Thermal Response and Degressive Reaction Study of Oxo-Biodegradable Plastic Products Exposed to Various Degradation Media







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In this work, three plastic film products commonly used as commodity thermoplastic articles were analysed with the aim of characterizing their thermal behaviour and stability. The test specimens were subjected to a series of analytical tests to confirm their biodegradable nature. The specimens ranged between 30 and 70??m in thickness and showed high concentrations of regulated metals, namely, lead (Pb), postchemical analysis which can lead to its migration to natural sinks. The specimens were also exposed to degressive media, namely, accelerated (UV induced) weathering and soil burial field testing. The weight loss measured exceeded 58% after soil burial indicating deterioration under natural environmental stressors. In addition, the thermal characterization campaign executed with the aim of determining the product’s thermal response followed internationally recognised experimental protocols for the determination of thermal stability. The methodology used followed the International Confederation for Thermal Analysis and Calorimetry (ICTAC) recommendation for thermal stability and the computation of kinetic parameters. The degradation reaction kinetics were also determined postexposure to degressive media. Thermogravimetric analysis coupled with differential scanning calorimetry heat flow analysis and Fourier infrared spectroscopy results was also used in studying the degradation behaviour of the specimens. Analytical kinetic estimation methods relying on model free solutions enabled the determination of the apparent activation energy () of the specimens postexposure to degradation media. A shift in the degradation mechanism was also detected after studying the kinetic parameters which showed a range of between 86.64 and 226.90?kJ?mol-1 depending on the type of specimens and exposure media. It can be concluded that the oxo-biodegradable films are well suited for thermal treatment in the future as discarded plastic solid waste (PSW) articles. This work also paves the way for developing national standards and future plans for societies burdened with PSW accumulation.


1. Introduction


The accumulation of micro- and macroplastic materials alike has led to an overwhelming impact on the environment. Plastics are noted to be an essential commodity to our daily functions. They are versatile with excellent compounding, mechanical, and insulation properties. They are also characterized with a nondegradable behaviour in open environments. However, the increasing demand of consumers forced various countries and authorities to start considering alternative strategies to mitigate plastic pollution. It is estimated that nondegradable plastics accumulate on a global basis (in natural sinks) at a rate of 25 million tonnes per annum [1]. The United States Environmental Protection Agency (USEPA) has announced that 236 million tonnes of municipal solid waste (MSW) was generated on US soil back in the year 2003. The plastic solid waste (PSW) fraction was estimated to be 11.3% of the total solid waste (SW) load. The majority of this PSW has accumulated in open/urban environment or landfilled [2].


The build-up of PSW leads to a number of adverse environmental and health-related effects. These could be summarised as heavy metal migration to natural sinks, gaseous emissions, and release of toxins and finally the loss of carbon from the nonrecoverable fraction of plastics in the form of energy and chemicals [3, 4]. Figure 1 summarises the main environmental burdens associated with plastic pollution and mismanagement. PSW could be treated and recycled using various techniques, following the PSW management hierarchy previously depicted in Al-Salem et al. [5]. These technologies can transform plastics into various market-grade products, chemicals, and fuel that can result in the reduction of the mismanaged portion of the plastics in industrial/use flow.


Figure 1: Associated environmental burdens of unsanitary landfilling and/or mismanaged plastics.


The majority of consumers’ demand on plastic resin is oriented towards polyolefin (PO) polymers. High-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE) alone occupy over 37% of the US market demand [2]. These grades are typically used for the packaging sector and are nondegradable by nature. Therefore, it is essential to start advocating techniques and technologies that can reduce the bulk of plastics by promoting degradation routes of plastics. Biodegradation is one of those techniques that compete with PSW recycling and recovery as an alternative route for plastic waste management. Biodegradation of plastics can be achieved via a number of methods. The biodegradability of the constituting polymers of the plastic product can be enhanced by blending them with natural substances (materials) such as starch, chitin, or cellulose [6]. These materials are termed as biobased plastics and classed as hydrobiodegradable polymers. Natural resources, food supplies, and vast land space are consumed during the production of the raw material for the compounding of hydrobiodegradables. This raises a claim on the fact that such processes utilize natural resources that are best suited for human consumption and urban development. The second method that renders a plastic material as a biodegradable one can be achieved by mixing prodegradants (i.e., prooxidants) with the polymer. These additives contain various transition metals such iron (Fe), manganese (Mn), calcium (Ca), and cobalt (Co) [7]. Their aim is to increase the rate of oxidation with air leading to the cleavage of the PO under heat and/or light.


There exist a number of obstacles in transforming common commodity PO plastics into biodegradable ones, leaving behind nothing but carbon dioxide (CO2), water, and humus. PO polymers are hydrophobic which makes them resistant to hydrolysis in the first place. This is due to their chemical structure being of a long-chain hydrocarbon (HC), namely, CH2. The insertion of hydrophilic groups as a result of an initial chemical or physical degradation will result in the insertion of hydrophilic groups on the polymer’s surface [1]. After which, microorganisms will start growing by using the polymeric backbone as a carbon source. In addition, PO manufacturers typically use antioxidants and light transforming (LT) additives during the manufacturing of commercial-grade resins. These are used to give the plastic articles strength and more integrity for various indoor/outdoor applications [8, 9]. Furthermore, PO polymers possess a high molecular weight (MW) that ranges between 4,000 and 28,000?g?mol-1 [1]. Biodegradable polymers will have a MW of 500?g?mol-1 or less [10]. Therefore, it is essential to consider the abovementioned factors when the development of a biodegradable polymer takes place. The addition of renewable or prooxidant additives must initiate the long polymeric chain breakdown. This will result in producing oligomers, dimmers, or monomers that can have these microorganisms feed on as a carbon and energy source.


Various countries have started implementing obligatory regulations that govern the use of biodegradable plastic products. A leading example is the United Kingdom (UK) where majority of plastic carrier bags used by consumers is of biodegradable nature. On the other hand, countries within the Middle East (ME) region have just started to take such matters in a more serious manner. A prime example is the State of Kuwait noted for being the highest MSW generation country in the world [11]. The MSW generation rate has reached an alarming rate of 5.7?kg per capita per day, out of which plastics make up over 16% [9, 12]. Past research work has focused on the behaviour of oxo-biodegradables in marine environments and soil burial conditions. The aim was typically devoted to assessing the toxicity level and impact on local environments. Markowicz et al. [13] have recently evaluated ten different grades of polymers including oxo-biodegradable films. The aim of their work was to determine the material’s applicability for composting and potential impact on the environment. The work showed the inapplicability of prooxidant-filled polymers with heavy metal content for industrial composting. Possible release of metals in the environment also shows possible alarming threat to materials postcomposting or treatment. This inapplicability for composting was also previously declared by Camann et al. [14]. Requejo and Pajarito [15] studied LDPE oxo-biodegradable films in water. The monitored parameters include pH level, total dissolved solids, and oxidation-reduction potential. The results indicated that over the period of 49 days of continuous monitoring, the water quality has drastically decreased. Various oxo-biodegradable grades were also tested by seawater immersion over a prolonged period of time (180 days) by Nazareth et al. [16]. Majority of tested samples showed no evidence of degradation by studying their infrared (IR) spectra including polyethylene (PE) plastic bags, which was deemed as a misleading choice for environmentally concerned consumers.


Ojeda et al. [17] examined commercial oxo-plastic products found on the Brazilian market postexposure to environmental conditions. The studied samples have shown a degree of mineralization after three months of incubation with compost especially with highly saturated environments. The conventional PE films exhibited a slow degradation behaviour. Nikoli? et al. [18] studied LLDPE formulations with nano titania under various environmental stressors including soil and influential parameters such as moisture and organic matter. It was noted that by comparison to poly(butylene adipate-co-terephthalate) (PBAT) (trade name Ecoflex), fungi play a major role in the biodegradation rather than bacteria. This was confirmed by the DNA analysis conducted. It was also concluded that photochemical reactions of PBAT are the dominating factor in the UV-induced degradation. By observing past results within this research area, very few have attempted to examine the alteration of thermal response and stability of such material (e.g., oxo-biodegradable plastics) postexposure to various environmental conditions. However, in majority of cases reviewed, it was noted that the local environment plays a major role in the alteration of properties with time. The recyclability of such materials in thermal units and the changes in thermal stability also play a crucial role in determining the optimal treatment method as a PSW end of pipe solution.


There are no governing standards or regulations for the use of biodegradable plastics within Kuwait. Therefore, and as a cornerstone baseline study, it is essential to initiate the work revealed in this paper. In this work, three biodegradable plastic bags were secured from local sources within the state. The main objective is to study and reveal the nature of the plastic products, whilst verifying the claims of the supplying companies. The plastic bags are exposed to various routes of degradation, in order to determine their biodegradability under various environmental conditions. This was conducted to aid local government authorities in standardising the practices for such commodity products. A secondary objective of the work revolves around determining the thermal response and applicability of these products for thermal and thermochemical recycling technologies (e.g., incineration, pyrolysis, and gasification). These technologies are arguably the most beneficial in terms of environmental burden reduction and ecumenical return rate [19, 20]. Therefore, it is of immense importance to establish a baseline of thermal characteristics for such products of which scientific literature lacks. The work shown in this communication paves the way for determining the behaviour of biodegradble plastic filmsunder various under various degradation media, designed to have governing standards on a state level.


2. Materials and Methods2.1. Sample Acquirement and Assessment


Three types of plastic bags used in the Kuwaiti market were secured with their main characteristics depicted in Table 1. The samples acquired were presumed to be environmentally friendly and biodegradable, which were secured from various sources within the state (Figure S1). The three types of the plastic bags are the most dominant degradable plastic types in the local market. The samples were cut into standard testing dimension specimens accepted by international protocols for thin plastic film characterization as described previously [21]. A reference polyethylene (PE) bag obtained from a commercial outlet source in the UK was also analysed for comparative assessment. The reference material was used as received and was clear/transparent in appearance with a reported melting point of 129°C. For confidentiality reasons, the company name was left out of this communication.


Table 1: Types and description of plastic bags considered for testing. Adapted from Al-Salem et al. [21].2.2. Fourier Transformed Infrared Spectrometry (FTIR)


An IS10 Thermo-Nicolet FTIR unit equipped with an attenuated reflectance (diamond) attachment was used for the analysis and the identification of the material’s chemical fingerprint. The test was conducted in accordance with ISO 10640 [22]. Samples were tested using 32 scans for the background and each individual spectrum in a spectral range of 4000 to 400?cm-1.


2.3. Crystallinity Measurements Using Differential Scanning Calorimetry (DSC) and Thermogravimetry


A Shimadzu DSC (Model 60 Plus Series) was used to test control and exposed samples using samples taken from the middle section of the film specimens. Aluminium oxide (Al2O3) crucibles were used for both samples and reference material experimental runs. Crystallinity measurements were determined using scans of the first and second heating cycles between 50 and 230°C based on the peak area of the heat flow curve between 60°C and 130°C with a nitrogen (N2) gas flow rate of 20?ml?min-1 and a heating rate of 10°C?min-1. The cooling rate was set at 15°C?min-1 in similar conditions [23, 24]. The second heating cycle was included in the crystallinity analysis following the initial heating run, as it will eliminate inherited effects of thermal histories and weathering on the specimens. The degree of crystallinity (%) was calculated from the software by dividing the melting enthalpy over the melting enthalpy of a 100% crystalline PE (293.6?g?J-1) for each heating cycle [25]. Thermal degradation of the samples was investigated using a Shimadzu TGA-50 thermobalance equipped with a data acquisition/analysis software (TA Instruments) set to record the data every second under five heating rates () (i.e., 5, 10, 15, 20, and 25°C?min-1). A constant flow of pure (99.99%) dry nitrogen with a flow rate of 50?ml?min-1 was maintained throughout the experiments. The measurements were conducted using samples from room temperature to 550°C made with triplicates showing high repeatability with standard deviation (std.) not exceeding 1% in accordance with the International Confederation for Thermal Analysis and Calorimetry (ICTAC) recommendations [26, 27] for nonisothermal (dynamic) thermogravimetry, to diminish sample size influences on the kinetics.


2.4. Evolved/Pyrolysis Gaseous Product Analysis Coupled with Gas Chromatography-Mass Spectrometry (GC/MS)


A NETZSCH (Tarsus F3 Model) Thermogravimetric Analyser (TGA) was used for the determination of the thermal decomposition properties of the samples and inorganic content and the analysis of the evolved gases. Approximately, 10?mg of specimens was cut out from the samples for the analysis. Samples were heated from room temperature (RT) to 900°C at a heating rate (?) of 20°C?min-1. The TGA unit was calibrated using standard reference materials (e.g., tin, indium, and zinc) as per ISO 11358 [28]. The TGA unit was coupled with the FTIR IS10 Thermo Nicolet for the evolved gas analysis of the samples. The coupling was achieved with the use of a heated thermogravimetric infrared (TG-IR) transfer line from Thermo Nicolet (Figure S2). Combining the TGA with an infrared spectrometer enables the accurate characterization of the evolved compounds. In other words, TGA can reveal how much (quantity) and FTIR can reveal what (quality) is being evolved from a specimen. The gas cell and the transfer line were heated at approximately 200°C, preventing any possible condensation of the evolved gases. The standard protocols of ASTM E2105 [29] were followed for the determination of volatiles evolved. Both the TGA and the FTIR units were controlled by a propriety software. FTIR scans were conducted with IR at a frequency of one scan per minute. The total absorption of the evolved gases analysed by the FTIR is represented by the Gram-Schmidt curve (GS). The GS curve is similar to the DTG (1st derivative TG) of the TGA thermogram. Specimens were heated from RT to 900°C, and the gases were analysed in real time. The Gram-Schmidt graph was collected. The additives in the plastic bags were also investigated using an Agilent 7820A GC-Agilent 5977E MS Single quadrupole for pyrolysis gas analysis following the principles of ISO 21396 [30]. The unit is equipped with a HP5MS column with length of 30?m, bore size of 0.25?mm, and a stationary phase of 0.25 microns. A splitless injector was used at 280°C, and the carrier gas used was hydrogen in a flow rate of 1.2?ml?min-1.


2.5. ICP-MS/Additive Analysis


Inductively coupled plasma mass spectrometry (ICP-MS) was conducted after preparing the samples via microwave digestion with concentrated nitric acid (HNO3) using ICP-OES Thermo ICAP 6000 series. The aim of the test was to determine the presence of inorganic metal additives (prodegradants) rooted within the plastic bags. General (total) metal screening including mercury (Hg), arsenic (As), cobalt (Co), calcium (Ca), nickel (Ni), and lead (Pb) was performed following ASTM D5673 [31] and ISO 11885 [32].


2.6. Degressive Media Exposure via Soil Burial and Accelerated Weathering


Soil burial tests in field conditions were carried out to study the degradation of the plastic bags in the natural environment. The site for the burial tests was chosen as one of the locations within the largest operating landfill sites in Kuwait (South 7th Ring Road Landfill—29.1707° latitude and 47.9085° longitude) where no presumed landfilling is to be conducted for the next two years [21]. The site also reflects the type of land used for recent waste management activities within the state as discussed with Kuwait Municipality (KM) which is responsible for such matters. Readers are referred to Figure S3 for a pictorial depiction of the procedure and the burial site. The soil properties and characteristics were previously shown in Al-Salem et al. [33]. Ten film samples were buried at a depth of 0.3?m, so as to allow microorganism attack in accordance with the procedure of Muthukumar et al. [34]. Samples were buried for a total of 30 days. The average water absorption and sample weight loss were estimated for the test specimens as per the following [35]:where is the average water absorption (%), is the remaining mass at time , is the mass of the dried sample, is the initial mass, and is the sample weight loss (%). Samples were removed from the soil at a specific interval () which is taken as 30 days in this work. The samples were carefully cleansed with distilled water and superficially dried. Samples were then dried under vacuum at 35°C until constant weight. Samples were washed, dried, and stored in a dark laboratory space at 23°C/50% relative humidity (RH) between the sample collection time and characterization. Accelerated (artificial) indoor weathering (ageing) tests are commonly used for studying material integrity and were conducted in this work to determine the photodegradation extent of the studied film samples. The specimens were also exposed to accelerated weathering (AW) in accordance with ASTM D 4329 [36]. Samples were mounted on the racks facing the ultraviolet (UV) lamps with no empty spaces in the panels. This is in order to maintain uniform repeatable test conditions. The cycle A procedure was used for general application durability testing (i.e., 8 hours of UV exposure at 60°C followed by 4 hours of condensation at 50°C). At the end of each continuous weathering test, the chamber was cooled to room temperature and the trays were set to rest on a flat surface for a minimum of 24 hours. Samples were laid to rest for a minimum of 72 hours before characterization following internationally recognised methodologies and laboratory testing protocols of weathering [8, 9, 36]. A minimum of four replicates were exposed to the different exposure durations in the QUV machine chamber. Ultraviolet (UV) lamp irradiance was also selected according to ASTM D 4329 [36], and the lamp type was set to be 0.68?W?m-2 (irradiance) for normal lamp operation, which was maintained for almost 5000 hours of operation. The irradiance sensor was calibrated every 400 hours of lamp operation during the UV cycle under normal test temperature. The equipment used was cleaned every 800 hours to remove scale deposits resulting from water evaporation during the condensation cycles [8, 9].


3. Results and Discussion3.1. Product Identification and Biodegradation (Mechanism) Route


At the first stage, the three studied products were assessed for their type and additive content. The IR spectra obtained for the three types of plastic bags were used to identify the base polymer material (Figure S4). A comparative spectrum for the reference material is also presented with the other three types of materials studied. It was noted that the spectra show a PO polymer (e.g., PE) signature [37]. This was based on the presence of the bands in the region of 2916?cm-1, 2849?cm-1, 1472?cm-1 and the range of 731 and 791?cm-1. These bands indicate functional groups associated with C-H stretching asymmetric and symmetric vibrations, CH2 scissoring vibrations, and CH2 rocking vibrations, respectively [16]. These signature bands were also noted and recognised by other researchers in the past as signatory reference bands for PE polymers [3840]. Additional peaks were also detected in the studied samples, between the 850?cm-1 and 1450?cm-1 region of each IR spectrum (Figure S4). The presence of the peaks is an indication of possible additives in the bags’ formulation. Samples of the type II material showed some peak differences when compared with the other two materials. No additional peaks were evident at the 1450?cm-1 region, and the peak at 850?cm-1 was significantly less intense when compared with the other sample bags. This indicates that the formulation of this bag type is different.


Prodegradants are typically mixed with a base PO polymer to compound a biodegradable plastic product. This is in order to accelerate the degradation process. These additives will accelerate the polymer’s oxidative degradation. Most common additives used for such products are stearate (St) complexes of transition metals such as zinc (ZnSt), copper (CuSt), silver (AgSt), cobalt (CoSt), nickel (NiSt), manganese (MnSt), chromium (CrSt), and vanadium (VSt) or alkaline earth metals such as magnesium (MgSt) and calcium (CaSt) [41]. The subtraction of the base polymer spectra obtained for the materials studied revealed that peaks correspond to calcium carbonate (CaCO3), matching the regions between 2800?cm-1 and 3000?cm-1 and 1200?cm-1 to 1500?cm-1. This indicates that the studied blends are of oxo-biodegradable type.


The three types of bags along with the reference PE were also examined for their heat flow properties using DSC analysis. The main thermogram properties are depicted in Figure S5. Two indicative melting points were noted with the heat flow analysis at around 110-113°C and 122-130°C, respectively, for type I of the samples studied. This indicates that the samples are of a commercial grade blend of polymers originating from PO materials. The melting points and thermogram properties indicate that samples of type I can be a blend of PE polymers and more precisely LLDPE and LDPE (Figure S5). This blend is a common practice with plastic compounders allowing better flow properties for thin polymeric films, which was reported previously in past investigations [9].


There was a noted melting peak between 125 and 129°C for type II of the studied samples with a collective area under the curve estimated at 120?J?g-1 (Figure S5). Type III showed a melting point at 131°C. The base PO polymer of both types is consistent with HDPE. The crystallinity measurements depicted at later stages of this communication also confirms these findings.


3.2. Evolved Gas Analysis and Metal Content


The Gram-Schmidt curve for the qualitative analysis of the evolved gases is presented in Figure S6. According to the Gram-Schmidt graphs for type I and type III bags, the decomposition took place in two steps. The first was mainly for the organic part whilst the second was for the CaCO3 (Figure S7). Type II samples showed one main decomposition peak at the organic loss region. The inorganic decomposition region was very small corresponding to the weight loss region (1.78?wt.%). From the analysis of the results, it became apparent that the type II samples showed different decomposition characteristics than the reference PE material. The weight loss at the organic region was 96% with no residue at 900°C (Figure S7). Type II also showed less weight loss at the CaCO3 region. By combining the analysis of the results, it can be noted that the samples contained 9.53%, 1.78%, and 11.89% of CaCO3 (inorganic content/additives) for types I, II, and III, respectively. This is also consistent with the weak traces of the IR signal compiled in Figure S8 for the three tested bags and past investigations depicting oxo-biodegradable bags [42]. IR spectra for the second decomposition step were similar for types I and III and the reference material. It was consistent with a CO2 by-product of the decomposition of calcium oxide or calcium carbonate (Figure S9). Type II showed weak traces of CO2 and stronger traces of water. This could be attributed to the amount of inorganic content used in compounding the biodegradable bag. This was also noted previously in past findings on commercial-grade oxo-biodegradable plastics [17, 18].


The total ion chromatographs (TIC) obtained from the GC/MS tests showed similar traces and were consistent with the traces from a standard PE sample (Figure S10). However, it should be noted that type II showed weaker traces compared with the rest of the samples. This could be attributed to the less amount of additives present in the formulation and the distribution of the alkane/carbon numbers in the PO base polymer [43]. A total metal content analysis was performed for the three tested materials and the reference plastic bag. Inductive couple plasma optical emission spectroscopy technique was conducted, and t

» Author: S. M. Al-Salem,1 A. Y. Al-Nasser,1 M. H. Behbehani,1 H. H. Sultan,1 H. J. Karam,1 M. H. Al-Wadi,1 A. T. Al-Dhafeeri,2 Z. Rasheed,1 and M. Al-Foudaree1

» Reference: International Journal of Polymer ScienceVolume 2019, Article ID 9612813, 15 pageshttps://doi.org/10.1155/2019/9612813

» Publication Date: 30/04/2019

» Source: S. M. Al-Salem,1 A. Y. Al-Nasser,1 M. H. Behbehani,1 H. H. Sultan,1 H. J. Karam,1 M. H. Al-Wadi,1 A. T. Al-Dhafeeri,2 Z. Rasheed,1 and M. Al-Foudaree1

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