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Home  > Publications  > Blog  > Never ending story: bisphenol A

Tuesday, 25 February 2020

Never ending story: bisphenol A

Thursday, 5 September 2013 | Posted by: Sonja Schulte, European Coatings Journal

During the last weeks I came across several times with the uncomfortable topic "bisphenol A". For example Sid Harris mentioned it in his article in Focus on Powder Coatings where he reported on the use of Schiff based replacement for bisphenol A. Harris pointed out, that is it not only feasible as a potential safe alternative to BPA, but it also offers considerable improvements in both mechanical and chemical resistance properties.Great news! Howeveri In the February issue of Chemical & Engineering News Melody M. Bomgardner reported on the fact, that none of the alternatives developed by the coatings industry works for all foods and beverages. But no reason to throw in the towel! The coatings industry is on a good way and the end consumer will be happy when at one day all products will be free of BPA.

For years the use of BPA is under debate .In January 2013, the US-State California proposed, for the second time to list bisphenol a as a cause of reproductive toxicity under a state law called Proposition 65. Even if the allowed dose is too high to require warning labels on most products, the proposal adds another reason that people might want to avoid this molecule. This affects the coatings industry strongly, especially in the food packaging market segment. Because of this longstanding discussion a lot of companies are working on alternatives for BPA as Melody M. Bomgardner found out:

Ongoing research

"A review of patent filings and regulatory approvals shows that dozens of substances are in the pipeline. They are being developed by paint firms including Valspar, PPG Industries, and AkzoNobel, and by chemical firms such as Eastman Chemical, Cytec Industries, and Dow Chemical. Companies working on replacements for BPA-based epoxies are not keen to talk about product development efforts in this sensitive area. However, Jonathan Mason, associate R&D director at Dow, agreed to answer questions by e-mail. He summed up the challenge facing the industry this way: "The lowest-price, best-performing solution today is epoxy.” In addition, he predicted that no one formulation in the initial group of alternatives will work across all food and beverage types. Instead, a variety of new technologies will be required." In her article one can also find this table showing that alternative coatings systems perform not as well as the BPA containing epoxy coatings.


Schiff bases as an alternative

So, what about the above mentioned Schiff base epoxy resins? In the July-August Issue of Progress in Organic coatings (Vol 76 (7-8), pp 1040-1045) researchers at Polish Univeristies and Engineering Insitutes published an article entitled "Organic coatings based on new Schiff Base expoy resins”. The article describes the preparation of both Schiff base compounds and the epoxy resins containing azomethine groups. This is followed by details of a wide range of test methods including NMR, DSC, Epoxy Equivalent EE, electrospray ionisation mass spectroscopy ESI-MS, together with mechanical tests on the coatings. The coatings were then tested by applying them to glass to test hardness and steel to test adhesion, flexibility and impact resistance. The tests showed that the coatings based on synthesized epoxy resins have very good adhesion to metal substrates. Cross-cut adhesion gave zero removal but the results of the pull-off testing varied with the structure of the Schiff base used and the final processing method. The best results were observed in the coating containing epoxy groups and many free hydroxyl groups. All other mechanical properties were good and comparable to the best of the standard epoxy systems. Additionally, these coatings have good thermokinetic properties.

Probably this is the route to BPA-free epoxy coatings. We´ll see.

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Sonja Schulte
European Coatings Journal
Sonja Schulte
Editor-in-Chief, Science & Technology
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