Influences of the crystallisation rate on thermal and barrier properties of polylactide acid (PLA) food packaging films G. Colomines1, S. Domenek2, V. Ducruet2, A. Guinault1 1
CNAM, laboratoire des matériaux industriels polymères, 292 rue Saint-Martin, case courrier 322, 75141 Paris cedex 03, France URL: http://www.cnam.fr/depts/gemme/polymeres/Recherche.htm e-mail:
[email protected];
[email protected]
2
UMR sciences de l’aliment et de l’emballage (1211), AgroParisTech, Massy, France URL: http://www.agroparistech.fr e-mail:
[email protected];
[email protected]
ABSTRACT: Poly(lactic acid) (PLA) films have a great interest for the food packaging. But the interaction between the food and the packaging must be investigated. The gas and aroma barrier properties of PLA were analysed and the influence of PLA crystallinity on these properties was studied because of processing conditions. The crystallinity seems to have no effect on helium and oxygen barrier properties but ethyl acetate as aroma compound has a plasticizing effect on the PLA film. Key words: Polylactide (PLA), crystallinity, film extrusion processing, gas barrier properties, aroma sorption
1 INTRODUCTION The use of suitable packaging by the food industry has become a topic of great interest because of their potential to maintain quality along the shelf life of food products. Nevertheless during the storage, transfer of compounds coming from food such as lipids or aroma compounds can interact with the packaging causing modification and deterioration of their gas barrier properties [1]. Driven by environmental concerns, new polymers are coming to the market based on renewable resources. One of the most prominent polymers among these materials is poly(lactic acid) (PLA), which is approved for food contact. For the optimization of the fabrication of food packaging based PLA, the control of its thermal, gas and aroma barrier properties, which might change with the crystallinity, is important. The aim of this work is to evaluate the influence of the PLA crystallinity on the thermal and barrier properties (oxygen, helium, ethyl acetate). In order to work on the properties of PLA in contact with food stuff, we investigated its thermal and barrier properties before and after sorption of ethyl acetate, an aroma compound chosen as a model. PLA films were obtained by flat die extrusion. The
crystallisation rate of the films was modified by compression-molding to obtain samples with specific crystallinity. The data were compared to an industrial sample (PLA Biophan). 2 MATERIALS AND METHODS 2.1 Materials Two PLA materials were studied: PLA Biophan film and PLA Biomer L9000. PLA Biophan 121 (Treofan) is provided directly in film form, it is composed of 3 layers: A core layer of white PLA and two external layers of sealable PLA. Its thickness is 30 micrometers and its OTR (oxygen transmission rate) is equal to 675 cm3/m2.day.bar at 23°C and 50% RH according to the Treofan datasheet. PLA Biomer L9000, being 100% in L conformation, was provided in pellets form. Ethyl acetate (EA) was provided by Sigma (purity 99.5 %). 2.2 Preparation of PLA films Extruded PLA Extruded PLA Biomer film was prepared by extrusion on a 30 mm diameter extruder, with a 33 L/D (Length on Diameter) barrel. 100 mm width flat die and chill roll equipment were used to
manufacture an 80 mm width film with a thickness of about 120 µm. The extruder temperature profile was defined at 210°C, in order to avoid unmelted particles and degradation. The roll temperature and roll speed were respectively fixed at 25°C and about 10m/min. Amorphous PLA In order to obtain an amorphous PLA film, the previous extruded film has been compressionmolded at 200°C and 3.4 Bar for five minutes then quenched in water at room temperature. Recrystallised PLA Because PLA can recrystallise during heating, the amorphous films were maintained at different temperatures and durations under compression. Then the samples were quenched in water to obtain various crystallinity rates. 2.3 Analysis Methods 2.3.a Differential scanning calorimetry (DSC) Two equipments were used: - a Pyris 1 (Perkin Elmer) to study the crystallinity of the samples (melting enthalpy of a 100% crystalline PLA = 93 J/g, [2]). Tests are performed at 10°C/min from 0 to 200°C. - a modulated DSC QSC 100 (TA Instruments) to study the glass transition. The heating scan was performed under sinusoidal temperature modulation with a heating rate of 2.5°C, a period of 40 s and a modulation of ± 0.265°C between 10 and 190°C. Measurements are duplicated. Each apparatus was equipped with an intracooler. Tests were performed under nitrogen atmosphere in hermetic aluminium pans. 2.3.b Oxygen and Helium permeability The direct measurement of the oxygen transmission rate was monitored at 23°C and 0% RH with a Systech 8001 apparatus. The helium transmission rate was measured at 23°C and 0% RH, by a specific analyser developed by Cnam, based also on the ISO 15105-2:2003. Measurements were duplicated. 2.3.c Sorption isotherm method The sorption isotherm of ethyl acetate (EA) was measured at 25°C and 0% RH, using an electronic microbalance, IGA-002, (Hiden Analytical, Warrington (UK) with a sensitivity of 0.2 µg; sample weight: 5-10 mg). Measurements were triplicated for each ethyl acetate activity.
3 RESULTS AND DISCUSSION 3.1 Effect of crystallinity on barrier properties 3.1.a Crystallinity measurement DSC study has shown that optimum crystallisation rate was obtained with the following recrystallisation conditions, 25 minutes at 92°C. Under these conditions, the film was not also able to recrystallise during the first heating in DSC. This film is named recrystallised PLA. Crystallinity measurements were performed on the amorphous PLA, Biophan film, extruded film and recrystallised film. The results are given in table 1. Table 1: Crystallinity characteristics of the PLA samples Film Amorphous PLA Extruded PLA PLA Biophan Recrystallised PLA
Crystallisation temperature (°C) 97.8 100.5 -
Crystallisation enthalpy (J/g) 31.9 33.4 -
Melting temperature (°C) 166.4 164.2 138.1 167.9
Melting enthalpy (J/g) 37.7 35.5 17.8 36.7
Crystallinity degree (%) 6 2 19 39
Surprisingly, the extruded film shows a low crystallinity degree as the amorphous film. This could indicate a too fast cooling stage for the extruded film. The Biophan film presents a medium crystallinity compared to the recrystallised film one, but its value is an average due of the multilayer structure. 3.1.b Gas barrier properties Helium and oxygen transmission values are given in the table 2. Results are compared with measurements performed on conventional packaging materials, low density Polyethylene (PE), polyethylene terephthalate (PET) and polystyrene (PS). Table 2: Gas barrier properties of the different PLA films and classical packaging materials Helium Oxygen transmission rate transmission rate cm3.cm/m2.jour.bar cm3.cm/m2.jour.bar Amorphous PLA 89 2.2 PLA Biophan 54 1.9 Extruded PLA 80 n.d* Recrystallised PLA 92 2.4 Low density Polyethylene 53 23.0 Polyethylene terephthalate 9 0.2 Polystyrene 180 17.0** * : Width of the sample was not sufficient to measure oxygen transmission rate **: bibliographic data [3] Material
Helium barrier properties of the four PLA films are of the same order, so that we can think that the oxygen transmission rates are also the same. Crystallinity degree does not seem to have an effect on the gas barrier properties. Compared to the
conventional packaging polymers, PLA is intermediate between the PET and the PS, which are respectively classified in medium and low barrier polymer family. 3.1.c Aroma sorption The kinetics of sorption allow us to calculate the solubility (Table 3) and diffusion (Table 4) coefficients of EA in PLA films in relation to the activity (p/p0) of ethyl acetate. SD(%) is the standard deviation of each measurement. Three activities are tested, 0.2, 0.5 and 0.9. Table 3: Solubility coefficients (S) of ethyl acetate in the PLA samples at different activities Activity Film Amorphous PLA Extruded PLA PLA Biophan
0.2 S×103 (kg.m3.Pa-1) 0.14 0.09 0.35
SD (%) 24.8 37.5 9.2
0.5 S×103 (kg.m3.Pa-1) 7.0 11.3 8.6
SD (%) 16.3 27.5 6
0.9 S×103 (kg.m3.Pa-1) 15.2 13.8 18.9
SD (%) 3.9 n.d. 3
Table 4: Diffusion coefficients (D) of ethyl acetate into the PLA samples at different activities Activity Film Amorphous PLA Extruded PLA PLA Biophan
0.2 D×1014 2 -1 (m .s ) 6.76 8.86 0.51
SD (%) 11.0 34.5 47.2
0.5 D×1014 2 -1 (m .s ) 8.42 6.54 2.13
SD (%) 5.0 18.9 36.7
0.9 D×1014 2 -1 (m .s ) 150.0 66.9 5.0
SD (%) 5.7 n.d. 28.9
The sorption isotherms of EA for the three PLA films show a typical shape (not shown). Below 0.2 aroma activity the sorption increase is low and is quite linear. Above 0.2, the sorption of the aroma compounds increases with increasing the activity of EA. The solubility coefficients of the three PLAs increase dramatically with an increase of the activity of EA from 0.2 up to 0.5. The main increase is obtained for the extruded PLA. At 0.5 activity, its solubility coefficient is 125-fold those measured at 0.2 This behaviour which was frequently described for semi-crystalline polymers such as PET and polyolefins reveals a plasticizing effect of the aroma compound onto the PLA films and could be explained by the Flory equation or the ENSIC model [4]. For the three PLAs, the diffusion coefficient increases slowly between 0.2 and 0.5 activity then increases sharply at 0.9 activity for Amorphous PLA and Extruded PLA. Diffusion coefficients seem to be linked to the degree of crystallinity of the PLA films regardless the activity of ethyl acetate. But this result may be validated with recrystallised PLA film because of the different nature of Biophan film.
The measurement of the glass transition temperature (Tg) of the PLA samples was carried out before and after contact with ethyl acetate (EA) at 0.5 activity for 3 days in an hermetic vessel. Tg has been measured during the second heating because of the impossibility to observe the Tg during the first heating. The results are given in table 5. Table 5: Influence of the aroma sorption on the film Tg Tg before EA Tg after EA Film sorption (°C) sorption (°C) Amorphous PLA 58.0 (+/-1) 38.5 (+/-1) Extruded PLA 59.2 (+/-0.5) 38.5 (+/-0.5) PLA Biophan 54.7 (+/-1) 39.7 (+/-1)
A net decrease of the Tg is observed due to the ethyl acetate sorption. So, we can conclude that the aroma compound has a plasticizing effect on PLA. It will be interesting to further characterize this effect directly after the aroma sorption by DMTA for example. The influence of the crystallinity of the sample on the plasticizing effect is in progress. 4 CONCLUSION To investigate the influence of PLA crystallinity on its barrier properties, we have manufactured PLA film with different crystallinity degrees by extrusion and by compression-molding. The results were systematically compared with the commercial PLA Biophan film. The degree of crystallinity seems to have no effect on the helium and oxygen barrier properties but the aroma barrier properties remain under investigation. However, Tg diminishes after the aroma sorption. Therefore, ethyl acetate has a plasticizing effect on the PLA regardless the degree of crystallinity. REFERENCES 1.
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