Alkyd paint and paint driers.†
1.1 Introduction
1.1.1 A brief history of paints and coatings[1-4] Paints and coatings occupy a prominent place in the cultural history of mankind. People have always been fascinated with colors and used paints to decorate and beautify themselves and their environment. Archaeologists found pigments and paint grinding equipment in Zambia (Southern Africa), thought to be between 350,000 and 400,000 years old. The oldest testimony of artistic activity was found in the south of France where some 30,000 years ago our prehistoric ancestors decorated the walls of their cave dwellings with stunning animal drawings. The ancient paints consisted of animal fat and colored earth or natural pigments such as ochre. Hence, they were based on the same principle as the paints that are used today - a binder and a coloring agent. Around 3000 BC, Egyptians began using varnishes and enamels made of beeswax, gelatin and clay – and later protective coatings of pitch and balsam to waterproof their wooden boats. About 1000 BC, the Egyptians created varnishes from Arabic gum. Independently, the ancient Asian cultures developed lacquers and varnishes and by the 2nd century BC, these were being used as coverings on a variety of buildings, artwork and furnishings in China, Japan and Korea. The ancient Chinese knew how to make black lacquer (the true predecessor of modern coatings) from the sap of the lacquer tree Rhus Vernicifera. In India, the secrete of the lac insect Coccus lacca was used to produce a clear coat to beautify and protect wooden surfaces and objects. The early Greeks and Romans also relied on paints and varnishes, adding colors to these coatings and applying them on homes, ships, and artwork. It was not until the industrial revolution and ensuing mass production of linseed oil, however, that the production of modern house-paints began. Over the decades, formulations of paints and coatings have become more and more sophisticated. Today, coatings not only protect and beautify the substrate. They also have functional properties: they are used as anti-skid surfaces, they can insulate or act as an electrical conductor, they can reflect or absorb light, etc. Paints and coatings play an indispensable role in our modern world and cover virtually everything we use from household appliances, buildings, cars, ships, airplanes to computers, microchips or printed circuit boards.
†
This chapter is based on: R. van Gorkum and E. Bouwman, Coord. Chem. Rev. 2005, in press
Chapter 1
1.1.2 Alkyd paint A modern paint is a complex mixture of components (Table 1.1).[5] Oil paint, the oldest form of modern paint, uses a binder that is Table 1.1: derived from a vegetable oil, such as obtained Typical paint composition from linseed or soya bean. In alkyd paint, the Component Weight-% binder is a synthetic resin, which is called an binder 30 alkyd resin. The term alkyd was coined in the organic solvent 27 early days and originates from the AL in water 10 polyhydric ALcohols and the CID (modified to pigments 19 KYD) in polybasic aCIDs. Hence, in a chemical extenders 12 sense the terms alkyd and polyester are additives 2 synonymous. Commonly, the term “Alkyd” is limited to polyesters modified with oils or fatty acids. A typical alkyd resin is prepared by heating for example linseed oil, phthalic acid anhydride and glycerol to obtain a fatty-acid containing polyester, as schematically shown Fig 1.1. Paints based on alkyd resin binders are usually solvent-borne paints, common solvents being white spirit (a mixture of saturated aliphatic and alicyclic C7-C12 hydrocarbons with a content of 15-20% (by weight) of aromatic C7-C12 hydrocarbons), or xylene. Although European legislation drives paint development towards water-borne systems, in order to reduce the amount of VOC’s (volatile organic compounds) in the atmosphere, solvent-borne paints often show a number of advantages over water-borne paints. Examples are: easier application properties, wider application and drying tolerance under adverse conditions (low temperature, high humidity) and a higher level of performance on difficult substrates, such as heavily stained or powdery substrates. As a result solvent-borne coatings will not be totally replaced by water-borne coatings in the foreseeable future, according to the paint industry.[1] Further important components of alkyd paint are pigments and extenders. The pigment is the substance that gives the paint color. Pigments are derived from natural or synthetic materials that have been ground into fine powders. Extenders are inert pigments used to extend or increase the bulk of a paint. Extenders are also used to adjust the consistency of a paint and to let down colored pigments of great tinting strength.[6] The last important category of components of alkyd paint comprises the additives. A large variety of coating additives is known, which have widely differing functions in a coating formulation. One of the most important groups of additives is that of the catalytically active additives, which includes the paint drying
O O CH
O C O CH2 O O O n
Figure 1.1: Example of an alkyd resin used as a binder compound in alkyd paint. The fatty acid chain shown is linoleic acid.
10
Alkyd Paint and paint driers
catalysts, or driers.[5] Driers are metal soaps or coordination compounds which accelerate paint drying, thus shortening the total drying time. Without driers, the drying time of alkyd paint would be over 24 hours, which is clearly undesirable for most applications. 1.1.3 The drying of alkyd paint[5] During the drying of alkyd paints several different stages can be identified. The first process is the physical drying of the paint. In this process, the solvent evaporates and a closed film forms through coalescence of the binder particles. Then chemical drying (also called oxidative drying) occurs, a lipid autoxidation process, which means that the paint dries by oxidation of the binder compound with molecular oxygen from the air. Autoxidation will be discussed in detail in the next section. During the drying process four overlapping phases can be discriminated: • • • • Induction period Hydroperoxide formation Hydroperoxide decomposition into free radicals Polymerisation / crosslinking
The induction period is the time between application of the paint to a surface and the start of dioxygen uptake by the paint film. The induction period occurs because the effects of solvent, anti-skinning agent and natural anti-oxidants that may be present in the alkyd resin must be overcome before the drying process can begin. Autoxidation of the unsaturated fatty-acid chains in the alkyd binder then gives rise to hydroperoxides with uptake of atmospheric oxygen. Decomposition of these hydroperoxides results in the formation of peroxide and alkoxide radicals. These radicals initiate the polymerisation of the unsaturated molecules of the binding medium. Polymerisation occurs through radical termination reactions forming cross-links, causing gelling of the film, which is followed by drying and hardening. The number of cross-linked sites that are formed determines the film hardness. Cross-link formation is irreversible; hence when a paint layer has dried it cannot be easily removed.
1.2 Metal-catalysed autoxidation
1.2.1 Autoxidation in general Autoxidation is the direct reaction of molecular oxygen with organic compounds under relatively mild conditions.[7] More specifically, autoxidation is described as the insertion of a molecule of oxygen into a C-H bond of a hydrocarbon chain to give an alkyl hydroperoxide.[8] Autoxidation and metal-catalysed autoxidation has been extensively studied for numerous substrates under various reaction conditions.[8-15] Generally, an induction time is observed after which the autoxidation reaction abruptly starts and rapidly attains a limiting, maximum oxidation rate.[12, 13, 16, 17] The reaction proceeds by a free-radical chain mechanism and can be described in terms of initiation, propagation and termination. Scheme 1.1 shows the generally accepted reaction steps. Initiation can occur via several different pathways, either metal mediated or not. Steps 1 and 2 in Scheme 1.1 are often proposed as the initiation steps in non-metal-
11
Chapter 1
Initiation. RH + Initiator → R + Initiator-H ROOH → RO• + •OH ROOH + M n+ •
Scheme 1.1: Radical chain reactions as occuring in metal-catalysed autoxidation.
mediated autoxidation.[5, 18] Step 1 is the reaction of some arbitrary initiating species with the substrate, directly forming a carbon-centred radical. Reaction 2 is the thermal homolytic decomposition of a hydroperoxide, which only plays a significant role at elevated temperatures. In metal-mediated initiation, the initiation of radical chains predominantly occurs through steps 3 to 6. Reactions 3 and 4 are the so-called HaberWeiss reactions, originally proposed for the decomposition of H2O2 by iron in aqueous media.[19] This set of reactions is the generally accepted mechanism by which hydroperoxides are decomposed in polar media by a metal with two stable oxidation states differing one electron. The situation is different in apolar media,[8, 14] however, as will be discussed in the next section. Reaction 4 generally is slower than reaction 3.[20] Initiation reaction 5a,b involves the activation of molecular oxygen by the metal in the lower oxidation state, forming a metal-superoxide complex (5a). This complex can then abstract a hydrogen atom from the substrate to form a hydroperoxide complex and a carbon radical (5b). This initiation pathway operates predominately in apolar media.[21] In the last initiation reaction in scheme 1.1, reaction 6, a carbon radical is formed by direct reaction of the higher-valent metal ion with the substrate.[8, 15, 22] The most important propagation reactions are reactions 7 and 8. Reaction 7 is extremely rapid (diffusion controlled, k ~ 109 l⋅mol–1⋅s–1) except at very low partial pressures of dioxygen (dissolved-oxygen concentration
