that the addition of Mimusops elengi seed powder to polypropylene has little effect on thermal properties.

| Morphological analysis
SEM micrographs of the polyethylene/sour cherry shell powder composites surface are shown in Figure 4. In Figure 4a, the film sheet containing no sour cherry shell powder shows a uniform surface.  background due to the filling of the holes in the polymer matrix. The effect of bio-composites as fillers in bio-composites containing other agricultural wastes has also been reported (Qian et al., 2018). Also in this figure, a relatively good distribution of sour cherry shell powder particles in the polymer sheet was observed. Figure 4c shows a composite film containing 5% by weight of sour cherry shells powder.
The particles are not well dispersed, and empty cavities are seen in the polymer matrix. Figure 4d shows composite containing 7.5% by weight of sour cherry shells powder. Polymer reinforced with high concentration of sour cherry shells powder (7.5%), causing agglomeration of particles in the composite film. This agglomeration reduces tension and elongation at the breaking point. According to Muniyadi et al.(2018),

| CON CLUS ION
In this work, polyethylene-based (low-density polyethylene 83%, linear low-density polyethylene 13%, and maleic anhydride polyethylene 4%) bio-composites reinforced with sour cherry shell powder at 0, 2.5, 5, and 7.5 wt % contents were prepared by melt blending in extruder. Addition 2.5% of sour cherry shell powder to polymer matrix improved mechanical and moisture absorption properties of the Bio-composite film compared to film without sour cherry shell powder. The presence of sour cherry shell improved the tensile properties of the composite by 26%, too. Sour cherry shell powder loading reduced the water absorption in the composites at 2.5% wt % and increased at 5 and 7.5 wt % contents. Morphological evaluation of the bio-composite showed that the sour cherry shell powder was well bonded to the matrix field.
The results of the present study showed that the developed bio-composites reinforced with sour cherry shell waste could be a new approach to use the value-added agricultural waste and reduce environmental pollution and less use of oil resources polymers. Based on the results, it is suggested that these composites can be used in the food industry such as stretch film, shrink film, and bags of fruit.

ACK N OWLED G M ENTS
The authors thank the Iranian Research Organization for Science & Technology (IROST) and Iran Polymer and Petrochemical Institute (IPPI) for technical and laboratory support. This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

CO N FLI C T O F I NTE R E S T
The authors declare that they do not have any conflict of interest.

E TH I C A L S TATEM ENT
Ethics approval was not required for this research.

DATA AVA I L A B I L I T Y S TAT E M E N T
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