Mar and scratch resistance of automotive clearcoats: development of new testing methods to improve coatings

Tribology Research: From Model Experiment to Industrial Problem G. Dalmaz et al. (Editors) 9 2001 Elsevier Science B.V. All rights reserved.

883

Mar and scratch resistance of automotive clearcoats 9 development of new testing methods to improve coatings p. BERTRAND_LAMBOTTE a, b, j. L. LOUBET a, C. VERPY b a Ecole Centrale de Lyon, UMR CNRS 5513, Laboratoire de Tribologie et Dynamique des Syst6mes, 36, avenue Guy de Collongue, BP 163, F-69131 ECULLY Cedex Patricia.Bertrand@ec-lyon. fr

b PSA Peugeot Citroen, Centre technique de V61izy, Direction Mat6riaux et Proc6d6s - Peinture, Route de Gisy, F-78943 VELIZY VILLACOUBLAY Cedex Automotive clearcoats are subjected to physical, chemical and mechanical damage among which scratch and mar damage play a key role. Car wash brushes are the standard environmental source of scratch and mar damage. Most car manufacturers and most of their suppliers have been using functional tests like rubbing the coating with an abrasive and measuring the level of damage. These tests are only a qualitative way to assess the scratch resistance of clearcoats. Keeping in mind that mechanical properties would be useful in characterising coatings, our method of investigation is based on the use of a stylus to deform the coating surface in a controlled manner. Previously, optical and scanning probe microscope imaging of tests panels brushed by an automated car wash simulator had revealed both plastic scratches and fractures. Then, we have tried to reproduce this kind of scratches by controlling mechanical parameters such as the applied load, the deformation and the strain rate. First, mechanical properties of the clearcoats are measured in indentation tests. Secondly, scratching tests providing greater strain rates allow to extend the indentation results. Finally, indentation tests at controlled temperature have been carried out. For the first time, the evolution of the hardness versus the strain rate at different temperatures has been measured, putting forward the temperature influence.

1

INTRODUCTION

Automotive coatings must provide excellent resistance to chemical and mechanical damage in order to maintain long-term appearance of vehicles and the owner's long-term satisfaction. Most automobiles today are coated with a basecoat/clearcoat coating system (Figure 1) where the basecoat provides the colour component and the clearcoat provides high gloss and protection attributes.

884

The most widely used products for automotive clearcoats are acrylic copolymers crosslinked with melamine resins. Two-component (2K) polyurethane coatings [1, 2, 3 ] are being increasingly used as well. A consequence of adapting two-component finishes is generally an investment in specialised two-component equipment. Since this is not practical everywhere, there are efforts undertaken to develop onecomponent urethane finishes with the same high quality as the two-component products. These kinds of coatings have to be such as to resist [4, 5] chemical agents, scratching, stone chipping, thermal cycling and UV radiation and also present high gloss (Figure 2). Car manufacturers try to define the best product able to resist most of the damages. That is a complex engineering phenomenon implying both chemical and mechanical properties of the product. As a consequence, two approaches to the problem are used : one chemical and the second mechanical.

Automotive clearcoatsdamage Marring

Stonechipping [ ~ ~

Chemical damage

l

_

_

I Deepscratches

'--Scratching

_

consequences such as allowing corrosion of the substrate to begin.

Mechanical damage

Stoneschipping

Scratching /

Figure 3 9 different kinds of mechanical damage on automotive clearcoats.

Marring is either caused by washer machines leading to shallow mars or by hand wiping leading to wide mars. The other difference between the two phenomena is the rate at which the mar is performed. It ranges from 1 to 2 m/s for the washer machine and only from 0.05 to 0.2 m/s for hand wiping [6] (Figure 4).

|

Weathering

_

Acidrain I ~ I

I

Airpollution Corrosion

Temperaturecycling] 1 UVRadiation

Figure 2 : classification of the different automotive clearcoats damage. Chemical damage and weathering may influence mechanical properties and scratching and stone chipping may influence corrosion resistance.

Let us concentrate on the mechanical aspect. Whereas marring only involves the extreme first layer of the coating (the clearcoat), chipping and scratching may affect the whole paint system (Figure 3). Actually, chipping and scratching rather refer to damage associated with serious

There is a great variety of scratching laboratory simulation tests [7, 8]. Most investigators rub a dry or wet abrasive over the clearcoat and assess the resultant damage by the

885

change in "brightness". A research test method that may enable details of the marring process to be elucidated is based on the use of a stylus to deform the clearcoat in a controlled manner. In the test, a cone-shaped stylus is drawn across the surface at a controlled rate under a fixed normal load. By using a series of cones which differ in sharpness, different levels of deformation are created in the coating [9, 10]. The purpose of our study is to contribute to the understanding of the mechanical parameters determining the mar resistance of automotive clearcoats, especially the mars brought about by washer machines.

2

EXPERIMENTAL

Three samples of automotive clearcoats (A, B, C) have been brushed by a washer machine simulator (Touzart et Matignon| Their chemical composition is reported in Table 1. Clearcoats A B C Table

2.1

Description

Composition Acrylic, melamine, 1 component styrene, polyester 2 components Acrylic, polyurethane Acrylic, melamine, 1 component styrene, polyurethane 1 : chemical composition of the 3 clearcoats A, B, C.

Appraisal of clearcoats brushed by a washer machine

2.1.1

Mar resistance

The combination of polyethylene brushes containing dirt in water gives rise to marring. The washer simulator uses an alumina abrasive solution to simulate dirt. Marring appears like a loss of gloss to the naked eye. Mar resistance is determined by measuring, in gloss units, the gloss loss of the clearcoats after being scratched. Clearcoats are by this way classified according to their mar resistance. The best one is clearcoat C and the worst is clearcoat B.

The samples have also been analysed with an atomic force microscope (Park Instrument| : on average, mars are between 0.05 and 2 lam deep and 5 and 10 gm wide. There are both ductile and brittle scratches : ductile scratches present no tearing while brittle scratches show irreversible rips. Consequently, heating can make ductile scratches disappear.

886

Continuous stiffness measurement is used : that means that we superimpose a small oscillation on the large-scale DC loading [12]. By observing the resultant displacement amplitude and the phase shift between the force and the displacement, we can thus infer the stiffness S and the damping of the contact at all displacements. Thus, we can measure the elastic modulus and the hardness as a continuous function of indenter displacement with only one indent. The frequency of the sinusoidal motion ranges from 5.10 -3 to 60 Hz. In the experiments reported in this paper, a 32 Hz frequency is used apart from the indention tests at controlled temperature for which the frequency is 45 Hz. The excitation amplitude is continuously adjusted such that the corresponding displacement amplitude remains constant at 1 nm. The displacement and force amplitudes, as well as the phase angle between the two, are monitored using a lock-in amplifier (frequency specific amplification). 9

Indentation

For each test, an indentation curve (Figure 9) is plotted : it represents the normal load P versus displacement (compliance method).

The experiments are conducted using a procedure in which the applied load P increases 1 dP constant) to reach a P dt exponentially ( --. maximum load of 15 mN. This type of loading is an essential requirement when working on homogeneous polymer materials as the strain rate remains constant during the test [ 13].

887

9

Scratching tests The

method consists in reproducing the

ductile scratches on the clearcoat while controlling the applied stress and recording the strain rate, the strain and the scratching depth. The scratching procedure was chosen for the similarity of its scratches with reality and for their reproducibility. The applied load P is maintained constant along the scratch at 15 mN and the scratching velocities of the different experiments range from 0.1 to 10 pm/s. 2.2.2

defined as following 9 ANt = 35.37.(h r, + h0) 2 (hr' is the plastic depth and h0 is the tip defect, Figure 10).

Tests at different temperatures

As we assume that molecular motions are concerned in scratch resistance, we have carried out the same indentation tests as described previously at different temperatures. We have performed these tests in Oakridge, TN, in the MTS laboratory. The Nano-Indenter was set in a "S-Series Temperature Chamber" commercialised by Thermotron| Its temperature ranges from-70~ to 177~ and its charge rate is 3 to 4 ~ Indentation tests were only performed on the best and the worst clearcoats C and B at 3 strain rates (0.05 s-~, 0.01 S-1, 0.005 S-1) at-IO~ O~ I O~ 20~ 35~ and 45~ 3

dP is the experimentally measured dh contact stiffness. For a Berkovich indenter, ANI is 11) and S = - -

DATA ANALYSIS

The

__1 E*

reduced

_--1--V

]

1-v 2 + ~m

Ei

modulus

is

defined

where Ei and vi are the

Em

Young's modulus and the Poisson ratio of the indenter and Em and Vm are the Young's modulus and the Poisson ratio of the material. Since the indenter is diamond, Ei >> Em and as a consequence

Indentation tests are performed to determine the Young's modulus and the indentation hardness of the clearcoats. Scratching tests provide the scratching hardness.

as the normal applied load P divided by ANt.

3.1

3.2

Indentation

The indentation strain rate is defined as the dh instantaneous descent rate of the indenter (--~t) divided by the displacement at that instant in time (h). It equals 1._P for homogeneous materials. The 2 P reduced Young's modulus is calculated from the following

equation

S = 2. E* . x/AN~ ~

where

AN, is the

normal projected contact area (Figure

by

E * z ~ Em 2 1-V

"

The indentation hardness is defined

m

Scratching In case of scratching, the normal projected

contact area equals ANs =

n .12

3

where I is the half

scratch width and the relationship between contact stiffness and l is the following : /

l S = 2.E* 9~[ANs x/r~~ = 2.E*. 4r~ .

The

projected

contact area Av is the projection in a vertical plan of the front area of the indenter in contact with the material. At is related to ANs by tan(65.3)= ANs Ar

889

A, at g =0.13 s-1 for clearcoat B and at g =0.02 s-1 for clearcoat C. 2.5 2.3

9 Indentation a Scratching

~'2.1 "~ 1.9 4r ~

= 0.02 s"

1.7 1.5 -~ -2.5

-2

-1.5 log(~)

-1

-0.5

Indentation (clearcoat C) log(H) = 0.15. log(~)+ 2.14 (r 2 = 0.992) Figure 13 : hardness data evaluated at differing strain rates in indentation and scratching tests. Norton-Hoff law fit equations corresponding to the indentation tests are reported. Hardness-strain rate data from indentation tests obey a power law relationship H oc~ ~ (Norton-Hoff type, where n is called the viscoplasticity index). Scratching hardness curves complement and extend indentation hardness results. However it seems that the scratching curve is shifted downwards. It means that Hs would be underestimated. We assume that some matter could flow back up behind the indenter increasing the contact area and reducing Hs (Figure 14).

The scratching hardness results do not obey a Norton-Hoff equation. Actually, the plot log(H)versus log(~) does not present a single straight line but 2 line segments of different slope. This "transition" appears at ~ =0.05 s-1 for clearcoat

The viscoplasticity index n from the NortonHoff law is a good means to evaluate the viscoplastic properties of the material since, if n~l, the material rather shows a viscous behaviour and if n >> 1, the material rather exhibits a plastic behaviour. The clearcoats were classified according to the value of this exponent. It tums out that mar resistance as depicted in Figure 5 is quite well correlated with the exponent n obtained in indentation experiments (Figure 15). Of course, this part of the curve concerns g ranging from 0.001 to 0.1 s-1 and while being washed by a washer machine, clearcoats are subjected to d ranging from 105 to 106 s -1 : it would rather concern the part of curve starting after the observed transition. Actually, the relative slope change between the 2 parts of the curve on both sides of the transition point should be quite the same for whatever clearcoat. It would explain why this exponent n corresponding to the part of the curve before the transition, is correlated so well with mar resistance of the clearcoats at high strain rates.

4.2

Indentation

tests

at

different

temperatures Figure 16 shows hardness versus strain rate at different temperatures" f r o m - 1 0 ~ to 35~ for

890

clearcoat B and from 0~ to 45~ for clearcoat C. For a given value of the strain rate, the hardness increases with decreasing temperature. 200

9

-IO~

9

0~

o

10 ~

9

20 ~

o

35 ~

9 O

D

[]

A n

300

O O

~"

g

~

O-

........

200

!

........

|

........

t

0.01

1E-3

1E-4

........

!

0.1

i 10

........

1

- - e - - -10 ~ 0 ~

--*--

7:

- - n - - 10 ~

o~O .

.

.

.

.

!

.

.

.

.

.

.

4--

20 ~

--o--

35 ~

9

.

0.01

0.'1

g

0~

n

10 ~

9

20 ~

o

35 ~

9

45 ~

n

100

9 0

100

n~_______-----n10

........ IE-5

| 1E-4

........

| 1E-3

........

|

........

0.01

i

0.1

........

|

1

.'

.......

t 10

Ya, o

-

-

-

-

~

~

a

--v-- 0 ~ - - n - - 10 ~ - - * - - 20 ~ --o--

Figure 17 9 time-temperature superposition curves of hardness data versus strain rate for clearcoats B and C. The temperature reference is 20 ~

35 ~

-a-- 45 ~ ,

|

0.01

.

.

.

.

.

.

Once more, these plots seem to present a

. 011

transition at ~ ~ 0 . 1 s -1 for clearcoat B and at Figure 16 9 hardness data versus strain rate at different temperatures for clearcoats B and C.

4.29 1

Time-temperature superposition curve

A time-temperature superposition curve has been plotted. The temperature reference To is 20~ That is to say that there is a function aT(T) called

shift factor

such that H ( ~ , T ) - H ( ~-%,To) . aT

~ 0.02 s -1 for clearcoat C. These results are well correlated with those observed in Figure 13 at room temperature.

We noted that the shift factor aT obeys an Arrhenius law" ln a r =

9

-

where Q is

the activation energy and R the perfect gas constant. Figure 18 shows ln(aT) versus 1/T. A linear fit gives the activation energy Q : 106 kJ for clearcoat B and 137 kJ for clearcoat C.

891

4

7

o

,

I

. -2

~

~

~, "

-4

I

-6 i -8

9

i

0.

.

,

, 0.0038

l n ( a r ) = _ 1 2 7 1 5 . 1 + 43.239 T

.

r 2 =0.951

5

I / T ( K -1)

4

eq

""

0

_=

-2

0.0032

0 . 0 0 3 4 ~

0.0036

-4 ln(a r ) = - 1 6 5 0 2 . 1- + 56.071 T r 2

= 0.998

1/T ( K -1)

Figure 18 9 ln(aT) versus 1/T. A linear fit gives the activation energy Q 9 106 kJ for clearcoat B and 137 kJ for clearcoat C.

4.2.2

The Eyring equation

The tests performed at controlled temperature can also be interpreted using an Eyring type activation theory [15]. The Eyring model assumes that the deformation of a polymer is a thermally activated rate process involving the motion of segments of chain molecules over potential barriers. The Eyring equation relates the tensile yield stress Cry (we will take the hardness H), the strain rate ~, the activation energy Q, the activation volume v and the temperature T :

H_ Q + T v-N-T v.N m

.

.

.

.

In 2

where N is the Avogadro constant.

Plots of (hardness/T) against log(strain rate) for a series of temperatures give a series of parallel straight lines (Figure 19) and allows one to calculate the activation volume v.

Figure 20 9 (hardness/T) against log(l/T). The slope of the curves equals Q/Nv which allows to calculate

Q.

892

The activation energy results (Table 3) are in good agreement with those given by the timetemperature superposition curves. The activation volumes correspond to those presented in literature.

Activation volume

Activation energy

v=0.3 nm 3

Q- 110 kJ/mole

v=0.3 nm 3

Q= 143 kJ/mole

Table 3 : activation volume and activation energy of clearcoats B and C determined from the Eyring equation.

4.2.3

Determination

of

the

transition

temperature at 1 Hz Knowing the activation energy and the strain rate at which the transition occurs at room temperature, we can calculate the transition temperature at a reference strain rate of 1 Hz using the Arrhenius equation.

Activation Energy (average) 108 kJ/mole 140 kJ/mole

Transition strain rate at Transition room temperature T~ temperature atlHz (23 ~ 0.10 s1 38~ 54~ 0.02 s-1

44~

40~

Table 4 : transition temperature at 1 Hz determined from the Arrhenius equation. Ta was determined in the PSA technical center with a Stressmeter [16] : the measurement was done at the beginning of the module drop. These results prompt us to think that this transition could be a 13 transition since the activation energies are in the correct order of magnitude. Moreover, that would mean that this transition and the ~ transition are quite well separated for clearcoat B whereas they are not for clearcoat C.

5

CONCLUSION

Automobile clearcoats are complex materials subjected to different damage of chemical and mechanical origins. For this reason, devising clearcoats resistant to most of the damage is a complicated challenge. Subsequently, the whole problem is decomposed in several less complicated studies : chemical resistance, weather resistance, chip resistance, scratch resistance, mar resistance... Next, results from the different studies are gathered and compared in order to formulate c learcoats resistant to most of the damage. Our study focuses on mar resistance and the present paper concentrates on 3 different automobile clearcoats. First, indentation and scratching tests are performed with a Berkovich indenter at room temperature. The corresponding strain rates range from 0.001 s-1 to 2 s-1. For each clearcoat, hardness results versus strain rate show a transition (slope change) at strain rates ranging from 0.01 to 0.1 s-1. Next, indentation tests are carried out at controlled temperature (from -10~ to 45~ With the help of a time-temperature superposition principle, the data of these tests are compared with the previous ones obtained at room temperature. The results are similar: there is still a transition in hardness results and it appears at the same strain rate. This transition obeys an Arrhenius law and the activation energies lead us to believe it could be a 13 transition. Furthermore, the study seems to show that this observed transition and the c~ transition are not separated in case of a mar resistant clearcoat whereas they are separated in case of a poorly resistant clearcoat. Other tests at controlled temperatures are planned to confirm these conclusions. Anyway, viscoplasticity and secondary transitions seem to play a key role in understanding scratching mechanisms.

6

ACKNOWLEDGEMENTS

The authors would like to thank MTS (Oakridge TN, USA) for access to the "temperature chamber" and also would like to express their gratitude to Louis David (PSA Peugeot Citroen, V61izy, Materials and Process Direction) for his

893

help in a fruitful discussions.

cooperation

and precious

REFERENCES 1. T. Engbert, M. Bock, O. Kirihara; High performance polyurethane clearcoats for automotive OEM ; Journal of the Japan society of color material, 72 (2), 102-107 (1999) 2. L. Kahl, R. Halpaap, C. Wamprecht; 2C-PU automotive OEM clear coats with improved etch and scratch resistance; Surface coatings International, n ~ 10, Vol 76, 1993 3. T. A. Potter, L. Kahl; 1K and 2K polyurethanes for automotive topcoats; International Congress and Exposition, Detroit, Michigan, March 1-5, 1993 4. R. A. Ryntz; Coating evolution in the automotive industry; Intemational Waterborne, High-solids and powder coatings symposium; Feb. 18-20, 1998, New Orleans, LA, USA 5. C. K. Schoff; Surface defects:diagnosis and cure; Journal of Coatings Technology, vol. 71, n~ January 1999 6. K. Shibato, S. Beseche, S. Sato; Studies on acid etch and scratch resistance of clearcoats for automotive industry; Paintindia, 1995, vol. 2, n~ 16-22 7. B. V. Gregorovich, I. Hazan; Environmental etch performance and scratch and mar of automotive clearcoats; Proc. 19th international conferance in oragnic coatings, science and technology, Athens, Greece, 1993

8. M. Hartman; Mar and scratch resistance of automotive clearcoats; Surcar 1997, Cannes 9. B. J. Briscoe; Isolated contact stress deformations of polymers: the basis for interpreting polymer tribology; Tribology International, 1998, vol. 31, n~ pp 121-126 10. V. Jardret, H. Zahouani, J. L. Loubet, T. G. Mathia; Understanding and quantification of elastic and plastic deformation during a scratch test; Wear 218 (1998) 8-14 11. W. C. Oliver, G. M. Pharr; An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments; J. Mater. Res., Vol. 7, 6, June 1992 12. S. Bec, A. Tonck, J. M. Georges, E. Georges, J. L. Loubet; Improvements in the indentation method with a surface force apparatus, Philosophical Magazine A, 1996, Vol. 74, 5, 1061-1072 13. B. N. Lucas, W. C. Oliver, G. M. Pharr, J. L. Loubet; Time-dependent deformation during indentation testing; Mat. Res. Soc. Symp. Proc. 436, P. 233, 1997 14. L. Odoni; Th6se de doctorat; Propri6t6s m6caniques et effets d'6chelle; 1999 15. I. M. Ward, D. W. Hadley; An introduction to the mechanical properties of solid polymers; John Wiley & Sons, 1993 16. D. Y. Perera; Stress phenomena in organic coatings; Paint and Coating Testing manual, Ch. 49, 14th Ed. of the Garner-Sward Handbook, Ed. Joseph V. Koleske, ASTM Manual Series 17, 1995