Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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Characterization, evaluation and testing of organic paint coatings 55

• Humidity

• Outdoor exposure

• Immersion tests.

4. Paint durability and weathering tests:

• Accelerated UV

• Color, gloss and chalking resistance.

Durability of a paint system greatly depends upon various conditions

including quality of paint systems, exposed conditions and other factors. Test

methods for developing and monitoring the performance of paints are

designed to simulate the conditions of usage. They are usually designed to

accelerate the degradation process to which paints are subjected. Another

facet of durability is the tendency to withstand abuse in various forms under

conditions of use, and in this case the requirements of different markets

dictate different methods of testing.

In order to evaluate the painted surfaces for properties such as weather-

ing resistance, light resistance, corrosion resistance and adhesive strength,

impact resistance and hardness by instrumental measurement or by

visual inspection, suitable well-documented standards are used that specify

the detailed procedure of evaluation. The most common standards for

characterizing and evaluating paint coatings are those of the following

organizations:

• American Society for Testing and Materials (ASTM)

• British Standards Institution (BSI)

• International Standards Organization (ISO)

• Steel Structure Painting Council (SSPC)

• National Association of Corrosion Engineers (NACE)

• Bureau of Indian Standards (BIS)

• German Standard (DIN).

The approach to characterize a particular coating depends upon its appli-

cation requirements. Characterization and evaluation of paint coatings can

further be divided into two parts:

• Tests carried out on coatings before they are applied to a substrate

• Tests carried out on coatings after they are applied to a substrate.

4.2 Tests carried out prior to application

In sections 4.2–4.10, tests that are performed on paint systems, such as

volume solid, viscosity, VOC content, etc., are discussed in detail.

56 High-performance organic coatings

4.2.1 Volume solid

Volume solid is the most important characteristic of a paint coating. The

will not evaporate, for example, resin (vehicle), pigment and other additives.

The volume solid of a coating is best described diagrammatically as shown

in Fig. 4.1.

% Volume solid 100=×

where volume of non-volatile components in coatingV

‘’

vvolume of ‘volatile’ components in coating

VVV=+

The % volume solid of a paint is important for the following reasons.

1. It is the measure of spreading capacity of a paint system.

volume solid of a coating will give a higher DFT per coat. Based upon

this, three different categories of paint systems are described:

• Conventional solvent-borne coatings with % volume solid of 40–

60%, usually giving a DFT of 50–70 μm.

• High build (HB) coatings with % volume solid above 80%, usually

giving a DFT of 80–100 μm.

• Solventless coatings with % volume solid 99–100%. These are usually

applied by airless spray gun to a DFT of 100–1000 μm, depending

upon the gun parameters.

3. It can be used to calculate paint coverage. How much paint is required

for a given surface area can be calculated from the % volume solid and

Coating container

Substrate Substrate

Volume of wet

coating (V)

Volume of dry

coating (V

)

= Volume of ‘non-volatile’ components in coating

= Volume of ‘volatile’ components in coating

V = V

+ V

= Volume of coating in container

% volume solids =

× 100

4.1 Illustration of % volume solid.

volume solid of a coating is simply defined as the amount of coating that

2. It decides the dry film thickness (DFT) of a paint coating. A higher

Characterization, evaluation and testing of organic paint coatings 57

the DFT required. This is given by a term called the theoretical coverage

rate, given by:

Theoretical coverage rate % volume solid 10 DFT=×/

However, the theoretical coverage rate is not the correct value to calculate

total coverage. Another term, called the practical coverage rate, is used

which takes care of several losses that usually occur during application of

of % volume solid. It is calculated according to ASTM D 2697 [1]. For this,

4.3 Identifying non-volatile matter (NVM)

– which could be resin, pigment and additives. This is calculated by a simple

gravimetric method as follows:

1. Weigh approximately 2 g of paint sample in a lid and spread it across.

2. Place it in an oven at 105°C for 3 h or 120°C for 1 h.

3. Calculate the weight retained as a percentage.

4.4 Density

Density is the mass per unit volume. However, in paint coatings, it is mea-

sured as weight per liter (WPL). For this, a standard calibrated cylinder of

converted to 1 liter is the WPL. This is determined as follows.

4.4.1 Density cup method

For measurement of density of paint coatings, density cups can be used.

Density cups are generally cylindrical in shape, which provides a large

steel covers have an upward slope to a small hole in the center to allow

excess sample materials to be expelled without entrapping air bubbles,

Measurement of density using density cups follows this sequence:

1. Weigh the thoroughly cleaned density cup and record the weight.

2. Fill the density cup with paint whose density is to be determined.

3. Put the cover on the density cup without any tilting. Care should be

taken to avoid any kind of bubble entrapment to get more accurate

results.

paint. These include distribution loss, loss due to anchor profile, etc. Before

going on to calculate the practical coverage, let us first study the calculation

one first has to calculate its NVM and density.

NVM is defined as the relative weight of total non-volatile matter in a paint

volume 100 ml is used and is filled with paint. The difference in weight

opening for easy filling, emptying and cleaning. The tightly fitted stainless

which in turn increases efficiency.

58 High-performance organic coatings

removed with absorbent cloth.

can be calculated using the following equation:

WPL=

weight of empty cup(g)−

vvolume of density cup (ml)

1000×

Usually, the density cups are of volume 100 ml.

•

there can be some error in charging of the batch.

• It can act as a check on the solid content of the paint.

Now to calculate the % volume solid, we need to calculate the weight of

paint in the wet condition. For this, the Archimedes Principle is used to

calculate the applied paint in dry and wet conditions. For simplicity, water

is used as the solvent, whose density is 1 g/l. The calculation is as follows:

• W

= weight of disc in air(g); W

= weight of disc in water (g)

• D = density of water (1 g/l)

• Volume of disc, G = (W

– W

)/D

• W

= weight of coated disc in air (g)

• W

= weight of coated disc in water (g)

• Volume of coated disc, H = (W

− W

)/D

• Volume of dried coating = F = H − G

• Volume of wet coating, V = (W

− W

)/(% NVM × WPL)

The % volume solid is then calculated as below:

% Volume solid

volume of dried coating

volume of wet coatin

100×

4.5 Practical coverage calculations

International Paint [2] devised a method to calculate the practical coverage.

For this, a paint system with a % volume solid of 80% is chosen as an

example. To apply one coat of DFT of 100 μm, theoretical coverage would

be 80 × 10/100 = 8 m

/l. This means that, to apply one coat of DFT of 100 μm,

with a paint of % volume solid of 80%, will require 1 liter of paint to cover

than this value. This is mainly because of the following reasons:

There are several kinds of losses that occur during paint application.

Weigh the filled density cup again.

After obtaining the weights of the empty and filled density cup, the WPL

weight of filled density cup()g

The significance of density (WPL) is as follows:

If density is not within the specification, then there is a good chance that

8 m . However, in practice one finds that the paint required is much more

These can be classified as apparent and actual losses.

4. Paint overflowing after putting the cover on the cup should be carefully

Characterization, evaluation and testing of organic paint coatings 59

4.5.1 Apparent losses

The quantity of paint used on a smooth surface is much less than the quan-

tity of paint used on a blasted (rough) surface. Depending upon the blast

Paint distribution losses

A good quantity of paint is lost when applied by different application

methods as given in Table 4.2. As can be seen, when applied by simple

techniques such as brush and roller, the paint loss is much less compared

to when it is applied by a spray process. Further, the loss is greater when

is an object of complex shape such as round, curved, etc.

4.5.2 Actual losses

The other losses are due to paint application, paint wastage from spillage,

paint handling such as retention in container, on brush or roller, or that

Table 4.1 Extra paint required as the surface roughness increases, as

suggested by International Paint [2]

Surface DFT loss*

Unblasted surface 0 0

Blasted with abrasives 0–50 μm 10 μm

Blasted with abrasives 50–100 μm 35 μm

Blasted with abrasives 100–150 μm 60 μm

Blasted with abrasives 150–300 μm 125 μm

Table 4.2 Distribution losses for various paint application methods [2]

Application method Type of structure Estimated loss

due to distribution

Brush and roller Simple structure 5%

Brush and roller Complex structure 10–15%

Spray Simple structure 20%

Spray Complex structure 40%

Blast profile

*DFT loss is the additonal DFT required to compensate for the blast profile.

Effect of blast profile during surface preparation

profile, additional paint will be required. Table 4.1 lists the extra paint

required for different anchor profiles of the substrate surface.

the object is a simple structure, such as a flat surface, compared to when it

60 High-performance organic coatings

remaining in the spray line, etc. Losses also occur due to premature gelling

during application (e.g. due to improper mixing ratio, high temper ature, etc.).

Further, there are fewer losses when the paint is one-component com-

pared to when it consists of two components. In a two-component system,

the loss due to crossing of pot life is often more. Estimated losses for one

component are 5% maximum, while in the case of two components, the

losses can be as high as 10%.

The difference between the theoretical and practical coverage can be illus-

trated using an example. Let us consider a two-component paint system

with % volume solid of 80% and apply two coats of 100 μm each on a

50 μm. The theoretical coverage is 80 × 10/200 = 4 m

/l. The practical cover-

age will be calculated as follows:

First coat

Required DFT 100 μm

10 μm

Loss due to distribution @ 40% (100 × 0.4) 40 μm

150 μm

Loss due to application @ 5% (150 × 0.05) 7.5 μm

Loss due to wastage @ 10% (150 × 0.1) 15 μm

172.5 μm

Second coat

Required DFT 100 μm

Nil

Loss due to distribution @ 40% (100 × 0.4) 40 μm

140 μm

Loss due to application @ 5% (140 × 0.05) 7 μm

Loss due to wastage @ 10% (140 × 0.1) 14 μm

161 μm

Extra paint required for the next coat is 61% more.

Total loss for two coats

72.5+61

66.75%==

complex structure, with a surface finish of Sa2- with an anchor profile of

Loss due to blast profile

Extra paint required for the first coat will be 72.5%.

Loss due to blast profile

Characterization, evaluation and testing of organic paint coatings 61

This means that 66.75% extra paint is required with respect to the theoreti-

cal quantity, i.e. 1.67 liters of paint is actually required instead of 1 liter.

Practical spreading rate

theoretical coverage

actual paint

rrequired

m liter m

==4167 2391/. . /

Overall loss factor =− × =(.) / .%4 2 39 100 4 40 25

Thus as seen above, the volume solid of the paint coating is an essential and

most important factor which not only helps to determine the spreading rate

of a paint system and DFT, it also helps in estimating the practical coverage

of the paint, which is very important for a user when ordering a paint

consignment.

4.6 Viscosity

of a paint system. It also decide whether a paint coating be applied by brush

or roller or by an air spray or airless spray. The viscosity can also be esti-

mated from its % volume solid. A paint with high volume solid has higher

viscosity than a paint with lower volume solid. Viscosity is modulated by

the selection of vehicle, binder, and its molecular weight; by the use of

of the type and amount of solvents to be employed [3].

Viscosity affects the mixing of the components of the coating during manu-

facture, its stability during storage, the ease with which it is applied to a sub-

of paints, viscosity measurements can give a lot of information about the

Viscosity can be measured by a variety of instruments and techniques. In

cup to a computer-controlled rotation viscometer, have been established for

the determination of viscosity. Using different viscometers, we can measure

the viscosity of samples at low, intermediate and high shear rates.

4.6.1 Ford cup

This is the simplest method, in which a given quantity of paint is allowed

Therefore the practical spreading rate can be defined as:

Utilization efficiency = 60%

thixotropes or rheological modifiers, and most importantly by the selection

causes of paint failures or difficulties one finds during paint application.

The viscosity of paint is an important property. It is the measure of the flow

strate, and the manner in which it flows out to form a smooth layer, free of

defects like runs and brush marks. Since many defects are related to the flow

the paint industry, a number of measurement methods, from a simple flow

to pass through a fixed size of orifice. Initially, the flow through the orifice

62 High-performance organic coatings

is the time in seconds which is used as a measure of the viscosity of the

paint. Low viscosity paints are those with viscosity less than 40 s. Brush

application can be possible up to a viscosity of 75 s. Paints with viscosity

more than 75 s are viscous paints and need to be applied by air spray or

airless spray.

4.6.2 Stormer viscometer

The Stormer viscometer is a rotation instrument used to determine

the viscosity of paints. This method is intended for paints applied by

brush or roller. It consists of a paddle-type rotor that is spun by an internal

motor, submerged into a cylinder of viscous substance. The rotor speed

can be adjusted by changing the amount of load supplied. The standard

tion of 200 revolutions per minute. The standard also includes charts which

enable the instrument weight and rotational speed data to be converted to

Krebs units (a unit of viscosity measure unique to this instrument) [4]. It is

also a convenient method for measuring the viscosity of a sample under

low shear conditions. Table 4.3 relates viscosity as expressed in different

units.

4.6.3 Rotational viscometer

number of different spindles. The choice of spindle and rotation speed

enables measurements to be carried out in the low to medium shear rate

range, i.e. 0.1 to 50 s

−1

The standard is intended for use with non-Newtonian samples and is

particularly applicable to thixotropic paints. Methods are described for

measuring the viscosity under constant and variable shear conditions.

4.6.4 Cone and plate viscometer

The cone and plate viscometer, as the name implies, consists of an electri-

cally driven shallow cone whose vertex touches a rigid temperature-

the plate, and the torque is measured either electronically or mechanically.

shear rate to be attained and this high shear rate corresponds to that pro-

duced during brushing, roller coating and spraying operations.

defines the consistency of a paint as the weight required to produce a rota-

This method uses a Brookfield-type rotational viscometer, which consists

of a variable speed, electrically driven shaft onto which can be fitted a

controlled plate. The test liquid fills the narrow gap between the cone and

The choice of cone geometry and speed of rotation enables a sufficient

is continuous. The point at which there is a break in the continuity of flow

Characterization, evaluation and testing of organic paint coatings 63

4.7 Volatile organic components

Volatile organic component (VOC) means any organic compound having

an initial boiling point less than or equal to 250°C, measured at a standard

pressure of 101.3 kPa. ‘VOC content’ means the mass of volatile organic

compounds, expressed in g/l, in the formulation of the product in its ready-

Table 4.3 Relationship between viscosity in different units (poise, seconds and

KU) [4]

Poise Ford cup (s) Stormer viscometer (KU)

0.01 Out of range Out of range

0.10 Out of range Out of range

0.15 Out of range Out of range

0.22 13.6 Out of range

0.32 15.3 Out of range

0.50 19.0 Out of range

0.65 22.0 Out of range

0.85 27.0 Out of range

1.00 30.0 Out of range

1.25 96.0 Out of range

1.40 40.0 Out of range

1.65 46.0 Out of range

2.00 50.0 Out of range

2.25 55.0 54

2.50 68.0 56

2.75 74.0 59

3.00 81.0 61

3.20 86.0 62

3.40 91.0 63

3.70 99.0 64

4.00 107.0 65

4.35 116.0 66

4.70 125.0 67

5.00 133.0 68

5.50 146.0 69

6.30 167.0 71

8.85 199.0 78

10.70 270.0 85

12.90 Out of range 95

17.60 Out of range 100

22.70 Out of range 105

27.00 Out of range 114

36.20 Out of range 126

46.30 Out of range 129

63.40 Out of range Out of range

98.50 Out of range Out of range

148.00 Out of range Out of range

64 High-performance organic coatings

to-use condition. VOCs are the major concern of environmental protection

agencies all over the world. In paint composition, organic solvent is the only

volatile component, which in standard paint formulations serves as the

carrier for paint pigment. When paint dries, the solvent (VOC) evaporates

and causes several health problems due to inhalation, and therefore is the

cause of great concern. Exposure to VOCs can trigger asthma attacks, create

throat and eye irritation, nausea and headache among other health-related

problems. Long-term exposure can lead to cancer and diseases of the kidney

and liver. Some of the more common VOCs used in paint as solvents and

preservatives include formaldehyde, xylene, toluene and benzene. VOC is

measured by subtracting the % non-volatile matter (NVM), calculated

above, from 100:

VOC NVM=−100 %

The most serious health effects caused by low concentrations of organic

compounds in the air do not result directly from the compounds themselves,

but from products of reactions that the compounds can undergo. The most

notorious ingredients which can cause health hazards are the reaction of

VOCs with nitrogen oxides, and sunlight. In the presence of ultraviolet light

from the sun, VOCs and nitrogen oxides react to produce ozone, a very

reactive form of oxygen that attacks lung tissue. Nitrogen oxides are pro-

duced by high-temperature combustion (such as from vehicle engines and

power plants) and are therefore present in most populated places. Any

additional VOCs will lead to more ozone, and thus more respiratory prob-

lems. The remedial measures are:

• Use paints with not more than 450 mg/l of VOC.

• Use solventless paint systems.

• Use waterborne paint formulations.

4.8 Pigment volume concentration

The paint industry describes the concentration of the vehicle and pigment

in a coating based upon volume instead of weight, because many of the

properties of coatings vary with volume and may be compared on this basis.

(in volume, measured in %) to the total volume of solids’. This is used to

measure the extent of binder that is available to surround and protect the

pigment.

The PVC is a comparison of the relative volumes (not weights) of total

pigment and binder, and is calculated as follows:

PVC%

volume of pigments

volume of pigments volume of binder

×× 100

Pigment volume concentration (or PVC) is defined as the ‘ratio of pigment