Cure Kinetic Model, Heat of Reaction, and Glass Transition Temperature of AS4/3501-6 Graphite–Epoxy Prepregs

Abstract

A new isothermally-based, cure kinetic model for the carbon graphite–epoxy AS4/3501-6 prepreg is presented using an industrially supplied prepreg rather than the neat epoxy resin. A two-stage model is used: first, rate-controlled (autocatalytic); then diffusion-controlled. A Differential Scanning Calorimeter (DSC) is employed to investigate the prepreg’s cure kinetics using both isothermal and dynamic DSC scans, although only isothermal DSC scan data is used to fit the model’s parameters. A Thermogravimetric Analyzer (TGA) is used to determine the mass fraction of the epoxy resin in the prepreg. The coefficients are provided for a prepreg having 37.4% resin mass fraction; they must be re-evaluated for prepregs having significantly different resin mass fractions. The ultimate heat of reaction of the resin incorporated within the prepreg is measured as 433.7 J/g (based upon a linear baseline) or 422.7 J/g (based upon a specific-heat-corrected baseline) using dynamic DSC scans at 20°C/min. This value is 8–17% less than the reported 472–508 J/g for the neat 3501-6 epoxy also measured using dynamic DSC scans at 20°C/min. Important to manufacturing applications, dynamic DSC scans reveal that the prepreg’s ultimate heat of reaction is sensitive to scan rate. Finally, the prepreg’s carbon fiber and/or sizing appears to suppress the curing reaction and decrease the ultimate heat of reaction over that for the neat epoxy resin. The glass transition temperature Tg (°C) of partially-cured AS4/3501-6 prepregs is measured using DSC and is expressed as a function of degree of cure: Tg = 22.9 exp(2.16α) – markedly similar to 3501-6 neat resin Tg data measured earlier using a dynamic mechanical analyzer. The small specimen DSC thermal scans provide an effective means to measure the Tg of completely- or partially-cured prepregs. The Tg of the 100%-cured prepreg was experimentally determined to be 199°C, slightly higher than the 193°C reported for the 100%-cured neat epoxy resin. The AS4 fibers play little or no role in the degree-of-cure dependence of the AS4/3501-6 prepreg’s Tg. Recommended for general purpose structural applications, the AS4/3501-6 prepreg is composed of continuous, PAN-based, AS4C carbon (graphite) fibers and an amine-cured epoxy resin system. The epoxy resin, 3501-6, is a tetraglycidyl diamino diphenyl methane (TGDDM) with diaminodiphenylsulfone (DDS) catalyzed with boron trifluoride monoethylamine. 3501-6 was developed to operate in atemperature environment of up to 350°F (177°C).

Direct Resistive Heating Simulation Tool for the Repair of Aerospace Structures through Composite Patches

“…The resulting heat transfer conduction equation is shown in (9), with the thermal inertia matrix expression provided in (10), the thermal stiffness matrix in (11), and the nodal flow vector in (12).…”

“…Resistive heating curing experimental work in aerospace applications with a very simple heat transfer model is presented by Rider, Wang, and Cao [7] and the prediction and measurement of electrical conductivity for carbon unidirectional layers are reported by Athanasopoulos and Kostopoulos [8]. The study of the cure kinetics for aerospace employed resins has been reported by Kim, Moon, and Howell [9] and Shin and Hahn [10]. They provide, through Differential Scanning Calorimetry (DSC) experimental work, values for the cure kinetics equations.…”

Thermo-oxidative degradation of graphite/epoxy composite laminates: Modeling and long-term predictions

“…The Polyacrylonitrile-based (PAN-based) continuous AS4 fibers have a carbon content of 94% [14]. The 3501-6 is a Bisphenol A Diglycidyl Ether (DGEBA) type epoxy resin that has been frequently used in aerospace industry [15].…”

“…The detailed composition of 3501-6 has been analyzed by Hou and Bai [16]. The composite material system is expected to be used at temperatures up to the recommended cure temperature of 177°C (350°F) [14]. The laminates in this study were cured in an autoclave using a heat rate of 5°C/min followed by a 3 hours hold at 177°C (350°F) at 206.8 kPa (30 psi) pressure.…”

Influence of Curing Process Interrupt on Ilss of Thin Carbon Epoxy Laminates

“…Even the part looks cured visually the process interruption at critical stages of the cure cycle affects the molecular network formation and the polymerization reaction. Springer et.al [1][2] made attempts to model the Heat of Reaction, Degree of Cure, and Viscosity of Hercules 3501-6 Resin, Mantel et.al [3] explained Cure Kinetics and Rheology Models for ICI Fiberite 977-3 and 977-2 Thermosetting Resins, Shamku et.al [4] explained about the rheological studies with the cure temperature for EPON 862/W Epoxy Used in Pultrusion, similarly Hubert and Kim et.al [5][6] explained the cure kinetics and viscosity model of the Hexcel 8552 and 3501-6 materials, these literatures show behavior of resin viscosity with the cure temperature and time. J.M Khenny and Gordana Bogoeva et.al, [7][8] explained the method for evaluating the cure kinetics for the epoxy resins.…”