The Effect of Oven-Heat Flux on Powder-Coating ... - ACS Publications

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Ind. Eng. Chem. Res. 2009, 48, 1638–1649

The Effect of Oven-Heat Flux on Powder-Coating Issues of Sheet Molding Compound Panels Soumendra K. Basu,*,† Bhavesh Shah,† and Hamid G. Kia† India Science Laboratory, General Motors R&D, ITPL, Whitefield Road, Bangalore, 560066, India, and Materials & Processes Laboratory, General Motors R&D, 30500 Mound Road, Warren, Michigan 48090

The effects of variations in the powder bake temperature profile on the paint popping defect of sheet molding compound (SMC) panels was investigated by subjecting powder-coated panels to various heating rates using IR heaters and convective ovens, and by using a mathematical model of heat transfer and curing reaction. It was found that overheating the SMC than that required for optimum powder curing can cause high rates of outgassing of absorbed moisture and, therefore, excessive popping. It was further found that higher quantity of moisture outgassing per unit time through the molten primer layer during a fast temperature ramp causes more popping compared to a slow temperature ramp. Therefore, it is expected that any plant that is experiencing paint popping would benefit from slowing down the rate of temperature rise of the panels during the initial 5-10 min of the heating cycle. 1. Introduction The automotive industry has been using increasing amounts of polymer-based lightweight materials, such as sheet molding compounds (SMC) and reinforced thermoplastics, in exterior panels and semistructural interior applications to enable enhanced fuel economy, superior drivability, corrosion resistance, and greater design and manufacturing flexibility.1 Simultaneously, because of environmental regulations on volatile organic compound emissions, the automotive industry is also moving toward increasing the use of solventless organic powder coatings, particularly for priming applications. Electrostatic spray of powder onto the parts reduces overspray waste, and almost 99% of the oversprayed powder can be effectively recycled.1 However, use of powder primers on plastic and especially SMC body panels2 has become a major challenge for the automotive industry because of a cosmetic defect in the finished product called paint popping.1-7 The paint popping is a surface defect on the coated substrates in the form of bubbles, pin holes, or craters that are up to few hundred micrometers in diameter. These defects on any automotive outer panel make the part cosmetically unacceptable, and result in a high rejection rate and repair costs.1 Though popping has been observed with conventional liquid primers,8 the defect becomes more severe when powder primers are used.1-7 It has been shown that the entrapped air and the absorbed moisture in the SMC that outgas during the heating in the oven cause popping.1,3 Figure 1 shows a schematic of the sequence of several processes involved in an automotive paint process.1,9 The heat treatment in the Electrocoat Paint Operation (ELPO) renders the SMC parts completely dry and moisture free. However, after the ELPO operation, till the powder primer application, the SMC parts are free to absorb moisture in the plant atmosphere. The amount of moisture absorbed increases with humidity level and exposure time. Therefore, the events after the ELPO oven up to the powder application are critical to the formation and extent of paint popping. The popping defect can be addressed by various methods including (1) changing the SMC composition to achieve low * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +91-9880518260. † General Motors, India. † General Motors, U.S.A.

moisture absorbing, low air entrapping formulations, (2) modifying the conductive coating, which is applied onto the SMC surface prior to powder application, to render a controlled moisture permeation, (3) developing low temperature cure powder coatings, and (4) modifying the powder bake profile to control the rate of moisture release. Several of these approaches have been tried separately and in combination, both at the laboratory and plant scales with various degrees of success. In a previous study,1 it was shown that commercial SMC products typically contain 0.5-1.5 wt % moisture, and all show some degree of popping irrespective of the choice of commercial conductive coatings. It was further demonstrated that even at completely dry conditions SMC panels showed popping due to the entrapped air present within the matrix microvoids. The popping at the dry condition was successfully eliminated by changing the low profile additive in the SMC formulation.3 Subsequently, low moisture absorbing SMC formulations were developed that absorbed only 0.3-0.4 wt % moisture.4 Panels made with these SMC formulations did not show paint popping in a straight-through paint operation. In a later effort, a conductive coating was developed that when combined with the low moisture absorbing SMC was able to eliminate popping in all laboratory trials.5 Plant trials for the above formulations showed no popping in the case of no line stoppage. However, in the case of extensive storage exceeding 2 weeks between ELPO treatment and powder application, the panels showed popping.6 It was further found that rapid heating of the panels using high intensity infrared (IR) heaters is detrimental as far as the popping defect is concerned. Extensive research on the effect of SMC and conductive coating composition on popping have resulted in significant

Figure 1. A schematic of the processes involved in an automotive paint process.

10.1021/ie801130g CCC: $40.75  2009 American Chemical Society Published on Web 12/23/2008

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1639

Figure 2. Categorization of the severity of the popping defects based on visual evaluation of the panels: severe, medium, minor, and no defects.

improvement in reducing the extent of popping defects. However, these significant advances are still not able to address the issue of popping in the case of extreme plant conditions, such as, extensive line stoppage and high rate of heating in the powder bake ovens. Because of extensive validation requirements involved in getting any paint certified for automotive usage, introducing a new low temperature cure, better pop resistant powder primer is a costly and long-term process. However, an alternative route of eliminating the popping defect that has not been extensively studied is modification of the powder bake temperature profile. The observation that rapid heating of SMC panels in plants using high intensity IR heaters causes excessive popping, shows that the modification of the heat treatment in the powder bake oven is a promising route for addressing the popping defect. It is therefore important to investigate whether the heating conditions in the powder bake ovens that are designed and optimized for sheet metal panels, cause overheating of SMC panels. The overheating of SMC panels more than what is required for melting and curing of the powder would cause the release of excess moisture from the panels and, therefore, increase popping. Furthermore, it is also important to understand the effect of different modes of heating (IR, convection, etc.), heat flux, moisture content, and primer viscosity on the extent of popping. This article discusses our effort to address the above concerns by investigating paint popping formation in powder-coated SMC panels subjected to various heating profiles using IR heaters and convective ovens, and by using a conventional heat transfer model for the panels during the powder bake process. 2. Experimental 2.1. Materials. A commercially available SMC based on toughened class A polyester resin (Continental Structural Plastics) and a commercially available sheet metal made of cold rolled steel were used to study the temperature profile of the panels in the convection oven and under the infrared heaters. A low moisture SMC (made by Meridian Automotive Systems in collaboration with Ashland Chemicals) was used for the powder priming experiments. An experimental conductive coating (developed by Redspot Paints and Varnishes Co., Inc.) was used on the SMC panels to enable powder priming of the panels. A commercially available polyester epoxy hybrid powder primer (PCV Envirocron 70104, PPG) was subsequently used to coat both SMC and sheet metal panels. 2.2. Procedures. 2.2.1. Preparation of Panels. For laboratory experiments, SMC panels (12 × 18 in.2) with 2.57 mm thickness and sheet metal panels (12 × 18 in.2) with 0.88 mm thickness were used. The procedures for molding, cleaning, and conditioning of the SMC panels have been discussed elsewhere.1 The sheet metal panels were used as received. For plant experiments, SMC panels with 2.5 mm thickness and sheet metal panels with 0.7 mm thickness were used. 2.2.2. Powder Priming. The SMC panels were prepared and dried at 110 °C for 24 h followed by the application of the

experimental conductive coating.1 The panels were subsequently dried again at 180 °C for 1 h followed by an exposure to 90% relative humidity (RH) at 40 °C for either 24 or 48 h. The preconditioning and conductive coating applications were not required for the sheet metal panels. Both SMC and sheet metal panels were then powder coated electrostatically using manual powder-coating equipment (SAMES, JRN406 gun with CRN456 controller operating at 50kV). The powder layer was subsequently cured either by infrared heating or by convective heating. For infrared curing the panels were transferred to a thermoformer (Drypoll thermoformer) equipped with a bank of 60 infrared heaters. The distance between the parts and the heaters was kept at 7 in. The intensity of the heaters was adjusted by controlling the power input. For convective heating a forced air convection batch oven (Despatch Industries, model RFD 2-19-2E) was used. 2.2.3. Popping Defect Characterization. After the completion of the powder bake, the panels were inspected for popping/ foaming with the naked eye and under a microscope. Based on visual estimation, the finish of the panels was rated as severe, medium, minor, or no defects. Figure 2 shows the representative optical micrographs and the corresponding defect ratings. Only the panels with no defects qualify as production ready. 2.2.4. Temperature Profile Measurement. In laboratory experiments the temperature profiles of SMC and sheet metal panels as well as the air temperature during infrared and convective heating were measured as a function of time using an oven tracker (Oven XL) made by Datapaq Inc. The temperature of the panels in the convective oven, which heats the panels equally from both sides, were measured on both sides, and the average value was reported. In contrast, the temperature of the panels under the IR heater, which heats the panels from front, was reported only for the back side. The temperature measured from the front side was not considered due to the possibility of the thermocouples being overheated because of direct IR radiation. However, for the purpose of validating the theoretical model, the measured temperatures from specially embedded thermocouples within the SMC panels close to the front and back surfaces of the panels were used. In the case of powder-coated panels, only the backside temperature was measured for both convective and IR heating. The temperature of the primer layer during curing was measured by embedding a thermocouple inside the primer. In the plant experiments the temperature profiles of the backside of the sheet metal roofs, as well as the oven air temperature during the entire process were measured for two different plants following the same procedure described above. 2.2.5. Powder Kinetics Measurement. The kinetics of the powder cure reaction was measured using a differential scanning calorimeter (DSC Q1000, TA instruments). Samples were prepared using crimped aluminum pans, and the typical range of the sample size was 9-12 mg. The experiments were run in scan mode at various heating rates ranging from 0.5 to 20 K/min. The data was recorded in the form of heat flow (J/g) versus

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time and temperature. The kinetic parameters were calculated assuming an autocatalytic reaction for powder cure as presented later. 2.2.6. Powder Viscosity Measurement. The viscosity profile of the powder was measured using rheometrics dynamic analyzer (RDA II). The experiments were run using 25 mm parallel plates in oscillatory mode at 10 Hz under controlled strain. Gap setting was kept at 0.1 mm for all experiments. Isothermal runs were carried out for 30 min at four different temperatures of 120, 140, 160, and 180 °C. Temperature ramp runs were carried out by heating the sample from 65 to 180 °C in variable durations of 5, 10, and 20 min followed by 30 min isothermal cure at 180 °C. Viscosity data were reported in Pa · s against time and temperature. 2.2.7. Moisture Release Measurement. Samples for moisture release experiment were prepared by first recording the dry weight of the SMC panels after heating the panels at 180 °C for 45 min. The panels were then kept at 40 °C, 90% RH environment for 2 days prior to recording the weight of the panels again. The samples out of the 90% RH environment were immediately transferred to a remote diffusion cell (MOCON, Minneapolis, MN) and the edges of the samples were sealed with a filled epoxy. The diffusion cell was equipped with flow lines that allow dry nitrogen gas to sweep both sides of the panel separately. The diffusion cell was then placed in an oven and was heated at different heating ramps. During heating, a nitrogen flow of 75 cc/min was used on both sides of the sample, and the relative humidity of the nitrogen coming off the panels, as well as the temperature of the panels were recorded as a function of time using Labview software (version 8.2). The amount of moisture outgassing from the SMC samples in g/min and the corresponding temperature were reported as a function of time.

Figure 3. A schematic representation of the heat transfer process during primer cure in the oven.

the front surface of the primer layer can receive heat by one or more of the following modes: (1) convective heating by ambient air, (2) radiative heating by oven walls, and (3) direct long wavelength IR heating by high intensity IR heaters. Long wavelength IR radiation has poor penetration into coating/panels and is typically considered as a surface heat flux.11,13 The backside of the panels can lose heat to the ambient by both convection and radiation. Apart from the heat transfers with the environment, the primer layer also generates heat (Qrxn) owing to its exothermic cure reaction. The heat transfer process for the thin, flat panels used in both laboratory and plant experiments can be modeled as a one-dimensional problem by neglecting any transfers in the in-plane directions. This is a standard assumption used for thin, flat geometries because typical edge effects do not propagate for more than 2-5 times the coating/panel thickness. The details of the model equations and solution procedure are presented in Appendix A.

3. Model A mathematical model for the heating of powder-primercoated automotive panels with simultaneous curing of the primer was considered on the basis of commonly practiced methods.10-14 The main purpose of the model was to investigate whether the SMC panels are overheated in the plant ovens that are typically designed and optimized for sheet metal panels. The possibility of overheating of the SMC panels becomes more significant when high intensity IR heating is used to melt and cure the powder layer. Since the SMC panels are subjected to IR heating from one side, that surface may be significantly overheated compared to the other surface because of the low thermal conductivity of the SMC. Under such heating conditions the effects of the primer layer, and the heat generated during curing, on the temperature of the SMC panel may also become important. The temperature of the SMC panels, especially that of the exposed surface of the panel, determines the amount of moisture outgassed from the panel. A lower temperature of the SMC panel is expected to cause lower moisture outgassing and, therefore, reduced popping. Figure 3 shows a schematic of the powder-coated panels and the possible modes of heat transfer between the panel and the plant or laboratory environment. The substrate and the powder primer layers are assumed to have a thickness of tS and tP, respectively. The temperature of the backside (the back surface) of the substrate at z ) 0 is denoted by TS, and that of the exposed side (the front surface) of the primer layer at z ) tS + tP is denoted by TP. The temperatures of the ambient and the oven walls are denoted by T∞ and TW, respectively. Depending upon the zone of the plant oven or the specific laboratory experiment,

4. Results and Discussion The values of the heat transfer coefficients and parameters that capture the temperature profile of SMC and sheet metal panels subjected to convective and direct high intensity IR heating were obtained by comparing the model predictions with the measurements from laboratory experiments. For this purpose the temperature of the backside of the panels were compared to that predicted by the model. The comparison was done without the presence of the primer layer in order to discount the effect of the exothermic reaction in the primer layer and study the thermal transients of the panels alone. The effect of the primer layer on panel temperature was investigated separately. Figure 4 shows the measured temperature profiles (black solid lines) and the model predictions (gray solid lines) for both SMC and sheet metal panels. The corresponding ambient air temperature was also measured (black dotted lines) and used as input to the model. The target powder bake temperature is also indicated in each figure by a narrow horizontal line. Figure 4 plots A and B show the results for force convective heating of SMC and sheet metal substrates in the absence of direct IR heating. A convective heat transfer coefficient in the range of 11.0-12.5 W/(m2 · K) produced an excellent match for both SMC and sheet metal panels. It should be noted that the value of the convective heat transfer coefficient is specific to the operating conditions used in the oven and provides a ballpark estimate that may vary to some extent with changes in operating conditions. Figure 4 plots C and D show the results for direct IR heating of SMC and sheet metal substrates in the presence of natural convection. The panels were subjected to IR radiation


Figure 4. Comparison of the measured temperature at the backside of the substrate (z ) 0) with the corresponding model predictions in absence of the primer and under laboratory conditions. The black solid lines indicate measured temperature, the gray solid lines indicate model estimation, and the black dotted lines indicate corresponding ambient temperature. The estimated values of surface IR heat flux and convective heat transfer coefficient obtained by fitting the model to the measured data for each case are also presented. The top two plots present forced convection heating in the absence of IR for (A) SMC and (B) sheet metal substrates and the bottom two plots present natural convection heating in the presence of IR for (C) SMC and (D) sheet metal substrates.

of different intensities by changing the power input to the IR heaters. A convective heat transfer coefficient in the range of 14.0-15.5 W/(m2 · K) produced the best match for both SMC and sheet metal panels at all IR intensities. The values of surface IR heat flux (QIR) that produced the best match for both SMC and sheet metal panels as well as showed a liner dependence on power input to the IR heaters (40%, 50%, 60%, 70%, and 80% of the maximum power) are presented in the figures. Figure 4 demonstrates that the model predictions can match the experimentally measured temperatures of the backside of both SMC and sheet metal panels at various heating conditions with the chosen values for the unknown heat transfer coefficients and parameters. As explained earlier, in case of convective heating, the temperatures of the front and the backside of the panels are expected to be identical and, therefore, matching the temperature of the backside alone is sufficient. In the case of IR heating from one side for thin panels with high thermal conductivity, such as sheet metal, almost identical temperatures is expected on both sides and, therefore, matching the temperature of the backside alone is sufficient. However, low thermal conductivity panels, such as SMC, can exhibit significant temperature difference between the front and the back surfaces. Therefore, the temperatures of both sides of the SMC panels subjected to IR heating were measured with specially embedded thermocouples close to the surface, and the model predictions were validated against them.

Figure 5 shows the model predictions for the temperature of the front (solid lines) and back (dashed lines) surfaces of SMC (black) and sheet metal (gray) panels in the absence of the powder coating for convective heating with or without IR. The values of convective heat transfer coefficient and surface IR heat flux were obtained from laboratory validation experiments (Figure 4) and are indicated in the figure. The corresponding ambient temperatures (black dotted lines), as measured in laboratory validation experiments, were also used for the model calculations. Figure 5A shows the results for convective heating in the absence of IR. As expected, the front and back surface temperatures for both SMC and sheet metal panels fall on top of each other. Furthermore, the temperature of sheet metal is always higher than SMC, indicating that the overheating of SMC does not occur under the convective heating conditions used in the laboratory experiments. Figure 5B shows the results for convective heating in the presence of IR for several surface IR heat flux values. Front and back surface temperatures for high thermal conductivity, low heat capacity sheet metal panels coincide very well, whereas, those for low thermal conductivity, high heat capacity SMC panels show significant difference. The difference in temperature between the front and back surfaces of the SMC panels rise with rise in the intensity of the IR heat flux. Moreover, upon prolonged exposure to IR heat flux, the front surface temperature of the SMC panels exceed that of the sheet metal panels and the crossover happens earlier as the


Figure 5. Comparison of the model predictions for temperatures of the front and backside surfaces of sheet metal and SMC substrates for (A) convective heating in absence of IR and (B) convective heating in the presence of IR both in absence of the primer and under laboratory conditions. The gray solid and dashed lines indicate the front and backside surface temperatures of sheet metal (which coincide), the black solid lines indicate the front surface temperature of SMC, the black dashed lines indicate the backside surface temperature of SMC, and the black dotted lines indicate the corresponding ambient temperature used for the model prediction. The values of surface IR heat flux and convective heat transfer coefficient used for each case are also presented.

Figure 6. Comparison of the model predictions for the temperature of the front surfaces of sheet metal and SMC substrates for (A) convective heating in absence of IR and (B) convective heating in presence of IR both in absence and presence of the primer and under laboratory conditions. The gray solid lines indicate front surface temperatures of sheet metal without primer film, the gray dashed lines indicate front surface temperatures of sheet metal with primer film, the black solid lines indicate the front surface temperature of SMC without primer film, the black dashed lines indicate the front surface temperature of SMC with primer film, and the black dotted lines indicate the corresponding ambient temperature used for the model predictions. The values of surface IR heat flux and convective heat transfer coefficient used for each case are also presented.

intensity of the IR heat flux increases. The overheating of the front surface of the SMC panels in plants, where operating conditions are fixed to keep the sheet metal temperatures within the required bound, can prevent optimal curing of the powder coating. Moreover, overheating of SMC panels can cause additional moisture release and, therefore, increase popping. In such cases, partially or completely switching off the IR heaters when SMC panels pass underneath would facilitate the optimal curing of the primer, save energy costs, and reduce popping. Figure 6 shows the effects of the presence of the primer layer, and the exothermic curing reaction within the layer, on the temperature of the front surface of SMC (black) and sheet metal (gray) panels for convective heating in the absence and presence of IR. The solid lines indicate the front surface temperatures in the absence of the primer layer, whereas, the dashed lines indicate the same in the presence of the primer layer. As before, the values of convective heat transfer coefficient, surface IR heat flux, and the corresponding ambient temperature (black dotted lines) were obtained from laboratory validation experiments (Figure 4), and are reported in this figure. Figure 6 plots A and B show the results for convective heating in the absence and presence of IR, respectively. In all cases, the estimated

temperatures of the front surface of the panels are only few degrees lower in presence of the primer layer. The thermal resistance of the primer layer and the heat generated during the exothermic reaction does not have any significant effect on the temperature of the front surface of the panels and, therefore, the primer layer is not considered in the plant heat transfer analysis presented below. Figure 7 shows the temperature measured from the backside of a sheet metal roof (black solid lines) and model predictions of the same (gray solid lines) as a function of the residence time in the plant oven for two different plants with two different initial heating rates. The corresponding ambient temperature (black dotted lines) and oven wall temperature (black dash-dot lines) were also measured and presented in the figure. The target powder bake temperature and the allowed upper and lower limits for ensuring the optimum cure are also indicated. As explained in detail in Appendix A, direct high intensity IR heaters are used only in the initial IR zone, where wall heating and high convective heating are not used. The wall heating and high convective heating are used in the intermediate still and air seal zones. In the last ambient zone the panels are allowed to cool to room temperature. The values of the parameters htop, hbot,


Figure 7. Comparison of the measured temperature at the backside of the vehicle roof with the corresponding model predictions for two sets of plant conditions: (A) slower heating rate and (B) faster heating rate. The black solid lines indicate measured temperature, the gray solid lines indicate model predictions, the black dotted lines indicate corresponding ambient temperature, and the black dash-dot lines indicate corresponding wall temperatures used for model predictions. Different zones of the oven, along with their subzones, are also indicated. Table 1. The Values of Residence Time, Wall Temperature, Surface Heat Flux, and Convective Heat Transfer Coefficients at the Front and Backside Surfaces for Each Subzone That Was Used for the Model Predictionsa still zones

IR

duration, s TW, K QIR, W/m2 htop, W/m2 · K hbot, W/m2 · K duration, s TW, K QIR, W/m2 htop, W/m2 · K hbot, W/m2 · K

air seal

I

II

III

I

360 294 2000 10 10

140 490 0 15 10

140 485 0 15 10

140 480 0 15 10

230 470 0 15 10

360 294 800 7 7

140 465 0 15 7

140 465 0 15 7

140 465 0 15 7

230 460 0 15 7

II

III

IV

V

230 460 0 15 10

225 450 0 15 10

225 450 0 15 10

230 460 0 15 7

225 460 0 15 7

225 460 0 15 7

VI

VII

ambient

Case A 230 460 0 15 10

580 294 0 10 10

Case B

a

230 460 0 15 7

190 455 0 15 7

190 455 0 15 7

200 294 0 7 7

Cases A and B refer to the two heating rates presented in Figure 7.

and QIR used in each zone to obtain the model estimates for both plants are presented in Table 1. The values of these parameters in each zone were chosen in the vicinity of the values obtained from the laboratory experiments to obtain the best possible match with the measured profiles. In the IR and ambient zones, where no forced convection is used, the values of htop and hbot are kept equal. In the intermediate zones, where forced convection is used, a higher value of htop was used. However, even in the intermediate zones the back side of the roof does not experience any force convection; therefore, hbot is kept equal to that used outside the forced convection zones. Figure 7 clearly shows, for two significantly different plant conditions, the model can very well estimate the temperatures of sheet metal panels provided proper values are chosen for the unknown parameters. The objective behind obtaining the values for the unknown parameters for accurate prediction of sheet metal temperature profiles was to create a virtual platform for treating the SMC panels (for which plant oven data was not available). Figure 8 compares the model predictions for the front side and the backside surface temperatures of SMC (black lines) and sheet metal (gray lines) panels for the plant conditions presented in Figure 7 and Table 1. The front side surface temperatures of both panels are shown by solid lines, whereas, those of the backside are shown by dashed lines. As in Figure 7, the measured ambient temperature and the oven wall temperature are shown by black dotted and dash-dot lines, respectively. As expected, the temperature of the backside of the sheet metal

panel coincides with that of the front surface, whereas, the temperature of the backside of the SMC panel lags that of the front surface. For both plant conditions, which have different heating rates and different intensities for incident IR heat flux, the temperature of the front surface of the SMC panel is always lower than that of the sheet metal panel. Therefore, at the representative plant conditions investigated here, the SMC panels do not get overheated and do not prevent optimum curing of the primer. Furthermore, at the representative plant conditions overheating of SMC is not responsible for primer popping. However, other plants that may use prolonged high intensity IR heating can suffer from SMC overheating as in the laboratory experiments shown in Figure 5. First order estimates for the rate of temperature rise and, therefore, the possibility of overheating of a given panel can be obtained from a lumped capacitance model of the heating process. Such models assume uniform temperature for the entire panel, and perform an overall heat balance with the environment. In the case of convective heating of a panel of thickness tP, density F, specific heat capacity CP, the overall transient heat balance is given by ∂ (FCPtPT) ) h(T - T∞) (1) ∂t The analytical solution of the above equation is given by9 T ) Tf - (Tf - Ti)e-t⁄τ

(2)


Figure 8. Comparison of model predictions for the temperatures of front and backside surfaces of sheet metal and SMC roofs for two sets of plant conditions: (A) slower heating rate and (B) faster heating rate. The gray solid lines indicate the front and backside temperatures of sheet metal (which fall on top of each other), the black solid lines indicate the front surface temperature of SMC, the black dashed lines indicate the backside surface temperature of SMC, the black dotted lines indicate the corresponding ambient temperature, and the black dash-dot lines indicate corresponding wall temperatures used for model predictions. Residence time and heating conditions presented in Table 1 (identical to Figure 7) were used.

where, Ti and Tf are the initial and the final temperatures and τ ) FCPtP/h is the time constant of the first order process. For convective heating in the absence of IR assuming a value of h ) 15 W/(m2 · K) yields τ ) 330 s for SMC and τ ) 165 s for SM. For convective heating in the presence of IR, τ can be redefined using a modified convective heat transfer coefficient (hmod) equivalent to the combined effects of convection and IR given by

(

hmod ) h

Qconv + QIR Qconv

)

(3)

Considering typical values Qconv ) 1000 W/m2 and QIR ) 2000 W/m2 yields hmod ) 45 W/(m2 · K), and τ ) 110 s for SMC and τ ) 55 s for SM. In either case sheet metal heats up twice as fast as SMC. This difference is primarily because the thickness of SMC is more than three times that of SM. The time constant of the lumped capacitance model, however, only presents information about the average temperature of the panels. Depending upon the thermal conductivity of the panel and heating conditions the surface temperature of the panels may be significantly different than the average temperature. The difference between the surface temperature and the average temperature of the panel can be estimated by evaluating the Biot number, Bi ) htP/k ) ∆Tinternal/∆Texternal, which gives the ratio of the temperature differences internal and external to the panel. Using a value of h ) 15-45 W/(m2 · K) yields Bi ) 1.3-3.9 × 10-1 for SMC and Bi ) 2.3-6.9 × 10-4 for SM. Three orders of magnitude higher Bi in SMC indicates higher internal temperature difference in SMC than in SM. This is why though the average temperature of SMC remains lower than that of SM, the surface temperature of SMC can overshoot that of sheet metal as observed in Figure 5. With progressive light-weighting of the vehicles for enhanced fuel economy, use of thinner body panels and low density materials such as low density SMC, carbon fiber SMC, and magnesium, etc. are becoming the standards for the industry. Vehicle lines are increasingly opting for multimaterial solutions to enable light-weighting, advanced styling, and agile manufacturing. New-age paint ovens, therefore, should be designed/ operated keeping in mind the heating time constants and internal resistances of each type of body panels made of different materials so that the paint process is material transparent. As far as the popping defect is concerned, lowering the temperature

Table 2. The Effect of the Intensity of the Surface IR Flux and Panel Preconditioning on the Degree of Popping panel preconditioning QIR, W/m2

24 h at 90% RH, 40 °C

48 h at 90% RH, 40 °C

2000 2650 3300 7500

no popping no popping minor popping severe popping

no popping minor popping medium popping severe popping

of the panels, which overheat due to high intensity IR radiation or other reasons, to the level required for the optimum curing of the primer should reduce the amount of moisture outgassing and therefore popping. However, for the panels that do not overheat, for example those in Figure 7, the rate at which they were heated to reach the target powder bake temperature may play a vital role in determining whether the panels would show popping. It is believed that the initial stage of primer cure (when the primer is melted and present as a viscous layer) is the most critical stage for developing popping defects.3 This is in harmony with observations in plants where the presence or absence of high intensity IR heating at the initial stage often translates into the presence or absence of popping defects in the finished products. To investigate the effect of IR heating on popping defects, powder-coated preconditioned SMC panels were subjected to IR heating of varying intensities, and the final coatings were inspected for popping defects. Table 2 shows the visual estimation of popping defects observed in powder-coated SMC panels subjected to IR heating at various intensities. The SMC panels prior to powder application were preconditioned by drying at 180 °C for 60 min, followed by exposure to a 90% RH, 40 °C environment for either 24 or 48 h. The IR intensities reported in the figure are based on the calibration of the power settings of the IR heater performed during laboratory validation experiments presented in Figure 4. As evident from Table 2, increasing the intensity of the IR heat flux causes the popping defect to worsen. Additionally, longer exposure to humid atmosphere during the preconditioning stage, which causes the amount of moisture absorbed in the panel to increase,1 further worsens the popping defect. Figure 9 shows the temperature profiles of front surfaces of SMC panels at various IR intensities that produced severe, medium, and no popping defects. The temperature profiles clearly show that the popping defect worsens as the rate of temperature rise increases. However, increasing the intensity


Figure 9. Temperature profiles of front surfaces of SMC panels subjected to direct IR heating that produced severe, medium, and no popping defects. The reported temperatures are those obtained from thermocouples embedded beneath the front surface of the SMC panel that were simultaneously subjected to the same heating conditions but were not coated with primer.

of the IR heat flux not only increases the rate of temperature rise, but also the final steady state temperature of the panel. To conclusively determine the effect of rate of temperature rise of the panels on popping defect, a series of controlled heating experiments were performed using a convective oven. Figure 10 shows the temperature profiles of front surfaces of SMC panels (solid lines) and the corresponding air temperatures (dotted lines) during convective heating at different temperature ramps. The extent of popping observed in the finished panels corresponding to each heating profile is also indicated in the figure. Figure 10A shows the temperature profiles corresponding to the cases where the oven temperature was initially kept at about 50 °C, and then ramped linearly to 180 °C within 20 min, 15 min, and instantaneously. Since all the panels were heated from identical initial temperatures to identical final temperatures, higher rate of temperature rise alone caused the panels to produce progressively worsening popping defects. This observation emphasizes that irrespective of the mode of heating, IR or convection, higher rate of temperature rise caused increased popping. For both modes of heating there exists a critical rate of temperature rise below which the panels did not exhibit

popping. To further investigate the effect of rate of temperature rise during the initial stage of heating (when the primer is mostly liquid) separately than the later stage (when the primer is mostly solid), the panels were subjected to separate heating ramps during the initial and the later stages while keeping the initial and final temperatures identical. Figure 10B shows the temperature profiles corresponding to the cases where the oven initially was kept at 50, 75, 100, and 180 °C, and then ramped linearly to 180 °C within 20 min. As evident from the temperature profiles, the panel which had the fastest rate of temperature rise at the initial stage, but the slowest rate of temperature rise at the later stage demonstrated the worst popping. The popping defect improved as the rate of temperature rise at the initial stage fell but that at the later stage rose. This clearly demonstrates that the rate of temperature rise at the initial stage of primer cure is critical as far as the popping defect is concerned. A higher rate of temperature rise at the initial stage (∼300 s) caused increased popping even if the rate of temperature rise at the later stage was proportionately lower. It is expected that higher rate of temperature rise would cause higher rate of moisture outgassing from the SMC substrate. At the initial stage of heating, when the primer is melted and is a viscous liquid, a higher rate of moisture outgassing would lead to excessive popping. To investigate the effect of rate of temperature rise on the rate of moisture outgassing from the SMC substrate, the amount of moisture released as a function of time was measured for various temperature ramps. Figure 11 shows the measured moisture release profiles and the corresponding SMC temperature profiles for two heating ramps. Figure 11A shows that for a given heating ramp the moisture release rises linearly at the initial stage, peaks at an intermediate stage when the temperature reaches close to the final steady state, and then falls as the panel is maintained at the final steady state temperature. The faster the rate of temperature rise, the faster is the rate of moisture release at the initial stage, and the appearance of the peak at the intermediate stage. Interestingly, when the temperature and moisture release profiles are plotted with respect to each other in Figure 11B, the profiles for both temperature ramps at the initial stage completely coincide. Therefore, within the range of the temperature ramps investigated here it can be concluded that the amount of moisture released when a panel is heated from one temperature to another would remain

Figure 10. Temperature profiles of front surfaces of SMC panels (solid lines) and the corresponding air temperatures (dotted lines) during convective heating that produced severe, medium, minor, and no popping defects: (A) oven kept at 325 °K and then ramped to 450 °K linearly within different time spans and (B) oven kept at different initial temperatures and then ramped to 450 °K within 20 min. The temperatures reported are those obtained from thermocouples embedded beneath the front surface of the SMC panel that were simultaneously subjected to the same heating conditions but were not coated with primer.


Figure 11. (A) Moisture release and the corresponding SMC temperature versus time and (B) moisture release versus SMC temperature at different heating rates (black, fast; gray, slow).

Figure 12. Primer viscosity profile as a function of time: (A) at various isothermal conditions and (B) corresponding to various linear temperature ramps and (C) primer viscosity as a function of temperature corresponding to various linear temperature ramps. Times indicated in plots B and C are those needed by the linear temperature ramps to reach the final temperature.

constant irrespective of the rate of heating. At the initial stage of heating, the rate of moisture release can therefore be assumed to be linearly proportional to the rate of temperature rise. Apart from the rate of moisture outgassing, the other important factor that depends on the rate of temperature rise, and has a significant bearing on the formation of the popping defect, is the viscosity of the primer layer. Figure 12 shows the primer viscosity profiles as a function of time for various thermal profiles. Figure 12A shows the

viscosity profiles for several isothermal conditions. At any isothermal condition the viscosity of the primer rises with time due to increase in the extent of cross-linking reaction. As the isothermal runs are carried out at progressively higher temperatures, the initial viscosity of the primer falls but the rate of rise of viscosity increases. The viscosity of the primer during a high temperature isothermal run starts low, but rises fast to cross those of lower temperature runs and reaches the final steady state value. Figure 12 plots B and C show the effects linear


temperature ramps have on the primer viscosity profiles. The complex effects of competing events of viscosity reduction due to temperature and viscosity build-up due to cross-linking chemical reaction can be observed from the figures. It can be seen that at lower temperature ramps, within the experimental range, the viscosity and the appearance of the minimum in the viscosity profile is predominantly determined by the temperature, whereas, at lower temperature ramps the effect of cross-linking also becomes prominent. Figures 12C and 11B collectively show that at a given temperature the viscosity of the primer layer and the amount of moisture released from the SMC panel does not vary with the rate of temperature rise. Therefore, corresponding to a slow and a fast heating ramp between identical sets of initial and final temperatures, equal quantities of moisture would release from the SMC panel and would outgas through primer layers of similar levels of viscosity. However, during the fast heating ramp the released moisture would have to outgas in a shorter time duration compared to the slow heating ramp. Therefore, higher quantity of moisture outgassing per unit time during the fast heating ramp causes more popping compared to the slow heating ramp. At the initial stage of the heating cycle, when the powder has not melted, the released moisture escapes through the porous powder layer without creating any popping defect. At the later stage of the heating process, when the primer layer has solidified significantly, the released moisture can permeate through, but would not be able to displace the high modulus primer layer and, therefore, would not create any popping defect. It is only at the intermediate stage of the heating process, when the primer layer is present as a viscous layer, the chances of popping becomes significant. Experiments with close observation of samples show that when the viscosity is low, the moisture transport through the bubble formation process does not cause defects as the primer levels and heals. However, as the crosslinking increases the viscosity of the primer layer, the bubbles can no longer escape without leaving a footprint behind. With further increase in viscosity the bubbles cannot escape at all, and are trapped within the primer layer. Therefore, the defect formation is a time sensitive mechanism and depends on the amount of moisture to be released, the rate of release, and the state of the primer at the time of moisture release. Figures 9-11 and Table 2 collectively show that irrespective of the mode of heating, a slower heating rate causes popping defects to improve, and below a critical heating rate no popping would be observed. The absolute value of the critical heating rate would depend on the amount of moisture present in the SMC panels, the humidity and the air flow conditions in the plant ovens and, therefore, can vary to some extent between plants. The critical heating rate required for eliminating popping defects in controlled laboratory convective oven experiments was about 6 °C/min, whereas the initial heating rate observed in the representative plants in Figure 7 are 15 °C/min and 17 °C/min for cases A and B, respectively. Both these plants showed popping when the SMC substrates contained significant quantities of moisture similar to preconditioned SMC panels used in the laboratory experiments.6 Therefore, any plant that is experiencing popping defect could potentially benefit from slowing down the rate of temperature rise during the initial 5 to 10 min of the heating cycle. This change of heating ramp should have no impact on total oven residence time or on manufacturing cost. In case the residence time at the constant temperature curing zone is reduced due to the enhanced ramping time, the temperature of the curing zone can be operated closer

Figure A1. Primer kinetic data: measured (solid line) vs model predictions (dotted line).

to the upper temperature bound specified by the coating manufacturer to attain the required degree of cure by the end of the priming oven process. 6. Conclusions The effect of powder bake temperature profile on the popping defect was investigated by subjecting the powder coated panels to various heating profiles using IR heaters and convective ovens. To investigate whether overheating of the SMC panels causes popping when the panels are subjected to the same heating conditions in the plant ovens that are typically designed and optimized for sheet metal panels, a transient heat transfer model of the powder baking process in the plant was used. The heat transfer parameters of the model were obtained by fitting model predictions to the laboratory experimental results. It was found that because of the lower thermal time constant, at any heating condition the average temperature of sheet metal rises faster than that of SMC. However, because of high specific heat capacity and low thermal conductivity of SMC compared to sheet metal, the temperature of the front surface of SMC can exceed that of sheet metal subjected to prolonged high intensity IR heating. Overshooting of the SMC temperature can cause enhanced outgassing of absorbed moisture and, therefore, excessive popping. Furthermore, the rate at which the SMC panels are heated plays a significant role in determining the extent of popping. Irrespective of the mode of heating, the slower rate of temperature rise of the substrate improves popping defect and beyond a critical heating rate, the popping defect can be eliminated. It was further found that the rate of temperature rise during the initial 5 to 10 min of powder bake is critical in determining the extent of popping irrespective of the rate of temperature rise at the later stages. The faster the temperature rise is at the initial stage, the higher is the quantity of moisture released per unit time. Higher quantity of moisture outgassing per unit time through the molten primer layer during a fast temperature ramp causes more popping compared to a slow temperature ramp. Therefore, it is expected that any plant that is experiencing popping defect would benefit from slowing down the rate of temperature rise of the substrates during the initial 5 to 10 min of the heating cycle. Appendix Appendix A The mathematical model of the heat transfer process between the powder-coated panel and the oven atmosphere as depicted

1648 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 Table A1. Physical Properties and Thicknesses of the Substrates and Primer sheet metal 3

density (F), Kg/m specific heat capacity (CP), J/(Kg · K) thermal conductivity (k), W/(m · K) emissivity (ε) thickness (laboratory), m thickness (plant), m

SMC

powder

7800 460

1850 1054

1255 1850

44.5

0.29

0.08

0.9 8.8 × 10-4 7.0 × 10-4

0.9 2.57 × 10-3 2.50 × 10-3

0.7 6 × 10-5 6 × 10-5

Table A2. Estimated Values of the Autocatalytic Kinetic Model Parameters for the Primer parameters

estimated values

k0, s-1 Ea, J/mol m n ∆Hrxn, J/Kg

1.64 × 107 81570 0.038 1.37 -32750

in Figure 3 is presented below. The temperature of the panel (0 e z e tS), which changes only because of conduction, can be modeled as ∂T ∂ ∂ (F C T) ) k (A1) ∂t S P,S ∂z S ∂z where, FS, CP,S, and kS are the density, specific heat capacity, and thermal conductivity of the substrate. To simplify the model of heat transfer to the primer layer, the melting of the powder and the change in thermo physical properties due to curing have been neglected. The temperature of the primer layer (tS e z e tS + tP) changes due to conduction and the heat generated from the curing reaction:

( )

∂T ∂ ∂ (F C T) ) k + FP∆Hrxnrrxn (A2) ∂t P P,P ∂z P ∂z where, FP, CP,P, and kP are the density, specific heat capacity, and thermal conductivity of the primer layer. ∆Hrxn and rrxn are the heat of complete reaction and the rate of reaction, respectively. The cure kinetics of the epoxy-based system can be represented using an autocatalytic model:13,15

( )

( )

Ea m dR ) -k0 exp R (1 - R)n (A3) dt RT where R is the degree of cure, k0 is the pre-exponential factor, Ea is the activation energy, R is the universal gas constant, and m and n are the reaction orders with (m + n) being the overall reaction order. Gerard et al.11 further showed that the kinetics of epoxy curing reaction only depends on the absolute temperature and not on the mode of heating. Therefore, the same cure kinetics model can be used for convective, radiative and direct IR heating modes. Equations (A1) through (A3) were solved simultaneously for T and R subject to the following heat flux-based boundary conditions. At the front surface of the primer layer (z ) tP + tS) the conductive heat flux into the primer layer is equal to the sum of incident heat flux from the IR heaters, wall radiation heat flux, and the air convection heat flux: rrxn )

-kP

dT 4 ) -QIR + σεP(T|z)t - TW4) + P+tS dz z)tP+tS htop(T|z)tP+tS - T∞) (A4)

where σ is the Stefan-Boltzmann constant, εP is the emissivity of the primer, and htop is the convective heat transfer coefficient above the substrate. In the above equation the value of QIR is

positive for incident IR flux onto the coating surface. At the interface between the primer layer and the substrate (z ) tS) the conductive heat flux is continuous: -kP

dT dT ) -kS z)tS dz z)tS dz

(A5)

At the back surface of the substrate (z ) 0) the conductive heat flux out of the substrate is equal to the sum of the radiative and convective heat fluxes to the ambient air: -kS

dT 4 ) -σεS(T|z)0 - T∞4) - hbot(T|z)0 - T∞) (A6) dz z)0

where εS is the emmisivity of the substrate, and htop is the convective heat transfer coefficient below the substrate. As initial conditions the degree of cure in the primer layer is assumed to be zero, and the temperature of the primer layer as well as the substrate is assumed to be at an initial temperature Tinit. R(tS e z e tS + tP, t ) 0) ) 0

(A7)

T(0 e z e tS + tP, t ) 0) ) Tinit

(A8)

The thermo-physical properties of the primer layer, SMC, and sheet metal were obtained either from the supplier datasheet or from in-house experiments and are reported in Table A1. The parameters k0, Ea, m, n, and ∆Hrxn of the autocatalytic kinetics were determined by minimizing the error between the solution of eq A3 and the degree of cure vs time data obtained from DSC experiments at different heating rates using Levenberg-Marquardt multivariate optimization. Figure A1 shows the excellent match obtained between the experimental results and the model predictions, and Table A2 lists the corresponding optimized set of kinetic parameters. The convective heat transfer coefficients above and below the panel (htop and hbot), and the incident IR heat flux (QIR) are unknown parameters. The typical values of convective heat transfer coefficient vary between 5 and 50 W/(m2 · K).9,10 Using these guideline values, model predictions were fitted against measured temperature profiles from separate convective and IR heating of SMC and sheet metal substrates at laboratory scale to obtain ballpark values for htop, hbot, and QIR. The ballpark values obtained from laboratory experiments were further used as initial guesses for matching the model predictions to the measured plant temperature profiles. The ambient, initial, and wall temperatures were measured and used as inputs to the model. The plant ovens are generally divided into separate zones, and one or more modes of heating are used in each zone. In the first zone of some plants, high intensity IR heaters are used to rapidly melt and initiate cure of the powder layer. In this zone high air flow is not used in order to avoid airborne contamination to the wet film. In the intermediate zones high intensity IR heaters are not used, whereas convective heating by air and radiative heating by oven walls are used to drive the curing reaction to completion. The wall temperatures in the intermediate zones are used as operational control variables to keep the temperature of the primer layer within a predefined upper and lower bounds required for optimum cure. In the final zone no heating is used, and the panels are allowed to cool to room conditions. Because of different operational conditions, the values of htop, hbot, QIR, TW, and T∞ vary along the length and the residence time in the oven. Therefore, these parameters are provided as a function of time; piecewise cubic interpolation was used for T∞ and linear interpolation was used for the rest. Finite element method with quadratic basis function was used to solve the coupled initial value differential equations A1-A8 using a commercial multiphysics software COMSOL. A variable


time-step, direct solver was used to perform the time integration to predict evaluation of the temperature across the substrate/ coating assembly thickness, and the degree of cure across the coating thickness. Literature Cited (1) Kia, H. G.; Shah, B.; Wathen, T. J.; Mitchell, H. A.; Berger, C. R. Powder priming of SMC, part I: assessment of the current technologies. J. Compos. Mater. 2006, 40, 1413. (2) Jacob, A. Stopping the pops: the final challenge for SMC. Reinf. Plast. 2002, (3) Kia, H. G.; Shah, B.; Wathen, T. J.; Mitchell, H. A.; Berger, C. R. Powder priming of SMC, part II: failure mechanism. J. Compos. Mater. 2006, 40, 1431. (4) Kia, H. G.; Wathen, T. J.; Shah, B.; Robbins, J. R.; Kleese, E. J.; Seats, R. L. New developments in powder priming of SMC. J. Exhib. Compos. Annu. Meet. 2006, (5) Kia, H. G.; Shah, B.; Mitchel, H. A.; Wathen, T. J.; Berger C. R. DeVelopment of ConductiVe Coating for Powder Priming of SMC; American Composites Manufacturers Association Annual Meeting, St. Louis, MO, October 18-20, 2006 [CD-ROM]. (6) Shah, B.; Kia, H. G. Plant Trials for Powder Priming of SMC; Society of Plastics Engineers Automotive Composites Conference and Exposition, Troy, MI, Sept 11-13, 2007 [CD-ROM]. (7) Abrams, L. M.; Castro, J. M. Powder coating of sheet molding compound (SMC) body panels. Polym. Compos. 2001, 22, 702.

(8) Vessot, S.; Andrieu, J.; Laurent, P. Curing study and optimization of a polyurethane-based model paint coated on sheet molding compound part I: polymerization and drying modeling. Drying Technol. 2000, 18, 199. (9) Dickie, R. A.; Bauer, D. R.; Ward, S. M.; Wagner, D. A. Modeling paint and adhesive cure in automotive applications. Prog. Org. Coat. 1997, 31, 209. (10) Lou, H. H.; Huang, Y. L. Integrated modeling and simulation for improved reactive drying of clearcoat. Ind. Eng. Chem. Res. 2000, 39, 500. (11) Blanc, D.; Laurent, P.; Andrieu, J.; Gerard, J. F. Convective and radiant (IR) curing of bulk and waterborne epoxy coatings as thin layers. Part II: Infrared curing. Polym. Eng. Sci. 1999, 39, 2487. (12) Deans, J.; Kogl, M. The curing of powder coatings using gaseous infrared heaters: An analytical model to assess the process thermal efficiency. Int. J. Therm. Sci. 2000, 39, 762. (13) Vechot, L.; Bombard, I.; Laurent, P.; Lieto, J. Experimental and modeling study of the radiative curing of a polyester-based coating. Int. J. Therm. Sci. 2006, 45, 86. (14) Blanc, D.; Vessot, S.; Laurent, P.; Gerard, J. F.; Andrieu, J. Study and modeling of coated car painting film by infrared or convective drying. Drying Technol. 1997, 15, 2303. (15) Kamal, M. R.; Sourour, S. Kinetics and thermal characterization of thermal cure. Polym. Eng. Sci. 1973, 13, 59.

ReceiVed for reView July 23, 2008 ReVised manuscript receiVed November 10, 2008 Accepted November 12, 2008 IE801130G

The Effect of Oven-Heat Flux on Powder-Coating ... - ACS Publications

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