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

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Mechanical degradation of organic paint coatings 125

phenomena in the laboratory because of the presence of several different

the degradation behaviour it is very important to obtain useful information

in a short time. Nevertheless it is not easy to carry out an accelerated test,

with results representative of the actual damage process.

The evaluation of the damage produced during laboratory tests is the

other critical point. The following shows some parameters that are con-

sidered representative of the damage produced but in many cases some

methodologies appear doubtful.

Finally, it is very important that the chosen laboratory test presents

high reproducibility to allow comparison of the data obtained in different

laboratories. Nevertheless, there does not exist an experimental test that

allows assessments of protective properties to be made in an exhaustive

way. This has caused the automotive industry to create a variety of tests

with the intention of simulating the abrasive damage during the service life

of the components [1, 2]. All these tests have some limitations, and they do

not determine a precise value of the protective capacity and only allow us

to perform comparative tests among the different coating systems, giving

rise to limited concepts of the abrasion behaviour [3]. Similarly, several

standards are created to simulate the abrasion and erosion conditions of

the components during service life. Nevertheless there is no generally

accepted test method for quantitative analysis of the abrasion or erosion

resistance.

7.2 Mar degradation

As previously introduced, mar resistance is the ability of a coating to resist

permanent deformation or fracture, resulting from the application of a

dynamic mechanical force. Physical damage that affects appearance is pro-

duced. The major contribution of mar damage is micro-scale scratches

caused by light abrasion. In this damage process there is the production of

gloss. Considering this aspect to obtain useful information about the mar

phenomenon it is important to choose a test which produces minimal

of the protection properties. To evaluate the experimental data it is neces-

sary to consider a property directly related to the aesthetic aspect.

The standard ASTM D6037 (Standard Test Methods for Dry Abrasion

Mar Resistance of High Gloss Coatings) considers the use of the Taber test

apparatus [4–6] to characterize the mar behaviour of paint systems. Two

abrasive wheels are rotated due to the rotation action of the samples. Other

details about this test will be introduced in the following part about abra-

sion resistance evaluation. In the case of mar degradation the CS10 abrasive

parameters which have an influence on degradation. Logically, to evaluate

micro-scale scratches which change the pattern of reflected light and reduce

damage which could influence the aspect of the paint without modification

126 High-performance organic coatings

a low damage action. To evaluate the produced damage, the level of gloss

is measured. With the same test apparatus other authors have produced

more intense damage by using a more abrasive means as the CS17 wheels

[7]. Figure 7.1 shows the Taber test apparatus.

Another possibility to produce surface damage is the ASTM D2486 stan-

dard (Standard Test Methods for Scrub Resistance of Wall Paints). This

standard is not dedicated to the study of mar damage of paint but neverthe-

less could be interesting because light surface damage is produced. In this

case, the abrasion apparatus consists of an abrasive brush. By using different

hardness and abrasive effects of the brushes it is possible to obtain

different levels of damage. Also from the literature it is possible to consider

different tests. A very good review of mar damage testing was made by

Osterhold and Wagner [8].

Several methods to produce aesthetic damage on paints are considered.

The most important test simulates the car wash action in the laboratory. A

brush with sprayed water and quartz powder is used on the coated panel.

A similar test is obtained using the Rota-Hub-Scratch-Tester where a rotat-

ing disc with the scratching medium produces the damage. A similar test

uses abrasive particles contained in the ‘Scotch Brite’ sponge [9, 10]. The

the presence of abrasive powder. Also, micro-indentation hardness is fre-

between the paint and abrasion resistance. Nevertheless, no clear correla-

tion has been highlighted. Other authors consider the micro- and nano-

scratch experiments [11–15]. In this case it is not easy to correlate the

experimental data with the mar phenomenon. Nevertheless, considering

techniques appear interesting. Considering the previously cited test methods

7.1 Taber test apparatus.

wheels or wheels fitted with abrasive paper are frequently used to simulate

Crockmeter produces linear damage using a rubber finger with and without

quently used to evaluate the hardness of the paints to find a correlation

that the mar damage is produced by a very superficial and low action, these

Mechanical degradation of organic paint coatings 127

it is possible to conclude that no one test is considered completely suitable

to simulate the mar damage phenomenon.

The system to evaluate the damage obtained in the laboratory is easier

using AFM technology or a roughness meter. Although a good mapping of

the surface state is obtained in this way, no direct correlation with the aes-

thetic appearance is easily obtained. For this, the measurement of a char-

acteristic that can be directly correlated with the optical nature appears

more interesting. The gloss measurement is then the most suitable tech-

nique. The measurements, carried out at a 60° angle, appear the most indica-

tive because it is an intermediate angle between a 20° angle, for which the

effect of wear is dominant, and an 85° angle, for which the effect of the

morphological texture is dominant on the gloss of the polymer coatings.

Figure 7.2 shows the glossmeter for the measurement of the gloss of a

surface.

7.3 Reduction of protection properties due to

mechanical damage

This type of damage is more dangerous because not only the aesthetic

There are two different phenomena present in this case: erosive wear, where

the velocity of abrasive in air or in solution is important; and abrasive wear,

where a material comes into contact with rugged or abrasive particles. When

the abrasive material abrades the softer material, two-body abrasive wear

geometry is present; when loose abrasive particles remove the softer mate-

rial, three-body abrasive wear geometry is present.

7.2 Glossmeter for measurement of gloss of a surface.

to derive. During damage action, a modification of the surface increasing

the roughness is present. The physical modifications could be easily checked

aspect is lost but there is the possibility of corrosion attack on the substrate.

Mechanical damage can significantly decrease the protection properties.

128 High-performance organic coatings

7.3.1 Erosive wear

Erosive wear is a very important damage phenomenon for the automobile

industry which currently lacks quantitative tests for assessing the durability

of paint systems under conditions of both impact and abrasion by hard

particles.

Some test methodologies are used [1, 16–22]. Frequently the erodent

air, or a centrifugal accelerator is used. Other different types of apparatus

to accelerate the abrasive materials are being developed [1]. Different abra-

[23, 24]. Normally, the damage produced is evaluated by optical and

microscopic observation or by considering the volume of paint removed

per gram of impacted sand. Another possibility is measuring the decrease

in thickness. However, in all cases, no direct indication concerning the

protection property loss is collected. This aspect will be considered.

The most used test is the methodology presented in the ASTM D968

standard (Standard Test Methods for Abrasion Resistance of Organic

Coatings by Falling Abrasive). Abrasive sand falls due to gravity through a

transparent glass tube on the painted panel which is placed at a 45° inclina-

tion. Figure 7.3 (a and b) shows the test apparatus.

Two abrasion means are considered in the standard: siliceous sand with

round grains, called Ottawa sand, and silicon carbide abrasive with edge

grains. Nevertheless it is possible to use the same test geometry with differ-

ent types of abrasive, to assess the quality of the tested paint and the service

life of painted components. Two litres of abrasive fall on the painted samples,

producing an elliptical damage area (25 mm × 30 mm), as reported in

Fig. 7.4. It is important to note that this area is not uniformly damaged with

differing reduction of protection properties. Following the standard, the test

ends when a 4 mm diameter area of substrate without paint is produced.

The abrasion resistance is obtained in litre/mil (0.001 inch) by dividing the

used abrasive volume (in litres) by the thickness of coating (in mil). As an

alternative, the abrasion resistance could be obtained by calculating the

weight of used abrasive (kg) against the thickness of coating (in mil). The

information about the change of the surface aspect. By comparing this test

with those where the abrasive is transported by a stream of gas or air it is

possible to observe that, in this case, the damage is less intense [25].

7.3.2 Abrasive wear

Several tests have been introduced to simulate the actual behaviour of paint

systems to abrasive wear. One test gives a variable tribological intensity and

sion material is used: natural abrasion sands and artificial abrasives, such as

gloss measurement after a fixed quantity of falling abrasive could give

particles are accelerated along a nozzle in a flowing stream of gas, usually

Mechanical degradation of organic paint coatings 129

a variable abraded area during testing. Considering this aspect, the most

frequently used test is outlined in the ASTM G 65 standard (Standard Test

Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel

Apparatus) [26–30]. A rubber-rimmed wheel slides against the surface of a

plane test sample in the presence of abrasive particles (Fig. 7.5a). During

abrasion the area of contact increases. As in previous tests the damage

produced is evaluated by measuring the mass or volume loss. Abrasive wear

is also tested by using a scrubbing brush that moves backwards and

(a)

(b)

7.3 Apparatus for falling abrasive test: (a) complete apparatus;

(b) particulars of the abrasion process.

130 High-performance organic coatings

forwards at a constant rate and force over a substrate [31]. In this case, the

damage is evaluated by measuring the weight loss or by the analysis of the

sample surface.

Other authors use the scratch test [17, 31–37]. The abrasive mechanism

normal load. Different materials are used to produce the abrasion action:

back and forth over the sample, is used. The damage produced is evaluated

ered. All these tests show an increase in the dimensions of the damaged

this case, tests that produce damage on a constant area with nominally

constant tribological intensity could be interesting.

The most important test is the Taber test shown in the ASTM D4060

standard (Standard Test Method for Abrasion Resistance of Organic

7.4 Typical morphology of damage produce by falling abrasive test.

Ball test

Pin test

Organic layer

Substrate

Organic layer

Substrate

Organic layer

Organic coating

(a) (b)

Substrate

Abrasive sand

Organic layer

Fibre test

Force

Substrate

7.5 Test geometry of (a) dry sand rubber wheel and (b) scratch tests.

consists of a cylindrical (or half-cylindrical) tool or a pin, a ball or a fibre

steel, diamond, carbide (Fig. 7.5b). In the case of fibre test, a circle of scour-

ing cloth composed of nylon fibres, which was rotated at 60 rpm and moved

by analysing the scratched surface using a profilometer or using optical and

microscopical techniques. In some cases, the friction coefficient is consid-

area with the consequent modification of the test parameters over time. In

tool. This tool is in sliding contact with the painted flat sheet under a varying

Mechanical degradation of organic paint coatings 131

Coatings by the Taber Abraser). This test was developed to evaluate the

resistance of organic coatings [4, 5], but it is also quite widely applied to

other coated systems, such as hard layers produced by PVD [38–40] or those

produced by sol-gel techniques [31, 41, 42] or coated polymeric materials

[1, 2, 5, 6, 23, 24, 43–46]. The coated sample rotates (60 rpm) and induces

rotation of two abrading grinders, causing mechanical degradation of the

protective layers [2, 23, 24] as shown in Fig. 7.1. To increase the action of

the grinders, a weight in the range 250–1000 g is imposed. The damaged area

consists of an annulus (Fig. 7.6). The abrasion resistance is evaluated by

considering the mass decrease (ΔW, in mg) after, usually, 1000 cycles. From

this value it is possible to obtain the wear (or Taber) index:

TI =Δ ×Wc1000/

where c is the number of cycles.

To evaluate the resistance of organic coatings, abrading grinders CS10

and CS17 are usually used. These are wheels (50 mm in diameter and 12 mm

materials as abrading parts [23, 24]. The abrasion wheels simulate the effect

of abrasive particles during the service life of components. Nevertheless the

abrasion produced using the Taber abrasimeter results in more uniform and

constant damage than that produced by free abrasive methods. The wheels

ciency decreases and to maintain a constant abrasive action it is necessary

to grind the wheels using an abrasive paper each 500–1000 cycles.

To produce more realistic damage some authors undertake the test using

rubber wheels instead of abrasive wheels [47–49]. Between the painted

sample and the wheels, different abrasive means with different characteris-

tics (granulometry, form and hardness of grains) are introduced. The need

testing apparatus as shown in Fig. 7.7 [47, 48].

7.6 Typical morphology of damage area produced by Taber test.

thick) made of a rubber matrix with tungsten carbide and other unspecified

reduce uniformly the thickness of paints. During the test the abrasion effi-

to confine the abrasive on the tested area requires a modification to the

132 High-performance organic coatings

7.3.3 Abrasive wear in presence of solution

In some cases the painted components are damaged by the action of a solu-

tion which contains the abrasive slurry. Several applications could be cited,

including marine environment, chemical plants and car washing machines.

In cases where the abrasive is contained in a solution, the abrasive effect

changes totally. The solution could act like lubricant, reducing the damage

action. Nevertheless the presence of water can produce damage in the paint

due to water uptake with adhesion loss, blister formation and swelling of

the paint. If the solution has an aggressive action against the organic coating

degradation could be observed.

In the literature there are studies of the characterization of the abrasive

effect of an abrasive neutral slurry using a particular test method. Some

authors use a scrubbing brush that moves backwards and forwards at a

constant rate and force over a substrate. A scrub paste consisting of water,

thickener, sand and detergent is used to simulate the car washing process

[50, 51]. Other authors use a ball rotating in a slurry of small silicon carbide

abrasive particles [17]. The ASTM G 105 standard (Standard Test Method

for Conducting Wet Sand/Rubber Wheel Abrasion Tests) works by abrading

a specimen with a slurry containing grit. The abrasive is introduced between

the test specimen and a rotating wheel with a neoprene rubber tyre. The

free abrasive [47].

7.7 Modification of Taber test geometry to study abrasion effect of

or the metallic substrate, a significant influence on mechanical–chemical

test specimen is pressed against the rotating wheel at a specified force.

Other authors have modified the Taber test abrasimeter to produce an

Mechanical degradation of organic paint coatings 133

action that contains the electrolyte and the abrasion types (Fig. 7.8)

[48, 49].

7.3.4 Limitations of test methodology and

measurement of the damage

is not optimal, particularly when the results are obtained in different labo-

ratories [23]. Considering the Taber test, the variability in the properties of

debris, is transferred to some extent to the wheel surface, despite measures

to remove it continually from the specimen. The use of a rubber wheels and

free abrasive could reduce this problem [47].

Some research has investigated whether it is possible to establish a cor-

relation between the data obtained with different test methods. Only

Chisholm et al. [24] compare the Taber test with the jet abrasion test (JAT)

2 3

Taber and the JAT, suggesting that the mechanisms of abrasion for the two

processes give similar results. Moreover, the great problem using the Taber

test, as with the other abrasion test methods, relates to the evaluation of

the damage and to checking the decrease in the protection properties. The

evaluation of damage is made only through mass loss measurements, without

rosion protection performances. This fact is very important because a sample

could present a good Taber index (low mass loss), but its corrosion protec-

tion properties could be notably decreased due to formation of defects.

Figure 7.9 shows an epoxy polyester powder coating after 1000 cycles of

Taber test using CS17 abrasive wheels and 1000 g of imposed load. It clearly

REFERENCE ELECTRODE

APPLIED LOAD SYSTEM

ELECTRICAL CONTACT

ROTATING PLATFORM

COATED SAMPLE

ELECTROLYTE WITH

ABRASIVE

RUBBER WHEELS

slurry and to enable electrochemical measurements without moving

sample from Taber platform [48, 49].

7.8 Modification of Taber test geometry to study effect of abrasive

Some test methodologies present deficiencies. The reproducibility of tests

using a stream of Al O particles. He finds a good correlation between the

the abrasive wheels during use, weathering effect and influence of the wear

considering the form of the damage, and the consequent influence on cor-

134 High-performance organic coatings

shows the presence of a deep defect, probably produced by a third body,

which reduces the protection properties of the coating. Nevertheless the

mass loss value is low. Considering only the mass loss or thickness decrease,

this painted sample presents very good abrasion resistance. On the contrary,

the protection properties are lost locally due the presence of this defect.

Considering this aspect, the only possibility is to use a method that allows

us to highlight the decrease in protection properties as a function of the

abrasion time. The electrochemical measurements and, in particular, elec-

trochemical impedance spectroscopy (EIS) are widely used to check the

protection performance of organic coatings [52]. This approach permits us

to obtain information without the necessity of knowing the reduction of

mass or the decrease of thickness. In particular this technique is very sensi-

tive to the presence of defects which reduce the protection performance of

an organic coating [52].

Ramamurthy et al. [3, 17] introduced the use of electrochemical measure-

ments to estimate the corrosion connected with the damage produced by

the impact test. Rossi et al. [5, 6, 47] introduced the use of EIS measure-

ments widely to check the damage produced by abrasion action in particu-

lar using the Taber test. By using this electrochemical method it is possible

to evaluate the damage trend with information about the decrease in pro-

tection properties (resistance of coating) and the thickness decrease trend

imposed weights is easily highlighted. Using EIS measurements and con-

sidering the coating resistance as representative of the protection proper-

ties of the paint system, to obtain results it is necessary to calculate a

threshold value to determine how protective the coating is. Frequently, for

normal organic layers, the values of the coating resistance equal to 10

Ω cm

could be considered as the limit of the protection properties [53]. Using

this approach, it is important to introduce the possibility of using the

7.9 Damaged surface of powder coating using Taber test (after 1000

cycles of abrasion using 1000 g weight and CS17 grinders) [5].

(capacity of coating). The influence of the type of abrasion wheels and