Thermal Degradation Of Carbon Fiber/Cyanate Ester Resin Composites Filled With Clay Silicate Nanoparticles Dr. Shawn Doh

1. Thermal Degradation Of Carbon Fiber/Cyanate Ester Resin Composites Filled With Clay Silicate Nanoparticles Dr. Shawn Doherty Univ. of Delaware – Center for Composite Materials

2. Introduction • Polymer matrix composites are being used in extreme operating environments • High temperature applications such as engine components and aircraft structures • Long term use in extreme environments leads to decrease in composite properties • Cyanate ester resins are promising thermosets for study due to excellent properties • High glass transition temperature (Tg) • Low moisture absorption • Good thermal stability • Low shrinkage

3. Introduction • Project Goal • Use nanoclay additives to reduce the rate of thermo-oxidative decomposition and microcracking of high-temperature polymer resins • Concept • Dispersed nanoparticles in the matrix present a barrier to the diffusion of oxygen that would slow down the decomposition of the resin under long term exposure • Inorganic nanoparticle additives would help overcome induced stresses during resin degradation preventing microcracking that accelerate thermo-oxidative degradation • Inorganic nanoparticles help prevent rapid microcrack growth • Addition of inorganic particles would reduce CTE of resin, minimizing mismatch between resin and carbon fibers

4. Outline of Work • Preparation of nanoclay/cyanate ester resin mixtures and composites • Examination the effects of nanoclays on the cure behavior of the cyanate ester resin • Examination of the effects of nanoclays on the structure and properties of the resin • Analysis of changes in mechanical behavior of nanoclay/resin composites at high temperatures

5. Preparation of Materials • Base resin • Modified cyanate ester resin (RS-9D) • Tg > 350 ºC • Maximum service temperature of 280 ºC • Nanoclays • Organically modified montmorillonite clay • Developed by Triton for high-temperature stability and solubility with cyanate ester resin

6. Preparation of Materials • Mixture of cyanate ester and clay • High-shear mixing setup, 10,000 rpm at 110 ºC for 10 minutes • Differerent weight percentages of nanoclay added • 2.5 and 5 wt% of nanoclay for each resin system • Cobalt-based catalyst added (1.5 wt%) to promote curing • Used for cure behavior studies • Composites made from resin mixtures and carbon fiber • Prepreg made using IM7 carbon fiber and each clay/resin mixture • Prepreg layers were pressed into ½” thick unidirectional panels • Fiber volume fraction of 57% • Used for mechanical testing and thermal aging studies

7. Effects on Cure Behavior - Rheological • Clay systems had lower cure temperatures and higher minimum viscosities than neat resin, which makes the resin less processable • Cyclic Amine and Aromatic Phosphonium had best performance • Aliphatic Phosphonium had worst performance • Higher clay content = worse performance

8. Effects on Cure Behavior - Rheological • Working temperatures determined by isothermal heating • Heterocyclic Amine had most accelerated cure, then Aromatic Phosphonium and Cyclic Amine • Clay acts like catalyst, curing at lower temperatures and shorter times 100 ºC 120 ºC 110 ºC

9. Neat Resin Aromatic Phosphonium —— no catalysts - - - - with catalysts - - - - no catalysts —— with catalysts Effects on Cure Behavior - Calorimetric • Since nanoclay may be acting as a catalyst, measurements were taken for each system both with and without the cobalt-based catalyst • In neat resin, catalyst lowered cure temperature and broadened peak • Addition of clay to catalyzed system lowered initial cure temperature further and narrowed curing temperature range • Addition of clay to catalyst-free systems lowered the peak temperature

10. With catalyst Without catalyst Organoclay type ΔH0 (J/g) Tmax(C) ΔH0 (J/g) Tmax(C) Neat 519 164 458 254 Heterocyclic Amine 395 147 420 195 Aromatic Phosphonium 468 187 429 192 Cyclic Amine 498 196 438 215 Aliphatic Phosphonium 442 174 437 187 Effects on Cure Behavior - Calorimetric • Clay reduced heat of cure in catalyzed systems compared to neat resin, up to 24% for Heterocyclic Amine(no change in non-catalyzed) • Polymerization initiated at lower T and occurs at faster rate, but prevented from reaching full conversion due to premature termination • Changes in cure kinetics of resin systems may lead to reduction in mechanical properties for clay systems, due to reduced cross-linking

11. Microscopic Analysis of Composite Structure • Some of the nanoclay resin systems were examined using transmission electron microscopy (TEM) to determine the distribution of particles in the resin • Clay layers can be seen at higher magnification and agglomerates at low magnification which indicates minimal particle separation or exfoliation of the clay

12. Effects of Thermal Aging – Weight Loss • Composite samples of each clay/resin system were aged for 51 days at 260 ºC and the weight loss was measured. • After 3 days, each had lost 0.8% of initial weight • After 1 month, neat resin had lost 5.6% while others had lost ~4.5% • After 51 days: • Neat: 12% • Largest: 5 wt% Cyclic Amine (10.5%) • Smallest: 2.5 wt% Aromatic Phosphonium (5.6 %) • Nanoclay particles are reducing the amount of thermal degradation in the resin system

13. Effects of Thermal Aging – Fracture Toughness • To measure the fracture toughness of the nanoclay composite systems, notched samples were cut according to ASTM D-5045 • Each specimen was 3” x ½” x ¼” with fiber direction // to the short dimension and the notch ¼” deep sharpened to a crack at the tip • Samples were tested in a 3-point bend test to measure displacement as a load was applied to the cracked region • Fracture toughness (KIC) was calculated for each of the systems using 5 replicates 1Hansen, Gillespie; Journal of Composites Technology and Research ; Vol.20; 1998

14. Effects of Thermal Aging – Fracture Toughness • After 250 hours, • Cyclic Amines had largest decrease (20%) • Heterocyclic Amine had almost no change • Neat resin had 16% loss of toughness • After 500 hours, no significant change in fracture toughness compared to 250 hours • Results consistent with weight loss: Largest weight loss corresponds to largest in fracture toughness loss (Neat and Cyclic Amine systems) • Variation in toughness for the control samples is likely due to variation in cure behavior due to the nanoclay components. control 250 hours 500 hours

15. Effects of Thermal Aging - Microcracking • Study on effects of nanoclays on crack propagation currently underway • Initial work examined the surface cracking of the aged composites to determine if the nanoclays had any visible difference • Surface cracks in aged neat composite were more pronounced than aged composite with nanoclay particles Neat cyanate ester composite after 500 hours of aging Aromatic phosphonium / cyanate ester composite after 500 hours of aging

16. Conclusions • Addition of organically modified nanoclay particles to cyanate ester resin will effect the properties and processability of the material • Clay particles will decrease the processing window of the resin prior to curing • Nanoclay decreases the cure temperature of the resin • Nanoclay shrinks the amount of time before the resin cures at a fixed temperature • Clay particles act as catalysts, increasing the rate of cure and decreasing the maximum cure temperature • Nanoclay/resin composites are more thermally stable than neat systems • Nanoclay composites had less weight loss after aging than neat system • Nanoclay composites had higher fracture toughness than neat system

17. Future Work • Work to improve the distribution and exfoliation of the clay particles in the resin system, since increasing the resin-clay interaction should improve properties • Continued study of the microcracking in order to quantify the penetration of cracks into the composite bulk. Factors to be considered: • Aging time • Nanoclay system • Both in-plane and transverse direction

18. Acknowledgements • Dr. Joseph Deitzel – CCM • Misaki Takemori – CCM • Touy Thiravong – CCM • Apoorva Shah – Triton Systems, Inc • Dr. Arjan Giaya – Triton Systems, Inc • Dr. Jack Gillespie – CCM • Dr. Dirk Heider – CCM