Paint and Surface Coatings

Theory and Practice

R Lambourne, T A Strivens

• Publisher: Woodhead Publishing
• Year: 1999
• ISBN:
  9781855737006
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

This second edition of an established and well received book has been carefully revised, in many instances by the original authors, and enlarged by the addition of two completely new chapters. These deal with the use of computers in the paint industry and with the increasingly important subject of health and safety. The chapter on pigments has also been re-written by an author new to this edition.It was the editor's intention in the first edition to provide science graduates entering the paint industry with a bridge between academia and the applied science and technology of paints. The great strength and appeal of this book remains that it deals with the technology of paints and surface coatings while also providing a basic understanding of the chemistry and physics of coatings.

• Extensive revision of first edition
• New chapter on computers and modelling
• New chapter on health and safety

1

Paint composition and applications — a general introduction

R. Lambourne

1.1 A short history of paint

Primitive men are credited with making the first paints about 25 000 years ago. They were hunters and cave dwellers and were probably inspired by the rock formations of their cave walls to outline and colour the shapes of the animals they hunted. It is possible that by creating these images they thought their power over their prey would be increased.

Chemical analysis of cave paintings discovered at Altamira (Spain) and Lascaux (France) show that the main pigments used by Palaeolithic artists were based upon iron and manganese oxides. These provide the three fundamental colours found in most cave paintings, namely black, red, and yellow, together with intermediate tints. Carbon from burnt wood, yellow iron carbonate, and chalk may also have been used. Surprisingly, there is no trace of a white pigment (the commonest pigment in use today) at Lascaux, where the natural colour of the rock was used as a pale background. However, white pigments do occur in some prehistoric paintings in Africa.

These earth pigments were ground to a fine powder in a pestle and mortar. Naturally hollowed stones are thought to have been used as mortars and bones as pestles, following the finds of such articles stained with pigments. The powdered pigments were probably mixed with water, bone marrow, animal fats, egg white, or vegetable sugars to form paints. They were applied by finger ‘dabbing’, or crudely made pads or brushes from hair, animal fur, or moss. Cave paintings have survived because of their sheltered positions deep in caves which were subsequently sealed off. These paints have very poor durability, the binders serving merely to make the pigments stick to the cave walls.

The Egyptians developed the art of paint-making considerably during the period circa 3000–600 BC. They developed a wider colour range of pigments which included the blues, lapis lazuli (a sodium silicate-sodium sulphide mixed crystal), and azurite (chemically similar to malachite). Red and yellow ochres (iron oxide), yellow orpiment (arsenic trisulphide), malachite green (basic copper carbonate), lamp-black, and white pigment gypsum (calcium sulphate) all came into use during this period. The first synthetic pigment, known today as Egyptian Blue, was produced almost 5000 years ago. It was obtained by calcining lime, sodium carbonate malachite, and silica at a temperature above 830°C. The Egyptians also developed the first lake pigments. These were prepared by precipitating soluble organic dyes onto an inorganic (mineral) base and ‘fixing’ them chemically to form an insoluble compound. A red dye obtained from the roots of the madder plant was used in the first instance. This is no longer used other than in artists' colours (‘rose madder’) because it fades rapidly on exposure to sunlight, and it has been replaced by alizarin. Lake pigments still, however, represent an important group of pigments today. Red lead was used in preservative paints for timber at this time, but was more extensively used by the Romans. The resins used were almost all naturally occurring gums; waxes which were applied molten as suitable solvents were unknown. Linseed and other drying oils were known, but there is no evidence that they were used in paints.

The Greeks and Romans in the period 600BC-AD400 almost certainly appreciated that paint could preserve as well as decorate objects. Varnishes incorporating drying oils were introduced during this period. However, it was not until the thirteenth century that the protective value of drying oils began to be recognized in Europe. During the Middle Ages much painting, especially on wood, was protected by varnishing. The varnish was made by dissolving suitable resins in hot linseed, hempseed, or walnut oil, all of which tend to darken with time.

By the late eighteenth century, demands for paints of all types had increased to such an extent that it became worthwhile for people to go into business to make paint and varnishes for others to use. In 1833, J W Neil advised varnish makers always to have an assistant present during the varnish making process, for safety. ‘Never do anything in a hurry or a flutter… a nervous or timorous person is unfit either for a maker or assistant, and the greatest number of accidents occur either through hurry, fear or drunkenness.' This admonition is indicative of the increase in scale of manufacture and the dangers of use of open-pan varnish kettles.

The industrial revolution had a major effect on the development of the paint industry. The increasing use of iron and steel for construction and engineering purposes resulted in the need for anti-corrosive primers which would delay or prevent rusting and corrosion. Lead- and zinc-based paints were developed to fulfil these needs. It is interesting to note that one of the simplest paints based upon red lead dispersed in linseed oil is still probably one of the best anti-corrosive primers for structural steel. Lead-based paints are being superseded not because better products have been produced, but because of the recognition of their toxicity and the hazards attendant upon their use.

An acceleration of the rate of scientific discovery had a growing impact on the development of paints from the eighteenth century to the present day. Prussian blue, the first artifical pigment with a known chemistry, was discovered in 1704. The use of turpentine as a paint solvent was first described in 1740. Metal driers, for speeding up the drying of vegetable oils, came into use about 1840.

The basis of formaldehyde resin chemistry was laid down between 1850 and 1890 although it was not used in paints until the twentieth century. Likewise, it was discovered in 1877 that nitrocellulose could be made safe to use as a plastic or film, by plasticizing it with camphor, but it was not until after the First World War that it was used in any significant amount in paints. The necessary impetus for this to happen came with the mass production of the motor car. Vast quantities of nitrocellulose were manufactured for explosives during the war. At the end of the war, with the decline in the need for explosives, alternative outlets for nitrocellulose needed to be found, and the mass production of motor cars provided the necessary market. The war had accelerated the exploitation of the discoveries of chemistry and the growth of the chemical industry. New coloured pigments and dyestuffs, manufactured synthetically, became available, and in 1918 a new white pigment, titanium dioxide, which was to replace white lead completely, was introduced. Titanium dioxide improved the whiteness and ‘hiding’ or obliterating power of paint, but when originally introduced it contributed to more rapid breakdown of paints in which it was used because of its photoactivity. Subsequent research has overcome this problem and ensured that the modern pigmentary forms of titanium dioxide can be used in any type of composition without suffering any disadvantage of this kind.

The most recent influences on coating developments are related to environmental considerations, and the need to conform to health and safety legislation. Cost/benefit relationships have also become more important in an increasingly competitive world market and have influenced formulation practice markedly.

Subsequent chapters of this book will be largely concerned with developments that have taken place in the twentieth century, of which most have occurred within the last fifty years.

1.2 Paint or surface coating?

The terms ‘paint’ and ‘surface coating’ are often used interchangeably. Surface coating is the more general description of any material that may be applied as a thin continuous layer to a surface. Purists regard the term ‘surface coating’ as tautological. However, it has been used widely in the UK and in North America to distinguish painting from other forms of surface treatment, such as electroplating, anodizing, and the lamination of polymer film onto a surface. Paint was traditionally used to describe pigmented materials as distinct from clear films which are more properly called lacquers or varnishes. We shall be most concerned with paint in the context of this book; but, as we shall see, modern painting processes may include composite systems in which a total paint system comprises several thin films, some, but not all, of which may be pigmented. We shall use both terms as appropriate to the context in which specific paint compositions are being discussed.

The purpose of paints and surface coatings is two-fold. They may be required to provide the solution to aesthetic or protective problems, or both. For example, in painting the motor car the paint will be expected to enhance the appearance of the car body in terms of colour and gloss, and if the body is fabricated out of mild steel it will be required to give protection against corrosion. If the body is formed from glass fibre reinforced plastic the paint will only be required for aesthetic purposes. There are obviously very sound economic reasons why it is attractive to colour only the outer surface of articles that might otherwise be self-coloured by using materials of fabrication, e.g. plastics that are pigmented, particularly if a wide choice of coloured effects is required. This topic will be developed in the chapters on paints for specific markets (Chapters 9–13).

In considering the nature of paints it will become abundantly clear that the relationship between the coating and the substrate is extremely important. The requirements for a paint that is to be applied to wood are different from those of a paint to be applied to a metal substrate. Moreover, the method by which the paint is applied and cured (or dried) is likely to be very different. In formulating a paint for a particular purpose it will be essential for the formulator to know the use to which the painted article is to be put, and physical or mechanical requirements are likely to be called for. He will also have to know how it is to be applied and cured. Thus, a paint for an item made from cast iron may call for good resistance to damage by impact (e.g. chipping), whilst a coating on a beer can will call for a high degree of flexibility. These different requirements will be described in Chapters 9–13 which will deal with specific areas of paint usage.

It has long been recognized that it is difficult, if not impossible, to meet the requirements of many painting processes by the use of a single coat of paint. If one lists the requirements of a typical paint system it is easy to see why. Many, if not all, of the following are likely to be required: opacity (obliteration); colour; sheen (gloss); smoothness (or texture); adhesion to substrate; specific mechanical or physical properties; chemical resistance; corrosion protection; and the all-embracing term ‘durability’. Durability is an important area that we shall return to in many contexts. The number of different layers that comprise the paint system will depend on the type of substrate and in what context the coated object is used. A typical architectural (gloss) paint system might consist of a ‘primer’, an ‘undercoat’, and a ‘topcoat’. All three are likely to be pigmented compositions, and it is probable that there will be more than one coat (or la...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• List of contributors
• Preface to first edition
• Preface to second edition
• 1: Paint composition and applications — a general introduction
• 2: Organic film formers
• 3: Pigments for paint
• 4: Solvents, thinners, and diluents
• 5: Additives for paint
• 6: The physical chemistry of dispersion
• 7: Particle size and size measurement
• 8: The industrial paint-making process
• 9: Coatings for buildings
• 10: Automotive paints
• 11: Automotive refinish paints
• 12: General industrial paints
• 13: The painting of ships
• 14: An introduction to rheology
• 15: The rheology of paints
• 16: Mechanical properties of paints and coatings
• 17: Appearance qualities of paint — Basic concepts
• 18: Specification and control of appearance
• 19: Durability testing
• 20: Computers and modelling in paint and resin formulating
• 21: Health and safety in the coatings industry
• Index