a)      Solvency, whether it is a solvent or a non-solvent for a given film former. This depends upon the film former and is not an independent property of a particular solvent.

b)      Viscosity or consistency.

c)      Boiling point or evaporation rate.

d)     Flash point.

e)      Chemical nature.

f)       Toxicity and smell.

g)      Cost.

These properties will be considered with reference to Table 2.

a)      Solvency

A solvent for a polymer is a liquid, the molecules of which are strongly attracted by the polymer molecules. The molecules of a non-solvent are only weakly attracted. Since the forces of attraction between polymer and solvent molecules are as strong as those between the polymer molecules, dissolving the polymer is like mixing a very viscous liquid with a very fluid one. There is no solubility limit. A true solvent is miscible with the polymer at all proportions.

It is not easy to look at the chemical composition of a polymer and predict solvents for it. A general guide is that like dissolves like, but this is often wrong and there is still no complete explanation for polymer solvency.

In 1955 an American paint chemist suggested that two factors govern the parameters of solvency, 1) The hydrogen bonding capacity of the liquid (hydrogen forms weak electrostatic bonds to oxygen, which give water its characteristic high boiling point) Hydrogen bonding group I weak (Hydrocarbons), II moderate (Ketones, Esters), III strong (alcohols, water), 2) The second parameter is calculated from the latent heat of evaporation.

From these two values a numerical value of this parameter is a measure of the attractive forces between the liquid molecules. This parameter is called the solubility parameter.

In determining which type of solvent to use and particularly in lacquer coatings, the strength of the solvent can have a marked effect on the viscosity of the solution and thus, the application solids, among the active solvents, a ketone is stronger than an ester, which means that for the same volume, a ketone will reduce the viscosity more. Additionally, more latent solvent and diluent could be used with ketone than with ester to balance flow, drying rates, etc.

With alkyd paints, aromatic hydrocarbons are stronger solvents than aliphatic.

Vẽ bảng (Table 2 Solvent properties)- Trang 5+6

b)      Viscosity/Consistency

Viscosity is a measure of the internal resistance to flow and is measured in units called poise. Flow cups are the most widely used measure of viscosity. The viscosity of a polymer.is a measure of forces of attraction operating between polymer molecules in solution.

If the polymer chains are long, this increases intermolecular forces and the possibility of mechanical tangling. As the solids of the solution rises, entanglements increase and viscosity rises. Lower molecular weight polymers would have a lower viscosity than a higher molecular weight (longer chains) at the same solids.

If we take a given sample of polymer of fixed molecular weight and dissolve it at the same solids in a variety of true solvents, the viscosity of the solutions will be proportional to the viscosities of the original solvents,

This is important because we can reduce paint viscosity without lowering solids or polymer molecular weight, simply by changing to a less viscous solvent if one is available.

Viscosities of emulsion paints in which the polymer is fully emulsified in a liquid, molecular entanglements do not occur, consequently the emulsion viscosity is independent of the polymer and at low solids, is the viscosity of the continuous phase (water). At higher concentrations the particles can scarcely move past one another, so the viscosity rises very quickly.

c)      Boiling Point or Evaporation Rate

The flow of paint on a vertical surface can be controlled by solvent evaporation. To get the right solvent balance we need to know the evaporation rates of thin films of solvents at normal temperatures.

Evaporation rates for solvents are available.

We are concerned with the evaporation of mixtures of liquids in the presence of polymers under varying atmospheric conditions. Evaporation rates of solvents in mixtures cannot be predicted from individual evaporation rates, attractive forces between molecules can delay evaporation, and attraction of polymer molecules will also delay evaporation. Because evaporation rates are no more than a guide, often boiling points or boiling range of solvents are used. It is usually true that liquids with low boiling points evaporate more rapidly at room temperature than those with high boiling points, but the boiling point will tell you when a liquid will boil, not how fast it will evaporate.

Solvents are graded roughly into three groups:

Low boilers are used in spraying paints, because they evaporate between gun and surface and give the necessary rise in solids and viscosity. High boilers are used to give flow and can be the sole solvent when the paint must be kept fluid for long periods, e.g. where brushing paint must be married in. Medium boilers can he used in all types of paint to give flow at first, followed by fairly quick set up. Most paints contain as much diluent as possible, since diluents are usually aliphatic hydrocarbons, which are much cheaper than the true solvents, the limit is decided by the fact that the polymer must be kept in solution in the can and at all stages of application and drying. For this reason the evaporation rates of diluents and solvents have to be balanced carefully.

To make sure that there is always enough true solvent present and particularly to ensure that the last molecules to evaporate are solvents. If there is too much diluent at any stage, the polymer precipitates and the film appears milky or low gloss.

d)     Flash point gives an indication of flammability or fire risk. The flash point is the lowest temperature at which enough vapour is given off to form a mixture of air and vapour immediately above the liquid, which can be ignited by 3 Spark or flame under specified conditions.

Most countries have regulations concerning the storage, transport and use of products containing flammable solvents.

Highly flammable materials are usually those having a flash point below a certain figure and they may also be required to support combustion. A water-borne paint may have a low flash point from a small amount of low boiling alcohol in the product, yet the paint might not support combustion.

The combination of flash point below 32°C and combustability is used in the UK Highly Flammable Liquids Regulations.

e)      Chemical Nature

It is easy to forget that solvents are chemicals and can react with other paint ingredients. For paint stability in the can this is undesirable and care in selecting solvents and can stabilisers are tested.

Alcohols with hydroxyl groups are reactive with additives such as isocyanate and are therefore not used as thinners in isocyanate paint systems.

f)       Toxicity and Smell

Safe working conditions are established in the UK by the Occupational Exposure Limits (OELs) set by the the Health and Safety Executive, so selection of solvents must conform to these regulations. Odour is of p more importance in household paints where strong smells are often masked by deodourisers, than paints used in industrial processes.

g)      Cost

Hydrocarbons (particularly aliphatic) are the cheapest solvents (apart from water). Some esters, less common ketones and nitro-paraffins are the most expensive.