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

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Mechanical degradation of organic paint coatings 135

electrochemical measurements after each abrasion step to obtain useful

data about the reduction of the protection properties of the layer as a func-

tion of the abrasion test parameters. For this goal it is necessary to formu-

late a test methodology to make the electrochemical measurements at each

abrasion step; after the electrochemical measurement it must be easy to

continue in the abrasion action. A new electrochemical cell could be intro-

duced like that shown in Fig. 7.10. Step-by-step the sample is taken from

the platform of the Taber test apparatus to carry out electrochemical mea-

surements. It is important that the electrochemical measurements and the

tion in the tested paints. It was demonstrated that the EIS measurement

used (Na

for example). In addition, a thermal treatment at low tem-

perature could eliminate the eventuality of water uptake effects [47, 54].

After the evaluation of the damage the sample could be replaced on the

Taber apparatus for the following abrasion step.

With this approach, considering the coating resistance, it is easy to obtain

information about the damage level of the coating. For example, using

the form of the grains and dimension, the hardness of the abrasives and the

imposed weights on the decrease of protection, as shown in Fig. 7.11. More

interesting could be the possibility of checking the damage produced con-

tinuously during the abrasive action. Nevertheless, it is not possible to

obtain good electrochemical data because of the noise produced by the

abrasion action during damage steps [49].

of an abrasive slurry permit us to carry out EIS measurements without

moving the coated samples from the abrasion apparatus. There is the pos-

sibility of carrying out electrochemical tests more frequently without intro-

ducing noise factors. For this purpose it necessary to use a slurry (water and

7.10 System to make electrochemical measurement step-by-step in

abrasion test.

contact with the electrolyte do not produce additional damage or modifica-

Nevertheless, the modification of the Taber test configuration and the use

results did not influence the damage when a non-aggressive electrolyte is

rubber wheels and free abrasive it is possible to evaluate the influence of

136 High-performance organic coatings

abrasive sand) to allow the electrochemical measurements to be made. A

non-aggressive solution such as Na

is indicated. This method allows us

to evaluate the abrasion effect of slurry with the presence of different

it is possible also to consider whether the presence of an aggressive solution

cal–chemical degradation phenomena. Considering different slurry solu-

ions in the substrate, such as chloride ions, or the action of organic solvents

that could interact with the coating’s organic matrix [55]. For example, Figs

paint of the pH of the slurry and the effect of different organic solvents

(methyl-ethyl-ketone and N-vinyl-2-pyrrolidone) on the decrease of prop-

erties of the paint.

7.4 Understanding damage morphology

In the abrasion tests the analysis of the damaged surface could provide

useful information that is complementary to the electrochemical test data.

The damaged area could be investigated using optical and scanning elec-

tronic microscopes. The environmental scanning electron microscope is

very useful, because it is possible to observe the damaged paint surface

without the sample preparation necessary using traditional SEM to create

a conductive surface.

180, 1000g

320, 1000g

600, 1000g

180, 250g

320, 250g

600, 250g

0 1000 2000 3000

Cycles

4000 5000

Resistance (Ω*cm

)

7.11 Coating resistance trend as function of cycles using abrasive

means with different dimension of grains (180 320 and 600 mesh) and

imposed weight (250 and 1000 g) [47].

abrasives (natural sands and artificial abrasives) [48, 49]. Using this approach

(for coating or for metallic substrate) could have an influence on mechani-

tions, it is possible to check the influence of the pH, the presence of aggressive

7.12 and 7.13 show the influence on the protection properties of a polyester

Mechanical degradation of organic paint coatings 137

Logically the damage morphology depends on the abrasive system used

and the characteristics of the organic coatings tested. Nevertheless it is pos-

sible to introduce the same considerations. Using the standard abrasive

wheels of the Taber test apparatus a, uniform reduction of the thickness

an increase of roughness of the sample surface is shown (Fig. 7.14).

Frequently at this time the presence of localized defects and scratches is

observed, which could reduce considerably the protection properties of the

paint. The action of the wheels produces a typical track orientation. In some

cases after several cycles it is possible to observe the formation of debris

coming from the consumption of the wheels and from the damage to the

paint [5]. Finally the consumption of the total thickness of the organic layer

is produced in some areas, uncovering the metallic substrate.

The use of abrasive sands or particles produces a different morphology in

terms of function of dimension and geometry of the grains. The hardness of

1.0E+11

1.0E+10

1.0E+09

1.0E+08

1.0E+07

1.0E+06

1.0E+05

0 200 400 600 800 1000

Cycles

(Ω* cm

)

1200

threshold limit

pH 3

pH 4

pH 7

pH 12

1400 1600 1800 2000

7.12 Trend of coating resistance as function of abrasion cycling for

abrasion test using aggressive slurry with different pH [55].

MEK B

MEK C

N-vinyl-2-pyrrolidone B

N-vinyl-2-pyrrolidone C

threshold limit

1,0E+09

1,0E+08

1,0E+07

(Ω* cm

)

0 50 100

Cycles

150 200

1,0E+06

1,0E+05

1,0E+04

7.13 Trend of coating resistance as function of abrasion cycling for

abrasion test using organic ketones [55].

after the first abrasion cycle is produced. Then, after about 1000–2000 cycles

138 High-performance organic coatings

with round grains produce uniform damage with a constant reduction of

thickness. The damage morphology appears similar to those produced using

Taber wheels. Increasing the dimension of the grains and using abrasives with

sharp edges gives localized damage with the formation of cut and scratches.

This type of wear mechanism, together with the growth of defects, is the

main cause of the loss of protection properties of the system. The grains of

sand are able to cut and snatch the paint, creating defects in the coating.

The resulting abrasion area is not uniform on all surface samples. A small

uniformly abraded; on the contrary, the internal zone was not uniformly

abraded, as clearly shown in Fig. 7.15. The internal zone is the result of the

superimposition of grooves with an ‘X’ shape, caused by the action of

the two wheels correlated to the difference of the relative speed between

the external and the internal faces of a wheel and the platform. Possibly,

7.14 Abraded surface after 1000 cycles of Taber test (CS17, 1000 g of

imposed weight) [5].

7.15 Optical micrograph of abraded sample surface using imposed

abrasive slurry [49].

load of 500 g, after 1100 cycles obtained by modified Taber test using

portion of the external zone of the damage produced by artificial sand is

sands could be shown to influence the intensity of the damage. Thin sands

Mechanical degradation of organic paint coatings 139

the difference in the damage morphology and extent is due to the different

amount of grains of sand in the two zones. In fact, the grains of sand could

roll towards the external zone because of the rotation of the platform. Also

the effect of the abrasive slurry produces a damage morphology very similar

to those produced by the use of abrasive without solution. The thickness of

the coating appeared not to be uniform on the abraded track. The bare

metal substrate is revealed where the coating is detached by the action of

the grains and also the metal substrate was heavily abraded by the sand

(Fig. 7.16) [49].

The use of an aggressive abrasive slurry could modify totally the damage

morphology. In cases where the solution is aggressive for the substrate,

considering the short time of the test, only an acceleration of the protection

properties is produced. No change in the damage morphology is evident.

Totally different is the case of an aggressive slurry for the organic matrix.

This is the case in the use of organic solvents like alcohols or ketones. In

this last case the effect on the protection system depends on the chemical

interaction between the solution and the organic materials. In some cases

it is possible to observe the formation of cracks in the paints, blistering and

swelling morphology or localized defects that produce uncovering of the

metallic substrate (Fig. 7.17) [55].

7.5 Conclusions

At the end of this chapter it is important to highlight some points about

the effects of abrasion action on a protective organic coating system.

Mechanical damage on paint could reduce the aesthetic aspect (mar) and

the corrosion protection properties. In the literature, several testing methods

are introduced and discussed. Besides, it is not easy to compare the experi-

mental data obtained using different tests.

7.16 SEM micrograph of abraded sample surface using imposed load

abrasive slurry [49].

of 500 g, after 1100 cycles obtained by modified Taber test using

140 High-performance organic coatings

The abrasion effect with the loss of protection properties is the most

important phenomenon. In the tests used to study the effect of mechanical

damage, considering thickness and mass loss, and analysis of the damaged

surface, it is not possible to evaluate completely the damage and the loss

of protection properties. It is necessary to formulate other methods to cor-

relate the damage with the decrease in protection properties. Using elec-

trochemical measurements and particularly electrochemical impedance

spectroscopy, it is possible to highlight the actual damage produced by

abrasive action and the decrease of protection properties of the organic

means (form, granulometry, chemical composition and hardness). In many

cases it is necessary to modify the testing apparatus to enable the electro-

chemical measurements to be made with more accuracy, avoiding noise

problems.

Finally, the use of neutral slurry, abrasive means in a solution, or an

aggressive slurry for the metal substrate or for organic matter allows us to

evaluate the mechanical–chemical degradation phenomenon.

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7.17 Abraded surface of samples using organic slurry (MEK) after 50

cycles [55].

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Part II

Types of organic coating

145

High-performance epoxies and solvent-less

epoxies for corrosion protection

S KANITKAR, Aditya Birla Chemicals (Thailand) Ltd.,

Epoxy Division, Thailand

8.1 Introduction

The word epoxy has become synonymous with anti-corrosion in today’s

industrial environment. Epoxy resin was invented by Dr Castan and Dr

Greenlee in 1936 and commercialized in 1948. Since then, every decade has

seen the evolution of new applications for epoxy resin, thereby keeping

epoxy on a continuous growth path for more than half a century. The

reasons for the success of epoxy resin lie in the unique combination of

properties that epoxies possess. In the following pages we will look in more

detail at the various properties and performance-related attributes of epoxy

resin.

8.2 Epoxy basics

8.2.1 Structure of epoxy resin

Epoxy resin refers broadly to the polymer that contains an epoxide ring (a

three-membered ring containing oxygen and carbon, also called an oxirane

ring and shown in Fig. 8.1). The three-membered ring requires that the

bond angle within the ring be about 60°, which is far from the normal single

bond angle of about 109°. These distortions from the preferred bond angle

raise the energy of the molecule and also bring the three-membered ring

under strain. The strain and the energy together make the oxirane ring very

active and cause it to undergo often exothermic reactions with a number

of functional groups.

There are a number of curing agents available to react with the epoxide

group. The most common of them are amines, mercaptans, amides, phenols,

isocyanates, anhydrides, acids, etc. Many of these curing agents start react-

ing immediately as soon as they are mixed with the epoxy resin and the

reaction proceeds rapidly even at room temperature.