Multi-Analytical Assessment of Bodied Drying Oil Varnishes and Their Use as Binders in Armour Paints

Francesca Caterina Izzo,Austin Nevin,Arja Källbom

doi:10.3390/heritage4040189

Abstract

The characteristics of commercially available refined and bodied linseed and tung oils, used as binders in the production of armour paints after historic recipes, are explored. Employed as anticorrosive paints mainly from the 1920s to 1960s, armour paints are greener alternatives that can be used for protection in industrial heritage conservation. Using a multi-analytical approach, chemical and physical properties of the fresh oils and solid films before and after accelerated ageing (ISO 16474-2:2013) were investigated to better understand which features are beneficial for the technical function of armour paints. Tests included measurements of density, the refractive index, insoluble impurities, alkaline impurities, the water content, the iodine value, the saponification value, the free fatty acid concentration, the acid value, the peroxide value and colour (Lovibond) and cold tests. The characterisation of the fresh oils using molecular analysis with FTIR and GC-MS revealed the complexity of the commercial formulations, for which additions of semi- and non-drying oils were detected. The results show that organic paint binders follow complex chemical reactions (such as oxidation and decrease of unsaturation being variable or swelling following water-immersion tests), with implications for their suitability for use in protection.

Highlights

Armour paint is applied to ferrous surfaces and consists of two layers of a primer of red-lead linseed oil paint and two layers of lamellar specularite and leafing aluminium pigments bound in bodied linseed and a tung stand oil mixture
The highest acid value was observed for the stand oil We expected the highest peroxide stand oil (WS-L), which was a result of the refinement process
It was observed that stand oil was able to etch the mild steel, since marks were seen visually in the application where the oil was applied with a pipette to the sheet before it was distributed over the surface

Summary

Introduction

This work aims to investigate the ageing characteristics of binders alone in order to reveal more information about their function as part of armour paints. The concept and ageing characteristics of aluminium-pigmented armour paint—a historical, anticorrosive paint type from the early 20th century—entail excellent protection for weather-exposed ferrous substrates [1,2]. Armour paints are of particular relevance for historical ferrous objects for which there is a need for sustainable and greener paint systems and a desire to replicate historical practices in applications. Armour paint is applied to ferrous surfaces and consists of two layers of a primer of red-lead linseed oil paint and two layers of lamellar specularite and leafing aluminium pigments bound in bodied linseed and a tung stand oil mixture (i.e., drying oil varnishes). Historical and replica paints (based on historical recipes) become highly oxidised, but natural ageing for four years in southern Sweden and accelerated ageing (according to ISO 16474-2:2013) were found to result in approximately equal ageing characteristics, and a high proportion of unsaturated fatty acids remained in the sub-surfaces of the replica paints [2]

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Conclusion