1. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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 (T g )  Low moisture absorption  Good thermal stability  Low shrinkage

3. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved Preparation of Materials  Base resin  Modified cyanate ester resin (RS-9D)  T g > 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. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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 Neat Resin —— no catalysts - - - - with catalysts Aromatic Phosphonium - - - - no catalysts —— with catalysts

10. © 2006 University of Delaware, All rights reserved 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 215438196498Cyclic Amine 195420147395Heterocyclic Amine 192429187468Aromatic Phosphonium 187437174442Aliphatic Phosphonium 254458164519Neat T max (C) ΔH 0 (J/g)T max (C)ΔH 0 (J/g)Organoclay type Without catalystWith catalyst

11. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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 1 Hansen, Gillespie; Journal of Composites Technology and Research ; Vol.20; 1998  Samples were tested in a 3- point bend test to measure displacement as a load was applied to the cracked region  Fracture toughness (K IC ) was calculated for each of the systems using 5 replicates

14. © 2006 University of Delaware, All rights reserved Effects of Thermal Aging – Fracture Toughness  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.  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 control 250 hours 500 hours

15. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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. © 2006 University of Delaware, All rights reserved 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