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Improving coating resistance to acetic acid sterilisation: An EIS approach
Progress in Organic Coatings 65 (2009) 30–36
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Improving coating resistance to acetic acid sterilisation: An EIS approach Arnoud de Vooys ∗ , Berend Boelen, Jan Paul Penning, Hans van der Weijde Corus Research, Development and Technology, Centre for Packaging Technology, P.O. Box 10.000, 1970 CA IJmuiden, The Netherlands
a r t i c l e
i n f o
Article history: Received 23 October 2007 Received in revised form 7 July 2008 Accepted 12 September 2008 Keywords: Acetic acid resistance PET PP EIS Corrosion Tg
a b s t r a c t In situ EIS spectroscopy was used to evaluate the performance of the inside coating of a deep-drawn polymer-coated steel can during sterilisation and pasteurisation with a 1% acetic acid solution. Acetic acid is a very common ingredient in filling goods and is a particularly aggressive substance for packaging materials. The in situ coating performance was correlated with polymer properties, like glass transition and thermal expansion. Three systems were tested: a PET coating, an improved PET coating, and a PP coating. The PET coating, which performed poorly and showed a relatively high postmortem corrosion rate of the substrate, showed a clear change in properties when heated above the polymer glass transition temperature (Tg ) and subsequently cooled down below Tg . Above Tg the coating resistance decreased with temperature and corrosion products were formed beneath the coating. On subsequent cooling below Tg , the coating develops microscopic cracks, enabling transfer of ionic species to the solution. Based on these observations two improvements were made to the PET coating. In a first approach, the PET coating was modified by blending with PEN, a polymer which is structurally related to PET but with enhanced properties such as a higher Tg , higher strength and toughness and improved barrier properties. The addition of PEN is found to suppress micro-cracking phenomena during cooling after sterilisation and thus prevents ion transfer to the solution. In a second approach, PET is replaced by PP, a polymer that has a much lower Tg and is more hydrophobic than PET on account of its apolar and flexible chain structure. Using this coating, the formation of corrosion products under the coating decreased and ion transfer to the solution was prevented. A clear relation between coating composition, in situ EIS behaviour and postmortem observations is shown in all three cases. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The packaging industry is an increasingly demanding market, driven by coating innovations, legislative pressure (e.g. REACH, VOC emissions, etc.), higher customer expectations and a globalizing market. An answer to these changes is the Protact® product line of steel coated with thermoplastic polymers, mainly poly(ethylene terephthalate) (PET) and polypropylene (PP). The polymer-coated steel is supplied to the canmaker as a pre-coated product, which eliminates the need for lacquering the cans. Since severe deformations occurring in the canmaking process can have a pronounced effect on the coating performance [1–5], it is of importance to consider the product performance of polymer-coated steel the finished can, i.e. after deformation. A main market for packaging is the food sector, where it is usually required to sterilise or pasteurise the food after filling and
∗ Corresponding author. Tel.: +31 251 494922; fax: +31 251 470344. E-mail address: [email protected] (A. de Vooys). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.09.004
closing in order to increase shelf life. In many cases, food products contain acetic acid, an aggressive component to which PET is known to have limited resistance, in particular at high concentrations in combination with elevated temperatures. Typical pasteurisation and sterilisation temperatures are above the glass transition temperature (Tg ) of PET (353 K, 80 ◦ C), where water uptake and permeability of the polymer become much higher [1,6]. Usually, products containing acetic acid require only pasteurisation to about 363 K (90 ◦ C), since the final pH is lower than 4. However, in some cases sterilisation may be required because the initial pH is lower than 4 while after the processing cycle the pH has increased. To evaluate these cases as well, the present research focuses on the “worst case scenario” of sterilisation at 394 K (121 ◦ C) for a prolonged time (1 h) with a relatively high concentration (1%, v/v) of non-buffered acetic acid. These conditions are more extreme than is encountered in practice, but provide strong discrimination between different coatings. When subjecting a container having a traditional PET coating to this worst case scenario, three types of degradation are observed: discoloration of the substrate, attributed to the formation of iron
A. de Vooys et al. / Progress in Organic Coatings 65 (2009) 30–36
Fig. 1. Equivalent circuit for fitting.
oxides and acetates underneath the coating, blister formation and an increased concentration of Fe ions in the solution. In the present study, the performance of polymer coatings was studied at the various stages of the sterilisation process using in situ electrochemical impedance spectroscopy (EIS). The aim of this work is to clarify the corrosion mechanism and to establish a link between corrosion performance and polymer properties, and to evaluate and improve the performance of Protact® polymer-coated steel for packaging of acetic acid containing media. Two possible improvements to the traditional PET coating were examined. In a first approach, the PET coating was modified by blending with poly(ethylene naphthalate) (PEN), a polymer which is structurally related to PET but with enhanced properties such as a higher Tg , higher strength and toughness and improved barrier properties [7,8]. In a second approach, PET is replaced by PP, a polymer that, due to its apolar and flexible chain structure, is much more hydrophobic and has a much lower Tg than PET. It is shown that both approaches lead to a considerable improvement in acetic acid resistance. 2. Experimental Polymer-coated steel was produced by direct extrusion at the pilot line facility of Corus RD&T in IJmuiden, using a standard ECCS (tinfree steel) substrate. For the reference PET coating, a standard bottle-grade PET resin was used as starting material. A modified PET coating was obtained by blending the afore-mentioned PET resin in 50/50 (w/w) ratio with a PEN homopolymer. For PP coatings, an injection moulding grade PP homopolymer was used as a starting material. In all cases, the polymer coating was about 20 m in thickness. Deep drawn cans of 65 mm in diameter were prepared by a two-stage redraw process to a total draw ratio of 2.77, which is comparable to standard two-piece food cans. All cans were filled with a 1% (v/v) acetic acid solution at ambient temperature with 5% headspace, closed and pasteurised at 368 K or sterilised at 395 K for 1 h. The temperature–time cycle during sterilisation or pasteurisation was measured in a separate can with a datalogger. The heating rate during the sterilisation cycle was 2.2 K/min up to ca. 353 K after which it slowly decreases as the temperature increases. The cans reached a temperature of 395 K after 90 min. The cooling rate was also 2.2 K/min to ca. 383 K, after which it slowly decreases. The temperature after 90 min was 313 K. The containers tested with EIS were equipped with an in situ EIS-sensor prior to pasteurisation or sterilisation. The system is
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described in detail elsewhere [9]. In short, a hole is drilled in the bottom of the can and a sensor is inserted. The sensor consists of the container as the working electrode, a platinum wire as the counter electrode and a silver wire as the reference electrode. A three-way radio plug in a PEEK casing was used to transfer the signal from the inside of the can to the potentiostat, allowing continuous measurements during the entire measurement. EIS spectra were taken throughout the sterilisation/ pasteurisation run at a rate of 1 spectrum/min, and 1 scan/5 min once the container had cooled down below 313 K. The frequency range was 100 kHz to 1 Hz when measured once per minute and 100 kHz to 0.1 Hz when measured once per 5 min. The acquisition time of the each measurement (53 s) was fast enough to assume (1) that the spectrum can be assigned to a single temperature (as the temperature varies maximal 2 K during the acquisition) and (2) that any distortions in the spectrum (related to variations in the system during the acquisition, especially at low frequencies due to the relatively long acquisition time) are minimal. The spectra were checked for distortions prior to analysis, and they were indeed low. The effect of distortions on the fitting procedure was checked separately. The fitting procedure was rather insensitive to the distortions: it yields an average value over the low frequency range. After cooling the cans were evaluated for surface discoloration and blister formation using a visual rating system. The iron content of the solution after 2 weeks storage of the sterilised cans at room temperature was determined with ICP-AES. For this purpose, cans which had not been equipped with an EIS sensor were used. The EIS spectra were recorded in triplo, while the discoloration, blister formation and iron pickup were determined in 10-fold. The spectra were analysed with a simplified Randalls circuit (Fig. 1), in which Rel is the electrolyte resistance, Ccoat the coating capacitance, Rcoat the coating resistance and Wdiff is the diffusion of ions. This circuit was chosen since the presence of each element can be directly deduced from the Bode plots, it provides an accurate fit in the frequency range measured (100 kHz to 0.1 Hz), and the four elements (especially Ccoat and Rcoat ) can be related to the processes occurring in the coating. The coating conductivity (=1/Rcoat , which reflects the coating porosity) is given instead of the coating resistance, since this term gives a better insight into the corrosion mechanisms and into the polymer physics. The values Ccoat , Rcoat and Wdiff showed a relatively large statistical error between reproduction runs (ca. 50% of the measured value). This is caused by small changes between the cans and variability in the breakdown of the coating which had a large effect on the values. However, all reproductions overlap when the variations are taken into account, i.e. the qualitative behaviour of the cans is the same and therefore the cans degrade according to the same mechanism. Secondly, there is no such variability in the temperature, the Tg for instance was reproducible within ±3 ◦ C. The run with the values closest to the average is given in the figures. The focus in
Fig. 2. Discoloration, blister formation and iron pickup of containers after test pack with sterilisation.
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A. de Vooys et al. / Progress in Organic Coatings 65 (2009) 30–36
Fig. 3. Blister after sterilisation on reference PET, arrows indicate cracks in the coating.
Fig. 4. Blister after sterilisation on PEN modified PET.
the results and discussion sections will be on the trends and the associated temperature ranges, without using the exact values of the measurements.
decreases with decreasing temperature, but the conductivity does not return to its original value observed in the heating curve. Once the coating has cooled down (Fig. 7) a second, low frequency term evolves with time and the medium frequency impedance drops with time. The PEN-modified PET coating showed a similar behaviour as the reference PET during heating up and cooling down of the can. The main difference with the reference PET is observed after cooling down (Fig. 8 compared to Fig. 6): the low frequency term does not emerge as in Fig. 7. The spectra of a PP coated container during sterilisation are qualitatively very similar to those of the PEN-modified PET coating, as shown in Figs. 9 and 10. Fitting of the data showed that for both PET coatings a strong increase in the coating conductivity is observed above ca. 375 K (Fig. 11a, from ca. 1.7 × 10−7 S to ca. 1.5 × 10−6 S for the reference PET, Fig. 11b, from ca. 9 × 10−7 S to ca. 5 × 10−6 S for the PENmodified PET). Based upon extrapolation a value of ca. 3.5 × 10−7 S respectively 1.2 × 10−6 S would be expected, which would be a factor 4 lower than the observed values. During cooling down the coating conductivity decreases linearly with temperature, but the original level is not reached. The low frequency term once the coating has cooled down can be fitted accurately with a Warburg element (Fig. 12), indicating that this term can be interpreted as diffusion of ions through the coating. The cans were pasteurised to 368 K to obtain more accurate data of the processes around Tg . The pasteurisation was repeated directly after the can had cooled down to 310 K, to check the reversibility
3. Results For the three coatings studied, substrate discoloration, blister formation and iron pickup as observed after sterilisation in 1% acetic acid are given in Fig. 2a–c. The reference PET coatings showed poor performance in all three cases. The PEN-modified PET coating showed a slight improvement with respect to blister formation and a strong improvement with respect to iron pickup. The PP coating showed a strong improvement in all three aspects. These results show that the three processes are not completely correlated, in particular there appears to be no correlation between the presence of blisters and the level of iron pickup. Postmortem pictures of the blisters (Figs. 3 and 4) showed that, in the reference PET coating, some of the blisters contained microscopic cracks, while in the case of PEN-modified PET the blisters were intact. In both cases, blisters showed corrosion products beneath the coating. The sterilisation cycle can be divided into three stages: heating up of the can, cooling down, and storage at room temperature once the can has cooled down. Examples of the EIS spectra in all three stages are given in Figs. 5–7 for the reference PET coating. The heating stage (Fig. 5) is accompanied by a decrease in the low frequency impedance, i.e. an increase in coating conductivity. During cooling down of the can (Fig. 6), the coating conductivity
Fig. 5. Bode plot of reference PET coating during heating up.
A. de Vooys et al. / Progress in Organic Coatings 65 (2009) 30–36
Fig. 6. Bode plot of reference PET coating during cooling down.
Fig. 7. Bode plot of reference PET coating after cooling.
Fig. 8. Bode plot of modified PET coating after cooling down.
Fig. 9. Bode plot of PP coating during heating up (the sudden decrease occurs near 390 K).
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Fig. 10. Bode plot of PP coating during and after cooling down.
Fig. 11. Coating conductivity during sterilisation, heating up closed diamonds, cooling down open circles: (a) reference PET; (b) modified PET.
of the processes. The coating capacity (Fig. 13a) of the reference PET showed a peak in the first heating run and an S-type behaviour in the subsequent runs. The S-type behaviour of the capacitance is similar to earlier results of epoxy and polyamide based lacquers [10], and is expected for any polymer going through the glass transition: the dielectric constant of the polymer, the water uptake and the coating thickness all increase in a rather broad, stepwise fashion. The PEN-modified PET did not show a peak in the coating capacitance (Fig. 13b), only the S-type behaviour. The onset of the capacitance change is at the same temperature for both coatings (ca. 340 K), but for the PEN-modified PET coating the transition extends to higher temperatures (360 K for the reference PET and 370 K for the PEN-modified PET). Thus, the glass transition region is slightly broadened by the addition of PEN, but Tg is not shifted significantly to higher values.
The coating conductivity of the reference PET coating during pasteurisation is shown in Fig. 14. In the first heating run, coating conductivity showed a small increase with temperature from the glass transition (340 K) upwards. In the second heating run, coating conductivity follows the first cooling run up to about 350 K, where it is linear with temperature. Once the second heating run reaches 350 K, the conductivity increases again compared to the first cooling run. The second cooling run drops again linearly in temperature below 350 K, but the entire curve is higher than the first cooling run. This result indicates that the conductivity increase below 350 K is reversible while above 350 K it is irreversible. The cans showed slight discoloration and no blisters after pasteurisation. Fitting of the PP data showed no changes in the coating capacitance throughout the measurement. The coating conductivity (Fig. 15) showed a linear increase with temperature until ca. 380 K, where a strong increase is observed. During cooling down after sterilisation hardly any recuperation is observed. The pasteurisation showed that the coating conductivity was linear with temperature and reversible throughout the cycle (Fig. 16). The pasteurised cans showed no discoloration or blister formation.
4. Discussion
Fig. 12. Warburg element in time after the container has cooled down, closed circles is reference PET, open triangles is modified PET.
Both during the sterilisation and pasteurisation cycles (Figs. 11 and 14–16) an increase in coating conductivity with temperature is seen. Upon cooling, the coating conductivity decreases but does not return to its original value, indicating that changes in coating conductivity with temperature have a reversible and an irreversible component. The reversible part is linear with temperature (especially Figs. 14 and 16) and is associated with the thermal expansion of
A. de Vooys et al. / Progress in Organic Coatings 65 (2009) 30–36
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Fig. 13. Coating capacitance during pasteurisation (two subsequent cycles): (a) reference PET; (b) modified PET.
the polymer, i.e. the density decreases with increasing temperature. This behaviour is expected both in the rubber state and the glass state [11]. A similar behaviour has been observed for polyamide and epoxy type lacquers [10], and is observed here for both polyesters and polyolefins, showing the general nature of this phenomenon. The existence of an irreversible component of changes in coating conductivity suggests that permanent paths for ionic transport are created through the coating. This process starts at ca. 360 K for PET (Fig. 14) and 380 K for PP coatings (Fig. 15). This is also the point where the steel substrate starts to show discoloration, especially for the PET coatings: coatings heated to 360 K (just above the Tg ) are slightly discoloured and PET coatings that are sterilised are strongly discoloured. The level of discoloration matches the level to which the coating conductivity has increased after the entire cycle; the pasteurised sample returns to a lower value (ca. 2 × 10−7 S, Fig. 14) than the sterilised sample (ca. 1 × 10−6 S, Fig. 11). Figs. 3 and 4 confirm that the discoloration is the result of the formation of oxidation products of the substrate; the polymer itself shows no colour change due to this process. It is therefore reasonable to infer that the two processes are linked: corrosion of the steel substrate, as evidences by discoloration, starts once ions from the solution can reach the polymer–steel interface due to increased permanent conductivity of the polymer. It is worth noting that the qualitative behaviour and the mechanism are similar, but the level of discoloration is strongly dependent on the coating system: the increase of discoloration is much greater for PET based coatings compared to PP based coatings, even though the irreversible increase in conductivity is similar. We tentatively assign this behaviour to differences in adhesion mechanism [12]. In PP, adhesion to the metal surface is achieved by modification of the polymer with maleic anhydride, which reacts with the metal
Fig. 14. Coating conductivity of reference PET during pasteurisation (two subsequent cycles).
surface hydroxyl group to establish a covalent bond across the polymer–metal interface. On the other hand, the adhesion of PET coatings is based on acid–base interactions and hydrogen bonds between the polymers carbonyl groups and the metal oxide surface. It is known that ions interact more strongly with the hydrogen bridges, and thus decrease the adhesion and subsequent increase the rate of corrosion [12]. Both the reference PET and the PEN-modified PET coatings showed a large number of blisters after the sterilisation cycle (Fig. 2b). However, the iron pickup does not correlate with the extent of blister formation since this is low for the PEN-modified PET coating (10 mg/l) and high for the reference PET coating (70 mg/l). Apparently, the PEN-modified coating is still relatively intact over the blisters preventing excessive iron pickup, in agreement with Figs. 3 and 4. The EIS spectra of the PEN-modified PET coating confirm that it is relatively intact: the shape of the spectrum during and after the cooling is typical for a slightly conductive coating (Fig. 8), and the coating conductivity remains at ca. 2 × 10−6 S (500 k impedance). Note that there is no indication of a second time constant or Warburg type element in Fig. 8 at the end of the experiment, which is reflected in a low value of the Wahrburg term after the fitting procedure (<10−6 S, Fig. 12). This is in strong contrast with the reference PET coating (Fig. 7), which shows a continuous rise in coating conductivity (1 × 106 M impedance for the first scan to 2 × 104 k for the last scan, i.e. an increase from 1 × 10−6 S to 5 × 10−4 S in time), and a significant Warburg type element evolving in time (from 4 × 10−6 S to 2 × 10−5 S, Fig. 12). The formation of blisters after sterilisation is not unique to acetic acid media, blisters have also been seen after sterilisation with NaCl solutions [1]. However, there is a profound difference in the shape of the EIS spectra, as well as in size and colour of the blisters. The
Fig. 15. Coating conductivity of PP during sterilisation.
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A. de Vooys et al. / Progress in Organic Coatings 65 (2009) 30–36
Fig. 16. Coating conductivity of PP during pasteurisation; closed circles first run, open diamonds second run: (a) heating part of cycle; (b) cooling part of cycle.
spectra during NaCl sterilisation typically show a characteristic low frequency time constant RQ with the order of the Q between 0.6 and 0.8. The conclusion of Ref. [1] was that during saline sterilisation corrosion occurs under a small coating defect, and propagation occurs via cathodic delamination, which is in line with what could be expected for organic coatings on steel [e.g. [13]]. Based on our present analysis, the sequence of events in the case of acetic acid sterilisation is thought to be as follows: 1. Ions permeate through the coating, as evidenced by the increasing coating conductivity. 2. The substrate surface corrodes, as evidenced by the discoloration. 3. The corrosion products create a blister under an almost intact coating, as evidenced by the low coating conductivity after blister formation. The blister formation is therefore a consequence of the corrosion at the polymer–steel interface, which also caused the discoloration, and not by cathodic delamination. The main difference between the reference PET and the PENmodified PET coatings is in the iron pickup after 2 weeks, in the evolution of a diffusion term with time for the reference PET coatings, and in the observation of microscopic cracks in the blisters. It is likely the terms are related to the same process, i.e. that the EIS term observed represents the diffusion of iron ions into the electrolyte. Significant iron pickup can only occur through relatively large pores in the coating, which can be linked to the observed micro-cracks. The EIS spectra suggest that cracking of the reference PET coating occurs during and after cooling down to ambient temperature. Crack formation in the coating is probably related to residual stresses from blister formation which are frozen by cooling down below Tg . The capacitance change with temperature (Fig. 13) is another indication that the glass transition for reference PET is less reversible than for the PEN-modified PET. The PP coating shows no glass transition in the relevant temperature region, and no blister formation is observed, and therefore no additional stresses are present which could lead to cracking of the coating. This, combined with a relatively low conductivity after cooling down (compared to the reference PET), explains the low iron pickup.
5. Conclusions The degradation of PET and PP coatings on steel during sterilisation in acetic acid media consists of three consecutive stages: discoloration of the substrate, blister formation and iron pickup. The discoloration is related to an increase in permeability of the coating, as evidenced by an irreversible increase in coating conductivity. This increase starts at 360 K for PET coatings and 380 K for PP coatings. Below these temperatures, changes in coating conductivity are reversible and are related to the physical (volume) expansion of the coating upon heating. The blisters are formed by corrosion of the substrate under an almost intact coating. This is in contrast to blister formation under saline conditions, which has a different mechanism (cathodic delamination). However, the increase in coating conductivity and blister formation is not the direct cause of iron pickup by the electrolyte. Iron pickup is related to the formation of microscopic cracks in the coating after cooling down to ambient temperature, due to residual thermal and mechanical stresses. Improving the coating can be done by modifying the coating to withstand the glass–rubber transition better. A strong increase in performance in all three aspects can be reached by using PP. References [1] B. Boelen, H. den Hartog, H. van der Weijde, Progress in Organic Coatings 50 (2004) 40–46. [2] A.C. Bastos, A.M.P. Simoes, Progress in Organic Coatings 46 (3) (2003) 220–227. [3] F. Deflorian, L. Fedrizzi, S. Rossi, Corrosion Science 42 (7) (2000) 1283–1301. [4] V. Lavaert, P. Praet, M. Moors, E. Wettinck, B. Verhegghe, Progress in Organic Coatings 39 (2–4) (2000) 157–165. [5] F. Deflorian, L. Fedrizzi, S. Rossi, Corrosion 55 (11) (1999) 1003–1011. [6] X. Zhang, B. Boelen, P. Beentjes, J. Mol, H. Terryn, H. de Wit, Progress in Organic Coatings 60 (4) (2007) 335–342. [7] A. Arkhireyeva, S. Hashemi, Polymer 43 (2) (2002) 289–300. [8] A. Tonneli, Polymer 43 (2) (2002) 637–642. [9] B. Boelen, R. Breur, H. den Hartog, H. van der Weijde, Proceedings Eurocorr., 2004. [10] J.V. Standish, H. Leidheiser Jr., Journal of Coating Technology 53 (678) (1981) 38–53. [11] D.W. van Krevelen, Properties of Polymers, third edition, Elsevier Science B.V., Amsterdam, 1990, pp. 88–94. [12] J.C. Bolger, A.S. Michaels, Proceedings Interface Conversion for Polymer Coatings, American Elsevier Publishing Company Inc., New York, 1968, pp. 3–61. [13] H. Leidheiser Jr., Mechanism of de-adhesion of organic coatings from metal surfaces, ACS Symposium Series 322 (1986) 124–135.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Improving coating resistance to acetic acid sterilisation: An EIS approach Arnoud de Vooys ∗ , Berend Boelen, Jan Paul Penning, Hans van der Weijde Corus Research, Development and Technology, Centre for Packaging Technology, P.O. Box 10.000, 1970 CA IJmuiden, The Netherlands
a r t i c l e
i n f o
Article history: Received 23 October 2007 Received in revised form 7 July 2008 Accepted 12 September 2008 Keywords: Acetic acid resistance PET PP EIS Corrosion Tg
a b s t r a c t In situ EIS spectroscopy was used to evaluate the performance of the inside coating of a deep-drawn polymer-coated steel can during sterilisation and pasteurisation with a 1% acetic acid solution. Acetic acid is a very common ingredient in filling goods and is a particularly aggressive substance for packaging materials. The in situ coating performance was correlated with polymer properties, like glass transition and thermal expansion. Three systems were tested: a PET coating, an improved PET coating, and a PP coating. The PET coating, which performed poorly and showed a relatively high postmortem corrosion rate of the substrate, showed a clear change in properties when heated above the polymer glass transition temperature (Tg ) and subsequently cooled down below Tg . Above Tg the coating resistance decreased with temperature and corrosion products were formed beneath the coating. On subsequent cooling below Tg , the coating develops microscopic cracks, enabling transfer of ionic species to the solution. Based on these observations two improvements were made to the PET coating. In a first approach, the PET coating was modified by blending with PEN, a polymer which is structurally related to PET but with enhanced properties such as a higher Tg , higher strength and toughness and improved barrier properties. The addition of PEN is found to suppress micro-cracking phenomena during cooling after sterilisation and thus prevents ion transfer to the solution. In a second approach, PET is replaced by PP, a polymer that has a much lower Tg and is more hydrophobic than PET on account of its apolar and flexible chain structure. Using this coating, the formation of corrosion products under the coating decreased and ion transfer to the solution was prevented. A clear relation between coating composition, in situ EIS behaviour and postmortem observations is shown in all three cases. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The packaging industry is an increasingly demanding market, driven by coating innovations, legislative pressure (e.g. REACH, VOC emissions, etc.), higher customer expectations and a globalizing market. An answer to these changes is the Protact® product line of steel coated with thermoplastic polymers, mainly poly(ethylene terephthalate) (PET) and polypropylene (PP). The polymer-coated steel is supplied to the canmaker as a pre-coated product, which eliminates the need for lacquering the cans. Since severe deformations occurring in the canmaking process can have a pronounced effect on the coating performance [1–5], it is of importance to consider the product performance of polymer-coated steel the finished can, i.e. after deformation. A main market for packaging is the food sector, where it is usually required to sterilise or pasteurise the food after filling and
∗ Corresponding author. Tel.: +31 251 494922; fax: +31 251 470344. E-mail address: [email protected] (A. de Vooys). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.09.004
closing in order to increase shelf life. In many cases, food products contain acetic acid, an aggressive component to which PET is known to have limited resistance, in particular at high concentrations in combination with elevated temperatures. Typical pasteurisation and sterilisation temperatures are above the glass transition temperature (Tg ) of PET (353 K, 80 ◦ C), where water uptake and permeability of the polymer become much higher [1,6]. Usually, products containing acetic acid require only pasteurisation to about 363 K (90 ◦ C), since the final pH is lower than 4. However, in some cases sterilisation may be required because the initial pH is lower than 4 while after the processing cycle the pH has increased. To evaluate these cases as well, the present research focuses on the “worst case scenario” of sterilisation at 394 K (121 ◦ C) for a prolonged time (1 h) with a relatively high concentration (1%, v/v) of non-buffered acetic acid. These conditions are more extreme than is encountered in practice, but provide strong discrimination between different coatings. When subjecting a container having a traditional PET coating to this worst case scenario, three types of degradation are observed: discoloration of the substrate, attributed to the formation of iron
A. de Vooys et al. / Progress in Organic Coatings 65 (2009) 30–36
Fig. 1. Equivalent circuit for fitting.
oxides and acetates underneath the coating, blister formation and an increased concentration of Fe ions in the solution. In the present study, the performance of polymer coatings was studied at the various stages of the sterilisation process using in situ electrochemical impedance spectroscopy (EIS). The aim of this work is to clarify the corrosion mechanism and to establish a link between corrosion performance and polymer properties, and to evaluate and improve the performance of Protact® polymer-coated steel for packaging of acetic acid containing media. Two possible improvements to the traditional PET coating were examined. In a first approach, the PET coating was modified by blending with poly(ethylene naphthalate) (PEN), a polymer which is structurally related to PET but with enhanced properties such as a higher Tg , higher strength and toughness and improved barrier properties [7,8]. In a second approach, PET is replaced by PP, a polymer that, due to its apolar and flexible chain structure, is much more hydrophobic and has a much lower Tg than PET. It is shown that both approaches lead to a considerable improvement in acetic acid resistance. 2. Experimental Polymer-coated steel was produced by direct extrusion at the pilot line facility of Corus RD&T in IJmuiden, using a standard ECCS (tinfree steel) substrate. For the reference PET coating, a standard bottle-grade PET resin was used as starting material. A modified PET coating was obtained by blending the afore-mentioned PET resin in 50/50 (w/w) ratio with a PEN homopolymer. For PP coatings, an injection moulding grade PP homopolymer was used as a starting material. In all cases, the polymer coating was about 20 m in thickness. Deep drawn cans of 65 mm in diameter were prepared by a two-stage redraw process to a total draw ratio of 2.77, which is comparable to standard two-piece food cans. All cans were filled with a 1% (v/v) acetic acid solution at ambient temperature with 5% headspace, closed and pasteurised at 368 K or sterilised at 395 K for 1 h. The temperature–time cycle during sterilisation or pasteurisation was measured in a separate can with a datalogger. The heating rate during the sterilisation cycle was 2.2 K/min up to ca. 353 K after which it slowly decreases as the temperature increases. The cans reached a temperature of 395 K after 90 min. The cooling rate was also 2.2 K/min to ca. 383 K, after which it slowly decreases. The temperature after 90 min was 313 K. The containers tested with EIS were equipped with an in situ EIS-sensor prior to pasteurisation or sterilisation. The system is
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described in detail elsewhere [9]. In short, a hole is drilled in the bottom of the can and a sensor is inserted. The sensor consists of the container as the working electrode, a platinum wire as the counter electrode and a silver wire as the reference electrode. A three-way radio plug in a PEEK casing was used to transfer the signal from the inside of the can to the potentiostat, allowing continuous measurements during the entire measurement. EIS spectra were taken throughout the sterilisation/ pasteurisation run at a rate of 1 spectrum/min, and 1 scan/5 min once the container had cooled down below 313 K. The frequency range was 100 kHz to 1 Hz when measured once per minute and 100 kHz to 0.1 Hz when measured once per 5 min. The acquisition time of the each measurement (53 s) was fast enough to assume (1) that the spectrum can be assigned to a single temperature (as the temperature varies maximal 2 K during the acquisition) and (2) that any distortions in the spectrum (related to variations in the system during the acquisition, especially at low frequencies due to the relatively long acquisition time) are minimal. The spectra were checked for distortions prior to analysis, and they were indeed low. The effect of distortions on the fitting procedure was checked separately. The fitting procedure was rather insensitive to the distortions: it yields an average value over the low frequency range. After cooling the cans were evaluated for surface discoloration and blister formation using a visual rating system. The iron content of the solution after 2 weeks storage of the sterilised cans at room temperature was determined with ICP-AES. For this purpose, cans which had not been equipped with an EIS sensor were used. The EIS spectra were recorded in triplo, while the discoloration, blister formation and iron pickup were determined in 10-fold. The spectra were analysed with a simplified Randalls circuit (Fig. 1), in which Rel is the electrolyte resistance, Ccoat the coating capacitance, Rcoat the coating resistance and Wdiff is the diffusion of ions. This circuit was chosen since the presence of each element can be directly deduced from the Bode plots, it provides an accurate fit in the frequency range measured (100 kHz to 0.1 Hz), and the four elements (especially Ccoat and Rcoat ) can be related to the processes occurring in the coating. The coating conductivity (=1/Rcoat , which reflects the coating porosity) is given instead of the coating resistance, since this term gives a better insight into the corrosion mechanisms and into the polymer physics. The values Ccoat , Rcoat and Wdiff showed a relatively large statistical error between reproduction runs (ca. 50% of the measured value). This is caused by small changes between the cans and variability in the breakdown of the coating which had a large effect on the values. However, all reproductions overlap when the variations are taken into account, i.e. the qualitative behaviour of the cans is the same and therefore the cans degrade according to the same mechanism. Secondly, there is no such variability in the temperature, the Tg for instance was reproducible within ±3 ◦ C. The run with the values closest to the average is given in the figures. The focus in
Fig. 2. Discoloration, blister formation and iron pickup of containers after test pack with sterilisation.
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Fig. 3. Blister after sterilisation on reference PET, arrows indicate cracks in the coating.
Fig. 4. Blister after sterilisation on PEN modified PET.
the results and discussion sections will be on the trends and the associated temperature ranges, without using the exact values of the measurements.
decreases with decreasing temperature, but the conductivity does not return to its original value observed in the heating curve. Once the coating has cooled down (Fig. 7) a second, low frequency term evolves with time and the medium frequency impedance drops with time. The PEN-modified PET coating showed a similar behaviour as the reference PET during heating up and cooling down of the can. The main difference with the reference PET is observed after cooling down (Fig. 8 compared to Fig. 6): the low frequency term does not emerge as in Fig. 7. The spectra of a PP coated container during sterilisation are qualitatively very similar to those of the PEN-modified PET coating, as shown in Figs. 9 and 10. Fitting of the data showed that for both PET coatings a strong increase in the coating conductivity is observed above ca. 375 K (Fig. 11a, from ca. 1.7 × 10−7 S to ca. 1.5 × 10−6 S for the reference PET, Fig. 11b, from ca. 9 × 10−7 S to ca. 5 × 10−6 S for the PENmodified PET). Based upon extrapolation a value of ca. 3.5 × 10−7 S respectively 1.2 × 10−6 S would be expected, which would be a factor 4 lower than the observed values. During cooling down the coating conductivity decreases linearly with temperature, but the original level is not reached. The low frequency term once the coating has cooled down can be fitted accurately with a Warburg element (Fig. 12), indicating that this term can be interpreted as diffusion of ions through the coating. The cans were pasteurised to 368 K to obtain more accurate data of the processes around Tg . The pasteurisation was repeated directly after the can had cooled down to 310 K, to check the reversibility
3. Results For the three coatings studied, substrate discoloration, blister formation and iron pickup as observed after sterilisation in 1% acetic acid are given in Fig. 2a–c. The reference PET coatings showed poor performance in all three cases. The PEN-modified PET coating showed a slight improvement with respect to blister formation and a strong improvement with respect to iron pickup. The PP coating showed a strong improvement in all three aspects. These results show that the three processes are not completely correlated, in particular there appears to be no correlation between the presence of blisters and the level of iron pickup. Postmortem pictures of the blisters (Figs. 3 and 4) showed that, in the reference PET coating, some of the blisters contained microscopic cracks, while in the case of PEN-modified PET the blisters were intact. In both cases, blisters showed corrosion products beneath the coating. The sterilisation cycle can be divided into three stages: heating up of the can, cooling down, and storage at room temperature once the can has cooled down. Examples of the EIS spectra in all three stages are given in Figs. 5–7 for the reference PET coating. The heating stage (Fig. 5) is accompanied by a decrease in the low frequency impedance, i.e. an increase in coating conductivity. During cooling down of the can (Fig. 6), the coating conductivity
Fig. 5. Bode plot of reference PET coating during heating up.
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Fig. 6. Bode plot of reference PET coating during cooling down.
Fig. 7. Bode plot of reference PET coating after cooling.
Fig. 8. Bode plot of modified PET coating after cooling down.
Fig. 9. Bode plot of PP coating during heating up (the sudden decrease occurs near 390 K).
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Fig. 10. Bode plot of PP coating during and after cooling down.
Fig. 11. Coating conductivity during sterilisation, heating up closed diamonds, cooling down open circles: (a) reference PET; (b) modified PET.
of the processes. The coating capacity (Fig. 13a) of the reference PET showed a peak in the first heating run and an S-type behaviour in the subsequent runs. The S-type behaviour of the capacitance is similar to earlier results of epoxy and polyamide based lacquers [10], and is expected for any polymer going through the glass transition: the dielectric constant of the polymer, the water uptake and the coating thickness all increase in a rather broad, stepwise fashion. The PEN-modified PET did not show a peak in the coating capacitance (Fig. 13b), only the S-type behaviour. The onset of the capacitance change is at the same temperature for both coatings (ca. 340 K), but for the PEN-modified PET coating the transition extends to higher temperatures (360 K for the reference PET and 370 K for the PEN-modified PET). Thus, the glass transition region is slightly broadened by the addition of PEN, but Tg is not shifted significantly to higher values.
The coating conductivity of the reference PET coating during pasteurisation is shown in Fig. 14. In the first heating run, coating conductivity showed a small increase with temperature from the glass transition (340 K) upwards. In the second heating run, coating conductivity follows the first cooling run up to about 350 K, where it is linear with temperature. Once the second heating run reaches 350 K, the conductivity increases again compared to the first cooling run. The second cooling run drops again linearly in temperature below 350 K, but the entire curve is higher than the first cooling run. This result indicates that the conductivity increase below 350 K is reversible while above 350 K it is irreversible. The cans showed slight discoloration and no blisters after pasteurisation. Fitting of the PP data showed no changes in the coating capacitance throughout the measurement. The coating conductivity (Fig. 15) showed a linear increase with temperature until ca. 380 K, where a strong increase is observed. During cooling down after sterilisation hardly any recuperation is observed. The pasteurisation showed that the coating conductivity was linear with temperature and reversible throughout the cycle (Fig. 16). The pasteurised cans showed no discoloration or blister formation.
4. Discussion
Fig. 12. Warburg element in time after the container has cooled down, closed circles is reference PET, open triangles is modified PET.
Both during the sterilisation and pasteurisation cycles (Figs. 11 and 14–16) an increase in coating conductivity with temperature is seen. Upon cooling, the coating conductivity decreases but does not return to its original value, indicating that changes in coating conductivity with temperature have a reversible and an irreversible component. The reversible part is linear with temperature (especially Figs. 14 and 16) and is associated with the thermal expansion of
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Fig. 13. Coating capacitance during pasteurisation (two subsequent cycles): (a) reference PET; (b) modified PET.
the polymer, i.e. the density decreases with increasing temperature. This behaviour is expected both in the rubber state and the glass state [11]. A similar behaviour has been observed for polyamide and epoxy type lacquers [10], and is observed here for both polyesters and polyolefins, showing the general nature of this phenomenon. The existence of an irreversible component of changes in coating conductivity suggests that permanent paths for ionic transport are created through the coating. This process starts at ca. 360 K for PET (Fig. 14) and 380 K for PP coatings (Fig. 15). This is also the point where the steel substrate starts to show discoloration, especially for the PET coatings: coatings heated to 360 K (just above the Tg ) are slightly discoloured and PET coatings that are sterilised are strongly discoloured. The level of discoloration matches the level to which the coating conductivity has increased after the entire cycle; the pasteurised sample returns to a lower value (ca. 2 × 10−7 S, Fig. 14) than the sterilised sample (ca. 1 × 10−6 S, Fig. 11). Figs. 3 and 4 confirm that the discoloration is the result of the formation of oxidation products of the substrate; the polymer itself shows no colour change due to this process. It is therefore reasonable to infer that the two processes are linked: corrosion of the steel substrate, as evidences by discoloration, starts once ions from the solution can reach the polymer–steel interface due to increased permanent conductivity of the polymer. It is worth noting that the qualitative behaviour and the mechanism are similar, but the level of discoloration is strongly dependent on the coating system: the increase of discoloration is much greater for PET based coatings compared to PP based coatings, even though the irreversible increase in conductivity is similar. We tentatively assign this behaviour to differences in adhesion mechanism [12]. In PP, adhesion to the metal surface is achieved by modification of the polymer with maleic anhydride, which reacts with the metal
Fig. 14. Coating conductivity of reference PET during pasteurisation (two subsequent cycles).
surface hydroxyl group to establish a covalent bond across the polymer–metal interface. On the other hand, the adhesion of PET coatings is based on acid–base interactions and hydrogen bonds between the polymers carbonyl groups and the metal oxide surface. It is known that ions interact more strongly with the hydrogen bridges, and thus decrease the adhesion and subsequent increase the rate of corrosion [12]. Both the reference PET and the PEN-modified PET coatings showed a large number of blisters after the sterilisation cycle (Fig. 2b). However, the iron pickup does not correlate with the extent of blister formation since this is low for the PEN-modified PET coating (10 mg/l) and high for the reference PET coating (70 mg/l). Apparently, the PEN-modified coating is still relatively intact over the blisters preventing excessive iron pickup, in agreement with Figs. 3 and 4. The EIS spectra of the PEN-modified PET coating confirm that it is relatively intact: the shape of the spectrum during and after the cooling is typical for a slightly conductive coating (Fig. 8), and the coating conductivity remains at ca. 2 × 10−6 S (500 k impedance). Note that there is no indication of a second time constant or Warburg type element in Fig. 8 at the end of the experiment, which is reflected in a low value of the Wahrburg term after the fitting procedure (<10−6 S, Fig. 12). This is in strong contrast with the reference PET coating (Fig. 7), which shows a continuous rise in coating conductivity (1 × 106 M impedance for the first scan to 2 × 104 k for the last scan, i.e. an increase from 1 × 10−6 S to 5 × 10−4 S in time), and a significant Warburg type element evolving in time (from 4 × 10−6 S to 2 × 10−5 S, Fig. 12). The formation of blisters after sterilisation is not unique to acetic acid media, blisters have also been seen after sterilisation with NaCl solutions [1]. However, there is a profound difference in the shape of the EIS spectra, as well as in size and colour of the blisters. The
Fig. 15. Coating conductivity of PP during sterilisation.
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Fig. 16. Coating conductivity of PP during pasteurisation; closed circles first run, open diamonds second run: (a) heating part of cycle; (b) cooling part of cycle.
spectra during NaCl sterilisation typically show a characteristic low frequency time constant RQ with the order of the Q between 0.6 and 0.8. The conclusion of Ref. [1] was that during saline sterilisation corrosion occurs under a small coating defect, and propagation occurs via cathodic delamination, which is in line with what could be expected for organic coatings on steel [e.g. [13]]. Based on our present analysis, the sequence of events in the case of acetic acid sterilisation is thought to be as follows: 1. Ions permeate through the coating, as evidenced by the increasing coating conductivity. 2. The substrate surface corrodes, as evidenced by the discoloration. 3. The corrosion products create a blister under an almost intact coating, as evidenced by the low coating conductivity after blister formation. The blister formation is therefore a consequence of the corrosion at the polymer–steel interface, which also caused the discoloration, and not by cathodic delamination. The main difference between the reference PET and the PENmodified PET coatings is in the iron pickup after 2 weeks, in the evolution of a diffusion term with time for the reference PET coatings, and in the observation of microscopic cracks in the blisters. It is likely the terms are related to the same process, i.e. that the EIS term observed represents the diffusion of iron ions into the electrolyte. Significant iron pickup can only occur through relatively large pores in the coating, which can be linked to the observed micro-cracks. The EIS spectra suggest that cracking of the reference PET coating occurs during and after cooling down to ambient temperature. Crack formation in the coating is probably related to residual stresses from blister formation which are frozen by cooling down below Tg . The capacitance change with temperature (Fig. 13) is another indication that the glass transition for reference PET is less reversible than for the PEN-modified PET. The PP coating shows no glass transition in the relevant temperature region, and no blister formation is observed, and therefore no additional stresses are present which could lead to cracking of the coating. This, combined with a relatively low conductivity after cooling down (compared to the reference PET), explains the low iron pickup.
5. Conclusions The degradation of PET and PP coatings on steel during sterilisation in acetic acid media consists of three consecutive stages: discoloration of the substrate, blister formation and iron pickup. The discoloration is related to an increase in permeability of the coating, as evidenced by an irreversible increase in coating conductivity. This increase starts at 360 K for PET coatings and 380 K for PP coatings. Below these temperatures, changes in coating conductivity are reversible and are related to the physical (volume) expansion of the coating upon heating. The blisters are formed by corrosion of the substrate under an almost intact coating. This is in contrast to blister formation under saline conditions, which has a different mechanism (cathodic delamination). However, the increase in coating conductivity and blister formation is not the direct cause of iron pickup by the electrolyte. Iron pickup is related to the formation of microscopic cracks in the coating after cooling down to ambient temperature, due to residual thermal and mechanical stresses. Improving the coating can be done by modifying the coating to withstand the glass–rubber transition better. A strong increase in performance in all three aspects can be reached by using PP. References [1] B. Boelen, H. den Hartog, H. van der Weijde, Progress in Organic Coatings 50 (2004) 40–46. [2] A.C. Bastos, A.M.P. Simoes, Progress in Organic Coatings 46 (3) (2003) 220–227. [3] F. Deflorian, L. Fedrizzi, S. Rossi, Corrosion Science 42 (7) (2000) 1283–1301. [4] V. Lavaert, P. Praet, M. Moors, E. Wettinck, B. Verhegghe, Progress in Organic Coatings 39 (2–4) (2000) 157–165. [5] F. Deflorian, L. Fedrizzi, S. Rossi, Corrosion 55 (11) (1999) 1003–1011. [6] X. Zhang, B. Boelen, P. Beentjes, J. Mol, H. Terryn, H. de Wit, Progress in Organic Coatings 60 (4) (2007) 335–342. [7] A. Arkhireyeva, S. Hashemi, Polymer 43 (2) (2002) 289–300. [8] A. Tonneli, Polymer 43 (2) (2002) 637–642. [9] B. Boelen, R. Breur, H. den Hartog, H. van der Weijde, Proceedings Eurocorr., 2004. [10] J.V. Standish, H. Leidheiser Jr., Journal of Coating Technology 53 (678) (1981) 38–53. [11] D.W. van Krevelen, Properties of Polymers, third edition, Elsevier Science B.V., Amsterdam, 1990, pp. 88–94. [12] J.C. Bolger, A.S. Michaels, Proceedings Interface Conversion for Polymer Coatings, American Elsevier Publishing Company Inc., New York, 1968, pp. 3–61. [13] H. Leidheiser Jr., Mechanism of de-adhesion of organic coatings from metal surfaces, ACS Symposium Series 322 (1986) 124–135.