Introduction

Polyethylene (PE) is one of the most widely used plastics in the world for a variety of several applications, mainly packaging, due to its low cost and excellent properties, including low strength, hardness and rigidity, as well as ductility and impact strength [1, 2]. PE types used commercially were high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), ultra-high molar weight polyethylene (UHMWPE), and ultra-low-density polyethylene (ULDPE) [3]. LDPE films are light and easy to use, making them attractive for industrial applications. They are odor-free, non-toxic and show good ductility and low water permeability, allowing their use in food packaging [4, 5]. Their degree of crystallinity influences properties such as tensile and impact strength, stiffness, and chemical resistance [5]. However, their linear chain and lack of functional groups lead a very slow degradation under natural conditions [1, 2]. Additionally, when discarded these materials occupy large areas, and when mechanically recycled require a significant amount of water for rinsing purposes [6,7,8]. In this context, there is a crucial environmental need to develop new methods and technologies that can make this polymer more degradable or biodegradable.

In general, polymer degradation or biodegradation is influenced by both abiotic and biotic factors. Abiotic term is referred to parameters such as mechanical stress, light and temperature, and biotic term is referred to the involvement of naturally microorganisms such bacteria, fungi and algae [9, 10]. Some studies report that chemical, thermal, or photo-oxidative treatment can facilitate LDPE biodegradation. These processes are capable of reducing the hydrophobicity and molar weight of LDPE, favoring degradation by microorganisms [11,12,13]. According to the literature, biodegradation can be considered as a secondary process where microorganisms use these oxidized products for conversion of H2O, CO2, and CH4 or energy storage products, such as polyhydroxy alkanoates [14, 15]. However, natural weathering caused by solar radiation, moisture, and wind leads to a slow oxidative degradation due to the formation of free radicals [16].

Considering biodegradation process of biodegradable polymers it can be divided in four steps: biodeterioration, depolymerization, bioassimilation, and mineralization. In biodeterioration step, there is microbial biofilm formation, which leads to polymer surface degradation, and consequently, its fragmentation into smaller particles. After that, microorganisms release extracellular enzymes, which catalyze the depolymerization of polymer chain into oligomers, dimers, or monomers. Bioassimilation occurs when these small particles and depolymerized compounds are absorbed by microbial cells producing primary and second metabolites. Finally, all these metabolites are degraded and mineralized into CO2, CH4, H2O, and N2 [9]. Many techniques can be used in combination to measure or quantify biodegradation process, such as: TGA, which measures thermal stability of a polymer, and a decrease in this parameter can be an indicator of polymer degradation; GPC, which measure molar weights of polymers (Mn and Mw), and a decrease in Mn can indicate chain scission; FTIR, which is normally used to evaluate and monitor chemical modifications in polymer structure; SEM, which can be used to observe formation of cracks and holes or biofilm formation (demonstrate microbial colonization); and standards methods, for the production of CO2 and/or CH4 measurements [17].

Another way of promoting LDPE biodegradation is by adding substances such as natural macromolecules, pro-oxidants compounds, or biomolecules to the matrix [18,19,20]. Pro-oxidants additives are organometallic which need oxidative degradation (ultraviolet radiation and/or heat) to be activated. After that, it allows molecular weight decrease and addition of oxygenated groups on PE surface, which are easily metabolized by microorganisms [17]. Biodegrading additives are composed since natural substances (enzymes or natural polymer) to synthetic polymers containing ester, hydroxyl or ether groups, which are prone to hydrolytic cleavage by microorganisms[19, 21] In our group’s previous work [22] degradation process in soil of polypropylene (PP) blends with 2–8% (w/w) of two biodegrading additives (organic and enzymatic) was evaluated over 6 months. Both additives had influence on PP degradation with 2% of additive (low percentage), indicating acceleration of this process by their action. Authors highlighted the need for further studies with enzymatic additive because it never has been reported in literature before. Moreover, in other work of our research group [23] , the degradation process in natural fresh water of the same PP blends was evaluated over 6 months. PP degradation also was favored by the use of enzymatic additive in fresh water, showing that blends could have less durability after use, which may lead to environmental benefits, since it is an alternative to minimize the effects caused by plastic waste. The main aim of all these additives is induce deterioration of PE structure after discard, to allow more access of microorganisms and their extracellular enzymes during the biodegradation process. Thus, this study aims to investigate and evaluate the changes in the chemical, thermal, mechanical and morphological properties of abiotically degradable LDPE blend prepared with an biodegrading additive (without metals) related to pure LDPE, after exposition to accelerated aging (abiotic degradation) and inoculation in soil (biodegradation).

Experimental

Materials

Low-density polyethylene (LDPE) was obtained in pellet form by Braskem (Brazil, product code PB608, MFI 30 g/10 min and density 0.915 g/cm3). Biodegrading additive was used in pellet form and it contained 95% LDPE and 5% of enzymatic compounds (India). Soil used in biodegradation assays (pH 8.03, 37.7% dry basis moisture content, 59.3% water holding capacity and C/N ratio of 22.7) was obtained from a landfill located in Porto Alegre city (Rio Grande do Sul state, Brazil). Commercial corn starch (~ 25% amylose and ~ 75% amylopectin) was purchased from Unilever Company (Brazil). Potassium hydroxide (KOH) and hydrochloric acid (HCl, 37%) were supplied by Vetec and Neon (Brazil), respectively. Glutaraldehyde and osmium tetroxide (OsO4) were supplied by EMS Company (USA). Phosphate buffer solution and acetone were supplied by Merck (Germany).

Biodegradable LDPE film preparation

LDPE/biodegrading additive blend (8% w/w) was prepared by injection, using a thermoplastic injector (Wittmann Battlefield, Austria) with a single screw speed of 350 mm/s and a temperature profile of 175 °C in the first zone, 170 °C in the middle, 165 °C in the last zone and 180 °C in the nozzle. Pure LDPE (Mn 10.0 kg/mol and Mw 96.0 kg/mol) and LDPE/additive blend (Mn 7.2 kg/mol and Mw 93.1 kg/mol) were denominated as PE0 and PE8, respectively. Films were prepared by hot-pressing at 110 °C under three tons for 2 min in a hydraulic press model MA098 (Marconi, Brazil). It presented approximately 0.20 mm of thickness, which was measured using a digital caliper ruler.

Abiotic degradation (accelerated weathering test)

A QUV/spray/240 accelerated weathering chamber (QLAB, USA) was used for abiotic degradation study (Fig. 1a), in accordance with ASTM G154-16. PE0 and PE8 films (95 × 65 mm, independent triplicate) were exposed to alternating cycles of 8 h of UV light (UVA lamps at 0.89 W/cm2 and 340 nm with an incident radiation beam at 90° and temperature of 60 °C) and 4 h of water spray (tap water) over 1000 h (42 days), at 40 °C and 50% relative humidity. To assess the effect of exposure over time, the aged samples were removed from the chamber every 200 h (8 days). Films were identified using the acronym PEX-YYY, where X designated the additive amount in the blend and YYY as the exposure time (h), e.g., PE0-600 (pure LDPE with 600 h exposure).

Fig. 1
figure 1

Scheme of accelerated weathering (a), soil exposition (b) and carbon mineralization assays

Full size image

Biodegradation assays

Biodegradation tests were carried out by two different methods (Fig. 1b, c) described below:

Soil exposition assay (biodeterioration evaluation)

Non-aged and aged (600 and 1000 h) PE0 and PE8 samples were exposed to soil for 60 days (composting conditions: 58 °C; 50% moisture), at a sample/soil ratio of 0.5 g/30 g, in a microbiological incubator (Brazil) according to ISO 14855-12.

Carbon mineralization assay (aerobic biodegradation evaluation)

Aerobic biodegradation test of non-aged and aged (800 h) PE8 samples was performed in accordance with ISO 17,556 and ASTM D5988-18 for 166 days (22 ± 1 °C) in soil (sample/soil ratio of 1 g/500 g), respectively. Commercial corn starch was used as positive control, and the system which contained only soil was used as blank. KOH solution was used to trap the produced CO2 and a 0.25 M HCl solution was used for the titration to quantify this production. For each system, it was used 20 mL of 0.5 M KOH solution and 50 mL of distilled water. KOH solutions of the systems were removed each 1 week, titrated with HCl and it was replaced by a new KOH solution and distilled water. According to the stoichiometry (Eqs. 1 and 3), produced CO2 was calculated (Eq. 4), where VHCl is the spent volume of HCl solution in the acid–base titration. Elemental analysis was performed for both materials to quantify C content, which was used to calculate theoretical CO2 (Eq. 5) based on stoichiometry of Eq. 2. Produced and theoretical CO2 were used to calculate carbon mineralization percentage (Eq. 6) for all samples.

$$\mathrm{C}+{\mathrm{O}}_{2} \to {\mathrm{CO}}_{2},$$
(1)
$$2 \mathrm{KOH}+{\mathrm{CO}}_{2} \to {K}_{2}{\mathrm{CO}}_{3}+ {\mathrm{H}}_{2}\mathrm{O},$$
(2)
$$\mathrm{KOH}+\mathrm{HCl} \to \mathrm{KCl}+ {\mathrm{H}}_{2}\mathrm{O},$$
(3)
$$\mathrm{mg} {\mathrm{CO}}_{2 \mathrm{produced}}=\left(\frac{0.5 \mathrm{M} \times 20 \mathrm{mL}}{0.25 \mathrm{M}}- {V}_{\mathrm{HCl}}\right) \times \frac{0.25 \mathrm{M}}{2} \times 44\frac{\mathrm{g}}{\mathrm{mol}},$$
(4)
$$\mathrm{mg} {\mathrm{CO}}_{2 \mathrm{theoretical} }=\frac{\mathrm{mc} \times 44 \frac{\mathrm{g}}{\mathrm{mol}}}{12 \frac{\mathrm{g}}{\mathrm{mol}}},$$
(5)
$$\mathrm{Carbon} \,\mathrm{mineralization} (\%)=\frac{{\mathrm{mg} {\mathrm{CO}}_{2 }\mathrm{produced}}_{\mathrm{test material}}-{\mathrm{mg} {\mathrm{CO}}_{2}\mathrm{produced}}_{\mathrm{Blank}}}{\mathrm{mg} {\mathrm{CO}}_{2}\mathrm{theoretical}}\times 100.$$
(6)

Characterizations

Thermal, mechanical, morphological, and spectroscopic characterizations were performed on the non-aged and aged PE0 and PE8 films.

Differential scanning calorimetry (DSC)

Thermal behavior analysis of the specimens was carried out in a calorimeter (TA Instruments, model Q20, USA) in the heat/cool/heat mode from − 90 to 200 °C, at rate of 10 °C/min, under nitrogen atmosphere (flow rate at 50 mL/min). Melting temperature was determined in the second heating step (in triplicate). Degree of crystallinity was calculated using melting enthalpy (ΔHm) of 286.6 J/g as reference, corresponding to 100% crystalline LDPE [24].

Thermogravimetric analysis (TGA)

Thermal stability was determined in an SDT Q600 device (TA Instruments, USA). Specimens were heated from room temperature to 600 °C under nitrogen atmosphere (flow rate at 100 mL/min) with heating rate 10 °C/min (in triplicate).

Fourier transformed infrared spectroscopy (FTIR)

FTIR spectra were obtained at room temperature in UATR mode, in the range of 4000–650 cm−1, on a Perkin Elmer spectrometer (Spectrum One model). The carbonyl index (CI) (in triplicate) was calculated according to the method described in literature, before and after the aging test, using formula shown in Eq. 7 [25].

$$\mathrm{CI}=\frac{{A}_{1715}}{{A}_{2870}},$$
(7)

where A1715 is the absorbance of the carbonyl group (C = O of acids, aldehydes, ketones) and A2870 is the symmetric stretching of methylene group (–CH2–).

Tensile test

Tensile tests were performed in a Q800 DMTA apparatus (TA Instruments, USA), cell load of 18 N, film tension mode at 25 °C and rate of 3 N/min, using samples measuring 0.20 mm (thickness) × 7 mm (length) × 3 mm (width). Young’s modulus of the specimens was determined at triplicate according to ASTM D638-14. Biodegraded samples could not be evaluated due the deterioration level after soil exposition.

Dynamic mechanical properties

Dynamic mechanical properties of specimens (7 mm × 3 mm × 0.2 mm) were analyzed in a Q800 DMTA apparatus (TA instruments, USA), in the multi-frequency mode. Samples were heated from − 100 to 90 °C, at rate of 3 °C/min, under an oscillating frequency of 1 Hz (in duplicate).

Field emission scanning electronic microscopy (FEG-SEM)

Surface morphology of the specimens coated with a thin layer of gold was assessed by FEG-SEM, using an FEI Inspect F50 scanning electron microscope (Japan) in secondary electron mode (SE). Biodeterioration (biofilm formation and microorganism morphology) on the sample surface was also evaluated and required sample preparation. To that purpose, samples were fixed with 25% glutaraldehyde in 0.2 M phosphate buffer solution for 24 h and then immersed in a solution content 2% of OsO4 in 0.2 M phosphate buffer (pH 7.0) for 30 min, followed by sequential dehydration in increasing concentrations of acetone, for 30 min each. Finally, the samples were left in 100% acetone for 1 h, and then air-dried at room temperature.

Elemental analysis

Carbon, hydrogen, and nitrogen contents of the non-aged and aged specimens were determined by elemental analysis in a TruSpec CHNS elemental analyzer (LECO, USA), equipped with infrared and thermal conductivity detectors. Films were analyzed in an O2 atmosphere at 950 °C.

Gel permeation chromatography

Number average molar weight (Mn) and weight average molar weight (Mw) and molar weight distribution were determined by GPC (in triplicate) in a Viscotek 350A HT—GPG system (Malvern Instruments, UK), equipped with columns set PLgel 20 mm Mixed-A 300 × 7.5 mm (Polymer Laboratories, UK). 1,2,4-Trichlorobenzene was used as eluent, with a flow rate of 0.8 mL/min at 135 °C. Polystyrene (Pressure Chemical, USA) with a molar weight of 2450 to 44.8 × 106 g/mol was used as calibration standard. The following Mark–Houwink constants were used in the calculations: K (PS) = 0.000121; α (PS) = 0.707; K (PE) = 0.000406; α (PE) = 0.725.

Statistical analysis

DSC and tensile test were assessed applying two-way repeated-measures ANOVA followed by Bonferroniposthoc test (Statistical Package for the Social Science, software version 20.0; SPSS, v.20.0). The significance level established in the present study was p < 0.05.

Results and discussion

Influence of accelerated weathering on the properties of LDPE blend

Structural characteristics

Incorporation of additive did not change the structural characteristics of pure LDPE based in FTIR analyses. The changes in structural characteristics of PE0 and PE8 during accelerated weathering test were evaluated by FTIR. Figure 2 shows the FTIR spectra of the non-aged and aged PE0 and PE8 films. The bands at 1373 and 910 cm−1 are assigned, respectively, to vibrations of the alkyl (CH3) and vinyl bending (HC = CH2) groups associated to PE structure. The appearance of bands at 1714 and 1176–1018 cm−1 assigned to vibrations of C = O stretching (carboxylic acid) and C–O stretching (alcohols), respectively, is due polymer photodegradation (Norrish type I and II reactions) in accelerated weathering conditions [2]. Moreover, an increase in band intensity was observed related to these groups with the increase of exposure time (600 and 1000 h), confirming the strong time effect on the photodegradation process. According to the literature, the exposition to abiotic factors contributes to biodegradation, since carbonyl group formation may occur due to polymer chain oxidation by exposure to UV light, leading to increase carbonyl index [26,27,28]. Similar behavior was observed in this work for both samples (Table 1). These surface changes have led to biofilm formation [29].

Fig. 2
figure 2

FTIR spectra in 2000 to 650 cm−1 wavenumber: a PE0 films and b PE8 films

Full size image
Table 1 Melting temperature (Tm), crystallinity degree (X%), Young’s modulus, stress and strain values at break for PE0 and PE8 films before and after accelerated aging
Full size table

Thermal properties

Table 1 shows melting temperatures (Tm) and crystallinity values (X) of non-aged and aged PE0 and PE8 films. Incorporation of the biodegrading additive in polymeric matrix did not alter significantly their thermal properties (PE0 and PE8, Table 1). In general, there was no significant interaction between the independent factors exposition times and the presence of additive (F = 2.19; p = 0.09). Considering independent variables, there was significant interaction only at 600 h when PE8 and PE0 groups were compared (\(\stackrel{-}{\Delta T}=2.06^\circ \mathrm{C}; \mathrm{standard} \mathrm{error}=0.46, p=0.01; F=20.03).\) Regarding the influence of exposure time, statistical analysis demonstrated that Tm did not vary significantly for isolated PE0 group. However, for PE8 groups, when 0 h is compared with the other exposition times, there were significant differences at 800 h (\(\stackrel{-}{\Delta T}=2.92^\circ \mathrm{C};p=0.026)\) and 1000 h \((\stackrel{-}{\Delta T}=3.20^\circ C; p=0.03)\). Moreover, comparing exposition times as 600 h (\(\stackrel{-}{\Delta T}=2.30^\circ C; p=0.02),\) there was significant variation. Tm data showed parametric distribution. Regarding the X% values, statistical analysis indicated that when PE0 group was evaluated individually, it did not exhibit significant variation over time. For PE8 group, it was observed a tendency of increasing crystallinity comparing 0 h and 800 h (\(\stackrel{-}{\Delta X}=1.25 \%; p=0.07).\) A comparison of the groups (PE0 x PE8) showed significant difference between values in 800 h (\(\stackrel{-}{\Delta X}=3.09 \%; F=229.59;p<0.001).\) X% data also showed parametric distribution.

Ojeda et al. [16] evaluated the thermal behavior of HDPE/LLDPE commercial blend containing a pro-oxidant additive over 1 year of natural weathering. They observed an increasing trend of the blend crystallinity and proposed that this increase results in decreasing molecular size, and thus decreasing chain mobility, dominating in the blend. Gulmine et al. [27] studied the accelerated weathering of PE samples in the Weather-Ometer and QUV machines (ASTM G53-96) for 800 h. It was observed an increase in the crystallinity, density and hardness, as well as a broadening of the melting peak of the PE samples degraded by accelerated aging. They attributed it to changes in crystallite sizes, molecular weight differences that are brought about by chain breaking and secondary recrystallization.

Thermogravimetric analysis was performed to evaluate the thermal degradation behavior of samples after exposure to accelerated weathering. Figure 3 shows the TG and DTG curves for non-aged and aged (600 and 1000 h) specimens. Non-aged PE0 (Fig. 3a, b) and PE8 (Fig. 3c, d) films displayed similar thermal behavior with only one step in the degradation process. The Table 2 presents the temperature at 10% weight loss (T10) values of PE0 and PE8 samples. This parameter was used to evaluate the beginning of the thermal event. For non-aged PE0 and PE8 samples, T10values were similar, while it was lower in aged (600 and 1000 h) PE0 and PE8 samples. Thermal behavior changes in the aged films can be attributed to photodegradation that alters polymer structural characteristics, as shown in FTIR analysis. Oxidative degradation of materials as polyethylene occurs due to chain scission by free radicals, generating compounds with shorter chains [30]. During the accelerated degradation, these degradation processes can lead to formation of smaller molecules lowering the initial degradation temperature in the thermal degradation. Corti et al. [20] reported lower degradation temperature values for LLDPE samples with pro-oxidant additives exposed to outdoor weathering.

Fig. 3
figure 3

TG and DTG curves of non-aged and aged PE0 and PE8 films: a TG curve of PE0, b DTG curve of PE0, c TG curve of PE8, and d DTG of PE8

Full size image
Table 2 Carbonyl index (CI), temperature at 10% weight loss (T10), Mn and Mw values, and melting temperature (Tm) of non-aged and aged samples before and after soil exposure
Full size table

Mechanical properties

Mechanical tests were performed to assess the mechanical behavior of samples after accelerated weathering (Table 1) to determine whether the material becomes stiffer or flexible with exposure time. After 1000 h of exposure, films were disintegrated and, therefore, could not be evaluated. Thus, only the 600 and 800 h exposure specimens were used. For PE0 and PE8 samples (non-aged and aged), Young’s modulus rose as exposure time increased, i.e., films became stiffer over time. This behavior is corroborated for decrease in the strain values (Table 1). A similar increase of Young’s modulus was observed for both specimens (approximately 100% over 800 h) when compared with non-aged samples. Besides that, PE8 samples exhibited higher Young’s modulus when compared to PE0 samples, indicating that the biodegrading additive modified the elastic behavior of the material. Statistical analysis was only performed for Young’s modulus (E) values, confirming the effect of exposure time of accelerated aging for PE0 and PE8. When PE0 and PE8 films were compared, there was a significant difference over the time (∆E = 16.317 MPa; F = 425.105; standard error = 0.791; p < 0.001). In an intragroup analysis, either for PE0 or PE8 groups, there were significant differences considering the comparison among 0 h and other exposition times (F = 1557.381; p = 0.019; and F = 1660.317; p = 0.018, respectively).Young’s modulus data also showed parametric distribution.

According to literature, from tan δ curves, it is possible to identify the transitions temperatures related to β-relaxation (branch relaxation) and α-transition, which are associated with molecular mobility and lamellar slip of the crystalline phase, respectively [28, 31]. Storage modulus (Fig. 4a) and loss modulus curves (Fig. 4b) indicate the effect of aging time the polymer chains, since these chains lose their ability of absorb and dissipate the energy during aging, leading to smaller displaced peaks. So, when these curves are evaluated, it is possible to observe that the incorporation of the additive reduced the PE chain mobility once there was a shift in β-relaxation value to a higher temperature [32]. The tan δ curves of PE0 (Fig. 4c) show peaks at − 21.9 °C (shoulder) and 64.6 °C (strong peak) corresponding to β-relaxation and α-transition, respectively. The same behavior was observed for non-aged PE8 (− 17.9 and 62.8 °C, related to β-relaxation and α-transition, respectively). The peaks for aged PE0 and PE8 films occurred at higher temperatures (2.2 and 72.8 °C for PE0; − 11.7 and 83.2 °C for PE8), indicating that aging reduces chain mobility due to changes in the amorphous phase of the material and increases crystallinity [32].

Fig. 4
figure 4

Storage modulus (a), loss modulus (b), and Tan δ (c) versus temperature curves for non-aged and aged PE0 and PE8 films

Full size image

Morphology

FESEM images (Fig. 5) show changes on the surface of PE0 and PE8 films over time during the aging process, with severe cracks in PE0-1000 and minor cracks in PE8-1000. This surface behavior (cracking) has been reported for LDPE samples after 800 h exposure in a QUV chamber [26, 27]. According to the literature, abiotic degradation occurs primarily in amorphous regions and reduces strain values, which is considered the main parameter for detecting chain scission and crosslinking during abiotic degradation [4, 16, 33,34,35,36]. Moreover, this behavior also was evidenced by the increase of crystallinity degree and Young’s modulus, as well as the decrease of loss modulus of PE8 samples after aging exposition times.

Fig. 5
figure 5

FE-SEM images of non-aged and aged samples before and after exposition to soil: a PE0 non-aged, b, c PE0 non-aged 60 days, d PE0-600, e, f PE0-600 60 days, g PE0-1000, h, i PE0-1000 60 days, j PE8 non-aged, k, l PE8 non-aged 60 days, m PE8-600, n, o PE8-600 60 days, p PE8-1000 and q, r PE8-1000 60 days. PE0 films present coccus (1), hyphae (2) and spores (3) morphology. Besides, PE8 films present bacillus (4) morphology. Figures a, b, d, e, g, h, j, k, m, n, p (mag. ×2000). Figures c, f, i, l, o and r (mag. ×10,000)

Full size image

Soil exposition assay (biodeterioration evaluation)

Morphology

FESEM images of PE0 and PE8 samples (Fig. 5) were obtained to assess biofilm formation after 60 days of soil exposure. Microorganism adhesion was observed on the surface of all samples (0, 600, and 1000 h). PE0 films (non-aged and aged) present similar microorganisms (coccus, hyphae, and spores). Besides these microorganisms, it was also observed the presence of bacillus in samples of PE8 (non-aged and aged). Additionally, the microbial community began to enter the micro-cracks present in aged films, invading the bulk polymer. Filamentous microorganisms develop their mycelia framework within the materials. The mechanical action of apices penetrating in the materials increases the pores size, which provokes cracks and due to this, the material resistance and durability are weakened [36].

Structural characteristics

FTIR analyses of PE0 and PE8 films after 60 days of soil exposure showed changes in structural characteristics (Fig. 6). Spectra of PE0 and PE8 films exhibited bands at 3697–3290 cm−1 (OH stretching), 1641 cm−1 (C = O stretching of secondary amides), 1556 cm−1 (N–H bending of secondary amides), 1100–1007 cm−1 (C–O bond and N–O stretching) and 910 cm−1 (H2C = CH2 group). PE0- and PE8-aged samples spectra showed two bands in the 1650–1500 cm−1 range, corresponding to C = O stretching and N–H bending of secondary amides and C = C stretching of the aromatic ring. Structural changes after exposure to microorganisms have also been reported in the literature and are related to consumption of oxygenated groups present in the polymer by the microorganisms [2, 37]. Table 2 shows the carbonyl indexes (CIs) for these samples after 60 days of soil exposure. In non-aged films, only PE8 displayed an increase in the carbonyl index (sixfold), reinforcing the finding that the additive facilitates the surface degradation. However, the CI declined in aged films. Comparison of the CIs of aged PE0 and PE8 films after 60 days’ soil exposure showed a sharper decline in 600 h in PE8, and 1000 h for PE0. A previous study also reported the decrease of CI in a microbiologic assay with HDPE and LDPE [38].

Fig. 6
figure 6

FTIR spectra of PE0 and PE8 samples (0 and 1000 h) before and after 60 days of soil exposure in the interval of wavenumber 2000 to 650 cm−1: a PE0, b PE0-1000, c PE8, and d PE8-1000

Full size image

Thermal properties

TGA analysis was performed on non-aged and aged PE0 and PE8 films after 60 days’ exposure to soil (Table 2). Non-aged PE0 and PE8 films obtained similar T10 values (≈ 440 °C) before and after soil exposure. Similar thermal behavior was observed for all the aged samples after soil exposure and both groups displayed a slight upward trend in T10.There were not changes in Tm values of the samples after soil exposition.

Molar weight evaluation

In the present study, molar weight of PE0 and PE8 samples declined with aging time related to non-aged samples (Table 2). However, when aging times were compared between them, there was no difference (PE0-600 and PE0-1000), with exception of PE8-600 and PE8-1000 that presented different values, being molar weight value of PE8-1000 greater. Even values seemed to be dissimilar and it should be reminded that assays were performed in independent triplicate. Albertsson and Karlsson [14] studied the degradation behavior of LDPE (Mn 37.2 kg/mol and Mw 196.5 kg/mol) and LDPE/pro-oxidant additive (Mn 37.5 kg/mol and Mw 192.9 kg/mol) in water medium at 95 °C for 45 days. The researchers observed a decrease in Mn and Mw values for LDPE (Mn 8.0 kg/mol and Mw 54.0 kg/mol) and LDPE/additive (Mn 3.2 kg/mol and Mw 19.4 kg/mol). They identified that low molar weight products (carboxylic acids, hydrocarbons, ketones, aldehydes, and alcohols) formed after the degradation were responsible for the molar weight decrease. Teymori et al. [39] studied the thermal aging at 80 °C of LDPE (Mn 22.3 kg/mol and Mw 257.8 kg/mol) over 42 days and found that Mn and Mw values decreased with longer exposure times, with Mn and Mw values of 9.1 and 33.8 kg/mol, respectively. Ojeda et al. [16] evaluated changes in molar weight of LDPE before (143 kg/mol) and after 280 days (42.9 kg/mol) of natural weathering exposition and also observed a decrease in Mw values. Related to soil exposition samples, it was possible observe an increase in molar weight values only for PE0-600. Santo et al. [1] exposed a LDPE previously UV-irradiated to a crude extracellular laccase from a bacterial strain Rhodococcusruber and observed a decrease in molar weight values before (Mn 40.4 kg/mol and Mw 116.5 kg/mol) and after exposition (Mn 34.2 kg/mol and Mw 93.6 kg/mol). Yang et al. [13] exposed PE to a two bacterial strains from the guts of plastic-eating wax worms and observed that Mn values did not change but Mw decreased, which Mw value was 90.0 kg/mol before, 82.5 kg/mol (strain 1) and 78.2 kg/mol (strain 2).

Carbon mineralization assay (aerobic biodegradation evaluation)

Carbon dioxide production and carbon mineralization

The results of elemental analysis showed a C content of 37.73% for positive reference material (starch), 90.62% for PE8 sample and 78.32% for PE8-800 samples, showing that aging process decreased C content in PE8 samples. Therefore, the accelerated weathering can alter the carbon amount available to microorganisms, since random chain scission and crosslinking reactions occur during this process [15].

Figure 7 shows the mg CO2produced and mineralization percentage results for the blank (soil only), positive control (corn starch), PE8 and PE8-800. The amount of cumulative CO2 produced after 166 days.

Fig. 7
figure 7

Produced CO2 (mg) curve for the blank (soil only), positive control (corn starch), PE8 and PE8-800 a, and mineralization percentage curve b for the positive control, PE8 and PE8-800 over 166 days

Full size image

(Fig. 7a) was higher in the aged sample with additive (5144.5 mg) compared to positive control (4825.9 mg) and non-aged PE8 (4740.5 mg). Final values of C mineralization (%) for positive control, PE8 and PE8-800 were 21.3, 6.7 and 20.3%, respectively. A trend of faster biodegradation was observed for the positive control when compared to the other samples (until 40 days, Fig. 7b). This result is important, since the positive control can be used to evaluate soil quality. A comparison of the amount of cumulative CO2 produced by PE8 and PE8-800 films at the end (166 days) indicated that accelerated weathering facilitated microbial attack, according to the literature [2, 40]. Moreover, PE8-800 sample showed a percentage of mineralization final value very similar to positive control, which is a biodegradable material and from renewable source. These results clearly demonstrate the effect of accelerated weathering on the LDPE biodegradation, as reported in the literature. Esmaeli et al. [12] studied CO2 evolution for LDPE in soil over 125 days (ASTM D5988-12). The authors found that samples that were in contact with the soil/selected microorganisms (Lysinibacillus xylanilyticus and Aspergillus niger) exhibited mineralization of 15.8% (non-aged) and 29.5% (aged), whereas those in contact with the soil alone displayed only 7.6% (non-aged) and 8.6% (aged).According to literature, these mineralization values may be attributed to the microbial assimilation of carbon present in the UV-modified samples or abiotic oxidation products [41]. Castro-Aguirre et al. [40] also obtained lower mineralization percentage values in non-aged and pure LDPE samples (6.8 ± 4.8%), as well as, for this sample in powder form (3.7 ± 2.5%) after exposing it to a compost at 58 °C during 60 days.

Structural characteristics

FTIR analysis of the PE8 and PE8-800 films after 166 days of aerobic biodegradation indicated changes in structural characteristics. For PE8 sample, bands were observed at 3697–3290 cm−1, attributed to –OH stretching, and at 1100–1007 cm−1 corresponding to the C–O bond and N–O stretching. Similar bands were found for the PE8-800 film, in addition to 1650–1500 cm−1, attributed to C = O stretching and N–H bending of secondary amides, and C = C stretching of the aromatic ring. These bands are associated with N and C mineralization. Under aerobic conditions, organic nitrogen can be transformed into NH3 or NH4+ during ammonification and then into nitrites (NO2−). Nitrites formed at the end of mineralization are converted into nitrates [15].

Molar weight evaluation

On the biodegradation process, changes in the material molar weight occur due to depolymerization and bioassimilation steps. As previously mentioned, in the soil exposition test, aging time influences the decline in molar weight. Mn values obtained after the aerobic biodegradation were 10.3 ± 1.0 and 3.1 ± 0.4 kg/mol for PE8 and PE8-800, respectively. Mw values obtained after the aerobic biodegradation were 82.9 ± 0.7 and 49.8 ± 4.4 kg/mol for PE8 and PE8-800, respectively. It showed the effects of aging exposure and soil microbiome. Fontanella et al. [18] evaluated abiotic degradation for 300 h thermal oxidation and biodegradation (Rhodococcus ruber) of pure LDPE and mixed with pro-oxidants additive. Although, they observed a decrease in molar weight after abiotic exposure, and the values did not change after biodegradation.

According to Morro et al. [10] the biodegradation process can complement the photodegradation. However, its efficacy depends on several factors, such as material composition, physical and chemical surface properties, and native microbiota. In addition, very small changes in these factors can result in large differences in material biodegradation behavior [10]. Likewise, changes in the biodegradation tests conditions (temperature, pH and moisture, as well as nutrition composition) can modify the material biodegradation behavior, due the change of amount and predominance of the microorganism species [42]. Gu [41] proposed that dominant group of microorganisms and degradation pathways associated to polymer degradation process generally are determined by environment conditions. Also, the biodeterioration and degradation of polymeric substrate rarely reach 100% and a little portion of this can be incorporated to microbial biomass, humus and others natural products.

Conclusion

Abiotic and biotic degradations of a LDPE blend in soil of South Brazil landfill were indications of the changes in chemical, thermal, mechanical and morphological properties of a biodegradable LDPE blend prepared with a biodegrading additive (without metals), after exposition to accelerated aging and inoculation in soil. The presence of additive favored degradation process, given that polar functional groups were incorporated into the chemical structure of LDPE. Biotic degradation was more accentuated in the aged films (with/without additive) exposed to soil when compared to their non-aged counterparts. Results obtained in the biodegradation assay are promising, once that it was reached in a natural soil from South Brazil landfill, which has its origin from the composting of urban solid waste and due to this, its composition and characteristics are the closest to reality. Moreover, aged material exhibits biodegradable behavior, once that reached the greater values of carbon mineralization. Combination of biodegrading additive and abiotic factors showed that LDPE blends became more attractive for soil microbiota adhesion, surface material biodeterioration and carbon mineralization. Therefore, biodegradable LDPE blends obtained in this work can be considered as an alternative for plastic packaging industry, reducing the effects caused by plastic waste on the environment.