For simple elementary substances, the smallest partical is called an atom. An atom is very small, a sheet of iron 1 micrometer thick would be 4300 atoms thick. These atoms combine to form the 92 elements found in nature, the latin names for these elements are abbreviated to one or two letters, e.g. O for Oxygen, C for Carbon, S for Sulpher, Hg for Mercury (Hydragyrum), Fe for Iron (Ferrum), Pb for Lead (Plumbum). The elements are listed in the periodic table, the elements listed in vertical columns behave similarly in a chemical sense.

The atoms of different elements are different in weight, size and behaviour. Atoms are composed of a number of different electrically charged or neutral particles. An atom consists of a nucleus, which contains relatively heavy particles, some of which are positively charged (protons) and some are neutral (neutrons), and this is surrounded by a number of relatively light negatively charged particles called electrons.

The electrons are in constant motion about the nucleus and the roughly spherical volume in which thay move is the volume of the atom. The electrical charges on protons and electrons are equal, but opposite in sign. The number of electrons is exactly balanced by protons, so atoms are electrically neutral. The atomic number of an element is the number of protons (or electrons) in one atom of that element.

Substance which are not elements are compounds. A compound consists of two or more atoms held together by chemical bonds. Compounds, like elements, can be represented by symbols, thus Water, H20, contains two atoms of hydrogen and one of oxygen per molecule. This symbol describles the chemical formula.

A mixture of elements, without chemical bonding between the differrent types of atoms is not a compound. Mixtures can often be seperated by simple physical means, and the propotion of elements in a mixture may vary considerably from sample to sample, in a given compound the proportions are always the same.

At any particular temperature, any substance exists as either a solid, a liquid or a gs. If the temperature is varied, the same substance may pass through all three states. Identical molecules possess forces of attraction for one another at close range. All these molecules possess energy received from their surroundings in the form of heat, light or radiation of from molecular collisions. The molecules are in a constant state of motion as a result of the enrgy they possess.

In the solid state, the forces of attraction are strong enough to overcome the motion of the molecules, so that they are closely packed in an ordered arrangement. Nevertheless, they are moving by vibrating backwards and forwards. As the solid state is heated, the vibrations become more vigorous, until the molecules have sufficient energy to overcome the attractive forces, the pattern of toms is borken and the solid melts.

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The arrangement of molecules in a solid.

THE LIQUID STATE

Althought the molecules can move about, they are still close together and the attractive forces prevent complete escape. The molecules at the surface of a liquid are not attracted in all direction since there are only occasional gas molecules and little pull in this direction. The attractive forces tend to draw surface molecules together and in towards the centre of the liquid. The liquid behaves as if it had a skin and the surface molecules are decribed as being under surface tention. Surface tension is expressed in dynes per c.m., Water=73 dynes/cm, Xylene=30, n-Hexane=18. Thus, a pin can float in water, nut not on Hexane.

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The arrangement of molecules in a liquid;surface tension.

A.    Surface molucules attracted to centre of liquid.

B.    Interior molucules attracted in all directions.

Viscosity is a measure of resistance which hinders the flow of the liquid \. The movement of one molecules is hindered by

a)     Collisions with other molecules.

b)    Physical entanglement with other molecules (due to irregularities of shape).

c)     Attraction of other molecules in the immediate viscosity.

Viscosity is measured in poise and is a measure of =    Stress

                                                                                  ---------------

                                                                                  Rate of Shear

The Viscosity varies appreciably with small temperature changes. A temperature rise provides more energy, molecules move faster, more able yo overcome attractive forced of neighbours, flow more easily and viscosity will drop.

As the temperature increases, energy is sharded out unequally between the molecules, so that at the surface some molecules are moving fast enough to escape the attraction of neighbours and fly off into air, this is known as evaporation.

Finally, all the surface molecules escape readily and boiling takes places. In the gaseous state, molecules are widely space and travelling in straight lines at several hundred miles an hour, so the forces of attraction come into play by chance encounters of molecules. The higher the temperatue, the more impact, hence gas pressure increase with temperature.

SOLUTIONS

If a solid and liquid are mixed, one of three things can happen:

a) If the attraction between liquid and solid is less than that between I solid molecules, the liquid will do no more than separate solid particles from one another. These particles will be dispersed

throughout the liquid by shaking, the result is a suspension of solid particles which will eventually settle out on standing.

b) If there is strong attraction between liquid and solid molecules, liquid molecules will penetrate the solid structure and solid molecules will break away and mingle with the liquid molecules.

Finally, the solid dissolves and a solution is formed.

c) A chemical reaction may occur, the products of which if a liquid and solid may behave as a) or b).

There is a limit to the amount of solid that will dissolve in a liquid to form a solution, increasing the temperature usually increased solubility (amount that will dissolve). A solution containing the maximum amount of solid is known as a saturated solution.

SUSPENSION

If the particles in solution are very small, 0.4 micrometers or smaller, ask the particles settle out they are constantly bombarded by the liquid molecules, which are in continual motion. Such collisions do not affect large particles, but small particles rebound slightly and the frequency of the collisions cause them to alter course and follow a zig-zag path, known as Brownian movement. Such random movement delays settlement.

The particle surface may also bare an electrical charge. When two particles come close together, the like charges repell one another, this also delays settlement. Such fine particles are said to be in a colloidal state. Colloidal suspension can be extremely stable and resist settling for long periods.

Another form of colloidal dispersion can be produced from two liquids that will not mix. Agitation will disperse one of them as fine droplets in the other, the product is an emulsion. The emulsion can be very stable if the dispersed droplets are electrically charged or have absorbed surfactants at their surfaces. Milk is an emulsion of fat in water.

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CHEMICAL BONDING

All chemicals are held together by chemical bonds and its the nature of these chemical bonds and the type of chemicals which give the characteristic properties of materials, different bonds vary in strength, flexibility and reactivity within the paint industry.

The manufacture of resins is all about making the right bonds. The drying process controls the formation of further chemical bonds and the breakdown of paint is often the breaking of Chemical bonds by UV light. The colour of pigments is produced by the type of bonds and the energy produced within the bonds.

Elements within the periodic-table are classified in groups of 1 to 7, these groups indicating the number of electrons available for bonding. All (apart from H and He) elements like to have a stable configuration of 8 outermost electrons, near enough to be considered as associated with that element. This can be achieved by viewing group one elements as being able to give up

an electron, and group 7 elements as being able to accept an electron, thus forming an ionic bond, and is best represented by the reaction of Na (sodium) and cl (chlorine). If these two elements are mixed a violent reaction occurs and sodium chloride (salt) is formed.

The reaction is described in chemical terms: Na + cl ->Nacl. Where Na and cl are the reactants, which are added together and the arrow indicating the direction of the reaction and Na cl being the products of the reaction.

Elements of group 3, A, 5 and 6 find it more difficult to give up or accept sufficient electrons to reach the stable configuration state of 8 electrons, but find it more easy to share their electrons. This process is described as co-valent bond type and is perhaps best described by the reaction of C (Carbon) and H (Hydrogen) to form CH4 (Methane), Carbon having four of its own electrons and sharing four Hydrogen electrons. Hydrogen having one electron and sharing one Carbon electron. The reaction may be described

C + AH --------------------------> CH4

Platinum Catalyst

One Carbon atom plus four Hydrogen atoms react in the presence of a Platinum catalyst to form Methane, which may be shown as

Vẽ hình trang 2/5                                               Each - (line) representing a chemical bond

A chemical equaiion states how many atoms of each substance are required and how many are produced, since in an equaiion the two sides must balance. In a chemical reaction matter is neither created or destroyed, it is simply converted into something else. Burning Carbon disappears by conversion into a transparent gas.

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The way in which the chemical bonds hold the elements together gives the ‘2 compound different physical properties as can be seen from pentane, which has dlfferent bo111ng points for straight chain pentane iso-perztane and cyclo pentane.

TABLE 1

ISOMERS OF PENTANE C5H12                            BOILING POINT

_ Pentane                    CH₃ – CH₂ – CH₂ – CH₂ – CH₃                                             36

Iso-Pentane                 CH₃ – CH – CH₂ – CH₃                                                         28

                                                 CH₃

Neo-Pentane                           CH₃

                                    CH₃     C         CH₃                                                                 9

                                                CH₃

Cyclo-Pentane                              CH₂                                                                                    50

                                    CH₂                             CH₂

                                                CH₂     CH₂

Similarly, the number of bonds affects itsphysical properties.

                                                                                    Boiling Point

Alkane Propane           CH₃ – CH₂ – CH₃                               -42℃

Alkane Propane           CH₃ – CH = CH₂                                -48℃

Alkane Propane           CH₃ – C  =_CH                                  -23℃

TABLE 2

FRACTIONATION OF CRUDE OIL

BOILING RANGE℃                       FRACTION                             C COMPONENT         USE

0 – 20                                      Nature gas                               C₁ – C₄                       Nature Gas

20 – 100                                  Light Petrol                              C₅ – C₇                       Solvents

70 – 90                                    Benzine                                    C₆ – C₇                        Dry Cleaning

80 – 120                                  Ligroin                                      C₆ – C₈                        Solvents

70 – 200                                  Petrol                                        C₆ – C₁₁                      Motor Fuel

200 – 300                                Paraffin oil (Kerosine)           C₁₂ – C₁₆                    Lighting

Above 300                              Gas Oil (Heavy Oil)                  C₁₂ – C₁₈                    Fuel Oil

                                                 Lubricating Oil                          C₁₆ – C₂₀                    Lubriciants

                                                 Grease Vaseline                      C₁₈ – C₂₂                    Phamaceutial

                                                Paraffin Wax                              C₂₀ – C₃₀                   Candles Wax paper

                                                Residues (Butumen)               C₃₀ – C₄₀                  Asphalt Tar

SOLID STATE

Pure, simple substances separate from solutions on cooling as crystals. These crystals have sharp melting points, which marks the precise boundary between solid and liquid.

The shape of a crystal is due to the pattern in which the atoms ions or molecules are arranged. Thus, crystals of common salt are always cubic.

Carbon is covalently bonded to four other carbon atoms in a titrahedrial arrangement, forming the very strong diamond shape. In many solids, the molecules are not arranged in any pattern at all. Instead they contain the random arrangement of molecules typical of liquids which appear to have become 'fixed“as a solid by cooling.

Since there is no pattern of molecules, this is known as amorphous (shapeless) solid. The molecules gradually become slower and more closely attracted to one another, until they are so closely packed that they completely impede one another and the substance will not flow. It is

impossible to say exactly when this occurs.

Examples of amorphous_solids are glass, tar and naturally occurring gums and resins, such resins form the basis of many early paints. It is the amorphous continuity that makes the resin film extremely suitable for protective surfaces.

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FIGURE 8: The structure of sodium chloride. The lines joining sodiums and chlorides are interested merely to show the relative positions of the jons. They are not meant to represent electron-pair bonds of the sort described in Section below. The interionic distance in the crystal is 281 pm (1 picometre (pm) = 10^-12m).

FIGURE 12:  The structure of diamond. Each carbon atom is surrounded by four others located at the cornoers of a tetrahedrom, as indicated bu the shading at the top. The C-Cbond length is 154 pm.

GIANT MOLECULES

Synthetic resins are polymers, poly-mer meaning many parts; Man~made giant molecules are made by linking together much smaller molecules (termed monomer molecules) frequently of just one type, like links in a chain and the process is called polymerisation. Polymers are classified according to the way in which the monomers link together.

Addition polymers, compounds with double bonds such as ethene are said to be unsaturated because they can take part in addition reactions to form two new single bonds.

… + CH₂ = CH₂ + CH₂ = CH₂ + CH₂ = CH₂ + ... Ethene

                                        ↓

     - CH₂ – CH₂ – CH₂ – CH₂ – CH₂ – CH₂ -      Polyethene

Polyethene or more commonly known as polythene, careful conditions are necessary for the reaction to occur, high pressure high temperature and platinum catalyst.

Other polymers produEed from additional polymerisation:

CF_{2 =}     CF_{2                                       →}             CF₂ – CF₂ – P.T.F.E.

Tetra Flouroethene

CH₂ = CH – o – CO – CH_{3               →}            P.V.A. (Polyvinyl Acetate)

5 Vinyl Acetate

CH₂ = CH Cl                                        _→             P.V.C. {Polyvinyl Chloride)

Vinyl Chloride

CH₂ = C(CH₃) – CO – O - CH3         _→             Polymethyl Methacrylate (Perspex)

Methyl Methacrylate

Usually the monomer (the starting compound) is liquid and can he polymerised with the use of an initiator, which produces the free radical which has sufficient energy to break one of the double bonds and cause the chain reaction to occur.

Organic peroxides are often used. The length of polymer chain can he controlled by the amount of initiator and chain transfer agents used (usually hydrogen which uSes up the free radical).

The polymer molecular size is described by its molecular weight. If a hydrogen atom is one, carbon is 12, nitrogen 1A, oxygen 16) and so on. The molecular weight is found by adding together the atomic weights of all the atoms in the molecule. A polymer weight may vary from 1000 to over a million, ethylene has a molecular weight of 28.

Each of the polymers listed so far form ens monomer only, these are called homo polymers.

Different mixtures of monomers can also be polymerised together to produce a co-polymer, which contains random polymerised monomers. Thus, poly methyl methacrylate is hard, brittle and insoluble in petrol, while polybutyl methacrylate is soft, flexible and soluble in petrol. A co-polymer, predominantly methyl methacrylate could remain insoluble in petrol (softened a bit) and yet be more flexible than polymethyl methacrylate.

Condensation Polymers

In addition, polymerisation molecules grow by the addition of another monomer unit without the loss of any atoms in the process. In condensation polymerisation, each growth reaction is accompanied by the production of small molecules (usually water} which is excluded from the polymer chain e.g.

CH₃ - C - OH + CH₃ - OH _→ CH₃ - C - O - CH₃ + H₂O

          ll                                                          ll

          O                                                         O

Acetic Acid + Methanol              Methyl Acetate Water

HO - O - C - R - C - O - OH + HO – R’ - OH

Di Basic Acrd                          Dy Hydric Alcohol

→ HO - O - C - R - C - O - O - R’ – OH = H₂O

                       PoryesterRegin

The esterifaction of one group from each molecule produces water and a product containing a caboxyl group at one end and a hydroxyl group at the other. As further reaction proceeds, each reaction produces another ester group and the restitant polymer is a polyester resin. Poly amides, epoxy and amino resins are also product this method.

Additional polmerisation is fast and can give very high yields, but condensationpolymerisation usually takes much longer and by-products must be continually removed, high molecular weights take considerably longer.

A molecule with the ability to react with one Other molecule cannot produce apolymer. If X and Y are reactive groups which will react, then R – X + R’ - Y -> R - XY – R’ and no further reaction is possible since X and Y are no longer reactive. A useful monomer must react with a minimum of two other molecules. In condensation polymerisation, this means two reactive groups per molecule. In additional polymerisation, the double bond has the ability to produce two free radicals.

The number of molecules with which a monomer molecule may combine in the polymerisation reaction is known as functionality. If the monomers present have a functionality of two, a linear polymer is produced (-A- has a functionality of two), then polymer -A-A-A-A- is produced. If small amounts of monomer with a functionality of three is included, branching can occur,(-B has a functionality of three).

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If a large amount of a trifunctional monomer is present, a network in three dimensions is formed.

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This polymer is said to be cross-linked.

In a linear polymer, there is rotation about the single bond. The polymer is flexible, heat sets the molecules moving and small liquid molecules may penetrate between the chains. Both will help to sort out the tangle, i.e. the polymer will melt or dissolve. The.same is true for lightly branched polymers of low molecular weight and are known as thermo plastic polymers.

In a cross-linked polymer, every chain is probably connected to at least three others, so thatgthe solid lump is one molecule. To separate the chains, we have to break several covalent bonds, thus heat will first soften, as the segments between cross-links get a limited freedom or movement, but the next stage is not melting, it is decomposition.

While liquid solvents can penetrate into the holes in the cage-like structure a causing it to swell, they cannot dissolve the polymer, since dissolving implies separating the molecules from one another. This is clearly impossible and the resin is described as a thermo setting polymer.

Crystallinityfcan occur in polymers where a uniform parallel arrangement of lengthy segments of polymer chains occur, marked crystallinity introduces a number of objectionable features into a polymer from a paint point of view.

1. Lack of transparency, due to reactive index differences between regions of polymer solid.

2. Sharp melting points, stoving paints required paints which flow to permit minor irregularities to flow out without causing sags.

3. Insolubility in all but the most polar solvents.

Thus, for paint purposes, largely amorphous polymers are required and they

tendancy to crystallise can be reduced by:' l

1. Making the arrangement of repeating units an irregular one, e.g. by co-polymerisation.

2. Introducing bulk side groups.

3. Spacing out the polar groups that provide the strong intermolecular

forces.

4. Introducing a high boiling point plasticiser to effect intermolecular forces and space polymer chains.