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Identification of Commercial Oxo-Biodegradable Plastics: Study of UV Induced Degradation in an Effort to Combat Plastic Waste Accumulation

End of life single-use items such as carrier bags constitute a large proportion of the litter found in marine and terrestrial environments. The main objective of the current work was to investigate the response of an oxo-biodegradable commercial plastic film product to photo-degradation using accelerated weathering, verifying the claim of its biodegradability and suitability as an eco-friendly product. This study is also geared towards the appropriateness of such products to reduce plastic waste accumulation in urban environments. The film samples were exposed to weathering as a means to determine the impact of UV induced oxo-biodegradation. Haze (%), light transmission (%) and the total change in colour (ΔE) were measured as indicators to the degradation profile of the polymeric materials, in addition to tensile pull mechanical properties and thermal stability. The melting peak indicates the melting point (Tm) of the polymer and with exposure to weathering it showed a slight decrease from 105 to 102 °C indicating that biodegradation mechanisms were triggered to a certain extent. The reduction in strain at rupture was also indicative for a loss in crystalline structure, coupled with Young’s modulus increase throughout the weathering exposure tests. Therefore, the material studied could be a candidate to mitigate the accumulation of plastic waste in open environments based on the results of this study whilst regulating controlled substances.


Introduction
Plastics are an appealing versatile material associated with low production cost compared to other classical materials (e.g., cement, wood and metal). Their beneficial properties have resulted in a worldwide annual production rate that exceeds 335 million tonnes [1]. About 50% of plastics are discarded after single use which creates major environmental burdens associated with accumulation and pollution [2]. Plastic carrier bags are one example of a single-use item and were first introduced on the market back in the 1970s and quickly became an essential item to our daily lives. Carrier bags typically consist of polyethylene (PE), which orients the petrochemical market towards polyolefin (PO) plastic film production. End of life single-use items such as carrier bags are frequently thrown away in the environment by consumers and contribute significantly to marine and terrestrial pollution. It was estimated that 98.6 billion carrier bags were introduced in the European Union (EU) market in 2010 and about 100 billion plastic bags were added ever since on an annual basis. Plastic bags are usually provided free of charge or for a very low charge when consumer products are purchased at outlets. However, few countries have started implementing stringent regulations against their use even if a consumer charge is applied. Within an EU context, the annual per capita usage of plastic bags exceeds 450 bags per person [3]. It is estimated that about 8 billion plastic shopping bags are used per annum in the United Kingdom, 4.3 billion in Australia, 9.8 billion in Hong Kong, 3.3 billion in Bangladesh, 100 billion in the United States and 300 billion to 1 trillion in China [4,5].
The majority of single use plastics is still disposed of in a non-sustainable way, i.e. without the recovery of energy or their recycling to useful products [6]. Uncontrolled incineration or mechanical tillage in the field have high environmental impacts and generate air and soil pollution [7]. Various types of plastic materials can be designed to be biodegradable, including polyesters, which typically possess a poor water-solubility. They are not directly bioavailable to organisms but can be degraded via an extracellular enzymatic process. Enzymes secreted by microorganisms initiate the first step in degradation by catalysing the hydrolysis of the ester bonds and degrading the polymer into oligomers and monomers. These lower molecular weight intermediates (≈ 500 g mol −1 ) show a much higher water solubility and can pass through the cell membrane for further intracellular metabolization by microorganisms [7].
Biodegradable plastics are subdivided in two categories depending on the manufacturing process and degradation mechanism. The first type are hydro-biodegradable plastics which are mainly produced from renewable raw materials such as starch but can still contain up to 50% of plastics derived from crude oil or aliphatic polyesters [8]. The second type of biodegradable polymers are oxo-biodegradables, which are based on petrochemical naphtha blends mixed with pro-oxidant additives. The latter have become popular nowadays for packaging and film applications [9]. Their degradation can be initiated by different factors such as UV light, moisture, heat and microorganisms in processes classified as photo-oxidative degradation, thermal degradation and biodegradation. The degradation of polymers can be assessed by measuring changes in physical properties, loss in molecular weight (MW), amount of carbon-dioxide evolved and by calculating microbial growth on the surface of the polymer after exposure to the natural conditions. To monitor this chemical reaction, evolution of certain products is typically monitored such as carbon dioxide [10]. Controversy exists as to the impact of products from oxo-biodegradation on the environment due to the release of metals and chemicals [8]. In contrast, hydro-biodegradables divert feedstock from natural food resources in their manufacturing process and are quite costly to produce. There is still no agreement in the scientific literature, as to which option would be best for environmental control pollutants release. Furthermore, it can also be deduced that the longterm fate of plastics in open and closed environments is in direct relation to the types of additive present, biodegradation route and climatic conditions [3,9].
There are several testing methods to assess degradation, i.e., by measuring MW, tensile properties, weight loss, extent of fragmentation, enzyme assays, biochemical oxygen demand and carbon dioxide production. To confirm that degradation has occurred, the combination of multiple tests is employed. The combination of the test procedure has to be selected according to the potential fields of polymer application and its way of disposal, such as compost, soil, marine and so on. Appendix A (Supplementary material) presents a review of the main studies that encompass testing procedure of biodegradability and impact of the surrounding environment.
Accumulation of plastic waste in the environment forces many industries to generate biodegradable products. Municipal solid waste (MSW) management through composting and replacement of conventional plastics with biodegradable ones is an environmentally friendly process because it decreases the output of waste to landfills, while saving energy and yielding valuable organic compounds and nutrients that can be used in agriculture [11]. Degradation of plastics is a complicated process as it is associated with release of large quantities of carbon dioxide into the environment together with other toxic compounds. It is estimated that 2.8 kg of carbon dioxide (CO 2 ) is released by the incineration of 1 kg of plastic [12]. The introduction of biodegradable films, therefore, requires an assessment of their biodegradability within both managed and natural environments using harmonized assessment criteria [13]. Nazareth et al. [14] experimentally evaluated alteration, and chemical and structural composition of selected plastic products. The aging experiments carried out by seawater immersion for 180 days showed no evidence of degradation for 4 out of the 6 studied samples indicating unequivocal green-washing.
The main objectives of the current work are to investigate the response of an oxo-biodegradable plastic product to photo degradation using accelerated weathering testing. The work provides a realistic evaluation on the applicability of commercial plastic grades available on the market and their true nature in terms of biodegradability and environmental friendliness. Additionally, information is generated on the appropriateness of this class of materials to reduce PSW accumulation in urban environments.

Acquirement of Materials
White to transparent commercial grade plastic bags acquired from a household goods franchise were used as test specimens with a measured thickness of 200 μm. The plastic bags were claimed to be of oxo-biodegradable nature of PE origin. The bags were cut into standard test specimens with a size of 20 × 1 cm as described in previous works [9,15]. Pigmentation of the bags due to store logo stamp was avoided in all test specimens used in this work as presented in Figure  S1 of the Supplementary Materials File.

Fourier Transformed Infrared Spectrometry (FTIR)
A JASCO FTIR 4700 unit equipped with an attenuated reflectance infrared spectroscopy (ATR) attachment was used for the analysis and the identification of the material's chemical fingerprint before exposure to accelerated weathering (irradiation). Samples were tested using 32 scans for the background and each individual spectrum in the range of 4000-400 cm −1 [16]. Storage and transportation of samples was done using a sealed hygienic plastic PE bag, which was always kept in dark room conditions (approximately 22-23 °C/50% relative humidity) as previously conducted in Al-Salem et al. [9].

X-ray Fluorescence (XRF) Workflow
The control (unexposed) samples were subjected to X-ray florescence (XRF) analysis in emission mode for possible heavy metal detection using a VANTA (M series)-Olympos Company XRF gun calibrated at laboratory conditions.

Accelerated Exposure (Aging) of Plastic Films
The test specimens were exposed to accelerated (ageing) weathering tests as per ASTM D 4329 to simulate the outdoor conditions and exposure of such materials to the natural environment [17]. The tests were conducted using a Q-lab QUV model machine equipped utilising UVA fluorescent bulbs. The protocol was used to set the exposure cycles for 8 continuous hours of UV exposure at 60 °C, followed by 4 h of condensation at 50 °C. A minimum of four replicates were used in each exposure rack with a lamp irradiance of 0.68 W m −2 . The tests were terminated after 20 days of continuous exposure after 40% of loaded material showed visible cracking and breakage.

Thermal Response and Stability
Thermal degradation of the samples was investigated using a Mettler-Toledo TGA 3 + Model coupled with StarE data acquisition/analysis software to record the data every second under a heating rate (β) of 15 °C min −1 . A constant flow of pure (99.99%) dry nitrogen gas (N 2 ) with a flow rate of 50 ml min −1 was maintained throughout the experiments. The measurements were conducted using 1 ± 0.1 mg samples from RT to 600 °C made with at least triplicates showing high repeatability with standard deviation (std) not exceeding 1% [18,19]. The crucibles used in all experimental runs were made of alumina oxide (Al 2 O 3 ) with a 70 μL size. Weight loss (%) and first derivative (DTG, % o C −1 ) were recorded constantly, and the onset (T os ), midset (T md ), inflection point (T if ) and maximum degradation temperatures (T f ) were recorded based on the thermogravimetric analysis conducted.

Crystallinity Estimation Using Differential Scanning Calorimetry (DSC)
A Netzsch DSC (Model 60 Plus Series) was used to test control and exposed specimens using approximately 3 ± 0.1 mg samples taken from the middle section of the film specimens. Al 2 O 3 crucibles were used for both samples and reference (no weight) experimental runs. Crystallinity measurements were determined using scans of the first and second heating cycle between 50 to 230 °C based on the peak area of the heat flow curve between 60 °C and 130 °C with a N 2 gas flowrate of 20 ml min −1 and a heating rate of 10 °C min −1 . Cooling rate was set at 15 °C min −1 in similar conditions in accordance with ISO 11,357-1 and ISO 11,357-3. The analysis in this work includes the second heating cycle, in order to determine the actual behaviour after eliminating the inherited effect of weathering on the samples. The degree of crystallinity (%) was calculated by dividing the melting enthalpy over the melting enthalpy of a 100% crystalline PE (293.6 g J −1 ) for both the first and second heating cycle, respectively [20].

Scanning Electron Microscopy (SEM)/Energy Dispersive Elemental Spectroscopy (EDS)
Scanning Electron Microscopy (SEM) was conducted to determine the extent of degradation and investigate microcracks on the surface of test specimens before and after exposure to accelerated weathering. The analysis was performed after coating the samples with pure gold (Au) to condition the plastic to a charging state appropriate for SEM imaging. A JEOL JSM 6010LA Model sputter coater was used with a 15 mA sputter current (60 s, approximately 3 nm thickness), was used. Argon (Ar) gas was used to purge the sputter coater system. A TESCAN-VEGA 3 model microscope equipped with a Bruker Energy Dispersive Elemental Spectroscopy (EDS) analysis system and software, was utilised in this study. Imaging was conducted using a voltage of 15 kV and EDS analysis was done on uncoated polymeric articles to avoid over charging the samples and obtaining realistic experimental results (see Figure S2 in the Supplementary Materials File).

Mechanical Properties Evaluation
The mechanical properties were evaluated at RT to determine the impact of exposing the samples to various degradation media following ASTM D 882. Experiments were conducted using a Tinuis Olsen (UK) 50 ST Model-Universal Testing Machine (50 kN load cell) set to a 500 mm min −1 test speed. Strain at break (elongation at break) was measured based on cross head displacement and a gauge of 100 mm. The measurement values obtained are the average of the replicate specimens stored directly from the machine's software (Horizon 10.2.5.0). The tests were conducted with a gauge grip separation of 100 mm as suggested by the experimental protocol followed and was also set to terminate at 75% of ultimate force. To assure accurate readings that reflects the true modulus of elasticity determined from the stress and strain curves, the grip separation was reduced to 25 mm for the accelerated weathering specimens at threshold limit of weathering exposure. This was done due to the fact that samples were shorter due to exposure to UV for prolonged periods of time in the weathering chamber similar to our previous work [9].

Haze and Light Transmission Measurements
A Diffusion System MF709 (spherical) haze meter (model BS 2782) was employed to measure the haze (%) and light transmission (%) at the midpoint of the studied samples, in accordance with ASTM D1003-13 for samples exposed to the degradation media. All experimental results conducted resulted from taking a minimum of two readings (runs) having > ± 5% error for each data replica.

Measurement of Colour Parameters and Total Change in Colour (∆E)
Standard colour parameters for light/dark (L*), red/green (a*), and yellow/blue (b*) post degradation were measured using a Shenzhen Technology 3NH Model colorimeter according to ASTM D 2244-11. Parameters measured were used to calculate the total change in colour (∆E) between control samples and each formulation thus [15]: where ∆L, ∆a and ∆b are the values of the difference between the control specimen measurements (control samples with no exposure) and those of each tested batch.

Identification of Plastic Material
To determine the exact polymer and additive content, ATR analysis was conducted on the specimens. ATR is a technique used for detecting the absorption in the mid-infrared region and was used to detect distinctive adsorption peaks of the plastic films studied. Several prominent peaks were identified. Figure 1 shows spectra obtained for control (preweathered) specimen. The prominent peaks identified in the range between 700 and 760 cm −1 correspond to the rocking vibration associated to the crystallinity of the polymer. The peak between 1460 and 1480 cm −1 relates to CH 2 bending vibration, while the band between 2850 and 2950 cm −1 associates with the asymmetric and symmetric CH stretching [16]. The major peak was around 1470 cm −1 which is a distinctive PE band [21]. The overall spectra of the sample and the aforementioned bands are also associated with oxobiodegradable PE spectra as confirmed by previous authors [22,23]. Oxo-pro-degradant additive peaks were also identified at 850 cm −1 , and 2800 to 3000 cm −1 and 1200 cm −1 to 1500 cm −1 which is typically associated with calcium carbonate (CaCO 3 ) additive for PE commercial grades [14,24]. Küpper et al. [25] analysed PE sample's ATR-spectrum, and in addition to the main 1470 cm −1 PE band, bands in the interval between 3400 and 3200 cm −1 and around 1600 cm −1 were also observed assigned to UV-stabilisers of the hindered amine stabiliser (HAS)-type. In the studied oxo-biodegradable PE sample, none of these bands were observed. The sample studied in this work is classified to be of PE origin as claimed by the manufacturer, and was confirmed to be so based on the peaks detected. In order to detect possible inclusion of typical oxo-biodegradable additives, the test samples were subjected to XRF analysis in emission mode for possible metal and additives detection ( Table 1). Analysis of the control PE bags specimens (0 days' exposure) indicated that metals from the first transition series were present in the polymer. Transition metal ions are the most widely reported pro-degradant additives used. They are added into the polymer matrix in order to weaken the carbon-carbon backbone and make it more degradable [26]. These additives catalyse the decomposition of hydro peroxides into free radicals. The most used transition metals include iron, cobalt and manganese. Iron accelerates photo-degradation while manganese and cobalt render the plastic more sensitive to thermal degradation. These metal ions are generally introduced at trace levels in the form of an organic complex [27]. In the present samples, manganese (10 ppm) and iron (314 ppm) were detected. Apart from transition metals also high levels of calcium were identified. The presence of calcium is related with common fillers and additives, such as calcium carbonate and calcium stearate [27]. One of the commercially available thermo-oxidative additives is the patented AddiFlex which works in synergy with CaCO 3 . The presence of CaCO 3 increases UV degradation by up to 66%, hence less additive is required. It has been claimed to be biodegradable, forming water, carbon dioxide and biomass [28]. Titanium was detected at 240 ppm which is usually found in a form of the titanium dioxide pigment and as catalytic residue from Ziegler-Natta catalysts and pigmentation used for PE bags by various manufacturers [27]. According to the 94/62/EC regulation for packaging materials, the sum of concentration levels of lead, cadmium, mercury and hexavalent chromium present in packaging or packaging components shall not exceed 100 ppm by weight [29]. Furthermore, chromium and lead are not to exceed 60 ppm [24]. From the regulated chemicals, only chromium was present in the studied PE bag at a level of 7 ppm, which is below the permissible limit. This means that it adheres to the regulation. Table 1 also reveals the evolution of the samples exposed to accelerated weathering as a function of exposure days. A decreasing trend was noticed with the majority of the samples as a function of weathering time (20 days). This shows that the UV irradiation took affect and triggered the degradation mechanism of the oxo-biodegradable samples. Nonetheless, iron, potassium and calcium were evidently at a slightly higher concentration to the control samples post 20 days of weathering. This shows that iron was the prevailing transition metal used in the oxoprodegradant additive used on the commercial bags, which was also increasing as a residue after the carbon-carbon cleavage takes place due to biodegradation. This phenomenon was also previously observed by other authors using similar analytical surface techniques [9] indicating filler residues post biodegradation (e.g., calcium and potassium). Therefore, it can be stated that the plastic film has experienced the degradation mechanism and based on the moderate losses of the metals; the mechanism was not drastically severe with respect to 20 days of accelerated UV exposure. This accelerated weathering duration should also be noted to be equivalent to over 1 year and three months in arid climatic conditions [9,15,17].

Effect of Weathering on Thermal Stability
Polymer weathering leads to free radicals' formation, which are combined with oxygen to form peroxides and hydroperoxide radicals. Additionally, alkanes, alkenes, ketones, aldehydes, alcohols, carboxylic acids, keto-acids, linear-esters and lactones are formed, which decrease the polymer's molar mass and hydrophobicity, leading to increased bioavailability for degradation. Abiotic degradation can be accelerated by the addition of certain organic salts of transition metals which generate free radicals during redox reaction. These radicals decompose previously generated hydroperoxide or generate hydrocarbon chains. Biodegradable plastics are a source of carbon and energy for micro-organisms. The biodegradation under aerobic conditions can be best described as per the following reaction [10]: where the carbon of the polymer is assimilated by microorganisms into biomass and further mineralized into carbon dioxide and water. Reaction a is the core biodegradation one whilst reaction b is the mineralisation pathway. UV accelerated weathering as per ASTM D 4329 was used on the plastic bag specimens to simulate the exposure to the natural environment. After the accelerated weathering the thermal response and stability were investigated. Weight loss (%), midset (T md ), inflection point (T if ) were recorded based on the thermogravimetric analysis conducted and are presented in Table 2.
According to the TGA thermograms (presented in the Supplementary Materials File within Figures S3 to S9), the materials exhibited single step weight loss. Table 2 presents  the onset temperature (T md ), midset temperature (T md ) (which is the temperature on the thermogram where 50% of weight loss occurs), inflection point (T if ), final maximum temperature (T f ), and weight loss before and after accelerated weathering. The midset temperature also decreased with weathering duration by some 10 °C. The decrease in the degradation temperature indicates a lower thermal stability of these materials [15]. The shift in the midset temperatures also points towards the fact that samples exposed to weathering by 20 days (480 continuous hours) are more brittle and have started to lose their amorphous region within the PE matrix which is a semi-crystalline polymer. A similar observation was also made previously by other authors [24]. Although the thermal decomposition of un-weathered (control) samples occurs with almost little residue remaining (≈ 19 wt%), the amount of residue is higher for samples that have undergone weathering. As can be seen from the table, the pre-exposure weight loss was 80% but after 20 days of accelerated weathering this value decreased to 38%. This suggests that the origin of the residue of these samples is mainly the additive, which may not completely decompose in the temperature range of these experiments due to its inorganic components [30]. It also indicates that samples are more brittle with weathering making them more susceptible to thermal degradation under thermogravimetry. This also confirms that the samples have undergone the desired UV triggered degradation mechanism due to pro-degradants presence. The degree of crystallinity was estimated using differential scanning calorimetry (DSC) by dividing the melting enthalpy over the melting enthalpy of a 100% crystalline PE (293.6 g J −1 ) for both the first and second heating cycle, respectively [20]. The values obtained are presented in Table 3, and the thermograms are presented in Figures S10 to S14. As it can be seen from the table with the accelerated weathering duration, the crystallinity decreases from 82 to 35% as calculated from the 2nd heating cycle. Guadagno et al. [31] has observed an increase in crystallinity for LLDPE from 37.9 to 53.8% over 300 h. The chain scission due to the photo-oxidation makes the amorphous chains more mobile and freeer to crystallise further, and this happens mainly when the degradation occurs simultaneously in many chains. Corti et al. [32] observed the increase in crystallinity took place after both the first and the second heating, but the increase was much more pronounced after the first heating. Ojeda et al. [27] has also observed that the reduction in molar mass was followed by an increase in crystallinity due to the higher freedom of motion of smaller polymer chains, which could be rearranged in more crystalline structures while mechanical properties fell to zero in about 3 to 4 months of sun exposure. The pre-exposed PE sample showed a melting endothermic peak at 105 °C (as shown in DSC graphs in the Supplementary Material File). The melting peak indicates the melting point (T m ) of the polymer and with exposure to weathering it showed a slight decrease from 105 to 102 °C. Overall, the thermal properties and stability of the plastic films showed a high resistance to thermal degradation. This also indicates that the film samples are stable due to the presence of the additive within the polymer matrix. The incremental decrease in the tested specimens (Table 3) shows that the samples are losing the crystalline region of the polymeric matrix but without a clear effect on the thermal stability.

Mechanical Properties
To determine their integrity and durability, the studied materials were exposed to weathering conditions and the mechanical properties were assessed before and after weathering. The mechanical profile of the samples conforms with commercial plastic film specifications, in terms of tensile pull properties [9]. This shows that the composition of the PE with the additives presents is suitable to be marketed as a carrier bag for consumer demand. Figure 2 shows the Young's modulus and ultimate strain measured for the studied samples with respect to the different weathering times. By comparative assessment, the sample that was not exposed to weathering had the lowest Young's modulus compared to samples exposed to weathering. This shows that the products' deformability and elasticity decrease with exposure time [15]. The Young's modulus for the studied samples is between 439 and 975 MPa, while the ultimate strain decreased from 618.75% to 13.89% over the 20 days weathering exposure. The Young's modulus and strain at break values are depicted in Table 4. Young's modulus is related to lower crystalline content of materials [33], which indicates that samples in this work have started to lose their structure with exposure time due to photo-degradation. Ojeda et al. [27] evaluated abiotic and biotic degradation of plastic bags of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) formulated with pro-oxidant additives. The studied materials were exposed to natural weathering and were analysed with respect to changes in mechanical and structural properties. In the study, the strain at fracture of the oxo-biodegradable PE decreased from 400 to 60% over the duration of 80-day natural weathering exposure. The stress at fracture decreased from 52 to 16 MPa over the 20 days weathering period. Based on the mechanical properties studies, it can be noted that the material conforms with typical HDPE commercial grade resins used in formulating oxo-biodegradable PE bags [9,24], and that there is a clear loss of plastic integrity as a function of weathering duration, which is in direct relation to the biodegradation mechanism the samples were subjected to. Figure 3 shows estimated haze and light transmission at different weathering durations. The sample that was not exposed to weathering showed a haze of 89.62% and after 20 days of weathering this value decreased slightly to 88.65%. Table 5 shows the estimated values of haze and light transmission at different weathering times. Increase in haze also indicates severe cross-linking in the polymeric matrix. This was previously observed in past studies. Al-Salem et al. [9,34] studied the abiotic and biotic degradation of pro-oxidant filled PE films, haze increased with respect to soil burial time where an increase of 63% was estimated by the twelfth month of field testing. While the inverse effect was noticed with light transmission as it decreased due to less light being scattered in samples that suffered loss of amorphous regions. The light transmission in this study did not change significantly, and even showed an increase from 67.67 to 70.68%. Al-Salem et al. [9] reported the degradation of prooxidant filled PE films and the total change in colour was assessed for white plastic bags of PE origin containing prooxidant filler. The total change in colour for the material studied after 19 days of UV exposure was 23, while in this study the total change in colour (ΔE) increased from 0.75 to 1.05 after 5 days and 20 days of weathering, respectively. The values for different weathering durations are presented    in Table 6. This indicates that a bleaching effect started to take place to a minimal degree in this work. Figure 4 depicts the micrographs obtained from the microscopic evaluation of the tested materials. The presence of the CaCO 3 filler and inorganic additives can be observed in the form of disunity zones in the matrix. After 15 days of exposure, dark spots were noticed on the sample, that could possibly be a biofilm formation on the surface. After 20 days of weathering exposure some micro cavities could be noticed. Ojeda et al. [27] exposed oxo-biodegradable films to open environments and noted biofilm formation on the surfaces of the materials tested. A similar observation was noticed by Bonhomme et al. [35]. The EDS analysis of the control samples and accelerated weathering ones are shown in Fig. 5. The samples' surfaces showed the presence of the inorganic ion of elemental calcium. A similar observation was previously made by other researchers in the field [14,27]. As indicated previously by XRF analysis, the tested samples contained calcium, which can likely be attributed to inorganic filler in the form of CaCO 3 . The metal elements were also observed to start diminishing and were not easily

Environmental Implications of Plastic Solid Waste Accumulation
Plastics accumulate in the environment at a rate of 25 million metric tons per annum. PE represents 64% of plastic materials produced as packaging and bottles, which are usually discarded after single use [36]. The major problem is that a large portion of plastics produced is used to make disposable packaging items or other short-lived products that are permanently discarded within a year of manufacture [37]. Single use plastic bags are one of the packaging materials that accumulate in the environment due to low biodegradability. Their accumulation generates plastic pollution and takes up landfill space. Plastic pollution is a recognised international problem that is associated with environmental and health concerns. 80% of the waste that accumulates on the shorelines, the ocean surface and seabed is plastic [38]. Plastic bags have a small mass and are usually contaminated, rendering their recycling economically not feasible [39]. Replacing plastic bags with biodegradable materials could help solve the plastic pollution problem. Due to plastic's resilience against degradation and its increased use and production in industry, the issue of plastic pollution has become a risk to global ecology. Plastic pollution arises from both terrestrial and marine sources. There is a continuous stream of pollutant plastic via: inappropriate dumping of domestic and industrial refuse and unintentional dumping through poorly contained and transported waste. Plastic accumulation in the marine environment poses a large threat to wildlife, the main dangers are associated with it are entanglement and ingestion of pollutants. Conventional plastics do not naturally degrade when released to the environment. This is not unexpected, as the main reasons for the widespread use of many polymers is their high stability and durability [40]. One of the attractive alternatives of dealing with accumulation problems is its degradation. Various polymers absorb UV and undergo photolytic, photo-oxidative, and thermo-oxidative reactions that result in their degradation. However, depending on the application, products need controlled stability as for example packaging products need durability instead of degradation. To increase the resistance and durability, the plastic products are produced so to increase their resistance to UV and thermal exposure Synthetic polymers are stable and are commonly used in packaging, food industry and etc. However, their stability is counterbalanced by polymers contributing to the demand for oil and resistance to biodegradation that leads to accumulation in the environment. The alternative to synthetic polymers are biodegradable polymers such as starch-based blends but their cost is higher and there is a lack of waste management infrastructure for such wastes. Further research is required to make the biodegradable polymers cost effective [41].
One production method of biodegradable plastics is to produce them based on conventional plastics with enhanced degradability, without compromising the material properties. Polymers with additional functional groups have been produced making them more susceptible to attack from microbial enzymes. However, biodegradation of such polymers is still relatively limited. There is still a requirement for substantial energy input in order for the degradation to initiate and it is uncertain whether these polymers are biodegraded or whether they disintegrate into smaller pieces [41]. There are limited scientific studies on oxo-biodegradable polymers and their degradability. They have to be validated as more environmental friendly alternatives to conventional plastics. Based on these needs, this study aimed to assess the degradation of commercial PE bags claiming to be oxobiodegradable. The findings of the study presented herein also indicate that the film samples have started to decompose and deteriorate with time equivalent to over one year in natural weathering [9]. However, judging by the fact that the samples were still showing mechanical integrity post 20 days of exposure, and did not appear to be deteriorated to a powdery or residue like form even after accelerated weathering, it could be well stated that the materials formulation can't perform as a waste accumulation or mitigation material. On the other hand, the samples were also within standards for metal content and could be classed as a safe plastic product based on eth European standards we have depicted. Nonetheless, further leaching tests mimicking samples deterioration in the environment and possible metal contamination is also a must. This also puts an argument based on the oxo-biodegradable plastic characterised here, that hydro-biodegradable materials could be more effective due to having a more soluble material (e.g. starch) within the polymer matrix itself. The readers are also referred to our review annexed in Appendix A (Supplementary material) for a comparative assessment to recent cases with starch based polymers response in open environments.
There have been previous studies that have also linked oxo-biodegradable plastics with poor biodeterioration and degradation under certain environmental conditions. Some studies have also linked such products with exceeding permissible limits of regulated substances [24]. Musiol et al. [42] studied the degradation behaviour of oxo-additive based PE commercial bags under abiotic and biotic stressors, and reported that minimal changes in terms of deterioration were observed due to occurrence of crosslinking triggered by the UV exposure. Greenwashing is a paramount concern that needs to be highlighted to consumers, where false claims of environmentally friendly properties are in need of exposure to concerned parties [14]. Eight different PP and poly lactic acid (PLA) biodegradable products were studied for their degressive behaviour by Viera et al. [43]. Greenwashing was confirmed in their study and appropriate labelling for products was also suggested. This is in-line to findings in our work were appropriate labelling for chromium is also recommended based on the results shown in the previous section. Such practices are essential to be in-line with regulations concerning biodegradable bags set by European Commission (EC) [44] and raise awareness of consumers regarding the fate of products in open environments [3].

Conclusion
The main objective of the work was to investigate the response of an oxo-biodegradable plastic product to photo degradation using accelerated weathering testing. Several prominent peaks were identified around 1470 cm −1 which is a distinctive PE band. From the regulated chemicals according to 94/62/EC, only chromium was present in the studied PE bag at a level of 7 ppm, which means that it adheres to the regulation. UVA accelerated weathering as per ASTM D 4329 was used on the plastic bag specimens to simulate the exposure to natural environment. After the accelerated weathering the thermal response and stability were investigated. According to the TGA thermograms the materials exhibited single step weight loss and the decrease in the degradation temperature in respect with weathering was identified that indicated a lower thermal stability of these materials. The thermal decomposition of un-weathered samples occurred with little residue remaining (19 wt%), the amount of residue was higher for samples that have undergone weathering. This suggests that the origin of the residue of these samples is mainly the additive, which may not completely decompose in the temperature range of these experiments due to its inorganic components. By comparative assessment the sample that was not exposed to weathering had the lowest Young's modulus. This shows that the products deformability and elasticity decreased with exposure time. The young's modulus for studied samples was between 439 and 975 MPa while the ultimate strain decreased from 618.75 to 13.89% over the 20 days weathering. The haze and light transmission (%) were measured to determine the loss of properties after exposure to weathering. The control sample showed the haze of 89.62% and after 20 days of weathering this value decreased slightly to 88.65%. The light transmission in this study did not change significantly, it showed an increase from 67.67 to 70.68% while the total change in colour (ΔE) increased from 0.75 to 1.05 after 5 days and 20 days of weathering respectively. SEM was conducted to determine the extent of degradation and investigate micro-cracks on the surface of test specimens before and after exposure to accelerated weathering. After 20 days of weathering exposure some micro cavities could be noticed. The current study showed that the commercial PE bag claiming to be oxo-biodegradable started to show signs of degradation after accelerated weathering. However, biodegradation of such polymers is still relatively limited. There is still a requirement for substantial energy input in order for the degradation to initiate and it is uncertain whether these polymers are biodegraded or whether they disintegrate into smaller pieces. There are limited scientific studies on oxo-biodegradable polymers and their degradability. They still have to be validated as more environmental alternatives to conventional plastics. Based on these needs, this study aimed to assess the degradation of commercial oxo-biodegradable PE bags.