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Analyzing variations in SMC formulations.

Analyzing Variations in SMC Formulations

An understanding of material properties and processing parameters is an essential part of process control efforts and optimization of molded part quality. As the processing of sheet molding compound (SMC) has evolved from a technological art to a science, empirical tests and indexes have been used extensively for characterizing the materials. Two test methods that offer a realistic evaluation of the compression molding process by providing quantitative information are a dielectric polymerization test and a squeeze-flow rheological test.

Dielectric analysis is the study of the capacitive and conductive properties of a material. SMC polymerizes when subjected to heat and pressure. Its electrical properties change during the polymerization process, and these changes can be used to characterize material variations. One parameter, the dielectric loss permittivity, e", is a measure of the energy required align dipoles and move ions. Ionic conductivity is a quantity proportional to the loss factor, and is considered to be a measure of the "fluidity" or the ease with which ionic impurities move through the material. An example of a typical loss factor response for polymerizing SMC is shown in Fig. 1.

A compression technique using a squeeze-flow rheometer to characterize SMC rheology was developed by the University of Akron and Premix, Inc. This device monitors the stress as the material is compressed between two platens. The upper platen is stationary and is connected to a load cell to measure force. The lower platen moves upward at a specified velocity and ceases motion at a present level of strain. During closure, stress builds with three important phenomena taking place: compaction of the material, transient material deformation, and steady-state flow. The viscosities are calculated from the flow regime. Following closure, the stress relaxation is monitored for a programmed period of time. Typical data are shown in Fig. 2.


The instrument used in this study is the Premix[R] Processability Tester, manufactured by Interlaken Technology Corp., which was modified for dielectric testing. Samples consisted of three plies of SMC, which were compressed to 3.175 mm at a closure velocity of 10 mm/sec. The tests were carried out at both room temperature and 150[degrees]C.

At the highest temperature, the samples were held in compression for two minutes while the polymerization was monitored by dielectric analysis. A

one-volt AC potential difference was applied across the sample. Impedance was measured with an HP 4284A LCR meter interfaced to a PC with data-acquisition and instrument-control software. The impedance is converted into conductance (the inverse of resistance), which is a measurement of how well a conductor transfers electrical charge. The loss factor is then determined by:


where: G=conductance; [C.sub.o]=the capacitance of air, 7.9 pF; and [omega]=2 x the experimental frequency, 1kHz.

A typical SMC formulation used is given below.

The glass fiber loading is based on total weight and was varied at 20%, 25%, and 30%. The different peroxides were used for the experiments shown in Fig. 6. The ratio of paste to thickener was 73:1 in all cases.

Rheological Data

At both 22[degrees]C and 150[degrees]C the apparent viscosity increased with percent glass (Fig. 3). There is some variation of the viscosity because of glass-fiber-crossover points--locations where randomly oriented glass fibers accumulate to such an extent as to affect the viscosity of the SMC. At a given fiber loading, a distribution of crossover occurrences is possible. It is apparent that with only a 5% difference in loading, some overlapping of the distributions occurs. More work is needed to define the statistics of the glass-fiber mat and the resulting effect on viscosity.

Useful comparisons of rheological data can be made by examining the power law index. The power law index is related to the slope of the line obtained from a plot of log viscosity vs. log shear rate. Figure 4 shows that sample temperature had a significant effect; the power law index is lower at room temperature than at the elevated temperature. This may be because at the higher temperature either the material has more Newtonian behavior, because of a temperature effect, or there is less shear thinning, because of enhanced slippage at the wall.

The values for the two temperatures diverge as the glass loading is increased. At the higher temperature, the increase of the power law index with percent glass is probably due to further enhancement of slippage with more fiber. At room temperature, where flow has a greater shear influence, increasing the fiber loading has the opposite effect because it decreases the volume fraction of the paste. The effective paste height in a sample of the same geometry is thus reduced, and the actual shear rate that the paste is subjected to is increased.

Dielectric Data

The increase in the loss factor maximums with fiber loading, shown in Fig. 5, may be due to either the effects of additives present on the glass or to a faster gel time at lower glass loadings. The latter would result from a reduction in heat transfer as fiber content increased. In fact, a correlation is seen between the time for e" maximum and the maximum value. This theory is supported by the comparison of the two peroxide systems shown in Fig. 6, where the faster decomposing peroxide (2,5 dimethyl-2,5-bis(2-ethyl hexoyl peroxy) hexane) resulted in a quicker gel time and a significantly lower e" maximum was reached.

Correlating Rheological

and Dielectric Data

Figure 7 shows that the loss maximum, e", increases with increasing viscosity, and hence glass fiber loading. Glass fibers tend to retard the flow of the SMC while at the same time giving the paste added strength. Therefore, the higher the glass concentration, the higher the viscosity. The increased levels of glass, which is an insulator, also act to retard heat transfer and slow down the gel time. This allows more time for ion movement, which contributes to the increased e" measurement. Using relationships similar to Fig. 7, e" values can be used to calculate material viscosities as they vary with changing fiber loading.
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Title Annotation:sheet molding compound
Author:Rosich, Michael; Allen, Paula
Publication:Plastics Engineering
Date:Apr 1, 1991
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