VACUUM INFUSION AND CURING OF CARBON FIBER/BENZOXAZINE COMPOSITES FOR HIGH TEMPERATURE APPLICATIONS

Transcription

1 VACUUM INFUSION AND CURING OF CARBON FIBER/BENZOXAZINE COMPOSITES FOR HIGH TEMPERATURE APPLICATIONS Amol Ogale 1, David Leach 2, Ehsan Barjasteh 2, Helder Barros-Abreu3, Dale Brosius 4, Jens Schlimbach 1 1 Quickstep GmbH, Rolf-Engel-Strasse 6.5, Ottobrunn, Germany; 2 Henkel Aerospace, 2850 Willow Pass Road, Bay Point CA 94565; 3 Henkel AG, Henkelstrasse, Dusseldorf, Germany; 4 Quickstep Composites LLC, 3251 McCall St., Dayton, OH ABSTRACT Fiber reinforced polymer composites are increasingly considered for replacing metals in high temperature-resistant components for aircraft or helicopters (service temperature >190 C). There are very limited resin options available to manufacture such composite components. One system of considerable interest is the family of benzoxazine resins, which offer high glass transition temperatures and excellent hot/wet performance, combined with room temperature storage and easy processing. The main objective of this work was to develop an energy efficient and manufacturing friendly Out-of-Autoclave (OoA) manufacturing technique for a commercially available high temperature benzoxazine resin. This paper presents the process development for the infusion of the benzoxazine resin into carbon fiber preforms, followed by subsequent cure and postcuring. Physical and thermal behavior, including mechanical performance after impact damage is reported. An integrally stiffened structure was produced as a demonstrator to evaluate the behavior of the benzoxazine resin in a complex infusion situation. The Quickstep OoA curing process was used to demonstrate highly efficient manufacture of the panels and demonstrator parts. 1. INTRODUCTION The use of liquid resin processes such as resin transfer molding (RTM) and vacuum assisted RTM (VARTM) has developed rapidly in recent years. There is a strong desire to use these processes more extensively in future aerospace structures because large and complex parts can be manufactured economically, and the costs of autoclave processing can be avoided. Despite the benefits of liquid resin processes, there are challenges to extending their use to primary structure and high temperature applications due to the limitations of the resin systems available. The aspects which must be balanced in the development of resins and processes include [1]: - Increased toughness - High temperature performance - Infusion conditions - Stable viscosity at infusion temperature - Cost-effective and controlled heating and cooling during processing Copyright 2014 by Henkel Corporation and Quickstep Holdings Ltd. Published by Society for the Advancement of Material and Process Engineering with permission

2 For this study, a commercial benzoxazine resin, optimized for use in infusion processing, was combined with a rapid heating and cooling manufacturing approach to produce high temperature resistant composite laminates and parts at significantly lower cycle times. Key mechanical strength properties were measured and are reported herein. The technique was then applied to a curved, integrally stiffened composite panel to demonstrate the ability of the process to produce complex parts suitable for use in the aerospace environment. 1.1 Materials for high temperature performance Epoxy, polyimide, polyurethane, bismaleimide (BMI), benzoxazine and cyanate ester thermosetting polymers are commonly used in high temperature composite applications; the specific material system is selected based on the application area. Nevertheless, processing of each resin type differs greatly and requires attention towards the issues of foaming, pot life, and the possibility to use as an infusion resin, including infusion temperature, curing temperature, etc. Being related to the chemistry of phenolic resins, benzoxazines offer excellent flame retardancy and very little shrinkage. In contrast to phenolic resins, they cure without elimination of volatiles and show significantly better properties of the cured material. New efforts in order to make the resins tougher by two-phase modification have shown noticeable success and have dry T g above 250 C and wet T g in the range of 200 C with G IC values 350 J/m 2 [2]. Henkel Corporation has introduced a number of commercial systems suitable for use in prepregs, adhesives and infusion processes [3]. For this study, LOCTITE BZ 9130 AERO resin was selected and characterized using the Quickstep process for the infusion processing. Key characteristics of the BZ 9130 resin include [4]: - Room temperature stability, requiring no refrigerated storage - A one-part system, so no mixing required by the processor - Broad processing window, suitable for large parts and complex shapes - Stable, low viscosity at infusion temperature - Low heat release during cure, reducing exotherm risk - High hot/wet property retention for higher service temperature applications Figure 1 shows the dynamic processing behavior of the BZ 9130 resin when heated at a rate of 2 ºC per minute. The viscosity drops into an infusion friendly range of 150 to 300 cps in the range of ºC. Figure 2 shows that this low viscosity window remains in place for over 90 minutes, suitable for the infusion of large or more complex structures typical of aircraft applications. A recommended cure cycle of 120 minutes at 185 ºC, followed by a free-standing post cure of 60 minutes at 232 ºC results in a dry T g via DMA (ASTM E1640, onset) of 255 ºC. After a 72 hour water boil, the resulting hot/wet T g by the same method is 196 ºC. Fracture toughness, or G IC, is 2195 J/m 2.

3 Figure 1. Dynamic viscosity of BZ 9130 AERO resin at heating rate of 2 ºC/min Figure 2. Isothermal viscosity of BZ 9130 AERO resin at various infusion temperatures

4 1.2 Environmentally friendly OoA processing Preform resin infusion technology is the main target of this study to avoid prepreg and autoclave processing. This alone is beneficial by eliminating needs of pre-impregnation processes, cold storage and wastage of specially treated materials. For the laminate and demonstrator curing the Quickstep TM process has been chosen. The Quickstep process is based on the principle of conduction heating. This process utilizes heat transfer fluid (HTF) to apply heat and pressure to the uncured component during processing. Quick transfer of heat energy (ramp rate of K/min and more) into the curing substrate (fiber and resin) is the heart of this technology [5]. Figure 3 shows the principle of Quickstep curing system. Figure 3. Schematics of Quickstep curing chambers Conventional mold heating, such as that in an oven or autoclave, causes uneven temperature distribution over the mold surface. The use of the fluid-based Quickstep process solves this problem, especially while achieving the fast ramp-up rates. Fast ramp-up via conventional heating techniques may also cause uneven temperature distribution over the laminate, which affects resin shrinkage and changes in polymer morphology [6]. A heated fluid and slightly pressurized Quickstep process (up to 0.8 bar) accurately achieves the required mold temperature alll over the surface with faster ramp rates and effective utilization of energy. This helps to modify the cure cycle and dwell time without affecting the physical and chemical nature of a polymeric resin. Figure 4 shows the Quickstep heating and cooling unit and the curing chambers. Figure 5 illustrates the concept for infusing consolidated preform in the Quickstep chamber using the VARTM process. By combining the cure kinetics of selected resin, OoA curing and online temperature monitoring during the process, the dwell times to complete the laminate curing can be shortened to a great extent. Ultimately, this also causes lower energy consumption during the manufacturing processes. Additionally, fewer molds are required for serial production and reduced machine work hours and maintenance, etc., resulting in lower costs.

5 Figure 4. Quickstep machine with curing chamber. Figure 5. Schematic of resin infusion within the Quickstep curing system. 2.1 Processing of test laminates 2. EXPERIMENTS A screening matrix of mechanical properties was developed by the authors to reflect key considerations for the design of aircraft laminates, including tension, compression, shear, open hole compression and compression after impact. The fabric chosen was a 370 gsm 5HS style using 6K HTS fiber. Preforms were produced using application of a compatible tackifying binder (5 %) supplied by Henkel, and then vacuum consolidated. Preforms of 5, 8 and 12 plies were prepared according to the particular tests performed.

6 Composite laminates were made using VARTM processing techniques. In VARTM, a double bag technique was used to provide high degree of vacuum within the preformed fabric. The plate with the double bag assembly was placed in the Quickstep chamber and pre-heated to 130 ± 5 ºC. The resin was preheated in the oven, at 110 ºC. 100 mbar vacuum was applied to both the inner and outer bags and the complete lay-up was evacuated. The resin line was then connected and the resin was infused into the reinforcement using a distribution medium. When resin flow without bubbles was seen at the outlet, the outlet port was closed and then full vacuum was applied to the outer bag. The temperature was than increased to 185 ºC at 1-3 ºC/min, and held at 185 ºC for 120 min. A free standing post cure was then performed by placing the specimen in oven for 60 min at 232 ºC. 2.2 Production of prototype demonstrator In order to demonstrate the capability of the resin and infusion/curing techniques on a scale larger than flat laminates, a demonstrator panel, approximately 1 m 2 was preformed and infused. This panel consists of 5 layers of 370 gsm 5HS carbon fabric for the skin plies, resulting in a skin thickness of 2 mm, onto which a series of four hat-shaped, or omega-type e stiffeners were placed using removable mandrels. The nominal thickness of each stiffener is 2 mm. The assembly was bagged as described earlier and placed into the Quickstep curing chamber for infusion and cure. Figure 6 shows the cure profile used to manufacture the integrally stiffened panel. Figure 7 shows the molded and trimmed stiffened panel. Figure 6. Infusionn cycle and parameters for stiffened panel demonstrator.

7 Figure 7. Hat-stiffened curved panel demonstrator produced using BZ 9130 resin. 3.1 Mechanical properties of flat laminates 3. RESULTS AND DISCUSSIONS After molding and post cure, the panels were shipped from Quickstep to Henkel for testing. Thickness measurements on the 5, 8 and 12 ply laminates taken showed average thicknesses of 2.0 mm, 3.2 mm and 4.8 mm, respectively. Mechanical properties were measured on the machined laminate specimens at room temperature and dry conditions and are presented in Table 1. Layup schedules and test methods for each measured property are also listed.

8 Table 1. Manufactured carbon fiber/benzoxazine composite laminate properties Fiber: HTS 5HS (6k) 370 gsm, Resin: BZ 9130 Property Specification Layup Test Temp. / Condition Mean Units CoV In-Plane Shear Strength ASTM D /-45] 2S RT Dry 91 MPa 2.3% Interlaminar Shear Strength Compression Strength, 0 ASTM D2344 [0] 8 RT Dry 67 MPa 5.1% ASTM D MPa 7.2% Compression Modulus, 0 ASTM D GPa 13.4% [0] 8 RT Dry Compression Strength, 90 ASTM D MPa 5.5% Compression Modulus, 90 ASTM D GPa 8.4% Tensile Strength, 0 ASTM D MPa 2.9% Tensile Modulus, 0 ASTM D GPa 5.4% [0] 5 RT Dry Tensile Strength, 90 ASTM D MPa 3.0% Tensile Modulus, 90 ASTM D GPa 5.2% OHC ASTM D /-45] 3S RT Dry 350 MPa 5.1% CAI ASTM D /-45] 3S RT Dry 174 MPa 1.2% Values for all properties except shear and OHC normalized to 58% fiber by volume 3.2 Fiber volume and laminate quality measurements Fiber volume on the laminates of the three thicknesses was determined via acid digestion as follows: 2 mm panel (5 ply) 56.5% 3.2 mm panel (8 ply) 57.1% 4.8 mm panel (12 ply) 55.4% Each of the above values were in the expected range for woven fabric panels produced using the VARTM process. Microscopy analysis was completed on the 2 mm and 4.8 mm laminates, as shown in Figures 8 and 9, respectively. The panels were found to be void-free using this technique.

9 Figure 8. Photomicrographs of 2 mm (5 ply laminate) Figure 9. Photomicrographs of 4.8 mm (12 ply laminate) 3.3 Demonstrator The hat-stiffened demonstrator panel was inspected for thickness and any evidence of dry spots. Visual inspection indicated no evidence of dry areas throughout the panel. Microscopy analysis of trimmed edges confirmed no visual internal voids in the panel or stiffeners. 3.4 Discussion The processing of the BZ 9130 resin proved to be infusion-friendly in the Quickstep process, with void-free panels produced with fiber volumes in the 55 to 57% range, which is considered good for a woven fabric laminate. Mechanical evaluation of the panels confirmed values similar or higher than that achieved in standard oven-based VARTM processing. The Quickstep process provided a faster heating and cooling of the laminate and uniform temperature during the infusion process. A complex demonstrator, featuring hat-section stiffeners, was infused and cured in a single operation, and showed uniform fiber/resin distribution throughout without dry or resin-rich areas.

10 4. CONCLUSIONS Panels from a high temperature benzoxazine infusion resin were produced using the Quickstep fluid-based heating and cooling process. The successful manufacture of a demonstrator (hatstiffened composite panel) validate the possibilities of the BZ 9130 resin and the Quickstep outof-autoclave process as suitable for the fabrication of aircraft structures. 5. REFERENCES 1. W H Li, J Gao, A Wong, D Leach, Moving the Boundaries of Liquid Resin Properties and Processing, SAMPE 2011 Long Beach, CA May 23-26, W H Li, A Wong, D Leach, Advances in Benzoxazine resins for Aerospace Applications, SAMPE 2010 Seattle, WA May 17-20, E Barjesteh, X Du, W H Li, Improvements in High Temperature Infusion Resin, SAMPE 2012 Baltimore, MD May 21-24, Product datasheet of LOCTITE BZ 9130 AERO benzoxazine resin: 5. Brosius, D., and Schlimbach, J. An advanced out-of-autoclave curing technology for prepregs and resin infusion. JEC Composites 36 (2007): Mold Temperature control effectiveness electric heat vs. pressurized water. [ ].