Investigation of Gas Release from Recycled Plastic Shopping Bags during Melting at Low Temperatures

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Introduction
Plastic waste is an environmental problem worldwide, and the recycling of this waste is problematic as toxic and fammable gases are released into the atmosphere when these materials are heated.Climate change and population growth make it necessary to fnd sustainable and environmentally friendly methods of recycling and to fnd new applications for used plastic [1].Undas et al. [2] found that 368 Mt of plastic was produced globally.According to Hopewell et al. [3], 4% of gas and oil produced worldwide is used to make plastic products discarded within a short period, and shopping bags form a large part of this waste.In 2019, worldwide plastic production was 460 t [4].Te OECD reports that only 9% of plastic worldwide is recycled [5], and in Europe, 9.4 MT was recycled in 2018 [6].Achillias et al. [7] state that 17% of the total plastic consumed in Western Europe is LDPE, whereas 11% is HDPE and 16% polypropylene (PP).LDPE has a lower density (0.917-0.930 g/ cm 3 ), is softer, and has more branching, whereas HDPE (density 0.944-0.965g/cm 3 ) is harder and has better heat resistance.Most virgin plastic bags are made from LDPE.Seventy-eight percent of all consumed plastics in Western Europe are thermoplastics.HDPE is a thermoplastic that can be melted and moulded many times.HDPE melting often involves high temperature pyrolysis in an oxygen-deprived atmosphere in reactors and the use of catalysts, which produce middle distillates like gasoline, kerosene, and diesel [8].According to Achillias et al. [7], the total hydrocarbon fraction of waste HDPE after pyrolysis at 450 °C was 3.28% weight on polymers, as shown in Table 1.Te highest compound was nC 4 (butane), followed by isopentane (iC 5 ), then propylene, and the aromatic C 6 hydrocarbon fraction.Pyrolysis, however, is not suitable if the material is destined for remoulding.
Currently, only a small percentage of the used plastic is recycled and reused, for example, as recycled plastic bags.In the European Union, companies that produce plastic shopping bags must now include at least 70% polyethylene waste [9].In this study, low-temperature melting to remould them to other useable products was investigated.Recycled plastic poses additional challenges for recycling, as the components of these recycled items are a mix of diferent, often contaminated, substances, which are often also additionally soiled during their use.It was investigated whether melting at a lower temperature reduces the overall risk by reducing the release of toxic, fammable, and greenhouse gases formed during the process.Aguado et al. [10] found that heating plastic at low temperatures results in three fractions: high calorifc value gas, condensable hydrocarbons, and waxes.In other studies of lower temperature melting of HDPE, run endothermically until 300 °C, DSC (diferential scanning calorimetry) and TGA (thermogravimetric analysis) test results showed that there was a loss of 0.5% of the initial mass of the polymer.Above 300 °C, an exothermic reaction starts thermal degradation, which reaches its highest point at 500 °C [11].In this study, FTIR-ATR was used to determine the composition of the recycled plastic bags, and then, the bags were cut in pieces and melted at diferent temperatures.From the gas that developed above the heated plastic, quantitative and qualitative gas analysis using GC-FID and GCMS analysis was conducted.Te results were then compiled into diferent compound classes, and the efect of rising temperatures on gas elution was evaluated.

Materials and Methods
Tree repetitions of samples of 0.5 g of supermarket shopping bags which contained 80% recycled material and were made in Malaysia were analysed.Te thickness of plastic was 0.55 mm thick.Tey were white and had red and yellow ink dots on them.Te samples were not washed before analysis.Te plastic bag samples were cut into small squares.FTIR was conducted using Shimadzu IRA Afnity-IS with a GladiATR10 attachment on the original, uncleaned bags.All colours, including white, were scanned separately.For both GC-FID and GCMS headspace analyses, the samples were measured into 20 mL HS vials and heated in an oven to 160 °C, 200 °C, and 250 °C for 30 minutes.All heating temperatures were above the melting point of HDPE at 130 °C.Each sample was performed in triplicate.Te shopping bag samples in the vials are shown in Figure 1; the plastic material melted at all three temperatures; however, at 250 °C, the selected material turned into a brown, charred waxy substance.
Te sample vials heated to temperature were used directly for compound screening.Te eluding gases were then analysed with GC-FID analysis and HP-SPME GCMS analysis.Te standard methods for gas analysis were used.All instrument parameters are described in Attachment 1.For GC-FID analysis, the samples were injected manually from the headspace of the vials into the instrument.GC-FID analysis was used to measure alkanes (saturated hydrocarbons) methane, acetylene, and ethylene.Gas chromatography using HP-SPME-GCMS was used for all other volatile compounds.SPME stands for solid-phase microextraction, an injection technique in GCMS analysis which uses an SPME fbre to adsorb volatile compounds, which are then dissolved into a bufer and automatically injected.Volatiles were adsorbed onto a SUPELCO SPME StableFlex fbre (50/ 30 μm DVB/CAR/PDMS).GCMS was Shimadzu GC-QP2010 ULTRA ™ (Shimadzu Corporation, Kyoto, Japan) with an MS detector, and the column was purchased from Agilent Technologies, ZB-WAX, with measurements of 60 m × 0.50 mm (ID) × 0.32 μm.

Results
FTIR analysis showed that the sample consisted mainly of HDPE (81% confdence).Other polyethylene compounds were also present, like LDPE, ethylene/propylene copolymer, calcium carbonate dyes, magnesium stearate, waxes, and ionomers, which are used to cross-link plastics to make them sturdier, for example, in plastic bottles.Figure 2 shows the peaks of the functional groups present in plastic.Tere also was an additional peak at 872 cm −1 , which indicates a group of carbonates (calcium carbonate, often used as an additive to HDPE to provide colour).Te diferent inks showed small diferences in transmission (not shown).
Table 2 shows the peaks of the FTIR spectrum of the white area of the shopping bags.Te wave numbers 2916.37 cm −1 and 2846.93 cm −1 corresponded to the HDPE library entry, but the peaks at 1458 cm −1 (CH 2 bending), 1458 cm −1 , and 1427 cm −1 (CH 2 bending) had lower transmittance, indicating that these functional groups increased compared to the library HDPE spectrum.Tey also merged, whereas the HDPE peaks were separated.
Te quantitative gas analysis for methane, ethylene, and acetylene was conducted against calibration curves prepared from standards; the results for the headspace analysis of the gas are shown in Figure 3. Te highest amount of quantifed gas was methane, although, at 160 °C, there was a bit more ethylene.Te methane emission from the bags at 160 °C was 0.0295 mg/g; at 250 °C degrees, it was rising to 0.334 mg/g; the ethylene concentration rose from 0.035 mg/g to 0.082 mg/g.In all samples, there was little acetylene (<0.004 mg/g sample).
Te qualitative GCMS scan revealed a variety of volatile compounds.Tese were identifed with peak integration and compound searching against the NIST14 library SI > 70.Relative abundance of the gases in the scan in percent was used to report the results.Relative abundance is a common way to report results from a qualitative GCMS scan and is only applicable for the given scan under the set conditions.Many gases were alkenes, alkanes, and fatty acids.Tis analysis identifed 146 peaks across three heating temperatures.Te samples contained four phthalates.Figure 4 shows the 10 most abundant gases in the scans at their respective temperature.Te most abundant gas at all heating temperatures was hexadecane.At all heating temperatures, eicosane, 1-nonadecene, nonanal, and isopropyl myristate occurred.Other than that, there was little similarity of gases among the 30 most abundant ones across all heating temperatures.
Figure 5 shows the compounds groups which were released by the plastic bags.For this compilation, only the 30 most abundant compounds from each heating temperature were selected.Tis selection accounted for 78-87% of all compounds.Alkanes were the dominant compound, amounting to 28% at 160 °C and increasing to 43% at 250 °C.Tis situation was reversed for alkenes, which decreased with rising temperatures (23% at 160 °C to 3% at 250 °C).Aromatic hydrocarbons increased to 10% of the total gas volume at 250 °C.Te largest group of methylated   compounds was methylated alkanes, which amounted to 10% of the gas volume.Additionally, there were methylated alkenes and esters.In Figure 6, the compound groups which were methylated are shown separately.
Long-chain fatty alcohols amounted to 5% of the gas, decreasing with rising temperatures.Te percentage of acids was 5%.Tey only occurred at 160 °C.
Te melted plastic bags released diferent phthalate compounds.Table 3 shows the phthalates in % discovered at diferent temperatures.
Aldehydes and organic compounds created by the oxidation of alcohol were well below 5% and decreased with rising temperatures.Table 4 shows the aldehydes produced.Acetaldehyde was not detected.Te detailed results from all gas analyses are provided in Attachments 1 and 2.

Discussion
Finding ways to reuse plastic waste safely has a high priority.One of the main contaminants is plastic bags, which are found in oceans [12] and landflls.Tey are a fre hazard and release toxins into the environment [13], and their degradation contributes to greenhouse gases [14,15].In 2016, the U.S. Environmental Protection Agency joined a retail industry campaign which intended to double plastic flm recycling to 2 billion pounds by 2020 [16].Recycled shopping bags are a step in the right direction, but they also need to be recycled when they reach the end of their lifetime.Recycled plastic bags are made up of many diferent materials and may contain contaminations from unknown sources and therefore pose additional challenges, as the components of these recycled items are a mix of diferent substances.Heating them for remoulding invariably releases greenhouse gases but less so at lower temperatures.Used plastic may be soiled by organic matter from their use, thereby creating a biohazard.Te microbes that grow there, as well as the organic matter, add to the load of greenhouse gases when these bags are heated; however, heating them may also sterilise the material, making it safe for future use, for example, for sealing grain to minimise losses after harvesting [17], for use as hospital gift bags for new mothers [18], or as Tedlar bags for gas storage [19].Treatments like colouring and plasticisers add another level of uncertainty to the process.In this study, it was investigated whether melting recycled plastic at a low temperature reduces the overall risk of recycling by reducing the release of toxic, fammable, and greenhouse gases formed during the process.We analysed some of the gases released from plastic shopping bags made from 80% recycled material to determine if the process was safe when the bags were melted at 160 °-250 °C for remoulding purposes.Tis temperature range falls into the endothermal stage of degradation [11].
FTIR analysis (Figure 2, Table 2) showed that the bags consisted primarily of HDPE and were coloured with ink which contained calcium carbonate.FTIR is a wellestablished method for the identifcation of polymers using infrared absorption bands which show functional chemical groups in the material.However, structural isomeric polymers like HDPE and LDPE are often hard to 4 Advances in Materials Science and Engineering distinguish.Chércoles et al. [20] and Nishikida and Coates [21] showed that compared to HDPE, LDPE had an additional peak at 1377 cm −1 , which represents CH 3 bending deformation.Tis peak was not present in our samples.While most shopping bags are made from LDPE, the recycled shopping bags in this study consisted primarily of HDPE, likely because plastic chips from many diferent hard plastic sources were used to produce these bags.Te qualitative GCMS scan (Figures 4-6) and (Tables 3  and 4) identifed 146 peaks across three heating temperatures.Tis shows that the bags consisted of many diferent compounds.Te ten most abundant gases are shown in Figure 4. Te main gas at all three temperatures was hexadecane (C 16 H 34 ), followed by eicosane (C 20 H 42 ).When compared to high temperature pyrolysis, longer chained hydrocarbons are much more prevalent.For example, Achilias et al. [7] found that at 450 °C, the most common hydrocarbon was nC 4 (butane), followed by isopentane (iC 5 ).One explanation for why there are diferent compounds across the diferent temperatures could be that at diferent temperatures, diferent chemical reactions took place between components.Tis could be due to the breaking up or formation of bonds, saturation, or methylation.Te results showed, for example, that with rising temperatures, unsaturated hydrocarbons (alkenes) were transformed into saturated compounds (alkanes).
Te analysis also showed that the plastic bags released methylated carbon compounds, mainly alkanes, alkenes, and esters, as can be seen in Figure 6.In the temperature range analysed, higher methane concentrations did not lead to higher methylation in the gases.Methylation can occur through methane exposure released from HDPE or from methane stemming from organic contaminants.Moulay [22] states that C-methylation with methane is an oxidative process.It would therefore not occur during pyrolysis in oxygen-deprived conditions; however, in this study, melting occurred in ambient air conditions.Microbes may also be present in the bags, producing methane gas from Advances in Materials Science and Engineering contaminants.According to Ghosh et al. [23], microorganisms can help break down the used plastic; however, this was not the aim of this project.
Yamashita et al. [24] state that aldehydes are one of the main fammable components eluding when plastic is melted, but in this study, their concentration was less than 5% of the total gas volume.Seven aldehydes eluded from the heated plastic bags, but acetaldehyde was not detected in the qualitative GCMS scan.According to Ullmann's Encyclopedia of Industrial Chemistry, aldehydes are valuable compounds.For example, nonanal, which was detected in the scan, is an essential marketable chemical used as a favour in food items and is also used in perfumes as it has a citrus-like scent.Food favours, resins, and rubber accelerators are made from pentanal.It has a nutty and fruity smell of fermented bread [25].As the quantities of these compounds were very small, commercialisation would be difcult.Acetaldehyde, which was only present in small quantities in the GCMS scan, can easily be reduced to ethane, a gas that could  have been present in larger quantities but was not quantitatively analysed in this scan.Ethane gas is an unsaturated hydrocarbon, the smallest alkene (C 2 H 4 ), and highly fammable and explosive [26].
Phthalates are commonly used as plasticisers [27] and not present in pure HDPE.Dibutyl phthalate (DBP) was the most abundant phthalate eluded from the bags, eluding at 3% at 160 °C and increasing to 3.5% at 200 °C.At 250 °C, it was not detected (Table 3).Like most phthalates, it has adverse health efects [28].Surprisingly, dibutyl isophthalate (DBP) and DEHP eluded at a low heating temperature of 160 °C, although the volatilisation temperature of phthalates is thought to be higher.Tis suggests that the actual temperature in the glass vial might have been higher.Te elution of diferent phthalate compounds at diferent temperatures from the recycled plastic bags leads to the conclusion that the composition of these plastic bags was not homogenous, as each part of the bags could have contained diferent recycled plastic pieces or virgin plastic.Other compounds were also present, including calcium carbonate ink, which was detected in the FTIR scan and may also have contained phthalates.Tis result highlights some of the additional challenges posed by the recycling of recycled plastic.
For GC-FID quantitative analysis (Figure 3), the same sample preparation as for GCMS was used.Te fammable gases quantifed with a standard were acetylene, ethylene, and methane.Tese gases represent only a small percentage of the total gas released, as the GCMS results, which were not quantifed, indicated that there were many other gases present.Te gas release from the quantifed volatiles was only 0.3 g/kg polymer or 0.03%.Te most abundant quantifed gas was methane.Methane is considered an asphyxiant at extremely high concentrations, as per Rim-Rukeh [29].Methane and ethylene are potent greenhouse gases [14,15], and it is reported that methane emissions are responsible for 25% of the current temperature rises worldwide [30].Finding ways to reduce it has high priority.Royer et al. (2017) [31] discovered that polyethylene produced the greenhouse gases such as methane and ethylene, when exposed to the sun, with the release increasing over time.Te methane gas produced could also stem from contamination of the initially recycled plastic pieces or from contamination of the shopping bags during use, as they were not cleaned before analysis.In our experiment, overall methane concentrations were very low.Te methane emission from the bags at 160 °C was 0.0295 mg/g.At 250 °C degrees, it was rising to 0.334 mg/g, eleven times higher; the ethylene concentration rose from 0.035 mg/g to 0.082 mg/g, around twice the initial amount, and acetylene remained at a very low value of 0.004 mg/g (Figure 3).If 1 kg of shopping bags was melted in a room, it would only lead to an air methane concentration of 0.02 ppm.When this methane release is compared to the methane released through pyrolysis at 450 °C [7], which was 0.01% weight on polymers, our fnding was only lower for the 160 °C temperature (0.003%).Tis result needs further investigation.Te American CDC's National Institute for Occupational Safety and Health (NIOSH) [32] states that the maximum recommended safe methane concentration for workers over 8 hours is 1,000 ppm (0.1%).Melting, however, should only be carried out using safety precautions, for example, an exhaust fan or a fume hood.
4.1.Summary.Te recycled bags consisted mainly of HDPE but also some LDPE, ethylene/propylene copolymer, calcium carbonate dyes, magnesium stearate, waxes, and ionomers.During heating at low temperatures, the gas fraction in the headspace consisted of a mix of methane, ethylene, acetylene, and many other, primarily long-chain aliphatic and aromatic hydrocarbons, esters and methylated compounds, aldehydes, and phthalates, as well as many other potentially hazardous and fammable gases.Reuse of nonrenewable fossil fuels is essential.It leads to reduced energy consumption, less landfll, and reduced CO 2 and other greenhouse gas emissions.Overall, melting of recycled plastic at lower temperatures and ambient oxygen levels can help further reduce the environmental impact of plastic production, as it leads to additional useable products, reduces gas emission, and is more energy efcient than high temperature pyrolysis.Although it would be better to frst recycle 91% plastic that is not currently recycled, reuse of recycled plastic can help reduce the environmental impact of plastic production even further by supporting a circular economy.

Conclusions
(1) Alkanes were the largest fraction of all compounds.
Compared to pyrolysis at higher temperatures, more long-chain hydrocarbons were released (>C 6 ).Alkane hexadecane, C 16 H 34 , a saturated aliphatic hydrocarbon, was the most abundant compound found in the GCMS scan, amounting to 28% at 160 °C and increasing to 43% at 250 °.Te lower the temperature, the lower the emission of greenhouse gases, which are often short-chain alkanes like methane.For example, the release of shortest alkane methane at 160 °C was 11 times lower than at 250 °C.(2) Alkenes decreased with rising temperatures (23% at 160 °C to 3% at 250 °C), as they were transformed into alkanes.(3) Methylated compounds, for example, methylated alkanes at 10%, were present at all temperatures.Methane and methylated compounds are released from plastic itself and contamination of the bags with organic matter.Heating the bags may remove some of the biohazard posed by organic matter and microbes, making the new articles safe to reuse.(4) Potentially useful substances like aldehydes were detected in small quantities.(5) Potentially toxic substances like phthalates and aromatic hydrocarbons like phenol and benzene were released in small quantities, but at signifcant temperature, the efect in the studied range could not be established.
Advances in Materials Science and Engineering (6) Safety measures during melting should include good ventilation and exhaust fans, as well as fre-fghting equipment on hand.

Figure 2 :
Figure 2: FTIR scan transmission results of the original white bag area (a) compared to the HDPE library entry (b).Te arrow denotes the calcium carbonate dye used.

Figure 4 :
Figure 4: Te ten most abundant gases in the plastic bags in % at diferent temperatures.

Figure 5 :Figure 6 :
Figure 5: Concentration of substance groups (%) eluded from plastic bags with increasing temperature, calculated from the 30 most abundant gases in the GCMS scan.

Table 2 :
FTIR-ATR peaks taken from the white area of the shopping bag.
Quantitative analysis of the fammable gases like methane, ethylene, and acetylene in bags (B) at 160 °C, 200 °C, and 250 °C.
Concentration in mg/g sample in the HP-SPME headspace measurement.

Table 3 :
Phthalates released at diferent temperatures in percent.

Table 4 :
Aldehydes occurring in the GCMS Scan.