Obtaining and studying properties of biodestructable composite films based on polyethylene

At present, polyolefins are a heavily produced thermoplastic, and they are used in different industries. Approximately one-third of produced plastics are used in the packing industry, and this volume will only be increased in the future.


Introduction
At present, polyolefi ns are a heavily used of thermoplastic, and they are used in different industries. Approximately onethird of produced plastics are used in the packaging industry, and this volume will only be increased in the future [1,2]. The advantages of plastic packaging, such as strength, fl exibility, moisture stability, comfort, safety and low cost, they also possess one disadvantage: they do not decompose in natural conditions, which causes considerable ecological problems, namely, plastic waste composes half of the domestic rubbish.
Polyethylene (PE) and Polypropylene (PP) are widely used polyolefi ns for the manufacturing of packaging materials [3]. Al-Salem, S., Lettieri P., Baeyens, M. J. [4] found that solid domestic waste lost its consumption properties early; their traditional methods of utilization involve dumping or burning. When they are burned, harmful compounds are emitted into the atmosphere. The use of two-component polymers for decomposition can be a solution to environmental pollution by polymer waste. Biodegradable polymer materials can be decomposed during a defi nite period of time to lowmolecular-weight compounds that can be used by different microorganisms under the action of different factors (e.g., the temperature, UV irradiation, water exposure and oxygen exposure). Natural polymers and their modifi ed based on starch cellulose, chitin [5,6] and polymers synthesized from renewable raw materials using biotechnological methods, namely, polyhydroxyalkanoates, polylactides, and degradable aliphatic polyesters from oil-chemical raw materials, such as polycaprolactam and polyesteramides [7][8][9], can be used as initial materials to obtain biodegradable polymer materials.
However, biodegradable polymers have disadvantages, such as relatively high cost, low mechanical strength, operational characteristics, and low thermostability, which have prevented the expansion of their widespread use in the packing industry.
At present, it is not possible to implement all demands for their use, which can introduce limits or exclude the use of biopolymers [10].
To overcome these limitations, the most effective methods involve the use of compositional materials based on polyolefi ns, including different fi llers, such as natural polymers [11][12][13], and the use of oxo-additions to initiate the decomposition of polyolefi ns in environmental conditions. The oxo-decomposition of plastics is based on the oxidation of polymer materials under the action of external environmental factors (e.g., UV irradiation and exposure to O 2 in air) and formation of low molecular weight compounds. In the literature, there are many investigations [14][15][16] of decomposable compositions of polyolefi ns and oxo-additives, which are comonomers with functional groups that promote photodestruction (e.g., vinylketones and CO) or photocatalysts (compounds of metals with a variable valence, like Fe, Ni, Co, and Mn) Additionally, to improve the biodegradability of polymers, investigations have been carried out that involve grafting hydrophilic compounds in the structure of synthetical polymers by copolymerization, which changes the chemical and physical properties of the initial polymers [17][18][19][20].
However, such mixtures are not fully biodegradable because under the action of natural factors, they decompose to form dispersion products, but at this point, their polymer base has remained undecomposed [10]. These materials did not decompose completely under natural conditions, but the structure of the packaging materials was destroyed. These fi llers decomposed completely, but the synthetic part was scattered as a powder, preventing the accumulation of large solid waste. Such materials can be referred to as 'biodegradable materials'.
The addition of natural fi llers enables biodegradability but also substitution of oil-based raw materials that are not renewed and a decrease in the cost of obtained products [29].
The aim of this work is to obtain biodegradable polymer fi lms based on PE and starch and different types of nanocellulose and to investigate their properties, physicochemical characteristics and biodegradability.

Materials
The materials included low-density linear PE from the Shurtan Gas Chemical Complex (Uzbekistan) at mark F-0320 with a melting fl ow index of 0.31 g/10 min (190°C, 2,16 kg); industrial sample of corn starch (St c ), potatoes starch (St p ), and rice starch (St r ); and nanocellulose (NC) obtained from cotton cellulose by acid hydrolysis. Their base compositions were the objects of this investigation.

Methods
The preparation of the compositions was carried out under the action of a high-temperature laboratory rotary mixer by Brabender. Vasilenko, A. Yu. [30] showed that this method is effective for obtaining polymer compositions with improved exploitation properties and provided the best distribution of the fi ller in the polymer matrix [31].
Fabrication of the components was carried out at a temperature of 190±2°C and rotor rotation of 30 turnovers/ min. In the mixing chamber, in accordance with the desired composition, the PE with fi ller was loaded and then mixed for 20 min. After mixing, the obtained composite was removed from the mixer and cooled in air at 25±2°C. The content of the fi ller in the composites was varied as follows (mass %): 5, 10, 20, 30 and 40.
The blends were compression molded into sheets 2 mm thick using 20 x 20 cm window frame molds in Hydraulic Press, Model CMP 30. The platens of the press were heated to 190°C. The window was placed between two glazed sheets, already sprayed with silicon mold release agent on the contact surface and fi lled with preweighed material. The assembly was then placed in the hydraulic press and initially heated for 6 min without applying any pressure to ensure uniform heat fl ow through the material. The temperature was maintained at 190°C for all the compositions of blends for 10 min at a pressure of 25 t, which was applied gradually during the fi rst 5 min. The sheet thus obtained was removed from the press after cooling to room temperature. Round fi lm samples with a diameter of 6 sm and thickness of 130±10 mkm were obtained.
An investigation of the structure of the obtained fi lms was carried out on an optical microscope by Motic. The transparency of the fi lms was determined on a SKF 120C fully automatic spectrophotometer. The melt fl ow index (MFI) of the samples was investigated on a DH-MI-BP tester. The mechanical properties of the fi lms were determined on a TM2101tensile testing machine. The rheology of the composites was investigated with a rheostat.
The biodegradation of the obtained samples was determined by two methods. In the fi rst method, fi lms under defi nite conditions were subjected to the action of a natural complex of soil microfl ora, and the stability of the microbiological decomposition as a function of the outer appearance and mechanical strength was determined. The testing mixture (soil) was prepared from sand horse manure and garden earth in equal quantities by mass. The soil before tests was sustained for 2 months at a temperature of 20±5°C and moisture of 30%.
Changes in the mass and outer appearance of the fi lm samples after exposure to water for a defi nite period of time were determined with the second method.

Results and discussion
Obtaining composites of PE and starch by thermomechanical mixing requires a highly uniform distribution of starch molecules in the volume of melted PE. The presence of starch in the volume of PE infl uenced the processing due to the transmission of the composite in the liquid state. Therefore, it is very important to investigate the behaviour of composites in the liquid state with air to determine the peculiarities of structural and phase transformations in the fl ow. Decreasing the temperature to 150°C resulted in the loss of fl uidity of the melt, and in the temperature range of 80°C, PE hardened. For this reason, the processing of PE as a liquid-solid body is important. The results of the PE starch composites in shear fl ow are presented as the dependence of the rate gradient () on the temperature (t, °C) (Figure 1). With a temperature above 140°C, samples started to fl ow, which indicates that it was melting. From 150°C to 180°C, the rate gradient monotonically increased for the PE from 0.56 to 1.2 c-1 and for the PE-starch composite from 0.5 to 1.1 c-1. Such a change in the frequency of the rate gradient with increasing temperature indicates that the fl uidity of the PE melt increased owing to decreasing intermolecular bonds under the action of heat on the moving fl ow. In the case of the composites, a decrease in the gradient was caused by the infl uence of the starch on the fl uidity, that is, by increasing the viscosity of the melt of this system. Decreasing the temperature was accompanied by a decrease in the velocity gradient, and at t=100°C, the fl uidity of the melts practically stopped. In the region of t80°C, the melts passed into the solid state. In general, in the temperature range from 145-175°C, the rate gradient of melting samples changed insignifi cantly. In this temperature interval, PE:starch can be processed as PE as a thermoplastic material.
An analysis of the MFI changes as a function of the PE:fi ller ratio and its nature allowed us to determine changes in the rheological parameters (Table 1). Table 1, increasing the content (mass %) of the fi llers in the composites decreased the MFI, that is, the compositions had a high viscosity. The lowest viscosity was observed for the composite with a fi ller content of 5 mass % in the polymer matrix. High MFI values were observed when a high fi ller content (40 mass %) of corn starch was used, and the dimensions and form of the fi ller particles obviously infl uenced the viscosity of the composites to a great degree. Composites containing a highly dispersed fi ller had a decreased viscosity.

As shown in
Optical investigations of the composite fi lms showed that with increasing fi ller content, an increased number of large particles (agglomerates) were present in the polymer matrix. After an increased time of component mixing, a uniform distribution of the fi ller in the matrix of PE was observed ( Figure 2).
The presence of starch in the composites infl uenced the transparency of the fi lms. It was shown that with increasing fi ller content in the composites, the transparency of the fi lms decreased (Table 2) Figure 3.
It was found that the index of strength limit (ISL) at tension was higher for compositions PE/St c in comparison with other samples at the same content of natural fi llers. A decrease in the strength characteristics of the compositional fi lms was observed following raw РЕ/St c РЕ/St p >РЕ/St r . In fi lms obtained by the addition of nanocellulose (NC), the same raw of decreasing mechanical strength was preserve ( Table 3).
The ability of composite materials based on polyolefi n and natural fi llers to decompose and be assimilated by microorganisms is dependent on the characteristics of the polymeric matrix and fi ller, which determines the structure of the fi nal product. For example, polyolefi ns with a low molecular mass that possess a branched structure and a low degree of crystallites have an improved biodegradability. However, to a considerable degree, the biodegradability of composites   One of the factors determining the low ability of polyolefi ns to biodegrade is their low ability to absorb water, which is necessary for the development of microorganisms. Film samples were tested for water action (Figure 4).
In all cases, a monotonic change in the mass was not observed. After 40-60 days of testing, the mass of fi lms    Citation: Turdikulov  The fi lms were also buried in soil, and then, the strength after 60 days were investigated ( Figure 5).
As s hown in Figure 5,   groups and mechanical destruction. Mastalygina Ye [32] showed that changes in the colour can be due to removal of fi llers that are soluble in water and can undergo hydrolysis; the colour change may also be due to the accumulation of melanintype pigments, which are products of the metabolism of certain moielialic fungi and bacteria.

Conclusion
Biodegradable compositional fi lms based on PE and natural fi llers (different starches and nanocellulose) were fabricated. The