-----PAINT CONSTITUENTS AND BASIC TECHNOLOGY-----
-----PAINT CONSTITUENTS AND BASIC TECHNOLOGY-----
Paint is a material which will change the Texture, Colour or
appearance and give some form of protection to the underlying
surface. Has been classified in many ways, e.g. by principle
method:
1. Barrier; - The material forms a thick impermeable layer of a
high electrical resistance, e.g. Urethane.
2. Passivation; - Causing a chemical reaction between the paint
constituents and the substrate, e.g. rust inhibitive primers.
3. Cathodic Protection; - Employs the bi-metallic principles by
using a less noble metal as pigmentation, e.g. zinc in zinc rich
primers. By function, e.g.:
Anti Fouling - To inhibit marine growth on ship hulls.
Road Marking - To give white or yellow lines on roads. Fire
Proofing - To provide resistance to fire & Anti corrosive.
Paints can be classified by binder type (the main constituent)
or by colour, and in some cases even by the pigment type. No matter
which identification system is used, all the paints contain the
basic ingredients.
a) Binder, b) Pigments and other additives, c) Solvent (where
applicable).
The chemical structure and constituents give the paints their
own individual properties. Paints are supplied as either liquids or
solids in powder form and can be subdivided into groups:i) Liquid
paints containing solvent the solvent does not relate solely to
Hydrocarbon solvents, but also includes water. Due to the modern
EPA (Environmental Protection Act) requirements manufacturers are
researching new paint technology vastly reduced amounts of volatile
organic compounds.
ii) Solvent free They are generally chemical curing materials
which require the mixing of two or more components and usually go
under the name of MCLs (Multi Complex Liquids).
iii) Powders Virtually solvent free MCLs, which are solid, at
room temperature. The powder can be applied onto a preheated
substrate at about 240 deg.C or onto thin plate electro statically
and post heated.
Binder: The binder is the main constituent of paint and is often
referred to as a film former. Other terms are vehicle and
non-volatile. Some major considerations of a binder are:
1. Ease of application (flow properties or viscosity).
2. Adhesion to the substrate for the expected life of
coating.
3. Resistance to abrasion.
4. Resistance to chemical attack according to environment.
5. Cohesive strength, its ability to hold together as a
film.
6. Dialectric strength.
7. Ability to resist the passage of water.
8. Ability to change from a liquid as applied, into a solid to
provide the above properties, and others, for the expected life of
the coating.
Several materials satisfy the criteria above for different
environmental conditions, among them are:-
a) Acrylic; Synthetic resins, can be used in H C solvents or
water. Good colour retention, good film properties. E.g. Urethane
modified Acrylic as used by BG.
b) Alkyd; A term derived from alcohol acid reaction, they have
low resistance to Alkalies. In common with most resins they are
brittle and need modification with oils.
c) Ashphaltic Bituments; Petroleum based materials,
thermoplastic, relatively inexpensive, known for water proof
properties, poor resistance to sunlight, very low resistance to
solvents.
d) Cellulose Resins; Synthetic material, not extensively used
for industrial coating but good example of reversible
materials.
e) Chlorinated Rubber; Organic resins derived from reaction of
rubber with chlorine. Very poor resistance to HC solvents.
f) Emulsions; Obviously not used for anti corrosive systems but
are included for other factors e.g. Drying mechanisms. g) Epoxies;
Synthetic organic resins, generally provide good chemical, solvent
and water resistance. Good exterior durability but are prone to
chalking. Epoxies- two pack, single pack, solvent free and solvent
borne.
h) Ethyl and Methyl Silicates; Inorganic materials with
excellent weathering, solvent and heat resistance.
i) Natural Oils; Many natural oils can be used in the paints
industry. They are mixed with resins to modify the film properties.
Some natural oils are Linseed Oil, Tung Oil ( also known as China
Wood oil), Soya oil, Tall oil and Safflower oil.
j) Natural Resins; Natural resins are brittle by nature and fast
drying. They need to be mixed with oils to modify some properties.
A mixture of oil and resin is known as "Oleoresinous". Examples of
natural resins are copals, dammars and coumarones. Natural resins
are not soluble in water.
k) Phenolic Resins; Made from phenol and formaldehyde, coal
derivatives, characterized by excellent adhesion properties and
resistance to heat and chemicals. Greater than 65 deg. C couldn't
be used. Commonly called hot drying oils.
l) Polyurethane's; can come in several forms, Moisture Curing,
two pack Polyurethane's, chemically curing and single pack.
Industrial coatings are mainly the first two. They produce an
excellent synthetic coating, with outstanding abrasion resistance,
chemical resistance, and good exterior gloss and colour retention
with a minimum of chalking.m) Silicones; Designed as high
temperature service materials for temperatures ideally above 150
deg. C service temperature. Usually carbon or aluminium pigmented,
they are used to seal inorganic zinc silicates or metal sprayed
surface.
n) Styrene; Styrene is sometimes referred to as a blinder and is
used to modify other properties. Styrene is referred to as a vinyl
type monometer and is used to cross-link the film.
o) Vinyl; Vinyl use a different solvent group and water.
Binder Solvent Groups and Compatibility:-
A solvent free binder, or a binder using a very weak solvent,
will cause very few problems when over coating another product.
Guide to binder solvent combinations as follows Solvent strength
in
descending order Common NamesBinders
WaterEmulsions PVC/PVA
Vinyl's, Acrylics other materials, e.g. Epoxy-Bitumins,
Polyurethanes,
Alkyds, Acrylated Rubbers.
Aliphatic HydrocarbonsWhite SpiritTurpentine, Turpentine
substitute, Solvent naphtha's, Hexanes upwards Natural oils,Natural
Resins,Alkyds,
Phenolics.
Aromatic HydrocarbonsXyleneToluene
BenzeneChlorinated rubber
KetonesAcetoneMethyl Ethyl Ketone,
Methyl ISO Butyl Ketone.Epoxy
Embolden name is the main one used.
Polyurethanes use ketones and esters with aromatic diluents. In
descending order down the table the solvent groups increase in
strength. It is not advisable to use a binder with a strong solvent
over an existing coating, which uses a weak solvent. For example,
Chlorinated rubber coated over an Alkyd would result in lifting and
wrinkling, but Alkyd over Chlorinated rubber would have no ill
effect.
Ethyl and Methyl Silicates do not appear on the list because
they are high (or low) temperature performance coatings. Usually
the only material suitable is Silicone. Ethyl and Methyl Silicates
will not adhere over any substrate other than bare, clean
steel.
Chlorinated Rubber used to be the main material for ambient
temperature usage for BGAS.
The advantages of using this material were:-
1. Because of the chlorine content, high resistance to mould
growth.2. Because of the chlorine, non-flammable after solvent
release.
3. Very resistant to chemical attack e.g. Acids and Alkalis.
4. Very high resistance to water Vapour transmission.
5. Materials is non toxic and provides a very durable film.
6. Very easily maintained, no abrasion needed, clean surface
only.
The Disadvantages are:-
1. Its position on solvent compatibility list shows low
resistance to solvents i.e. only resistant to
Aliphatics and water.
2. Low temperature tolerance, 65 deg. C Max.
3. Spray application resulted in 'cobwebs'.
Polymers:-One of the properties expected of a binder is to
change from a liquid into a solid to form a film.
The word polymer means literally many parts, poly = many, mer =
single unit or part. Mer (meras GK) can be a single atom, or a
molecule and can be described as being " a string or structure of
repeated units", and polymerisation is the "joining together of a
string or structure of repeated units". In the case of most paints
the main constituents of the polymers are: H, C, N, O, Cl.
(Hydrogen, Carbon, Nitrogen, Oxygen, Chlorine).The main three
polymer types are Linear, Branched and Cross-Linked.Oils:-
Natural oils (Vegetable oils) are produced from seeds of a
plant, well known examples being linseed, castor, olive, coconut,
soya and tung oil. In order to be usable as a paint binder the oil
must be of a type that will combine with oxygen, i.e. it must be
"unsaturated". A saturated oil cannot be used as a binder because
it will not solidify by polymerisation to form a film. Therefore
oil van be divided into three groups:A) Drying Oils; which have
three sets of double bonds along the carbon backbone, and react
with oxygen readily at ambient temperature.
B) Semi Drying Oils; have on or two sets of double bonds, and
may need addition heat, or some other catalyst to promote
oxidation.
C) Non Drying Oils; will not oxidize and therefore cannot be
used as binders. Instead these are used as plasticisers in paint
formulation, to modify properties of a resin.
Oils and resins are mixed to give a binder with modified
properties -:
Long Oil paint; more than 60% oil to resin, elastic, slower
drying properties suitable for domestic
applications, decorative materials.
Medium Oil paint; between 45-60% oil to resin.
Short Oil paint; less than 45% oil to resin, faster drying
material, suitable for steelwork. More brittle with
shorter over coating time. Pigments:-
Pigments have many properties and characteristics. They are
derived from many sources, animal, vegetable, mineral and
synthetically produced, and can be in a wide variety of particle
sizes and shapes. Pigments used in paints must remain as solid
particles within the vehicle (the binder plus the solvent if a
solvent is used), and not dissolve. If it dissolves it is known as
dye, not a pigment.Pigment particles contribute to the paint films
strength cohesively, its abrasion resistance, durability, opacity,
in some cases impermeability and resistance to ultra violet rays.
Some pigment particles are as small as 1/10 th micron. They are
subdivided into groups:
Rust Inhibitive Pigments. Anticorrosive; Rust inhibitive
pigments are added into primers to protect the steel substrate by
passivation. Typical material in the category are:- Red lead*,
Calcium plumbate*, Coal tar*, Zinc Chromate*, Zinc phosphate,
Barium metaborate and Zinc phosphosilicate.
Zinc phosphate is the most commonly used. The four marked with
asterisk (*) are toxic and restricted in use.Red lead is a basic
inhibitor and works in the presence of fatty acid esters in natural
oils and resins only.
Metallic Pigments; Metallic pigments are also used on a steel
substrate to protect the steel, but this time by cathodic
protection. If a metal which is less noble than steel, (more
electronegative) is included in the film, and an electrolyte e.g.
water passes through the film, contacting substrate and pigment
particles, then a circuit can be engaged whereby the pigment will
receive the Hydroxyl ions thus suffer corrosion in preference to
the steel substrate. In order to satisfy this requirement the metal
pigment must be below the position of steel on the galvanic list.
The two must amenable metals to satisfy this are, Zinc and
Aluminium.
Zinc is the better of the two for galvanic protection but
Aluminium is excellent for Solar protection, reflecting the Ultra
Violet A and B.Opaque pigments;- Coloured pigments. Opaque pigments
are inert particles with excellent light scattering properties in
order to give covering power, (opacity) and colour.
1. Carbon-------------------------------------Black.
2. Compound of Cobalt--------------------Blue.
3. Compound of Chromium--------------Greens, Yellows and
Oranges.
4. Compound of Iron-----------------------Browns, reds and
Yellows.
5. Compound of Calcium------------------Reds and Yellows.
6. Titanium Dioxide-------------------------White.
Extender Pigments;- known simply as extenders or fillers,
provide some of the main properties expected of the film, such as
adhesion, cohesion, film strength and durability. Have a role in
application and flow, leveling, and other mechanical properties of
the film, and are an aid to inter coat adhesion and can reduce
gloss. Materials used as extenders are :
Clays-------------------Kaolin, China clay.
Chalk------------------Calcium carbonate.
Talcum----------------Magnesium silicate.
State Flour-----------Aluminium silicate.
Laminar Pigments;- Plate like pigments such as MIO (Micaceous
Iron Oxide), Aluminium Flake, Glass Flake, Mica and Graphite
provide excellent barriers. MIO sometimes known as 'Specular
Haematite' is widely specified.PVC;- The pigment to binder ratio is
a very important factor in the design and manufacture of paint and
is known as the 'Pigment Volume Concentration'. There is an ideal
pigment binder ratio, which varies from paint to paint, pigment to
pigment, and this is known as CPVC "Critical Pigment Volume
Concentration".
CPVC is defined in BS 2015 as the particular value of the
pigment volume concentration at which the voids between the solid
particles that are nominally touching are just filled with binder
and in the region of which certain properties are changed
markedly.i) Too much binder to solids ratio, would give a film of
good gloss properties, but poor covering power (opacity) and with a
tendency to blister (low cohesive strength).
ii) A film with lower gloss properties but greater cohesive
strength and just enough resin to encapsulate each particle, giving
good resistance to water permeation.
iii) When CPVC is exceeded and all particles are not wetted, the
film would be porous, low in cohesive strength and
adhesion.Solvents;- solvents are added to paints to reduce the
viscosity and ease application properties. If a solvent evaporates
too quickly the film will not dry eventually, if it evaporates too
slowly, drying will be protracted and on vertical surfaces the
paint is likely to Sag. The four important properties of a Solvent
are:-
a. Solvent strength;- Low molecular weight solvents are stronger
than high molecular weight solvents and strong binders such as
Epoxies and Polyurethanes, need strong solvents to 'cut' or
separate the molecules. Hence Ketones and Aromatics are used for
these materials. Natural resins don't have the same attraction
between the molecules and therefore need weaker solvents, higher
molecular weight, such as Aliphatics.b. Evaporation Rate;- The
evaporation rate governs at what point the Polymerisation
starts.
c. Flash Point;- The flash point of a solvent is a safety
consideration. Roughly defined as "The minimum temperature of the
solvent at which the vapours given off are flammable if a source of
ignition is introduced". The higher the flash point, the safer the
solvent.
d. Toxicity;- Solvents are substances hazardous to health and
therefore have predetermined concentrations to which humans can be
safely exposed. These limits are expressed in parts per million,
ppm.
Other Additives:-
Other than the main constituents of a paint viz, binder,
solvent, pigment, and extenders, there are approximately fifty
other materials which can be added to give other, or alter existing
properties. They can be grouped into Aids to Manufacture, Aids to
Storage, Aids to Application, Aids to Film Formation, Aids to Film
Curing, and others. Among them being;-i) Anti Setting Agents;- is
an aid to shelf life. It is a thixotrope, a thickener, which also
allows a higher film thickness. Thixotropic paints are jelly
paints, non drip, and if stirred change to normal liquid
consistency. Thixotropic agents are Bentones and Waxes, and help
keep solid particulate constituents in dispersion within the paint,
i.e. stop settlement.
ii) Plasticisers;- A plasticiser basically gives paint
flexibility and reduce brittleness and therefore needs to be
compatible with the binder, and have low volatility in order to
stay in the film for a long time. Alkyd resin was extensively used
in Chlorinated rubber binders, but for natural resins and their
isomers Non Drying Oils are used, saturated oils, which will not
polymerise. Castor oil, Coconut oil and some Palm oils fall into
this category.
iii) Drier;- Also known as Oxidants, used in oxidizing oils and
resins. They are heavy metal salts, rich in oxygen, which are added
to the paint during manufacture. Common salts are Octoates or
Naphthanates of Cobalt, Manganese and Zirconium e.g. Cobalt
Naphthanate. The acids producing the salts from the heavy metals
are Octoic Acid and Naphthanic Acid.iv) Anti Skinning;- Also known
as Anti Oxidants. These are added to oxidizing paints to retard the
formation of a skin on the surface of the paint. If a skin forms it
cannot be stirred back into solution, and must be removed. Because
the anti oxidant works against the oxidant they are added in very
small controlled amounts and are liquids usually. E.g. Methyl Ethyl
Ketoxime.
--:SOLUTIONS AND DISPERSIONS:-- Solutions:-
A Solvent is a liquid, which will dissolve another material,
liquid or solid.
A Solute is the material dissolved by the solvent.
A Solution is the resulting liquid. Salt and water, sugar and
water are Solutions, a Binder and
Solvent are also a Solution.
Dispersions:-
A paint consists of solid particles suspended in the vehicle,
where there is no solubility, so a paint is a
Dispersion. A dispersion can be either a solid or liquid
dispersed with another liquid, where there is
no solubility.
A Suspension:-
A suspension is when fine particulate solids, e.g. pigment and
extenders are dispersed within a liquid,
the vehicle. Ideally after the manufacturing process, each
particle should be completely wetted by the
vehicle. However because the pigment particles are so small,
they cluster together to form
agglomerates or aggregates. In some paints, especially gloss,
the size of these aggregates is a very
important factor and so has to be checked. The aggregate size is
known as Degree of Dispersion of
Fineness of Grind. An Emulsion:-
An emulsion is a liquid dispersed in another liquid when there
is no solubility. In vinyl or acrylic
Emulsion, very tiny droplets of resin are suspended within
water, which can now be seen to be a
non-solvent. In an emulsion water is a carrier, not a solvent.
Water is called the continuous phase, and
oil/resin is called the dispersed phase.
