A-level Applied Science/Colour Chemistry/Paint/Solvents

Solvents

In paint, the solvent serves to adjust the viscosity of the paint. It is volatile and does not become part of the paint film.

Solvents are also known as diluents and thinners.

Water is a common solvent, used in emulsion ('latex') paints.

White spirit (also known as Stoddard solvent, mineral spirit or petroleum spirit) is a paraffin-derived clear, transparent liquid which is a common organic solvent used in painting and decorating. It is used in alkyd ('gloss') paints.

It is a mixture of saturated aliphatic and alicyclic C7 to C12 hydrocarbons with a maximum content of 25% of C7 to C12 alkyl aromatic hydrocarbons.

White spirit is used as an extraction solvent, as a cleaning solvent, as a degreasing solvent and as a solvent in aerosols, paints, wood preservatives, lacquers, varnishes, and asphalt products. In western Europe about 60% of the total white spirit consumption is used in paints, lacquers and varnishes. White spirit is the most widely used solvent in the paint industry. It is a typical VOC.

Turpentine substitute is a mineral-based replacement for the vegetable-based organic solvent turpentine. It is a hydrotreated naphtha - a light distillate of petroleum - which forms a clear transparent liquid at room temperature. It is a complex mixture of highly refined hydrocarbon distillates mainly in the C9-C16 range. The liquid is highly volatile and the vapours are flammable.

As the name suggests it is a widely available and cheaper substitute for turpentine. It is commonly used as an organic solvent in painting and decorating, for thinning oil based paint and cleaning brushes. Also known as turps substitute, mineral turpentine, or just turps, causing confusion with genuine turpentine. It is a very dangerous inhalant.

Industrial paints use mixtures of dimethylbenzene, methylbenzene, alcohols, esters and ketones. Other typical solvents include glycol ethers and volatile low-molecular weight synthetic resins. Hammerite^TM, for example, uses a mixture of acetone (propanone, a ketone), hydrotreated naphtha (turpentine substitute - hydrocarbon) and butyl acetate (ester).

VOCs

Volatile organic compounds (VOCs) are organic compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere. A wide range of carbon-based molecules, such as aldehydes, ketones, and hydrocarbons are VOC's.

Sources of VOCs

Common artificial sources of VOCs include paint thinners, dry cleaning solvents, and some constituents of petroleum fuels (e.g. gasoline and natural gas). Trees are also an important biological source of VOC. It is also known that trees emit large amounts of VOCs especially isoprene and terpenes. Significant biological sources of methane are termites, cows (ruminants) and cultivation (estimated emissions 15, 75 and 100 million tons per year respectively).

Environmental Impacts

VOCs are sometimes accidentally released into the environment, where they can become soil and groundwater contaminants. Vapours of VOCs escaping into the air contribute to air pollution.

The aromatic compounds benzene, toluene and xylene are suspected carcinogens. 1,3-butadiene is another dangerous compound which is often associated with industrial uses.

Some VOCs also react with nitrogen oxides in the air in the presence of sunlight to form ozone. Ozone poses a health threat in the lower atmosphere by causing respiratory problems. In addition high concentrations of low level ozone can damage crops and buildings.

Contribution to Indoor Air Pollution

Many VOCs found around the house, such as paint strippers and wood preservatives, contribute to sick building syndrome because of their high vapour pressure. VOC's are often used in paint, plastics, and cosmetics.

Terminology and legal definitions

There are a number of different ways to collectively refer to those chemical compounds that participate in photochemical reactions. That is, those that react with other pollutants, in the presence of sunlight, to form ozone.

Some of the more common terms are:

• NMHC — Non-Methane Hydrocarbons
• NMOG — Non-Methane Organic Gasses
• ROG — Reactive Organic Gasses
• TOG — Total Organic Gasses
• TVOG — Total Volatile Organic Compounds
• VOC — Volatile Organic Compounds

While all these terms are used, it is not always clear which pollutants are included in each term. The term "VOC" has the advantage of having a precise definition codified by the US EPA in the Code of Federal Regulations (CFR).

It is worth noting that the CFR definition is, in many respects, more a matter of policy than a matter of science. Because the CFR has characterized a compound as having "negligible photochemical reactivity" it does not necessarily imply that it is, at any particular time, less reactive than those compounds which are not on the list. Since first establishing the list of exempt compounds in 1977, the EPA has added several to the list, and frequently has several petitions undergoing review.