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Sorption of organic solvents by packaging materials: polyethylene terephthalate and TOPAS®
Polymer Testing 24 (2005) 395–402 www.elsevier.com/locate/polytest
Material Behaviour
Sorption of organic solvents by packaging materials: polyethylene terephthalate and TOPASw M. Limam, L. Tighzert*, F. Fricoteaux, G. Bureau Centre d’Etudes et de Recherche en Mate´riaux et Emballage (CERME), ESIEC, Esplanade Roland Garros, BP 1029, 51686 Reims Cedex 2, France Received 24 July 2004; accepted 1 September 2004
Abstract The sorption of organic solvents (1-butanol and 1-octanol) by polyethylene terephthalate and TOPASw (cyclic olefin copolymers or COC) was investigated at 3, 23 and 42 8C and was studied by coupling two techniques (in an off-line way): supercritical fluid extraction to GC/MS. Supercritical fluid extraction (SFE) is widely perceived as a technique for the extraction of low to moderately polar compounds. The results obtained enable us to determine the corresponding diffusion coefficients and to show the relation existing between the diffusion coefficient and the following factors: the temperature, the size of diffusing molecules and the crystallinity degree of packaging materials. Therefore, the diffusions of 1-butanol and 1-octanol are less important in PET than in TOPASw (completely amorphous). q 2004 Published by Elsevier Ltd. Keywords: Polyethylene terephthalate; COC; Coefficient of diffusion; GC/MS; Supercritical fluid extraction
1. Introduction Thanks to their outstanding properties, the use of plastics as packaging materials has grown exponentially in the last few decades. Polyethylene terephthalate (PET) is widely used in containers for food, beverages, pharmaceutical products and even cosmetic products [1,2]. TOPASw, a new cyclic olefin copolymer, is primarily used in blisters or in optical applications [3]. The diffusion of molecules through polymer membranes is an important phenomenon in many different areas of science and engineering. For example, the diffusivity in polymer films and membranes [4,5] is important in the use of polymers in packaging applications. In fact, when a plastic packaging material is in contact with any product mass transfer occurs: migration of additives, necessary for * Corresponding author. Tel./fax: 33 3 26 91 37 64. E-mail address: [email protected] (L. Tighzert). 0142-9418/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.polymertesting.2004.09.004
the stability and the processing of the packaging, into the packed product could occur and, conversely, the product components (small molecules, aromas, flavour,.) could be sorbed by the polymer [6,7]. In literature [8–14], the transport properties of small molecules in polymers (sorption of solvent) depends on several factors: 1. the chemical structure of the packed product: acid, amide, ester, etc. 2. characteristics of the polymer (the container): molecular weight, degree of crystallinity, glass transition temperature,. 3. the temperature of the environment 4. the size, the shape and the polarity of diffusing molecules [12]. These factors essentially control the solubility and the degree of swelling of the diffusing molecules and influence
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the rate at which these molecules are sorbed and transported into the polymer [15]. Several studies [15–18] were carried out with the aim of determining the sorption of various solvents in the PET by employing different analytical techniques such as: IRTF [8,15–16], extraction liquid–solid/GC/FID [17], Soxhlet extraction coupled to GC/MS [2,18]. Some works on the diffusion of organic solvents into PET have been published. For example, the sorption of methanol in polyethylene terephthalate [19–21] showed that this phenomenon is accompanied by swelling of the polymer (change of Tg and the degree of crystallinity). The sorption of a series of alcohols (methanol, ethanol, propanol, butanol and isopropanol) in amorphous PET showed that the sorption rate decreased with increasing size of alcohol molecule. The sorption of organic solvents into PET dramatically depends on the chemical microstructure. That means that the sorption varies according to whether if it is amorphous or oriented [13]. Some authors [15,22,23] compared the sorption of water to the sorption of methanol into several samples of PET (with different degrees of crystallinity). Thanks to the technique of FTIR-ATR, Sammon et al. [15] determined that the diffusion coefficient varied according to the crystallinity of polymer: it decreased with increase in crystallinity. Moreover, the sorption of methanol into PET was accompanied by significant swelling and crystallisation. The diffusion rate of methanol was faster than that of water due to the swelling. It was shown that the diffusion of small molecules follows Fick’ law:
Fig. 2. Synthesis of TOPASw.
to undertake this study was headspace gas chromatography with detection FID (HSGC). Escobal et al. noticed that highly crystalline polymers showed less absorption of organic solvents than polymers with important amorphous zones [24,25]. 1.1. Cyclic olefin copolymers
where D0 is a constant, R the perfect gas constant in J molK1 KK1, T the temperature in K and Ea the activation energy in J molK1. The sign (K) indicates that flow goes from high to low concentration of diffusion molecules. Escobal et al. [14] studied the sorption of two solvents: ethanol and ethyl acetate into various packaging materials. They have evaluated the absorption of solvents by the most used polymeric materials and have determined the influence of several parameters on the absorption of these solvents with particular attention to polymer crystallinity, thickness of the film and the immersion time. The technique employed
Cyclic olefin copolymers (COC) are engineering thermoplastics with a combination of properties [26]. They form a new and interesting group of materials suitable for high performance optical, medical, electrical, packaging and other applications. One of these products is TOPASw (Fig. 1) commonly used in flexible films. The cyclic olefin copolymers are obtained by the copolymerization of an olefin with a cyclic olefin in the presence of a metallocene catalyst. In the case of TOPASw, 2-norbornene is the cyclic olefin used (Fig. 2). The 2-norbornene reacts with ethylene in the presence of a metallocene catalyst [3,27–29] to form TOPASw, an amorphous and transparent compound. Several types of COCs are obtained by the copolymerization of an olefin with a cyclic olefin in the presence of different other metallocene catalysts (Fig. 3). All cyclic olefin copolymers (COCs) are completely amorphous. This morphology is due to the steric hindrance created by the incorporation of norbornene in their chains. In fact, norbornene is much bulkier than ethylene and has rigid bridged-ring structure that prevents crystallization [26]. Thus, the COCs are transparent and this has a direct consequence on their optical properties, which are excellent. They also present other properties [27–32] such as good stability, low density, good resistance to acids and bases, moderate permeability to gases, low resistance to nonpolar solvents (heptane, toluene,.) and to halogenous solvents (methylene chloride, for example), excellent transparency, strong water barrier (four times higher than in the case of
Fig. 1. The chemical structure of TOPASw.
Fig. 3. Different chemical structures of COCs (E, ethylene; N, norbornene).
J Z KD grad C Z KD dC=dx where C is the concentration of the diffusing species, grad C is the gradient of the concentration and D the diffusion coefficient in m2 sK1 strongly depends on the temperature: D Z D0 expðKEa =RTÞ
M. Limam et al. / Polymer Testing 24 (2005) 395–402 Table 1 Characteristics of the COC Parameters
397
5 8C minK1, then cooled to 25 8C at 10 8C.minK1 and heated to 270 8C at 5 8C minK1. Value
Density (g cm ) Water absorption (%) Water vapor permeability (g mK2 jK1) Elongation at break (%) Hardness (shore D) Galss transition temperature (8C) Resistivity (U mK1) at 23 8C Thermal conductivity (W mK1 KK1) at 20 8C Permittivity (F/m) K3
1.02 !0.01 0.02–0.04 3–10 89 70–180 O1016 0.16 2.35
PET), good resistance to polar solvents (ketones, methanol, ethanol,.). Some other properties of the COCs are given in Table 1. The COCs can be used in two ways in packaging films. First, an individual layer of COC used in a multilayer structure. In this case, the moisture barrier will be the highest and the COC layer will contribute significantly to film stiffness [26]. In the case of very thick COC layers, the films can be thermoformed for pharmaceutical blister packs. Second, a more widely applicable way to use COCs in packaging would be as a blend in polyolefins to accomplish specific modifications of the film. The aim of our study is to compare the behavior of the PET of various degrees of crystallinity, in contact with organic solvents, with that of an amorphous compound like TOPASw. The diffusion coefficient of both materials is calculated.
2. Experimental 2.1. Materials Two different samples of polyethylene terephthalate were kindly provided by PET POWER (amber PET and transparent PET) and by ALCAN PACKAGING (PET 22391-013) and a sample of COC was kindly provided by TICONA (TOPASw 8007). This grade of COC is used in the majority of the applications in films, bottles and moulded articles. The solvents employed were 1-octanol (ACROS ORGANICS 98%), dichloromethane (RP NORMAPURTM AR 99.5%), ethanol (RP NORMAPURTM AR 99.5%), 1-butanol (RP NORMAPURTM AR 99.5%). 2.2. Characterization of the various samples by DSC A TA Instrument differential scanning calorimeter (2920 modulate DSC) interfaced to a PC and operating under a nitrogen flow of 60 ml minK1 was used to determine the thermal characteristics of PET and TOPASw 8007. Samples were cut from sheets using a weight between 10 and 16 mg. An empty aluminium pan was used as reference. The samples were heated from 25 to 270 8C at a rate of
2.3. Sorption of solvents into packaging materials: immersion study Each sample of PET and TOPASw 8007 was cut into 2.7 cm!4 cm pieces. Each sheet was weighed and put in a glass cell filled with solvent. Three immersion temperatures were selected: 3, 23 and 42 8C and the cells were shielded from light. The immersion times varied from 1 h to 60 days. Prior to the analysis of amounts of absorbed solvent, samples were wiped, weighed and cut into very small pieces, and finally placed in the supercritical fluid extraction cell. 2.4. Supercritical fluid extraction (SFE) Recent studies [33–36], have shown that, first, the use of supercritical fluids as extraction media provides a powerful alternative to traditional extraction methods. In addition, they showed that CO2 at high pressure acts like common organic solvents in its ability to swell and plasticize polymers. The extraction was carried out with a Roucaire SFX 2-10 apparatus equipped with a syringe pump Model 260D. The extraction cell, made from stainless-steel and containing PET or TOPASw 8007 pellets, was placed in the extraction compartment and was filled with CO2. The temperature was maintained at 45 8C and the CO2 underwent compression to 150 bar. The extraction was considered finished after 75 min. During the experiment, the extract was recovered in 20 ml of dichloromethane. At the end of the extraction, a large proportion of dichloromethane was lost by evaporation. Hence, the obtained extract was put into a flask and recovery completed with 10 ml of dichloromethane in the presence of an internal standard (nonane). Finally, 1 ml of this solution was injected in the gas chromatograph equipped with a mass spectrometer detector. 2.5. Analysis of the extract by GC/MS The chromatographic analysis was carried out using a Varian STAR 3400CX gas chromatograph equipped with an automatic injection system (Varian 8200CX) and a mass spectrometer detector (Varian SATURN 3). A capillary column (Equity1, 30 m!0.53 mm i.d., 3.0 mm; Sigma-Aldrich) was used for separation with the following oven temperature program: 40 8C for 1 min, increased to 200 8C at 7 8C minK1 and further increased to 220 8C at 20 8C minK1. The rate of the carrier gas Helium is 1 ml minK1. The mass spectrometer was used in the following conditions: detector at 140 8C; injector at 100 8C, transfer line at 230 8C and ionic trap at 140 8C.
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Fig. 4. DSC analysis of amber PET.
Fig. 6. DSC analysis of PET 22391-013.
the crystallization temperature measured at each step of the DSC temperature program. The degree of crystallinity is defined as [37,38]:
3. Results 3.1. Characterization of PET and TOPASw 8007 3.1.1. Polyethylene terephthalate The DSC thermograms of the PET samples (Figs. 4–6) show that they have different thermal behaviour, thus different morphologies and/or structure and composition. During the first heating, both amber PET and transparent PET (Figs. 4–5) did not show the crystallization peak about 130 8C, but this exotherm peak is clearly seen in the case of PET 22391-013 (Fig. 6). Moreover, on cooling from the melt, the transparent PET shows a crystallization peak at about 145 8C, while the two other PETs crystallize at much higher temperature, about 200 8C. This result means that the transparent PET crystallizes with more difficulty than both other PETs. In fact, during the second heating the residual crystallization is seen at about 140 8C (Fig. 5). This exotherm is absent in both amber PET and PET 22391013. All results are reported in Table 2 where we note Tg the glass transition temperature, DHc the enthalpy of crystallization, Tf the melting temperature, DHf the enthalpy of fusion measured at the melting temperature (Tf) and Tc
Xc Z DHf =DHf0 where Xc is the crystalline fraction, DHf is the enthalpy of fusion measured at the melting temperature (Tf) and DHf0 is the enthalpy of fusion of the totally crystalline sample (DHf0 Z 117:6 J gK1). According to Table 2, the amber PET and PET 22391013 are the most and the least crystalline, respectively. The crystalline ratio is calculated by taking account of the fusion enthalpy; but these results would be verified and confirmed by using MDSCw, which permits to distinguish the nonreversing signal (crystallization) from the reversing signal (fusion). In fact, Thomas [39] has found that all of the melting that occured during the MDSCw experiment was the result of crystallization that occured during the experiment. In other words, his results showed that the crystallization starts at about 100 8C and continues until the sample is fully Table 2 Thermodynamic characteristics of the PET studied samples
First heating Tg (8C) DHc (J gK1) Tf (8C) DHf (J gK1) Cooling Tc (8C) DHc (J gK1) Second heating Tg (8C) DHc (J gK1) Tf (8C) DHf (J gK1) Degree of crystallinity (%) Fig. 5. DSC analysis of transparent PET.
Amber PET
Transparent PET
PET 22391-013
68 7 253 52
69 – 244 47
76 24 253 44
189 44
151 13
186 47
79 – 253 40 44
79 10 244 40 40
78 – 254 41 37
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Table 4 Diffusion coefficients of the 1-butanol in PET samples PET samples
Temperature (8C) 3
22391-013 (XcZ37%) ( cm2 sK1) Transparent PET (XcZ40%) ( cm2 sK1) Amber PET (XcZ44%) ( cm2 sK1)
23
42
7.57!10
K8
1.67!10
4.97!10K8
5.74!10K9
7.69!10K9
1.52!10K8
3.70!10K9
6.62!10K9
1.01!10K8
K9
Fig. 7. DSC analysis of TOPASw 8007.
melted at 275 8C. This crystallization exotherm is masked by the melting peak. The studied PET may or may not present this phenomenon. Hence, we use the obtained crystallinity ratios (Table 2) to relate to the diffusion constant calculated later. 3.1.2. Cyclic olefin copolymers: TOPASw 8007 The sample of TOPASw 8007 studied underwent the same temperature program as the sample of PET. The thermogram obtained (Fig. 7) shows the presence of glass transition temperatures and the lack of fusion peaks. These results confirm the amorphous nature, the main characteristic, of the COC as mentioned in literature [27–29]. 3.2. Determination of the diffusion coefficients The data obtained after the GC/MS analysis were treated by software called Experimental Diffusion FIT (EXDIF). This software was developed by the Swiss Federal Office of Public Health [40]. The mathematical model used is represented by the following relation [11]: " # N Mt 8 X 1 ð2n C 1Þ2 p2 Z1K 2 exp K Dt MN p nZ0 ð2n C 1Þ2 L2
where Mt and MN are the quantities of the diffusing substance, at time t and at the equilibrium, respectively, D is the diffusion coefficient and L the thickness of polymer. 3.2.1. Polyethylene terephthalate The different values of the diffusion coefficients are reported in Tables 3 and 4 for the several systems of three PET samples in 1-octanol and 1-butanol. All of these data are represented graphically to demonstrate the relation between the diffusion coefficients and parameters like temperature and crystallinity of the polymer [8–14]. Figs. 8 and 9 show that: 1. the diffusion coefficient decreases when the degree of crystallinity increases 2. the diffusion coefficient decreases when the size of the sorbed molecule increases 3. for each solvent (1-octanol or 1-butanol), when the temperature increases the diffusion coefficient increases also. Similar results were also given in literature [12,15, 22–25]. These conclusions confirm those obtained by a FTIR-ATR study [15] about the diffusion in two systems
Table 3 Diffusion coefficients of the 1-octanol in PET samples PET samples
Temperature (8C) 3
23
42
22391-013 (XcZ37%) ( cm2 sK1) Transparent PET (XcZ40%) ( cm2 sK1) Amber PET (XcZ44%) ( cm2 sK1)
5.67!10K9
9.25!10K9
1.31!10K8
2.86!10K9
5.90!10K9
9.90!10K9
1.60!10K9
4.16!10K9
8.41!10K9 Fig. 8. Diffusion coefficients of PET versus crystallinity degree at different temperatures.
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Fig. 9. Diffusion coefficients versus temperature of PET with various degrees of crystallinity.
(H2O/PET and CH3OH/PET): the observed phenomenon depends on the polarity of the liquid and the crystallinity of the polymer [14]. In addition, the diffusion of the solvents is strongly favored by the presence of amorphous zones in polymers [24].
Fig. 10. Sorption profile M* (t)Zf(t) of the 1-octanol in the TOPASw 8007 at TZ42 8C.
3.2.2. Cyclic olefin copolymers: TOPASw 8007 Table 5 summarizes the values of the diffusion coefficients determined in the two systems studied: (TOPASw 8007/1-butanol) and (TOPASw 8007/1-octanol) at 23 and 42 8C. Figs. 10–13 show the sorption profiles of the TOPASw. We observe that the phenomenon of sorption depends on several parameters (Figs. 10–13): 1 the crystallinity and the interactions between the simulant and the polymer structure 2 the operating conditions such as: time of immersion, temperature, agitation,. According to Table 5, the diffusion coefficients of each solvent in the COC are much higher than those obtained in the case of the various PET samples. This result derives from the morphology difference between PET and COC, which are semi-crystalline and amorphous, respectively. This confirms well the relationship existing between the degree of crystallinity and the diffusion coefficient [15,22–25]. Thus, polymers are considered as being a barrier against solvent diffusion are those with a high ratio of crystallinity.
Fig. 11. Sorption profile M* (t)Zf(t) of the 1-octanol in the TOPASw 8007 at TZ23 8C.
Table 5 Diffusion coefficients of 1-butanol and 1-octanol in the COC at various temperatures Solvents
Temperature (8C) 23
42
1-Butanol ( cm2 sK1) 1-Octanol ( cm2 sK1)
9.06!10K7 3.58!10K7
1.04!10K6 7.83!10K7
Fig. 12. Sorption profile M* (t)Zf(t) of 1-butanol in the TOPASw 8007 at TZ42 8C.
M. Limam et al. / Polymer Testing 24 (2005) 395–402
Fig. 13. Sorption profile M* (t)Zf(t) of 1-butanol in the TOPASw 8007 at TZ23 8C.
4. Conclusion The combination of supercritical fluid extraction and gas chromatography–mass spectrometry is an effective way for the determination organic solvents sorbed by the polymeric materials used as pharmaceutical packaging. Highly crystalline polymers show lower sorption of organic solvents than amorphous polymers. Among the parameters which affect the process of retention of these solvents, the interaction solvent–polymer structure stands out as the most important, but it is necessary to consider the influence of other external parameters such as the immersion time, the temperature, the film thickness,. In conclusion, the PET is confirmed to be a good barrier to alcohols such as butanol and octanol in comparison to COC.
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Material Behaviour
Sorption of organic solvents by packaging materials: polyethylene terephthalate and TOPASw M. Limam, L. Tighzert*, F. Fricoteaux, G. Bureau Centre d’Etudes et de Recherche en Mate´riaux et Emballage (CERME), ESIEC, Esplanade Roland Garros, BP 1029, 51686 Reims Cedex 2, France Received 24 July 2004; accepted 1 September 2004
Abstract The sorption of organic solvents (1-butanol and 1-octanol) by polyethylene terephthalate and TOPASw (cyclic olefin copolymers or COC) was investigated at 3, 23 and 42 8C and was studied by coupling two techniques (in an off-line way): supercritical fluid extraction to GC/MS. Supercritical fluid extraction (SFE) is widely perceived as a technique for the extraction of low to moderately polar compounds. The results obtained enable us to determine the corresponding diffusion coefficients and to show the relation existing between the diffusion coefficient and the following factors: the temperature, the size of diffusing molecules and the crystallinity degree of packaging materials. Therefore, the diffusions of 1-butanol and 1-octanol are less important in PET than in TOPASw (completely amorphous). q 2004 Published by Elsevier Ltd. Keywords: Polyethylene terephthalate; COC; Coefficient of diffusion; GC/MS; Supercritical fluid extraction
1. Introduction Thanks to their outstanding properties, the use of plastics as packaging materials has grown exponentially in the last few decades. Polyethylene terephthalate (PET) is widely used in containers for food, beverages, pharmaceutical products and even cosmetic products [1,2]. TOPASw, a new cyclic olefin copolymer, is primarily used in blisters or in optical applications [3]. The diffusion of molecules through polymer membranes is an important phenomenon in many different areas of science and engineering. For example, the diffusivity in polymer films and membranes [4,5] is important in the use of polymers in packaging applications. In fact, when a plastic packaging material is in contact with any product mass transfer occurs: migration of additives, necessary for * Corresponding author. Tel./fax: 33 3 26 91 37 64. E-mail address: [email protected] (L. Tighzert). 0142-9418/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.polymertesting.2004.09.004
the stability and the processing of the packaging, into the packed product could occur and, conversely, the product components (small molecules, aromas, flavour,.) could be sorbed by the polymer [6,7]. In literature [8–14], the transport properties of small molecules in polymers (sorption of solvent) depends on several factors: 1. the chemical structure of the packed product: acid, amide, ester, etc. 2. characteristics of the polymer (the container): molecular weight, degree of crystallinity, glass transition temperature,. 3. the temperature of the environment 4. the size, the shape and the polarity of diffusing molecules [12]. These factors essentially control the solubility and the degree of swelling of the diffusing molecules and influence
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the rate at which these molecules are sorbed and transported into the polymer [15]. Several studies [15–18] were carried out with the aim of determining the sorption of various solvents in the PET by employing different analytical techniques such as: IRTF [8,15–16], extraction liquid–solid/GC/FID [17], Soxhlet extraction coupled to GC/MS [2,18]. Some works on the diffusion of organic solvents into PET have been published. For example, the sorption of methanol in polyethylene terephthalate [19–21] showed that this phenomenon is accompanied by swelling of the polymer (change of Tg and the degree of crystallinity). The sorption of a series of alcohols (methanol, ethanol, propanol, butanol and isopropanol) in amorphous PET showed that the sorption rate decreased with increasing size of alcohol molecule. The sorption of organic solvents into PET dramatically depends on the chemical microstructure. That means that the sorption varies according to whether if it is amorphous or oriented [13]. Some authors [15,22,23] compared the sorption of water to the sorption of methanol into several samples of PET (with different degrees of crystallinity). Thanks to the technique of FTIR-ATR, Sammon et al. [15] determined that the diffusion coefficient varied according to the crystallinity of polymer: it decreased with increase in crystallinity. Moreover, the sorption of methanol into PET was accompanied by significant swelling and crystallisation. The diffusion rate of methanol was faster than that of water due to the swelling. It was shown that the diffusion of small molecules follows Fick’ law:
Fig. 2. Synthesis of TOPASw.
to undertake this study was headspace gas chromatography with detection FID (HSGC). Escobal et al. noticed that highly crystalline polymers showed less absorption of organic solvents than polymers with important amorphous zones [24,25]. 1.1. Cyclic olefin copolymers
where D0 is a constant, R the perfect gas constant in J molK1 KK1, T the temperature in K and Ea the activation energy in J molK1. The sign (K) indicates that flow goes from high to low concentration of diffusion molecules. Escobal et al. [14] studied the sorption of two solvents: ethanol and ethyl acetate into various packaging materials. They have evaluated the absorption of solvents by the most used polymeric materials and have determined the influence of several parameters on the absorption of these solvents with particular attention to polymer crystallinity, thickness of the film and the immersion time. The technique employed
Cyclic olefin copolymers (COC) are engineering thermoplastics with a combination of properties [26]. They form a new and interesting group of materials suitable for high performance optical, medical, electrical, packaging and other applications. One of these products is TOPASw (Fig. 1) commonly used in flexible films. The cyclic olefin copolymers are obtained by the copolymerization of an olefin with a cyclic olefin in the presence of a metallocene catalyst. In the case of TOPASw, 2-norbornene is the cyclic olefin used (Fig. 2). The 2-norbornene reacts with ethylene in the presence of a metallocene catalyst [3,27–29] to form TOPASw, an amorphous and transparent compound. Several types of COCs are obtained by the copolymerization of an olefin with a cyclic olefin in the presence of different other metallocene catalysts (Fig. 3). All cyclic olefin copolymers (COCs) are completely amorphous. This morphology is due to the steric hindrance created by the incorporation of norbornene in their chains. In fact, norbornene is much bulkier than ethylene and has rigid bridged-ring structure that prevents crystallization [26]. Thus, the COCs are transparent and this has a direct consequence on their optical properties, which are excellent. They also present other properties [27–32] such as good stability, low density, good resistance to acids and bases, moderate permeability to gases, low resistance to nonpolar solvents (heptane, toluene,.) and to halogenous solvents (methylene chloride, for example), excellent transparency, strong water barrier (four times higher than in the case of
Fig. 1. The chemical structure of TOPASw.
Fig. 3. Different chemical structures of COCs (E, ethylene; N, norbornene).
J Z KD grad C Z KD dC=dx where C is the concentration of the diffusing species, grad C is the gradient of the concentration and D the diffusion coefficient in m2 sK1 strongly depends on the temperature: D Z D0 expðKEa =RTÞ
M. Limam et al. / Polymer Testing 24 (2005) 395–402 Table 1 Characteristics of the COC Parameters
397
5 8C minK1, then cooled to 25 8C at 10 8C.minK1 and heated to 270 8C at 5 8C minK1. Value
Density (g cm ) Water absorption (%) Water vapor permeability (g mK2 jK1) Elongation at break (%) Hardness (shore D) Galss transition temperature (8C) Resistivity (U mK1) at 23 8C Thermal conductivity (W mK1 KK1) at 20 8C Permittivity (F/m) K3
1.02 !0.01 0.02–0.04 3–10 89 70–180 O1016 0.16 2.35
PET), good resistance to polar solvents (ketones, methanol, ethanol,.). Some other properties of the COCs are given in Table 1. The COCs can be used in two ways in packaging films. First, an individual layer of COC used in a multilayer structure. In this case, the moisture barrier will be the highest and the COC layer will contribute significantly to film stiffness [26]. In the case of very thick COC layers, the films can be thermoformed for pharmaceutical blister packs. Second, a more widely applicable way to use COCs in packaging would be as a blend in polyolefins to accomplish specific modifications of the film. The aim of our study is to compare the behavior of the PET of various degrees of crystallinity, in contact with organic solvents, with that of an amorphous compound like TOPASw. The diffusion coefficient of both materials is calculated.
2. Experimental 2.1. Materials Two different samples of polyethylene terephthalate were kindly provided by PET POWER (amber PET and transparent PET) and by ALCAN PACKAGING (PET 22391-013) and a sample of COC was kindly provided by TICONA (TOPASw 8007). This grade of COC is used in the majority of the applications in films, bottles and moulded articles. The solvents employed were 1-octanol (ACROS ORGANICS 98%), dichloromethane (RP NORMAPURTM AR 99.5%), ethanol (RP NORMAPURTM AR 99.5%), 1-butanol (RP NORMAPURTM AR 99.5%). 2.2. Characterization of the various samples by DSC A TA Instrument differential scanning calorimeter (2920 modulate DSC) interfaced to a PC and operating under a nitrogen flow of 60 ml minK1 was used to determine the thermal characteristics of PET and TOPASw 8007. Samples were cut from sheets using a weight between 10 and 16 mg. An empty aluminium pan was used as reference. The samples were heated from 25 to 270 8C at a rate of
2.3. Sorption of solvents into packaging materials: immersion study Each sample of PET and TOPASw 8007 was cut into 2.7 cm!4 cm pieces. Each sheet was weighed and put in a glass cell filled with solvent. Three immersion temperatures were selected: 3, 23 and 42 8C and the cells were shielded from light. The immersion times varied from 1 h to 60 days. Prior to the analysis of amounts of absorbed solvent, samples were wiped, weighed and cut into very small pieces, and finally placed in the supercritical fluid extraction cell. 2.4. Supercritical fluid extraction (SFE) Recent studies [33–36], have shown that, first, the use of supercritical fluids as extraction media provides a powerful alternative to traditional extraction methods. In addition, they showed that CO2 at high pressure acts like common organic solvents in its ability to swell and plasticize polymers. The extraction was carried out with a Roucaire SFX 2-10 apparatus equipped with a syringe pump Model 260D. The extraction cell, made from stainless-steel and containing PET or TOPASw 8007 pellets, was placed in the extraction compartment and was filled with CO2. The temperature was maintained at 45 8C and the CO2 underwent compression to 150 bar. The extraction was considered finished after 75 min. During the experiment, the extract was recovered in 20 ml of dichloromethane. At the end of the extraction, a large proportion of dichloromethane was lost by evaporation. Hence, the obtained extract was put into a flask and recovery completed with 10 ml of dichloromethane in the presence of an internal standard (nonane). Finally, 1 ml of this solution was injected in the gas chromatograph equipped with a mass spectrometer detector. 2.5. Analysis of the extract by GC/MS The chromatographic analysis was carried out using a Varian STAR 3400CX gas chromatograph equipped with an automatic injection system (Varian 8200CX) and a mass spectrometer detector (Varian SATURN 3). A capillary column (Equity1, 30 m!0.53 mm i.d., 3.0 mm; Sigma-Aldrich) was used for separation with the following oven temperature program: 40 8C for 1 min, increased to 200 8C at 7 8C minK1 and further increased to 220 8C at 20 8C minK1. The rate of the carrier gas Helium is 1 ml minK1. The mass spectrometer was used in the following conditions: detector at 140 8C; injector at 100 8C, transfer line at 230 8C and ionic trap at 140 8C.
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Fig. 4. DSC analysis of amber PET.
Fig. 6. DSC analysis of PET 22391-013.
the crystallization temperature measured at each step of the DSC temperature program. The degree of crystallinity is defined as [37,38]:
3. Results 3.1. Characterization of PET and TOPASw 8007 3.1.1. Polyethylene terephthalate The DSC thermograms of the PET samples (Figs. 4–6) show that they have different thermal behaviour, thus different morphologies and/or structure and composition. During the first heating, both amber PET and transparent PET (Figs. 4–5) did not show the crystallization peak about 130 8C, but this exotherm peak is clearly seen in the case of PET 22391-013 (Fig. 6). Moreover, on cooling from the melt, the transparent PET shows a crystallization peak at about 145 8C, while the two other PETs crystallize at much higher temperature, about 200 8C. This result means that the transparent PET crystallizes with more difficulty than both other PETs. In fact, during the second heating the residual crystallization is seen at about 140 8C (Fig. 5). This exotherm is absent in both amber PET and PET 22391013. All results are reported in Table 2 where we note Tg the glass transition temperature, DHc the enthalpy of crystallization, Tf the melting temperature, DHf the enthalpy of fusion measured at the melting temperature (Tf) and Tc
Xc Z DHf =DHf0 where Xc is the crystalline fraction, DHf is the enthalpy of fusion measured at the melting temperature (Tf) and DHf0 is the enthalpy of fusion of the totally crystalline sample (DHf0 Z 117:6 J gK1). According to Table 2, the amber PET and PET 22391013 are the most and the least crystalline, respectively. The crystalline ratio is calculated by taking account of the fusion enthalpy; but these results would be verified and confirmed by using MDSCw, which permits to distinguish the nonreversing signal (crystallization) from the reversing signal (fusion). In fact, Thomas [39] has found that all of the melting that occured during the MDSCw experiment was the result of crystallization that occured during the experiment. In other words, his results showed that the crystallization starts at about 100 8C and continues until the sample is fully Table 2 Thermodynamic characteristics of the PET studied samples
First heating Tg (8C) DHc (J gK1) Tf (8C) DHf (J gK1) Cooling Tc (8C) DHc (J gK1) Second heating Tg (8C) DHc (J gK1) Tf (8C) DHf (J gK1) Degree of crystallinity (%) Fig. 5. DSC analysis of transparent PET.
Amber PET
Transparent PET
PET 22391-013
68 7 253 52
69 – 244 47
76 24 253 44
189 44
151 13
186 47
79 – 253 40 44
79 10 244 40 40
78 – 254 41 37
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Table 4 Diffusion coefficients of the 1-butanol in PET samples PET samples
Temperature (8C) 3
22391-013 (XcZ37%) ( cm2 sK1) Transparent PET (XcZ40%) ( cm2 sK1) Amber PET (XcZ44%) ( cm2 sK1)
23
42
7.57!10
K8
1.67!10
4.97!10K8
5.74!10K9
7.69!10K9
1.52!10K8
3.70!10K9
6.62!10K9
1.01!10K8
K9
Fig. 7. DSC analysis of TOPASw 8007.
melted at 275 8C. This crystallization exotherm is masked by the melting peak. The studied PET may or may not present this phenomenon. Hence, we use the obtained crystallinity ratios (Table 2) to relate to the diffusion constant calculated later. 3.1.2. Cyclic olefin copolymers: TOPASw 8007 The sample of TOPASw 8007 studied underwent the same temperature program as the sample of PET. The thermogram obtained (Fig. 7) shows the presence of glass transition temperatures and the lack of fusion peaks. These results confirm the amorphous nature, the main characteristic, of the COC as mentioned in literature [27–29]. 3.2. Determination of the diffusion coefficients The data obtained after the GC/MS analysis were treated by software called Experimental Diffusion FIT (EXDIF). This software was developed by the Swiss Federal Office of Public Health [40]. The mathematical model used is represented by the following relation [11]: " # N Mt 8 X 1 ð2n C 1Þ2 p2 Z1K 2 exp K Dt MN p nZ0 ð2n C 1Þ2 L2
where Mt and MN are the quantities of the diffusing substance, at time t and at the equilibrium, respectively, D is the diffusion coefficient and L the thickness of polymer. 3.2.1. Polyethylene terephthalate The different values of the diffusion coefficients are reported in Tables 3 and 4 for the several systems of three PET samples in 1-octanol and 1-butanol. All of these data are represented graphically to demonstrate the relation between the diffusion coefficients and parameters like temperature and crystallinity of the polymer [8–14]. Figs. 8 and 9 show that: 1. the diffusion coefficient decreases when the degree of crystallinity increases 2. the diffusion coefficient decreases when the size of the sorbed molecule increases 3. for each solvent (1-octanol or 1-butanol), when the temperature increases the diffusion coefficient increases also. Similar results were also given in literature [12,15, 22–25]. These conclusions confirm those obtained by a FTIR-ATR study [15] about the diffusion in two systems
Table 3 Diffusion coefficients of the 1-octanol in PET samples PET samples
Temperature (8C) 3
23
42
22391-013 (XcZ37%) ( cm2 sK1) Transparent PET (XcZ40%) ( cm2 sK1) Amber PET (XcZ44%) ( cm2 sK1)
5.67!10K9
9.25!10K9
1.31!10K8
2.86!10K9
5.90!10K9
9.90!10K9
1.60!10K9
4.16!10K9
8.41!10K9 Fig. 8. Diffusion coefficients of PET versus crystallinity degree at different temperatures.
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Fig. 9. Diffusion coefficients versus temperature of PET with various degrees of crystallinity.
(H2O/PET and CH3OH/PET): the observed phenomenon depends on the polarity of the liquid and the crystallinity of the polymer [14]. In addition, the diffusion of the solvents is strongly favored by the presence of amorphous zones in polymers [24].
Fig. 10. Sorption profile M* (t)Zf(t) of the 1-octanol in the TOPASw 8007 at TZ42 8C.
3.2.2. Cyclic olefin copolymers: TOPASw 8007 Table 5 summarizes the values of the diffusion coefficients determined in the two systems studied: (TOPASw 8007/1-butanol) and (TOPASw 8007/1-octanol) at 23 and 42 8C. Figs. 10–13 show the sorption profiles of the TOPASw. We observe that the phenomenon of sorption depends on several parameters (Figs. 10–13): 1 the crystallinity and the interactions between the simulant and the polymer structure 2 the operating conditions such as: time of immersion, temperature, agitation,. According to Table 5, the diffusion coefficients of each solvent in the COC are much higher than those obtained in the case of the various PET samples. This result derives from the morphology difference between PET and COC, which are semi-crystalline and amorphous, respectively. This confirms well the relationship existing between the degree of crystallinity and the diffusion coefficient [15,22–25]. Thus, polymers are considered as being a barrier against solvent diffusion are those with a high ratio of crystallinity.
Fig. 11. Sorption profile M* (t)Zf(t) of the 1-octanol in the TOPASw 8007 at TZ23 8C.
Table 5 Diffusion coefficients of 1-butanol and 1-octanol in the COC at various temperatures Solvents
Temperature (8C) 23
42
1-Butanol ( cm2 sK1) 1-Octanol ( cm2 sK1)
9.06!10K7 3.58!10K7
1.04!10K6 7.83!10K7
Fig. 12. Sorption profile M* (t)Zf(t) of 1-butanol in the TOPASw 8007 at TZ42 8C.
M. Limam et al. / Polymer Testing 24 (2005) 395–402
Fig. 13. Sorption profile M* (t)Zf(t) of 1-butanol in the TOPASw 8007 at TZ23 8C.
4. Conclusion The combination of supercritical fluid extraction and gas chromatography–mass spectrometry is an effective way for the determination organic solvents sorbed by the polymeric materials used as pharmaceutical packaging. Highly crystalline polymers show lower sorption of organic solvents than amorphous polymers. Among the parameters which affect the process of retention of these solvents, the interaction solvent–polymer structure stands out as the most important, but it is necessary to consider the influence of other external parameters such as the immersion time, the temperature, the film thickness,. In conclusion, the PET is confirmed to be a good barrier to alcohols such as butanol and octanol in comparison to COC.
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