QUANTIFICATION OF CURING STRESSES IN PAINTS AND COATINGS

Transcription

1 QUANTIFICATION OF CURING STRESSES IN PAINTS AND COATINGS Moavin Islam, Ph.D., C.Eng., John Repp, P.E., James A. Ellor, P.E., and Brad Shaw, P.E. Corrpro Companies, Inc., Ocean City Research Corporation, Ocean City, New Jersey, USA Abstract: The present paper describes a research effort to quantify the internal stresses that develop in a coating system during curing and exposure. Several common overcoating systems (typically used for bridge structures) were investigated that included a poly silicone alkyd, an acrylic, a moisture-cured urethane, and an epoxy. The work reported in this paper constitutes part of an NCHRP-IDEA project 1 the main objective of which was to design and demonstrate an innovative field device to assess the risk of overcoating failures. Work related to the design and development of the adhesion tester is the subject of another paper to be published later. Two methods of measuring cure-induced stresses in coatings were examined: (a) Deflection measurements using a capacitive transducer and (b) Direct measurements using a miniature surface mounted fiber optic (FO) strain gage. The latter technique was chosen for conducting the experiments because of accuracy and ease of use. The results indicated that the ambient relative humidity (RH) has a significant effect on the type and magnitude of stresses developed by different types of coatings during curing and aging. The stresses measured during this program were found to be as high as 9 to 11 MPa (13 to 19 psi), similar to cure stresses reported in the literature. INTRODUCTION Rehabilitation of deteriorated painted structures (such as bridge components) is necessary to prolong their service life. This rehabilitation typically entails either complete coating removal through abrasive blasting and reapplying a new coating system or overcoating the substrate with a maintenance coating system. In recent years, overcoating has become the preferred alternative 1 Adhesion Tool for Overcoating Risk-Reduction Analysis, Contract No. NCHRP-74, 22 Copyright 23 by SSPC: The Society for Protective Coatings since the cost incurred can be nearly % less than complete removal and replacement and the reduction in worker exposure to hazardous materials (such as lead containing paints) is significantly reduced. However, premature failure of overcoating systems has occurred, often during or immediately after a winter season. In such instances, a coating system intended to provide 1-1 years of life is damaged in the first few years. Figure 1 shows a typical overcoating failure on a bridge structure. Such failures negate the cost advantages of overcoating, making it less attractive to state departments of transportation (DOT s) and other structure owners. The predominant mode reported for such catastrophic overcoating failure is initial paint cracking and subsequent delamination due to the development of in-plane coating tensile stress. In service, as the coating begins to crack and deform, the applied shear stress actually becomes a peel stress. Thus, the characteristic appearance of failed coating systems in the delaminated sections is curled towards the topcoat. Strains are produced in coatings because of shrinkage, due either to solvent evaporation or the chemical changes of cross-linking (1). During the cure process, solvents and other volatile materials flash-off leaving behind a solid, cured film. During the cure process the coating film changes in volume, which causes the coating to pull-back on itself and effectively shrink. This is the cause of thin spots on edges or sharp corners and can induce internal shear stress and strain. Figure 2 shows this schematically. As additional coats are added, the total shear acting on the coating is increased. Energy stored in a coating by virtue of its internal strain increases as the coating thickness increases and, at a particular thickness, becomes sufficient to overcome the work of adhesion at the interface so that the coating

2 spontaneously peels off (2). Although delamination is often not observed immediately after overcoating, prolonged exposure to other stress inducing phenomenon (i.e., weather events and structure vibrations, which are assumed to be additive) can cause the early onset of this type of coating failure. A system targeted to last 1+ years may fail within the first few years of service, with no warning signs. For existing bridge structures, the substrate over which the overcoat system is applied is typically an aged, lead oil-alkyd coating system. Long-term cross-linking of these systems produces high internal stress and a brittle coating. The overcoat system transfers any curing or exposure-induced stress to this oil-alkyd system. These stresses act to disbond the existing, weakly adherent, alkyd system from the steel substrate. The present paper describes a research effort to quantify the internal stresses that develop in a coating system during curing and exposure. Several common overcoating systems (typically used for bridge structures) were investigated that included a poly silicone alkyd, an acrylic, a moisture-cured urethane, and an epoxy (see Table 1 for further details on the coating materials). The work reported here constitutes part of a NCHRP-IDEA project the main objective of which was to design and demonstrate an innovative field device to assess the risk of overcoating failures. Work related to the design and development of the adhesion tester is the subject of another paper to be published later. EXPERIMENTAL WORK During this program two methods of measuring cured-induced stresses in coatings were examined: (a) Deflection measurements using a capacitive transducer and (b) Direct measurements using a miniature surface mounted fiber optic (FO) strain gage. The latter technique was ultimately chosen for conducting the final set of experiments because of accuracy and ease of use. The fundamental bases for these two techniques are given below. Off the shelf commercial devices were available for both techniques. 29 Coating Stress Measurement Techniques (a) Deflection Method. Coating materials applied at a uniform thickness to one side of a thin steel foil were evaluated. Figure 3 shows a sketch of this apparatus. During cure the position of the foil is continually measured to a fixed, non-contact probe. Changes in relative position to the probe are used to calculate a deflection. At the completion of testing the stress is determined using the equation: 3 DEd σ =, 3δI 3 ( d + δ )(1 υ) Where: σ = internal stress, MPa D = deflection, mm E = substrate elastic modulus, MPa d = substrate thickness, mm δ = coating thickness, mm l = length of panel, mm ν = substrate Poison ratio Referring to Figure 3, a deflection to the left indicates a shrinking stress while a deflection to the right indicates an expansive stress. A shrinking type stress would impart a tensile, peeling action on the undercoat and possible subsequent failure. An expansive stress would put the undercoat into compression. A capacitive proximity sensor system manufactured by Capacitec, Inc. of Ayer, MA was used to measure the capacitance between the probe and the target (a coated steel shim), which is calibrated to convert the capacitance to distance. (b) Direct Measurements. A direct measurement of the stress induced by the coating material was determined using a miniature fiber optic (FO) strain gage manufactured by Luna Innovations of Blacksburg, VA. Figure 4 shows a schematic diagram of the FO strain gage, which is fabricated by inserting two optical fibers into a silica capillary tube. The length of the strain gage is 4 mm and the diameter can be in the range of 8 to 3 µm. The FO device is a reflective type fiber optic sensor meaning that the same optical fiber serves as both input and output to the strain gage element. During fabrication, the Input/Output and

3 Reflector optical fibers are joined to the inside of the silica capillary tube, either with an adhesive or by directly fusing the fibers to the tube. The distance between the Input/Output and Reflector attachment points in the silica capillary tube define the gage length (L) of the device. Operation of the FO gage revolves around monitoring the air gap s which is the distance between the Input/Output optical fiber and the Reflector (see Figure 4) with a special fiber optic measurement device. When a stress is applied at the surface of the base material it creates an elongation or compression of the strain gage. An elongation of the strain gage indicates an expansive (compressive) stress in the coating while a compression of the strain gage indicates a shrinking (tensile) stress in the coating. As the FO strain gage is stressed its optical properties change as signaled by changing values of s. This change is used to determine the change in the gage length, which when divided by the initial gage length gives the strain, by the following equation: Where: L ε =, L ε = strain L = the change in gage length, mm L = the original gage length Knowing the strain induced on a material, the stress induced along that surface can be determined. This is calculated based on the modulus of elasticity, which can be found for many materials. The stress is calculated using the following equation: Where: σ = εg, σ = internal stress, MPa ε = strain G = modulus of elasticity, MPa Test Specimens and Coating Application The test specimens were.-inch wide by 12-inch long by.6-inch thick steel (feeler gage) shims as the substrate on which the selected coatings were applied at a thickness of about mils (wet film thickness). This type of specimen has been found to be ideal for determining internal coating stress in the laboratory (3,4). During coating application and curing, environmental conditions (temperature and relative humidity) were monitored. These were used to ensure proper cure conditions and to determine their possible influences on coating stress. The miniature FO gages were oriented along the longitudinal axis of the steel shim as shown in Figure. The gages were bonded to the specimen surface (after recommended surface preparation) using an M-Bond 2 adhesive (a commercially available adhesive designed for strain gage applications). Following attachment of the FO strain gage and complete cure of the adhesive, a coating film was brush-applied over the entire length of the shim material and allowed to cure while oriented horizontally on edge. This was done to minimize the effects gravity would have on the strain reading, allowing for the measurement of the true strain induced by the coating material. Throughout cure, discrete measurements of the strain were recorded to determine stress as a function of time. RESULTS AND DISCUSSION For the four overcoat systems listed in Table 1, duplicate tests were conducted but at different time periods (during February through June). It was noted that although the room temperature remained more or less constant to within a few degrees of 2 o C, the ambient relative humidity (RH) changed significantly during the different time periods (from as low as 1% in February to over 6% in early June). No attempts were made to control the relative humidity; hence the data are represented in four sets to reflect this variability in RH. 1. Set-1, conducted in February/March the four coating systems were tested for 24 to 48 hours (except for the epoxy, which was tested for about 7 hours). 26

4 2. Set-2, conducted in early April/May was a repeat of the 1 st set but all 4 systems were tested for at least 2 hours. 3. Set-3, conducted in early May and early June to gather longer-term data (up to about 12 hours) for the Set-1 specimens. 4. Set-4, conducted in early June to collect longer-term data for the specimens from Set- 2. Figures 6 through 9 show plots of the stresses for each coating system for the first set of specimens (Set-1). Figures 1 through 13 represent data from Set-2 specimens while Figures 14 and 1 show longer-term coating stress data for Set-3 for the urethane and epoxy systems respectively. (Note: Data could not be obtained for the alkyd and acrylic systems since the FO strain gages attached to the specimens were damaged during handling). Figures 16 through 19 represent the long-term data from Set-4. Figures 6 through 13 show the variation of the curing stress for the four coating materials during the first hours after application. It is important to note the difference in stress patterns displayed by the coatings depending on the ambient relative humidity. Specific observations are as follows: The Set-1 specimens, tested when the RH was in the range of 1-2%, developed positive (expansive) stresses to varying magnitudes from slightly above to about 7 MPa [ to 11 psi] (see Figures 6 through 9). This implies that the coating is imparting a compressive stress on the substrate (i.e. undercoat material), which may not be detrimental. The Set-2 specimens, where the RH was in the range of 4-4%, showed different behaviors. The acrylic and urethane developed significantly large [about 9 MPa (13 psi) to about 11 MPa (19 psi)], negative (shrinking) stresses (see Figures 11 and 12) while the alkyd and epoxy, showed positive (expansive) stresses in the range of about 1-4 MPa [14 8 psi] (see Figures 1 and 13). It may be mentioned that similar ranges of curing stresses have been reported in the literature (2,3). Negative stresses imply that the coating is imparting a tensile (curling) stress to the substrate. If this curling stress exceeds the adhesive strength between the coating and the undercoat, then the coating material could be disbonded from the undercoat. Additionally, if the curling stress exceeds the adhesive strength of undercoating material, then the undercoat could disbond from the substrate. Data from the longer-term tests (Set-3 and Set-4, Figures 14-19) where the RH increased to over 6% the following observations can be made: Shrinking (negative) stresses developed by the acrylic and urethane coatings became less negative than observed at lower RH values, The expansive (positive) stresses displayed by the alkyd and epoxy became less positive and crossed over to slightly negative values with an increase in RH. This behavior suggests that the development of stress within a coating is dependent on RH. Variations in humidity (specifically increases) can result in a higher likelihood of shrinking stresses particularly for acrylics and urethane systems in the RH range of 4-%. CONCLUSIONS Based on this testing the following conclusions can be made: The ambient RH has an effect on the type and magnitude of stresses developed by different types of coatings during curing and aging. Large negative (shrinking) stresses were measured for acrylic and urethane systems in the RH range of 4-4% followed by a shift towards less negative stresses at RH values of 6-6%. The alkyd and epoxy systems showed positive (expansive) stresses in the RH range of 4-4% with a shift towards less positive values at higher RH conditions. Under certain RH conditions, acrylic and urethane coatings may develop sufficient shrinking stresses to cause disbondment of the undercoat as is sometimes encountered in the field on painted bridge components. 261

5 REFERENCES 1. S.G. Croll, Journal of Coatings Technology, Volume 3, No. 672, January 1981, pp S.G. Croll, Journal of Coatings Technology, Volume 2, No. 66, June 198, pp S.G Croll, J.Oil Color Chem. Assoc., Vol. 63, 198, p S.G Croll, private communication March, 21 ACKNOWLEDGEMENTS This work was carried out as part of an NCHRP- IDEA Project under Contract No. NCHRP

6 Table 1. Coating systems used in the test program Manufacturer Name Chemistry Applied WFT Keeler & Long Poly Silicone Enamel Alkyd -6 mils Benjamin Moore DTM Acrylic Gloss M-28 Acrylic -6 mils Wasser MC Luster MC Urethane -6 mils Carboline Carbomastic 9 Epoxy 9-1 mils Figure 1. Typical overcoating failure on a bridge structure 263

7 Clamp CURE PROCESS Coating is applied to the existing aged material as a liquid. Coating cures, creating a solidified film that develops internal stresses. The aged system (less adherent to the substrate) disbonds under the heightened stress conditions. Figure 2. Shrinkage of an overcoat system during curing Coating Thin steel foil Shrinking (tensile) stress on substrate Deflection Expansive (compressive) stress on substrate Figure 3. Coating stress determination by deflection measurement 264

8 Silica Capillary Tube Air Gap Input/Output Optical Fiber R 1 R 2 Reflector Adhesive S Gage Length (L) Figure 4. Miniature Fiber Optic Strain Gage Steel shim Strain gage Longitudinal axis Figure. Placement and orientation of miniature Fiber Optic Gage on test specimen 26

9 1 Set-1 - Alkyd 3 Stress (MPA) Elapsed Time (hours Figure 6. Coating stress measurements for an alkyd system obtained with the FO strain gage technique (Set-1). 1 Set-1 - Acrylic Temperture (C) and Relative Humidity Figure 7. Coating stress measurements for an acrylic system obtained with the FO strain gage technique (Set-1). 266

10 1 Set-1- Urethane Figure 8. Coating stress measurements for a urethane system obtained with the FO strain gage technique (Set-1). 1 Set-1 - Epoxy Figure 9. Coating stress measurements for carbomastic epoxy system obtained with the FO strain gage technique (Set-1). 267

11 1 Set-2- Alkyd Figure 1. Coating stress measurements for an alkyd system obtained with the FO strain gage technique (Set-2). 1 Set-2 - Acrylic Figure 11. Coating stress measurements for an acrylic system obtained with the FO strain gage technique (Set-2). 268

12 1 Set-2- Urethane Elpased Time (hours) Figure 12. Coating stress measurements for a urethane system obtained with the FO strain gage technique (Set-2). 1 Set-2 - Epoxy Figure 13. Coating stress measurements for a carbomastic epoxy system obtained with the FO strain gage technique (Set-2). 269

13 1 Set-3 Urethane Figure 14. Coating stress measurements (long-term data) for an alkyd system obtained with the FO strain gage technique (continuation of Set-1 specimens). 1 Stress (low) Stress (high) Temperature Relative Humidity Set-3 Epoxy Figure 1 Coating stress measurements (long-term data) for an epoxy system obtained with the FO strain gage technique (continuation of Set-1 specimens). 27

14 1 Stess Low Stress High Temp RH Set-4 Alkyd Temperature (F) and Relative Humidity Figure 16. Coating stress measurements (long-term data) for an alkyd system obtained with the FO strain gage technique (continuation of Set-2 specimens). 1 8 Stess Low Stress High Temp RH Set-4 Acrylic Temperature (F) and Relative Humidity Figure 17. Coating stress measurements (long-term data) for an acrylic system obtained with the FO strain gage technique (continuation of Set-2 specimens). 271

15 1 8 Set-4 Urethane Figure 18. Coating stress measurements (long-term data) for a urethane system obtained with the FO strain gage technique (continuation of Set-2 specimens). 1 Set-4 Epoxy Figure 19. Coating stress measurements (long-term data) for an epoxy system obtained with the FO strain gage technique (continuation of Set-2 specimens). 272