INTRODUCTION

At present, wrapping films receive a special attention: materials are selected carefully; information placed on labels of wrapping is analyzed, etc. Polymer wrapping materials based on polyethylene are the most widespread owing to cost efficiency, light weight, compactness, and adequate strength [1]. Various images and texts are applied on the polymer films, mainly by means of gravure, flexographic, or screen printing. High quality of printings and firm fixation of colors on the polymer film is guaranteed by treatment of the surface by corona discharge. For this reason, there is a series of requirements that must be met during the printing processes, including mechanical strength to bending, fracture, and extension. Mechanical properties of polymer films were traditionally estimated in regard to such parameters as ultimate tensile strength σt, relative extension at rupture εr, and elastic modulus E [2].

The objective of the present study was to investigate the parameters of mechanical properties of wrapping polyethylene films and to select wrapping material with optimal properties.

EXPERIMENTAL

Ten common sets of polyethylene film samples (bags) with thicknesses ranging from 0.01 to 0.05 mm (three samples of each set) were investigated.

The type of polymer material was determined via IR spectrometry using a Nickolet Avatar 380 IR Fourier analyzer. The obtained spectra were processed by means of the device’s software OMNIC ver. 6.1a. The investigated materials were attributed to polyethylene by determining matching spectra with maxima at 2923.9, 2856.0, 1469.7, and 729.6 cm–1 with polyethylene spectra according to reference databases.

Static extension tests were performed according to the Russian Federation State Standard GOST 14236-81 for polymer films and film materials with thickness of up to 1 mm [3] under a deformation rate of 10 mm/min using an INSTRON 5982 universal testing machine, which complied with GOST 28840-90 [4]. The polyethylene samples with size of 20 × 150 mm had a working length of 100 mm. The relative measurement error was less than 1% of the measured values. The software package Bluehill 3 was used to control the testing process and to process the data.

RESULTS AND DISCUSSION

A polyethylene spectrum from the IR spectral library Hummel Polymer and Additives is shown in Fig. 1. All the spectra of the investigated film samples experimentally obtained via the IR Fourier analyzer demonstrated complete conformity with this spectrum.

Fig. 1.
figure 1

Library spectrum of polyethylene.

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Stress-strain curves were obtained as a result of the static tension tests. The general form of these curves for all the investigated samples was the same and is depicted in Fig. 2 by example of polyethylene samples with a thickness of 0.05 mm. The average values of mechanical parameters for each set of samples obtained during the tests are provided in Table 1. One can see that the mechanical properties of the polymer samples selected for the tests differed substantially.

Fig. 2.
figure 2

Dependences for the sample with thickness of 0.05 mm in coordinates: (a) load–transition; (b) stress–deformation.

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Table 1.   Results of tensile tests of wrapping polyethylene film
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Regression analysis of the results of investigation was performed by means of the application software package Statgraphics Centurion XV.I in order to establish general dependences of the mechanical parameters on the initial thickness of the samples. The following dependences of the mechanical properties of polyethylene films on the initial sample thickness were determined:

$$\begin{gathered} E = - 8619.99 + 2{\kern 1pt} {\kern 1pt} 124{\kern 1pt} 350H - 168{\kern 1pt} {\kern 1pt} 419{\kern 1pt} 000{{H}^{2}} \\ + \,\,5903790000{{H}^{3}} - 93{\kern 1pt} {\kern 1pt} 616{\kern 1pt} 700{\kern 1pt} 000{{H}^{4}} \\ + \,\,547{\kern 1pt} 512{\kern 1pt} 000{\kern 1pt} 000{{H}^{5}}; \\ {{\sigma }_{{\text{F}}}} = 14.075 + 404.058H - 163527{{H}^{2}} \\ + \,\,8{\kern 1pt} 644{\kern 1pt} 370{{H}^{3}} - 165{\kern 1pt} 979{\kern 1pt} 000{{H}^{4}} + 1{\kern 1pt} 075{\kern 1pt} 420{\kern 1pt} 000{{H}^{5}}; \\ {{\sigma }_{{\text{t}}}} = 159.305 - 24877.4H + 1623350{{H}^{2}} \\ - \,\,49{\kern 1pt} 529{\kern 1pt} 400{{H}^{3}} + 711{\kern 1pt} 729{\kern 1pt} 000{{H}^{4}} - 3{\kern 1pt} 892{\kern 1pt} 080{\kern 1pt} 000{{H}^{5}}. \\ \end{gathered} $$

The values of mechanical properties of the polymer materials were calculated according to these dependences on the initial thickness of the samples (Table 2). Good agreement was observed when these calculated data were compared to the experimental results (Figs. 3–5).

Table 2.   Calculated mechanical properties of polyethylene films
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Fig. 3.
figure 3

Dependences of Young’s modulus on initial thickness.

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Fig. 4.
figure 4

Dependences of conditional fluidity limit on initial thickness.

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Fig. 5.
figure 5

Dependences of ultimate tensile strength values on initial thickness.

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In order to select the optimal initial thickness, the Solver add-in of Microsoft Excel was applied using the generalized reduced gradient for the case of maximum strength value under limiting values: σF ≥ 6 MPa and E ≥ 1000 MPa. The results are shown in Table 3. It was established that the optimal value of thickness of polyethylene film was approximately 0.04 mm.

Table 3.   Optimal initial thickness with maximum strength value and the following limiting values: E ≥ 1000 MPa and σF ≥ 6 MPa
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The developed models can be applied for calculation of mechanical properties (strength, limit of stretching strain, extension, etc.) of films and other polymer materials under various conditions. This is crucial since, knowing the required size of the polymer wrapping, including bags made of polyethylene (as in our case) and the number of required articles, the product unit cost can be determined at a preset polymer thickness [5]. The obtained data can also be useful for development of low-waste technologies for processing materials with application of advanced nanocomposite coatings, lubricants, and polymer operating materials [5–13].

CONCLUSIONS

It has been established that the optimal thickness of a polyethylene wrapping film (bag) is about 0.04 mm.

Under conditions of the same sets and size of polyethylene bags, one should select bags with greater thickness and, therefore, higher extension strength.