Stress Development and Weathering of Organic Coatings - American

Chapter 21

Stress Development and Weathering of Organic Coatings Dan Y. Perera, Patrick Schutyser, and D. Vanden Eynde

Downloaded by UNIV LAVAL on October 25, 2015 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch021

Coatings Research Institute (CoRI), Ave. P. Holoffe, 1342 Limelette, Belgium Regardless of the mechanism involved in the development of stress in organic coatings due to weathering, when high hygrothermal stresses are induced they can, alone or in combination with fatigue processes, cause coating degradation, such as cracking and/or detachment. It is suggested in this paper that hygroscopic stress participates in formation and/or enlargement of pathways in the coating, which enable the transport of the electrolyte to the metallic substrate provoking its corrosion.

Protection and decoration, two main functions of an organic coating, are directly dependent on the coating resistance to environmental degradation. Competitive and/or legislative pressures to produce environmentally and user friendly, durable coatings is generating abundant research. In attempting to develop such coatings it is important to be able to predict their service life. To reach this aim at least two conditions are necessary: (i) to have a methodology for predicting the long term weatherability (1), and (ii) to understand the main facets of coating degradation (2). To reach this understanding a variety of techniques, such as spectroscopy and measurement of mechanical properties, gloss, transport of water, oxygen and ions (3,4), could be used. A number of studies (5,9) contributed significantly to elucidating the chemical changes occurring in certain coatings during weathering, and to the development of tests for monitoring the rate of photo-oxidation. Physical aging, another process that can affect coating durability, is the relaxation process of a material in glassy state arising as a consequence of its nonequilibrium condition (10-12). In the coating's approach towards equilibrium, its density increases thus inducing important changes in its mechanical, thermal and electrical properties, some of which, e.g., increase in hardness, can decrease the coating's durability. The importance of electrochemical aspect of corrosion in the comprehension of the degradation of organic coating used to protect metallic substrates has also to be mentioned (13,14). If chemical changes in coating composition due to the action of UV radiation, moisture and temperature are unquestionable factors in the degradation of most of organic coatings, it is now accepted that stresses arising in a coating are also playing an

© 1999 American Chemical Society In Service Life Prediction of Organic Coatings; Bauer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

323

324 important role in this process (15,16). There is also evidence that the stress accelerates the photochemical and thermo-oxidation degradations (17). This paper focuses mainly on the role of stress in the failure of organic coatings exposed to weathering and wet conditions. Stress phenomena If pure mechanical effects, e.g., stone chipping, are neglected, the main causes inducing stress in a coating are (18) fdm formation, and temperature (T) and relative humidity (RH) variations, producing, respectively, internal (S ), thermal (S ), and hygroscopic (S ) stresses. In practice, these stresses interact resulting in a low or high total stress (S ): F

T

H


tot

F

T

S =S ±S ±S

H

(1)

tot

The positive and the negative signs, arbitrarily chosen, indicate the coating tendency to contract (tensile stress) and expand (compressive stress), respectively. Since during film formation the coating is practically always shrinking, S is positive. Equation (1) indicates the possibility of a high tensile stress at low T and RH, and a high compressive stress at high T and RH. Other situations are discussed in detail in references 18 and 19. It could be shown that in an identical environment, the stress arising in a coating is dependent on the coating previous history. For example, the stress in a coating cured and kept at room climatic conditions (e.g. 21°C and 50% RH) will be different from that in a coating brought to a temperature above the glass transition temperature (T ) for sufficient time. In the first case S = S - S and in the latter one S = S - S . This is so because at T>T , S could relax, and the cooling develops S . It is important to add that a high stress reduces adhesion (20-22), and favors the formation and propagation of cracks in a coating. This is especially true for any coating in the glassy region where the stress cannot easily relax. The interdependence between stress and adhesion (21) is expressed by: F

F

g

T

tot

H

tot

H

F

T

g

p - cS e

(2)

where P, c, S and e are the elastic energy acting against adhesion, coating thickness, stress and strain, respectively. Therefore, the higher the stress magnitude (S) arising in the coating, the higher is the coating tendency to detach from the substrate. Failure of organic coatings by loss of cohesion can occur at relatively insignificant stress levels, far below the tensile strength. One of the reasons for this is the presence of stress concentrations in the film due to coating heterogeneity that can initiate fissuring and/or cracking. Experimental Stress. The thermal tensile and the maximum hygroscopic compressive stresses measurements were made with CoRI-Stressmeter, Braive Instruments, Liege, Belgium (18,19), and an experimental device that allows stress measurement under liquid water (23,24).The thermal tensile stress was measured by cooling the samples at a rate of 0.4 °C/min, under dry conditions,froma temperature above T to one below it (i.e., free of g

In Service Life Prediction of Organic Coatings; Bauer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

325 physical aging).The maximum hygroscopic compressive stress was determined by exposing the samples to various RH or water. In this study the samples consisted of coated, pre-calibrated stainless steel substrates. Materials. The materials used in this study were an acrylic/polyurethane, an acrylic/melamine, a polyester/melamine, a polyester/TGIC powder coating, a polyisobutyl methacrylate (Plexigum), a high T acrylic latex, a fast degrading epoxy, and a polyurethane. Accelerated weathering tests. These tests were carried out in a QUV apparatus by submitting the samples to continuous exposure of UV-A or UV-B at 70°C, or alternating dry/wet cycles of 4 hours of UV-B at 60°C, and 4 hours condensation at 40° or 50°C. Downloaded by UNIV LAVAL on October 25, 2015 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch021

g

Hygrothermal stress and weathering If during weathering a coating undergoes photo-initiated oxidation, hydrolysis, and/or thermal degradation, it is also exposed to an increasing hygrothermal stress that often results in coating cracking and/or delamination. Figure 1 shows the stress (S) dependence on T and RH of a 40 um thick acrylic/melamine coating exposed for 7 days,

Figure 1. Stress (S) dependence on temperature (T) at 0 %RH (left), and on relative humidity (RH) at 21°C (right) for a 40 um thick acrylic/melamine coating weathered in a QUV apparatus during 7 days. (1) = continuous UV-A or UV-B exposure; (2) = alternating cycles (4 hours UV-B at 60°C, and 4 hours condensation at 50°C).


326 in a QUV apparatus, to different weathering conditions. The results indicate not only that, as expected, the type of weathering affects the magnitude of stress as a result of different chemical reactions, but also the great sensitivity of the stress method to detect changes in organic coatings. For many organic coatings important changes in the S = f(T) and S = f(RH) are observed even after a few hours of weathering. Figure 2, describing S = f(T) and S = f(RH) for an acrylic/polyurethane coating about 90 pm thick after different periods of weathering, demonstrates that weathering strongly affects the stress development of this coating. The displacement of the curves S = f(T) to higher temperatures, and the increase in the slope of the curves S = f(RH) indicates an increase of T and of the coating hydrophilicity, respectively, with the duration of weathering (15,16). These properties induce a high tensile stress under dry conditions, and a high compressive stress under wet conditions which, in combination with the fatigue process due to alternation in T and RH occurring during weathering, eventually causes the cracking of the coating. The increase of T with the duration of weathering observed with many coatings can have one or more causes acting separately or, more likely, simultaneously. Such causes could be loss of residual solvents, cure completion (25), and above all, transformation of flexible segments into less flexible ones (15,16). This last cause results from a chain scission due to the photo-oxidation reactions occurring during weathering. This is confirmed by the change of the slope of the curves describing S = f(RH) (see the right side of Figure 2), an indication of an increase in polar functional groups.


g

g

S(MPa)

T(°C)

RH(%)

Figure 2. Stress (S) dependence on temperature (T) at 0 %RH (left), and on relative humidity (RH) at 21°C (right) for a 90 pm thick acrylic/polyurethane coating after different periods of exposure (h, hours) in a QUV apparatus. Weathering conditions: alternating cycles (4h, 60°C, UV-B and 4h, 40°C, condensation). Cracking of the coating occurs between 1032 and 1368 h.



327

Figure 3. Stress (S) dependence on temperature (T) at 0 %RH (left), and on relative humidity (RH) at 21°C (right) for an about 75 urn thick polyester/TGIC powder coating after different periods of exposure (h, hours) in a QUV apparatus. No cracking of the coating occurs during 1306 hours of exposure.

It is important to add that there were also cases where the coating is cracking without a significant increase of T . For these cases an important vertical upward displacement of curves S = f(T) to high tensile stress values takes place. One possible explanation of this process would be that weathering induces the formation of new, photo-oxidized products with higher cross-link density. Figure 3 illustrates the case of an organic coating with good weathering resistance, i.e., a polyester/TGIC powder coating. Contrary to acrylic/melamine coatings used in this study, this powder coating is practically unaffected by the accelerated weathering for the period of time investigated. Indeed, for this coating S = f(T) and S = f(RH) are not changing or changing very little with weathering. The results obtained here are in agreement with the behavior of these coatings under tropical conditions. While the acrylic/polyurethane coatings investigated are cracking after a few years, the powder coating is still undamaged after the same period of time. It is important to note that in Figures 1 -3 the stress was calculated by considering the total film thickness, i.e., as if the whole thickness (40, 75 or 90 fim) is submitted to the UV aggressive action. In reality, the weathered layer is certainly much thinner, at least during the first stages of aging. This implies that the coating can be considered at least as a bi-layer system. The mathematical equations developed to calculate the stress in each layer of a multicoat system (26) can also be used to determine the stress in the g


328 weathered layer more accurately [see equation (3) for a bi-layer system]. Such calculations indicate that in most cases the stress developed in the weathered layer is much higher during the first period of weathering than those presented in Figures 1-3, and references 15 and 16.

c (f+2c,+c ) 6(1 -v )

where:

2

t, c c: : R,, R : l9

2


2

E , v: s

s

2

s

R

2

(3)

R

x

thickness of the substrate, layer 1 and layer 2, respectively; radius of curvature of the bent coated substrate with one and two layers, respectively; elastic modulus and the Poisson's ratio of the substrate, respectively

Hygroscopic stress and failure of coating/metal systems A case often encountered in practice (i.e., in natural and accelerated weathering) is the stress arising during immersion of a coated substrate in water or its exposure to a high RH. Previous studies (18,23) indicate that for a great number organic coatings the stress development as a function of time can be represented schematically by the curves shown in Figure 4. These curves indicate that the immersion of a coated substrate in water induces a hygroscopic compressive stress (arbitrarily chosen as negative values, i.e., curve 1) that, after a period of time, starts to decrease (curve 2).

S

H 0 2

(1) 0

Figure 4. Schematic representation of the stress dependence on time during immersion of a coating in water and exposure to 50 %RH, at 21°C; X = initial stress


329 The exposure of the coatings to 50% RH induces first the development of a hygroscopic tensile stress (curve 3) that also eventually decreases (curve 4). If no delamination occurs, these decreases (curves 2 and 4) are due to stress relaxation, the relaxation rate being related to the real T of the coating immersed in water or exposed to 50% RH, respectively. The lower the T the faster is the relaxation process. The determination of the maximum hygroscopic compressive and tensile stresses, as well as the time necessary to reach them, is useful for a theoretical interpretation of the stress results and from practical point of view. For example, the consideration of these times in programming the wet/dry cycles, with/without UV, will accelerate the deterioration of coatings, thus reducing the time necessary for their selection. Another important fact is the role of the hygroscopic stress in the failure of coating/ metal systems exposed to electrolyte or water (27). Examples of the hygroscopic compressive stress development in five organic coatings of different nature are shown in figure 5. Alternative Current (AC)-impedance, wet adhesion, water uptake and thermal analysis measurements were also carried out. The examination of the data collected indicated only one good correlation, namely between the hygroscopic stress and ACimpedance measurements. The coating which provided the best protection against corrosion, in this case the powder coating, developed the lowest hygroscopic compressive stress and had the highest impedance moduli at low frequency. g


g

x A o • •

m™

A

A

Powder coat. Plexigum Latex Polyureth. Epoxy

—

— J

*

^

-4H 0

^

^

'

0

0

0

"

0

'

.

• o

—

1

1

1

4

8

12

-r

16

1

20

1

24

time(h)

Figure 5. Stress (S) as a function of time (hour, h) at 21°C for five coatings immersed in water.


330


It is important to add that a coating which develops a relatively high hygroscopic compressive stress, still has a satisfactory behavior if the stress relaxes relatively fast. The good correlation between the values of hygroscopic stress and ACimpedance signifies that the stress contribution to coating degradation can be significant. The heterogeneous nature of coatings with regions of various degree of hydrophilicity induces local stresses when exposed to a high RH. This is a consequence of a difference in the swelling between the hydrophilic and hydrophobic regions. We suggest that these hygroscopic local stresses contribute to the enlargement of pathways providing the passage for the electrolyte to reach the substrate, thus causing corrosion in agreement with the model discussed in the reference 13. Conclusions Chemical changes occurring during weathering can induce high hygrothermal stresses that, alone or in combination with fatigue processes due to variation in temperature and relative humidity, can provoke coating cracking and/or loss of adhesion. It is suggested that the hygroscopic stress participates in formation and/or enlargement of pathways in the coating, which enables the transport of the electrolyte to the substrate provoking its corrosion. The consideration of the time necessary to reach the maximum tensile and compressive stresses should result in efficiency improvements in the accelerated weathering tests. The measurement of stress as a function of temperature and relative humidity is also a sensitive way to detect early changes in an organic coating due to weathering. Acknowledgement The active participation of Dr. Tinh Nguyen from National Institute of Standards and Technology, NIST (USA), in part of the project is acknowledged. Literature cited 1. Martin, J.W., Saunders, S.C., Floyd, F.L. and Wineburg, J.P., NIST Building Science Series 172, "Methodologies for Predicting the Service Lives of Coating Systems", U.S. Department of Commerce, Washington, DC 20402-9325, 1994. 2. Bauer, D.R., J. Coat. Technol.,1997, 69, N° 864, 85. 3. Prog. Org. Coat., 1987, 15, No.3, 8 papers. 4. Symposium on "Durability of Coatings", 1993, ACS (PMSE Div.), 68, 26 papers. 5. Gerlock, J.L. and Bauer, D.R., J. Polym. Sci. Polym. Lett., 1984, 22, 447. 6. Bauer, D.R., Briggs, L.M. and Gerlock, J.L., Polym. Sci.Polym. Ed, 1986, 24, 1651. 7. Bauer, D.R., Mielewski, D.F. and Gerlock, J.L., Polym. Deg. Stab., 1992, 38, 57. 8. Bauer, D.R., ACS (PMSE Div.), 1993, 68, 62. 9. van der Ven, L.G.J. and Hofman, L.H., ACS (PMSE Div.), 1993, 68, 64. 10. Struik, L.C.E., Physical Aging in Amorphous Polymers and Other Materials; Elsevier, Amsterdam, Holland, 1978. 11. Perera, D.Y. and Schutyser, P., 22th FATIPEC Congress, Budapest, Hungary, 1994, 1,25. 12. Perera, D.Y. and Schutyser, P., Prog. Org. Coat., 1994, 24, 299. 13. Nguyen, T., Hubbard, J.B. and Pommersheim, J.M., J. Coat. Technol., 1996, 68, No.855, 45.


331 14. 15. 16. 17. 18.


19. 20. 21. 22. 23. 24. 25. 26.

Piens, M . and Verbist, R., in Leidheiser, H . Jr., (ed.), Corrosion Control by Organic Coatings, Natl. Assoc. Corros. Eng., Houston, TX, 1981 (p. 32). Oosterbroek, M . , Lammers, R.J., van der Ven, L.G.J. and Perera, D.Y.., J.CoatTechnol., 1991, 63, No.797, 55. Perera, D.Y. and Oosterbroek, M . , J.CoatTechnol., 1994, 66, No.833, 83. White, J.R. and Rapoport, N.Y., Trends in Polym. Sci., 1994, 2, No.6, 197. Perera, D.Y., in Koleske, J.V.(ed.), Paint and Coating Testing Manual, 14 edition of the Gardner-Sward Handbook, A S T M (MNL 17), 1995 (p. 585). Perera; D.Y. and Vanden Eynde, D., J.CoatTechnol., 1987, 59, No.748, 55. Croll,S.G., in Mittal, K . L . (ed.), Adhesion Aspects of Polymeric Coatings, Plenum, 1983, (p. 107). Perera, D.Y., Prog. Org. Coat., 1996, 28, 21. De Deurwaerder, H.L., 23 FATIPEC Congress, Brussels, Belgium, 1996, vol.A, 1. Perera; D.Y. and Vanden Eynde, 20 FATIPEC Congress, Nice, France, 1990, 125. Perera; D.Y. and Vanden Eynde, D., J.CoatTechnol., 1981, 53, No.677, 39. Hill, L.W., Kerzeniowski, H.M., Ojunga-Andrew, M . and Wilson, R.C., 19 Intern. Conf. Org. Coat. Sci. and Technol., Athens, Greece, 1993, 225. Boerman, A.E. and Perera, D.Y. to be published in J. Coat. Technol.. Perera, D.Y. and Nguyen, T., EUROCOAT 96, Genova, Italy, 1996, vol. 1, 2 or 3 (p.1); or Double Liaison, No; 489, 1996, 22 and 66.

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Stress Development and Weathering of Organic Coatings - American

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