DETERMINATION OF LEVELS OF LEAD IN PAINTS TO WHICH YOUNG CHILDREN AND ADULTS ARE EXPOSED
CHAPTER ONE
INTRODUCTION
1.1 History of paints
Paints have been manufactured since prehistoric times. It is used to decorate, protect and prolong the life of natural and synthetic materials and act as a barrier against environmental conditions. They are used for coloring and protecting many surfaces including; houses, cars, furniture, road markings etc. Each of these different applications requires different sorts of paint (Pappa, 2011). For centuries paint was made of essentially lead. Ancient Greeks made paint by treating lead with vinegar. Later white-lead paste was mixed up then linseed oil, turpentine; drier and colored
pigments were added in oil. White lead (PbCO3) was produced during the 4th century BC, the process is described by PLINY the elder, VITRUVIUS and the ancient Greek author THEOPHRASTUS (Hoevel, 1985).
The traditional method of making the pigment was called the "stack process". Hundreds or thousands of earthenware pots were designed so that the vinegar and lead were in separate compartments, but the lead was in contact with the vapor of the vinegar. The lead was usually coiled into a spiral and placed on a ledge inside the pot. The pot was loosely covered with a grid of lead, which allowed the carbon dioxide formed by the fermentation of the tan bark or the dung to circulate in the pot. Each layer of pots was covered by a new layer of tan, then another layer of pots. The heat created by the fermentation, acetic acid vapor and the carbon dioxide within the stack did their work and within a month, the lead coils were covered with a crust of white lead. This crust was separated from the lead, washed and ground for pigment. This was an extremely dangerous process for the work men. Despite the risk, the pigment was very popular with artists because of its density and opacity, a small amount could cover a large surface. It is widely used
by artists until the 19th century when it was replaced by zinc white and titanium white because of the danger caused for both the work men and the inhabitants of houses painted with lead colors (Thompson, 1956).
Such leaded-paint had greater hiding power, was easy to work with, stuck where it was applied and weathered well. Unfortunately, it also poisoned people, by skin absorption, respiration or ingestion. Today, paints with more than 0.6% lead are banned for residential use because of its toxicity (Martin and Griswold, 2007).
There has also been a shift away from oil plant as the base for paint. This began during World War II, when linseed oil and the solvents that cut it were scarce. By the mid-50s, synthetic replacements were outperforming natural ingredients. Today, practically all paints consist of some form of synthetic resins or polymers. Modern solvent-thinned paints still work like the old oil-based paints, only more effectively; alkyds have replaced most or all of the natural oils. Alkyd formulations are comparatively low in cost and have excellent color retention, durability and flexibility. Paint is essentially composed of a binder, solvent and pigment (Pullaro, 2009).
1.1.1 Binders in paint (resins)
The binder is used to hold the pigment to the surface. The binder is a polymeric substance and is either dissolved in the paint or suspended in it by emulsifiers. An example of binder is Linseed oil. Linseed oil is a mixture of triglycerides of long chain carboxylic acids (Talbert et al., 2014). The three (3) most common binders used include; alkyd resins, vinyl and acrylic emulsions, epoxy resins;
- Alkyd resins: They are the most common resins to be used in solvent-based paints. They are basically polyesters and are used for both air-drying and heat-cured paints.
- Vinyl and acrylic emulsions: These are emulsions in water and are the most common water-based binders for use in household paint.
- Epoxy resins: These resins are based on polymers containing the simple organic compound “oxirane” (ethylene oxide). A variety of other components are added to give a wide range of properties (Brewer, 1964).
1.1.2 Solvents in paint
A solvent that dries evenly is usually used. The binder is usually soluble in it but not so soluble that it won't polymerize. The solvent used in emulsion paint is water and for resin-based paints, a variety of organic compounds such as turpentine are used (Doublas, 2004).
1.1.3 Pigments in paint
This gives the paint its color and also protects the surface underneath from corrosion and weathering as well as hold the paint together. Both the organic and the inorganic surfaces are used. Special pigments can be used to give metallic finishes (e.g. for car bodies), to be hard wearing (for road markings). Organic paints are preferred as they tend to be brighter and more stable. However, inorganic pigments are cheaper and are heat and light stable. An example of
organic pigment is Azo dye. Azo dyes all contain azo group, -N=N-, which links two SP" hybridized carbon atoms. Azo dyes give bright, high intensity colors, the general formula for making an azo dye requires two organic compounds; a coupling component and a diazo component (Clark, 2014).
Typically, inorganic pigments are crystals of metal compounds (often oxides). Transition metals have their bonding electrons in d orbital when the metal is bonded to groups of atoms, this can cause the d orbitals to split into two levels of energy. The electrons in these orbitals can then absorb energy to move between the two levels. The energy gap between the levels determines the wavelength of light absorbed. As not all of the light hitting the compound is reflected, this means they are colored (Talbert et al., 2014).
