MANUFACTURE OF PAINT
Supervised by: prof.Dr Farag Abd El-hai
Prepared by: Mohamed Abu Bakr Elgharib
Faculty of science
Applied Chemistry Branch
I would like to express my gratitude for everyone who
helped me during the graduation project starting with
endless thanks for my supervisor prof.Dr/Farag Abd El-
hai who didn’t keep any effort in encouraging me to do a
great job, providing me with valuable information and
advices to be better each time. Thanks for the continuous
support and kind communication which had a great effect
regarding to feel interesting about what I am working on.
TABLE OF CONTENTS
i. Abstract 3
ii. Introduction 3
iii. Classification of paints 5
iv. Paint raw materials 6
v. Factors influencing the paint formula 6
vi. General Rules on Drawing up Formulations 7
vii. Material Flow in a Paint Factory 10
viii. Theory of Dispersion 10
ix. Production of Coating Materials 16
x. Quality control in paint industry 29
xi. Paint defects and application problems 32
xii. Arabic summary 33
xiii. References 37
xiv. Communication information 38
The manufacture of coating materials is a technical process which has been optimized
from economic and environmental perspectives and which is composed of numerous
Transportation, metering, homogenization in agitators and dispersers with subsequent
separation processes by sieving, filtration and centrifugation are the most important
processes during the journey from raw material to coating material.
All the production stages are subject to scrutiny by relevant laboratories. Formulations
developed by the labs address not only the economic aspects but also the quality
standards and environmental regulations.
The specific task for a paint manufacturer–manufacturing a semi-finished product
which will be converted into a coating at a later.
The central task of pigment dispersion forms part of the complex sequence of wetting,
thorough moistening of the agglomerates with the film forming agent solution and the
subsequent mechanical stress in viscous laminar flow gradients.
When it comes to rapid and complete dispersion, the pigment type, size and form are
variables in just the same way as the polarity, size and form of the polymer molecules.
The latter play a fundamental role in shaping the rheological properties and the
surface tension of the film forming agent solution.
The pigment stabilization necessary if pigmented coating materials are to be usable
can be achieved by coating with ions and the associated formation of electrical double
layers or with sufficiently long mobile and well solvated polymer chains with as
complete a coating as possible.
Optimum conditions can only be identified to a limited extent by theoretical
considerations. Experimentation is therefore still the indispensable way of determining
the optimum grinding conditions.
The task of coating technology is to provide surface protection, decorative finishes and
numerous special functions for commodities and merchandise by means of organic
coatings. Many everyday products are only made usable and thus saleable because of
their surface treatment.
To achieve this, relevant coating
formulations, their production plant, the
coating material and suitable coating
processes for the product must be available.
However, the quality to be achieved by means
of the coating process is not the only function
of the coating material used. The object to be
painted or coated itself with its specific
material and design and an appropriate
application process are further variables
which play a significant role. Also taken into
account as the framework defining the conditions in which work is carried out from
development to application.
Two of the most important of the many functions which coatings have to meet are
protection and decoration. Other noteworthy features are the informative tasks and the
achievement of special physical effects.
The conspicuousness of emergency service vehicles, the camouflaging of military
equipment, and road or airport markings are just some of the informative tasks required
Floors and steps can be made nonslip by means of rough or high grip coatings, thereby
increasing their utility value.
By contrast, surface friction can be reduced by use of smooth coatings to produce a high
degree of nonadhesiveness.
Flammable materials can be rendered safe by means of flame retardant coatings.
Antibacterial coatings help maintain sterile surfaces in production and storage facilities in
dairies and breweries or prevent the growth of barnacles and algae on ships’ hulls.
In the electrical engineering sector insulating coatings provide effective and lasting
insulation for wire, windings and condenser materials.
On the other hand, conductive coatings can be used to make insulating substrates
electrically conductive or even to print electrical circuits.
Furthermore, organic coatings can help to reduce noise pollution. Acoustical insulation
coatings for machines and underbody protection coatings for passenger cars are examples
Fig.1 Factors determining the quality of coatings.
iii. Classification of paints:
Classification of paints by physical type:
1) Solvent-borne paints contain up to 80% of solid constituents (binders, pigments and
additives) dispersed in the organic solvent. Solvent-borne paints dry fast and may contain
a wide range of binders. The main disadvantages of the solvent-borne paints are their
toxicity and combustibility.
2) Water-borne paints contain water as the paint solvent. Waterborne paints are non-
toxic and noncombustible but they are characterized by long drying time due to slow
evaporation rate of water.
Water-borne paints based on water-soluble binders contain low molecular
weight polymeric binders dispersed in water in form of true solutions. Water-
soluble binders contain up to 15% of organic oxygen containing solvents soluble in
water (alcohols, glycol ethers, etc.).
Water-borne paints based on polymer dispersions (Emulsion paints)
contain 50-60% of high molecular weight polymeric binders dispersed in water in
form of Colloids. Emulsion paint contain up to 5% of organic oxygen containing
solvents soluble in water (alcohols, glycol ethers, etc.).
3) High-solids paints (Low VOC paints) contain more than 80% of solid constituents
(binders, pigments) dispersed in an organic solvent. VOC - volatile organic
4) Powder coatings are obtained from powdered resin, particles of which are attracted
by the electrostatic force to the substrate surface (electrodeposition). No solvent is
involved in the process therefore powder coatings produce no/low toxic waste. The
main disadvantage of powder coatings is high cost of equipment.
5) Radiation curable coatings are formed from a mixture of prepolymers, monomers
and additives, which is cured under ultra-violet radiation. Radiation curable coatings
harden fast and contain no solvents. The main disadvantage is relatively high cost.
Classification of painting products
by their functions:
1) Paint - colored non-transparent
2) Varnish - transparent or semi-
transparent protective coating. A
varnish is made of binder, solvent
and additives. Some varnishes
contain small amounts of pigment.
3) Enamel - hard protective coating
with glossy finish.
4) Primer - the first coating applied to the
surface in order to enhance the adhesion of the final paint (topcoat) and to seal the
substrate surface. Primer may be formulated to impart additional protection to the
substrate (e.g. anti-rust primer for steel substrates).
iv. PAINT RAW MATERIALS:
Paints is made up of numerous components such as:
1) Resins or Binder.
v. Factors influencing
the paint formula:
The paint and coating raw
materials manufactured out of
them meet the varied
requirements of practical
applications. The formulations
which have to be developed for
this contain not only detailed
information on the raw
materials to be used, but also
provide precise details on the
Fig.2 Typical composition of coatings.
Fig.3 Factors influencing the paint formula.
quantitative proportions, the sequence in which the formulation ingredients are to be
combined and the settings for the machines to be used in production.
The correct selection of the raw materials must be based on accurate knowledge of the
interaction in the product used for its later purpose, knowledge of the processing methods
and the available plant and machinery at the paint manufacturer. The varying
requirements of the processor or end user relating to final product quality, in conjunction
with the environmental demands, the application processes and the prices to be achieved
in the market place are variables which have to be covered by a diverse range of raw
materials. The composition to be selected for a coating material is therefore subject to
preconditions, requirements and external influences.
Bearing in mind that in the automotive sector alone the number of producible
formulations together with the necessary raw materials is hugely increased as a result of
the desired variety of colors and effects, it is no wonder that major paint factories have to
have many thousands of formulations on hand.
vi. General Rules on Drawing up Formulations:
1. Purpose and Quality:
If the customer merely wishes decorative goals to be met, more attention will have to be
paid to the pigmentation and the selection of the effect materials than to the other raw
materials. With many pigments a high gloss requires a low pigment level, which results
in an inadequate hiding power.
In order to achieve a large amount of room for manoeuvre in terms of color and gloss a
certain separation of functions in the form of two coats has become accepted The hiding
power is provided by a base coat, irrespective of gloss requirements, while the gloss itself
is delivered by an additional clear coat on top of the base coat.
If the purpose of the paint or coating is the protection of commodities, the selection
criteria are aimed initially at the film forming agent. Special demands are made on low
water vapor and ion permeability for coatings subject to long term outdoor exposure for
the protection of metal objects.
Further selection criteria include excellent adhesion under extreme climatic conditions
combined with the physical and chemical effect of anticorrosive pigments. If, for
example, zinc dust is used for active corrosion control, the pigmentation level is a
decisive factor. Only at weight concentrations of more than 90% do the metal particles
touch each other, thereby permitting their protective effect to develop.
2. Production Resources, Application Systems and Object to be Painted:
When it comes to pigment dispersion formulations for the manufacture of colored coating
materials where dissolvers are used differ considerably from those produced in agitator
mills. The pigment-binder ratio and the rheological properties have to be adapted to the
particular machines by use of appropriate solvents and additives.
The processing systems in operation at the paint customer also have a significant impact
on the formulation. It is obvious that roller requires different material properties from the
spray guns or dip baths of industrial scale paint processors.
If more complex objects incorporating cavities with difficult access have to be coated,
dip-coating processes are the only alternative to the standard spray application. If the
latest electrocoating process is chosen, all the formulation ingredients have to be matched
to the electrochemical precipitation process.
Watersoluble resins can be made to coagulate on the objects to be painted, which are
connected to form electrodes, by means of changes in the pH value, and these determine
3. Function in Paint System:
Primers, whose main task is corrosion control, are provided with resins from the raw
material side which feature excellent adhesion even under extreme climatic conditions.
Aromatic epoxy resins, aromatic polyurethanes or modifications of these can therefore be
used because of their good adhesion, despite the lack of light fastness, as no account
needs to be taken of the lack of light fastness in the aromatic structural elements. These
are always optically covered by a further functional layer, the primer surfacer. This
second layer provides everything needed to make the topcoat optically attractive while at
the same time meeting all the demands on mechanical-technological properties
Primer surfacers therefore have to prime the surface, i.e. even out any unevenness and
be well sandable so that its surface can be smoothed, if required. Primer surfacers also
take on the function of protection against stone chipping as an intermediate coat in
conjunction with the topcoat. With this purpose in mind, resins have to be selected whose
glass temperature is in the operating temperature range. Furthermore, extenders have to
be used which create predetermined breaking points under extreme impact loads. This
prevents the coating from separating from the substrate.
The light fastness requirements on the primer surfacer coat are not as rigorous as those
As with the primer coat, epoxy ester resin and polyurethanes are the standard film
forming agents for the functional intermediate coating. The tasks of the last coat, the
topcoat, are quite different. The emphasis here is on the visual properties, provided that
the mechanical-technological properties have been addressed. A high level of gloss and
pronounced optical color effects are the usual requirements for the topcoat.
The functionability of the coating depends also on good adhesion to the primer surfacer
and a high degree of hardness with good wear and scratch resistance at the same time.
When it comes to selecting the raw materials for topcoats, resistance to sunlight and
outdoor exposure, but also to chemicals from industrial emissions and natural
atmospheric effects such as tree resins and bird droppings, are the primary considerations,
in addition to the abovementioned factors.
Topcoat formulations differ significantly, therefore, from the abovementioned items.
Weather resistant alkyd-melamine resin or acrylic-melamine resin paints or
polyurethanes based on polyester or acrylic resins are the standard film forming agents.
By contrast with primer surfacers, the principle with single film topcoats is to use the
minimum necessary amount of pigment.
4. Cost Effectiveness and Availability:
The demands for cost effective production have a particular impact on formulation
In the manufacture of pigmented paints the incorporation of pigments in the paint
formulation causes the greatest production input. The goal, therefore, is to identify the
combination of pigments, film forming agents, additives and solvents which permits
rapid dispersion of the pigments with the highest possible pigment component and thus
the smallest possible volume.
In addition to manufacturers’ demands relating to cost effective production the processor
also requires coating materials which enable large areas to be covered at a low
consumption rate. Paints always have a high yield if a high hiding power with a low dry
film density at the same time makes it possible to apply thin film thicknesses.
5. Occupational Health, Safety and Environmental Protection Regulations:
Paint raw materials are regularly replaced by others for occupational health, safety and
environmental protection reasons as new knowledge becomes available.
However, great efforts are still required to achieve emission free, physiologically
harmless paint technology which is reflected in the paint types and their formulations.
The replacement of volatile organic solvents by water, the reduction of solvents by
increasing the solid content, or the replacement of volatile solvents by reactive diluents,
but also the complete omission of solvents in the use of powder coatings has brought new
formulation principles to the process of developing paints.
Novel film forming agents, new additives and physiologically less harmful pigments are
superseding the existing range of raw materials continuously.
Environmentally polluting emissions of organic solvents and additives containing heavy
metals are being avoided by means of new environmentally compatible painting
vii. Material Flow in a Paint Factory:
Unlike plants operating continuous production processes the paint and coatings industry
has to operate in batch production mode as this is the only way of handling the
production demands of the variety of products.
Since customer orders differ in terms not only of the formulation itself but also of the
order quantity, the plants must have an appropriate range of production systems.
Fig.4 Process map for paint manufacturing.
viii. Theory of Dispersion:
1) Pigment Specific Properties for Dispersion Processes:
Whereas the manufacture of clearcoats is limited to the mixing of various film forming
agents with solvents and additives, the production process of pigmented systems is based
on complex wetting and stabilization processes.
The interaction of the pigments with the incident light results in selective shares being
removed from the white light and the remaining being scattered as widely as possible.
The first property results in Chroma, while the second is the prerequisite for good hiding
power. The ability to produce Chroma is proportional to the pigment surface area which
can interact with the light.
This means that for a specified pigment quantity the degree of selective absorption
increases with decreasing particle size up to a discrete stage. This stage is reached when
the particles are completely penetrated by light.
The scattering power increases with the number of dipoles in the pigments which can be
excited by light. For this reason the scattering power increases initially with increasing
However, this increase only continues as long as all the dipoles can interact in each
pigment particle with the incident light. But this is only possible up to a specific particle
size for each pigment. Above this critical size the dipoles inside the pigment become
optically inactive. Furthermore, interference occurs, resulting ultimately in a decrease in
scattering power. There is, therefore, an optimum particle size or particle diameter for the
Such small particles tend to combine to form larger units as a function of their shape and
polarity. If the primary particles are in contact with each other along edges and at corners,
the conglomerations with diameters of more than 100 µm can become very large, though
the forces of attraction remain relatively low. Such particles are termed agglomerates.
If the primary particles are connected via common surfaces, the forces of attraction
increase to such an extent that separation via mechanical means is only possible with
difficulty. However, the volume of such aggregates is much lower. The cavities in the
agglomerates and the cavities in the pigment powder in the delivery form can represent
up to 75% of the overall volume, depending on the pigment form and size. The pigment
volume concentration (PVC) is then only 25%.
The forces of
attraction among the
are an addition of
Coulomb forces, van-
and hydrogen bridge
bonds, depending on
structure. The range
and strength of the
forces of attraction
are very greatly
influenced by the
type of interaction.
2) Wetting and Decomposition of Agglomerates:
Wetting of the pigments is a
surface energy process. The
matching of the surface tension
of the pigment to be dispersed
with the interacting binder
solution is thus the decisive
fundamental prerequisite for
successful pigment processing.
The surface tension is the
surface related work which has
to be performed to form the
surface which is newly created
by the wetting of the pigments.
Thorough wetting of the pigment
surfaces by resin solutions is
only possible if the energy gain
Fig.5 Scheme of dispersing and stabilizing of pigments.
Fig.6 Schemes of the most important dispersing equipments.
resulting from the interaction of fluid and pigment is greater than the work which has to
be performed to increase the surface area of the liquid.
The wetting of the outer agglomerate is therefore followed by the penetration of the resin
solution into the cavities of the agglomerates.
Various features are used to describe the material specific dispersion properties of
pigments. These include the oil absorption value and the wetting volume.
The type and quantity of solvents and solvent combinations also have a bearing on
viscosity and surface tension, while also determining the configuration of the resin
molecules which are in the form of coils.
Since the low molecular solvent molecules have much greater mobility than the film
forming agents, they also penetrate the agglomerate cavities more quickly. They generate
a preliminary wetting of the pigment surfaces, termed pseudo wetting, during dispersion.
The solvent type and the film forming agent polarity must therefore be matched to each
other such that the pseudowetting by solvents is then converted to real stabilization by
replacing them with film forming agent molecules. Only part of the agglomerates can be
destroyed by the physicochemical interactions of the pigments with the resin solution.
Complete conversion to the primary particle with the best possible pigment wetting at the
same time can only be achieved by the additional transmission of mechanical forces to
Numerous dispersers have been developed to transmit shear forces to pigment
agglomerates by laminar flows of viscous fluids. The most important are dissolvers,
which are suitable for easily dispersible pigments, three roll mills for highly viscous
pastes and the very effective agitator mills for universal use. A small number of ball mills
are also in use, though these are uneconomic by comparison with agitator mills and
3) Stabilization of Pigment Dispersions:
The uniform distribution of the pigments brought about by the dispersion process is only
a feasible technical solution if the distribution state is also retained during formulation,
storage, processing and the subsequent film formation. The reagglomeration of the
primary particles would result in a reduction in gloss and a change in the tinting strength.
Other side effects would include color shifts in pigment mixtures, specks and
Stabilization by means of electrical double layers This is a relatively simple method of
describing surface energies when there is a polar, i.e. ion forming environment, which
makes it a useful method for practical operations. The mathematical correlations which
are known by the term DLVO theory show that a pigment dispersion is stable if the
repulsive forces acting on the surface as a result of ion adsorption exceed the van-der-
Waals’ forces of attraction.
VT = VA + VR
The total potential surface energy VT is composed of the attraction VA and the electrically
induced repulsion VR energy levels.
Steric stabilization while the DLVO theory allows calculations to describe conditions
with a good fit when assessing pigment stabilization in aqueous media, this method is
ineffective in a nonpolar environment.
To permit stabilization, the pigment must be surrounded by polymer chains which are
fixed punctually but which otherwise can move freely. When two particles stabilized in
this way approach, the mobility of the polymer chains and thus the entropy are reduced.
The selection of a suitable solvent is also important when stabilizing pigments. Because
of the greater mobility of the solvent molecules the primary wetting of the pigment
cavities is initially achieved by solvent molecules. A subsequent complete wetting
conversion is only achieved if a gain in free enthalpy is identified through the wetting
process of an adequate concentration of film forming agent molecules.
The use of relatively nonpolar solvents enhances the immediate wetting process with
polar binder molecules and at the same time the wetting conversion of the already
adsorbed solvent components.
Mixed stabilization the process of correlating theoretical considerations on the
stabilization of pigment surfaces with practical operations is made more difficult by the
fact that different stabilization
mechanisms can overlap. Mixed
forms of particle stabilization were
discovered during the calculation of
electrically stabilized gold sols after
the addition of nonionic
polyethylene glycol. Heller and
Pugh, for example, discovered that
the existing stabilization of
electrically stabilized gold sols can
be further improved by adding
Fig.7 Mixed stabilizations of pigments and their effect on the zeta-potential.
The zeta-potential, which is an indicator of the stability of pigment distributions.
Stabilization limits despite the best possible stabilization the primary particles can
approach the range of the forces of attraction if the kinetic energy is sufficiently high, for
example at relatively high temperatures. This leads to pigment flocculation.
Flocculation as a result of solvent loss and inadequate covering of the pigment surfaces
can occur despite optimum dispersion and the best possible stabilization if the material to
be dispersed is wrongly handled when lacquering pigment concentrates.
Pigment shock and solvent shock in particular are commonly encountered in coating
Fig.8 Pigment shock by differences in solvent concentrations of dispersion and lacquer.
Fig.9 Solvent shock by unprofessional addition of solvents to the dispersion.
4) Optimum Mill Base Formulation:
This is done by determining the solid content of a film forming agent solution at which a
specified pigment quantity can be converted into a flowable mixture with the minimum
amount of resin solution.
The pigment absorption capability is quantified using indicators for better evaluation. An
important indicator for determining the optimum mill base composition is the yield value.
This is defined as the volume of a film forming agent solution which is required to
convert a specified pigment quantity into a flowable state. The characterizing feature of
this property is that the mixture flows in a continuous thread from a glass rod.
If the compositions of the ternary
mixtures of film forming agent, solvent
and pigment determined from the
individual yield points are plotted in a
concentration triangle, the ranges of the
flowable mixtures can be attractively
presented in graphical form.
If the two ends of the yield point curve
are extended to the triangle sides and the
area of the supercritical pigmentation is
hatched, the area enclosed by the curve
with the exception of the hatched,
supercritical area contains all the flowable
Super critically pigmented mill base compositions lie above the critical pigment volume
concentration (CPVC), at which the volume of the film forming agent is lower than that
of the spaces between the pigments. The CPVC can be calculated from the bulk volume
of the pigments and must be converted into the critical pigment weight concentrations
(CPWC) for entering in the concentration triangle.
ix. Production of Coating Materials:
The formulations drawn up by the laboratory must be supplemented by further detailed
standards of operation and test specifications for large scale production. The latter refer
equally to technical processing requirements and to quality.
Manufacturing instructions contain information on the order in which raw materials are
added, machine settings and temperatures for homogenization, dispersion and filtration
systems, information on process monitoring and interim tests. Information on approval
tests, instructions for filling methods, the container type and size to be chosen, together
with safety information and labeling on the containers must be clearly specified in the
Modern make-to-order production can then use one of three methods:
i. Direct dispersion
ii. Production of pastes with later lacquering
iii. The production of mixing paints
Fig.10 concentration triangle of film forming agent, solvent
and pigment to demonstrate the flow curves.
While the mixing paint principle has won through for the very varied color requirements
of decorative and automotive refinishing paints, paste production and direct dispersion
are commonly used for production for large scale users.
Mixing paints are coating materials with a standardized color and tinting strength which
mostly contain only one pigment and are turned into the final product by simply stirring
together, without the addition of a binder. They are stored by paint users such as dealers
in specially developed mixing machines and converted into the end products in
accordance with given formulations.
Direct dispersions and products made from pastes are manufactured by the paint
manufacturer and sold as ready-to-use paint. Paste production of individual pigments has
proved its worth, despite the expense of storing the standardized pastes, because of the
advantages of the more effective and thus more economical dispersion of batches for
grinding. The individual pigment pastes are combined in particular mixing ratios as per
the color requested by the processor, with completion to form the coating material as per
the specification involving the addition of film forming agent solutions and additives.
1) Agitation and Agitators:
Visitors to a paint plant will see the basic mechanical process engineering operations to
combine materials via agitation in the dissolving of resins, the mixing of liquids, the
premixing of material to be dispersed or the tinting of color pastes.
In all cases the aim is to eliminate inhomogeneities in material mixtures. These can reveal
themselves in differences in concentration, density or color. Temperature differentials or
different refractive indices can also be used to characterize the mixing quality.
The homogenization of liquids or liquid-solid mixtures is carried out by generating flows
which are as irregular as possible with the aim of achieving the desired degree of
homogenization as quickly as possible with
minimum energy input.
If, because of excessively high viscosities, random
shifts in position can no longer be achieved by free
product flow but only by means of pressure or shear
forces, this is termed kneading.
The task of homogenizing free flowing material
mixtures is achieved by agitation using vertical and
horizontal flows. These are generated by rotating
agitator blades. Numerous types of agitator have Fig.11 Characteristic flow in a stirring vessel.
proved their practical value, depending on the type of material and the desired degree of
Homogenization which has to be carried out under economic conditions is best achieved
if horizontal and vertical circular motions are present in the mixing vessel at the same
time. When homogenizing materials of different density it is recommended that vertical
flow be used because of the tendency towards sedimentation of the heavier components.
Wherever possible, movement of the entire amount of material together in the mixing
vessel, which occurs frequently during the agitation process, should be avoided because
of the unwanted separation caused by centrifugal forces.
The mixing time is short if the components to be mixed undergo a large number of
changes of location. This can take the form of movement of the agitator itself or of
material flows generated by the agitator. They can be achieved by impact, flow around
obstacles, crossing directions of flow and speed differentials at the interfaces of parallel
Two different agitator designs have become established. Either the entire contents of the
vessel are circulated by means of a large agitator surface area and slow material motionor
small partial quantities are transported at high speed by means of impact and mixing
eddies with a small agitator surface area and a high rotational speed. The first concept is
represented by blade agitators, and the second by propeller agitators. At high agitation
speeds the occurrence of conical eddies can be observed which entrain air into the
mixture. To avoid this, baffles are fitted in the agitators or the agitator shafts are
A broad range of different consistencies has to be covered in the paint and coatings
industry when homogenizing materials. The viscosity of the material to be agitated is
therefore adjusted continually by altering the rotational speed or by means of gear
mechanisms and interchangeable agitator blades.
The anchor mixer is an old design which is unsuitable from an economic
perspective. It rotates slowly and is now only still used in resin reactors for better heat
transfer and to eliminate vapour bubbles. It is not suitable for homogenization tasks
because of its lack of vertical flow.
The blade agitator is also a simple agitator design. It is mainly used to maintain
already homogenized mixtures. Because of the size of the agitator blades there is the
danger of radial motion being induced in the agitated material. Such motion can be
prevented or reduced by means of baffles. Vertical flow is not very pronounced in blade
agitators. Flow is different in the beater or the modern variant, the multistage impulse
agitator both achieve intensive vertical motion.
The propeller agitator moves part of the mill base at high speed. It entrains the
axially and discharges it in turn axially, this ensures good vertical flow, and the undesired
circular motion can be prevented by rotating rings. The propeller agitator copes with a
broad range of consistencies and has therefore become accepted in practical paint
production operations as a universal machine. Its disadvantage is that the generated flow
is often insufficiently turbulent, as a result of which long mixing times have to be
Safety regulations must be observed when operating agitators and other mixing
equipment. Explosion protection systems, rigorous earthing of all agitators and vessels,
Fig.12 Different agitator types.
and the fitting of grilles in vessel openings are important measures to take to ensure
accident free operation.
2) Dispersersing and Dispersers:
The task of dispersers is to separate pigment agglomerates in viscous fluids from each
other by the transmission of shear forces.
At the same time the conditions required for the stabilizing covering of the pigment
surfaces which is also necessary are created by suitable raw material selection and
formulation. The shear forces generated in the machines by flow are enhanced by
supplementary rotation of the particles by centrifugal forces.
In addition, there are also tensile and compressive loads at work which help in breaking
down the agglomerates. Since the transmission of shear forces right to the level of the
primary particle must be possible, flow states must be ensured which enable the
transmission of forces irrespective of particle size.
The machines used for pigment dispersal are varied, and there is a wide range of designs.
Nonetheless, all have the same operating principle. They transmit high shear forces in
laminar flow gradients to separate the agglomerates. The most important machines used
commonly in large scale production are dissolvers, agitator mills and roll mills.
Dissolvers: Are disc agitators which can transmit shear stresses to pigments under
certain conditions which are sufficiently high to ensure dispersion.
The necessary conditions for this are met if the agitator disc and the agitator vessel are
matched in size and this is complemented by appropriate agitator disc sizes and shapes,
an adequate rotational speed, an optimized fill level and adapted mill base formulations.
Even when all the conditions are met, the shear forces transmitted by dissolvers are not
high. Dissolvers are therefore only suitable for the dispersion of easily dispersed
pigments, such as for dispersion paints and decorative paints. Different dispersion
systems must be used if requirements are more demanding.
Dissolvers are then only used for preliminary homogenization or preliminary dispersion.
This enables the throughputs of the actual dispersion systems to be significantly
increased. To generate the necessary laminar flow the sizes of the vessels and agitator
discs and the fill level and disc configuration must, as mentioned above, be matched to
Optimum dispersion results are achieved if the vessel diameter is 2.5 to 3 times the size
of the disc diameter, and the distance from the agitator disc to the base corresponds to 0.5
and the fill level to 2 disc diameters.
Under such conditions dispersions are
possible once the tangential speed UA at the
periphery of the disc has reached 24 m/s.
The limit speed is therefore a function of
the disc size.
Apart from conventional dissolvers
numerous special designs will be found in
paint plants. They differ in the type and
number of agitator discs and in the
configuration of the agitator vessels. Of the
multishaft machines, the twinshaft dissolver
is the most commonly encountered
disperser. It has agitator discs which are
offset in height, mounted on two separate
shafts, and thus better agitation properties.
Fig.14 Schemes of different dissolver types.
Rotor-stator agitators (inline dissolvers) have also proved successful for high speed
homogenization, particularly for the manufacture of waterbased paints. Because of the
high rotational speed of the rotor fast and effective distribution of the formulation
ingredients is achieved with these, particularly when systems with a high pseudoplasticity
Fig.13 Scheme of flow in dissolvers and pictures of dissolver
equipment and a dissolver disc.
have to be processed. They
feature small dispersion
chambers and are mainly used
for continuous processes.
The principle of dispersion by
means of friction rolls or roll
mill is implemented in single-
roll, two-roll, three-roll and
multiroll mills. The only one of
any significance for the paint and coatings industry is the three-roll mill which has
become less important because of its relatively low throughput by comparison with other
types of dispersing systems.
Its use is now limited to the manufacture of fillers, highly viscous printing inks and,
because of its gentle treatment of the pigment surfaces, to the dispersion of sensitive
surface treated pigments and temperature sensitive pastes. Three-roll mills consist in
essence of three metal or plastic cylinders configured one after the other. The middle roll
has a fixed mount, while the two outer ones are pressed against the middle one with
pressures of up to 1,000 bars. High shear forces can be transmitted by viscous liquids in
the narrow gaps between the rolls as a result of the different rotational speeds of the
A speed ratio of 1:3:9 yields the best and most cost effective dispersing results, as has
been repeatedly confirmed in numerous experiments.
The rolls themselves generally have an
extremely hard, 1 cm thick outer
lining. Beneath this there are softer
elements on the inside. These enable
better heat transfer to the cooling hoses
inside the rolls.
Before a three-roll mill is used, the mill
base should undergo preliminary
dispersion in a dissolver or kneader,
depending on its consistency. The mill
base is then transferred to a feed device
Fig.16 Picture of a three-roll mill.
Fig.15 Scheme and function of an inline dissolver.
between the first two rolls. The mill base is drawn into the gap as it adheres to the wall of
Because of the shear stress in the direction of flow and the diminishing gap opening a
pressure is generated in addition to the shear load which promotes dispersion. This
external pressure also causes pressure rises inside the agglomerates which may still
contain air and promotes the decomposition processes.
The width and height of the pressure profile depends on the diameter of the rolls and the
viscosity of the mill base. It is all the narrower, the thinner the mill base and the smaller
the roll diameter. The effect is enhanced by the shear of the material, caused by the
differing roll speeds.
The pressure is at its maximum shortly before the narrowest part of the gap. This is
followed directly by a zone of under pressure, as a result of which a sudden pressure drop
causes pigment wetting which is advantageous for the mill base.
The mill base is then separated at the exit of the gap and distributed onto both rolls in a
ratio corresponding to the rotational speed. With a speed ratio of 1:3 an appropriate
amount of material is transferred to the second roll.
Corresponding processes also take place between the second and third rolls. The
dispersed mill base adhering to the third roll is then removed from the roll by a doctor
blade and transferred to a storage container. As with the other disperser types, the
viscosity of the material to be dispersed must be adjusted to the three-roll process. If the
viscosity is too low, this results in spraying in the filling gap and in inadequate shear
forces; high viscosities, on the other hand, do not cause problems by comparison.
However, because of the high energy dissipation and the corresponding heat buildup,
appropriate measures must be taken to ensure good cooling or supplementary high
boiling liquids must be added to prevent the rolls from drying out. The latter would cause
irrevocable damage in the form of grooves in the roll surface.
Another application nowadays for the three-roll mill is the flushing of pigments. The
pigment cake, which is generally aqueous, still wet and therefore not yet or only slightly
agglomerated during pigment production, is not dried using the otherwise normal process
during flushing, but is converted from the aqueous to the organic phase by treating it with
organic binder solutions in kneaders.
The intention of the subsequent roll dispersion is less to decompose the agglomerates but
more to remove the residual water completely. Consequently, the rolls are not cooled, but
are heated with steam to ensure faster removal of the water. This method enables high
tinting strengths and good fineness levels to be achieved with pigments that are hard to
disperse. Carbon black pastes manufactured in
this way, for example, are used to produce haze
free, glossy, deep black topcoats.
Ball mills: the ball mills have been displaced
more and more by agitator mills because of their
poor economy. Their use is now limited to just a
few applications in dispersing. Ball mills are
hollow cylinders which rotate about a horizontal
axis in operation. They are filled to about 40%
of their volume with mill base and grinding
media, generally porcelain or steel balls with a
diameter of 20 – 30 mm, in a given ratio. As a result of the relative motion of the
grinding balls passing each other at different speeds, friction surfaces are generated
which enable the dispersion of the pigments. Above a certain rotational speed the moving
grinding media are lifted and cause an additional unwanted impact load on the pigment
agglomerates as they fall back down. This results in decomposition of the primary
particle, which is associated with deterioration in the optical power of the pigments. The
drum is therefore set to rotate more slowly for roll friction.
The actual dispersion process occurs whenever grinding media pass each other at
different speeds and generate the shear stress necessary for dispersion at the point where
they are closest together.
Because of their closed design, however, ball mills
have the benefit of being able to use low boilers as
solvents. A further advantage of ball mills worth
mentioning is their low maintenance during the
The attritor: This consists of an upright milling
vessel in which 3 – 5 mm thick balls are artificially
moved by means of an agitator. The mill base is fed
into the mill chamber at the bottom and drawn off
again at the top. The cycle carries on continually
until the required particle fineness or tinting strength
has been achieved.
Fig.17 Function of a three-roll mill and pressure
profile in the milling gap.
Fig.18 Function of a ball mill.
The continuing shortcomings of a high level of wear and the
high power consumption were largely eradicated by
changing the shape and the type of the agitation tools and by
making the grinding media even smaller. The attritor became
slimmer and taller, with the outcome that it was possible to
achieve adequate dispersion after one pass. In principle,
therefore, the agitator mill had been invented.
Agitator mills: which were usually open and fitted with
wedge wire sieves were increasingly replaced by closed
systems. By a closed system we mean a disperser which
enables the separation of the mill base from the grinding
media by means of friction gaps. Such closed agitator mills
bring numerous additional benefits.
Dispersion can take place at increased pressure, thereby
making higher throughputs possible. Material feed or
material throughput can be varied by means of infinitely
adjustable pumps in
order to achieve better
adaptation of the
disperser to material
specific features in
many areas. As with
ball mills, steps must
be taken to provide an
system in agitator
Dyno Mill: Dyno Mill is mainly used in ink industry, paint making plant, food plant
and medicine industries.
Fig.19 From ball mills via attritor to
Fig.20 Scheme of an agitator mill.
Basket Mill: The basket mill is a submersible
milling unit that will achieve particle size reduction
without the use of hard to clean pumps, hoses, and
tanks. The basket mill allows a greater amount of
material to pass through the milling chamber more
often resulting in a narrower particle size distribution
and stronger pigment strength in a shorter amount of
Sand Mill: It uses a different type of impeller and
it is charged with glass or ceramic as a grinding
media. Pigments are ground by shear and attrition and the finesse of the dispersion is
determined by the length of time the paste is in the mill, or residency time. This mill
consists of a high-speed shaft with one or two solid disc impellers rotating off center in a
cylindrical tank with a cone-shaped bottom, filled with small grinding beads.
3) Separation Processes in the Manufacture of Paint:
In the paint manufacturing industry
numerous separation processes also
have to be carried out during the
production of pigmented and non-
pigmented coating materials. Examples
include the partial or complete removal
of solid particles from liquids, the
separation of grinding media in the
wedge wire sieves of agitator mills, the
recovery of hard agglomerates or the
removal of foreign matter before the
coating materials which are ready for
dispatch are transferred to their relevant containers. The separating operations are carried
out in sieveing and filtration units or by accelerated sedimentation in centrifuges.
Sieving and filtration Sieving or screening is a sizing process and is used for
fractionating mixtures of solids. This can take place in the gaseous phase or in a liquid
phase. The principle of separation by sieves lies in offering up the solid particles to be
separated as frequently as possible to the openings of the sieve surface.
The aim is to permit smaller particles to pass through, while holding larger ones back.
Fig.22 Different forms of sieves.
Fig 21 basket mill.
The sieves with their defined openings are made of metal, organic fibres or plastics.
These are divided into perforated, woven or clamped sieve surfaces, depending on the
design. To ensure a consistent long-term separating effect it is important to ensure that
the sieve design does not permit any shift, i.e. any enlargement of the openings.
The necessary stability in the sieve surface is achieved by the use of monofilament yarn,
a fixed weave in the fabric and nonswelling materials.
It is also important in practice to use materials which are not attacked by the material
being separated. Depending on the material to be separated, solvent, acid, alkali or
bacteria resistant sieve fabrics should be used.
When dealing with hot materials ensure that the heat resistance of the used materials is
High specific separation performance is achieved when all particles are offered up to the
sieve openings as frequently as possible by inducing movement of the material to be
separated. The frequency with which material is offered up can be increased by
horizontal or vertical motion and by vibration.
Filtration is the separation of solid particles from suspensions with the aid of porous
filter media. Strictly speaking, therefore, the filtration process is the complete separation
of solids (filter cake) from the liquid phase (filtrate).
Depending on the purpose of the filtration process, the treatment of the liquid is termed
clarification, and separation of the solids is termed cake filtration.
Because of the small size of the pigments the term ”filtration” is applied when fine and
extremely fine sieves are used although the dispersed pigment is still in the filtrate.
Whether such processes are termed sieveing or filtration in the paint and coatings
industry depends merely on the degree of fineness of the separation system.
Filtration processes are divided into surface filtration, cake filtration and deep bed
filtration, depending on the separation method employed.
Surface or sieve filtration is a method of solids separation which obeys the laws of
sieveing. It is used when only small quantities of solids have to be separated by means of
fine sieve bags with pore sizes of up to just a few micrometers (clarification).
Cake filtration is a separation process in which the solid cake accumulating on the
partition also serves as a filter aid at the same time. It enables the isolation of large
quantities of solids. With a pure surface filtration system all the solid particles are larger
than the openings in the perforated plate. With cake filtration this is only partially true. In
the course of the filtration process the solids form a cake which then enables the
separation of finer particles. In both processes the solids are deposited above the filter
Fig.23 Principles of filtration techniques.
The principle of deep bed filtration is quite different. This uses thicker filter media with
larger pores than the diameter of the solids to be separated. As a result these penetrate the
labyrinth of the channels in the filter medium where they are lodged either mechanically
in relatively narrow side passages or adsorptively on the internal walls of the capillaries.
Because of the fineness of the pore labyrinth deep bed filtration is not generally carried
out with pigmented paints.
Deep bed filtration is mainly used for purifying clearcoats and binder solutions. The
mechanical choking of the channels, by contrast with purely adsorptive separation, results
in a decrease in filter performance. On the other hand, the adsorptively retained particles
can be transported more or less quickly through the channels if filtered for long enough,
depending on the size of the attractive forces, with the result that deep bed filters can
”blow” after excessively long use. Deep bed filtration allows even relatively small gel
particles, i.e. deformable particles, to be separated if their diameter is small enough.
Larger gel particles, on the other hand, choke the pores of the filter medium and result in
a rapid drop in filter performance. To prevent blockages in the fine filter channels,
filter aids are
frequently used to
Fig.24 Scheme and picture of a plate-and-frame filter press.
The main differences between the plate-and-frame filter press
and the chamber filter press are its larger chamber volume of
the former and the option of carrying out flushing and washing
operations. It is therefore used for filtering suspensions with a
high solid content, such as in cake filtration for recovering
pigment powder during the manufacture of pigments.
The reasons for the increasing importance of bag filters
are essentially commercial. Higher throughputs, lower costs for
rigging, lower material costs and less waste merit particular
Sedimentation Solid particles of different sizes can be
separated from the liquid phase selectively or completely if
there is a difference in density between the liquid and dispersed phases. With gravity
sedimentation the acceleration due to gravity g and, in the opposite direction, the
frictional forces of the fluids act on the solid particles.
x. Quality control in paint industry:
Quality control is a set of procedures carried out to ensure that a manufactured product
and performed service adheres to a defined set of criteria and standard
values, before, during and after manufacturing, to ensure customer satisfaction and
conformance with statutory regulations.
The raw materials, manufacturing process and finished products undergo stringent QC
1. Paint manufacturing process:
thing should be ensuring that raw materials are available.
Then, batch sheet is issued, based on the quantity of raw materials available & sales
Next thing is to ensure that the container and HSD and Grinding machine is very
Manufacturing instruction has to be adhered to in order to achieve the desired
2. Water based paints manufacturing process:
Water based paints is processed in a HSD (high-speed dispersion) tank, in which a
circular, toothed blade attached to a rotating shaft agitates the mixture of pigment,
Fig.25 bag filter equipment.
extenders, wetting & dispersing agents, little quantity of water, and defoamer until
the pigment particles are fully dispersed.
Once the dispersion is certified okay, by the Quality Control, the temperature of the
mixture is controlled to 300▫C, before adding the binder & other raw materials
remaining in the Batch sheet.
The paint is then mixed and sampled to the laboratory to check Quality Control
parameters (viscosity, S.G (Specific gravity), Colour, Opacity, drying, texture -
consistency, gloss or sheen, NVC (nonvolatile content) etc.) to ensure conformance
with the set standard.
Thinning & Tinting occurs.
Once the paint is certified okay by QC, it is then PASSED & PACKED as finished
product. However, QC ensures that packaging containers are properly labelled, free
from dirt & that products are packed to level.
3. Oil-based paint manufacturing process:
The first step in making oil-based paint involves mixing the pigment and fillers with
little resin, little solvent, wetting and dispersing agent to form a paste.
It is then routed into a sand mill or grinding machine (a large cylinder that agitates
and grinds the pigment and filler particles, making them smaller and dispersing them
throughout the mixture).
After about 30minutes, the fineness of grind is checked by the Quality Control
Personnel. If okay, the paint is discharged & made-up. At this stage, the remaining
raw materials yet to be added are added.
Thinning & tinting then starts.
Quality Control parameters (viscosity, S.G, Colour, Opacity, drying, gloss, NVC etc.)
are checked to ensure conformance with the set standard.
Once the paint is certified okay by QC, it is then PASSED & PACKED as finished
However, QC has to ensure that packaging containers are properly labelled, free from
dirt and that products are packed to level.
4. In-process/finished product analysis:
PH: 7.5 – 9.0
Dispersion & Fineness of Grind
Viscosity determined by Ford Cup (seconds) for low viscous products and
Rotothinner (Poises) for highly viscous products.
Bleed resistance/ flocculation
The Texture of the paint is determined by applying it on the wall using a Texcote
roller to check for sagging.
Color – using spectrophotometer
Opacity or Hiding power is measured by painting it over a black surface and a white
surface. The ratio of coverage on the black surface to coverage on the white surface is
Gloss or Sheen is measured by determining the amount of reflected light given off a
painted surface, using a Gloss meter.
Adhesion is tested by making a crosshatch on a dried paint surface. A piece of tape is
applied to the crosshatch, and then pulled off. A good paint will remain on the surface.
Resistance to soapy water is tested by a machine that rubs a soapy brush over the
paint's surface. – Wet Abrasion Scrub Tester
Weathering and Resistance of the color to fading is determined by exposing a
portion of a painted surface to outdoor conditions i.e. sunlight, water, extreme
temperature, humidity, and comparing the amount of fading to a painted surface that
was not exposed.
Coarse particle and foreign matter – stick, rope, sack etc.
Flash Point is the temperature at which the mixture of the paint vapour and air can
ignite in the presence of a spark. The higher the flash point, the safer a solvent-based
paint is considered for storage.
xi. Paint defects and application problems:
1 Settling Low dispersion
2 Paint Separation Incompatibility
3 Foaming Mixing at high speed, insufficient defoamer.
4 Foul smell& Mould growth Micro-organisms
5 Sagging, no texture/ pattern Too much water, sand omitted
6 Low viscosity Excess solvent
7 High Viscosity Insufficient solvent
8 High Specific gravity Insufficient solvent
9 Low Specific gravity Excess solvent, foaming
11 Foreign matter Adding foreign contaminants without manufacturers
specification (lead to film defect)
11 Chalking (is the progressive
powdering of the paint film
on the painted surface).
Polymer degradation of the paint matrix, due to
exposure from UV radiation.
12 Erosion (Erosion is a very
due to external agents like rainfall
13 Peeling/Blistering Improper surface treatment before application&
dampness present in the substrate.
14 Cracking When paint coatings are not allowed to cure/dry
completely before the next coat is applied.
15 Pigment Flocculation The pigment, after dispersion, reverts to a greater or
lesser degree, when rubbed. (Colour change)
16 Tacking/ not drying Insufficient drier
شكل3خطالمائي األساس ذات البويات إنتاج.
 Prof. Dr. A. Goldschmidt and Dr. H.-J. Streitberger Wiedenbrück, BASF
Handbook on Basics of Coating Technology ,July 2007.
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xiv. communication information:
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