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Processability Study of In-Mold Coating for Sheet Molding Compound Compression Molded Parts.

INTRODUCTION

In recent years, the processing of thermosetting resins has received increased attention from automotive and aerospace industries as excellent choices for lightweight materials. Unlike the processing of thermoplastic materials, the processing of thermosetting resins is accompanied by chemical reaction. As the material reacts, it transforms from low viscosity material to a solid; this transformation is referred to as the curing process [1,2]. The curing behavior of thermosetting resins affects the physical, chemical, and mechanical properties of the final product [3-5]. Therefore, the processing of thermosetting resins requires understanding the effect of chemical reaction on the rheology of the reacting system.

The in-mold coating (IMC) resin is a thermosetting liquid that when injected onto the surface of the part cures and bonds to provide a smooth conductive part. To make the coating conductive, carbon black (CB) is added to the IMC formulation [6]. In the IMC process, when the sheet molding compound (SMC) has cured adequately (part stiff enough so that the mold can be opened without surface blemishes), the mold is slightly opened and the required coating volume is injected onto the surface of the SMC while the part is still in the mold [7, 8]. Once the coating solidifies by chemical reaction, the mold is opened, and the part removed as shown schematically in Fig. 1.

In order to optimize the coating process, an understanding of the rheological behavior of the reactive mixture is needed. The time available for flow is limited by the time at which the viscosity starts to increase. On the other hand, the time when the part can be removed from the mold is given by the time at which the initially liquid resin has undergone the transition to solid, and its mechanical strength is large enough so that the part can be removed without surface blemishes. Figure 2 shows the typical rheological changes that the IMC undergoes during the IMC process [6]. The changes in viscosity([eta]) and elastic modulus (G')are shown as a function of conversion and reaction time. During the initial period, the viscosity remains low. After all inhibitor is consumed, viscosity increases nearly vertically (gel point)([[alpha].sub.gel]). Just before the gel point, the elastic modulus starts to rise until it reaches a value sufficiently high (cure time) where the mold can be opened without damaging the coating. Many studies have shown the cure behavior and rheological changes of thermosetting resins demonstrating chemorheological modeling [9-12]. The rheological changes of the reactive systems during curing are directly related to the change from the liquid state to solid state at its fully cured condition [13, 14]. However, very limited information is available between the chemorheology of reactive systems and the relation between flow and cure time during actual molding.

This study investigated the curing and rheological behavior of a commercial IMC system and established its relationship to flow time and cure time by carrying out IMC experiments for SMC compression molding at varying mold opening times and different molding temperatures.

EXPERIMENTAL

Materials

The commercial IMC resin provided by OMNOVA Solutions Inc. was used in this study. A liquid, heat-activated conductive IMC is designed to enhance and make the surface conductive for subsequent electrostatic painting of fiber-reinforced polymeric composites. In particular, for SMC compression, molded exterior body panels were used in the automotive and trucking industries. The generic composition of IMC resin is given in Table 1; the specific details are proprietary. The IMC contains unsaturated oligomers and monomers to give adequate hardness and adhesion to SMC substrates. Hydroxypropyl methacrylate (HPMA) and styrene are used as diluents to decrease the viscosity. Talc is used to improve hardness and decrease shrinkage. A cobalt compound is employed as an accelerator. The conductive filler used in IMC resin is commercially available CB. This resin contains 2.8% of CB by weight to make the coating conductive. The inhibitor, benzoquinone is used to provide shelf-life and increase flow time. The initiator used in this study is 1.0 wt% of tert-butyl peroxybenzoate (TBPB). TBPB is the recommended organic peroxide at molding temperatures in the range of 130[degrees]C-160[degrees]C, which is the typical molding temperature range of compression molded SMC process.

Rheological Experiment

The rheological behavior of IMC was measured with a TA ARES-G2 rheometer using 25 mm disposable parallel plates with a gap of 1 mm. Both steady and dynamic shear tests were conducted under isothermal conditions at 100[degrees]C, 110[degrees]C, and 120[degrees]C. Steady shear viscosity was measured as a function of reaction time at a shear rate of 0.1 [s.sup.-1]. The time at which the viscosity approaches a very large value is nearly independent of shear rate [15]. Dynamic shear flow measurements were performed at a frequency of 1 Hz and a strain of 0.1%. The linear viscoelastic region was determined through a strain sweep test. Strain sweeps were carried out at 1 Hz, and a strain of 0.1% was used to ensure that the sample was within the linear viscoelastic region. The complex viscosity, elastic modulus, and loss modulus were measured as a function of the reaction time.

Differential Scanning Calorimetry Experiment

Differential scanning calorimetry (DSC) measurements were performed using a TA Q20 differential scanning calorimeter under the same conditions as the rheological experiments. Isothermal tests were performed at 100[degrees]C, 110[degrees]C, and 120[degrees]C. A dynamic scan was performed at a heating rate of 10[degrees]C x [min.sup.-1] from 30[degrees]C to 250[degrees]C to determine the total heat released during the curing reaction.

SMC/IMC Molding

IMC experiments during SMC compression molding were conducted at OMNOVA solutions technical center using a 400 ton Hoesch press with a steam-heated flat plate mold of 431.8 x 558.8 mm (17 x 22 inches). The pressing force during compression molding was 187 ton. A 1275 g SMC charge was used. The mold temperatures were 137[degrees]C, 150[degrees]C, and 155[degrees]C. After the SMC part was solid enough, the mold was slightly opened (approximately 0.5 mm) and the coating material was injected using an EMC2 Inc. IMC injection unit. The IMC coated SMC parts were cured at various times, to experimentally obtain the cure time (minimum required mold opening time) by visual inspection.

Surface Roughness Measurement

To determine the minimum mold opening time required of molded IMC parts in a more quantitative way, at each molding temperature, the surface quality of molded parts was evaluated by using a Wyko NT9100 optical profilometer (Bruker AXS Inc.). The surface roughness ([R.sub.a]) reported by the device of every molded part was measured at 10 different locations and averaged. These measurements allowed us to better quantify the minimum required mold opening time than visual inspection of molded parts.

Gel Test

Viscosity rise experiments at temperatures above 130[degrees]C are not feasible as the time at which the viscosity starts to rise is too short. In order to experimentally evaluate the time available for flow at typical SMC molding temperatures (140[degrees]C-160[degrees]C), isothermal gel test measurements were carried out at those temperatures. In this experiment, the coating was quickly deposited onto a hot plate and a thin circular aluminum plate 50 mm in diameter rotated by hand. The gel time was identified as the time at which the plate cannot be moved. The temperature between hot plate and aluminum plate was measured by a thermocouple.

RESULTS AND DISCUSSION

Figure 3 shows the viscosity versus reaction time in steady shear at three different temperatures. From Fig. 3, we can see that the viscosity remains nearly constant until the time where it rises very sharply. At lower temperatures, the viscosity starts at a higher value but takes longer to rise due to the slower reaction rate. This sharp rise is associated with gelation, which corresponds to the forming of an infinite network. The coating can no longer flow after this point. Thus, the time available for flow is limited by the gel point [16].

In the IMC process, the mold can be opened and the part demolded, when the coating has enough integrity, so that the opening occurs without damaging the part surface. The elastic modulus during cure was investigated in the oscillatory shear mode. As the reaction time increases, G' increases, until it reaches a nearly constant plateau, as shown in Fig. 4.

In order to predict the time available for flow and the needed mold opening time, we need to relate viscosity and elastic modulus to extent of reaction or conversion instead of reaction time. To measure the extent of reaction, we used DSC. The basic assumption for applying DSC to predict the extent of reaction is that the dimensionless reaction rate, d[alpha]/dt, is proportional to the measured heat flow, dH/dr.

d[alpha]/dt = 1/[H.sub.total] dH/dt, (1)

where if isothermal DSC is used, the total heat for the curing reaction, [H.sub.total], is given by

[H.sub.total] = [H.sub.i] + [H.sub.r], (2)

where [H.sub.i] is the heat evolved during the isothermal DSC experiments and [H.sub.r] is the residual heat of the curing reaction after isothermal cure, which can be measured by scanning the isothermally cured samples.

The degree of cure as a function of time is given by:

[alpha](t) = total, [[integral].sup.t.sub.0] (dH/dt) dt. (3)

The heat flows during isothermal runs at the same three temperatures at which the rheological properties were measured. The measured heat flow of a typical isothermal scan of IMC resin is shown in Fig. 5. The baseline is a straight tangent line to the horizontal portion of the isothermal DSC curve. The inhibition time was selected as the cross point of baseline and DSC curve where the heat flow starts to increase. In the initial stage, the heat flow remains constant. After all inhibitor is consumed, the heat flow starts to increase. The reaction rate reaches a maximum and then decreases. By integration of the area under the curve, the heat flow during isothermal run was calculated.

In order to predict conditions others than the measured ones, we need to develop a mathematical model to predict the extent of reaction as a function of time and temperature (cure model). The typical IMC compounds have vinyl groups that react by the free radical mechanism. A series of kinetic models for free radical polymerization of IMC coating materials have been developed to describe the reaction mechanism [17-19]. In this study, the free radical polymerization model was used to determine the inhibition time, that is, the time before the thermosetting reaction starts, which for the case of one initiator as used in this study is given by the following equation:

[mathematical expression not reproducible] (4)

where [t.sub.z] is the inhibition time, [k.sub.do] is the frequency factor for initiator decomposition, [C.sub.zo] is the initial concentration of inhibitor, [C.sub.IO] is the initial concentration of initiator, [E.sub.d] is the activation energy of decomposition, R is the ideal gas constant, and T is the temperature.

After the inhibitor is consumed, the curing reaction starts. To predict the conversion versus time and temperature, we used the autocatalytic model [20, 21], a phenomenological model, as it gave us the best results. The parameters of the cure model were fit using the differential scanning calorimeter data [22]. The model is represented by Equation 5.

d[alpha]/dt = ([[alpha].sub.0] + [k.sub.p][[alpha].sup.m])[([[alpha].sub.max] - [alpha]).sup.n] where [[alpha].sub.0] = 0.001, (5)

where [k.sub.p] is the kinetic rate constant and assumed to have an Arrhenius temperature dependence, m and n are reaction orders and [[alpha].sub.max] is maximum conversion. The parameters of the model are given in Table 2. Figure 6 shows good agreement between predictions of the model and experimental data. However, a more relevant test of the model is to compare against data from a DSC scan, over a broad temperature range including the typical SMC molding temperatures. The model does a good job predicting the conversion during the DSC scan [22].

From the rheological experiments, we obtain viscosity and elastic modulus versus time and from the DSC measurements, we obtain the experimental conversion versus time. Combining both, we can obtain both viscosity and modulus versus conversion. Figure 7a shows measured viscosity versus conversion at 120[degrees]C. As can be seen in the figure, the viscosity increases as a step function as soon as the conversion becomes non-zero (extremely low), contrary to most typical reactive systems, such as the epoxy system discussed for reference in the Supporting Information, where the viscosity increases sharply near the predicted gel point conversion. Figure 7b shows elastic modulus versus conversion at 120[degrees]C.

It is important to note that the steady shear viscosity increases sharply as soon as the inhibitor is consumed, that is, as soon as the conversion becomes non-zero. Thus, the time available for flow (flow time) is limited by the time that it takes for the inhibitor to be consumed (inhibition time). Steady shear measurements can characterize only the initial portion of a thermoset's viscosity range. Near the gel point, the steady shear viscosity increases rapidly and becomes unmeasurable due to stiffening the sample fractures. The reduced viscosities divided by the initial value from steady shear measurements for all temperatures for IMC as function of conversion do not collapse into a single curve as the epoxy system shown in the Supporting Information, but the viscosity rises to infinity at a very low conversion value.

As discussed, the viscosity of IMC starts to increase rapidly as soon as the inhibitor is consumed. The inhibition time cannot be measured above 120[degrees]C using either the rheometer or DSC due to the time it takes to load the sample. Therefore, the gel test was performed to measure the gel time of IMC at temperatures above 120[degrees]C. For all conditions, five measurements were done. The experimental results are compared to the predicted inhibition time in Table 3. From the table, we can see that the predicted inhibition time agrees well with the measured gel times.

IMC experiments for SMC compression molding were conducted. Mold opening time was varied at several mold temperatures, to experimentally obtain the minimum mold opening time, by visual inspection. For example, Fig. 8 shows IMC coated compression molded SMC parts cured at a mold temperature of 137[degrees]C for 100 and 150 s. At a mold temperature of 137[degrees]C, the coating is under cured for 100 s of cure time. It is fully cured for 150 s cure time. A sample is considered fully cure if it does not have any visible surfaced blemishes. The results are summarized in the first three columns of Table 4. Because this is a qualitative determination, the values of surface roughness have a relatively large variability and are only used to reinforce the selected conversion for opening the mold.

In order to have a more accurate determination of the cure time, the surface quality of molded parts was evaluated using the optical profilometer. Increasing the mold opening time, the measured surface roughness decreased in all mold temperatures. The measured surface roughness at different mold temperatures and varying cure time are summarized in the last column of Table 4. These measurements allow for a better quantification of the minimum cure time than the visual inspection of molded parts.

Figure 9 shows measured surface roughness versus predicted conversion at different mold temperatures. At a conversion of 0.9, surface roughness was between 250 and 300 nm for all temperatures and visual observation also agreed with these parts being defect free.

The ultimate goal is how to estimate the flow and mold opening times using the cure model developed from the DSC measurements. The time available for flow is defined as the inhibition time. As shown previously, the steady shear viscosity nearly becomes infinity as soon as any reaction is detected. As shown in Fig. 10, the elastic modulus at a conversion of 0.9 has reached a nearly constant plateau for all temperatures. This is in good agreement with the value at which the part can be considered fully cured by visual inspection as well as the value at which a plateau of surface roughness is reached. The minimum mold opening time obtained from SMC/IMC moldings and the time to reach 0.9 conversion predicted by the cure model are in good agreement as shown in Table 5. Figure 11 compares predicted flow and mold opening times with experimental values from gel test and SMC/IMC molding experiments for the relevant temperature range.

CONCLUSIONS

In this work, the rheological behavior of a commercial IMC system was investigated using rheometer in both steady shear and dynamic oscillation mode. DSC data were used to obtain the degree of cure or conversion as a function of reaction time. The viscosity and elastic modulus were correlated with the degree of cure from the DSC measurements. It was found that gel conversion of IMC was extremely low, 0.01. This phenomenon is very different with the case of a standard epoxy resin system given in the Supporting Information as reference, which has 0.75 gel conversion. The viscosity of IMC resin starts to increase as soon as all inhibitor is consumed; therefore, the maximum time available for flow is limited by the inhibition time. It was observed that the elastic modulus at a conversion of 0.9 reached a nearly constant plateau for all temperatures.

IMC/SMC molding tests were conducted at varying mold temperatures and several mold opening times. Minimum acceptable mold opening time was determined by investigating surface quality using optical profilometer. Good agreement between the inhibition time and experimental flow time and between the predicted time to reach 90% conversion and the minimum mold opening time during IMC was obtained.

We expect that the approach presented in this article can be applied to not only IMC resin but also other thermosetting systems. However, the selection of conversion level needed to open the mold, and may vary from system to system. In our experience with most thermoset systems, conversion is always around 0.9. The selection of the time available for flow as the inhibition time will in general be adequate for most heat-activated systems. However, for mixing activated systems, such as urethanes or the epoxy system, it is best defined in the Supporting Information as the time needed for the initial viscosity to raise to a given level, usually about 10 times its initial value.

ACKNOWLEDGMENTS

The authors would like to acknowledge the financial support of OMNOVA Solutions Inc.

REFERENCES

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Seunghyun Ko (iD), (1) Xilian Ouyang, (2) Elliott J. Straus, (3) L. James Lee, (2) Jose M. Castro (1)

(1) Department of Integrated Systems Engineering, The Ohio State University, Columbus, Ohio, 43210

(2) Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, Ohio, 43210

(3) Omnova Solutions Inc, Akron, Ohio, 44305

Additional Supporting Information may be found in the online version of this article.

Correspondence to: Jose M. Castro; e-mail: castro.38@osu.edu

DOI 10.1002/pen.25167

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Schematic representation of the stages of IMC during the SMC process. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. Schematic representation of the change in viscosity and elastic modulus for a typical IMC system during cure.

Caption: FIG. 3. Viscosity versus reaction time in steady shear test. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 4. Elastic modulus versus reaction time in dynamic shear test. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. A typical isothermal DSC curing curve of IMC resin. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Comparison between isothermal DSC tests and cure model. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. (a) Viscosity versus conversion (b) elastic modulus (G') versus conversion of IMC at 120[degrees]C. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 8. IMC coated SMC parts at 137[degrees]C. (a) IMC cure time: 100 s; (b) IMC cure time: 150 s. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Measured surface roughness versus conversion at varying mold temperature. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. Reduced G' versus conversion for IMC. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 11. Predicted flow and mold opening time for varying temperatures compared with gel test and SMC/IMC molding experiments. The two dashed lines shown for the experimental results indicate maximum and minimum values measured. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Generic formulation of coating material.

                       Commercial IMC resin

                            Epoxy
                            Styrene
                            HPMA
                             Talc
                            Cobalt
                             CB
                           Inhibitor
                           Initiator

TABLE 2. Parameters for cure model.

Parameters                   IMC system

m                              0.9998
n                              0.7095
[k.sub.p0] (1/s)              1.73E+07
[E.sub.p] (J/mol)             6.63E+04
[[alpha].sub.max]    0.0024 x r(kelvin) - 0.0022
[k.sub.do] (1/s)              2.65E+13
[E.sub.d] (J/mol)             1.32E+05

TABLE 3. Predicted inhibition and gel time
for varying mold temperatures.

Mold temperature   Inhibition time (s)   Gel time (s)
([degrees]C)

120                        118             114-134
130                       47.5              47-60
135                       27.1                --
140                       15.5              17-21
145                        8.9                --
150                        4.6               5-7
155                        2.9                --

TABLE 4. Visual inspection and measured surface roughness
at varying mold temperatures and cure time.

Mold temperature   Cure time of   Visual inspection   [R.sub.a] (nm)
([degrees]C)        IMC (sec)     of molding parts

137                     80             Uncured
                       100           Under cured           938
                       110           Under cured           422
                       120              Cured              273
                       150              Cured              243
150                     50           Under cured           360
                        60           Under cured           262
                        65              Cured              241
                        70              Cured              216
155                     35           Under cured           406
                        40              Cured              247
                        45              Cured              277

TABLE 5. Minimum mold opening time from SMC/IMC moldings
and predicted time to reach a conversion of 0.9.

Mold
temperature    Minimum mold opening time   Mold opening time
([degrees]C)   from SMC/IMC molding (s)    from cure model (s)

137                     110-120                    125
150                      60-65                     59
155                      35-40                     44
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Author:Ko, Seunghyun; Ouyang, Xilian; Straus, Elliott J.; Lee, L. James; Castro, Jose M.
Publication:Polymer Engineering and Science
Article Type:Case study
Geographic Code:1USA
Date:Aug 1, 2019
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