Photodegradation and Biodegradation of Poly(Lactic)

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Photodegradation and Biodegradation of Poly(Lactic) Acid Containing Orotic Acid as a Nucleation Agent Jan Salač, Jana Šerá, Martin Jurča, Vincent Verney, Adam Marek, Marek Koutny

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Jan Salač, Jana Šerá, Martin Jurča, Vincent Verney, Adam Marek, et al.. Photodegradation and Biodegradation of Poly(Lactic) Acid Containing Orotic Acid as a Nucleation Agent. Materials, MDPI, 2019, 12 (3), pp.481. 10.3390/ma12030481. hal-02190364

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Article Photodegradation and Biodegradation of Poly(Lactic) Acid Containing Orotic Acid as a Nucleation Agent

Jan Salaˇc 1 , Jana Šerá 1, Martin Jurˇca 1, Vincent Verney 2 , Adam A. Marek 3 and Marek Koutný 1,*

1 Department of Environmental Protection Engineering, Faculty of Technology, Tomas Bata University, Vavreˇckova275, 760 01 Zlín, Czech Republic; [email protected] (J.S.); [email protected] (J.Š.); [email protected] (M.J.) 2 Institut de Chimie de Clermont-Ferrand, Université Clermont Auvergne, CNRS, F-63000 Clermont–Ferrand, France; [email protected] 3 Department of Organic Chemical Technology and Petrochemistry, Silesian University of Technology, 44100 Gliwice, Poland; [email protected] * Correspondence: Correspondence: [email protected]; Tel.: +420-576-031-208

Received: 11 January 2019; Accepted: 2 February 2019; Published: 4 February 2019

Abstract: Orotic acid is a natural heterocyclic compound that acts as a nucleation agent in poly(lactic acid) (PLA). PLA materials with increasing orotic acid content were prepared and characterized. It was found that crystallinity of about 28% was reached with 0.3% content of the agent. Further enhancement in the content of the agent did not provoke any additional signiﬁcant increase of crystallinity. Subsequently, it was investigated whether the orotic acid content affected photodegradation of PLA and, in the next phase, its biodegradation. The results of rheological measurements showed that the compound slightly accelerates photodegradation of the material, which was accompanied by the cleavage of PLA chains. Previous photodegradation was shown to accelerate the subsequent biodegradation by shortening the lag phase of the process, where the explanation is probably in the reduction of the polymer molecular weight during the photodegradation. Moreover, the presence of orotic acid in both initial and photodegraded samples was found to inﬂuence biodegradation positively by shortening the lag phase and increasing the observed maximal rate of the biodegradation.

Keywords: poly(lactic acid); orotic acid; crystallinity; photodegradation; biodegradation; compost

1. Introduction More than 180 million tons of plastic waste are produced every year; a part of it enters the environment and causes problems due to its accumulation and omnipresence [1–3]. Currently, poly(lactide) acid (PLA) appears to be the most available biodegradable alternative to conventional polymers [4]. PLA is made through a combination of biotechnological and chemical processes, starting from renewable resources. PLA is biodegradable at higher temperatures only, approximately above 55 ◦C, which corresponds to biological composting processes [5]. On the other hand, PLA is not regarded as biodegradable in the soil environment [6]. PLA can find a number of applications, especially in the agricultural and food sectors [7,8]. The material properties of neat PLA are often not very favorable for some applications but they can be modified by various approaches known in polymer science [9]. One of the limiting features is a relatively high glass transition temperature of about 58 ◦C, making the material rather brittle at normal temperatures and the corresponding heat deflection temperature, which means that the PLA specimens tend to soften above a temperature of about 60 ◦C[10].

Materials 2019, 12, 481; doi:10.3390/ma12030481 www.mdpi.com/journal/materials Materials 2019, 12, 481 2 of 12

PLA materials that are processed in the form of melt in the conventional plastic processing technologies have usually very low crystallinity because the crystallization proceeds extremely slowly with crystal growth rate of less than 6 µm·min−1 [11]. For practical reasons, PLA materials usually cannot be annealed for a sufficiently long time during the production process. However, it was shown that a higher crystallinity could increase the elastic module up to about 25% and also considerably improve the heat deflection temperature [12,13]. The crystallinity of many polymer materials can be increased by the addition of nucleation agents, which provide initiation centres for the crystallization and also can increase the crystallization rate [14]. Various nucleation agents were examined with PLA, including some inorganic nucleating substances such as talc and clay [15–18] with some level of success. The general problem of the inorganic nucleation agents is their incompatibility with the polymer matrix and thus problems with even dispersion and prevention of their aggregation during the melt processing. Organic compounds with better solubility in the polymer matrix were also tested, including N,N-ethylene-bis(12- hydroxylstearamide) [19], and N,N0,N”-tricyclohexyl-1,3,5-benzene-tricarboxylamide (TMC-328) [20], where the latter was proven to be a very efficient nucleation agent, also able to improve the barrier properties of the material. The ideal nucleation agent for PLA should also preserve its positive properties, especially biodegradability. Consequently, the compound must be nontoxic and biodegradable. Orotic acid (OA) appears to be a particularly good candidate. OA is a natural intermediate in the biosynthesis of pyrimidines [21–23] and was previously investigated and proved to be a relatively potent nucleation agent in PLA [24], though not so effective as some synthetic compounds e.g., N,N-(ethane-1,2-diyl)bis(N-phenoxylalamide) [25]. OA was also seen to improve the crystallization of polyhydroxy buthyrate [26]. This study aimed to verify the capacity of OA to act as a nucleation agent in PLA and especially to investigate whether the degradation of the resulting materials is altered. In particular, the photodegradation in controlled laboratory conditions and biodegradation during laboratory composting tests were studied. These processes are especially important for outdoor applications of PLA-based materials. It has been previously shown that crystallinity can affect the hydrolysis of ester bonds in PLA [5,27,28]; hence, in this case, some changes in the degradation processes could be expected. The influence of OA itself on the photodegradation, the biodegradation and the combination of both could be quite unpredictable and should be examined experimentally.

2. Materials and Methods

2.1. Preparation of Materials The polymer PLA2002D was obtained from NatureWorks® Ingeo™ (Minnetonka, MN, USA); its weight average molecular weight (MW) was about 120 kg·mol−1. Orotic acid (Figure1) was from Sigma-Aldrich (St. Louis, MO, USA). It was in the form of ﬁne powder, the grains were of uneven shape, where the majority of particle sizes was in the interval from 7 to 30 µm. The distribution of the particle cross-section arreas was evaluated with optical microscopy using ImageJ 1.5 software (Wayne Rasband, National Institutes of Health, Bethesda, MD, USA) (Figure S1 in Supplementary Materials). OA has melting point of about 345 ◦C so it was still solid at the processing temperature. First, PLA masterbatch with 5% (w/w) of OA was prepared by melting and mixing the components in the two-screw laboratory mini-extruder DSM Xplore 15 (Xplore Instruments BV, Sittard, The Netherlands) at 190 ◦C for 5 min. Then materials containing 0.1, 0.3 and 0.5% (w/w) of OA were prepared under the same conditions by mixing the masterbatch and neat PLA. Finally, the materials were dried in the vacuum oven at 60 ◦C for 24 h. Sample ﬁlms about 100 µm thick were prepared by compression molding at 180 ◦C and 20 MPa for 1 min. Then the hot plates were removed from the press and let to cool down slowly on air for about 2 h. Materials 2018, 11, x FOR PEER REVIEW 3 of 12 in the backscattered electron mode. Measurements were carried out on samples without prior metallization using a special sample holder for Phenom Pro. Image analysis of the orotic acid particles was done with the ImageJ 1.5 software (Wayne Rasband, National Institutes of Health, Bethesda, MD, USA). The size of individual particles was determined from threshold-adjusted SEM images.

2.2.2. Polarized Light Microscopy PLA film materials were observed with polarized light microscopy ECLIPSE i50 (Nikon, Tokyo, Materials 2019, 12, 481 3 of 12 Japan).

Figure 1. Orotic acid. Figure 1. Orotic acid. 2.2. Scanning Electron and Optical Microscopy 2.3. Differential Scanning Calorimetry (DSC) 2.2.1. Scanning Electron Microscopy (SEM) For DSC measurement, Mettler Toledo DSC822 instrument (Mettler Toledo, Columbus, OH, USA)Particles was used. of OAThe andtemperature PLA film range materials was wereset from analyzed 25 °C withto 200 the °C Phenom with the Pro heating (Thermo rate Fisherof 10 K·minScientific,−1 in Waltham,air atmosphere. MA, USA)After heating, SEM. The the samples sampleswere were observedcooled down at the at the acceleration same rate voltageand then, of 10finally, kV in the the heating backscattered cycle was electron repeated. mode. The Measurements degree of crystallinity were carried was outdetermined on samples from without the cooling prior tracemetallization with theusing help of a specialthe following sample equation: holder for Phenom Pro. Image analysis of the orotic acid particles was done with the ImageJ 1.5 software (Wayne Rasband, National Institutes of Health, Bethesda, MD, USA). The size of individual particles was determined∆ −∆ from threshold-adjusted SEM images. χ = 100 % (1) ∆ 1 − 2.2.2. Polarized Light Microscopy 100 PLA film materials were observed with polarized light microscopy ECLIPSE i50 (Nikon, where ΔHm is the heat of fusion, ΔHc is the cold crystallization enthalpy and ΔH0m is the tabulated Tokyo, Japan). theoretical heat of fusion for 100% crystalline PLA homopolymer (93.1 J·g−1) [29]. The measurements were2.3. Differential done twice Scanning with identical Calorimetry results (DSC) with the precision needed in this study (±1%).

2.4. MeltFor DSCRheology measurement, Study Mettler Toledo DSC822 instrument (Mettler Toledo, Columbus, OH, USA) was used. The temperature range was set from 25 ◦C to 200 ◦C with the heating rate of 10 K·min−1 in Changes in the material on the molecular level were investigated by melt viscoelasticity air atmosphere. After heating, the samples were cooled down at the same rate and then, ﬁnally, the experiments in oscillatory shearing mode using an advanced rheometry (ARES Rheometric Scientific heating cycle was repeated. The degree of crystallinity was determined from the cooling trace with the T&A Instruments, New Castle, DE, USA) in the two parallel plate geometry (diameter 25 mm, gap help of the following equation: between plates of 1 mm). Intially, strain amplitude values were optimized for each sample to ensure that the ∆Hm − ∆Hc measurements were done withinχ c the=  the linear viscoelastic ×region.100 Then, dynamic stresses were (1) %wt f iller 0 −1 recorded at the optimal strain aplitude within∆Hm the1 − frequency100 interval from 0.1 to 100 rad·s at 180 °C. The oscillatory frequency sweep test was applied to obtain resulting stress σ*(t) for an applied 0 wheresinusoidal∆Hm strainis the γ*(t). heat The of fusion, complex∆H dynamicc is the cold viscosity crystallization η*(ω) can enthalpythen be derived: and ∆H m is the tabulated theoretical heat of fusion for 100% crystalline PLA homopolymer (93.1 J·g−1)[29]. The measurements were done twice with identical results withη*( theω) precision = σ*(t)γ*(t) needed in this study (±1%). (2) η*(ω) = η′(ω) − j η″(ω) (3) 2.4. Melt Rheology Study 2 whereChanges η′(ω) is the in theloss materialviscosity onandthe η″(ω molecular) the storage level viscosity were and investigated “j” is the complex by melt number viscoelasticity j = −1. experimentsData were in oscillatoryplotted in the shearing complex mode plane using as so-called an advanced Cole-Cole rheometry graphs (ARES [30,31]. Rheometric The relaxation Scientiﬁc in theT&A entaglement Instruments, polymer New Castle, melt is DE, manifested USA) in in the the two form parallel of semi-circular plate geometry traces (diameter of the measurement 25 mm, gap valuesbetween during plates the of 1oscillatory mm). shearing. The Newtonian zero-shear viscosity η0, then corresponds to Intially, strain amplitude values were optimized for each sample to ensure that the measurements were done within the the linear viscoelastic region. Then, dynamic stresses were recorded at the optimal strain aplitude within the frequency interval from 0.1 to 100 rad·s−1 at 180 ◦C. The oscillatory frequency sweep test was applied to obtain resulting stress σ*(t) for an applied sinusoidal strain γ*(t). The complex dynamic viscosity η*(ω) can then be derived:

η*(ω) = σ*(t)γ*(t) (2) Materials 2019, 12, 481 4 of 12

η*(ω) = η0(ω) − j η”(ω) (3) where η0(ω) is the loss viscosity and η”(ω) the storage viscosity and “j” is the complex number j2 = −1. Data were plotted in the complex plane as so-called Cole-Cole graphs [30,31]. The relaxation in the entaglement polymer melt is manifested in the form of semi-circular traces of the measurement values during the oscillatory shearing. The Newtonian zero-shear viscosity η0, then corresponds to the intersection between the extrapolated semi-circle and the X-axis and is proportional to MW of the polymer: a η0 ∝ K·(Mw) (4) where K and a are constants and a was determined to be in the interval 3.4–3.6 [32,33].

2.5. Photooxidation Accelerated photo-ageing was performed in the ATLAS SEPAP 12/24 instrument (Atlas Material Testing Technology, Mount Prospect, IL, USA) [34] in air at 60 ◦C. The device is equipped with four 400 W mercury lamps (Mazda type MA 400) at each corner of the oven. UV-light radiation with wavelengths below 300 nm is ﬁltered out by the glass envelopes of the sources. 2 cm × 2 cm ﬁlm fragments were irradiated on aluminum holders on a rotating carousel. Films were exposed to the irradiation and temperature for up to 100 h.

2.6. Spectroscopic Characterization Transmission UV-VIS spectra of the 100 µm PLA films were recorded in Shimadzu UV-2600 spectrometer (Shimadzu, Kyoto, Japan) in the wavelength interval from 200 nm to 800 nm. Fourier transform infrared (FTIR) spectra of the film samples in the transmission mode were obtained in Thermo Scientific Nicolet 6700 instrument (Thermo Fisher Scientific, Waltham, MA, USA).

2.7. Biodegradation Testing Biodegradation tests [33] were realized in 500 mL flasks with septa mounted on the stoppers. The flasks contained polymer samples (50 mg), mature compost (5 g of dry weight, pH 7.1) and perlite (5 g). The flasks were incubated at 58 ◦C. Head space gas was sampled at appropriate intervals through the septum with a gas-tight syringe (100 µL), and injected into a gas chromatography (GC) instrument (Agilent 7890, Agilent Technologies, Santa Clara, CA, USA) equipped with Porapak Q (1.829 m length, 80/100 MESH) and 5A molecular sieve (1.829 m length, 60/80 MESH) columns connected in series and a thermal conductivity detector (carrier gas helium, flow 53 mL·min−1, column temperature 60 ◦C) to determine the concentration of CO2. The percentage of net mineralization with respect to the carbon content of the initial samples was calculated. Three parallel flasks were run for each sample, along with four blanks. The experimental data were used to derive basic parameters of the biodegradation curves with the help of a mathematical model adapted from [35]:

Cco2Max Cco2Max Cco2 = h i − h i (5) 1 + exp 2 + 4k (C − t) 1 + exp 2 + 4k C Cco2Max Cco2Max

where CCO2Max is the maximal level of evolved CO2 carbon expressed as the % of the maximal theoretical −1 value; k represents the maximal rate of CO2 production (days ) and C is the length of the lag phase (days). The modeling was done in open source software Gnuplot (version 5.2, Thomas Williams, Colin Kelley). Materials 20192018,, 1211,, 481x FOR PEER REVIEW 5 of 12

3.2. Thermal Properties 3. Results and Discussion The basic thermal properties of the PLA samples with different OA contents and especially their 3.1.tendency Preparation to crystallize of Samples were estimated in DSC experiments. The samples were first heated; here, the glass transition temperature and melting behavior was determined (Figure 2A, Table 1). Samples PLA containing OA was prepared in a twin-screw extruder. First, a masterbatch containing 5% containing OA exhibited two distinct melting peaks. This phenomenon was already described [36] (w/w) of OA was made, then the individual samples with lower OA concentrations were prepared and explained by the presence of two types of crystals with identical α morphology but a different through the second extrusion process, including processing neat PLA reference material with zero level of organization [37]. OA presence also caused a small positive shift in the glass transition concentration of OA. Thus, all samples finally had the identical processing history. Sample films were temperature. During the cooling cycle, marked crystallization peaks were recorded in samples prepared by compression molding and then cooled slowly on air to facilitate the crystallization of containing OA (Figure 2B). The crystallization temperature deduced was a bit higher than previously the polymer. reported, which can be explained by a different type of PLA employed; however, the recorded 3.2.enthalpy Thermal of Propertiescrystallization appeared to be identical [24]. It was thus proven that OA acted as a nucleation agent. The degree of crystallization increased with the increasing content of OA, but it was The basic thermal properties of the PLA samples with different OA contents and especially their evident that a further increase of the OA content above 0.3% did not lead to a marked increase in tendency to crystallize were estimated in DSC experiments. The samples were first heated; here, the crystallinity (Table 1). glass transition temperature and melting behavior was determined (Figure2A, Table1). Samples containing OA exhibitedTable two 1. distinctDSC-derived melting parameters peaks. Thisfor PLA phenomenon with the OA was content. already described [36] and explained by the presence of two types of crystals with identical α morphology but a different level of organizationMaterial [37]. OA presenceTg (°C) alsoTm1 caused(°C) T am2 small (°C) positiveTc (°C) shift Hg in (J·g the−1·K glass−1) transitionHc (J·g−1) temperature.χc (%) During thePLA cooling cycle,57 marked 153 crystallization nd peaksnd were recorded0.53 in samples0 containing0 OA (FigurePLA2B). + 0.1% The crystallizationOA 58 temperature 150 deduced156 was 99 a bit higher 0.40 than previously 1.8 reported, 2 whichPLA can + be0.3% explained OA by 59 a different 148 type of 155 PLA employed; 102 however, 0.29 the recorded 26.2 enthalpy 28 of crystallizationPLA + 0.5% appeared OA to 59 be identical 149 [24]. It 156 was thus 103 proven that 0.26 OA acted as 28.6 a nucleation 31 agent. The degreePLA + 5.0% of crystallization OA 59 increased 150 with 155the increasing 110 content 0.27 of OA, but 31.8it was evident 34 that a furtherTg, glass increase transition ofthe temperature; OA content Tm1 and above Tm2 0.3%, melting did temperature; not lead to T ac, markedtemperature increase of crystallization; in crystallinity (TableH1g)., enthalpy of glass transition; Hc, enthalpy of crystallization; χc, degree of crystallinity.

Figure 2. Differential scanning calorimetry. (A) Cooling traces; (B) heating traces with heat/cooling Figure 2. Differential scanning calorimetry. (A) Cooling traces; (B) heating traces with heat/cooling rate 10 K·min−1. rate 10 K·min−1.

The crystallization and uniform distribution of the nucleation agent was qualitatively confirmed by optical polarization microscopy (Figure 3). The important extent of the crystallization is clearly visible in the presence of OA; even distribution of the crystallites is also evident.

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Table 1. DSC-derived parameters for PLA with the OA content.

H H Material T (◦C) T (◦C) T (◦C) T (◦C) g c χ (%) g m1 m2 c (J·g−1·K−1) (J·g−1) c PLA 57 153 nd nd 0.53 0 0 PLA + 0.1% OA 58 150 156 99 0.40 1.8 2 PLA + 0.3% OA 59 148 155 102 0.29 26.2 28 PLA + 0.5% OA 59 149 156 103 0.26 28.6 31 PLA + 5.0% OA 59 150 155 110 0.27 31.8 34

Tg, glass transition temperature; Tm1 and Tm2, melting temperature; Tc, temperature of crystallization; Hg, enthalpy of glass transition; Hc, enthalpy of crystallization; χc, degree of crystallinity.

The crystallization and uniform distribution of the nucleation agent was qualitatively conﬁrmed by optical polarization microscopy (Figure3). The important extent of the crystallization is clearly

Materialsvisible in2018 the, 11 presence, x FOR PEER of REVIEW OA; even distribution of the crystallites is also evident. 6 of 12

(a) (b)

FigureFigure 3. 3. PLAPLA films ﬁlms without without OA OA ( (aa)) and and with with 0.3% 0.3% of of OA OA ( (bb)) as as obtained obtained by by polarization polarization microscopy. microscopy.

3.3. Photodegradation Photodegradation and and Subsequent Analysis The photodegradation of of PLA-based PLA-based materials materials by by sun sun irradiation irradiation plays plays an an important important role, role, especially in agricultural applications. The The photodegradation photodegradation promotes promotes changes in the materials that can influence inﬂuence their properties during its life cycle. To To estimate such effects, a series of PLA materials with different OA contents (0.0%, 0.1%, 0.3%, and 0.5%) were subjected to the controlled artificial artiﬁcial photodegradation experiment.experiment. SeveralSeveral increasing increasing time time periods periods of of the the exposure exposure were were chosen, chosen, where where the themaximal maximal period period of 100 of hours 100 hours should should be equivalent be equiva tolent about to three about months three ofmonths sun irradiation of sun irradiation outdoors, outdoors,during the during summer the season summer in the season temperate in the climatic temper zoneate climatic [38]. Changes zone [38]. in the Changes material in were the evaluated material werewith theevaluated help of FTIRwith andthe help UV-VIS of FTIR spectroscopy. and UV-VIS It was spectroscopy. suggested [39 It] thatwas thesuggested photodegradation [39] that the of photodegradationPLA proceeds with of the PLA usual proceeds radical with mechanism the usual beginning radical mechanism with the abstraction beginning of with a tertiary the abstraction hydrogen ofatom a tertiary from a hydrogen PLA chain atom with from the formation a PLA chain of a radical,with the which formation then reactsof a radical, with oxygen which tothen form reacts peroxide with oxygenand subsequently to form peroxide hydroperoxide. and subsequently The photolysis hydroperox of theide. hydroperoxide The photolysis then of the triggers hydroperoxide the cleavage then of triggersthe PLA the chain. cleavage Only subtleof the PLA changes chain. were Only previously subtle changes reported were in FTIR previously spectra reported upon photodegradation in FTIR spectra uponof PLA photodegradation [40]. Here, no evident of spectralPLA [40]. changes Here, in theno courseevident of photodegradationspectral changes could in the be discernedcourse of in photodegradationFTIR spectra (Figure could S2 in be Supplementary discerned in FTIR Materials), spectra even (Figure in the S2 casein Supplementary of the maximal Materials), irradiation even time in(100 the h), case which of the was maximal chosen to irradiation correspond time to about(100 h), one which season was of outdoorchosen to exposure correspond and toto beabout relevant one seasonto some of agricultural outdoor exposure applications. and to The be surfacerelevant of to the some samples agricultural was subjected applications. to an inspection The surface with of SEM, the samplesbut any structuralwas subjected changes to oran erosion inspection were with detected SEM, (Figure but any S3 instructural Supplementary changes Materials) or erosion after were the detected (Figure S3 in Supplementary Materials) after the photodegradation. On the other hand, the aromatic character of OA was revealed in UV-VIS spectra of the materials (Figure 4), where the aromatic component of the compound provoked a broad peak at about 280 nm. It could be seen that after 20 h of exposure, the OA peak was eliminated, providing an evidence of compound photodegradation.

Materials 2019, 12, 481 7 of 12 photodegradation. On the other hand, the aromatic character of OA was revealed in UV-VIS spectra of the materials (Figure4), where the aromatic component of the compound provoked a broad peak at about 280 nm. It could be seen that after 20 h of exposure, the OA peak was eliminated, providing an Materials 2018, 11, x FOR PEER REVIEW 7 of 12 evidence of compound photodegradation.

Figure 4. UV-VIS spectra of PLA materials containing different contents of OA before and Figure 4. UV-VIS spectra of PLA materials containing different contents of OA before and after after photodegradation. photodegradation. The photodegraded samples were further evaluated in melt rheology experiments using The photodegraded samples were further evaluated in melt rheology experiments using plate- plate-plate geometry. Data for the PLA materials with the increasing content of OA were displayed in plate geometry. Data for the PLA materials with the increasing content of OA were displayed in the the form of Cole-Cole plots (Figure5) as typical semicircle traces in the complex plane. It is evident that form of Cole-Cole plots (Figure 5) as typical semicircle traces in the complex plane. It is evident that the storage modulus decreases with the increasing content of OA for non-irradiated samples; hence, OA the storage modulus decreases with the increasing content of OA for non-irradiated samples; hence, provides some level of lubrication in the PLA matrix. The intersections of the semicircles with “x” axis OA provides some level of lubrication in the PLA matrix. The intersections of the semicircles with correspond to the zero-shear viscosity (η0), which is proprotional to MW of the polymer (Equation (4)). “x” axis correspond to the zero-shear viscosity (η0), which is proprotional to MW of the polymer Comparing the traces for the increasing irradiation times, it is obvious that the photodegradation was (Equation (4)). Comparing the traces for the increasing irradiation times, it is obvious that the accompanied by polymer chain scissions and resulted in a marked decrease in its MW. photodegradation was accompanied by polymer chain scissions and resulted in a marked decrease Deriving the zero shear viscosities and plotting them against the period of photooxidation in its MW. (Figure6), the kinetic of the zero shear viscosity decrease and the corresponding decrease in MW Deriving the zero shear viscosities and plotting them against the period of photooxidation of the polymer is much better discernable. In the initial phase, the rate of the zero shear viscosity (Figure 6), the kinetic of the zero shear viscosity decrease and the corresponding decrease in MW of decrease corresponded well with the content of OA, with the gradual increase of the initial rate up to the polymer is much better discernable. In the initial phase, the rate of the zero shear viscosity AO content of 0.3%. Thus, in the early phase of the photodegradation, OA acted as a photosensitizer decrease corresponded well with the content of OA, with the gradual increase of the initial rate up to and promoted a higher rate of chain scissions. In consequence, it can be seen that the rate of the zero AO content of 0.3%. Thus, in the early phase of the photodegradation, OA acted as a photosensitizer shear viscosity decrease and so the corresponding decrease of MW was proportional to the content of and promoted a higher rate of chain scissions. In consequence, it can be seen that the rate of the zero OA. Employing Equation (4) and the published values of the constants [30,31], it could be estimated shear viscosity decrease and so the corresponding decrease of MW was proportional to the content that MW of the material with 0.3% OA content dropped to about 80 kg·mol−1 at the end of the period of OA. Employing Equation (4) and the published values of the constants [30,31], it could be observed in Figure5. In the later phase of the photodegradation, OA was probably degraded already, estimated that MW of the material with 0.3% OA content dropped to about 80 kg·mol−1 at the end of as could be assumed from UV-VIS spectra (Figure4), and the zero shear viscosity for all materials with the period observed in Figure 5. In the later phase of the photodegradation, OA was probably different contents of OA leveled at about 0.3 Pa·s−1 (Figure6). degraded already, as could be assumed from UV-VIS spectra (Figure 4), and the zero shear viscosity for all materials with different contents of OA leveled at about 0.3 Pa·s−1 (Figure 6).

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Figure 5. Cole-Cole plot for PLA materials with different contents of OA. Figure 5. Figure 5. Cole-Cole plot for PLA materials withwith differentdifferent contentscontents ofof OA.OA.

Figure 6. Dependence of the zero shear viscosity of PLA materials with different contents of OA on the periodFigure of6. photodegradation.Dependence of the zero shear viscosity of PLA materials with different contents of OA on the period of photodegradation. Figure 6. Dependence of the zero shear viscosity of PLA materials with different contents of OA on the period of photodegradation.

Materials 2018, 11, x FOR PEER REVIEW 9 of 12

3.4. Biodegradation in Compost Selected samples of PLA materials both before and after photodegradation were subjected to biodegradation in the compost environment at 58 °C. The material with 0.3% OA content was selected because it shows high crystallinity at a reasonable content of the additive and thus is the most relevant for a real application. The aim of the experiment was to investigate the influence of the OA presence, and eventually of the increased crystallinity induced by OA during the biodegradation. Furthermore, the experiment should also show the impact of the previous photodegradation on the biodegradation. It is well accepted [41] that the biodegradation of PLA is preceded by the chemical hydrolysis of ester

Materialsbonds in2019 the, 12 ,polymer, 481 which is connected with the decrease of MW to the level accessible by9 ofthe 12 enzymatic systems of microorganisms [26]. Hence, the photodegradation, which was proven to decrease MW, can support somewhat faster biodegradation. 3.4. BiodegradationThere were no in significant Compost differences between the biodegradation curves of photodegraded and initialSelected PLA materials samples (Figure of PLA 7). materials All samples both reached beforeand almost after full photodegradation mineralization, i.e., were metabolization subjected to biodegradationof carbon in the in material the compost into carbon environment dioxide, at 58in about◦C. The 90 material days. Judging with 0.3% just OAby sight, content non-irradiated was selected becausesamples itexhibited shows high longer crystallinity lag phases at a reasonable on the be contentginning of theof additivethe process. and thusAfter is thea more most relevantdetailed forcomparison a real application. and the deduction The aim of of the the experiment kinetic parame wasters to investigate of the biodegradation the inﬂuence curves, of the OAit was presence, found andthat eventuallyphotodegraded of the samples increased exhibited crystallinity faster induced minerali byzation OA during (Table the 2). biodegradation.It was especially Furthermore, evident that thethe experimentlag phases were should shorter also showfor the the photodegraded impact of the previoussamples, photodegradationwhich is consistent on with the the biodegradation. drop in MW, Itdemonstrated is well accepted in rheological [41] that themeasurements. biodegradation As already of PLA mentioned, is preceded a bylag thephase chemical is a typical hydrolysis feature ofof esterPLA biodegradation, bonds in the polymer, during whichwhich isrelative connected molecular with theweight decrease has to of decrease MW to to the about level 30,000, accessible driven by theby the enzymatic abiotic hydrolysis, systems of before microorganisms the CO2 evolution [26]. Hence, from thethe photodegradation,biological processes which can be was observed proven [41]. to decreaseMW reduction MW, can during support the somewhat photodegradation faster biodegradation. thus tend to shorten the lag phase during the biodegradation.There were noOn signiﬁcantthe other hand, differences as seen between from the the rheological biodegradation measurements curves of and photodegraded UV-VIS and FTIR and initialspectra, PLA the materialsphotosensitization (Figure7). caused All samples by the reached OA presence almost seemed full mineralization, to be very weak i.e., and metabolization transient. of carbonThe maximal in the material rate of into the carbonsubsequent dioxide, CO2 in production about 90 days. appeared Judging to be just almost by sight, identical non-irradiated between samplesthe initial exhibited and photodegraded longer lag samples, phases which on the suggested beginning that of the the photodegradation process. After adid more not influence detailed comparisonthe process through and the deductionother factors, of thee.g., kinetic the formation parameters of inhibiting of the biodegradation compounds. curves, it was found that photodegradedThe comparison samples of the corresponding exhibited faster samples mineralization with and (Table without2). It wasOA especiallyis interesting. evident Both that lag thephases lag phasesand maximal were shorter rates suggeste for thed photodegraded that OA presence samples, supported which fa isster consistent biodegradation. with the In drop some in MW,previous demonstrated studies, it inwas rheological outlined measurements.that crystallinity As itself already did mentioned,not retard abiodegradation lag phase is a typical[27,28]. featureMoreover, of PLAas shown biodegradation, previously [33], during during which the relative initial lag molecular phase, the weight cold crystallization has to decrease induced to about by chain scissions tended to increase the crystallinity considerably, up to about 30% and thus probably 30,000, driven by the abiotic hydrolysis, before the CO2 evolution from the biological processes can be observedthe crystallinities [41]. MW in reduction all samples during were theequalized photodegradation at the end of thus the tendlag phase. to shorten Another the lagmechanism phase during then theshould biodegradation. be sought to Onexplain the other the a hand,cceleration as seen of from the biodegradation, the rheological measurementse.g., catalysis of and the UV-VIS ester bond and FTIRhydrolysis spectra, in thePLA photosensitization by OA or by degradation caused by pr theoducts OA presenceof OA formed seemed by to the be photodegradation. very weak and transient.

Figure 7. Biodegradation of PLA materials in compost at 58 ◦C.

The maximal rate of the subsequent CO2 production appeared to be almost identical between the initial and photodegraded samples, which suggested that the photodegradation did not inﬂuence the process through other factors, e.g., the formation of inhibiting compounds. The comparison of the corresponding samples with and without OA is interesting. Both lag phases and maximal rates suggested that OA presence supported faster biodegradation. In some previous studies, it was outlined that crystallinity itself did not retard biodegradation [27,28]. Moreover, as shown previously [33], during the initial lag phase, the cold crystallization induced by chain scissions Materials 2019, 12, 481 10 of 12 tended to increase the crystallinity considerably, up to about 30% and thus probably the crystallinities in all samples were equalized at the end of the lag phase. Another mechanism then should be sought to explain the acceleration of the biodegradation, e.g., catalysis of the ester bond hydrolysis in PLA by OA or by degradation products of OA formed by the photodegradation.

Table 2. Kinetics parameters of the biodegradation obtained from Equation (5).

Parameter PLA PLA + 0.3% OA UV (h) 0 75 0 75 a CO2_Max (%) 86 ± 2.5 95 ± 1.1 85 ± 2.1 112 ± 5.1 k b (day−1) 1.9 ± 0.09 1.8 ± 0.04 1.4 ± 0.04 1.5 ± 0.05 C c, (days) 28.4 ± 1.2 18.5 ± 0.57 19.9 ± 0.92 14.7 ± 1.4 R2 0.991 0.994 0.996 0.991 a Maximal level of mineralization; b maximal rate of mineralization; c duration of the lag phase, values ± standard deviations.

4. Conclusions With growing pressure to restrict conventional plastic, at least in some applications, there is the growing need to seek alternatives for materials in various biodegradable plastics. A very challenging task is then the effort to improve the properties of the latter materials to meet the demands for particular applications and at the same time preserve wanted properties e.g., above all, biodegradability. In the study, it has been proven that OA can be employed as a relatively potent crystallization agent in PLA, where the improved crystallinity could be very important for certain applications. Although OA can effectively absorb UV irradiation, it was not identiﬁed to be a strong photosensitization agent in given concentrations, and degraded rather rapidly by the action of UV irradiation in the employed concentrations. The photodegradation, regardless of the OA presence, shortened the lag phase of the PLA biodegradation, where the mechanism lay most probably in the reduction of MW during the photodegradation. Moreover, it was shown that OA presence did not negatively interfere with the biodegradability of PLA, which is the crucial and most valuable property of this material. OA itself or the crystallinity induced by the OA presence did not retard the biodegradation; even so, the presence of OA seemed to shorten the lag phase and increase the observed maximal rates of the biodegradation.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/12/3/481/s1, Figure S1: (a) SEM micrographs of OA powder, (b) Threshold sharpened images of OA grains used for the image analysis, (c) Distribution of cross-section areas of OA grains. Figure S2: FTIR spectra of the investigated PLA materials with different contents of OA, both before and after 20 h of the photodegradation. Figure S3: SEM of PLA materials before and after photodegradation. PLA (a), PLA after 75 h of photodegradation (b), PLA with 0.3% of OA (c), PLA with 0.3% of OA after 75 hours of photodegradation (d). Author Contributions: Investigation, J.S., J.Š., M.J., V.V. and M.K.; Methodology, V.V. and A.M.; Writing—original draft, J.S. and M.K. Funding: Financial support from Tomas Bata University in Zlín, Internal Grant Agency (projects IGA/FT/2018/009 and IGA/FT/2019/011) Conﬂicts of Interest: The authors declare no conﬂict of interest.

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