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

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Characterization, evaluation and testing of organic paint coatings 75

ported on a frame at 45° to the stream of the sand. The Visual observation

is made to decide the extent of coating failure.

4.14.3 Hardness

Hardness is the resistance of a coating to a mechanical force, such as pressure,

rubbing or scratching. The hardness test of painted surfaces is one of the most

the requirements, there are various methods for testing hardness. These

include pendulum, scratching, and indentation measuring methods.

Pendulum hardness

This method evaluates hardness by measuring the damping time of an oscil-

lating pendulum. The pendulum rests with two stainless steel balls on the

coating surface. A physical relation exists between oscillation time, ampli-

tude and the geometric dimensions of the pendulum. The viscoelastic

behavior of the coating determines its hardness. There are two types of

• The Konig pendulum

• The Persoz pendulum.

Both pendulums have spherical balls which rest on the coating under test

and form the fulcrum. Both employ the same principle, i.e. the softer the

coating, the more the pendulum oscillations are damped and the shorter

amount. The two pendulums differ in shape, mass and oscillation time and

there is no general relationship between the results obtained using the two

pieces of equipment. The test simply involves noting the time in seconds

for the amplitude of swing to decrease from either 6° to 3° (Konig pendu-

lum) or 12° to 4° (Persoz pendulum) [24].

Indentation hardness

This method is suitable for coatings with plastic deformation behavior.

ASTM D 1474 is used for testing the indentation hardness of organic coatings

Table 4.4 Abrasive wheels for different types of coating

Coating type to be tested Type of wheel

Coatings of all types including those resistant to general

handling

CS 10

CS 17Floor coatings resistant to pedestrian traffic

Floor coatings resistant to vehicle trafficH 22

important parameter in evaluating the quality of paint film. Depending on

apparatus defined in this test:

the time needed for the amplitude of oscillation to be reduced by a specified

76 High-performance organic coatings

[25]. The apparatus used in this test is known as the Buchholz indenter. It

consists of a sharp-edged wheel (made of hardened tool steel) which is

ratus rests on a horizontal surface, the effective load on the indenter is 500 g.

The test involves placing the indenter on the coating and leaving it for

cule is then used, together with a light source, to measure the length in

millimeters of the shadow cast by the indentation. The value obtained when

100 is divided by this length is known as the Buchholz indentation resis-

tance of the coating.

Buchholz indentation resistance

indentation length

100

Coatings with elastic deformation behavior should not be evaluated with

this test method, because after removal of the intrument an elastic coating

will show no or very little indentation.

Shore hardness

This is a simple indentation technique in which a sharp indenter is allowed

to penetrate the coating. Depending upon the softness or hardness of

the coating, the indenter penetrates or resists penetration. This is read

directly from an analog or digital reading. There are two kinds of Shore

hardness scale, A and D. For paint coatings usually Shore D is used,

while for soft rubber coatings Shore A is used. There is a relationship

between Shore A and Shore D hardness, as given in Fig. 4.6, which gives

a nomograph scale between Shore A and Shore D along with Rockwell

hardness [26].

Pencil hardness tester

substrate in terms of drawing leads or pencil leads as per ASTM Standard

D 3363 [27]. The pencil hardness test is one of the simplest forms of hard-

ness test. Although some people may consider it to be ‘low tech’, using

pencils to test hardness is still a method used by many coating manufactur-

degrees of hardness to scratch the coating, which then determines its hard-

ness. This test procedure is quite easy to do but still gives reliable and

A complete set of pencils will have the following gradings, starting with

the hardest: 9H, 8H, 7H, 6H, 5H, 4H, 3H, 2H, F, HB, B, 2B, 3B, 4B, 5B, 6B,

7B, 8B and 9B (the 9B is the softest pencil; it contains the most graphite

and the least amount of clay). The ‘H’ stands for ‘hardness’ and the ‘B’

mounted in a rectangular block of metal fitted with two feet. When this appa-

30 seconds. A high magnification microscope fitted with an eyepiece grati-

This test method measures the film hardness of an organic coating on a

ers and some finishing shops. The test uses special pencils with different

reproducible results as it depends mainly on the pencil specifications.

Characterization, evaluation and testing of organic paint coatings 77

stands for ‘blackness’, which comes from the amount of graphite used in

the softer pencils. Artists’ pencils are made with leads that range in hardness

from 9H (very hard) to 9B (very soft). A holder is provided with pencil sets

across the surface of the coating under test. Pencils are pushed into the

test procedure starts with the hardest pencil and continues using progres-

sively softer pencils until the hardest pencil that does not mark the coating

has been established. This is termed the ‘pencil hardness’ of the coating. The

• a permanent indentation of the surface of the coating, and/or

• a scratch or rupture of the surface of the coating.

Durometer A

Durometer D

Rockwell R

150 — — Phenolics

— Acrylics

— Polycarbonate

— Nylon

— Polypropylene

— Auto tire treads

— Inner tubes

— Rubber bands

— Polystyrene

110 —

100 —

90 —

70 —

50 —

120 —

70 —

60 —

50 —

40 —

30 —

80 —

130 —

140 —

95 —

60 —

50 —

40 —

30 —

20 —

70 —

80 —

90 —

Papermaking rolls —

Metal-forming wiper dies —

Nonspark hammers —

Solid truck tires —

Metal-forming die pads —

ldler rolls —

Abrasive-handling pads —

Silk screen wiper blades —

Door seals —

Can tester pads —

Printing rolls —

Polyurethane

elastomers

Hardness

scales

Conventional

plastics and rubbers

4.6 Relationship between Shore A and Shore D hardness along with

Rockwell hardness [26].

which allows a pencil to be held at a 45° angle, while being pushed firmly

sample and the coating hardness is identified by the trace generated. The

standard defines ‘marking’ of the coating as:

78 High-performance organic coatings

4.14.4 Impact resistance

It is very important for many paints to know the effect of impact of a

falling object. ASTM D 2794 provides a falling weight test [28]. Here a

falling weight of 4.26 kg is allowed to fall on the coating from a height

of 50 cm. If the impact area does not show any crack, the coating passes

the test.

4.14.5 Flexibility of coating (ASTM D 522)

Flexibility of the coating is measured using a conical mandrel test [29]. A

the conical mandrel. The mandrel is then rotated, which turns the coating

if it does not show any crack on its surface.

4.14.6 Gloss

Gloss is often described as the attribute responsible for the shiny or lustrous

appearance of a coating. Gloss measurement is essential where an esthetic

which directs a light (that has a similar wavelength range to that of the

20°

60°

Angles of observation

for gloss readings

85°

4.7 Schematic representation of measurement of gloss of coated

sample.

steel specimen of thickness less than 1 mm is coated and fixed vertically in

appearance of the coating finish is required. Gloss is a measurement of the

human eye) at a specific angle to the test surface and measures the amount

into several angles ranging from 0 to 360°. The coating is considered flexible

light reflectance of a paint surface. Gloss is determined using a gloss meter

of reflection. The percentage of the beam that is reflected at the same angle

is measured by a photocell. Completely specular light reflection (perfect

gloss) would be 100% whereas completely diffuse light reflection (matt or

dead flat) would be 0% (Fig. 4.7).

Paint on flat, uniform surface

Characterization, evaluation and testing of organic paint coatings 79

the ability of the surface to bounce back varying amounts of light beamed

to describe the ‘depth of gloss’ and is used to describe gloss and semigloss

paints. The reading at 60° is the measurement of gloss referred to most

satin, semigloss and gloss will have sheen and gloss values falling into the

ranges shown in Table 4.5. The gloss level of a coating is based on

the pigment level in the paint. Pigment reduces the shininess or gloss of

the binder.

One of the most important properties of coatings is gloss retention. A

coating that has maximum gloss retention will remain unchanged for a long

duration. PVDF coatings have very long gloss retention. It has been found

that the gloss retention for PVDF coatings is more than 85% after 10 years

(outdoor exposures) or 4000 h (accelerated weathering).

the fading in color or the tendency to change to some other color. A por-

table spectrophotometer is the most suitable system to measure both the

gloss retention and the color change (䉭E) in a single analysis. The lower is

the value of 䉭E, the better is the color retention of the coating.

4.14.7 Holiday test

Holidays are nothing but very small regions on the coated surface where

the paint failed to adhere. Their dimensions range from fractions of mil-

limeters to a few millimeters. They are caused by several means:

• Air and solvent entrapment that results in the formation of voids or

•

coating or lining dries.

Table 4.5 Gloss ranges for various paints with different gloss measuring

devices

Type of paint 20° Gloss 60° Gloss 85° Gloss (sheen)

Gloss 20–90 70–95+ –

Semigloss 5–45 25–75 –

Satin – 5–25 10–40

Eggshell – 2–15 5–25

Flat – 0–10 0–15

The classification of paints according to gloss ratings depends on

Another important property of coatings is the color change. This defines

bubbles in the film.

Pinholes caused by air or solvent migration through the films as the

on it, and these readings show the relative reflectivity of the coated surface

as compared with a smooth, flat mirror. The gloss reading at 20° serves

often, and is used with all but dead-flat paints. The 85° reading describes

the ‘sheen’ of flat, eggshell and satin paints. Paints described as flat,

80 High-performance organic coatings

• Poor application and drying conditions are usually responsible for exces-

sive air bubble and pinholes. Sharp irregularities and unevenness in the

surface, such as edges, corners and rough or porous welds, may also

produce thin spots or porosity in the coatings.

• Physical damage after application, such as that caused by nicks and

scrapes from equipment and personnel, is also a frequent cause of

holidays.

It is very important to measure these holidays before the coated component

is passed for actual use. A pinhole can be the easy path for the environment

to penetrate the coating, reach the substrate and cause corrosion. These,

material is sent for use.

A usual way of testing for pinholes is by subjecting the coated surface to

a high voltage between the substrate and a scanning electrode (in the form

of a brush, broom, ring, etc.). When the electrode scans over the coated

surface, it creates a spark at a place where there is a break (holiday). The

voltage applied is not arbitrary; it varies with the thickness of the coating.

The relationship between the applied voltage and the coating thickness as

per ASTM D 5162 [30] is given by the following:

• If the coating thickness is less than 1.016 mm then the test voltage V is

given by

VMT= ()

where is the coating thickness and 3294 if is in m

TMT= mm.

• If the coating thickness is more than 1.016 mm then the test voltage V

is given by

VKT= ()

where 7843 if is in mm.

KT=

Table 4.6 gives the test voltage for coatings of various thicknesses.

The electrical potential of low-voltage holiday detectors is usually under

20 mils in thickness applied over steel substrates. High voltages are avail-

able from 900 V up to 45 000 V for testing coatings or linings applied over

both steel and concrete substrates from 20 mils to over 100 mils thickness.

Voltage output can be adjusted to the requirements established for various

types of coatings and linings and their recommended thickness. Most high-

voltage holiday detectors operate on the pulse principle in which a short-

duration pulse of high-voltage electricity is applied to the coating.

The surface of the coating to be inspected should be dry and free of oil,

grease, dirt or other contaminants. Prior to holiday detection, the protective

therefore, must be detected first and must be plugged before the coated

90 volts and they are normally used for testing non-conductive films up to

Characterization, evaluation and testing of organic paint coatings 81

coating should be dried or cured in accordance with the manufacturer’s

by the buyers.

If no uncoated area is available to ground to on the coated side of the sub-

strate, the ground wire should be attached to the uncoated side of the metal

substrate. The ground wire should be held to the substrate by adhesive tape,

clip or other suitable means. Where a test coating is applied to a concrete

substrate, the ground wire should be attached to the reinforcing steel. The

concrete surface must be damp or otherwise conductive in order to com-

plete the circuit.

coating and moved across the surface at a suitable speed. During holiday

testing, complete surface contact between the electrode and the coated

surface must be achieved. Activation of an audible signal implies the pres-

ence of holidays, pinholes or voids; when using a high-voltage detector, a

spark will also be seen when a holiday, pinhole or void is detected. Various

kinds of electrodes are used to scan the coated surface. The ring type is used

to scan a big pipe in the shortest possible time (less than a minute).

After the detection of a holiday, the holiday should be marked by circling

the spot with grease-free chalk. If several holidays are detected in one

general area, a circle should be made around the whole areas. Spot areas

should be touched up and larger areas recoated in accordance with the

cured, it should be retested. In extreme cases of pinholes, in the coating or

lining should be removed and reapplied.

However, precautions should be taken that the voltage should not be too

high, otherwise holes would likely be burnt through the coating or linings

Table 4.6 Test voltage for holiday detection for coatings of various thicknesses

Thickness (mil) Thickness (mm) Inspection voltage (KV)

8–12 0.20–0.31 1.5

13–18 0.32–0.46 2.0

19–30 0.47–0.77 2.5

31–40 0.78–1.03 4.0

41–60 1.04–1.54 5.0

61–80 1.55–2.04 7.5

81–100 2.05–2.55 10.0

101–125 2.56–3.19 12.0

126–160 3.20–4.07 15.0

161–200 4.08–5.09 20.0

201–250 5.10–6.35 25.0

printed instructions, the project specifications, or the procedure permitted

The ground wire should be firmly attached to the substrate to be tested.

specifications and the manufacturer’s instructions. After the repair is fully

The sensor electrode should be pressed flat against the surface of the

82 High-performance organic coatings

to the metal, and the coating would have to be repaired. This would be

expensive and time consuming and could cause serious delays.

4.14.8 Cathodic disbondment test (as per ASTM G8)

A cathodic disbondment test estimates the extent of disbondment around

an already damaged coating [31]. The test is carried out at room tempera-

ture or at 65°C for a period of 28 days. The coated test sample is made to

be the cathode, and platinum is used for the anode. A suitable current is

passed into the cell so that there is a constant voltage of −1.5 V between a

reference calomel electrode and the cathode. To start with, an intentional

hole of 3–5 mm is made on the coated sample, which is dipped into a solu-

tion of 3.5% NaCl. After 28 days, the test is stopped and the coating next

to the intentional hole is physically removed. The corroded area around the

intentional hole (called the disbonded area) is measured and converted into

millimeters radius.

4.14.9 Water permeability test

One of the most important properties of coatings is their permeability for

water vapor [32]. Deterioration of a coating takes place when water vapor

or moisture from the environment enters the coating, diffuses into the

substrate, the corrosion process begins and deterioration of the coating

starts. Thus water permeation basically measures the life of a coating. ASTM

D 1653 is used to determine water permeability.

quarters of its height. It is then weighed and kept in a desiccator which is

phosphorus pentaoxide. The desiccant initially absorbs all the moisture in

the desiccator. Now in order to establish equilibrium between the desicca-

tor environment and the aluminum cup, water from the cup migrates out

which is measured after every 24 h. The experiment continues for 5–7 days

or more till a constant weight loss value is reached. This value is the perme-

ability of the coating expressed in mg/m

/24 h.

4.15 Summary

This chapter has discussed the most important aspects of paint coatings,

characterization, evaluation and various testing procedures. It covers topics

such as coverage calculation, which is important not only for the paint

coating and finally reaches the substrate. Once the moisture reaches the

In this method, free film of the coating is prepared, which is kept as a lid

on a specially fabricated aluminum cup filled with water up to three-

filled with a moisture absorbant such as anhydrous calcium chloride or

through the free film of the coating. This results in weight loss of the cup,

Characterization, evaluation and testing of organic paint coatings 83

applicator and supervisor to account for inventory, but also for the user for

ordering the correct quantity of paint to be used for a job. Although there

are many testing and evaluation methods and procedures based on different

national and international standards, those most commonly used are

reader, paint applicator, supervisor and user to know the basic principles

of various testing procedures.

4.16 References

1. ASTM ‘Standard test method for volume non volatile matter in clear or pig-

mented coatings’, ASTM D-2697-03, Annual Book of ASTM Standards, 2003,

Vol. 06.01.

2. Akzonobel (covering calculations), www.international-pc.com/pc/pds/the_

uk.pdf

3. J. Boxall and J.A. Von Fraunhofer, Paint Formulation, Principles and Practice,

Industrial Press, New York, 1981, p. 16.

4. Viscosity Conversion Chart, AOM-America L.L.C., Bethlehem, PA, USA.

pigment volume concentration (PVC) on the properties of an epoxy coating.

Part I. Thermal and mechanical properties’ Progress in Organic Coatings, 50(1),

2004, pp. 62–67.

6. W.A. Zisman, ‘Surface energetics of wetting, spreading, and adhesion’, Journal

of Paint Technology, 44, no. 564, 1972, pp. 42–57.

7. C.M. Hansen, ‘Characterization of surfaces by spreading liquids’, Journal of

Paint Technology, 42, no. 550, 1970, pp. 660–664.

top coatings at room temperature’, ASTM D 1640, Annual Book of ASTM

Standards, 2003, Vol. 06.01.

9. ASTM, ‘Standard practice for operating salt spray (fog) apparatus,’ ASTM B-

117-03, Annual Book of ASTM Standards. 2003, Vol. 06.02.

10. ASTM, ‘Standard practice for testing water resistance of coatings in 100% rela-

tive humidity’, ASTM D-2247-02, Annual Book of ASTM Standards, 2002, Vol.

06.02.

Annual Book of ASTM Standards, 2002, Vol. 03.02.

12. ASTM, ‘Standard test method for evaluating degree of blistering of paints’,

ASTM D-714, Annual Book of ASTM Standards, 1994, Vol. 06.01.

13. N.D. Cremer, ‘Prohesion compared to salt spray and outdoors: cyclic method of

accelerated corrosion testing’, Proc. Conf. Federation of Societies for Coatings

Technology, 1 Paint Show, 1980.

14. Technical Bulletin F-8144, Q-Panel Co., 1993.

15. ASTM, ‘Standard test method for specular gloss’, ASTM D-523, Annual Book

of ASTM Standards, 1999, Vol. 06.01.

16. ASTM, ‘Standard test method for calculation of color differences from instru-

mentally measured color coordinates’, ASTM D-2244, Annual book of ASTM

Standards, 2007, Vol. 06.01.

covered in this chapter. The idea of this chapter is to appraise the first-time

8. ASTM, ‘Standard test methods for drying, curing or film formation of organic

11. ASTM, ‘Standard practice for modified salt spray (fog) testing,’ASTM G-85,

5. M.T. Rodríguez, J.J. Gracenea, A.H. Kudama, and J.J. Suay, ‘The influence of

84 High-performance organic coatings

17. ASTM, ‘Standard test methods for evaluating degree of chalking of exterior

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18. ASTM, ‘Standard test method for measuring adhesion by tape test’, ASTM D-

3359, Annual Book of ASTM Standards, 1997, Vol. 06.01.

19. ASTM, ‘Standard method for pull off strength of coatings using portable adhe-

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06.02.

20. BSI, ‘Paints and varnishes pull-off test for adhesion’, BSI BS EN ISO 4624,

British Standards Institution, 20 August 2003.

21. BS 3900, British Standard Methods of Test for Paints, 1979.

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taber abrader’, ASTM D-4060-01, Annual Book of ASTM Standards, 2001, Vol.

06.01.

23. ASTM, ‘Standard test methods for abrasion resistance of organic coatings by

falling abrasive’, ASTM D-968, Annual Book of ASTM Standards, 2001, Vol.

06.01.

24. B. Persoz, The Hardness Pendulum PPVEA, 1945, p. 194.

25. ASTM, ‘Standard test methods for indentation hardness of organic coatings’,

ASTM D-1474, Annual Book of ASTM Standards, 2002, Vol. 06.01.

26. Shore hardness conversion chart (A to D), www.machinist-materials.com

05, Annual Book of ASTM Standards, 2005.

28. ASTM, ‘Standard test method for resistance of organic coating to the effects of

rapid deformation (impact)’, ASTM D-2794, Annual Book of ASTM Standards,

1993, Vol. 06.01.

29. ASTM, ‘Standard test methods for mandrel bent test of attached organic coat-

ings’, ASTM D-522, Annual Book of ASTM Standards, 1993, Vol. 06.01.

30. ASTM, ‘Practice for discontinuity (holiday) testing of non-conducting protec-

tive coating on metallic substrate’, ASTM D-5162, Annual Book of ASTM

Standards, 1991, Vol. 06.02.

31. ASTM, ‘Standard test methods for cathodic disbonding of pipeline coatings’,

ASTM G-8, Annual Book of ASTM Standards, 1996, Vol. 06.02.

32. ASTM, ‘Standard test methods for water vapor transmission of organic coating

paint films’, ASTM D-4214, Annual Book of ASTM Standards, 2007, Vol.

27. ASTM, ‘Standard test method for film hardness by pencil test’, ASTM D-3363-

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