Effect of gamma irradiation on physicochemical properties of commercial poly(lactic acid) clamshell for food packaging

Author’s Accepted Manuscript Effect of gamma irradiation on physicochemical properties of commercial poly(lactic acid) clamshell for food packaging Tomás J. Madera-Santana, R. Meléndrez, Gerardo González-García, Patricia Quintana-Owen, Suresh D. Pillai www.elsevier.com/locate/radphyschem

PII: DOI: Reference:

S0969-806X(16)30044-5 http://dx.doi.org/10.1016/j.radphyschem.2016.02.001 RPC7058

To appear in: Radiation Physics and Chemistry Received date: 11 April 2015 Revised date: 1 February 2016 Accepted date: 2 February 2016 Cite this article as: Tomás J. Madera-Santana, R. Meléndrez, Gerardo GonzálezGarcía, Patricia Quintana-Owen and Suresh D. Pillai, Effect of gamma irradiation on physicochemical properties of commercial poly(lactic acid) clamshell for food packaging, Radiation Physics and Chemistry, http://dx.doi.org/10.1016/j.radphyschem.2016.02.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of gamma irradiation on physicochemical properties of commercial poly(lactic acid) clamshell for food packaging Tomás J. Madera-Santanaa,*, R. Meléndrezb, Gerardo González-Garcíac, Patricia QuintanaOwenc, Suresh D. Pillaid a

Centro de Investigación en Alimentación y Desarrollo, A.C. CTAOV. Km. 0.6 Carr. a La

Victoria. A.P. 1735. 83304. Hermosillo, Sonora, México. b

Departamento de Investigación en Física. Universidad de Sonora. A.P. 5-088. 83190.

Hermosillo, Sonora. México. c

Centro de Investigación y Estudios Avanzados del IPN Unidad Mérida. Laboratorio Nacional

de Nano Biomateriales. A.P. 73, Cordemex. 97310 Mérida, Yucatán. México. d

National Centre for Electron Beam Research. Texas A&M University. College Station. Texas.

USA. * Corresponding author. Tel. +52-662-289-2400; fax: +52-662-280-0422. E-mail address: [email protected] (T. J. Madera-Santana).

ABSTRACT Poly(lactic acid) (PLA) is a well-known biodegradable polymer with strong potential application in food packaging industry. In this paper, samples of PLA clamshell for tomatoes packaging were exposed with 60CO γ-ray’s source (1.33 MeV) at different dose levels (0, 10, 60, 150, 300, and 600 kGy), at room temperature and in presence of air. The physicochemical properties of neat PLA and sample exposed to gamma irradiation were investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis

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(TGA), scanning electron microscopy (SEM) and tensile measurements. Results show as the dose increases, the molecular weight (Mw), melting temperature (Tm), tensile strength and elongation at break decreased. However, the tensile modulus increased with increasing doses. The surface of PLA clamshells was degraded (scratches and minor cracks) when samples were exposed to doses greater than 60 kGy. Keywords Poly(lactic acid); Gamma-irradiation; Thermal properties; XRD; Physico-chemical properties 1. Introduction Food irradiation is an effective technology for reducing postharvest food losses and ensuring hygienic quality of foods. It is known that gamma irradiation is the most widely-used cold sterilization technique because of its high penetration power (George et al., 2007). The food is packaged during irradiation processing of foods, and a wide variety of packaging configurations such as single or multi-layered films, trays, clamshells, etc. are employed in food packaging. It is well known, that ionizing radiation produces physical and chemical changes in packaging materials. Traditionally, petroleum-derived plastics have been used as food packaging materials. However, the growing accumulation of petro-polymers in the environment is a global environmental issue and has spurred interest in replacing these with biodegradable polymers from renewable resources (Mohanty et al., 2002; Wang et al., 2007). Among the group of biodegradable polymers, polyesters and their derives, poly(-caprolactone), poly(3-hydroxy butyric acid), polymalic acid and polylactic acid have been studied extensively (Lunt, 1998; Nugroho et al., 2001). The poly(lactic acid) (PLA) offers the potential for an attractive combination of mechanical properties, thermal plasticity, high degree of transparency, biocompatibility and cost. Therefore, PLA has tremendous potential for several applications in

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the biomedical, household, and agricultural industries. The US Food and Drug Administration have approved the use of PLA for food contact and is widely used in rigid and flexible food packaging (FDA, 2005). There are published studies reporting on the thermal, hydrolytic or oxidative stability of this polymer, which has been shown to undergo random chain scission (Nugroho et al., 2001; Zaidi et al., 2013). Moreover, Gupta et al. (1982) have demonstrated that the isothermal degradation of PLA is a random chain scission in which two kinetically independent units take part in the degradation. There are, however, only few reports on radiation-induced degradation of PLA, even though it is known to be highly sensitive to ionizing radiation (Nugroho et al., 2001; Yovcheva et al., 2013). Some of these papers have reported the optical thermal, morphological, structural among other properties of modified PLA (blends, crosslinked, filled with nanoparticles, etc.) (Zaidi et al., 2013; Razavi et al., 2014). To our knowledge, the physical properties of PLA clamshells designed for food packaging when subjected to different doses of gamma irradiation have not been reported so far. In this research, laminates of clamshell basedon PLA have been subjected to different doses, from 0 to 600 kGy. The molecular characteristics, mechanical, thermal, structural and morphological properties were studied as a function of irradiation dose and the mechanism of radiation induced degradation is discussed. 2. Experimental 2.1 Materials Clamshells were manufactured from PLA pellets (NatureWorks LLC. Ingeo™ biopolymer 2003D), and were used as received. The material has a number average molecular weight (Mn) of 150,500 g/mol, melt flow rate of 5.92 g/10 min and density of 1.2 g/cm. The PLA samples were cut from clamshells into 5x5 cm2 sheets having the thickness of 400 μm.

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2.2 Methods 2.2.1. Gamma irradiation exposure test The PLA samples were irradiated in a self-contained gamma research irradiator Gammacell 220 Excel (GC-220E) of MDS Nordion (4.9 kCi). Target doses of 0, 10, 60, 150, 300 and 600 kGy were performed at 3.86 kGy/h dose rate by a 60Co- source, under ambient conditions (25 °C). 2.2.2. Optical properties Film opacity was determined using procedures described by Zhang and Han (2006). Briefly, the film samples were cut into rectangles and placed on the internal side of a spectrometer cell. The absorbance spectrum (200-800 nm) was recorded for each sample using a Varian Cary-50 Bio UV-visible spectrophotometer (Palo Alto, CA). The opacity of the PLA samples was determined by measuring the absorbance at 600 nm (A600). The opacity at unit light path length was calculated using the following equation described by Han and Floros (1997): (1) where A600 is the absorbance at 600 nm, and the thickness T is in mm. The measurement was performed in three replicates for each film, and the average value is reported. The irradiated samples were measured for their color values. Color was read using the Commission International de L’ Eclairage (CIE) Lab parameters (L*, a*, b*) with a spectrophotometer-colorimeter Konica Minolta model Chroma Meter CR-300 (Ramsey, NJ). The scanner was calibrated with a white standard tile (Y=93.2, X=0.3133, Y=0.3192). In this coordinate system, the L* value is a measure of the lightness (brightness), ranging from 0 (black) to 100 (white); the a* value is a measure of the redness, ranging from -100 (green) to +100 (red),

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and the b* value is a measure of the yellowness, ranging from -100 (blue) to +100 (yellow). The color differences (E) were calculated by the following equation: √(

)

(

)

(

)

(2)

where L*= L*-Lo, a*= a*- ao, and b*= b* - bo. Lo, ao and bo represent the color parameter values of the standard, and L*, a* and b* represent the color parameters of the sample. Measurements were performed by placing the PLA sample over the standard. All of the samples were analyzed by recording six measurements of each sample. 2.2.3. Mechanical properties The irradiated samples were cut into rectangular pieces (6x1 cm) and evaluated using an universal testing machine United model SSTM-5 (Houtivigton Beach, CA). The thickness of each specimen was measured using a micrometer Mitutoyo Digimatic MDC-1SB (Kawasaki, JP) with a precision range between 0.001 mm and average values were determined. The initial grip separation was set at 30 mm, and the crosshead speed was set at 10 mm/min. A minimum of five specimens were used to determine the average of mechanical parameters (tensile strength, elongation at break, and tensile modulus). 2.2.4. Structural analysis The infrared spectra in the attenuated total reflection mode (FTIR-ATR) were obtained at room temperature using Thermo Nicolet spectrometer model Nexus 670-FTIR (Madison, WI). The samples were analyzed in the range of 4000-600 cm-1, with 4 cm-1 of resolution, and 100 scans. To determine the crystalline structure of the irradiated samples, a sample of 1x1 cm2 was placed in a sample holder for X-ray diffractometry. The X-ray diffraction patterns were recorded in the reflexion mode in an angular range of 5-60° (2θ) at room temperature using a diffractometer Siemens model D5000 (Karlsruhe, Germany), with a Bragg Brentano geometry

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and monochromatic CuKα radiation ( = 1.5418 Å), at 34 kV, and 20 mA. The molecular weights of PLA samples were determined by gel permeation chromatography (GPC) an Agilent PL-GPC 50 system (Santa Clara, CA) was used in combination with a differential refractive index detector Agilent 1260 Infinity. The column set consisted of two PLgel columns of 5 m Mixed D with 300 mm of length and 7.5 mm of diameter. The eluent was analytical grade tetrahydrofuran (THF) at a flow rate of 1 mL/min at 40 °C. PLA samples were previously dissolved in THF for 24 under stirring. Solutions were filtered through 0.45 μm stainless steel frits and heated at 40 °C for 1 h prior to injection. The quantitative 1H solution NMR spectra were recorded at 25 °C on a Varian/Agilent 600 MHz Premium Compact NMR spectrometer (Santa Clara, CA) at 599.78 MHz, 8.7 µs, 10 s and 1.5 s for 1H frequency, π/2 pulse, recycle delay and acquisition time respectively. Deuterium-chloroform 99.8% (CDCl3) from Cambridge Isotope Laboratories, Inc. was used as solvent and chemical shifts (δ) were determined relative internal TMS (1H, δ 0.00). 2.2.5. Thermal analysis The differential scanning calorimetry (DSC) thermograms of irradiated samples were performed on a Discovery DSC from TA Instruments (New Castle, DE) under nitrogen atmosphere. A sample of 5 mg was heated to 200 °C at 10 °C/min, isothermal for 2 min, and cooled down to 25 °C at 50 °C/min, and reheated to 200°C at 10 °C/min. From second thermogram of each sample the parameters: glass transition temperature (Tg), specific heat change (ΔCp), enthalpies of melting (ΔHm) and cold crystallization (ΔHc), temperatures of cold crystallization (Tc) and melting (Tm) were determined. The percentage of crystallinity (c) was calculated by using Eq. (3), ΔH°m is the enthalpy of melting for 100% crystalline PLA (93.6 J/g) (Tsuji et al., 2004). 6

c

(3)

Thermal decomposition analyses were performed using a thermogravimetric analyzer (TGA) model Discovery TGA from TA Instruments (New Castle, DE). Approximately, 10 mg of sample were heated from 50 to 600 °C at 10 °C/min under nitrogen atmosphere with a flow rate of 60 mL/min, the residue was evaluated as the residual weight at 600 °C. 2.2.6. Film microstructure The morphological defects were observed by environmental scanning electron microscopy (ESEM) from FEI Corp. model Philips XL30 ESEM-FEG (Hillsboro, OR), with an accelerating voltage of 5 kV and magnification of 1000X. A sample of 5x10 mm2 was fixed on the support using a double side adhesive tape. The samples were coated with a thin layer of Au-Pd using a sputter machine Quorum model QI5OR-ES (Sussex, UK) prior to scanning. 2.2.7. Statistical analysis The experimental data was statistically analyzed by one-way analysis of variance (ANOVA) using NCSS 2007 LLC (Kaysville, UT) and the statistical significance of each mean property was determined (p˂0.05) to evaluate the effect of absorbed doses on the optical and mechanical properties. 3. Results and discussion 3.1. Optical properties The absorption of light energy by polymeric materials in UV and visible regions involves transition of electrons in σ, π and n-orbitals from the ground state to higher energy states. Therefore, UV/Vis spectroscopy has become a suitable tool to investigate these electronic transitions and particular properties such as transparency. The optical absorption edge in the UVvisible regime of neat and irradiated PLA clamshell samples at various doses is shown in Fig. 1. 7

The absorption edge is slightly shifted towards the higher wavelength side, as the irradiation dose was increased. The absorption edge lies within the length region of 265-383 nm. This shift in the absorption edge could be correlated with the formation of conjugated bonds showing the possibility of formation of carbon clusters. Furthermore, this type of transition occurs in compounds due to bond cleavage and reconstruction (Siddhartha et al., 2012). The constant absorption region of un-irradiated sample is attributed to both superposition of absorption and scattering of UV rays. However, the rises of curve did not increase in the crystallinity of the PLA samples as the absorbed dose was increased. The exception was sample irradiated with 600 kGy, where a significant increase in crystallinity was corroborated by DSC and XRD analysis. Opacity is an important physical property of packaging items, because it provides seethrough property and prevents light transmission. Table 1 shows the opacity of PLA samples at different doses. The opacity as measured by Ec. (1) shows that this property increases as the absorbed dose reaches up to 300 kGy. However, samples with a dose of 600 kGy showed a drastic decrease. This is probably due to microcracks produced by the chain scission of PLA molecule, which allowed that the transmission of UV beam through the sample. The rectangular coordinates (L*, a* and b*) and the total color difference (E) of PLA samples at different doses are shown in Table 1. The neat PLA sample was transparent without any color, while the samples exposed to higher doses (>150 kGy) were less transparent with a slight yellowish tint, which is clearly shown in the result of surface color parameters and transparency of the PLA samples. The luminosity values (L*) of PLA samples up to dose of 150 kGy did not shown significant difference (p>0.05). However, at doses of 300 and 600 kGy, the L* parameter decreased significantly (p˂0.05) compared with the un-irradiated samples. The a* parameter showed a significant (p˂0.05) decrease as the dose was increased. In contrast, for b*

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parameter, it showed a significant (p˂0.05) increase at 150 kGy of absorbed dose, after which a remarkably decrease was observed. From the results, the decrease in L* values with increase in b* values corroborates that a yellowish tint appeared in the PLA samples. Increasing doses resulted in significant (p<0.05) increase in total color difference (E) from 0.84±0.11 to 8.42±0.43. However, it is important to note that yellowish tint was evident when the sample maintains structural integrity (up to 150 kGy). A higher dose, the sample gets fragmented and the color turns to white-grayish color. This result suggests that functional groups of PLA chain molecule are related to the color of the sample. 3.2. Mechanical Properties Irradiation usually produces notable effects in the mechanical properties of polymers (Razavi et al., 2014). These effects differ depending on whether crosslinking or degradation takes place due to irradiation. Table 2 shows the tensile strength, elongation at break and tensile modulus of neat and irradiated PLA clamshell samples. The samples with doses over 60 kGy were unable to be tested due to their extreme fragility. Irradiation of PLA samples produced a significant (p˂0.05) decrease in tensile strength and elongation at break values. The brittleness of PLA is one of its major drawbacks and this problem is accentuated after exposure to gamma irradiation. In contrast, the tensile modulus (stiffness) of PLA increased significantly (p˂0.05) with gamma irradiation of 60 kGy, when compared with the control (Table 2). The tensile modulus of pristine PLA was found to decrease at 10 kGy, but exposure to a dose of 60 kGy resulted in significant increase of the tensile modulus was observed. Nugroho et al. (2001) reported for irradiated PLA a decrease in tensile strength and elongation at break, although the authors did not reported the stiffness values. The results of mechanical properties indicate that chain-scission could be occurring at the folded or loosely looped chain molecules in the amorphous regions of PLA.

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Furthermore, the hydro-peroxidative chain process initiates the scissoning of the main PLA backbone chain, thereby reducing the PLA’s molecular weight and severely diminishing the intermolecular forces. The dominant chain scission causes the long polymer chains and crosslinked structures to be break down into shorter chains and smaller structures, thereby decreasing the crosslinking density. Short chains have better mobility and little restriction from the rigid crosslinking networks. 3.3. Structural properties Fig. 2 (with offset for comparison) the FTIR spectra of PLA samples at irradiation dose of 0, 10, 150, and 600 kGy is shown. In Fig. 2(a) the region 4000-2700 cm-1 in the spectra are observed particular bands located at 3506 cm-1, that corresponds to –OH groups in alcohols, hydro-peroxides or carboxylic acids. This band becomes intense and broad in samples with doses of 150 and 600 kGy, respectively. It is attributed to the formation of hydroperoxide derivatives which degrade in compounds containing a carboxylic acid and diketone end groups. Moreover, the three bands situated at 2995, 2945 and 2875 cm-1 are attributed to the saturated CH3 stretching bands and symmetric/asymmetric deformation bands of –CH of PLA, respectively (Auras et al., 2004). In Fig. 2(b) the spectra region between 2400 and 600 cm-1 is shown. A strong absorbance observed at 1756 cm-1 confirms the presence of the ester via the carbonyl stretch (C=O). The bands at 1455 and 1392 cm-1 are attributed to the saturated C–H stretch. The bands at 970, 880, 760 cm-1 are attributed to the deformation vibration of =CH groups (Arrieta et al., 2013). As can be observed in Figs. 2(a) and 2(b), the changes in the absorption spectra peaks of PLA samples can be attributed to chain scission mechanism and consequent oxidation reactions induced by irradiation treatment, reveled by marked modification in intensity of the absorption bands of FTIR spectra. However, other authors have reported that gamma irradiation

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induced neither any detectable change in intensity or any wavenumber shift of FTIR spectra peaks of PLA, despite the observed effects of the ionizing irradiation on mechanical properties of PLA (Kodama et al., 2007; Razavi et al., 2014). These contradictory results could be attributed to the gamma irradiation dose that was employed in this study (up to 600 kGy) compared to lower doses (~ 50 kGy) used in previous studies. The XRD patterns of control and irradiated PLA samples are presented in Fig. 3, while the parameters are provided in Table 3. The XRD patterns of the samples are characterized by halos extending in the 2 range from 3-60°. The main diffraction peak occurs at 2 = 16.51° and 16.63° in samples with the absorbed doses of 0 and 600 kGy, respectively. It can be seen that the dose at 600 kGy increases the diffraction peak intensity, and the formation of small diffraction peaks at 14.76°, 19.04°, 22.36°, 29.04°, and 33.68°. The increase in the peak intensity indicates that gamma-irradiation produces an increase in the degree of polymer crystallinity. Peaks at 2 = 14.7, 16.6, 18.9 and 22° are associated to homopolymer crystals, and these peak positions are in good agreement with the reported values (Zhang et al., 2008). XRD data summarized in Table 3, showed that the peak located at 16.6° is attributed to the 200/110 plane reflection of the PLA crystallites. As expected, gamma irradiation leads to deformation of PLA structure. It is well documented in literature that the broadening of the peak is associated with the decrease in average crystallite size (Rosenberg et al., 1992; Kumar et al., 2012). However, our results followed a contrary behavior; it could be due to a self rearrange of fragmented macromolecules on the surface of the already existing crystals. The macromolecular chains have great mobility and are liberated from the amorphous region due to scission in the chain. In this sense, the chemiocrystallization phenomenon could be occurring in this system, similar behavior was reported for poly(hydroxybutyrate) which is a biodegradable polyester from bacterial origin

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(Oliveira et al., 2006). The crystallinity was calculated using the method reported by Stribeck without take into account the amorphous halo (Stribeck, 2007). The % of crystallinity showed in Table 3 follow a decrease tendency, as the absorbed dose increase up to 150 kGy. Afterwards, the percentage of crystallinity of PLA increases significantly, it is due to gamma irradiation involves the phenomenon called nucleation (Oliveira et al., 2006). A common effect observed in irradiated polymers is the scission of weak bonds in the macromolecule chains; it is because the energy is absorbed by the exposed polymer chains (Miao et al., 2009). Fig. 4(a) shows the number and weight average molecular weight (Mn and Mw) of irradiated samples as a function of dose and this decrease in the molecular weight occurred at dose between 10 to 150 kGy. The decrease in molecular weight is due to backbone main chain scission, where some long polymeric backbone chains break into shorter chains. The excited state dissipates part of this excess of energy by bond scission and produce alkyl free radicals in PLA. Assuming that this process takes place under ambient conditions, oxygen diffuses into the PLA where it reacts with the alkyl free radicals to produce peroxyl free radicals. The peroxyl free radicals could cause chain scission through hydrogen abstraction, although the decrease in molecular by chain scission through hydrogen abstraction is less pronounced in comparison with the main chain scission. Fig. 4(b) shows the polydispersity index (Mw/Mn) of PLA against irradiation dose. The results show the increase in polydispersity index of PLA as the gamma dose is increased up to 150 kGy. At doses over 150 kGy, the polydispersity index decreases; it means that the Mw decreases faster that Mn with the absorbed dose. Moreover, the Mn,0/Mn,t ratio of molecular weight of the un-irradiated sample (Mn,0) to molecular irradiated sample (Mn,t) shows the degree of degradation as a function of dose, and it can be observed in Fig. 4(b). The deviation of Mn,t

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from Mn,0 values implies that a proper degradation has occurred, which results in an increase of Mn,0/Mn,t ratio. Gamma irradiation causes two opposite process, the breaking and formation of polymer bonds as a result of molecular chain scission and crosslinking in the polymer. It means that the chain scission causes one molecule to become in two, and the crosslinking causes two molecules to become in one (Hill et al., 1996). The radiation chemical yield for chain scission (Gs) and crosslinking (Gx), where both parameters are defined as the number of such reactions (radiolysis events) caused by the absorption of 100 eV of irradiation energy, therefore the extent of chain scission and crosslinking during irradiation can be calculated approximately using the following equations: ( (

)

(4) )

(5)

Where Mw,0 and Mn,0 are the weight and number average molecular weight of polymer before irradiation. Mw,t and Mn,t are the weight and number average molecular weight after irradiation. D is the absorbed dose (kGy). If the ratio of Gs/Gx is greater than four, it indicates that the chain scission dominates the process. In Figure 4(c) are shown the plots of (

) and (

) against D, where both follow a linear relationship, with slope values of 5.793x10-7 and 8.319x10-7, respectively. The values of Gs and Gx of commercial PLA clamshells were calculated using Eqs. (4) and (5), was found to be 0.959 and 0.401, respectively. The ratio of Gs/Gx is 2.39, it means that the chain scission occurs, tough there might have been some crosslink in polymer chains. Other authors have reported the Gs value of poly(L-lactic acid) irradiated is 0.83 (Nugtoho et al., 2001), Vargas et al. (2009) have reported for PLA from 13

thermoformed cup drinks cups Gs values of 0.52 and 2.18, and for polypropylene (PP) the Gs values irradiated in air and vacuum have been reported to be 3.1 and 0.65, respectively (Kagiya et al., 1985). In Fig. 5 is shown the quantitative 1H NMR spectra of the control and the gamma irradiation effect on the PLA structure at different doses. An increase of the dose over the PLA samples produced an increase of the CH ending group. It indicates an increment of the backbone scission and a decrease in the numerical molecular weight (Mn), as can be seen in the insert of Fig. 5. These results are in good agreement with GPC analysis. 3.4. Thermal properties In Table 4 the thermal parameters obtained from DSC thermograms are shown. The decrease in the glass transition temperature (Tg), cold crystallization (Tc) and melting temperature (Tm) of PLA samples with the increase in dose is evident. At high irradiation dose (≤ 300 kGy) the enthalpy of melting (Hm) and crystallinity (Hc) increases; it could be related with chemiocrystallization phenomenon. It means that the increase in degree of polymer crystallinity during the irradiation process is a result of liberation of macromolecular fragments (Oliveira et al., 2006). Moreover, the remaining PLA chains could have high mobility and reorganization capacity themselves to lead an increase in the crystallinity as can be seen in Table 4. When the percentage of crystallinity determined by XRD was compared to the percentage of crystallinity of the DSC experiment in PLA sample at 600 kGy, it was observed that the results follow similar tendency. The thermal stability of PLA samples was studied by thermogravimetric analysis (TGA). The TGA profiles of copolymer irradiated at various doses are shown in Fig. 6(a). It can be observed that all the PLA samples decompose in only one stage and it involves the degradation of lactic

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acid and leads to complete polymer volatilization. In Table 5 are shown the corresponding temperature of degradation for 5, 25, 50 and 75% mass loss. We can see that 5 and 25% mass loss temperature (T5 and T25) for control PLA are 329 °C and 351 °C, respectively. At 60 kGy irradiation, T5 and T25 decrease to 313 °C and 349 °C, and for 600 kGy these are 256 °C and 319 °C, respectively. The decomposition temperature was calculated using the derivative form of TGA (DTGA). Where the differentials of TGA values were calculated using a central finite difference method and results are shown in Fig. 6(b). The DTGA curves clearly show the maximum temperature for decomposition (Tmax) of the PLA tends to decrease as the dose is increased. The results of TGA analysis are summarized in Table 5, it can be seen that thermal stability of PLA samples decreased as the dose was increased and it is due to decrease in molecular weight of PLA. 3.5. Film microstructure In order to examine the influence of irradiation on the microstructure, the surface morphology of PLA samples was analyzed by SEM. Figs. 7(a), 7(b) and 7(c) shows the SEM microphotographs of PLA films at 0, 300 and 600 kGy, respectively. The inserted photograph belongs to the PLA samples after irradiation exposition. Apparently, the samples with low doses (≤60 kGy) showed homogeneous and smooth surface microstructure with apparent compact structural integrity. The control PLA sample in Fig. 7(a) exhibits a regular and smooth surface, but after gamma irradiation of 300 kGy (Fig. 7(a)) and 600 kGy (Fig. 7(b)) the film surface is obviously degraded. It is characterized by an opaque and rough surface, the appearance of lines or scratches on its surface of different sizes that reveal the crack formation. Moreover, the photographs in the insets show the morphological changes at macroscale. 4. Conclusions

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The PLA clamshell samples when subjected to different doses resulted in chain scission and oxidative degradation in the amorphous regions, as a consequence the average molecular weight (Mn and Mw) decreased. The optical properties, such as opacity and color of PLA samples were affected by the irradiation process due to the formation of unstable chromatic groups that introduces the conjugation of carbonyl groups. Colorimetry revealed that gamma irradiation caused an increase of opacity and color difference of PLA samples. The irradiated samples turned to yellowish tint (at 150 kGy) and to white-grayish color (≥300 kGy) in the fragmented sample. The mechanical properties were greatly dependent on the dose in the samples; the tensile strength and elongation at break are decreased greatly, whereas the elastic modulus increased slightly. The effect of the dose on the thermal stability found that the mass (%) and decomposition temperature decreased as the dose increased. The scission and oxidation of PLA chains was considerably intensified at high doses, generating molecular species with higher mobility and induces the formation of crosslinking in the PLA matrix. The increase of percentage of crystallinity in PLA at high dose is attributed to the phenomenon called chemiocrystallization. Evident changes in properties, caused by gamma irradiation in PLA clamshell, were observed at doses ≥60 kGy. Acknowledgments This research was financed by Fundación Produce Sonora under project contract No. 262012-0021. SEM, NMR and XRD measurements were performed at LANNBIO Cinvestav Mérida, under support from projects FOMIX-Yucatán 2008-108160 and CONACYT LAB-200901 No. 123913. Technical help is acknowledging to MSC. D. Aguilar-Treviño and Mrs. A. Cristóbal for their assistance for XRD and SEM analyses, respectively. Dr. González-García gratefully acknowledges for postdoctoral scholarship from CONACYT.

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References Arrieta, M.P., López, J., Ferrándiz, S., Peltzer, M.A., 2013. Characterization of PLA-limonene blends for food packaging applications. Polymer Testing 32(4), 760-768. Auras, R., Harte, B., Selke, S., 2004. An overview of polylactides as packaging materials. Macromolecular Bioscience 4(9), 835-864. FDA. Inventory of effective Food Contact Substance (FCS) 2005. George, J., Kumar, R., Sajeevkumar, V.A., Sabapathy, S.N., Vaijapurkar, S.G., Kumar, D., Kchawahha, A., Bawa, A.S., 2007. Effect of -irradiation on commercial polypropylene based mono and multi-layered retortable food packaging materials. Radiation Physics and Chemistry 76(7), 1205-1212. Gupta, M.C., Desmukh, V.G., 1982. Thermal oxidative degradation of polylactic acid. Colloids and Polymer Science 260(5), 514-517. Han, J.H., Floros, J.D., 1997. Casting antimicrobial packaging films and measuring their physical properties and antimicrobial activity. Journal of Plastics of Films and Sheeting. 13(4), 287-298. Hill, D.J.T., Milne, K.A., O’Donnell, J.H., Pomery, P.J., A recent advance in the determination of scission and crosslinking yields of gamma ray irradiated polymers. In: Irradiation of polymers, fundamentals and technological applications. Clough RL, Shalaby SW (Eds). ACS Symposium series 620. Washington DC, 1996:74-80. Kagiya, T., Nishimoto, S., Watanabe, Y., Kato. M., 1985. Importance of the amorphous fraction of polypropylene in the resistance to radiation-induced oxidative degradation. Polymer Degradation and Stability 12(3), 261-275.

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Kodama, Y., Machado, L.D.B., Giovedi, C., Nakayama, K., 2007. Gamma radiation effect on structural properties of PLLA/PCL blends. Nuclear Instruments and Methods in Physics and Research Section B 265(1), 294-299. Kumar, V., Ali, Y., Sonkawade, R.G., Dhaliwal, A.S., 2012. Effect of gamma irradiation on the properties of plastic bottle sheet. Nuclear Instruments and Methods in Physics and Research Section B 287(1), 10-14. Lunt, J., 1998. Large-scale production, properties and commercial applications of polylatic acid polymer. Polymer Degradation and Stability 57(1-3), 145-152. Miao, P., Zhao, C., Xu, G., Fu, Q., Tang, W., Zeng, K., Wang, Y., Zhou, H., Yang, G., 2009. Degradation of poly(D,L-lactic acid)-b-polyethylene glycol-b-poly(D,L-lactic acid) copolymer by electron beam radiation. Journal of Applied Polymer Science 112(5), 29812987. Mohanty, S.K., Misra, M., Drzal, L.T., 2002. Sustainable biocomposite from renewable resources: Opportunities and challenges in the green materials world. Journal of Polymer and the Environment 10(1-2), 19-26. Nugroho, P., Mitomo, H., Yoshii, F., Kume, T., 2001. Degradation of polylactic acid by irradiation. Polymer Degradation and Stability 72(2), 337-343. Oliveira, L.M., Araujo, E.S., Guedes, S.M.L., 2006. Gamma irradiation effects on poly(hydroxyburtyrate). Polymer Degradation and Stability 91(9), 2157-2162. Razavi, S.M., Dadbin, S., Frounchi, M., 2014. Effect of gamma ray on poly(lactic acid)/polyvinyl acetate-co-vinyl alcohol) blends as biodegradable food packaging films. Radiation Physics and Chemistry 96(1), 12-18.

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Rosenberg, Y,, Siegmann, A,, Narkis, M., Shkolnik, S., 1992. Low dose -irradiation of some fluoropolymers: Effect of polymer chemical structure. Journal of Applied of Polymer Science 45(5), 783-795. Siddhartha, W., Aarya, S., Dev, K., Raghuvanshi, S.K., Krishna, J.B.H., Wahab, M.A., 2012. Effect of gamma radiation on the structural and optical properties of polyethyleneterephtalate (PET) polymer. Radiation Physics and Chemistry 81(4), 458-462. Stribeck, N. X-ray scattering of soft matter. Springer Laboratory Series. Germany. 2007, p. 102. Tsuji, H., Ikarashi, K., Fukuda, N., 2004. Poly(L-lactide): XII. Formation, growth and morphology of crystalline residues as extended chain crystallites through hydrolysis of poly(L-lactide) films in phosphate-buffered solution. Polymer Degradation and Stability 84(3), 515-523. Vargas, LF, Welt, B.A., Pullammanappallil, P., Texeira, A.A., Balaban, M.O., Beatty, C.L., 2009. Effect of electron beam treatments on degradation kinetics of PLA plastic waste under backyard composting conditions. Packaging Technology and Science 22(2), 97-106. Wang, H.Y., Huang, M.F., 2007. Preparation, characterization and performances of biodegradable thermoplastic starch. Polymers for Advanced Technologies 18(11), 910-915. Yovcheva, T., Marudova, M., Viraneva, A., Gencheva, E., Balabanov, N., Mekishev, G., 2013. Effect of gamma-irradiation on the electrical properties of poly (L-lactide). Journal of Applied Polymer Science 128(1), 139-144. Zaidi, L., Bruzaud, S., Kaci, M., Bourmaud, A., Gautier, N., Grohens, Y., 2013. The effects of gamma irradiation on the morphology and properties of polylactide/Closite 30B nanocomposites. Polymer Degradation and Stability 98(1), 348-355.

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Zhang, J., Tashiro, K., Tsuji, H., Domb, A.J., 2008. Disorder-to-order phase transition and multiple melting behavior of poly(L-lactide) investigated by simultaneous measurements of WAXD and DSC. Macromolecules 41(4), 1352-1357. Zhang ,Y, J.H., Han J.H., 2006. Plasticization of pea starch films with monosaccharides and polyols. Journal of Food Science 71(6), E253-E261. Figure 1. UV/vis spectra of PLA clamshell samples subjected to gamma-irradiation at different doses. Figure 2. FTIR spectra of PLA clamshell samples in ATR mode at different absorbed doses. Spectra in the wavenumber ranges of 4000-2700 cm-1 (a), and 2400-600 cm-1 (b). Figure 3. XRD patterns of PLA clamshell samples at different absorbed doses. The intensity has been plotted in log scale for clarity. Figure 4. Number and weight average molecular weight (Mn and Mw, respectively) of PLA clamshell samples as a function of dose (a), ratio of Mn,0 / Mn,t and polydispersity index as a function of dose (b), and Plots of (

) and (

) against dose (c).

Figure 5. Quantitative 1H NMR spectra of PLA control (a), PLA irradiated at 10 kGy (b), 60 kGy (c), 150 kGy (d), 300 kGy (e); and 600 kGy (f). Insert: Effect of the gamma irradiation on number molecular weight (Mn) of PLA (g). Figure 6. TGA thermograms (a), and corresponding derivatives (b) of PLA clamshell samples at different absorbed doses. Figure 7. Microphotographs of PLA clamshell samples exposed to absorbed doses of 0 kGy (a), 300 kGy (b), and 600 kGy (c). Inset: Photographs of corresponding samples.

Table 1. Transparency and color parameters of PLA clamshell samples subjected to different doses Opacity

L*

a*

b*

E

0

0.350 (0.01) a

96.85 (0.29) a

5.07 (0.02) a

-3.41 (0.10) a

0.84 (0.11) a

10

0.452 (0.02) b

96.91 (0.20) a

5.09 (0.05) a,b

-3.36 (0.13) a,b

0.80 (0.22) a

60

0.638 (0.03) c

96.78 (0.22 a

5.01 (0.03) b

-3.45 (0.06) a,c

0.90 (0.22) a

Doses (kGy)

20

150

0.760 (0.09) d

96.66 (0.20) a

5.03 (0.01) b

-3.63 (0.12) c

1.01 (0.21) a

300

0.825 (0.13) d

94.64 (0.29) b

4.95 (0.16) b,c

-3.17 (0.28) d

3.06 (0.83) b

600

0.452 (0.06) b

89.71 (0.88) c

4.20 (0.21) c

-1.00 (0.34) e

8.42 (0.43) c

Values between parentheses indicate standard deviation. Different letter in the same column indicate significant difference (p<0.05).

Table 2. Mechanical parameters of PLA clamshell samples subjected to gamma irradiation Parameter Doses

Tensile strength

Elongation at break

Tensile modulus

(kGy)

(MPa)

(%)

(MPa)

0

51.7 (4.34) a

240.6 (39.7) a

839.1 (24.04) a

10

43.2 (4.22) a,b

155.2 (29.9) b

828.4 (15.93) a,b

60

40.0 (3.08) b

25.7 (6.91) c

882.4 (23.10) b

Values between parentheses indicate standard deviation. Different letter in the same column indicated significant difference (p<0.05).

Table 3. X-ray diffraction parameters of PLA clamshell samples at different doses Dose

Peak position

FWHM

Crystallite size

Crystallinity

(kGy)

2 (°)

 (°)

L (Å)

XRD (%)

0

16.51

0.398

201

46.0

10

16.68

0.386

208

42.7

60

16.51

0.352

228

44.5

150

16.60

0.357

225

40.6

300

16.49

0.310

259

45.3

600

16.63

0.229

352

68.7

Table 4. Thermal parameters of PLA clamshell at different doses Dose

Tg

Tc

∆Hc

Tm

∆Hm

c

(kGy)

(°C)

(oC)

(J/g)

(oC)

(J/g)

(%)

21

0

58.19

119.28

22.7

148.60

36.38

39.38

10

62.65

90.87

25.21

151.62

34.39

36.74

60

60.36

91.84

29.52

152.28

34.97

37.36

150

56.16

82.82

30.97

147.04

30.98

33.11

300

51.56

80.59

31.58

140.52

36.27

38.75

600

51.65

-

-

92.42

50.35

53.80

Table 5. Results of TGA data of PLA clamshell irradiated at different doses Doses

Temperature of mass loss (°C)

Maximum temperature for

(kGy)

T5%

T25%

T50%

T75%

decomposition, Tmax (°C)

0

329

351

358

370

371

10

330

351

361

367

360

60

313

349

362

371

367

150

288

339

359

367

366

300

284

325

357

364

364

600

256

319

348

362

363

Doses (kGy): 0 10 60 150 300 600

Absorbance (a.u.)

0.6

0.3

0.0 300

400

500

600

Wavelength (nm)

700

800

22

Absorbance (a.u.)

Figure 1

Doses (kGy): 0 10 150 600

(a)

4000

3800

3600

3400

3200

3000

2800

-1

Absorbance (a.u.)

Wavenumber (cm )

Doses (kGy): 0 10 150 600

(b)

2400 2200 2000 1800 1600 1400 1200 1000

800

600

-1

Wavenumber (cm )

23

Figure 2

Intensity, a.u.

10000 Doses (kGy): 0 10 60 150 300 600

1000

100

10 0

10

20

30

40

50

60

2 , degree

24

Figure 3

Average molecular weight (g/mol)

90000 80000

Mn Mw

70000 60000 50000

(a)

40000 30000 20000 10000 0 0

100

200

300

400

500

600

Dose (kGy) 2.6

30 Polydispersity index Mn,0 / Mn,t

25

2.2

20

2.0

15

1.8

10

(b)

1.6

Mn,0 / Mn,t ratio

Polydispersity index

2.4

5

1.4

0 0

100

200

300

400

500

600

Dose (kGy) 4.0

5

-4

4

3.0 2.5

3 2.0 2

1.5

-5

[1/Mw,t - 1/Mw,0] x 10 (mol/g)

[1/Mn,t - 1/Mn,0] x 10 (mol/g)

3.5

1.0

(c)

1

0.5 0

0.0 0

100

200

300

Dose (kGy)

400

500

600

25

Figure 4

Figure 5 26

100

Doses (kGy): 0 10 60 150 300 600

90 80

Mass (%)

70 60 50

(a)

40 30 20 10 0 100

200

300

400

500

o

Temperature ( C)

Derivative mass/temperature

0.0 -0.5 -1.0

-2.5

Doses (kGy): 0 10 60 150 300 600

-3.0

(b)

-1.5 -2.0

-3.5 200

240

280

320

360

400

440

o

Temperature ( C)

27

(a)

(b)

(c)

Figure 7

Figure 6 28

HIGHLIGHTS 

The gamma irradiation effects on PLA clamshells were studied.



DSC, XRD, NMR and FTIR analysis were used for PLA clamshell characterization.



The Mw, Tm, tensile strength and elongation at break of the irradiated PLA clamshells decreased.



The tensile modulus increased with increasing gamma doses.



The Surface of PLA clamshell showed scratches and minor cracks.

29