Studies of Adhesion and Disbonding of Coatings by Scanning

Chapter 9

Studies of Adhesion and Disbonding of Coatings by Scanning Acoustic Microscopy

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Downloaded by RUTGERS UNIV on May 30, 2018 | https://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch009

J. D. Crossen , J. M . Sykes , G. A. D. Briggs , and J. P. Lomas

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Department of Materials, University of Oxford, Oxford ΟΧ1 3PH, England A.M.TEC Ltd., P.O. Box 501, Warrington WA4 2JP, United Kingdom 2

The effect of water exposure on the adhesion of an epoxy/polyamide lacquer to mild and stainless steel substrates has been examined. Loss of adhesion is rapid, falling to steady levels within 24 hours. Adhesion is partially regained after drying but maximum values recorded are significantly less than the dry strength. Changes occurring at the interface during exposure and recovery have been examined in-situ by Scanning Acoustic Microscopy (SAM) and Time-of-Flight SAM (TOFSAM). During exposure, patches of disbonding at the interface are observed growing rapidly for the first few hours. TOFSAM reveals that after initial disbonding, the coating swells above the disbonded region prior to growth of micro-blisters. During growth, there is little further lateral disbonding. In general, blister growth stops after a period of time. It has been found that drying leads to the disappearance of the majority of the micro-blisters. Observations suggest coating heterogeneities on a microscopic scale. It has long been recognised that exposure to water of organic coatings on metal surfaces can lead to reduction in adhesion (7,2). In the case of epoxy coatings, this deterioration is particularly rapid (5), leading to failure at the interface between the epoxy and the substrate in an adhesion test (4,5). It has been demonstrated that adhesion loss is partly (7), or fully (3) recoverable if the coating is allowed to dry out. In previous work (6), it was proposed that the rapid adhesion loss of an epoxypolyamide lacquer to mild steel during exposure to distilled water was in part due to the formation of discrete regions of micro-blistering at the interface observed by Scanning Acoustic Microscopy (SAM). Drying out samples showed that adhesion recovered rapidly but to a value below the original dry strength. SAM examination revealed that the majority of the blisters disappeared, but a few air-filled micro-voids were left at the interface. It was believed that these voids prevented full adhesion 106

©1998 American Chemical Society

Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

107 recovery. Adhesion testing was carried out on mild steel samples but SAM examination was on stainless steel samples to avoid any influence of corrosion during longer exposures. The adhesion test method used, lap-shear testing, is defect-sensitive and therefore is affected by the presence of blisters after drying. Additional work has therefore been carried out to examine more fully the adhesion behaviour on both types of steel substrate in order to further explore the link between adhesion strength nd blistering. In the current work, 90° peel-tesing has been used to measure adhesion on mild steel and stainless steel substrates because it is less sensitive to the presence of defects at the interface. It will therefore give a better indication of any more general reduction in interfacial bond strength. The work has also examined blister growth on both substrates in more detail by using Time-of-Flight SAM (TOFSAM) to follow the change in blister dimensions during water exposure.


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Experimental Procedure Adhesion Testing. Substrate samples of size 100 χ 25 χ 0.8mm were cutfromtype R mild steel Q-panels and austenitic stainless steel sheets. The test-strips were then vapour degreased in 1,1,1 trichloroethane. A water-break test indicated that the substrates were free from any organic contaminants i.e. complete wetting of the surface, corresponding to a contact angle approaching zero. No other surface preparation was carried out. The coating studied was an unpigmented 1:1 (by weight) mixture of Epikote-828 type epoxy resin and a Versamid amino polyamide curing agent, thinned with a 3:1 xylene.butanol mixture. The solvent mixture was added in equal amounts to each component prior to mixing in order to reduce viscosity and make preparation easier. The coating was applied on one side of the samples only by flood-spinning for ten seconds and the samples cured at room temperature for 14 days in a dust-free environment prior to testing. Thefinallacquer thickness was 20 ± Ιμπι. To examine adhesion reduction during exposure to distilled water, samples were immersed at 30°C for set periods of time prior to 90° peel-testing in quadruplicate. The testing was carried out on a simple rig where weight was added via a pulley system until a 1.5 χ 4cm strip scribed along the coating started to peel continuously. The weight required was then converted to a peel strength using a simple formula. To measure the recovery of adhesion during drying, samples were exposed to distilled water at 30°C for 24 hours then left to dry out in a desiccator for set periods of time prior to testing. Scanning Acoustic Microscopy/Time-of-Flight SAM. Sub-surface imaging with a Scanning Acoustic Microscope (SAM) at a resolution approaching that of an optical microscope is possible because the ultrasonic waves generated in the lens are able to propagate below the surface of an optically opaque sample and be brought to a focus within the solid. In addition, the microscope has a confocal imaging system, giving



108 enhanced depth discrimination in favour of the plane being imaged. Discontinuities such as thin cracks or small unbonded regions at an interface can excite strong contrast in the image due to differences in acoustic wave/sample interaction detected by the lens. Since the lens is generally coupled to the sample by water, true in-situ examination in water-disbondment experiments is therefore possible. The technique has been used to examine organic coatings in the past (6-9) and it is evidentfromthis work that the SAM can be used to either study the polymer film itself or to examine processes at the metal-coating interface. Time-of-flight scanning acoustic microscopy (TOFSAM) is a technique in which extremely short acoustic pulses are sent through the lens to the specimen. The time interval between echoes from the upper and lower surfaces of a coating or defects within it can be measured as the pulsed signal is scanned along a line and changes due to swelling or blistering can be analysed quantitatively: the time being easily converted to a distance if the acoustic velocity is known. The resultant data is a measure of the signal intensity, S, as a function of time, t, and position, y, and is thus referred to as an S(t,y) scan. Full details of TOFSAM and other, more general aspects of scanning acoustic microscopy are given by Briggs (10) and Briggs and Hoppe (11). Stainless steel and mild steel samples were cutfromprepared peel-test specimens and examined on the O X S A M scanning acoustic microscope operating at a frequency of 300-350MHz and using distilled water at 30°C as the acoustic coupling medium. The focus was adjusted so that the interface between the coating and metal was imaged. Micrographs and S(t,y) scans were then taken after increasing lengths of exposure time to record the processes occurring at the interface. For longer exposure times (up to 4 weeks), the areas being observed were marked, the samples removed from the microscope and then immersed in distilled water, again at 30°C During this time, the samples were periodically re-examined. Results and Discussion Wet Adhesion Testing. For both stainless and mild steel samples, reduction in lacquer peel strength was found to be rapid (Figure 1), leading to apparently interfacial failure within 20 minutes exposure. It can be concluded, therefore, that water had penetrated the coating rapidly and disrupted adhesion uniformly. Adhesion reduction continued for about 24 hours exposure by which point residual adhesion levels, approximately 3% of the measured dry strength for stainless steel samples and 4% for mild steel samples, had been reached. Residual levels remained constant even after prolonged exposure. Surprisingly, the reduction in adhesion is proportionately greater than that measured during lap-shear testing. For longer term exposure, a visible interfacial water layer could be seen evaporatingfromthe exposed metal surface after peeling. The rapid loss of adhesion observed corresponds well with the work of Ruggeri and Beck (3) who examined the adhesion of epoxy under similar conditions using a mechanical cutting device. For coatings of similar thickness, they found that reduction



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Figure 1. Reduction in lacquer peel strength on mild and stainless steel substrates during exposure to distilled water at 30°C.


110 of adhesion was rapid and within 30 minutes, the measured value had fallen to approximately 38% of the original dry strength. Other work by Walker (1) verifies the extreme sensitivity of epoxy-polyamide to water. Using a direct pull-off adhesion test, he found that 50% of the original adhesion was lost within 1 hour of exposure. Again, this is in good agreement with the current work.


Scanning Acoustic Microscopy/Time-of-flight SAM during water exposure. In previous work (6) it was shown that small, distinct patches of bright contrast, corresponding to discrete patches of disbonding at the coating-stainless steel interface, become visible after approximately 30 minutes exposure to water. Figures 2-5 show SAM micrographs of the coating-stainless steel interface during the first few hours of exposure. It should be noted, that the development of disbonded areas does not occur until significant adhesion loss has been measured by peel-testing. During the initial exposure period, the disbonds grow rapidly for the first 2-3 hours, after which point the rate of lateral spreading decreases and, in the majority of cases, becomes negligible. After 2 weeks exposure, (Figure 6), it can be seen that areas of bright contrast cover approximately 50-55% of the total surface area. However, it is also apparent that some regions of the coating still remain in good contact with the substrate (uniform dark contrast). Little further change is observed after 4 weeks exposure. S(t,y) scans taken across the bright patches after 2 weeks exposure indicate that there is a waterfilledregion between the polymer and the metal. A typical scan, Figure 7, taken across the area marked on Figure 6 shows three groups of fringes, marked (i)(iii), which in essence display a cross-section of the sample and can be interpreted as follows: (i) Thefirstset of fringes corresponds to acoustic wave reflectionfromthe surface of the coating. As can be clearly seen, the time taken becomes shorter above the disbonded region, revealing the profile of a micro-blister, as the pulse measurement proceeds across the region. (ii) The second set of reflections arefromthe lower surface of the coating which is no longer in contact with the metal. The 180° phase change which can be seen (the fringes are now black-white-black) occurs when acoustic waves travelfroma higher velocity medium to a lower velocity medium, in this casefromthe epoxy to the fluid in the blister. (iii) The third (and most intense) set offringescorresponds to reflectionfromthe metal surface. This requires a medium within the blister (water) to transmit the acoustic waves. During the first few hours of exposure, S(t,y) scans taken across bright patches show swelling in the coating (raising of the upper surface), but no significant separation of the coatingfromthe substrate. It is also possible to estimate the heights of the micro-blisters by counting fringes arisingfrominterference between acoustic waves within the blister (visible in Figure 6). Each additional fringe corresponds to an increase in height of λ/2 (2.5μπι with a



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Figures 2 and 3. SAM micrographs of the lacquer-stainless steel interface at the start of exposure and after exposure for 30 minutes. (Reproduced with permissionfromref. 6. Copyright 1995 The Electrochemical Society.)



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Figures 4 and 5. SAM micrographs of the lacquer-stainless steel interface after exposure for 1 hour and 4 hours. (Reproduced with permission from ref. 6. Copyright 1995 The Electrochemical Society.)



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Figures 6 and 7. SAM micrograph of the lacquer-stainless steel interface after exposure for 2 weeks and corresponding S(t,y) scan across the microblister indicated. (Reproduced with permission from ref. 6. Copyright 1995 The Electrochemical Society.)



114 microscope operating frequency of 300MHz). For larger blisters, the estimated value can be verified by examination of S(t,y) scans. Having calculated the height, blister volumes can subsequently be determined by assuming that the blisters have the form of a spherical or ellipsoidal cap. Figure 8 shows a typical result (blister diameter increase, i.e. the rate of lateral disbonding, is also plotted). It should be noted that the measured heights of the blisters is very small compared to the diameters. Four basic stages in blister growth have been identified: 1) rapid lateral disbonding at the interface. 2) localised swelling in the coating above the disbonded regions (revealed by S(t,y) scans) before the development of a blister. 3) Increase in blister height accompanied by a small amount of additional disbonding at the periphery, with approximately linear increase in volume. This is illustrated in Figures 9-14, showing SAM micrographs and S(t,y) scans across the micro-blister indicated after increasing time of exposure. The increasing height (and thus volume) can be seen from S(t,y) scans and the increasing number offringesvisible on the micrographs. Note that there is little further growth of the surrounding micro-blisters. 4) In the majority of cases, volume increase slows down and blister growth stops. It has also been found that similar processes occur on mild steel samples, although the micro-blistering is less severe. In the case of mild steel, both blistering and loss of adhesion might conceivably be linked to the effects of corrosion, specifically generation of alkali, but for stainless steel there is, as expected, apparently no corrosion and therefore osmosis is the most likely mechanism of blister initiation. The fact that patches of disbondment occur and blisters form at certain sites at the interface suggests that coating adhesion is particularly weak or susceptible to attack by water at these points. Although adhesion in the regions between blisters is also reduced by water exposure to extremely low levels, the blister volume continues to increase without significant additional disbonding at the periphery of the blister. According to van der Meer-Lerk and Heertjes (72) blister growth will continue so long as the internal blister pressure generates sufficient stress at the edge of the blister to detach the coating from the substrate. In the current work, it is clear that most of the peeling off at the edge of the disbond occurs prior to the formation of what could be considered a blister. It is therefore apparent that some other process, perhaps swelling of the coating, must be operative during blister growth. This is currently being investigated further. It should be noted that localised swelling during exposure is only seen above the disbonded regions so it does appear that the nature of the polymer at these points is different from the coating as a whole. The heterogeneous nature of epoxy/polyamide films has been well documented in the past (73) and in the current work, differences in coating properties appear to exist on a microscopic scale. It is therefore also reasonable to suppose that variation in the polymer from point to point should be reflected in changes in the properties of the polymer-metal interface.



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exposure time (hours)

Figure 8. shown.

Rate of micro-blister volume increase. Blister diameters are also



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Figures 9 and 10. SAM micrograph of the lacquer-stainless steel interface after exposure for 23 hours and corresponding S(t,y) scan across the microblister indicated.



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119 Recovery of Adhesion. Figure 15 shows data obtained during the recovery process for each substrate. Adhesion is regained to some extent after drying but the level reached after 1 month is still significantly less than the lowest dry strength measured prior to exposure. Both substrates show similar behaviour, and a high degree of interfacial failure is also noted even after prolonged drying. It is therefore apparent that some process occurs at the interface which prevents full recovery of adhesion during drying. According to Ruggeri and Beck (5) irreversible adhesion loss only occurred when samples were exposed to a corrosive environment. However, since no corrosion was observed on the coated stainless steel samples studied in the current work, some other process must be responsible. As shown previously (6), drying leads to the disappearance of the majority of the micro-blisters. Figure 16 shows the coating-stainless steel interface after 4 weeks exposure followed by 24 hours drying (same area shown wet in Figure 6). An S(t,y) scan across the area marked, Figure 17, again shows three sets of reflected fringes. In this case however, there is a break in the reflectionfromthe metal surface immediately beneath the blister. This indicates that there is no transmission of acoustic waves through the underside of the coating to the metal (in Figure 7, the reflectionfromthe metal surface is continuous) and it can thus be concluded that these regions are airfilled voids. From SAM examination, it would therefore appear that recovery had not occurred at these points due to irreversible breaking of the adhesive bonding and localised permanent deformation of the coating. Similar observations have been made on mild steel samples. While these areas undoubtedly contribute to the fact that full adhesion is not regained during drying, they represent a smallfractionof the total interfacial area. Peel-test results indicate an additional, more general reduction in recovered adhesion in areas showing recovered bonding to the substrate.

Conclusions 1) Peel strength of a epoxy/polyamide coating on both mild and stainless steel samples is reduced rapidly by exposure to distilled water. However, some adhesion is retained even after prolonged exposure. 2) Peel strength is partially regained after recovery with both substrate types but values are significantly less than before water exposure. 3) SAM examination reveals that localised growth of micro-blisters at the coating/metal interface occurs in much the same way on both mild and stainless steel samples. Disappearance of the majority of micro-blisters during drying has also been observed with both substrates. 4) SAM and Time-of-Flight SAM show that micro-blister growth starts with disbonding at the interface, followed by swelling of the coating and then growth in height by water ingress without significant increase in diameter. After an initially rapid rate of volume increase, blister growth slows down then stops.



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Figure 15. Increase in lacquer peel strength on mild and stainless steel substrates during recovery after 24 hours exposure to distilled water at 30°C.



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Figures 16 and 17. SAM micrograph of the lacquer-stainless steel interface after exposure for 4 weeks then drying for 24 hours and corresponding S(t,y) scan across the area indicated. (Reproduced with permission from ref. 6. Copyright 1995 The Electrochemical Society.)


122 In addition to a link with osmosis caused by soluble contamination at the interface, other factors, such as localised swelling of the coating appear to influence the blister growth. 5) 3) and 4) reveal the existence of microscopic coating heterogeneities.


Acknowledgments The authors would like to thank Professor B. Cantor for the provision of Laboratory facilities, EPSRC and Courtaulds Coatings forfinancialsupport and Dr. L.M. Callow for invaluable discussion. Literature Cited

(1) Walker, P. Off. Digest 1965, 37, pp1561. (2) James, D.M. J.O.C.C.A 1956, 39, pp39. (3) Ruggeri, R.T.; Beck, T.R. In Adhesion Aspects of Polymeric Coatings; Mittal, K.L. Ed.; Plenum Press, N.Y., 1983; pp329. (4) Kinloch, A.J. J.Adhesion 1979, 10, pp193. (5) Gettings,M.;Baker, F.S.; Kinloch, A.J. J.Appl.Poly. Sci. 1977, 21, pp2375. (6) Crossen, J.D.; Sykes, J.M.; Knauss, D.; Briggs, G.A.D.; Lomas, J.P., In Proc. of the Symp. "Advances in Corrosion Protection by Organic Coatings II", 1994; Scantlebury, J.D.; Kendig, M., Eds.; Electrochem. Soc. Inc., 1995, Vol 95-13, pp274. (7) Sinton, A.M.; Briggs, G.A.D.; Tsukahara, Y. In Acoustical Imaging; Shimizu, H., Chubachi, N., Kushibiki, J., Eds.; Plenum Press, N.Y., 1988, Vol. 17; pp87-95. (8) Addison, R.C.; Kendig, M.W.; Jeanjaquet, S.J. In Acoustical Imaging; Shimizu, H., Chubachi, N., Kushibiki, J., Eds.; Plenum Press, N.Y., 1988, Vol. 17; pp143-152. (9) Kendig, M.; Addison, R.; Jeanjaquet, S. J. Elect. Chem. Soc. 1990, 107, No.9 pp2690. (10) Briggs, A. In Acoustic Microscopy, Clarendon Press, Oxford, 1992. (11) Briggs, G.A.D.; Hoppe, M.; In Images of Materials; Williams, D.B.; Pelton, A.R.; Gronsky, R. Eds.; Oxford Univ.Press, N.Y., 1991; pp114-141. (12) van der Meer-Lerk, L.A.; Heertjes, P.M. J.O.C.C.A 1979, 62, pp256. (13) Mayne, J.E.O.; Scantlebury, J.D. Br. Polym. J. 1970, 2, pp240.


Studies of Adhesion and Disbonding of Coatings by Scanning

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