The Effect of Weathering on the Stress Distribution and Mechanical

Chapter 22

The Effect of Weathering on the Stress Distribution and Mechanical Performance of Automotive Paint Systems

Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: April 15, 1999 | doi: 10.1021/bk-1999-0722.ch022

M . E . Nichols and C. A. Darr Ford Motor Company, P.O. Box 2053, M D 3182 SRL, Dearborn, MI 48121

The sources of stress in complete automotive paint systems have been identified and measured as a function of weathering. The main sources of stress are thermal expansion coefficient mismatch, humidity expansion mismatch, and densification of the clearcoat. Stresses generally increase during weathering due to a slow densification of the clearcoat and increasing water absorption and desorption stresses. Finite element analysis (FEA) was used to compute the stress distribution in full paint systems. Stresses are typically in-plane and highest in the primer and clearcoat. Stresses approaching those required to propagate cracks can be attained in weathered paint systems. The presence offlaws,either cracks or incipient delaminations, will lead to large stress concentrations that can give rise to peeling forces not present in coatings without cracks.

Modern automotive paint systems are highly complex, multilayer structures where each layer performs multiple functions. For example, the primary role of the clearcoat is to enhance appearance by maintaining a high level of gloss. However, the clearcoat must also screen the underlying layersfromharmful ultraviolet radiation, which is accomplished by doping the clearcoat with ultraviolet light absorbers. While each individual layer is engineered to possess certain chemical and physical properties, many aspects of paint performance depend on system properties and interactions between layers. Hindered amine light stabilizers (HALS), for example, are often added to the clearcoat, but during cure the HALS can migrate to the basecoat thereby distributing the additive and coupling the basecoat performance to the composition of the clearcoat (1). Similarly chip resistance is related to the brittleness of the clearcoat, the stiffness of the underlying layers, and the geometry of the vehicle structure (2,3). While potential weathering-induced failures on vehicles usually have their origin in chemical composition changes brought on by photooxidation (4-10), the failures that mayfinallybe observed are typically mechanical in appearance: cracking of the clearcoat or delamination of one layerfromanother. These failure

332

© 1999 A m e r i c a n C h e m i c a l Society

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


333 modes are driven by two separate events, the changing toughness of the material or interface where the crack may grow and the stresses the material or interface experiences during exposure (see Figure 1). While quantification of the fracture toughness orfractureenergy of structural materials and interfaces is becoming common, only recently has thefracturebehavior of thin polymerfilms,and in particular automotive coatings, been explored (11,12). This has been due to the difficulty in handling and preparing samples and also to the lack of theoretical understanding of thefracturemechanics of thinfilms.However, these problems have been recently addressed (13-16). The question of the stress distribution in thinfilmsis equally important when considering their mechanical failure, be they single or multilayer systems. The most brittle material orfilmwill not crack without a stress to drive the crack tip forward. Stresses can arise in coatingsfroma number of sources. A mismatch in the thermal expansion coefficients between the coating and substrate (or between coating layers) will cause stresses to arise when the temperature is changed. Changes in the humidity of the environment can cause swelling and plastisization of a coating leading to changes in it's stiffness and dimensions. These changes can lead to stresses in coatings adhering to substrates as can a slow increase in the density of the coating. For a simple single layer coating on a substrate, the stresses produced by any of the above phenomena can be simply calculated given the appropriate material constants for both the substrate and the coating, i.e. the coefficient of thermal expansion, the density change, or the amount of swelling and the elastic modulus of the coating and substrate. A number of techniques also exist to experimentally measure the stress in coatings on substrates. The classical method of applying a coating to a thin metal shim has been used widely, and a commercial instrument is available (17-19). For small deflections the use of Corcoran's equation is quite satisfactory (20). In addition, the use of interference fringes (21), Raman scattering (22), andfluorescentprobes (23) can be used to measure the stresses in coatings. A limited number of reports have been published regarding the changes in stress as a function of weathering. Perera et. al. monitored the stresses as a function of humidity and weathering and found the stresses in most coating systems to increase with weathering and decrease with humidity (24,25). Perera ascribed most of the stress increase to an increase in T . Both Perera and Shiga have noted that stress relaxation, that is the decay of stress over time, is important in quantifying the stress response to any environmental perturbation (23,25). For multilayer systems the problem of deducing the stress in individual layers is much more difficult. Knowledge of the stress in an individual layerfromdata taken in multilayer systems is essentially impossible using beam deflection techniques because only information on the overall or mean stress in the coating system is accessible. Thefluorescentprobe technique is an exception and appears promising, as it can probe the stresses in each layer of a coating system (23). However, the stress range over which current probes are effective is limited. By measuring the relevant material properties of isolated, individual layers, the stresses in a multilayer system can be calculated. However, because no closed form g



Figure 1. Two possible modes of crack propagation in basecoat/clearcoat paint systems. Delamination (upper), where the clearcoat peel off of the basecoat. Cracking (lower), where small cracks form in the clearcoat and propagate down to the basecoat and spread laterally in the clearcoat.



335 analytical solution for the stresses in each layer of a multilayer system with arbitrary geometry exists, one must resort to numerical methods to calculate stresses in individual layers. A number of techniques exist for doing this including finite element analysis (FEA) and boundary element methods (BEM) (26,27). The main hurdle to applying such methods is the time involved in generating the material properties for each layer of a multilayer system. By knowing the stresses in various layers of a modern automotive paint system one could more accurately comment on the likelihood of potential failures in any given layer or at any interface. Also, by knowing which material properties play the most important role in deterrriining the stresses, better coating systems could be designed which could more effectively manage environmental inputs. In this paper we report on our initial attempts at quantifying the stresses in various layers of a modern automotive paint system as function of weathering and comment on how these stresses relate to the material properties that likely govern potential failure mechanisms. Experimental Materials. The thermoelastic constants of three different clearcoats (clearcoats B, C, and D), two basecoats, and one monocoat (Monocoat A) were measured. All of these coatings were acrylic/melamine based. In addition an epoxy based e-coat and polyester based primer coat were studied. For humidity-stress testing an additional clearcoat (E) based on acrylic/silane chemistry was also examined. All of the coatings were prepared as isolated films. No multilayer laminates were made. The coatings were applied to substrates, either tin-plated steel, poly(tetraflouroethylene), or aluminum, with a Byrd applicator and cured for the recommended times and the recommended temperatures. Freefilmswere made by removing the coating from the substrate by peeling or my forming an amalgam of the tin with mercury. Free films were die cut into strips for testing. Accelerated Weathering. All clearcoats and basecoats were weathered for various times in a QUV weathering chamber (QUV Co.). FS-340 nm bulbs were used as the light source. The temperature was held constant at 40°C and the dew point at 25°C. These conditions correspond to the "near ambient" exposure used by Bauer et. al (4). Dynamic Mechanical Analysis (DMA) and Thermomechanical Analysis (TMA) The elastic modulus as a function of temperature was measured in tension for all the free coatingfilmsusing a Polymer Laboratories DMTA. The coefficient of thermal expansion (CTE) was measured using the same instrument in TMA mode. In this configuration, thefilmis held in tension under a negligible load as the temperature is changed. The instrument adjusts the length of thefilmto maintain zero load. The CTE is the change in length divided by both the change in temperature and the originalfilmlength. CTE measurements were made as a function of weathering on


336 each basecoat and clearcoat. CTE measurements were also made on unweathered primer and e-coat. Density. The density of each clearcoat was measured as a function of weathering using the method of Guy-Lussac. Small pieces of clearcoat were immersed in a beaker containing an aqueous solution of ZnCl. The concentration of the solution was adjusted until the piece of clearcoat was perfectly suspended in the middle of the solution, indicating a match in densities of the solution and clearcoat. The solution was then transferred to a Guy-Lussac flask (the exact volume and mass of which had been previously determined). Thefilledflaskwas weighed and the mass and then density of the solution calculated. The density of polymers can be measured accurately to ±0.002 g/cm using this technique.


3

Humidity Stresses. Changes in the internal stress of coatings produced by changes in humidity were measured using a custom built loadframe.The small load frame was equipped with a sensitive load cell and signal conditioner (Transducer Techniques, Temecula CA) as shown schematically in Fig. 2. The loadframewas placed inside a UV2 weathering chamber (Atlas Electric Co.) with the load cell and strain adjustment screw outside the chamber. The chamber temperature was maintained at a constant 40°C. Afreeclearcoatfilmwas subjected to a small strain, by adjusting the screw at the top of the loadframe.The load as a function of time was recorded with a strip chart recorder. A dry atmosphere (0°C dew point) in the chamber was produced by purging the chamber with nitrogen gas. A dew point of approximately 25°C was produced byfillingthe bottom of the chamber with water. The stress in the coating as a function of dew point was recorded. All stresses were allowed to relax to relatively steady-state values before environmental conditions were changed. Weathering was accomplished by taking thefreefilmsand mounting them with tape on an aluminum panel. Thesefilmswere then exposed in the QUV and removed periodically for the stress measurements described above. For a given clearcoat, the exact same specimen was used for each measurement at different weathering times. In this manner, the humidity induced stresses could be measured as a function of weathering. Modeling of Stresses in multilayer Coatings. A number of methods exist to approximate the stresses that arise in thin multilayered structures due to changes in temperature. Vilms and Kerps recognized previous attempts to solve these problems were plagued by inconvenient calculation schemes and cluttered nomenclature (28). Their analysis addresses these issues and is improved upon by Townsend et. al (29). Suhir has furthered the work by extending the calculations to real structures withfinitedimensions, taking into account the nontrivial edge effects (30). Suresh has accounted for plasticity in one or more of the layers (31). All of the methods used to calculate the stresses in each layer rely on the same fundamental principle: the different thermal expansion coefficients of each material will cause each layer to seek a new equilibrium size when the temperature is changed. If the layer is constrained by adhering to another layer(s), a stress will



337

Figure 2. Apparatus for measuring the humidity induced stresses in clearcoats.



338 arise in the film. If the substrate is not irifinitely thick compared to thefilmson top of it, a curvature of the substrate/film structure will ensue as the forces and moments on the composite structure must sum to zero. The approach is general. Differential thermal expansion is just the most common subset of circumstances under which stresses arise in multilayer thinfilms.This can easily be extended to account for changes in dimensions due to moisture induced swelling or the gradual change in specific volume of the various layers due to physical or chemical aging. However, summing the effectfrommoisture, thermal expansion and densification proved cumbersome using analytical techniques. In addition, the analytical solutions are restricted to simple planar geometries, and the introduction offlawscannot be accounted for. For these reasons, the stress distributions were determined using finite element analysis (FEA). Commercial FEA software (ANSYS, Ansys Corp.) was used. 8-node quadrilateral elements were employed for the analysis. Unlike analytical solutions which can only deal with simple geometries, FEA analysis can solve structural mechanics equations for complex geometries with complex constitutive behavior. This is accomplished by breaking down complex geometries into smaller "elements" on which a computer can solve the standard set of differential equations for displacements and stresses. In addition material nonlinearities can be accounted for as well as the summation of various stress sources. However, a number of simplifying assumptions were made in the FEA analysis presented in this paper to prevent the details of the analysisfromobscuring the general conclusions. It was assumed that (1) the thermal expansion (or humidity expansion) coefficient is constant over the relevant temperature range, (2) the elastic modulus remains constant over the same temperature range, and (3) the effects of relaxations can be accounted for by simply using an appropriately smaller modulus. The validity of these assumptions will be addressed later in this paper. It should be emphasized that the FEA technique does not require these assumptions, and that the added complexity of the required analysis would not dramatically change the results, but would significantly increase the analysis time.

Results Thennoelastic constants ofIndividual Layers. Table I shows the measured thermal expansion coefficients and elastic modulus for each of the clearcoats and basecoats, along with the primer and e-coat that were common to each system. Thermal expansion coefficients were generally higher in the clearcoat and basecoat and lower in the primer, e-coat and the monocoat topcoat. The modulus of the primer was higher than the other coatings because of its substantially higher pigment concentration. Table 1 also shows the changes in CTE and modulus for each coating layer as a function of weathering. Humidity Stresses in Single Layers. Stresses will result in coatings when the ambient humidity is changed. The coatings will take up water and swell when the


339 Table I. Thermoelastic constants of coatings. Note: All Moduli (E) are in units of GPa. AllCTE(a)areinunitsof°C" xlO" . Subscripts refer to weathering time in hours.


1

Material CCB BCB CCC CCD BCD Monocoat A Primer E-coat Steel

E 1.6 1.4 2.1 1.9 1.9 2.1 3.2 0.8 205 0

6

Oo

E1500

Cti500

E3000

CC3000

84 95 109 123 106 77 82 63 10

1.8 1.1 2.2 2.5 2.0 2.6

74 70 101 102 92 70

2.1 1.2 2.5 2.3 2.1 2.3

109 74 108 102 92 77

humidity is raised and will shrink and stiffen when the humidity is lowered. The humidity response of a typical clearcoat is shown in Figure 3, where the stress as function of time is shown. The dew points at various times throughout the test are shown on the graph. The measurements were made at a constant dry bulb air temperature of 40°C. The stress in the clearcoat initially relaxes due to viscoelastic effects. At point A the dew point is lowered to 0°C and the stress goes up. When the dew point is quickly raised (point B), the stress quickly drops. A plateau level is eventually attained (C). As the dew point slowly decreases, the stress slowly rises (D to E) until at point E it reaches its former level just before point A. We have defined the stress response to humidity as the difference in stress between points E and C. By normalizing the stress difference (E to C) by the change in dew point (in this case 25°C) the humidity response can be quantified for arbitrary changes in dew point. This is shown for three different clearcoats in Figure 4. The results have been normalized to changes in stress per one °C change in dew point. Clearcoat E is quite sensitive to changes in humidity. The fully stabilized version of clearcoat D (contains HALS and UVA) is quite insensitive to changes in humidity while the unstabilized version (no HALS or UVA) is more sensitive. Densification Stresses. The density of all the clearcoats changes with weathering time. Table II shows the density of the clearcoats as a function of weathering time. Because most of the chemical composition changes occur in the clearcoat, the density of the other layers was not monitored. Multilayer Stresses. All of the stress profiles for multilayer systems were done using the same thickness numbers for the coatings: 25 [im e-coat, 25 urn primer, 25 \im basecoat, and 50 urn clearcoat. The analysis assumes a biaxial stress state in the



15

Time (Hours)

10

B

20

Figure 3. Typical stress response of clearcoat to changing humidity. Dewpoints: Before A, 5°C; A, 0°C; B, 25°C; C, 2S°C; D, 25-G°C; E, 0°C.

O.OE+00

1.0E+06

2.0E+06

3.0E+06 -

4.0E+06

5.0E+06


25


0

•

•

500

—J

Exposure Time (Hrs.)

1500

1

Ac/Mel (D) - Stabilized

1000

1

•

•

Ac/Mel (D) - No HALS

Ac/Sil (E) - Stabilized •

2000

1

•

•

•

Figure 4. Change in humidity/stress response with weathering for three clearcoats.

20000

40000

60000

80000


2500

342

Table H Density (g/cm ) of three aciyhc/melamine clearcoats as a function of accelerated weathering time.


Clearcoat Monocoat A Clearcoat B Clearcoat C Clearcoat D

Density(t=0) 1.127 1.148 1.20 1.165

Density(t=1500 hrs) Density(t=3000 hrs) 1.132 1.131 1.162 1.155 1.205 1.196 1.174 1.172

Table HI. Maximum calculated stresses in each layer of a complete unweathered and weathered paint system. Stresses do not account for relaxation effects, which would reduce values by roughly 50%. Moduli and CTE values detennined at 10°C dew point.

Coating Clearcoat Basecoat Primer E-coat

Stress Max. t=0 hrs (MPa) 8.8 8.8 17.2 3.2

Stress Max. t=3000 hrs. (MPa) 15.5 5.7 17.2 3.2

coating system, as would be the case on a largefractionof the vehicle. This enhances the stress over the values observed in a simple beam deflection experiment by a factor of l/(l-v) due to Poisson's effects. Figure 5 shows the stress distribution in a BC/CC paint system after the temperature has dropped 50°C to 25°C. The temperature is uniform throughout the paint system and the different grey-scales signify different contours of stress. The top of the clearcoat is at the top of thefigureand the analysis extends down approximately 350 um into the paint and steel. The thermoelastic constants chosen represent the unweathered paint system B as in Table I. The stresses rangefrom17 MPa in the primer to only 3 MPa in the e-coat. The stresses shown are the in-plane stresses. The normal stresses (not shown) are comparatively uniform throughout the coating system and approach zero. Stresses are highest in the primer due to its high modulus. Figure 6 shows the in-plane stress distribution for a system whose thermoelastic constants correspond to weathered paint system B. Changes occur mostly in the clearcoat and basecoat regions. Table III lists the maximum stress in each layer of the coating for the two weathered and unweathered coating systems.



Figure 5. In-plane stress distribution for a unweathered full paint system on coolingfrom70°C to 20°C. Grey-scales correspond to different stress levels. Stresses are as in Table 3.



Figure 6. In-plane stress distribution for a full paint system weathered 3000 Hrs and cooledfrom70°C to 20°C. Stresses as are in Table 3.


345 Discussion


The combination of increasing stresses and changing mechanical properties lay at the root of most mechanical failures of paint systems. The changing mechanical properties are largely driven by the weathering induced chemical composition changes of the various coating layers. The details of the chemical changes play a key role in determining the mode and location of any possible mechanical failures (8). However, even the most brittle materials and interfaces will not fail in the absence of stress. At least three sources of stress have been identified: thermal expansion mismatch, humidity expansion mismatch, and densification. The coefficient of thermal expansion results indicate that, depending on the formulation of the clearcoat, the CTE can vary between clearcoats by up to 50%. Larger CTEs will always lead to higher stresses given the same modulus, but typically, coatings with high CTEs will have lower elastic moduli, as the two properties are not fully independent on a molecular scale. The thermal expansion coefficientfirstdecreases and then increases with weathering for all three clearcoats studied. This mirrors changes that have been observed for other properties, such as crosslink density. Similar results were observed for the basecoats, withfirsta decrease then increase in CTE. Much smaller changes were observed in the room temperature modulus of the coatings as a function of weathering. This is not surprising as well below T the modulus of most polymers is quite constant and similar to each other. Changes in modulus at elevated temperatures, where changes in crosslink density have occurred, are much more pronounced (32). The changes in density exhibited by the clearcoats can varyfromnear zero to almost 1.5% after 3000 hours of weathering. Assuming all of the density change is due to a collapse of volume and not an increase in mass, a coating constrained on a substrate will experience a tensile stress as the density increases. The change in any linear dimension is the cubed root of the change in the density so a change of 1.5% in the volume will lead to a roughly 0.5% change in the linear dimension. For clearcoats with typical elastic moduli this will lead to a strain of almost 10 MPa in the unrelaxed state. However, this strain will relax in the same manner as the thermal strain. The exact origin of this densification has been studied extensively in many polymers and recently for a number of coatings. If the densification is reversible it is termed physical aging and is due to the amorphous polymer slowly progressing towards its equilibrium specific volume (33-35). This densification proceeds more quickly at temperatures close to, but below, the polymer's glass transition temperature (T ). When the polymer is exposed to a temperature above its T it will recover to a higher specific volume, thus the process is reversible. Chemical aging can also cause densification and occurs when the polymer undergoes chemical composition changes, due to degradation or continued curing, over the course of time. These changes are not reversible as they resultfromthe rupture and formation of chemical bonds. In many cases these chemical changes can lead to a decrease in the specific volume of a polymer. This is because most curing reactions are volume reducing (with the exception of expanding monomers) and most degradation reactions will increase the polarity of the network increasing the likelihood of g

g


g

346 hydrogen bonding. While volume decrease effects probably dominate the density increase phenomena, photooxidation can lead to the incorporation of oxygen which can initially lead to an increase in density. This is likely offset in the latter stages of photooxidation as small molecules such as C0 , methanol, and formaldehyde are produced (36). If the density increase is indeed due to volume reduction, not mass increase, tensile stresses will occur. However, this assumption has not been proven. Most coatings as they weather will decrease in thickness due to the loss of the most degraded material near the surface. The density of the material left behind is the crucial parameter. Assessing its density is easy; confirming that it is indeed due to volume collapse and not mass increase is difficult due to the concomitant thickness loss. The complicationsfromstress relaxation are common to both the density and thermal stress measurements and are due to the viscoelastic nature of polymers. Because the elastic modulus is a function offrequencyor time, a given strain will produce a constantly decreasing stress with time. For crosslinked polymers this stress will eventually plateau. For coatings well below their T this may take an experimentally unrealizable time, but for coatings near T the relaxation processes can occur quite rapidly, making it unlikely that large stresses can be supported by coatings near their T . Some estimate must be made for thefractionof stress on a coating that will remain after reasonably long periods of time. We accomplish this by comparing the residual stress in coatings after curing to the stress values calculatedfrommeasured moduli and CTE values. For clearcoat B, for example, the residual stress measured using the shim bending technique was 2.0 MPa. From CTE and moduli values for brass and the clearcoat and assuming stress does not build up until after the polymer is cooled below its T (55°C). The stress can be calculated using


2

g

g

g

g

a = EATAa

(1)

where E is the elastic modulus of the clearcoat, AT is the temperature change, and Act is the difference in thermal expansion coefficients between brass and the clearcoat. The calculated value is approximately 4.0 MPa. The ratio between the two being approximately 1:2. This ratio appears to hold for most clearcoats. Therefore, as an estimate, we use a factor of two as the amount by which the stress will relax during normal service conditions. Analytically this can be affected by using a modulus value one half of the measured value. Of course in-service vehicles see constantly changing temperatures and humidities. During hot cycles the stress will be much lower and during cold cycles the stress can be higher. These temperature extremes, where the material properties could be quite different from the room temperature properties used in this analysis, can be accounted for using FEA techniques. In addition, the transient stresses set up when a paint system transitionsfromone environment to another will be highly dependent on diffusion and on the viscoelastic nature of the polymer coatings. If required, these effects could also be accounted for in the analysis. In addition, the increase in T can lead to higher stresses on cooling a coatingfromabove T , due to the larger difference g

g


347 between T (below which stresses can be supported) and the ambient temperature (19). Weathering has a much more pronounced effect on the humidity induced stresses. The majority of chemical changes that can occur in clearcoats during weathering produce polar or hydrophilic groups in the network. The production of these groups increases the clearcoat's tendency to absorb moisture. This is also the main reason the T of coatings tend to increase as weathering progresses. Some coatings are much more susceptible to these chemical changes than other coatings. For example, in Figure 4 the acrylic/silane clearcoat becomes quite sensitive to moisture as weathering progresses, while the conventional, stabilized acrylic/melamine clearcoat changes comparatively little. However, the unstabilized version of the clearcoat D shows a greater humidity/stress response as weathering proceeds, in accordance with the greater amount of photooxidation occurring in the clearcoat and a greater proportion of polar species being produced. The acrylic/silane clearcoat likely shows an increasing sensitivity to moisture as weathering progresses due to the enhanced network formation these coatings undergo after exposure to moisture. The changes in dew point that are experienced either in a weatherometer or outdoors rarely exceed 25°C. Thus, the humidity induced stresses in a clearcoat can rangefrom0.25 MPa to up to 2 MPa. These stresses are the fully relaxed stresses as the experiments were performed after waiting for the clearcoats to come to stress plateaus. These values would be enhanced by transient effects. Perera has shown that large stress overshoots can occur when coatings are takenfromwet to dry environments very quickly (37). Also in multilayer coatings, moisture will diffuse from the surfacefirstleaving the underlying layers swollen, givingriseto enhanced surface stresses. In Fig. 3 the effects of these transient can be seen as a stress undershoot and then reverse relaxation to a higher stress when the humidity is quickly increased. While not intuitive, this reverse relaxation is viscoelastically permitted and is related to different rates of stress and volume relaxation occurring in the polymer (38). After measuring the stress in individual layers, the stressesfromeach source can be summed (assuming simple additivity) and then computed for the entire paint system. The harshest conditions would be cold dry conditions after some weathering has occurred, so as to induce a stress due to densification also. Weathering is assumed to effect the modulus and CTE of the clearcoat and basecoat only and densification of the clearcoat only. The assumption used, that the modulus and CTE of the coatings are independent of temperature, is obviously incorrect, but since we are only trying to calculate stresses at the temperature at which the CTE and modulus were measured, we have taken them to be constant (after dividing the modulus by 2 to correct for relaxation effects). Figure 7 shows the stress distribution that would be brought about by a 50°C drop in temperature and a 25°C drop in dew point, along with a 1% increase in density. Stresses are highest in the cleacoat. These stresses are the fully relaxed stresses, unlike those in Figs. 5 and 6 where the stresses are the initial stresses. These stresses will be sustained for a significant amount of time providing the opportunity for mechanical damage. The g


g



Figure 7. In-plane stresses for a weathered paint system after relaxation. See text for environmental conditions. Stresses shown on plot.



349 initial stresses in the clearcoat would be approximately twice as high, approaching 40 MPa for some coatings. These stresses would also be magnified due to the inhomogeneous nature of the degradation. Because UV light absorbers are used in all commercial clearcoats, a gradient of photooxidation exists in the clearcoat. This gradient in photooxidation will set up a gradient in the thermomechanical properties also, with changes being greatest at the surface. This gradient will only increase the surface stresses over what has been calculated using average layer properties here. As noted earlier, the stresses that exist in intact paint systems are almost exclusively biaxial in the plane of the paint system. Only near the edges and near very sharp radii of curvature are any significant normal or shear stresses incurred. However, paint can delaminate. Because delamination stresses do not typically occur for intact systems, the question must be asked where do delamination stresses arisefrom?Certainly absorption of moisture, particularly if it is trapped at interfaces can give rise to capillary pressure and some normal stresses. More likely however,flawsin the clearcoat will provide the opportunity for normal stresses. It has been shown analytically that the normal and shear stresses drop away exponentially to zerofroma crack or edge, but that very close to theseflawsthe normal and shear stresses are quite high (30). Figure 8 shows some initial FEA results on the normal stress distribution in a paint system around a lOpm crack in a clearcoat that has been cooled 50°C. For an uncracked clearcoat the normal stresses approach zero. However, the stresses are quite high in this cracked clearcoat (depending on the radius of curvature of the tip) - in this case up to 30 MPa. The shear stresses are likewise high in the tip region. In addition, if a small delaminated zone is introduced near the crack tip (Figure 9), the peeling forces are clearly seen, as is the deflection of the clearcoat awayfromthe surface. In this case the delaminated clearcoat initially laid with no adhesion on the surface of the basecoat for three crack tip radii awayfromthe normal crack. As the temperature was reduced the clearcoat peeled away in the delaminated region and large stresses at the tip of the delamination were produced which would drive the delamination outward. Knowledge of the stress distribution in paint systems is crucial to designing accelerated weathering tests and understanding the differences between failures in outdoor exposures and accelerated weathering. To more closely mimic outdoor exposure (with Florida as the standard comparator) accelerated test conditions must not greatly distort the weathering chemistry and the test must produce similar mechanical failures as are observed outdoors. Current weatherometers can closely mimic outdoor chemistry by using borosilicate inner and outerfilteredxenon arc light, but the mechanical failures are often incorrect because the stresses produced in weatherometers are different than those produced outdoors. It is not uncommon for test panels to remain intact inside a weatherometer but crack when they are removed due to the rapid drying and cooling which sets up large thermal and humidity stresses gradients. By understanding the stress distribution in paint systems - and how heat, moisture, and age effect the stress distribution - better accelerated weathering protocols should be realized.



Figure 8. Normal stress distribution around 10mm crack in clearcoat. Note high stresses (-50 MPa) near crack tip. Stresses shown on plot.



Figure 9. Normal stress distribution near incipient delamination. Note high stresses ahead of delaminationfront(-20 MPa). Stresses shown on


352 Final remarks must be made regarding the interaction between stresses and relevant material properties. Because most possible failure mechanisms in clearcoat/basecoat systems involve either crack propagation in the clearcoat or at the clearcoat/basecoat interface, thefractureenergy of these materials and interfaces is important. Thefractureenergy is the amount of mechanical energy required to propagate a crack in a material or at an interface and is a direct measure of the brittleness of a material. For example, thefractureenergy of silica glass and mild steel are is approximately 6 J/m and 12,000 J/m respectively. For typical unweathered automotive clearcoats thefractureenergy can rangefrom15-350 J/m , which would lead to failure stresses on the order of20-40 MPa for these same clearcoats at nominalfilmthicknesses (11). Weathering typically embrittles materials lowering theirfractureenergy significantly, by over half in some cases. The failure stresses required for weathered clearcoats can be easily reached through a combination of the stresses outlined in this paper. This increasing embrittlement coupled with increasing stresses can lead to mechanical failure. 2

2


2

Conclusions Asidefromexternally applied impacts and loads, the main sources of stress in complete automotive paint systems have been identified to be: thermal expansion mismatch, humidity expansion mismatch, and densification. These stresses have been measured for individual layers as a function of accelerated weathering time. In general, stresses increase with weathering, due mainly to continued densification of the clearcoat and increased stress sensitivity to moisture. Thermal expansion mismatch stresses, while large, change relatively little with weathering. The stress distribution in complete paint systems, as calculated byfiniteelement analysis, shows uniform in-plane stresses in uncracked coatings. Stresses are often highest in the primer or cleacoat due to their thermoelastic properties. Cracks in the clearcoat will lead to large normal stresses that can lead to delamination. These normal stresses are typically not present in uncracked coating systems. The stresses present in weathered paint systems approach those that will propagate cracks in many of these systems. References 1. Haacke, G., Andrawes, F. F., and Campbell, B. H., J. Coat. Tech., 1996, 68, 57. 2. Ryntz, R.A.,Ramamurthy, A. C., and Holubka, J. W., J.Coat.Tech., 1995, 67, 23. 3. Oosterbroek, M., Proc. XVth International Conference on Organic Coatings Science and Technology, 1989, Athens. 4. Bauer, D. R., Mielewski, D. F., and Gerlock, J. L., Polym. Deg. and Stab., 1992, 38, 57. 5. Bauer, D. R., Gerlock, J. L., and Mielewski, D. F., Polym. Deg. and Stab., 1993, 41, 9. 6. Dickie, R.A.,J. Coat. Tech., 1994, 66, 29. 7. Wypich, G., Handbook of Material Weathering, ChemTec Pub., Toronto, 1995. 8. Gerlock, J. L., Prater, T. J., Kaberline, S. L., and deVries, J. E., Polym. Deg. and Stab., 1995, 47, 405.



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The Effect of Weathering on the Stress Distribution and Mechanical

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