This paper presents an analytical and experimental investigation of cure cycle effect on carbon-fiber reinforced high-temperature polymer composite structures molded by vacuum assisted resin transfer molding (VARTM). The molded composite structure consists of AS4-8 harness carbon-fiber fabrics and a high-temperature polymer (Cycom 5250-4-RTM). Thermal and resin cure analysis is performed to model the cure cycle of the VARTM process. The temperature and cure variations with time are determined by solving the three-dimensional transient energy and species equations within the composite part. Several case studies were investigated by the developed analytical model. The same cases were also experimentally investigated to determine the ultimate tensile strength for each case. This study helps in developing a science based technology for the VARTM process for the understanding of the process behavior and the effect of the cure cycle on the properties of the molded high-temperature polymer composites.
Nonautoclave molding technology, such as vacuum assisted resin transfer molding (VARTM), offers a more cost effective manufacturing process for composite materials than the resin transfer molding (RTM) and autoclave/prepreg technique. However, its potential for fabricating high-temperature polymer composites needs to be explored. Process variables, including the maximum cure temperature, cure time, and postcure cycles have significant effects on the resulting composite [
Kim and Daniel [
The present paper introduces an analytical and experimental investigation of cure cycle effect on high-temperature polymer composite structures molded by vacuum assisted resin transfer molding (VARTM). Thermal and resin cure analysis is performed to model the cure cycle of the VARTM process. The temperature and cure variations with time are determined by solving the three-dimensional transient energy and species equations within the composite part. Several case studies were investigated by the developed analytical model. The same cases were also experimentally investigated to determine the ultimate tensile strength for each case.
The current analytical investigation aims at modeling the cure cycle of Cycom 5250-4-RTM resin molded by VARTM. Thermal and resin cure analysis is performed to determine the temperature and cure variations with time in the molded composite structure. The three-dimensional transient energy and species equations are coupled and solved simultaneously to determine the degree of cure as a function of the temperature and the time during the cure cycle as follows:
A MATLAB code was written, using the finite difference technique to solve for the thermal and cure histories in the cure cycle [
Kinetic parameters for 5250-4-RTM resin.
Arrhenius constant, |
Arrhenius constant, |
Activation energy, |
Activation energy, |
Heat of reaction, |
Order of reaction, |
---|---|---|---|---|---|
0.815 | 40.2 | 29.3 | 37.3 | 249.8 | 2 |
The developed model was used to get complete temperature and cure histories of the molded composites. Several case studies were investigated by the developed model. Table
Some cases used to produce composite panel by VARTM.
Case number | Maximum cure temperature (°C) | Heating rate (°C/min) | Postcure time at 227°C (h) |
---|---|---|---|
Case 1 | 194 | 1.39 | 2 |
Case 2 | 194 | 0.56 | 2 |
Case 3 | 183 | 1.39 | 2 |
Case 4 | 205 | 1.39 | 2 |
Figure
Cure and temperature histories for Case 1.
Figure
Distribution of degree of cure through the panel thickness along the length of the composite panel after 1 hour of the cure cycle.
Distribution of degree of cure through the panel thickness along the width of the composite panel after 1 hour of the cure cycle.
Distribution of temperature through the panel thickness along the length of the composite panel at the end of cure cycle.
Distribution of degree of cure through the panel thickness along the length of the composite panel at the end of cure cycle.
Another case was studied, Case 2, with a low heating rate of 0.56°C/min, but with the same cure temperature and postcure time as in Case 1. The complete temperature and cure histories for Case 2 are shown in Figure
Cure and temperature histories for Case 2.
Cure history comparison for Cases 1 and 2.
The effect of maximum cure temperature on the trend of degree of cure was studied through the comparison of two cases, 3 and 4, with the same heating rate and postcure time but with different maximum cure temperature. In Case 3 the maximum cure temperature was 183°C, while it was 205°C in Case 4. Figure
Cure history comparison for Cases 3 and 4.
The presented analytical cases were also investigated experimentally to determine the maximum tensile strength for each case. A two-layer laminate was molded by VARTM process. The materials used are AS4-8H carbon-fiber fabric and a high-temperature polymer called Cycom 5250-4-RTM. Full description of the experimental setup was reported in a previous publication by the authors of [
Table
Process parameters effect on tensile strength for each case.
Case number | Maximum cure temperature (°C) | Heating rate (°C/min) | Postcure time at 227°C (h) | Number of samples tested | Maximum tensile strength (MPa) |
---|---|---|---|---|---|
Case 1 | 194 | 1.39 | 2 | 5 | 856 |
Case 2 | 194 | 0.56 | 2 | 5 | 825 |
Case 3 | 183 | 1.39 | 2 | 5 | 884 |
Case 4 | 205 | 1.39 | 2 | 5 | 817 |
During cure and postcure stages, the cross-linking mechanism involves two main steps. These two steps are carbon-carbon double bonds (C=C) opening and dehydration of hydroxyl groups. The first step improves the composite mechanical properties, while the second step deteriorates the mechanical properties. As the maximum cure temperature increases, the effects of the rate of dehydration become more pronounced as compared with the mechanism of carbon-carbon double bonds (C=C) opening. Since curing the resin at a high temperature, for the same time as at a low temperature, increases the possibility of water molecules diffusion out of the polymer, which leads to more defects. While at a low maximum cure temperature, within the range used in the experimental design, the carbon-carbon double bonds (C=C) opening dominates the mechanism of cross linking, which improves the composite mechanical properties. Decreasing the heating rate form Case 1 to Case 2 increases the cure cycle time by an hour which, as well, increases the polymer dehydration, which deteriorates the mechanical properties. So, curing time, which is 4 hours in this study, at a low maximum cure temperature of 183°C with 1.39°C/min heating rate is enough to reach the maximum degree of cure based on carbon-carbon double bonds (C=C) opening mechanism, which improves the properties.
This paper presents an analytical and experimental investigation of cure cycle effect on carbon-fiber reinforced polymer composites molded by VARTM. The investigated composite structure was composed of high-temperature polymer (Cycom 5250-4-RTM) and eight harness woven fiber fabrics. The findings of this study show that, within the range used for each parameter in the experimental design, a maximum cure temperature of 183°C, a postcure time of 2 hours at 227°C, and a heating rate of 1.67°C/min helped to produce composite structures with high ultimate tensile strength tested at room temperature. The developed analytical model can determine the degree of cure as functions of time and temperature during the cure cycle, as well as, the distribution of degree of cure and temperatures within the part thickness at any time of the cure cycle. This analytical-experimental investigation provides capability for robust process design and characterization of process induced damage. This study helps in developing a science based technology for the VARTM process for the understanding of the process behavior and the effect of the cure cycle on the properties of the molded composites.
The author acknowledges that there is no any kind of conflict of interests regarding this paper.
The author wishes to acknowledge the support of the Airtech Advanced Materials Group, Hexcel Corporation, and Cytec Engineered Materials.