Pigments

Pigments have to satisfy certain conditions such as opacity, strength of colour and spreading rate, and they must not fade with exposure to light (Table 18.6). Pigments should neither melt nor dissolve in the binders or solvents used in the paint. Not all pigments can be used in all paints; for example, pigments in a lime based paint have to be compatible with lime. White pigment is the most popular and represents about 90% of all pigments used. Pigments can be inorganic or organic. There are two types of inorganic pigments: earth pigment and mineral pigment.

Earth pigment occurs ready-to-use in certain types of earth. It is composed of the decaying products of particular types of stone, and has good durability. Extraction involves washing the earth. Water is added and the mixture is stirred and left. When all of the earth has settled, the water is poured off and the uppermost layer of fine earth is treated in the same way. This is done five or six times. The earth is then

Table 18.6 Pigments in house paint

Pigment

Constituents

Comments

White pigments:

Chalk

Calcium carbonate

From natural resources; not very strong in oil paint

Glass white

Ground, recycled glass

At an experimental stage

Lead white

Lead carbonate or lead chromate

Highly toxic; also used as siccative

Titanium white

Titanium oxide

Most widely used white pigment

Zinc white

Zinc oxide

Toxic; fungicidal effect; usually produced from recycled zinc

Red pigments:

Cadmium red

Cadmium selenide

Highly toxic

Chrome red

Lead chromate

Highly toxic

Iron oxide

Ferrous oxide (Fe II)

Originally an earth pigment

Red lead

Lead tetroxide

Highly toxic, anticorrosive

Red ochre

Hydrated ferric oxide (Fe III)

Originally an earth pigment

Brown pigments:

Burnt sienna

Hydrated ferric oxide, silicic acid

Originally an earth pigment

Umber

Clay containing oxides of iron and manganese

Partly still prepared as an earth pigment, but mostly now prepared synthetically from ferric oxide

Yellow pigments:

Cadmium yellow

Cadmium sulphide

Highly toxic

Chrome yellow

Lead chromate

Highly toxic

Naples yellow

Lead (II) antimonate

Highly toxic

Yellow ochre

Hydrated ferric oxide (Fe III)

Originally an earth pigment

Zinc yellow

Zinc chromate

Toxic

Green pigments:

Chrome green

Chromium (III) oxide

Toxic

Green earth

Ferric (Fe III) and ferrous (Fe II) oxides

Originally an earth pigment

Zinc green

Mixture of zinc yellow and prussian blue

Toxic

Blue pigments:

Cobalt blue

Cobalt (II) aluminate

Occurs naturally as a mineral; slightly toxic; used as siccative

Manganese blue

Barium manganate

Toxic

Ultramarine

Sodium aluminosilicate

Occurs naturally as the mineral lazurite. Prepared synthetically with a mixture of kaolin, soda, sodium sulphate, sulphur, resin, charcoal and quartz

Prussian blue

Iron (III) ferrocyanide

Toxic

Table 18.6 (Continued)

Black and grey pigments: Bone black

Carbon, calcium phosphate

Prepared by charring different organic materials, animal bones and wood

Carbon black

Carbon

Prepared from amorphous carbon which occurs from burning oil and tar products

Slate grey

Slate flour

Seldom used; obtained through grinding and making a paste of the slate ground in a mortar, adding water. It is finally dried and the binder is added.

Mineral pigment is obtained by cleaning natural minerals. Synthetic mineral pigments are extracted by combustion (e.g. zinc white), calcination (e.g. ultramarine) or precipitation in a solution (e.g. chrome yellow). Compared with the natural earth colours, the synthetic variations are more homogenous. With the exception of umber, most inorganic pigments today are produced synthetically, which therefore often makes it difficult to restore colours in ancient buildings.

Organic pigments are less durable than the inorganic pigments. One natural organic pigment is coal black, which is made of charcoal preferably from willow, beech or maple. Organic pigments are not normally used nowadays for painting buildings, with the exception of some blues and greens.

Many mineral pigments are based on limited or very limited reserves. The production of pigments normally involves high energy consumption and pollution. This is particularly the case for cadmium, chrome, manganese and lead products; pollution occurs in the factory environment and when the waste is deposited in the surroundings. The production of white pigments also causes a great deal of pollution, particularly in the case of titanium white, where the energy use is also exceptionally high.

Pigments are relatively well bound within paints, and they are less chemically active. However, where paint is applied by spraying, it is spread into the air as small droplets and pigments can be inhaled. Welding of painted objects, scraping, sanding or removing paint with hot air can all cause the same problem. Pigments containing chrome are strongly oxidizing and thereby irritating and damaging to the respiratory system. Zinc chromate can also cause chrome allergy. Chrome, cadmium and lead compounds are, amongst other things, strongly carcinogenic. Ferric oxides can be considered relatively harmless.

In the indoor environment pigments are normally harmless if they are well bound with the paint and not too exposed to wear and tear. Children have, however, been poisoned by licking painted surfaces. Alkylphenol ethoxylates (such as nonylphenol) are often used in pigment pastes as a dispersal agent. These are thought to be harmful environmental estrogens.

Energy recycling of painted products can lead to emissions of toxic pigment vapours. Material painted with products containing heavy metals represent a considerable pollution hazard and must be treated

18.2.4 Other additives as special waste. The same is true of zinc white, whereas titanium white is not a problem as a waste product (Figure 18.2).

Many other additives are used, depending upon the type of paint and where it is to be used.

Anti-corrosion agents are used to prevent rust from being formed in the paint tin itself or when painting metal surfaces such as nails, etc. Traditionally they contain chrome and lead compounds. In water-based paints a mixture of sodium benzoate and sodium nitrite is used in proportions of 10:1, and makes up around 0.5% of the paint.

Anti-skinning agents are added to stop a skin forming on top of the paint in the tin. The substances used are butyraldoxime and methy-lethylketoxime, added in proportions of 0.1-0.4%.

Dispersing agents are used in dispersion paints to keep the particles spread throughout the suspension. Much used are soylecithin, poly-acrylates and alkylphenol etoxylates (such as nonylphenol).

Drying agents (siccatives) are added to various oil paints to shorten their drying time, particularly in linseed oil paints. Common siccatives are found in zirconium, cobalt salts and manganese, added in proportions from 0.02 to 0.1% of the dry content in the binder. Lead salts were once used in proportions between 0.5 and 1.0% of the dry content. Alternative siccatives are natural drying oils like cedar oil and oil from the Chinese tung tree, which should be added in proportions between 2 and 10%. Addition of pulverized lime will also reduce the drying time.

Drying retardants are added to water-based latex paints. They help reduce the evaporation of water while painting is taking place, and usually consist of glycols and glycol ethers. For a long period after painting is complete, glycols can be emitted and irritate the respiratory system.

Emulsifying agents, mostly as natural proteins, are used in emulsion paints to promote the suspension of for example oil in water.

Fibre materials can be added to paint to make it tougher and provide reinforcement on difficult surfaces. Glass fibre and polyester fibre are often used.

18.2

Indication of a colour scale with pigments having less environmental risk. Source: Jordfarg EF.

18.2

Indication of a colour scale with pigments having less environmental risk. Source: Jordfarg EF.

Fillers are simple, colourless materials with the primary function of economizing and spreading the paint, and in some cases of improving the opacity. They also make the paint more matt. Important fillers are kaolin, dolomite, talcum, sand, fossil meal, diabase, baryte and calcite. In the traditional earth colours, neutral clays were used as fillers.

Film-forming agents are much used in water-based synthetic paints in quantities from 1-5%, usually as triethylene glycol n-butylether, poly-thylene glycol, butyl glycol and 2,2,4-trimethyl-1,3 pentandiol monoiso-butyrat (Texanol).

Foam reducers, usually glycerides, are often added to water-based paints so that the paint does not froth.

Fungicides are often necessary to prevent the paint from attack by mould during storage and after application. The least toxic alternatives are lime and metal sulphates such as alum and ferrous sulphate, which are used in many paints with organic or even mineral binders. Some pigments also have preservative capacities. Paints with 50% zinc white are not attacked by mould. Water-based synthetic paints and some paints using organic solvents can contain fungicides based on chlorinated hydrocarbons. Up to the end of the 1970s, the very toxic poly-chlorinated biphenyls (PCBs) were used. Common fungicides today are isothiazolinones (e.g. Kathon), 2-brom-2-nitro-1,3-propandiol (Bronopol), N-(trichlorometyltio)ftalimid (Folpet), diclofluanide, tolylfluanide, tetra-chloroisophthalodinitrile (Chlorothalonil), methyl 2-benzimidazolecarba-mate (Carbendazim), formaldehyde and tributyl tin. Fungicides typically make up 0.5 to 3% of the paint mixture. All fungicides are more or less volatile and can cause problems in the indoor environment. They can irritate the mucous membranes and, in some cases, trigger allergies. Several of the fungicides are also potent environmental toxins, including carcinogens and mutagens.

PH-regulating agents can be added to water-based plastic paints to increase the pH value and thereby reduce the chance of mould growth. Ammonia or triethylamine are often used.

Perfume is added to a few water-based paints, mostly to neutralize the unpleasant smells from chemicals such as amines.

Softeners are used in water-based synthetic paints. Dibutylphthalate (DBP) and dietylhexylphthalate (DEHP) are much used, and even chloroparaffins in certain products. Softeners are released from the painted surface for a long time after it has been applied, and can be both irritating to the mucous membranes and cause allergies. In many modern paints the need for softeners are avoided by using a copolymer binder.

Thickeners are added to water-based plastic paints to give the paint a slow flowing consistency. Water soluble cellulose glue or derivatives of polyurethane and polyacrylate are used for this.

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