Apoorva Ambarkar - Microstructure of packaging coatings

Presentation - pdf

Apoorva Ambarkar*‡§, Steve Edmondson‡, Keimpe van den Berg§
‡: School of Materials, The University of Manchester, Manchester, M13 9PL, England, UK
§ :AkzoNobel Coatings B.V, Sassenheim, 2171 AJ, The Netherlands

Organic coating systems play an important role in the protection of metallic surfaces from corrosion. It is often observed that coatings without an apparent defect, fail unexpectedly during use, allowing corrosion to occur in specific regions and not uniformly across the surface. Thus, fundamental understanding of organic coating microstructure may hold the key to understanding failure mechanisms in intact coatings. Despite prior efforts to characterize relevant coating microstructures, direct correlations to performance remain limited. To obtain further insight requires advanced characterization tools that allow simultaneous mapping of multiple properties across the surface, such as crosslinking, stiffness/hardness, water uptake etc. AFM-IR, a hybrid technique between AFM (Atomic Force Microscopy) and infrared spectroscopy, allows advanced characterization of organic coatings by mapping their infrared absorptions across the underlying microstructure.

 In this study, two variants of BPANI (bisphenol A non-intended) food-can coating that are polyester based were chosen; Type A (chemical resistant type coating) and Type B (flexible type coating). Polyester based coating such type A & Type B are complex systems consisting of polyester as binder and several crosslinkers such as phenolics, Benzoguanamines, isocyanate etc., By studying their microstructure, it was discovered that Type B coating appears to be phase separated while Type A coating appears homogenous. Efforts were made to understand this distinction in the two variants, as it may be contributing in poor chemical resistance of Type B coating. AFM IR was used to map the chemical absorbance in the different phases of Type B coatings. The near industrial systems give a truer picture of microstructure but also introduce several complexities due to the complex chemistry involved between the binders & crosslinkers. Data processing was used to resolve height complexities and improve data interpretation. Simplified formulation made by varying the molar ratio of functional group between binder and crosslinkers further revealed crucial information on phase separation. AFM IR allowed identifying the phases in model systems and onset of phase separation. AFM IR technique has been useful in understanding chemical heterogeneity in complex systems.

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From  Dr Tolutope Siyanbola : What is the ratio of your polyol to that of your crosslinker (IPDI)?
Answer: in the full formulation the isocyanate is 4.5 time that of the polyester in terms of functional groups, by weight composition the polymer is 70% and the isocyanate is 16%.

From  Dr Tolutope Siyanbola : Did you carryout any formulation with aromatic crosslinker?
Answer: No, we haven’t.  

From  Koray Yıldırım : Thank you very much Apoorva, great presentation, very enlighting. Do you think in some afm images at the beginning of your presentation, distrupted morphologies are caused by air release? Did you use any levelling agent?
Answer: there are no additives in the formulation. It may be what the question suggest, we are not using levelling agent but it doesn’t come out in the stiffness value. Maybe solvent evaporation.