characterization of different types of lacquers used in food packaging

Acta Alimentaria, Vol. 36 (1), pp. 27–37 (2007) DOI: 10.1556/AAlim.36.2007.1.5

CHARACTERIZATION OF DIFFERENT TYPES OF LACQUERS USED IN FOOD PACKAGING: LACQUER ADHESION TESTS A. NINČEVIĆa*, A. PEZZANIb and G. SQUITIERIb a Faculty

of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, 10000 Zagreb. Croatia b Stazione Sperimentale per l’Industria delle Conserve Alimentari, Via Nazionale, 121/123, 84012 Angri (SA). Italy (Received: 22 September 2005; accepted: 28 October 2006)

The application of organic coatings is widely used in the production of metallic food containers to protect metal against corrosion and to avoid metal-food contact. The protective action of a lacquer film is determined by its physical-chemical characteristics, the method of application and its compatibility with the packed products. The goal of this study was to investigate the protective action of three groups of coatings (water based lacquers, UV cured lacquers and conventional epoxyphenolic lacquers) applied on four lots of different metallic substrate. In characterization, the methods used are dry adhesion, wet adhesion and wet adhesion after sterilization treatments of samples in food imitating solutions. The results show that the best characterization of lacquers is given by the last method, which allows better differentiation between lacquers/substrate systems. Keywords: coatings, tinplates, food packaging, adhesion

Tinplate has a long and successful record as a packaging material. Its special characteristics, e.g. rigidity, tightness and impermeability are suitable for preservation and protection of food products. Despite the increasing use by the canning industry, food products may become contaminated with tin and iron during the corrosion process (CATALÁ et al., 1998; BASTIDAS et al., 2000). Development of corrosion depends on many factors such as composition of metallic material and composition of food. In order to stabilize the tinplate surface and to prevent corrosion during metal-food contact, one of most commonly used alternative is the application of organics coatings (lacquers). Frequently used coatings in the food canning industry are conventional

* To whom correspondence should be addressed. Fax: +385–1-4836-083; e-mail: [email protected] 0139-3006/$ 20.00 © 2007 Akadémiai Kiadó, Budapest

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NINČEVIĆ et al.: LACQUER ADHESION TEST

lacquers with high quantities of organic solvent (or solvents), up to 80% of total weight. The innovative technologies on which research is being concentrated are those using lacquers with low solvent content (BUSATO, 2002; BARILLI et al., 2003; FUREY, 2004), i.e. low volatile organic compounds (low VOC). The technologies based on the partial or total elimination of organic solvent include water-based lacquers, high-solid lacquers, UV (ultra violet) cured lacquers and powder-based lacquers. In this research we studied UV-cured lacquers, water-based lacquers and standard epoxyphenolic lacquers as reference. Their behaviour was evaluated in order to find best tinplate/lacquers system, which can be applied to the vegetable canning industry. To this purpose, different metallic substrates and three above mentioned lacquers groups were investigated. This part of the work shows the results of metallic substrate composition and lacquer adhesion on metallic substrates in test solutions, which simulate food environment. 1. Materials and methods 1.1. Characterisation of metallic materials The following metallic materials were analysed: a) Tinplate D5.6/2.8 (type 3), b) Tinplate D5.6/2.8 (type 4), c) Tinplate D11.2/2.8 (type 6) and d) Tin free steel (type 5). The properties of each tinplate (passivation layer) and tin free steel (TFS) were determined using electrochemical galvanostatic method. Total chromium (CrT) and metallic chromium (CrM) content in TFS layer were determined applying current density of 1 mA cm–2 on the sample (specimen) in a solution of 53 g l–1 sodium hydrogen carbonate (NaHCO3). Before CrM analysis, specimens were washed in boiling 7.5 mol l–1 sodium hydroxide (NaOH) solution for 1/2 min (ITALIAN NORM 2004b). The CrT content (BRITTON, 1975) in the tinplates was analysed on the specimen in a phosphate buffer solution at pH 7.4, 8 g l–1 sodium dihydrogen phosphate (NaH2PO4×2H2O) and 9.5 g l–1 di-sodium hydrogen phosphate (Na2HPO4), applying current density of 25 µA cm–2. The CrM content in the tinplates (AUBRUM & PENNERA, 1976) was determined after washing the specimens in boiling 1.0 mol l–1 NaOH solution for 1 min. Analysis was performed applying current density of 50 µA cm–2 in solution of 50 g l–1 NaH2PO4×2H2O, which was brought to pH 5.0 by adding 10 mol l–1 NaOH. The chromium oxide (CrOx) content as Cr (III) in the TFS and tinplates was determined as the difference between CrT and CrM. The tin content of tinplates (ITALIAN NORM, 2004a) was analysed applying current density of 4.2 mA cm–2 on specimens in solution of 1 mol l–1 hydrochloric acid (HCl). Before measurements, the specimens were degreased by acetone. All experiments were carried out in a standard three electrodes configuration, using platinum foil as counter electrode and saturated calomel (SCE) as reference electrode. The whole system was linked to a 273 EG&G potentiostat/galvanostat. Metals content of each sheet type are reported in Table 1 and 2.

Acta Alimentaria 36, 2007


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Table 1. Chromium content in TFS and tinplates

Sheet TFS D5.6/2.8 D5.6/2.8 D11.2/2.8

Chromium content (mg m–2) Internal surface External surface CrM CrOx CrT CrM CrOx 57.4 1.1 62.8 59.9 2.9 4.6 1.2 5.4 3.2 2.2 4.1 3.6 7.9 4.1 3.8 5.0 2.4 5.9 3.8 2.1

Type CrT 58.5 5.8 7.7 7.4

5 3 4 6

Table 2. Tin content in tinplates

Sheet

Type

D5.6/2.8 D5.6/2.8 D11.2/2.8

3 4 6

Tin content (g m–2) Internal surface External surface Total tin Tin in alloy Free tin Total tin Tin in alloy Free tin 6.27 2.15 4.12 2.89 0.93 1.96 5.17 1.03 4.14 2.17 1.01 1.16 10.44 0.96 9.48 2.47 0.93 1.54

1.2. Characterization of lacquers 1.2.1. Lacquer types. Sheets from TFS and tinplates for making the cans were coated using three groups of lacquers (Table 3). Their behaviour was examined using different experimental tests. Table 3. Analysed lacquers types (lacquer code name) Lacquers (surface) Water based (internal) Water based (internal) UV-cured (external) UV-cured (external) Epoxyphenolic (internal) Epoxyphenolic (external)

Body, ends INT-A INT-B UV-A UV-B INT RIF RIF

Stripe INST 1 INST 2 EXST 1 EXST 2 INST RIF RIF ST

1.2.2. Dry and wet adhesion test. Adhesion of lacquer was determined under dry and wet adhesion conditions (MONTANARI et al., 1989). Dry adhesion was performed using a sheet of polyamide adhesive tape (2.54 cm by 7 cm), which was pressed onto the samples lacquered surface and then removed after 2 min (cross-cut test). For wet adhesion, samples were immersed in a pH 3.0 aerated, citrate buffer and were polarized at –2.0 V v.s. SCE for 30 min. A sheet of adhesive film was put on lacquered, dried samples (20 °C) and after 2 min it was removed. In both tests, the adhesion was measured by the quantity of lacquers, which it detaches, and if the adhesion is good, there should be no detaching. For dry adhesion, the area removed was evaluated by standardised scale between 0–10 (ranging from complete intactness to total


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detachment). The results of wet adhesion test were expressed as a percentage of total detachment to complete intactness (0–100%); the higher the value obtained, the better the adhesion. 1.2.3. Adhesion test in food imitating solutions. Adhesion properties were also tested on the three lots of sterilised (at 120 °C for 1 h in autoclave) samples under these three conditions: specimens without incision (the specimens as such), specimens with incision before sterilization (pre-incision) and specimens with incision after sterilization (post-incision). The sterilization consists of total immersion of specimens in foodimitating solutions (internal and external). The internal chemical solutions used were: a) 5 g l–1 acetic acid (C2H4O2) and 50 mg l–1 sodium meta-bisulphite (Na2S2O5) at pH value of 3.05, b) 5 g l–1 C2H4O2, 15 g l–1 lactic acid (C3H6O4) and 10 mol l–1 NaOH for adjustment of pH to 3.87, c) 10 g l–1 citric acid (C6H8O7) and 20 mg l–1 sodium nitrate (NaNO3) at pH value of 3.95, d) 10 g l–1 C6H8O7 and 20 g l–1 ascorbic acid (C6H8O6) at pH value of 3.96, e) 10 g l–1 C6H8O7, 13 g l–1 sodium chloride (NaCl) and 10 mol l–1 NaOH for adjustment of pH to 4.00, f) 3 g l–1 L(–) cysteine (C3H7O2N) and 0.2 mol l–1 sodium monophosphate (Na3PO4) at pH value of 6.21, g) distilled water. The external chemical solutions used were: a) water, b) 10 g l–1 NaCl, c) 0.1 mol l–1 NaH2PO4 at pH value of 8.64, d) 0.1 mol l–1 NaHCO3 at pH value of 8.83, e) 0.1 mol l–1 Na2HPO4 at pH value of 9.76. After sterilization in internal and external solutions, an adhesive tape (2.54 cm by 7 cm) was applied to the surface of dried sample for 2 min. At the end of this time, the adhesion was measured as a percentage of removed lacquers (see 1.2.2). 2. Results and discussion 2.1. Dry and wet adhesion tests Adhesion is a phenomenon, which involves the lacquer-substrate interface (MONTANARI et al., 1989; BARILLI et al., 2003). One of the major factors affecting adhesion and coating performance is the surface quality of the metallic plates. Different amounts of tin, tin oxide and trivalent chromium oxide and chromium metal compounds can be formed on the substrate surface during the passivation treatment (BASTIDAS et al., 2000). This process provides elimination of tin oxide formed during the production of metallic plate. If there is high content of chromium and its compounds in the passivation layer, the tin oxide grown is reduced and lacquer adhesion becomes better (MONTANARI et al., 1989). In this work, the results of metallic composition (Table 1 and 2) were correlated with results of lacquer adhesion to find the explanation of lacquers’ behaviour. Since the results of dry test showed excellent adhesion for all analysed lacquers, a totally different evaluation was obtained with wet adhesion test (Table 4).



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Table 4. Wet adhesion test of lacquers on internal and external surface of TFS and tinplates Adhesion (%) Sheet types

Lacquer types INT-RIF INT-A INT-B RIF UV-A UV-B

3 80 60 80 100 20 0

4 60 80 80 100 0 0

5 100 100 100 100 100 100

6 80 80 80 100 20 0

The results of internal surface show that TFS is the best metallic substrate for all applied lacquers. The results of type 3 confirm that low chromium content is connected with less adhesion of lacquers, especially for INT-A type. Type 4 shows similar adherence behaviour as type 3. For type 4, we expected better adhesion, because in the passivation layer higher chromium content is present (7.7 mg m–2). The best adhesion was found for type 6, compared with the two other tinplates. Probably, high chromium content (7.4 mg m–2) during lacquer adhesion also influences high tin content (10.44 g m–2). The results of external surface show that TFS is also the substrate with the highest adhesion for all external lacquers. The type 3 and 6, with low chromium and higher tin contents (Table 1 and 2), show better adhesion for UV-A lacquer than type 4. In this case, the higher chromium content (7.9 mg m–2) does not provide good adhesion result. This confirms that not only metallic composition and its content, but also other factors are colligated with unexplained behaviour of lacquers. We can propose that probably surface roughness (pores and scratches of varying size) of the substrate, thickness of the metallic coating, lacquer type, which is not compatible with metallic plate type, may influence lacquer adhesion positively or negatively. In the case of stripe lacquers, which were applied only on sheet type 3, obtained results show excellent behaviour for both adhesion tests (Table 5). Table 5. Dry and wet adhesion tests of internal and external stripe lacquers on tinplate D5.6/2.8 (types 3) Lacquer code name INST 1 INST 2 INST RIF EXST 1 EXST 2 RIF ST

Dry adhesion 0 0 0 0 0 0

Wet adhesion (%) 100 100 100 100 100 100

2.2. Adhesion tests of lacquers in food imitating solutions 2.2.1. Adhesion test of internal lacquers in food imitating solutions. The INT-B lacquer has excellent adhesion (100%) for all sheets types in all internal food imitating solutions in three sterilization tests. The same effect was observed for INT-RIF lacquer. The


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behaviour of both of them was similar for all experimental tests and will not be discussed further. Only with lactic acid solution (pH=3.87), specimens treated with preincision sterilization gave adhesion of 20% for INT-RIF lacquer. A

B

C

Fig. 1. Adhesion of INT-A lacquer on: A) without incision, B) pre-incision and C) post-incision sterilization treated specimens in internal food imitating solutions. □: pH=3.05; etc.



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In the case of INT-A lacquer, all sterilized specimens, under three conditions, gave relatively similar results. It is obvious that specimens without incision (Fig. 1A) showed the highest adhesion percentage when compared with two other types of pre-incision (Fig. 1B) and post-incision (Fig. 1C) specimens. The best behaviour of INT-A lacquer for specimens without incision was found in solutions with pH values 6.21, 3.95, 4.00 and distilled water. Good adhesion was also obtained in the solutions with pH values of 3.05 and 3.96, while solution with pH value of 3.87 gave the worst results. The experimental data also show that the excellent metallic substrate for INT-A lacquer is type 4 followed by type 6 tinplates. With this kind of lacquer, TFS has less adherence. In the cases of pre and post-incision specimens, the best adhesion was found in solutions with pH values of 3.95 and 6.21, while the worst in solutions with pH values of 3.96 and 3.87. Under these test conditions , the best behaviour of adherence was found in tinplates substrate types 4 and 6, while in TFS it was the worst. The obtained results have also shown that the same lacquer type could be more or less adhesive in the solutions with very similar pH-values (3.87, 3.95 and 3.96). This considerable difference of lacquer adhesion is associated with different chemical composition of food-imitating solutions. For example, the components of solutions with pH-values of 3.87 and 3.96 are aggressive for INT-A lacquer. Its adhesion is very low. 2.2.2. Adhesion test of external lacquers in food imitating solutions. The UV-B lacquer offers the highest adhesion (100%) in all external solutions for all lots of sterilization treated specimens. The RIF lacquer (Table 6) shows excellent adhesion on type 4, 5 and 6 sheet plates and worst adhesion of 20% on type 3 in solutions with pH values of 8.64, 8.83 and 9.76. Poor results were obtained with the same lacquer in solutions on pre and postincision treated specimens. (Table 6). Table 6. Adhesion of RIF lacquer to specimens in external food imitating solutions without incision, or pre and post-incision sterilization treated ones Adhesion (%)

External solutions NaCl Water pH=8.64 pH=8.83 pH=9.76

Without incision 3 100 100 20 20 20

4 100 100 100 100 100

5 100 100 100 100 100

Pre- and post incision Sheet types 6 3 100 100 100 100 100 20 100 20 100 20

4 100 100 60 40 20

5 100 100 100 100 100

6 100 100 20 20 0

The behaviour of the last analysed lacquer (UV-A), on the specimens without incision, was identical to UV-B type (100% of adhesion). The only difference was the result of 40% adhesion for type 3 in solution with pH value of 9.76. The specimens with pre and post-incision showed the same adherent behaviour (Table 7). The best adhesion was found on type 6, slightly worse on types 5 (pH=9.76) and 4 (pH=8.83). The worst adherent substrate was type 3, particularly in the solutions with pH values of 8.83 and 9.76.


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A

B

C

Fig. 2. Adhesion of stripe lacquers on: A) without incision, B) pre-incision and C) post-incision sterilization treated specimens in external food imitating solutions



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Table 7. Adhesion of UV-A lacquer on the pre and post-incision sterilisation treated specimens in external food imitating solutions External solutions NaCl Water pH=8.64 pH=8.83 pH=9.76

Adhesion (%) Sheet types 3 100 100 80 20 20

4 100 100 100 60 100

5 100 100 100 100 80

6 100 100 100 100 100

2.2.3. Adhesion of stripe lacquers in internal and external food imitating solutions. All internal stripe lacquers show quite a good adhesion of 80% under the conditions of three sterilization treatments. The results of external measurements have shown that lacquers adhesion decrease in following order: EXST 1, EXST 2 and RIF ST for the specimens without incision (Fig. 2A). The same order for lacquers was found for preincision specimens (Fig. 2B), but with smaller values of adhesion. In the case of postincision specimens (Fig. 2C), RIF ST lacquer gave the best adhesion, while EXST 2 and especially EXST 1 showed the least adhesive and worst protective properties of metallic substrates. 3. Conclusions The adhesion tests, e.g. dry adhesion, wet adhesion and wet adhesion in internal and external food imitating solutions were applied as a tool to classify lacquers to be used on tinplates and TFS for making the cans in vegetable industry. Sterilization, an important variable in the packaging process, was involved in characterization. . 3.1. Dry and wet adhesion tests Water-based lacquers, UV-cured lacquers and epoxyphenolic lacquers have shown excellent dry adhesion on all samples taken into consideration. Wet adhesion of water based lacquers (internal surface) showed that INT-B type provides the best results on all substrate types. Wet adhesion of UV-cured lacquers showed very poor results. Reference, RIF type, was the best lacquer in external groups of coatings. Wet adhesion of all internal and external stripe lacquers provided excellent results (100% of adhesion). Dry and wet adhesion results have also shown that the best adhesive substrate is TFS for all internal and external lacquers, and D5.6/2.8 (type 3) for all stripe lacquers.


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3.2. Adhesion tests of lacquers in food imitating solutions 3.2.1. Adhesion test of internal lacquers in food imitating solutions. It is obvious that INT-B lacquer presented the best resistance in contact with internal food imitating solutions. Its adhesion was 100%. The pre and post-incision sterilization treatments gave very limited adhesion for INT-A lacquer, especially in food solutions with very low pH values, 3.87 and 3.96. The chemical compositions of these solutions and conditions of sterilization treatments are the factors, which are associated with adhesion. Obviously, the aggressiveness of these solutions makes detachment of lacquer from substrate easier. 3.2.2. Adhesion test of external lacquers in food imitating solutions. The best wet adhesion result was found for UV-B cured lacquers in all external food imitating solutions. Similar result was obtained with UV-A cured lacquer in sterilization test without incision of samples. The pre and post-incision treatments provide worse adhesion results, especially in solutions with higher pH values (8.64, 8.83 and 9.76). The use of UV-A lacquer in mentioned solutions is not recommended. Obviously, the augmentation of pH values influences reduction of lacquer adhesion. This conclusion is also confirmed with the behaviour of RIF (epoxyphenolic) lacquer after sterilization treatments with pre and post-incision samples. 3.2.3. Adhesion of stripe lacquers in internal and external food imitating solutions. All water based stripe lacquers have shown quite good adhesion (80%) in contact with all internal food imitating solutions. The same results were obtained with reference lacquers as well. The UV cured stripe lacquers in contact with external food imitating solutions have shown better adhesion results in sterilization treatments without incision and preincision, compared with reference epoxyphenolic lacquer. In general, all external stripe lacquers have shown very poor adhesion results. It is evident that D5.6/2.8 (type 3) is not an adequate choice as substrate. In conclusion, the dry adhesion test of all different samples showed no difference. The wet adhesion measurement was a more useful tool to study the behaviour of every different interaction between substrate-lacquer systems. Finally, the most sensitive test that allows differentiation between lacquers and tinplate is wet adhesion after sterilization treatment of samples in contact with food simulates. This test provides meaningful information about lacquer-substrate quality in the short term. References AUBRUN, P. & PENNERA, G.A. (1976): Coulometric determination of metallic chromium in passivation films on tinplates. Rev. Metall., 73, 745–752. BARILLI, F., FRAGNI, R., GELATI, S. & MONTANARI, A. (2003): Study on the adhesion of different types of lacquers used in food packaging. Prog. Org. Coat., 46, 91–96. BASTIDAS, J.M., CABAÑES, J.M., CATALÁ, R. (2000): Corrosion behavior of lacquered tinplate cans in contact with cockles (Cardium edulis) in brine solution. Corrosion, 56, 429–432.



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BRITTON, S.C. (1975): Examination of the layer produced by chromate passivation treatments of tinplates. Brit. Corros. J., 10, 85–90. BUSATO, F. (2002): Powder and waterborne coatings 2001–2010 – Is past growth sustainable? New technological developments and the impact on future markets, a world overview. Macromol. Symp., 187, 17–21. CATALÁ, R., CABAÑES, J.M. & BASTIDAS, J.M. (1998): An impedance study on the corrosion properties of lacquered tinplates cans in contact with tuna and mussles in pickled sauce. Corr. Sci., 40, 1455–1467. FUREY, G. (2004): Innovation revives UV coatings for food cans. Canmaker, 17, 29–30. ITALIAN NORM (2004a): Electrochemical method for determining tin coatings mass. UNI EN 10202, Appendix D, pp. 24–27. ITALIAN NORM (2004b): Methods for determinations of metallic chromium and chromium oxide on the surface of electrolytic chromium and chromium oxide coated steel. UNI EN 10202, Appendix E, pp. 28–34. MONTANARI, A., MILANESE, G. & CASSARÀ, A. (1989): Techniques for evaluating the organic coatings used to protect metal cans. Ind. Conserve, 64, 332–342.
