HIGH-TEMPERATURE FINISHES FOR POLYIMIDE MATRIX COMPOSITES

Transcription

1 HIGH-TEMPERATURE FINISHES FOR POLYIMIDE MATRIX COMPOSITES Ronald E. Allred, Andrea E. Hoyt Haight, Jan-Michael Gosau, and Jeremy P. Barlow Adherent Technologies, Inc. Albuquerque, NM ABSTRACT High-temperature polymer matrix composites (PMCs) are used for a variety of jet engine parts, including the fan case, stators, fan ducts, augmentor ducts, external flaps, the lift fan system, the fan inlet case, roll posts, and engine-airframe integration parts. These parts amount to 600 to 1000 pounds per engine. The current baseline composite system for the joint strike fighter (JSF) engine is T carbon fiber with HTS sizing in an AFR-PE-4 fluorinated polyimide matrix. There are several problems with the current fiber-sizing combination, including lack of adequate protection of the fibers during weaving, poor property translation in the composite, poor fibermatrix interfacial adhesion, and poor wetting during prepregging and composite processing. A new fiber-sizing combination is needed to alleviate the problems with the current baseline material. Such a development will result in lowering costs while improving overall performance. Adherent Technologies, Inc. is developing a new, more compatible sizing (finish) for T650-35/AFR-PE-4 composites. Interfacial mechanical property data as well as the performance of the new fiber finish system in accelerated thermo-oxidative aging experiments will be discussed. KEYWORDS: Interface/Surface Analysis, Resins/Materials Phenylethynyl Imide, Sizing/Coupling Agents 1.1 Composites in Military Jet Engines 1. INTRODUCTION A typical jet engine cross section is shown in Figure 1. High-temperature polymer matrix composites (PMCs) are used for a variety of jet engine parts, including the fan case, stators, fan ducts, augmentor ducts, external flaps, the lift fan system, the fan inlet case, roll posts, and engine-airframe integration parts. These parts amount to 600 to 1000 pounds per engine. The current baseline composite system for the joint strike fighter (JSF) engine is T carbon fiber with HTS sizing in an AFR-PE-4 fluorinated polyimide matrix. Copyright 2007 by Adherent Technologies, Inc. Published by Society for the Advancement of Material and Process Engineering with permission.

2 Figure 1. Appearance of generic military jet engine (courtesy Pratt & Whitney) There are several problems with the current fiber-sizing combination, including lack of adequate protection of the fibers during weaving, poor property translation in the composite, poor fibermatrix interfacial adhesion, and poor wetting during prepregging and composite processing. Most of these problems are interrelated and caused by the HTS sizing [1]. A new fiber-sizing combination is needed to alleviate the problems with the current baseline material. Such a development will result in lowering costs while improving overall performance. Cost reductions will result from minimizing damage during weaving, which reduces scrap, fraying, and fuzzing of the fabrics used in composite fabrication. Cost reductions will also result from better wetting, which also reduces scrap and prepregging costs, and improved property translation, allowing lighter weight parts. The overall goal of the Phase II program is to develop a new, more compatible sizing (finish) for T650-35/AFR-PE-4 composites. 1.2 Reactive Finish Concept In this work, we distinguish a finish as an adhesion-promoting chemistry applied to a fiber surface as opposed to a sizing, which is applied as a handling aid. Enhanced interfacial properties are a direct result of forming chemical bonds with the fiber surface and matrix resin through the use of reactive coupling agents. These coupling agents will bond directly with the basal plane crystallites on the carbon-fiber surface in addition to edge and defect sites. Those bonds maximize interface environmental durability [2-4]. This approach has been shown to improve the interface sensitive mechanical properties of carbon/polyimide composites [2-4]. The reactive finish concept for bonding to AFR-PE-4 polyimide resins is shown in Figure 2, where the unsaturation shown as double C-C bonds will be phenylethynl groups. When the finish is applied and reacted to the carbon fiber, it leaves residual unsaturated groups on the fiber surface that can subsequently chemically bond with the polyimide matrix resin during composite fabrication. These reactive finishes typically are composed of four or more ingredients including a reactive coupling agent, a film-forming polymer carrier, a surfactant, and a solvent. Mixtures of some of these ingredients are often necessary for solubility and processing reasons.

3 Figure 2. Reactive finish concept Proper selection of the polymer or prepolymer carrier in the finish formulation results in a uniform nanometer scale ( nm) coating on the fiber surface that is chemically bonded to both sides of the interface. Figure 3 depicts the generic structure of the reactive coupling agents used in the reactive finishes reported in References 2-4. Figure 3. Reactive coupling agent structure

4 Finish Application 2. MATERIALS AND EXPERIMENTAL APPROACH Three different fiber finish concentrations were applied using solutions of Maverick s J1 polymer with our reactive coupling agent. The J1 resin is a RTM version of AFR-PE-4. The finish was applied to Cytec Thornel T k tow from methyl ethyl ketone (MEK) using Adherent Technologies fiber finishing line [5]. Finish solution concentrations evaluated were 0.01%, 0.1%, and 1% by weight in MEK. Prepreg Fabrication Prepregs using the finished fibers described above were prepared by Patz Materials and Technologies (Benicia, CA). Prepregs were also fabricated using unsized T fiber and Cytec s HTS sized T fiber for comparison. The AFR-PE-4 resin in ethanol was supplied as 60% solids from Maverick Corp. The resin solution was concentrated to 69% before the prepregging operation. Seventy feet of 6.5-inch wide high-quality prepreg were fabricated for each fiber under consideration. The prepregs were frozen and shipped to ATI. Laminate Fabrication and Characterization The 70 foot long 6.5 inch wide prepregs fabricated in the previous quarter were made into 6.5 inch x 7.0 inch stacks of 12 plies each for each of the 5 different fiber surface chemistries and shipped to the University of Dayton Research Institute (UDRI) for autoclave molding. The molding setup used is shown in Figure 4. Figure 4. Autoclave molding setup used at UDRI Ultrasonic C-scans of these plates confirmed that they were of high quality.

5 Mechanical Testing The laminates were then cut into mechanical samples using a waterjet. Polished cross-sections were prepared on some of the 90 flexure samples to observe the fiber and void distributions. Flexural specimens of 0 and 90 were tested according to ASTM D a. Short beam shear testing was conducted according to ASTM D Specimen dimensions were based on a nominal value of in. (d) for panel thickness. Table I lists specimen dimensions for the three tests. Table I. Specimen Dimensions for 12-Ply Carbon/AFR-PE-4 Panels Test Type 0 flexure 90 flexure Short Beam Shear Span, in. 2.0 (l/d =32) 0.5 (l/d = 8) 0.25 (l/d = 4) Width, in Length, in (8 d) Crosshead velocities were 0.2 in./min for 0 flexure, 0.05 in./min for 90 flex, and 0.05 in./min for short beam shear. The average value of the span calculated from the thickness of each specimen for a given test was used for all five specimens. Individual width and thickness measurements were used to compute stress for each specimen.

6 3. RESULTS AND DISCUSSION Cross-sections of the control unsized and HTS sized laminates are given in Figure 5. Figure 5. Cross-sections of unsized (top) and HTS sized (bottom) T650-35/AFR-PE-4 laminates (10x left, 20x right) The laminates show good fiber distribution throughout with very low void content. Similar cross-sections for the laminates with the reactive finishes are shown in Figure 6.

7 Figure 6. Cross-sections of laminates with reactive finished T650-35/AFR-PE-4. (finish concentrations: top 0.01%, middle 0.1%, bottom 1.0%) (10x left, 20x right)

8 Overall, the laminates were deemed to be of high enough quality for mechanical testing. Five specimens each were removed for the three types of panels for 0 flexure, 90 flexure, and short beam shear testing. The width and thickness of each specimen were measured at the midpoint and tabulated. All the laminate types exhibited elastic to failure behavior in 0 flexure and were very brittle. Resultant average mechanical properties from five specimens and the coefficients of variation (standard deviation divided by the mean) for each data set are given in Table II for longitudinal flexure tests. All of the laminate types show very similar properties in longitudinal flexure, which is an indication that they were all of high quality. Table II. Longitudinal (0 ) Flexural Properties of T650-35/AFR-PE-4 Laminates Laminate Type Ultimate Strength (psi) Young's Modulus (psi) Deflection at Failure (in.) Unsized (12 ply) 265, x HTS Size (12 ply) 259, x ATI-9307-J1 Finish 0.01% (12 ply) ATI-9307-J1 Finish 0.1% (12 ply) ATI-9307-J1 Finish 1.0% (12 ply) 266, x , x , x Properties in transverse flexure are given in Table III. The data in Table III are also average values for five specimens of each laminate type. Transverse flexure is extremely sensitive to interfacial tensile strength and has been effectively used to study high-temperature aging effects [6]. Again, the data in transverse flexure for all the laminate types is similar, which would be expected from the longitudinal results.

9 Table III. Transverse (90 ) Flexural Properties of T650-35/AFR-PE-4 Laminates Laminate Type Ultimate Strength (psi) Young's Modulus (psi) Deflection at Failure (in.) Unsized (12 ply) 24, x HTS Size (12 ply) 22, x ATI-9307-J1 Finish 0.01% (12 ply) 22, x ATI-9307-J1 Finish 0.1% (12 ply) 23, x ATI-9307-J1 Finish 1.0% (12 ply) 24, x Interface dominated tests under room temperature, dry conditions often do not exhibit a wide range of properties due to interface modification. Average short beam shear strengths and statistical variations for five specimens from each laminate type are given in Table IV. Table IV. Short Beam Shear Strengths of T650-35/AFR-PE-4 Laminates Laminate Type Shear Strength (psi) Standard Deviation (psi) Coefficient of Variation (%) Unsized (12 ply) 16, HTS Size (12 ply) 15, ATI-9307-J1 Finish 0.01% (12 ply) ATI-9307-J1 Finish 0.1% (12 ply) ATI-9307-J1 Finish 1.0% (12 ply) 16, , , The short beam shear data show the same trend as the transverse and longitudinal flexure data with all the laminate types displaying similar properties. It should be noted that over 16 ksi is a good short beam shear strength for aerospace composites, especially with an untoughened matrix

10 resin. As such, all five types of laminates exhibit good shear strengths. The lower coefficients of variation for the reactive finished laminates may be an indication of better interfacial bonding, but that is difficult to speculate from this data. 4. CONCLUSIONS 5. ACKNOWLEDGMENTS The authors would like to acknowledge the guidance from Dr. Fred Arnold at WPAFB, the technical point of contact for the study. Dr. Charles Watson and Mr. Charles Logan at Pratt & Whitney provided much needed insight into composite applications in jet engines and current problems with existing materials. This work was funded by NAVAIR under their Small Business Innovation Research (SBIR) program. 6. REFERENCES 1. Charles Watson, Pratt & Whitney, private communications to R.E. Allred during the Phase I program. 2. R. E. Allred and J. K. Sutter, Fiber Finish for Improving Thermo-Oxidative Stability of Polyimide Matrix Composites, Proc. 42nd Intl SAMPE Symp. and Exhib., Anaheim, CA, May 1997, pp R. E. Allred, R. E. Jensen, B. W. Gordon, T. A. Donnellan, T. Williams, Jr., R. C. Cochran, and K. W. Miller, Fiber Finishes for Improving Galvanic Resistance of Imide-Based, Composites, Proc. 43rd Intl SAMPE Symp. and Exhib., Anaheim, CA, May 1998, pp R. E. Allred, S. P. Wesson, E. Eugene Shin, L. Inghram, L. McCorkle, D. Papadopoulos, D. Wheeler, and J. K. Sutter, The Influence of Sizings on the Durability of High-Temperature Polymer Composites, J. High Performance Polymers, 15, 4, 2003, pp R. E. Allred, S. P. Wesson, A. E. Hoyt, and J. Whitehead, Reactive Finishes for Improving Adhesion in Carbon/Vinyl Ester Laminates, Proc. 49th Intl. SAMPE Symp. and Exhib., Long Beach, CA, May 17-20, K. J. Bowles, Transverse Flexural Tests as A Tool for Assessing Damage to PMR-15 Composites from Isothermal Aging in Air at Elevated Temperatures, SAMPE Quarterly, 24 (2), January 1993, pp