Comparison of Physical Properties and Degradation of Polylactic Acid and Polypropylene Facemask Layer

Abstract

The current 3-ply facemasks are produced mostly using polypropylene (PP) as the main plastic layer that is a non-biodegradable polymer. The objective of this study was to fabricate a polylactic acid (PLA) layer, a biodegradable polymer, using the electrospinning process. Comparison on the degradation trend of PLA layer and PP layer obtained from commercial 3-ply face mask (CFM) layer in seawater, river water, soil, and compost environment for a total of 60 days was observed. Results showed that the electrospinning method can produce PLA layers with similar thickness with current CFM layer, narrow distribution of fiber diameter, and slightly dense surface morphology comparable to the CFM. Synthesized PLA fiber layer shows 68.6%, 39.8%, and 415.7% improvement in tensile strength, Young’s modulus, and elongation at break, respectively, compared to CFM samples. Meanwhile, for the degradation study, PLA samples show more significant changes on its morphology corresponding to the degradation process that occurred on its fibers in the seawater, river water, and soil environment, whereas for the weight change study, only the soil environment shows significant changes after 45 days. In contrast, CFM samples did not show any significant changes corresponding to degradation activity.

Introduction

The novel coronavirus (COVID-19) pandemic has started to spread rapidly in the early 2020; COVID-19 spreads quickly from person to person through cough or respiratory droplets, bodily fluids, and contaminated surfaces [1, 2]. In the wake of the COVID-19 pandemic, facemasks have become a global healthcare requirement, which is particularly relevant in the early phase when there is a scarcity of information about infection potency and transmission. Filtering facemasks are a non-pharmaceutical method of reducing inhalation exposure to airborne contaminants linked to health risks and thereby avoiding the spread of infectious diseases.

3-Ply facemask consists of three layers, namely cover layer (outer layer), filter layer (middle layer), and shell layer (inner layer) that are commonly composed of nonwoven fabric made from polypropylene (PP) fibers. PP is widely used in the production of facemask. High molecular weight synthetic PP fibers containing large carbon–carbon bond are generally resistant to biodegradation because microbes are not accessible to them due to their hydrophobic nature [3]. It was previously reported that the hydrophobicity of PP makes it difficult for bacteria to attach [4]. PP is hydrophobic in nature due to its lack of chemical functionalities and non-polarity. However, due to its properties of hydrophobicity, high molecular weight, and high surface roughness, PP is difficult to degrade in the aquatic environment [5]. A previous study also found that the most influential degradation process for PP is photooxidative degradation, which requires PP to be exposed to sunlight and oxygen to produce polar carbonyl groups, esters, ketones, and acids, which will improve the hydrophilic characteristic of PP. Due to a combination of factors such as high temperature, ultraviolet exposure, hydrophobicity, and pH change, the thermoplastic polymer fraction of this plastic content can disintegrate into smaller particles, known as microplastics (MPs) [6]. As a result, a significant portion of PP ends up in the environment as MPs. Thus, scientists worldwide are currently seeking to develop a new layer made up of biodegradable fiber layer as an action to overcome the rising environmental problems and increase the efficiency of the filtration of viruses.

Biodegradable polymers are polymers that are gaining main attention by researchers in the development of a new facemask to reduce environmental issues. Polylactic acid (PLA) is a promising candidate for the development of the medical face mask filter layer due to its excellent properties. PLA has exceptional tensile properties, making it ideal for biomedical applications [7]. PLA also has strong biodegradability and biocompatibility, making it suitable for use as a bioresorbable polymer in a wide range of biomedical applications. PLA can be made from carbon dioxide, wheat, corn, and rice, among other renewable resources. The breakdown products of PLA are also safe for people and the environment. PLA requires between 25 and 55% less energy to manufacture than petroleum-based polymers.

Electrospinning is currently one of the best techniques for fabricating nonwoven filter layers made up of continuous fibers with diameters as small as a few nanometers, with large surface-to-volume ratios, high porosity, and a variable pore-size distribution as compared to conventional spun bond and melt blown techniques [8–10]. Electrospun filter membranes have shown promising results for filtration applications due to their controlled processing, which allows for customized material structure for specific applications [11].

Motivated by the research on biodegradable polymers and degradation of face mask, the present research aimed to fabricate PLA layer using the electrospinning method. Physical (surface morphology, fiber diameter, and fiber thickness) and tensile properties and degradation of PLA layer and PP commercial face mask (CFM) layer were compared. Although many studies on the degradation of PLA have been published, there is a limited amount of work on the degradation of PLA in the form of membrane, and comparison on degradation of PLA with CFM layer using PP was made. In addition, the synthesized PLA fiber layers were compared with the CFM filtration layer for their degradation behavior in four different conditions and durations. The four sources of nature, namely seawater, river water, soil, and compost, were selected as suitable for biological degradation. The degradation was evaluated based on morphology and weight change for duration of 15, 30, 45, and 60 days.

Materials and Methods

Raw Materials

PLA was supplied by NatureWorks (Minnetonka, MN, USA). PLA consists of 96% l-lactide and 4% d-lactide contents. PLA has an average molecular weight of 60,000 g/mol, density of 1.24 g/cm³, melt flow index of 4–8 g/10 min at 210 °C, glass transition temperature of 60 °C–65 °C, and melting temperature of 190 °C–210 °C. Two solvents, which are dichloromethane (DCM) and N,N-dimethylformamide (DMF), supplied by Merck (Darmstadt, Germany), were used to dissolve PLA.

Synthesizing PLA Fiber Layer Using Electrospinning Method

For the fabrication of the PLA fiber layer, the concentration of the solution was fixed to 12.5% w/v of the PLA, and two solvents were used to dissolve the PLA pellets. The ratio of DCM and DMF solvents was fixed at 70:30 for 50 mL solution. For the processing of PLA, the electrospinning parameters are set as shown in Table 1.

Table 1.

The parameters used to fabricate PLA layer

Parameters	Description
Voltage	10 kV
Solution stirring duration	48 h
Flow rate	1.5 mL/h
Distance	9 cm
Electrospinning time	60 min

Comparison on Degradation of Synthesized PLA and CFM Filtration Layer

The synthesized PLA fiber layers were compared to the CFM filtration layer with respect to physical (surface morphology, fiber diameter, and fiber thickness) and tensile properties. Degradation behavior of synthesized PLA and CFM filtration layer in four different conditions (seawater, river water, soil, and compost) was evaluated. The selected sources are also based on the real situation where facemasks are most commonly found to be improperly and unethically disposed by citizens. For the degradation test, there are a total of 48 PP samples of middle layer of commercial facemask, and 48 PLA fiber layers were used for the current study. The dimension of the sample was set to 4 × 4 cm. The duration setup for this degradation test is for 15, 30, 45, and 60 days, where three samples were used for each duration. For the seawater and river water setup, the samples were placed inside a small plastic container that has three partitions. The small containers were placed in a big clear container. Each small plastic container represents each duration. In contrast, for soil, the samples were buried 20 cm deep below the ground, and for compost, the samples were buried 20 cm deep inside a plastic container. The setup for the degradation tests in all the four conditions is as shown in Fig. 1.

Fig. 1.

Degradation test setup in a seawater and river water, b soil, and c compost condition

Characterization Methods

Tabletop scanning electron microscope (SEM), Hitachi TM3000, and field emission SEM, Zeiss Supra 35 VP were used to characterize the morphology of the PLA and CFM filtration layers. For the fiber diameter measurement, ImageJ software was used to analyze the images obtained from the SEM analysis. In this observation, the diameter of the fiber was identified. The number of the fiber (n) selected for this measurement was n = 100. A minimum of four SEM images were taken to obtain a total of n = 100. For weight change, analytical balance with four decimal points was used to weigh the weight of those samples. Then, the initial weight of those samples was subtracted with the final weight of each respective sample to calculate the degradation percentage [12].

where W_o is the initial weight of the materials and W_f is the weight after treatment over time. Micrometer screw gage (Mitutoyo tube micrometer series) was used to measure the thickness of each sample including the CFM layer.

The tensile test was performed by INSTRON 3366 Universal Testing Machine. Six samples were tested: three from the filtration layer of commercial facemask and another three from the synthesized PLA layer. The preparation of the samples was done according to the ASTM D882-12 standard. Those samples were cut into 60 mm × 5 mm for length and width, with a gage length of 30 mm. The pulling speed of this test was set to 50 mm/min, and the load used was 100 N [13]. With this testing, the tensile strength, Young’s modulus, and elongation at break were obtained.

Results and Discussion

Comparison Between CFM and Synthesized PLA Fiber Layer

In Fig. 2, the CFM filtration layer has thicker diameter than the PLA fiber layer, and the PLA fiber layer has denser packed layer with more uniform fiber. Figure 3 shows the distribution diameter and thickness of the PLA and CFM fiber layers. Fiber diameter measurement shows that the fiber diameter of synthesized PLA is smaller than that of CFM filtration layer. As can be seen, the distribution frequencies for the diameter of PLA fiber layer are more uniform than those of CFM filtration layer, where PLA fiber layer shows a narrower bell curve. Dense structure results in small distance between those fibers, where those fibers are closer to each other than the large distance between the fibers in the CFM filtration layer. According to Chung [14], when the distance between the fibers is closer to each other or equal to the size of the particle, better interception will occur during the particle filtration process. It is believed that packed fiber structure will result in excellent filtration efficiencies [15]. Apart from filtration efficiencies, the air permeability of the facemask and filter layers play an important role which is related to the comfort properties of the wearer. The breathability of the facemask is evaluated by pressure differential measurements. The air flow pressure is measured from the inner and outer layer and the pressure difference is calculated. A lower pressure difference indicates better breathability performance. Generally, the filter layer made up of smaller fiber have better breathability due to the voids present between the thinner fibers. It can be seen from Fig. 2, that the electrospun PLA fibers have shorter diameter which might lead to better breathability as compared to CFM filtration layers [16–18]. Based on the error bar in Fig. 3b, the thickness of CFM and PLA’s filtration layer is slightly similar.

Fig. 2.

Surface morphology of a CFM filtration layer and b synthesized PLA fiber layers

Fig. 3.

a Distribution curves of CFM and PLA fiber diameters, b average thickness of the PLA and CFM fiber layers

Figure 4 shows the samples used for tensile testing, and Fig. 5 shows the comparison of stress–strain curve on the synthesized PLA fiber layer and the CFM filtration layer made up of PP. The tensile strength test shows significant difference between the types of the plastic used as can be seen in Fig. 5. Based on the stress–strain curve in Fig. 5a, the synthesized PLA fiber layer samples show higher UTS than the CFM filtration layer. In Fig. 5b, the results show that the average tensile strength of the PLA fiber layer is higher than that of the CFM filtration layer, which is 6.54 and 3.88 MPa, respectively. The results obtained from the Young’s modulus also show that the PLA fiber layer has higher average Young’s modulus value, which is 187.27 MPa with lower standard deviation than the CFM filtration layer, which is 133.93 MPa with high standard deviation of 26.11. Based on the elongation at break, PLA fiber layer has higher elongation percentage, which is 124.07%, than CFM, where the elongation was only 24.06%. Higher tensile strength of PLA fiber layer might be due to the PLA fibers randomly welded together during the electrospinning process, reaching the collector and causing the union of fibers to form stronger connections [19].

Fig. 4.

Samples used for tensile testing a CFM filtration layer and b PLA fiber layer

Fig. 5.

a Stress–strain curve, b tensile strength and Young’s modulus, and c elongation at break of PLA fiber layer and CFM filtration layer

PLA fiber layer has higher Young’s modulus and elongation at break, which means that the PLA fiber layer is more resistant to deformation under load and more ductile than the CFM filtration layer respectively. The higher value of tensile strength is related to the fiber diameter, morphology, and fiber density. The nanofibers studies indicate that the mechanical properties of the fibrous mat is significantly dependent on the fiber diameter as it was found that the lower diameter leads to higher tensile values [20]. A uniform and aligned fiber with higher fiber density will lead to higher tensile strength, and hence, electrospun fibers will have higher mechanical properties as compared to spun bond CFM filter layers [21].

Degradation Study of CFM Filtration Layer and PLA Fiber Layer

Four degradation conditions were selected for the degradation study of CFM and PLA filtration layers. Seawater, river water, soil, and compost were used as degradation environment, and degradation was monitored for 15, 30, 45, and 60 days. After each corresponding day, the samples were taken out and characterized by surface morphology and weight change.

Degradation Effect in Seawater

Figure 6 shows the samples that have been degraded in seawater for 15, 30, 45, and 60 days. Physical appearance shows that the longer the degradation time, the color of the sample changes. Figure 7 shows the surface morphology of CFM and PLA samples degraded for 15, 30, 45, and 60 days, taken with SEM at a magnification of 3000 × . The surface morphology of the samples was analyzed to determine the degradation behavior on the fiber morphology with time.

Fig. 6.

Physical appearance on the surface of the a CFM filtration layer and b PLA fiber layer after degradation study in sea water on each respective duration

Fig. 7.

Surface morphology of the samples after degradation study in sea water at a–d 15, 30, 45 and 60 days, for CFM filtration layer and e–h 15, 30, 45 and 60 days, for PLA fiber layer, respectively

Based on the visual examination, no clear tear or crack was found on the surface of all samples. However, as the duration increased, more contamination was found on the surface of the samples. For the CFM filtration layer, the samples were contaminated by algae formation (Fig. 7a), green color and it became visible after 30 days of degradation study duration. In contrast, for the PLA fiber layer, the samples had less changes with respect to the color on its surface, but those samples were also contaminated by algae formation; however, the formation of algae does not necessarily indicate degradation of polymer samples since there are no concrete evidence which states that, micro-algae degrade microplastics [22]. Furthermore, PLA samples had changes with respect to dimension, where those samples were shrinking in size if compared to the initial dimension. Based on the surface morphology analysis, the contamination on the fiber (Fig. 7b), yellow circle can be clearly seen as early as 15 days of study, and the algae formation increases with times. For the CFM samples, some small voids were found on the fiber for the sample duration of 45 days as can be seen in Fig. 7c (red circle), and some cracks were observed on the fiber for the sample duration of 60 days as can be seen in Fig. 7d. Meanwhile for the PLA sample, the contamination that formed on the fiber was in a thin layer form that is quite different from the CFM sample that is thicker as can be seen in Fig. 7f. For the study duration of 45 and 60 days, some fibers were observed to be torn apart as a sign of the degradation process.

Figure 8 shows the line graph generated from the data obtained on the weight of the sample before and after the study. As shown in the figure, the weight of the CFM samples shows no significant decrease although those samples were left to degrade up to 60 days. It was found that the weight was increased, and the increment was due to the algae formation on the surface and the fiber of the samples. A slight increase in weight is observed for the sample duration of 15 and 30 days; however, the weight of the samples for 45 and 60 days was increased abruptly. In Fig. 8b, the weight of the PLA fiber layer also was increased, but the increased amount is lesser than that of the CFM filtration layer. The highest increase in weight was observed for the PLA sample after 60 days of soaking, where the weight is increased to 20.5% = + 0.0068 g. Meanwhile, for 45 days of study, the weight of the sample increased to 12.4% = + 0.0043 g, and the sample for 15 and 30 days only showed 0.3% increment. Although the analysis on the surface morphology of the PLA samples shows that the samples soaking for 45 and 60 days have some torn fiber due to degradation, the amount of algae formed on the fiber layer increased as the study duration prolonged, resulting in the increase on the weight of the samples.

Fig. 8.

Weight of the a CFM samples, b PLA fiber layer before and after degraded in seawater

Degradation Effect in River Water

Figure 9 shows the changes of the samples used for the degradation study in the river water for soaking time of 15–60 days. Meanwhile, Fig. 10 shows the surface morphology of the samples under SEM with 3000 × magnification. Visual observation in Fig. 9 shows that the samples do not have any sign of tear and crack instead only a brown color contaminant was seen on the surface of the samples. The brown algae formation gradually increased with respect to the area as the duration of the study prolonged and it started to become obvious after 45 days of study on the CFM sample. Meanwhile, for the PLA samples, the contamination on the samples was not as obvious as that on CFM samples. The brown algae formation in the river water samples is not as distinct as the algae formation in the seawater samples. Algae formation is highly dependent on the nutrient sources in the surrounding environment, and it was assumed that river water contains lower nutrition based on the amount of the algae formed on the samples. Dimension of PLA samples changes where the samples shrank after soaking. Based on the observation on the image of the surface morphology of the samples, there was no clear sign of defects on the fiber of both CFM and PLA samples for the samples with duration of 15 and 30 days. Sample of 45 days for both CFM and PLA showed some minor cracks on some of the fiber. However, there was no obvious sign that degradation occurred on the CFM samples for all study duration, which made it different with the PLA samples where the PLA sample for 60 days of study showed a clear sign of torn on its fiber as can be seen in Fig. 10h.

Fig. 9.

Physical appearance on the surface of the a CFM filtration layer and b PLA fiber layer after degradation study in river water on each respective duration

Fig. 10.

Surface morphology of the CFM filtration layer after degradation study in river water at a-d 15, 30, 45 and 60 days, for CFM filtration layer and e–h 15, 30, 45 and 60 days, for PLA fiber layer, respectively

Figure 11 shows the data plotted for the weight of the samples before and after the degradation study for each duration. Based on the figure, all the CFM samples show no significant decrease on the weight of the samples after the degradation study for each duration. The sample with duration of 15 days shows the least increases of its weight, whereas the weight was increased abruptly for the samples of 30, 45, and 60 days. The increment of the weight is believed to be due to the formation of the brown algae that have contaminated the surface and inside of the fiber of the sample. Meanwhile, based on Fig. 11b, three out of four samples show a minimal decrease on the weight of the PLA samples. Only the sample for 60 days of study shows an increase on the weight of the sample, in which the weight was increased to 3% (+ 0.0011 g), whereas the other samples decreased in weight for 45, 30, and 15 days of study with 0.2%, 0.9%, and 0.9%, respectively. The increase of weight on the sample for 60 days was believed to be due to longer study duration and more contamination on the sample as more algae were formed and contaminated the fiber.

Fig. 11.

Weight of the a CFM samples, b PLA fiber layer before and after degraded in river water

Degradation Effect in Soil

For the CFM samples, there were no significant physical changes on the samples except for the contamination of soil on the sample (brown color) as shown in Fig. 12a. However, for the PLA samples, immense changes were observed on the physical appearance of the samples as the study duration becomes longer as shown in Fig. 12b. The same samples were then examined under the SEM with a magnification of 3000 × to analyze the surface morphology of the samples after the degradation study as shown in Fig. 13.

Fig. 12.

Physical appearance on the surface of the a CFM filtration layer and b PLA fiber layer after degradation study in soil on each respective duration

Fig. 13.

Surface morphology of the CFM filtration layer after degradation study in soil at a–d 15, 30, 45 and 60 days, for CFM filtration layer and e–h 15, 30, 45 and 60 days, for PLA fiber layer, respectively

Based on the physical observation on those samples, there was no crack or tear found on the CFM samples. Those samples were only contaminated with soil on its surface and inside the fiber. However, at duration of 60 days, the physical appearance of the sample does not differ much with the other duration of study. For the PLA samples, the physical appearance of the samples started to show degradation at the start of 30 days of study with minor voids. As the study duration becomes longer, which is 45 and 60 days, more area of the samples were degraded with more obvious tear and voids. Apart from that, as seen in Fig. 13, the fiber of the CFM samples was not affected as there was no significant sign of degradation such as cracks, voids, or tear on its fiber for all the samples. The only visible defect to the fiber is the contamination of soil on the fibers of the samples. Kijchavengkul et al. [23] found that degradation by burying in soil takes a longer time where the degradation starts to show after week 24 of their study. Meanwhile, for the PLA samples, the fiber started to tear apart with the sample for 30 days of study, and the tear becomes more obvious on 45 and 60 days of study. The morphology of PLA samples is in agreement with the study by Weng et al. [24], where they reported that PLA started to degrade after 1 month of burying in soil where many corrosive holes appeared on the surface of the samples.

Figure 14 shows the line graph generated from the data obtained on the weight of the sample before and after the study. Based on the analysis of the figure, the weight of the CFM samples increases after the degradation study. The increase in weight was due to the soil contamination on the samples, and the weight increased higher for those samples with longer degradation study. The weight for CFM sample with 60 days of study increased to 8.5% (0.0029 g), which is the highest. In contrast, CFM sample for 45 days of study was increased in weight at 4.5% (0.0015 g), sample for 30 days increased at 3.2% (0.0011 g), and sample for 15 days increased at 2.1% (0.0007 g). Meanwhile, for the PLA samples, for duration of 15, and 30 days, the weight of the samples was increased due to the soil contamination inside the fiber layer as seen in Fig. 14. The weight of the samples was increased at 2.9% (+ 0.0009 g) and 4.5% (+ 0.0012 g) for 15 and 30 days, respectively. However, the weight of the samples for 45 and 60 days started to decrease as more area of the sample was degraded, leaving many holes on the samples. The weight of the sample for 45 days of study was decreased at 6.9% (−0.0026 g) and 7.9% (−0.0023 g) for the sample of 60 days of study.

Fig. 14.

Weight of the a CFM samples, b PLA fiber layer before and after degraded in soil

Degradation Effect in Compost

Figure 15 shows the physical comparison of the samples for the CFM and PLA samples buried under compost at four study durations. All the samples show no major defect such as crack and hole except only contamination on the surface of the sample. The size dimension of the samples also did not have many changes compared among the samples themselves. Figure 16 shows the surface morphology of the samples taken using SEM at 3000 × magnification.

Fig. 15.

Physical appearance on the surface of the a CFM filtration layer and b PLA fiber layer after degradation study in compost on each respective duration

Fig. 16.

Surface morphology of the CFM filtration layer after degradation study in compost at a–d 15, 30, 45 and 60 days, for CFM filtration layer and e–h 15, 30, 45 and 60 days, for PLA fiber layer, respectively

Based on the morphology analysis on CFM and PLA samples, the fibers of those samples do not show major defects or any sign of degradation. Most of the images taken show that the fibers were only contaminated with foreign particles coming from the compost. There were some defects detected on the CFM sample for 60 days of study as seen in the red circle in Fig. 16d. Meanwhile, there were a lot of contamination of foreign particles spotted sticking on the fiber of the PLA sample for 60 days of study.

Figure 17 shows the data plotted for the weight of the samples before and after the degradation study for each duration. All samples showed no significant decrease in weight. For the CFM samples, sample for 60 days of study shows a large increment on the weight of the sample, which is 16.9% (+ 0.0056 g), whereas the rest were only small increments, 4.6% (+ 0.0015 g) for 45 days and 3.6% (+ 0.0012 g) for both 30 and 15 days. In contrast, for the PLA samples, the increment in weight was lesser than for the CFM samples. For sample of 60 days, the weight was increased only at 2.4% (+ 0.0008 g) and 1.4% (+ 0.0005 g) for 45 and 30 days, respectively, and 1.2% (+ 0.0004 g) for 15 days of study. Although both types of samples increased in weight, the increment on the PLA samples was incredibly lesser than that on the CFM samples.

Fig. 17.

Weight of the a CFM samples, b PLA fiber layer before and after degraded in compost

Based on the analyses, for CFM filtration layer, no degradation process is observed for all four conditions. The results only show that the samples were contaminated with algae for the test in seawater and river water and soil with foreign particle for the test in soil and compost. The result is in accordance with other findings where CFM filtration layer is known to be made by PP, and PP is a non-biodegradable polymer. According to Arkatkar et al. [4], PP is composed of a continuous chain with repetitive methylene, which is known to be hydrophobic, and makes it resistant to degradation. The hydrophobicity of PP makes it also difficult for bacteria to attach. Veerappapillai and Muthukumar [25] stated that PP composed of carbon chain makes it difficult to be directly degraded. Khoironi et al. [6] also stated that due to the hydrophobicity, high molecular weight, and high surface roughness of PP, it is hard to degrade in an aquatic environment. In contrast, for the samples of PLA fiber layer, results showed that the PLA fiber was degraded in the seawater, river water, and soil. Based on the surface morphology, it can be clearly seen that the fibers were torn apart starting from 45 days for seawater, after 60 days for river water, and from 30 days for soil. This shows that the synthesized PLA fiber layer degraded faster when buried in soil than in other conditions.

In the study, it is observed that the PLA fiber layer in soil starts to show the degradation process after 30 days. The finding by Rudnik and Briassoulis [26] reported that the biodegradation of PLA under soil conditions is a complex process, and that the degradation rate is relatively slow. According to Martucci and Ruseckaite [27], PLA-degrading microorganisms are not widely distributed in the natural environment, and thus PLA is less susceptible to microbial attack in the natural biotic medium than other microbial and aliphatic polyesters. This explains the relatively small amount of weight change results in all the conditions in this study. This was also agreed by Fu et al. [28] where they found that the PLA started to show significant degradation process on the PLA sample after 6 months of incubation in freshwater. Chamas et al. [29] also stated that a slow hydrolysis on the PLA ester linkage can occur when exposed to moisture at low temperature at least 30 °C. In addition, Zhang et al. [15] stated that PLA will degrade under the influences of heat, water, oxygen, and enzymes. Furthermore, Weng et al. [24] stated that PLA is first hydrolyzed, and that the hydrolysate products are then decomposed by microorganisms into carbon dioxide and water. According to Fu et al. [28], PLA macromolecules decompose mostly at the ends of molecules by a zipper-like undoing mechanism, which is controlled by the molecular weight, crystallinity, purity, and stabilizers of PLA.

The degradation mechanism of plastics is highly dependent on the polymer functional groups and degrading environment. The degradation mechanism usually involves hydrolysis, photooxidative degradation and thermal oxidative degradation [30]. The hydrolysis and photooxidative reaction are more significant in sea and river water since they are exposed to sunlight resulting in shorter carboxylic acid and alcohol terminated chains formation [5]. The plastics buried in soil and compost experience a lower photooxidative and hydrolysis degradation; however, thermal oxidative reaction is more significant since the plastic are less exposed to sunlight and oxygen. In case of photooxidative mechanism, the degradation initiates due to the extraction of hydrogen from the carbon backbone, generating a reactive carbon-based alkyl radical. The radical chain reaction further progresses with O₂ reaction forming peroxy radical which abstracts the hydrogen atom from the subsequent polymer chain to form hydroperoxide and a new alkyl radical. After which the bond scission (O–O) in the hydroperoxide occurs that leads to formation of alkoxy and hydroxyl radicals which abstracts another hydrogen from the polymeric chain forming new alkyl free radical; finally, the termination of reaction occurs through free radical recombination. The thermal degradation reaction also follows free radical mechanism in which the plastics are broken down into small molecule carboxylic acids, aldehydes, and other oxygenates [31, 32].

Conclusion

It was found that the synthesized PLA fiber layer produced using the electrospinning method shows consistent fiber diameter, finer surface morphology, and higher tensile properties than the CFM filtration layer. Degradation study shows that synthesized PLA fiber layer shows more significant degradation process based on the analysis on the visual inspection on the physical appearance and the surface morphology of the PLA samples. Based on the weight change on the degraded samples, the results show no significant decrease on all the CFM samples, but instead some large increments were observed especially on the samples of 60 days of study in all conditions. Meanwhile, for PLA sample, an insignificant trend in weight change was observed. However, samples buried in soil after 45 and 60 days of study show changes in weight that indicate degradation starts to occur. The morphology of PLA samples clearly shows significant changes on the fiber where some of the fibers started to tear apart and eroded as the duration of degradation study increased. This shows that there was degradation process that occurred for the PLA samples. However, for the CFM samples, no significant changes can be observed on its fiber except for contamination by algae and foreign particles.

Funding

AUN/SEED-Net/JICA, 304/PBAHAN/6050453/A119, M. Mariatti.

Data availability

Data will be made available on request.