Multianalytical Assessment of Armour Paints—The Ageing Characteristics of Historic Drying Oil Varnish Paints for Protection of Steel and Iron Surfaces in Sweden

Abstract

The characteristics of armour paints, historically used to protect ferrous industrial heritage, are explored. Amour paints contain lamellar and highly reflexive pigments of micaceous iron oxide (MIO) and metallic, leafing aluminium, bound in linseed oil and linseed oil–tung oil mixtures, on an inhibitive and soap-forming linseed oil primer (red lead). It is the first study of the binding media used for historical armour paints and investigates the chemical and physical ageing of armour paints using a multianalytical approach. Naturally aged examples are compared to accelerated aged replica armour paint, and to historical paints. The ageing and degradation reactions are assessed by complementary GC–MS and FTIR, together with measurements of wettability, hardness and surface colour. The historical paint formulations include linseed oils and alkyd binders. The results confirm that the leafing effect of aluminium pigments results in only a small concentration of binder at the surface: the paints studied reflect light and form a strong chemical and physical barrier. Linseed oils and tung oil mixtures have been proven to be suitable for the production of armour paints, but the evaluation of ageing and assessment of physical changes will require further investigation.

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

“…The results are reported in Figure 4 and Table 3 and show that the modern linseed oil varnish (OH-0), as well as both the manganese oxide varnishes (WHT-0, P-0), became milky after a very short time when immersed in distilled water; indeed, the appearance of the solid films began to change after only 1.5 h. In contrast, when 20 wt% tung oil was added to these varnishes (WT-0), the water absorption decreased significantly and was comparable to the stand oil (WS-0) and litharge varnish: their appearance was almost unaffected after more than 24 h. The results indicated that fresh films based on stand oils, any varnish with the addition of 20 wt% tung oil, and litharge varnish had high resistance to water exposure in immersion tests. They also indicated that the good performance of OH as binder in aged replica armour paint, as previously reported [2], is a result of the lamellar pigments' ability to delay the deterioration of the organic binder.…”

“…The solid films were, despite this, expected to contain some inhomogeneities. When the binder is mixed with pigment, the effects of the autophobicity of the air-blown varnish (OH) are less pronounced, as described in previous work [1,2].…”

“…The following commercially available drying oils were investigated to compare the influence of air blowing, heat bodying and the presence of stand oil in paints: (1) a modern, air-blown linseed oil varnish (OH), (2) a heat-bodied manganese oxide varnish (WHT), (3) a linseed stand oil with a viscosity of 50 dPa•s (WS) and (4) a mixture of (2) + 20 wt% bodied tung oil (WT). Details of the binders can be found in supplement A in [2]. Additional binders were also assessed using water immersion tests and included an oil boiled at high temperature with litharge addition (L), pyrolusite varnish (P) and a manganese borate varnish (MB).…”

“…Armour paints are paint systems that have shown remarkable performance and have lasted outdoors for over 85 years, as exemplified by the Oat Mill in Stockholm [1]. 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.…”

“…In its original concept, 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). With ageing, 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].…”