SAMPE Journal -September/October 2011 - 6

Feature Article

Winner of the SAMPE 2011 Outstanding Technical Paper Award

Out-Time Effects on VBO (Vacuum Bag Only) Prepreg and Laminate Properties
L.K. Grunenfelder, S.R. Nutt University of Southern California, Los Angeles, CA Abstract Changes in VBO (vacuum bag only) prepreg properties were tracked as a function of room temperature aging time (out-time). A modulated differential scanning calorimetry method was used to characterize the prepreg, and changes in prepreg tack levels were examined using an energy of separation technique. Laminates were cured from prepreg at various levels of aging using both traditional autoclave processing and low-pressure vacuum bag only (VBO) techniques. Cured laminates were examined using ultrasound scanning, mechanical properties were tested, and laminates were sectioned for examination of microstructure. Laminate quality as a function of out-time is discussed in terms of prepreg properties and manufacturing technique. Introduction Composite parts for commercial aircraft are traditionally manufactured using high-pressure autoclave processing. In recent decades, however, the use of composite parts for aircraft secondary and primary structure has increased sharply, and aircraft markets have grown, creating pressure to increase production rates. To meet the growing demand for composite aircraft parts and to allow for the production of large composite components (i.e. wings and fuselage) alternative processing methods will be required. There are several drawbacks to autoclave processing, including a large capital investment, cycle time, the high cost of the nitrogen gas used to pressurize the vessel, size limitations, and poor energy efficiency1-3. New out-of-autoclave processing methods have been developed to address these issues. One such method is vacuum bag only (VBO) processing of prepregs, a technique which uses atmospheric pressure alone to consolidate parts1,4,5. VBO processing presents a potential solution for the manufacture of larger parts at faster rates using conventional lay-up and placement tools. However, before VBO materials and methods can be used on primary structure, the quality of VBO processed parts must be shown to be equivalent to that of autoclave cured parts. The elimination of high external pressures in the cure cycle eliminates safeguards in the manufacturing process, resulting in the need for strict protocols in the lay-up and cure of VBO parts. One such protocol that remains to be determined is the useful out-life of VBO prepreg materials. An issue relevant to any composite processing method is ambient aging of material. The resin in prepregs is unstable at room temperature, so to prevent advancement of cure, material is stored in freezers until used. However, for the manufacture of large parts, lay-up can take several days6,7. It is thus important to understand how material characteristics will change with prolonged exposure to ambient conditions. Several out-time studies have been performed on a range of prepreg material systems6-16. Most of these studies have focused on autoclave processing6-15, though vacuum bag only processing has been examined as well . What has yet to be
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explored is the difference in part quality for the same material cured using both autoclave and VBO techniques. Aging will impact processing and final properties, and it is possible that the quality of VBO laminates will have a different dependence on out-time than that of autoclave processed parts. This work presents an evaluation of prepreg properties and associated laminate quality as a function of not only aging time but also processing technique. Characterization of prepreg is aimed at determining a simple method to estimate prepreg age, and laminate testing provides a means of relating this age to anticipated laminate quality. Experimentation Prepreg Characterization Modulated Differential Scanning Calorimetry A modulated differential scanning calorimetry (MDSC) technique was utilized to track changes in the resin system as a function of ambient aging time. MDSC is a powerful thermal analysis technique that results the separation of two distinct thermal transition types: reversing and nonreversing17. This separation is accomplished through a method of modulated heating, a sinusoidal temperature profile that is applied on top of the standard temperature ramp. Reversing reactions (i.e. glass transition) are heating rate dependent, and thus change based on the application of a sinusoidal heating and cooling cycle17. Nonreversing reactions, on the other hand, are kinetic events that are dependent only on absolute temperature, and cannot be altered by cyclic heating and cooling (i.e. crystallization, exothermic cure reactions)17. The separation of heat flow into these two components allows for the identification of subtle transitions. MDSC was performed on prepreg samples sealed in aluminum DSC pans using an instrument capable of modulation and cooling to subambient temperatures (TA Instruments Q2000). Precut samples were aged in open DSC pans, then sealed and tested. During testing, a temperature modulation of ±1º C every 60 seconds was applied to a temperature ramp from -90ºC to 280ºC at 3ºC/min. The same
SAMPE Journal, Volume 47, No. 5, September/October 2011

SAMPE Journal -September/October 2011

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