Influence of UV irradiation on the degradation of composites based on polyethylene

. The study of photo degradation of polymers is an important task of the present. In particular, understanding the kinetics of these processes helps to improve the properties of polymeric materials. In this article, films based on low-density polyethylene with a content of 10-50 wt.% natural rubber were studied when exposed to ultraviolet radiation. It has been established that the addition of natural rubber to polyethylene promotes the onset of photo degradation of the resulting composite material upon irradiation at a wavelength of 254 nm. At the same time, no changes occur in the structure of 100% polyethylene under similar conditions.


Introduction
Currently, more than 150 million tons of petroleum-based synthetic polymers are produced annually. Some polymers, such as low density polyethylene (LDPE), are used in a wide variety of fields of science, technology and economy due to their good physical and chemical properties. However, LDPE practically does not decompose after disposal, which leads to the accumulation of this polymer in the environment and disrupts the ecosystem.
The properties of polymers change with time due to various physical, chemical, thermal factors and their combinations [1][2][3]. In recent years, there has been increased interest in assessing the service life of polymeric materials exposed to environmental factors. This problem is the change in the physical and chemical properties of materials after exposure to factors such as ultraviolet radiation, humidity and temperature. Degradation causes abrupt changes in the structure, which affect the mechanical properties [4,5]. The non-degradable nature of synthetic polymers along with stringent environmental laws and regulations have forced the industry to look for other sustainable and biodegradable polymer materials [6,7].
Taking into account the various decomposition processes, it is possible to distinguish between photodegradation, thermal-oxidative degradation and biodegradation of polymer waste. Among these processes, photocatalysis stands out, which is characterized as an environmentally friendly process and is able to decompose organic pollutants and then convert them into carbon dioxide, water and mineral acids, is one of the promising decomposition processes, since this process has a significant potential for the decomposition of plastic waste. An important aspect of plastic decomposition using photocatalysis is the formation of two components -hydroxyl and superoxide radicals [8]. These two species are powerful oxidizing agents that can initiate the process of plastic degradation, leading to the degradation of the polymer structure of the plastic [9].
The main reactions that occur during the photooxodegradation of polyethylene are well studied, see, for example, references [10][11][12]. The main degradation mechanism is the chain splitting reaction in the amorphous phase [13,14], where oxygen easily diffuses. Photodegradation of polyethylene results in random chain cleavage and photooxidation, which leads to secondary crystallization and the formation of various degradation products such as carboxylic acids, ketones and aldehydes [15,16].
At the same time, the main factors causing polymer chain breakage under biodegradation conditions are microorganisms, enzymes and water. In hydrophobic polymers, which include, for example, polyolefins, polyesters, polysiloxanes, it is possible to form an ordered structure stabilized by hydrogen bonds of water molecules [17][18][19][20].
Since polyethylene is one of the most used polymers, it is extremely important to study the possibility of giving it the properties of accelerated degradation in the natural environment. One of the options for creating degradable polyethylene can be the addition of a biodegradable polymer. In this case, rubber can be used as the second component of the mixture to improve the degradable properties of the polymeric material. Natural rubber (NR), epoxidized natural rubber and nitrile rubber (NBR) offer a unique combination of strength, flexibility, biocompatibility and biodegradability [21][22][23][24].
As noted in [25], an important property required for the production of a heat shrink film is the crystallization of the polymer during stretching. It is crystallization that controls the shrinkage of the finished product (film). The outstanding properties of unvulcanized natural rubber also include deformation-induced crystallization during the stretching process. The authors propose to use the composition of LDPE/NR as a shrink film [26].
The aim of this work is to study the effect of ultraviolet radiation on polymeric materials based on polyethylene with natural rubber and polylactide.

Methodology and materials of the experiment
The object of the study were mixtures based on low-density polyethylene and natural rubber (NR, brand SVR-3L, Vietnam). The content of NR in the mixtures was 10, 20, 30, 40, 50 wt.%. All the mixtures were prepared using Plasticorder PLD-651 (Brabender, Germany) plastic extruder in an argon atmosphere at the temperature of (140 ± 2) °C. Film samples were obtained on a press at a temperature of (140 ± 2) °C on a cellophane substrate, followed by quenching in water at (20 ± 2) °C. As a result, round-shaped film samples with a diameter of 7 cm and a thickness of (120 ± 10) μm were obtained. Mixtures of LDPE/PLA (PLA -Nature works 4032D, USA) were obtained by the same way. Mixing and pressing temperature -(180 ± 2) °C.
Methods of structural analysis. The study of the structure of the material was carried out on an optical microscope Olympus BX3M-PSLED at magnification of 50x and 200x.
DSC analysis was performed using a DSC 214 Polyma (Netzsch, Germany) at a heating rate of 10 deg/min and a sample weight of (10 ± 0.1) mg. Melting heat of an ideal polyethylene crystal ∆Нm * = 293 J/g [27].
TGA analysis. The thermal characteristics were measured in an argon atmosphere using a TGA/DSC 3+ thermogravimetric analyzer (Mettler Toledo, Switzerland). All samples were heated from 25 to 700 °C at a heating rate of 20 °C/min. IR analysis. The infrared spectra of the samples were recorded on a Lumos Bruker FTIR spectrometer (Germany) at T = (23 ± 2)°C in the wavenumber range 4000 ≤ ν ≤ 600 cm-1 in reflected light by the method of multiple frustrated total internal reflection.
UV radiation. The resistance of samples to photodegradation was studied using a VL-6.LC ultraviolet radiation source from Viber Lourmat (France). The radiation wavelength was 254 nm for 250 hours (LDPE/NR) and 150 hours (LDPE/PLA).

The results of experiments and discussion
Polyolefins are high molecular weight hydrophobic polymers that do not degrade under the influence of abiotic and biotic factors. UV irradiation of polyethylene for 16 days prior to incubation in soil for 10 years has been shown to release < 0.5 % carbon by mass compared to unirradiated polyethylene which produced < 0.2% CO2 over the same period [28].
Photodegradation involves the natural ability of most polymers to gradually react with atmospheric oxygen in the presence of light. The photodegradation mechanism involves the absorption of ultraviolet light, which then leads to the formation of free radicals. Then the autoxidation process takes place, which ultimately leads to the degradation of the polymer. It is believed that the instability of polyolefins is observed due to the presence of impurities (carbonyl and hydroperoxide groups) that are formed during the manufacture or processing of polyolefin products.
The addition of a filler to the polymer matrix leads to significant changes in the morphology of the composite material and the macromolecular mobility of the boundary layers, which in the future may affect the degradation of the material as a whole. It was shown in [29] that with an increase in the NR content in the PE matrix, a more uniform scattering of domains occurs with a decrease in their size.
As a result of the study of mixtures of LDPE/NR exposure to UV irradiation with a wavelength of 254 nm for 250 hours, it was found that there is a change in the properties of the compositions. The melting temperature of LDPE in mixtures decreases by 1.5-2 °C, while the degree of crystallinity of LDPE increases by almost 4-8 %, which may be due to the process of polyethylene oxidation, a change in the crystal structure and the degradation of the amorphous part due to the presence of NR in mixtures ( Table 1). The characteristics of pure LDPE did not change significantly, which indicates the resistance of this polymer to UV radiation.
In this work, polyethylene and its composites before and after different exposure times to ultraviolet radiation were analyzed using the IR spectroscopy method, which is one of the main spectral methods for studying the chemical structure. Figures 1 and 2 show the IR spectra of LDPE/NR compositions containing 30 and 50 wt.% NR, before and after UV irradiation (λ = 254 nm, 250 hours). It was found that changes occur in the structure of both polymers. In the IR spectra, it is possible to distinguish regions related to the vibrations of the vinyl group (-C=C), this is the range from 1000 cm -1 to 900 cm -1 , the peaks of the carbonyl group (-C=O) in the range from 1800 cm -1 to 1500 cm -1 and peaks of the hydroxyl group (-OH) in the range from 3600 cm -1 to 3100 cm -1 . The region 1660-1640 cm -1 refers to bending vibrations of N-H and C-N amide groups. One of the structural bands of natural rubber is 840 cm -1 , which refers to the C-H-out-of-plane bending vibration of the C(CH3)=CH group. The decrease in this band after irradiation indicates the degradation of the NR.  2. IR spectra of 50LDPE/50NR film before (1) and after (2) exposure to 250 hours of UV radiation (λ = 254 nm).
The impact of UV radiation is also reflected in the thermal stability of the compositions. As an example, the TGA and DTG curves of the composition with 30 wt.% NR in the mixture are shown (Fig. 3, 4). According to the results of the experiment, after exposure to UV radiation, the process of weight loss is accelerated. Figure 4b clearly shows two peaks at about 355 °C and 460 °C in the original sample, which correspond to Tmax of the degradation of NR and LDPE, respectively. It should be noted that exposure to UV radiation changes the NR peak, which practically disappears, while the LDPE peak shifts to 470 °C, which may be due to the degradation of the amorphous phase and an increase in the proportion of crystalline. For LDPE, as in the case of LDPE/NR samples, Tmax increases by almost 15 °C. At the same time, a shift in the degradation peak of LDPE indicates the degradation of the amorphous part of the polymer. Despite the fact that, according to TGA data, LDPE has a higher degradation temperature, photooxidation begins in it.
PLA is a potential candidate for wide application in various fields of industry. LDPE/PLA are promising due to the susceptibility of PLA to accelerated disintegration under the influence of aggressive environmental factors. In the articles [30][31][32], the authors report on the exposure of PLA to UV irradiation. The Norrish II mechanism was the most possible degradation route [33].
The method DSC showed that the melting point of PLA decreases by 20-24 ° C, the degree of crystallinity -by 15-22%. Changes in thermal properties are also recorded by the TGA method (Fig. 5).
The degradation temperature of the PLA decreases by 50° C, which indicates the degradation of both the amorphous and crystalline parts of the PLA. As for LDPE/NR samples, Tmax of LDPE increases by almost 15° C. At the same time, the displacement of the LDPE degradation peak indicates the degradation of the amorphous part of the polymer. a b Fig. 5. TGA (a) and DTG (b) of 70LDPE/30NR film before (1) and after (2) exposure to 150 hours of UV radiation.

Conclusions
In this study, the process of photodegradation of mixed LDPE/NR and LDPE/PLA compositions is considered. It has been found that exposure of NR to UV radiation can accelerate the degradation of LDPE. At the initial stage, the degradation of the NR matrix and PLA matrix occurs, and then this entails the degradation of the composites. These data are confirmed by both IR spectroscopy: a change in the characteristic bands of the mixture components is noted, and DSC: changes in the thermophysical characteristics are observed. For example, in LDPE/NR compositions, an increase in the degree of crystallinity occurs due to more active degradation of NR. Due to the degradation PLA, there is a significant decrease in the melting temperature and the degree of crystallinity, which leads to embrittlement of the entire material. Based on the results of the study, it can be concluded that UV irradiation in LDPE/NR samples first destroys the biodegradable component of NR, and then the LDPE matrix.