18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS INVESTIGATION OF VISCOELASTICITY AND CURE SHRINKAGE IN AN EPOXY RESIN DURING PROCESSING T. Shimizu 1* , H. Koinuma 1 , K. Nagai 1 1 Mitsubishi Heavy Industries, Ltd., Nagoya, Japan * Corresponding author (takayuki_shimizu@mhi.co.jp) Keywords : Process modeling, viscoelasticity, cure shrinkage of the rheometer due to correlate the viscoelastic 1. Introduction Process-induced deformation in composite parts properties. affects their qualities and demands a lot of efforts for the compensation. 3 Test Results and Discussion Many researchers have developed process modeling or simulation tools for predicting the deformation [1- 3.1 Resin Viscoelasticity 3]. They are effective for cost and time saving in the Storage and loss stiffness data measured with the development of composite parts. It is, however, MR-300 are shown in Fig.1. The two measurement difficult to establish the accurate modeling due to data for each temperature conditions were almost the complexity of the input parameters, especially same. All the storage stiffness developed over changing material properties during cure processing. 12MPa could not be measured due to the limit of the Cure shrinkage and changing elastic state of a matrix apparatus. resin during processing are well known parameters The storage and loss stiffness data were time and having an impact on the result of process modeling. temperature dependant. The resin degree of cure was In this work, investigation of the cure shrinkage and calculated using the cure kinetics investigated by viscoelasticity of an epoxy resin has been carried out Dykeman[4]. The stiffness vs. the degree of cure, α , for upgrading the prediction methods of process- curves at all test cases are almost same. An example induced deformation on composite parts. of the results, at the ramp rate of 2.0°C/min., is shown in Fig.2. The gel point was determined as the storage stiffness exceeds the loss stiffness. The 2 Experiments average gel point of the degree of cure for all test cases was 0.50 ( α =0.50) to be the gel point of this 2.1 Resin viscoelasticity resin. The viscoelastic properties of an epoxy resin in a The linear plots of the same data in Fig.2 are shown CFRP, T800S/3900-2B, were measured with a in Fig.3 to investigate the development of the elastic rotational rheometer, Rheology Co. Ltd MR-300 modulus. After the gel point, the storage stiffness Soliquidmeter, at three ramp rates in temperature begins to rise, and shoots up from around α =0.76 within the practical range, as 0.5°C/min., 2.0°C/min. just before the loss stiffness begins to drop. Another and 2.8°C/min. up to 180°C and holding the test case, at the ramp rate of 0.5°C/min., also shows temperature until the viscosity developed to the the similar curves as seen in Fig.3. The glass apparatus’s limit. Constant amplitude of 0.5° and transition temperatures were also calculated and frequency of 0.5Hz were used for all measurements. shown in Fig.3. Because a resin elastic modulus The measurements were conducted at least twice in depends on the glass transition. The glass the same condition due to confirm the reliability of temperatures after α =0.76 are almost same in these the obtained data. cases. It means that only the degree of cure affects 2.2 Cure shrinkage the elastic modulus within the practical ramp rates and the hold temperature of 180°C. The cure shrinkages of the resin were measured with As mentioned above, the final stiffness after a dilatometer, Shibayama Scientific Co. Ltd S701, at vitrification could not be measured. And storage and the completely same conditions in temperature that
loss stiffness measured with a rotational rheometer The specific volume after subtracting the thermal generally depend on the measurement frequency. It expansion vs. the degree of cure curves show same is difficult to determine the elastic modulus shape as shown in Fig.6. Some researchers have quantitatively. But due to implement the elastic reported that the specific volumes were linear [2, 5] modulus in process simulation, the fitting curve of or quadratic[6] to the degree of cure. But the curves the storage stiffness after gelation described in in Fig.4 do not fit the reported equations and have an equation (1) was calculated. inflection point near the gel point ( α gel =0.50). Based on the test results, the cure shrinkage equation is proposed with modification of the quadratic G’ r =0, α < α gel equation as follows: G’ r =A 1 exp{ K 1 ( α - α gel ) n } , α gel ≤ α < α vir (1) V r =0, α < α 1 G’ r =A 2 exp{ K 2 ( α - α vir )} , α ≥ α vir gel -A) α s1 2 , α 1 ≤ α < α gel V r =A α s1 +(V r Where, the A 1 , K 1 , n, A 2 and K 2 are constants and the gel +B α s2 +(V r inf -V r gel -B) α s2 2 , α gel ≤ α < α 2 V r =V r (2) α gel and α vir are degrees of cure at the gelation and inf , α ≥ α 2 vitrification. V r = V r Each parameters of the equation (1) were determined α s1 =( α - α 1 )/( α gel - α 1 ) with a least-square method to the test data and shown in Table 1. The fitting curve is shown in α s2 =( α - α gel )/( α 2 - α gel ). Fig.4 and good agreement with the measurement data. The elastic moduli used in process simulation gel and V r inf are volumetric cure Where, the V r were obtained by multiplication of this stiffness shrinkage at gel point and complete cured. The α s1 tailored to the deformation of specimens by cure and α s2 are degrees of cure at that the shrinkage shrinkage shown in sub section 3.3. begins and stops. The A and B are nonlinear factors 3.2 Cure shrinkage before and after gel point. Each parameters of the equation (2) were determined Changes of the specific volume during process at with a least-square method to all the test data and three temperature conditions are shown in Fig.5. For shown in Table 2. The fitting curve using the two measurements at each temperature conditions, parameters is also shown in Fig.6 and good the first runs showed always higher volume than the agreement with the measurement data can be seen. second runs. The causes of the differences were not ascertained completely. One of the speculations is 3.3 Determination of the elastic moduli the variety of each batch because different batches Heating tests of asymmetric lay-up plates were of the resin were used for first runs and second runs. conducted for determination of resin cure-induced Or the effects of deforming action for the test setup deflection[7]. The specimens, length of 150mm and might be different between first runs and second 250mm, and lay-up of [0 4 /90 4 ] and [0 8 /90 8 ] were runs. used. The width of specimens were one tenth of the Although the differences of the absolute volume, the length. The test setup are shown in Fig.7. The timing of the peak volume and the amount of specimens were heated up from room temperature to reduction from the peak volume were almost same at near the cure temperature, 180°C in an each conditions. In this work, the cure shrinkage, V r , environmental chamber and the deflections were was obtained by subtracting the effect of thermal measured by a scale at about every 20°C up. expansion from the measured specific volume. A example of the test results is shown in Fig.8. The The thermal expansion lines were set with the two deflections at 180°C are calculated from linear volumes at 90°C and 110°C for the case of extrapolation of the measured data. Table 3 shows 0.5°C/min. and at 90°C and 150°C for the cases of the deflections of all specimens at room temperature 2.0°C/min. and 2.8°C/min.. and at 180°C.
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