Professor James Economy's Group
Fiber Reinforced Thick Section Copolyester Composites
Thick section composite laminates have typically been expensive and difficult to manufacture because of problems with porosity, residual strains and the large exothermic reaction of epoxies. Further, mismatches between the coefficient of thermal expansion of the fiber and matrix can lead to residual strains as a result of shrinkage and thermal stress during cure. Also, in bismaleimides, any volatile products that form during the cure process have a potential to form voids between plies, which can lead to delimitation. Typically, a stage curing process has been used to reduce the effect of these problems, but it requires multiple cure processes and is time consuming. More recently, a method in which a temperature gradient is applied through the thickness has been used to cure thick composites without stage curing.
The copolyester materials developed here can eliminate these problems. Through interchain transesterification reactions, individual plies can be bonded with each other in the solid state through heat and pressure (as seen the background in the Polymer section). Hence, little or no volatile products are given off, residual strains are localized to the individual plies and there is no need to worry about exothermic reactions. Furthermore, oure Liquid Crystal Polymer Thermoset (LCP-TS), can potentially match the coefficient of thermal expansion of carbon fibers at the fiber/matrix interface. This is important in high temperature composite applications where many thermal cycles are experienced. Small strains at the interface can lead, over many heating and cooling cycles, to fatigue crack growth and delaminating.
Our copolyesters excel compared to current resins. They have a high glass transition temperature of about 250oC, compared to high grade epoxies which can reach temperatures of about 175o Celsius, and high thermal stability (degradation in air does not initiate until about 350oC). Past research has also shown high lap shear strengths for temperatures up to 300oC. For thin bond thicknesses of about 4 micrometers, lap shear strengths of about 3000 Psi have been observed.
Current work is in the primary stages: studies optimizing the time and temperature required to cure and bond lamina, literature research on different methods to coat fibers and fabrics with the solid polyester powders (at room temperature) and manufacturing thin and thick laminates. Since we are dealing with a new type of thermosetting copolyester resin, these processes need to be characterized if they are to be viable for use in structural composites.
In the very near future, the tensile properties and flexure properties will be tested. Dynamic Mechanical Analysis (DMA) of solid state bonded specimens has been done on 45% fiber volume fraction composites made from plain woven carbon fibers. The storage modulus at room temperature averages about 35 GPa in the 0 direction (for 3 ply samples).