Surface modification of Cr(III) packaging substrates for enhanced adhesion via citric acid processing
Introduction
Metal packaging is an important means by which food can be stored in modern society, resulting in a global market of around $110 Bn in 2017 [1,2]. Steel based metal packaging boasts many benefits, in that it is robust, recyclable, achieves many years of shelf life, reducing waste for both producer and consumer, and has established products with whole supply chain integrity [3,4]. Traditionally, steel packaging has been produced either using tinplate, having a layer of tin approximately 0.2-2µm thick or electro chromium coated steel (ECCS) which has a 10-30 nm layer of chromium/chromium oxide, produced using a Cr(VI) electroplating process [5].
The steel packaging industry, as with other European industries, faces the challenge of removing Cr(VI) from industrial processes due to legislative moves, driven by health implications surrounding Cr(VI) species [6]. To this end, novel methods to electroplate steel with chromium species have be developed, using Cr(III) species in the electroplating process [7], which replace the Cr(VI) species used in the production of ECCS. The novel substrate is one that requires the same level of characterisation as the well-established ECCS. To this end, previous work carried out using iterations of this novel developmental Cr(III) derived substrate showed that it is less resistant to corrosion when compared to the Cr(VI) derived substrate [8].
The primary usage for this substrate is to be for coated applications where an organic lacquer is applied and cured prior to formation and filling of a packaging can. The coating provides a protective layer during handling, can forming, filling and cooking processes. Bisphenol-A (BPA) has been a key monomer in the formation for epoxyphenolic lacquers, providing a flexible and versatile coating. However, owing to legislative moves in some European territories, BPA must be removed from food contact materials [9]. Coating chemistry technologies based around polyester (BPANI - BPA non-intent) have been developed [10,11], which have been found to be perform poorly compared to the incumbent epoxy phenolic materials, [12, 13].
Following filling, the can contents is cooked and sterilized under temperature and pressure. Under this aggressive environment the novel, developmental Cr(III) substrate performs less favourably than ECCS [14]. When the substrate is subjected to the sterilisation process, corrosion is induced at the surface of the substrate, owing to the aggressive chemical environment that is contained within the can at high temperature and pressure [15] which leads to failure at the lacquer / substrate interface. There is significant interaction between the foodstuff (through controlled simulant) and the substrate/coating system, showing the novel Cr(III) substrate is a promising alternative to ECCS but with development required to show equivalent performance [14]. A feature of this previous work is the advantage of using citric acid in the sterilisation process, seemingly mitigating the effects of sodium chloride [12,14] in improving the lacquer / substrate adhesion. This was attributed to an increase in chromium oxide species on the surface and a reduction of iron species compared with the pre-sterilisation surface chemistry.
Citric acid has been examined as a pre-treatment or passivation technique for stainless steel [16], as a replacement for nitric acid. The purpose of the passivation technique is to remove anodic surface contaminants such as iron compounds from the surface through chemical dissolution, leading to the formation of a dense nanometre thick chrome oxide layer [17], improving corrosion resistance. Thin oxide films of cerium oxide [18,19], praseodymium [20] and Neodymium [21] result in similar improved corrosion performance of steel coated substrates. These however require the deposition of another metal oxide to the steel surface, as opposed to oxidation of the metal which is already present. In situ oxidation of the chrome offers other advantages. Chrome has many years of proven food safety, it has proven economic scalability and the deposited chrome metal forms part of the physical barrier between the foodstuff and steel substrate. Treatment of the surface using citric acid is particularly interesting for food packaging application due to it being present in the juices of citrus fruits, it is inherently food safe. Due to its biological source and biodegradable nature, it is a more sustainable and less toxic alternative to nitric acid.
There is a potential benefit of applying a citric acid based pre-treatment on the novel Cr(III) plated steel substrate during manufacturing in order to enhance the substrate / lacquer interface integrity. The aim of the investigation is three-fold. Firstly, it aimed to understand the mechanism by which the adhesion of an organic lacquer is improved by a citric acid based pre-treatment. Secondly the study aimed to identify the interrelationship between the citric acid pre-treatment and other foodstuff simulants. Finally, it aimed to assess the feasibility of applying a citric acid pre-treatment for Cr(III) coated steel packaging substrates. This was achieved by studying surface chemistry, the electrochemical and corrosion properties, topological properties as well as the macro scale adhesion properties that have the greatest impact for the can manufacturers and fillers.
Section snippets
Materials
Cr(III) electroplated substrate was supplied as a developmental substrate from TATA steel packaging, using Cr(III) in the electroplating bath rather than the Cr(VI) species used in ECCS production. The developmental substrate was produced on a full-scale electroplating line during parameter optimisation tests and is hence not commercially available. ECCS used as a reference material was also supplied by TATA. Neither substrate has free Cr(III) or Cr(VI) present on the surface after the
Results
The strategy employed in the study was to firstly explore the interaction between the presence of citric acid as a passivation treatment in the simulant solution and the presence of increasing levels of NaCl. This would identify the envelope of passivation that could be obtained within the sterilisation process. These findings then guided the study of the effectiveness of citric acid pre-treatment and its operational process window which was established by macro adhesion measurements coupled
Discussion
Given the processing times required in practice are typically in the order to seconds in order to maintain production speeds of 100m/min (1.7 m/s) then the application of this technique to Cr(III) coated packaging steel at the point of manufacture may be impractical. A 3m tank would result in an exposure time of less than 2 seconds which is insufficient to produce any positive improvement in performance. There would be little room for increasing the reaction rate in order to reduce exposure
Conclusion
The treatment of next generation packaging steels based on Cr(III) deposition chemistry with citric acid has been shown to improve the adhesion of lacquers during the subsequent sterilisation process. Through surface oxidation the adhesion of the lacquers is enhanced, but there exists an optimum window of citric acid treatment which is dependent on both the acid concentration and temperature. Under optimum conditions, adhesion can be significantly improved but excessive treatment can lead to
CRediT authorship contribution statement
Andrew Allman: Conceptualization, Formal analysis, Investigation, Methodology, Writing - original draft. Jordan Whiteside: Formal analysis, Investigation, Methodology. Eifion Jewell: Conceptualization, Methodology, Writing - review & editing, Supervision. Chris Griffiths: Formal analysis, Investigation. Neil McMurray: Supervision. Arnoud de Vooys: Supervision, Resources, Writing - review & editing.
Declaration of Competing Interest
None.
Acknowledgements
The authors would like to thank Crown Packaging, EPSRC (grant ID EP/L015099/1), TATA Steel, and the Welsh European Funding Office (WEFO) for supporting the work described in this article.
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