MIL-HDBK-17-1F Volume 1,Chapter 5 Prepreg Materials Characterization CHAPTER 5 PREPREG MATERIALS CHARACTERIZATION 5.1 INTRODUCTION The processability and properties of high performance composites depend upon the composition of the fiber/resin preimpregnated materials (prepregs)from which they are manufactured.In general,pre- pregs consist of "modified"or surface-treated glass,graphite,or aramid fibers impregnated with 28-60 weight-percent of a reactive and chemically-complex thermoset resin formulation or a thermoplastic resin. A typical thermoset resin formulation may contain,for example,several different types of epoxy resins, curing agents,diluents,rubber modifiers,thermoplastic additives,accelerators or catalysts,residual sol- vents,and inorganic materials,plus various impurities and synthetic by-products.Furthermore,such res- ins are often "staged"or partially reacted during the prepregging process and may undergo compositional changes during transport,handling,and storage.Although less likely to undergo compositional changes polymer molecular weight(MW),molecular weight distribution(MWD),and crystalline morphology have major effects on the processability and properties of thermoplastic prepregs and composites.Inadvertent or minor changes in resin composition may cause problems in processing and have deleterious effects on the performance and long-term properties of composites. Modern analytical techniques and detailed knowledge relating to fibers,fiber surface treatments,and resin types and formulations are needed to characterize prepregs and composite materials.Characteri- zation involves the identification and quantification of the fiber,fiber surface,and major resin components and should include information about the presence of impurities or contaminants.For thermoset resins and composites,characterization should include a description of the nature and extent of the prepreg resin reaction and the thermal/rheological and thermal/mechanical behavior.In the case of thermoplas- tics,the polymer molecular weight distribution,crystallinity,and time/temperature viscosity profile should also be analyzed.However.few laboratories are equipped or have the knowledgeable technical person- nel to characterize prepregs and composites completely,and few studies have been published describing how variations in fiber type and resin chemistry/morphology affect the physical properties and long-term performance of composites.Also,until recently,prepreg compositions were considered proprietary,proc- essing conditions were only recommended,and acceptance was based primarily upon mechanical testing of fabricated specimens.The purpose of this chapter is to provide an overview of characterization tech- niques and,more specifically,to address the application of state-of-the-art techniques for the chemical and physical characterization of resins and prepreg materials used in the manufacture of high perform- ance organic matrix composites. 5.2 CHARACTERIZATION TECHNIQUES-OVERVIEW According to a recent survey(Reference 5.2(a)),the most widely utilized techniques for the charac- terization and quality assurance of composite material precursors are- 1.High Performance Liquid Chromatography (HPLC) 2.Infrared(IR)Spectroscopy 3. Thermal Analysis 4.Rheological Analysis HPLC and IR spectroscopy provide the capability for rapid screening and quality control fingerprinting of individual resin constituents as well as of the prepreg resin and,therefore,may be used advanta- geously by both the prepregger and composite manufacturer(References 5.2(b)-(f)).Thermal analytical techniques,such as thermal gravimetrical analysis(TGA),differential thermal analysis(DTA),differential scanning calorimetry (DSC),thermal mechanical analysis (TMA),dynamic mechanical analysis (DMA), and torsional braid analysis (TBA)are not strictly chemical analysis techniques;however,they provide useful information relating to the composition and processability of resins (Reference 5.2(g)).Similarly, rheological and dielectric techniques are used frequently to evaluate the chemoviscosity properties of 5-1MIL-HDBK-17-1F Volume 1, Chapter 5 Prepreg Materials Characterization 5-1 CHAPTER 5 PREPREG MATERIALS CHARACTERIZATION 5.1 INTRODUCTION The processability and properties of high performance composites depend upon the composition of the fiber/resin preimpregnated materials (prepregs) from which they are manufactured. In general, pre￾pregs consist of "modified" or surface-treated glass, graphite, or aramid fibers impregnated with 28-60 weight-percent of a reactive and chemically-complex thermoset resin formulation or a thermoplastic resin. A typical thermoset resin formulation may contain, for example, several different types of epoxy resins, curing agents, diluents, rubber modifiers, thermoplastic additives, accelerators or catalysts, residual sol￾vents, and inorganic materials, plus various impurities and synthetic by-products. Furthermore, such res￾ins are often "staged" or partially reacted during the prepregging process and may undergo compositional changes during transport, handling, and storage. Although less likely to undergo compositional changes, polymer molecular weight (MW), molecular weight distribution (MWD), and crystalline morphology have major effects on the processability and properties of thermoplastic prepregs and composites. Inadvertent or minor changes in resin composition may cause problems in processing and have deleterious effects on the performance and long-term properties of composites. Modern analytical techniques and detailed knowledge relating to fibers, fiber surface treatments, and resin types and formulations are needed to characterize prepregs and composite materials. Characteri￾zation involves the identification and quantification of the fiber, fiber surface, and major resin components and should include information about the presence of impurities or contaminants. For thermoset resins and composites, characterization should include a description of the nature and extent of the prepreg resin reaction and the thermal/rheological and thermal/mechanical behavior. In the case of thermoplas￾tics, the polymer molecular weight distribution, crystallinity, and time/temperature viscosity profile should also be analyzed. However, few laboratories are equipped or have the knowledgeable technical person￾nel to characterize prepregs and composites completely, and few studies have been published describing how variations in fiber type and resin chemistry/morphology affect the physical properties and long-term performance of composites. Also, until recently, prepreg compositions were considered proprietary, proc￾essing conditions were only recommended, and acceptance was based primarily upon mechanical testing of fabricated specimens. The purpose of this chapter is to provide an overview of characterization tech￾niques and, more specifically, to address the application of state-of-the-art techniques for the chemical and physical characterization of resins and prepreg materials used in the manufacture of high perform￾ance organic matrix composites. 5.2 CHARACTERIZATION TECHNIQUES - OVERVIEW According to a recent survey (Reference 5.2(a)), the most widely utilized techniques for the charac￾terization and quality assurance of composite material precursors are - 1. High Performance Liquid Chromatography (HPLC) 2. Infrared (IR) Spectroscopy 3. Thermal Analysis 4. Rheological Analysis HPLC and IR spectroscopy provide the capability for rapid screening and quality control fingerprinting of individual resin constituents as well as of the prepreg resin and, therefore, may be used advanta￾geously by both the prepregger and composite manufacturer (References 5.2(b) - (f)). Thermal analytical techniques, such as thermal gravimetrical analysis (TGA), differential thermal analysis (DTA), differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), dynamic mechanical analysis (DMA), and torsional braid analysis (TBA) are not strictly chemical analysis techniques; however, they provide useful information relating to the composition and processability of resins (Reference 5.2(g)). Similarly, rheological and dielectric techniques are used frequently to evaluate the chemoviscosity properties of